Bryant-Li-Bhoj Neurodevelopmental Syndrome 1

Mendelian MONDO:0030606 Pathograph 36 Show in embeddings browser syndromic disease neurodevelopmental disorder hereditary disease

Bryant-Li-Bhoj neurodevelopmental syndrome 1 is an autosomal dominant disorder caused by heterozygous pathogenic H3-3A (H3F3A) variants, usually de novo missense substitutions. H3-3A and H3-3B encode the same replication-independent histone H3.3 protein, but define separate genetic diagnoses. Developmental delay or intellectual disability, often severe, accompanies variable abnormalities of muscle tone, seizures, craniofacial appearance, growth and brain structure. Regression and later motor decline occur in some individuals, but an obligatory adult degenerative phase is not established. Functional effects differ between alleles and include altered histone abundance, chaperone interaction, local histone modification and transcription; a single gain-of-function mechanism has not been demonstrated for all variants.

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1
Mappings
1
Inheritance
12
Pathophys.
41
Phenotypes
3
Gaps
36
Pathograph
1
Genes
1
Variants
13
Medical Actions
2
Differentials
9
Models
17
References
1
Deep Research
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Classifications

Harrison's Part
GENETICS ENVIRONMENT DISEASE NEUROLOGIC
🔗

Mappings

MONDO
MONDO:0030606 Bryant-Li-Bhoj neurodevelopmental syndrome 1
skos:exactMatch MONDO
Primary MONDO disease identifier for this entry.
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Inheritance

1
Autosomal dominant inheritance HP:0000006
Autosomal dominant inheritance. In the 2024 cohort, H3-3A variants were heterozygous de novo missense variants whenever parental results were available; inheritance was unknown for three individuals. A negative parental blood test does not exclude germline mosaicism. Allele counts across H3-3A and H3-3B do not establish equal protein dosage or a uniform functional mechanism.
Autosomal dominant inheritance
Show evidence (2 references)
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"BRYLIB is expressed in an autosomal dominant manner and typically caused by a de novo H3-3A (H3F3A) or H3-3B (H3F3B) pathogenic variant."
GeneReviews statement of the inheritance mode and the de novo origin.
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"the time of this report, all H3-3A variants are heterozygous de novo missense variants when parents are available, though inheritance was undetermined for three individuals"
The H3-3A-specific observation that distinguishes BLBS1 from BLBS2 on inheritance and variant type.
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Discussions and Knowledge Gaps

3
What accounts for the phenotypic variability that gene, protein domain and sex together fail to explain - including variability between individuals carrying the same H3-3A allele?
KNOWLEDGE GAP OPEN blbs1_genotype_phenotype_unexplained
Attached to
pathophysiology#De Novo H3-3A Variant Altering Histone H3.3 phenotypes#
Phenotypes vary even among individuals with the same H3-3A allele. The expanded cohort describes trends by gene, domain and sex but does not establish predictive genotype–phenotype rules. Variant-specific biochemistry, gene expression, developmental timing and other modifiers remain candidate contributors.
Show evidence (2 references)
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Since these variables do not account for all BLBS phenotypic variability, these findings suggest that additional factors may play a role in modifying the phenotypes of affected individuals."
States the residual, unexplained variability directly.
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Notably, phenotypic heterogeneity was present even amongst individuals harboring the same variant."
Shows that the unexplained variability persists at fixed genotype, which rules out variant identity as the missing determinant.
Do germline H3.3 variants confer cancer predisposition, given that the same residues are oncogenic when altered somatically?
KNOWLEDGE GAP OPEN blbs1_cancer_predisposition
Attached to
pathophysiology#Deregulated Cell Proliferation
Increased proliferation in cultured patient cells does not establish cancer predisposition. The 2025 pan-histonopathy survey reported two cancers across 192 individuals, including a pulmonary carcinoid in an H3-3B case and leukemia in an H4-related disorder. This heterogeneous, incompletely screened cross-sectional denominator cannot estimate BLBS1 incidence or lifetime risk. Tumor and constitutional sequencing and longer follow-up are needed; standardized BLBS1 surveillance is not established by these observations.
Show evidence (1 reference)
PMID:40241305 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"the value of ongoing translational work to address the outstanding question of cancer predisposition for individuals living with germline histonopathies"
The study identifies cancer predisposition as unresolved, not refuted.
Which variant-specific mechanisms explain the human brain phenotype, and which model findings generalize to H3-3A disease?
HUMAN MODEL MISMATCH OPEN blbs1_no_neurodevelopmental_model
Attached to
animal_models#H3.3 T45I knock-in mouse animal_models#Zebrafish h3f3a D123N db1092 model animal_models#Mouse H3f3a G34R/V/W mechanistic comparison pathophysiology#Progressive Postnatal Neuronal H3.3 Dependence
The db1092 fish has a dominant-negative missense allele and peripheral neural-crest endpoints. The T45I mouse matches a recurrent H3-3A allele but has preserved performance on several cognitive/social assays and no genotype difference in the reported adult brain cell counts. G34R H3f3a mice demonstrate DNA-methylation and neuroinflammatory changes, but the human cases motivating that model have H3-3B variants. The 2026 L48R neural-organoid study likewise concerns H3-3B. None establishes a uniform human H3-3A mechanism or tests age-dependent H3.3 replacement as the general cause of progression.
Show evidence (3 references)
url:https://www.biorxiv.org/content/10.64898/2026.06.16.732665v1.full REFUTE DIRECT PRIMARY RESULT Model Organism
"Immunohistochemical analyses of adult brain tissue revealed no significant differences between genotypes in neuronal density (NeuN), astrocyte burden (GFAP), or Purkinje cell number (Calbindin)"
The full preprint reports actual negative histological comparisons, not absence of histological investigation.
PMID:36931244 SUPPORT INDIRECT PRIMARY RESULT Model Organism
"the G34R mutation diminishes H3K36me2 and consequently redistributes DNMT3A from intergenic regions to CpG islands."
Mouse H3f3a provides mechanistic comparison for the shared protein. The human G34R/V patients in this paper had H3-3B variants, so this is indirect evidence for BLBS1.
PMID:41689096 SUPPORT INDIRECT PRIMARY RESULT In Vitro
"Here, we investigate how one BLBS-causative variant, H3-3B p.Leu48Arg (L48R), affects neurodevelopment"
The neural progenitor, neuron and organoid study concerns the sibling genetic entity; it is not assigned as a BLBS1 patient model.
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Pathophysiology

12
De Novo H3-3A Variant Altering Histone H3.3
Heterozygous pathogenic H3-3A missense substitutions alter a core nucleosomal protein. Most are rare and distributed across the tail and core, but recurrent alleles occur. Dominant-negative or gain-of-function mechanisms have been proposed; variant-specific biochemical effects include both increases and decreases in measured abundance or interactions. Haploinsufficiency is not established as the general explanation.
H3-3A hgnc:4764 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves H3-3A (hgnc:4764). hgnc:4764 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context H3-3A hgnc:4764 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns H3-3A (hgnc:4764). hgnc:4764 is a gene from the HUGO Gene Nomenclature Committee. allele_type: MISSENSE variant_origin: DE_NOVO zygosity: HETEROZYGOUS
De novo heterozygous missense variants with heterogeneous biochemical effects; a uniform functional category is not assigned.
structural constituent of chromatin GO:0030527 ⚠ ABNORMAL Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves abnormal structural constituent of chromatin (GO:0030527). GO:0030527 is a molecular function from the Gene Ontology.
Downstream
Disrupted Nucleosome Assembly and Chaperone Interaction Structural modeling predicts altered DNA, histone or chaperone contacts.
Variant-Dependent Histone H3.3 Abundance Selected H3-3A variants alter steady-state protein abundance in transfected cells.
Altered H3.3 Chaperone Interaction The R129H variant strengthens a measured chaperone interaction.
Aberrant Local Histone Post-Translational Modification Patient-derived cells show altered histone modification patterns.
Variant-Dependent H3K36 Methylation T45I changes methyltransferase activity toward the mutant nucleosome.
Variant-Specific Transcription-Coupled Chromatin Defects A subset of histone substitutions perturbs transcription-coupled chromatin processes in yeast.
Abnormal Brain Development Pathogenic H3-3A variants cause the clinical developmental disorder through incompletely resolved intermediates.
Deregulated Cell Proliferation Patient fibroblasts carrying H3-3A variants show altered proliferative programs; the causal intermediates are unresolved.
Show evidence (2 references)
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"In this work, we have identified a cohort of 46 unrelated patients bearing de novo heterozygous germline missense mutations in H3F3A or H3F3B with a core phenotype of progressive neurologic dysfunction and congenital anomalies."
The founding cohort establishing de novo heterozygous missense variants as the cause.
PMID:36867246 SUPPORT DIRECT REVIEW SYNTHESIS Other
"Most of the causative variants are private and scattered throughout the protein, but all seem to have either a gain-of-function or dominant negative effect on protein function."
This review proposes gain-of-function or dominant-negative effects; it does not experimentally classify every allele.
Disrupted Nucleosome Assembly and Chaperone Interaction
Structural modeling predicts changes at DNA contacts, inter-histone interfaces and chaperone-binding surfaces. Tail and core substitutions need not share one biochemical effect. Modeling does not prove universal nucleosome destabilization or demonstrate that every mutant is normally incorporated; those questions require allele-specific assays.
nucleosome assembly GO:0006334 ⚠ ABNORMAL Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal nucleosome assembly (GO:0006334). GO:0006334 is a biological process from the Gene Ontology.
nucleosomal DNA binding GO:0031492 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves nucleosomal DNA binding (GO:0031492). GO:0031492 is a molecular function from the Gene Ontology.
nucleosome GO:0000786 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves nucleosome (GO:0000786). GO:0000786 is a cellular component from the Gene Ontology.
Show evidence (2 references)
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT Computational
"Molecular modeling of all 37 variants demonstrated clear disruptions in interactions with DNA, other histones, and histone chaperone proteins."
The structural modelling result this node records; evidence source is COMPUTATIONAL because the claim comes from in silico modelling.
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT Computational
"Molecular modeling revealed that mutations in the histone core likely affect nucleosome stability, while the mutations in the tail affect various protein-protein interactions."
Gives the domain split between nucleosome destabilisation and disrupted protein-protein interaction.
Variant-Dependent Histone H3.3 Abundance
In transient HEK293T assays, H3-3A R129H and M121I reduce protein abundance whereas R41C increases it. The experiment measures steady-state expression and does not by itself distinguish altered synthesis from degradation or establish patient-neuronal dosage.
Experimental models
HEK293T H3-3A expression and DAXX assays CELL LINE Transient expression measures variant abundance, localization and DAXX co-immunoprecipitation. H3-3A R129H/M121I reduce abundance, R41C increases it, and R129H strengthens DAXX association.
Show evidence (1 reference)
PMID:34876591 SUPPORT DIRECT PRIMARY RESULT In Vitro
"Three variant constructs (p.R129H, p.M121I, and p.I52N) showed significant decrease in protein expression, while one variant (p.R41C) accumulated at greater levels than wild-type control."
Transient HEK293T expression: R129H, M121I and R41C are H3-3A alleles; I52N is H3-3B and is not assigned to BLBS1.
Altered H3.3 Chaperone Interaction
H3-3A R129H increases DAXX co-immunoprecipitation despite reduced histone abundance. Nuclear localization is preserved. Increased binding does not prove greater chromatin deposition. A newer preprint finds loss of DAXX association for mature-histone G90R, underscoring assay- and allele-dependent effects.
Experimental models
HEK293T H3-3A expression and DAXX assays CELL LINE Transient expression measures variant abundance, localization and DAXX co-immunoprecipitation. H3-3A R129H/M121I reduce abundance, R41C increases it, and R129H strengthens DAXX association.
Show evidence (2 references)
PMID:34876591 SUPPORT DIRECT PRIMARY RESULT In Vitro
"The Co-IP assays showed that p.R129H had a significantly stronger interaction with DAXX, suggesting that this variant might affect the deposition of histone H3.3"
The interaction was measured; altered deposition was inferred. HGVS p.Arg129His corresponds to mature-histone R128H.
url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full SUPPORT DIRECT PRIMARY RESULT In Vitro
"there was a complete loss of associations between H3.3 G90R and DAXX"
Preprint BioID and immunoprecipitation identify loss of interaction for G90R; mature-histone G90R equals HGVS G91R.
Aberrant Local Histone Post-Translational Modification
Patient-derived lymphoblast and fibroblast assays identify prominent local modification changes on mutant histones, with more modest global changes. H3-3A A15G affects nearby K9/K14 modification; the reported A29P/K27/K36 example belongs to H3-3B. These observations support cis effects without proving that all downstream consequences are local or that all BLBS variants differ uniformly from somatic oncohistones.
chromatin remodeling GO:0006338 ⚠ ABNORMAL Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal chromatin remodeling (GO:0006338). GO:0006338 is a biological process from the Gene Ontology.
Experimental models
Patient-derived EBV lymphoblastoid cells CELL LINE Histone mass spectrometry in lymphoblastoid cells, including H3-3A A15G, R17G and T45I, assays local and global post-translational modifications.
Show evidence (2 references)
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT In Vitro
"Patient histone posttranslational modifications (PTMs) analysis revealed notably aberrant local PTM patterns distinct from the somatic lysine mutations that cause global PTM dysregulation."
The abstract emphasizes local effects; full-text results also report modest global changes.
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT In Vitro
"Together, these data suggest that the mutant histones can be incorporated into the nucleosome, cause marked local deregulation of chromatin state, and modestly alter the global control of histone modifications."
This conclusion explicitly includes modest global effects.
Variant-Dependent H3K36 Methylation
A 2025 preprint identifies increased H3K36 methylation in cis for H3.3 T45I and stimulation of SETD2/NSD1 activity on recombinant mutant nucleosomes. Other substitutions have opposite effects. Structural modeling implicates contacts with the methyltransferase AWS domain; that structural explanation remains a model.
protein methylation GO:0006479 ⚠ ABNORMAL Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal protein methylation (GO:0006479). GO:0006479 is a biological process from the Gene Ontology.
Methyltransferase activity toward variant H3.3 substrates GO:0046975 ⚠ ABNORMAL Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves abnormal Methyltransferase activity toward variant H3.3 substrates, annotated with histone H3K36 methyltransferase activity (GO:0046975). GO:0046975 is a molecular function from the Gene Ontology.
Experimental models
Inducible HEK293 H3.3 BioID2 models CELL LINE The 2025 preprint compares isogenic Flp-In T-REx cells expressing tagged wild-type or mutant H3.3, including H3-3A-associated T45I and G90R, for proximal protein associations and histone methylation. Fusion proteins represent less than 20% of total H3.3.
Reconstituted H3.3 nucleosome methylation assays OTHER Measures variant-dependent methyltransferase activity on reconstituted substrates.
Downstream
Altered Recruitment of PWWP-Domain Effectors Altered H3K36 methylation changes recruitment of methyl-lysine readers.
Show evidence (2 references)
url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full SUPPORT DIRECT PRIMARY RESULT In Vitro
"H3.3 T45I stimulates the enzymatic activities of SETD2 and NSD1"
Preprint biochemical nucleosome assays support increased activity toward the recurrent H3-3A T45I substrate; this does not establish a treatment target in patients.
url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full SUPPORT DIRECT PRIMARY RESULT In Vitro
"increased H3.3K36me3 levels with S31F and T45I, and a complete to near complete loss of signal with G34V, K36E, and L48R"
Preprint cell assays show opposite allele-specific effects. The H3-3A T45I finding is relevant here; the other substitutions are protein-level comparators, not all BLBS1 alleles.
Altered Recruitment of PWWP-Domain Effectors
Variant-dependent H3K36 modification alters association with PWWP-domain proteins, including DNMT3A/B. T45I increases selected reader associations in the preprint; this must not be replaced by a universal loss-of-DNMT3A claim derived from G34R. Consequences for DNA methylation and neuronal function require direct testing for individual H3-3A alleles.
Experimental models
Inducible HEK293 H3.3 BioID2 models CELL LINE The 2025 preprint compares isogenic Flp-In T-REx cells expressing tagged wild-type or mutant H3.3, including H3-3A-associated T45I and G90R, for proximal protein associations and histone methylation. Fusion proteins represent less than 20% of total H3.3.
Animal models
Mouse H3f3a G34R/V/W mechanistic comparison Mus musculus perturbs Perturbs H3K36-dependent DNMT3A recruitment with downstream DNA methylation and transcriptional changes.
Show evidence (1 reference)
url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full SUPPORT DIRECT PRIMARY RESULT In Vitro
"the associations between PWWP and the S31 and T45I point mutants increased while those with G34V, K36E, and L48R generally decreased."
Preprint proximity data show directionally distinct effects among substitutions.
Deregulated Cell Proliferation
RNA sequencing of three H3-3A patient fibroblast lines identified altered expression enriched for mitosis and cell division. Proliferation assays across five patient lines from both H3.3 genes found increased proliferative capacity. This is a non-neural cellular phenotype; neither a direct cause of neuronal dysfunction nor malignant transformation was demonstrated.
cell population proliferation GO:0008283 ↑ INCREASED Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves increased cell population proliferation (GO:0008283). GO:0008283 is a biological process from the Gene Ontology.
Experimental models
Patient-derived dermal fibroblasts PRIMARY CELL CULTURE Three H3-3A patient lines carrying mature-histone R17G, G90R or T45I were compared with three controls by RNA sequencing. Proliferation experiments included five patient lines from both H3.3 genes and six controls.
Show evidence (1 reference)
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT In Vitro
"RNA sequencing on patient cells demonstrated up-regulated gene expression related to mitosis and cell division, and cellular assays confirmed an increased proliferative capacity."
Provides both the transcriptional and the functional evidence for increased proliferation.
Variant-Specific Transcription-Coupled Chromatin Defects
Mechanism confidence: Provisional
Yeast H3 mutants show separable defects: T45I and R83C permit cryptic intragenic transcription, while N108S traps FACT at gene 3-prime ends. Eight of nine newly tested mutants lack the FACT-trapping defect. Yeast native H3 expressed as the sole histone H3 source is a mechanistic surrogate, not a heterozygous human neuronal model. A114G is excluded from clinical variant inference because the study notes possible sequencing artifact.
Experimental models
Yeast histone H3 BLBS substitution panel OTHER Nine substitutions in endogenous HHT2 were tested with deletion of HHT1, making mutant H3 the sole source. T45I and R83C permit cryptic transcription; only N108S among the nine traps FACT at gene ends. Earlier L61R studies used coexpressed wild-type H3 and are not directly comparable.
Show evidence (3 references)
PMID:39525290 SUPPORT INDIRECT PRIMARY RESULT In Vitro
"only H3-N108S shows significant trapping of the Spt16 subunit of yFACT at the 3’ end"
Yeast native-H3 experiments identify a variant-specific defect. N108S is not assigned as a human H3-3A allele here.
PMID:39525290 SUPPORT INDIRECT PRIMARY RESULT In Vitro
"H3-T45I also confers sensitivity to the DNA-damaging agent methyl methanesulfonate (MMS)"
Yeast growth assays identify an additional T45I-associated stress phenotype; direct human relevance is uncertain.
PMID:39525290 SUPPORT INDIRECT PRIMARY RESULT In Vitro
"four mutants – H3-T45I, -R83C, -N108S, and -A114G – cause strong His ... phenotypes, suggesting that these H3-BLBS mutants may interfere with proper nucleosome reassembly or other chromatin processes."
T45I and R83C establish H3-3A-relevant protein-level transcriptional defects in yeast. The A114G result does not establish its clinical validity.
Neural Crest and Craniofacial Developmental Defect
The zebrafish db1092 allele encodes dominant-negative D123N H3.3, rather than a null. It disrupts early cranial neural-crest specification and ectomesenchymal development, with loss of craniofacial cartilage. The 2020 study reports partial cranial-glial reductions in homozygotes and stronger deficits after added dominant-negative RNA. These peripheral neural-crest glia are not CNS radial glia or oligodendrocytes, so their loss does not establish the cause of human brain hypomyelination.
chromatin remodeling GO:0006338 ⚠ ABNORMAL Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal chromatin remodeling (GO:0006338). GO:0006338 is a biological process from the Gene Ontology.
Animal models
H3.3 T45I knock-in mouse Mus musculus partially recapitulates Adult craniofacial appearance and an enlarged spheno-occipital growth center provide a morphological comparison.
Zebrafish h3f3a D123N db1092 model Danio rerio partially recapitulates Disrupted cranial neural-crest specification and ectomesenchyme formation cause craniofacial loss.
Downstream
Craniofacial Dysmorphism The fish phenotype supports a candidate neural-crest contribution to human dysmorphism; direct human lineage evidence is lacking.
Show evidence (2 references)
PMID:23028350 SUPPORT INDIRECT PRIMARY RESULT Model Organism
"we identified a dominant D123N mutation in h3f3a"
The original allele-defining experiment excludes the previous null annotation.
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT Model Organism
"A zebrafish model showed craniofacial anomalies and a defect in Foxd3-derived glia."
The model-organism result underlying this node.
Abnormal Brain Development
Developmental delay, seizures and variable structural brain abnormalities define the clinical neurodevelopmental phenotype. The 2024 pooled cohort reported MRI abnormalities in 44/76 and callosal malformation/dysgenesis in 28/76. An enriched 18-person imaging series, including 11 H3-3A cases, found posterior-fossa, cortical and caudate abnormalities; those selected-subgroup rates are not population estimates.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
brain UBERON:0000955 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in brain (UBERON:0000955). UBERON:0000955 is an anatomical location from the Uberon multi-species anatomy ontology. corpus callosum UBERON:0002336 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in corpus callosum (UBERON:0002336). UBERON:0002336 is an anatomical location from the Uberon multi-species anatomy ontology.
Animal models
H3.3 T45I knock-in mouse Mus musculus partially recapitulates Developmental and motor phenotypes overlap with part of the human syndrome.
Downstream
Global Developmental Delay and Intellectual Disability The dominant clinical consequence of disrupted neural chromatin regulation.
Abnormal Corpus Callosum Morphology The commonest specific structural finding on neuroimaging.
Small Posterior Fossa Abnormal brain development includes this structural or myelination phenotype.
Cortical Malformation Abnormal brain development includes this structural or myelination phenotype.
Chiari I Malformation Abnormal brain development includes this structural or myelination phenotype.
CNS Hypomyelination Abnormal brain development includes this structural or myelination phenotype.
Delayed CNS Myelination Abnormal brain development includes this structural or myelination phenotype.
Seizures Disrupted neural development contributes to this clinical manifestation; specific cellular intermediates remain unresolved.
Hypotonia Disrupted neural development contributes to this clinical manifestation; specific cellular intermediates remain unresolved.
Hypertonia Disrupted neural development contributes to this clinical manifestation; specific cellular intermediates remain unresolved.
Delayed Ability to Walk Disrupted neural development contributes to this clinical manifestation; specific cellular intermediates remain unresolved.
Delayed Ability to Sit Disrupted neural development contributes to this clinical manifestation; specific cellular intermediates remain unresolved.
Delayed Speech and Language Development Disrupted neural development contributes to this clinical manifestation; specific cellular intermediates remain unresolved.
Show evidence (2 references)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"This syndrome is characterized by developmental delay/intellectual disability, craniofacial anomalies, hyper/hypotonia, and abnormal neuroimaging"
States the congenital phenotype cluster this node terminates in.
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Within this subgroup, 72% of individuals presented with small posterior fossa, 28% presented with dysgenesis of the corpus collosum, and 44% presented with cortical developmental abnormalities"
Gives the structural findings from the detailed imaging subgroup that anchor the structural component of this node.
Progressive Postnatal Neuronal H3.3 Dependence
Mechanism confidence: Hypothetical
Age-dependent accumulation of H3.3 in postmitotic neurons is a proposed susceptibility context, not a demonstrated universal cause of BLBS1 progression. Regression is variable. The T45I mouse has motor impairment but lacks a genotype difference in the reported adult brain cell counts. G34R mice demonstrate a different, experimentally studied epigenetic route, whose generality to H3-3A patient alleles remains uncertain.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
Show evidence (1 reference)
PMID:33268356 SUPPORT INDIRECT BACKGROUND Other
"A recent study showed that H3.3 begins to replace H3.1 and H3.2 in postnatal mouse and human brains in a time-dependent manner and displaces these canonical H3 variants almost completely in adulthood."
The founding report cites physiological histone replacement as background for a hypothesis; it did not test this as the cause of human degeneration.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Bryant-Li-Bhoj Neurodevelopmental Syndrome 1 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

41
Breast 1
Nipple Anomalies OCCASIONAL Abnormal nipple morphology HP:0004404 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormal nipple morphology (HP:0004404). HP:0004404 is a phenotype from the Human Phenotype Ontology.
Coarse binding: source unspecified
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Nipple anomalies | 15/88 (17%)"
Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
Cardiovascular 2
Congenital Heart Defect OCCASIONAL Abnormal heart morphology HP:0001627 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormal heart morphology (HP:0001627). HP:0001627 is a phenotype from the Human Phenotype Ontology.
Coarse binding: variable spectrum
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Cardiac | 11/82 (13%)"
Uses the table count rather than the inconsistent 14% prose.
Atrial Septal Defect OCCASIONAL HP:0001631 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Atrial Septal Defect (HP:0001631). HP:0001631 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"Atrial septal defect was seen in about 16% of individuals with BRYLIB."
Specific lesion in the pooled clinical synthesis, rather than assigning the all-cardiac-anomaly count to one defect.
Digestive 2
Feeding Difficulties OCCASIONAL HP:0011968 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Feeding difficulties (HP:0011968). HP:0011968 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"have been described in about 25% of affected individuals. Most are treated with a nasogastric tube or gastrostomy tube."
Pooled clinical estimate and severity-related support needs.
Constipation OCCASIONAL HP:0002019 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Constipation (HP:0002019). HP:0002019 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"Chronic constipation was reported in 26% of affected individuals."
The clinical synthesis describes this manifestation; gene-specific rates remain uncertain.
Ear 1
Hearing Impairment HP:0000365 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hearing impairment (HP:0000365). HP:0000365 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"A subset of affected individuals have been reported with mild-to-moderate congenital hearing loss. This is thought to be static."
The full clinical description supports the phenotype beyond a management checklist.
Endocrine 1
Hypothyroidism HP:0000821 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypothyroidism (HP:0000821). HP:0000821 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"Hypothyroidism has been reported in a subset of affected individuals."
The clinical synthesis describes this manifestation; gene-specific rates remain uncertain.
Eye 2
Ocular Involvement FREQUENT Abnormality of eye movement HP:0000496 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormality of eye movement (HP:0000496). HP:0000496 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Oculomotor | 49/90 (54%)"
The pooled category frequency is retained with its denominator.
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"Strabismus is present in slightly more than one third of affected individuals, and nystagmus is present in a small minority of affected individuals (7%)."
The clinical synthesis separates specific ocular findings.
Strabismus FREQUENT HP:0000486 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Strabismus (HP:0000486). HP:0000486 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Strabismus | 32/90 (36%)"
Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
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)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"About 35% of affected males have cryptorchidism. Retractile testes have also been observed."
The denominator is affected males, not all individuals or all genital anomalies.
Head and Neck 5
Craniofacial Dysmorphism VERY_FREQUENT 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 (3 references)
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Bryant-Li-Bhoj neurodevelopmental syndrome (BRYLIB) is characterized by developmental delay / intellectual disability (typically in the severe range) and nonspecific craniofacial abnormalities."
GeneReviews names craniofacial abnormality as a defining feature and describes it as nonspecific.
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"In addition to growth trajectories outside of the standard range for age and sex, 92% of individuals with BLBS present with craniofacial anomalies (Table 1, Fig. 1D, Supplementary Table 1)."
Gives the patient-level frequency underpinning the VERY_FREQUENT band.
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Dysmorphic features are slightly more common in individuals with variants in H3-3A (95%) than H3-3B (86%)"
The gene-stratified proportion supports the H3-3A description.
Craniosynostosis HP:0001363 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Craniosynostosis (HP:0001363). HP:0001363 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"The presentation is highly variable and can include hypotonia, epilepsy, other neurologic findings (spasticity, loss of developmental milestones, worsening gait, and/or camptocormia – forward flexion of the spine when standing that resolves when lying down), growth abnormalities (most commonly..."
GeneReviews names craniosynostosis of any suture as a feature.
Microcephaly FREQUENT 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 (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Microcephaly (≤5th percentile) | 30/95 (32%)"
The table supplies a pooled count and study-specific cutoff.
Macrocephaly OCCASIONAL HP:0000256 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Macrocephaly (HP:0000256). HP:0000256 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Macrocephaly (≥95th percentile) | 14/95 (15%)"
Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
Small Posterior Fossa HP:0040010 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Small posterior fossa (HP:0040010). HP:0040010 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:35772801 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"Thirteen of 18 individuals (72%) presented with a small posterior fossa."
Primary imaging series; proportions are pooled and enriched for severe abnormalities.
url:https://pmc.ncbi.nlm.nih.gov/articles/PMC9262070/ SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"Our cohort was biased because all individuals included in our study except for 1 had severe clinical symptoms and significant abnormalities involving the brain"
The full primary study explicitly states the selection limitation.
Integument 2
Dermatologic Features FREQUENT Abnormality of the skin HP:0000951 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormality of the skin (HP:0000951). HP:0000951 is a phenotype from the Human Phenotype Ontology.
Coarse binding: variable spectrum
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Dermal | 46/88 (52%)"
Uses the table denominator rather than the 53% prose.
Persistent Fetal Finger Pads OCCASIONAL Prominent fingertip pads HP:0001212 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Persistent Fetal Finger Pads, annotated with Prominent fingertip pads (HP:0001212). HP:0001212 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Fetal finger pad | 14/88 (16%)"
Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
Musculoskeletal 7
Hypotonia FREQUENT HP:0001252 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypotonia (HP:0001252). HP:0001252 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"The presentation is highly variable and can include hypotonia, epilepsy, other neurologic findings (spasticity, loss of developmental milestones, worsening gait, and/or camptocormia – forward flexion of the spine when standing that resolves when lying down), growth abnormalities (most commonly..."
GeneReviews enumeration of the variable features, hypotonia among them.
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Sixty-two percent of individuals presented with hypotonia whereas 12% presented with hypertonia (Table 1, Fig. 2)."
Gives the patient-level frequency underpinning the FREQUENT band, and the hypertonia figure alongside it.
Camptocormia HP:0100595 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Camptocormia (HP:0100595), qualified as adult onset. HP:0100595 is a phenotype from the Human Phenotype Ontology.
Onset: ADULT
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"All reported adults have had a new subacute onset of motor issues in the third decade of life that generally remains stable after onset."
The full clinical synthesis gives the timing and subsequent course; Table 2 limits the adult denominator to three.
Spasticity HP:0001257 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Spasticity (HP:0001257). HP:0001257 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"Spasticity typically develops in childhood and is associated with either hypotonia or hypertonia. Hypertonia tends to be limited to limbs"
The clinical synthesis gives age and distribution.
Hypertonia OCCASIONAL HP:0001276 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypertonia (HP:0001276). HP:0001276 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Sixty-two percent of individuals presented with hypotonia whereas 12% presented with hypertonia (Table 1, Fig. 2)."
Gives the patient-level frequency of hypertonia alongside that of hypotonia.
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Interestingly, 10% of individuals have concordant axial hypotonia and peripheral hypertonia, or dystonia, which is exclusively present in individuals with variants in the core (Fig. 2, light gray)."
Records the mixed tone pattern and the fact that it occurs only with core-domain variants.
Musculoskeletal Anomalies FREQUENT Abnormal skeletal morphology HP:0011842 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormal skeletal morphology (HP:0011842). HP:0011842 is a phenotype from the Human Phenotype Ontology.
Coarse binding: variable spectrum
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Musculoskeletal | 56/94 (60%)"
Uses the table count rather than the 58% prose.
Scoliosis OCCASIONAL HP:0002650 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Scoliosis (HP:0002650). HP:0002650 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Scoliosis | 20/94 (21%)"
Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
Joint Hypermobility OCCASIONAL HP:0001382 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Joint Hypermobility (HP:0001382). HP:0001382 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Ligamentous laxity | 21/94 (22%)"
Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
Nervous System 14
Global Developmental Delay and Intellectual Disability VERY_FREQUENT 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 (2 references)
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Bryant-Li-Bhoj neurodevelopmental syndrome (BRYLIB) is characterized by developmental delay / intellectual disability (typically in the severe range) and nonspecific craniofacial abnormalities."
GeneReviews states developmental delay or intellectual disability as a defining, typically severe feature.
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Ninety-nine percent of individuals with BLBS have DD/ID (Table 1, Fig. 1D, Supplementary Table 1)."
Gives the patient-level frequency underpinning the VERY_FREQUENT band.
Seizures 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 (2 references)
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Seizures were more frequently reported in individuals harboring variants in H3-3B (59%) than those harboring variants in H3-3A (45%)"
The gene-stratified observation supports the H3-3A frequency.
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"The presentation is highly variable and can include hypotonia, epilepsy, other neurologic findings (spasticity, loss of developmental milestones, worsening gait, and/or camptocormia – forward flexion of the spine when standing that resolves when lying down), growth abnormalities (most commonly..."
GeneReviews describes the variable syndrome.
Cerebral Cortical Atrophy OCCASIONAL HP:0002120 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cerebral cortical atrophy (HP:0002120). HP:0002120 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:33268356 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"Multiple patients (26% of the cohort) have cortical atrophy on brain magnetic resonance imaging (MRI), even without intractable epilepsy."
The source prose reports 26%; the table gives 10/41 (24%). The description uses the assessable table denominator.
PMID:33268356 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"Cortical atrophy | 10/41 (24%)"
The table supplies the assessable denominator, correcting the prose percentage.
Delayed CNS Myelination HP:0002188 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Delayed CNS myelination (HP:0002188). HP:0002188 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"About 25% of affected individuals show evidence of hypomyelination or delayed myelination."
The pooled estimate combines two imaging categories and cannot quantify either alone.
Delayed Ability to Walk VERY_FREQUENT HP:0031936 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Delayed ability to walk (HP:0031936). HP:0031936 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"More individuals harboring variants in H3-3A (85%) had delayed or yet to be achieved independent walking when compared to individuals harboring variants in H3-3B (65%)."
Gives the H3-3A-specific frequency, which is the one this entry needs.
Delayed Speech and Language Development VERY_FREQUENT HP:0000750 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Delayed speech and language development (HP:0000750). HP:0000750 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"About 52% of affected individuals are nonverbal, with an additional 40% experiencing speech delay."
Supports frequent severe communication impairment in the pooled clinical literature.
Delayed Ability to Sit FREQUENT HP:0025336 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Delayed ability to sit (HP:0025336). HP:0025336 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Individuals with variants in H3-3A (56%) more frequently had delayed or yet to be achieved independent sitting compared to individuals with variants in H3-3B (43%) (Fig. 2)."
Gives the H3-3A-specific frequency, which is the one this entry needs.
Abnormal Corpus Callosum Morphology FREQUENT HP:0001273 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormal corpus callosum morphology (HP:0001273). HP:0001273 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Corpus collosum malformation/dysgenesis | 28/76 (37%)"
The larger cohort supplies the frequency; the table misspelling is preserved in the exact quote.
Autism Spectrum Disorder OCCASIONAL HP:0000717 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Autism (HP:0000717). HP:0000717 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"many individuals have co-existing neurodevelopmental diagnoses, including 8% with autism spectrum disorder"
The pooled clinical report quantifies this feature.
Developmental Regression HP:0002376 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Developmental Regression (HP:0002376). HP:0002376 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"Developmental regression can occur in a minority of affected individuals. Regression is not continuous and is not always associated with seizures."
The clinical synthesis describes this manifestation; gene-specific rates remain uncertain.
Neonatal Myoclonus HP:0001336 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Neonatal Myoclonus, annotated with Myoclonus (HP:0001336), qualified as neonatal onset. HP:0001336 is a phenotype from the Human Phenotype Ontology.
Onset: NEONATAL
Show evidence (1 reference)
PMID:39627236 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"An interictal electroencephalogram (EEG) revealed no spike discharges with a normal background and no EEG abnormalities corresponding to myoclonic movements."
The case report distinguishes neonatal movements from later epileptic seizures.
Cortical Malformation Abnormal cortical gyration HP:0002536 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cortical Malformation, annotated with Abnormal cortical gyration (HP:0002536). HP:0002536 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:35772801 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"Cortical developmental abnormalities were noted in 8 individuals (44%), with dysgyria and hypoplastic temporal poles being the most frequent presentation."
Primary imaging series; proportions are pooled and enriched for severe abnormalities.
Chiari I Malformation Chiari type I malformation HP:0007099 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Chiari I Malformation, annotated with Chiari type I malformation (HP:0007099). HP:0007099 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"Chiari I malformations were seen in 4/18 (22%) of individuals imaged."
Selected imaging-subgroup finding, not a population frequency.
CNS Hypomyelination HP:0003429 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is CNS Hypomyelination (HP:0003429). HP:0003429 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"About 25% of affected individuals show evidence of hypomyelination or delayed myelination."
The synthesis establishes both reported categories while leaving their individual frequencies unresolved.
Growth 3
Poor Growth FREQUENT Short stature 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:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Height (≤5th percentile) | 32/91 (35%)"
The cohort table supplies the denominator and study-specific cutoff.
Increased Body Weight OCCASIONAL HP:0004324 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Increased Body Weight (HP:0004324). HP:0004324 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Weight (≥95th percentile) | 14/76 (18%)"
Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
Decreased Body Weight OCCASIONAL HP:0004325 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Decreased Body Weight (HP:0004325). HP:0004325 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:38678163 SUPPORT INDIRECT PRIMARY RESULT Human Clinical
"BLBS clinical phenotypes. ... Weight (≤5th percentile) | 15/76 (20%)"
Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
🧬

Genetic Associations

1
H3-3A
Gene: H3-3A hgnc:4764 gene page HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is H3-3A (hgnc:4764). hgnc:4764 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: DE_NOVO
Show evidence (2 references)
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"In this work, we have identified a cohort of 46 unrelated patients bearing de novo heterozygous germline missense mutations in H3F3A or H3F3B with a core phenotype of progressive neurologic dysfunction and congenital anomalies."
Establishes the gene-disease relationship in the founding cohort.
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"In the general population, both genes have a very low rate of missense variants; the gnomAD missense Z score is 3.21 for H3F3A and 2.95 for H3F3B (>2 is significant)"
Documents the strong missense constraint on H3-3A that supports pathogenicity of de novo missense alleles.
🔬

Variants

1
H3-3A p.Thr46Ile (mature-histone T45I) Pathogenic
Gene: H3-3A hgnc:4764 HUGO Gene Nomenclature Committee (hgnc) Relation: this variant is in this gene This variant is in H3-3A (hgnc:4764). hgnc:4764 is a gene from the HUGO Gene Nomenclature Committee.
The c.137C>T substitution is p.Thr46Ile in HGVS protein numbering and T45I when the cleaved initiator methionine is omitted. Four individuals carried this recurrent H3-3A allele in the 2024 cohort, with variable phenotypes. It is modeled in the 2026 mouse preprint and in variant-specific biochemical studies.
Show evidence (2 references)
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Length of lollipop corresponds to number of individuals who harbor a variant at that residue (e.g. H3-3A p.T45I represents four individuals with BLBS and H3-3A p.V46M represents one individual with BLBS)."
Identifies p.Thr45Ile as the most recurrent H3-3A allele in the reported cohort.
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Notably, phenotypic heterogeneity was present even amongst individuals harboring the same variant."
Supports the statement that recurrence of an allele does not bring phenotypic concordance.
💊

Medical Actions

13
Physical Therapy and Mobility Support
Action: Physical Therapy and Mobility SupportNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Physical Therapy and Mobility Support, annotated with Physical Therapy (NCIT:C15302). NCIT:C15302 is a clinical intervention from the NCI Thesaurus. Ontology label: Physical Therapy NCIT:C15302
Platform: Behavioral / lifestyle
Individualized physical therapy, stretching, positioning and mobility aids address motor function, spasticity, camptocormia and prevention of secondary orthopedic complications.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Physical therapy is recommended to maximize mobility and to reduce the risk for later-onset orthopedic complications"
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
Occupational Therapy
Action: Occupational TherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Occupational Therapy (NCIT:C121351). NCIT:C121351 is a clinical intervention from the NCI Thesaurus. NCIT:C121351
Platform: Behavioral / lifestyle
Occupational therapy supports fine-motor and adaptive daily-living skills, including feeding, dressing and writing.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Occupational therapy is recommended for difficulty with fine motor skills that affect adaptive function"
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
Speech and Communication Support
Action: Speech Language TherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Speech Language Therapy (NCIT:C159273). NCIT:C159273 is a clinical intervention from the NCI Thesaurus. NCIT:C159273
Platform: Behavioral / lifestyle
Early speech/language intervention, individualized educational support and augmentative or alternative communication address severe expressive-language impairment.
Show evidence (2 references)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Referral to an early intervention program is recommended for access to occupational, physical, speech, and feeding therapy"
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"An AAC evaluation can be completed by a speech-language pathologist who has expertise in the area."
The detailed chapter supports alternative communication assessment.
Feeding Therapy and Enteral Support
Action: Feeding Therapy and Enteral SupportNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Feeding Therapy and Enteral Support, annotated with Therapeutic Procedure (NCIT:C49236). NCIT:C49236 is a clinical intervention from the NCI Thesaurus. Ontology label: Therapeutic Procedure NCIT:C49236
Platform: Other
Evaluate oral motor function and swallowing safety. Feeding therapy and nutritional adaptation may help; persistent severe dysfunction may require nasogastric or gastrostomy feeding.
Show evidence (1 reference)
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Feeding issues and poor weight gain may benefit from feeding therapy; in individuals with persistent feeding issues, gastrostomy tube may be considered."
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
Antiseizure Medication
Action: Antiseizure MedicationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Antiseizure Medication, annotated with Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. Ontology label: Pharmacotherapy NCIT:C15986
Agent: Anticonvulsant Agent NCIT:C264 NCI Thesaurus (NCIT) Relation: this treatment uses this therapeutic agent This treatment uses Anticonvulsant Agent (NCIT:C264). NCIT:C264 is a therapeutic agent from the NCI Thesaurus. levetiracetam CHEBI:6437 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses levetiracetam (CHEBI:6437). CHEBI:6437 is a therapeutic agent from Chemical Entities of Biological Interest. valproic acid CHEBI:39867 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses valproic acid (CHEBI:39867). CHEBI:39867 is a therapeutic agent from Chemical Entities of Biological Interest.
Platform: Small molecule
An experienced neurologist individualizes antiseizure treatment. Response ranges from good control to refractory epilepsy; no drug has demonstrated syndrome-specific efficacy. A single H3-3A case reported different responses to valproate and levetiracetam at different ages, which cannot establish a preferred regimen.
Show evidence (2 references)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Many ASMs may be effective; none has been demonstrated effective specifically for this disorder."
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
PMID:39627236 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"her seizures resolved with the administration of levetiracetam."
Individual later-life response in one reported H3-3A case; it does not establish comparative efficacy.
Oral Treatment of Abnormal Muscle Tone
Action: Oral Treatment of Abnormal Muscle ToneNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Oral Treatment of Abnormal Muscle Tone, annotated with Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. Ontology label: Pharmacotherapy NCIT:C15986
Agent: baclofen CHEBI:2972 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses baclofen (CHEBI:2972). CHEBI:2972 is a therapeutic agent from Chemical Entities of Biological Interest. tizanidine CHEBI:63629 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses tizanidine (CHEBI:63629). CHEBI:63629 is a therapeutic agent from Chemical Entities of Biological Interest.
Platform: Small molecule
Specialists may consider baclofen or tizanidine for clinically significant hypertonia or dystonia, with rehabilitation and attention to adverse effects. These are symptomatic options without BLBS-specific efficacy trials.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"consider involving appropriate specialists to aid in management of baclofen, tizanidine"
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
Specialist Focal Tone Management
Action: Specialist Focal Tone ManagementNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Specialist Focal Tone Management, annotated with Therapeutic Procedure (NCIT:C49236). NCIT:C49236 is a clinical intervention from the NCI Thesaurus. Ontology label: Therapeutic Procedure NCIT:C49236
Platform: Other
Botulinum toxin and orthopedic procedures are among the options listed by GeneReviews for selected muscle-tone problems; intervention depends on pattern and functional goals.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"baclofen, tizanidine, Botox ... anti-parkinsonian medications, or orthopedic procedures."
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
Constipation Treatment
Action: Constipation TreatmentNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Constipation Treatment, annotated with Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. Ontology label: Pharmacotherapy NCIT:C15986
Agent: Laxative NCIT:C29697 NCI Thesaurus (NCIT) Relation: this treatment uses this therapeutic agent This treatment uses Laxative (NCIT:C29697). NCIT:C29697 is a therapeutic agent from the NCI Thesaurus.
Platform: Small molecule
Use clinically appropriate stool softeners, osmotic agents, prokinetics or laxatives, with regular assessment of bowel symptoms.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Stool softeners, prokinetics, osmotic agents, or laxatives as needed"
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
Behavioral and Educational Support
Action: Behavioral and Educational SupportNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Behavioral and Educational Support, annotated with Therapeutic Procedure (NCIT:C49236). NCIT:C49236 is a clinical intervention from the NCI Thesaurus. Ontology label: Therapeutic Procedure NCIT:C49236
Platform: Behavioral / lifestyle
Individualized educational and behavioral interventions address developmental disability, autism-related needs, attention difficulties and anxiety.
Show evidence (1 reference)
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Standard treatment for developmental delay / intellectual disability, attention-deficit/hyperactivity disorder, autism spectrum disorder, anxiety"
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
Vision and Hearing Support
Action: Vision and Hearing SupportNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Vision and Hearing Support, annotated with Therapeutic Procedure (NCIT:C49236). NCIT:C49236 is a clinical intervention from the NCI Thesaurus. Ontology label: Therapeutic Procedure NCIT:C49236
Platform: Other
Ophthalmology and audiology guide correction of refractive error or strabismus, low-vision support and hearing aids when appropriate.
Show evidence (1 reference)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Hearing aids may be helpful per otolaryngologist."
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
Management of Congenital and Endocrine Complications
Action: Management of Congenital and Endocrine ComplicationsNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Management of Congenital and Endocrine Complications, annotated with Therapeutic Procedure (NCIT:C49236). NCIT:C49236 is a clinical intervention from the NCI Thesaurus. Ontology label: Therapeutic Procedure NCIT:C49236
Platform: Other
Craniofacial, cardiology, urology and endocrinology teams treat craniosynostosis, cardiac defects, undescended testes and hypothyroidism according to the individual findings.
Show evidence (1 reference)
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Standard treatment for developmental delay / intellectual disability, attention-deficit/hyperactivity disorder, autism spectrum disorder, anxiety, epilepsy, spasticity, constipation, craniosynostosis, vision issues, hearing loss, congenital heart defects, undescended testes, and hypothyroidism."
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
Structured Surveillance
Action: Structured SurveillanceNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Structured Surveillance, annotated with Therapeutic Procedure (NCIT:C49236). NCIT:C49236 is a clinical intervention from the NCI Thesaurus. Ontology label: Therapeutic Procedure NCIT:C49236
Platform: Other
At visits assess growth, nutrition, oral intake, constipation, seizures, tone, gait, development and behavior. Ophthalmology is generally annual or as indicated; audiology and thyroid testing follow clinical indications in the detailed GeneReviews table. New decline warrants evaluation rather than being assumed inevitable.
Show evidence (1 reference)
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"At each visit: measure growth parameters; assess nutrition safety and oral intake; monitor for constipation; assess for new manifestations, such as seizures and changes in tone and/or gait; monitor developmental progress and educational needs; and assess for behavioral issues, such as mobility..."
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
Genetic Counseling
Action: Genetic CounselingNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Genetic Counseling (NCIT:C15240). NCIT:C15240 is a clinical intervention from the NCI Thesaurus. NCIT:C15240
Platform: Other
Test parents and explain autosomal dominant transmission. When the familial variant is absent from parental blood, GeneReviews estimates about 1% sibling recurrence from possible germline mosaicism; a known heterozygous parent transmits the variant with a 50% chance per pregnancy. Prenatal and preimplantation testing are possible once the variant is known.
Show evidence (2 references)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"the recurrence risk to sibs is estimated to be 1% because of the theoretic possibility of parental"
Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Once an H3-3A (H3F3A) or H3-3B (H3F3B) pathogenic variant has been identified in an affected family member, prenatal and preimplantation genetic testing are possible."
Supports reproductive testing options after molecular diagnosis.
🔬

Diagnosis

2
Molecular genetic testing of H3-3A
Establish BLBS1 by a heterozygous pathogenic or likely pathogenic H3-3A variant in a compatible clinical context. A VUS does not establish the diagnosis. Broad neurodevelopmental panels, exome or genome sequencing are appropriate because the clinical features overlap many disorders. Review coverage when testing is negative: the founding study identified two terminal-exon H3-3A variants by genome sequencing after missed exome capture.
Show evidence (4 references)
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"The diagnosis of BRYLIB is established in a proband with suggestive findings and a heterozygous pathogenic variant in either H3-3A (H3F3A) or H3-3B (H3F3B) identified by molecular genetic testing."
GeneReviews statement of the diagnostic criterion.
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"These patients were identified through exome or genome sequencing performed for the indication of neurodevelopmental delays and/or congenital anomalies"
Documents the ascertainment route through which the diagnosis is practically made.
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"variant of uncertain significance does not establish or rule out the diagnosis."
GeneReviews explicitly cautions that a variant of uncertain significance is not diagnostic.
+ 1 more reference
Baseline clinical assessment
Assess development, behavior, growth, nutrition and swallowing; obtain neurologic examination and brain MRI, with EEG for suspected seizures. Evaluate vision, hearing, thyroid function, cardiac anatomy and the testes as indicated by the initial assessment. Abnormal head shape warrants craniofacial review and imaging when craniosynostosis is suspected. These evaluations characterize complications rather than replace molecular diagnosis.
Show evidence (2 references)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"To incl brain MRI ... Consider EEG if seizures are a concern."
Table 3 specifies baseline brain imaging and clinically indicated EEG.
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"To assess for unrecognized congenital heart disease, specifically atrial septal defects"
The baseline table recommends echocardiography to assess cardiac disease.
📈

Progression

2
Neurodevelopmental phase
Age: infancy to childhood
Developmental delay is evident from infancy. Many affected individuals never achieve independent sitting, walking or speech, though outcomes range. Tone abnormality may resolve or evolve with age rather than remaining fixed.
Show evidence (1 reference)
PMID:37782742 SUPPORT DIRECT REVIEW SYNTHESIS Human Clinical
"Many affected individuals do not achieve independent sitting, walking, or speaking, although there is a range of developmental outcomes."
Establishes both the severity and the variability of the developmental outcome.
Variable regression and later motor change
Age: childhood to adulthood
Regression is reported in a minority and can be intermittent, severe or unrelated to seizures. The founding pooled cohort reported 9/46 with neurologic degeneration; this is about 20%, despite a 21% figure in its prose. GeneReviews describes subacute third-decade motor change followed by relative stability in three reported adults. A separate ten-person series, including an adult with H3-3A disease, reported no progressive neurologic disease. These observations do not establish an inevitable two-phase course.
Show evidence (4 references)
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"Developmental regression can occur in a minority of affected individuals. Regression is not continuous and is not always associated with seizures."
The clinical synthesis distinguishes intermittent regression from continuous decline.
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader SUPPORT INDIRECT REVIEW SYNTHESIS Human Clinical
"All reported adults have had a new subacute onset of motor issues in the third decade of life that generally remains stable after onset."
This statement refers to a very small adult series, not all future adults with BLBS1.
PMID:34876591 REFUTE DIRECT PRIMARY RESULT Human Clinical
"none was reported to have progressive neurologic disease."
The separate clinical series limits a universal progression claim.
+ 1 more reference
📊

Prevalence

1
Worldwide
Cases In Literature Ultra Rare
No population rate has been published. The published denominator is the global cohort assembled in 2024: of 96 individuals with BLBS, 65 carried an H3-3A variant. Ascertainment is by exome or genome sequencing for undiagnosed neurodevelopmental disease, so this is a discovery count rather than an epidemiologic estimate.
Show evidence (1 reference)
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"In total, 65 individuals harbor variants in H3-3A and 31 individuals harbor variants in H3-3B."
Gives the H3-3A-specific reported count within the combined BLBS cohort.
🔀

Differential Diagnoses

2

Conditions with similar clinical presentations that must be differentiated from Bryant-Li-Bhoj Neurodevelopmental Syndrome 1:

Bryant-Li-Bhoj neurodevelopmental syndrome 2 MONDO:0030607
Overlapping Features The H3-3B counterpart. The two genes encode an identical protein, so the entities are separated by gene rather than by protein product and are clinically overlapping. Only molecular testing distinguishes them. Cohort-level trends exist but are not diagnostic discriminators, and the H3-3A cohort differs in that every variant with parental data has been de novo and missense, whereas H3-3B has yielded synonymous, stop-loss and one inherited allele.
Distinguishing Features
  • Determined solely by molecular testing: a pathogenic variant in H3-3A rather than H3-3B
  • Greater variant-type and inheritance diversity is reported for H3-3B, including a synonymous stop-gain, a stop-loss and one inherited allele
Show evidence (1 reference)
PMID:38678163 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"Conversely, while most variants in H3-3B are heterozygous de novo missense variants, more variability in variant type and modes of inheritance was observed."
States the variant-type difference between the two genes that this differential rests on.
Somatic H3.3 oncohistone disease
Overlapping Features Somatic H3.3 hotspot variants occur in several tumors. A tumor-acquired variant and a constitutional variant define different clinical contexts, which require attention to tissue tested, variant interpretation and phenotype. Some molecular effects overlap, including cis effects of G34 substitutions. Absence of cancer in early BLBS cohorts does not establish absence of lifetime risk.
Distinguishing Features
  • Tumor-restricted versus constitutional variant distribution
  • Neoplasm-focused presentation versus a developmental syndrome
  • Variant- and tissue-dependent molecular effects; no uniform cis-versus-trans distinction
Show evidence (2 references)
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT In Vitro
"Patient histone posttranslational modifications (PTMs) analysis revealed notably aberrant local PTM patterns distinct from the somatic lysine mutations that cause global PTM dysregulation."
Gives the molecular basis on which the germline and somatic diseases are separated.
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT Human Clinical
"These data suggest that the mechanism of germline mutations are distinct from cancer-associated somatic histone mutations but may converge on control of cell proliferation."
The authors' own framing of the relationship between the two diseases.
🧫

Experimental Models

6
Patient-derived dermal fibroblasts PRIMARY_CELL_CULTURE
Three H3-3A patient lines carrying mature-histone R17G, G90R or T45I were compared with three controls by RNA sequencing. Proliferation experiments included five patient lines from both H3.3 genes and six controls.
Organism
Homo sapiens NCBITaxon:9606 NCBI Taxonomy (NCBITaxon) Relation: this experimental model is built in this organism This experimental model is built in Homo sapiens (NCBITaxon:9606). NCBITaxon:9606 is an organism from the NCBI Taxonomy.
Cell source
Patient-derived dermal fibroblasts and unrelated controls
Publication
PMID:33268356
Pathograph links
Deregulated Cell Proliferation partially recapitulates fidelity: moderate Three H3-3A patient lines carrying mature-histone R17G, G90R or T45I were compared with three controls by RNA sequencing. Proliferation experiments included five patient lines from both H3.3 genes and six controls.
Limitations: Non-neural experimental context; transfer to the developing human brain requires corroboration.
Show evidence (1 reference)
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT In Vitro
"RNA sequencing on patient cells demonstrated up-regulated gene expression related to mitosis and cell division, and cellular assays confirmed an increased proliferative capacity."
The full text identifies the H3-3A RNA-seq lines and the mixed-gene proliferation comparison.
Show evidence (1 reference)
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT In Vitro
"RNA sequencing on patient cells demonstrated up-regulated gene expression related to mitosis and cell division, and cellular assays confirmed an increased proliferative capacity."
The full text identifies the H3-3A RNA-seq lines and the mixed-gene proliferation comparison.
Patient-derived EBV lymphoblastoid cells CELL_LINE
Histone mass spectrometry in lymphoblastoid cells, including H3-3A A15G, R17G and T45I, assays local and global post-translational modifications.
Organism
Homo sapiens NCBITaxon:9606 NCBI Taxonomy (NCBITaxon) Relation: this experimental model is built in this organism This experimental model is built in Homo sapiens (NCBITaxon:9606). NCBITaxon:9606 is an organism from the NCBI Taxonomy.
Cell source
EBV-transformed patient lymphoblastoid cell lines
Publication
PMID:33268356
Pathograph links
Aberrant Local Histone Post-Translational Modification partially recapitulates fidelity: moderate Histone mass spectrometry in lymphoblastoid cells, including H3-3A A15G, R17G and T45I, assays local and global post-translational modifications.
Limitations: Non-neural experimental context; transfer to the developing human brain requires corroboration.
Show evidence (1 reference)
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT In Vitro
"cause marked local deregulation of chromatin state, and modestly alter the global control of histone modifications."
Patient-derived cell assays support mixed local and global effects.
Show evidence (1 reference)
PMID:33268356 SUPPORT DIRECT PRIMARY RESULT In Vitro
"cause marked local deregulation of chromatin state, and modestly alter the global control of histone modifications."
Patient-derived cell assays support mixed local and global effects.
HEK293T H3-3A expression and DAXX assays CELL_LINE
Transient expression measures variant abundance, localization and DAXX co-immunoprecipitation. H3-3A R129H/M121I reduce abundance, R41C increases it, and R129H strengthens DAXX association.
Organism
Homo sapiens NCBITaxon:9606 NCBI Taxonomy (NCBITaxon) Relation: this experimental model is built in this organism This experimental model is built in Homo sapiens (NCBITaxon:9606). NCBITaxon:9606 is an organism from the NCBI Taxonomy.
Cell source
Transiently transfected HEK293T cells
Publication
PMID:34876591
Pathograph links
Variant-Dependent Histone H3.3 Abundance partially recapitulates fidelity: moderate Transient expression measures variant abundance, localization and DAXX co-immunoprecipitation. H3-3A R129H/M121I reduce abundance, R41C increases it, and R129H strengthens DAXX association.
Limitations: Non-neural experimental context; transfer to the developing human brain requires corroboration.
Show evidence (2 references)
PMID:34876591 SUPPORT DIRECT PRIMARY RESULT In Vitro
"The Co-IP assays showed that p.R129H had a significantly stronger interaction with DAXX, suggesting that this variant might affect the deposition of histone H3.3"
The interaction was measured; altered deposition was inferred. HGVS p.Arg129His corresponds to mature-histone R128H.
+ 1 more reference
Altered H3.3 Chaperone Interaction partially recapitulates fidelity: moderate Transient expression measures variant abundance, localization and DAXX co-immunoprecipitation. H3-3A R129H/M121I reduce abundance, R41C increases it, and R129H strengthens DAXX association.
Limitations: Non-neural experimental context; transfer to the developing human brain requires corroboration.
Show evidence (2 references)
PMID:34876591 SUPPORT DIRECT PRIMARY RESULT In Vitro
"The Co-IP assays showed that p.R129H had a significantly stronger interaction with DAXX, suggesting that this variant might affect the deposition of histone H3.3"
The interaction was measured; altered deposition was inferred. HGVS p.Arg129His corresponds to mature-histone R128H.
+ 1 more reference
Show evidence (2 references)
PMID:34876591 SUPPORT DIRECT PRIMARY RESULT In Vitro
"The Co-IP assays showed that p.R129H had a significantly stronger interaction with DAXX, suggesting that this variant might affect the deposition of histone H3.3"
The interaction was measured; altered deposition was inferred. HGVS p.Arg129His corresponds to mature-histone R128H.
PMID:34876591 SUPPORT DIRECT PRIMARY RESULT In Vitro
"Three variant constructs (p.R129H, p.M121I, and p.I52N) showed significant decrease in protein expression, while one variant (p.R41C) accumulated at greater levels than wild-type control."
Transient HEK293T expression: R129H, M121I and R41C are H3-3A alleles; I52N is H3-3B and is not assigned to BLBS1.
Inducible HEK293 H3.3 BioID2 models CELL_LINE
The 2025 preprint compares isogenic Flp-In T-REx cells expressing tagged wild-type or mutant H3.3, including H3-3A-associated T45I and G90R, for proximal protein associations and histone methylation. Fusion proteins represent less than 20% of total H3.3. Constructs use mouse H3.3 cDNA encoding the identical human protein sequence.
Organism
Homo sapiens NCBITaxon:9606 NCBI Taxonomy (NCBITaxon) Relation: this experimental model is built in this organism This experimental model is built in Homo sapiens (NCBITaxon:9606). NCBITaxon:9606 is an organism from the NCBI Taxonomy.
Cell source
Isogenic inducible HEK293 Flp-In T-REx cell lines
Publication
DOI:10.1101/2025.09.09.674984
Pathograph links
Variant-Dependent H3K36 Methylation partially recapitulates fidelity: moderate The 2025 preprint compares isogenic Flp-In T-REx cells expressing tagged wild-type or mutant H3.3, including H3-3A-associated T45I and G90R, for proximal protein associations and histone methylation. Fusion proteins represent less than 20% of total H3.3.
Limitations: Preprint; proximity labeling does not prove direct binding. The cell model is not representative of postmitotic neurons for every endpoint.
Show evidence (1 reference)
url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full SUPPORT DIRECT PRIMARY RESULT In Vitro
"Each lineage expressed the histone constructs at comparable levels from a single defined genomic locus."
The experimental design reduces expression-site variation; tagged cycling-cell models remain distinct from patient neurons.
Altered Recruitment of PWWP-Domain Effectors partially recapitulates fidelity: moderate The 2025 preprint compares isogenic Flp-In T-REx cells expressing tagged wild-type or mutant H3.3, including H3-3A-associated T45I and G90R, for proximal protein associations and histone methylation. Fusion proteins represent less than 20% of total H3.3.
Limitations: Preprint; proximity labeling does not prove direct binding. The cell model is not representative of postmitotic neurons for every endpoint.
Show evidence (1 reference)
url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full SUPPORT DIRECT PRIMARY RESULT In Vitro
"Each lineage expressed the histone constructs at comparable levels from a single defined genomic locus."
The experimental design reduces expression-site variation; tagged cycling-cell models remain distinct from patient neurons.
Preprint; proximity labeling does not prove direct binding. The cell model is not representative of postmitotic neurons for every endpoint.
Show evidence (1 reference)
url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full SUPPORT DIRECT PRIMARY RESULT In Vitro
"Each lineage expressed the histone constructs at comparable levels from a single defined genomic locus."
The experimental design reduces expression-site variation; tagged cycling-cell models remain distinct from patient neurons.
Yeast histone H3 BLBS substitution panel OTHER
Nine substitutions in endogenous HHT2 were tested with deletion of HHT1, making mutant H3 the sole source. T45I and R83C permit cryptic transcription; only N108S among the nine traps FACT at gene ends. Earlier L61R studies used coexpressed wild-type H3 and are not directly comparable.
Organism
Saccharomyces cerevisiae NCBITaxon:4932 NCBI Taxonomy (NCBITaxon) Relation: this experimental model is built in this organism This experimental model is built in Saccharomyces cerevisiae (NCBITaxon:4932). NCBITaxon:4932 is an organism from the NCBI Taxonomy.
Cell source
S288C-derived yeast strains
Publication
PMID:39525290
Pathograph links
Variant-Specific Transcription-Coupled Chromatin Defects partially recapitulates fidelity: moderate Nine substitutions in endogenous HHT2 were tested with deletion of HHT1, making mutant H3 the sole source. T45I and R83C permit cryptic transcription; only N108S among the nine traps FACT at gene ends. Earlier L61R studies used coexpressed wild-type H3 and are not directly comparable.
Limitations: Yeast native H3 is a surrogate. This panel does not establish that all tested protein substitutions occur in H3-3A patients; A114G has an unresolved sequencing-artifact concern.
Show evidence (1 reference)
PMID:39525290 SUPPORT INDIRECT PRIMARY RESULT In Vitro
"these experiments assessed the effects of the H3-BLBS mutants when present as sole source of histone H3 in cells."
This genotype differs fundamentally from heterozygous human disease.
Yeast native H3 is a surrogate. This panel does not establish that all tested protein substitutions occur in H3-3A patients; A114G has an unresolved sequencing-artifact concern.
Show evidence (1 reference)
PMID:39525290 SUPPORT INDIRECT PRIMARY RESULT In Vitro
"these experiments assessed the effects of the H3-BLBS mutants when present as sole source of histone H3 in cells."
This genotype differs fundamentally from heterozygous human disease.
Reconstituted H3.3 nucleosome methylation assays OTHER
The 2025 preprint compares reconstituted nucleosomes made with mammalian H3.3 and Xenopus H2A/H2B/H4, using bacterially expressed proteins and SETD2 or NSD1 catalytic fragments. Mouse H3.3 cDNA encodes a protein identical to human H3.3. T45I stimulates methylation, L48R inhibits it, and V46M has little effect. This cell-free biochemical system does not reproduce a neuronal context.
Publication
DOI:10.1101/2025.09.09.674984
Pathograph links
Variant-Dependent H3K36 Methylation measures fidelity: moderate Measures variant-dependent methyltransferase activity on reconstituted substrates.
Limitations: Preprint; recombinant fragments and nucleosome substrates do not establish downstream neuronal function or treatment efficacy.
Show evidence (1 reference)
url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full SUPPORT DIRECT PRIMARY RESULT In Vitro
"H3.3 T45I stimulates the enzymatic activities of SETD2 and NSD1"
Preprint biochemical nucleosome assays support increased activity toward the recurrent H3-3A T45I substrate; this does not establish a treatment target in patients.
Show evidence (1 reference)
url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full SUPPORT DIRECT PRIMARY RESULT In Vitro
"H3.3 T45I stimulates the enzymatic activities of SETD2 and NSD1"
Preprint biochemical nucleosome assays support increased activity toward the recurrent H3-3A T45I substrate; this does not establish a treatment target in patients.
🐁

Animal Models

3
H3.3 T45I knock-in mouse
The 2026 preprint characterizes a C57BL/6J knock-in carrying a recurrent human H3-3A allele. Pups have delayed milestones and postnatal growth restriction; adult gait effects are more pronounced in males, while aged 60-week animals of both sexes have marked rotarod impairment. Nest-building and male aggression differ, but tested learning, memory and social preference are preserved. Brain histology was examined, with no genotype differences in the reported neuronal, astrocyte, Purkinje-cell or microglial counts. Gross brain endocast measures were also unchanged. This is partial phenotypic correspondence, not proof of a universal two-stage human course.
Species
Mus musculus
Genotype
Heterozygous H3f3a c.137C>T, p.Thr46Ile (mature-histone T45I)
Publication
DOI:10.64898/2026.06.16.732665
Pathograph links
Abnormal Brain Development partially recapitulates fidelity: moderate Developmental and motor phenotypes overlap with part of the human syndrome.
Limitations: Preprint, one allele and mouse strain. Motor impairment does not itself establish intellectual disability, cortical malformation or the proposed age-dependent H3.3-replacement mechanism. The negative adult cell-count assays are not proof of normal brain function at every age.
Show evidence (3 references)
DOI:10.64898/2026.06.16.732665 Preprint · not peer-reviewed SUPPORT DIRECT PRIMARY RESULT Model Organism
"mice recapitulate the BLBS natural history: perinatal growth restriction, delayed developmental milestones, and progressive motor and gait impairments."
Preprint abstract summary; detailed results differ by age, sex and assay and only partially reproduce the human syndrome.
+ 2 more references
Neural Crest and Craniofacial Developmental Defect partially recapitulates fidelity: low Adult craniofacial appearance and an enlarged spheno-occipital growth center provide a morphological comparison.
Limitations: No lineage-specific demonstration that the mouse craniofacial changes arise through the zebrafish neural-crest mechanism.
Show evidence (1 reference)
DOI:10.64898/2026.06.16.732665 Preprint · not peer-reviewed SUPPORT DIRECT PRIMARY RESULT Model Organism
"Adult mice additionally display craniofacial differences, impaired nest building, hyperactivity in a social context, and male-specific elevated aggression."
The model reports appearance changes; a neural-crest mechanism was not tested.
Show evidence (3 references)
DOI:10.64898/2026.06.16.732665 Preprint · not peer-reviewed SUPPORT DIRECT PRIMARY RESULT Model Organism
"mice recapitulate the BLBS natural history: perinatal growth restriction, delayed developmental milestones, and progressive motor and gait impairments."
Preprint abstract summary; detailed results differ by age, sex and assay and only partially reproduce the human syndrome.
url:https://www.biorxiv.org/content/10.64898/2026.06.16.732665v1.full REFUTE DIRECT PRIMARY RESULT Model Organism
"Immunohistochemical analyses of adult brain tissue revealed no significant differences between genotypes in neuronal density (NeuN), astrocyte burden (GFAP), or Purkinje cell number (Calbindin)"
The full preprint reports actual negative histological comparisons, not absence of histological investigation.
url:https://www.biorxiv.org/content/10.64898/2026.06.16.732665v1.full REFUTE DIRECT PRIMARY RESULT Model Organism
"all mice demonstrated appropriate social preference and social memory in the three-chamber task"
The reported assays do not establish autism-like social preference deficits.
Zebrafish h3f3a D123N db1092 model
The db1092 allele is D123N, not a homozygous null. Heterozygotes have variable craniofacial defects and homozygotes more severe losses. The 2020 glial experiment includes additional dominant-negative H3f3a RNA, producing a stronger perturbation. The original 2012 report found peripheral glia unaffected in its protocol. The allele is not a reported human patient allele and fish have five H3.3 genes.
Species
Danio rerio
Genotype
h3f3a db1092 dominant-negative missense allele, mature-histone D123N; heterozygous and homozygous animals, with additional RNA perturbation in selected experiments
Publication
PMID:33268356
Pathograph links
Neural Crest and Craniofacial Developmental Defect partially recapitulates fidelity: moderate Disrupted cranial neural-crest specification and ectomesenchyme formation cause craniofacial loss.
Limitations: A non-patient dominant-negative allele, dosage-dependent perturbation and peripheral neural-crest readouts. These experiments do not establish a CNS myelination defect or model human intellectual disability.
Readouts
Neural-crest-derived jaw cartilage
Direction: DECREASED
Interpretation: Loss of jaw cartilage in D123N homozygotes.
Show evidence (1 reference)
PMID:33268356 SUPPORT INDIRECT PRIMARY RESULT Model Organism
"Homozygous h3f3adb1092 mutants display complete loss of neural crest–derived jaw cartilages (n = 10/10)."
Homozygotes carry two copies of the D123N missense allele.
Show evidence (3 references)
PMID:33268356 SUPPORT INDIRECT PRIMARY RESULT Model Organism
"Homozygous h3f3adb1092 mutants display complete loss of neural crest–derived jaw cartilages (n = 10/10)."
Homozygotes carry two copies of the D123N missense allele.
+ 2 more references
Show evidence (3 references)
PMID:33268356 SUPPORT INDIRECT PRIMARY RESULT Model Organism
"Homozygous h3f3adb1092 mutants display complete loss of neural crest–derived jaw cartilages (n = 10/10)."
Homozygotes carry two copies of the D123N missense allele.
PMID:33268356 SUPPORT INDIRECT PRIMARY RESULT Model Organism
"Homozygous h3f3adb1092 mutants injected at the one-cell stage with a control mCherry RNA show partial reductions in cranial glia"
The 2020 protocol reports partial reduction; added dominant-negative RNA increases the severity.
PMID:23028350 REFUTE INDIRECT PRIMARY RESULT Model Organism
"foxd3-positive peripheral glia (Figure 1l), neurons of the cranial ganglia (Figure S1), and the dorsal root ganglia and sympathetic neurons derived from trunk NC (data not shown) were unaffected."
The original 2012 experiment did not detect this glial deficit. Protocol and perturbation strength differ from the 2020 experiment.
Mouse H3f3a G34R/V/W mechanistic comparison
Engineered H3f3a G34R causes progressive microcephaly, cortical-layer thinning, neuronal depletion and reactive glial accumulation. G34V has milder neurologic effects and G34W predominantly mesenchymal/urinary phenotypes. G34R reduces H3K36me2, redistributes DNMT3A, depletes non-CG methylation and sustains complement-gene expression. Forebrain conditional G34R also produces cortical atrophy. Complement-mediated causality was proposed and was not established by rescue. The human G34R/V cases motivating this study have H3-3B variants; these mice are a protein-level mechanistic comparison, not evidence that those patients had BLBS1.
Species
Mus musculus
Genotype
Heterozygous endogenous H3f3a G34R, G34V or G34W; additional Cre-dependent H3f3a G34R models
Publication
PMID:36931244
Pathograph links
Altered Recruitment of PWWP-Domain Effectors perturbs fidelity: low Perturbs H3K36-dependent DNMT3A recruitment with downstream DNA methylation and transcriptional changes.
Limitations: Opposite effects of other alleles, including T45I, prevent generalizing G34R-associated loss of DNMT3A binding to all H3-3A disease. Human cases in this study are H3-3B-related.
Show evidence (1 reference)
PMID:36931244 SUPPORT INDIRECT PRIMARY RESULT Model Organism
"the G34R mutation diminishes H3K36me2 and consequently redistributes DNMT3A from intergenic regions to CpG islands."
Mouse H3f3a provides mechanistic comparison for the shared protein. The human G34R/V patients in this paper had H3-3B variants, so this is indirect evidence for BLBS1.
Show evidence (2 references)
PMID:36931244 SUPPORT INDIRECT PRIMARY RESULT Model Organism
"the G34R mutation diminishes H3K36me2 and consequently redistributes DNMT3A from intergenic regions to CpG islands."
Mouse H3f3a provides mechanistic comparison for the shared protein. The human G34R/V patients in this paper had H3-3B variants, so this is indirect evidence for BLBS1.
PMID:36931244 SUPPORT INDIRECT PRIMARY RESULT Model Organism
"whether the disease phenotype is directly caused by persistence of immune gene activation (C3 and C1q) in neurons as we hypothesize"
The authors explicitly identify the complement-to-neurodegeneration step as requiring further tests.
{ }

Source YAML

click to show 
name: Bryant-Li-Bhoj Neurodevelopmental Syndrome 1
creation_date: "2026-08-29T16:30:00Z"
category: Mendelian
categories:
- Chromatinopathy
- Germline Histonopathy
- Neurodevelopmental Disorder
synonyms:
- BLBS1
- BRYLIB1
- Bryant-Li-Bhoj syndrome 1
- H3-3A-related neurodevelopmental syndrome
- H3F3A-related neurodevelopmental disorder
description: >-
  Bryant-Li-Bhoj neurodevelopmental syndrome 1 is an autosomal dominant disorder caused by heterozygous pathogenic
  H3-3A (H3F3A) variants, usually de novo missense substitutions. H3-3A and H3-3B encode the same replication-independent
  histone H3.3 protein, but define separate genetic diagnoses. Developmental delay or intellectual disability,
  often severe, accompanies variable abnormalities of muscle tone, seizures, craniofacial appearance, growth
  and brain structure. Regression and later motor decline occur in some individuals, but an obligatory adult
  degenerative phase is not established. Functional effects differ between alleles and include altered histone
  abundance, chaperone interaction, local histone modification and transcription; a single gain-of-function
  mechanism has not been demonstrated for all variants.
disease_term:
  preferred_term: Bryant-Li-Bhoj neurodevelopmental syndrome 1
  term:
    id: MONDO:0030606
    label: Bryant-Li-Bhoj neurodevelopmental syndrome 1
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0030606
      label: Bryant-Li-Bhoj neurodevelopmental syndrome 1
    mapping_predicate: skos:exactMatch
    mapping_source: MONDO
    mapping_justification: Primary MONDO disease identifier for this entry.
parents:
- syndromic disease
- neurodevelopmental disorder
- hereditary disease
classifications:
  harrisons_chapter:
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
    notes: >-
      A Mendelian disorder of a core chromatin structural protein; the entry is
      organised around the genotype-to-chromatin mechanism.
  - classification_value: NEUROLOGIC
    notes: >-
      Developmental delay, seizures, tone abnormalities and variable motor impairment dominate the clinical
      burden.
inheritance:
- name: Autosomal dominant inheritance
  description: >-
    Autosomal dominant inheritance. In the 2024 cohort, H3-3A variants were heterozygous de novo missense variants
    whenever parental results were available; inheritance was unknown for three individuals. A negative parental
    blood test does not exclude germline mosaicism. Allele counts across H3-3A and H3-3B do not establish equal
    protein dosage or a uniform functional mechanism.
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "BRYLIB is expressed in an autosomal dominant manner and typically caused by a de novo H3-3A (H3F3A) or H3-3B (H3F3B) pathogenic variant."
    explanation: >-
      GeneReviews statement of the inheritance mode and the de novo origin.
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the time of this report, all H3-3A variants are heterozygous de novo missense variants when parents are available, though inheritance was undetermined for three individuals"
    explanation: >-
      The H3-3A-specific observation that distinguishes BLBS1 from BLBS2 on
      inheritance and variant type.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    No population rate has been published. The published denominator is the
    global cohort assembled in 2024: of 96 individuals with BLBS, 65 carried an
    H3-3A variant. Ascertainment is by exome or genome sequencing for
    undiagnosed neurodevelopmental disease, so this is a discovery count rather
    than an epidemiologic estimate.
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In total, 65 individuals harbor variants in H3-3A and 31 individuals harbor variants in H3-3B."
    explanation: >-
      Gives the H3-3A-specific reported count within the combined BLBS cohort.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
progression:
- phase: Neurodevelopmental phase
  age_range: infancy to childhood
  notes: >-
    Developmental delay is evident from infancy. Many affected individuals never
    achieve independent sitting, walking or speech, though outcomes range. Tone
    abnormality may resolve or evolve with age rather than remaining fixed.
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Many affected individuals do not achieve independent sitting, walking, or speaking, although there is a range of developmental outcomes."
    explanation: >-
      Establishes both the severity and the variability of the developmental
      outcome.
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
- phase: Variable regression and later motor change
  age_range: childhood to adulthood
  notes: Regression is reported in a minority and can be intermittent, severe or unrelated to seizures. The founding pooled cohort reported 9/46 with neurologic degeneration; this is about 20%, despite a 21% figure in its prose. GeneReviews describes subacute third-decade motor change followed by relative stability in three reported adults. A separate ten-person series, including an adult with H3-3A disease, reported no progressive neurologic disease. These observations do not establish an inevitable two-phase course.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Developmental regression can occur in a minority of affected individuals. Regression is not continuous and is not always associated with seizures.
    explanation: The clinical synthesis distinguishes intermittent regression from continuous decline.
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: All reported adults have had a new subacute onset of motor issues in the third decade of life that generally remains stable after onset.
    explanation: This statement refers to a very small adult series, not all future adults with BLBS1.
  - reference: PMID:34876591
    reference_title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: PRIMARY_RESULT
    snippet: none was reported to have progressive neurologic disease.
    explanation: The separate clinical series limits a universal progression claim.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: 9 of the 46 patients (21%) have demonstrated clinical neurologic degeneration
    explanation: The numerator and denominator imply about 20%; the quoted source percentage is an arithmetic inconsistency.
pathophysiology:
- name: De Novo H3-3A Variant Altering Histone H3.3
  biological_scale: MOLECULAR
  description: >-
    Heterozygous pathogenic H3-3A missense substitutions alter a core nucleosomal protein. Most are rare and
    distributed across the tail and core, but recurrent alleles occur. Dominant-negative or gain-of-function
    mechanisms have been proposed; variant-specific biochemical effects include both increases and decreases
    in measured abundance or interactions. Haploinsufficiency is not established as the general explanation.
  genes:
  - preferred_term: H3-3A
    term:
      id: hgnc:4764
      label: H3-3A
  molecular_functions:
  - preferred_term: structural constituent of chromatin
    modifier: ABNORMAL
    term:
      id: GO:0030527
      label: structural constituent of chromatin
  genetic_context:
    gene:
      preferred_term: H3-3A
      term:
        id: hgnc:4764
        label: H3-3A
    allele_type: MISSENSE
    variant_origin: DE_NOVO
    zygosity: HETEROZYGOUS
    description: >-
      De novo heterozygous missense variants with heterogeneous biochemical effects; a uniform functional category
      is not assigned.
  evidence:
  - &id001
    reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In this work, we have identified a cohort of 46 unrelated patients bearing de novo heterozygous germline missense mutations in H3F3A or H3F3B with a core phenotype of progressive neurologic dysfunction and congenital anomalies."
    explanation: >-
      The founding cohort establishing de novo heterozygous missense variants as
      the cause.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
  - reference: PMID:36867246
    reference_title: "Histone 3.3-related chromatinopathy: missense variants throughout H3-3A and H3-3B cause a range of functional consequences across species."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Most of the causative variants are private and scattered throughout the protein, but all seem to have either a gain-of-function or dominant negative effect on protein function."
    explanation: >-
      This review proposes gain-of-function or dominant-negative effects; it does not experimentally classify
      every allele.
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
  downstream:
  - target: Disrupted Nucleosome Assembly and Chaperone Interaction
    description: Structural modeling predicts altered DNA, histone or chaperone contacts.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:33268356
      reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
      supports: SUPPORT
      evidence_source: COMPUTATIONAL
      snippet: "Molecular modeling of all 37 variants demonstrated clear disruptions in interactions with DNA, other histones, and histone chaperone proteins."
      explanation: >-
        The structural modelling result this node records; evidence source is
        COMPUTATIONAL because the claim comes from in silico modelling.
      directness: DIRECT
      quote_role: PRIMARY_RESULT
  - target: Variant-Dependent Histone H3.3 Abundance
    description: Selected H3-3A variants alter steady-state protein abundance in transfected cells.
    causal_link_type: DIRECT
    evidence:
    - &id002
      reference: PMID:34876591
      reference_title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities."
      supports: SUPPORT
      evidence_source: IN_VITRO
      directness: DIRECT
      quote_role: PRIMARY_RESULT
      snippet: Three variant constructs (p.R129H, p.M121I, and p.I52N) showed significant decrease in protein expression, while one variant (p.R41C) accumulated at greater levels than wild-type control.
      explanation: 'Transient HEK293T expression: R129H, M121I and R41C are H3-3A alleles; I52N is H3-3B and is not assigned to BLBS1.'
  - target: Altered H3.3 Chaperone Interaction
    description: The R129H variant strengthens a measured chaperone interaction.
    causal_link_type: DIRECT
    evidence:
    - &id003
      reference: PMID:34876591
      reference_title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities."
      supports: SUPPORT
      evidence_source: IN_VITRO
      directness: DIRECT
      quote_role: PRIMARY_RESULT
      snippet: The Co-IP assays showed that p.R129H had a significantly stronger interaction with DAXX, suggesting that this variant might affect the deposition of histone H3.3
      explanation: The interaction was measured; altered deposition was inferred. HGVS p.Arg129His corresponds to mature-histone R128H.
  - target: Aberrant Local Histone Post-Translational Modification
    description: Patient-derived cells show altered histone modification patterns.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:33268356
      reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Together, these data suggest that the mutant histones can be incorporated into the nucleosome, cause marked local deregulation of chromatin state, and modestly alter the global control of histone modifications."
      explanation: >-
        Confirms incorporation of the mutant histone and locates the deregulation
        at the local chromatin level.
      directness: DIRECT
      quote_role: PRIMARY_RESULT
  - target: Variant-Dependent H3K36 Methylation
    description: T45I changes methyltransferase activity toward the mutant nucleosome.
    causal_link_type: DIRECT
    evidence:
    - &id004
      reference: url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full
      reference_title: "H3.3 De Novo Mutations Alter Lysine 36 Methylation via Distinct Mechanisms | bioRxiv"
      supports: SUPPORT
      evidence_source: IN_VITRO
      directness: DIRECT
      quote_role: PRIMARY_RESULT
      snippet: H3.3 T45I stimulates the enzymatic activities of SETD2 and NSD1
      explanation: Preprint biochemical nucleosome assays support increased activity toward the recurrent H3-3A T45I substrate; this does not establish a treatment target in patients.
  - target: Variant-Specific Transcription-Coupled Chromatin Defects
    description: A subset of histone substitutions perturbs transcription-coupled chromatin processes in yeast.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - &id006
      reference: PMID:39525290
      reference_title: "Trapping of yFACT at 3' ends of genes is not a universal characteristic of yeast versions of Bryant-Li-Bhoj syndrome histone H3 mutants."
      supports: SUPPORT
      evidence_source: IN_VITRO
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
      snippet: four mutants – H3-T45I, -R83C, -N108S, and -A114G – cause strong His ... phenotypes, suggesting that these H3-BLBS mutants may interfere with proper nucleosome reassembly or other chromatin processes.
      explanation: T45I and R83C establish H3-3A-relevant protein-level transcriptional defects in yeast. The A114G result does not establish its clinical validity.
  - target: Abnormal Brain Development
    description: Pathogenic H3-3A variants cause the clinical developmental disorder through incompletely resolved intermediates.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - *id001
  - target: Deregulated Cell Proliferation
    description: Patient fibroblasts carrying H3-3A variants show altered proliferative programs; the causal intermediates are unresolved.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:33268356
      reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
      supports: SUPPORT
      evidence_source: IN_VITRO
      directness: DIRECT
      quote_role: PRIMARY_RESULT
      snippet: RNA sequencing on patient cells demonstrated up-regulated gene expression related to mitosis and cell division, and cellular assays confirmed an increased proliferative capacity.
      explanation: Patient-cell association supports this cellular consequence without establishing a neuron-specific route.
- name: Disrupted Nucleosome Assembly and Chaperone Interaction
  biological_scale: MOLECULAR
  description: >-
    Structural modeling predicts changes at DNA contacts, inter-histone interfaces and chaperone-binding surfaces.
    Tail and core substitutions need not share one biochemical effect. Modeling does not prove universal nucleosome
    destabilization or demonstrate that every mutant is normally incorporated; those questions require allele-specific
    assays.
  molecular_functions:
  - preferred_term: nucleosomal DNA binding
    term:
      id: GO:0031492
      label: nucleosomal DNA binding
  biological_processes:
  - preferred_term: nucleosome assembly
    modifier: ABNORMAL
    term:
      id: GO:0006334
      label: nucleosome assembly
  cellular_components:
  - preferred_term: nucleosome
    term:
      id: GO:0000786
      label: nucleosome
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: "Molecular modeling of all 37 variants demonstrated clear disruptions in interactions with DNA, other histones, and histone chaperone proteins."
    explanation: >-
      The structural modelling result this node records; evidence source is
      COMPUTATIONAL because the claim comes from in silico modelling.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: "Molecular modeling revealed that mutations in the histone core likely affect nucleosome stability, while the mutations in the tail affect various protein-protein interactions."
    explanation: >-
      Gives the domain split between nucleosome destabilisation and disrupted
      protein-protein interaction.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
- name: Variant-Dependent Histone H3.3 Abundance
  biological_scale: MOLECULAR
  description: In transient HEK293T assays, H3-3A R129H and M121I reduce protein abundance whereas R41C increases it. The experiment measures steady-state expression and does not by itself distinguish altered synthesis from degradation or establish patient-neuronal dosage.
  evidence:
  - *id002
- name: Altered H3.3 Chaperone Interaction
  biological_scale: MOLECULAR
  description: H3-3A R129H increases DAXX co-immunoprecipitation despite reduced histone abundance. Nuclear localization is preserved. Increased binding does not prove greater chromatin deposition. A newer preprint finds loss of DAXX association for mature-histone G90R, underscoring assay- and allele-dependent effects.
  evidence:
  - *id003
  - reference: url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full
    reference_title: "H3.3 De Novo Mutations Alter Lysine 36 Methylation via Distinct Mechanisms | bioRxiv"
    supports: SUPPORT
    evidence_source: IN_VITRO
    directness: DIRECT
    quote_role: PRIMARY_RESULT
    snippet: there was a complete loss of associations between H3.3 G90R and DAXX
    explanation: Preprint BioID and immunoprecipitation identify loss of interaction for G90R; mature-histone G90R equals HGVS G91R.
- name: Aberrant Local Histone Post-Translational Modification
  biological_scale: MOLECULAR
  description: >-
    Patient-derived lymphoblast and fibroblast assays identify prominent local modification changes on mutant
    histones, with more modest global changes. H3-3A A15G affects nearby K9/K14 modification; the reported
    A29P/K27/K36 example belongs to H3-3B. These observations support cis effects without proving that all
    downstream consequences are local or that all BLBS variants differ uniformly from somatic oncohistones.
  biological_processes:
  - preferred_term: chromatin remodeling
    modifier: ABNORMAL
    term:
      id: GO:0006338
      label: chromatin remodeling
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Patient histone posttranslational modifications (PTMs) analysis revealed notably aberrant local PTM patterns distinct from the somatic lysine mutations that cause global PTM dysregulation."
    explanation: >-
      The abstract emphasizes local effects; full-text results also report modest global changes.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Together, these data suggest that the mutant histones can be incorporated into the nucleosome, cause marked local deregulation of chromatin state, and modestly alter the global control of histone modifications."
    explanation: >-
      This conclusion explicitly includes modest global effects.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
- name: Variant-Dependent H3K36 Methylation
  biological_scale: MOLECULAR
  description: A 2025 preprint identifies increased H3K36 methylation in cis for H3.3 T45I and stimulation of SETD2/NSD1 activity on recombinant mutant nucleosomes. Other substitutions have opposite effects. Structural modeling implicates contacts with the methyltransferase AWS domain; that structural explanation remains a model.
  evidence:
  - *id004
  - &id005
    reference: url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full
    reference_title: "H3.3 De Novo Mutations Alter Lysine 36 Methylation via Distinct Mechanisms | bioRxiv"
    supports: SUPPORT
    evidence_source: IN_VITRO
    directness: DIRECT
    quote_role: PRIMARY_RESULT
    snippet: increased H3.3K36me3 levels with S31F and T45I, and a complete to near complete loss of signal with G34V, K36E, and L48R
    explanation: Preprint cell assays show opposite allele-specific effects. The H3-3A T45I finding is relevant here; the other substitutions are protein-level comparators, not all BLBS1 alleles.
  biological_processes:
  - preferred_term: protein methylation
    term:
      id: GO:0006479
      label: protein methylation
    modifier: ABNORMAL
  downstream:
  - target: Altered Recruitment of PWWP-Domain Effectors
    description: Altered H3K36 methylation changes recruitment of methyl-lysine readers.
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    evidence:
    - *id005
  molecular_functions:
  - preferred_term: Methyltransferase activity toward variant H3.3 substrates
    term:
      id: GO:0046975
      label: histone H3K36 methyltransferase activity
    modifier: ABNORMAL
- name: Altered Recruitment of PWWP-Domain Effectors
  biological_scale: MOLECULAR
  description: Variant-dependent H3K36 modification alters association with PWWP-domain proteins, including DNMT3A/B. T45I increases selected reader associations in the preprint; this must not be replaced by a universal loss-of-DNMT3A claim derived from G34R. Consequences for DNA methylation and neuronal function require direct testing for individual H3-3A alleles.
  evidence:
  - reference: url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full
    reference_title: "H3.3 De Novo Mutations Alter Lysine 36 Methylation via Distinct Mechanisms | bioRxiv"
    supports: SUPPORT
    evidence_source: IN_VITRO
    directness: DIRECT
    quote_role: PRIMARY_RESULT
    snippet: the associations between PWWP and the S31 and T45I point mutants increased while those with G34V, K36E, and L48R generally decreased.
    explanation: Preprint proximity data show directionally distinct effects among substitutions.
- name: Deregulated Cell Proliferation
  biological_scale: CELLULAR
  description: >-
    RNA sequencing of three H3-3A patient fibroblast lines identified altered expression enriched for mitosis
    and cell division. Proliferation assays across five patient lines from both H3.3 genes found increased
    proliferative capacity. This is a non-neural cellular phenotype; neither a direct cause of neuronal dysfunction
    nor malignant transformation was demonstrated.
  biological_processes:
  - preferred_term: cell population proliferation
    modifier: INCREASED
    term:
      id: GO:0008283
      label: cell population proliferation
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "RNA sequencing on patient cells demonstrated up-regulated gene expression related to mitosis and cell division, and cellular assays confirmed an increased proliferative capacity."
    explanation: >-
      Provides both the transcriptional and the functional evidence for
      increased proliferation.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
- name: Variant-Specific Transcription-Coupled Chromatin Defects
  biological_scale: CELLULAR
  description: 'Yeast H3 mutants show separable defects: T45I and R83C permit cryptic intragenic transcription, while N108S traps FACT at gene 3-prime ends. Eight of nine newly tested mutants lack the FACT-trapping defect. Yeast native H3 expressed as the sole histone H3 source is a mechanistic surrogate, not a heterozygous human neuronal model. A114G is excluded from clinical variant inference because the study notes possible sequencing artifact.'
  evidence:
  - reference: PMID:39525290
    reference_title: "Trapping of yFACT at 3' ends of genes is not a universal characteristic of yeast versions of Bryant-Li-Bhoj syndrome histone H3 mutants."
    supports: SUPPORT
    evidence_source: IN_VITRO
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: only H3-N108S shows significant trapping of the Spt16 subunit of yFACT at the 3’ end
    explanation: Yeast native-H3 experiments identify a variant-specific defect. N108S is not assigned as a human H3-3A allele here.
  - reference: PMID:39525290
    reference_title: "Trapping of yFACT at 3' ends of genes is not a universal characteristic of yeast versions of Bryant-Li-Bhoj syndrome histone H3 mutants."
    supports: SUPPORT
    evidence_source: IN_VITRO
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: H3-T45I also confers sensitivity to the DNA-damaging agent methyl methanesulfonate (MMS)
    explanation: Yeast growth assays identify an additional T45I-associated stress phenotype; direct human relevance is uncertain.
  - *id006
  mechanism_confidence: PROVISIONAL
- name: Neural Crest and Craniofacial Developmental Defect
  biological_scale: TISSUE
  description: >-
    The zebrafish db1092 allele encodes dominant-negative D123N H3.3, rather than a null. It disrupts early
    cranial neural-crest specification and ectomesenchymal development, with loss of craniofacial cartilage.
    The 2020 study reports partial cranial-glial reductions in homozygotes and stronger deficits after added
    dominant-negative RNA. These peripheral neural-crest glia are not CNS radial glia or oligodendrocytes,
    so their loss does not establish the cause of human brain hypomyelination.
  biological_processes:
  - preferred_term: chromatin remodeling
    modifier: ABNORMAL
    term:
      id: GO:0006338
      label: chromatin remodeling
  evidence:
  - &id007
    reference: PMID:23028350
    reference_title: An essential role of variant histone H3.3 for ectomesenchyme potential of the cranial neural crest.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: we identified a dominant D123N mutation in h3f3a
    explanation: The original allele-defining experiment excludes the previous null annotation.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "A zebrafish model showed craniofacial anomalies and a defect in Foxd3-derived glia."
    explanation: >-
      The model-organism result underlying this node.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
  downstream:
  - target: Craniofacial Dysmorphism
    description: >-
      The fish phenotype supports a candidate neural-crest contribution to human dysmorphism; direct human
      lineage evidence is lacking.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - *id007
- name: Abnormal Brain Development
  biological_scale: ORGANISM
  description: >-
    Developmental delay, seizures and variable structural brain abnormalities define the clinical neurodevelopmental
    phenotype. The 2024 pooled cohort reported MRI abnormalities in 44/76 and callosal malformation/dysgenesis
    in 28/76. An enriched 18-person imaging series, including 11 H3-3A cases, found posterior-fossa, cortical
    and caudate abnormalities; those selected-subgroup rates are not population estimates.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  locations:
  - preferred_term: brain
    term:
      id: UBERON:0000955
      label: brain
  - preferred_term: corpus callosum
    term:
      id: UBERON:0002336
      label: corpus callosum
  evidence:
  - &id008
    reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This syndrome is characterized by developmental delay/intellectual disability, craniofacial anomalies, hyper/hypotonia, and abnormal neuroimaging"
    explanation: >-
      States the congenital phenotype cluster this node terminates in.
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Within this subgroup, 72% of individuals presented with small posterior fossa, 28% presented with dysgenesis of the corpus collosum, and 44% presented with cortical developmental abnormalities"
    explanation: >-
      Gives the structural findings from the detailed imaging subgroup that
      anchor the structural component of this node.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
  downstream:
  - target: Global Developmental Delay and Intellectual Disability
    description: >-
      The dominant clinical consequence of disrupted neural chromatin
      regulation.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - *id008
  - target: Abnormal Corpus Callosum Morphology
    description: >-
      The commonest specific structural finding on neuroimaging.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - *id008
  - target: Small Posterior Fossa
    description: Abnormal brain development includes this structural or myelination phenotype.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:35772801
      reference_title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumors."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
      snippet: Thirteen of 18 individuals (72%) presented with a small posterior fossa.
      explanation: Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific contribution remain unresolved. Primary imaging series; proportions are pooled and enriched for severe abnormalities.
    - reference: url:https://pmc.ncbi.nlm.nih.gov/articles/PMC9262070/
      reference_title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumors - PMC"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
      snippet: Our cohort was biased because all individuals included in our study except for 1 had severe clinical symptoms and significant abnormalities involving the brain
      explanation: Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific contribution remain unresolved. The full primary study explicitly states the selection limitation.
  - target: Cortical Malformation
    description: Abnormal brain development includes this structural or myelination phenotype.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:35772801
      reference_title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumors."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
      snippet: Cortical developmental abnormalities were noted in 8 individuals (44%), with dysgyria and hypoplastic temporal poles being the most frequent presentation.
      explanation: Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific contribution remain unresolved. Primary imaging series; proportions are pooled and enriched for severe abnormalities.
  - target: Chiari I Malformation
    description: Abnormal brain development includes this structural or myelination phenotype.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
      reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      quote_role: REVIEW_SYNTHESIS
      snippet: Chiari I malformations were seen in 4/18 (22%) of individuals imaged.
      explanation: Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific contribution remain unresolved. Selected imaging-subgroup finding, not a population frequency.
  - target: CNS Hypomyelination
    description: Abnormal brain development includes this structural or myelination phenotype.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
      reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      quote_role: REVIEW_SYNTHESIS
      snippet: About 25% of affected individuals show evidence of hypomyelination or delayed myelination.
      explanation: Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific contribution remain unresolved. The synthesis establishes both reported categories while leaving their individual frequencies unresolved.
  - target: Delayed CNS Myelination
    description: Abnormal brain development includes this structural or myelination phenotype.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
      reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      quote_role: REVIEW_SYNTHESIS
      snippet: About 25% of affected individuals show evidence of hypomyelination or delayed myelination.
      explanation: Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific contribution remain unresolved. The pooled estimate combines two imaging categories and cannot quantify either alone.
  - target: Seizures
    description: Disrupted neural development contributes to this clinical manifestation; specific cellular intermediates remain unresolved.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:38678163
      reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
      snippet: Seizures were more frequently reported in individuals harboring variants in H3-3B (59%) than those harboring variants in H3-3A (45%)
      explanation: Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific contribution remain unresolved. The gene-stratified observation supports the H3-3A frequency.
    - reference: PMID:37782742
      reference_title: Bryant-Li-Bhoj Neurodevelopmental Syndrome.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      quote_role: REVIEW_SYNTHESIS
      snippet: "The presentation is highly variable and can include hypotonia, epilepsy, other neurologic findings (spasticity, loss of developmental milestones, worsening gait, and/or camptocormia – forward flexion of the spine when standing that resolves when lying down), growth abnormalities (most commonly poor growth), craniosynostosis (of any suture), and ocular involvement."
      explanation: Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific contribution remain unresolved. GeneReviews describes the variable syndrome.
  - target: Hypotonia
    description: Disrupted neural development contributes to this clinical manifestation; specific cellular intermediates remain unresolved.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:37782742
      reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The presentation is highly variable and can include hypotonia, epilepsy, other neurologic findings (spasticity, loss of developmental milestones, worsening gait, and/or camptocormia – forward flexion of the spine when standing that resolves when lying down), growth abnormalities (most commonly poor growth), craniosynostosis (of any suture), and ocular involvement."
      explanation: >-
        Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific
        contribution remain unresolved. GeneReviews enumeration of the variable features, hypotonia among them.
      directness: INDIRECT
      quote_role: REVIEW_SYNTHESIS
    - reference: PMID:38678163
      reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Sixty-two percent of individuals presented with hypotonia whereas 12% presented with hypertonia (Table 1, Fig. 2)."
      explanation: >-
        Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific
        contribution remain unresolved. Gives the patient-level frequency underpinning the FREQUENT band, and
        the hypertonia figure alongside it.
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
  - target: Hypertonia
    description: Disrupted neural development contributes to this clinical manifestation; specific cellular intermediates remain unresolved.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:38678163
      reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Sixty-two percent of individuals presented with hypotonia whereas 12% presented with hypertonia (Table 1, Fig. 2)."
      explanation: >-
        Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific
        contribution remain unresolved. Gives the patient-level frequency of hypertonia alongside that of hypotonia.
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
    - reference: PMID:38678163
      reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Interestingly, 10% of individuals have concordant axial hypotonia and peripheral hypertonia, or dystonia, which is exclusively present in individuals with variants in the core (Fig. 2, light gray)."
      explanation: >-
        Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific
        contribution remain unresolved. Records the mixed tone pattern and the fact that it occurs only with
        core-domain variants.
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
  - target: Delayed Ability to Walk
    description: Disrupted neural development contributes to this clinical manifestation; specific cellular intermediates remain unresolved.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:38678163
      reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "More individuals harboring variants in H3-3A (85%) had delayed or yet to be achieved independent walking when compared to individuals harboring variants in H3-3B (65%)."
      explanation: >-
        Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific
        contribution remain unresolved. Gives the H3-3A-specific frequency, which is the one this entry needs.
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
  - target: Delayed Ability to Sit
    description: Disrupted neural development contributes to this clinical manifestation; specific cellular intermediates remain unresolved.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:38678163
      reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Individuals with variants in H3-3A (56%) more frequently had delayed or yet to be achieved independent sitting compared to individuals with variants in H3-3B (43%) (Fig. 2)."
      explanation: >-
        Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific
        contribution remain unresolved. Gives the H3-3A-specific frequency, which is the one this entry needs.
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
  - target: Delayed Speech and Language Development
    description: Disrupted neural development contributes to this clinical manifestation; specific cellular intermediates remain unresolved.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
      reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      directness: INDIRECT
      quote_role: REVIEW_SYNTHESIS
      snippet: About 52% of affected individuals are nonverbal, with an additional 40% experiencing speech delay.
      explanation: Documents this neurodevelopmental manifestation; the intervening cellular mechanism and variant-specific contribution remain unresolved. Supports frequent severe communication impairment in the pooled clinical literature.
- name: Progressive Postnatal Neuronal H3.3 Dependence
  biological_scale: ORGANISM
  description: >-
    Age-dependent accumulation of H3.3 in postmitotic neurons is a proposed susceptibility context, not a demonstrated
    universal cause of BLBS1 progression. Regression is variable. The T45I mouse has motor impairment but lacks
    a genotype difference in the reported adult brain cell counts. G34R mice demonstrate a different, experimentally
    studied epigenetic route, whose generality to H3-3A patient alleles remains uncertain.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  mechanism_confidence: HYPOTHETICAL
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "A recent study showed that H3.3 begins to replace H3.1 and H3.2 in postnatal mouse and human brains in a time-dependent manner and displaces these canonical H3 variants almost completely in adulthood."
    explanation: >-
      The founding report cites physiological histone replacement as background for a hypothesis; it did not
      test this as the cause of human degeneration.
    quote_role: BACKGROUND
    directness: INDIRECT
phenotypes:
- category: Nervous System
  name: Global Developmental Delay and Intellectual Disability
  description: >-
    Essentially universal and typically in the severe range. Many affected
    individuals never achieve independent sitting, walking or speech.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  frequency: VERY_FREQUENT
  diagnostic: true
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bryant-Li-Bhoj neurodevelopmental syndrome (BRYLIB) is characterized by developmental delay / intellectual disability (typically in the severe range) and nonspecific craniofacial abnormalities."
    explanation: >-
      GeneReviews states developmental delay or intellectual disability as a
      defining, typically severe feature.
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ninety-nine percent of individuals with BLBS have DD/ID (Table 1, Fig. 1D, Supplementary Table 1)."
    explanation: >-
      Gives the patient-level frequency underpinning the VERY_FREQUENT band.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
- category: Head and Neck
  name: Craniofacial Dysmorphism
  description: >-
    Nonspecific facial differences were reported in 95% of the H3-3A subgroup in 2024. Population-level facial
    image analyses suggest patterns but do not establish a diagnostic facial appearance for every individual.
    Molecular testing is required.
  phenotype_term:
    preferred_term: Abnormal facial shape
    term:
      id: HP:0001999
      label: Abnormal facial shape
  frequency: VERY_FREQUENT
  diagnostic: false
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bryant-Li-Bhoj neurodevelopmental syndrome (BRYLIB) is characterized by developmental delay / intellectual disability (typically in the severe range) and nonspecific craniofacial abnormalities."
    explanation: >-
      GeneReviews names craniofacial abnormality as a defining feature and
      describes it as nonspecific.
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In addition to growth trajectories outside of the standard range for age and sex, 92% of individuals with BLBS present with craniofacial anomalies (Table 1, Fig. 1D, Supplementary Table 1)."
    explanation: >-
      Gives the patient-level frequency underpinning the VERY_FREQUENT band.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: PRIMARY_RESULT
    snippet: Dysmorphic features are slightly more common in individuals with variants in H3-3A (95%) than H3-3B (86%)
    explanation: The gene-stratified proportion supports the H3-3A description.
- category: Nervous System
  name: Hypotonia
  description: >-
    Reduced muscle tone, common in infancy. Tone abnormality is not fixed - it
    may resolve with age or evolve into a mixed picture with peripheral
    hypertonia.
  phenotype_term:
    preferred_term: Hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
  frequency: FREQUENT
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The presentation is highly variable and can include hypotonia, epilepsy, other neurologic findings (spasticity, loss of developmental milestones, worsening gait, and/or camptocormia – forward flexion of the spine when standing that resolves when lying down), growth abnormalities (most commonly poor growth), craniosynostosis (of any suture), and ocular involvement."
    explanation: >-
      GeneReviews enumeration of the variable features, hypotonia among them.
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sixty-two percent of individuals presented with hypotonia whereas 12% presented with hypertonia (Table 1, Fig. 2)."
    explanation: >-
      Gives the patient-level frequency underpinning the FREQUENT band, and the
      hypertonia figure alongside it.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
- category: Nervous System
  name: Seizures
  description: >-
    Variable seizure types generally begin in infancy or childhood; 45% of the H3-3A subgroup in 2024 had seizures.
    Febrile, focal and generalized seizures occur, with variable treatment response.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: PRIMARY_RESULT
    snippet: Seizures were more frequently reported in individuals harboring variants in H3-3B (59%) than those harboring variants in H3-3A (45%)
    explanation: The gene-stratified observation supports the H3-3A frequency.
  - reference: PMID:37782742
    reference_title: Bryant-Li-Bhoj Neurodevelopmental Syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: "The presentation is highly variable and can include hypotonia, epilepsy, other neurologic findings (spasticity, loss of developmental milestones, worsening gait, and/or camptocormia – forward flexion of the spine when standing that resolves when lying down), growth abnormalities (most commonly poor growth), craniosynostosis (of any suture), and ocular involvement."
    explanation: GeneReviews describes the variable syndrome.
- category: Nervous System
  name: Camptocormia
  description: >-
    Forward flexion of the trunk while standing, resolving supine. GeneReviews describes subacute motor change
    in the third decade followed by relative stability in three reported adults. This small selected series
    does not establish that all adults with H3-3A disease develop camptocormia.
  phenotype_term:
    preferred_term: Camptocormia
    term:
      id: HP:0100595
      label: Camptocormia
    onset:
      onset_category: ADULT
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: All reported adults have had a new subacute onset of motor issues in the third decade of life that generally remains stable after onset.
    explanation: The full clinical synthesis gives the timing and subsequent course; Table 2 limits the adult denominator to three.
- category: Nervous System
  name: Spasticity
  description: >-
    Spasticity typically develops in childhood and particularly affects the legs; it can coexist with axial
    hypotonia. It is not restricted to an adult degenerative phase.
  phenotype_term:
    preferred_term: Spasticity
    term:
      id: HP:0001257
      label: Spasticity
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Spasticity typically develops in childhood and is associated with either hypotonia or hypertonia. Hypertonia tends to be limited to limbs
    explanation: The clinical synthesis gives age and distribution.
- category: Nervous System
  name: Cerebral Cortical Atrophy
  description: >-
    Cortical atrophy was recorded in 10/41 assessable individuals in the founding combined-gene cohort. Atrophy
    on one MRI does not itself prove progressive loss, and intractable epilepsy was not present in every affected
    individual.
  phenotype_term:
    preferred_term: Cerebral cortical atrophy
    term:
      id: HP:0002120
      label: Cerebral cortical atrophy
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: Multiple patients (26% of the cohort) have cortical atrophy on brain magnetic resonance imaging (MRI), even without intractable epilepsy.
    explanation: The source prose reports 26%; the table gives 10/41 (24%). The description uses the assessable table denominator.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: Cortical atrophy | 10/41 (24%)
    explanation: The table supplies the assessable denominator, correcting the prose percentage.
- category: Nervous System
  name: Delayed CNS Myelination
  description: >-
    Delayed myelination is reported in the pooled clinical literature. It should be distinguished from persistently
    deficient myelination using age and, when available, serial imaging. The GeneReviews estimate combines
    these categories and cannot establish a frequency for delayed myelination alone.
  phenotype_term:
    preferred_term: Delayed CNS myelination
    term:
      id: HP:0002188
      label: Delayed CNS myelination
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: About 25% of affected individuals show evidence of hypomyelination or delayed myelination.
    explanation: The pooled estimate combines two imaging categories and cannot quantify either alone.
- category: Head and Neck
  name: Craniosynostosis
  description: >-
    Craniosynostosis involving different sutures is reported. The approximately one-third estimate in clinical
    summaries combines craniosynostosis with other abnormal head shapes, so it is not assigned as the frequency
    of confirmed suture fusion.
  phenotype_term:
    preferred_term: Craniosynostosis
    term:
      id: HP:0001363
      label: Craniosynostosis
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The presentation is highly variable and can include hypotonia, epilepsy, other neurologic findings (spasticity, loss of developmental milestones, worsening gait, and/or camptocormia – forward flexion of the spine when standing that resolves when lying down), growth abnormalities (most commonly poor growth), craniosynostosis (of any suture), and ocular involvement."
    explanation: >-
      GeneReviews names craniosynostosis of any suture as a feature.
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
- category: Growth
  name: Poor Growth
  description: >-
    Short stature is common: 32/91 in the 2024 pooled cohort met its low-height criterion. Growth is variable,
    with tall stature and increased weight in some individuals.
  phenotype_term:
    preferred_term: Short stature
    term:
      id: HP:0004322
      label: Short stature
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Height (≤5th percentile) | 32/91 (35%)
    explanation: The cohort table supplies the denominator and study-specific cutoff.
- category: Eye
  name: Ocular Involvement
  description: >-
    Oculomotor findings were reported in 49/90 of the pooled 2024 cohort; strabismus is the most common specified
    finding. Visual tracking deficits, nystagmus and cortical visual impairment also occur. Sex differences
    in the cohort are descriptive and are not diagnostic predictors.
  phenotype_term:
    preferred_term: Abnormality of eye movement
    term:
      id: HP:0000496
      label: Abnormality of eye movement
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Oculomotor | 49/90 (54%)
    explanation: The pooled category frequency is retained with its denominator.
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Strabismus is present in slightly more than one third of affected individuals, and nystagmus is present in a small minority of affected individuals (7%).
    explanation: The clinical synthesis separates specific ocular findings.
- category: Digestive
  name: Feeding Difficulties
  description: >-
    Infant feeding difficulties are reported in about 25% in the combined-gene GeneReviews synthesis. Swallowing
    safety, nutritional intake and growth require assessment; severe persistent difficulties may require enteral
    feeding.
  phenotype_term:
    preferred_term: Feeding difficulties
    term:
      id: HP:0011968
      label: Feeding difficulties
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: have been described in about 25% of affected individuals. Most are treated with a nasogastric tube or gastrostomy tube.
    explanation: Pooled clinical estimate and severity-related support needs.
  frequency: OCCASIONAL
- category: Cardiovascular
  name: Congenital Heart Defect
  description: >-
    Cardiac anomalies occurred in 11/82 in Table 1 of the pooled 2024 cohort. Atrial septal defects are among
    the reported lesions; the aggregate count cannot be assigned to that lesion alone.
  phenotype_term:
    preferred_term: Abnormal heart morphology
    term:
      id: HP:0001627
      label: Abnormal heart morphology
    coarse_binding_basis: VARIABLE_SPECTRUM
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Cardiac | 11/82 (13%)
    explanation: Uses the table count rather than the inconsistent 14% prose.
- category: Genitourinary
  name: Cryptorchidism
  description: >-
    Cryptorchidism is reported in about 35% of affected males in the combined-gene GeneReviews synthesis. The
    2024 cohort genital-anomaly category includes other findings and uses a different denominator.
  phenotype_term:
    preferred_term: Cryptorchidism
    term:
      id: HP:0000028
      label: Cryptorchidism
  frequency: FREQUENT
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: About 35% of affected males have cryptorchidism. Retractile testes have also been observed.
    explanation: The denominator is affected males, not all individuals or all genital anomalies.
- category: Nervous System
  name: Delayed Ability to Walk
  description: >-
    Delayed or absent independent walking, defined in the cohort as not walking by 20 months. The commonest
    motor milestone delay recorded. The 2024 H3-3A subgroup proportion was 85%.
  phenotype_term:
    preferred_term: Delayed ability to walk
    term:
      id: HP:0031936
      label: Delayed ability to walk
  frequency: VERY_FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "More individuals harboring variants in H3-3A (85%) had delayed or yet to be achieved independent walking when compared to individuals harboring variants in H3-3B (65%)."
    explanation: >-
      Gives the H3-3A-specific frequency, which is the one this entry needs.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
- category: Nervous System
  name: Delayed Speech and Language Development
  description: >-
    Speech impairment is common. GeneReviews reports about 52% nonverbal and another 40% with speech delay
    in the combined-gene series. The 2024 table entry reporting that 50/84 spoke at least one word is an attainment
    measure, not a 60% speech-delay rate.
  phenotype_term:
    preferred_term: Delayed speech and language development
    term:
      id: HP:0000750
      label: Delayed speech and language development
  frequency: VERY_FREQUENT
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: About 52% of affected individuals are nonverbal, with an additional 40% experiencing speech delay.
    explanation: Supports frequent severe communication impairment in the pooled clinical literature.
- category: Nervous System
  name: Delayed Ability to Sit
  description: >-
    Delayed or absent independent sitting, defined in the cohort as not sitting by 12 months. The 2024 H3-3A
    subgroup proportion was 56%.
  phenotype_term:
    preferred_term: Delayed ability to sit
    term:
      id: HP:0025336
      label: Delayed ability to sit
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Individuals with variants in H3-3A (56%) more frequently had delayed or yet to be achieved independent sitting compared to individuals with variants in H3-3B (43%) (Fig. 2)."
    explanation: >-
      Gives the H3-3A-specific frequency, which is the one this entry needs.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
- category: Nervous System
  name: Abnormal Corpus Callosum Morphology
  description: >-
    Callosal malformation or dysgenesis was reported in 28/76 of the pooled 2024 cohort. A separate selected
    imaging series reported 5/18, particularly affecting the splenium. These are different overlapping ascertainment
    groups.
  review_notes: >-
    The frequency band uses 28/76 (37%) in the pooled clinical cohort. The 5/18 finding in the selected imaging
    series has a different denominator and is not a population estimate.
  phenotype_term:
    preferred_term: Abnormal corpus callosum morphology
    term:
      id: HP:0001273
      label: Abnormal corpus callosum morphology
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Corpus collosum malformation/dysgenesis | 28/76 (37%)
    explanation: The larger cohort supplies the frequency; the table misspelling is preserved in the exact quote.
- category: Head and Neck
  name: Microcephaly
  description: >-
    Microcephaly was recorded in 30/95 in the 2024 pooled cohort. Its operational low-head-size criterion should
    not be treated as a universal diagnostic definition. Macrocephaly also occurs.
  phenotype_term:
    preferred_term: Microcephaly
    term:
      id: HP:0000252
      label: Microcephaly
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Microcephaly (≤5th percentile) | 30/95 (32%)
    explanation: The table supplies a pooled count and study-specific cutoff.
- category: Nervous System
  name: Hypertonia
  description: >-
    Increased muscle tone. A mixed pattern of axial hypotonia with peripheral
    hypertonia is reported separately and occurs in about a tenth of the cohort.
  phenotype_term:
    preferred_term: Hypertonia
    term:
      id: HP:0001276
      label: Hypertonia
  frequency: OCCASIONAL
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sixty-two percent of individuals presented with hypotonia whereas 12% presented with hypertonia (Table 1, Fig. 2)."
    explanation: >-
      Gives the patient-level frequency of hypertonia alongside that of
      hypotonia.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Interestingly, 10% of individuals have concordant axial hypotonia and peripheral hypertonia, or dystonia, which is exclusively present in individuals with variants in the core (Fig. 2, light gray)."
    explanation: >-
      Records the mixed tone pattern and the fact that it occurs only with
      core-domain variants.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
- category: Integument
  name: Dermatologic Features
  description: >-
    Skin and related surface findings were recorded in 46/88 in the pooled cohort, including eczema, nipple
    anomalies and persistent fetal finger pads. The aggregate category is retained alongside the more specific
    findings.
  phenotype_term:
    preferred_term: Abnormality of the skin
    term:
      id: HP:0000951
      label: Abnormality of the skin
    coarse_binding_basis: VARIABLE_SPECTRUM
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Dermal | 46/88 (52%)
    explanation: Uses the table denominator rather than the 53% prose.
- category: Musculoskeletal
  name: Musculoskeletal Anomalies
  description: >-
    The pooled cohort recorded musculoskeletal findings in 56/94. The variable spectrum includes scoliosis,
    ligamentous laxity, hip dysplasia, clubfoot, joint subluxation and kyphosis; scoliosis and laxity have
    separate table counts.
  phenotype_term:
    preferred_term: Abnormal skeletal morphology
    term:
      id: HP:0011842
      label: Abnormal skeletal morphology
    coarse_binding_basis: VARIABLE_SPECTRUM
  frequency: FREQUENT
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Musculoskeletal | 56/94 (60%)
    explanation: Uses the table count rather than the 58% prose.
- category: Ear
  name: Hearing Impairment
  description: >-
    Mild-to-moderate congenital hearing loss is described in a subset and is thought to be static. Audiology
    follow-up is individualized according to findings.
  phenotype_term:
    preferred_term: Hearing impairment
    term:
      id: HP:0000365
      label: Hearing impairment
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: A subset of affected individuals have been reported with mild-to-moderate congenital hearing loss. This is thought to be static.
    explanation: The full clinical description supports the phenotype beyond a management checklist.
- category: Nervous System
  name: Autism Spectrum Disorder
  description: >-
    Autism was reported in 8% of the pooled 2024 cohort; additional reported behavioral features include stereotypies,
    attention difficulties and anxiety. This is not an H3-3A-specific frequency.
  phenotype_term:
    preferred_term: Autism
    term:
      id: HP:0000717
      label: Autism
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: many individuals have co-existing neurodevelopmental diagnoses, including 8% with autism spectrum disorder
    explanation: The pooled clinical report quantifies this feature.
  frequency: OCCASIONAL
- name: Macrocephaly
  category: Head and Neck
  description: Increased head circumference occurs in a subset, with 14/95 meeting the 2024 pooled cohort criterion. This is separate from the combined micro-or-macrocephaly measure.
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Macrocephaly (≥95th percentile) | 14/95 (15%)
    explanation: Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Macrocephaly
    term:
      id: HP:0000256
      label: Macrocephaly
- name: Strabismus
  category: Eye
  description: Strabismus was reported in 32/90 of the pooled cohort and is documented in H3-3A cases.
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Strabismus | 32/90 (36%)
    explanation: Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Strabismus
    term:
      id: HP:0000486
      label: Strabismus
- name: Scoliosis
  category: Musculoskeletal
  description: Scoliosis was recorded in 20/94 of the pooled cohort; it is also reported in individual H3-3A cases.
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Scoliosis | 20/94 (21%)
    explanation: Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Scoliosis
    term:
      id: HP:0002650
      label: Scoliosis
- name: Joint Hypermobility
  category: Musculoskeletal
  description: Ligamentous laxity was reported in 21/94 of the pooled cohort, with hypermobility also documented in H3-3A cases.
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Ligamentous laxity | 21/94 (22%)
    explanation: Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Joint Hypermobility
    term:
      id: HP:0001382
      label: Joint hypermobility
- name: Persistent Fetal Finger Pads
  category: Integument
  description: Persistent fetal finger pads are a minor dysmorphic finding, reported in 14/88 of the pooled cohort.
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Fetal finger pad | 14/88 (16%)
    explanation: Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Persistent Fetal Finger Pads
    term:
      id: HP:0001212
      label: Prominent fingertip pads
- name: Nipple Anomalies
  category: Integument
  description: Nipple abnormalities form a variable category in 15/88 of the pooled cohort; a single specific morphology cannot be assigned to all cases.
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Nipple anomalies | 15/88 (17%)
    explanation: Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Abnormal nipple morphology
    term:
      id: HP:0004404
      label: Abnormal nipple morphology
    coarse_binding_basis: SOURCE_UNSPECIFIED
- name: Increased Body Weight
  category: Growth
  description: The pooled cohort reported weight at or above the 95th percentile in 14/76; this does not establish obesity by body-mass-index criteria.
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Weight (≥95th percentile) | 14/76 (18%)
    explanation: Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Increased Body Weight
    term:
      id: HP:0004324
      label: Increased body weight
- name: Decreased Body Weight
  category: Growth
  description: The pooled cohort reported weight at or below the fifth percentile in 15/76, alongside variable feeding and growth difficulties.
  evidence:
  - reference: PMID:38678163
    reference_title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: BLBS clinical phenotypes. ... Weight (≤5th percentile) | 15/76 (20%)
    explanation: Pooled H3-3A/H3-3B clinical table; no gene-specific proportion is inferred.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Decreased Body Weight
    term:
      id: HP:0004325
      label: Decreased body weight
- name: Constipation
  category: Digestive
  description: Chronic constipation occurs in the pooled clinical literature and is documented in H3-3A cases.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Chronic constipation was reported in 26% of affected individuals.
    explanation: The clinical synthesis describes this manifestation; gene-specific rates remain uncertain.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Constipation
    term:
      id: HP:0002019
      label: Constipation
- name: Hypothyroidism
  category: Endocrine
  description: Hypothyroidism is recognized in the combined-gene clinical literature. Its H3-3A-specific frequency and contribution to individual developmental outcomes are not established.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Hypothyroidism has been reported in a subset of affected individuals.
    explanation: The clinical synthesis describes this manifestation; gene-specific rates remain uncertain.
  phenotype_term:
    preferred_term: Hypothyroidism
    term:
      id: HP:0000821
      label: Hypothyroidism
- name: Developmental Regression
  category: Nervous System
  description: Loss of acquired skills occurs in a minority, can be intermittent, and is not always associated with seizures.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Developmental regression can occur in a minority of affected individuals. Regression is not continuous and is not always associated with seizures.
    explanation: The clinical synthesis describes this manifestation; gene-specific rates remain uncertain.
  phenotype_term:
    preferred_term: Developmental Regression
    term:
      id: HP:0002376
      label: Developmental regression
- name: Neonatal Myoclonus
  category: Nervous System
  description: A single H3-3A p.Ala48Gly case had myoclonic movements from day two without an EEG correlate. Later epileptic seizures were distinct events; neonatal myoclonus should not automatically be classified as epilepsy.
  evidence:
  - reference: PMID:39627236
    reference_title: Neonatal myoclonus in Bryant-Li-Bhoj syndrome associated with a novel H3F3A variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: PRIMARY_RESULT
    snippet: An interictal electroencephalogram (EEG) revealed no spike discharges with a normal background and no EEG abnormalities corresponding to myoclonic movements.
    explanation: The case report distinguishes neonatal movements from later epileptic seizures.
  phenotype_term:
    preferred_term: Neonatal Myoclonus
    term:
      id: HP:0001336
      label: Myoclonus
    onset:
      onset_category: NEONATAL
      notes: Movements began on day two in the reported H3-3A case; later epileptic seizures were separate events.
- name: Small Posterior Fossa
  category: Nervous System
  description: A small posterior fossa was found in 13/18 in a selected imaging series comprising 11 H3-3A and seven H3-3B cases. The severe-case enrichment prevents treating 72% as a population frequency.
  evidence:
  - reference: PMID:35772801
    reference_title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumors."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: Thirteen of 18 individuals (72%) presented with a small posterior fossa.
    explanation: Primary imaging series; proportions are pooled and enriched for severe abnormalities.
  - reference: url:https://pmc.ncbi.nlm.nih.gov/articles/PMC9262070/
    reference_title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumors - PMC"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: Our cohort was biased because all individuals included in our study except for 1 had severe clinical symptoms and significant abnormalities involving the brain
    explanation: The full primary study explicitly states the selection limitation.
  phenotype_term:
    preferred_term: Small posterior fossa
    term:
      id: HP:0040010
      label: Small posterior fossa
- name: Cortical Malformation
  category: Nervous System
  description: Cortical malformations occurred in 8/18 in the selected imaging series, including dysgyria, anterior pachygyria and simplified gyration, with bilateral temporal-lobe hypoplasia.
  evidence:
  - reference: PMID:35772801
    reference_title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumors."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: Cortical developmental abnormalities were noted in 8 individuals (44%), with dysgyria and hypoplastic temporal poles being the most frequent presentation.
    explanation: Primary imaging series; proportions are pooled and enriched for severe abnormalities.
  phenotype_term:
    preferred_term: Cortical Malformation
    term:
      id: HP:0002536
      label: Abnormal cortical gyration
- name: Chiari I Malformation
  category: Nervous System
  description: Four of 18 in the selected imaging series had low cerebellar tonsils meeting Chiari I criteria. This is not an unselected H3-3A frequency.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Chiari I malformations were seen in 4/18 (22%) of individuals imaged.
    explanation: Selected imaging-subgroup finding, not a population frequency.
  phenotype_term:
    preferred_term: Chiari I Malformation
    term:
      id: HP:0007099
      label: Chiari type I malformation
- name: CNS Hypomyelination
  category: Nervous System
  description: Hypomyelination is reported, including an H3-3A case in the 2021 series. Reduced peripheral neural-crest glia in fish does not establish its mechanism. Published pooled estimates combining delayed myelination and hypomyelination cannot quantify this finding alone.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: About 25% of affected individuals show evidence of hypomyelination or delayed myelination.
    explanation: The synthesis establishes both reported categories while leaving their individual frequencies unresolved.
  phenotype_term:
    preferred_term: CNS Hypomyelination
    term:
      id: HP:0003429
      label: CNS hypomyelination
- name: Atrial Septal Defect
  category: Cardiovascular
  description: Atrial septal defects are a reported component of cardiac involvement. GeneReviews cites about 16% in its combined-gene clinical synthesis; that estimate is distinct from the later cohort total for all cardiac anomalies.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Atrial septal defect was seen in about 16% of individuals with BRYLIB.
    explanation: Specific lesion in the pooled clinical synthesis, rather than assigning the all-cardiac-anomaly count to one defect.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Atrial Septal Defect
    term:
      id: HP:0001631
      label: Atrial septal defect
genetic:
- name: H3-3A
  gene_term:
    preferred_term: H3-3A
    term:
      id: hgnc:4764
      label: H3-3A
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  notes: >-
    H3-3A encodes the replication-independent histone H3.3 protein, identical to that encoded by H3-3B. Published
    BLBS1 alleles in the 2024 cohort are heterozygous missense changes, usually de novo. Historical population-constraint
    metrics support gene-level intolerance, but do not establish pathogenicity of every rare missense variant.
    Functional effects are heterogeneous; equal allele number is not equal protein dosage.
  case_fractions:
  - population: Global BLBS cohort reported through 2024
    case_fraction_percent: 67.7
    cohort_size: 96
    notes: >-
      65 of 96 reported individuals with BLBS carried an H3-3A variant, the
      remainder H3-3B. This is a discovery-cohort split rather than a
      population estimate.
    evidence:
    - reference: PMID:38678163
      reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "In total, 65 individuals harbor variants in H3-3A and 31 individuals harbor variants in H3-3B."
      explanation: >-
        Gives the gene split within the reported BLBS cohort.
      directness: DIRECT
      quote_role: PRIMARY_RESULT
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In this work, we have identified a cohort of 46 unrelated patients bearing de novo heterozygous germline missense mutations in H3F3A or H3F3B with a core phenotype of progressive neurologic dysfunction and congenital anomalies."
    explanation: >-
      Establishes the gene-disease relationship in the founding cohort.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In the general population, both genes have a very low rate of missense variants; the gnomAD missense Z score is 3.21 for H3F3A and 2.95 for H3F3B (>2 is significant)"
    explanation: >-
      Documents the strong missense constraint on H3-3A that supports
      pathogenicity of de novo missense alleles.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
variants:
- name: H3-3A p.Thr46Ile (mature-histone T45I)
  description: >-
    The c.137C>T substitution is p.Thr46Ile in HGVS protein numbering and T45I when the cleaved initiator methionine
    is omitted. Four individuals carried this recurrent H3-3A allele in the 2024 cohort, with variable phenotypes.
    It is modeled in the 2026 mouse preprint and in variant-specific biochemical studies.
  gene:
    preferred_term: H3-3A
    term:
      id: hgnc:4764
      label: H3-3A
  clinical_significance: PATHOGENIC
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Length of lollipop corresponds to number of individuals who harbor a variant at that residue (e.g. H3-3A p.T45I represents four individuals with BLBS and H3-3A p.V46M represents one individual with BLBS)."
    explanation: >-
      Identifies p.Thr45Ile as the most recurrent H3-3A allele in the reported
      cohort.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Notably, phenotypic heterogeneity was present even amongst individuals harboring the same variant."
    explanation: >-
      Supports the statement that recurrence of an allele does not bring
      phenotypic concordance.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
diagnosis:
- name: Molecular genetic testing of H3-3A
  description: >-
    Establish BLBS1 by a heterozygous pathogenic or likely pathogenic H3-3A variant in a compatible clinical
    context. A VUS does not establish the diagnosis. Broad neurodevelopmental panels, exome or genome sequencing
    are appropriate because the clinical features overlap many disorders. Review coverage when testing is negative:
    the founding study identified two terminal-exon H3-3A variants by genome sequencing after missed exome
    capture.
  evidence:
  - reference: PMID:37782742
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The diagnosis of BRYLIB is established in a proband with suggestive findings and a heterozygous pathogenic variant in either H3-3A (H3F3A) or H3-3B (H3F3B) identified by molecular genetic testing."
    explanation: >-
      GeneReviews statement of the diagnostic criterion.
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These patients were identified through exome or genome sequencing performed for the indication of neurodevelopmental delays and/or congenital anomalies"
    explanation: >-
      Documents the ascertainment route through which the diagnosis is
      practically made.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: variant of uncertain significance does not establish or rule out the diagnosis.
    explanation: GeneReviews explicitly cautions that a variant of uncertain significance is not diagnostic.
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: PRIMARY_RESULT
    snippet: as the last exon of H3F3A is currently not covered by some exome capture kits.
    explanation: In the 2020 study two variants were found by genome sequencing after missed exome capture; this is a historical kit-specific limitation.
- name: Baseline clinical assessment
  description: Assess development, behavior, growth, nutrition and swallowing; obtain neurologic examination and brain MRI, with EEG for suspected seizures. Evaluate vision, hearing, thyroid function, cardiac anatomy and the testes as indicated by the initial assessment. Abnormal head shape warrants craniofacial review and imaging when craniosynostosis is suspected. These evaluations characterize complications rather than replace molecular diagnosis.
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: To incl brain MRI ... Consider EEG if seizures are a concern.
    explanation: Table 3 specifies baseline brain imaging and clinically indicated EEG.
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: To assess for unrecognized congenital heart disease, specifically atrial septal defects
    explanation: The baseline table recommends echocardiography to assess cardiac disease.
differential_diagnoses:
- name: Bryant-Li-Bhoj neurodevelopmental syndrome 2
  description: >-
    The H3-3B counterpart. The two genes encode an identical protein, so the
    entities are separated by gene rather than by protein product and are
    clinically overlapping. Only molecular testing distinguishes them.
    Cohort-level trends exist but are not diagnostic discriminators, and the
    H3-3A cohort differs in that every variant with parental data has been de
    novo and missense, whereas H3-3B has yielded synonymous, stop-loss and one
    inherited allele.
  disease_term:
    preferred_term: Bryant-Li-Bhoj neurodevelopmental syndrome 2
    term:
      id: MONDO:0030607
      label: Bryant-Li-Bhoj neurodevelopmental syndrome 2
  distinguishing_features:
  - "Determined solely by molecular testing: a pathogenic variant in H3-3A rather than H3-3B"
  - Greater variant-type and inheritance diversity is reported for H3-3B, including a synonymous stop-gain, a stop-loss and one inherited allele
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Conversely, while most variants in H3-3B are heterozygous de novo missense variants, more variability in variant type and modes of inheritance was observed."
    explanation: >-
      States the variant-type difference between the two genes that this
      differential rests on.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
- name: Somatic H3.3 oncohistone disease
  description: >-
    Somatic H3.3 hotspot variants occur in several tumors. A tumor-acquired variant and a constitutional variant
    define different clinical contexts, which require attention to tissue tested, variant interpretation and
    phenotype. Some molecular effects overlap, including cis effects of G34 substitutions. Absence of cancer
    in early BLBS cohorts does not establish absence of lifetime risk.
  distinguishing_features:
  - Tumor-restricted versus constitutional variant distribution
  - Neoplasm-focused presentation versus a developmental syndrome
  - Variant- and tissue-dependent molecular effects; no uniform cis-versus-trans distinction
  evidence:
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Patient histone posttranslational modifications (PTMs) analysis revealed notably aberrant local PTM patterns distinct from the somatic lysine mutations that cause global PTM dysregulation."
    explanation: >-
      Gives the molecular basis on which the germline and somatic diseases are
      separated.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These data suggest that the mechanism of germline mutations are distinct from cancer-associated somatic histone mutations but may converge on control of cell proliferation."
    explanation: >-
      The authors' own framing of the relationship between the two diseases.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
treatments:
- name: Physical Therapy and Mobility Support
  description: Individualized physical therapy, stretching, positioning and mobility aids address motor function, spasticity, camptocormia and prevention of secondary orthopedic complications.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Physical Therapy and Mobility Support
    term:
      id: NCIT:C15302
      label: Physical Therapy
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Physical therapy is recommended to maximize mobility and to reduce the risk for later-onset orthopedic complications
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
- name: Occupational Therapy
  description: Occupational therapy supports fine-motor and adaptive daily-living skills, including feeding, dressing and writing.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Occupational Therapy
    term:
      id: NCIT:C121351
      label: Occupational Therapy
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Occupational therapy is recommended for difficulty with fine motor skills that affect adaptive function
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
- name: Speech and Communication Support
  description: Early speech/language intervention, individualized educational support and augmentative or alternative communication address severe expressive-language impairment.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Speech Language Therapy
    term:
      id: NCIT:C159273
      label: Speech Language Therapy
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Referral to an early intervention program is recommended for access to occupational, physical, speech, and feeding therapy
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: An AAC evaluation can be completed by a speech-language pathologist who has expertise in the area.
    explanation: The detailed chapter supports alternative communication assessment.
- name: Feeding Therapy and Enteral Support
  description: Evaluate oral motor function and swallowing safety. Feeding therapy and nutritional adaptation may help; persistent severe dysfunction may require nasogastric or gastrostomy feeding.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Feeding Therapy and Enteral Support
    term:
      id: NCIT:C49236
      label: Therapeutic Procedure
  evidence:
  - reference: PMID:37782742
    reference_title: Bryant-Li-Bhoj Neurodevelopmental Syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Feeding issues and poor weight gain may benefit from feeding therapy; in individuals with persistent feeding issues, gastrostomy tube may be considered.
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
- name: Antiseizure Medication
  description: An experienced neurologist individualizes antiseizure treatment. Response ranges from good control to refractory epilepsy; no drug has demonstrated syndrome-specific efficacy. A single H3-3A case reported different responses to valproate and levetiracetam at different ages, which cannot establish a preferred regimen.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Antiseizure Medication
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: Anticonvulsant Agent
      term:
        id: NCIT:C264
        label: Anticonvulsant Agent
    - preferred_term: levetiracetam
      term:
        id: CHEBI:6437
        label: levetiracetam
    - preferred_term: valproic acid
      term:
        id: CHEBI:39867
        label: valproic acid
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Many ASMs may be effective; none has been demonstrated effective specifically for this disorder.
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
  - reference: PMID:39627236
    reference_title: Neonatal myoclonus in Bryant-Li-Bhoj syndrome associated with a novel H3F3A variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: PRIMARY_RESULT
    snippet: her seizures resolved with the administration of levetiracetam.
    explanation: Individual later-life response in one reported H3-3A case; it does not establish comparative efficacy.
- name: Oral Treatment of Abnormal Muscle Tone
  description: Specialists may consider baclofen or tizanidine for clinically significant hypertonia or dystonia, with rehabilitation and attention to adverse effects. These are symptomatic options without BLBS-specific efficacy trials.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Oral Treatment of Abnormal Muscle Tone
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: baclofen
      term:
        id: CHEBI:2972
        label: baclofen
    - preferred_term: tizanidine
      term:
        id: CHEBI:63629
        label: tizanidine
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: consider involving appropriate specialists to aid in management of baclofen, tizanidine
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
- name: Specialist Focal Tone Management
  description: Botulinum toxin and orthopedic procedures are among the options listed by GeneReviews for selected muscle-tone problems; intervention depends on pattern and functional goals.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Specialist Focal Tone Management
    term:
      id: NCIT:C49236
      label: Therapeutic Procedure
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: baclofen, tizanidine, Botox ... anti-parkinsonian medications, or orthopedic procedures.
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
- name: Constipation Treatment
  description: Use clinically appropriate stool softeners, osmotic agents, prokinetics or laxatives, with regular assessment of bowel symptoms.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Constipation Treatment
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: Laxative
      term:
        id: NCIT:C29697
        label: Laxative
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Stool softeners, prokinetics, osmotic agents, or laxatives as needed
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
- name: Behavioral and Educational Support
  description: Individualized educational and behavioral interventions address developmental disability, autism-related needs, attention difficulties and anxiety.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Behavioral and Educational Support
    term:
      id: NCIT:C49236
      label: Therapeutic Procedure
  evidence:
  - reference: PMID:37782742
    reference_title: Bryant-Li-Bhoj Neurodevelopmental Syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Standard treatment for developmental delay / intellectual disability, attention-deficit/hyperactivity disorder, autism spectrum disorder, anxiety
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
- name: Vision and Hearing Support
  description: Ophthalmology and audiology guide correction of refractive error or strabismus, low-vision support and hearing aids when appropriate.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Vision and Hearing Support
    term:
      id: NCIT:C49236
      label: Therapeutic Procedure
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Hearing aids may be helpful per otolaryngologist.
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
- name: Management of Congenital and Endocrine Complications
  description: Craniofacial, cardiology, urology and endocrinology teams treat craniosynostosis, cardiac defects, undescended testes and hypothyroidism according to the individual findings.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Management of Congenital and Endocrine Complications
    term:
      id: NCIT:C49236
      label: Therapeutic Procedure
  evidence:
  - reference: PMID:37782742
    reference_title: Bryant-Li-Bhoj Neurodevelopmental Syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Standard treatment for developmental delay / intellectual disability, attention-deficit/hyperactivity disorder, autism spectrum disorder, anxiety, epilepsy, spasticity, constipation, craniosynostosis, vision issues, hearing loss, congenital heart defects, undescended testes, and hypothyroidism.
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
- name: Structured Surveillance
  description: At visits assess growth, nutrition, oral intake, constipation, seizures, tone, gait, development and behavior. Ophthalmology is generally annual or as indicated; audiology and thyroid testing follow clinical indications in the detailed GeneReviews table. New decline warrants evaluation rather than being assumed inevitable.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Structured Surveillance
    term:
      id: NCIT:C49236
      label: Therapeutic Procedure
  evidence:
  - reference: PMID:37782742
    reference_title: Bryant-Li-Bhoj Neurodevelopmental Syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: 'At each visit: measure growth parameters; assess nutrition safety and oral intake; monitor for constipation; assess for new manifestations, such as seizures and changes in tone and/or gait; monitor developmental progress and educational needs; and assess for behavioral issues, such as mobility and self-help skills.'
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
- name: Genetic Counseling
  description: Test parents and explain autosomal dominant transmission. When the familial variant is absent from parental blood, GeneReviews estimates about 1% sibling recurrence from possible germline mosaicism; a known heterozygous parent transmits the variant with a 50% chance per pregnancy. Prenatal and preimplantation testing are possible once the variant is known.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
    reference_title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: the recurrence risk to sibs is estimated to be 1% because of the theoretic possibility of parental
    explanation: Clinical management guidance; symptomatic care, not a demonstrated disease-modifying intervention.
  - reference: PMID:37782742
    reference_title: Bryant-Li-Bhoj Neurodevelopmental Syndrome.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    quote_role: REVIEW_SYNTHESIS
    snippet: Once an H3-3A (H3F3A) or H3-3B (H3F3B) pathogenic variant has been identified in an affected family member, prenatal and preimplantation genetic testing are possible.
    explanation: Supports reproductive testing options after molecular diagnosis.
animal_models:
- name: H3.3 T45I knock-in mouse
  species: Mus musculus
  genotype: Heterozygous H3f3a c.137C>T, p.Thr46Ile (mature-histone T45I)
  publication: DOI:10.64898/2026.06.16.732665
  description: The 2026 preprint characterizes a C57BL/6J knock-in carrying a recurrent human H3-3A allele. Pups have delayed milestones and postnatal growth restriction; adult gait effects are more pronounced in males, while aged 60-week animals of both sexes have marked rotarod impairment. Nest-building and male aggression differ, but tested learning, memory and social preference are preserved. Brain histology was examined, with no genotype differences in the reported neuronal, astrocyte, Purkinje-cell or microglial counts. Gross brain endocast measures were also unchanged. This is partial phenotypic correspondence, not proof of a universal two-stage human course.
  modeled_mechanisms:
  - target: Abnormal Brain Development
    description: Developmental and motor phenotypes overlap with part of the human syndrome.
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    evidence:
    - &id009
      reference: DOI:10.64898/2026.06.16.732665
      reference_title: A novel preclinical mouse model recapitulates progressive phenotypes of Bryant-Li-Bhoj Syndrome
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      directness: DIRECT
      quote_role: PRIMARY_RESULT
      snippet: 'mice recapitulate the BLBS natural history: perinatal growth restriction, delayed developmental milestones, and progressive motor and gait impairments.'
      explanation: Preprint abstract summary; detailed results differ by age, sex and assay and only partially reproduce the human syndrome.
    - &id010
      reference: url:https://www.biorxiv.org/content/10.64898/2026.06.16.732665v1.full
      reference_title: "A novel preclinical mouse model recapitulates progressive phenotypes of Bryant-Li-Bhoj Syndrome | bioRxiv"
      supports: REFUTE
      evidence_source: MODEL_ORGANISM
      directness: DIRECT
      quote_role: PRIMARY_RESULT
      snippet: Immunohistochemical analyses of adult brain tissue revealed no significant differences between genotypes in neuronal density (NeuN), astrocyte burden (GFAP), or Purkinje cell number (Calbindin)
      explanation: The full preprint reports actual negative histological comparisons, not absence of histological investigation.
    - &id011
      reference: url:https://www.biorxiv.org/content/10.64898/2026.06.16.732665v1.full
      reference_title: "A novel preclinical mouse model recapitulates progressive phenotypes of Bryant-Li-Bhoj Syndrome | bioRxiv"
      supports: REFUTE
      evidence_source: MODEL_ORGANISM
      directness: DIRECT
      quote_role: PRIMARY_RESULT
      snippet: all mice demonstrated appropriate social preference and social memory in the three-chamber task
      explanation: The reported assays do not establish autism-like social preference deficits.
    limitations: Preprint, one allele and mouse strain. Motor impairment does not itself establish intellectual disability, cortical malformation or the proposed age-dependent H3.3-replacement mechanism. The negative adult cell-count assays are not proof of normal brain function at every age.
  - target: Neural Crest and Craniofacial Developmental Defect
    description: Adult craniofacial appearance and an enlarged spheno-occipital growth center provide a morphological comparison.
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    evidence:
    - reference: DOI:10.64898/2026.06.16.732665
      reference_title: A novel preclinical mouse model recapitulates progressive phenotypes of Bryant-Li-Bhoj Syndrome
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      directness: DIRECT
      quote_role: PRIMARY_RESULT
      snippet: Adult mice additionally display craniofacial differences, impaired nest building, hyperactivity in a social context, and male-specific elevated aggression.
      explanation: The model reports appearance changes; a neural-crest mechanism was not tested.
    limitations: No lineage-specific demonstration that the mouse craniofacial changes arise through the zebrafish neural-crest mechanism.
  evidence:
  - *id009
  - *id010
  - *id011
- name: Zebrafish h3f3a D123N db1092 model
  species: Danio rerio
  genotype: h3f3a db1092 dominant-negative missense allele, mature-histone D123N; heterozygous and homozygous animals, with additional RNA perturbation in selected experiments
  publication: PMID:33268356
  description: The db1092 allele is D123N, not a homozygous null. Heterozygotes have variable craniofacial defects and homozygotes more severe losses. The 2020 glial experiment includes additional dominant-negative H3f3a RNA, producing a stronger perturbation. The original 2012 report found peripheral glia unaffected in its protocol. The allele is not a reported human patient allele and fish have five H3.3 genes.
  modeled_mechanisms:
  - target: Neural Crest and Craniofacial Developmental Defect
    description: Disrupted cranial neural-crest specification and ectomesenchyme formation cause craniofacial loss.
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    evidence:
    - &id012
      reference: PMID:33268356
      reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
      snippet: Homozygous h3f3adb1092 mutants display complete loss of neural crest–derived jaw cartilages (n = 10/10).
      explanation: Homozygotes carry two copies of the D123N missense allele.
    - &id013
      reference: PMID:33268356
      reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
      snippet: Homozygous h3f3adb1092 mutants injected at the one-cell stage with a control mCherry RNA show partial reductions in cranial glia
      explanation: The 2020 protocol reports partial reduction; added dominant-negative RNA increases the severity.
    - &id014
      reference: PMID:23028350
      reference_title: An essential role of variant histone H3.3 for ectomesenchyme potential of the cranial neural crest.
      supports: REFUTE
      evidence_source: MODEL_ORGANISM
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
      snippet: foxd3-positive peripheral glia (Figure 1l), neurons of the cranial ganglia (Figure S1), and the dorsal root ganglia and sympathetic neurons derived from trunk NC (data not shown) were unaffected.
      explanation: The original 2012 experiment did not detect this glial deficit. Protocol and perturbation strength differ from the 2020 experiment.
    limitations: A non-patient dominant-negative allele, dosage-dependent perturbation and peripheral neural-crest readouts. These experiments do not establish a CNS myelination defect or model human intellectual disability.
    readouts:
    - name: Neural-crest-derived jaw cartilage
      target: Neural Crest and Craniofacial Developmental Defect
      direction: DECREASED
      interpretation: Loss of jaw cartilage in D123N homozygotes.
      evidence:
      - *id012
  evidence:
  - *id012
  - *id013
  - *id014
- name: Mouse H3f3a G34R/V/W mechanistic comparison
  species: Mus musculus
  genotype: Heterozygous endogenous H3f3a G34R, G34V or G34W; additional Cre-dependent H3f3a G34R models
  publication: PMID:36931244
  description: Engineered H3f3a G34R causes progressive microcephaly, cortical-layer thinning, neuronal depletion and reactive glial accumulation. G34V has milder neurologic effects and G34W predominantly mesenchymal/urinary phenotypes. G34R reduces H3K36me2, redistributes DNMT3A, depletes non-CG methylation and sustains complement-gene expression. Forebrain conditional G34R also produces cortical atrophy. Complement-mediated causality was proposed and was not established by rescue. The human G34R/V cases motivating this study have H3-3B variants; these mice are a protein-level mechanistic comparison, not evidence that those patients had BLBS1.
  modeled_mechanisms:
  - target: Altered Recruitment of PWWP-Domain Effectors
    description: Perturbs H3K36-dependent DNMT3A recruitment with downstream DNA methylation and transcriptional changes.
    relationship: PERTURBS
    fidelity: LOW
    evidence:
    - &id015
      reference: PMID:36931244
      reference_title: Single substitution in H3.3G34 alters DNMT3A recruitment to cause progressive neurodegeneration.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      directness: INDIRECT
      quote_role: PRIMARY_RESULT
      snippet: the G34R mutation diminishes H3K36me2 and consequently redistributes DNMT3A from intergenic regions to CpG islands.
      explanation: Mouse H3f3a provides mechanistic comparison for the shared protein. The human G34R/V patients in this paper had H3-3B variants, so this is indirect evidence for BLBS1.
    limitations: Opposite effects of other alleles, including T45I, prevent generalizing G34R-associated loss of DNMT3A binding to all H3-3A disease. Human cases in this study are H3-3B-related.
  evidence:
  - *id015
  - reference: PMID:36931244
    reference_title: Single substitution in H3.3G34 alters DNMT3A recruitment to cause progressive neurodegeneration.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: whether the disease phenotype is directly caused by persistence of immune gene activation (C3 and C1q) in neurons as we hypothesize
    explanation: The authors explicitly identify the complement-to-neurodegeneration step as requiring further tests.
discussions:
- discussion_id: blbs1_genotype_phenotype_unexplained
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    What accounts for the phenotypic variability that gene, protein domain and
    sex together fail to explain - including variability between individuals
    carrying the same H3-3A allele?
  attaches_to:
  - pathophysiology#De Novo H3-3A Variant Altering Histone H3.3
  - phenotypes#
  rationale: >-
    Phenotypes vary even among individuals with the same H3-3A allele. The expanded cohort describes trends
    by gene, domain and sex but does not establish predictive genotype–phenotype rules. Variant-specific biochemistry,
    gene expression, developmental timing and other modifiers remain candidate contributors.
  evidence:
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Since these variables do not account for all BLBS phenotypic variability, these findings suggest that additional factors may play a role in modifying the phenotypes of affected individuals."
    explanation: >-
      States the residual, unexplained variability directly.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
  - reference: PMID:38678163
    reference_title: "Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Notably, phenotypic heterogeneity was present even amongst individuals harboring the same variant."
    explanation: >-
      Shows that the unexplained variability persists at fixed genotype, which
      rules out variant identity as the missing determinant.
    directness: DIRECT
    quote_role: PRIMARY_RESULT
- discussion_id: blbs1_cancer_predisposition
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Do germline H3.3 variants confer cancer predisposition, given that the same
    residues are oncogenic when altered somatically?
  attaches_to:
  - pathophysiology#Deregulated Cell Proliferation
  rationale: >-
    Increased proliferation in cultured patient cells does not establish cancer predisposition. The 2025 pan-histonopathy
    survey reported two cancers across 192 individuals, including a pulmonary carcinoid in an H3-3B case and
    leukemia in an H4-related disorder. This heterogeneous, incompletely screened cross-sectional denominator
    cannot estimate BLBS1 incidence or lifetime risk. Tumor and constitutional sequencing and longer follow-up
    are needed; standardized BLBS1 surveillance is not established by these observations.
  evidence:
  - reference: PMID:40241305
    reference_title: Coupling deep phenotypic quantification with next-generation phenotyping for 192 individuals with germline histonopathies.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: the value of ongoing translational work to address the outstanding question of cancer predisposition for individuals living with germline histonopathies
    explanation: The study identifies cancer predisposition as unresolved, not refuted.
- discussion_id: blbs1_no_neurodevelopmental_model
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: >-
    Which variant-specific mechanisms explain the human brain phenotype, and which model findings generalize
    to H3-3A disease?
  attaches_to:
  - animal_models#H3.3 T45I knock-in mouse
  - animal_models#Zebrafish h3f3a D123N db1092 model
  - animal_models#Mouse H3f3a G34R/V/W mechanistic comparison
  - pathophysiology#Progressive Postnatal Neuronal H3.3 Dependence
  rationale: >-
    The db1092 fish has a dominant-negative missense allele and peripheral neural-crest endpoints. The T45I
    mouse matches a recurrent H3-3A allele but has preserved performance on several cognitive/social assays
    and no genotype difference in the reported adult brain cell counts. G34R H3f3a mice demonstrate DNA-methylation
    and neuroinflammatory changes, but the human cases motivating that model have H3-3B variants. The 2026
    L48R neural-organoid study likewise concerns H3-3B. None establishes a uniform human H3-3A mechanism or
    tests age-dependent H3.3 replacement as the general cause of progression.
  evidence:
  - *id010
  - *id015
  - reference: PMID:41689096
    reference_title: "A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: Here, we investigate how one BLBS-causative variant, H3-3B p.Leu48Arg (L48R), affects neurodevelopment
    explanation: The neural progenitor, neuron and organoid study concerns the sibling genetic entity; it is not assigned as a BLBS1 patient model.
references:
- reference: DOI:10.64898/2026.06.16.732665
  title: A novel preclinical mouse model recapitulates progressive phenotypes of Bryant-Li-Bhoj Syndrome
  findings:
  - statement: 2026 T45I knock-in mouse preprint; a recurrent human H3-3A allele with partial phenotypic correspondence.
- reference: PMID:23028350
  title: An essential role of variant histone H3.3 for ectomesenchyme potential of the cranial neural crest.
  findings:
  - statement: Original db1092 D123N allele, dominant-negative nucleosome interference and neural-crest specificity.
- reference: PMID:33268356
  title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
  findings:
  - statement: Founding mixed-gene cohort, patient-cell PTMs and proliferation, and zebrafish neural-crest experiments.
- reference: PMID:34876591
  title: "De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities."
  findings:
  - statement: Additional H3-3A/H3-3B cases and variant-specific expression and DAXX assays.
- reference: PMID:35772801
  title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumors."
  findings:
  - statement: Selected 18-person MRI study; full text identifies 11 H3-3A cases and severe-case enrichment.
- reference: PMID:36867246
  title: "Histone 3.3-related chromatinopathy: missense variants throughout H3-3A and H3-3B cause a range of functional consequences across species."
  findings:
  - statement: Review of heterogeneous H3.3 variant effects; mechanistic proposals remain qualified.
- reference: PMID:36931244
  title: Single substitution in H3.3G34 alters DNMT3A recruitment to cause progressive neurodegeneration.
  findings:
  - statement: H3f3a G34 comparison models identify DNMT3A redistribution and a candidate neuroinflammatory route; human cases have H3-3B variants.
- reference: PMID:37782742
  title: Bryant-Li-Bhoj Neurodevelopmental Syndrome.
  findings:
  - statement: GeneReviews clinical overview, management and genetic counseling.
  tags:
  - GeneReviews
- reference: PMID:38678163
  title: Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
  findings:
  - statement: Expanded 96-person cohort with gene-stratified and pooled clinical counts.
- reference: PMID:39525290
  title: "Trapping of yFACT at 3' ends of genes is not a universal characteristic of yeast versions of Bryant-Li-Bhoj syndrome histone H3 mutants."
  findings:
  - statement: Yeast mutant panel shows non-universal FACT trapping and distinct transcriptional defects.
- reference: PMID:39627236
  title: Neonatal myoclonus in Bryant-Li-Bhoj syndrome associated with a novel H3F3A variant.
  findings:
  - statement: One H3-3A case with neonatal myoclonus, later epilepsy and normal MRI.
- reference: PMID:40241305
  title: Coupling deep phenotypic quantification with next-generation phenotyping for 192 individuals with germline histonopathies.
  findings:
  - statement: Pan-histonopathy comparison and unresolved cancer risk; cancer counts are not BLBS1-specific incidence.
- reference: PMID:41689096
  title: "A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation."
  findings:
  - statement: H3-3B L48R neural models delimit the scope of extrapolation to H3-3A disease.
- reference: url:https://pmc.ncbi.nlm.nih.gov/articles/PMC9262070/
  title: "Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumors - PMC"
  findings:
  - statement: Full imaging study provides subgroup composition, specific MRI patterns and ascertainment limitations.
- reference: url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full
  title: "H3.3 De Novo Mutations Alter Lysine 36 Methylation via Distinct Mechanisms | bioRxiv"
  findings:
  - statement: Full 2025 preprint identifies opposite variant effects on H3K36 methylation and PWWP-effector associations.
- reference: url:https://www.biorxiv.org/content/10.64898/2026.06.16.732665v1.full
  title: "A novel preclinical mouse model recapitulates progressive phenotypes of Bryant-Li-Bhoj Syndrome | bioRxiv"
  findings:
  - statement: Full 2026 preprint supplies positive motor findings and negative histological, cognitive and social-assay results.
- reference: url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
  title: "Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf"
  findings:
  - statement: Full GeneReviews chapter and tables support clinical distinctions, management and recurrence-risk counseling.
  tags:
  - GeneReviews
notes: >-
  This entry is restricted to H3-3A-related BLBS1; H3-3B-related BLBS2 is a separate entry. Most clinical studies
  combine the two genes. Pooled proportions below retain their cohort, denominator and ascertainment limitations
  and are not H3-3A-specific population frequencies. Historical histone residue numbering omits the initiator
  methionine; where used, it is distinguished from HGVS numbering. Evidence from engineered protein variants
  and the mouse H3f3a locus does not automatically establish that the corresponding allele occurs in human
  H3-3A. The reviewed literature does not establish a disease-modifying therapy or a BLBS1-specific cancer-risk
  estimate.
experimental_models:
- name: Patient-derived dermal fibroblasts
  organism:
    preferred_term: Homo sapiens
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  description: Three H3-3A patient lines carrying mature-histone R17G, G90R or T45I were compared with three controls by RNA sequencing. Proliferation experiments included five patient lines from both H3.3 genes and six controls.
  evidence: &id016
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: IN_VITRO
    directness: DIRECT
    quote_role: PRIMARY_RESULT
    snippet: RNA sequencing on patient cells demonstrated up-regulated gene expression related to mitosis and cell division, and cellular assays confirmed an increased proliferative capacity.
    explanation: The full text identifies the H3-3A RNA-seq lines and the mixed-gene proliferation comparison.
  modeled_mechanisms:
  - target: Deregulated Cell Proliferation
    description: Three H3-3A patient lines carrying mature-histone R17G, G90R or T45I were compared with three controls by RNA sequencing. Proliferation experiments included five patient lines from both H3.3 genes and six controls.
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    evidence: *id016
    limitations: Non-neural experimental context; transfer to the developing human brain requires corroboration.
  experimental_model_type: PRIMARY_CELL_CULTURE
  publication: PMID:33268356
  cell_source: Patient-derived dermal fibroblasts and unrelated controls
- name: Patient-derived EBV lymphoblastoid cells
  organism:
    preferred_term: Homo sapiens
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  description: Histone mass spectrometry in lymphoblastoid cells, including H3-3A A15G, R17G and T45I, assays local and global post-translational modifications.
  evidence: &id017
  - reference: PMID:33268356
    reference_title: "Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients."
    supports: SUPPORT
    evidence_source: IN_VITRO
    directness: DIRECT
    quote_role: PRIMARY_RESULT
    snippet: cause marked local deregulation of chromatin state, and modestly alter the global control of histone modifications.
    explanation: Patient-derived cell assays support mixed local and global effects.
  modeled_mechanisms:
  - target: Aberrant Local Histone Post-Translational Modification
    description: Histone mass spectrometry in lymphoblastoid cells, including H3-3A A15G, R17G and T45I, assays local and global post-translational modifications.
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    evidence: *id017
    limitations: Non-neural experimental context; transfer to the developing human brain requires corroboration.
  experimental_model_type: CELL_LINE
  publication: PMID:33268356
  cell_source: EBV-transformed patient lymphoblastoid cell lines
- name: HEK293T H3-3A expression and DAXX assays
  organism:
    preferred_term: Homo sapiens
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  description: Transient expression measures variant abundance, localization and DAXX co-immunoprecipitation. H3-3A R129H/M121I reduce abundance, R41C increases it, and R129H strengthens DAXX association.
  evidence: &id018
  - *id003
  - *id002
  modeled_mechanisms:
  - target: Variant-Dependent Histone H3.3 Abundance
    description: Transient expression measures variant abundance, localization and DAXX co-immunoprecipitation. H3-3A R129H/M121I reduce abundance, R41C increases it, and R129H strengthens DAXX association.
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    evidence: *id018
    limitations: Non-neural experimental context; transfer to the developing human brain requires corroboration.
  - target: Altered H3.3 Chaperone Interaction
    description: Transient expression measures variant abundance, localization and DAXX co-immunoprecipitation. H3-3A R129H/M121I reduce abundance, R41C increases it, and R129H strengthens DAXX association.
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    evidence: *id018
    limitations: Non-neural experimental context; transfer to the developing human brain requires corroboration.
  experimental_model_type: CELL_LINE
  publication: PMID:34876591
  cell_source: Transiently transfected HEK293T cells
- name: Inducible HEK293 H3.3 BioID2 models
  organism:
    preferred_term: Homo sapiens
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  description: The 2025 preprint compares isogenic Flp-In T-REx cells expressing tagged wild-type or mutant H3.3, including H3-3A-associated T45I and G90R, for proximal protein associations and histone methylation. Fusion proteins represent less than 20% of total H3.3. Constructs use mouse H3.3 cDNA encoding the identical human protein sequence.
  evidence: &id019
  - reference: url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full
    reference_title: "H3.3 De Novo Mutations Alter Lysine 36 Methylation via Distinct Mechanisms | bioRxiv"
    supports: SUPPORT
    evidence_source: IN_VITRO
    directness: DIRECT
    quote_role: PRIMARY_RESULT
    snippet: Each lineage expressed the histone constructs at comparable levels from a single defined genomic locus.
    explanation: The experimental design reduces expression-site variation; tagged cycling-cell models remain distinct from patient neurons.
  modeled_mechanisms:
  - target: Variant-Dependent H3K36 Methylation
    description: The 2025 preprint compares isogenic Flp-In T-REx cells expressing tagged wild-type or mutant H3.3, including H3-3A-associated T45I and G90R, for proximal protein associations and histone methylation. Fusion proteins represent less than 20% of total H3.3.
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    evidence: *id019
    limitations: Preprint; proximity labeling does not prove direct binding. The cell model is not representative of postmitotic neurons for every endpoint.
  - target: Altered Recruitment of PWWP-Domain Effectors
    description: The 2025 preprint compares isogenic Flp-In T-REx cells expressing tagged wild-type or mutant H3.3, including H3-3A-associated T45I and G90R, for proximal protein associations and histone methylation. Fusion proteins represent less than 20% of total H3.3.
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    evidence: *id019
    limitations: Preprint; proximity labeling does not prove direct binding. The cell model is not representative of postmitotic neurons for every endpoint.
  notes: Preprint; proximity labeling does not prove direct binding. The cell model is not representative of postmitotic neurons for every endpoint.
  experimental_model_type: CELL_LINE
  publication: DOI:10.1101/2025.09.09.674984
  cell_source: Isogenic inducible HEK293 Flp-In T-REx cell lines
- name: Yeast histone H3 BLBS substitution panel
  organism:
    preferred_term: Saccharomyces cerevisiae
    term:
      id: NCBITaxon:4932
      label: Saccharomyces cerevisiae
  description: Nine substitutions in endogenous HHT2 were tested with deletion of HHT1, making mutant H3 the sole source. T45I and R83C permit cryptic transcription; only N108S among the nine traps FACT at gene ends. Earlier L61R studies used coexpressed wild-type H3 and are not directly comparable.
  evidence: &id020
  - reference: PMID:39525290
    reference_title: "Trapping of yFACT at 3' ends of genes is not a universal characteristic of yeast versions of Bryant-Li-Bhoj syndrome histone H3 mutants."
    supports: SUPPORT
    evidence_source: IN_VITRO
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    snippet: these experiments assessed the effects of the H3-BLBS mutants when present as sole source of histone H3 in cells.
    explanation: This genotype differs fundamentally from heterozygous human disease.
  modeled_mechanisms:
  - target: Variant-Specific Transcription-Coupled Chromatin Defects
    description: Nine substitutions in endogenous HHT2 were tested with deletion of HHT1, making mutant H3 the sole source. T45I and R83C permit cryptic transcription; only N108S among the nine traps FACT at gene ends. Earlier L61R studies used coexpressed wild-type H3 and are not directly comparable.
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    evidence: *id020
    limitations: Yeast native H3 is a surrogate. This panel does not establish that all tested protein substitutions occur in H3-3A patients; A114G has an unresolved sequencing-artifact concern.
  notes: Yeast native H3 is a surrogate. This panel does not establish that all tested protein substitutions occur in H3-3A patients; A114G has an unresolved sequencing-artifact concern.
  experimental_model_type: OTHER
  publication: PMID:39525290
  cell_source: S288C-derived yeast strains
- name: Reconstituted H3.3 nucleosome methylation assays
  experimental_model_type: OTHER
  description: The 2025 preprint compares reconstituted nucleosomes made with mammalian H3.3 and Xenopus H2A/H2B/H4, using bacterially expressed proteins and SETD2 or NSD1 catalytic fragments. Mouse H3.3 cDNA encodes a protein identical to human H3.3. T45I stimulates methylation, L48R inhibits it, and V46M has little effect. This cell-free biochemical system does not reproduce a neuronal context.
  publication: DOI:10.1101/2025.09.09.674984
  modeled_mechanisms:
  - target: Variant-Dependent H3K36 Methylation
    description: Measures variant-dependent methyltransferase activity on reconstituted substrates.
    relationship: MEASURES
    fidelity: MODERATE
    evidence:
    - *id004
    limitations: Preprint; recombinant fragments and nucleosome substrates do not establish downstream neuronal function or treatment efficacy.
  evidence:
  - *id004
📚

References & Deep Research

References

17
DOI:10.64898/2026.06.16.732665
A novel preclinical mouse model recapitulates progressive phenotypes of Bryant-Li-Bhoj Syndrome
1 finding
2026 T45I knock-in mouse preprint; a recurrent human H3-3A allele with partial phenotypic correspondence.
PMID:23028350
An essential role of variant histone H3.3 for ectomesenchyme potential of the cranial neural crest.
1 finding
Original db1092 D123N allele, dominant-negative nucleosome interference and neural-crest specificity.
PMID:33268356
Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients.
1 finding
Founding mixed-gene cohort, patient-cell PTMs and proliferation, and zebrafish neural-crest experiments.
PMID:34876591
De novo variants in H3-3A and H3-3B are associated with neurodevelopmental delay, dysmorphic features, and structural brain abnormalities.
1 finding
Additional H3-3A/H3-3B cases and variant-specific expression and DAXX assays.
PMID:35772801
Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumors.
1 finding
Selected 18-person MRI study; full text identifies 11 H3-3A cases and severe-case enrichment.
PMID:36867246
Histone 3.3-related chromatinopathy: missense variants throughout H3-3A and H3-3B cause a range of functional consequences across species.
1 finding
Review of heterogeneous H3.3 variant effects; mechanistic proposals remain qualified.
PMID:36931244
Single substitution in H3.3G34 alters DNMT3A recruitment to cause progressive neurodegeneration.
1 finding
H3f3a G34 comparison models identify DNMT3A redistribution and a candidate neuroinflammatory route; human cases have H3-3B variants.
PMID:37782742
Bryant-Li-Bhoj Neurodevelopmental Syndrome.
1 finding
GeneReviews clinical overview, management and genetic counseling.
PMID:38678163
Expanded phenotypic spectrum of neurodevelopmental and neurodegenerative disorder Bryant-Li-Bhoj syndrome with 38 additional individuals.
1 finding
Expanded 96-person cohort with gene-stratified and pooled clinical counts.
PMID:39525290
Trapping of yFACT at 3' ends of genes is not a universal characteristic of yeast versions of Bryant-Li-Bhoj syndrome histone H3 mutants.
1 finding
Yeast mutant panel shows non-universal FACT trapping and distinct transcriptional defects.
PMID:39627236
Neonatal myoclonus in Bryant-Li-Bhoj syndrome associated with a novel H3F3A variant.
1 finding
One H3-3A case with neonatal myoclonus, later epilepsy and normal MRI.
PMID:40241305
Coupling deep phenotypic quantification with next-generation phenotyping for 192 individuals with germline histonopathies.
1 finding
Pan-histonopathy comparison and unresolved cancer risk; cancer counts are not BLBS1-specific incidence.
PMID:41689096
A novel iPSC model of Bryant-Li-Bhoj neurodevelopmental/neurodegenerative syndrome demonstrates the role of histone H3.3 in chromatin dynamics, neuronal differentiation, and maturation.
1 finding
H3-3B L48R neural models delimit the scope of extrapolation to H3-3A disease.
url:https://pmc.ncbi.nlm.nih.gov/articles/PMC9262070/
Brain Abnormalities in Patients with Germline Variants in H3F3: Novel Imaging Findings and Neurologic Symptoms Beyond Somatic Variants and Brain Tumors - PMC
1 finding
Full imaging study provides subgroup composition, specific MRI patterns and ascertainment limitations.
url:https://www.biorxiv.org/content/10.1101/2025.09.09.674984v1.full
H3.3 De Novo Mutations Alter Lysine 36 Methylation via Distinct Mechanisms | bioRxiv
1 finding
Full 2025 preprint identifies opposite variant effects on H3K36 methylation and PWWP-effector associations.
url:https://www.biorxiv.org/content/10.64898/2026.06.16.732665v1.full
A novel preclinical mouse model recapitulates progressive phenotypes of Bryant-Li-Bhoj Syndrome | bioRxiv
1 finding
Full 2026 preprint supplies positive motor findings and negative histological, cognitive and social-assay results.
url:https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/?report=reader
Bryant-Li-Bhoj Neurodevelopmental Syndrome - GeneReviews® - NCBI Bookshelf
1 finding
Full GeneReviews chapter and tables support clinical distinctions, management and recurrence-risk counseling.

Deep Research

1

Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.

Evaluations and curation notes (2)

Review H3-3A mechanisms, natural history, clinical care and experimental models against cached primary sources · 2026-10-01T11:29:08Z · View source

Reviewed the complete existing entry, prior history, deep-research report, clinical cohorts and full GeneReviews chapter. Corrected the db1092 zebrafish allele from null to dominant-negative D123N and removed the peripheral-glia-to-CNS-myelination inference. Replaced uniform gain-of-function and obligatory adult degeneration claims with variant-specific, qualified mechanisms and natural history. Added measured H3-3A abundance and DAXX effects, T45I H3K36/PWWP findings, and yeast transcriptional results with model limitations. Read both new preprints in full; recorded negative mouse histological and cognitive results. Used the G34R H3f3a study as an explicitly indirect mechanistic comparison because its human cases carry H3-3B variants. Corrected pooled versus gene-specific phenotype denominators and added specific imaging, growth, musculoskeletal, ocular and endocrine findings. Expanded management, baseline assessment and genetic counseling; qualified cancer-risk evidence. H3-3B neural organoids remain outside the BLBS1 model set. All 161 evidence snippets and 178 titles pass automated cache validation; schema and ontology checks pass.

Create: Bryant-Li-Bhoj Neurodevelopmental Syndrome 1 · 2026-08-29T15:57:52Z · View source

De novo curation of BLBS1 (MONDO:0030606, H3-3A) from stubs/Bryant-Li-Bhoj_Neurodevelopmental_Syndrome_1.yaml, using a claude_code deep-research report. The sibling entry Bryant-Li-Bhoj_Neurodevelopmental_Syndrome_2 (H3-3B) already existed and explicitly recorded BLBS1 as a differential diagnosis rather than folding it in, so this entry reciprocates that differential. just preflight-dr returned WARN because the report mentions H3-3B 27 times against 34 for H3-3A; the report discusses both entities, and its H3-3B-specific content (the p.Leu48Arg iPSC hyper-deposition model, the inherited p.Asn108Ser allele, the synonymous stop-gain and stop-loss alleles) was excluded rather than folded in. The GeneReviews baseline required by the disorder-creation skill was fetched (PMID:37782742) and its Clinical Characteristics, Management and Surveillance sections were cross-referenced into phenotypes and treatments; the top-level references block carries the GeneReviews tag. Mechanism is a seven-node chain from the de novo H3-3A variant through disrupted nucleosome assembly and aberrant local histone post-translational modification to deregulated proliferation, the neural crest craniofacial defect, and the neurodevelopmental plus degenerative brain phenotype. The zebrafish h3f3a model is curated here with modeled_mechanisms, readouts and an explicit limitations note, because h3f3a is the H3-3A ortholog. Three ontology CURIEs suggested by the report were wrong and were replaced after checking the ontologies directly: HP:0031808 is Decreased total basophil count rather than Camptocormia (correct HP:0100595), CL:0002608 is hippocampal neuron rather than radial glial cell (correct CL:0000681), and HP:0007256 is Abnormal pyramidal sign rather than Spasticity (correct HP:0001257). No conforms_to is declared against epigenetic_machinery_neurodevelopmental_dysregulation because that module's entry node is haploinsufficiency and this disease is explicitly not haploinsufficiency; the sibling entry makes the same choice. Validated with just validate (61/61 snippets verified), just validate-terms, check-entity-refs, check-duplicate-keys, check-folded-hyphens, check-snippet-length, check-title-snippets, check-snippet-grading, and the batched just validate-disorders. After the first CI run, the branch was rebased onto main to pick up #10003, which retired supports: PARTIAL; the three PARTIAL evidence items were migrated to supports: SUPPORT with directness: INDIRECT, which is what each of them was actually recording - a quote from which the curated claim follows by an inference step rather than one that asserts it. A second round addressed the reviewer's content findings: nine phenotypes were added from Table 1 of the 2024 cohort paper (delayed walking, delayed sitting, delayed speech, corpus callosum morphology, microcephaly, hypertonia, dermatologic features, musculoskeletal anomalies, hearing impairment, autism); three over-specific bindings were generalised to what their sources actually support (ocular involvement from HP:0000486 Strabismus to HP:0000496, congenital heart defect from HP:0001631 to HP:0001627, musculoskeletal from HP:0002650 Scoliosis to HP:0011842); every table-row snippet was replaced with the corresponding prose sentence after `just check-snippet-length` correctly rejected bare table cells as non-propositional; the bundled 'Abnormal Brain Development and Progressive Neurologic Dysfunction' node was split into a congenital node and a postnatal-H3.3-dependence node; and the h3-3a p.T45I knock-in mouse (DOI:10.64898/2026.06.16.732665, a preprint located via Europe PMC after PubMed returned nothing) was curated with three modeled_mechanisms links, so the HUMAN_MODEL_MISMATCH discussion now argues from two models failing in opposite directions rather than asserting that no in vivo model exists. The DOI prefix is in the reference validator's skip_prefixes, so those seven snippets were verified by hand against the cache.

Claude Code ▸
Bryant-Li-Bhoj Neurodevelopmental Syndrome 1 (BRYLIB1): Comprehensive Research Report
claude-haiku-4-5-20251001, claude-sonnet-5 20 citations 2026-08-29T14:50:25.608289

Bryant-Li-Bhoj Neurodevelopmental Syndrome 1 (BRYLIB1): Comprehensive Research Report

1. Disease Information

Overview. Bryant-Li-Bhoj neurodevelopmental syndrome (BLBS/BRYLIB) is a rare, clinically variable neurodevelopmental (and, in a subset of adults, neurodegenerative) disorder first delineated in 2020 and caused by heterozygous germline variants in one of the two genes that encode replication-independent histone H3.3: H3-3A (formerly H3F3A) and H3-3B (formerly H3F3B). Variants in H3-3A define Bryant-Li-Bhoj neurodevelopmental syndrome 1 (BRYLIB1); variants in H3-3B define the molecularly and clinically similar BRYLIB2. The condition was named for the discovery team (Bryant, Li, Bhoj) in the founding report ("Histone H3.3 beyond cancer: Germline mutations in Histone 3 Family 3A and 3B cause a previously unidentified neurodegenerative disorder in 46 patients," Science Advances, 2020). GeneReviews describes it as "characterized by developmental delay/intellectual disability... and nonspecific craniofacial abnormalities," with a highly variable, gene-dosage-sensitive presentation (GeneReviews, NBK595206, updated 2023) [https://www.ncbi.nlm.nih.gov/sites/books/NBK595206/].

Key identifiers: - OMIM (phenotype): #619720 — BRYANT-LI-BHOJ NEURODEVELOPMENTAL SYNDROME 1; BRYLIB1 (https://www.omim.org/entry/619720) - OMIM (gene): 601128 — H3 HISTONE, FAMILY 3A; H3F3A / H3-3A (https://omim.org/entry/601128) - Related disorder: OMIM #619721 — BRYLIB2 (H3-3B, gene 601058), the sister condition - MONDO: MONDO:0030606 (Bryant-Li-Bhoj neurodevelopmental syndrome 1), confirmed via ClinGen curation (https://search.clinicalgenome.org/kb/conditions/MONDO:0030606) - HGNC: H3-3A, HGNC:4764 - ClinGen gene-disease validity: Definitive (Syndromic Disorders GCEP, classification date 2024-03-19) - Orphanet: no distinct ORPHA number could be confirmed in this search (the entity is very recently named, 2022 OMIM addition; it may still be tracked in Orphanet under a broader "H3.3-related chromatinopathy" heading rather than a dedicated ORPHA code) - Synonyms: BRYLIB1; Bryant-Li-Bhoj syndrome (BLBS); H3.3-related neurodevelopmental disorder; H3-3A-related neurodevelopmental disorder; (historically, before nosologic separation from cancer-associated somatic H3.3 mutations) "H3F3A germline neurodevelopmental disorder"

Data provenance: Nearly all current knowledge derives from aggregated case-series/cohort resources (GeneReviews, the 2020 founding cohort of 46 patients, and a 2024 expansion to 96 total individuals) built from clinician-submitted, ClinVar-deposited, and literature case reports — not from a population-level EHR resource. This is disease-level/aggregated-cohort evidence, not raw individual-patient EHR data.

2. Etiology

Primary cause — genetic, autosomal dominant, de novo. BRYLIB1 is caused by heterozygous, essentially always de novo pathogenic variants in H3-3A. GeneReviews states plainly: "BRYLIB is an autosomal dominant disorder typically caused by a de novo pathogenic variant." Parental testing in reported cohorts has confirmed de novo origin in essentially all cases; the 2024 expansion cohort reported the field's first exception — a maternally inherited H3-3B variant (p.Asn108Ser) in a mildly affected mother and child, establishing that (at least for H3-3B) transmission from a mosaic or mildly affected parent is possible (Layo-Carris et al., Eur J Hum Genet 2024;32(8):928–937, PMID: 38678163).

Genetic risk factors. The disease-causing variant itself is the risk factor; there is no described susceptibility-locus or polygenic contribution. gnomAD constraint metrics show H3-3A and H3-3B are highly intolerant to variation (missense z-scores of 3.16 and 2.88, respectively), consistent with the observation that even single heterozygous substitutions are clinically impactful (Layo-Carris et al. 2024).

Modifier factors / phenotype drivers. The affected gene (H3-3A vs H3-3B), the protein domain hit (N-terminal tail, aa 1–43, vs. histone-fold core, aa 44–135), and sex all statistically associate with different symptom sub-profiles (see Genotype-Phenotype Correlations, section 4/6), but the authors explicitly caution that "the current stratification by sex, affected gene, or affected protein domain does not account for all phenotypic variation observed," implicating unidentified modifiers — possibly epigenetic or gene-environment interaction effects, since "histone biology sits at the genetics-epigenetics interface" (Layo-Carris et al. 2024).

Environmental/lifestyle risk factors. None identified; this is a Mendelian single-gene disorder with no established environmental etiologic contribution.

Protective factors. None reported.

Somatic vs. germline distinction (important etiologic contrast). Identical or adjacent missense substitutions occurring somatically in H3-3A (notably p.Gly34Arg/Val and p.Lys27Met) are well-known oncogenic drivers of pediatric diffuse midline glioma and giant cell tumor of bone. In BRYLIB1/2, the same or neighboring residues are altered germline, and critically, no oncologic complications have been reported in the BLBS germline cohort to date, despite two individuals carrying germline p.Gly34Arg/Val substitutions identical to oncogenic somatic hotspots (Bryant et al. 2020; Layo-Carris et al. 2024). This is mechanistically informative: PTM/chromatin dysregulation in the germline, congenital setting differs qualitatively from the global histone-code disruption caused by somatic "oncohistone" mutations.

3. Phenotypes

Frequencies below combine GeneReviews (n=57), the founding 2020 cohort (n=46), and the 2024 expanded cohort of 96 total individuals (58 previously reported + 38 new; Layo-Carris et al. 2024, PMID 38678163), which is the most complete phenotype table available.

Phenotype Frequency HPO suggestion
Global developmental delay / intellectual disability (moderate–severe) 99% (94/95) HP:0012758 (Developmental regression) / HP:0001263 (Global developmental delay) / HP:0001249 (Intellectual disability)
Poor/absent speech, delayed speech (no words by 20 mo) 60% (50/84) HP:0000750 (Delayed speech and language development)
Delayed walking (>20 mo) 79% (59/75) HP:0002505 / HP:0031936 (Delayed ability to walk)
Delayed sitting (>12 mo) 51% (33/65) HP:0025336 (Delayed ability to sit)
Craniofacial dysmorphism (nonspecific pattern) 88–92% HP:0001999 (Abnormal facial shape)
Hypotonia 62–72% (often resolves/evolves with age) HP:0001252 (Hypotonia)
Hypertonia / peripheral spasticity (partly age-evolving) 12–19% HP:0001276 (Hypertonia) / HP:0007256 (Spasticity)
Oculomotor abnormalities (strabismus, nystagmus) 53–54% (strabismus alone 36%) HP:0000486 (Strabismus) / HP:0000639 (Nystagmus)
Seizures (variable types, childhood onset) 47–49% (febrile seizures ~20% of these) HP:0001250 (Seizure)
Abnormal brain MRI (composite) 58% HP:0002060 (Abnormal cerebral morphology)
— Small posterior fossa 72% (in detailed-MRI subgroup) HP:0025265
— Corpus callosum malformation 37% (28/76) HP:0002079 (Hypoplasia of the corpus callosum)
— Delayed/hypomyelination, dilated ventricles subset HP:0002188 (Delayed CNS myelination), HP:0002119 (Ventriculomegaly)
Short stature 35–39% HP:0004322 (Short stature)
Microcephaly 32–33% HP:0000252 (Microcephaly)
Macrocephaly 15% HP:0000256 (Macrocephaly)
Craniosynostosis / abnormal head shape 32% HP:0001363 (Craniosynostosis)
Musculoskeletal anomalies (incl. scoliosis 21%) 60% HP:0002650 (Scoliosis)
Dermatologic features 52% HP:0000951 (Abnormality of the skin)
Congenital heart defects (esp. atrial septal defect) 13–19% HP:0001631 (Atrial septal defect)
Genital anomalies / cryptorchidism (males) 20–35% HP:0000028 (Cryptorchidism)
Hearing loss subset HP:0000365 (Hearing impairment)
Feeding problems common in infancy HP:0011968 (Feeding difficulties)
Neurobehavioral abnormalities (autism, ADHD, happy demeanor, stereotypies) ~50% in one cohort HP:0000717 (Autism), HP:0000733 (Stereotypy)
Camptocormia / new-onset motor decline in adulthood 100% of reported adults (subacute onset, 3rd decade of life, then stabilizes) HP:0031808 (Camptocormia)
Gait abnormality / ataxic gait up to universal in some reports HP:0002317 (Unsteady gait)

Onset/progression/severity. Developmental delay is evident from infancy/early childhood; the reported age range at evaluation spans 10 weeks to 39 years, so the natural history now extends well into adulthood. Notably, this is one of the few histone-related NDDs with a documented adult-onset neurodegenerative component: "all reported adults have had a new subacute onset of motor issues in the third decade of life that generally remains stable after onset," manifesting principally as camptocormia (progressive bent-spine posture) — the feature that anchors the "neurodegenerative" half of the syndrome's Science Advances title. Hypotonia can resolve or evolve into a mixed axial-hypotonia/peripheral-hypertonia pattern with age; this mixed pattern was found to occur exclusively in individuals with variants in the histone core domain (see Section 6).

Quality of life impact. Not separately quantified with EQ-5D/SF-36 instruments in the literature reviewed; qualitatively, impact is substantial given near-universal moderate-to-severe intellectual disability, motor delay, and (in a subset) adult-onset progressive motor decline, feeding difficulties, and seizures requiring ongoing neurological management.

4. Genetic/Molecular Information

Causal gene: H3-3A (HGNC:4764; OMIM 601128), chromosome 1q42.12, encoding replication-independent histone H3.3. (BRYLIB2 is caused by H3-3B, OMIM 601058, chromosome 17q25.1 — the two genes encode an essentially identical H3.3 protein and produce clinically overlapping disease.)

Variant spectrum: - BRYLIB1 (H3-3A): overwhelmingly de novo heterozygous missense variants, identified in the founding cohort in 33 unrelated patients, "found by whole-exome or genome sequencing, occurred throughout the gene" (OMIM #619720, citing Bryant et al. 2020). - BRYLIB2 (H3-3B): greater variant diversity, including a synonymous variant acting as a cryptic stop-gain in a non-canonical transcript (p.Val117Val), a stop-loss variant (p.Cys136ext9, a 2-nt deletion ablating the stop codon), and the one known inherited variant (p.Asn108Ser). - Across the combined 96-individual cohort: 70 unique causative variants*, distributed throughout the H3.3 protein rather than clustered at one or two hotspots — in contrast to the sharply localized cancer "oncohistone" hotspots (K27, G34, K36). - Representative variants: H3-3A p.Thr45Ile (T45I) — the most recurrently reported variant, now used to generate a mouse model (see Section 15); H3-3A p.Thr46Ile; H3-3B p.Leu48Arg (L48R) — used to generate an iPSC model; H3-3A/B p.Gln125Arg (shared, 8 individuals, discordant phenotypes); germline p.Gly34Arg/Val in H3-3B (mirrors the somatic oncohistone hotspot but with no oncologic phenotype). - ClinVar entry example: NM_002107.7(H3-3A):c.137C>T (p.Thr46Ile), classified pathogenic/likely pathogenic for BRYLIB1 (https://www.ncbi.nlm.nih.gov/clinvar/RCV001823766/).

Variant classification (ACMG/AMP): Pathogenic/likely pathogenic missense (and the rarer truncating/stop-loss/cryptic-splice) variants; interpreted per standard ACMG/AMP criteria plus gene-specific de novo and functional evidence used by the ClinGen Syndromic Disorders GCEP, which rated the H3-3A–BRYLIB1 relationship Definitive.

Allele frequency: Not present (or present only as ultra-rare singleton entries) in gnomAD/population databases, consistent with strong purifying selection (missense z-scores >2.8 for both genes) and complete penetrance of de novo disease alleles.

Somatic vs. germline: BRYLIB1/2 are strictly germline disorders; the same H3-3A/H3-3B residues altered somatically (K27M, G34R/V, K36M) instead cause pediatric high-grade/diffuse midline glioma and giant cell tumor of bone — an important differential/molecular contrast, not a subtype of BRYLIB1.

Functional consequences. Molecular modeling of the founding cohort's 37 variants "demonstrated clear disruptions in interactions with DNA, other histones, and histone chaperone proteins" (Bryant et al. 2020). Patient-derived histone post-translational modification (PTM) profiling showed "notably aberrant local PTM patterns distinct from the somatic lysine mutations that cause global PTM dysregulation" — i.e., germline variants perturb the histone code locally/regionally rather than globally. RNA-seq on patient cells showed "up-regulated gene expression related to mitosis and cell division, with increased proliferative capacity." A specific structural mechanism identified for one recurrent variant (H3-3A p.Arg129His) was "significantly stronger interaction with DAXX," the H3.3-specific histone chaperone, implicating disrupted, chaperone-mediated H3.3 deposition (Tsuchiya et al./de novo-variant cohort, PMC8651650). For H3-3B p.Leu48Arg, iPSC modeling indicates the variant "increases H3-3B expression, resulting in the hyper-deposition of H3.3 into the nucleosome" (see Section 6).

Modifier genes: None formally established; phenotype correlates statistically with affected gene and protein domain (tail vs. core) rather than with a distinct modifier locus (see Section 6).

Epigenetic information: This is fundamentally an epigenetic-machinery disorder — the causal protein is a core chromatin component. Aberrant local histone PTM deposition and altered chromatin accessibility (documented directly by ATAC-seq/multi-omic profiling in iPSC-derived neural models) are central to pathogenesis rather than secondary.

Chromosomal abnormalities: Not a copy-number/structural disorder; disease is driven by single-nucleotide/small-indel variants in H3-3A. Gene-targeted deletion/duplication analysis detection rate is not established (no cases reported to date attributable to CNV).

5. Environmental Information

No environmental, lifestyle, or infectious contributing factors have been identified or proposed for BRYLIB1 beyond the theoretical, unproven suggestion in the 2024 expanded-cohort paper that "gene-environment interactions" might help explain residual phenotypic variability not accounted for by gene/domain/sex stratification — this is explicitly speculative rather than evidence-based (Layo-Carris et al. 2024).

6. Mechanism / Pathophysiology

Molecular substrate. H3.3 is a replication-independent histone variant (as opposed to the replication-dependent canonical H3.1/H3.2) that is deposited onto DNA by the DAXX/ATRX and HIRA histone chaperone complexes, especially at transcriptionally active regions, telomeres, and pericentric heterochromatin. Two molecules of each core histone (H2A, H2B, H3, H4) assemble into the octamer around which ~146 bp of DNA wraps to form the nucleosome; H3.3 is essential for chromatin compaction, early embryonic development, and cell-lineage commitment. Suggested GO terms: GO:0000786 (nucleosome), GO:0031507 (heterochromatin formation), GO:0006335 (DNA replication-independent nucleosome assembly), GO:0016575 (histone deacetylation)/relevant PTM GO terms, GO:0006338 (chromatin remodeling).

Causal chain (as currently understood): 1. Trigger (molecular/upstream): De novo missense (or rare truncating/inherited) variant in H3-3A alters the H3.3 protein at a residue involved in DNA contact, chaperone binding (e.g., DAXX interaction, strengthened for p.R129H), or inter-histone (H3-H4/H2A-H2B) contacts within the nucleosome. 2. Molecular consequence: Disrupted or altered-affinity interactions with DNA, chaperones, and neighboring histones; in some variants (e.g., L48R) increased H3-3B expression and hyper-deposition of H3.3 into nucleosomes rather than simple loss-of-function. 3. Chromatin-level consequence: Aberrant, locally restricted (not globally dysregulated, unlike somatic oncohistones) deposition of histone PTMs, producing a disrupted local "histone code." 4. Gene-regulatory consequence: Genome-wide, cell-type-specific dysregulated gene expression and altered chromatin accessibility — documented in iPSC-derived neural progenitor cells, forebrain neurons, and organoids as changes affecting genes governing "neuronal fate, adhesion, neurotransmission, and excitatory/inhibitory balance" (Journal of Translational Medicine 2026 iPSC paper, PMC11382994). Patient fibroblasts additionally show up-regulated mitosis/cell-division transcriptional programs and increased proliferative capacity (Bryant et al. 2020). 5. Cellular consequence: Altered proportions of radial glia versus mature neuronal populations in organoids; decreased spontaneous electrical activity in L48R-mutant forebrain-organoid neurons by patch-clamp electrophysiology, indicating impaired neuronal maturation/functional network formation. 6. Organismal/clinical consequence: Global developmental delay/intellectual disability, dysmorphic craniofacial features, hypotonia/hypertonia, seizures, structural brain anomalies (hypomyelination, corpus callosum dysgenesis, small posterior fossa), and — in the neurodegenerative arm of the phenotype — adult-onset progressive camptocormia/motor decline, presumably reflecting cumulative disruption of H3.3's dominant role in post-mitotic neuronal chromatin maintenance over decades (H3.3 becomes >93% of total neuronal H3 in mature neurons, versus ~31% of the H3 pool during early neurodevelopment).

Upstream vs. downstream framing: Upstream = the germline histone variant itself (a cell-autonomous, constitutive lesion present from the zygote onward, not one triggered by an external event). Midstream = chromatin/PTM/accessibility dysregulation. Downstream = neurodevelopmental (congenital) phenotypes plus a distinct, temporally separate downstream neurodegenerative phenotype (adult camptocormia) that appears to require decades of cumulative dysfunction in postmitotic neurons — consistent with the biological observation that H3.3 dependence increases sharply as neurons mature and exit the cell cycle.

Cell types/processes implicated: Neural progenitor cells, radial glia, cortical/forebrain excitatory neurons (maturation and electrophysiological function), cranial neural crest cells (craniofacial dysmorphism, per zebrafish data), and — per RNA-seq of patient fibroblasts — mitotically active non-neural cells showing increased proliferation. Suggested CL terms: CL:0000047 (neural progenitor cell/stem cell), CL:0002608 (radial glial cell), CL:0000540 (neuron)/CL:0000598 (pyramidal neuron), CL:0002500 (cranial neural crest cell).

Genotype-phenotype correlation (protein-domain level, from the 96-individual 2024 cohort): - N-terminal tail variants (aa 1–43): associated with undergrowth (44%), abnormal neuroimaging (63%), hypotonia (73%), dermal features (64%), delayed sitting (59%). - Histone-fold core variants (aa 44–135): associated with the mixed axial-hypotonia/peripheral-hypertonia pattern (exclusively seen with core variants), higher rates of overweight (25% vs 7%) and cardiac anomalies (17% vs 7%), and more often normal height. - Gene-level: H3-3A variants trend toward more craniofacial dysmorphism (95% vs 86%) and more delayed walking (85% vs 65%); H3-3B variants trend toward higher seizure prevalence (59% vs 45%), more macrocephaly (53% vs 43%), and more genital anomalies (28% vs 17%). - Sex: males show more delayed walking and speech delay; females show more oculomotor dysfunction; seizure prevalence and overall developmental-delay severity do not differ significantly by sex. - Critically, identical variants produce discordant phenotypes (e.g., four individuals with H3-3A p.Thr45Ile, and eight individuals sharing p.Gln125Arg across both genes, show substantial phenotypic variability), so genotype-phenotype rules are statistical trends, not deterministic.

Advanced/omics technologies used to date: multi-omic (transcriptomic + chromatin-accessibility/ATAC-like) profiling of iPSC-derived 2D neural progenitor cells and forebrain neurons; single-cell/organoid-level immunofluorescence characterization of 3D dorsal forebrain organoids; patch-clamp single-cell electrophysiology. No published single-cell RNA-seq atlas, spatial transcriptomics, or CRISPR functional-genomics screen specific to BLBS was identified in this search.

7. Anatomical Structures Affected

Organ level: Primary — central nervous system (brain: cortex, corpus callosum, posterior fossa/cerebellum, white matter); craniofacial skeleton (skull shape, craniosynostosis). Secondary — cardiovascular system (atrial septal defect and other congenital heart defects), musculoskeletal system (scoliosis, other skeletal anomalies, adult-onset camptocormia of the spine), genitourinary system (cryptorchidism, other genital anomalies), integumentary system (dermatologic features), auditory system (hearing loss), ocular system (strabismus, nystagmus, other oculomotor dysfunction), endocrine system (hypothyroidism reported in the sister Rahman-syndrome literature and surveilled for in BLBS management protocols).

Body systems involved: nervous, musculoskeletal, cardiovascular, ocular, auditory, endocrine, genitourinary, integumentary — a genuinely multisystem chromatinopathy centered on but not limited to neurodevelopment. Suggested UBERON terms: UBERON:0000955 (brain), UBERON:0002336 (corpus callosum), UBERON:0002037 (cerebellum), UBERON:0002298 (brainstem/posterior fossa structures), UBERON:0003128 (skull).

Tissue/cell level: cerebral cortical neurons and their progenitors (radial glia); cranial neural crest-derived craniofacial mesenchyme; cardiac septal tissue; ocular extraocular muscle/oculomotor control circuitry.

Subcellular level: the nucleus/chromatin is the primary subcellular compartment affected — the nucleosome itself (GO Cellular Component: GO:0000786 nucleosome; GO:0000785 chromatin; GO:0005694 chromosome) is the direct molecular substrate of disease.

Localization/lateralization: No lateralization pattern reported; craniofacial and brain anomalies are typically bilateral/midline (e.g., corpus callosum, posterior fossa). Camptocormia is axial/midline (spine).

8. Temporal Development

Onset: Congenital/early childhood for the neurodevelopmental component (developmental delay evident in infancy); distinctly adult-onset (third decade) for the neurodegenerative motor component (camptocormia).

Progression: Two temporally distinct phases are now recognized — (1) a static-to-improving neurodevelopmental phase in childhood (hypotonia can resolve with age in some individuals), followed by (2) a subacute-onset, then stabilizing neurodegenerative motor phase beginning in early adulthood, universal among reported adults, that "generally remains stable after onset" rather than being relentlessly progressive. Seizures, when present, begin in childhood with variable, sometimes treatment-refractory, course.

Patterns: Developmental regression is reported in a minority of individuals, ranging mild to severe; no spontaneous full remission is described. No defined "critical period" for intervention has been established in the literature, though early developmental intervention (birth–age 3) is uniformly recommended in management guidance.

9. Inheritance and Population

Epidemiology: Ultra-rare; 96 individuals reported in the peer-reviewed literature as of the 2024 expanded cohort (58 previously known + 38 new), combined across BRYLIB1 (H3-3A, 65 individuals) and BRYLIB2 (H3-3B, 31 individuals). No population-based prevalence or incidence estimate (per 100,000) has been established; the condition is almost certainly ascertainment-limited (identified via exome/genome sequencing for undiagnosed neurodevelopmental disorders) rather than reflecting a stable epidemiologic denominator.

Inheritance pattern: Autosomal dominant, essentially always de novo; one confirmed maternally-inherited H3-3B case is the sole reported exception to date (2024 cohort). GeneReviews recurrence-risk guidance: for parents of an isolated proband, recurrence risk to siblings is ~1% (accounting for possible parental germline mosaicism) if parental testing is negative; risk to offspring of an affected individual is not yet established because few affected individuals have reached reproductive age.

Penetrance: Reported cases are essentially fully penetrant (all carriers described to date are symptomatic), though the founder mild inherited case (mother + child, p.Asn108Ser) suggests a spectrum extending toward milder/possibly under-ascertained presentations.

Expressivity: Markedly variable, even among carriers of the identical variant (documented explicitly for p.Thr45Ile and p.Gln125Arg) — this is one of the most striking features of the disorder and is explicitly flagged by the authors as only partially explained by gene, domain, or sex.

Genetic anticipation: Not reported/applicable (not a repeat-expansion disorder).

Germline mosaicism: Formally invoked as the explanation for the ~1% empiric sibling recurrence risk quoted by GeneReviews, though not directly demonstrated in a specific reported family to date in this search.

Founder effects / consanguinity: None reported; disease arises from de novo mutation, not from population-specific founder alleles or recessive consanguinity-driven inheritance.

Population demographics: No described ethnic, geographic, or ancestry-specific enrichment; cohorts to date are drawn from international, largely clinical-exome-sequencing-ascertained populations (US, European, and at least one Chinese cohort reported separately — see PMC ResearchGate "Bryant-Li-Bhoj neurodevelopmental syndrome: a case report in China and literature review"). Sex ratio: no marked male:female skew reported, though some phenotype features differ by sex in frequency (Section 6).

10. Diagnostics

Diagnostic criteria (GeneReviews): "The phenotypic features associated with Bryant-Li-Bhoj neurodevelopmental syndrome are not sufficient to diagnose this condition clinically." Diagnosis requires suggestive clinical findings plus identification of a heterozygous pathogenic/likely-pathogenic variant in H3-3A or H3-3B by molecular genetic testing.

Testing approach/order: 1. Chromosomal microarray analysis (initial broad screen, primarily to exclude a CNV-based alternative diagnosis) 2. Intellectual disability multigene panel including H3-3A/H3-3B 3. Exome or genome sequencing (most productive; used to identify the great majority of reported cases) 4. Single-gene sequential testing is explicitly "rarely useful and typically NOT recommended," given the phenotype's non-specificity.

Detection rate: Sequence analysis has detected essentially all reported pathogenic variants (39/39 H3-3A cases and 18/18 H3-3B cases in the cohort GeneReviews cites); detection rate for gene-targeted deletion/duplication (CNV) analysis is unknown/unestablished, since no CNV-mediated cases have been reported.

Imaging: Brain MRI is the key structural imaging modality — findings include small posterior fossa, corpus callosum hypoplasia/dysgenesis, delayed/hypomyelination, cortical dysplasia in a subset, and (in some individuals) leukoencephalopathy-pattern white-matter change.

Differential diagnosis: Because the phenotype (developmental delay + nonspecific dysmorphism + variable hypotonia/seizures) overlaps broadly with hundreds of other Mendelian intellectual-disability disorders, GeneReviews directs clinicians to the OMIM autosomal dominant, autosomal recessive, and X-linked intellectual developmental disorder phenotypic series for full differential consideration. Notably, close molecular differentials include the H1.4-linker-histone disorder Rahman syndrome (HIST1H1E/H1-4) — a distinct, separately named chromatinopathy causing overlapping intellectual disability, overgrowth, and dysmorphic facies, but mechanistically and molecularly separate (linker histone H1 vs. core nucleosomal histone H3.3) and must not be conflated with BRYLIB1/2 despite superficial resemblance and shared "histone-disorder" framing in review literature.

Genetic testing modalities: Standard clinical exome/genome sequencing is the primary and most productive tool; chromosomal microarray, karyotyping, FISH, and mitochondrial DNA testing are not primary diagnostic tools for this specific condition (used only to exclude alternative etiologies). No repeat-expansion or specialized epigenomic clinical test is part of the standard diagnostic pathway at this time, although research-grade histone PTM profiling and chromatin-accessibility assays have been used investigationally (see Section 6).

Screening: No newborn-screening, population carrier-screening, or cascade-screening program exists (as expected for an ultra-rare, almost-always de novo disorder); prenatal/preimplantation genetic testing is available once a familial pathogenic variant is identified.

11. Outcome/Prognosis

Survival/mortality: "It is unknown whether life span in BRYLIB is abnormal" (GeneReviews); several adults with the condition have now been reported, and life span does not appear to be markedly shortened by the condition itself based on current (still limited) natural-history data.

Morbidity/function: Chronic disability driven by moderate-to-severe intellectual disability, motor delay, and (in adulthood) progressive camptocormia; no tumors have been reported in individuals with germline H3.3 variants (an important reassurance point distinguishing this from the somatic oncohistone literature).

Disease course: Predominantly non-regressive/stable neurodevelopmental phenotype through childhood for most individuals, with a distinct adult-onset subacute motor decline (camptocormia) that then plateaus; a minority show frank developmental regression of variable severity.

Prognostic factors: No formal prognostic biomarker or scoring system has been developed; affected gene/domain and (to a lesser extent) sex have only modest, non-deterministic predictive value for symptom profile as discussed above.

12. Treatment

No disease-modifying or curative treatment exists. Management is entirely multidisciplinary and supportive, per GeneReviews Table 4 ("Treatment of Manifestations," https://www.ncbi.nlm.nih.gov/books/NBK595206/table/brylib.T.bryantlibhoj_neurodevelopmental_1/):

  • Developmental/educational support: Early intervention (age 0–3), developmental preschool/special education, individualized education plans, physical/occupational/speech therapy. NCIT: NCIT:C15302 (Physical Therapy), NCIT:C15747 (Supportive Care), NCIT:C49236 (Therapeutic Procedure).
  • Seizure management: Standardized anti-seizure medication under an experienced neurologist; multiple agent options exist, some individuals show treatment-refractory seizures. NCIT: NCIT:C15986 (Pharmacotherapy) with an anticonvulsant therapeutic_agent.
  • Feeding/nutrition: Feeding therapy; gastrostomy tube placement for persistent feeding dysfunction; ongoing nutritional monitoring. NCIT: NCIT:C15447 (Dietary Intervention).
  • Spasticity management: Physical therapy, positioning devices, antispasticity medications.
  • Craniosynostosis: Surgical correction when clinically indicated. NCIT: NCIT:C16186 (Orthopedic Surgical Procedure) or NCIT:C15329 (Surgical Procedure).
  • Ophthalmology: Ongoing care for strabismus/visual impairment.
  • Audiology: Hearing-loss surveillance and intervention.
  • Cardiology: Evaluation/management of congenital heart defects.
  • Urology: Consultation for cryptorchidism.
  • Endocrinology: Management of hypothyroidism when present.

Surveillance protocol (GeneReviews): at each visit — growth parameters, nutrition/oral-intake safety, constipation screening, new seizure or gait-change assessment, developmental-progress monitoring, behavioral screening; annually/as indicated — ophthalmology exam, audiology exam, thyroid function testing; and (per the 2024 expanded-cohort recommendations) repeat neuroimaging to track the progressive/neurodegenerative component and ongoing genitourinary surveillance.

Experimental/precision-medicine pipeline (very early stage, no clinical trials yet identified): The two most important recent developments are disease models built specifically to enable future targeted-therapy development: - iPSC model (H3-3B p.Leu48Arg): 2D neural progenitor cells, 2D forebrain neurons, and 3D dorsal forebrain organoids, characterized by multi-omic profiling, immunofluorescence, and patch-clamp electrophysiology (Journal of Translational Medicine, Feb 2026; PMC11382994, PMID 39253491). The authors explicitly frame this model as "a crucial step towards preclinical development and testing of targeted therapies." - Mouse model (H3-3A p.Thr45Ile), 2026 preprint: the first in vivo preclinical model, recapitulating "perinatal growth restriction, delayed developmental milestones, and progressive motor and gait impairments," plus adult craniofacial differences, impaired nest building, social-context hyperactivity, and male-specific elevated aggression (bioRxiv, June 2026, https://www.biorxiv.org/content/10.64898/2026.06.16.732665v1). This model is positioned to enable in vivo therapeutic testing going forward.

No NCT-registered clinical trial for a targeted BLBS therapy was identified in this search as of August 2026; the field remains at the mechanistic/preclinical-model stage.

13. Prevention

No primary, secondary, or tertiary prevention strategy exists beyond standard reproductive genetic counseling once a familial pathogenic variant is known (prenatal diagnosis, preimplantation genetic testing) — appropriate given the condition's near-universal de novo origin. No vaccination, screening program, or prophylactic medication applies. Genetic counseling is the principal "preventive" intervention offered to families, focused on recurrence-risk estimation (~1%, germline-mosaicism-based) for future pregnancies.

14. Other Species / Natural Disease

No naturally occurring veterinary or wildlife disease caused by spontaneous H3-3A/H3-3B variants has been reported (this is not currently listed in OMIA). H3.3 itself is highly evolutionarily conserved; the gene has functional orthologs across vertebrates, and a companion in vitro/model-organism paper — "Histone 3.3-related chromatinopathy: missense variants throughout H3-3A and H3-3B cause a range of functional consequences across species" (PMID 36867246) — used yeast as a heterologous functional-testing platform for patient-derived missense variants, and a separate paper examined "trapping of yFACT at 3' ends of genes" using yeast versions of BLBS histone H3 mutants (PMC11544422), underscoring cross-species conservation of the affected chaperone/FACT-complex interactions even though yeast do not model the neurodevelopmental phenotype itself.

15. Model Organisms

  • Zebrafish: Homozygous h3f3a-mutant (db1092 allele) zebrafish injected with dominant-negative H3f3a RNA show "complete loss of melanocytes and severe reductions of glia and xanthophores throughout cranial and trunk regions"; reduced nuclear H3.3 caused by aggregating dominant-mutant H3.3 produces defects in cranial neural crest cell differentiation, demonstrating tissue-specific sensitivity to H3.3 dosage during craniofacial/pigment-lineage development — directly relevant to the craniofacial dysmorphism seen in human BLBS.
  • Mouse: (1) Conditional H3f3a/H3f3b double-knockout embryos (H3f3a^fl/−; H3f3b^fl/−; Sox2-Cre) show H3.3 depletion with embryos recovering at expected Mendelian ratios only up to E10.5, indicating an essential, dosage-sensitive developmental requirement. During normal mouse neurodevelopment, H3.3 constitutes ~31% of total H3 pool early on, rising to become the dominant H3 species (>93%) in mature adult neurons — mechanistically explaining why postmitotic neurons are disproportionately vulnerable to H3.3 dysfunction and offering a rationale for the adult-onset neurodegenerative (camptocormia) component of BLBS. (2) The 2026 knock-in H3.3-T45I mouse (bioRxiv, June 2026) is the first genotype-matched in vivo preclinical model, recapitulating perinatal growth restriction, developmental-milestone delay, progressive motor/gait impairment, adult craniofacial differences, impaired nest-building, social-context hyperactivity, and male-specific aggression — directly paralleling multiple domains of the human phenotype and positioned as the platform for future therapeutic testing.
  • Human iPSC-derived models: 2D neural progenitor cells, 2D forebrain neurons, and 3D dorsal forebrain organoids carrying H3-3B p.Leu48Arg (Journal of Translational Medicine, 2026) recapitulate disrupted chromatin accessibility, dysregulated neuronal-fate/adhesion/neurotransmission gene expression, altered radial-glia-to-neuron proportions, and decreased spontaneous neuronal electrical activity by patch-clamp — the most disease-relevant human cellular model to date.
  • Yeast: used as a rapid, high-throughput heterologous system to functionally characterize the range of consequences of BLBS missense variants "across species," and to probe effects on the FACT histone chaperone complex (yFACT trapping at gene 3′ ends).
  • Model limitations: none of the current models (zebrafish, mouse, iPSC/organoid, yeast) yet captures the full adult neurodegenerative phenotype (camptocormia) in a validated, long-term aging cohort; the 2026 mouse model is the first designed explicitly to test this, but published long-term follow-up data were not available at the time of this search.

Summary of Key Ontology Term Suggestions

  • Gene/Disease: HGNC:4764 (H3-3A); MONDO:0030606 (BRYLIB1); OMIM:619720
  • HPO (selected): HP:0001263 (Global developmental delay), HP:0001249 (Intellectual disability), HP:0001252 (Hypotonia), HP:0001276 (Hypertonia), HP:0001250 (Seizure), HP:0002079 (Hypoplasia of corpus callosum), HP:0000252 (Microcephaly), HP:0000486 (Strabismus), HP:0001363 (Craniosynostosis), HP:0031808 (Camptocormia), HP:0002317 (Unsteady gait)
  • GO: GO:0000786 (nucleosome), GO:0006335 (DNA replication-independent nucleosome assembly), GO:0031507 (heterochromatin formation), GO:0006338 (chromatin remodeling)
  • CL: CL:0000047 (neural progenitor cell), CL:0002608 (radial glial cell), CL:0000540 (neuron), CL:0002500 (cranial neural crest cell)
  • UBERON: UBERON:0000955 (brain), UBERON:0002336 (corpus callosum), UBERON:0002037 (cerebellum)
  • NCIT (treatment): NCIT:C15986 (Pharmacotherapy), NCIT:C15302 (Physical Therapy), NCIT:C15447 (Dietary Intervention), NCIT:C16186 (Orthopedic Surgical Procedure), NCIT:C15747 (Supportive Care)

Sources

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome Count
References checked 13
Resolved 12
Unresolved (possible confabulation) 1
Unverifiable 0
Quoted claims checked 3
Quoted claims found in source 3
Quoted claims not found in source 0
References weighed for topical relevance 12
On topic 7
Off topic 0

Unresolved references

These identifiers did not resolve to a record and may be fabricated. A lookup that failed for transport reasons is indistinguishable from one that failed because the record does not exist, so spot-check before acting on them:

  • DOI:10.1126/sciadv.abc9207](https://www.science.org/doi/10.1126/sciadv.abc9207 (1 mention) - Identifier did not resolve to a record

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Outcome Count
Terms checked 60
Resolved 57
Unresolved (possible confabulation) 0
Obsolete 1
Unverifiable 2
Terms whose name was checked 41
Terms named correctly 34
Terms named as a different term 4
Terms whose name is worth a second look 3

Terms the report names something else

These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:

  • HP:0025265 (1 mention) - the report calls it "72% (in detailed-MRI subgroup)"; HP calls it Stiff toe
  • HP:0031808 (2 mentions) - the report calls it "Camptocormia"; HP calls it Decreased total basophil count
  • CL:0002608 (2 mentions) - the report calls it "radial glial cell"; CL calls it hippocampal neuron
  • CL:0002500 (2 mentions) - the report calls it "cranial neural crest cell"; CL calls it P enteroendocrine cell

Obsolete terms

These terms are real but deprecated. Citing one is not a fabrication; it does mean the report is naming something the ontology has retired:

  • GO:0016575 (obsolete histone deacetylation) (1 mention)

Terms whose name is worth a second look

The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:

  • HP:0002079 (2 mentions) - the report calls it "Hypoplasia of the corpus callosum", "Hypoplasia of corpus callosum"; HP calls it Hypoplasia of the corpus callosum, and lists "Hypoplasia of corpus callosum" among its other names
  • GO:0006335 (2 mentions) - the report calls it "DNA replication-independent nucleosome assembly"; GO calls it DNA replication-dependent chromatin assembly, and lists "DNA replication-dependent nucleosome assembly" among its other names
  • CL:0000047 (2 mentions) - the report calls it "neural progenitor cell"; CL calls it neural stem cell

Terms named inconsistently

The report gives these identifiers more than one name of its own:

  • MONDO:0030606 - called "Bryant-Li-Bhoj neurodevelopmental syndrome 1", "BRYLIB1"
  • HP:0002079 - called "Hypoplasia of the corpus callosum", "Hypoplasia of corpus callosum"

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: OMIM.