H3F3B Gene — H3.3 Histone B

Overview

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    GADD45A["GADD45A"] -->|"associated with"| H3F3B["H3F3B"]
    JUN["JUN"] -->|"associated with"| H3F3B["H3F3B"]
    HSPA8["HSPA8"] -->|"associated with"| H3F3B["H3F3B"]
    CEBPB["CEBPB"] -->|"associated with"| H3F3B["H3F3B"]
    HSPA5["HSPA5"] -->|"associated with"| H3F3B["H3F3B"]
    DDIT3["DDIT3"] -->|"associated with"| H3F3B["H3F3B"]
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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.^1Citation 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.^2Citation The dual role of H3F3B in both brain development and brain tumors makes it a unique gene of interest in neuroscience[“^3”][^4].

Molecular Biology

Gene Structure

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].

Protein Structure

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

Expression Patterns

H3F3B is expressed in most tissues, with particularly high expression in:

In the brain, H3F3B expression is:

• Highest in cerebral cortex, hippocampus, and cerebellum

• Dynamically regulated during development

• Activity-dependent in mature neurons

Function

Chromatin Deposition and Regulation

H3.3B is incorporated into chromatin via distinct chaperone complexes:

1. 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]

2. 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.

Role in Neuronal Function

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

DNA Repair

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

Disease Associations

Bryant-Li-Bhoj Syndrome

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:

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

Rett Syndrome

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

Neurodevelopmental Disorders

H3F3B variants are associated with other neurodevelopmental conditions:

• Autism spectrum disorder

• Intellectual disability without syndrome diagnosis

• Epileptic encephalopathies

Mechanisms of Pathogenesis

Epigenetic Dysregulation

The primary mechanism by which H3F3B mutations cause disease involves epigenetic dysregulation:

1. Loss-of-Function Mutations: Reduce H3.3 availability for chromatin remodeling

2. Dominant-Negative Mutations (K27M): Hijack epigenetic machinery

3. Altered Post-Translational Modifications: Affect histone mark distribution

Transcriptional Dysregulation

H3F3B mutations lead to:

• Aberrant gene expression programs

• Failure to maintain neuronal transcriptional identity

• Dysregulation of synaptic plasticity genes

• Impaired activity-dependent responses

Cell Cycle Dysregulation

In tumors, H3.3 mutations affect:

• Cell cycle checkpoint control

• Stem cell maintenance

• Differentiation blockade

• Proliferation signaling

Interaction Network

H3F3B interacts with multiple protein complexes:

Research Models

Cell Lines

• HEK293T: For studying H3.3 deposition mechanisms

• Neural progenitor cells: Differentiation studies

• Glioma cell lines: K27M mutation effects

• iPSC-derived neurons: Disease modeling

Animal Models

• Zebrafish: Knockout models show brain development defects

• Mouse: Conditional knockouts reveal neuronal function requirements

• Xenopus: Developmental studies

Therapeutic Approaches

1. Epigenetic Drugs:

   • HDAC inhibitors

   • EZH2 inhibitors

   • DNA methyltransferase inhibitors

2. Immunotherapy:

   • H3K27M peptide vaccines

   • CAR-T cells targeting mutant H3

3. Gene Therapy:

   • Wild-type H3F3B delivery

   • Allele-specific approaches

Key Publications

1. 40987316: H3.1 vs H3.3 in cancer. Trends Cancer, 2025.

2. 40696776: H3F3B variant and paroxysmal dyskinesia. Neurology, 2026.

3. 39627236: Bryant-Li-Bhoj syndrome with H3F3A variant. Neurology, 2024.

4. 38678163: Expanded Bryant-Li-Bhoj phenotype. Am J Hum Genet, 2024.

5. 38238293: Gatad2b and neurodevelopment. Nat Commun, 2024.

6. 37814296: H3.3 in brain development. Dev Cell, 2023.

7. 37170146: H3K27M glioma therapy. Nat Med, 2023.

8. 36656301: ATRX-DAXX and H3.3. Nat Rev Cancer, 2022.

9. 35970867: H3.3 in neurodevelopment. Neuron, 2022.

10. 34771425: H3F3B mutations in neurodevelopment. Am J Hum Genet, 2021.

See Also

• H3F3A Gene

• H3.3 Protein

• Bryant-Li-Bhoj Syndrome

• Glioma

• Diffuse Intrinsic Pontine Glioma

• Epigenetics in Neurodegeneration

• Chromatin Remodeling

• Histone Modifications

• Rett Syndrome