Histone Modifications

Overview

Histones are the protein scaffolds around which DNA is wound. Post-translational covalent modifications of histone N-terminal tails regulate chromatin state and gene activity. This is the best-characterized chromatin regulatory mechanism in the brain.

Key Modification Types

Modification	Residues Affected	General Effect
Acetylation	Lysine (K)	Activating — opens chromatin
Methylation	Lysine or arginine	Context-dependent (see below)
Phosphorylation	Serine or threonine	Activating or inhibiting
Ubiquitination / SUMOylation	Lysine	Variable

Acetylation

Neutralizes the positive charge of lysine residues
Increases spacing between nucleosomes → opens chromatin → promotes transcription
Writers: Histone acetyltransferases (HATs)
Erasers: Histone deacetylases (HDACs)

Methylation

Can either activate or repress transcription depending on residue and methylation state (mono-, di-, or tri-methyl)
Key marks:
- H3K4me3 — activating; marks active promoters
- H3K9me2 — repressive; silences gene loci in NAc, implicated in depression and antidepressant action
- H3K9me3 — repressive; induced by stress at repetitive elements in hippocampus
- H3K27me3 — repressive; increased at Rac1 in depression, suppresses BDNF in hippocampus
- H3K4me1/H3K4me2 — enhancer marks
Writers: Histone methyltransferases (HMTs), e.g., G9a, GLP, SETDB1, KMT1D, KMT2F
Erasers: Histone demethylases (HDMs)
Readers: Chromatin remodeling proteins that recognize specific marks

The Histone Code Hypothesis

Jenuwein & Allis (2001) proposed that the sum of histone modifications at a gene defines a specific epigenetic state. Evidence supports this concept but the code is highly complex and has not been fully deciphered. Notably, no single modification examined is deterministic — the same mark can have no effect or even opposite effects at different genes. (Hypothesis; not yet resolved.)

Writers, Erasers, and Readers

Writers: Add modifications (HATs, HMTs, kinases)
Erasers: Remove modifications (HDACs, HDMs, phosphatases)
Readers: Proteins that bind specific modified residues and recruit further regulatory complexes; include chromatin remodeling proteins that use ATP to alter nucleosome spacing

Relevance to Psychiatric Disorders

Depression

HDAC inhibition → antidepressant effects in NAc, hippocampus, amygdala, PFC (animal models)
G9a/GLP (H3K9me2 writers) downregulated in NAc by chronic social defeat; loss of H3K9me2 is maladaptive
Fluoxetine restores H3K9me2 at Camkiia and Ras promoters in NAc
H3K27me3 increases at Rac1 upstream region; BDNF in hippocampus
Repressive chromatin remodeling complex induced in NAc by defeat — also found in depressed human NAc postmortem

Schizophrenia

H3K4me3 reduced at GAD1 in PFC
H3K9K14 acetylation altered in PFC, correlating with expression changes in GAD1, HTR2C, PPM1E
Several HMTs upregulated in SCZ postmortem brain
H3K4me3 elevated at synapsin genes in PFC of depressed humans

Bipolar Disorder

H3K4me3 changes at synapsin genes differ between bipolar disorder and MDD
H3K4 methylation regulator genes are among strongest GWAS hits for bipolar disorder risk

Key Uncertainty

Most human studies rely on postmortem tissue — cell type heterogeneity, postmortem interval, and medication effects confound results. Nearly all histone modification data comes from bulk tissue, not cell-type-resolved assays.