Hypothesis

The core circadian repressor REV-ERBα drives a daily wave of HDAC3 recruitment to the MLH1 promoter, promoting histone deacetylation and sustaining mismatch repair (MMR) capacity in colonic epithelial cells. With age, dampened REV-ERBα rhythm reduces HDAC3 occupancy, leading to promoter hyperacetylation, a repressive chromatin switch, and epigenetic silencing of MLH1. This mechanistic link explains how circadian erosion precipitates MMR decline, microsatellite instability, and early‑onset colorectal cancer (CRC).

Mechanistic Rationale

• REV-ERBα binds heme and recruits the NCOR1/HDAC3 corepressor complex in a ligand‑dependent manner.
• HDAC3 activity is NAD+‑sensitive, coupling cellular metabolic state to chromatin state.
• Circadian oscillation of NAD+ levels (driven by NAMPT) thus gates HDAC3 enzymatic output.
• When REV-ERBα expression peaks (subjective night), HDAC3 deacetylates H3K27ac at the MLH1 promoter, maintaining an open, transcription‑permissive state.
• Loss of this rhythm (e.g., via shift‑like lighting or Bmal1 KO) yields sustained acetylation, recruitment of DNMT3A, and promoter methylation that silences MLH1.

Testable Predictions

1. Chromatin immunoprecipitation‑sequencing (ChIP‑seq) for REV-ERBα and HDAC3 in mouse colonic crypts will show anti‑phase peaks at the MLH1 promoter that coincide with low H3K27ac and high MLH1 mRNA.
2. Pharmacological inhibition of HDAC3 (e.g., with RGFP966) or genetic knockdown of Rev-erbα will increase MLH1 promoter acetylation, reduce MLH1 transcription, and lower MMR activity measured by a fluorescent reporter assay.
3. Restoring NAD+ levels with NR supplementation in aged mice will rescue HDAC3 activity, decrease MLH1 promoter methylation, and improve MMR efficiency despite low Rev-erbα expression.
4. Human colon biopsies from early‑onset CRC patients with reported circadian disruption will display elevated MLH1 promoter acetylation and methylation relative to age‑matched controls.

Potential Challenges and Controls

• Off‑target effects of HDAC3 inhibitors: use catalytic‑dead HDAC3 rescue experiments.
• Circadian confounding: perform experiments under constant darkness to isolate cell‑autonomous rhythms.
• Microbiome influence: include germ‑free or antibiotic‑treated cohorts to verify that the effect is intrinsic to epithelial clock.

By positioning the circadian repressor REV-ERBα as a rhythmic gatekeeper of HDAC3‑mediated promoter epigenetics, this hypothesis directly connects the erosion of the circadian firewall to the age‑dependent loss of mismatch repair, offering a clear, falsifiable path forward.