Histones and Epigenetic Modifications
The proteins that pack DNA and decide which genes are active
Definition
Histones are basic proteins (positively charged) around which DNA wraps to be packaged in the nucleus: 147 base pairs of DNA wrap around an octamer of histones (H2A, H2B, H3, H4 — two copies of each) forming the nucleosome, the basic unit of chromatin. Reversible chemical modifications on histone tails (acetylation, methylation, phosphorylation, ubiquitination) constitute the 'histone code' that determines which genes are active or silenced at any given moment — the second main level of epigenetic regulation alongside DNA methylation.
Detailed explanation
Key histone modifications and their effects:
Acetylation (HAT vs HDAC): adding an acetyl group to lysines neutralises the positive charge, relaxes chromatin packaging, and allows transcription. Histone acetyltransferases (HAT) are 'activators'; histone deacetylases (HDAC, including sirtuins) are 'repressors'. β-hydroxybutyrate (ketosis) inhibits class I HDACs; resveratrol activates SIRT1.
Histone methylation (HMT vs HDM): variable effect depending on position. H3K4me3 = transcriptional activation; H3K9me3 = silencing (heterochromatin); H3K27me3 = developmental silencing (Polycomb).
With ageing, the 'histone landscape' becomes disorganised: progressive loss of active marks, aberrant expansion of heterochromatin towards genes that should be active, and vice versa. David Sinclair calls this 'epigenetic information loss' and proposes it is the primary driver of ageing.
Strategies to preserve/restore the epigenetic landscape: sirtuin activation (NAD+ IV, resveratrol, exercise, caloric restriction), controlled HDAC inhibition (ketosis), partial reprogramming with Yamanaka factors (OSK — Oct4, Sox2, Klf2 — without c-Myc, in pre-clinical phase), and maintaining high NAD+ levels that ensure sirtuin function.
Scientific sources
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