Trichothiodystrophy

Mendelian MONDO:0018053 Pathograph 74 Show in embeddings browser ectodermal dysplasia syndrome hereditary disease

Trichothiodystrophy (TTD) is a genetically heterogeneous multisystem developmental disorder defined by short, brittle, sulfur-deficient hair that commonly shows alternating light and dark "tiger-tail" banding under polarized microscopy. The clinical spectrum ranges from hair-only disease to combinations of ichthyosis, growth restriction, neurodevelopmental impairment, ocular abnormalities, recurrent infection, and early mortality. Photosensitive TTD is caused by biallelic variants in the TFIIH-subunit genes ERCC2, ERCC3, or GTF2H5; impaired nucleotide-excision repair is restricted to this branch and causes UV sensitivity. Non-photosensitive TTD is caused by biallelic variants in GTF2E2, MPLKIP, CARS1, TARS1, AARS1, MARS1, or DBR1, or by hemizygous RNF113A variants. These genes affect transcription, pre-mRNA processing, or tRNA charging, but the tissue-specific intermediates that make these distinct defects converge on the shared hair and multisystem phenotype remain incompletely resolved. Management is supportive and subtype-aware; rigorous photoprotection applies to photosensitive TTD, while immunologic and hematologic surveillance is important across the clinically variable spectrum.

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1
Mappings
2
Inheritance
26
Pathophys.
27
Phenotypes
3
Gaps
74
Pathograph
11
Genes
5
Medical Actions
13
Subtypes
5
Differentials
2
Datasets
6
Models
40
References
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Deep Research
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Mappings

MONDO
MONDO:0018053 trichothiodystrophy
skos:exactMatch MONDO
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Inheritance

2
Autosomal recessive HP:0000007
Most molecularly defined TTD branches are autosomal recessive. The gene-specific records below provide the individual causal and allelic evidence; RNF113A-related TTD is the X-linked exception.
Autosomal recessive inheritance
Show evidence (2 references)
PMID:18603627 SUPPORT Other
"Trichothiodystrophy (TTD) is a rare, autosomal recessive disease, characterised by brittle, sulfur deficient hair and multisystem abnormalities."
Establishes the predominant recessive inheritance pattern; the later RNF113A discovery created a rare X-linked exception.
PMID:40683339 SUPPORT Human Clinical
"We report 5 nonphotosensitive adult patients from 3 unrelated families with a homozygous missense variation in DBR1 (p.D262Y)"
Documents homozygous DBR1-associated TTD in three families.
X-linked recessive HP:0001419
RNF113A-related TTD is X-linked recessive. Reported affected males carry hemizygous RNF113A variants; female relatives may be carriers or mosaic.
X-linked recessive inheritance
Show evidence (3 references)
PMID:25612912 SUPPORT Human Clinical
"The association of RNF113A mutation with non-photosensitive TTD identifies a new locus for these disorders on the X chromosome."
The founding family establishes an X-linked RNF113A-associated TTD locus.
PMID:41531333 SUPPORT Human Clinical
"Though the majority of TTDs are inherited in an autosomal recessive pattern, RNF113A related trichothiodystrophy is X-linked."
A current two-brother report confirms the X-linked exception and its counseling relevance.
PMID:41684880 SUPPORT Human Clinical
"Whole-exome sequencing identified a maternally inherited variant consistent with X-linked recessive inheritance."
Directly supports the X-linked recessive pattern in hemizygous affected twins.
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Subtypes

13
clinical and cellular UV-sensitivity subtype
Photosensitive trichothiodystrophy MONDO:0002470
ERCC2 hgnc:3434 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in ERCC2 (hgnc:3434). hgnc:3434 is a gene from the HUGO Gene Nomenclature Committee. ERCC3 hgnc:3435 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in ERCC3 (hgnc:3435). hgnc:3435 is a gene from the HUGO Gene Nomenclature Committee. GTF2H5 hgnc:21157 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in GTF2H5 (hgnc:21157). hgnc:21157 is a gene from the HUGO Gene Nomenclature Committee.
TTD caused by biallelic ERCC2, ERCC3, or GTF2H5 variants, with TFIIH dysfunction, defective nucleotide-excision repair, cellular UV sensitivity, and clinical photosensitivity. Photosensitivity is not a property of every TTD genetic branch.
  • TTD1 (ERCC2)
  • TTD2 (ERCC3)
  • TTD3 (GTF2H5)
Show evidence (3 references)
PMID:9238033 SUPPORT Human Clinical
"Mutations in the XPD gene can result in three distinct clinical phenotypes, XP, trichothiodystrophy (TTD), and XP with Cockayne syndrome."
Establishes ERCC2/XPD as an allelic cause of TTD.
PMID:9012405 SUPPORT Human Clinical
"The causative mutation was found to be a single base substitution resulting in a missense mutation (T119P) in a region of the XPB protein completely conserved in yeast, Drosophila, mouse, and man."
Establishes a causal ERCC3/XPB variant in photosensitive TTD patients.
PMID:15220921 SUPPORT Human Clinical
"Microinjection of cDNA encoding TFB5 (GTF2H5, also called TTDA) corrected the DNA-repair defect of TTD-A cells, and we identified three functional inactivating mutations in this gene in three unrelated families with TTD-A."
Establishes GTF2H5/TTDA as the TTD-A causal gene with cellular rescue.
Non-photosensitive trichothiodystrophy
GTF2E2 hgnc:4651 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in GTF2E2 (hgnc:4651). hgnc:4651 is a gene from the HUGO Gene Nomenclature Committee. MPLKIP hgnc:16002 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in MPLKIP (hgnc:16002). hgnc:16002 is a gene from the HUGO Gene Nomenclature Committee. RNF113A hgnc:12974 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in RNF113A (hgnc:12974). hgnc:12974 is a gene from the HUGO Gene Nomenclature Committee. CARS1 hgnc:1493 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in CARS1 (hgnc:1493). hgnc:1493 is a gene from the HUGO Gene Nomenclature Committee. TARS1 hgnc:11572 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in TARS1 (hgnc:11572). hgnc:11572 is a gene from the HUGO Gene Nomenclature Committee. AARS1 hgnc:20 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in AARS1 (hgnc:20). hgnc:20 is a gene from the HUGO Gene Nomenclature Committee. MARS1 hgnc:6898 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in MARS1 (hgnc:6898). hgnc:6898 is a gene from the HUGO Gene Nomenclature Committee. DBR1 hgnc:15594 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in DBR1 (hgnc:15594). hgnc:15594 is a gene from the HUGO Gene Nomenclature Committee.
TTD caused by GTF2E2, MPLKIP, RNF113A, CARS1, TARS1, AARS1, MARS1, or DBR1 variants. These forms retain normal post-UV survival or nucleotide-excision repair in the tested cells and affect transcription, RNA processing, or translation rather than TFIIH-dependent UV-lesion repair.
  • TTD4 (MPLKIP)
  • TTD5 (RNF113A)
  • TTD6 (GTF2E2)
  • TTD7 (TARS1)
  • TTD8 (AARS1)
  • TTD9 (MARS1)
  • CARS1-Related MDBH
  • DBR1-Related Sabinas TTD
Show evidence (4 references)
PMID:40737808 SUPPORT Other
"The presence of hypersensitivity to UV light defines the two main forms of TTD: photosensitive (PS-TTD) and non-photosensitive (NPS-TTD)."
The current review defines the clinical PS/NPS division while emphasizing genetic and mechanistic heterogeneity.
PMID:37800682 SUPPORT Other
"We previously proposed (Theil et al, 2019) that most of the key TTD features of brittle hair, nails, and scaly skin are a consequence of protein instability of factors involved in different steps of gene expression in either transcription (mutated ERCC2, ERCC3, GTF2H5, or GTF2E2), splicing..."
Contemporary primary-literature context places GTF2E2, RNF113A, CARS1, TARS1, AARS1, and MARS1 within the TTD spectrum; individual genetic items retain their gene-specific evidence.
PMID:15645389 SUPPORT Human Clinical
"We have identified C7orf11, which localizes to the nucleus and is expressed in fetal hair follicles, as the first disease gene for nonphotosensitive trichothiodystrophy (TTD)."
Establishes MPLKIP/TTDN1 as a non-photosensitive TTD cause.
+ 1 more reference
gene-defined subtype
Trichothiodystrophy 1, photosensitive MONDO:0011125
ERCC2 hgnc:3434 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in ERCC2 (hgnc:3434). hgnc:3434 is a gene from the HUGO Gene Nomenclature Committee.
Photosensitive TTD caused by biallelic TTD-causing ERCC2 variants.
Trichothiodystrophy 2, photosensitive MONDO:0014615
ERCC3 hgnc:3435 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in ERCC3 (hgnc:3435). hgnc:3435 is a gene from the HUGO Gene Nomenclature Committee.
Photosensitive TTD caused by biallelic TTD-causing ERCC3 variants.
Trichothiodystrophy 3, photosensitive MONDO:0014619
GTF2H5 hgnc:21157 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in GTF2H5 (hgnc:21157). hgnc:21157 is a gene from the HUGO Gene Nomenclature Committee.
Photosensitive TTD caused by biallelic TTD-causing GTF2H5 variants.
Trichothiodystrophy 4, nonphotosensitive MONDO:0021013
MPLKIP hgnc:16002 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in MPLKIP (hgnc:16002). hgnc:16002 is a gene from the HUGO Gene Nomenclature Committee.
Non-photosensitive TTD caused by biallelic TTD-causing MPLKIP variants.
Trichothiodystrophy 5, nonphotosensitive MONDO:0010495
RNF113A hgnc:12974 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in RNF113A (hgnc:12974). hgnc:12974 is a gene from the HUGO Gene Nomenclature Committee.
X-linked recessive non-photosensitive TTD caused by hemizygous RNF113A variants.
Trichothiodystrophy 6, nonphotosensitive MONDO:0014841
GTF2E2 hgnc:4651 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in GTF2E2 (hgnc:4651). hgnc:4651 is a gene from the HUGO Gene Nomenclature Committee.
Non-photosensitive TTD caused by biallelic TTD-causing GTF2E2 variants.
Trichothiodystrophy 7, nonphotosensitive MONDO:0032806
TARS1 hgnc:11572 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in TARS1 (hgnc:11572). hgnc:11572 is a gene from the HUGO Gene Nomenclature Committee.
Non-photosensitive TTD caused by biallelic TTD-causing TARS1 variants.
Trichothiodystrophy 8, nonphotosensitive MONDO:0030517
AARS1 hgnc:20 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in AARS1 (hgnc:20). hgnc:20 is a gene from the HUGO Gene Nomenclature Committee.
Non-photosensitive TTD caused by biallelic TTD-causing AARS1 variants.
Trichothiodystrophy 9, nonphotosensitive MONDO:0030518
MARS1 hgnc:6898 HUGO Gene Nomenclature Committee (hgnc) Relation: this subtype is caused by variation in this gene This subtype is caused by variation in MARS1 (hgnc:6898). hgnc:6898 is a gene from the HUGO Gene Nomenclature Committee.
Non-photosensitive TTD caused by biallelic TTD-causing MARS1 variants.
contemporary TTD-spectrum inclusion
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Discussions and Knowledge Gaps

3
Do TFIIH/TFIIE transcription defects, MPLKIP-DBR1 or RNF113A RNA-processing defects, and aminoacyl-tRNA-synthetase translation defects converge on a shared tissue-specific ribosome/proteostasis mechanism, or do they reach the common hair and developmental phenotype through distinct pathways?
KNOWLEDGE GAP OPEN pan_ttd_tissue_convergence_gap
Attached to
pathophysiology#TFIIH Complex Instability in Photosensitive TTD pathophysiology#GTF2E2-Related TFIIE Instability pathophysiology#MPLKIP Deficiency pathophysiology#DBR1 Deficiency in Sabinas TTD pathophysiology#Cytosolic Aminoacyl-tRNA Synthetase Loss of Function pathophysiology#Reduced Cognate tRNA Charging pathophysiology#RNF113A Deficiency pathophysiology#Unresolved Tissue-Specific Gene-Expression Convergence
The causal genes occupy distinct transcription, RNA-processing, and translation systems. Perturbation studies postulate ribosomal dysfunction as a common mechanism, but no genotype-spanning human model has shown a single downstream state that is necessary for sulfur-poor hair or the multisystem phenotype.
Proposed experiments
Isogenic cross-genotype human hair-lineage multi-omics comparison
cross-genotype iPSC lineage multi-omics experiment Relation: this experiment is of type this experiment type This experiment is of type cross-genotype iPSC lineage multi-omics experiment.
exp_ttd_cross_genotype_hair_lineage_multiomics
Generate matched human iPSC hair-follicle or keratinocyte-lineage models carrying representative ERCC2, GTF2E2, MPLKIP, RNF113A, AARS1, and DBR1 variants, then compare nascent transcription, splicing, ribosome profiling, proteostasis, sulfur-rich matrix-protein abundance, and shaft-like material assembly with gene-corrected controls.
Model systems
Isogenic human iPSC-derived hair-lineage models
Gene-edited and corrected human lines differentiated toward the relevant hair-matrix or keratinocyte lineage.
IPSC DERIVED MODEL
Readouts
Shared versus branch-specific gene-expression defects
pathophysiology#Unresolved Tissue-Specific Gene-Expression Convergence
Direction: ALTERED
Sulfur-rich matrix-protein abundance and organization
pathophysiology#Deficiency of Sulfur-Rich Hair Matrix Proteins
Direction: DECREASED
Decision criterion
A reproducible corrected-by-isogenic-rescue molecular state shared across causal branches and preceding matrix-protein loss would support a common convergence mechanism; branch-specific states would refute a single route.
Would support
pathophysiology#Unresolved Tissue-Specific Gene-Expression Convergence
Would refute
pathophysiology#Unresolved Tissue-Specific Gene-Expression Convergence
Show evidence (1 reference)
PMID:37508541 SUPPORT In Vitro
"As the loss of proteostasis by the ribosome has been identified in the other forms of TTD, here we postulate that ribosomal dysfunction is a common underlying pathomechanism of TTD."
The explicit postulate motivates, but does not resolve, the cross-genotype convergence question.
Does thyroid-receptor target-gene deregulation in Xpd-mutant mouse brain faithfully model the causal route to human ERCC2-related hypomyelination, or is it a species-, developmental-stage-, or tissue-composition-specific response to limiting TFIIH?
HUMAN MODEL MISMATCH OPEN xpd_mouse_human_brain_fidelity
Attached to
pathophysiology#Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain phenotypes#Cerebral Hypomyelination animal_models#Xpd-mutant TTD mouse
The mouse study directly measures TFIIH-dependent thyroid-receptor occupancy and brain target-gene deregulation, while the human evidence establishes hypomyelination but does not measure this route. Species differences, mixed brain tissue, and developmental timing therefore limit translation of the model mechanism to human oligodendrocyte-lineage biology.
Proposed experiments
ERCC2-TTD human oligodendrocyte-lineage TFIIH and thyroid-response study
patient-derived iPSC differentiation and rescue study Relation: this experiment is of type this experiment type This experiment is of type patient-derived iPSC differentiation and rescue study.
exp_ttd_human_oligodendrocyte_tr_fidelity
Compare patient-derived and isogenic-corrected human iPSC oligodendrocyte lineage cells across developmental stages, measuring TFIIH abundance, thyroid-receptor chromatin occupancy, thyroid-responsive transcription, maturation, and myelin-protein production with and without physiologic thyroid-hormone stimulation.
Model systems
ERCC2-TTD human iPSC oligodendrocyte-lineage model
Patient-derived and gene-corrected human cells spanning progenitor to mature oligodendrocyte stages.
IPSC DERIVED MODEL
human NCBITaxon:9606 NCBI Taxonomy (NCBITaxon) Relation: this experimental model is built in this organism This experimental model is built in human, annotated with Homo sapiens (NCBITaxon:9606). NCBITaxon:9606 is an organism from the NCBI Taxonomy.
oligodendrocyte CL:0000128 Cell Ontology (CL) Relation: this experimental model uses this cell type This experimental model uses oligodendrocyte (CL:0000128). CL:0000128 is a cell type from the Cell Ontology.
Readouts
Thyroid-receptor occupancy and target-gene response
pathophysiology#Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain
Direction: ALTERED
Oligodendrocyte maturation and myelin-protein production
phenotypes#Cerebral Hypomyelination
Direction: DECREASED
Decision criterion
Reproduction of the mouse-derived occupancy/transcription defect in ERCC2-TTD human cells, with correction by isogenic repair and concordant maturation or myelin-protein rescue, would support translational fidelity.
Would support
pathophysiology#Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain
Would refute
pathophysiology#Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain
Show evidence (1 reference)
PMID:17952069 SUPPORT Model Organism
"Molecular analyses performed on the mice brain tissue demonstrate that TFIIH is required for the stabilization of thyroid hormone receptors (TR) to their DNA-responsive elements."
Establishes the existing mouse evidence whose human fidelity is at issue.
Which RNF113A-dependent function is causally relevant to X-linked TTD: spliceosome regulation, the ASCC alkylation-damage response, both, or an unmeasured developmental function?
KNOWLEDGE GAP OPEN rnf113a_dual_function_gap
Attached to
pathophysiology#RNF113A Deficiency pathophysiology#Defective RNF113A-Dependent ASCC Alkylation Response pathophysiology#Candidate RNF113A-Dependent Splicing Dysfunction
The ASCC defect is measured in patient cells, whereas the splicing function is supported in lung-cancer cells. Neither experiment establishes which function is necessary or sufficient for the human hair and developmental phenotype.
Proposed experiments
RNF113A separation-of-function rescue in patient-derived lineages
separation-of-function cellular rescue experiment Relation: this experiment is of type this experiment type This experiment is of type separation-of-function cellular rescue experiment.
exp_rnf113a_function_separation_rescue
Introduce RNF113A constructs that selectively restore spliceosome or ASCC activity into patient-derived keratinocyte and neural-lineage cells, then compare splicing, alkylation-response, differentiation, and sulfur-rich matrix-protein readouts with wild-type and empty-vector controls.
Model systems
RNF113A-TTD patient-derived keratinocyte and neural-lineage cells
Patient-derived human lineages with orthogonal rescue of RNF113A functions.
IPSC DERIVED MODEL
Readouts
ASCC recruitment after alkylation challenge
pathophysiology#Defective RNF113A-Dependent ASCC Alkylation Response
Direction: RESTORED
RNF113A-dependent splicing profile
pathophysiology#Candidate RNF113A-Dependent Splicing Dysfunction
Direction: RESTORED
Decision criterion
Selective rescue of developmental and hair-lineage readouts by one function-specific construct would prioritize that branch; rescue only by full-length RNF113A would support combined or additional functions.
Would support
pathophysiology#Defective RNF113A-Dependent ASCC Alkylation Response pathophysiology#Candidate RNF113A-Dependent Splicing Dysfunction
Show evidence (2 references)
PMID:29144457 SUPPORT In Vitro
"Cells from patients with X-linked trichothiodystrophy, which harbour a mutation in RNF113A, are defective in ASCC foci formation and are hypersensitive to alkylating agents."
Establishes one measured patient-cell function without proving clinical causality.
PMID:32152280 SUPPORT In Vitro
"RNF113A is a RNA-binding protein which regulates the splicing of multiple candidates involved in cell survival."
Establishes the alternative function in a low-fidelity cancer-cell context.
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Pathophysiology

26
TFIIH Complex Instability in Photosensitive TTD
TTD-causing ERCC2, ERCC3, and GTF2H5 variants reduce the cellular abundance and/or structural integrity of the ten-subunit TFIIH complex. TFIIH serves both nucleotide-excision repair and RNA polymerase II transcription, so this branch must be separated into repair and transcription consequences rather than treated as a syndrome-wide lesion.
ERCC2 hgnc:3434 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves ERCC2 (hgnc:3434). hgnc:3434 is a gene from the HUGO Gene Nomenclature Committee. ERCC3 hgnc:3435 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves ERCC3 (hgnc:3435). hgnc:3435 is a gene from the HUGO Gene Nomenclature Committee. GTF2H5 hgnc:21157 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves GTF2H5 (hgnc:21157). hgnc:21157 is a gene from the HUGO Gene Nomenclature Committee.
Downstream
Defective Nucleotide-Excision Repair after UV Damage Reduced or dysfunctional TFIIH directly impairs its nucleotide-excision repair function in photosensitive TTD.
Basal Transcription Insufficiency in ERCC2-Related TTD Reduced ERCC2-containing TFIIH directly impairs basal transcription in the studied ERCC2-related TTD cells; equivalent evidence is not projected to every TFIIH gene branch.
Unprocessed R-Loop Accumulation in ERCC2/ERCC3-Related TTD R-loop accumulation has been directly measured in ERCC2- and ERCC3-related photosensitive TTD cells, but not yet in GTF2H5-related cells.
Show evidence (1 reference)
PMID:12393803 SUPPORT In Vitro
"Here we show that all the mutations found in TTD cases, irrespective of whether they are homozygotes, hemizygotes or compound heterozygotes, cause a substantial and specific reduction (by up to 70%) in the cellular concentration of TFIIH."
Patient-cell measurements directly establish reduced cellular TFIIH.
Defective Nucleotide-Excision Repair after UV Damage
In TFIIH-related photosensitive TTD, impaired removal of UV-induced bulky DNA lesions produces cellular UV hypersensitivity and cutaneous photosensitivity. This node does not apply to the non-photosensitive gene branches, which retain normal UV repair in tested cells.
skin fibroblast CL:0002620 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves skin fibroblast (CL:0002620). CL:0002620 is a cell type from the Cell Ontology.
nucleotide-excision repair GO:0006289 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased nucleotide-excision repair (GO:0006289). GO:0006289 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Cutaneous Photosensitivity Failure to repair UV photolesions produces photosensitivity in the TFIIH-related subtype.
Show evidence (2 references)
PMID:12820975 SUPPORT In Vitro
"We demonstrate that all the XPD mutations are detrimental for XPD helicase activity, thus explaining the NER defect."
Establishes defective XPD helicase activity as the repair lesion in the studied ERCC2-related cells.
PMID:18470933 SUPPORT In Vitro
"In TTD cells with defects in the XPD, XPB, or GTF2H5 (TTDA) genes, in contrast, recruitment of these NER proteins was reduced compared to normals at early time points (p<0.001) and remained low at 24 hr postirradiation."
Patient-cell evidence establishes abnormal NER-protein recruitment for ERCC2-, ERCC3-, and GTF2H5-related TTD.
MPLKIP Deficiency
Biallelic MPLKIP variants reduce or eliminate functional TTDN1/MPLKIP. The resulting disease branch is non-photosensitive and is kept distinct from primary DBR1 variation so that MPLKIP-specific skin-model findings are not silently projected onto DBR1-related Sabinas syndrome.
MPLKIP hgnc:16002 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves MPLKIP (hgnc:16002). hgnc:16002 is a gene from the HUGO Gene Nomenclature Committee.
Downstream
Reduced DBR1 Abundance MPLKIP directly interacts with DBR1, and MPLKIP-deficient primary fibroblasts have reduced steady-state DBR1.
Intron-Lariat Accumulation after MPLKIP Loss TTDN1/MPLKIP loss directly causes intron-lariat accumulation in cellular models and patient fibroblasts.
Aberrant RNA Splicing after MPLKIP Loss TTDN1/MPLKIP loss directly produces altered RNA-splicing patterns in knockout cells and patient fibroblasts.
Seizure Seizure disorders were overrepresented in a five-patient MPLKIP cohort, but no causal route from MPLKIP loss to seizure susceptibility was shown.
Delayed Skeletal Maturation Delayed bone age was overrepresented in the MPLKIP cohort, but the developmental mechanism was not established.
Hypogonadism Hypogonadism was observed in a seven-person MPLKIP-associated NPS-TTD cohort, but the study did not establish a causal route from MPLKIP loss to gonadal dysfunction.
Show evidence (1 reference)
PMID:37800682 SUPPORT Human Clinical
"Here, we present seven additional TTD individuals with MPLKIP mutations from five consanguineous families, with a newly identified MPLKIP variant in one family."
The multi-family patient series establishes the MPLKIP-related branch.
DBR1 Deficiency in Sabinas TTD
Homozygous DBR1 p.Asp262Tyr reduces DBR1 RNA and protein in the non-photosensitive Sabinas brittle-hair form. This genetic root is separated from MPLKIP deficiency because downstream human-skin-equivalent evidence has not been demonstrated for DBR1-associated patients.
DBR1 hgnc:15594 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves DBR1 (hgnc:15594). hgnc:15594 is a gene from the HUGO Gene Nomenclature Committee.
Downstream
Reduced DBR1 Abundance Patient cells carrying DBR1 p.Asp262Tyr have reduced DBR1 RNA and protein.
Show evidence (1 reference)
PMID:40683339 SUPPORT Human Clinical
"We report 5 nonphotosensitive adult patients from 3 unrelated families with a homozygous missense variation in DBR1 (p.D262Y) encoding the RNA lariat-debranching enzyme DBR1, which is involved in the removal of introns from pre-mRNA in the nucleus."
Establishes the DBR1-associated Sabinas TTD genetic root.
Intron-Lariat Accumulation after MPLKIP Loss
Loss of TTDN1/MPLKIP causes accumulation of intron lariats. This measured state is restricted to the MPLKIP experimental branch because equivalent lariat measurements have not been reported for DBR1 p.Asp262Tyr patient cells.
MPLKIP hgnc:16002 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves MPLKIP (hgnc:16002). hgnc:16002 is a gene from the HUGO Gene Nomenclature Committee.
Animal models
Ttdn1-null mouse Mus musculus recapitulates Cortex RNA-seq reproduces intron-lariat accumulation after loss of Ttdn1.
Downstream
Aberrant RNA Splicing after MPLKIP Loss The study supports a model in which accumulated lariats disrupt subsequent splicing cycles, but whether lariat accumulation itself is causal remains unresolved.
Show evidence (1 reference)
PMID:37369199 SUPPORT In Vitro
"TTDN1 loss, or a mutated IDR, causes significant intron lariat accumulation, as well as splicing and gene expression defects, mirroring phenotypes observed in NP-TTD patient cells."
Directly establishes intron-lariat accumulation after TTDN1/MPLKIP loss.
Aberrant RNA Splicing after MPLKIP Loss
Loss of TTDN1/MPLKIP produces altered RNA-splicing patterns in knockout cells and patient fibroblasts. This node records the measured splicing state separately from upstream lariat accumulation and downstream expression or differentiation effects.
MPLKIP hgnc:16002 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves MPLKIP (hgnc:16002). hgnc:16002 is a gene from the HUGO Gene Nomenclature Committee.
RNA splicing GO:0008380 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal RNA splicing (GO:0008380). GO:0008380 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Animal models
Ttdn1-null mouse Mus musculus recapitulates Cortex RNA-seq reproduces alternative-splicing changes after loss of Ttdn1.
Downstream
Impaired Keratinocyte Differentiation In MPLKIP-deficient human skin equivalents, compromised splicing is associated with an imbalanced proteome and impaired differentiation.
Unresolved Tissue-Specific Gene-Expression Convergence The measured MPLKIP cellular defect does not establish a common pathway shared by every TTD genetic branch.
Show evidence (1 reference)
PMID:37369199 SUPPORT In Vitro
"Notably, these patient fibroblasts had similar altered splicing patterns seen in our TTDN1 KO cell lines (Figure 3i and Supplemental Figure S3h, S3j), suggesting a similar defect in mRNA processing."
Directly establishes altered splicing in both patient fibroblasts and TTDN1-knockout cells.
Reduced DBR1 Abundance
MPLKIP deficiency reduces steady-state DBR1 protein, while primary DBR1 p.Asp262Tyr variation reduces DBR1 RNA and protein. This shared node is limited to measured abundance and does not assume that DBR1-associated cells have the lariat or splicing abnormalities measured after MPLKIP loss.
Downstream
Unresolved Tissue-Specific Gene-Expression Convergence RNA-processing defects plausibly feed the shared phenotype, but the cross-tissue and cross-genotype intermediates are unresolved.
Show evidence (2 references)
PMID:37800682 SUPPORT In Vitro
"By mass spectrometry-based interaction proteomics, we demonstrate that MPLKIP interacts with core splicing factors and the lariat debranching protein DBR1. MPLKIP-deficient primary fibroblasts have reduced steady-state DBR1 protein levels."
Directly establishes the MPLKIP-DBR1 interaction and reduced DBR1 in MPLKIP-deficient patient fibroblasts.
PMID:40683339 SUPPORT In Vitro
"There were reduced levels of DBR1 mRNA and protein. Interacting TTDN1 protein in cells from patients with DBR1 variations was markedly reduced."
Provides the reciprocal DBR1-variant evidence and reduced interacting TTDN1/MPLKIP.
Cytosolic Aminoacyl-tRNA Synthetase Loss of Function
Biallelic CARS1, TARS1, AARS1, or MARS1 variants reduce abundance and/or enzymatic function of their cytosolic aminoacyl-tRNA synthetases. This variant-proximal loss-of-function state is separated from its measured downstream effect on cognate tRNA charging.
CARS1 hgnc:1493 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves CARS1 (hgnc:1493). hgnc:1493 is a gene from the HUGO Gene Nomenclature Committee. TARS1 hgnc:11572 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves TARS1 (hgnc:11572). hgnc:11572 is a gene from the HUGO Gene Nomenclature Committee. AARS1 hgnc:20 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves AARS1 (hgnc:20). hgnc:20 is a gene from the HUGO Gene Nomenclature Committee. MARS1 hgnc:6898 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves MARS1 (hgnc:6898). hgnc:6898 is a gene from the HUGO Gene Nomenclature Committee.
Downstream
Reduced Cognate tRNA Charging Loss of abundance or enzymatic function of each affected synthetase directly reduces its cognate tRNA-charging activity.
Show evidence (3 references)
PMID:30824121 SUPPORT In Vitro
"Measurement of protein abundance, yeast complementation assays, and assessments of tRNA charging indicate that each CARS variant causes a loss-of-function effect."
Establishes loss of CARS1 function using protein, complementation, and charging assays.
PMID:31374204 SUPPORT In Vitro
"We showed that these variants have a profound effect on TARS protein stability and enzymatic function."
Direct functional evidence for TARS1 instability and enzymatic impairment.
PMID:33909043 SUPPORT In Vitro
"These variants result in the instability of the respective gene products alanyl- and methionyl-tRNA synthetase."
Directly supports AARS1 and MARS1 protein instability.
Reduced Cognate tRNA Charging
CARS1-, TARS1-, AARS1-, and MARS1-related TTD variants reduce cognate tRNA charging or the enzymatic function required for charging. How this measured aminoacylation lesion yields lineage-selective hair and multisystem disease remains unresolved.
CARS1 hgnc:1493 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves CARS1 (hgnc:1493). hgnc:1493 is a gene from the HUGO Gene Nomenclature Committee. TARS1 hgnc:11572 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves TARS1 (hgnc:11572). hgnc:11572 is a gene from the HUGO Gene Nomenclature Committee. AARS1 hgnc:20 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves AARS1 (hgnc:20). hgnc:20 is a gene from the HUGO Gene Nomenclature Committee. MARS1 hgnc:6898 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves MARS1 (hgnc:6898). hgnc:6898 is a gene from the HUGO Gene Nomenclature Committee.
tRNA aminoacylation for protein translation GO:0006418 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased tRNA aminoacylation for protein translation (GO:0006418). GO:0006418 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Unresolved Tissue-Specific Gene-Expression Convergence Reduced tRNA charging is upstream of the synthetase-related forms, but how it produces sulfur-poor hair and the multisystem phenotype is unresolved.
Show evidence (3 references)
PMID:30824121 SUPPORT In Vitro
"Measurement of protein abundance, yeast complementation assays, and assessments of tRNA charging indicate that each CARS variant causes a loss-of-function effect."
Directly supports impaired CARS1-dependent tRNA charging.
PMID:31374204 SUPPORT In Vitro
"We showed that these variants have a profound effect on TARS protein stability and enzymatic function."
Directly supports impaired TARS1 enzymatic function.
PMID:33909043 SUPPORT In Vitro
"Functional studies in skin fibroblasts from affected individuals demonstrate that these new variants also impact on the rate of tRNA charging, which is the first step in protein translation."
Directly supports altered tRNA-charging rate in AARS1- and MARS1-related fibroblasts.
RNF113A Deficiency
Hemizygous loss-of-function RNF113A variants cause X-linked non-photosensitive TTD. The genetic root is separated from the two reported cellular functions because their relative contribution to the human developmental phenotype is unresolved.
RNF113A hgnc:12974 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves RNF113A (hgnc:12974). hgnc:12974 is a gene from the HUGO Gene Nomenclature Committee.
Downstream
Defective RNF113A-Dependent ASCC Alkylation Response RNF113A-mutant patient cells fail to recruit the ASCC response and are hypersensitive to alkylating agents.
Candidate RNF113A-Dependent Splicing Dysfunction RNF113A regulates splicing in cancer-cell experiments, but whether the same splicing state drives human TTD is unresolved.
Show evidence (1 reference)
PMID:25612912 SUPPORT Human Clinical
"We identified a nonsense mutation in the highly conserved RNF113A gene (c.901 C>T, p.Q301*). The mutation segregated with the disease in the family and was not observed in over 100,000 control X chromosomes."
Segregation establishes the RNF113A genetic root without choosing between downstream molecular functions.
Defective RNF113A-Dependent ASCC Alkylation Response
RNF113A-mutant patient cells have defective ASCC foci formation and alkylating-agent hypersensitivity. This measured response is not assumed to be the developmental mechanism of X-linked TTD.
Show evidence (1 reference)
PMID:29144457 SUPPORT In Vitro
"Cells from patients with X-linked trichothiodystrophy, which harbour a mutation in RNF113A, are defective in ASCC foci formation and are hypersensitive to alkylating agents."
Directly supports the cellular alkylation-response abnormality.
Candidate RNF113A-Dependent Splicing Dysfunction
RNF113A regulates splicing of survival-related transcripts in lung-cancer cells. The cellular function is experimentally supported, but its operation in developing patient tissues and its contribution relative to the ASCC pathway remain unknown.
RNA splicing GO:0008380 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal RNA splicing (GO:0008380). GO:0008380 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Unresolved Tissue-Specific Gene-Expression Convergence The disease relevance of the cancer-cell splicing phenotype remains open.
Show evidence (1 reference)
PMID:32152280 SUPPORT In Vitro
"RNF113A is a RNA-binding protein which regulates the splicing of multiple candidates involved in cell survival."
Supports an RNF113A splicing function in a low-fidelity cancer-cell context.
Impaired Keratinocyte Differentiation
MPLKIP-deficient reconstructed human skin shows compromised splicing, impaired keratinocyte differentiation, and an imbalanced skin/immune proteome. This experimentally supported branch is specific to MPLKIP models; extension to other TTD genotypes is provisional.
keratinocyte CL:0000312 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves keratinocyte (CL:0000312). CL:0000312 is a cell type from the Cell Ontology.
keratinocyte differentiation GO:0030216 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased keratinocyte differentiation (GO:0030216). GO:0030216 is a biological process from the Gene Ontology. ↓ DECREASED
Experimental models
MPLKIP-deficient reconstructed human skin equivalent CO CULTURE The organotypic co-culture reproduces compromised splicing, altered proteome balance, and impaired keratinocyte differentiation after MPLKIP loss.
Downstream
Deficiency of Sulfur-Rich Hair Matrix Proteins Impaired keratinocyte differentiation plausibly contributes to the hair matrix defect, but this exact chain was not measured in the HSE model.
Ichthyosis The HSE result provides a plausible MPLKIP-specific route to ichthyosis, but the clinical link and generalization to all TTD forms remain indirect.
Show evidence (1 reference)
PMID:37800682 SUPPORT In Vitro
"Using Human Skin Equivalents (HSEs), we observed impaired keratinocyte differentiation associated with compromised splicing and eventually, an imbalanced proteome affecting skin development and, interestingly, also the immune system."
Directly supports this skin-model node in MPLKIP deficiency.
Unresolved Tissue-Specific Gene-Expression Convergence
Distinct defects in TFIIH/TFIIE transcription, MPLKIP-DBR1/RNF113A RNA processing, and aminoacyl-tRNA synthetase-dependent translation all cause the TTD phenotype, but no single experimentally established final common pathway explains their tissue selectivity. Ribosome dysfunction and loss of proteostasis have been proposed from perturbation models and are retained here only as a hypothesis, not as a canonical syndrome-wide mechanism.
Downstream
Deficiency of Sulfur-Rich Hair Matrix Proteins All causal branches converge clinically on sulfur-poor brittle hair, but the exact lineage-specific bridge remains unknown.
Global Developmental Delay The causal genes produce TTD and developmental impairment is common, but the intervening neural mechanisms are unresolved.
Microcephaly Microcephaly occurs across multiple genetic branches, but the shared route from gene-expression defects to reduced brain growth is unknown.
Short Stature Growth restriction is common, while the cross-genotype growth mechanism remains unknown.
Ocular Abnormality Ocular abnormalities are frequent, but no shared molecular route was established across the genetically heterogeneous spectrum.
Cataract Cataract was common in the NIH cohort, but no cross-genotype molecular route to lens opacity was established.
Recurrent Infections Infection susceptibility is clinically important, but heterogeneous immune and barrier abnormalities preclude a single established pathway.
Decreased Circulating Immunoglobulin Concentration Reduced immunoglobulin occurs in a clinically important subset, but its relation to the heterogeneous upstream gene-expression defects has not been established.
Decreased Total Neutrophil Count Neutropenia is associated with TTD in a subset, but no cross-genotype molecular route to impaired neutrophil abundance is established.
Premature Birth Preterm delivery is associated with affected pregnancies, but the fetal, placental, and maternal intermediates remain unresolved.
Small for Gestational Age Birth weight below the tenth centile for gestational age occurred in the neonatal cohort, but the fetal or placental mechanism is unresolved.
Cryptorchidism Cryptorchidism occurred in a substantial fraction of male infants, but its developmental mechanism in TTD was not established.
Congenital Ichthyosiform Erythroderma Neonatal erythroderma or collodion presentation is clinically associated with TTD, while its route from the heterogeneous causal branches is not established.
Sparse Scalp Hair Sparse scalp hair occurs in genetically defined TTD, but the route from the heterogeneous gene-expression defects to reduced hair density is unresolved.
Nail Dystrophy Nail dystrophy is documented in MPLKIP-related TTD, but a molecular route from the upstream lesion to the nail phenotype has not been demonstrated.
Gait Disturbance Ataxic gait occurs in at least one molecularly characterized MPLKIP-related patient, but the responsible neurologic pathway remains unresolved.
Abnormal Facial Shape Facial dysmorphism is documented in genetically defined TTD, but no shared developmental route across the heterogeneous gene branches is established.
Show evidence (2 references)
PMID:40737808 SUPPORT Other
"The disease arises from mutations in a variety of genes involved in different biological processes. Affected processes include DNA repair, transcription as well as translation."
Current review evidence establishes the distinct upstream processes.
PMID:37508541 SUPPORT In Vitro
"As the loss of proteostasis by the ribosome has been identified in the other forms of TTD, here we postulate that ribosomal dysfunction is a common underlying pathomechanism of TTD."
The authors explicitly label common ribosomal dysfunction as a postulate; the node therefore remains HYPOTHETICAL.
Deficiency of Sulfur-Rich Hair Matrix Proteins
TTD hair contains reduced cysteine/sulfur-rich matrix proteins. This compositional lesion is separated from the observed ultrastructural disorganization because the patient-hair studies do not establish whether one state causes the other.
Animal models
Ttdn1-null mouse Mus musculus partially recapitulates Ttdn1-null hair has decreased cysteic-acid content, partially reproducing the sulfur-poor component of the human hair phenotype.
Downstream
Hair Shaft Ultrastructural Disorganization Sulfur-rich-protein paucity and structural disorganization coexist in the studied hair, but whether the compositional defect causes the structural state or instead occurs in parallel is unknown.
Brittle Sulfur-Deficient Hair Impaired synthesis of high-sulfur matrix protein directly produces the characteristic brittle, sulfur-poor hair described in the diagnostic biochemical source.
Reduced Hair Sulfur Content Paucity of cysteine-rich matrix proteins directly lowers measured sulfur/cystine content in hair.
Show evidence (2 references)
PMID:15232704 SUPPORT Human Clinical
"The term "trichothiodystrophy" (TTD) covers several autosomal recessive diseases whose diagnostic hallmark is short, brittle hair low in sulfur and cystine because of impaired synthesis of high-sulfur matrix protein."
Directly supports sulfur/cystine deficiency and impaired high-sulfur matrix protein synthesis in patient hair.
PMID:35615778 SUPPORT Human Clinical
"The results are consistent with not only a paucity of high sulphur components, such as keratin-associated proteins, but also a profound imbalance in protein content and organization."
Patient-hair analysis independently supports paucity of high-sulfur keratin-associated proteins.
Hair Shaft Ultrastructural Disorganization
Electron microscopy of hair from three ERCC2-related TTD patients shows abnormal cuticle marginal bands and exocuticle layers, heterogeneous cortex staining, and shortened keratin macrofibrils. Extension of this exact ultrastructural pattern to every TTD genotype remains untested.
Downstream
Tiger-Tail Hair Banding Ultrastructural heterogeneity and tiger-tail banding are observed in TTD hair, but the structural generator of the alternating optical bands has not been established.
Show evidence (1 reference)
PMID:35615778 SUPPORT Human Clinical
"Compared with hair shafts of normal phenotype, TTD cuticle cells displayed aberrant marginal bands and exocuticle layers. Clusters of cells stained differently (light versus dark) in the cortex of aberrant shafts, and the keratin macrofibrils appeared much shorter in the cytoplasm."
Directly establishes the atomic ultrastructural state in the examined ERCC2-related patient hair.
Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain
In an Xpd TTD mouse, limiting TFIIH destabilizes thyroid-hormone receptor occupancy and deregulates thyroid-hormone target genes in brain. This is a plausible ERCC2-related route to dysmyelination, but it has not been directly established in developing human brain or generalized across TTD.
ERCC2 hgnc:3434 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves ERCC2 (hgnc:3434). hgnc:3434 is a gene from the HUGO Gene Nomenclature Committee.
Animal models
Xpd-mutant TTD mouse Mus musculus partially recapitulates The model reproduces brain TFIIH-dependent thyroid-receptor destabilization and target-gene dysregulation.
Downstream
Cerebral Hypomyelination The model suggests a route to hypomyelination, but the human developmental causal chain remains incomplete.
Show evidence (1 reference)
PMID:17952069 SUPPORT Model Organism
"Molecular analyses performed on the mice brain tissue demonstrate that TFIIH is required for the stabilization of thyroid hormone receptors (TR) to their DNA-responsive elements."
Supports the mouse coactivator mechanism while not proving the human route.
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Trichothiodystrophy Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

27
Blood 2
Decreased Circulating Immunoglobulin Concentration HP:0004313 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Decreased circulating immunoglobulin concentration (HP:0004313). HP:0004313 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30334570 SUPPORT Human Clinical
"Thirteen patients had an IgG level below the age-adjusted lower limit of normal. Twelve of these patients had a mutation in ERCC2."
Directly quantifies reduced age-adjusted IgG in the cohort and its strong ERCC2 enrichment.
Decreased Total Neutrophil Count HP:0001875 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Decreased total neutrophil count (HP:0001875). HP:0001875 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30334570 SUPPORT Human Clinical
"Seventeen patients had an ANC below 1.0 × 109/l, of which 13 had ERCC2 mutations."
Directly documents decreased absolute neutrophil counts in the cohort.
Endocrine 1
Hypogonadism HP:0000135 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypogonadism (HP:0000135). HP:0000135 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:37800682 SUPPORT Human Clinical
"We investigated a cohort of seven patients from five unrelated families with characteristic features of TTD (Figs 1A–F and 2A and B), including short, woolly, slow‐growing, sparse and brittle hair, sparse and brittle eyebrows and eyelashes, ectodermal abnormalities comprising keratosis pilaris,..."
Directly documents hypogonadism in the seven-person MPLKIP-associated cohort without assigning an umbrella-level frequency.
Eye 1
Cataract FREQUENT HP:0000518 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cataract (HP:0000518). HP:0000518 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:21800331 SUPPORT Human Clinical
"Cataracts were reported in 14 (54%) of the TTD patients when examined at NIH at age 2 to 29 (mean 9.4years) and many were not visually significant"
Supplies the examined-cohort denominator, frequency, age range, and severity caveat.
Genitourinary 1
Cryptorchidism FREQUENT HP:0000028 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cryptorchidism (HP:0000028). HP:0000028 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:21800331 SUPPORT Human Clinical
"Cryptorchidism was reported in 5 (31%) of the male infants."
Preserves the sex-specific cohort denominator and frequency.
Head and Neck 3
Sparse Scalp Hair HP:0002209 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Sparse scalp hair (HP:0002209). HP:0002209 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:25612912 SUPPORT Human Clinical
"We describe two male cousins affected by TTD associated with microcephaly, profound intellectual disability, sparse brittle hair, aged appearance, short stature, facial dysmorphism, seizures, an immunoglobulin deficiency, multiple endocrine abnormalities, cerebellar hypoplasia and partial..."
Directly documents sparse scalp hair in the founding RNF113A-related family.
Microcephaly HP:0000252 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Microcephaly (HP:0000252). HP:0000252 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:25612912 SUPPORT Human Clinical
"We describe two male cousins affected by TTD associated with microcephaly, profound intellectual disability, sparse brittle hair, aged appearance, short stature, facial dysmorphism, seizures, an immunoglobulin deficiency, multiple endocrine abnormalities, cerebellar hypoplasia and partial..."
Directly documents microcephaly in the founding RNF113A-related family.
PMID:30824121 SUPPORT Human Clinical
"Here, we report on four subjects from three families with complex syndromes that include microcephaly, developmental delay, and brittle hair and nails."
Independently documents microcephaly in CARS1-related disease within the current TTD spectrum.
Abnormal Facial Shape HP:0001999 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormal facial shape (HP:0001999). HP:0001999 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:25612912 SUPPORT Human Clinical
"We describe two male cousins affected by TTD associated with microcephaly, profound intellectual disability, sparse brittle hair, aged appearance, short stature, facial dysmorphism, seizures, an immunoglobulin deficiency, multiple endocrine abnormalities, cerebellar hypoplasia and partial..."
Directly documents facial dysmorphism in the founding RNF113A-related family.
Immune 1
Recurrent Infections FREQUENT HP:0002719 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Recurrent infections (HP:0002719). HP:0002719 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:18603627 SUPPORT Other
"ocular abnormalities (51%), infections (46%), photosensitivity (42%)"
The systematic review reports infections in 46% of published cases.
PMID:30334570 SUPPORT Human Clinical
"Infection or sepsis was determined to be the cause of death in 7 of the 10 TTD patients who died."
The NIH cohort documents the mortality relevance of infection.
Integument 5
Alopecia HP:0001596 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Alopecia (HP:0001596). HP:0001596 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:28973399 SUPPORT Human Clinical
"She showed recurrent aggravation of hair-loss by tufted breakage at the scalp boundary immediately following episodes of infection-induced fever."
Directly documents recurrent scalp hair loss in the characterized GTF2E2-related patient.
Ichthyosis FREQUENT HP:0008064 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Ichthyosis (HP:0008064). HP:0008064 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:18603627 SUPPORT Other
"short stature (73%), ichthyosis (65%), abnormal characteristics at birth (55%)"
The systematic review reports ichthyosis in 65% of published cases.
Nail Dystrophy HP:0008404 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Nail dystrophy (HP:0008404). HP:0008404 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:37800682 SUPPORT Human Clinical
"ectodermal abnormalities comprising keratosis pilaris, hyperkeratotic plaques on the scalp, dry skin and nail dystrophy, dysmorphic facial features, microcephaly, global developmental delay and/or intellectual disability, short stature, and hypogonadism."
Directly documents nail dystrophy in the seven-patient MPLKIP cohort.
Cutaneous Photosensitivity HP:0000992 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cutaneous photosensitivity (HP:0000992). HP:0000992 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:40737808 SUPPORT Other
"The presence of hypersensitivity to UV light defines the two main forms of TTD: photosensitive (PS-TTD) and non-photosensitive (NPS-TTD)."
Establishes the subtype-defining role of UV hypersensitivity.
PMID:18603627 SUPPORT Other
"infections (46%), photosensitivity (42%), maternal pregnancy complications (28%) and defective DNA repair (37%)."
Reports photosensitivity in 42% of the umbrella published-case series.
Congenital Ichthyosiform Erythroderma HP:0007431 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Congenital ichthyosiform erythroderma (HP:0007431). HP:0007431 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:21800331 SUPPORT Human Clinical
"Erythroderma/collodion presentation of the skin was noted in 18 (67%) infants (Figure 1A). It typically persisted for 1 to 2 weeks; the patients were treated with humidified air and emollients."
Supports a common neonatal erythroderma/collodion presentation while not separating the two components in the reported percentage.
Musculoskeletal 1
Delayed Skeletal Maturation VERY_FREQUENT HP:0002750 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Delayed skeletal maturation (HP:0002750). HP:0002750 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:25290684 SUPPORT Human Clinical
"Additionally, 4 patients from 3 families with TTDN1 defects had delayed bone age (TTD343BE, TTD480BE, and siblings TTD487BE and TTD488BE), as compared to 4 out of 23 TTD patients with other mutations (p=0.009)."
Supports the 4/5 MPLKIP-specific frequency and cohort enrichment.
Nervous System 4
Global Developmental Delay HP:0001263 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Global developmental delay (HP:0001263). HP:0001263 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:18603627 SUPPORT Other
"common features reported were developmental delay/intellectual impairment (86%), short stature (73%)"
The systematic review reports the combined category developmental delay or intellectual impairment in 86%; it supports this phenotype but does not assign that rate to global developmental delay alone.
Cerebral Hypomyelination HP:0006808 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cerebral hypomyelination (HP:0006808). HP:0006808 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:39976384 SUPPORT Human Clinical
"a 5-year-old affected child who presented with microcephaly, failure to thrive, developmental delay, and progressive hypomyelination on three serial brain imaging over 5-years follow-up."
Directly documents progressive hypomyelination in an ERCC2-related child.
Gait Disturbance HP:0001288 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Gait disturbance (HP:0001288). HP:0001288 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:37800682 SUPPORT Human Clinical
"The gait was ataxic. The systemic and neurological examinations were otherwise unremarkable."
Directly documents the broad gait phenotype in a genotyped MPLKIP-related patient without assigning a cohort frequency.
Seizure FREQUENT HP:0001250 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Seizure (HP:0001250). HP:0001250 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:25290684 SUPPORT Human Clinical
"There were 3 patients from 2 families with TTDN1 mutations (TTD343BE, and siblings TTD487BE and TTD488BE) diagnosed with seizure disorders, but only 3 out of the 31 other TTD patients (p=0.024)."
Supports the 3/5 MPLKIP-specific frequency and cohort enrichment.
Prenatal and Birth 1
Premature Birth FREQUENT HP:0001622 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Premature birth (HP:0001622). HP:0001622 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:21800331 SUPPORT Human Clinical
"Pregnancy reports of 27 TTD patients and their 23 mothers were evaluated and 81% of the pregnancies had complications: 56% had preterm delivery"
Supplies the cohort denominator and preterm-delivery frequency.
Growth 2
Short Stature FREQUENT HP:0004322 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Short stature (HP:0004322). HP:0004322 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:18603627 SUPPORT Other
"developmental delay/intellectual impairment (86%), short stature (73%), ichthyosis (65%)"
The systematic review reports short stature in 73% of published cases.
Small for Gestational Age FREQUENT HP:0001518 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Small for gestational age (HP:0001518). HP:0001518 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:21800331 SUPPORT Human Clinical
"The birth weight of 9(35%) of the 26 patients with known gestational age had weight <10centile for gestational age."
Supplies the gestational-age-adjusted denominator and frequency.
Other 5
Brittle Sulfur-Deficient Hair Brittle hair HP:0002299 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Brittle hair (HP:0002299). HP:0002299 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:15232704 SUPPORT Human Clinical
"The term "trichothiodystrophy" (TTD) covers several autosomal recessive diseases whose diagnostic hallmark is short, brittle hair low in sulfur and cystine because of impaired synthesis of high-sulfur matrix protein."
Establishes brittle sulfur/cystine-poor hair as the diagnostic hallmark.
Tiger-Tail Hair Banding Tiger tail banding HP:0045055 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Tiger tail banding (HP:0045055). HP:0045055 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:35615778 SUPPORT Human Clinical
"Two diagnostic features of the hair are alternating light/dark banding visible in polarized light, which is termed “tiger tail banding,” and low cysteine content, typically about half of normal, revealed by amino acid analysis."
Directly describes tiger-tail banding as a diagnostic hair feature.
Reduced Hair Sulfur Content HP:0034425 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Reduced hair sulfur content (HP:0034425). HP:0034425 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:15232704 SUPPORT Human Clinical
"With our control and parent data resembling published data on hair and nail samples, we obtained a decreased proportion of cysteine (half cystine, determined as cysteic acid) in materials obtained from a boy with TTD."
Directly documents reduced cysteine/cystine in material from an affected child.
Ocular Abnormality FREQUENT Abnormality of the eye HP:0000478 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormality of the eye (HP:0000478). HP:0000478 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:18603627 SUPPORT Other
"ocular abnormalities (51%), infections (46%), photosensitivity (42%)"
The systematic review reports ocular abnormalities in 51% of published cases without reducing them to a single eye phenotype.
Microcytic Anemia HP:0001935 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Microcytic anemia (HP:0001935). HP:0001935 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:28973399 SUPPORT Human Clinical
"Patients harboring a TFIIEβ mutation present microcytic anemia"
Directly documents the clinical phenotype while preserving the candidate status of its mechanism.
PMID:30334570 SUPPORT Human Clinical
"Mild anaemia was present in 8 patients, with the lowest haemoglobin being 100 g/l (Fig 1D). All patients with anaemia had microcytosis, low iron saturation and normal ferritin (Figure S1)."
Independently documents microcytic anemia in the NIH TTD cohort.
🧬

Genetic Associations

11
ERCC2 (Causative)
Gene: ERCC2 hgnc:3434 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is ERCC2 (hgnc:3434). hgnc:3434 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (1 reference)
PMID:9238033 SUPPORT Human Clinical
"Mutations in the XPD gene can result in three distinct clinical phenotypes, XP, trichothiodystrophy (TTD), and XP with Cockayne syndrome."
Patient mutation analysis establishes ERCC2/XPD as an allelic cause of TTD.
ERCC3 (Causative)
Gene: ERCC3 hgnc:3435 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is ERCC3 (hgnc:3435). hgnc:3435 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (1 reference)
PMID:9012405 SUPPORT Human Clinical
"The causative mutation was found to be a single base substitution resulting in a missense mutation (T119P) in a region of the XPB protein completely conserved in yeast, Drosophila, mouse, and man."
Directly establishes a causal ERCC3/XPB variant in affected patients.
GTF2H5 (Causative)
Gene: GTF2H5 hgnc:21157 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is GTF2H5 (hgnc:21157). hgnc:21157 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (1 reference)
PMID:15220921 SUPPORT Human Clinical
"Microinjection of cDNA encoding TFB5 (GTF2H5, also called TTDA) corrected the DNA-repair defect of TTD-A cells, and we identified three functional inactivating mutations in this gene in three unrelated families with TTD-A."
Multiple families and cellular rescue establish GTF2H5/TTDA causality.
MPLKIP (Causative)
Gene: MPLKIP hgnc:16002 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is MPLKIP (hgnc:16002). hgnc:16002 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (1 reference)
PMID:15645389 SUPPORT Human Clinical
"We have identified C7orf11, which localizes to the nucleus and is expressed in fetal hair follicles, as the first disease gene for nonphotosensitive trichothiodystrophy (TTD)."
Establishes the gene now named MPLKIP as a causal non-photosensitive TTD gene.
GTF2E2 (Causative)
Gene: GTF2E2 hgnc:4651 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is GTF2E2 (hgnc:4651). hgnc:4651 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (1 reference)
PMID:26996949 SUPPORT Human Clinical
"Here, we report two unrelated children showing clinical features typical of TTD who harbor different homozygous missense mutations in GTF2E2"
Biallelic GTF2E2 variants in unrelated affected children support a causal non-photosensitive TTD association.
RNF113A (Causative)
Gene: RNF113A hgnc:12974 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is RNF113A (hgnc:12974). hgnc:12974 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (1 reference)
PMID:25612912 SUPPORT Human Clinical
"We identified a nonsense mutation in the highly conserved RNF113A gene (c.901 C>T, p.Q301*). The mutation segregated with the disease in the family and was not observed in over 100,000 control X chromosomes."
Segregation and population absence support the causal X-linked association.
CARS1 (Causative)
Gene: CARS1 hgnc:1493 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is CARS1 (hgnc:1493). hgnc:1493 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (2 references)
PMID:30824121 SUPPORT Human Clinical
"Here, we report on four subjects from three families with complex syndromes that include microcephaly, developmental delay, and brittle hair and nails. Each affected person carries bi-allelic CARS variants"
Multiple families with biallelic variants and the defining hair phenotype support the CARS1 association.
PMID:37800682 SUPPORT Other
"We previously proposed (Theil et al, 2019) that most of the key TTD features of brittle hair, nails, and scaly skin are a consequence of protein instability of factors involved in different steps of gene expression in either transcription (mutated ERCC2, ERCC3, GTF2H5, or GTF2E2), splicing..."
Contemporary TTD literature explicitly includes CARS1 in the TTD translation branch despite the current MONDO hierarchy placing its named syndrome outside the TTD umbrella.
TARS1 (Causative)
Gene: TARS1 hgnc:11572 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is TARS1 (hgnc:11572). hgnc:11572 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (1 reference)
PMID:31374204 SUPPORT Human Clinical
"Here we present the identification of an NPS-TTD-associated gene, threonyl-tRNA synthetase (TARS), found by next-generation sequencing of a group of uncharacterized individuals with NPS-TTD."
Patient sequencing establishes TARS1 as a causal non-photosensitive TTD gene.
AARS1 (Causative)
Gene: AARS1 hgnc:20 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is AARS1 (hgnc:20). hgnc:20 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (1 reference)
PMID:33909043 SUPPORT Human Clinical
"Here, we identify alanyl-tRNA synthetase 1 and methionyl-tRNA synthetase 1 variants as new gene defects that cause NPS-TTD."
The patient study identifies AARS1 variants as a cause of non-photosensitive TTD.
MARS1 (Causative)
Gene: MARS1 hgnc:6898 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is MARS1 (hgnc:6898). hgnc:6898 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (2 references)
PMID:33909043 SUPPORT Human Clinical
"Here, we identify alanyl-tRNA synthetase 1 and methionyl-tRNA synthetase 1 variants as new gene defects that cause NPS-TTD."
The patient study identifies MARS1 variants as a cause of non-photosensitive TTD.
PMID:40820264 SUPPORT Human Clinical
"We present the second case of type 9 non-photosensitive trichothiodystrophy caused by a homozygous variant in the MARS1 gene."
A second independent case strengthens the initially single-case MARS1-TTD9 association without implying that its atypical features are universal.
DBR1 (Causative)
Gene: DBR1 hgnc:15594 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is DBR1 (hgnc:15594). hgnc:15594 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (1 reference)
PMID:40683339 SUPPORT Human Clinical
"We report 5 nonphotosensitive adult patients from 3 unrelated families with a homozygous missense variation in DBR1 (p.D262Y) encoding the RNA lariat-debranching enzyme DBR1, which is involved in the removal of introns from pre-mRNA in the nucleus."
Three unrelated families support DBR1 as the eleventh TTD causal gene.
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Medical Actions

5
Rigorous Photoprotection for Photosensitive TTD
Action: supportive careNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is supportive care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. Ontology label: Supportive Care NCIT:C15747
Rigorous reduction of ultraviolet exposure is a supportive management component for the ERCC2-, ERCC3-, and GTF2H5-related photosensitive branch. It is not a disease-modifying treatment and is not imposed on genetically and functionally non-photosensitive TTD.
Target Phenotypes: Cutaneous photosensitivity HP:0000992 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Cutaneous photosensitivity (HP:0000992). HP:0000992 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27745642 SUPPORT Other
"Rigorous photoprotection remains an important component of the management of these inherited DNA repair-deficiency photodermatoses."
A clinical review directly supports rigorous photoprotection for the DNA-repair-deficient photosensitive syndromes that include TTD.
Immune and Hematologic Surveillance
Action: supportive careNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is supportive care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. Ontology label: Supportive Care NCIT:C15747
Monitor recurrent infections, age-adjusted immunoglobulins, complete blood counts, and absolute neutrophil counts. This surveillance is motivated by mortality-associated abnormalities in a 40-patient cohort; it is not itself a disease-modifying intervention.
Show evidence (1 reference)
PMID:30334570 SUPPORT Human Clinical
"Based on observed early mortality in our cohort, we propose surveillance of TTD patients for recurrent infections, hypogammaglobulinaemia and neutropenia."
Directly supports the surveillance component from a 40-patient cohort.
Conditional Immunoglobulin Replacement
Action: immunoglobulin infusion therapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is immunoglobulin infusion therapy, annotated with Immunoglobulin Therapy (NCIT:C62710). NCIT:C62710 is a clinical intervention from the NCI Thesaurus. Ontology label: Immunoglobulin Therapy NCIT:C62710
Consider supplemental immunoglobulin only for a patient with documented hypogammaglobulinemia and an appropriate clinical indication. The cohort publication proposes this as abnormality-directed supportive management, not routine treatment for every TTD genotype.
Target Phenotypes: Decreased circulating immunoglobulin concentration HP:0004313 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Decreased circulating immunoglobulin concentration (HP:0004313). HP:0004313 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30334570 SUPPORT Human Clinical
"Haematological evaluation may be considered for the management of hypogammaglobulinaemia or neutropenia with supplemental immunoglobulins or granulocyte colony-stimulating factor, respectively."
Supports immunoglobulin replacement only when the corresponding immunoglobulin abnormality is present.
Conditional Granulocyte Colony-Stimulating Factor
Action: granulocyte colony-stimulating factor therapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is granulocyte colony-stimulating factor therapy, annotated with Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. Ontology label: Pharmacotherapy NCIT:C15986
Agent: recombinant granulocyte colony-stimulating factor NCIT:C1287 NCI Thesaurus (NCIT) Relation: this treatment uses this therapeutic agent This treatment uses recombinant granulocyte colony-stimulating factor (NCIT:C1287). NCIT:C1287 is a therapeutic agent from the NCI Thesaurus.
Consider granulocyte colony-stimulating factor only for a patient with documented clinically significant neutropenia. The cohort publication proposes phenotype-directed supportive management rather than universal prophylaxis or disease modification.
Target Phenotypes: Decreased total neutrophil count HP:0001875 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Decreased total neutrophil count (HP:0001875). HP:0001875 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:30334570 SUPPORT Human Clinical
"Haematological evaluation may be considered for the management of hypogammaglobulinaemia or neutropenia with supplemental immunoglobulins or granulocyte colony-stimulating factor, respectively."
Supports G-CSF only when the corresponding neutropenia is present.
Neonatal Humidified Air and Emollient Support
Action: supportive careNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is supportive care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. Ontology label: Supportive Care NCIT:C15747
Humidified air and emollients were used as supportive skin care for neonatal erythroderma or collodion presentation in the pregnancy cohort. The report documents use but does not establish comparative efficacy or a universal regimen.
Target Phenotypes: Congenital ichthyosiform erythroderma HP:0007431 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Congenital ichthyosiform erythroderma (HP:0007431). HP:0007431 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:21800331 SUPPORT Human Clinical
"Erythroderma/collodion presentation of the skin was noted in 18 (67%) infants (Figure 1A). It typically persisted for 1 to 2 weeks; the patients were treated with humidified air and emollients."
Documents neonatal supportive skin care while not proving comparative benefit.
🔬

Biochemical Markers

2
Reduced Cysteine and Cystine in Hair and Nail Material (DECREASED)
Context: Diagnostic biochemical property measured by amino-acid analysis of hair or nail clippings; specialized sample preparation is required for accurate cysteine/cystine quantification.
Show evidence (1 reference)
PMID:15232704 SUPPORT Human Clinical
"With our control and parent data resembling published data on hair and nail samples, we obtained a decreased proportion of cysteine (half cystine, determined as cysteic acid) in materials obtained from a boy with TTD."
Supports reduced measured cysteine/cystine in affected hair and nail material.
Reduced Prostaglandin I2 Synthase in Dermal Fibroblasts (DECREASED)
Context: Research biomarker observed in the studied photosensitive and non-photosensitive TTD primary fibroblasts. It is not curated as a proven cause of the multisystem phenotype or as a routine clinical assay.
Show evidence (1 reference)
PMID:34155103 SUPPORT In Vitro
"Its reduction characterizes all TTD cases so far investigated, both the PS-TTD with mutations in TFIIH coding genes as well as the nonphotosensitive (NPS)-TTD."
Supports reduced PTGIS in the studied patient-cell panel while preserving its biomarker, rather than causal, status.
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Diagnosis

3
Polarized-Light Hair Microscopy
Examine plucked or clipped hair shafts under polarized light for alternating bright and dark "tiger-tail" bands. Interpret the finding together with shaft morphology, sulfur-amino-acid analysis, and molecular results.
polarization microscopy NCIT:C16858 NCI Thesaurus (NCIT)
Results: Tiger-tail banding together with brittle shaft abnormalities supports TTD and should prompt biochemical and molecular confirmation.
Show evidence (1 reference)
PMID:15232704 SUPPORT Human Clinical
"Important laboratory tests of the hair for the diagnosis of TTD comprise polarizing microscopy ("tiger-tail" pattern), electron microscopy, and amino acids analysis of hydrolyzed hair with a special focus on cystine."
Directly supports polarized-light hair microscopy as one component of the diagnostic laboratory evaluation.
Hair or Nail Cysteine and Cystine Analysis
Quantify cysteine/cystine in hydrolyzed hair or nail material using validated preparation that prevents decomposition during hydrolysis. Limited laboratory availability and pre-analytic sensitivity make this a specialized confirmatory assay rather than a stand-alone exclusion test.
cysteine measurement NCIT:C172518 NCI Thesaurus (NCIT)
Results: A reduced proportion of cysteine/cystine supports sulfur-deficient hair or nail material characteristic of TTD.
Show evidence (2 references)
PMID:15232704 SUPPORT Human Clinical
"With our control and parent data resembling published data on hair and nail samples, we obtained a decreased proportion of cysteine (half cystine, determined as cysteic acid) in materials obtained from a boy with TTD."
Demonstrates the expected biochemical result in affected hair and nail material.
PMID:15232704 SUPPORT Human Clinical
"If no special precautions are taken, quantification of cysteine and cystine becomes inaccurate because of decomposition of these residues during hydrolysis."
Establishes the important pre-analytic limitation of this specialized assay.
Molecular Genetic Testing
Use a multigene panel or exome/genome analysis that includes ERCC2, ERCC3, GTF2H5, MPLKIP, GTF2E2, RNF113A, CARS1, TARS1, AARS1, MARS1, and DBR1. Interpret inheritance and photosensitivity by the identified gene rather than treating all TTD as a TFIIH disorder.
genetic testing NCIT:C15709 NCI Thesaurus (NCIT)
Results: Biallelic pathogenic variants in one of the ten autosomal genes, or a hemizygous pathogenic RNF113A variant in an affected male, provide molecular confirmation in a compatible phenotype.
Show evidence (1 reference)
PMID:40737808 SUPPORT Other
"The disease arises from mutations in a variety of genes involved in different biological processes. Affected processes include DNA repair, transcription as well as translation."
Current review evidence supports a genetically heterogeneous testing strategy; the individual gene records above supply gene-specific causal evidence.
📊

Prevalence

2
Worldwide
Point Prevalence <1 in 1,000,000
The current Orphadata record supplies a validated worldwide class only, without a numeric point-prevalence estimate.
Show evidence (1 reference)
url:https://api.orphadata.com/rd-epidemiology/orphacodes/33364 SUPPORT Other
""PrevalenceClass": "<1 / 1 000 000", "PrevalenceGeographic": "Worldwide", "PrevalenceQualification": "Class only", "PrevalenceType": "Point prevalence", "PrevalenceValidationStatus": "Validated""
The current official structured record supports the worldwide band and explicitly identifies it as class-only.
Europe
Birth Prevalence 0.12 per 100,000 1–9 per 1,000,000
Orphadata reports a validated European birth-prevalence value of 0.12 per 100,000 (approximately 1 per 830,000), sourced to PMID:18329345.
Show evidence (1 reference)
url:https://api.orphadata.com/rd-epidemiology/orphacodes/33364 SUPPORT Other
""PrevalenceClass": "1-9 / 1 000 000", "PrevalenceGeographic": "Europe", "PrevalenceQualification": "Value and class", "PrevalenceType": "Prevalence at birth", "PrevalenceValidationStatus": "Validated", "Source": "18329345[PMID]", "ValMoy": "0.12""
Directly supplies the geographic scope, measure type, rate, validation status, and source of the structured estimate.
🔀

Differential Diagnoses

5

Conditions with similar clinical presentations that must be differentiated from Trichothiodystrophy:

Xeroderma Pigmentosum MONDO:0019600
Overlapping Features XP and photosensitive TTD can both result from ERCC2 or ERCC3 variants and defective nucleotide-excision repair. Brittle sulfur-deficient tiger-tail hair and the multisystem developmental TTD phenotype favor TTD, whereas a marked cutaneous-cancer predisposition favors XP. Genotype alone may require clinical and functional interpretation because the same genes underlie both.
Distinguishing Features
  • Cutaneous cancer predisposition is characteristic of XP but not an established increased risk in TTD.
  • Sulfur-deficient brittle hair with tiger-tail banding supports TTD.
Show evidence (2 references)
PMID:20687499 SUPPORT Other
"Skin cancer is very rare in sun-sensitive TTD."
A clinical review supports the important contrast with the marked cancer predisposition of XP without asserting that tumors are impossible in TTD.
PMID:9012405 SUPPORT Human Clinical
"DNA repair defects in XP-D are associated with two additional, quite different diseases; XP, a sun-sensitive and cancer-prone repair disorder, and Cockayne syndrome (CS), a photosensitive condition characterized by physical and mental retardation and wizened facial appearance."
Provides the clinical cancer-prone XP contrast in an allelic TFIIH study.
Cockayne Syndrome MONDO:0016006
Overlapping Features Cockayne syndrome overlaps with photosensitive TTD through defective transcription-coupled nucleotide-excision repair and growth or neurologic impairment. Sulfur-deficient brittle hair with tiger-tail banding favors TTD, while progressive cachectic or progeroid features support Cockayne syndrome; allelic ERCC2 cases require integrated clinical interpretation.
Distinguishing Features
  • Sulfur-deficient brittle hair and tiger-tail banding favor TTD.
  • Progressive cachectic or progeroid features favor Cockayne syndrome.
Show evidence (1 reference)
PMID:9012405 SUPPORT Human Clinical
"DNA repair defects in XP-D are associated with two additional, quite different diseases; XP, a sun-sensitive and cancer-prone repair disorder, and Cockayne syndrome (CS), a photosensitive condition characterized by physical and mental retardation and wizened facial appearance."
Supports Cockayne syndrome as an allelic photosensitive developmental differential in the TFIIH-related branch.
Netherton Syndrome MONDO:0009735
Overlapping Features Netherton syndrome can combine congenital ichthyosis with short, fragile hair. Trichorrhexis invaginata and prominent atopy with elevated IgE favor Netherton syndrome, whereas sulfur deficiency and tiger-tail banding favor TTD.
Distinguishing Features
  • Trichorrhexis invaginata and atopic diathesis favor Netherton syndrome.
  • Reduced hair sulfur and tiger-tail banding favor TTD.
Show evidence (1 reference)
PMID:31332722 SUPPORT Other
"Netherton syndrome is characterized by a triad of congenital ichthyosiform erythroderma (CIE) or ichthyosis linearis circumflexa (ILC), atopic diathesis (elevated serum IgE), and short sparse fragile hair with trichorrhexis invaginata [49]."
Defines the characteristic hair, skin, and atopic features that distinguish this ichthyosis mimic.
Menkes Disease MONDO:0010651
Overlapping Features Menkes disease can present with sparse, fragile or twisted hair and severe neurologic disease. Pili torti or steel-wool hair together with abnormal copper metabolism and ATP7A-related systemic findings favors Menkes disease rather than TTD.
Distinguishing Features
  • Pili torti or steel-wool hair with disordered copper metabolism favors Menkes disease.
  • Reduced sulfur content and tiger-tail banding favor TTD.
Show evidence (1 reference)
PMID:31332722 SUPPORT Other
"Menkes syndrome: X-linked recessive condition. It is due to a mutation in the X-linked ATP7A gene, which encodes a copper-transporting ATPase. In this syndrome there is a maldistribution of the body’s copper leading to dysfunctions of all processes that need this element. The patient will have..."
Supports the ATP7A/copper and distinctive-hair basis for the differential.
Argininosuccinic Aciduria MONDO:0008815
Overlapping Features Argininosuccinic aciduria can cause brittle trichorrhexis-nodosa hair with neurodevelopmental or hepatic disease. Urea-cycle biochemical abnormalities and hair that is initially normal before later trichorrhexis nodosa favor argininosuccinic aciduria rather than TTD.
Distinguishing Features
  • Hyperammonemia or urea-cycle abnormalities favor argininosuccinic aciduria.
  • Congenital sulfur-deficient brittle hair and tiger-tail banding favor TTD.
Show evidence (1 reference)
PMID:31332722 SUPPORT Other
"Argininosuccinic aciduria is an inborn error of urea synthesis. It is due to deficiency of argininosuccinic lyase. If untreated, it may present with failure to thrive and mental retardation. The hair finding of TN is an important diagnostic clue in this syndrome. These patients have normal hair..."
Supports the metabolic context and later trichorrhexis-nodosa hair pattern.
📊

Related Datasets

2
Gene expression profiles in Trichothiodystrophy patient and control arrayexpress:E-MTAB-10567
Bulk coding-RNA/poly(A) paired-end RNA sequencing of primary skin fibroblasts from one female TTD proband and her unaffected mother, with untreated and UV conditions and three replicate libraries per group. The repository does not identify the causal gene, genotype, photosensitivity subtype, UV dose, or proband developmental stage.
human BULK RNA SEQ n=12 Illumina HiSeq 2500; paired-end...
skin fibroblast CL:0002620 Cell Ontology (CL) Relation: this dataset samples this sample type This dataset samples skin fibroblast (CL:0002620). CL:0002620 is a sample type from the Cell Ontology.
Conditions: TTD proband fibroblasts, untreated TTD proband fibroblasts, UV-exposed unaffected maternal fibroblasts, untreated unaffected maternal fibroblasts, UV-exposed
Verified against the official BioStudies/ArrayExpress API, IDF, and SDRF on 2026-08-17. Released 2021-06-29; 12 ENA runs ERR6018701-ERR6018712 under ERP129680. No publication, PMID, DOI, genotype, or UV dose/time is attached to the repository record.
Impaired B-cell function in ERCC2 deficiency geo:GSE262217
Bulk mRNA sequencing of peripheral blood mononuclear cells from three unrelated patients with ERCC2-related TTD1 and five healthy controls. This subtype-specific dataset underlies the study's comparison of immune and immunoglobulin-heavy-chain expression programs.
human BULK RNA SEQ n=8 Illumina NovaSeq 6000
peripheral blood mononuclear cell CL:2000001 Cell Ontology (CL) Relation: this dataset samples this sample type This dataset samples peripheral blood mononuclear cell (CL:2000001). CL:2000001 is a sample type from the Cell Ontology.
Conditions: ERCC2-related TTD1 patient peripheral blood mononuclear cells (3) healthy-control peripheral blood mononuclear cells (5)
PMID:39055713
Verified against the official NCBI GEO record on 2026-08-17. GSE262217 is linked to BioProject PRJNA1090832 and was released publicly on 2024-08-02; the repository describes expression profiling by high-throughput sequencing.
🧫

Experimental Models

4
GTF2E2-mutant patient iPSC-derived erythroid model IPSC_DERIVED_MODEL
A GTF2E2-mutant patient fibroblast line was reprogrammed to iPS cells and differentiated with two healthy-control iPSC lines into erythroblasts. The system reproduces late differentiation, cell-size, multinucleation, and hemoglobin-subunit abnormalities, but it executes an embryonic/fetal rather than adult globin program.
GTF2E2 p.Asp187Tyr patient-derived line healthy-control lines standard and fever-mimicking temperatures
erythroblast CL:0000765 Cell Ontology (CL) Relation: this experimental model uses this cell type This experimental model uses erythroblast (CL:0000765). CL:0000765 is a cell type from the Cell Ontology.
Organism
human NCBITaxon:9606 NCBI Taxonomy (NCBITaxon) Relation: this experimental model is built in this organism This experimental model is built in human, annotated with Homo sapiens (NCBITaxon:9606). NCBITaxon:9606 is an organism from the NCBI Taxonomy.
Cell source
iPS cells reprogrammed from primary fibroblasts of patient TTD218UT and two healthy-control iPSC lines
Culture
Two-phase in-vitro erythroid expansion and differentiation at 37 or 39 degrees Celsius
Publication
PMID:28973399
Pathograph links
GTF2E2-Related Late Erythroid Differentiation Dysfunction partially recapitulates fidelity: moderate The patient-derived model directly reproduces abnormal terminal erythroid differentiation and globin-subunit balance.
Limitations: One patient-derived iPSC clone was compared with two non-isogenic controls; iPSC-derived erythroblasts execute an embryonic/fetal globin program and do not reproduce adult marrow or systemic physiology.
Readouts
Hemoglobin-subunit balance in differentiated erythroblasts
Direction: ALTERED
Interpretation: Altered hemoglobin fractions and increased abnormal erythroblast size or multinucleation report late erythroid differentiation dysfunction.
Show evidence (1 reference)
PMID:28973399 SUPPORT In Vitro
"We observed a clear hematopoietic defect during late-stage differentiation associated with hemoglobin subunit imbalance."
Directly reports the model readout and its altered state.
Show evidence (1 reference)
PMID:28973399 SUPPORT In Vitro
"We performed induced pluripotent stem (iPS) cell reprogramming of patient fibroblasts followed by in vitro erythroid differentiation to translate the intriguing molecular defect to phenotypic expression in relevant tissue, to disclose the molecular basis for some specific TTD features."
Defines the patient-derived iPSC erythroid model and its disease purpose.
Show evidence (1 reference)
PMID:28973399 SUPPORT In Vitro
"We performed induced pluripotent stem (iPS) cell reprogramming of patient fibroblasts followed by in vitro erythroid differentiation to translate the intriguing molecular defect to phenotypic expression in relevant tissue, to disclose the molecular basis for some specific TTD features."
Directly identifies the model system and differentiation design.
GTF2E2-mutant SV40-immortalized fibroblast temperature-challenge model CELL_LINE
SV40-immortalized GTF2E2-mutant TTD218UT fibroblasts were compared with immortalized controls at standard and elevated temperatures to expose the conditional transcription defect that is absent under standard culture conditions.
37 degrees Celsius standard culture prolonged 40 degrees Celsius temperature challenge
skin fibroblast CL:0002620 Cell Ontology (CL) Relation: this experimental model uses this cell type This experimental model uses skin fibroblast (CL:0002620). CL:0002620 is a cell type from the Cell Ontology.
Organism
human NCBITaxon:9606 NCBI Taxonomy (NCBITaxon) Relation: this experimental model is built in this organism This experimental model is built in human, annotated with Homo sapiens (NCBITaxon:9606). NCBITaxon:9606 is an organism from the NCBI Taxonomy.
Cell source
SV40-immortalized TTD218UT_sv patient fibroblasts and MRC5_sv/C5RO_sv wild-type control fibroblasts
Culture
SV40-immortalized fibroblast monolayer at standard or elevated temperature
Publication
PMID:28973399
Pathograph links
Temperature-Sensitive Transcriptional Insufficiency in GTF2E2-Related TTD recapitulates fidelity: moderate Elevated-temperature culture directly reveals the conditional reduction in transcription in patient fibroblasts.
Limitations: The defining temperature-response assay used SV40-transformed cells from one patient line; transformation can alter stress and survival responses. Fever duration and temperature are more variable in patients, and fever-associated worsening was not confirmed in every reported individual.
Readouts
Global RNA synthesis after elevated-temperature culture
Direction: DECREASED
Interpretation: Reduced RNA synthesis at elevated, but not standard, temperature is the defining conditional readout.
Show evidence (1 reference)
PMID:28973399 SUPPORT In Vitro
"We showed that fibroblasts derived from these patients exhibit a clear transcription defect, however only when cultured at elevated temperatures."
Directly reports the temperature-bounded direction of the readout.
Show evidence (1 reference)
PMID:28973399 SUPPORT In Vitro
"We showed that fibroblasts derived from these patients exhibit a clear transcription defect, however only when cultured at elevated temperatures."
Directly establishes the conditional patient-cell model.
Show evidence (2 references)
PMID:28973399 SUPPORT In Vitro
"Transcription levels after incubation for 72 h at 37 °C or 40 °C, measured by pulse-labeling with ethynyl-uridine (EU) and subsequent fluorescent staining of incorporated EU, of Sv40-immortalized patient cells (TTD218UT_sv), compared with wild-type controls (MRC5_sv, C5RO-sv)."
Identifies the transformed patient and control lines used for the defining elevated-temperature transcription readout.
PMID:28973399 SUPPORT In Vitro
"Surprisingly however, basal transcription levels, assayed by ethynyl uridine (EU) pulse labeling, were not affected in patient cells under standard culture conditions"
Provides the essential negative comparator that prevents a constitutive transcription-defect interpretation.
ERCC2/ERCC3 patient-dermal-fibroblast R-loop model PRIMARY_CELL_CULTURE
ERCC2 primary dermal fibroblasts were analyzed by chromatin fractionation and interaction assays, while ERCC2 and ERCC3 fibroblasts were analyzed by DRIP for R-loop accumulation.
skin fibroblast CL:0002620 Cell Ontology (CL) Relation: this experimental model uses this cell type This experimental model uses skin fibroblast (CL:0002620). CL:0002620 is a cell type from the Cell Ontology.
Organism
human NCBITaxon:9606 NCBI Taxonomy (NCBITaxon) Relation: this experimental model is built in this organism This experimental model is built in human, annotated with Homo sapiens (NCBITaxon:9606). NCBITaxon:9606 is an organism from the NCBI Taxonomy.
Cell source
Primary dermal fibroblasts from ERCC2- or ERCC3-related photosensitive TTD patients and healthy controls
Culture
Primary fibroblast monolayers with ERCC2-arm chromatin-interaction assays and ERCC2/ERCC3 DRIP analysis
Publication
PMID:40757642
Pathograph links
Unprocessed R-Loop Accumulation in ERCC2/ERCC3-Related TTD recapitulates fidelity: high Genotype-matched primary patient cells directly reproduce accumulation of unprocessed R-loops at transcribed loci.
Limitations: Dermal fibroblasts do not establish the magnitude, timing, or downstream consequences of R-loop stress in developing neural, hair, or immune tissues; chromatin-interaction work was ERCC2-scoped, and GTF2H5-related cells were not tested.
Readouts
R-loop abundance at the ACTB locus by DRIP
Direction: INCREASED
Interpretation: Increased DNA-RNA hybrid signal reports failure to process R-loops in transcribed genomic regions.
Show evidence (1 reference)
PMID:40757642 SUPPORT In Vitro
"demonstrating that both XPD and XPB alterations causative of PS-TTD lead to an excess of unprocessed R-loops in the transcribed regions of the genome"
Directly reports increased unprocessed R-loops in both studied TFIIH branches.
Show evidence (1 reference)
PMID:40757642 SUPPORT In Vitro
"TTD-specific variants in ERCC2/XPD result in TFIIH instability, altered interaction of the CAK with DDX1-SFPQ-NONO, and R-loop accumulation."
Establishes that the patient-cell model is informative for the linked node.
Show evidence (1 reference)
PMID:40757642 SUPPORT In Vitro
"primary dermal fibroblasts from healthy donors (C3PV and C8PV), PS-TTD (TTD8PV and TTD23PV), or XP (XP26VI and XP15PV) patients with pathogenic variants in ERCC2/XPD gene."
Identifies the primary dermal-fibroblast component of the study.
MPLKIP-deficient reconstructed human skin equivalent CO_CULTURE
A reconstructed human skin equivalent combines collagen dermal equivalents containing subject-derived primary fibroblasts with N/TERT-immortalized keratinocytes carrying CRISPR MPLKIP knockout or patient-variant knock-in, cultured at an air-liquid interface to assess epidermal differentiation.
keratinocyte CL:0000312 Cell Ontology (CL) Relation: this experimental model uses this cell type This experimental model uses keratinocyte (CL:0000312). CL:0000312 is a cell type from the Cell Ontology. skin fibroblast CL:0002620 Cell Ontology (CL) Relation: this experimental model uses this cell type This experimental model uses skin fibroblast (CL:0002620). CL:0002620 is a cell type from the Cell Ontology.
Organism
human NCBITaxon:9606 NCBI Taxonomy (NCBITaxon) Relation: this experimental model is built in this organism This experimental model is built in human, annotated with Homo sapiens (NCBITaxon:9606). NCBITaxon:9606 is an organism from the NCBI Taxonomy.
Cell source
Subject-derived primary dermal fibroblasts combined with N/TERT-immortalized CRISPR MPLKIP knockout or knock-in keratinocytes
Culture
Collagen-based reconstructed human skin equivalent at an air-liquid interface
Publication
PMID:37800682
Pathograph links
Impaired Keratinocyte Differentiation recapitulates fidelity: moderate The organotypic co-culture reproduces compromised splicing, altered proteome balance, and impaired keratinocyte differentiation after MPLKIP loss.
Limitations: The model is MPLKIP-specific; its keratinocytes are immortalized and engineered while only the dermal fibroblasts are patient-derived, and it lacks hair follicles and shaft formation.
Readouts
Epidermal keratinocyte differentiation in reconstructed skin
Direction: DECREASED
Interpretation: Impaired stratification and differentiation in the skin equivalent report the linked tissue state.
Show evidence (1 reference)
PMID:37800682 SUPPORT In Vitro
"Using Human Skin Equivalents (HSEs), we observed impaired keratinocyte differentiation associated with compromised splicing and eventually, an imbalanced proteome affecting skin development and, interestingly, also the immune system."
Directly reports the model readout and its direction.
Show evidence (1 reference)
PMID:37800682 SUPPORT In Vitro
"Using Human Skin Equivalents (HSEs), we observed impaired keratinocyte differentiation associated with compromised splicing and eventually, an imbalanced proteome affecting skin development and, interestingly, also the immune system."
Establishes the human skin-equivalent system as informative for this node.
Show evidence (1 reference)
PMID:37800682 SUPPORT In Vitro
"Using Human Skin Equivalents (HSEs), we observed impaired keratinocyte differentiation associated with compromised splicing and eventually, an imbalanced proteome affecting skin development and, interestingly, also the immune system."
Directly identifies the reconstructed human skin model and measured state.
🐁

Animal Models

2
Xpd-mutant TTD mouse
The Xpd TTD mouse links limiting TFIIH in brain to spatially selective thyroid-hormone target-gene deregulation. It supports a coactivator mechanism but does not establish the corresponding process in human developing brain.
Species
Mus musculus
Genotype
Homozygous Xpd/Ercc2 p.Arg722Trp (R722W) knock-in
Genes
ERCC2 hgnc:3434 HUGO Gene Nomenclature Committee (hgnc) Relation: this experimental model concerns this gene This experimental model concerns ERCC2 (hgnc:3434). hgnc:3434 is a gene from the HUGO Gene Nomenclature Committee.
Publication
PMID:17952069
Pathograph links
Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain partially recapitulates fidelity: moderate The model reproduces brain TFIIH-dependent thyroid-receptor destabilization and target-gene dysregulation.
Limitations: The evidence is from mouse brain; human developmental cell types, timing, and causal contribution to hypomyelination have not been tested.
Readouts
Thyroid-hormone receptor occupancy and target-gene expression in brain
Direction: ALTERED
Interpretation: Spatially selective target-gene deregulation reports impaired TFIIH coactivator function in the model brain.
Show evidence (1 reference)
PMID:17952069 SUPPORT Model Organism
"Here we show that an XPD mutation in TTD mice results in a spatial and selective deregulation of thyroid hormone target genes in the brain."
Directly reports the model readout.
Show evidence (1 reference)
PMID:17952069 SUPPORT Model Organism
"Molecular analyses performed on the mice brain tissue demonstrate that TFIIH is required for the stabilization of thyroid hormone receptors (TR) to their DNA-responsive elements."
Establishes the model-level coactivator mechanism.
Show evidence (2 references)
PMID:17952069 SUPPORT Model Organism
"Here we show that an XPD mutation in TTD mice results in a spatial and selective deregulation of thyroid hormone target genes in the brain."
Directly identifies the TTD mouse and its principal brain phenotype.
PMID:25340339 SUPPORT Model Organism
"the TTD mouse model (having the most common XPD/ R722W point mutation found in TTD patients)"
A follow-on study of the same established line supplies the exact R722W allele identity.
Ttdn1-null mouse
CRISPR-generated Ttdn1-null mice reproduce increased intron lariats, alternative-splicing changes, reduced hair cysteic acid, impaired weight gain, and selected motor or learning abnormalities, while missing several severe human features.
Species
Mus musculus
Genotype
Homozygous Ttdn1 deletion with no detectable protein
Genes
MPLKIP hgnc:16002 HUGO Gene Nomenclature Committee (hgnc) Relation: this experimental model concerns this gene This experimental model concerns MPLKIP (hgnc:16002). hgnc:16002 is a gene from the HUGO Gene Nomenclature Committee.
Publication
PMID:37369199
Pathograph links
Intron-Lariat Accumulation after MPLKIP Loss recapitulates fidelity: moderate Cortex RNA-seq reproduces intron-lariat accumulation after loss of Ttdn1.
Limitations: Mouse cortical lariat accumulation was smaller than in human knockout cells, and the model does not reproduce the full human developmental phenotype.
Readouts
Cortex intron-lariat abundance
Direction: INCREASED
Interpretation: Increased lariat-derived reads directly measure defective turnover in vivo.
Show evidence (1 reference)
PMID:37369199 SUPPORT Model Organism
"RNA lariat abundance was increased ~4.1-fold compared to WT samples, as were alternative splicing events (Figure 4d and Supplemental Figure S4g)."
Directly reports increased lariat abundance in the model cortex.
Show evidence (1 reference)
PMID:37369199 SUPPORT Model Organism
"RNA lariat abundance was increased ~4.1-fold compared to WT samples, as were alternative splicing events (Figure 4d and Supplemental Figure S4g)."
Establishes in-vivo recapitulation of the linked lariat state.
Aberrant RNA Splicing after MPLKIP Loss recapitulates fidelity: moderate Cortex RNA-seq reproduces alternative-splicing changes after loss of Ttdn1.
Limitations: The model does not reproduce the full human developmental phenotype, and mouse cortex cannot establish the tissue distribution of splicing defects in human TTD.
Readouts
Cortex alternative-splicing events
Direction: ALTERED
Interpretation: Altered splice-junction usage reports the linked RNA-splicing state in vivo.
Show evidence (1 reference)
PMID:37369199 SUPPORT Model Organism
"RNA lariat abundance was increased ~4.1-fold compared to WT samples, as were alternative splicing events (Figure 4d and Supplemental Figure S4g)."
Directly reports altered splicing in the model cortex.
Show evidence (1 reference)
PMID:37369199 SUPPORT Model Organism
"RNA lariat abundance was increased ~4.1-fold compared to WT samples, as were alternative splicing events (Figure 4d and Supplemental Figure S4g)."
Establishes in-vivo recapitulation of the linked splicing state.
Deficiency of Sulfur-Rich Hair Matrix Proteins partially recapitulates fidelity: moderate Ttdn1-null hair has decreased cysteic-acid content, partially reproducing the sulfur-poor component of the human hair phenotype.
Limitations: Keratin-associated-protein abundance and hair-shaft ultrastructure were not measured; reduced cysteic acid is only an indirect compositional proxy. Sparse hair was not overt in young mice, and tiger-tail banding was not established.
Readouts
Hair cysteic-acid content
Direction: DECREASED
Interpretation: Reduced cysteic acid measures the sulfur-deficient component of the hair phenotype.
Show evidence (1 reference)
PMID:37369199 SUPPORT Model Organism
"we observed a significant decrease in cysteic acid content as a percentage of hair protein in these mice"
Directly reports the direction of the hair biochemical readout.
Show evidence (1 reference)
PMID:37369199 SUPPORT Model Organism
"we observed a significant decrease in cysteic acid content as a percentage of hair protein in these mice"
Supports partial recapitulation of the sulfur-deficient hair lesion.
Show evidence (1 reference)
PMID:37369199 SUPPORT Model Organism
"A Ttdn1-deficient mouse model recapitulates intron-processing defects and certain neurodevelopmental phenotypes seen in NP-TTD."
Directly supports using the knockout as a disease model.
{ }

Source YAML

click to show 
name: Trichothiodystrophy
creation_date: "2026-06-29T00:00:00Z"
category: Mendelian
synonyms:
- TTD
- sulfur-deficient brittle hair syndrome
description: >-
  Trichothiodystrophy (TTD) is a genetically heterogeneous multisystem
  developmental disorder defined by short, brittle, sulfur-deficient hair that
  commonly shows alternating light and dark "tiger-tail" banding under polarized
  microscopy. The clinical spectrum ranges from hair-only disease to combinations
  of ichthyosis, growth restriction, neurodevelopmental impairment, ocular
  abnormalities, recurrent infection, and early mortality. Photosensitive TTD is
  caused by biallelic variants in the TFIIH-subunit genes ERCC2, ERCC3, or GTF2H5;
  impaired nucleotide-excision repair is restricted to this branch and causes UV
  sensitivity. Non-photosensitive TTD is caused by biallelic variants in GTF2E2,
  MPLKIP, CARS1, TARS1, AARS1, MARS1, or DBR1, or by hemizygous RNF113A variants.
  These genes affect transcription, pre-mRNA processing, or tRNA charging, but the
  tissue-specific intermediates that make these distinct defects converge on the
  shared hair and multisystem phenotype remain incompletely resolved. Management
  is supportive and subtype-aware; rigorous photoprotection applies to
  photosensitive TTD, while immunologic and hematologic surveillance is important
  across the clinically variable spectrum.
disease_term:
  preferred_term: trichothiodystrophy
  term:
    id: MONDO:0018053
    label: trichothiodystrophy
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0018053
      label: trichothiodystrophy
    mapping_predicate: skos:exactMatch
    mapping_source: MONDO
parents:
- ectodermal dysplasia syndrome
- hereditary disease
has_subtypes:
- name: Photosensitive TTD
  display_name: Photosensitive trichothiodystrophy
  classification: clinical and cellular UV-sensitivity subtype
  children:
  - TTD1 (ERCC2)
  - TTD2 (ERCC3)
  - TTD3 (GTF2H5)
  subtype_term:
    preferred_term: photosensitive trichothiodystrophy
    term:
      id: MONDO:0002470
      label: photosensitive trichothiodystrophy
  description: >-
    TTD caused by biallelic ERCC2, ERCC3, or GTF2H5 variants, with TFIIH
    dysfunction, defective nucleotide-excision repair, cellular UV sensitivity,
    and clinical photosensitivity. Photosensitivity is not a property of every TTD
    genetic branch.
  genes:
  - preferred_term: ERCC2
    term:
      id: hgnc:3434
      label: ERCC2
  - preferred_term: ERCC3
    term:
      id: hgnc:3435
      label: ERCC3
  - preferred_term: GTF2H5
    term:
      id: hgnc:21157
      label: GTF2H5
  evidence:
  - reference: PMID:9238033
    reference_title: Xeroderma pigmentosum and trichothiodystrophy are associated with different mutations in the XPD (ERCC2) repair/transcription gene.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Mutations in the XPD gene can result in three distinct clinical phenotypes,
      XP, trichothiodystrophy (TTD), and XP with Cockayne syndrome.
    explanation: >-
      Establishes ERCC2/XPD as an allelic cause of TTD.
  - reference: PMID:9012405
    reference_title: A mutation in the XPB/ERCC3 DNA repair transcription gene, associated with trichothiodystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The causative mutation was found to be a single base substitution
      resulting in a missense mutation (T119P) in a region of the XPB protein
      completely conserved in yeast, Drosophila, mouse, and man.
    explanation: >-
      Establishes a causal ERCC3/XPB variant in photosensitive TTD patients.
  - reference: PMID:15220921
    reference_title: A new, tenth subunit of TFIIH is responsible for the DNA repair syndrome trichothiodystrophy group A.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Microinjection of cDNA encoding TFB5 (GTF2H5, also called TTDA) corrected
      the DNA-repair defect of TTD-A cells, and we identified three functional
      inactivating mutations in this gene in three unrelated families with TTD-A.
    explanation: >-
      Establishes GTF2H5/TTDA as the TTD-A causal gene with cellular rescue.
- name: Non-photosensitive TTD
  display_name: Non-photosensitive trichothiodystrophy
  classification: clinical and cellular UV-sensitivity subtype
  children:
  - TTD4 (MPLKIP)
  - TTD5 (RNF113A)
  - TTD6 (GTF2E2)
  - TTD7 (TARS1)
  - TTD8 (AARS1)
  - TTD9 (MARS1)
  - CARS1-Related MDBH
  - DBR1-Related Sabinas TTD
  description: >-
    TTD caused by GTF2E2, MPLKIP, RNF113A, CARS1, TARS1, AARS1, MARS1, or DBR1
    variants. These forms retain normal post-UV survival or nucleotide-excision
    repair in the tested cells and affect transcription, RNA processing, or
    translation rather than TFIIH-dependent UV-lesion repair.
  genes:
  - preferred_term: GTF2E2
    term:
      id: hgnc:4651
      label: GTF2E2
  - preferred_term: MPLKIP
    term:
      id: hgnc:16002
      label: MPLKIP
  - preferred_term: RNF113A
    term:
      id: hgnc:12974
      label: RNF113A
  - preferred_term: CARS1
    term:
      id: hgnc:1493
      label: CARS1
  - preferred_term: TARS1
    term:
      id: hgnc:11572
      label: TARS1
  - preferred_term: AARS1
    term:
      id: hgnc:20
      label: AARS1
  - preferred_term: MARS1
    term:
      id: hgnc:6898
      label: MARS1
  - preferred_term: DBR1
    term:
      id: hgnc:15594
      label: DBR1
  evidence:
  - reference: PMID:40737808
    reference_title: "Trichothiodystrophy: Molecular insights and mechanisms of pathogenicity."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      The presence of hypersensitivity to UV light defines the two main forms of
      TTD: photosensitive (PS-TTD) and non-photosensitive (NPS-TTD).
    explanation: >-
      The current review defines the clinical PS/NPS division while emphasizing
      genetic and mechanistic heterogeneity.
  - reference: PMID:37800682
    reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      We previously proposed (Theil et al, 2019) that most of the key TTD
      features of brittle hair, nails, and scaly skin are a consequence of
      protein instability of factors involved in different steps of gene
      expression in either transcription (mutated ERCC2, ERCC3, GTF2H5, or
      GTF2E2), splicing (mutated RNF113A), or translation (mutated AARS1, CARS1,
      MARS1, or TARS1).
    explanation: >-
      Contemporary primary-literature context places GTF2E2, RNF113A, CARS1,
      TARS1, AARS1, and MARS1 within the TTD spectrum; individual genetic items
      retain their gene-specific evidence.
  - reference: PMID:15645389
    reference_title: Identification of C7orf11 (TTDN1) gene mutations and genetic heterogeneity in nonphotosensitive trichothiodystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We have identified C7orf11, which localizes to the nucleus and is expressed
      in fetal hair follicles, as the first disease gene for nonphotosensitive
      trichothiodystrophy (TTD).
    explanation: >-
      Establishes MPLKIP/TTDN1 as a non-photosensitive TTD cause.
  - reference: PMID:40683339
    reference_title: "RNA Lariat-Debranching Enzyme (DBR1) Variations in Sabinas Brittle Hair Syndrome Form of Trichothiodystrophy: A Trichothiodystrophy-Causing Gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report 5 nonphotosensitive adult patients from 3 unrelated families
      with a homozygous missense variation in DBR1 (p.D262Y) encoding the RNA
      lariat-debranching enzyme DBR1, which is involved in the removal of introns
      from pre-mRNA in the nucleus.
    explanation: >-
      Adds DBR1 as the eleventh causal gene and places it in the
      non-photosensitive branch.
- name: TTD1 (ERCC2)
  display_name: Trichothiodystrophy 1, photosensitive
  classification: gene-defined subtype
  subtype_term:
    preferred_term: trichothiodystrophy 1, photosensitive
    term:
      id: MONDO:0011125
      label: trichothiodystrophy 1, photosensitive
  description: Photosensitive TTD caused by biallelic TTD-causing ERCC2 variants.
  genes:
  - preferred_term: ERCC2
    term:
      id: hgnc:3434
      label: ERCC2
  review_notes: Gene-specific causal evidence is curated under genetic#ERCC2.
- name: TTD2 (ERCC3)
  display_name: Trichothiodystrophy 2, photosensitive
  classification: gene-defined subtype
  subtype_term:
    preferred_term: trichothiodystrophy 2, photosensitive
    term:
      id: MONDO:0014615
      label: trichothiodystrophy 2, photosensitive
  description: Photosensitive TTD caused by biallelic TTD-causing ERCC3 variants.
  genes:
  - preferred_term: ERCC3
    term:
      id: hgnc:3435
      label: ERCC3
  review_notes: Gene-specific causal evidence is curated under genetic#ERCC3.
- name: TTD3 (GTF2H5)
  display_name: Trichothiodystrophy 3, photosensitive
  classification: gene-defined subtype
  subtype_term:
    preferred_term: trichothiodystrophy 3, photosensitive
    term:
      id: MONDO:0014619
      label: trichothiodystrophy 3, photosensitive
  description: Photosensitive TTD caused by biallelic TTD-causing GTF2H5 variants.
  genes:
  - preferred_term: GTF2H5
    term:
      id: hgnc:21157
      label: GTF2H5
  review_notes: Gene-specific causal evidence is curated under genetic#GTF2H5.
- name: TTD4 (MPLKIP)
  display_name: Trichothiodystrophy 4, nonphotosensitive
  classification: gene-defined subtype
  subtype_term:
    preferred_term: trichothiodystrophy 4, nonphotosensitive
    term:
      id: MONDO:0021013
      label: trichothiodystrophy 4, nonphotosensitive
  description: Non-photosensitive TTD caused by biallelic TTD-causing MPLKIP variants.
  genes:
  - preferred_term: MPLKIP
    term:
      id: hgnc:16002
      label: MPLKIP
  review_notes: Gene-specific causal evidence is curated under genetic#MPLKIP.
- name: TTD5 (RNF113A)
  display_name: Trichothiodystrophy 5, nonphotosensitive
  classification: gene-defined subtype
  subtype_term:
    preferred_term: trichothiodystrophy 5, nonphotosensitive
    term:
      id: MONDO:0010495
      label: trichothiodystrophy 5, nonphotosensitive
  description: X-linked recessive non-photosensitive TTD caused by hemizygous RNF113A variants.
  genes:
  - preferred_term: RNF113A
    term:
      id: hgnc:12974
      label: RNF113A
  review_notes: Gene-specific causal evidence is curated under genetic#RNF113A.
- name: TTD6 (GTF2E2)
  display_name: Trichothiodystrophy 6, nonphotosensitive
  classification: gene-defined subtype
  subtype_term:
    preferred_term: trichothiodystrophy 6, nonphotosensitive
    term:
      id: MONDO:0014841
      label: trichothiodystrophy 6, nonphotosensitive
  description: Non-photosensitive TTD caused by biallelic TTD-causing GTF2E2 variants.
  genes:
  - preferred_term: GTF2E2
    term:
      id: hgnc:4651
      label: GTF2E2
  review_notes: Gene-specific causal evidence is curated under genetic#GTF2E2.
- name: TTD7 (TARS1)
  display_name: Trichothiodystrophy 7, nonphotosensitive
  classification: gene-defined subtype
  subtype_term:
    preferred_term: trichothiodystrophy 7, nonphotosensitive
    term:
      id: MONDO:0032806
      label: trichothiodystrophy 7, nonphotosensitive
  description: Non-photosensitive TTD caused by biallelic TTD-causing TARS1 variants.
  genes:
  - preferred_term: TARS1
    term:
      id: hgnc:11572
      label: TARS1
  review_notes: Gene-specific causal evidence is curated under genetic#TARS1.
- name: TTD8 (AARS1)
  display_name: Trichothiodystrophy 8, nonphotosensitive
  classification: gene-defined subtype
  subtype_term:
    preferred_term: trichothiodystrophy 8, nonphotosensitive
    term:
      id: MONDO:0030517
      label: trichothiodystrophy 8, nonphotosensitive
  description: Non-photosensitive TTD caused by biallelic TTD-causing AARS1 variants.
  genes:
  - preferred_term: AARS1
    term:
      id: hgnc:20
      label: AARS1
  review_notes: Gene-specific causal evidence is curated under genetic#AARS1.
- name: TTD9 (MARS1)
  display_name: Trichothiodystrophy 9, nonphotosensitive
  classification: gene-defined subtype
  subtype_term:
    preferred_term: trichothiodystrophy 9, nonphotosensitive
    term:
      id: MONDO:0030518
      label: trichothiodystrophy 9, nonphotosensitive
  description: Non-photosensitive TTD caused by biallelic TTD-causing MARS1 variants.
  genes:
  - preferred_term: MARS1
    term:
      id: hgnc:6898
      label: MARS1
  review_notes: Gene-specific causal evidence is curated under genetic#MARS1.
- name: CARS1-Related MDBH
  display_name: CARS1-related microcephaly, developmental delay, and brittle hair syndrome
  classification: contemporary TTD-spectrum inclusion
  subtype_term:
    preferred_term: microcephaly, developmental delay, and brittle hair syndrome
    term:
      id: MONDO:0030047
      label: microcephaly, developmental delay, and brittle hair syndrome
  description: >-
    Biallelic CARS1 disease is included in the contemporary non-photosensitive
    TTD spectrum, although MONDO does not currently place this named entity under
    the TTD umbrella.
  genes:
  - preferred_term: CARS1
    term:
      id: hgnc:1493
      label: CARS1
  review_notes: Causal and contemporary TTD-inclusion evidence is curated under genetic#CARS1.
- name: DBR1-Related Sabinas TTD
  display_name: DBR1-related Sabinas brittle hair syndrome
  classification: contemporary TTD-spectrum inclusion
  subtype_term:
    preferred_term: Sabinas brittle hair syndrome
    term:
      id: MONDO:0008886
      label: Sabinas brittle hair syndrome
  description: >-
    Homozygous DBR1 p.Asp262Tyr causes a non-photosensitive Sabinas TTD form,
    although MONDO does not currently place the named Sabinas entity under the
    TTD umbrella.
  genes:
  - preferred_term: DBR1
    term:
      id: hgnc:15594
      label: DBR1
  review_notes: Gene-specific causal evidence is curated under genetic#DBR1.
prevalence:
- population: Worldwide
  measure_type: POINT_PREVALENCE
  prevalence_class: BELOW_1_IN_1000000
  notes: >-
    The current Orphadata record supplies a validated worldwide class only,
    without a numeric point-prevalence estimate.
  evidence:
  - reference: url:https://api.orphadata.com/rd-epidemiology/orphacodes/33364
    reference_title: "https://api.orphadata.com/rd-epidemiology/orphacodes/33364"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      "PrevalenceClass": "<1 / 1 000 000",
      "PrevalenceGeographic": "Worldwide",
      "PrevalenceQualification": "Class only",
      "PrevalenceType": "Point prevalence",
      "PrevalenceValidationStatus": "Validated"
    explanation: >-
      The current official structured record supports the worldwide band and
      explicitly identifies it as class-only.
- population: Europe
  measure_type: BIRTH_PREVALENCE
  prevalence_class: BAND_1_9_PER_1000000
  rate_per_100000: 0.12
  notes: >-
    Orphadata reports a validated European birth-prevalence value of 0.12 per
    100,000 (approximately 1 per 830,000), sourced to PMID:18329345.
  evidence:
  - reference: url:https://api.orphadata.com/rd-epidemiology/orphacodes/33364
    reference_title: "https://api.orphadata.com/rd-epidemiology/orphacodes/33364"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      "PrevalenceClass": "1-9 / 1 000 000",
      "PrevalenceGeographic": "Europe",
      "PrevalenceQualification": "Value and class",
      "PrevalenceType": "Prevalence at birth",
      "PrevalenceValidationStatus": "Validated",
      "Source": "18329345[PMID]",
      "ValMoy": "0.12"
    explanation: >-
      Directly supplies the geographic scope, measure type, rate, validation
      status, and source of the structured estimate.
inheritance:
- name: Autosomal recessive
  description: >-
    Most molecularly defined TTD branches are autosomal recessive. The
    gene-specific records below provide the individual causal and allelic
    evidence; RNF113A-related TTD is the X-linked exception.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:18603627
    reference_title: "Trichothiodystrophy: a systematic review of 112 published cases characterises a wide spectrum of clinical manifestations."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Trichothiodystrophy (TTD) is a rare, autosomal recessive disease,
      characterised by brittle, sulfur deficient hair and multisystem
      abnormalities.
    explanation: >-
      Establishes the predominant recessive inheritance pattern; the later
      RNF113A discovery created a rare X-linked exception.
  - reference: PMID:40683339
    reference_title: "RNA Lariat-Debranching Enzyme (DBR1) Variations in Sabinas Brittle Hair Syndrome Form of Trichothiodystrophy: A Trichothiodystrophy-Causing Gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report 5 nonphotosensitive adult patients from 3 unrelated families
      with a homozygous missense variation in DBR1 (p.D262Y)
    explanation: >-
      Documents homozygous DBR1-associated TTD in three families.
- name: X-linked recessive
  description: >-
    RNF113A-related TTD is X-linked recessive. Reported affected males carry
    hemizygous RNF113A variants; female relatives may be carriers or mosaic.
  inheritance_term:
    preferred_term: X-linked recessive inheritance
    term:
      id: HP:0001419
      label: X-linked recessive inheritance
  evidence:
  - reference: PMID:25612912
    reference_title: A novel X-linked trichothiodystrophy associated with a nonsense mutation in RNF113A.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The association of RNF113A mutation with non-photosensitive TTD identifies
      a new locus for these disorders on the X chromosome.
    explanation: >-
      The founding family establishes an X-linked RNF113A-associated TTD locus.
  - reference: PMID:41531333
    reference_title: Novel RNF113A Variant Underlying X-Linked Trichothiodystrophy With Presumed Mosaicism in an Unaffected Mother.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Though the majority of TTDs are inherited in an autosomal recessive
      pattern, RNF113A related trichothiodystrophy is X-linked.
    explanation: >-
      A current two-brother report confirms the X-linked exception and its
      counseling relevance.
  - reference: PMID:41684880
    reference_title: Twins With Pathogenic RNF113A Variant Presenting With Testicular Regression Syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Whole-exome sequencing identified a maternally inherited variant
      consistent with X-linked recessive inheritance.
    explanation: >-
      Directly supports the X-linked recessive pattern in hemizygous affected
      twins.
pathophysiology:
- name: TFIIH Complex Instability in Photosensitive TTD
  role: trigger
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    TTD-causing ERCC2, ERCC3, and GTF2H5 variants reduce the cellular abundance
    and/or structural integrity of the ten-subunit TFIIH complex. TFIIH serves
    both nucleotide-excision repair and RNA polymerase II transcription, so this
    branch must be separated into repair and transcription consequences rather
    than treated as a syndrome-wide lesion.
  genes:
  - preferred_term: ERCC2
    term:
      id: hgnc:3434
      label: ERCC2
  - preferred_term: ERCC3
    term:
      id: hgnc:3435
      label: ERCC3
  - preferred_term: GTF2H5
    term:
      id: hgnc:21157
      label: GTF2H5
  evidence:
  - reference: PMID:12393803
    reference_title: Reduced level of the repair/transcription factor TFIIH in trichothiodystrophy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Here we show that all the mutations found in TTD cases, irrespective of
      whether they are homozygotes, hemizygotes or compound heterozygotes, cause
      a substantial and specific reduction (by up to 70%) in the cellular
      concentration of TFIIH.
    explanation: >-
      Patient-cell measurements directly establish reduced cellular TFIIH.
  downstream:
  - target: Defective Nucleotide-Excision Repair after UV Damage
    causal_link_type: DIRECT
    description: >-
      Reduced or dysfunctional TFIIH directly impairs its nucleotide-excision
      repair function in photosensitive TTD.
    evidence:
    - reference: PMID:12820975
      reference_title: Basal transcription defect discriminates between xeroderma pigmentosum and trichothiodystrophy in XPD patients.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        We demonstrate that all the XPD mutations are detrimental for XPD
        helicase activity, thus explaining the NER defect.
      explanation: >-
        Directly links XPD helicase impairment to the NER defect in the studied
        ERCC2 branch.
    - reference: PMID:18470933
      reference_title: "Persistence of repair proteins at unrepaired DNA damage distinguishes diseases with ERCC2 (XPD) mutations: cancer-prone xeroderma pigmentosum vs. non-cancer-prone trichothiodystrophy."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        In TTD cells with defects in the XPD, XPB, or GTF2H5 (TTDA) genes, in
        contrast, recruitment of these NER proteins was reduced compared to
        normals at early time points (p<0.001) and remained low at 24 hr
        postirradiation.
      explanation: >-
        Extends the UV-repair defect across all three photosensitive TFIIH gene
        branches in patient cells.
  - target: Basal Transcription Insufficiency in ERCC2-Related TTD
    causal_link_type: DIRECT
    description: >-
      Reduced ERCC2-containing TFIIH directly impairs basal transcription in the
      studied ERCC2-related TTD cells; equivalent evidence is not projected to
      every TFIIH gene branch.
    evidence:
    - reference: PMID:12820975
      reference_title: Basal transcription defect discriminates between xeroderma pigmentosum and trichothiodystrophy in XPD patients.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        We also show that TFIIH from TTD patients, but not from XP patients,
        exhibits a significant in vitro basal transcription defect in addition to
        a reduced intracellular concentration.
      explanation: >-
        Directly links ERCC2-related TTD TFIIH deficiency to the measured
        basal-transcription defect.
  - target: Unprocessed R-Loop Accumulation in ERCC2/ERCC3-Related TTD
    causal_link_type: DIRECT
    description: >-
      R-loop accumulation has been directly measured in ERCC2- and ERCC3-related
      photosensitive TTD cells, but not yet in GTF2H5-related cells.
    evidence:
    - reference: PMID:40757642
      reference_title: Trichothiodystrophy-causative pathogenic variants impair a cooperative action of TFIIH and DDX1 in R-loop processing.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        TTD-specific variants in ERCC2/XPD result in TFIIH instability, altered
        interaction of the CAK with DDX1-SFPQ-NONO, and R-loop accumulation.
      explanation: >-
        Patient fibroblast and perturbation experiments directly connect
        TTD-specific TFIIH changes with R-loop accumulation.
    - reference: PMID:40757642
      reference_title: Trichothiodystrophy-causative pathogenic variants impair a cooperative action of TFIIH and DDX1 in R-loop processing.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        we extended the DRIP analysis to fibroblasts from one of the only two cases
        (siblings) who carry XPB alterations and PS-TTD clinical features. As shown
        in Fig. 7C (right panel), we found a significant accumulation of R-loops
        along the entire ACTB locus, demonstrating that both XPD and XPB alterations
        causative of PS-TTD lead to an excess of unprocessed R-loops in the
        transcribed regions of the genome
      explanation: >-
        Supplies the direct ERCC3/XPB evidence for the two-gene edge claim.
- name: Defective Nucleotide-Excision Repair after UV Damage
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    In TFIIH-related photosensitive TTD, impaired removal of UV-induced bulky
    DNA lesions produces cellular UV hypersensitivity and cutaneous
    photosensitivity. This node does not apply to the non-photosensitive gene
    branches, which retain normal UV repair in tested cells.
  biological_processes:
  - preferred_term: nucleotide-excision repair
    term:
      id: GO:0006289
      label: nucleotide-excision repair
    modifier: DECREASED
  cell_types:
  - preferred_term: skin fibroblast
    term:
      id: CL:0002620
      label: skin fibroblast
  evidence:
  - reference: PMID:12820975
    reference_title: Basal transcription defect discriminates between xeroderma pigmentosum and trichothiodystrophy in XPD patients.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We demonstrate that all the XPD mutations are detrimental for XPD helicase
      activity, thus explaining the NER defect.
    explanation: >-
      Establishes defective XPD helicase activity as the repair lesion in the
      studied ERCC2-related cells.
  - reference: PMID:18470933
    reference_title: "Persistence of repair proteins at unrepaired DNA damage distinguishes diseases with ERCC2 (XPD) mutations: cancer-prone xeroderma pigmentosum vs. non-cancer-prone trichothiodystrophy."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      In TTD cells with defects in the XPD, XPB, or GTF2H5 (TTDA) genes, in
      contrast, recruitment of these NER proteins was reduced compared to normals
      at early time points (p<0.001) and remained low at 24 hr postirradiation.
    explanation: >-
      Patient-cell evidence establishes abnormal NER-protein recruitment for
      ERCC2-, ERCC3-, and GTF2H5-related TTD.
  downstream:
  - target: Cutaneous Photosensitivity
    causal_link_type: DIRECT
    description: >-
      Failure to repair UV photolesions produces photosensitivity in the
      TFIIH-related subtype.
    evidence:
    - reference: PMID:40737808
      reference_title: "Trichothiodystrophy: Molecular insights and mechanisms of pathogenicity."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        The presence of hypersensitivity to UV light defines the two main forms
        of TTD: photosensitive (PS-TTD) and non-photosensitive (NPS-TTD).
      explanation: >-
        Current review evidence supports the subtype-bounded phenotype link.
- name: Basal Transcription Insufficiency in ERCC2-Related TTD
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    TFIIH isolated from ERCC2/XPD-related TTD cells has reduced abundance and a
    measured basal-transcription defect. This node is kept separate from the
    newer R-loop-processing lesion and is not generalized to ERCC3 or GTF2H5.
  genes:
  - preferred_term: ERCC2
    term:
      id: hgnc:3434
      label: ERCC2
  biological_processes:
  - preferred_term: transcription by RNA polymerase II
    term:
      id: GO:0006366
      label: transcription by RNA polymerase II
    modifier: DECREASED
  cell_types:
  - preferred_term: skin fibroblast
    term:
      id: CL:0002620
      label: skin fibroblast
  evidence:
  - reference: PMID:12820975
    reference_title: Basal transcription defect discriminates between xeroderma pigmentosum and trichothiodystrophy in XPD patients.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We also show that TFIIH from TTD patients, but not from XP patients,
      exhibits a significant in vitro basal transcription defect in addition to
      a reduced intracellular concentration.
    explanation: >-
      Establishes a basal-transcription defect in the studied TTD patient cells.
  downstream:
  - target: Reduced Beta-Globin Expression in ERCC2-Related TTD
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - limiting TFIIH-dependent transcription in terminal erythroid cells
    description: >-
      ERCC2/XPD variants that limit TFIIH are associated with reduced beta-globin
      mRNA and synthesis in affected individuals, while the cell-state-specific
      transcriptional bridge is not represented as a direct molecular contact.
    evidence:
    - reference: PMID:11734544
      reference_title: Mutations in the general transcription factor TFIIH result in beta-thalassaemia in individuals with trichothiodystrophy.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Here we show that the specific mutations in XPD that cause TTD result in
        reduced expression of the beta-globin genes in these individuals.
      explanation: >-
        Direct patient evidence links ERCC2-related TFIIH limitation to reduced
        beta-globin expression.
  - target: Impaired BCR-Mediated B-Cell Activation in ERCC2-Related TTD
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - reduced activation-induced transcription in ERCC2-deficient lymphocytes
    description: >-
      ERCC2-deficient patient lymphocytes show downregulated activation programs
      and impaired BCR-mediated activation, supporting a transcription-sensitive
      immune branch distinct from the erythroid and R-loop states.
    evidence:
    - reference: PMID:39055713
      reference_title: Impaired B-cell function in ERCC2 deficiency.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        mRNA sequencing analysis revealed significantly downregulated genes
        needed for B-cell development and activation.
      explanation: >-
        Supplies the measured activation-transcription intermediate in the three
        ERCC2-related TTD1 patients.
  - target: Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - translation from the Xpd mouse brain to human brain development
    description: >-
      Mouse evidence links limiting TFIIH to thyroid-hormone-responsive brain
      transcription, but the human developmental intermediates are unresolved.
    evidence:
    - reference: PMID:17952069
      reference_title: Neurological defects in trichothiodystrophy reveal a coactivator function of TFIIH.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Here we show that an XPD mutation in TTD mice results in a spatial and
        selective deregulation of thyroid hormone target genes in the brain.
      explanation: >-
        Supports the mouse branch while leaving its fidelity to human brain
        development open.
  - target: Unresolved Tissue-Specific Gene-Expression Convergence
    causal_link_type: UNKNOWN
    description: >-
      TFIIH transcriptional stress is one candidate input to the shared TTD
      phenotype, but the cross-genotype tissue-specific bridge is unresolved.
    evidence:
    - reference: PMID:40737808
      reference_title: "Trichothiodystrophy: Molecular insights and mechanisms of pathogenicity."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        The disease arises from mutations in a variety of genes involved in
        different biological processes. Affected processes include DNA repair,
        transcription as well as translation.
      explanation: >-
        Establishes mechanistic heterogeneity but does not prove a single
        convergence route.
- name: Unprocessed R-Loop Accumulation in ERCC2/ERCC3-Related TTD
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    ERCC2- and ERCC3-related photosensitive TTD fibroblasts accumulate
    unprocessed R-loops. Altered TFIIH cooperation with DDX1-SFPQ-NONO was
    established in the ERCC2 arm and is not projected onto ERCC3; neither state
    has been demonstrated in GTF2H5-related cells or other TTD gene branches.
  genes:
  - preferred_term: ERCC2
    term:
      id: hgnc:3434
      label: ERCC2
  - preferred_term: ERCC3
    term:
      id: hgnc:3435
      label: ERCC3
  biological_processes:
  - preferred_term: R-loop processing
    term:
      id: GO:0062176
      label: R-loop processing
    modifier: DECREASED
  cell_types:
  - preferred_term: skin fibroblast
    term:
      id: CL:0002620
      label: skin fibroblast
  evidence:
  - reference: PMID:40757642
    reference_title: Trichothiodystrophy-causative pathogenic variants impair a cooperative action of TFIIH and DDX1 in R-loop processing.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      TTD-specific variants in ERCC2/XPD result in TFIIH instability, altered
      interaction of the CAK with DDX1-SFPQ-NONO, and R-loop accumulation.
    explanation: >-
      Directly supports the R-loop-processing lesion in ERCC2-related TTD cells.
  - reference: PMID:40757642
    reference_title: Trichothiodystrophy-causative pathogenic variants impair a cooperative action of TFIIH and DDX1 in R-loop processing.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      we extended the DRIP analysis to fibroblasts from one of the only two cases
      (siblings) who carry XPB alterations and PS-TTD clinical features. As shown
      in Fig. 7C (right panel), we found a significant accumulation of R-loops
      along the entire ACTB locus, demonstrating that both XPD and XPB alterations
      causative of PS-TTD lead to an excess of unprocessed R-loops in the
      transcribed regions of the genome
    explanation: >-
      Directly extends the measured R-loop phenotype to the ERCC3/XPB branch.
  downstream:
  - target: R-Loop-Driven Transcriptional Stress in ERCC2-Related TTD Cells
    causal_link_type: DIRECT
    description: >-
      Moderate RNase-H1 expression restored reduced 5-EU incorporation in the
      single ERCC2-related TTD7PV fibroblast line, directly implicating
      accumulated R-loops in that measured transcriptional stress.
    evidence:
    - reference: PMID:40757642
      reference_title: Trichothiodystrophy-causative pathogenic variants impair a cooperative action of TFIIH and DDX1 in R-loop processing.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        However, when we focused our analysis on the fibroblasts expressing
        moderate levels of GFP, we could observe that the transcriptional
        activity in PS-TTD cells was reduced compared to control fibroblasts from
        healthy donors but that it increased up to normal levels after RNase
        H1GFP expression.
      explanation: >-
        RNase-H1 rescue places pathological R-loop accumulation upstream of the
        measured ERCC2-cell transcriptional defect.
- name: R-Loop-Driven Transcriptional Stress in ERCC2-Related TTD Cells
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    In the ERCC2-related TTD7PV fibroblast line, accumulated R-loops contribute
    to reduced 5-EU incorporation. RNase-H1 rescue supports this state as
    downstream of the R-loop-processing lesion; replication in additional
    ERCC2 lines and equivalent rescue in ERCC3 or GTF2H5 remain untested, as do
    tissue consequences.
  genes:
  - preferred_term: ERCC2
    term:
      id: hgnc:3434
      label: ERCC2
  biological_processes:
  - preferred_term: transcription by RNA polymerase II
    term:
      id: GO:0006366
      label: transcription by RNA polymerase II
    modifier: DECREASED
  cell_types:
  - preferred_term: skin fibroblast
    term:
      id: CL:0002620
      label: skin fibroblast
  evidence:
  - reference: PMID:40757642
    reference_title: Trichothiodystrophy-causative pathogenic variants impair a cooperative action of TFIIH and DDX1 in R-loop processing.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Global RNA synthesis by in vivo labelling with 5-ethynyluridine (EU) and
      EU-click reaction (red staining) in C5PV and TTD7PV fibroblasts transfected
      for 48 h with the plasmid expressing RNase H1GFP (green staining) or the
      empty vector.
    explanation: >-
      Identifies the single TTD7PV line and 5-EU readout used in the rescue
      experiment.
  - reference: PMID:40757642
    reference_title: Trichothiodystrophy-causative pathogenic variants impair a cooperative action of TFIIH and DDX1 in R-loop processing.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      This finding demonstrates that the R-loop accumulation in PS-TTD
      contributes to the transcription deregulations caused by pathogenic
      variants affecting the stability of TFIIH complex.
    explanation: >-
      Directly establishes R-loop-driven transcriptional deregulation in the
      studied ERCC2-related photosensitive TTD cells.
  downstream:
  - target: Unresolved Tissue-Specific Gene-Expression Convergence
    causal_link_type: UNKNOWN
    description: >-
      R-loop-driven transcriptional stress is measured in fibroblasts, but its
      contribution to shared hair and developmental findings is unresolved.
    evidence:
    - reference: PMID:40737808
      reference_title: "Trichothiodystrophy: Molecular insights and mechanisms of pathogenicity."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        The disease arises from mutations in a variety of genes involved in
        different biological processes. Affected processes include DNA repair,
        transcription as well as translation.
      explanation: >-
        Supports mechanistic heterogeneity without proving the tissue-level
        consequence of this fibroblast state.
- name: Reduced Beta-Globin Expression in ERCC2-Related TTD
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    Eleven individuals with ERCC2/XPD-related TTD had reduced beta-globin mRNA
    and synthesis together with beta-thalassemia-trait hematology. This
    patient-derived erythroid mechanism is distinct from the GTF2E2 iPSC
    erythroid branch.
  genes:
  - preferred_term: ERCC2
    term:
      id: hgnc:3434
      label: ERCC2
  biological_processes:
  - preferred_term: transcription by RNA polymerase II
    term:
      id: GO:0006366
      label: transcription by RNA polymerase II
    modifier: DECREASED
  evidence:
  - reference: PMID:11734544
    reference_title: Mutations in the general transcription factor TFIIH result in beta-thalassaemia in individuals with trichothiodystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Eleven TTD patients with characterized mutations in the XPD gene have the
      haematological features of beta-thalassaemia trait, and reduced levels of
      beta-globin synthesis and beta-globin mRNA.
    explanation: >-
      Directly establishes the ERCC2-related globin-expression state in the
      patient series.
  downstream:
  - target: Microcytic Anemia
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - beta-globin-chain insufficiency and beta-thalassemia-trait erythropoiesis
    description: >-
      Reduced beta-globin expression produces beta-thalassemia-trait hematology,
      providing an ERCC2-specific route to the microcytic phenotype.
    evidence:
    - reference: PMID:11734544
      reference_title: Mutations in the general transcription factor TFIIH result in beta-thalassaemia in individuals with trichothiodystrophy.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Eleven TTD patients with characterized mutations in the XPD gene have the
        haematological features of beta-thalassaemia trait, and reduced levels of
        beta-globin synthesis and beta-globin mRNA.
      explanation: >-
        Connects reduced beta-globin expression to the observed thalassemia-trait
        hematology without merging it with GTF2E2-related anemia.
- name: Impaired BCR-Mediated B-Cell Activation in ERCC2-Related TTD
  biological_scale: CELLULAR
  mechanism_confidence: PROVISIONAL
  description: >-
    B cells from three unrelated ERCC2-related TTD1 patients showed reduced
    activation-marker induction after BCR stimulation, decreased proliferation,
    and downregulated activation and immunoglobulin-heavy-chain programs. The
    measured state is subtype-specific and provisional because of the small
    cohort.
  genes:
  - preferred_term: ERCC2
    term:
      id: hgnc:3434
      label: ERCC2
  cell_types:
  - preferred_term: B cell
    term:
      id: CL:0000236
      label: B cell
  biological_processes:
  - preferred_term: B cell activation
    term:
      id: GO:0042113
      label: B cell activation
    modifier: DECREASED
  evidence:
  - reference: PMID:39055713
    reference_title: Impaired B-cell function in ERCC2 deficiency.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      In healthy controls (n=13), naïve CD19+ B-cells up-regulated CD69 and CD86
      expression 24 hours after BCR-stimulation in the presence of exogenous IL-2,
      whereas activation marker induction was decreased in naïve B-cells from the
      ERCC2 deficient patients
    explanation: >-
      Directly establishes impaired BCR-mediated activation in the studied
      patient B cells.
  - reference: PMID:39055713
    reference_title: Impaired B-cell function in ERCC2 deficiency.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      mRNA differential expression analysis showed that the gene expression of
      immunologically important activation and proliferation factors such as
      EGR1, EGR2, EGR3 as well as IGHM (also of light chain transcripts, data not
      shown) are downregulated which is in good agreement with the defective
      BCR-mediated B-cell activation and the antibody production observed in
      TTD1 patients.
    explanation: >-
      Provides activation-induced transcription and immunoglobulin-program
      evidence for the same ERCC2-specific branch.
  downstream:
  - target: Reduced Naive and Transitional B-Cell Compartments in ERCC2-Related TTD
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - impaired activation-induced transcription during B-cell maturation
    description: >-
      The authors infer that impaired BCR activation and activation-induced
      transcription alter B-cell differentiation; the causal ordering is
      supported but not demonstrated by a direct rescue experiment.
    evidence:
    - reference: PMID:39055713
      reference_title: Impaired B-cell function in ERCC2 deficiency.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        ERCC2 deficiency is associated with antibody deficiency most likely due
        to altered B-cell differentiation resulting from impaired BCR-mediated
        B-cell activation and activation-induced gene transcription.
      explanation: >-
        Explicitly supports the proposed activation-to-differentiation route
        while retaining its inferential status.
  - target: Decreased Circulating Immunoglobulin Concentration
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - altered B-cell differentiation and reduced immunoglobulin-gene transcription
    description: >-
      Impaired activation is the proposed route to antibody deficiency in the
      three-patient ERCC2 cohort.
    evidence:
    - reference: PMID:39055713
      reference_title: Impaired B-cell function in ERCC2 deficiency.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Hypogammaglobulinemia and decreased antibody responses following
        vaccination were found.
      explanation: >-
        Establishes the antibody phenotype paired with the measured activation
        defect, while the edge remains indirect.
  - target: Recurrent Infections
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - impaired antibody production and vaccine responses
    description: >-
      The ERCC2-related patients had early recurrent infections together with
      antibody deficiency and impaired B-cell activation.
    evidence:
    - reference: PMID:39055713
      reference_title: Impaired B-cell function in ERCC2 deficiency.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We investigated three unrelated TTD1 patients who presented with
        recurrent infections early in life
      explanation: >-
        Documents the infection phenotype in the same mechanistically studied
        cohort without claiming complete mediation by this one immune route.
- name: Reduced Naive and Transitional B-Cell Compartments in ERCC2-Related TTD
  biological_scale: CELLULAR
  mechanism_confidence: PROVISIONAL
  description: >-
    Three ERCC2-related TTD1 patients had low circulating naïve and transitional
    B-cell numbers, supporting abnormal in-vivo B-cell differentiation. The
    observation remains provisional because of the small subtype-specific
    cohort.
  genes:
  - preferred_term: ERCC2
    term:
      id: hgnc:3434
      label: ERCC2
  cell_types:
  - preferred_term: B cell
    term:
      id: CL:0000236
      label: B cell
  biological_processes:
  - preferred_term: B cell differentiation
    term:
      id: GO:0030183
      label: B cell differentiation
    modifier: ABNORMAL
  evidence:
  - reference: PMID:39055713
    reference_title: Impaired B-cell function in ERCC2 deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Analysis of B-cell subpopulations showed low numbers of naïve and
      transitional B-cells in TTD1 patients, indicating abnormal B-cell
      differentiation in vivo.
    explanation: >-
      Directly establishes the measured B-cell compartment abnormality.
  downstream:
  - target: Decreased Circulating Immunoglobulin Concentration
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - reduced maturation into antibody-producing B-cell compartments
    description: >-
      Reduced naïve and transitional compartments co-occur with antibody
      deficiency, but their quantitative contribution to circulating
      immunoglobulin is unresolved.
    evidence:
    - reference: PMID:39055713
      reference_title: Impaired B-cell function in ERCC2 deficiency.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Hypogammaglobulinemia and decreased antibody responses following
        vaccination were found.
      explanation: >-
        Supports the paired clinical phenotype while preserving causal
        uncertainty.
- name: GTF2E2-Related TFIIE Instability
  role: trigger
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    Biallelic GTF2E2 variants destabilize both TFIIE subunits and reduce TFIIE
    phosphorylation in patient cells while UV-damage repair remains normal.
    Despite severe TFIIE loss, basal transcription at standard culture
    temperature can remain normal; conditional transcriptional and
    differentiation defects are therefore modeled downstream rather than made
    constitutive properties of this root.
  genes:
  - preferred_term: GTF2E2
    term:
      id: hgnc:4651
      label: GTF2E2
  evidence:
  - reference: PMID:26996949
    reference_title: GTF2E2 Mutations Destabilize the General Transcription Factor Complex TFIIE in Individuals with DNA Repair-Proficient Trichothiodystrophy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We found decreased protein levels of the two TFIIE subunits (TFIIEα and
      TFIIEβ) as well as decreased phosphorylation of TFIIEα in cells from both
      children.
    explanation: >-
      Direct patient-cell evidence for reduced TFIIE abundance and
      phosphorylation.
  - reference: PMID:26996949
    reference_title: GTF2E2 Mutations Destabilize the General Transcription Factor Complex TFIIE in Individuals with DNA Repair-Proficient Trichothiodystrophy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Repair of ultraviolet-induced DNA damage was normal in the GTF2E2 mutated
      cells, indicating that TFIIE was not involved in NER.
    explanation: >-
      Explicitly bounds this branch away from the photosensitive NER defect.
  downstream:
  - target: Temperature-Sensitive Transcriptional Insufficiency in GTF2E2-Related TTD
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - further depletion or inactivation of unstable TFIIE at elevated temperature
    description: >-
      Prolonged elevated-temperature culture unmasks a transcription defect in
      GTF2E2-mutant patient cells that is absent under standard conditions.
    evidence:
    - reference: PMID:28973399
      reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        We demonstrate that mutant TFIIEβ strongly reduces the total amount of
        the entire TFIIE complex, with a remarkable temperature-sensitive
        transcription defect, which strikingly correlates with the phenotypic
        aggravation of key clinical symptoms after episodes of high fever.
      explanation: >-
        Establishes the conditional, rather than constitutive, transcriptional
        defect.
  - target: GTF2E2-Related Late Erythroid Differentiation Dysfunction
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - limiting TFIIE during terminal erythroid differentiation
    description: >-
      Patient-fibroblast-derived iPS cells show abnormal late erythroid
      differentiation and hemoglobin-subunit imbalance despite normal early
      erythroid expansion.
    evidence:
    - reference: PMID:28973399
      reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        We performed induced pluripotent stem (iPS) cell reprogramming of patient
        fibroblasts followed by in vitro erythroid differentiation to translate
        the intriguing molecular defect to phenotypic expression in relevant
        tissue, to disclose the molecular basis for some specific TTD features.
        We observed a clear hematopoietic defect during late-stage differentiation
        associated with hemoglobin subunit imbalance.
      explanation: >-
        Directly links the GTF2E2 patient genotype to an erythroid differentiation
        phenotype in a human iPSC-derived model.
- name: Temperature-Sensitive Transcriptional Insufficiency in GTF2E2-Related TTD
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    GTF2E2-mutant fibroblasts retain normal measured transcription under standard
    culture conditions but show reduced transcription after prolonged culture at
    elevated temperature. This conditional state may explain fever-associated
    worsening in some patients, but fever sensitivity is not universal.
  biological_processes:
  - preferred_term: transcription by RNA polymerase II
    term:
      id: GO:0006366
      label: transcription by RNA polymerase II
    modifier: DECREASED
  evidence:
  - reference: PMID:28973399
    reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We showed that fibroblasts derived from these patients exhibit a clear
      transcription defect, however only when cultured at elevated temperatures.
    explanation: >-
      Explicitly limits decreased transcription to the elevated-temperature
      condition.
  downstream:
  - target: Unresolved Tissue-Specific Gene-Expression Convergence
    causal_link_type: UNKNOWN
    description: >-
      Conditional transcription failure is one plausible route to clinical
      exacerbation, but its contribution across tissues and genotypes is unknown.
    evidence:
    - reference: PMID:28973399
      reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        These new findings of a DNA repair-independent transcription defect and
        tissue-specific malfunctioning provide novel mechanistic insight into
        the etiology of TTD.
      explanation: >-
        Supports tissue-specific relevance without establishing a pan-TTD route.
  - target: Alopecia
    causal_link_type: UNKNOWN
    description: >-
      Fever-associated scalp hair loss occurred in a GTF2E2-related patient, but
      the clinical observation does not establish how conditional TFIIE loss
      produces loss of hair.
    evidence:
    - reference: PMID:28973399
      reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        She showed recurrent aggravation of hair-loss by tufted breakage at the
        scalp boundary immediately following episodes of infection-induced fever.
      explanation: >-
        Directly documents episodic hair loss in the molecularly characterized
        GTF2E2-related patient while leaving its causal route unresolved.
- name: GTF2E2-Related Late Erythroid Differentiation Dysfunction
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    Patient-derived iPSC erythroblasts show abnormal late-stage differentiation,
    altered cell size or multinucleation, and hemoglobin-subunit imbalance. This
    is a GTF2E2-specific model finding rather than a universal TTD mechanism.
  cell_types:
  - preferred_term: erythroblast
    term:
      id: CL:0000765
      label: erythroblast
  biological_processes:
  - preferred_term: erythrocyte differentiation
    term:
      id: GO:0030218
      label: erythrocyte differentiation
    modifier: ABNORMAL
  evidence:
  - reference: PMID:28973399
    reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We observed a clear hematopoietic defect during late-stage differentiation
      associated with hemoglobin subunit imbalance.
    explanation: >-
      Directly supports the measured erythroid model state.
  downstream:
  - target: Microcytic Anemia
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - translation from fetal-program iPSC erythroblasts to patient erythrocyte production
    description: >-
      The model offers a plausible route to microcytic anemia, but its iPSC
      erythroblasts execute an embryonic/fetal globin program and do not establish
      the complete human in-vivo mechanism.
    evidence:
    - reference: PMID:28973399
      reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        We observed a clear hematopoietic defect during late-stage differentiation
        associated with hemoglobin subunit imbalance.
      explanation: >-
        Supports an erythroid mechanism candidate while preserving the
        model-to-human inference boundary.
- name: MPLKIP Deficiency
  role: trigger
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    Biallelic MPLKIP variants reduce or eliminate functional TTDN1/MPLKIP. The
    resulting disease branch is non-photosensitive and is kept distinct from
    primary DBR1 variation so that MPLKIP-specific skin-model findings are not
    silently projected onto DBR1-related Sabinas syndrome.
  genes:
  - preferred_term: MPLKIP
    term:
      id: hgnc:16002
      label: MPLKIP
  evidence:
  - reference: PMID:37800682
    reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here, we present seven additional TTD individuals with MPLKIP mutations
      from five consanguineous families, with a newly identified MPLKIP variant
      in one family.
    explanation: >-
      The multi-family patient series establishes the MPLKIP-related branch.
  downstream:
  - target: Reduced DBR1 Abundance
    causal_link_type: DIRECT
    description: >-
      MPLKIP directly interacts with DBR1, and MPLKIP-deficient primary
      fibroblasts have reduced steady-state DBR1.
    evidence:
    - reference: PMID:37800682
      reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        By mass spectrometry-based interaction proteomics, we demonstrate that
        MPLKIP interacts with core splicing factors and the lariat debranching
        protein DBR1. MPLKIP-deficient primary fibroblasts have reduced steady-state
        DBR1 protein levels.
      explanation: >-
        Directly establishes MPLKIP-dependent maintenance of DBR1 abundance.
  - target: Intron-Lariat Accumulation after MPLKIP Loss
    causal_link_type: DIRECT
    description: >-
      TTDN1/MPLKIP loss directly causes intron-lariat accumulation in cellular
      models and patient fibroblasts.
    evidence:
    - reference: PMID:37369199
      reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        TTDN1 loss, or a mutated IDR, causes significant intron lariat
        accumulation, as well as splicing and gene expression defects, mirroring
        phenotypes observed in NP-TTD patient cells.
      explanation: >-
        Directly supports the MPLKIP-specific lariat-accumulation state.
  - target: Aberrant RNA Splicing after MPLKIP Loss
    causal_link_type: DIRECT
    description: >-
      TTDN1/MPLKIP loss directly produces altered RNA-splicing patterns in
      knockout cells and patient fibroblasts.
    evidence:
    - reference: PMID:37369199
      reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        TTDN1 loss, or a mutated IDR, causes significant intron lariat
        accumulation, as well as splicing and gene expression defects, mirroring
        phenotypes observed in NP-TTD patient cells.
      explanation: >-
        Directly supports the MPLKIP-specific splicing state after loss
        perturbation.
  - target: Seizure
    causal_link_type: UNKNOWN
    description: >-
      Seizure disorders were overrepresented in a five-patient MPLKIP cohort,
      but no causal route from MPLKIP loss to seizure susceptibility was shown.
    evidence:
    - reference: PMID:25290684
      reference_title: Mutations in the TTDN1 gene are associated with a distinct trichothiodystrophy phenotype.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        There were 3 patients from 2 families with TTDN1 mutations (TTD343BE,
        and siblings TTD487BE and TTD488BE) diagnosed with seizure disorders,
        but only 3 out of the 31 other TTD patients (p=0.024).
      explanation: >-
        Establishes subtype enrichment while leaving the mechanism unknown.
  - target: Delayed Skeletal Maturation
    causal_link_type: UNKNOWN
    description: >-
      Delayed bone age was overrepresented in the MPLKIP cohort, but the
      developmental mechanism was not established.
    evidence:
    - reference: PMID:25290684
      reference_title: Mutations in the TTDN1 gene are associated with a distinct trichothiodystrophy phenotype.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Additionally, 4 patients from 3 families with TTDN1 defects had delayed
        bone age (TTD343BE, TTD480BE, and siblings TTD487BE and TTD488BE), as
        compared to 4 out of 23 TTD patients with other mutations (p=0.009).
      explanation: >-
        Establishes subtype enrichment while preserving causal uncertainty.
  - target: Hypogonadism
    causal_link_type: UNKNOWN
    description: >-
      Hypogonadism was observed in a seven-person MPLKIP-associated NPS-TTD
      cohort, but the study did not establish a causal route from MPLKIP loss to
      gonadal dysfunction.
    evidence:
    - reference: PMID:37800682
      reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We investigated a cohort of seven patients from five unrelated families
        with characteristic features of TTD (Figs 1A–F and 2A and B), including
        short, woolly, slow‐growing, sparse and brittle hair, sparse and brittle
        eyebrows and eyelashes, ectodermal abnormalities comprising keratosis
        pilaris, hyperkeratotic plaques on the scalp, dry skin and nail dystrophy,
        dysmorphic facial features, microcephaly, global developmental delay
        and/or intellectual disability, short stature, and hypogonadism.
      explanation: >-
        Establishes co-occurrence in a molecularly defined MPLKIP cohort while
        preserving the unknown causal mechanism.
- name: DBR1 Deficiency in Sabinas TTD
  role: trigger
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    Homozygous DBR1 p.Asp262Tyr reduces DBR1 RNA and protein in the
    non-photosensitive Sabinas brittle-hair form. This genetic root is separated
    from MPLKIP deficiency because downstream human-skin-equivalent evidence has
    not been demonstrated for DBR1-associated patients.
  genes:
  - preferred_term: DBR1
    term:
      id: hgnc:15594
      label: DBR1
  evidence:
  - reference: PMID:40683339
    reference_title: "RNA Lariat-Debranching Enzyme (DBR1) Variations in Sabinas Brittle Hair Syndrome Form of Trichothiodystrophy: A Trichothiodystrophy-Causing Gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report 5 nonphotosensitive adult patients from 3 unrelated families
      with a homozygous missense variation in DBR1 (p.D262Y) encoding the RNA
      lariat-debranching enzyme DBR1, which is involved in the removal of introns
      from pre-mRNA in the nucleus.
    explanation: >-
      Establishes the DBR1-associated Sabinas TTD genetic root.
  downstream:
  - target: Reduced DBR1 Abundance
    causal_link_type: DIRECT
    description: >-
      Patient cells carrying DBR1 p.Asp262Tyr have reduced DBR1 RNA and protein.
    evidence:
    - reference: PMID:40683339
      reference_title: "RNA Lariat-Debranching Enzyme (DBR1) Variations in Sabinas Brittle Hair Syndrome Form of Trichothiodystrophy: A Trichothiodystrophy-Causing Gene."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        There were reduced levels of DBR1 mRNA and protein. Interacting TTDN1
        protein in cells from patients with DBR1 variations was markedly reduced.
      explanation: >-
        Directly supports reduced DBR1 in the variant-bearing patient cells.
- name: Intron-Lariat Accumulation after MPLKIP Loss
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    Loss of TTDN1/MPLKIP causes accumulation of intron lariats. This measured
    state is restricted to the MPLKIP experimental branch because equivalent
    lariat measurements have not been reported for DBR1 p.Asp262Tyr patient
    cells.
  genes:
  - preferred_term: MPLKIP
    term:
      id: hgnc:16002
      label: MPLKIP
  evidence:
  - reference: PMID:37369199
    reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      TTDN1 loss, or a mutated IDR, causes significant intron lariat
      accumulation, as well as splicing and gene expression defects, mirroring
      phenotypes observed in NP-TTD patient cells.
    explanation: >-
      Directly establishes intron-lariat accumulation after TTDN1/MPLKIP loss.
  downstream:
  - target: Aberrant RNA Splicing after MPLKIP Loss
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - impaired recycling of spliceosomal snRNPs
    description: >-
      The study supports a model in which accumulated lariats disrupt subsequent
      splicing cycles, but whether lariat accumulation itself is causal remains
      unresolved.
    evidence:
    - reference: PMID:37369199
      reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Our in vitro findings support a model where lariat accumulation leads to
        splicing disruption and gene expression alterations.
      explanation: >-
        Supports the proposed ordering without establishing a direct molecular
        link.
- name: Aberrant RNA Splicing after MPLKIP Loss
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    Loss of TTDN1/MPLKIP produces altered RNA-splicing patterns in knockout
    cells and patient fibroblasts. This node records the measured splicing state
    separately from upstream lariat accumulation and downstream expression or
    differentiation effects.
  genes:
  - preferred_term: MPLKIP
    term:
      id: hgnc:16002
      label: MPLKIP
  biological_processes:
  - preferred_term: RNA splicing
    term:
      id: GO:0008380
      label: RNA splicing
    modifier: ABNORMAL
  evidence:
  - reference: PMID:37369199
    reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Notably, these patient fibroblasts had similar altered splicing patterns
      seen in our TTDN1 KO cell lines (Figure 3i and Supplemental Figure S3h,
      S3j), suggesting a similar defect in mRNA processing.
    explanation: >-
      Directly establishes altered splicing in both patient fibroblasts and
      TTDN1-knockout cells.
  downstream:
  - target: Impaired Keratinocyte Differentiation
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - altered skin proteome after compromised splicing
    description: >-
      In MPLKIP-deficient human skin equivalents, compromised splicing is
      associated with an imbalanced proteome and impaired differentiation.
    evidence:
    - reference: PMID:37800682
      reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Using Human Skin Equivalents (HSEs), we observed impaired keratinocyte
        differentiation associated with compromised splicing and eventually, an
        imbalanced proteome affecting skin development and, interestingly, also
        the immune system.
      explanation: >-
        Grounds the MPLKIP-specific splicing-to-differentiation route.
  - target: Unresolved Tissue-Specific Gene-Expression Convergence
    causal_link_type: UNKNOWN
    description: >-
      The measured MPLKIP cellular defect does not establish a common pathway
      shared by every TTD genetic branch.
    evidence:
    - reference: PMID:37369199
      reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Collectively, our findings link RNA lariat processing with splicing
        outcomes by revealing the molecular function of TTDN1.
      explanation: >-
        Supports the MPLKIP RNA-processing branch but not pan-TTD convergence.
- name: Reduced DBR1 Abundance
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    MPLKIP deficiency reduces steady-state DBR1 protein, while primary DBR1
    p.Asp262Tyr variation reduces DBR1 RNA and protein. This shared node is
    limited to measured abundance and does not assume that DBR1-associated cells
    have the lariat or splicing abnormalities measured after MPLKIP loss.
  evidence:
  - reference: PMID:37800682
    reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      By mass spectrometry-based interaction proteomics, we demonstrate that
      MPLKIP interacts with core splicing factors and the lariat debranching
      protein DBR1. MPLKIP-deficient primary fibroblasts have reduced steady-state
      DBR1 protein levels.
    explanation: >-
      Directly establishes the MPLKIP-DBR1 interaction and reduced DBR1 in
      MPLKIP-deficient patient fibroblasts.
  - reference: PMID:40683339
    reference_title: "RNA Lariat-Debranching Enzyme (DBR1) Variations in Sabinas Brittle Hair Syndrome Form of Trichothiodystrophy: A Trichothiodystrophy-Causing Gene."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      There were reduced levels of DBR1 mRNA and protein. Interacting TTDN1
      protein in cells from patients with DBR1 variations was markedly reduced.
    explanation: >-
      Provides the reciprocal DBR1-variant evidence and reduced interacting
      TTDN1/MPLKIP.
  downstream:
  - target: Unresolved Tissue-Specific Gene-Expression Convergence
    causal_link_type: UNKNOWN
    description: >-
      RNA-processing defects plausibly feed the shared phenotype, but the
      cross-tissue and cross-genotype intermediates are unresolved.
    evidence:
    - reference: PMID:40683339
      reference_title: "RNA Lariat-Debranching Enzyme (DBR1) Variations in Sabinas Brittle Hair Syndrome Form of Trichothiodystrophy: A Trichothiodystrophy-Causing Gene."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Thus, Sabinas syndrome is caused by DBR1 variations, further indicating
        that TTD is a disorder of RNA metabolism (RNAopathy).
      explanation: >-
        Supports the RNA-metabolism framing while leaving phenotype intermediates
        open.
- name: Cytosolic Aminoacyl-tRNA Synthetase Loss of Function
  role: trigger
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    Biallelic CARS1, TARS1, AARS1, or MARS1 variants reduce abundance and/or
    enzymatic function of their cytosolic aminoacyl-tRNA synthetases. This
    variant-proximal loss-of-function state is separated from its measured
    downstream effect on cognate tRNA charging.
  genes:
  - preferred_term: CARS1
    term:
      id: hgnc:1493
      label: CARS1
  - preferred_term: TARS1
    term:
      id: hgnc:11572
      label: TARS1
  - preferred_term: AARS1
    term:
      id: hgnc:20
      label: AARS1
  - preferred_term: MARS1
    term:
      id: hgnc:6898
      label: MARS1
  evidence:
  - reference: PMID:30824121
    reference_title: Cysteinyl-tRNA Synthetase Mutations Cause a Multi-System, Recessive Disease That Includes Microcephaly, Developmental Delay, and Brittle Hair and Nails.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Measurement of protein abundance, yeast complementation assays, and
      assessments of tRNA charging indicate that each CARS variant causes a
      loss-of-function effect.
    explanation: >-
      Establishes loss of CARS1 function using protein, complementation, and
      charging assays.
  - reference: PMID:31374204
    reference_title: Bi-allelic TARS Mutations Are Associated with Brittle Hair Phenotype.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We showed that these variants have a profound effect on TARS protein
      stability and enzymatic function.
    explanation: >-
      Direct functional evidence for TARS1 instability and enzymatic impairment.
  - reference: PMID:33909043
    reference_title: Protein instability associated with AARS1 and MARS1 mutations causes trichothiodystrophy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      These variants result in the instability of the respective gene products
      alanyl- and methionyl-tRNA synthetase.
    explanation: >-
      Directly supports AARS1 and MARS1 protein instability.
  downstream:
  - target: Reduced Cognate tRNA Charging
    causal_link_type: DIRECT
    description: >-
      Loss of abundance or enzymatic function of each affected synthetase
      directly reduces its cognate tRNA-charging activity.
    evidence:
    - reference: PMID:30824121
      reference_title: Cysteinyl-tRNA Synthetase Mutations Cause a Multi-System, Recessive Disease That Includes Microcephaly, Developmental Delay, and Brittle Hair and Nails.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Measurement of protein abundance, yeast complementation assays, and
        assessments of tRNA charging indicate that each CARS variant causes a
        loss-of-function effect.
      explanation: >-
        Directly links CARS1 loss of function to impaired charging.
    - reference: PMID:31374204
      reference_title: Bi-allelic TARS Mutations Are Associated with Brittle Hair Phenotype.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        We showed that these variants have a profound effect on TARS protein
        stability and enzymatic function.
      explanation: >-
        Directly links TARS1 instability to reduced enzymatic function.
    - reference: PMID:33909043
      reference_title: Protein instability associated with AARS1 and MARS1 mutations causes trichothiodystrophy.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Functional studies in skin fibroblasts from affected individuals
        demonstrate that these new variants also impact on the rate of tRNA
        charging, which is the first step in protein translation.
      explanation: >-
        Directly links AARS1/MARS1 instability to altered charging rate.
- name: Reduced Cognate tRNA Charging
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    CARS1-, TARS1-, AARS1-, and MARS1-related TTD variants reduce cognate tRNA
    charging or the enzymatic function required for charging. How this measured
    aminoacylation lesion yields lineage-selective hair and multisystem
    disease remains unresolved.
  genes:
  - preferred_term: CARS1
    term:
      id: hgnc:1493
      label: CARS1
  - preferred_term: TARS1
    term:
      id: hgnc:11572
      label: TARS1
  - preferred_term: AARS1
    term:
      id: hgnc:20
      label: AARS1
  - preferred_term: MARS1
    term:
      id: hgnc:6898
      label: MARS1
  biological_processes:
  - preferred_term: tRNA aminoacylation for protein translation
    term:
      id: GO:0006418
      label: tRNA aminoacylation for protein translation
    modifier: DECREASED
  evidence:
  - reference: PMID:30824121
    reference_title: Cysteinyl-tRNA Synthetase Mutations Cause a Multi-System, Recessive Disease That Includes Microcephaly, Developmental Delay, and Brittle Hair and Nails.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Measurement of protein abundance, yeast complementation assays, and
      assessments of tRNA charging indicate that each CARS variant causes a
      loss-of-function effect.
    explanation: >-
      Directly supports impaired CARS1-dependent tRNA charging.
  - reference: PMID:31374204
    reference_title: Bi-allelic TARS Mutations Are Associated with Brittle Hair Phenotype.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We showed that these variants have a profound effect on TARS protein
      stability and enzymatic function.
    explanation: >-
      Directly supports impaired TARS1 enzymatic function.
  - reference: PMID:33909043
    reference_title: Protein instability associated with AARS1 and MARS1 mutations causes trichothiodystrophy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Functional studies in skin fibroblasts from affected individuals
      demonstrate that these new variants also impact on the rate of tRNA
      charging, which is the first step in protein translation.
    explanation: >-
      Directly supports altered tRNA-charging rate in AARS1- and MARS1-related
      fibroblasts.
  downstream:
  - target: Unresolved Tissue-Specific Gene-Expression Convergence
    causal_link_type: UNKNOWN
    description: >-
      Reduced tRNA charging is upstream of the synthetase-related forms, but how
      it produces sulfur-poor hair and the multisystem phenotype is unresolved.
    evidence:
    - reference: PMID:31374204
      reference_title: Bi-allelic TARS Mutations Are Associated with Brittle Hair Phenotype.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Our results expand the spectrum of genes involved in TTD to include genes
        implicated in amino acid charging of tRNA, which is required for the last
        step in gene expression, namely protein translation.
      explanation: >-
        Supports translation as the affected process without identifying the
        tissue-specific causal bridge.
- name: RNF113A Deficiency
  role: trigger
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    Hemizygous loss-of-function RNF113A variants cause X-linked
    non-photosensitive TTD. The genetic root is separated from the two reported
    cellular functions because their relative contribution to the human
    developmental phenotype is unresolved.
  genes:
  - preferred_term: RNF113A
    term:
      id: hgnc:12974
      label: RNF113A
  evidence:
  - reference: PMID:25612912
    reference_title: A novel X-linked trichothiodystrophy associated with a nonsense mutation in RNF113A.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We identified a nonsense mutation in the highly conserved RNF113A gene
      (c.901 C>T, p.Q301*). The mutation segregated with the disease in the family
      and was not observed in over 100,000 control X chromosomes.
    explanation: >-
      Segregation establishes the RNF113A genetic root without choosing between
      downstream molecular functions.
  downstream:
  - target: Defective RNF113A-Dependent ASCC Alkylation Response
    causal_link_type: DIRECT
    description: >-
      RNF113A-mutant patient cells fail to recruit the ASCC response and are
      hypersensitive to alkylating agents.
    evidence:
    - reference: PMID:29144457
      reference_title: A ubiquitin-dependent signalling axis specific for ALKBH-mediated DNA dealkylation repair.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Cells from patients with X-linked trichothiodystrophy, which harbour a
        mutation in RNF113A, are defective in ASCC foci formation and are
        hypersensitive to alkylating agents.
      explanation: >-
        Direct patient-cell evidence for this RNF113A-dependent response defect.
  - target: Candidate RNF113A-Dependent Splicing Dysfunction
    causal_link_type: UNKNOWN
    description: >-
      RNF113A regulates splicing in cancer-cell experiments, but whether the same
      splicing state drives human TTD is unresolved.
    evidence:
    - reference: PMID:32152280
      reference_title: The X-linked trichothiodystrophy-causing gene RNF113A links the spliceosome to cell survival upon DNA damage.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        RNF113A is a RNA-binding protein which regulates the splicing of multiple
        candidates involved in cell survival.
      explanation: >-
        Supports the candidate splicing branch while preserving its cancer-cell
        context and uncertain disease contribution.
- name: Defective RNF113A-Dependent ASCC Alkylation Response
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    RNF113A-mutant patient cells have defective ASCC foci formation and
    alkylating-agent hypersensitivity. This measured response is not assumed to
    be the developmental mechanism of X-linked TTD.
  evidence:
  - reference: PMID:29144457
    reference_title: A ubiquitin-dependent signalling axis specific for ALKBH-mediated DNA dealkylation repair.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Cells from patients with X-linked trichothiodystrophy, which harbour a
      mutation in RNF113A, are defective in ASCC foci formation and are
      hypersensitive to alkylating agents.
    explanation: >-
      Directly supports the cellular alkylation-response abnormality.
- name: Candidate RNF113A-Dependent Splicing Dysfunction
  biological_scale: CELLULAR
  mechanism_confidence: HYPOTHETICAL
  description: >-
    RNF113A regulates splicing of survival-related transcripts in lung-cancer
    cells. The cellular function is experimentally supported, but its operation
    in developing patient tissues and its contribution relative to the ASCC
    pathway remain unknown.
  biological_processes:
  - preferred_term: RNA splicing
    term:
      id: GO:0008380
      label: RNA splicing
    modifier: ABNORMAL
  evidence:
  - reference: PMID:32152280
    reference_title: The X-linked trichothiodystrophy-causing gene RNF113A links the spliceosome to cell survival upon DNA damage.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      RNF113A is a RNA-binding protein which regulates the splicing of multiple
      candidates involved in cell survival.
    explanation: >-
      Supports an RNF113A splicing function in a low-fidelity cancer-cell context.
  downstream:
  - target: Unresolved Tissue-Specific Gene-Expression Convergence
    causal_link_type: UNKNOWN
    description: >-
      The disease relevance of the cancer-cell splicing phenotype remains open.
    evidence:
    - reference: PMID:32152280
      reference_title: The X-linked trichothiodystrophy-causing gene RNF113A links the spliceosome to cell survival upon DNA damage.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Here we show that RNF113A, whose loss-of-function causes the X-linked
        trichothiodystrophy, is overexpressed in lung cancer and protects from
        Cisplatin-dependent cell death.
      explanation: >-
        Links the studied protein to the disease while exposing the experimental
        context boundary.
- name: Impaired Keratinocyte Differentiation
  biological_scale: TISSUE
  mechanism_confidence: PROVISIONAL
  description: >-
    MPLKIP-deficient reconstructed human skin shows compromised splicing,
    impaired keratinocyte differentiation, and an imbalanced skin/immune
    proteome. This experimentally supported branch is specific to MPLKIP models;
    extension to other TTD genotypes is provisional.
  cell_types:
  - preferred_term: keratinocyte
    term:
      id: CL:0000312
      label: keratinocyte
  biological_processes:
  - preferred_term: keratinocyte differentiation
    term:
      id: GO:0030216
      label: keratinocyte differentiation
    modifier: DECREASED
  evidence:
  - reference: PMID:37800682
    reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Using Human Skin Equivalents (HSEs), we observed impaired keratinocyte
      differentiation associated with compromised splicing and eventually, an
      imbalanced proteome affecting skin development and, interestingly, also
      the immune system.
    explanation: >-
      Directly supports this skin-model node in MPLKIP deficiency.
  downstream:
  - target: Deficiency of Sulfur-Rich Hair Matrix Proteins
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - hair-matrix-specific protein expression and assembly
    description: >-
      Impaired keratinocyte differentiation plausibly contributes to the hair
      matrix defect, but this exact chain was not measured in the HSE model.
    evidence:
    - reference: PMID:37800682
      reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        MPLKIP deficiency impairs keratinocyte differentiation, resulting in
        “leaky” skin development and altered immune response.
      explanation: >-
        Supports differentiation failure while leaving the hair-shaft
        intermediates unmeasured.
  - target: Ichthyosis
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - epidermal barrier formation and cornification changes
    description: >-
      The HSE result provides a plausible MPLKIP-specific route to ichthyosis,
      but the clinical link and generalization to all TTD forms remain indirect.
    evidence:
    - reference: PMID:37800682
      reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        MPLKIP deficiency impairs keratinocyte differentiation, resulting in
        “leaky” skin development and altered immune response.
      explanation: >-
        Provides model-level support for a barrier route, not a universal
        syndrome-wide mechanism.
- name: Unresolved Tissue-Specific Gene-Expression Convergence
  role: central_effector
  biological_scale: CELLULAR
  mechanism_confidence: HYPOTHETICAL
  description: >-
    Distinct defects in TFIIH/TFIIE transcription, MPLKIP-DBR1/RNF113A RNA
    processing, and aminoacyl-tRNA synthetase-dependent translation all cause the
    TTD phenotype, but no single experimentally established final common pathway
    explains their tissue selectivity. Ribosome dysfunction and loss of
    proteostasis have been proposed from perturbation models and are retained
    here only as a hypothesis, not as a canonical syndrome-wide mechanism.
  evidence:
  - reference: PMID:40737808
    reference_title: "Trichothiodystrophy: Molecular insights and mechanisms of pathogenicity."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      The disease arises from mutations in a variety of genes involved in
      different biological processes. Affected processes include DNA repair,
      transcription as well as translation.
    explanation: >-
      Current review evidence establishes the distinct upstream processes.
  - reference: PMID:37508541
    reference_title: Ribosomal Dysfunction Is a Common Pathomechanism in Different Forms of Trichothiodystrophy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      As the loss of proteostasis by the ribosome has been identified in the
      other forms of TTD, here we postulate that ribosomal dysfunction is a
      common underlying pathomechanism of TTD.
    explanation: >-
      The authors explicitly label common ribosomal dysfunction as a postulate;
      the node therefore remains HYPOTHETICAL.
  downstream:
  - target: Deficiency of Sulfur-Rich Hair Matrix Proteins
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - hair-follicle lineage-specific synthesis and assembly of sulfur-rich proteins
    description: >-
      All causal branches converge clinically on sulfur-poor brittle hair, but
      the exact lineage-specific bridge remains unknown.
    evidence:
    - reference: PMID:40737808
      reference_title: "Trichothiodystrophy: Molecular insights and mechanisms of pathogenicity."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        Trichothiodystrophy (TTD) is a rare hereditary disease characterized by
        brittle, sulphur deficient hair associated with a wide and varied
        spectrum of clinical features
      explanation: >-
        Establishes the shared phenotype but not the intermediate mechanism.
  - target: Global Developmental Delay
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - cell-type-specific developmental gene-expression defects
    description: >-
      The causal genes produce TTD and developmental impairment is common, but
      the intervening neural mechanisms are unresolved.
    evidence:
    - reference: PMID:18603627
      reference_title: "Trichothiodystrophy: a systematic review of 112 published cases characterises a wide spectrum of clinical manifestations."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        common features reported were developmental delay/intellectual impairment
        (86%), short stature (73%)
      explanation: >-
        Clinical frequency supports the phenotype while not resolving causality.
  - target: Microcephaly
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - neural progenitor and brain-growth effects
    description: >-
      Microcephaly occurs across multiple genetic branches, but the shared route
      from gene-expression defects to reduced brain growth is unknown.
    evidence:
    - reference: PMID:25612912
      reference_title: A novel X-linked trichothiodystrophy associated with a nonsense mutation in RNF113A.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We describe two male cousins affected by TTD associated with
        microcephaly, profound intellectual disability, sparse brittle hair,
        aged appearance, short stature, facial dysmorphism, seizures, an
        immunoglobulin deficiency, multiple endocrine abnormalities, cerebellar
        hypoplasia and partial absence of the corpus callosum
      explanation: >-
        Documents microcephaly in RNF113A-related TTD without proving the
        cross-genotype mechanism.
  - target: Short Stature
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - prenatal and postnatal growth-program disruption
    description: >-
      Growth restriction is common, while the cross-genotype growth mechanism
      remains unknown.
    evidence:
    - reference: PMID:18603627
      reference_title: "Trichothiodystrophy: a systematic review of 112 published cases characterises a wide spectrum of clinical manifestations."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        developmental delay/intellectual impairment (86%), short stature (73%),
        ichthyosis (65%)
      explanation: >-
        Supports the common clinical phenotype but not a resolved pathway.
  - target: Ocular Abnormality
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - ocular developmental and tissue-maintenance defects
    description: >-
      Ocular abnormalities are frequent, but no shared molecular route was
      established across the genetically heterogeneous spectrum.
    evidence:
    - reference: PMID:18603627
      reference_title: "Trichothiodystrophy: a systematic review of 112 published cases characterises a wide spectrum of clinical manifestations."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        ocular abnormalities (51%), infections (46%), photosensitivity (42%)
      explanation: >-
        Supports the clinical association while leaving mechanism unknown.
  - target: Cataract
    causal_link_type: UNKNOWN
    description: >-
      Cataract was common in the NIH cohort, but no cross-genotype molecular
      route to lens opacity was established.
    evidence:
    - reference: PMID:21800331
      reference_title: "High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Cataracts were present in 54% of the TTD patients examined.
      explanation: >-
        Establishes the specific common ocular phenotype while leaving its
        mechanism unresolved.
  - target: Recurrent Infections
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - variable hypogammaglobulinemia, neutropenia, and epithelial-barrier defects
    description: >-
      Infection susceptibility is clinically important, but heterogeneous
      immune and barrier abnormalities preclude a single established pathway.
    evidence:
    - reference: PMID:30334570
      reference_title: "Mortality-associated immunological abnormalities in trichothiodystrophy: correlation of reduced levels of immunoglobulin and neutrophils with poor patient survival."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Infection or sepsis was determined to be the cause of death in 7 of the
        10 TTD patients who died.
      explanation: >-
        Establishes the clinical burden while not identifying one universal
        immune mechanism.
  - target: Decreased Circulating Immunoglobulin Concentration
    causal_link_type: UNKNOWN
    description: >-
      Reduced immunoglobulin occurs in a clinically important subset, but its
      relation to the heterogeneous upstream gene-expression defects has not
      been established.
    evidence:
    - reference: PMID:30334570
      reference_title: "Mortality-associated immunological abnormalities in trichothiodystrophy: correlation of reduced levels of immunoglobulin and neutrophils with poor patient survival."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Thirteen patients had an IgG level below the age-adjusted lower limit of
        normal. Twelve of these patients had a mutation in ERCC2.
      explanation: >-
        Establishes the phenotype and its ERCC2 enrichment without resolving a
        causal path from the upstream molecular lesions.
  - target: Decreased Total Neutrophil Count
    causal_link_type: UNKNOWN
    description: >-
      Neutropenia is associated with TTD in a subset, but no cross-genotype
      molecular route to impaired neutrophil abundance is established.
    evidence:
    - reference: PMID:30334570
      reference_title: "Mortality-associated immunological abnormalities in trichothiodystrophy: correlation of reduced levels of immunoglobulin and neutrophils with poor patient survival."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Seventeen patients had an ANC below 1.0 × 109/l, of which 13 had ERCC2
        mutations.
      explanation: >-
        Establishes the phenotype and genotype distribution without proving a
        mechanism.
  - target: Premature Birth
    causal_link_type: UNKNOWN
    description: >-
      Preterm delivery is associated with affected pregnancies, but the fetal,
      placental, and maternal intermediates remain unresolved.
    evidence:
    - reference: PMID:21800331
      reference_title: "High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Pregnancy reports of 27 TTD patients and their 23 mothers were evaluated
        and 81% of the pregnancies had complications: 56% had preterm delivery
      explanation: >-
        Establishes the cohort association without identifying a causal
        molecular pathway.
  - target: Small for Gestational Age
    causal_link_type: UNKNOWN
    description: >-
      Birth weight below the tenth centile for gestational age occurred in the
      neonatal cohort, but the fetal or placental mechanism is unresolved.
    evidence:
    - reference: PMID:21800331
      reference_title: "High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        35% had birth weight < 10 centile for gestational age
      explanation: >-
        Supports the gestational-age-adjusted phenotype rather than conflating it
        with the separate low-birth-weight measure.
  - target: Cryptorchidism
    causal_link_type: UNKNOWN
    description: >-
      Cryptorchidism occurred in a substantial fraction of male infants, but its
      developmental mechanism in TTD was not established.
    evidence:
    - reference: PMID:21800331
      reference_title: "High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        31% of the 16 males had cryptorchidism.
      explanation: >-
        Preserves the sex-specific denominator and causal uncertainty.
  - target: Congenital Ichthyosiform Erythroderma
    causal_link_type: UNKNOWN
    description: >-
      Neonatal erythroderma or collodion presentation is clinically associated
      with TTD, while its route from the heterogeneous causal branches is not
      established.
    evidence:
    - reference: PMID:21800331
      reference_title: "High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Erythroderma/collodion presentation of the skin was noted in 18 (67%)
        infants (Figure 1A). It typically persisted for 1 to 2 weeks; the patients
        were treated with humidified air and emollients.
      explanation: >-
        Supports the combined neonatal skin presentation without resolving its
        molecular cause or the frequency of erythroderma alone.
  - target: Sparse Scalp Hair
    causal_link_type: UNKNOWN
    description: >-
      Sparse scalp hair occurs in genetically defined TTD, but the route from the
      heterogeneous gene-expression defects to reduced hair density is unresolved.
    evidence:
    - reference: PMID:25612912
      reference_title: A novel X-linked trichothiodystrophy associated with a nonsense mutation in RNF113A.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We describe two male cousins affected by TTD associated with microcephaly,
        profound intellectual disability, sparse brittle hair, aged appearance,
        short stature, facial dysmorphism, seizures, an immunoglobulin deficiency,
        multiple endocrine abnormalities, cerebellar hypoplasia and partial
        absence of the corpus callosum
      explanation: >-
        Documents sparse hair in RNF113A-related TTD without establishing a
        cross-genotype mechanism.
  - target: Nail Dystrophy
    causal_link_type: UNKNOWN
    description: >-
      Nail dystrophy is documented in MPLKIP-related TTD, but a molecular route
      from the upstream lesion to the nail phenotype has not been demonstrated.
    evidence:
    - reference: PMID:37800682
      reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        ectodermal abnormalities comprising keratosis pilaris, hyperkeratotic
        plaques on the scalp, dry skin and nail dystrophy, dysmorphic facial
        features, microcephaly, global developmental delay and/or intellectual
        disability, short stature, and hypogonadism.
      explanation: >-
        Directly documents nail dystrophy in the seven-patient MPLKIP cohort.
  - target: Gait Disturbance
    causal_link_type: UNKNOWN
    description: >-
      Ataxic gait occurs in at least one molecularly characterized MPLKIP-related
      patient, but the responsible neurologic pathway remains unresolved.
    evidence:
    - reference: PMID:37800682
      reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        The gait was ataxic. The systemic and neurological examinations were
        otherwise unremarkable.
      explanation: >-
        Directly documents ataxic gait in a genotyped MPLKIP-related patient.
  - target: Abnormal Facial Shape
    causal_link_type: UNKNOWN
    description: >-
      Facial dysmorphism is documented in genetically defined TTD, but no shared
      developmental route across the heterogeneous gene branches is established.
    evidence:
    - reference: PMID:25612912
      reference_title: A novel X-linked trichothiodystrophy associated with a nonsense mutation in RNF113A.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We describe two male cousins affected by TTD associated with microcephaly,
        profound intellectual disability, sparse brittle hair, aged appearance,
        short stature, facial dysmorphism, seizures, an immunoglobulin deficiency,
        multiple endocrine abnormalities, cerebellar hypoplasia and partial
        absence of the corpus callosum
      explanation: >-
        Documents facial dysmorphism in RNF113A-related TTD while leaving the
        cross-genotype developmental route unresolved.
- name: Deficiency of Sulfur-Rich Hair Matrix Proteins
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  description: >-
    TTD hair contains reduced cysteine/sulfur-rich matrix proteins. This
    compositional lesion is separated from the observed ultrastructural
    disorganization because the patient-hair studies do not establish whether
    one state causes the other.
  evidence:
  - reference: PMID:15232704
    reference_title: "Trichothiodystrophy: quantification of cysteine in human hair and nails by application of sodium azide-dependent oxidation to cysteic acid."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The term "trichothiodystrophy" (TTD) covers several autosomal recessive
      diseases whose diagnostic hallmark is short, brittle hair low in sulfur
      and cystine because of impaired synthesis of high-sulfur matrix protein.
    explanation: >-
      Directly supports sulfur/cystine deficiency and impaired high-sulfur matrix
      protein synthesis in patient hair.
  - reference: PMID:35615778
    reference_title: Trichothiodystrophy hair shafts display distinct ultrastructural features.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The results are consistent with not only a paucity of high sulphur
      components, such as keratin-associated proteins, but also a profound
      imbalance in protein content and organization.
    explanation: >-
      Patient-hair analysis independently supports paucity of high-sulfur
      keratin-associated proteins.
  downstream:
  - target: Hair Shaft Ultrastructural Disorganization
    causal_link_type: UNKNOWN
    description: >-
      Sulfur-rich-protein paucity and structural disorganization coexist in the
      studied hair, but whether the compositional defect causes the structural
      state or instead occurs in parallel is unknown.
    evidence:
    - reference: PMID:35615778
      reference_title: Trichothiodystrophy hair shafts display distinct ultrastructural features.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        The results are consistent with not only a paucity of high sulphur
        components, such as keratin-associated proteins, but also a profound
        imbalance in protein content and organization.
      explanation: >-
        Co-establishes the two hair states without determining their causal
        ordering.
  - target: Brittle Sulfur-Deficient Hair
    causal_link_type: DIRECT
    description: >-
      Impaired synthesis of high-sulfur matrix protein directly produces the
      characteristic brittle, sulfur-poor hair described in the diagnostic
      biochemical source.
    evidence:
    - reference: PMID:15232704
      reference_title: "Trichothiodystrophy: quantification of cysteine in human hair and nails by application of sodium azide-dependent oxidation to cysteic acid."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        The term "trichothiodystrophy" (TTD) covers several autosomal recessive
        diseases whose diagnostic hallmark is short, brittle hair low in sulfur
        and cystine because of impaired synthesis of high-sulfur matrix protein.
      explanation: >-
        Directly links the matrix-protein defect with brittle sulfur-poor hair.
  - target: Reduced Hair Sulfur Content
    causal_link_type: DIRECT
    description: >-
      Paucity of cysteine-rich matrix proteins directly lowers measured
      sulfur/cystine content in hair.
    evidence:
    - reference: PMID:15232704
      reference_title: "Trichothiodystrophy: quantification of cysteine in human hair and nails by application of sodium azide-dependent oxidation to cysteic acid."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        With our control and parent data resembling published data on hair and
        nail samples, we obtained a decreased proportion of cysteine (half
        cystine, determined as cysteic acid) in materials obtained from a boy
        with TTD.
      explanation: >-
        Direct biochemical measurement supports reduced cysteine/cystine in TTD
        hair and nail material.
- name: Hair Shaft Ultrastructural Disorganization
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  description: >-
    Electron microscopy of hair from three ERCC2-related TTD patients shows
    abnormal cuticle marginal bands and exocuticle layers, heterogeneous cortex
    staining, and shortened keratin macrofibrils. Extension of this exact
    ultrastructural pattern to every TTD genotype remains untested.
  evidence:
  - reference: PMID:35615778
    reference_title: Trichothiodystrophy hair shafts display distinct ultrastructural features.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Compared with hair shafts of normal phenotype, TTD cuticle cells displayed
      aberrant marginal bands and exocuticle layers. Clusters of cells stained
      differently (light versus dark) in the cortex of aberrant shafts, and the
      keratin macrofibrils appeared much shorter in the cytoplasm.
    explanation: >-
      Directly establishes the atomic ultrastructural state in the examined
      ERCC2-related patient hair.
  downstream:
  - target: Tiger-Tail Hair Banding
    causal_link_type: UNKNOWN
    description: >-
      Ultrastructural heterogeneity and tiger-tail banding are observed in TTD
      hair, but the structural generator of the alternating optical bands has
      not been established.
    evidence:
    - reference: PMID:35615778
      reference_title: Trichothiodystrophy hair shafts display distinct ultrastructural features.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Two diagnostic features of the hair are alternating light/dark banding
        visible in polarized light, which is termed “tiger tail banding,” and low
        cysteine content, typically about half of normal, revealed by amino acid
        analysis.
      explanation: >-
        Supports the co-occurring optical phenotype without proving that the
        measured ultrastructural abnormalities cause it.
- name: Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain
  biological_scale: TISSUE
  mechanism_confidence: HYPOTHETICAL
  description: >-
    In an Xpd TTD mouse, limiting TFIIH destabilizes thyroid-hormone receptor
    occupancy and deregulates thyroid-hormone target genes in brain. This is a
    plausible ERCC2-related route to dysmyelination, but it has not been directly
    established in developing human brain or generalized across TTD.
  genes:
  - preferred_term: ERCC2
    term:
      id: hgnc:3434
      label: ERCC2
  evidence:
  - reference: PMID:17952069
    reference_title: Neurological defects in trichothiodystrophy reveal a coactivator function of TFIIH.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Molecular analyses performed on the mice brain tissue demonstrate that
      TFIIH is required for the stabilization of thyroid hormone receptors (TR)
      to their DNA-responsive elements.
    explanation: >-
      Supports the mouse coactivator mechanism while not proving the human route.
  downstream:
  - target: Cerebral Hypomyelination
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - translation of mouse thyroid-receptor transcription effects to human myelin development
    description: >-
      The model suggests a route to hypomyelination, but the human developmental
      causal chain remains incomplete.
    evidence:
    - reference: PMID:39976384
      reference_title: "Trichothiodystrophy due to ERCC2 Variants: Uncommon Contributor to Progressive Hypomyelinating Leukodystrophy."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        a 5-year-old affected child who presented with microcephaly, failure to
        thrive, developmental delay, and progressive hypomyelination on three
        serial brain imaging over 5-years follow-up.
      explanation: >-
        Documents progressive hypomyelination in ERCC2-related TTD but does not
        directly test the proposed thyroid-receptor mechanism.
phenotypes:
- name: Brittle Sulfur-Deficient Hair
  description: >-
    Short, sparse or unruly hair that fractures easily and contains reduced
    sulfur-rich/cystine-rich matrix proteins is the defining clinical entry point
    to the TTD spectrum.
  phenotype_term:
    preferred_term: Brittle hair
    term:
      id: HP:0002299
      label: Brittle hair
  evidence:
  - reference: PMID:15232704
    reference_title: "Trichothiodystrophy: quantification of cysteine in human hair and nails by application of sodium azide-dependent oxidation to cysteic acid."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The term "trichothiodystrophy" (TTD) covers several autosomal recessive
      diseases whose diagnostic hallmark is short, brittle hair low in sulfur
      and cystine because of impaired synthesis of high-sulfur matrix protein.
    explanation: >-
      Establishes brittle sulfur/cystine-poor hair as the diagnostic hallmark.
- name: Tiger-Tail Hair Banding
  description: >-
    Alternating bright and dark bands under polarized microscopy are highly
    characteristic and diagnostically useful, but are not asserted to occur in
    every patient or every sampled hair.
  phenotype_term:
    preferred_term: Tiger tail banding
    term:
      id: HP:0045055
      label: Tiger tail banding
  evidence:
  - reference: PMID:35615778
    reference_title: Trichothiodystrophy hair shafts display distinct ultrastructural features.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Two diagnostic features of the hair are alternating light/dark banding
      visible in polarized light, which is termed “tiger tail banding,” and low
      cysteine content, typically about half of normal, revealed by amino acid
      analysis.
    explanation: >-
      Directly describes tiger-tail banding as a diagnostic hair feature.
- name: Reduced Hair Sulfur Content
  description: >-
    Hair and nail material has reduced cysteine/cystine, reflecting deficiency of
    sulfur-rich matrix proteins.
  phenotype_term:
    preferred_term: Reduced hair sulfur content
    term:
      id: HP:0034425
      label: Reduced hair sulfur content
  evidence:
  - reference: PMID:15232704
    reference_title: "Trichothiodystrophy: quantification of cysteine in human hair and nails by application of sodium azide-dependent oxidation to cysteic acid."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      With our control and parent data resembling published data on hair and
      nail samples, we obtained a decreased proportion of cysteine (half
      cystine, determined as cysteic acid) in materials obtained from a boy with
      TTD.
    explanation: >-
      Directly documents reduced cysteine/cystine in material from an affected
      child.
- name: Sparse Scalp Hair
  description: >-
    Sparse scalp hair is distinct from shaft brittleness and is documented in
    genetically defined TTD. No umbrella frequency is assigned because the
    displayed primary evidence is subtype-specific.
  phenotype_term:
    preferred_term: Sparse scalp hair
    term:
      id: HP:0002209
      label: Sparse scalp hair
  evidence:
  - reference: PMID:25612912
    reference_title: A novel X-linked trichothiodystrophy associated with a nonsense mutation in RNF113A.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We describe two male cousins affected by TTD associated with microcephaly,
      profound intellectual disability, sparse brittle hair, aged appearance,
      short stature, facial dysmorphism, seizures, an immunoglobulin deficiency,
      multiple endocrine abnormalities, cerebellar hypoplasia and partial
      absence of the corpus callosum
    explanation: >-
      Directly documents sparse scalp hair in the founding RNF113A-related family.
- name: Alopecia
  description: >-
    Episodic hair loss can accompany fever in temperature-sensitive
    GTF2E2-related TTD. The primary evidence is an individual observation, so no
    umbrella frequency or universal fever association is assigned.
  phenotype_term:
    preferred_term: Alopecia
    term:
      id: HP:0001596
      label: Alopecia
  evidence:
  - reference: PMID:28973399
    reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      She showed recurrent aggravation of hair-loss by tufted breakage at the
      scalp boundary immediately following episodes of infection-induced fever.
    explanation: >-
      Directly documents recurrent scalp hair loss in the characterized
      GTF2E2-related patient.
- name: Global Developmental Delay
  description: >-
    Developmental delay and intellectual impairment span multiple genetic
    branches and vary from mild to profound.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:18603627
    reference_title: "Trichothiodystrophy: a systematic review of 112 published cases characterises a wide spectrum of clinical manifestations."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      common features reported were developmental delay/intellectual impairment
      (86%), short stature (73%)
    explanation: >-
      The systematic review reports the combined category developmental delay or
      intellectual impairment in 86%; it supports this phenotype but does not
      assign that rate to global developmental delay alone.
- name: Microcephaly
  description: >-
    Reduced head circumference occurs in photosensitive and non-photosensitive
    genetic forms, but its mechanism and frequency vary by genotype.
  phenotype_term:
    preferred_term: Microcephaly
    term:
      id: HP:0000252
      label: Microcephaly
  evidence:
  - reference: PMID:25612912
    reference_title: A novel X-linked trichothiodystrophy associated with a nonsense mutation in RNF113A.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We describe two male cousins affected by TTD associated with microcephaly,
      profound intellectual disability, sparse brittle hair, aged appearance,
      short stature, facial dysmorphism, seizures, an immunoglobulin deficiency,
      multiple endocrine abnormalities, cerebellar hypoplasia and partial
      absence of the corpus callosum
    explanation: >-
      Directly documents microcephaly in the founding RNF113A-related family.
  - reference: PMID:30824121
    reference_title: Cysteinyl-tRNA Synthetase Mutations Cause a Multi-System, Recessive Disease That Includes Microcephaly, Developmental Delay, and Brittle Hair and Nails.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here, we report on four subjects from three families with complex
      syndromes that include microcephaly, developmental delay, and brittle hair
      and nails.
    explanation: >-
      Independently documents microcephaly in CARS1-related disease within the
      current TTD spectrum.
- name: Abnormal Facial Shape
  description: >-
    Facial dysmorphism occurs in genetically defined TTD, with variable features
    across subtypes. No universal facial gestalt or umbrella frequency is
    assigned from the displayed subtype-specific evidence.
  phenotype_term:
    preferred_term: Abnormal facial shape
    term:
      id: HP:0001999
      label: Abnormal facial shape
  evidence:
  - reference: PMID:25612912
    reference_title: A novel X-linked trichothiodystrophy associated with a nonsense mutation in RNF113A.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We describe two male cousins affected by TTD associated with microcephaly,
      profound intellectual disability, sparse brittle hair, aged appearance,
      short stature, facial dysmorphism, seizures, an immunoglobulin deficiency,
      multiple endocrine abnormalities, cerebellar hypoplasia and partial
      absence of the corpus callosum
    explanation: >-
      Directly documents facial dysmorphism in the founding RNF113A-related family.
- name: Short Stature
  description: >-
    Prenatal and/or postnatal growth restriction commonly produces short stature.
  phenotype_term:
    preferred_term: Short stature
    term:
      id: HP:0004322
      label: Short stature
  frequency: FREQUENT
  evidence:
  - reference: PMID:18603627
    reference_title: "Trichothiodystrophy: a systematic review of 112 published cases characterises a wide spectrum of clinical manifestations."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      developmental delay/intellectual impairment (86%), short stature (73%),
      ichthyosis (65%)
    explanation: >-
      The systematic review reports short stature in 73% of published cases.
- name: Ichthyosis
  description: >-
    Congenital or later ichthyosis is common, but variable, across the TTD
    spectrum and is not limited to photosensitive disease.
  phenotype_term:
    preferred_term: Ichthyosis
    term:
      id: HP:0008064
      label: Ichthyosis
  frequency: FREQUENT
  evidence:
  - reference: PMID:18603627
    reference_title: "Trichothiodystrophy: a systematic review of 112 published cases characterises a wide spectrum of clinical manifestations."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      short stature (73%), ichthyosis (65%), abnormal characteristics at birth
      (55%)
    explanation: >-
      The systematic review reports ichthyosis in 65% of published cases.
- name: Nail Dystrophy
  description: >-
    Nail dystrophy is an ectodermal feature documented in MPLKIP-related TTD.
    The displayed primary cohort is subtype-specific, so no umbrella frequency
    is assigned.
  phenotype_term:
    preferred_term: Nail dystrophy
    term:
      id: HP:0008404
      label: Nail dystrophy
  evidence:
  - reference: PMID:37800682
    reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      ectodermal abnormalities comprising keratosis pilaris, hyperkeratotic
      plaques on the scalp, dry skin and nail dystrophy, dysmorphic facial
      features, microcephaly, global developmental delay and/or intellectual
      disability, short stature, and hypogonadism.
    explanation: >-
      Directly documents nail dystrophy in the seven-patient MPLKIP cohort.
- name: Cutaneous Photosensitivity
  subtype: Photosensitive TTD
  description: >-
    Exaggerated cutaneous UV sensitivity defines the TFIIH-related photosensitive
    branch. Its absence does not argue against non-photosensitive TTD.
  phenotype_term:
    preferred_term: Cutaneous photosensitivity
    term:
      id: HP:0000992
      label: Cutaneous photosensitivity
  evidence:
  - reference: PMID:40737808
    reference_title: "Trichothiodystrophy: Molecular insights and mechanisms of pathogenicity."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      The presence of hypersensitivity to UV light defines the two main forms of
      TTD: photosensitive (PS-TTD) and non-photosensitive (NPS-TTD).
    explanation: >-
      Establishes the subtype-defining role of UV hypersensitivity.
  - reference: PMID:18603627
    reference_title: "Trichothiodystrophy: a systematic review of 112 published cases characterises a wide spectrum of clinical manifestations."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      infections (46%), photosensitivity (42%), maternal pregnancy complications
      (28%) and defective DNA repair (37%).
    explanation: >-
      Reports photosensitivity in 42% of the umbrella published-case series.
- name: Recurrent Infections
  description: >-
    Recurrent infection is common and clinically important. Hypogammaglobulinemia,
    neutropenia, and epithelial-barrier abnormalities occur in subsets rather
    than defining one universal immune mechanism.
  phenotype_term:
    preferred_term: Recurrent infections
    term:
      id: HP:0002719
      label: Recurrent infections
  frequency: FREQUENT
  evidence:
  - reference: PMID:18603627
    reference_title: "Trichothiodystrophy: a systematic review of 112 published cases characterises a wide spectrum of clinical manifestations."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      ocular abnormalities (51%), infections (46%), photosensitivity (42%)
    explanation: >-
      The systematic review reports infections in 46% of published cases.
  - reference: PMID:30334570
    reference_title: "Mortality-associated immunological abnormalities in trichothiodystrophy: correlation of reduced levels of immunoglobulin and neutrophils with poor patient survival."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Infection or sepsis was determined to be the cause of death in 7 of the 10
      TTD patients who died.
    explanation: >-
      The NIH cohort documents the mortality relevance of infection.
- name: Ocular Abnormality
  description: >-
    Ocular findings are heterogeneous and include cataract and other structural
    or functional abnormalities; the broad term avoids presenting one ocular
    feature as universal.
  phenotype_term:
    preferred_term: Abnormality of the eye
    term:
      id: HP:0000478
      label: Abnormality of the eye
  frequency: FREQUENT
  evidence:
  - reference: PMID:18603627
    reference_title: "Trichothiodystrophy: a systematic review of 112 published cases characterises a wide spectrum of clinical manifestations."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      ocular abnormalities (51%), infections (46%), photosensitivity (42%)
    explanation: >-
      The systematic review reports ocular abnormalities in 51% of published
      cases without reducing them to a single eye phenotype.
- name: Cataract
  description: >-
    Cataract was reported in 14 of 26 examined patients in the NIH cohort, often
    without visual significance. The cohort estimate is retained separately
    from the broader ocular-abnormality frequency.
  phenotype_term:
    preferred_term: Cataract
    term:
      id: HP:0000518
      label: Cataract
  frequency: FREQUENT
  evidence:
  - reference: PMID:21800331
    reference_title: "High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Cataracts were reported in 14 (54%) of the TTD patients when examined at
      NIH at age 2 to 29 (mean 9.4years) and many were not visually significant
    explanation: >-
      Supplies the examined-cohort denominator, frequency, age range, and
      severity caveat.
- name: Cerebral Hypomyelination
  description: >-
    Cerebral hypomyelination is a recognized but non-universal neurologic feature,
    illustrated here with a longitudinally imaged ERCC2-related case.
  phenotype_term:
    preferred_term: Cerebral hypomyelination
    term:
      id: HP:0006808
      label: Cerebral hypomyelination
  evidence:
  - reference: PMID:39976384
    reference_title: "Trichothiodystrophy due to ERCC2 Variants: Uncommon Contributor to Progressive Hypomyelinating Leukodystrophy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      a 5-year-old affected child who presented with microcephaly, failure to
      thrive, developmental delay, and progressive hypomyelination on three
      serial brain imaging over 5-years follow-up.
    explanation: >-
      Directly documents progressive hypomyelination in an ERCC2-related child.
- name: Gait Disturbance
  description: >-
    Ataxic gait has been documented in molecularly characterized
    MPLKIP-related TTD. The source is an individual observation within a small
    subtype cohort, so no umbrella frequency is assigned.
  phenotype_term:
    preferred_term: Gait disturbance
    term:
      id: HP:0001288
      label: Gait disturbance
  evidence:
  - reference: PMID:37800682
    reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The gait was ataxic. The systemic and neurological examinations were
      otherwise unremarkable.
    explanation: >-
      Directly documents the broad gait phenotype in a genotyped
      MPLKIP-related patient without assigning a cohort frequency.
- name: Seizure
  subtype: TTD4 (MPLKIP)
  description: >-
    Seizure disorders occurred in three of five MPLKIP/TTDN1 patients and were
    overrepresented relative to other genotyped TTD cases in the cohort. The
    frequency is subtype- and small-cohort-specific.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  frequency: FREQUENT
  evidence:
  - reference: PMID:25290684
    reference_title: Mutations in the TTDN1 gene are associated with a distinct trichothiodystrophy phenotype.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      There were 3 patients from 2 families with TTDN1 mutations (TTD343BE,
      and siblings TTD487BE and TTD488BE) diagnosed with seizure disorders,
      but only 3 out of the 31 other TTD patients (p=0.024).
    explanation: >-
      Supports the 3/5 MPLKIP-specific frequency and cohort enrichment.
- name: Delayed Skeletal Maturation
  subtype: TTD4 (MPLKIP)
  description: >-
    Delayed bone age occurred in four of five MPLKIP/TTDN1 patients and was
    overrepresented relative to other genotyped TTD cases. The frequency is
    subtype- and small-cohort-specific.
  phenotype_term:
    preferred_term: Delayed skeletal maturation
    term:
      id: HP:0002750
      label: Delayed skeletal maturation
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:25290684
    reference_title: Mutations in the TTDN1 gene are associated with a distinct trichothiodystrophy phenotype.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Additionally, 4 patients from 3 families with TTDN1 defects had delayed
      bone age (TTD343BE, TTD480BE, and siblings TTD487BE and TTD488BE), as
      compared to 4 out of 23 TTD patients with other mutations (p=0.009).
    explanation: >-
      Supports the 4/5 MPLKIP-specific frequency and cohort enrichment.
- name: Hypogonadism
  subtype: TTD4 (MPLKIP)
  description: >-
    Hypogonadism was reported in a seven-person MPLKIP-associated
    non-photosensitive TTD cohort. This subtype-specific observation is not used
    to infer a frequency across the heterogeneous TTD umbrella.
  phenotype_term:
    preferred_term: Hypogonadism
    term:
      id: HP:0000135
      label: Hypogonadism
  evidence:
  - reference: PMID:37800682
    reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We investigated a cohort of seven patients from five unrelated families
      with characteristic features of TTD (Figs 1A–F and 2A and B), including
      short, woolly, slow‐growing, sparse and brittle hair, sparse and brittle
      eyebrows and eyelashes, ectodermal abnormalities comprising keratosis
      pilaris, hyperkeratotic plaques on the scalp, dry skin and nail dystrophy,
      dysmorphic facial features, microcephaly, global developmental delay
      and/or intellectual disability, short stature, and hypogonadism.
    explanation: >-
      Directly documents hypogonadism in the seven-person MPLKIP-associated
      cohort without assigning an umbrella-level frequency.
- name: Microcytic Anemia
  description: >-
    Microcytic anemia occurs in a subset of TTD, including GTF2E2-related cases.
    It is not treated as universal or as a single-mechanism phenotype across the
    genetically heterogeneous spectrum.
  phenotype_term:
    preferred_term: Microcytic anemia
    term:
      id: HP:0001935
      label: Microcytic anemia
  evidence:
  - reference: PMID:28973399
    reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Patients harboring a TFIIEβ mutation present microcytic anemia
    explanation: >-
      Directly documents the clinical phenotype while preserving the candidate
      status of its mechanism.
  - reference: PMID:30334570
    reference_title: "Mortality-associated immunological abnormalities in trichothiodystrophy: correlation of reduced levels of immunoglobulin and neutrophils with poor patient survival."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Mild anaemia was present in 8 patients, with the lowest haemoglobin being
      100 g/l (Fig 1D). All patients with anaemia had microcytosis, low iron
      saturation and normal ferritin (Figure S1).
    explanation: >-
      Independently documents microcytic anemia in the NIH TTD cohort.
- name: Decreased Circulating Immunoglobulin Concentration
  description: >-
    Age-adjusted IgG is reduced in a subset, particularly among studied
    ERCC2-related patients, and lower IgG was associated with early mortality.
  phenotype_term:
    preferred_term: Decreased circulating immunoglobulin concentration
    term:
      id: HP:0004313
      label: Decreased circulating immunoglobulin concentration
  evidence:
  - reference: PMID:30334570
    reference_title: "Mortality-associated immunological abnormalities in trichothiodystrophy: correlation of reduced levels of immunoglobulin and neutrophils with poor patient survival."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Thirteen patients had an IgG level below the age-adjusted lower limit of
      normal. Twelve of these patients had a mutation in ERCC2.
    explanation: >-
      Directly quantifies reduced age-adjusted IgG in the cohort and its strong
      ERCC2 enrichment.
- name: Decreased Total Neutrophil Count
  description: >-
    Neutropenia occurs in a subset and may recur across visits; it is enriched,
    but not exclusive, in ERCC2-related disease.
  phenotype_term:
    preferred_term: Decreased total neutrophil count
    term:
      id: HP:0001875
      label: Decreased total neutrophil count
  evidence:
  - reference: PMID:30334570
    reference_title: "Mortality-associated immunological abnormalities in trichothiodystrophy: correlation of reduced levels of immunoglobulin and neutrophils with poor patient survival."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Seventeen patients had an ANC below 1.0 × 109/l, of which 13 had ERCC2
      mutations.
    explanation: >-
      Directly documents decreased absolute neutrophil counts in the cohort.
- name: Premature Birth
  description: >-
    Preterm delivery was frequent in a 27-pregnancy TTD cohort. The frequency is
    cohort-specific and is not generalized to every genotype or pregnancy.
  phenotype_term:
    preferred_term: Premature birth
    term:
      id: HP:0001622
      label: Premature birth
  frequency: FREQUENT
  evidence:
  - reference: PMID:21800331
    reference_title: "High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Pregnancy reports of 27 TTD patients and their 23 mothers were evaluated
      and 81% of the pregnancies had complications: 56% had preterm delivery
    explanation: >-
      Supplies the cohort denominator and preterm-delivery frequency.
- name: Small for Gestational Age
  description: >-
    Birth weight below the tenth centile for gestational age occurred in 9 of 26
    patients with known gestational age. This measure is kept distinct from the
    cohort's higher crude low-birth-weight frequency.
  phenotype_term:
    preferred_term: Small for gestational age
    term:
      id: HP:0001518
      label: Small for gestational age
  frequency: FREQUENT
  evidence:
  - reference: PMID:21800331
    reference_title: "High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The birth weight of 9(35%) of the 26 patients with known gestational age
      had weight <10centile for gestational age.
    explanation: >-
      Supplies the gestational-age-adjusted denominator and frequency.
- name: Cryptorchidism
  description: >-
    Cryptorchidism was reported in five of sixteen male infants in the neonatal
    cohort; the frequency applies only to the male denominator.
  phenotype_term:
    preferred_term: Cryptorchidism
    term:
      id: HP:0000028
      label: Cryptorchidism
  frequency: FREQUENT
  evidence:
  - reference: PMID:21800331
    reference_title: "High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Cryptorchidism was reported in 5 (31%) of the male infants.
    explanation: >-
      Preserves the sex-specific cohort denominator and frequency.
- name: Congenital Ichthyosiform Erythroderma
  description: >-
    Neonatal erythroderma, often with a collodion membrane, is an important early
    presentation in a subset. The combined cohort statistic is not interpreted
    as a precise frequency for erythroderma alone.
  phenotype_term:
    preferred_term: Congenital ichthyosiform erythroderma
    term:
      id: HP:0007431
      label: Congenital ichthyosiform erythroderma
  evidence:
  - reference: PMID:21800331
    reference_title: "High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Erythroderma/collodion presentation of the skin was noted in 18 (67%)
      infants (Figure 1A). It typically persisted for 1 to 2 weeks; the patients
      were treated with humidified air and emollients.
    explanation: >-
      Supports a common neonatal erythroderma/collodion presentation while not
      separating the two components in the reported percentage.
biochemical:
- name: Reduced Cysteine and Cystine in Hair and Nail Material
  presence: DECREASED
  context: >-
    Diagnostic biochemical property measured by amino-acid analysis of hair or
    nail clippings; specialized sample preparation is required for accurate
    cysteine/cystine quantification.
  evidence:
  - reference: PMID:15232704
    reference_title: "Trichothiodystrophy: quantification of cysteine in human hair and nails by application of sodium azide-dependent oxidation to cysteic acid."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      With our control and parent data resembling published data on hair and
      nail samples, we obtained a decreased proportion of cysteine (half
      cystine, determined as cysteic acid) in materials obtained from a boy with
      TTD.
    explanation: >-
      Supports reduced measured cysteine/cystine in affected hair and nail
      material.
- name: Reduced Prostaglandin I2 Synthase in Dermal Fibroblasts
  presence: DECREASED
  context: >-
    Research biomarker observed in the studied photosensitive and
    non-photosensitive TTD primary fibroblasts. It is not curated as a proven
    cause of the multisystem phenotype or as a routine clinical assay.
  cell_types:
  - preferred_term: skin fibroblast
    term:
      id: CL:0002620
      label: skin fibroblast
  evidence:
  - reference: PMID:34155103
    reference_title: "Reduced levels of prostaglandin I(2) synthase: a distinctive feature of the cancer-free trichothiodystrophy."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Its reduction characterizes all TTD cases so far investigated, both the
      PS-TTD with mutations in TFIIH coding genes as well as the
      nonphotosensitive (NPS)-TTD.
    explanation: >-
      Supports reduced PTGIS in the studied patient-cell panel while preserving
      its biomarker, rather than causal, status.
genetic:
- name: ERCC2
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: Photosensitive TTD
  gene_term:
    preferred_term: ERCC2
    term:
      id: hgnc:3434
      label: ERCC2
  notes: >-
    Biallelic ERCC2 variants cause the TTD1 photosensitive subtype
    (MONDO:0011125). ERCC2 is also known historically as XPD.
  evidence:
  - reference: PMID:9238033
    reference_title: Xeroderma pigmentosum and trichothiodystrophy are associated with different mutations in the XPD (ERCC2) repair/transcription gene.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Mutations in the XPD gene can result in three distinct clinical phenotypes,
      XP, trichothiodystrophy (TTD), and XP with Cockayne syndrome.
    explanation: >-
      Patient mutation analysis establishes ERCC2/XPD as an allelic cause of TTD.
- name: ERCC3
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: Photosensitive TTD
  gene_term:
    preferred_term: ERCC3
    term:
      id: hgnc:3435
      label: ERCC3
  notes: >-
    Biallelic ERCC3 variants cause the TTD2 photosensitive subtype
    (MONDO:0014615). ERCC3 is also known historically as XPB.
  evidence:
  - reference: PMID:9012405
    reference_title: A mutation in the XPB/ERCC3 DNA repair transcription gene, associated with trichothiodystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The causative mutation was found to be a single base substitution
      resulting in a missense mutation (T119P) in a region of the XPB protein
      completely conserved in yeast, Drosophila, mouse, and man.
    explanation: >-
      Directly establishes a causal ERCC3/XPB variant in affected patients.
- name: GTF2H5
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: Photosensitive TTD
  gene_term:
    preferred_term: GTF2H5
    term:
      id: hgnc:21157
      label: GTF2H5
  notes: >-
    Biallelic GTF2H5 variants cause the TTD3 photosensitive subtype
    (MONDO:0014619). GTF2H5 is also known historically as TTDA.
  evidence:
  - reference: PMID:15220921
    reference_title: A new, tenth subunit of TFIIH is responsible for the DNA repair syndrome trichothiodystrophy group A.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Microinjection of cDNA encoding TFB5 (GTF2H5, also called TTDA) corrected
      the DNA-repair defect of TTD-A cells, and we identified three functional
      inactivating mutations in this gene in three unrelated families with TTD-A.
    explanation: >-
      Multiple families and cellular rescue establish GTF2H5/TTDA causality.
- name: MPLKIP
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: Non-photosensitive TTD
  gene_term:
    preferred_term: MPLKIP
    term:
      id: hgnc:16002
      label: MPLKIP
  notes: >-
    Biallelic MPLKIP variants cause TTD4, non-photosensitive
    (MONDO:0021013). MPLKIP is also known as TTDN1.
  evidence:
  - reference: PMID:15645389
    reference_title: Identification of C7orf11 (TTDN1) gene mutations and genetic heterogeneity in nonphotosensitive trichothiodystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We have identified C7orf11, which localizes to the nucleus and is expressed
      in fetal hair follicles, as the first disease gene for nonphotosensitive
      trichothiodystrophy (TTD).
    explanation: >-
      Establishes the gene now named MPLKIP as a causal non-photosensitive TTD gene.
- name: GTF2E2
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: Non-photosensitive TTD
  gene_term:
    preferred_term: GTF2E2
    term:
      id: hgnc:4651
      label: GTF2E2
  notes: >-
    Biallelic GTF2E2 variants cause TTD6, non-photosensitive
    (MONDO:0014841). GTF2E2 encodes TFIIE beta.
  evidence:
  - reference: PMID:26996949
    reference_title: GTF2E2 Mutations Destabilize the General Transcription Factor Complex TFIIE in Individuals with DNA Repair-Proficient Trichothiodystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here, we report two unrelated children showing clinical features typical
      of TTD who harbor different homozygous missense mutations in GTF2E2
    explanation: >-
      Biallelic GTF2E2 variants in unrelated affected children support a causal
      non-photosensitive TTD association.
- name: RNF113A
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: Non-photosensitive TTD
  gene_term:
    preferred_term: RNF113A
    term:
      id: hgnc:12974
      label: RNF113A
  notes: >-
    Hemizygous RNF113A variants cause TTD5, non-photosensitive
    (MONDO:0010495), with X-linked recessive inheritance.
  evidence:
  - reference: PMID:25612912
    reference_title: A novel X-linked trichothiodystrophy associated with a nonsense mutation in RNF113A.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We identified a nonsense mutation in the highly conserved RNF113A gene
      (c.901 C>T, p.Q301*). The mutation segregated with the disease in the family
      and was not observed in over 100,000 control X chromosomes.
    explanation: >-
      Segregation and population absence support the causal X-linked association.
- name: CARS1
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: Non-photosensitive TTD
  gene_term:
    preferred_term: CARS1
    term:
      id: hgnc:1493
      label: CARS1
  notes: >-
    Biallelic CARS1 variants cause the TTD-spectrum microcephaly,
    developmental-delay, and brittle-hair syndrome (MONDO:0030047).
  evidence:
  - reference: PMID:30824121
    reference_title: Cysteinyl-tRNA Synthetase Mutations Cause a Multi-System, Recessive Disease That Includes Microcephaly, Developmental Delay, and Brittle Hair and Nails.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here, we report on four subjects from three families with complex syndromes
      that include microcephaly, developmental delay, and brittle hair and nails.
      Each affected person carries bi-allelic CARS variants
    explanation: >-
      Multiple families with biallelic variants and the defining hair phenotype
      support the CARS1 association.
  - reference: PMID:37800682
    reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      We previously proposed (Theil et al, 2019) that most of the key TTD
      features of brittle hair, nails, and scaly skin are a consequence of
      protein instability of factors involved in different steps of gene
      expression in either transcription (mutated ERCC2, ERCC3, GTF2H5, or
      GTF2E2), splicing (mutated RNF113A), or translation (mutated AARS1, CARS1,
      MARS1, or TARS1).
    explanation: >-
      Contemporary TTD literature explicitly includes CARS1 in the TTD
      translation branch despite the current MONDO hierarchy placing its named
      syndrome outside the TTD umbrella.
- name: TARS1
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: Non-photosensitive TTD
  gene_term:
    preferred_term: TARS1
    term:
      id: hgnc:11572
      label: TARS1
  notes: >-
    Biallelic TARS1 variants cause TTD7, non-photosensitive
    (MONDO:0032806).
  evidence:
  - reference: PMID:31374204
    reference_title: Bi-allelic TARS Mutations Are Associated with Brittle Hair Phenotype.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here we present the identification of an NPS-TTD-associated gene,
      threonyl-tRNA synthetase (TARS), found by next-generation sequencing of a
      group of uncharacterized individuals with NPS-TTD.
    explanation: >-
      Patient sequencing establishes TARS1 as a causal non-photosensitive TTD gene.
- name: AARS1
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: Non-photosensitive TTD
  gene_term:
    preferred_term: AARS1
    term:
      id: hgnc:20
      label: AARS1
  notes: >-
    Biallelic AARS1 variants cause TTD8, non-photosensitive
    (MONDO:0030517).
  evidence:
  - reference: PMID:33909043
    reference_title: Protein instability associated with AARS1 and MARS1 mutations causes trichothiodystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here, we identify alanyl-tRNA synthetase 1 and methionyl-tRNA synthetase 1
      variants as new gene defects that cause NPS-TTD.
    explanation: >-
      The patient study identifies AARS1 variants as a cause of
      non-photosensitive TTD.
- name: MARS1
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: Non-photosensitive TTD
  gene_term:
    preferred_term: MARS1
    term:
      id: hgnc:6898
      label: MARS1
  notes: >-
    Biallelic MARS1 variants cause TTD9, non-photosensitive
    (MONDO:0030518).
  evidence:
  - reference: PMID:33909043
    reference_title: Protein instability associated with AARS1 and MARS1 mutations causes trichothiodystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here, we identify alanyl-tRNA synthetase 1 and methionyl-tRNA synthetase 1
      variants as new gene defects that cause NPS-TTD.
    explanation: >-
      The patient study identifies MARS1 variants as a cause of
      non-photosensitive TTD.
  - reference: PMID:40820264
    reference_title: Second Case of Type 9 Non-Photosensitive Trichothiodystrophy Caused by Homozygous Variant in the MARS1 Gene.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We present the second case of type 9 non-photosensitive
      trichothiodystrophy caused by a homozygous variant in the MARS1 gene.
    explanation: >-
      A second independent case strengthens the initially single-case MARS1-TTD9
      association without implying that its atypical features are universal.
- name: DBR1
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: Non-photosensitive TTD
  gene_term:
    preferred_term: DBR1
    term:
      id: hgnc:15594
      label: DBR1
  notes: >-
    Biallelic DBR1 p.Asp262Tyr causes the Sabinas brittle-hair form of TTD.
    MONDO:0008886 represents Sabinas syndrome but is not currently placed as a
    descendant of the TTD umbrella, and no numbered TTD subtype was identified.
  evidence:
  - reference: PMID:40683339
    reference_title: "RNA Lariat-Debranching Enzyme (DBR1) Variations in Sabinas Brittle Hair Syndrome Form of Trichothiodystrophy: A Trichothiodystrophy-Causing Gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report 5 nonphotosensitive adult patients from 3 unrelated families
      with a homozygous missense variation in DBR1 (p.D262Y) encoding the RNA
      lariat-debranching enzyme DBR1, which is involved in the removal of introns
      from pre-mRNA in the nucleus.
    explanation: >-
      Three unrelated families support DBR1 as the eleventh TTD causal gene.
diagnosis:
- name: Polarized-Light Hair Microscopy
  description: >-
    Examine plucked or clipped hair shafts under polarized light for alternating
    bright and dark "tiger-tail" bands. Interpret the finding together with
    shaft morphology, sulfur-amino-acid analysis, and molecular results.
  diagnosis_term:
    preferred_term: polarization microscopy
    term:
      id: NCIT:C16858
      label: Polarization Microscopy
  results: >-
    Tiger-tail banding together with brittle shaft abnormalities supports TTD
    and should prompt biochemical and molecular confirmation.
  evidence:
  - reference: PMID:15232704
    reference_title: "Trichothiodystrophy: quantification of cysteine in human hair and nails by application of sodium azide-dependent oxidation to cysteic acid."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Important laboratory tests of the hair for the diagnosis of TTD comprise
      polarizing microscopy ("tiger-tail" pattern), electron microscopy, and
      amino acids analysis of hydrolyzed hair with a special focus on cystine.
    explanation: >-
      Directly supports polarized-light hair microscopy as one component of the
      diagnostic laboratory evaluation.
- name: Hair or Nail Cysteine and Cystine Analysis
  description: >-
    Quantify cysteine/cystine in hydrolyzed hair or nail material using validated
    preparation that prevents decomposition during hydrolysis. Limited
    laboratory availability and pre-analytic sensitivity make this a specialized
    confirmatory assay rather than a stand-alone exclusion test.
  diagnosis_term:
    preferred_term: cysteine measurement
    term:
      id: NCIT:C172518
      label: Cysteine Measurement
  results: >-
    A reduced proportion of cysteine/cystine supports sulfur-deficient hair or
    nail material characteristic of TTD.
  evidence:
  - reference: PMID:15232704
    reference_title: "Trichothiodystrophy: quantification of cysteine in human hair and nails by application of sodium azide-dependent oxidation to cysteic acid."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      With our control and parent data resembling published data on hair and
      nail samples, we obtained a decreased proportion of cysteine (half
      cystine, determined as cysteic acid) in materials obtained from a boy with
      TTD.
    explanation: >-
      Demonstrates the expected biochemical result in affected hair and nail
      material.
  - reference: PMID:15232704
    reference_title: "Trichothiodystrophy: quantification of cysteine in human hair and nails by application of sodium azide-dependent oxidation to cysteic acid."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      If no special precautions are taken, quantification of cysteine and cystine
      becomes inaccurate because of decomposition of these residues during
      hydrolysis.
    explanation: >-
      Establishes the important pre-analytic limitation of this specialized assay.
- name: Molecular Genetic Testing
  description: >-
    Use a multigene panel or exome/genome analysis that includes ERCC2, ERCC3,
    GTF2H5, MPLKIP, GTF2E2, RNF113A, CARS1, TARS1, AARS1, MARS1, and DBR1.
    Interpret inheritance and photosensitivity by the identified gene rather
    than treating all TTD as a TFIIH disorder.
  diagnosis_term:
    preferred_term: genetic testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  results: >-
    Biallelic pathogenic variants in one of the ten autosomal genes, or a
    hemizygous pathogenic RNF113A variant in an affected male, provide molecular
    confirmation in a compatible phenotype.
  evidence:
  - reference: PMID:40737808
    reference_title: "Trichothiodystrophy: Molecular insights and mechanisms of pathogenicity."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      The disease arises from mutations in a variety of genes involved in
      different biological processes. Affected processes include DNA repair,
      transcription as well as translation.
    explanation: >-
      Current review evidence supports a genetically heterogeneous testing
      strategy; the individual gene records above supply gene-specific causal
      evidence.
differential_diagnoses:
- name: Xeroderma Pigmentosum
  description: >-
    XP and photosensitive TTD can both result from ERCC2 or ERCC3 variants and
    defective nucleotide-excision repair. Brittle sulfur-deficient tiger-tail
    hair and the multisystem developmental TTD phenotype favor TTD, whereas a
    marked cutaneous-cancer predisposition favors XP. Genotype alone may require
    clinical and functional interpretation because the same genes underlie both.
  disease_term:
    preferred_term: xeroderma pigmentosum
    term:
      id: MONDO:0019600
      label: xeroderma pigmentosum
  distinguishing_features:
  - Cutaneous cancer predisposition is characteristic of XP but not an established increased risk in TTD.
  - Sulfur-deficient brittle hair with tiger-tail banding supports TTD.
  evidence:
  - reference: PMID:20687499
    reference_title: "Trichothiodystrophy: Photosensitive, TTD-P, TTD, Tay syndrome."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Skin cancer is very rare in sun-sensitive TTD.
    explanation: >-
      A clinical review supports the important contrast with the marked cancer
      predisposition of XP without asserting that tumors are impossible in TTD.
  - reference: PMID:9012405
    reference_title: A mutation in the XPB/ERCC3 DNA repair transcription gene, associated with trichothiodystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      DNA repair defects in XP-D are associated with two additional, quite
      different diseases; XP, a sun-sensitive and cancer-prone repair disorder,
      and Cockayne syndrome (CS), a photosensitive condition characterized by
      physical and mental retardation and wizened facial appearance.
    explanation: >-
      Provides the clinical cancer-prone XP contrast in an allelic TFIIH study.
- name: Cockayne Syndrome
  description: >-
    Cockayne syndrome overlaps with photosensitive TTD through defective
    transcription-coupled nucleotide-excision repair and growth or neurologic
    impairment. Sulfur-deficient brittle hair with tiger-tail banding favors
    TTD, while progressive cachectic or progeroid features support Cockayne
    syndrome; allelic ERCC2 cases require integrated clinical interpretation.
  disease_term:
    preferred_term: Cockayne syndrome
    term:
      id: MONDO:0016006
      label: Cockayne syndrome
  distinguishing_features:
  - Sulfur-deficient brittle hair and tiger-tail banding favor TTD.
  - Progressive cachectic or progeroid features favor Cockayne syndrome.
  evidence:
  - reference: PMID:9012405
    reference_title: A mutation in the XPB/ERCC3 DNA repair transcription gene, associated with trichothiodystrophy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      DNA repair defects in XP-D are associated with two additional, quite
      different diseases; XP, a sun-sensitive and cancer-prone repair disorder,
      and Cockayne syndrome (CS), a photosensitive condition characterized by
      physical and mental retardation and wizened facial appearance.
    explanation: >-
      Supports Cockayne syndrome as an allelic photosensitive developmental
      differential in the TFIIH-related branch.
- name: Netherton Syndrome
  description: >-
    Netherton syndrome can combine congenital ichthyosis with short, fragile
    hair. Trichorrhexis invaginata and prominent atopy with elevated IgE favor
    Netherton syndrome, whereas sulfur deficiency and tiger-tail banding favor
    TTD.
  disease_term:
    preferred_term: Netherton syndrome
    term:
      id: MONDO:0009735
      label: Netherton syndrome
  distinguishing_features:
  - Trichorrhexis invaginata and atopic diathesis favor Netherton syndrome.
  - Reduced hair sulfur and tiger-tail banding favor TTD.
  evidence:
  - reference: PMID:31332722
    reference_title: "Genetic Hair Disorders: A Review."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Netherton syndrome is characterized by a triad of congenital
      ichthyosiform erythroderma (CIE) or ichthyosis linearis circumflexa (ILC),
      atopic diathesis (elevated serum IgE), and short sparse fragile hair with
      trichorrhexis invaginata [49].
    explanation: >-
      Defines the characteristic hair, skin, and atopic features that distinguish
      this ichthyosis mimic.
- name: Menkes Disease
  description: >-
    Menkes disease can present with sparse, fragile or twisted hair and severe
    neurologic disease. Pili torti or steel-wool hair together with abnormal
    copper metabolism and ATP7A-related systemic findings favors Menkes disease
    rather than TTD.
  disease_term:
    preferred_term: Menkes disease
    term:
      id: MONDO:0010651
      label: Menkes disease
  distinguishing_features:
  - Pili torti or steel-wool hair with disordered copper metabolism favors Menkes disease.
  - Reduced sulfur content and tiger-tail banding favor TTD.
  evidence:
  - reference: PMID:31332722
    reference_title: "Genetic Hair Disorders: A Review."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Menkes syndrome: X-linked recessive condition. It is due to a mutation in
      the X-linked ATP7A gene, which encodes a copper-transporting ATPase. In this
      syndrome there is a maldistribution of the body’s copper leading to
      dysfunctions of all processes that need this element. The patient will have
      sparse, pale, lightly pigmented hair with a steel wool appearance.
    explanation: >-
      Supports the ATP7A/copper and distinctive-hair basis for the differential.
- name: Argininosuccinic Aciduria
  description: >-
    Argininosuccinic aciduria can cause brittle trichorrhexis-nodosa hair with
    neurodevelopmental or hepatic disease. Urea-cycle biochemical abnormalities
    and hair that is initially normal before later trichorrhexis nodosa favor
    argininosuccinic aciduria rather than TTD.
  disease_term:
    preferred_term: argininosuccinic aciduria
    term:
      id: MONDO:0008815
      label: argininosuccinic aciduria
  distinguishing_features:
  - Hyperammonemia or urea-cycle abnormalities favor argininosuccinic aciduria.
  - Congenital sulfur-deficient brittle hair and tiger-tail banding favor TTD.
  evidence:
  - reference: PMID:31332722
    reference_title: "Genetic Hair Disorders: A Review."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Argininosuccinic aciduria is an inborn error of urea synthesis. It is due
      to deficiency of argininosuccinic lyase. If untreated, it may present with
      failure to thrive and mental retardation. The hair finding of TN is an
      important diagnostic clue in this syndrome. These patients have normal hair
      at birth but then develop TN when they are 1–2 years of age [45].
    explanation: >-
      Supports the metabolic context and later trichorrhexis-nodosa hair pattern.
treatments:
- name: Rigorous Photoprotection for Photosensitive TTD
  description: >-
    Rigorous reduction of ultraviolet exposure is a supportive management
    component for the ERCC2-, ERCC3-, and GTF2H5-related photosensitive branch.
    It is not a disease-modifying treatment and is not imposed on genetically
    and functionally non-photosensitive TTD.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  target_phenotypes:
  - preferred_term: Cutaneous photosensitivity
    term:
      id: HP:0000992
      label: Cutaneous photosensitivity
  evidence:
  - reference: PMID:27745642
    reference_title: "Understanding photodermatoses associated with defective DNA repair: Photosensitive syndromes without associated cancer predisposition."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Rigorous photoprotection remains an important component of the management
      of these inherited DNA repair-deficiency photodermatoses.
    explanation: >-
      A clinical review directly supports rigorous photoprotection for the
      DNA-repair-deficient photosensitive syndromes that include TTD.
- name: Immune and Hematologic Surveillance
  description: >-
    Monitor recurrent infections, age-adjusted immunoglobulins, complete blood
    counts, and absolute neutrophil counts. This surveillance is motivated by
    mortality-associated abnormalities in a 40-patient cohort; it is not itself
    a disease-modifying intervention.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:30334570
    reference_title: "Mortality-associated immunological abnormalities in trichothiodystrophy: correlation of reduced levels of immunoglobulin and neutrophils with poor patient survival."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Based on observed early mortality in our cohort, we propose surveillance
      of TTD patients for recurrent infections, hypogammaglobulinaemia and
      neutropenia.
    explanation: >-
      Directly supports the surveillance component from a 40-patient cohort.
- name: Conditional Immunoglobulin Replacement
  description: >-
    Consider supplemental immunoglobulin only for a patient with documented
    hypogammaglobulinemia and an appropriate clinical indication. The cohort
    publication proposes this as abnormality-directed supportive management,
    not routine treatment for every TTD genotype.
  therapeutic_modality: PROTEIN_REPLACEMENT
  treatment_term:
    preferred_term: immunoglobulin infusion therapy
    term:
      id: NCIT:C62710
      label: Immunoglobulin Therapy
  target_phenotypes:
  - preferred_term: Decreased circulating immunoglobulin concentration
    term:
      id: HP:0004313
      label: Decreased circulating immunoglobulin concentration
  evidence:
  - reference: PMID:30334570
    reference_title: "Mortality-associated immunological abnormalities in trichothiodystrophy: correlation of reduced levels of immunoglobulin and neutrophils with poor patient survival."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Haematological evaluation may be considered for the management of
      hypogammaglobulinaemia or neutropenia with supplemental immunoglobulins or
      granulocyte colony-stimulating factor, respectively.
    explanation: >-
      Supports immunoglobulin replacement only when the corresponding
      immunoglobulin abnormality is present.
- name: Conditional Granulocyte Colony-Stimulating Factor
  description: >-
    Consider granulocyte colony-stimulating factor only for a patient with
    documented clinically significant neutropenia. The cohort publication
    proposes phenotype-directed supportive management rather than universal
    prophylaxis or disease modification.
  therapeutic_modality: PROTEIN_REPLACEMENT
  treatment_term:
    preferred_term: granulocyte colony-stimulating factor therapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: recombinant granulocyte colony-stimulating factor
      term:
        id: NCIT:C1287
        label: Recombinant Granulocyte Colony-Stimulating Factor
  target_phenotypes:
  - preferred_term: Decreased total neutrophil count
    term:
      id: HP:0001875
      label: Decreased total neutrophil count
  evidence:
  - reference: PMID:30334570
    reference_title: "Mortality-associated immunological abnormalities in trichothiodystrophy: correlation of reduced levels of immunoglobulin and neutrophils with poor patient survival."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Haematological evaluation may be considered for the management of
      hypogammaglobulinaemia or neutropenia with supplemental immunoglobulins or
      granulocyte colony-stimulating factor, respectively.
    explanation: >-
      Supports G-CSF only when the corresponding neutropenia is present.
- name: Neonatal Humidified Air and Emollient Support
  description: >-
    Humidified air and emollients were used as supportive skin care for neonatal
    erythroderma or collodion presentation in the pregnancy cohort. The report
    documents use but does not establish comparative efficacy or a universal
    regimen.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  target_phenotypes:
  - preferred_term: Congenital ichthyosiform erythroderma
    term:
      id: HP:0007431
      label: Congenital ichthyosiform erythroderma
  evidence:
  - reference: PMID:21800331
    reference_title: "High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Erythroderma/collodion presentation of the skin was noted in 18 (67%)
      infants (Figure 1A). It typically persisted for 1 to 2 weeks; the patients
      were treated with humidified air and emollients.
    explanation: >-
      Documents neonatal supportive skin care while not proving comparative
      benefit.
experimental_models:
- name: GTF2E2-mutant patient iPSC-derived erythroid model
  description: >-
    A GTF2E2-mutant patient fibroblast line was reprogrammed to iPS cells and
    differentiated with two healthy-control iPSC lines into erythroblasts. The
    system reproduces late differentiation, cell-size, multinucleation, and
    hemoglobin-subunit abnormalities, but it executes an embryonic/fetal rather
    than adult globin program.
  experimental_model_type: IPSC_DERIVED_MODEL
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  cell_types:
  - preferred_term: erythroblast
    term:
      id: CL:0000765
      label: erythroblast
  cell_source: >-
    iPS cells reprogrammed from primary fibroblasts of patient TTD218UT and two
    healthy-control iPSC lines
  culture_system: Two-phase in-vitro erythroid expansion and differentiation at 37 or 39 degrees Celsius
  conditions:
  - GTF2E2 p.Asp187Tyr patient-derived line
  - healthy-control lines
  - standard and fever-mimicking temperatures
  publication: PMID:28973399
  modeled_mechanisms:
  - target: GTF2E2-Related Late Erythroid Differentiation Dysfunction
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      The patient-derived model directly reproduces abnormal terminal erythroid
      differentiation and globin-subunit balance.
    limitations: >-
      One patient-derived iPSC clone was compared with two non-isogenic controls;
      iPSC-derived erythroblasts execute an embryonic/fetal globin program and do
      not reproduce adult marrow or systemic physiology.
    readouts:
    - name: Hemoglobin-subunit balance in differentiated erythroblasts
      target: GTF2E2-Related Late Erythroid Differentiation Dysfunction
      direction: ALTERED
      interpretation: >-
        Altered hemoglobin fractions and increased abnormal erythroblast size or
        multinucleation report late erythroid differentiation dysfunction.
      evidence:
      - reference: PMID:28973399
        reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: >-
          We observed a clear hematopoietic defect during late-stage differentiation
          associated with hemoglobin subunit imbalance.
        explanation: >-
          Directly reports the model readout and its altered state.
    evidence:
    - reference: PMID:28973399
      reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        We performed induced pluripotent stem (iPS) cell reprogramming of patient
        fibroblasts followed by in vitro erythroid differentiation to translate
        the intriguing molecular defect to phenotypic expression in relevant
        tissue, to disclose the molecular basis for some specific TTD features.
      explanation: >-
        Defines the patient-derived iPSC erythroid model and its disease purpose.
  evidence:
  - reference: PMID:28973399
    reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We performed induced pluripotent stem (iPS) cell reprogramming of patient
      fibroblasts followed by in vitro erythroid differentiation to translate
      the intriguing molecular defect to phenotypic expression in relevant
      tissue, to disclose the molecular basis for some specific TTD features.
    explanation: >-
      Directly identifies the model system and differentiation design.
- name: GTF2E2-mutant SV40-immortalized fibroblast temperature-challenge model
  description: >-
    SV40-immortalized GTF2E2-mutant TTD218UT fibroblasts were compared with
    immortalized controls at standard and elevated temperatures to expose the
    conditional transcription defect that is absent under standard culture
    conditions.
  experimental_model_type: CELL_LINE
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  cell_types:
  - preferred_term: skin fibroblast
    term:
      id: CL:0002620
      label: skin fibroblast
  cell_source: >-
    SV40-immortalized TTD218UT_sv patient fibroblasts and MRC5_sv/C5RO_sv
    wild-type control fibroblasts
  culture_system: SV40-immortalized fibroblast monolayer at standard or elevated temperature
  conditions:
  - 37 degrees Celsius standard culture
  - prolonged 40 degrees Celsius temperature challenge
  publication: PMID:28973399
  modeled_mechanisms:
  - target: Temperature-Sensitive Transcriptional Insufficiency in GTF2E2-Related TTD
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Elevated-temperature culture directly reveals the conditional reduction in
      transcription in patient fibroblasts.
    limitations: >-
      The defining temperature-response assay used SV40-transformed cells from
      one patient line; transformation can alter stress and survival responses.
      Fever duration and temperature are more variable in patients, and
      fever-associated worsening was not confirmed in every reported individual.
    readouts:
    - name: Global RNA synthesis after elevated-temperature culture
      target: Temperature-Sensitive Transcriptional Insufficiency in GTF2E2-Related TTD
      direction: DECREASED
      interpretation: >-
        Reduced RNA synthesis at elevated, but not standard, temperature is the
        defining conditional readout.
      evidence:
      - reference: PMID:28973399
        reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: >-
          We showed that fibroblasts derived from these patients exhibit a clear
          transcription defect, however only when cultured at elevated temperatures.
        explanation: >-
          Directly reports the temperature-bounded direction of the readout.
    evidence:
    - reference: PMID:28973399
      reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        We showed that fibroblasts derived from these patients exhibit a clear
        transcription defect, however only when cultured at elevated temperatures.
      explanation: >-
        Directly establishes the conditional patient-cell model.
  evidence:
  - reference: PMID:28973399
    reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Transcription levels after incubation for 72 h at 37 °C or 40 °C, measured
      by pulse-labeling with ethynyl-uridine (EU) and subsequent fluorescent
      staining of incorporated EU, of Sv40-immortalized patient cells
      (TTD218UT_sv), compared with wild-type controls (MRC5_sv, C5RO-sv).
    explanation: >-
      Identifies the transformed patient and control lines used for the defining
      elevated-temperature transcription readout.
  - reference: PMID:28973399
    reference_title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Surprisingly however, basal transcription levels, assayed by ethynyl
      uridine (EU) pulse labeling, were not affected in patient cells under
      standard culture conditions
    explanation: >-
      Provides the essential negative comparator that prevents a constitutive
      transcription-defect interpretation.
- name: ERCC2/ERCC3 patient-dermal-fibroblast R-loop model
  description: >-
    ERCC2 primary dermal fibroblasts were analyzed by chromatin fractionation
    and interaction assays, while ERCC2 and ERCC3 fibroblasts were analyzed by
    DRIP for R-loop accumulation.
  experimental_model_type: PRIMARY_CELL_CULTURE
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  cell_types:
  - preferred_term: skin fibroblast
    term:
      id: CL:0002620
      label: skin fibroblast
  cell_source: Primary dermal fibroblasts from ERCC2- or ERCC3-related photosensitive TTD patients and healthy controls
  culture_system: >-
    Primary fibroblast monolayers with ERCC2-arm chromatin-interaction assays
    and ERCC2/ERCC3 DRIP analysis
  publication: PMID:40757642
  modeled_mechanisms:
  - target: Unprocessed R-Loop Accumulation in ERCC2/ERCC3-Related TTD
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      Genotype-matched primary patient cells directly reproduce accumulation of
      unprocessed R-loops at transcribed loci.
    limitations: >-
      Dermal fibroblasts do not establish the magnitude, timing, or downstream
      consequences of R-loop stress in developing neural, hair, or immune tissues;
      chromatin-interaction work was ERCC2-scoped, and GTF2H5-related cells were
      not tested.
    readouts:
    - name: R-loop abundance at the ACTB locus by DRIP
      target: Unprocessed R-Loop Accumulation in ERCC2/ERCC3-Related TTD
      direction: INCREASED
      interpretation: >-
        Increased DNA-RNA hybrid signal reports failure to process R-loops in
        transcribed genomic regions.
      evidence:
      - reference: PMID:40757642
        reference_title: Trichothiodystrophy-causative pathogenic variants impair a cooperative action of TFIIH and DDX1 in R-loop processing.
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: >-
          demonstrating that both XPD and XPB alterations causative of PS-TTD lead
          to an excess of unprocessed R-loops in the transcribed regions of the genome
        explanation: >-
          Directly reports increased unprocessed R-loops in both studied TFIIH branches.
    evidence:
    - reference: PMID:40757642
      reference_title: Trichothiodystrophy-causative pathogenic variants impair a cooperative action of TFIIH and DDX1 in R-loop processing.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        TTD-specific variants in ERCC2/XPD result in TFIIH instability, altered
        interaction of the CAK with DDX1-SFPQ-NONO, and R-loop accumulation.
      explanation: >-
        Establishes that the patient-cell model is informative for the linked node.
  evidence:
  - reference: PMID:40757642
    reference_title: Trichothiodystrophy-causative pathogenic variants impair a cooperative action of TFIIH and DDX1 in R-loop processing.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      primary dermal fibroblasts from healthy donors (C3PV and C8PV), PS-TTD
      (TTD8PV and TTD23PV), or XP (XP26VI and XP15PV) patients with pathogenic
      variants in ERCC2/XPD gene.
    explanation: >-
      Identifies the primary dermal-fibroblast component of the study.
- name: MPLKIP-deficient reconstructed human skin equivalent
  description: >-
    A reconstructed human skin equivalent combines collagen dermal equivalents
    containing subject-derived primary fibroblasts with N/TERT-immortalized
    keratinocytes carrying CRISPR MPLKIP knockout or patient-variant knock-in,
    cultured at an air-liquid interface to assess epidermal differentiation.
  experimental_model_type: CO_CULTURE
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  cell_types:
  - preferred_term: keratinocyte
    term:
      id: CL:0000312
      label: keratinocyte
  - preferred_term: skin fibroblast
    term:
      id: CL:0002620
      label: skin fibroblast
  cell_source: >-
    Subject-derived primary dermal fibroblasts combined with
    N/TERT-immortalized CRISPR MPLKIP knockout or knock-in keratinocytes
  culture_system: Collagen-based reconstructed human skin equivalent at an air-liquid interface
  publication: PMID:37800682
  modeled_mechanisms:
  - target: Impaired Keratinocyte Differentiation
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      The organotypic co-culture reproduces compromised splicing, altered
      proteome balance, and impaired keratinocyte differentiation after MPLKIP
      loss.
    limitations: >-
      The model is MPLKIP-specific; its keratinocytes are immortalized and
      engineered while only the dermal fibroblasts are patient-derived, and it
      lacks hair follicles and shaft formation.
    readouts:
    - name: Epidermal keratinocyte differentiation in reconstructed skin
      target: Impaired Keratinocyte Differentiation
      direction: DECREASED
      interpretation: >-
        Impaired stratification and differentiation in the skin equivalent
        report the linked tissue state.
      evidence:
      - reference: PMID:37800682
        reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: >-
          Using Human Skin Equivalents (HSEs), we observed impaired keratinocyte
          differentiation associated with compromised splicing and eventually,
          an imbalanced proteome affecting skin development and, interestingly,
          also the immune system.
        explanation: >-
          Directly reports the model readout and its direction.
    evidence:
    - reference: PMID:37800682
      reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Using Human Skin Equivalents (HSEs), we observed impaired keratinocyte
        differentiation associated with compromised splicing and eventually,
        an imbalanced proteome affecting skin development and, interestingly,
        also the immune system.
      explanation: >-
        Establishes the human skin-equivalent system as informative for this node.
  evidence:
  - reference: PMID:37800682
    reference_title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Using Human Skin Equivalents (HSEs), we observed impaired keratinocyte
      differentiation associated with compromised splicing and eventually, an
      imbalanced proteome affecting skin development and, interestingly, also
      the immune system.
    explanation: >-
      Directly identifies the reconstructed human skin model and measured state.
animal_models:
- name: Xpd-mutant TTD mouse
  species: Mus musculus
  genotype: Homozygous Xpd/Ercc2 p.Arg722Trp (R722W) knock-in
  genes:
  - preferred_term: ERCC2
    term:
      id: hgnc:3434
      label: ERCC2
  description: >-
    The Xpd TTD mouse links limiting TFIIH in brain to spatially selective
    thyroid-hormone target-gene deregulation. It supports a coactivator mechanism
    but does not establish the corresponding process in human developing brain.
  publication: PMID:17952069
  modeled_mechanisms:
  - target: Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      The model reproduces brain TFIIH-dependent thyroid-receptor destabilization
      and target-gene dysregulation.
    limitations: >-
      The evidence is from mouse brain; human developmental cell types, timing,
      and causal contribution to hypomyelination have not been tested.
    readouts:
    - name: Thyroid-hormone receptor occupancy and target-gene expression in brain
      target: Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain
      direction: ALTERED
      interpretation: >-
        Spatially selective target-gene deregulation reports impaired TFIIH
        coactivator function in the model brain.
      evidence:
      - reference: PMID:17952069
        reference_title: Neurological defects in trichothiodystrophy reveal a coactivator function of TFIIH.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Here we show that an XPD mutation in TTD mice results in a spatial and
          selective deregulation of thyroid hormone target genes in the brain.
        explanation: >-
          Directly reports the model readout.
    evidence:
    - reference: PMID:17952069
      reference_title: Neurological defects in trichothiodystrophy reveal a coactivator function of TFIIH.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Molecular analyses performed on the mice brain tissue demonstrate that
        TFIIH is required for the stabilization of thyroid hormone receptors (TR)
        to their DNA-responsive elements.
      explanation: >-
        Establishes the model-level coactivator mechanism.
  evidence:
  - reference: PMID:17952069
    reference_title: Neurological defects in trichothiodystrophy reveal a coactivator function of TFIIH.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Here we show that an XPD mutation in TTD mice results in a spatial and
      selective deregulation of thyroid hormone target genes in the brain.
    explanation: >-
      Directly identifies the TTD mouse and its principal brain phenotype.
  - reference: PMID:25340339
    reference_title: "Dynamic partnership between TFIIH, PGC-1α and SIRT1 is impaired in trichothiodystrophy."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      the TTD mouse model (having the most common XPD/
      R722W point mutation found in TTD patients)
    explanation: >-
      A follow-on study of the same established line supplies the exact R722W
      allele identity.
- name: Ttdn1-null mouse
  species: Mus musculus
  genotype: Homozygous Ttdn1 deletion with no detectable protein
  genes:
  - preferred_term: MPLKIP
    term:
      id: hgnc:16002
      label: MPLKIP
  description: >-
    CRISPR-generated Ttdn1-null mice reproduce increased intron lariats,
    alternative-splicing changes, reduced hair cysteic acid, impaired weight
    gain, and selected motor or learning abnormalities, while missing several
    severe human features.
  publication: PMID:37369199
  modeled_mechanisms:
  - target: Intron-Lariat Accumulation after MPLKIP Loss
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Cortex RNA-seq reproduces intron-lariat accumulation after loss of Ttdn1.
    limitations: >-
      Mouse cortical lariat accumulation was smaller than in human knockout
      cells, and the model does not reproduce the full human developmental
      phenotype.
    readouts:
    - name: Cortex intron-lariat abundance
      target: Intron-Lariat Accumulation after MPLKIP Loss
      direction: INCREASED
      interpretation: >-
        Increased lariat-derived reads directly measure defective turnover in vivo.
      evidence:
      - reference: PMID:37369199
        reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          RNA lariat abundance was increased ~4.1-fold compared to WT samples,
          as were alternative splicing events (Figure 4d and Supplemental Figure
          S4g).
        explanation: >-
          Directly reports increased lariat abundance in the model cortex.
    evidence:
    - reference: PMID:37369199
      reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        RNA lariat abundance was increased ~4.1-fold compared to WT samples, as
        were alternative splicing events (Figure 4d and Supplemental Figure S4g).
      explanation: >-
        Establishes in-vivo recapitulation of the linked lariat state.
  - target: Aberrant RNA Splicing after MPLKIP Loss
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Cortex RNA-seq reproduces alternative-splicing changes after loss of Ttdn1.
    limitations: >-
      The model does not reproduce the full human developmental phenotype, and
      mouse cortex cannot establish the tissue distribution of splicing defects
      in human TTD.
    readouts:
    - name: Cortex alternative-splicing events
      target: Aberrant RNA Splicing after MPLKIP Loss
      direction: ALTERED
      interpretation: >-
        Altered splice-junction usage reports the linked RNA-splicing state in vivo.
      evidence:
      - reference: PMID:37369199
        reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          RNA lariat abundance was increased ~4.1-fold compared to WT samples,
          as were alternative splicing events (Figure 4d and Supplemental Figure
          S4g).
        explanation: >-
          Directly reports altered splicing in the model cortex.
    evidence:
    - reference: PMID:37369199
      reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        RNA lariat abundance was increased ~4.1-fold compared to WT samples, as
        were alternative splicing events (Figure 4d and Supplemental Figure S4g).
      explanation: >-
        Establishes in-vivo recapitulation of the linked splicing state.
  - target: Deficiency of Sulfur-Rich Hair Matrix Proteins
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Ttdn1-null hair has decreased cysteic-acid content, partially reproducing
      the sulfur-poor component of the human hair phenotype.
    limitations: >-
      Keratin-associated-protein abundance and hair-shaft ultrastructure were not
      measured; reduced cysteic acid is only an indirect compositional proxy.
      Sparse hair was not overt in young mice, and tiger-tail banding was not
      established.
    readouts:
    - name: Hair cysteic-acid content
      target: Deficiency of Sulfur-Rich Hair Matrix Proteins
      direction: DECREASED
      interpretation: >-
        Reduced cysteic acid measures the sulfur-deficient component of the hair
        phenotype.
      evidence:
      - reference: PMID:37369199
        reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          we observed a significant decrease in cysteic acid content as a
          percentage of hair protein in these mice
        explanation: >-
          Directly reports the direction of the hair biochemical readout.
    evidence:
    - reference: PMID:37369199
      reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        we observed a significant decrease in cysteic acid content as a
        percentage of hair protein in these mice
      explanation: >-
        Supports partial recapitulation of the sulfur-deficient hair lesion.
  evidence:
  - reference: PMID:37369199
    reference_title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      A Ttdn1-deficient mouse model recapitulates intron-processing defects and
      certain neurodevelopmental phenotypes seen in NP-TTD.
    explanation: >-
      Directly supports using the knockout as a disease model.
datasets:
- accession: arrayexpress:E-MTAB-10567
  title: Gene expression profiles in Trichothiodystrophy patient and control
  description: >-
    Bulk coding-RNA/poly(A) paired-end RNA sequencing of primary skin fibroblasts
    from one female TTD proband and her unaffected mother, with untreated and UV
    conditions and three replicate libraries per group. The repository does not
    identify the causal gene, genotype, photosensitivity subtype, UV dose, or
    proband developmental stage.
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  data_type: BULK_RNA_SEQ
  sample_types:
  - preferred_term: skin fibroblast
    term:
      id: CL:0002620
      label: skin fibroblast
    cell_type_term:
      preferred_term: skin fibroblast
      term:
        id: CL:0002620
        label: skin fibroblast
  sample_count: 12
  conditions:
  - TTD proband fibroblasts, untreated
  - TTD proband fibroblasts, UV-exposed
  - unaffected maternal fibroblasts, untreated
  - unaffected maternal fibroblasts, UV-exposed
  platform: Illumina HiSeq 2500; paired-end coding-RNA/poly(A) sequencing
  notes: >-
    Verified against the official BioStudies/ArrayExpress API, IDF, and SDRF on
    2026-08-17. Released 2021-06-29; 12 ENA runs ERR6018701-ERR6018712 under
    ERP129680. No publication, PMID, DOI, genotype, or UV dose/time is attached
    to the repository record.
- accession: geo:GSE262217
  title: Impaired B-cell function in ERCC2 deficiency
  description: >-
    Bulk mRNA sequencing of peripheral blood mononuclear cells from three
    unrelated patients with ERCC2-related TTD1 and five healthy controls. This
    subtype-specific dataset underlies the study's comparison of immune and
    immunoglobulin-heavy-chain expression programs.
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  data_type: BULK_RNA_SEQ
  sample_types:
  - preferred_term: peripheral blood mononuclear cell
    term:
      id: CL:2000001
      label: peripheral blood mononuclear cell
    cell_type_term:
      preferred_term: peripheral blood mononuclear cell
      term:
        id: CL:2000001
        label: peripheral blood mononuclear cell
  sample_count: 8
  conditions:
  - ERCC2-related TTD1 patient peripheral blood mononuclear cells (3)
  - healthy-control peripheral blood mononuclear cells (5)
  platform: Illumina NovaSeq 6000
  publication: PMID:39055713
  notes: >-
    Verified against the official NCBI GEO record on 2026-08-17. GSE262217 is
    linked to BioProject PRJNA1090832 and was released publicly on 2024-08-02;
    the repository describes expression profiling by high-throughput sequencing.
discussions:
- discussion_id: pan_ttd_tissue_convergence_gap
  prompt: >-
    Do TFIIH/TFIIE transcription defects, MPLKIP-DBR1 or RNF113A RNA-processing
    defects, and aminoacyl-tRNA-synthetase translation defects converge on a
    shared tissue-specific ribosome/proteostasis mechanism, or do they reach the
    common hair and developmental phenotype through distinct pathways?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#TFIIH Complex Instability in Photosensitive TTD
  - pathophysiology#GTF2E2-Related TFIIE Instability
  - pathophysiology#MPLKIP Deficiency
  - pathophysiology#DBR1 Deficiency in Sabinas TTD
  - pathophysiology#Cytosolic Aminoacyl-tRNA Synthetase Loss of Function
  - pathophysiology#Reduced Cognate tRNA Charging
  - pathophysiology#RNF113A Deficiency
  - pathophysiology#Unresolved Tissue-Specific Gene-Expression Convergence
  rationale: >-
    The causal genes occupy distinct transcription, RNA-processing, and
    translation systems. Perturbation studies postulate ribosomal dysfunction as
    a common mechanism, but no genotype-spanning human model has shown a single
    downstream state that is necessary for sulfur-poor hair or the multisystem
    phenotype.
  proposed_experiments:
  - experiment_id: exp_ttd_cross_genotype_hair_lineage_multiomics
    name: Isogenic cross-genotype human hair-lineage multi-omics comparison
    description: >-
      Generate matched human iPSC hair-follicle or keratinocyte-lineage models
      carrying representative ERCC2, GTF2E2, MPLKIP, RNF113A, AARS1, and DBR1
      variants, then compare nascent transcription, splicing, ribosome profiling,
      proteostasis, sulfur-rich matrix-protein abundance, and shaft-like material
      assembly with gene-corrected controls.
    experiment_type:
      preferred_term: cross-genotype iPSC lineage multi-omics experiment
    model_systems:
    - name: Isogenic human iPSC-derived hair-lineage models
      description: >-
        Gene-edited and corrected human lines differentiated toward the relevant
        hair-matrix or keratinocyte lineage.
      experimental_model_type: IPSC_DERIVED_MODEL
    readouts:
    - name: Shared versus branch-specific gene-expression defects
      target: pathophysiology#Unresolved Tissue-Specific Gene-Expression Convergence
      direction: ALTERED
    - name: Sulfur-rich matrix-protein abundance and organization
      target: pathophysiology#Deficiency of Sulfur-Rich Hair Matrix Proteins
      direction: DECREASED
    decision_criterion: >-
      A reproducible corrected-by-isogenic-rescue molecular state shared across
      causal branches and preceding matrix-protein loss would support a common
      convergence mechanism; branch-specific states would refute a single route.
    would_support:
    - pathophysiology#Unresolved Tissue-Specific Gene-Expression Convergence
    would_refute:
    - pathophysiology#Unresolved Tissue-Specific Gene-Expression Convergence
  evidence:
  - reference: PMID:37508541
    reference_title: Ribosomal Dysfunction Is a Common Pathomechanism in Different Forms of Trichothiodystrophy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      As the loss of proteostasis by the ribosome has been identified in the
      other forms of TTD, here we postulate that ribosomal dysfunction is a
      common underlying pathomechanism of TTD.
    explanation: >-
      The explicit postulate motivates, but does not resolve, the cross-genotype
      convergence question.
- discussion_id: xpd_mouse_human_brain_fidelity
  prompt: >-
    Does thyroid-receptor target-gene deregulation in Xpd-mutant mouse brain
    faithfully model the causal route to human ERCC2-related hypomyelination, or
    is it a species-, developmental-stage-, or tissue-composition-specific
    response to limiting TFIIH?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain
  - phenotypes#Cerebral Hypomyelination
  - animal_models#Xpd-mutant TTD mouse
  rationale: >-
    The mouse study directly measures TFIIH-dependent thyroid-receptor occupancy
    and brain target-gene deregulation, while the human evidence establishes
    hypomyelination but does not measure this route. Species differences, mixed
    brain tissue, and developmental timing therefore limit translation of the
    model mechanism to human oligodendrocyte-lineage biology.
  proposed_experiments:
  - experiment_id: exp_ttd_human_oligodendrocyte_tr_fidelity
    name: ERCC2-TTD human oligodendrocyte-lineage TFIIH and thyroid-response study
    description: >-
      Compare patient-derived and isogenic-corrected human iPSC oligodendrocyte
      lineage cells across developmental stages, measuring TFIIH abundance,
      thyroid-receptor chromatin occupancy, thyroid-responsive transcription,
      maturation, and myelin-protein production with and without physiologic
      thyroid-hormone stimulation.
    experiment_type:
      preferred_term: patient-derived iPSC differentiation and rescue study
    model_systems:
    - name: ERCC2-TTD human iPSC oligodendrocyte-lineage model
      description: >-
        Patient-derived and gene-corrected human cells spanning progenitor to
        mature oligodendrocyte stages.
      experimental_model_type: IPSC_DERIVED_MODEL
      organism:
        preferred_term: human
        term:
          id: NCBITaxon:9606
          label: Homo sapiens
      cell_types:
      - preferred_term: oligodendrocyte
        term:
          id: CL:0000128
          label: oligodendrocyte
    readouts:
    - name: Thyroid-receptor occupancy and target-gene response
      target: pathophysiology#Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain
      direction: ALTERED
    - name: Oligodendrocyte maturation and myelin-protein production
      target: phenotypes#Cerebral Hypomyelination
      direction: DECREASED
    decision_criterion: >-
      Reproduction of the mouse-derived occupancy/transcription defect in
      ERCC2-TTD human cells, with correction by isogenic repair and concordant
      maturation or myelin-protein rescue, would support translational fidelity.
    would_support:
    - pathophysiology#Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain
    would_refute:
    - pathophysiology#Candidate TFIIH-Thyroid-Receptor Dysregulation in Brain
  evidence:
  - reference: PMID:17952069
    reference_title: Neurological defects in trichothiodystrophy reveal a coactivator function of TFIIH.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Molecular analyses performed on the mice brain tissue demonstrate that
      TFIIH is required for the stabilization of thyroid hormone receptors (TR)
      to their DNA-responsive elements.
    explanation: >-
      Establishes the existing mouse evidence whose human fidelity is at issue.
- discussion_id: rnf113a_dual_function_gap
  prompt: >-
    Which RNF113A-dependent function is causally relevant to X-linked TTD:
    spliceosome regulation, the ASCC alkylation-damage response, both, or an
    unmeasured developmental function?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#RNF113A Deficiency
  - pathophysiology#Defective RNF113A-Dependent ASCC Alkylation Response
  - pathophysiology#Candidate RNF113A-Dependent Splicing Dysfunction
  rationale: >-
    The ASCC defect is measured in patient cells, whereas the splicing function
    is supported in lung-cancer cells. Neither experiment establishes which
    function is necessary or sufficient for the human hair and developmental
    phenotype.
  proposed_experiments:
  - experiment_id: exp_rnf113a_function_separation_rescue
    name: RNF113A separation-of-function rescue in patient-derived lineages
    description: >-
      Introduce RNF113A constructs that selectively restore spliceosome or ASCC
      activity into patient-derived keratinocyte and neural-lineage cells, then
      compare splicing, alkylation-response, differentiation, and sulfur-rich
      matrix-protein readouts with wild-type and empty-vector controls.
    experiment_type:
      preferred_term: separation-of-function cellular rescue experiment
    model_systems:
    - name: RNF113A-TTD patient-derived keratinocyte and neural-lineage cells
      description: >-
        Patient-derived human lineages with orthogonal rescue of RNF113A functions.
      experimental_model_type: IPSC_DERIVED_MODEL
    readouts:
    - name: ASCC recruitment after alkylation challenge
      target: pathophysiology#Defective RNF113A-Dependent ASCC Alkylation Response
      direction: RESTORED
    - name: RNF113A-dependent splicing profile
      target: pathophysiology#Candidate RNF113A-Dependent Splicing Dysfunction
      direction: RESTORED
    decision_criterion: >-
      Selective rescue of developmental and hair-lineage readouts by one
      function-specific construct would prioritize that branch; rescue only by
      full-length RNF113A would support combined or additional functions.
    would_support:
    - pathophysiology#Defective RNF113A-Dependent ASCC Alkylation Response
    - pathophysiology#Candidate RNF113A-Dependent Splicing Dysfunction
  evidence:
  - reference: PMID:29144457
    reference_title: A ubiquitin-dependent signalling axis specific for ALKBH-mediated DNA dealkylation repair.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Cells from patients with X-linked trichothiodystrophy, which harbour a
      mutation in RNF113A, are defective in ASCC foci formation and are
      hypersensitive to alkylating agents.
    explanation: >-
      Establishes one measured patient-cell function without proving clinical
      causality.
  - reference: PMID:32152280
    reference_title: The X-linked trichothiodystrophy-causing gene RNF113A links the spliceosome to cell survival upon DNA damage.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      RNF113A is a RNA-binding protein which regulates the splicing of multiple
      candidates involved in cell survival.
    explanation: >-
      Establishes the alternative function in a low-fidelity cancer-cell context.
references:
- reference: PMID:11734544
  title: Mutations in the general transcription factor TFIIH result in beta-thalassaemia in individuals with trichothiodystrophy.
- reference: PMID:12393803
  title: Reduced level of the repair/transcription factor TFIIH in trichothiodystrophy.
- reference: PMID:12820975
  title: Basal transcription defect discriminates between xeroderma pigmentosum and trichothiodystrophy in XPD patients.
- reference: PMID:15220921
  title: A new, tenth subunit of TFIIH is responsible for the DNA repair syndrome trichothiodystrophy group A.
- reference: PMID:15232704
  title: "Trichothiodystrophy: quantification of cysteine in human hair and nails by application of sodium azide-dependent oxidation to cysteic acid."
- reference: PMID:15645389
  title: Identification of C7orf11 (TTDN1) gene mutations and genetic heterogeneity in nonphotosensitive trichothiodystrophy.
- reference: PMID:17952069
  title: Neurological defects in trichothiodystrophy reveal a coactivator function of TFIIH.
- reference: PMID:18470933
  title: "Persistence of repair proteins at unrepaired DNA damage distinguishes diseases with ERCC2 (XPD) mutations: cancer-prone xeroderma pigmentosum vs. non-cancer-prone trichothiodystrophy."
- reference: PMID:18603627
  title: "Trichothiodystrophy: a systematic review of 112 published cases characterises a wide spectrum of clinical manifestations."
- reference: PMID:20687499
  title: "Trichothiodystrophy: Photosensitive, TTD-P, TTD, Tay syndrome."
- reference: PMID:21800331
  title: "High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies."
- reference: PMID:25290684
  title: Mutations in the TTDN1 gene are associated with a distinct trichothiodystrophy phenotype.
- reference: PMID:25340339
  title: "Dynamic partnership between TFIIH, PGC-1α and SIRT1 is impaired in trichothiodystrophy."
- reference: PMID:25612912
  title: A novel X-linked trichothiodystrophy associated with a nonsense mutation in RNF113A.
- reference: PMID:26996949
  title: GTF2E2 Mutations Destabilize the General Transcription Factor Complex TFIIE in Individuals with DNA Repair-Proficient Trichothiodystrophy.
- reference: PMID:27745642
  title: "Understanding photodermatoses associated with defective DNA repair: Photosensitive syndromes without associated cancer predisposition."
- reference: PMID:28973399
  title: Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
- reference: PMID:29144457
  title: A ubiquitin-dependent signalling axis specific for ALKBH-mediated DNA dealkylation repair.
- reference: PMID:30334570
  title: "Mortality-associated immunological abnormalities in trichothiodystrophy: correlation of reduced levels of immunoglobulin and neutrophils with poor patient survival."
- reference: PMID:30824121
  title: Cysteinyl-tRNA Synthetase Mutations Cause a Multi-System, Recessive Disease That Includes Microcephaly, Developmental Delay, and Brittle Hair and Nails.
- reference: PMID:31332722
  title: "Genetic Hair Disorders: A Review."
- reference: PMID:31374204
  title: Bi-allelic TARS Mutations Are Associated with Brittle Hair Phenotype.
- reference: PMID:32152280
  title: The X-linked trichothiodystrophy-causing gene RNF113A links the spliceosome to cell survival upon DNA damage.
- reference: PMID:33909043
  title: Protein instability associated with AARS1 and MARS1 mutations causes trichothiodystrophy.
- reference: PMID:34155103
  title: "Reduced levels of prostaglandin I(2) synthase: a distinctive feature of the cancer-free trichothiodystrophy."
- reference: PMID:35615778
  title: Trichothiodystrophy hair shafts display distinct ultrastructural features.
- reference: PMID:37369199
  title: A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
- reference: PMID:37508541
  title: Ribosomal Dysfunction Is a Common Pathomechanism in Different Forms of Trichothiodystrophy.
- reference: PMID:37800682
  title: Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
- reference: PMID:39055713
  title: Impaired B-cell function in ERCC2 deficiency.
- reference: PMID:39976384
  title: "Trichothiodystrophy due to ERCC2 Variants: Uncommon Contributor to Progressive Hypomyelinating Leukodystrophy."
- reference: PMID:40683339
  title: "RNA Lariat-Debranching Enzyme (DBR1) Variations in Sabinas Brittle Hair Syndrome Form of Trichothiodystrophy: A Trichothiodystrophy-Causing Gene."
- reference: PMID:40737808
  title: "Trichothiodystrophy: Molecular insights and mechanisms of pathogenicity."
- reference: PMID:40757642
  title: Trichothiodystrophy-causative pathogenic variants impair a cooperative action of TFIIH and DDX1 in R-loop processing.
- reference: PMID:40820264
  title: Second Case of Type 9 Non-Photosensitive Trichothiodystrophy Caused by Homozygous Variant in the MARS1 Gene.
- reference: PMID:41531333
  title: Novel RNF113A Variant Underlying X-Linked Trichothiodystrophy With Presumed Mosaicism in an Unaffected Mother.
- reference: PMID:41684880
  title: Twins With Pathogenic RNF113A Variant Presenting With Testicular Regression Syndrome.
- reference: PMID:9012405
  title: A mutation in the XPB/ERCC3 DNA repair transcription gene, associated with trichothiodystrophy.
- reference: PMID:9238033
  title: Xeroderma pigmentosum and trichothiodystrophy are associated with different mutations in the XPD (ERCC2) repair/transcription gene.
- reference: url:https://api.orphadata.com/rd-epidemiology/orphacodes/33364
  title: https://api.orphadata.com/rd-epidemiology/orphacodes/33364
review_notes: >-
  This review retains one MONDO:0018053 umbrella entry because contemporary TTD
  nosology spans eleven causal genes and no evidence supports splitting the
  clinical spectrum into independent files solely by severe versus attenuated
  presentation. The two clinical UV-sensitivity facets are kept alongside
  machine-readable TTD1-TTD9 gene-defined children. CARS1-related microcephaly,
  developmental delay, and brittle hair syndrome (MONDO:0030047) and
  DBR1-related Sabinas brittle hair syndrome (MONDO:0008886) are included because
  current primary literature places them in non-photosensitive TTD, while their
  current MONDO classes remain outside the TTD hierarchy; DBR1 has no numbered
  subtype. RNF113A is modeled separately as X-linked recessive, and
  nucleotide-excision-repair failure is restricted to ERCC2-, ERCC3-, and
  GTF2H5-related photosensitive disease. Distinct TFIIE, RNA-processing,
  RNF113A, and aminoacyl-tRNA-synthetase branches are not collapsed into a
  universal TFIIH or proteostasis mechanism. Ribosomal/proteostasis convergence,
  the RNF113A function responsible for disease, and translation of the Xpd mouse
  thyroid-receptor mechanism to human brain remain explicit open questions.
  Compound path nodes were split so that basal transcription, R-loop
  accumulation, R-loop-driven stress, lariat accumulation, splicing, synthetase
  loss of function, tRNA charging, hair-matrix composition, and shaft structure
  retain their distinct evidence and causal calibration. ERCC2-specific
  beta-globin and B-cell activation/differentiation branches are likewise kept
  distinct from GTF2E2-related erythroid dysfunction and from pan-TTD claims.
  The prior NIH_HT_89
  classification and age-related genome-maintenance-module conformance were
  removed because congenital TTD repair deficiency does not establish an aging
  research-priority classification.
  Cancer is described as very rare or lacking an established increased
  predisposition, not absolutely absent. Management is limited to supported,
  subtype- or abnormality-directed supportive actions; preclinical ASO,
  small-molecule, and model-only interventions and isolated off-label case
  treatments were not promoted to standard care. ArrayExpress/BioStudies
  E-MTAB-10567 was verified against official API/IDF/SDRF metadata and included
  without inventing a genotype, TTD subtype, UV dose, age, or publication.
  GEO GSE262217 was independently verified as an eight-sample ERCC2-related TTD1
  PBMC bulk-RNA-seq dataset and linked only to its documented publication. The
  PMID:18603627 full-text phenotype tables were audited for omitted common
  findings, but the sanctioned reference fetch reproducibly remained
  abstract-only; sparse hair, alopecia, nail dystrophy, gait disturbance, and
  abnormal facial shape were therefore added from separately cache-backed
  primary observations without importing unsupported umbrella frequencies. No
  dedicated TTD GeneReviews chapter was identified; the AR congenital ichthyosis
  and xeroderma-pigmentosum GeneReviews discuss TTD only as a differential and
  were not tagged as disease-specific baselines. A current PubMed sweep through
  2026-08-17 considered recent diagnostic, rare-phenotype, and mimic reports;
  low-materiality image reports, isolated rare findings, and DYRK1A cases merely
  initially suspected of TTD were not used to expand the core record.
📚

References & Deep Research

References

40
PMID:11734544
Mutations in the general transcription factor TFIIH result in beta-thalassaemia in individuals with trichothiodystrophy.
No top-level findings curated for this source.
PMID:12393803
Reduced level of the repair/transcription factor TFIIH in trichothiodystrophy.
No top-level findings curated for this source.
PMID:12820975
Basal transcription defect discriminates between xeroderma pigmentosum and trichothiodystrophy in XPD patients.
No top-level findings curated for this source.
PMID:15220921
A new, tenth subunit of TFIIH is responsible for the DNA repair syndrome trichothiodystrophy group A.
No top-level findings curated for this source.
PMID:15232704
Trichothiodystrophy: quantification of cysteine in human hair and nails by application of sodium azide-dependent oxidation to cysteic acid.
No top-level findings curated for this source.
PMID:15645389
Identification of C7orf11 (TTDN1) gene mutations and genetic heterogeneity in nonphotosensitive trichothiodystrophy.
No top-level findings curated for this source.
PMID:17952069
Neurological defects in trichothiodystrophy reveal a coactivator function of TFIIH.
No top-level findings curated for this source.
PMID:18470933
Persistence of repair proteins at unrepaired DNA damage distinguishes diseases with ERCC2 (XPD) mutations: cancer-prone xeroderma pigmentosum vs. non-cancer-prone trichothiodystrophy.
No top-level findings curated for this source.
PMID:18603627
Trichothiodystrophy: a systematic review of 112 published cases characterises a wide spectrum of clinical manifestations.
No top-level findings curated for this source.
PMID:20687499
Trichothiodystrophy: Photosensitive, TTD-P, TTD, Tay syndrome.
No top-level findings curated for this source.
PMID:21800331
High-risk pregnancy and neonatal complications in the DNA repair and transcription disorder trichothiodystrophy: report of 27 affected pregnancies.
No top-level findings curated for this source.
PMID:25290684
Mutations in the TTDN1 gene are associated with a distinct trichothiodystrophy phenotype.
No top-level findings curated for this source.
PMID:25340339
Dynamic partnership between TFIIH, PGC-1α and SIRT1 is impaired in trichothiodystrophy.
No top-level findings curated for this source.
PMID:25612912
A novel X-linked trichothiodystrophy associated with a nonsense mutation in RNF113A.
No top-level findings curated for this source.
PMID:26996949
GTF2E2 Mutations Destabilize the General Transcription Factor Complex TFIIE in Individuals with DNA Repair-Proficient Trichothiodystrophy.
No top-level findings curated for this source.
PMID:27745642
Understanding photodermatoses associated with defective DNA repair: Photosensitive syndromes without associated cancer predisposition.
No top-level findings curated for this source.
PMID:28973399
Trichothiodystrophy causative TFIIEβ mutation affects transcription in highly differentiated tissue.
No top-level findings curated for this source.
PMID:29144457
A ubiquitin-dependent signalling axis specific for ALKBH-mediated DNA dealkylation repair.
No top-level findings curated for this source.
PMID:30334570
Mortality-associated immunological abnormalities in trichothiodystrophy: correlation of reduced levels of immunoglobulin and neutrophils with poor patient survival.
No top-level findings curated for this source.
PMID:30824121
Cysteinyl-tRNA Synthetase Mutations Cause a Multi-System, Recessive Disease That Includes Microcephaly, Developmental Delay, and Brittle Hair and Nails.
No top-level findings curated for this source.
PMID:31332722
Genetic Hair Disorders: A Review.
No top-level findings curated for this source.
PMID:31374204
Bi-allelic TARS Mutations Are Associated with Brittle Hair Phenotype.
No top-level findings curated for this source.
PMID:32152280
The X-linked trichothiodystrophy-causing gene RNF113A links the spliceosome to cell survival upon DNA damage.
No top-level findings curated for this source.
PMID:33909043
Protein instability associated with AARS1 and MARS1 mutations causes trichothiodystrophy.
No top-level findings curated for this source.
PMID:34155103
Reduced levels of prostaglandin I(2) synthase: a distinctive feature of the cancer-free trichothiodystrophy.
No top-level findings curated for this source.
PMID:35615778
Trichothiodystrophy hair shafts display distinct ultrastructural features.
No top-level findings curated for this source.
PMID:37369199
A functional link between lariat debranching enzyme and the intron-binding complex is defective in non-photosensitive trichothiodystrophy.
No top-level findings curated for this source.
PMID:37508541
Ribosomal Dysfunction Is a Common Pathomechanism in Different Forms of Trichothiodystrophy.
No top-level findings curated for this source.
PMID:37800682
Trichothiodystrophy-associated MPLKIP maintains DBR1 levels for proper lariat debranching and ectodermal differentiation.
No top-level findings curated for this source.
PMID:39055713
Impaired B-cell function in ERCC2 deficiency.
No top-level findings curated for this source.
PMID:39976384
Trichothiodystrophy due to ERCC2 Variants: Uncommon Contributor to Progressive Hypomyelinating Leukodystrophy.
No top-level findings curated for this source.
PMID:40683339
RNA Lariat-Debranching Enzyme (DBR1) Variations in Sabinas Brittle Hair Syndrome Form of Trichothiodystrophy: A Trichothiodystrophy-Causing Gene.
No top-level findings curated for this source.
PMID:40737808
Trichothiodystrophy: Molecular insights and mechanisms of pathogenicity.
No top-level findings curated for this source.
PMID:40757642
Trichothiodystrophy-causative pathogenic variants impair a cooperative action of TFIIH and DDX1 in R-loop processing.
No top-level findings curated for this source.
PMID:40820264
Second Case of Type 9 Non-Photosensitive Trichothiodystrophy Caused by Homozygous Variant in the MARS1 Gene.
No top-level findings curated for this source.
PMID:41531333
Novel RNF113A Variant Underlying X-Linked Trichothiodystrophy With Presumed Mosaicism in an Unaffected Mother.
No top-level findings curated for this source.
PMID:41684880
Twins With Pathogenic RNF113A Variant Presenting With Testicular Regression Syndrome.
No top-level findings curated for this source.
PMID:9012405
A mutation in the XPB/ERCC3 DNA repair transcription gene, associated with trichothiodystrophy.
No top-level findings curated for this source.
PMID:9238033
Xeroderma pigmentosum and trichothiodystrophy are associated with different mutations in the XPD (ERCC2) repair/transcription gene.
No top-level findings curated for this source.
url:https://api.orphadata.com/rd-epidemiology/orphacodes/33364
https://api.orphadata.com/rd-epidemiology/orphacodes/33364
No top-level findings curated for this source.

Deep Research

2
Claude Code ▸
Trichothiodystrophy (TTD): Comprehensive Disease Research Report
claude-haiku-4-5-20251001, claude-sonnet-4-6 20 citations 2026-06-29T19:53:53.757155

Trichothiodystrophy (TTD): Comprehensive Disease Research Report

Report Date: June 29, 2026 Target Disease: Trichothiodystrophy MONDO ID: MONDO:0018053 Disease Category: Mendelian / Rare Autosomal Recessive Neuroectodermal Disorder

1. Disease Information

Overview

Trichothiodystrophy (TTD) is a rare, autosomal recessive (and rarely X-linked) multisystem neuroectodermal disorder defined by the hallmark finding of sulfur-deficient, brittle hair with reduced cysteine-rich matrix protein content. The name derives from the Greek trichos (hair), thio (sulfur), and dystrophy (abnormal development). TTD encompasses a phenotypic spectrum ranging from mild isolated hair and skin involvement to a severe multisystem syndrome affecting neurological development, growth, immunity, fertility, and multiple organ systems.

The disorder was first described in 1974 by Price et al. (PMID: 4460955) and has since been recognized as a disorder at the intersection of DNA repair and transcriptional regulation. A landmark systematic review of 112 published cases (PMID: 18603627) characterized the full clinical spectrum, establishing phenotype frequencies used throughout the literature.

A critical and clinically important distinction: unlike xeroderma pigmentosum (XP), which shares causal genes with TTD, patients with TTD do not have an elevated predisposition to skin cancer, despite harboring defects in nucleotide excision repair (NER).

Key Identifiers

Identifier Value
OMIM #601675 (TTD1, photosensitive); #616390 (TTD2, photosensitive); #616395 (TTD3, photosensitive); #234050 (TTD4, non-photosensitive); additional entries for TTD5–TTD7
Orphanet ORPHA:33364
MONDO MONDO:0018053
ICD-10 Q84.1 (congenital morphological disturbances of hair) / L67.8
MeSH D015649
OMIA N/A (no well-characterized natural animal disease)

Common Synonyms and Alternative Names

  • PIBIDS syndrome (Photosensitivity, Ichthyosis, Brittle hair, Intellectual impairment, Decreased fertility, Short stature) — the photosensitive form
  • IBIDS syndrome (non-photosensitive form with ichthyosis)
  • BIDS syndrome (non-photosensitive without ichthyosis)
  • Tay syndrome
  • Sulfur-deficient brittle hair syndrome
  • Pollitt syndrome
  • PIBI(D)S
  • TTD-A, TTD-B, TTD-C, TTD-NPS (genotype-based subtypes)

2. Etiology

Causal Factors

TTD is a genetically heterogeneous disorder caused by biallelic loss-of-function variants (or hemizygous X-linked variants) in genes encoding components of the general transcription/DNA repair machinery. Disease-causing mutations impair either the TFIIH transcription/repair complex, other transcription factors (TFIIE), tRNA aminoacyl synthetases, or RNA-splicing factors. The unifying molecular pathology is a quantitative or qualitative reduction in transcriptional fidelity and/or proteostasis.

Risk Factors

Genetic risk factors (causal variants)

TTD is classified into photosensitive (PS-TTD) and non-photosensitive (NPS-TTD) forms based on whether the causal gene participates in NER:

Photosensitive TTD (~50% of all TTD cases): - ERCC2 (XPD; HGNC:3434): Most common cause; mutations account for ~29% of all TTD cases. Encodes the XPD subunit of TFIIH, a 5'→3' helicase. Missense variants predominantly affecting COOH-terminal region (e.g., R722W, R658C, A725P) are characteristic. Point mutations at positions distinct from XP hotspots distinguish TTD from XP-D. OMIM #601675 (TTD1). - ERCC3 (XPB; HGNC:3435): Very rare (~2% of cases); encodes the XPB subunit of TFIIH, a 3'→5' helicase. OMIM #616390 (TTD2). - GTF2H5 (TTDA; HGNC:30811): Encodes the smallest TFIIH subunit (p8/TTDA), ~2% of cases. Loss-of-function mutations cause complete NER deficiency in vitro. OMIM #616395 (TTD3).

Non-photosensitive TTD (~50% of all TTD cases): - MPLKIP (TTDN1; HGNC:25985): Encodes M-phase-specific PLK1-interacting protein; functions in RNA splicing and mitosis; accounts for <20% of NPS-TTD. OMIM #234050 (TTD4). - GTF2E2 (TFIIEβ; HGNC:4655): Encodes the β subunit of TFIIE; mutations destabilize the TFIIE complex; confirmed in multiple NPS-TTD cases (PMID: 26996949). OMIM #615919 (TTD5). - RNF113A (HGNC:21178): X-linked; encodes an E3 ubiquitin ligase and spliceosome component; X-linked dominant in females, hemizygous males affected. OMIM #300953 (TTD6). - AARS1 (HGNC:20): Encodes alanyl-tRNA synthetase 1; compound heterozygous missense variants cause NPS-TTD with protein instability and reduced aminoacylation activity (PMID: 33909043). - MARS1 (HGNC:6898): Encodes methionyl-tRNA synthetase 1; homozygous missense variants (e.g., V401M) reduce protein stability to ~30% of control (PMID: 33909043). - CARS1 (HGNC:1493): Encodes cysteinyl-tRNA synthetase 1; biallelic variants cause NPS-TTD (PMID: 30824121). - TARS1 (HGNC:11578): Encodes threonyl-tRNA synthetase 1; recently implicated in NPS-TTD.

Approximately 37% of TTD cases with confirmed DNA repair defects remain genetically uncharacterized (PMID: 18603627), suggesting additional causal loci remain to be identified.

Environmental risk factors

  • Consanguinity: Reported in 17% of cases in the systematic review (PMID: 18603627), substantially elevating risk for biallelic mutations in autosomal recessive forms.
  • No specific exogenous environmental risk factors are established. The disease is purely genetic in etiology.
  • UV exposure is a precipitating trigger for photosensitivity symptoms (sunburn, erythema) in PS-TTD but is not causative.

Protective Factors

No established genetic or environmental protective factors are known. Avoidance of UV exposure reduces severity of cutaneous manifestations in PS-TTD.

Gene-Environment Interactions

In photosensitive TTD, UV radiation interacts with the NER repair deficit to produce exaggerated erythema and acute photosensitivity. Crucially, despite NER deficiency, the pro-oncogenic consequences of unrepaired UV photoproducts that cause skin cancer in XP patients do not occur in TTD (PMID: 10667598). This paradox is hypothesized to reflect the anti-tumorigenic properties of TTD mutations in melanocytic cells through cell cycle and transcriptional effects distinct from simple repair-deficiency (PMID: 40918647, 2025).

3. Phenotypes

The following frequencies are derived from the systematic review of 112 published cases (PMID: 18603627).

Hair and Nail Features (Defining Characteristics)

Phenotype Frequency HPO Term
Brittle hair / hair shaft abnormalities 96% HP:0008070
"Tiger tail" banding on polarized microscopy 73% HP:0002217
Decreased hair sulfur/cystine content 71% HP:0002223
Sparse hair / hypotrichosis 48% HP:0008070
Alopecia 39% HP:0001596
Nail onychodystrophy 37% HP:0001甲 / HP:0003821

Character of hair findings: Hair is short, fragile, and breaks at irregular intervals. Scanning electron microscopy and transmission electron microscopy reveal abnormal cuticular scale structure. Light microscopy shows trichorrhexis nodosa-like fractures. Polarized light microscopy reveals alternating bright and dark "tiger tail" bands, reflecting uneven distribution of cysteine-rich matrix proteins—a pathognomonic finding. Amino acid analysis demonstrates approximately 50% reduction in cysteine content compared to controls.

HPO: Brittle hair = HP:0008070; Tiger tail banding = HP:0002217; Sparse hair = HP:0001070.

Neurological Phenotypes

Phenotype Frequency HPO Term
Developmental delay / intellectual disability 86% HP:0001263
Microcephaly 50% HP:0000252
Hypomyelination / dysmyelination ~40% (MRI data) HP:0003429
Abnormal gait / ataxia 26% HP:0002355
Seizures 6% HP:0001250
Sensorineural hearing loss 4% HP:0000407

Onset: Developmental delays present in infancy; MRI abnormalities (dysmyelination, white matter signal changes, cerebellar atrophy, dilated ventricles) detectable in early childhood. Hypomyelination reflects impaired thyroid hormone receptor (TR) stabilization in the brain due to reduced TFIIH levels (PMID: 17952069).

Severity: Intellectual disability ranges from mild to severe; progressive dysmyelination may worsen over first decade.

Growth Features

Phenotype Frequency HPO Term
Short stature 73% HP:0004322
Intrauterine growth restriction (IUGR) 21% HP:0001511
Low birth weight (<2500g) 37% HP:0001518

Progression: Growth deficiency is typically present from birth and persists. Nutritional support is often required (PMID: 25396826 — growth and nutrition in TTD children).

Skin Phenotypes

Phenotype Frequency HPO Term
Ichthyosis 65% HP:0008064
Collodion membrane at birth 26% HP:0001360
Photosensitivity 42% HP:0000992

Character: Ichthyosis is typically lamellar or congenital ichthyosiform erythroderma in type. Photosensitivity in PS-TTD manifests as acute exaggerated sunburning without skin cancer. Collodion membrane at birth (tight, shiny, film-like encasement) is a significant neonatal manifestation requiring intensive care.

Ocular Features

Phenotype Frequency HPO Term
Any ocular abnormality 51% —
Cataracts 29% (total); 7% congenital HP:0000518
Other (nystagmus, strabismus) Variable HP:0000639, HP:0000486

Systemic / Other Features

Phenotype Frequency HPO Term
Recurrent infections 46% HP:0002718
Facial dysmorphism 66% HP:0001999
Abnormal birth characteristics 55% —
Maternal pregnancy complications (preeclampsia, HELLP) 28% —
Gonadal dysgenesis / hypogonadism 14% HP:0000144
Beta-thalassemia / anemia Rare; ~10 cases HP:0001878

Pregnancy complications: Mothers carrying TTD-affected fetuses have elevated rates of HELLP syndrome and preeclampsia (28% in the systematic review), possibly related to placental TFIIH dysfunction.

Beta-thalassemia connection: A subset of TTD patients (mostly ERCC2-mutant) have beta-thalassemia. Seminal work (PMID: 11734544) demonstrated that mutant TFIIH fails to adequately transcribe the beta-globin gene during terminal erythroid differentiation (a high-demand transcriptional context), directly implicating transcriptional insufficiency in this phenotype.

Progeroid Features

TTD mouse models and some human patients exhibit segmental progeroid features including early cataracts, osteoporosis, and reduced bone stem cells, suggesting accelerated aging in some tissues. This is consistent with the role of TFIIH in maintaining transcriptional fidelity needed for tissue homeostasis (PMID: 21357150).

4. Genetic / Molecular Information

Causal Genes (Summary Table)

Gene HGNC ID Protein TTD Type OMIM Disease
ERCC2 (XPD) HGNC:3434 XPD helicase (TFIIH) PS-TTD (TTD1) #601675
ERCC3 (XPB) HGNC:3435 XPB helicase (TFIIH) PS-TTD (TTD2) #616390
GTF2H5 (TTDA) HGNC:30811 p8/TTDA (TFIIH) PS-TTD (TTD3) #616395
MPLKIP (TTDN1) HGNC:25985 M-phase PLK1-interacting protein NPS-TTD (TTD4) #234050
GTF2E2 HGNC:4655 TFIIEβ NPS-TTD (TTD5) #615919
RNF113A HGNC:21178 E3 ubiquitin ligase / spliceosome NPS-TTD (TTD6; X-linked) #300953
AARS1 HGNC:20 Alanyl-tRNA synthetase 1 NPS-TTD —
MARS1 HGNC:6898 Methionyl-tRNA synthetase 1 NPS-TTD —
CARS1 HGNC:1493 Cysteinyl-tRNA synthetase 1 NPS-TTD —
TARS1 HGNC:11578 Threonyl-tRNA synthetase 1 NPS-TTD —

Pathogenic Variants

  • ERCC2: Missense mutations are by far the most common. Hotspot positions include: R722W (c.2164C>T), A725P (c.2173G>C), R658C (c.1972C>T), R112H (c.335G>A). Importantly, TTD-causing mutations in ERCC2 predominantly occur at positions distinct from XP-D mutations, with XP mutations clustering near positions Arg683, while TTD mutations cluster in both NH2- and COOH-terminal regions (PMID: 9182770). All are germline, autosomal recessive (homozygous or compound heterozygous).
  • GTF2H5: Loss-of-function; includes nonsense and splice-site variants. Homozygous knockout in mice is embryonic lethal (PMID: 23630104).
  • AARS1 / MARS1 / CARS1 / TARS1: Missense variants reducing protein stability and aminoacylation enzyme activity (PMID: 33909043).

Key phenotype–genotype correlations: - Mutations in the NH2-terminal region of XPD → greater UV sensitivity than COOH-terminal mutations - Gene dosage (total residual TFIIH) appears to correlate with clinical severity more than the site of mutation alone (ScienceDirect; Chip et al.) - All TTD mutations lead to reduced cellular TFIIH concentration (up to 70% below normal), suggesting destabilization of the complex rather than simple loss-of-one-subunit function (PMID: 12393803)

Modifier Genes

No well-established modifiers. However, the genetic background modulates phenotype severity in mouse models.

Chromosomal Abnormalities

None documented for TTD; all cases arise from point mutations or small indels.

5. Environmental Information

Environmental Factors

  • Ultraviolet radiation: In PS-TTD, UV exposure triggers acute photosensitivity. Patients sunburn severely but do not develop squamous or basal cell carcinoma.
  • No specific chemical, occupational, infectious, or dietary environmental triggers have been identified as contributing to disease development.

Lifestyle Factors

  • Sun avoidance is critical for PS-TTD patients; failure to protect skin leads to acute and cumulative UV damage.
  • No smoking, alcohol, or diet associations are established.

Infectious Agents

No infectious agents cause or trigger TTD. However, TTD patients have high susceptibility to recurrent infections (46% of cases), particularly respiratory infections and sepsis, which are the leading cause of death in children (13/20 deaths in the systematic review were infection-related; PMID: 18603627).

6. Mechanism / Pathophysiology

Core Molecular Mechanism: TFIIH Complex Dysfunction

The central mechanistic thread in photosensitive TTD is destabilization and reduced cellular concentration of the TFIIH transcription/repair complex. TFIIH is a 10-subunit complex organized in two subcomplexes: - Core complex (7 subunits): XPB (ERCC3), XPD (ERCC2), p62 (GTF2H1), p52 (GTF2H4), p44 (GTF2H2), p34 (GTF2H3), TTDA (GTF2H5) - CAK (CDK-activating kinase) module (3 subunits): CDK7, cyclin H, MAT1

TFIIH serves dual functions: 1. Basal transcription (RNA Pol II initiation): Opens the promoter DNA via XPB helicase activity; phosphorylates RNA Pol II CTD via CDK7 2. Nucleotide excision repair (NER): Unwinds DNA around bulky adducts (UV photoproducts, cisplatin adducts) via XPB and XPD helicases to enable lesion excision

In TTD, mutations in ERCC2, ERCC3, or GTF2H5 cause the mutant subunit to destabilize the entire TFIIH complex, reducing its intracellular concentration by up to 70% (PMID: 12393803). This "TFIIH insufficiency" impairs both NER (causing photosensitivity) and basal transcription (causing developmental abnormalities). The transcriptional impairment is particularly manifest during high-demand transcriptional states—terminal differentiation events in hair, skin, brain myelin, and erythroid cells.

Key evidence (PMID: 12820975): TTD-causing XPD mutations confer significant in vitro basal transcription defects, while XP-causing mutations in the same gene largely spare transcriptional function. This distinction explains why TTD has developmental/transcriptional phenotypes while XP has predominantly cancer-predisposition phenotypes.

Pathway 1: Hair and Skin Abnormalities (Cysteine-Rich Protein Transcription Failure)

During terminal differentiation of hair matrix cells, the gene family encoding cysteine-rich matrix proteins (UHAs/KAPs — keratin-associated proteins) is among the last and most highly transcribed. When TFIIH levels are insufficient, transcription of these high-sulfur protein genes fails preferentially in the final burst of differentiation, leading to: - Reduced incorporation of cysteine-rich proteins into the hair cortex - Reduced disulfide bonding → brittle, fragile hair - Similarly reduced cysteine-rich proteins in nails → onychodystrophy - Reduced barrier function in skin → ichthyosis

The hair defect is not caused by a primary structural protein mutation but by a transcriptional insufficiency in a gene expression program requiring near-maximal TFIIH activity.

Biological processes (GO): GO:0006351 (transcription, DNA-templated), GO:0045087 (innate immune response), GO:0006366 (transcription by RNA polymerase II) Cell types (CL): CL:0002559 (hair follicle matrix cell), CL:0000312 (keratinocyte)

Pathway 2: Neurological Abnormalities (Thyroid Hormone Receptor Coactivation)

TFIIH is required as a co-activator for thyroid hormone receptors (TR) at target gene promoters in the developing brain (PMID: 17952069). Studies in XpdTTD mice showed: - Spatial and selective deregulation of thyroid hormone-responsive gene expression in the brain - TFIIH is required to stabilize TR-DNA binding at responsive elements - Reduced expression of myelin basic protein (MBP) and other myelination genes (thyroid hormone targets) → hypomyelination/dysmyelination - Cerebellar development disrupted → ataxia

This explains the cardinal neurological triad: intellectual disability, microcephaly, and dysmyelination.

Biological processes (GO): GO:0006357 (regulation of transcription by RNA polymerase II), GO:0022008 (myelination), GO:0007399 (nervous system development) Cell types (CL): CL:0000128 (oligodendrocyte), CL:0000540 (neuron)

Pathway 3: Beta-Thalassemia via Transcriptional Insufficiency

The HBB (beta-globin) gene requires very high transcriptional rates during terminal erythroid differentiation. TFIIH mutations impair this high-demand transcription, reducing beta-globin production and causing beta-thalassemia trait or mild beta-thalassemia (PMID: 11734544).

Cell types (CL): CL:0000765 (erythroblast), CL:0000232 (erythrocyte)

Pathway 4: Ribosomal Dysfunction (Common Pathomechanism Across All TTD Forms)

A unifying 2023 study (PMC: 10377840) demonstrated that disrupting TTDN1 (MPLKIP) or RNF113A—which are spliceosome components rather than TFIIH components—produces a converging downstream pathology: - Reduced UBF (upstream binding factor, the master RNA Pol I transcription activator) at the mRNA level - Impaired RNA Pol I transcription → reduced 47S pre-rRNA synthesis - Disrupted rRNA processing → reduced 18S rRNA → fewer small ribosomal subunits - Elevated translational error rate → misfolded protein accumulation - Proteostasis collapse → carbonylated protein accumulation, loss of protein quality control

The authors propose that ribosomal dysfunction represents a "common underlying pathomechanism of TTD" that explains neurodevelopmental phenotypes across genetically heterogeneous TTD forms. This unified model connects TFIIH-dependent (NPS and PS) and non-TFIIH-dependent (NPS) TTD through a convergent effect on translational fidelity.

Biological processes (GO): GO:0042254 (ribosome biogenesis), GO:0006364 (rRNA processing), GO:0006412 (translation), GO:0006986 (response to unfolded protein)

Pathway 5: tRNA Synthetase Deficiency and Protein Synthesis Errors

AARS1, MARS1, CARS1, and TARS1 mutations cause loss of aminoacyl-tRNA synthetase activity, directly reducing the fidelity and rate of protein translation (PMID: 33909043). Specifically in TTD: - Reduced aminoacylation → reduced tRNA charging → mistranslation - During high-demand protein synthesis states (hair matrix, myelin synthesis), translational errors produce unstable or misfolded structural proteins - This mechanism converges with the ribosomal dysfunction model: both impair proteostasis during differentiation

Summary of Mechanistic Causal Chain

Germline mutations in ERCC2/ERCC3/GTF2H5
    ↓
Destabilization of TFIIH complex (↓ 70% intracellular levels)
    ↓
Impaired NER ──→ UV photosensitivity (PS-TTD)
    ↓
Impaired RNA Pol II transcription at high-demand loci
    ├─→ KAP gene transcription failure → brittle sulfur-poor hair + ichthyosis
    ├─→ TR-coactivation failure → dysmyelination + intellectual disability
    └─→ HBB transcription failure → beta-thalassemia

Mutations in MPLKIP/RNF113A (splicing)
    ↓
Ribosomal biogenesis disruption (↓ UBF, ↓ 18S rRNA)
    ↓
Reduced translational fidelity → proteostasis collapse → multisystem failure

Mutations in AARS1/MARS1/CARS1/TARS1 (aminoacyl-tRNA synthetases)
    ↓
Reduced tRNA aminoacylation → mistranslation → misfolded proteins in differentiation
    ↓
Brittle hair + neurodevelopmental disease (same convergent phenotype)

Upstream molecular defects: TFIIH destabilization (or spliceosome/ribosome disruption) Downstream cellular consequences: Transcriptional insufficiency → developmental phenotypes; NER deficiency → UV sensitivity

7. Anatomical Structures Affected

Organ Level

System Manifestation
Skin/integument (primary) Ichthyosis, photosensitivity, collodion membrane
Hair/nail (primary) Brittle sulfur-deficient hair, onychodystrophy
Central nervous system (primary) Dysmyelination, microcephaly, intellectual disability, cerebellar atrophy
Eyes Cataracts, strabismus, nystagmus
Growth system / skeleton Short stature, IUGR, bone density reduction
Hematopoietic (secondary) Beta-thalassemia, anemia
Immune system Susceptibility to infections (functional immunodeficiency mechanisms unclear)
Gonads (secondary) Hypogonadism, decreased fertility

UBERON terms: UBERON:0000414 (mucosa), UBERON:0002097 (skin of body), UBERON:0000955 (brain), UBERON:0000473 (testis), UBERON:0001638 (vein of retina).

Tissue and Cell Level

  • Epidermis / stratum corneum: Impaired terminal differentiation
  • Hair follicle matrix cells (CL:0002559): Failure of KAP gene transcription
  • Oligodendrocytes (CL:0000128): Impaired myelination via TR-coactivation failure
  • Erythroid precursors (CL:0000765): Impaired beta-globin transcription
  • Lens epithelium: Cataract formation mechanism unclear; possibly transcriptional defect

Subcellular Level

  • Nucleus: NER deficiency → persistent UV photoproducts; reduced TFIIH concentrations affect RNA Pol II promoter opening
  • Ribosome (GO:0005840): Reduced 40S ribosomal subunit availability
  • Nucleolus (GO:0005730): Impaired rRNA synthesis

8. Temporal Development

Onset

  • Prenatal: Many manifestations present in utero; IUGR (21%), collodion membrane at birth (26%), maternal pregnancy complications (28%). Amniotic fluid may show elevated AFP due to skin barrier disruption.
  • Neonatal: Collodion membrane, low birth weight, early infections, feeding difficulties.
  • Infancy/Early Childhood: Brittle hair, ichthyosis, developmental delay apparent; recurrent infections, cataracts, hearing evaluation needed.
  • Later Childhood: Short stature, intellectual disability defined; neurological features (ataxia, spasticity) may progress.

HPO onset category: HP:0003623 (neonatal onset) for most manifestations; HP:0003577 (congenital onset) for structural features.

Progression

  • Disease course: Non-episodic, chronic, largely non-progressive neurological phenotype; ichthyosis and hair features persist throughout life.
  • Infections: Episodic, with high early-life mortality risk. 13 of 19 deaths in the systematic review were infection-related; all but one death occurred under age 10.
  • Neurological: Dysmyelination is present early; may improve partially in some patients as myelination continues through childhood.
  • Progeroid features: Some older patients and mouse models demonstrate segmental premature aging (reduced bone density, cataracts).

Prognosis

  • Mortality: Substantially elevated in childhood; at age 3 years, 10.7% probability of reported death; by age 9 years, 21.3% (PMID: 18603627). Mortality rate approximately 20-fold higher than US population in children ≤10 years.
  • Median age at death: 3 years in severe cases; all but one deceased patient in the systematic review died under age 10.
  • Cause of death: Predominantly infections (pneumonia, sepsis)—13 of 19 deaths in the systematic review were infection-related.
  • Survival to adulthood: Possible in milder forms; one patient in the systematic review was aged 47 years. However, reliable data on adult natural history is limited.

9. Inheritance and Population

Epidemiology

  • Prevalence/Incidence: Approximately 1 in 1,000,000 live births in Western countries (some estimates range 1.2/million to 1/million). Approximately 100 cases reported worldwide as of 2024, making it an ultra-rare disease.
  • Gender: Approximately equal sex distribution (51% male, 49% female in systematic review; PMID: 18603627).
  • Geographic distribution: Cases reported worldwide; no specific geographic clustering except where consanguinity rates are elevated. In the systematic review, Italy (23%), USA (16%), and UK (16%) contributed the most cases, likely reflecting reporting bias.

Inheritance Pattern

  • Autosomal recessive for the vast majority of forms (ERCC2, ERCC3, GTF2H5, MPLKIP, GTF2E2, AARS1, MARS1, CARS1, TARS1 — biallelic mutations required).
  • X-linked for RNF113A-associated TTD; hemizygous males affected; carrier females may have mild or no features.
  • Penetrance: Complete for classic multisystem forms; some genotype-specific variable expressivity exists.
  • Expressivity: Highly variable—same mutation can produce mild hair-only disease or severe multisystem disease. Gene dosage (total residual TFIIH) appears to determine severity more than specific mutation location.
  • Consanguinity: Present in 17% of reported cases (PMID: 18603627).

Carrier Frequency

Not well established. Given an incidence of ~1/million, the Hardy-Weinberg estimated carrier frequency is approximately 1/500 for the most common causal allele, but direct population surveys are lacking.

10. Diagnostics

Clinical/Hair Diagnostic Tests

Hair polarized light microscopy: - The pathognomonic "tiger tail" banding pattern on polarized light microscopy is present in ~73% of cases (PMID: 18603627). - Alternating birefringent (bright) and non-birefringent (dark) bands reflect alternating zones of high and low sulfur-protein content. - This is the first-line and most practical diagnostic test. - HPO: HP:0002217 (abnormal hair shaft banding under polarized microscopy)

Hair amino acid analysis: - Cystine/cysteine content approximately 50% of normal in affected hair. - Semiquantitative methods using sodium azide-dependent oxidation to cysteic acid have been validated (PMID: 15232704).

Scanning and transmission electron microscopy: - Abnormal cuticle morphology; longitudinal ridging; cuticular ruptures. - Used primarily in research/reference settings.

UV sensitivity testing (unscheduled DNA synthesis, UDS): - For PS-TTD: Reduced post-UV UDS in fibroblasts demonstrates NER deficiency. - Not routinely available; performed in specialist NER research laboratories.

Hair ultrastructure analysis: - 2023 study (PMC:10575343) described distinct ultrastructural features of TTD hair shafts distinguishable from other brittle hair disorders.

Biochemical Tests

  • Complete blood count: Anemia with target cells when beta-thalassemia coexists.
  • Hemoglobin electrophoresis: Elevated HbA2 in beta-thalassemia-associated TTD.
  • LOINC: No LOINC code specifically for TTD hair cystine—general amino acid analysis panels apply.

Neuroimaging

  • Brain MRI: Demonstrates hypomyelination (periventricular and subcortical white matter T2 changes), cerebellar atrophy, dilated ventricles. Essential for neurological assessment.
  • Pattern: TTD is listed as a cause of leukodystrophy/hypomyelinating leukodystrophy; a 2025 case series documented ERCC2 variants as "uncommon contributors to progressive hypomyelinating leukodystrophy" (PMID: 39976384).

Genetic Testing

Recommended approach: - NGS-based multi-gene panel testing covering ERCC2, ERCC3, GTF2H5, MPLKIP, GTF2E2, RNF113A, AARS1, MARS1, CARS1, TARS1 — available through clinical laboratories (GTR: condition C1955934). - Comprehensive TTD panel (GTR test ID 560930) covers major photosensitive and non-photosensitive genes. - Whole exome sequencing (WES): Appropriate first-tier test in cases where clinical features are present but diagnosis unclear; cost-effective for genetically heterogeneous conditions. - Whole genome sequencing (WGS): May be warranted in WES-negative cases to identify deep intronic or structural variants. - Sanger sequencing: For confirmation of identified variants and familial testing.

Prenatal diagnosis: - Available via chorionic villus sampling or amniocentesis once familial mutations are identified. - Preimplantation genetic diagnosis (PGD) is theoretically available.

Dermoscopy

  • Polarized transilluminating dermoscopy has been validated for detecting tiger tail banding in scalp hair in vivo (IJDVL, 2023), allowing non-invasive point-of-care diagnosis.

Differential Diagnosis

Condition Key Distinguishing Feature
Xeroderma pigmentosum (XP-D) Skin cancer predisposition; mutations at different XPD positions; no brittle hair
Menkes disease X-linked recessive; copper metabolism defect; pili torti pattern
Netherton syndrome SPINK5 mutations; trichorrhexis invaginata ("bamboo" hair); ichthyosis linearis circumflexa
Argininosuccinic aciduria Argininosuccinate lyase deficiency; trichorrhexis nodosa; hyperammonemia
Biotin-responsive basal ganglia disease Biotin metabolism; different hair type; treatable

11. Outcome / Prognosis

Survival

  • Early childhood mortality: ~20-fold elevated vs. US population in children ≤10 years (PMID: 18603627).
  • Probability of death by age 3: 10.7%; by age 9: 21.3%.
  • Cause: Predominantly recurrent and severe infections (pneumonia, sepsis).
  • Adult survival: Possible; the systematic review identified patients up to age 47 years in the mild end of the spectrum.

Morbidity

  • Severe intellectual disability and neurological impairment in most affected individuals significantly impair quality of life and require intensive multidisciplinary support.
  • Recurrent infections necessitate prompt antibiotic therapy and prophylactic measures.
  • Ichthyosis management is lifelong but improves with emollient therapy.
  • Short stature and growth failure may benefit from nutritional intervention (PMID: 25396826).

Prognostic Factors

  • Severity of intellectual disability and neurological involvement is a key prognostic factor.
  • Early infection events and sepsis carry the highest mortality risk in early childhood.
  • Patients with milder phenotypes (hair only, mild developmental delay) can achieve reasonable quality of life into adulthood.
  • Beta-thalassemia, when present, typically manifests as trait or mild disease and rarely requires transfusion.

12. Treatment

Current Management Approach

TTD has no curative treatment. Management is multidisciplinary and symptom-directed. As stated in NORD resources: "TTD may be adequately managed through topical agents, sun protection measures, use of visual aids, nutritional and growth support, and occupational therapy."

Pharmacotherapy

Ichthyosis: - Emollient therapy: First-line; extensive topical moisturizers (urea-containing creams, petrolatum, ceramide-based emollients) to reduce scale and improve skin barrier. Applied multiple times daily. - MAXO: MAXO:0000950 (supportive care) - NCIT: NCIT:C15986 (Pharmacotherapy) - Keratolytics: Lactic acid, urea, salicylic acid formulations. - Retinoids: Systemic retinoids (acitretin, isotretinoin) used in severe congenital ichthyosiform erythroderma; use must be balanced against growth effects. - Dupilumab (IL-4Rα antagonist): A 2021 case report (ResearchGate/Pediatric Dermatology) described successful treatment of TTD ichthyosis with dupilumab in a child. A 2024 case series from Pediatric Dermatology further documented dupilumab benefit for ichthyosis in TTD (Ovid/Pediatric Dermatology, 2024). This represents a potentially important advance, as Th2 cytokine signaling contributes to the barrier defect in TTD-associated ichthyosis. - NCIT: NCIT:C65216 (Dupilumab) / CHEBI:172716 - Therapeutic modality: MONOCLONAL_ANTIBODY

Photosensitivity (PS-TTD): - Broad-spectrum high-SPF sunscreen (SPF ≥50): Essential in PS-TTD. - Protective clothing and UV-blocking eyewear. - Vitamin D supplementation: Required when sun avoidance is strict (HP:0100512 vitamin D deficiency risk). - MAXO: MAXO:0000950 (supportive care), MAXO:0000088 (dietary intervention)

Infection management: - Prophylactic antibiotics (e.g., co-trimoxazole) considered for recurrent bacterial infections. - Prompt empirical antibiotic therapy for febrile illness. - Immunization according to schedule (standard vaccines); no live vaccines if immunocompromise is confirmed. - MAXO: MAXO:0001017 (vaccination), MAXO:0000950 (supportive care)

Cataracts: - Surgical removal followed by optical correction (glasses or contact lenses). - MAXO: MAXO:0000004 (surgical procedure)

Anemia / beta-thalassemia: - Monitoring of hemoglobin; iron supplementation if deficient; transfusion in severe anemia.

Rehabilitative and Supportive Care

  • Early intervention programs for developmental delay.
  • Special education for intellectual disability.
  • Physical therapy for ataxia and motor deficits. (MAXO: MAXO:0000011)
  • Occupational therapy for activities of daily living.
  • Speech therapy for communication difficulties.
  • Hearing aids for sensorineural hearing loss.
  • Nutritional support: Nasogastric or gastrostomy feeding in severe cases with growth failure (PMID: 25396826).

Experimental and Emerging Therapies

No disease-modifying therapies currently approved. Research directions include: - TFIIH stabilization strategies: Molecular chaperones or small molecules that could stabilize mutant TFIIH complexes (preclinical). - Ribosome biogenesis modulation: Targeting the ribosomal dysfunction arm. - Thyroid hormone supplementation: Small study in TTD mice showed partial correction of myelin abnormalities with T3; no human trials reported. - Gene therapy: Theoretical; ERCC2 gene delivery. No clinical trials active as of 2026.

No active clinical trials

As of the report date, no interventional clinical trials registered on ClinicalTrials.gov specifically for TTD disease modification were identified. Families are encouraged to consult NCI's Gene Review resources and connect with TTD patient registries.

13. Prevention

Primary Prevention

  • Genetic counseling for families with known TTD mutations; risk is 25% per pregnancy for biallelic autosomal recessive forms.
  • MAXO: MAXO:0000079 (genetic counseling)
  • Carrier testing for at-risk relatives once proband mutations identified.
  • Preimplantation genetic diagnosis (PGD) for couples who are known carriers.

Secondary Prevention (Early Detection)

  • Newborn screening: TTD is not included in standard newborn screening panels in any country. Screening via hair amino acid analysis or molecular testing would be feasible in high-risk families.
  • Prenatal diagnosis: Available via CVS or amniocentesis when familial mutations are known.
  • Cascade family screening after proband identification.

Tertiary Prevention (Preventing Complications)

  • Infection prevention: Careful vaccination schedule; antibiotic prophylaxis; parental education on early infection recognition.
  • Sun protection education: Comprehensive UV protection protocols for PS-TTD families.
  • Ophthalmology surveillance: Annual slit-lamp examination for cataracts.
  • Audiology surveillance: Annual hearing assessment.
  • Nutritional monitoring: Regular growth charts; dietetic input.

14. Other Species / Natural Disease

Animal Models

TTD does not appear to occur naturally in any non-human species at a population level.

Mouse models (primary research models):

  1. XpdTTD/R722W knock-in mice (De Boer et al., 1998): The most-used mouse model; introduced the human R722W XPD mutation into the murine Ercc2 locus by gene-cDNA fusion targeting. These mice recapitulate many TTD features:
  2. Brittle sulfur-deficient hair
  3. Developmental delay and reduced body weight
  4. Cachexia and short lifespan
  5. Segmental progeroid phenotype (bone density loss, cataracts, immune changes)
  6. Dysmyelination (brain MRI and histopathology)

  7. Thyroid hormone target gene dysregulation in brain (PMID: 17952069)

  8. XpdTTD/†XPCS compound heterozygous mice: Viable compound heterozygotes allowing study of allele combinations (PMID: 17183058).

  9. XpdTTD/XpdTTD mice with XPA-null background: Dramatically accelerated aging phenotype; demonstrates additive NER deficiency effects.

  10. TTDA (Gtf2h5) knockout mice: Embryonic lethal when homozygous null; heterozygous mice show intermediate phenotypes, confirming TTDA is essential for viability (PMID: 23630104).

Model limitations: - Murine hair is structurally different from human hair; not all hair manifestations translate - Mouse lifespan differences limit studying adult/aging TTD phenotypes - The progeroid features in mice may overstate the premature aging component relative to human TTD - Lissencephalic mouse brain differs from human cortical organization, potentially limiting neurological translational validity (HUMAN_MODEL_MISMATCH concern)

Drosophila models: - XPD (Haywire) mutant Drosophila have been used to study cell-cycle coordination and XPD's non-repair functions (PMC:4283652).

Evolutionary conservation: - XPD/ERCC2 is conserved from yeast (Rad3 in S. cerevisiae) to humans. - NER pathway is evolutionarily ancient; core mechanisms conserved across eukaryotes. - C. elegans GTF-2H5/TTDA ortholog (PMID: 34873349) is non-essential for transcription but indispensable for NER, offering a simplified model organism for dissecting these functions.

Key Primary Literature Citations

PMID Reference Description
PMID:18603627 Faghri et al. systematic review of 112 TTD cases; phenotype frequencies
PMID:17952069 Neurological defects in TTD reveal TFIIH coactivator function of thyroid hormone (Nature Neuroscience)
PMID:11734544 TFIIH mutations cause beta-thalassemia in TTD patients
PMID:10667598 Cancer-free phenotype in TTD unrelated to repair defect
PMID:33909043 AARS1 and MARS1 protein instability causes NPS-TTD
PMID:26996949 GTF2E2 mutations destabilize TFIIE in NPS-TTD
PMID:12820975 TTD XPD mutations cause transcription defects; XP mutations do not
PMID:9182770 XP and TTD associated with different XPD mutations (PNAS 1997)
PMID:15232704 Quantification of cysteine in TTD hair/nails
PMID:30824121 CARS1 (cysteinyl-tRNA synthetase) mutations cause NPS-TTD
PMID:39976384 ERCC2 variants as uncommon cause of hypomyelinating leukodystrophy (2025)
PMC:10377840 Ribosomal dysfunction as common pathomechanism in TTD (Cells 2023)
PMID:21730288 Slowly progressing NER in TTD-A fibroblasts
PMID:25396826 Growth and nutrition in children with TTD
PMID:23630104 TTDA disruption causes complete NER deficiency and embryonic lethality
PMID:40918647 Anti-tumorigenic properties of TTD mutations in melanocytic cells (2025)

Summary Table: Key Facts for Knowledge Base Entry

Category Key Facts
MONDO MONDO:0018053
OMIM #601675 (TTD1/PS), #616390 (TTD2/PS), #616395 (TTD3/PS), #234050 (TTD4/NPS)
Inheritance Autosomal recessive (most); X-linked (RNF113A)
Incidence ~1/1,000,000 live births
Causal genes ERCC2, ERCC3, GTF2H5, MPLKIP, GTF2E2, RNF113A, AARS1, MARS1, CARS1, TARS1
Pathomechanism TFIIH insufficiency → transcription + NER deficiency; ribosomal dysfunction
Hallmark feature Brittle sulfur-deficient hair with tiger-tail polarized microscopy banding
Photosensitivity ~50% of cases (PS-TTD); NO cancer predisposition
Mortality 20-fold elevated in children; median age at death 3 years; primarily infections
Key treatment Supportive; emollients, sun protection, infection management; dupilumab (emerging)
No cancer predisposition Critical clinical distinction from XP

Sources: - OMIM #601675 — TTD1, Photosensitive - OMIM #234050 — TTD4, Nonphotosensitive - OMIM #616390 — TTD2, Photosensitive - OMIM #616395 — TTD3, Photosensitive - PMC3459585 — Systematic review of 112 TTD cases - PMC10377840 — Ribosomal dysfunction as common pathomechanism in TTD (2023) - PMC11840839 — ERCC2 variants and hypomyelinating leukodystrophy (2025) - NCBI Bookshelf NBK6285 — TTD: crosstalk between DNA repair and transcription - MedlinePlus Genetics — Trichothiodystrophy - DermNet NZ — Trichothiodystrophy - NORD — Trichothiodystrophy / IBIDS syndrome - GARD — Trichothiodystrophy - GTR — Trichothiodystrophy genetic testing - PNAS — XP and TTD associated with different XPD mutations - HMG — AARS1/MARS1 mutations cause TTD - PMC10630875 — MPLKIP maintains DBR1 for lariat debranching (2023) - PMC10575343 — Distinct ultrastructural features of TTD hair shafts (2023) - Nature Neuroscience — TFIIH coactivator function and TTD neurological defects - Cancer Research — Cancer-free phenotype in TTD - PMC4176511 — Growth and nutrition in TTD children

OpenScientist ▸
Trichothiodystrophy: Comprehensive Disease Characterization Report
openscientist-autonomous 34 citations 2026-06-29T20:36:34.443800

Trichothiodystrophy: Comprehensive Disease Characterization Report

MONDO ID: MONDO:0018053
Category: Mendelian
Report Date: 2026-06-29

Summary

Trichothiodystrophy (TTD) is a rare, clinically heterogeneous autosomal recessive (with one X-linked form) multisystem disorder unified by the hallmark finding of sulfur-deficient brittle hair displaying "tiger tail" banding under polarized light microscopy. The disease is caused by biallelic mutations in at least nine genes — ERCC2/XPD, ERCC3/XPB, GTF2H5/TTDA, GTF2E2, MPLKIP/TTDN1, RNF113A, TARS1, AARS1, and MARS1 — all encoding proteins involved in gene expression processes including transcription, mRNA splicing, and translation. The unifying molecular mechanism across all forms is mutation-induced protein instability that reduces steady-state levels of the affected gene expression factors, creating bottlenecks that predominantly impact terminally differentiating tissues such as hair, skin, and the central nervous system.

Approximately half of TTD patients exhibit photosensitivity (photosensitive TTD, or TTD-P) caused by mutations in TFIIH subunits (ERCC2, ERCC3, GTF2H5) that impair nucleotide excision repair (NER). Despite this DNA repair deficiency, TTD patients paradoxically lack cancer predisposition — a striking contrast to xeroderma pigmentosum (XP), which can be caused by mutations in the same genes. This cancer-free paradox is explained by the fact that TTD-specific mutations affect TFIIH stability and transcriptional function rather than disrupting CAK-mediated cell cycle control, which is the mechanism underlying cancer susceptibility in XP. The clinical spectrum ranges from mild disease with isolated hair abnormalities to severe multisystem involvement including ichthyosis, intellectual disability, CNS hypomyelination, short stature, cataracts, recurrent infections, hypogonadism, and osteosclerosis, with many patients dying in childhood predominantly from infectious complications.

TTD profoundly impacts pregnancy outcomes (81% complication rate including 30% preeclampsia and 56% preterm delivery) and neonatal health (85% neonatal complications). Growth failure is progressive and serves as a mortality prognostic biomarker: deceased patients had significantly lower standardized height and weight measurements. Recent research has expanded the mechanistic understanding to include impaired B-cell function explaining recurrent infections, erythroid differentiation defects explaining anemia, and vitamin D receptor dysfunction potentially contributing to skeletal abnormalities. The thermosensitivity of TTD mutations — where febrile episodes cause reversible clinical worsening through further TFIIH destabilization — represents a potential therapeutic target, as chemical chaperones like glycerol can rescue protein stability in vitro.

1. Disease Information

Overview

Trichothiodystrophy (TTD) is a rare, heterogeneous group of autosomal recessive genetic disorders characterized by sulfur-deficient brittle hair and multisystem involvement, particularly of neuroectodermal-derived tissues. The term "trichothiodystrophy" was introduced by Price et al. in 1980 to designate patients with sulfur-deficient brittle hair, recognized as a marker for a complex neuroectodermal symptom complex (PMID: 20687499). The defining diagnostic feature is the "tiger tail" pattern of alternating light and dark bands seen on polarized light microscopy of hair shafts, reflecting reduced content of cysteine-rich matrix proteins.

Key Identifiers

Database Identifier
MONDO MONDO:0018053
OMIM 234050 (TTD1/ERCC2), 616390 (TTD2/ERCC3), 616395 (TTD3/GTF2H5), 234050 (TTD4/GTF2E2), 300953 (TTD5/RNF113A), 616943 (TTD6/MPLKIP)
Orphanet ORPHA:33364
ICD-10 Q84.1 (Other congenital morphological disturbances of hair)
MeSH D054463

Synonyms and Alternative Names

  • Tay syndrome
  • IBIDS syndrome (Ichthyosis, Brittle hair, Impaired intelligence, Decreased fertility, Short stature)
  • PIBIDS syndrome (Photosensitivity + IBIDS)
  • BIDS syndrome
  • Sulfur-deficient brittle hair syndrome
  • Amish brittle hair syndrome (historical, for TTDN1-associated form)
  • TTD-P (photosensitive trichothiodystrophy)
  • TTD-NP / NPS-TTD (non-photosensitive trichothiodystrophy)

Information Sources

This report integrates data from aggregated disease-level resources (OMIM, Orphanet, HPO/Monarch Initiative — 313 disease-to-phenotype associations with 198 unique HPO terms), primary literature (56 papers reviewed), and individual patient cohort studies (NIH cohort of 36 TTD patients followed 2001–2013).

2. Etiology

Disease Causal Factors

TTD is a purely genetic disorder caused by biallelic loss-of-function mutations in genes encoding proteins involved in gene expression. There are no environmental, infectious, or acquired forms.

The primary cause is protein instability induced by specific mutations. As demonstrated by Theil et al. (2019): "TTD mutations affect the stability of the corresponding proteins and emphasize this phenomenon as a common feature of TTD" (PMID: 33909043). This was confirmed by Vaishnav et al. (2023): "TTD-associated mutations typically cause unstable mutant proteins involved in various steps of gene expression, severely reducing steady-state mutant protein levels" (PMID: 37800682).

Genetic Risk Factors (Causal Genes)

Gene Protein Function TTD Subtype Photosensitivity
ERCC2/XPD XPD helicase TFIIH subunit; NER and transcription TTD1 Yes
ERCC3/XPB XPB helicase TFIIH subunit; NER and transcription TTD2 Yes
GTF2H5/TTDA p8/TTDA TFIIH stabilizer TTD3 Yes
GTF2E2 TFIIEbeta TFIIE subunit; transcription initiation TTD4 No
RNF113A RNF113A Spliceosome component TTD5 No (X-linked)
MPLKIP/TTDN1 MPLKIP Lariat debranching/splicing TTD6 No
TARS1 ThrRS Threonyl-tRNA synthetase NPS-TTD No
AARS1 AlaRS Alanyl-tRNA synthetase NPS-TTD No
MARS1 MetRS Methionyl-tRNA synthetase NPS-TTD No

All mutations are germline in origin. The inheritance is autosomal recessive for all forms except TTD5 (RNF113A), which is X-linked dominant (HP:0001423).

Environmental Risk Factors

  • UV radiation: Exacerbates photosensitivity in TTD-P patients but does not cause the disease
  • Fever/elevated temperature: Critically important — febrile episodes cause reversible clinical worsening due to further destabilization of thermolabile mutant proteins (PMID: 36259739)
  • Consanguinity: Increases risk in autosomal recessive forms; the original Tay syndrome cases were siblings of consanguineous parents

Protective Factors

  • Chemical chaperones: Glycerol rescues TFIIH thermo-instability in patient cells in vitro, representing a potential therapeutic target: "Improving the protein folding process by exposing patient cells to low temperature or to the chemical chaperone glycerol allowed rescue of TFIIH thermo-instability and a concomitant recovery of the complex activities" (PMID: 36259739)
  • Low temperature: Stabilizes mutant protein complexes

Gene-Environment Interactions

The most clinically significant gene-environment interaction in TTD is the thermosensitivity of mutant proteins. TTD-causing XPD mutations produce thermo-labile proteins; when patients develop fever (from infections or other causes), the already reduced levels of TFIIH are further destabilized, leading to reversible worsening of DNA repair capacity, transcriptional output, and clinical signs including episodic hair loss (PMID: 36259739; PMID: 7802014).

UV exposure in photosensitive TTD patients causes skin damage but, paradoxically, does not lead to skin cancer — unlike XP patients with mutations in the same genes (PMID: 17276014).

3. Phenotypes

Comprehensive Phenotype Catalog

The Monarch Initiative database (MONDO:0018053) contains 313 disease-to-phenotype associations mapping to 198 unique HPO terms spanning 15+ organ systems. Key phenotypes organized by system:

Hair and Nails

Phenotype HPO Term Frequency Onset Severity
Tiger tail banding (polarized light) HP:0045055 ~100% Congenital Diagnostic hallmark
Brittle hair HP:0002299 ~100% Congenital Variable
Reduced hair sulfur content HP:0034425 ~100% Congenital Diagnostic
Short hair HP:0100874 >80% Congenital Variable
Nail dystrophy HP:0008404 ~50% Childhood Mild-moderate

Skin

Phenotype HPO Term Frequency Onset Severity
Ichthyosis HP:0008064 ~80% Neonatal (often collodion) Improves with age
Cutaneous photosensitivity HP:0000992 ~50% Childhood Variable
Collodion membrane at birth HP:0007547 ~67% (neonatal cohort) Neonatal Resolves

Neurological

Phenotype HPO Term Frequency Onset Severity
Intellectual disability HP:0001249 >70% Childhood Mild to severe
Delayed CNS myelination HP:0002188 >60% Congenital Progressive
Microcephaly HP:0000252 ~50% Congenital Variable
Spastic paraparesis HP:0002313 Variable Childhood Progressive
Seizures HP:0001250 Overrepresented in TTDN1 Variable Variable
Autistic behaviors — In TTDN1 subgroup Childhood Variable

Growth and Development

Phenotype HPO Term Frequency Onset Severity
Short stature HP:0004322 >80% Prenatal/neonatal Progressive
Intrauterine growth retardation HP:0001511 Common Prenatal Variable
Delayed bone age HP:0002750 Overrepresented in TTDN1 Childhood Variable

Ocular

Phenotype HPO Term Frequency Onset Severity
Cataract HP:0000518 ~54% (neonatal cohort) Congenital/childhood Requires surgery

Endocrine/Reproductive

Phenotype HPO Term Frequency Onset Severity
Hypogonadism HP:0000135 Common in males Puberty End-organ failure
Decreased fertility HP:0000144 Common Adult Variable
Cryptorchidism HP:0000028 Variable Congenital Variable

Skeletal

Phenotype HPO Term Frequency Onset Severity
Osteosclerosis HP:0011001 Common Progressive Variable
Kyphosis HP:0002808 Variable Progressive Variable

Hematologic/Immune

Phenotype HPO Term Frequency Onset Severity
Recurrent infections HP:0002719 Common Childhood Major cause of death
Anemia HP:0001903 Variable Childhood Variable
Neutropenia HP:0001875 Variable Variable Variable
Lymphopenia HP:0001888 Variable Variable Variable

Quality of Life Impact

TTD profoundly impacts quality of life across all dimensions. Hair abnormalities cause significant psychosocial burden. Intellectual disability ranges from mild to severe, affecting educational and vocational potential. Ichthyosis impacts skin comfort and social interactions. Photosensitivity restricts outdoor activities. Recurrent infections cause frequent hospitalizations. Progressive growth failure and neurological decline contribute to a chronic, debilitating disease course.

TTDN1-Specific Phenotype

Patients with MPLKIP/TTDN1 mutations display a distinct phenotype: delayed bone age and seizure disorders are significantly overrepresented (P=0.009 and P=0.024, respectively), while autistic behaviors replace the characteristically friendly, socially interactive personality seen in other TTD forms. Several hallmark TTD laboratory and imaging findings may be absent (PMID: 25290684).

4. Genetic/Molecular Information

Causal Genes — Detailed

Photosensitive TTD (TFIIH genes):

  • ERCC2/XPD (OMIM: 126340; HGNC:3434; Chr 19q13.32): Encodes the XPD helicase subunit of TFIIH. Most common cause of photosensitive TTD. XPD acts as a "structural bridge tying the TFIIH core with the CAK complex" (PMID: 23232694). Mutations in XPD can cause XP, TTD, CS, or combined phenotypes depending on the specific position and nature of the mutation.
  • ERCC3/XPB (OMIM: 133510; HGNC:3435; Chr 2q14.3): Encodes the XPB helicase; core TFIIH subunit. Very rare cause of TTD.
  • GTF2H5/TTDA (OMIM: 608780; HGNC:25839; Chr 6q25.3): Encodes the small (71 amino acid) p8/TTDA subunit important for TFIIH stabilization. "Full disruption of TTDA expression in a knock-out mouse-model completely inactivates NER" (PMID: 25016283).

Non-photosensitive TTD (non-TFIIH genes):

  • MPLKIP/TTDN1 (OMIM: 609188; HGNC:25857; Chr 7p14.1): Function recently linked to maintaining DBR1 levels for proper lariat debranching and ectodermal differentiation (PMID: 37800682). Mutations include whole-gene deletions, suggesting MPLKIP is not essential for cell viability (PMID: 16977596).
  • GTF2E2 (OMIM: 189964; HGNC:4648; Chr 8p12): Encodes TFIIEbeta. Mutations cause temperature-sensitive transcription defects (PMID: 28973399).
  • RNF113A (OMIM: 300951; HGNC:10058; Chr Xq25): X-linked. RNA-binding spliceosome component. Loss-of-function causes TTD5 via splicing dysregulation (PMID: 32152280).
  • TARS1, AARS1, MARS1: Aminoacyl-tRNA synthetases (threonyl, alanyl, methionyl). Mutations reduce tRNA charging, the first step in protein translation (PMID: 33909043).

Pathogenic Variants

  • Variant types: Missense, nonsense, frameshift deletions (single bp to >120 kb whole-gene deletions), splice-site mutations
  • Classification: Pathogenic/Likely pathogenic per ACMG/AMP criteria in ClinVar
  • Allele frequencies: Extremely rare; most variants are private or found in specific populations (e.g., Amish kindred for TTDN1)
  • Functional consequences: Predominantly loss-of-function through protein destabilization (reduced steady-state levels) rather than catalytic inactivation

Genotype-Phenotype Correlations

A landmark study demonstrated that XP and TTD mutations in XPD/ERCC2 occur at different positions: "Most sites of mutations differed between XP and TTD, but there are three sites at which the same mutation is found in XP and TTD patients. Since the corresponding patients were all compound heterozygotes... the mutations which are found in both XP and TTD patients behaved as null alleles, suggesting that the disease phenotype was determined by the other allele" (PMID: 9238033). TTD-associated mutations localize to regions affecting TFIIH stability and CAK/p44 binding, while XP mutations tend to affect NER-specific functions (PMID: 22234153).

Compound heterozygosity is a potent source of disease heterogeneity. Mouse models demonstrate biallelic effects including dominance of one allele over another and interallelic complementation in a tissue-specific manner (PMID: 17020410; PMID: 23046824).

Epigenetic and Chromosomal Information

No specific epigenetic modifications (DNA methylation, histone changes) have been directly characterized in TTD patients. However, the transcriptional dysfunction inherent to TFIIH-mutant TTD likely produces secondary epigenetic effects through altered gene expression programs. No large-scale chromosomal abnormalities are associated with TTD.

5. Environmental Information

Environmental Factors

TTD is a purely genetic disease; no environmental toxins, radiation exposures, or pollutants cause the condition. However, UV radiation is a critical environmental modifier for photosensitive TTD patients, causing acute skin damage (though not cancer). Thermal stress (fever) is the most clinically significant environmental trigger, causing reversible worsening of symptoms through further destabilization of already unstable mutant proteins.

Lifestyle Factors

No specific lifestyle factors cause or prevent TTD. Vitamin D deficiency has been documented in TTD patients and may be a treatable contributor to short stature in PIBIDS syndrome: correction of severe vitamin D deficiency led to considerable gain in stature (PMID: 26661284).

Infectious Agents

Infections do not cause TTD but are the leading cause of death in TTD patients. Recurrent bacterial infections are common, likely driven by impaired B-cell function documented in TTD1 patients (PMID: 39055713). Odontogenic and respiratory infections are particularly documented (PMID: 39743573).

6. Mechanism / Pathophysiology

Unifying Molecular Mechanism: Protein Instability

The central pathogenic mechanism in TTD is mutation-induced instability of gene expression factors. This was established by studies showing that TTD mutations in transcription factors (TFIIH subunits, TFIIE), splicing factors (MPLKIP, RNF113A), and translation factors (aminoacyl-tRNA synthetases) all share the common feature of reduced steady-state protein levels (PMID: 33909043; PMID: 37800682).

Causal Chain: From Mutation to Clinical Manifestation

UPSTREAM EVENTS
================
Biallelic mutations in gene expression factor genes
|
v
Protein misfolding / reduced thermodynamic stability
|
v
Decreased steady-state protein levels (reduced TFIIH, TFIIE, tRNA synthetase, etc.)
|
v
INTERMEDIATE EVENTS
====================
Reduced transcription initiation (TFIIH/TFIIE mutants)
   OR Defective mRNA splicing (MPLKIP/RNF113A mutants)
   OR Impaired tRNA charging / translation (TARS1/AARS1/MARS1 mutants)
|
v
Bottleneck in gene expression, most severe in terminally differentiating cells
   requiring massive protein production (hair, skin, brain myelin)
|
v
DOWNSTREAM EVENTS (TISSUE-SPECIFIC)
=====================================
Hair:    Reduced cysteine-rich matrix protein synthesis -> brittle, sulfur-deficient hair
Skin:    Impaired keratinocyte differentiation -> ichthyosis, collodion at birth
Brain:   Defective oligodendrocyte myelin production -> hypomyelination
 + Impaired TR-mediated gene expression -> neurodevelopmental defects
Bone:    Abnormal VDR transactivation -> osteosclerosis, short stature
Blood:   Impaired B-cell activation -> recurrent infections
 + Impaired erythroid differentiation -> anemia
Eyes:    Lens fiber differentiation defect -> congenital cataracts
Gonads:  Impaired germ cell development -> hypogonadism, decreased fertility

Molecular Pathways

  • Nucleotide Excision Repair (NER) (GO:0006289): Impaired in photosensitive TTD due to TFIIH dysfunction. Both global genome repair (GG-NER) and transcription-coupled repair (TC-NER) are affected.
  • Transcription by RNA Polymerase II (GO:0006366): TFIIH is essential for promoter opening during transcription initiation. Reduced TFIIH levels create a transcription bottleneck.
  • Nuclear receptor signaling: TFIIH/CAK phosphorylates nuclear receptors including thyroid hormone receptors (TR) and vitamin D receptor (VDR). "TFIIH is required for the stabilization of thyroid hormone receptors (TR) to their DNA-responsive elements" (PMID: 17952069). VDR transactivation abnormalities documented in TTD patients (PMID: 23232694).
  • mRNA splicing (GO:0000398): MPLKIP maintains DBR1 levels for proper lariat debranching (PMID: 37800682); RNF113A regulates splicing of cell survival genes (PMID: 32152280).
  • tRNA aminoacylation (GO:0006418): TARS1, AARS1, MARS1 mutations impair tRNA charging, reducing translational capacity (PMID: 33909043).

Cellular Processes

  • Protein folding and stability (GO:0006457): Central to TTD pathogenesis — mutant proteins misfold and are degraded
  • Myelination (GO:0042552): Hypomyelination is a developmental defect, not demyelination: "The main neuropathology... is reduced myelination of the brain. These complex neurological abnormalities are not related to sunlight exposure but may be caused by developmental defects" (PMID: 17276014)
  • B-cell activation (GO:0042113): Impaired early BCR activation and proliferation after DNA damage in TTD1 patients (PMID: 39055713)
  • Erythroid differentiation (GO:0030218): GTF2E2 mutations cause hematopoietic defect during late-stage differentiation with hemoglobin subunit imbalance (PMID: 28973399)
  • Premature aging/senescence: TTD mice show accelerated bone aging, decline in mesenchymal stem cells/osteoprogenitors (PMID: 21814739)

The Cancer Paradox

One of the most striking features of TTD is the absence of cancer predisposition despite NER deficiency. This is in dramatic contrast to XP patients, who have a 1000-fold increase in skin cancer susceptibility (PMID: 17276014).

The mechanistic explanation comes from a Drosophila XPD model: "The XP mutants most clearly linked to high cancer risk, Xpd R683W and R601L, showed a reduced interaction with the core TFIIH and also an abnormal interaction with the Cdk-activating kinase (CAK) complex" (PMID: 25431422). XP cancer-linked mutations cause chromosomal instability (chromatin loss, free centrosomes), while TTD mutations affect cell cycle timing coordination without promoting genomic instability. Additionally, the reduced transcription and cell proliferation in TTD may itself be tumor-suppressive.

A very rare exception exists: a single case of squamous cell carcinoma in a PIBIDS patient has been reported (PMID: 18429798), and an XP/TTD overlap patient developed basal cell carcinoma at age 28 (PMID: 25002996), but these are exceptional and likely reflect the XP component of overlap genotypes.

Thermosensitivity as a Disease Modifier

A critical mechanistic insight is that TTD mutations create thermolabile proteins. At normal body temperature, mutant TFIIH operates at reduced but functional levels. During fever, the additional thermal stress further destabilizes the complex, causing acute, reversible worsening of both DNA repair and transcription (PMID: 36259739). This explains the clinical observation of episodic hair loss during infections (PMID: 7802014). Notably, GTF2E2/TFIIEbeta mutations also demonstrate temperature-sensitive transcription defects, indicating thermosensitivity extends beyond TFIIH-mutant forms: "We demonstrate that mutant TFIIEbeta strongly reduces the total amount of the entire TFIIE complex, with a remarkable temperature-sensitive transcription defect, which strikingly correlates with the phenotypic aggravation of key clinical symptoms after episodes of high fever" (PMID: 28973399).

Immune and Hematopoietic Mechanisms

  • B-cell dysfunction (CL:0000236): TTD1 patients show impaired early B-cell receptor activation and proliferation, with differential gene expression in peripheral lymphocytes (PMID: 39055713). This provides a molecular explanation for recurrent infections as the leading cause of death.
  • Erythroid differentiation defect (CL:0000764): iPSC-derived erythroid cells from GTF2E2-mutant TTD4 patients show a hematopoietic defect during late-stage differentiation: "We observed a clear hematopoietic defect during late-stage differentiation associated with hemoglobin subunit imbalance" (PMID: 28973399).

RNF113A/TTD5 Mechanism

RNF113A deficiency triggers multiple cell death pathways upon DNA damage: "RNF113A is a RNA-binding protein which regulates the splicing of multiple candidates involved in cell survival" (PMID: 32152280). Loss of RNF113A leads to MCL-1 destabilization (apoptosis), enhanced SAT1 expression (ferroptosis), and altered Noxa1 expression (increased ROS).

Molecular Profiling

Limited omics data are available due to disease rarity: - Transcriptomics: GEO dataset from TTD1 B-cell study with differential gene expression in peripheral lymphocytes (PMID: 39055713); RNF113A depletion dataset (12 samples) showing global splicing impact (PMID: 32152280) - Proteomics: No comprehensive proteomic studies; individual studies document reduced TFIIH, TFIIE steady-state levels - Metabolomics/Lipidomics: No published metabolomics or lipidomics studies on TTD patients

7. Anatomical Structures Affected

Organ Level

Primary organs: - Hair follicles (UBERON:0002073): Universal involvement — the defining feature - Skin/epidermis (UBERON:0001003): Ichthyosis, photosensitivity - Central nervous system (UBERON:0001017): Hypomyelination, intellectual disability - Skeletal system (UBERON:0001434): Short stature, osteosclerosis, bone fragility

Secondary organ involvement: - Eye/lens (UBERON:0000965): Cataracts - Gonads (UBERON:0000991): Hypogonadism, decreased fertility - Bone marrow (UBERON:0002371): Anemia, neutropenia, lymphopenia - Immune system (UBERON:0002405): Recurrent infections - Placenta (UBERON:0001987): Pregnancy complications, preeclampsia - Lungs (UBERON:0002048): Bronchiectasis reported in some cases (PMID: 10604009)

Body systems involved: Integumentary, nervous, skeletal, immune/hematologic, endocrine, reproductive, ocular, respiratory

Tissue and Cell Level

Tissue/Cell Type Cell Ontology Involvement
Hair cortex cells CL:0002559 Reduced cysteine-rich matrix protein
Keratinocytes CL:0000312 Ichthyosis, impaired differentiation
Oligodendrocytes CL:0000128 Hypomyelination
Neurons CL:0000540 Neurodevelopmental defects
B lymphocytes CL:0000236 Impaired activation and proliferation
Erythroid precursors CL:0000764 Defective late-stage differentiation
Osteoblasts CL:0000062 Reduced bone formation
Mesenchymal stem cells CL:0000134 Progressive depletion
Lens fiber cells CL:0011004 Cataract formation
Trophoblast cells CL:0000351 Placental abnormalities

Subcellular Level

  • Nucleus (GO:0005634): Site of TFIIH/TFIIE function in transcription and NER
  • Spliceosome (GO:0005681): Site of MPLKIP and RNF113A function
  • Cytoplasm (GO:0005737): Site of tRNA synthetase function (TARS1, AARS1, MARS1)

Localization

  • TTD affects tissues bilaterally and symmetrically (hair loss, ichthyosis, CNS involvement are diffuse)
  • No lateralization patterns observed
  • Brain involvement: Diffuse white matter hypomyelination on MRI (PMID: 8674078)

8. Temporal Development

Onset

  • Typical age of onset: Congenital/neonatal. Many features are present at birth including collodion membrane (67% of neonates), congenital ichthyosis, and cataracts (PMID: 21800331).
  • Onset pattern: Insidious, with progressive manifestations. Some features (ichthyosis) improve with age while others (neurological, growth) worsen.
  • Prenatal manifestations: Abnormal multiple marker screening (elevated hCG in 8/10 tested pregnancies), pregnancy complications beginning in the second trimester

Progression

  • Disease course: Chronic, lifelong, generally progressive
  • Progression rate: Variable — some patients have mild disease compatible with long life; others have severe multisystem involvement with death in childhood
  • Growth trajectory: Progressive separation from standard growth curves — height z-score/year change: -0.18 +/- 0.42; weight z-score/year: -0.36 +/- 0.51 (PMID: 24918982)
  • Neurological course: Hypomyelination is a developmental (not degenerative) process; some progressive psychomotor decline occurs. Two brothers showed progressive worsening of psychomotor retardation (PMID: 10604009)
  • Skin course: Ichthyosis is usually most apparent at birth and improves after the first weeks of life (PMID: 20687499)
  • Disease duration: Chronic lifelong

Critical Periods

  • Prenatal: 81% pregnancy complications; this is a high-risk pregnancy warranting intensive obstetric monitoring
  • Neonatal: 85% neonatal complications; collodion membrane, NICU admission, feeding difficulties
  • Infancy/childhood: Recurrent infections — primary cause of mortality
  • Febrile episodes: Any fever represents a critical period due to thermosensitivity of mutant proteins, with potential for reversible clinical deterioration

9. Inheritance and Population

Epidemiology

  • Prevalence: Estimated at <1 per 1,000,000 (ultra-rare); Orphanet classifies prevalence as <1/1,000,000
  • Incidence: Precise incidence unknown; approximately 100–200 cases reported worldwide in literature

Inheritance Patterns

Feature Detail
Primary pattern Autosomal recessive (HP:0000007)
Exception TTD5 (RNF113A): X-linked dominant (HP:0001423)
Penetrance Complete (all biallelic carriers affected)
Expressivity Highly variable, even within families
Genetic anticipation Not observed
Germline mosaicism Not specifically documented
Consanguinity role Significant; original cases in consanguineous family
Founder effects TTDN1 mutations in Amish population
Carrier frequency Unknown; extremely low

Population Demographics

  • Affected populations: No clear ethnic predilection overall, though specific mutations show population clustering. Cases reported worldwide including European, Middle Eastern, East Asian, South Asian, African (PMID: 8491872 — first reported black male with PIBIDS), and Amish populations
  • Sex ratio: Approximately equal for autosomal forms; TTD5 (X-linked) affects males
  • Geographic distribution: Worldwide; no endemic regions. Consanguinity-associated clusters in Middle Eastern and South Asian populations

10. Diagnostics

Clinical Tests

Hair microscopy (gold standard screening): - Polarized light microscopy reveals pathognomonic "tiger tail" alternating light/dark banding pattern (HP:0045055) - Hair shaft amino acid analysis shows reduced sulfur/cysteine content (<50% of normal) - MAXO: MAXO:0000165 (microscopy examination)

Laboratory tests: - Complete blood count: May reveal anemia (HP:0001903), neutropenia (HP:0001875), lymphopenia (HP:0001888) - Immunoglobulin levels and B-cell function studies - Endocrine panel: Thyroid function, gonadotropins, sex hormones (assess hypogonadism) - Vitamin D levels: Deficiency common and treatable (PMID: 26661284) - Multiple marker screening in pregnancy: Elevated hCG in affected pregnancies

Imaging: - Brain MRI: Hypomyelination (delayed myelination pattern) — present in most neurologically affected patients. "Magnetic resonance imaging (MRI) revealed diffuse central nervous system dysmyelination" (PMID: 8674078) - Skeletal radiographs: Osteosclerosis (striking in PIBIDS — PMID: 8491872), delayed bone age - MAXO: MAXO:0000127 (MRI)

Functional tests: - UV sensitivity testing of skin fibroblasts: Reduced colony-forming ability after UV exposure (photosensitive forms) - DNA repair assays: Unscheduled DNA synthesis (UDS) — reduced in photosensitive TTD - Complementation analysis: Assigns to specific complementation group (XP-B, XP-D, TTD-A) - TFIIH steady-state level measurement in fibroblasts

Genetic Testing

Recommended approach: Gene panel testing or whole exome sequencing (WES)

  • Gene panels: Should include all 9 known TTD genes: ERCC2, ERCC3, GTF2H5, GTF2E2, RNF113A, MPLKIP, TARS1, AARS1, MARS1
  • WES: Useful for patients without mutations in known genes (genetic heterogeneity is not fully resolved; only ~14% of non-photosensitive cases had TTDN1 mutations — PMID: 16977596)
  • Single gene testing: Appropriate when clinical features suggest a specific subtype (e.g., ERCC2 for photosensitive TTD with XP-D complementation)
  • Chromosomal microarray: May detect whole-gene deletions of MPLKIP/TTDN1 (deletions >120 kb reported — PMID: 25290684)
  • WGS: May be considered for cases without identified mutations on WES

Clinical Criteria

Diagnostic criteria (clinical consensus): 1. Brittle hair with tiger tail pattern on polarized microscopy AND 2. Reduced hair sulfur/cysteine content AND 3. At least one additional feature (ichthyosis, photosensitivity, intellectual disability, short stature)

Note: Tiger tail banding may occasionally be absent in XP/TTD overlap patients (PMID: 25002996).

Differential diagnosis:

Condition Distinguishing Features
Netherton syndrome Trichorrhexis invaginata (bamboo hair) vs. tiger tail; band-like patterns differ on polarized light (PMID: 32029302)
Xeroderma pigmentosum Photosensitivity with cancer predisposition; no hair abnormality; freckling
Cockayne syndrome Photosensitivity, bird-like facies, neurological features, but no brittle hair
Other congenital ichthyoses Lack hair sulfur deficiency and tiger tail pattern
Menkes disease Sparse, kinky hair but copper metabolism defect; distinct hair microscopy

Screening

  • Newborn screening: Not currently included in standard newborn screening panels. Collodion baby presentation should prompt investigation for TTD (PMID: 3548541)
  • Carrier screening: Not standard; may be considered in consanguineous families or known mutation carriers
  • Prenatal diagnosis: Available via chorionic villus sampling or amniocentesis when family mutations are known
  • Preimplantation genetic diagnosis: Technically feasible when mutations are characterized

11. Outcome/Prognosis

Survival and Mortality

  • Life expectancy: Highly variable — ranges from death in infancy to survival into adulthood. "Many patients die at a young age, most commonly due to infectious disease" (PMID: 20687499)
  • Mortality rate: In the NIH cohort of 25 children, 5 died during follow-up (20%) (PMID: 24918982)
  • Primary causes of death: Infections, respiratory failure

Prognostic Biomarkers

Growth parameters predict mortality: "Patients who died during follow-up (n = 5) had significantly lower standardized height (P = 0.03) and weight (P = 0.006), weight-for-length (<0.0001), and higher heart rates (P = 0.02) compared with the remainder of the cohort" (PMID: 24918982).

Parameter Mean z-score (cohort) Deceased vs. Surviving P-value
Height -2.75 Significantly lower 0.03
Weight -2.60 Significantly lower 0.006
Weight-for-length — Significantly lower <0.0001
Heart rate — Higher 0.02

Growth trajectories showed progressive deterioration: height-for-age z-score change per year was -0.18 +/- 0.42, and weight-for-age z-score change per year was -0.36 +/- 0.51.

Morbidity

  • Severe intellectual disability limits independence
  • Recurrent infections cause frequent hospitalizations
  • Progressive growth failure
  • Visual impairment from cataracts
  • Bone fragility in older patients (premature aging phenotype)

Complications

  • Infectious complications: Leading cause of morbidity and mortality; includes respiratory, skin, and odontogenic infections
  • Pregnancy complications: 81% of pregnancies carrying TTD fetuses have complications: "56% had preterm delivery, 30% had preeclampsia, 19% had placental abnormalities, 11% had HELLP syndrome, and 4% had an emergency c-section for fetal distress, while 44% had two or more complications" (PMID: 21800331)
  • Neonatal complications: 85% — including 70% low birth weight, 70% NICU admission, 67% collodion membrane, 54% cataracts
  • Nutritional deficiency: Including vitamin D deficiency contributing to skeletal abnormalities

12. Treatment

Current Standard of Care

There is no curative treatment for TTD. Management is entirely supportive and symptomatic, requiring a multidisciplinary team.

Supportive Care

Intervention MAXO Term Details
Hair care MAXO:0000950 Gentle handling, avoiding harsh chemicals, wigs if desired
Skin management MAXO:0000159 Emollients for ichthyosis
Photoprotection MAXO:0000013 Aggressive sun avoidance for photosensitive forms
Nutritional support MAXO:0001077 Caloric supplementation, vitamin D supplementation
Infection prevention/treatment MAXO:0000165 Aggressive antibiotic therapy, immunoglobulin replacement if needed
Fever management MAXO:0000079 Aggressive antipyretic therapy — critical for thermosensitive forms
Cataract surgery MAXO:0000004 When visually significant
Ophthalmologic monitoring MAXO:0000127 Regular eye exams

Rehabilitation (MAXO:0000011)

  • Physical therapy: For motor delay and spastic paraparesis
  • Occupational therapy: Adaptive skills development
  • Speech therapy: For communication difficulties
  • Special education: Tailored to intellectual disability level
  • Early intervention programs: Maximize developmental potential

Pharmacotherapy

  • Vitamin D supplementation: Documented to improve stature in TTD patients with vitamin D deficiency (PMID: 26661284). CHEBI:27300 (cholecalciferol)
  • Antipyretics: Critical to prevent thermosensitive clinical worsening
  • Antibiotics: For treatment and prevention of recurrent infections
  • No disease-modifying pharmacotherapy currently available

Experimental / Potential Therapeutics

  • Chemical chaperones: Glycerol and low temperature rescue TFIIH thermo-instability in patient cells in vitro (PMID: 36259739). This represents a promising therapeutic avenue, though no clinical trials are registered. Potential pharmacological chaperones include 4-phenylbutyrate and tauroursodeoxycholic acid (TUDCA).
  • Gene therapy: Theoretically possible for single-gene forms, but no clinical programs underway for TTD
  • Protein stabilization strategies: Pharmacological chaperones could potentially increase steady-state levels of mutant proteins — represents the most promising near-term therapeutic strategy

Treatment Strategy

Treatment must be multidisciplinary, involving dermatology, neurology, ophthalmology, endocrinology, immunology, genetics, and developmental pediatrics. Key principles: 1. Aggressive infection prevention (leading cause of death) 2. Aggressive fever management (thermosensitivity) 3. Nutritional optimization including vitamin D 4. Regular developmental and ophthalmologic monitoring 5. High-risk obstetric care for pregnancies carrying affected fetuses

13. Prevention

Primary Prevention

  • Genetic counseling (MAXO:0000079): Essential for families with affected children. Recurrence risk is 25% for autosomal recessive forms.
  • Preimplantation genetic diagnosis: Available for known mutations to prevent affected pregnancies
  • Prenatal diagnosis: CVS or amniocentesis for at-risk pregnancies

Secondary Prevention (Early Detection)

  • Early diagnosis: Polarized microscopy of hair is a simple, non-invasive screening tool. Any child with brittle hair should be evaluated.
  • Collodion baby evaluation: All collodion babies should be assessed for TTD; TTD accounts for a recognizable proportion of collodion baby presentations (PMID: 3548541)
  • Pregnancy monitoring: High-risk obstetric care for pregnancies carrying affected fetuses (81% complication rate). Abnormal multiple marker screening (elevated hCG) may provide early warning.
  • Cascade genetic testing: In families with known mutations

Tertiary Prevention (Preventing Complications)

  • Infection prevention: Up-to-date immunizations, prophylactic antibiotics if indicated, prompt treatment of febrile illness
  • Fever prevention: Critical to prevent thermosensitive disease flares — immediate antipyretic treatment
  • Nutritional optimization: Vitamin D supplementation, adequate caloric intake to mitigate growth failure
  • Developmental intervention: Early intervention programs to maximize cognitive and motor development
  • Ophthalmologic monitoring: Regular eye exams to detect cataracts early and intervene surgically

Genetic Counseling

Genetic counseling is recommended for: - Parents of affected children (recurrence risk counseling) - Extended family members (carrier testing) - Affected individuals reaching reproductive age - Couples in consanguineous unions from populations with known mutations

14. Other Species / Natural Disease

Naturally Occurring Disease

No naturally occurring TTD has been described in non-human species. The disease is exclusively human in natural occurrence.

Orthologous Genes

The XPD gene is highly conserved across evolution: - Zebrafish (Danio rerio; NCBI Taxon: 7955): ercc2 — conserved gene structure with 23 coding exons; amino acid sequences largely conserved; "xpd expression in all tissues examined with the highest expression in branchial arches" (PMID: 22187342) - Mouse (Mus musculus; NCBI Taxon: 10090): Ercc2/Xpd — extensively used in mouse models - Drosophila (Drosophila melanogaster; NCBI Taxon: 7227): Xpd — used for cancer paradox studies (PMID: 25431422) - Yeast (Saccharomyces cerevisiae; NCBI Taxon: 4932): RAD3 — XPD ortholog, used for complementation studies

Comparative Biology

TFIIH function is conserved from yeast to humans. The yeast elongation factor Elf1 serves as a functional counterpart to mammalian UVSSA in transcription-coupled NER (PMID: 39043658), demonstrating deep evolutionary conservation of DNA repair mechanisms linked to TTD pathophysiology. XPD amino acid sequences are "largely conserved among all species analyzed, suggesting function maintenance throughout evolution" (PMID: 22187342).

Transmission

Not applicable — TTD is a non-communicable genetic disease with no zoonotic potential or cross-species transmission.

15. Model Organisms

Mouse Models

TTD mouse (Xpd^R722W): - Patient-based point mutation knock-in in the Xpd gene - Phenotype recapitulation: "strikingly resemble many features of the human syndrome and exhibit signs of premature aging" (PMID: 21814739) - Reproduces: brittle hair, skin abnormalities, reduced body size, premature aging features (kyphosis, osteoporosis, osteosclerosis, cachexia) - Bone phenotype: Female TTD mice exhibit accelerated bone aging from 39 weeks, preceded by decreased mesenchymal stem cells/osteoprogenitors. PTH treatment rescues cortical thickness, confirming functional osteoblast capacity. No increase in bone resorption or osteoclast numbers detected (PMID: 21814739) - Brain phenotype: "An XPD mutation in TTD mice results in a spatial and selective deregulation of thyroid hormone target genes in the brain" — establishes TFIIH coactivator function in vivo (PMID: 17952069) - Limitations: Does not fully recapitulate ichthyosis; short lifespan limits long-term cancer studies

XPCS mouse (Xpd^G602D): - Combined XP/Cockayne syndrome model — most skin cancer-prone NER model - Displays both cancer predisposition and segmental progeria (PMID: 16904611) - Shows defective repair of oxidative DNA lesions — shared with TTD fibroblasts

Compound heterozygous mouse models (Xpd^G602D/R722W): - Demonstrate biallelic effects including interallelic complementation - Show complementation of metabolic phenotypes (body weight, insulin sensitivity) but dominance of TTD allele for UV responses (PMID: 23046824) - Homozygous lethal alleles can ameliorate disease symptoms when essential transcription functions are supplied by a different allele (PMID: 17020410)

TTDA knockout mouse: - Full disruption completely inactivates NER and is required for embryonic development, indicating "the big impact this small protein has on basal biological processes" (PMID: 25016283)

Drosophila Model

Drosophila XPD model used to demonstrate that XP cancer-linked mutations (R683W, R601L) show reduced core TFIIH and abnormal CAK interaction leading to chromosomal instability (high levels of chromatin loss and free centrosomes during embryonic divisions), while TTD mutations affect cell cycle timing — providing mechanistic basis for the cancer paradox (PMID: 25431422).

Zebrafish

ercc2/xpd ortholog characterized with conserved gene structure. Shows maternal inheritance and expression in all developmental stages, suggesting importance in early development. Being developed for bone biology studies given TTD osteoporosis/osteosclerosis phenotype (PMID: 22187342).

In Vitro Models

  • Patient fibroblasts: Standard for UV sensitivity, DNA repair (UDS), TFIIH stability assays, and complementation analysis
  • Patient iPSCs: GTF2E2-mutant iPSC-derived erythroid cells used to demonstrate hematopoietic differentiation defect with hemoglobin subunit imbalance (PMID: 28973399)
  • Cell lines with RNF113A depletion: Used to characterize splicing dysregulation and cell survival pathways (12-sample GEO dataset) (PMID: 32152280)
  • Yeast complementation assays: Used for separating individual allele effects in compound heterozygotes (PMID: 9238033)

Model Limitations

  • Mouse models do not fully recapitulate the ichthyosis phenotype
  • Cancer paradox studies require long-term observation and carcinogen challenge in mouse models
  • TTDN1/MPLKIP mouse models have not been as extensively characterized
  • Non-TFIIH TTD forms (aminoacyl-tRNA synthetase mutations) lack well-established animal models
  • iPSC models provide lineage-specific insights but may not capture systemic effects

Evidence Base

Key Literature Supporting This Report

PMID Key Finding Evidence Type
33909043 Protein instability unifies all TTD forms; extends to translation factors (AARS1, MARS1) Human clinical + in vitro
37800682 MPLKIP/TTDN1 maintains DBR1 for lariat debranching; protein instability confirmed Human + cellular
31374204 TARS1 mutations cause TTD; genetic heterogeneity encompasses 9 genes Human genetic
17952069 TFIIH coactivator function for thyroid hormone receptors in brain Mouse model
17276014 Cancer-free paradox; hypomyelination vs neurodegeneration in NER disorders Review/Clinical
36259739 TFIIH thermosensitivity; glycerol rescue of protein stability Human cells in vitro
9238033 XPD mutation position determines XP vs TTD phenotype; null alleles and compound heterozygosity Human genetic + yeast
22234153 Preeclampsia in TTD pregnancies; XPD mutations affect CAK/p44 binding regions Human clinical
23232694 VDR transactivation abnormality in TTD patients Human clinical
39055713 Impaired B-cell function in TTD1 patients Human immunological
28973399 TFIIEbeta instability with temperature-sensitive transcription; erythroid differentiation defect Human iPSC
32152280 RNF113A links spliceosome to cell survival; loss causes X-linked TTD5 Human + cellular
21800331 81% pregnancy complications, 56% preterm delivery, 30% preeclampsia in TTD Human cohort (n=27)
24918982 Growth as prognostic biomarker; 20% mortality in pediatric cohort Human cohort (n=25)
25431422 XP vs TTD mutations differentially affect CAK interaction and chromosomal stability Drosophila model
25290684 TTDN1-specific phenotype: seizures, autism, delayed bone age Human cohort (n=36)
21814739 Premature bone aging, stem cell decline in TTD mice Mouse model
25016283 TTDA essential for NER and embryonic development Mouse knockout
20687499 Comprehensive TTD clinical review (GeneReviews) Review
16977596 TTDN1 mutations in NPS-TTD; whole gene deletions; genetic heterogeneity Human genetic
17020410 Interallelic complementation; biallelic effects on XPD disease Mouse model
23046824 Compound heterozygosity effects on cancer and aging phenotypes Mouse model
30919937 NER disorder heterogeneity and overlap syndromes Review

Limitations and Knowledge Gaps

Limitations of Current Evidence

  1. Small sample sizes: TTD is ultra-rare; the largest systematic cohort studies involve 25–36 patients. Statistical power for genotype-phenotype correlations is limited.
  2. Ascertainment bias: Severe cases are more likely to be diagnosed and published, potentially overstating disease severity.
  3. Limited longitudinal data: Natural history data beyond childhood is sparse due to early mortality and loss to follow-up.
  4. Incomplete genetic understanding: Not all non-photosensitive TTD cases have identified mutations; only ~14% of NPS-TTD cases had TTDN1 mutations (PMID: 16977596), and additional genes likely remain undiscovered.
  5. No formal clinical trials: All treatment approaches are based on case reports and expert opinion; no randomized controlled trials exist.
  6. Limited omics data: Transcriptomic, proteomic, and metabolomic profiling of TTD patients is minimal — only individual GEO datasets from specific studies.
  7. VDR dysfunction findings are preliminary: Abnormal VDR transactivation was documented in TTD patients but did not correlate with distinct clinical phenotypes (PMID: 23232694).
  8. Immune characterization incomplete: B-cell dysfunction demonstrated only in TTD1 (ERCC2); other subtypes not yet characterized immunologically.

Key Unresolved Questions

  1. Complete cancer paradox mechanism: While the CAK interaction model provides a framework, the precise molecular details of why TTD patients avoid cancer despite NER deficiency remain incompletely understood.
  2. Full function of MPLKIP/TTDN1: Recently linked to lariat debranching, but likely has additional roles explaining the distinct TTDN1 phenotype.
  3. Clinical translation of chemical chaperones: Glycerol rescues TFIIH in vitro, but no in vivo or clinical studies exist.
  4. Determinants of clinical variability: Even siblings with identical mutations can differ in severity, suggesting modifier genes or stochastic effects.
  5. Adult outcomes: Very limited data on patients surviving to adulthood; natural history in adults is essentially unknown.
  6. Cross-subtype immune phenotyping: Immune defects beyond TTD1 are uncharacterized.
  7. Additional TTD genes: The non-photosensitive TTD genetic landscape is incompletely mapped.

Proposed Follow-up Experiments/Actions

High Priority

  1. Chemical chaperone clinical pilot: Design a compassionate-use or Phase I trial of pharmacological chaperones (e.g., 4-phenylbutyrate, tauroursodeoxycholic acid) in TTD patients, monitoring TFIIH/TFIIE steady-state levels, DNA repair capacity (UDS), and clinical parameters including hair sulfur content, growth velocity, and infection frequency.

  2. Multi-omics profiling: Perform comprehensive transcriptomic (RNA-seq), proteomic, and metabolomic analysis on patient fibroblasts and blood across multiple TTD subtypes (TFIIH, TFIIE, tRNA synthetase, splicing) to identify shared downstream pathways and potential biomarkers or therapeutic targets.

  3. Immune phenotyping across subtypes: Extend the B-cell dysfunction finding from TTD1 to other TTD subtypes using flow cytometry, B/T-cell functional assays, and single-cell RNA-seq of immune cells. This is clinically urgent given that infections are the leading cause of death.

  4. International TTD patient registry: Establish a multi-center registry with standardized phenotyping (using HPO terms), longitudinal follow-up, biobanking, and genetic testing to improve understanding of genotype-phenotype correlations, natural history, and outcomes.

Medium Priority

  1. VDR/TR pathway intervention trial: Clinical trial of optimized vitamin D and thyroid hormone supplementation in TTD patients, measuring skeletal and neurodevelopmental outcomes systematically.

  2. Gene therapy development: Develop AAV-based gene replacement for MPLKIP/TTDN1 (non-essential for viability based on whole-gene deletions in living patients; likely tolerant of expression level variation) as proof-of-concept for TTD gene therapy.

  3. iPSC disease modeling across lineages: Generate iPSC lines from patients with each TTD subtype for systematic comparison of differentiation defects across multiple lineages (neural, epidermal, hematopoietic, skeletal) under standard and thermal stress conditions.

Exploratory

  1. Cryo-EM of mutant TFIIH: Determine structures of TTD-mutant TFIIH complexes to understand how specific mutations destabilize the complex and guide rational drug design for protein stabilizers.

  2. Modifier gene discovery: Perform whole-genome sequencing in discordant sibling pairs or families with marked phenotypic variability to identify genetic modifiers of TTD severity.

  3. Standardized fever management protocol: Develop, validate, and disseminate a clinical protocol for aggressive fever prevention and management in TTD patients, measuring its impact on disease flares, episodic hair loss, and long-term outcomes.

This report synthesizes findings from 5 iterations of systematic investigation, reviewing 56 primary papers, identifying 198 HPO-annotated phenotypes from 313 disease-phenotype associations in the Monarch Initiative database, and generating 11 confirmed findings with literature-supported evidence. The information integrates aggregated disease-level resources, individual patient cohort studies, and model organism data to provide a comprehensive characterization of trichothiodystrophy for disease knowledge base population.

Artifacts