Hypothesis

Aging promotes excessive H3K79 methylation by DOT1L, which stabilizes nucleosomes at plasticity gene loci and reduces transcriptional responsiveness, independent of histone acetylation levels.

Mechanistic Rationale

• DOT1L is recruited to chromatin via interaction with HDAC2/3 complexes that increase with age.
• H3K79me2/3 creates a binding platform for heterochromatin protein 1 (HP1) and Polycomb repressive complex 2 (PRC2), reinforcing a closed chromatin state.
• This methylation acts as a rheostat: when acetylation drops, DOT1L activity rises to compensate, locking genes in a repressive configuration.
• Experimental evidence: DOT1L inhibition in neurons increases H3K9ac and H4K12ac at Nr4a2 and c-fos promoters, enhancing dendritic spine density.

Testable Predictions

1. Epigenetic profiling – Aged mouse hippocampus will show elevated H3K79me2 at synaptic gene promoters compared to young controls (ChIP‑seq).
2. Pharmacological rescue – Treatment with the selective DOT1L inhibitor EPZ‑5676 will reduce H3K79me2, increase chromatin accessibility (ATAC‑seq), and restore Nr4a2/c‑fos transcription without altering nuclear acetyl-CoA levels.
3. Behavioral outcome – Aged mice receiving EPZ‑5676 will perform better in reversal learning and novel object recognition tasks, matching the cognitive rescue seen with ACC1 inhibitors.
4. Genetic validation – Conditional knockout of Dot1l in forebrain excitatory neurons will prevent age‑dependent decline in LTP and spine density, even in the presence of low acetyl-CoA.

Falsifiability

If DOT1L inhibition fails to reduce H3K79me2, does not increase transcription of plasticity genes, or does not improve cognitive performance despite confirmed target engagement, the hypothesis is falsified. Conversely, if acetyl-CoA restoration alone (e.g., via CMS121) normalizes H3K79me2 levels, then DOT1L activity may be downstream of acetyl-CoA, challenging the proposed independence.

Broader Impact

This reframes cognitive aging as a competition between acetyl‑CoA‑driven acetylation and DOT1L‑driven methylation, suggesting that combinatorial epigenetic therapies (acetyl‑CoA boosters + DOT1L inhibitors) could more effectively restore flexibility than either approach alone.