THOC2-Related Intellectual Disability: Disease Characteristics Report

Executive summary

THOC2-related intellectual disability is an ultra-rare, predominantly X-linked neurodevelopmental disorder caused by pathogenic or likely pathogenic germline variants in THOC2, which encodes the largest subunit of the nuclear TREX transcription–mRNA-export complex. Intellectual disability and speech/language impairment are central; hypotonia, gait or cerebellar-type motor abnormalities, seizures, behavioral abnormalities, short stature, microcephaly, low birth weight, and later truncal obesity occur variably. The best-supported mechanism is partial loss of THOC2/TREX function. A major 2024 study refined this to a causal chain of abnormal R-loop homeostasis, DNA damage, cell-cycle disruption and apoptosis during neurodevelopment, followed by neuronal migration, axonal, synaptic and network dysfunction. Evidence remains limited to small cohorts, patient cells and one hypomorphic mouse model; prevalence, longitudinal prognosis, validated biomarkers and disease-modifying treatments are unknown. (kumar2015thoc2mutationsimplicate pages 1-3, kumar2018severeneurocognitiveand pages 1-3, bhattacharjee2024compromisedtranscriptionmrnaexport pages 1-2, bhattacharjee2024compromisedtranscriptionmrnaexport pages 13-14)

The following table summarizes the most actionable evidence and its limitations.

Table: This table compacts the strongest retrieved evidence on THOC2-related intellectual disability, including inheritance, core phenotypes, variant classes, 2024 mechanism data, diagnostics, and major gaps. It is designed for rapid knowledge-base curation while clearly separating established findings from limited or absent evidence.

Evidence base and source provenance

The principal human evidence comprises a 2015 American Journal of Human Genetics study of four multigenerational families with 20 affected individuals and a 2018 Human Mutation expansion containing additional de novo and inherited cases. The principal recent advance is the February 2024 Nature Communications study using a hypomorphic mouse, neural stem cells, primary neurons and fibroblasts from an affected person. Thus, most clinical information is aggregated disease-level information abstracted from research cohorts, pedigrees and case reports—not EHR-derived population data. (kumar2015thoc2mutationsimplicate pages 1-3, kumar2018severeneurocognitiveand pages 1-3, bhattacharjee2024compromisedtranscriptionmrnaexport pages 1-2)

Key publications are:

• Kumar et al., “THOC2 Mutations Implicate mRNA-Export Pathway in X-Linked Intellectual Disability,” published August 2015, DOI: https://doi.org/10.1016/j.ajhg.2015.05.021; PMID: 26166475. (kumar2015thoc2mutationsimplicate pages 1-3)
• Kumar et al., “Severe neurocognitive and growth disorders due to variation in THOC2, an essential component of nuclear mRNA export machinery,” published June 2018, DOI: https://doi.org/10.1002/humu.23557; PMID should be verified against PubMed before database ingestion. (kumar2018severeneurocognitiveand pages 5-6)
• Bhattacharjee et al., “Compromised transcription-mRNA export factor THOC2 causes R-loop accumulation, DNA damage and adverse neurodevelopment,” published February 2024, DOI: https://doi.org/10.1038/s41467-024-45121-5; PubMed record: https://pubmed.ncbi.nlm.nih.gov/?term=10.1038%2Fs41467-024-45121-5. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 1-2)

Representative exact abstract statements include: “We implicated the X-chromosome THOC2 gene, which encodes the largest subunit of the highly-conserved TREX (Transcription-Export) complex, in a clinically complex neurodevelopmental disorder with intellectual disability as the core phenotype” and “Overall, we suggest that perturbed R-loop homeostasis… and DNA damage-associated functional alterations are at the root of THOC2 syndrome.” (bhattacharjee2024compromisedtranscriptionmrnaexport pages 1-2)

1. Disease information

Definition and nomenclature

The condition is a Mendelian neurodevelopmental syndrome in which damaging THOC2 variants compromise an essential RNA-processing/export factor. Preferred practical label: THOC2-related neurodevelopmental disorder or THOC2-related intellectual disability. Literature alternatives include THOC2-associated intellectual disability, THOC2 syndrome, X-linked intellectual disability due to THOC2, and historically X-linked intellectual disability 12/MRX12 for one linked family. Because phenotypes extend beyond cognition, “THOC2-related neurodevelopmental disorder” is the broadest label. (kumar2015thoc2mutationsimplicate pages 1-3, kumar2018severeneurocognitiveand pages 3-5, bhattacharjee2024compromisedtranscriptionmrnaexport pages 1-2)

Identifiers

• Gene: THOC2; the retrieved literature establishes X-chromosomal localization but did not independently validate HGNC/NCBI identifiers.
• OMIM: MRX12 is the historical family designation. The exact current OMIM disease and gene numbers should be verified directly in OMIM before knowledge-base release; they were not returned by the available evidence tools.
• MONDO: no disease-specific MONDO identifier was recoverable in the searches; Open Targets also returned no matching disease entity. Do not assign an unverified MONDO ID.
• Orphanet: no disease-specific ORPHA identifier was established.
• ICD-10/ICD-11 and MeSH: there is no retrieved evidence of a THOC2-specific code. Use broader intellectual-developmental-disorder and genetic-syndrome coding as locally appropriate, without implying molecular specificity.

2. Etiology, risk, protection and gene–environment interaction

The primary cause is a germline pathogenic or likely pathogenic THOC2 variant, usually hemizygous in a male. Both maternally inherited X-linked variants and de novo variants occur; an affected female with a de novo missense variant demonstrates that disease is not male-exclusive. (kumar2018severeneurocognitiveand pages 1-3, kumar2018severeneurocognitiveand pages 6-8)

Established genetic risk factors are damaging missense substitutions and splice-altering variants that destabilize THOC2, reduce protein abundance or produce C-terminal truncation. Skewed X-chromosome inactivation appears to protect many heterozygous females: clinically unaffected mothers had reported skewing of approximately 94%, 98:2% or 99.9:0.1%. This is a plausible protective modifier rather than a quantified guarantee of nonpenetrance. (kumar2018severeneurocognitiveand pages 6-8, kumar2018severeneurocognitiveand pages 26-27, kumar2018severeneurocognitiveand pages 8-10)

No environmental, infectious, dietary, occupational or lifestyle cause has been demonstrated. No protective diet, exposure, medication or genetic modifier other than the observed association with favorable X-inactivation has been validated. There are no established gene–environment interactions. Family history raises prior probability in inherited families but is not required because de novo disease occurs. (kumar2018severeneurocognitiveand pages 1-3, kumar2018severeneurocognitiveand pages 6-8)

3. Phenotypes

Core clinical spectrum

In the 2015 families, all affected males had intellectual disability ranging from borderline to severe. Frequently reported associated findings were speech delay, short stature, elevated BMI or adult-onset truncal obesity, seizures, tremor and gait disturbance. Truncal obesity was especially noted among older males in two of four families, suggesting an age-related feature rather than a universal congenital manifestation. (kumar2015thoc2mutationsimplicate pages 1-3, kumar2018severeneurocognitiveand pages 3-5)

Among seven established individuals characterized in the 2018 series, all had at least moderate intellectual disability; 3/7 were non-verbal, 2/7 non-ambulatory, 4/7 had infantile hypotonia, 2/7 tremor, 4/7 behavioral problems, 1/7 autism spectrum disorder, 1/7 confirmed seizures and 1/7 suspected seizures. Low birth weight occurred in 3/7, microcephaly in 2/7 and short stature in 2/7. These are referral-cohort frequencies, not population estimates. (kumar2018severeneurocognitiveand pages 5-6)

Suggested HPO annotations include:

• Intellectual disability—HP:0001249; global developmental delay—HP:0001263.
• Speech delay—HP:0000750; absent speech—HP:0001344.
• Infantile hypotonia—HP:0008947 or generalized hypotonia—HP:0001290.
• Abnormal gait—HP:0001288; ataxic gait—HP:0002066; tremor—HP:0001337; nystagmus—HP:0000639.
• Seizure—HP:0001250; epileptic encephalopathy—HP:0200134.
• Autism—HP:0000717; behavioral abnormality—HP:0000708; hyperactivity—HP:0000752.
• Microcephaly—HP:0000252; short stature—HP:0004322; low birth weight—HP:0001518; truncal obesity—HP:0001956.
• Non-ambulation may be represented by inability to walk—HP:0002540.

Imaging and other manifestations

In 2018, MRI was normal in 3/5 examined individuals. Abnormal findings in 2/5 included cortical gyral abnormalities, corpus-callosum hypoplasia, reduced brainstem volume, ventricular dilation, delayed myelination and periventricular white-matter lesions. Earlier reports described mild ventriculomegaly, gliosis, inferior cerebellar-vermis dysplasia and cervical-cord compression in selected individuals. There is therefore no established pathognomonic imaging signature. (kumar2018severeneurocognitiveand pages 5-6, kumar2015thoc2mutationsimplicate pages 1-3)

Suggested HPO terms include hypoplasia of the corpus callosum (HP:0002079), ventriculomegaly (HP:0002119), delayed CNS myelination (HP:0002188) and abnormal cerebral white matter morphology (HP:0002500). Cerebellar-type signs may occur despite a structurally unremarkable cerebellum. (kumar2018severeneurocognitiveand pages 8-10)

Onset, course and quality of life

Onset is developmental and generally evident in infancy or childhood through hypotonia, delayed milestones, speech delay or cognitive impairment. Available cohorts do not define formal stages, annual progression or remission. Intellectual and adaptive impairments appear chronic and lifelong; adult truncal obesity may emerge later. Non-verbal and non-ambulatory status in some children indicates substantial effects on communication, mobility, education, caregiving needs and independence. No THOC2-specific EQ-5D, SF-36, PROMIS or caregiver-burden study was found. (kumar2018severeneurocognitiveand pages 5-6, kumar2018severeneurocognitiveand pages 3-5)

4. Genetic and molecular information

Causal gene and variant spectrum

THOC2 is the established causal gene. The 2015 study identified four segregating missense variants: c.937C>T (p.Leu313Phe), c.1313T>C (p.Leu438Pro), c.2399T>C (p.Ile800Thr), and c.3034T>C (p.Ser1012Pro). They affected conserved residues, were absent from more than 60,000 reference individuals in the then-current 1000 Genomes/ExAC datasets and produced a combined pedigree LOD score of 8.1. Two destabilized THOC2 and TREX partners. (kumar2015thoc2mutationsimplicate pages 1-3)

The 2018 expansion added established missense changes including p.Tyr517Cys, p.Thr696Ile, p.Gly713Asp and p.His1187Tyr, plus splice variants including c.4450-2A>G and c.3503+4A>C, which generated abnormal C-terminal products. p.Tyr517Cys shortened measured protein turnover from approximately eight hours for wild type to three hours. These variants were assessed as pathogenic or likely pathogenic using segregation, rarity, ACMG criteria and functional evidence. (kumar2018severeneurocognitiveand pages 6-8, kumar2018severeneurocognitiveand pages 5-6, kumar2018severeneurocognitiveand pages 8-10)

Reported VUS, which must not be represented as confirmed causes, include p.Arg77Cys, p.Ser1108Leu, p.Arg1121Gly and p.Asn1261His. Their rarity, conservation and computational predictions were insufficient without stronger segregation or functional evidence. (kumar2018severeneurocognitiveand pages 10-11)

All reported disease variants are germline. No somatic THOC2 mechanism is implicated in this syndrome. The dominant molecular theme is partial loss of function or hypomorphism; complete loss is expected to be poorly tolerated because THOC2 is essential. Some truncated products may exert additional dominant-negative effects, but this is variant-specific and not a universal disease mechanism. (kumar2018severeneurocognitiveand pages 10-11, kumar2018severeneurocognitiveand pages 8-10)

No validated modifier gene, disease-specific methylation episignature, repeat expansion, aneuploidy or recurrent large chromosomal rearrangement was found. Historical reference-database absences should be rechecked in current gnomAD before clinical classification.

5. Environmental information

No toxin, radiation, pollution, occupation, smoking, alcohol, diet or exercise exposure is known to cause or materially modify THOC2-related intellectual disability. No bacterial, viral, fungal or parasitic trigger is implicated. Ordinary environmental and educational context may affect functional attainment, as in other developmental disabilities, but this is not a demonstrated molecular gene–environment interaction.

6. Mechanism and pathophysiology

Upstream molecular defect

THOC2 is the largest subunit of the conserved nuclear TREX complex, which links transcription, mRNA processing and export while helping preserve genome stability. Earlier cellular studies supported variant-dependent THOC2/TREX destabilization and disturbed RNA export. The 2024 work showed that a hypomorphic exon 37–38 deletion can leave bulk mRNA export relatively intact while profoundly disturbing R-loop and genome homeostasis. (kumar2015thoc2mutationsimplicate pages 1-3, bhattacharjee2024compromisedtranscriptionmrnaexport pages 1-2, bhattacharjee2024compromisedtranscriptionmrnaexport pages 14-16)

Current causal model

The best-supported chain is:

Pathogenic THOC2 variant → compromised THOC2/TREX function → unresolved RNA:DNA hybrid R-loops → replication/transcription-associated DNA damage → G2/M checkpoint disturbance and neural-stem-cell apoptosis → reduced or premature neural progenitor differentiation and altered cortical development → impaired neuronal migration, axon growth, dendritic-spine/synapse maturation and network activity → intellectual, speech and motor phenotypes. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 6-9, bhattacharjee2024compromisedtranscriptionmrnaexport pages 10-11, bhattacharjee2024compromisedtranscriptionmrnaexport pages 13-14)

R-loop staining was RNase-H sensitive in mutant neural stem cells and patient fibroblasts. RNase H1 overexpression significantly reduced both R-loop burden and DNA damage (reported p<0.0001), providing experimental evidence that R-loops are upstream contributors rather than merely downstream markers. Mutant cells also showed elevated γ-H2AX, comet-assay damage, G2/M abnormalities and apoptosis. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 14-16, bhattacharjee2024compromisedtranscriptionmrnaexport pages 13-14)

Cellular, anatomical and multi-omic consequences

The 2024 model showed reduced PAX6-positive cortical ventricular-zone thickness, premature neural-stem-cell differentiation, reduced cortical plate and corpus-callosum thickness, impaired migration, 32% shorter primary axons, fewer mature dendritic spines, reduced SYN1–PSD95 synaptic puncta and abnormal electrophysiological network activity. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 6-9, bhattacharjee2024compromisedtranscriptionmrnaexport pages 10-11)

Embryonic day 18.5 showed the largest transcriptomic disruption, involving transcription, cell cycle, cell death and cognition-related genes. Dysregulated neurodevelopmental genes included SYNGAP1, HUWE1, SHANK3, DLG4, KDM5C and CTNND1. Proteomics identified 421 dysregulated proteins enriched for translation, peptide biosynthesis and mRNA-catabolic processes. No validated metabolomic, lipidomic, single-cell or spatial-transcriptomic signature has been reported. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 6-9, bhattacharjee2024compromisedtranscriptionmrnaexport pages 13-14)

Suggested ontology terms:

• GO biological process: mRNA export from nucleus (GO:0006406), RNA processing (GO:0006396), regulation of transcription by RNA polymerase II, DNA-damage response (GO:0006974), cell-cycle checkpoint signaling, apoptotic process (GO:0006915), neurogenesis (GO:0022008), neuron migration (GO:0001764), axon development (GO:0061564) and synapse organization (GO:0050808).
• GO cellular component: nucleus (GO:0005634), nuclear speck, TREX complex, neuronal projection, dendritic spine and synapse.
• Cell Ontology: neural stem cell (CL:0000047), radial glial cell (CL:0000681), neuron (CL:0000540), cortical neuron and hippocampal neuron where a more specific supported term is available.

Immune dysregulation, inflammation, metabolic disease and primary mitochondrial dysfunction are not established components.

7. Anatomical structures affected

The central nervous system is primary, particularly the developing cerebral cortex and hippocampal/cortical neuronal systems examined experimentally. The cortical ventricular zone, cortical plate, corpus callosum, axons, dendritic spines and synapses are implicated. Variable human MRI findings also involve cerebral white matter, brainstem, ventricles and occasionally cerebellar vermis. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 6-9, bhattacharjee2024compromisedtranscriptionmrnaexport pages 10-11, kumar2018severeneurocognitiveand pages 5-6)

Suggested UBERON annotations include brain (UBERON:0000955), cerebral cortex (UBERON:0000956), hippocampus (UBERON:0002421), corpus callosum (UBERON:0002336), brainstem (UBERON:0002298), cerebellum (UBERON:0002037) and spinal cord (UBERON:0002240) where directly supported. No consistent lateralization has been reported.

At the subcellular level, the nucleus is central because THOC2/TREX controls transcription-coupled RNA processing/export and R-loop homeostasis; downstream effects involve axonal, dendritic and synaptic compartments. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 14-16, bhattacharjee2024compromisedtranscriptionmrnaexport pages 10-11)

8. Temporal development

The disorder is congenital in genetic origin and pediatric in clinical recognition. Infantile hypotonia, low birth weight or microcephaly may be early findings; developmental, speech and cognitive abnormalities emerge as milestones are missed. Growth impairment can occur during childhood, while truncal obesity was particularly observed in older males. (kumar2018severeneurocognitiveand pages 5-6, kumar2018severeneurocognitiveand pages 3-5)

The 2024 model indicates a critical prenatal neurodevelopmental period: THOC2 was abundant at E14.5/E18.5, with marked transcriptomic disruption at E18.5, reduced progenitor-zone thickness and subsequent cortical/synaptic deficits. This supports early developmental vulnerability but does not establish a human therapeutic window. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 6-9, bhattacharjee2024compromisedtranscriptionmrnaexport pages 11-13)

No relapsing-remitting pattern, spontaneous remission or defined end stage is known. Longitudinal natural-history cohorts are absent.

9. Inheritance and population

Inheritance is X-linked. Hemizygous males are predominantly affected, whereas heterozygous mothers are commonly unaffected, plausibly because of highly skewed X-inactivation. De novo disease can affect either sex, and a severely affected female with de novo p.Tyr517Cys was reported. Expressivity in males ranges from borderline ID to severe non-verbal/non-ambulatory disease. Formal penetrance, germline-mosaicism frequency, anticipation, founder effects, carrier frequency and consanguinity effects are unknown. (kumar2015thoc2mutationsimplicate pages 1-3, kumar2018severeneurocognitiveand pages 8-10, kumar2018severeneurocognitiveand pages 6-8)

No incidence or prevalence estimate exists. The literature consists of a 2015 cohort of 20 affected people in four families and a small number of additional established cases in 2018. Cases arose from multiple countries, arguing against a known geographically restricted population, but the sample is too small for demographic conclusions. The observed male predominance reflects X-linked biology and ascertainment rather than a measured population sex ratio. (kumar2015thoc2mutationsimplicate pages 1-3, kumar2018severeneurocognitiveand pages 27-27)

10. Diagnostics

Clinical recognition and differential diagnosis

Suspect the disorder in a male with unexplained developmental delay/intellectual disability and speech impairment, especially when accompanied by hypotonia, abnormal gait, tremor/ataxia, seizures, short stature, microcephaly, truncal obesity or an X-linked pedigree. The phenotype is not sufficiently distinctive for clinical diagnosis alone. (kumar2015thoc2mutationsimplicate pages 3-4, kumar2015thoc2mutationsimplicate pages 1-3)

Important differential categories include other X-linked intellectual-developmental disorders; epileptic encephalopathies; cerebral-palsy-like genetic motor disorders; chromosomal copy-number disorders; metabolic causes of developmental delay; and other RNA-processing/TREX disorders, including THOC6-associated Beaulieu–Boycott–Innes syndrome. Distinction requires molecular testing.

Recommended testing strategy

1. Trio WES or WGS is the preferred broad approach for an undiagnosed neurodevelopmental disorder. Both were successful in published THOC2 cases. (kumar2018severeneurocognitiveand pages 5-6)
2. A comprehensive intellectual-disability/epilepsy panel that includes THOC2 is reasonable when sequencing depth and copy-number calling are adequate.
3. Confirm candidate variants and segregation by Sanger sequencing; establish de novo status with parental testing where possible. (kumar2018severeneurocognitiveand pages 6-8, kumar2018severeneurocognitiveand pages 5-6)
4. For intronic or splice-region variants, perform patient RNA/cDNA analysis, RT-PCR or RNA sequencing. Published studies used cDNA PCR, RT-qPCR, western blotting and protein-stability assays to establish consequences. (kumar2018severeneurocognitiveand pages 6-8, kumar2018severeneurocognitiveand pages 3-5)
5. Interpret missense variants conservatively under ACMG/AMP criteria. Rarity and computational prediction alone do not convert a VUS into a diagnosis. (kumar2018severeneurocognitiveand pages 10-11)
6. CMA remains useful for unexplained developmental disability or suspected CNV, but it will usually miss small THOC2 sequence variants. Karyotype/FISH, mitochondrial testing and repeat-expansion assays are not THOC2-specific first-line tests unless the broader phenotype indicates them.

There is no validated biochemical, circulating, proteomic or imaging biomarker. MRI, EEG, vision assessment and growth/endocrine testing are phenotype-directed rather than diagnostic. No newborn-screening assay or standardized disease-specific clinical criteria exist.

11. Outcome and prognosis

Published cases demonstrate survival through childhood and into adulthood, but no five- or ten-year survival estimates, life-expectancy analyses or disease-specific mortality rates are available. Complete THOC2 loss is predicted to be poorly tolerated, whereas observed hypomorphic variants are compatible with survival. (kumar2018severeneurocognitiveand pages 10-11)

Morbidity is principally lifelong neurodevelopmental disability. Prognosis is variable: some individuals have borderline or mild ID, while others are non-verbal, non-ambulatory or have epileptic encephalopathy and cortical visual impairment. Motor dysfunction, seizures, behavioral abnormalities and growth problems increase care burden. No validated molecular prognostic biomarker or genotype-based outcome calculator exists. (kumar2018severeneurocognitiveand pages 5-6, kumar2015thoc2mutationsimplicate pages 1-3)

12. Treatment and current applications

There is no approved THOC2-specific disease-modifying therapy, gene therapy, RNA therapy, cell therapy or targeted pharmacotherapy, and the clinical-trial search found no relevant disease-specific interventional trial. No response-rate or adverse-event series exists.

Current real-world management is multidisciplinary and phenotype-directed:

• early developmental intervention, special education and neuropsychological assessment;
• speech-language therapy, including augmentative and alternative communication for non-verbal individuals;
• physical and occupational therapy for hypotonia, gait, coordination and adaptive skills;
• standard antiseizure therapy guided by seizure type and EEG;
• behavioral, autism and psychiatric assessment and treatment;
• nutrition, weight and growth surveillance;
• ophthalmology for visual impairment and audiology as clinically indicated;
• brain/spine MRI, neurology or orthopedic evaluation when focal signs, gait deterioration or suspected structural disease are present;
• endocrine evaluation for poor growth. One reported boy with growth-hormone deficiency received replacement, but this is treatment of a documented comorbidity, not THOC2-directed therapy. (kumar2018severeneurocognitiveand pages 5-6, kumar2015thoc2mutationsimplicate pages 3-4)

Potential NCIt intervention concepts include Genetic Counseling, Physical Therapy, Occupational Therapy, Speech Therapy, Special Education, Anticonvulsant Therapy and Growth Hormone Replacement Therapy. Exact NCIt codes should be resolved against the current NCIt release.

Experimentally, RNase H1 rescue of R-loops provides target-validation evidence but is not a human therapy. Translation would require safe, cell- and developmentally appropriate correction of R-loop homeostasis without disrupting essential RNA biology. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 14-16)

13. Prevention

There is no lifestyle, vaccine, environmental or drug-based primary prevention. Primary genetic prevention options require identification of a familial pathogenic variant and nondirective counseling. These may include carrier testing, cascade testing, prenatal diagnosis and preimplantation genetic testing for monogenic disease. Because de novo variants occur, a negative family history does not eliminate risk. Residual recurrence risk from parental germline mosaicism should be discussed even when parental blood testing is negative, although its THOC2-specific frequency is unknown. (kumar2018severeneurocognitiveand pages 1-3, kumar2018severeneurocognitiveand pages 6-8)

Secondary prevention consists of prompt genomic diagnosis and early developmental, communication, seizure, vision, mobility and growth interventions. Tertiary prevention includes seizure control, contracture/fall prevention, weight management and support for communication and adaptive functioning. Population newborn or carrier screening is not currently established.

14. Other species and natural disease

No naturally occurring veterinary THOC2 syndrome, breed predisposition, zoonotic potential or cross-species transmission is established. The disorder is genetic and non-infectious. THOC2 orthologs are evolutionarily conserved, consistent with the essential role of TREX-mediated RNA biology. Ortholog-specific NCBI Gene and NCBI Taxonomy identifiers should be retrieved directly before database ingestion.

Relevant experimental species include Mus musculus (NCBI Taxonomy 10090), Danio rerio (7955) and Drosophila melanogaster (7227). Zebrafish Thoc2 is essential for embryonic development, and depletion in Drosophila S2 cells impairs mRNA export; these findings support conserved essentiality but do not constitute naturally occurring animal disease. (kumar2015thoc2mutationsimplicate pages 1-3, bhattacharjee2024compromisedtranscriptionmrnaexport pages 1-2)

15. Model organisms

Mouse

The strongest model is the hemizygous Thoc2Δ/Y hypomorphic mouse carrying an exon 37–38 deletion modeled on a human variant. It recapitulated smaller size/weight and deficits in spatial learning, working memory, fine motor control and sensorimotor function. Birth rate was reduced by approximately 33%. Morris water maze, Barnes maze, Y-maze, beam-walking and pasta-handling tests demonstrated cognitive and motor phenotypes; hyperactivity and reduced anxiety-like behavior were also reported. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 11-13, bhattacharjee2024compromisedtranscriptionmrnaexport pages 4-5)

The model reproduced developmental pathology—R-loop accumulation, DNA damage, neural-stem-cell apoptosis, reduced cortical structures and impaired neuronal maturation—and is suitable for studying R-loop rescue, developmental timing and synaptic consequences. Its chief limitation is that one hypomorphic deletion cannot represent every missense or splice variant, female X-inactivation, or the full human phenotypic range. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 6-9, bhattacharjee2024compromisedtranscriptionmrnaexport pages 10-11, bhattacharjee2024compromisedtranscriptionmrnaexport pages 13-14)

Cellular models

Patient dermal fibroblasts reproduced R-loop accumulation and comet-assay DNA damage. Mouse neural stem cells, neurospheres and primary cortical/hippocampal neurons modeled progenitor survival, differentiation, migration, axonal development, synapses and electrophysiology. HEK293T and patient-derived cells were used for protein localization, abundance and cycloheximide-chase assays. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 14-16, bhattacharjee2024compromisedtranscriptionmrnaexport pages 10-11, kumar2018severeneurocognitiveand pages 5-6)

Zebrafish and Drosophila

Zebrafish studies support embryonic essentiality, while Drosophila S2-cell depletion supports conserved THO/TREX-dependent mRNA export. These are useful for rapid functional testing but do not reproduce the complete human syndrome. (kumar2015thoc2mutationsimplicate pages 1-3, bhattacharjee2024compromisedtranscriptionmrnaexport pages 1-2)

No disease-specific rat, organoid, human iPSC-neuron, CRISPR-screen, single-cell or spatial-transcriptomic model was identified in the retrieved literature.

Knowledge gaps and expert assessment

The 2024 work materially changes current understanding: THOC2 disease should not be represented solely as a generic “mRNA-export defect.” At least for the modeled hypomorphic deletion, defective R-loop resolution and genome stability are central and experimentally rescuable upstream events, while overt bulk nuclear mRNA retention was not observed. The authors themselves identify nuclear/cytoplasmic RNA sequencing as a needed next step to determine whether selected transcripts nevertheless have export defects. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 14-16, bhattacharjee2024compromisedtranscriptionmrnaexport pages 4-5)

For knowledge-base curation, clinical frequencies should be labeled small-cohort observations, not population frequencies. VUS must remain separate from established variants. Likewise, mouse rescue data should be annotated as preclinical target-validation evidence rather than treatment evidence. Highest-priority research needs are an international natural-history registry, systematic female-carrier phenotyping and X-inactivation studies, current ClinVar/gnomAD aggregation, variant-specific functional assays, patient iPSC-derived neural models, transcript-compartment profiling, and development of safe R-loop/genome-stability biomarkers and interventions.

References

1. (kumar2015thoc2mutationsimplicate pages 1-3): Raman Kumar, Mark A. Corbett, Bregje W.M. van Bon, Joshua A. Woenig, Lloyd Weir, Evelyn Douglas, Kathryn L. Friend, Alison Gardner, Marie Shaw, Lachlan A. Jolly, Chuan Tan, Matthew F. Hunter, Anna Hackett, Michael Field, Elizabeth E. Palmer, Melanie Leffler, Carolyn Rogers, Jackie Boyle, Melanie Bienek, Corinna Jensen, Griet Van Buggenhout, Hilde Van Esch, Katrin Hoffmann, Martine Raynaud, Huiying Zhao, Robin Reed, Hao Hu, Stefan A. Haas, Eric Haan, Vera M. Kalscheuer, and Jozef Gecz. Thoc2 mutations implicate mrna-export pathway in x-linked intellectual disability. American journal of human genetics, 97 2:302-10, Aug 2015. URL: https://doi.org/10.1016/j.ajhg.2015.05.021, doi:10.1016/j.ajhg.2015.05.021. This article has 93 citations and is from a highest quality peer-reviewed journal.

2. (kumar2018severeneurocognitiveand pages 1-3): Raman Kumar, Alison Gardner, Claire C. Homan, Evelyn Douglas, Heather Mefford, Dagmar Wieczorek, Hermann-Josef Lüdecke, Zornitza Stark, Simon Sadedin, Catherine Bearce Nowak, Jessica Douglas, Gretchen Parsons, Paul Mark, Lourdes Loidi, Gail E. Herman, Theresa Mihalic Mosher, Meredith K. Gillespie, Lauren Brady, Mark Tarnopolsky, Irene Madrigal, Jesús Eiris, Laura Domènech Salgado, Raquel Rabionet, Tim M. Strom, Naoko Ishihara, Hidehito Inagaki, Hiroki Kurahashi, Tracy Dudding-Byth, Elizabeth E. Palmer, Michael Field, and Jozef Gecz. Severe neurocognitive and growth disorders due to variation in thoc2, an essential component of nuclear mrna export machinery. Human Mutation, 39:1126-1138, Jun 2018. URL: https://doi.org/10.1002/humu.23557, doi:10.1002/humu.23557. This article has 31 citations and is from a domain leading peer-reviewed journal.

3. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 1-2): Rudrarup Bhattacharjee, Lachlan A. Jolly, Mark A. Corbett, Ing Chee Wee, Sushma R. Rao, Alison E. Gardner, Tarin Ritchie, Eline J. H. van Hugte, Ummi Ciptasari, Sandra Piltz, Jacqueline E. Noll, Nazzmer Nazri, Clare L. van Eyk, Melissa White, Dani Fornarino, Cathryn Poulton, Gareth Baynam, Lyndsey E. Collins-Praino, Marten F. Snel, Nael Nadif Kasri, Kim M. Hemsley, Paul Q. Thomas, Raman Kumar, and Jozef Gecz. Compromised transcription-mrna export factor thoc2 causes r-loop accumulation, dna damage and adverse neurodevelopment. Nature Communications, Feb 2024. URL: https://doi.org/10.1038/s41467-024-45121-5, doi:10.1038/s41467-024-45121-5. This article has 16 citations and is from a highest quality peer-reviewed journal.

4. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 13-14): Rudrarup Bhattacharjee, Lachlan A. Jolly, Mark A. Corbett, Ing Chee Wee, Sushma R. Rao, Alison E. Gardner, Tarin Ritchie, Eline J. H. van Hugte, Ummi Ciptasari, Sandra Piltz, Jacqueline E. Noll, Nazzmer Nazri, Clare L. van Eyk, Melissa White, Dani Fornarino, Cathryn Poulton, Gareth Baynam, Lyndsey E. Collins-Praino, Marten F. Snel, Nael Nadif Kasri, Kim M. Hemsley, Paul Q. Thomas, Raman Kumar, and Jozef Gecz. Compromised transcription-mrna export factor thoc2 causes r-loop accumulation, dna damage and adverse neurodevelopment. Nature Communications, Feb 2024. URL: https://doi.org/10.1038/s41467-024-45121-5, doi:10.1038/s41467-024-45121-5. This article has 16 citations and is from a highest quality peer-reviewed journal.

5. (kumar2018severeneurocognitiveand pages 3-5): Raman Kumar, Alison Gardner, Claire C. Homan, Evelyn Douglas, Heather Mefford, Dagmar Wieczorek, Hermann-Josef Lüdecke, Zornitza Stark, Simon Sadedin, Catherine Bearce Nowak, Jessica Douglas, Gretchen Parsons, Paul Mark, Lourdes Loidi, Gail E. Herman, Theresa Mihalic Mosher, Meredith K. Gillespie, Lauren Brady, Mark Tarnopolsky, Irene Madrigal, Jesús Eiris, Laura Domènech Salgado, Raquel Rabionet, Tim M. Strom, Naoko Ishihara, Hidehito Inagaki, Hiroki Kurahashi, Tracy Dudding-Byth, Elizabeth E. Palmer, Michael Field, and Jozef Gecz. Severe neurocognitive and growth disorders due to variation in thoc2, an essential component of nuclear mrna export machinery. Human Mutation, 39:1126-1138, Jun 2018. URL: https://doi.org/10.1002/humu.23557, doi:10.1002/humu.23557. This article has 31 citations and is from a domain leading peer-reviewed journal.

6. (kumar2018severeneurocognitiveand pages 6-8): Raman Kumar, Alison Gardner, Claire C. Homan, Evelyn Douglas, Heather Mefford, Dagmar Wieczorek, Hermann-Josef Lüdecke, Zornitza Stark, Simon Sadedin, Catherine Bearce Nowak, Jessica Douglas, Gretchen Parsons, Paul Mark, Lourdes Loidi, Gail E. Herman, Theresa Mihalic Mosher, Meredith K. Gillespie, Lauren Brady, Mark Tarnopolsky, Irene Madrigal, Jesús Eiris, Laura Domènech Salgado, Raquel Rabionet, Tim M. Strom, Naoko Ishihara, Hidehito Inagaki, Hiroki Kurahashi, Tracy Dudding-Byth, Elizabeth E. Palmer, Michael Field, and Jozef Gecz. Severe neurocognitive and growth disorders due to variation in thoc2, an essential component of nuclear mrna export machinery. Human Mutation, 39:1126-1138, Jun 2018. URL: https://doi.org/10.1002/humu.23557, doi:10.1002/humu.23557. This article has 31 citations and is from a domain leading peer-reviewed journal.

7. (kumar2018severeneurocognitiveand pages 8-10): Raman Kumar, Alison Gardner, Claire C. Homan, Evelyn Douglas, Heather Mefford, Dagmar Wieczorek, Hermann-Josef Lüdecke, Zornitza Stark, Simon Sadedin, Catherine Bearce Nowak, Jessica Douglas, Gretchen Parsons, Paul Mark, Lourdes Loidi, Gail E. Herman, Theresa Mihalic Mosher, Meredith K. Gillespie, Lauren Brady, Mark Tarnopolsky, Irene Madrigal, Jesús Eiris, Laura Domènech Salgado, Raquel Rabionet, Tim M. Strom, Naoko Ishihara, Hidehito Inagaki, Hiroki Kurahashi, Tracy Dudding-Byth, Elizabeth E. Palmer, Michael Field, and Jozef Gecz. Severe neurocognitive and growth disorders due to variation in thoc2, an essential component of nuclear mrna export machinery. Human Mutation, 39:1126-1138, Jun 2018. URL: https://doi.org/10.1002/humu.23557, doi:10.1002/humu.23557. This article has 31 citations and is from a domain leading peer-reviewed journal.

8. (kumar2018severeneurocognitiveand pages 26-27): Raman Kumar, Alison Gardner, Claire C. Homan, Evelyn Douglas, Heather Mefford, Dagmar Wieczorek, Hermann-Josef Lüdecke, Zornitza Stark, Simon Sadedin, Catherine Bearce Nowak, Jessica Douglas, Gretchen Parsons, Paul Mark, Lourdes Loidi, Gail E. Herman, Theresa Mihalic Mosher, Meredith K. Gillespie, Lauren Brady, Mark Tarnopolsky, Irene Madrigal, Jesús Eiris, Laura Domènech Salgado, Raquel Rabionet, Tim M. Strom, Naoko Ishihara, Hidehito Inagaki, Hiroki Kurahashi, Tracy Dudding-Byth, Elizabeth E. Palmer, Michael Field, and Jozef Gecz. Severe neurocognitive and growth disorders due to variation in thoc2, an essential component of nuclear mrna export machinery. Human Mutation, 39:1126-1138, Jun 2018. URL: https://doi.org/10.1002/humu.23557, doi:10.1002/humu.23557. This article has 31 citations and is from a domain leading peer-reviewed journal.

9. (kumar2018severeneurocognitiveand pages 5-6): Raman Kumar, Alison Gardner, Claire C. Homan, Evelyn Douglas, Heather Mefford, Dagmar Wieczorek, Hermann-Josef Lüdecke, Zornitza Stark, Simon Sadedin, Catherine Bearce Nowak, Jessica Douglas, Gretchen Parsons, Paul Mark, Lourdes Loidi, Gail E. Herman, Theresa Mihalic Mosher, Meredith K. Gillespie, Lauren Brady, Mark Tarnopolsky, Irene Madrigal, Jesús Eiris, Laura Domènech Salgado, Raquel Rabionet, Tim M. Strom, Naoko Ishihara, Hidehito Inagaki, Hiroki Kurahashi, Tracy Dudding-Byth, Elizabeth E. Palmer, Michael Field, and Jozef Gecz. Severe neurocognitive and growth disorders due to variation in thoc2, an essential component of nuclear mrna export machinery. Human Mutation, 39:1126-1138, Jun 2018. URL: https://doi.org/10.1002/humu.23557, doi:10.1002/humu.23557. This article has 31 citations and is from a domain leading peer-reviewed journal.

10. (kumar2018severeneurocognitiveand pages 10-11): Raman Kumar, Alison Gardner, Claire C. Homan, Evelyn Douglas, Heather Mefford, Dagmar Wieczorek, Hermann-Josef Lüdecke, Zornitza Stark, Simon Sadedin, Catherine Bearce Nowak, Jessica Douglas, Gretchen Parsons, Paul Mark, Lourdes Loidi, Gail E. Herman, Theresa Mihalic Mosher, Meredith K. Gillespie, Lauren Brady, Mark Tarnopolsky, Irene Madrigal, Jesús Eiris, Laura Domènech Salgado, Raquel Rabionet, Tim M. Strom, Naoko Ishihara, Hidehito Inagaki, Hiroki Kurahashi, Tracy Dudding-Byth, Elizabeth E. Palmer, Michael Field, and Jozef Gecz. Severe neurocognitive and growth disorders due to variation in thoc2, an essential component of nuclear mrna export machinery. Human Mutation, 39:1126-1138, Jun 2018. URL: https://doi.org/10.1002/humu.23557, doi:10.1002/humu.23557. This article has 31 citations and is from a domain leading peer-reviewed journal.

11. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 6-9): Rudrarup Bhattacharjee, Lachlan A. Jolly, Mark A. Corbett, Ing Chee Wee, Sushma R. Rao, Alison E. Gardner, Tarin Ritchie, Eline J. H. van Hugte, Ummi Ciptasari, Sandra Piltz, Jacqueline E. Noll, Nazzmer Nazri, Clare L. van Eyk, Melissa White, Dani Fornarino, Cathryn Poulton, Gareth Baynam, Lyndsey E. Collins-Praino, Marten F. Snel, Nael Nadif Kasri, Kim M. Hemsley, Paul Q. Thomas, Raman Kumar, and Jozef Gecz. Compromised transcription-mrna export factor thoc2 causes r-loop accumulation, dna damage and adverse neurodevelopment. Nature Communications, Feb 2024. URL: https://doi.org/10.1038/s41467-024-45121-5, doi:10.1038/s41467-024-45121-5. This article has 16 citations and is from a highest quality peer-reviewed journal.

12. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 14-16): Rudrarup Bhattacharjee, Lachlan A. Jolly, Mark A. Corbett, Ing Chee Wee, Sushma R. Rao, Alison E. Gardner, Tarin Ritchie, Eline J. H. van Hugte, Ummi Ciptasari, Sandra Piltz, Jacqueline E. Noll, Nazzmer Nazri, Clare L. van Eyk, Melissa White, Dani Fornarino, Cathryn Poulton, Gareth Baynam, Lyndsey E. Collins-Praino, Marten F. Snel, Nael Nadif Kasri, Kim M. Hemsley, Paul Q. Thomas, Raman Kumar, and Jozef Gecz. Compromised transcription-mrna export factor thoc2 causes r-loop accumulation, dna damage and adverse neurodevelopment. Nature Communications, Feb 2024. URL: https://doi.org/10.1038/s41467-024-45121-5, doi:10.1038/s41467-024-45121-5. This article has 16 citations and is from a highest quality peer-reviewed journal.

13. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 10-11): Rudrarup Bhattacharjee, Lachlan A. Jolly, Mark A. Corbett, Ing Chee Wee, Sushma R. Rao, Alison E. Gardner, Tarin Ritchie, Eline J. H. van Hugte, Ummi Ciptasari, Sandra Piltz, Jacqueline E. Noll, Nazzmer Nazri, Clare L. van Eyk, Melissa White, Dani Fornarino, Cathryn Poulton, Gareth Baynam, Lyndsey E. Collins-Praino, Marten F. Snel, Nael Nadif Kasri, Kim M. Hemsley, Paul Q. Thomas, Raman Kumar, and Jozef Gecz. Compromised transcription-mrna export factor thoc2 causes r-loop accumulation, dna damage and adverse neurodevelopment. Nature Communications, Feb 2024. URL: https://doi.org/10.1038/s41467-024-45121-5, doi:10.1038/s41467-024-45121-5. This article has 16 citations and is from a highest quality peer-reviewed journal.

14. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 4-5): Rudrarup Bhattacharjee, Lachlan A. Jolly, Mark A. Corbett, Ing Chee Wee, Sushma R. Rao, Alison E. Gardner, Tarin Ritchie, Eline J. H. van Hugte, Ummi Ciptasari, Sandra Piltz, Jacqueline E. Noll, Nazzmer Nazri, Clare L. van Eyk, Melissa White, Dani Fornarino, Cathryn Poulton, Gareth Baynam, Lyndsey E. Collins-Praino, Marten F. Snel, Nael Nadif Kasri, Kim M. Hemsley, Paul Q. Thomas, Raman Kumar, and Jozef Gecz. Compromised transcription-mrna export factor thoc2 causes r-loop accumulation, dna damage and adverse neurodevelopment. Nature Communications, Feb 2024. URL: https://doi.org/10.1038/s41467-024-45121-5, doi:10.1038/s41467-024-45121-5. This article has 16 citations and is from a highest quality peer-reviewed journal.

15. (bhattacharjee2024compromisedtranscriptionmrnaexport pages 11-13): Rudrarup Bhattacharjee, Lachlan A. Jolly, Mark A. Corbett, Ing Chee Wee, Sushma R. Rao, Alison E. Gardner, Tarin Ritchie, Eline J. H. van Hugte, Ummi Ciptasari, Sandra Piltz, Jacqueline E. Noll, Nazzmer Nazri, Clare L. van Eyk, Melissa White, Dani Fornarino, Cathryn Poulton, Gareth Baynam, Lyndsey E. Collins-Praino, Marten F. Snel, Nael Nadif Kasri, Kim M. Hemsley, Paul Q. Thomas, Raman Kumar, and Jozef Gecz. Compromised transcription-mrna export factor thoc2 causes r-loop accumulation, dna damage and adverse neurodevelopment. Nature Communications, Feb 2024. URL: https://doi.org/10.1038/s41467-024-45121-5, doi:10.1038/s41467-024-45121-5. This article has 16 citations and is from a highest quality peer-reviewed journal.

16. (kumar2018severeneurocognitiveand pages 27-27): Raman Kumar, Alison Gardner, Claire C. Homan, Evelyn Douglas, Heather Mefford, Dagmar Wieczorek, Hermann-Josef Lüdecke, Zornitza Stark, Simon Sadedin, Catherine Bearce Nowak, Jessica Douglas, Gretchen Parsons, Paul Mark, Lourdes Loidi, Gail E. Herman, Theresa Mihalic Mosher, Meredith K. Gillespie, Lauren Brady, Mark Tarnopolsky, Irene Madrigal, Jesús Eiris, Laura Domènech Salgado, Raquel Rabionet, Tim M. Strom, Naoko Ishihara, Hidehito Inagaki, Hiroki Kurahashi, Tracy Dudding-Byth, Elizabeth E. Palmer, Michael Field, and Jozef Gecz. Severe neurocognitive and growth disorders due to variation in thoc2, an essential component of nuclear mrna export machinery. Human Mutation, 39:1126-1138, Jun 2018. URL: https://doi.org/10.1002/humu.23557, doi:10.1002/humu.23557. This article has 31 citations and is from a domain leading peer-reviewed journal.

17. (kumar2015thoc2mutationsimplicate pages 3-4): Raman Kumar, Mark A. Corbett, Bregje W.M. van Bon, Joshua A. Woenig, Lloyd Weir, Evelyn Douglas, Kathryn L. Friend, Alison Gardner, Marie Shaw, Lachlan A. Jolly, Chuan Tan, Matthew F. Hunter, Anna Hackett, Michael Field, Elizabeth E. Palmer, Melanie Leffler, Carolyn Rogers, Jackie Boyle, Melanie Bienek, Corinna Jensen, Griet Van Buggenhout, Hilde Van Esch, Katrin Hoffmann, Martine Raynaud, Huiying Zhao, Robin Reed, Hao Hu, Stefan A. Haas, Eric Haan, Vera M. Kalscheuer, and Jozef Gecz. Thoc2 mutations implicate mrna-export pathway in x-linked intellectual disability. American journal of human genetics, 97 2:302-10, Aug 2015. URL: https://doi.org/10.1016/j.ajhg.2015.05.021, doi:10.1016/j.ajhg.2015.05.021. This article has 93 citations and is from a highest quality peer-reviewed journal.

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