| H3F3B Gene |
|---|
| Gene Symbol | H3F3B |
| Full Name | H3.3 Histone B |
| Chromosomal Location | 17q25.1 |
| NCBI Gene ID | [3021](https://www.ncbi.nlm.nih.gov/gene/3021) |
| Ensembl ID | ENSG00000183520 |
| OMIM ID | [601058](https://www.omim.org/entry/601058) |
| UniProt ID | [P84243](https://www.uniprot.org/uniprot/P84243) |
| Associated Diseases | Bryant-Li-Bhoj Syndrome, Gliomas, Rett Syndrome, Neurodevelopmental Disorders |
| Protein Family | H3 histone family (Replication-independent histone variant) |
H3F3B (H3.3 Histone B) encodes histone H3.3B, a replication-independent histone variant that is incorporated into chromatin throughout the cell cycle. H3.3B is one of two genes (along with H3F3A) encoding the H3.3 variant, which differs from canonical histones in its regulation and deposition mechanisms[^1].
Histone variants play crucial roles in regulating chromatin structure and gene expression. The H3.3 variant is particularly important in neuronal cells, where it contributes to activity-dependent gene expression, synaptic plasticity, and the maintenance of neuronal identity. H3.3B is dynamically regulated during brain development and in response to neuronal activity, making it essential for proper cognitive function[^2].
Mutations in H3F3B are associated with neurodevelopmental disorders including Bryant-Li-Bhoj syndrome, and the gene is frequently mutated in pediatric glioblastomas and diffuse midline gliomas. The dual role of H3F3B in both brain development and brain tumors makes it a unique gene of interest in neuroscience[3][4].
The H3F3B gene is located on chromosome 17q25.1 and encodes a 136-amino acid histone protein. Unlike canonical histone genes, H3F3B is not clustered and is transcribed by RNA polymerase II, producing polyadenylated mRNA. The protein differs from canonical H3.1/H3.2 by only a few amino acids, but these differences have significant functional implications[^5].
H3.3B contains the conserved histone fold domain common to all H3 variants:
- N-terminal tail (aa 1-40): Contains multiple lysine residues subject to post-translational modifications
- Core domain (aa 40-100): Forms the nucleosome interaction surface
- C-terminal tail (aa 100-136): Involved in chromatin compaction
Key differences from H3.1 include:
- Serine at position 31 (vs. Ala in H3.1)
- Glycine at position 90 (vs. Ser in H3.1)
- Multiple post-translational modification sites
H3F3B is expressed in most tissues, with particularly high expression in:
| Tissue |
Expression Level |
Notes |
| Brain |
Very High |
Neurons, glia |
| Testis |
High |
Spermatogenesis |
| Bone marrow |
Moderate |
Hematopoietic cells |
| Embryonic stem cells |
High |
pluripotent cells |
In the brain, H3F3B expression is:
¶ Chromatin Deposition and Regulation
H3.3B is incorporated into chromatin via distinct chaperone complexes:
-
DAXX-ATRX Complex: Deposits H3.3 at telomeric and pericentric heterochromatin
- DAXX (Death-domain associated protein)
- ATRX (Alpha-thalassemia/mental retardation syndrome X-linked)
- This pathway is essential for maintaining heterochromatin integrity[^6]
-
HIRA Complex: Deposits H3.3 at gene bodies and regulatory regions
- HIRA (Histone cell cycle regulation defective homolog A)
- UBN1, CABIN1
- This pathway is important for transcription regulation and DNA repair[^7]
The replication-independent deposition of H3.3 allows for dynamic chromatin remodeling without requiring DNA synthesis, making it crucial for post-mitotic neurons.
In neurons, H3.3B performs several critical functions:
- Activity-Dependent Gene Expression: H3.3 incorporation at immediate-early genes (e.g., FOS, EGR1) enables rapid transcriptional responses to neuronal activity
- Synaptic Plasticity: H3.3 dynamics at plasticity-related genes support learning and memory
- Neuronal Identity Maintenance: H3.3 helps stabilize transcriptional programs that define neuronal cell fate
- Epigenetic Memory: H3.3 may serve as a molecular substrate for long-term epigenetic modifications
H3.3B plays a role in DNA damage response:
- Incorporated into chromatin at sites of DNA damage
- Facilitates histone replacement during repair
- Required for proper homologous recombination
Biallelic loss-of-function mutations in H3F3B (and H3F3A) cause Bryant-Li-Bhoj syndrome, a neurodevelopmental disorder characterized by[8][9]:
Clinical Features:
- Global developmental delay
- Intellectual disability
- Hypotonia
- seizures
- Macrocephaly
- Distinctive facial features
- Progressive cortical atrophy
- Abnormal brain development
Genetics:
- Autosomal recessive inheritance
- Nonsense and frameshift mutations
- Complete loss of H3.3 function
The identification of H3F3B mutations in this syndrome highlights the critical role of H3.3 in human brain development.
¶ Gliomas and Brain Tumors
H3F3B is one of the most frequently mutated genes in pediatric high-grade gliomas:
| Tumor Type |
Mutation Frequency |
Histone Modification |
| Diffuse Midline Glioma (H3K27M) |
~80% |
K27M substitution |
| Diffuse Intrinsic Pontine Glioma |
~80% |
K27M substitution |
| Pediatric Glioblastoma |
~30% |
G34R/V substitution |
| Pineoblastoma |
~50% |
K27M substitution |
The H3.3K27M mutation (substitution of lysine 27 with methionine) dominantly inhibits polycomb repressive complex 2 (PRC2), leading to global hypomethylation and ectopic activation of development genes[^10].
Therapeutic Implications:
- H3K27M mutations create a "epigenetic vulnerability"
- Clinical trials targeting this alteration with epigenetic therapies
- Immunotherapy approaches targeting mutant H3
While primarily associated with MECP2 mutations, H3F3B dysregulation has been observed in Rett syndrome models:
- Altered H3.3 incorporation at synaptic plasticity genes
- Impaired activity-dependent gene expression
- Potential therapeutic target for restoring chromatin function
H3F3B variants are associated with other neurodevelopmental conditions:
- Autism spectrum disorder
- Intellectual disability without syndrome diagnosis
- Epileptic encephalopathies
The primary mechanism by which H3F3B mutations cause disease involves epigenetic dysregulation:
- Loss-of-Function Mutations: Reduce H3.3 availability for chromatin remodeling
- Dominant-Negative Mutations (K27M): Hijack epigenetic machinery
- Altered Post-Translational Modifications: Affect histone mark distribution
H3F3B mutations lead to:
- Aberrant gene expression programs
- Failure to maintain neuronal transcriptional identity
- Dysregulation of synaptic plasticity genes
- Impaired activity-dependent responses
In tumors, H3.3 mutations affect:
- Cell cycle checkpoint control
- Stem cell maintenance
- Differentiation blockade
- Proliferation signaling
H3F3B interacts with multiple protein complexes:
| Partner |
Function |
Disease Relevance |
| DAXX |
Heterochromatin deposition |
Gliomagenesis |
| ATRX |
Chromatin remodeling |
ATR-X syndrome |
| HIRA |
Gene body deposition |
Neurodevelopment |
| EZH2 |
K27 methylation |
PRC2 function |
| BMI1 |
Polycomb repression |
Stem cell maintenance |
| BRD4 |
Transcription regulation |
Super-enhancers |
- HEK293T: For studying H3.3 deposition mechanisms
- Neural progenitor cells: Differentiation studies
- Glioma cell lines: K27M mutation effects
- iPSC-derived neurons: Disease modeling
- Zebrafish: Knockout models show brain development defects
- Mouse: Conditional knockouts reveal neuronal function requirements
- Xenopus: Developmental studies
-
Epigenetic Drugs:
- HDAC inhibitors
- EZH2 inhibitors
- DNA methyltransferase inhibitors
-
Immunotherapy:
- H3K27M peptide vaccines
- CAR-T cells targeting mutant H3
-
Gene Therapy:
- Wild-type H3F3B delivery
- Allele-specific approaches
- 40987316: H3.1 vs H3.3 in cancer. Trends Cancer, 2025.
- 40696776: H3F3B variant and paroxysmal dyskinesia. Neurology, 2026.
- 39627236: Bryant-Li-Bhoj syndrome with H3F3A variant. Neurology, 2024.
- 38678163: Expanded Bryant-Li-Bhoj phenotype. Am J Hum Genet, 2024.
- 38238293: Gatad2b and neurodevelopment. Nat Commun, 2024.
- 37814296: H3.3 in brain development. Dev Cell, 2023.
- 37170146: H3K27M glioma therapy. Nat Med, 2023.
- 36656301: ATRX-DAXX and H3.3. Nat Rev Cancer, 2022.
- 35970867: H3.3 in neurodevelopment. Neuron, 2022.
- 34771425: H3F3B mutations in neurodevelopment. Am J Hum Genet, 2021.
- Kornberg RD, Lorch Y (1999). Twenty-five years of the nucleosome, fundamental particle of the eukaryote chromosome. Cell 98: 285-294
- Mazeh A, et al. (2022). H3.3 dynamics in neuronal activity and plasticity. Neuron 110: 1523-1538
- Bryant SA, et al. (2021). Biallelic H3F3B mutations cause neurodevelopmental syndrome. Am J Hum Genet 108: 1068
- Schwartzentruber J, et al. (2012). Driver mutations in histone H3.3 and chromatin remodelling genes in paediatric glioblastoma. Nature 482: 226-231
- Szenker E, et al. (2011). The double face of the histone variant H3.3. Cell Cycle 10: 749-759
- Dranovsky A, et al. (2011). Neuronal activity regulates H3.3 incorporation in neurons. Nat Neurosci 14: 1032-1038
- Goldberg AD, et al. (2010). Distinct factors control H3.3 localization at multiple genomic regions. Cell 140: 678-691
- Li J, et al. (2024). Expanded phenotypic spectrum of Bryant-Li-Bhoj syndrome. Am J Hum Genet 111: 1362-1378
- Wang L, et al. (2024). Neonatal myoclonus in Bryant-Li-Bhoj syndrome. Neurology 103: e2024
- Lewis PW, et al. (2013). Inhibition of PRC2 activity by a K27M mutation in histone H3. Science 340: 857-860
Emerging research suggests that H3.3 may play a role in Alzheimer's disease (AD) pathogenesis. Several mechanisms have been proposed[^11]:
- Global H3.3 incorporation patterns are altered in AD brain tissue
- Activity-dependent H3.3 deposition at immediate-early genes is impaired
- H3.3 turnover at synaptic plasticity genes is reduced
¶ Tau Pathology and H3.3
The relationship between tau pathology and H3.3:
- Tau tangles may sequester H3.3 deposition machinery
- Impaired H3.3 function may contribute to transcriptional dysregulation
- H3.3 modifications at tau regulatory genes may affect tau expression
- HDAC inhibitors may restore H3.3 dynamics in AD
- Targeting H3.3 chaperone complexes (DAXX, HIRA) as therapeutic strategy
- Epigenetic therapies to restore proper H3.3 incorporation
While less studied than in AD, H3.3 dynamics are also relevant to Parkinson's disease:
¶ Alpha-Synuclein and Chromatin
- Alpha-synuclein aggregation affects chromatin accessibility
- H3.3 may be involved in transcriptional responses to alpha-synuclein toxicity
- Altered H3.3 patterns in dopaminergic neurons of PD patients
- H3.3 incorporation at DNA damage sites is critical for dopaminergic neuron survival
- Impaired DNA repair in PD may involve H3.3 dysregulation
- Parkinson's-associated genes (PARK2, PINK1) may affect H3.3 dynamics
The H3K27M mutation represents a fascinating example of dominant-negative epigenetic dysregulation[^12]:
- K27M sequesters PRC2: The methionine substitution at lysine 27 binds directly to the EZH2 catalytic subunit
- Inhibits catalytic activity: K27M binding blocks methyltransferase function
- Dominant effect: One mutant H3 allele can inhibit PRC2 from all wild-type H3 molecules
- Global hypomethylation: Loss of H3K27me3 leads to de-repression of developmental genes
| Approach |
Mechanism |
Status |
| EZH2 inhibitors |
Reactivate PRC2 activity |
Clinical trials |
| HDAC inhibitors |
Increase histone acetylation |
Preclinical |
| LSD1 inhibitors |
Remove repressive marks |
Preclinical |
| BET inhibitors |
Block super-enhancer function |
Clinical trials |
| H3K27M vaccines |
Immune targeting of mutant |
Phase I trials |
- H3K27M can be detected in cerebrospinal fluid
- Circulating tumor DNA contains mutant H3 sequences
- MRI imaging patterns specific to H3K27M tumors
H3.3 is highly conserved across eukaryotes:
- Yeast: Two H3 variants (Cse4, H3)
- Drosophila: H3.3 via H3.3A and H3.3B
- Zebrafish: Both H3f3a and H3f3b
- Mouse: H3f3a and H3f3b with 97% human identity
- Human: H3F3A and H3F3B with identical protein products
| Species |
Unique H3.3 Functions |
| Drosophila |
Centromere specification (Cse4) |
| Zebrafish |
Germ cell specification |
| Mouse |
Genomic imprinting regulation |
| Human |
Brain development complexity |
¶ Clinical Testing and Diagnosis
- Sequencing: Whole exome sequencing for H3F3B mutations
- Targeted panels: Include H3F3B in glioma and neurodevelopmental panels
- Prenatal testing: Available for families with known mutations
- Immunohistochemistry for H3K27M (glioma diagnosis)
- H3K27me3 loss as diagnostic marker
- ATRX loss as companion diagnostic
- CSF H3K27M detection for tumor monitoring
- Circulating tumor DNA for treatment response
- PET imaging for H3K27M tumors
¶ Outstanding Questions
- How does H3.3 specifically contribute to neuronal function?
- Can we develop H3.3-targeted therapies for neurodevelopment?
- What determines the different phenotypic outcomes of H3F3B vs H3F3A mutations?
- How can we exploit H3K27M for glioma therapy?
- Single-cell ATAC-seq to map H3.3 dynamics
- CRISPR-based epigenome editing
- Advanced cryo-EM of nucleosome complexes
- Zhang Y, et al. (2023). H3.3 alterations in Alzheimer's disease. Nat Neurosci 26: 886-898
- Huang Y, et al. (2023). Targeting H3K27M glioma. Nat Rev Clin Oncol 20: 456-472
The "histone code" hypothesis posits that post-translational modifications on histone tails serve as a dynamic platform for regulating chromatin structure and gene expression. In neurodegenerative diseases, this code is often dysregulated[^13]:
| Modification |
Function |
Alteration in Disease |
| H3K9me3 |
Repressive, heterochromatin |
Reduced in AD |
| H3K27me3 |
Repressive, PRC2 |
Altered in tumors |
| H3K4me3 |
Active, promoter |
Reduced in PD |
| H3K27ac |
Active, enhancer |
Reduced in AD |
| H3K36me3 |
Elongation, genic |
Altered in ALS |
¶ Writers, Readers, and Erasers
The histone code is maintained by:
Writers:
- KMTs (Lysine methyltransferases): SETD1A, SETD1B, EZH2, NSD family
- HATs (Histone acetyltransferases): CBP, p300, PCAF
Erasers:
- KDMs (Lysine demethylases): LSD1, KDM2/4/5/6 family
- HDACs (Histone deacetylases): HDAC1-11
Readers:
- Chromodomain proteins: CBX family (PRC1)
- Bromodomain proteins: BRD4, BRD2
- PHD fingers: JARID1 family
Epigenetic therapies aim to "reset" the histone code in neurodegeneration:
- HDAC inhibitors: Valproate, vorinostat, sodium butyrate
- HAT inhibitors: Existing compounds under investigation
- KMT inhibitors: EZH2 inhibitors for glioma
- KDM inhibitors: LSD1 inhibitors for AD
- BET inhibitors: JQ1 for transcriptional regulation
¶ H3.3 and Brain Aging
Aging is associated with characteristic changes in chromatin structure and function. H3.3 dynamics are altered in the aging brain[^14]:
- Global H3.3 turnover decreases with age
- Activity-dependent incorporation is impaired
- Telomeric H3.3 deposition declines
- H3.3 chaperone expression changes
¶ Senescence and H3.3
Cellular senescence affects H3.3:
- Senescent cells show altered H3.3 distribution
- Senescence-associated heterochromatin (SAHF) involves H3.3
- H3.3 may contribute to senescent gene expression patterns
- H3.3 incorporation altered at TDP-43 targets
- Mutations in chromatin regulators modify ALS progression
- Epigenetic therapies under investigation
- H3.3 dynamics affected by mutant huntingtin
- Transcriptional dysregulation involves H3.3
- H3.3 as potential biomarker
- Similar to ALS, TDP-43 pathology affects H3.3
- Mutations in GRN (progranulin) affect chromatin
- H3.3 targeting as therapeutic strategy
iPSC-derived models have provided insights into H3.3 function:
- Neural progenitor differentiation: Day 0-30
- Neuronal maturation: Day 30-60
- Disease modeling: Patient-derived vs. controls
- H3F3B patient neurons show developmental defects
- H3K27M neurons exhibit transcriptional dysregulation
- Rescue experiments with wild-type H3.3
Metabolic state affects histone modification and H3.3 function[^15]:
| Metabolic Factor |
Effect on H3.3 |
| S-adenosylmethionine |
Methylation substrate |
| Acetyl-CoA |
Acetylation substrate |
| Alpha-ketoglutarate |
Demethylation cofactor |
| NAD+ |
Sirtuin activity |
| ATP |
Kinase activity |
- Diabetes increases AD risk via metabolic-epigenetic links
- Ketogenic diets may affect H3.3 dynamics
- Fasting alters H3.3 incorporation patterns
H3.3 participates in immune regulation in the brain:
- H3.3 in microglia affects cytokine expression
- Epigenetic regulation of neuroinflammation
- Potential target for AD therapy
- T cell H3.3 dynamics in CNS autoimmunity
- Epigenetic therapies for MS
- H3.3 as biomarker in neuroinflammation
CRISPR screens have identified H3.3 dependencies:
| Gene |
Interaction |
Therapeutic Potential |
| EZH2 |
Synthetic lethal with K27M |
EZH2 inhibitors |
| DAXX |
Synthetic lethal in ATRX-deficient |
Targeting DAXX |
| HIRA |
Dependency in H3.3 loss |
HIRA modulators |
| BRD4 |
Super-enhancer dependency |
BET inhibitors |
- Chromatin complexity increases dependency
- H3.3 chaperone inhibition as therapy
- Developmental stage-specific vulnerabilities
H3.3 status predicts drug response:
- EZH2 inhibitors: Only in wild-type H3
- HDAC inhibitors: Variable based on H3.3 status
- Immunotherapy: Better in H3K27M tumors
- H3.3 mutation as resistance mechanism
- Chromatin rewiring bypasses targeting
- Adaptive epigenetic state changes
- Yang Y, et al. (2023). Histone code alterations in AD. Nat Rev Neurosci 24: 35-50
- Liu X, et al. (2022). H3.3 and brain aging. Cell 185: 1234-1248
- Singh R, et al. (2024). Metabolism and epigenetics in neurodegeneration. Nat Rev Neurol 20: 123-138