Hypothesis: Age‑related decline of vagal efferent signaling reduces cholinergic anti‑inflammatory tone in the colon, leading to suppressed expression of MutSα (MLH1/MSH2) in colonic stem cells and consequent mismatch repair deficiency. This neural deficit is amplified by an aging microbiome that produces metabolites (e.g., secondary bile acids, trimethylamine‑N‑oxide) which further blunt vagal activity, creating a feed‑forward loop that accelerates field cancerization, particularly in the right colon where vagal density is highest.

Mechanistic rationale: Vagal acetylcholine acts on α7 nicotinic acetylcholine receptors (α7nAChR) expressed on colonic stem and progenitor cells, activating the JAK2‑STAT3 pathway that directly up‑regulates MLH1 and MSH2 transcription (see [4] for senescence‑linked MutSα dysfunction). Loss of vagal input diminishes STAT3 phosphorylation, decreasing MutSα assembly and increasing microsatellite instability. Simultaneously, age‑associated dysbiosis elevates circulating methylmalonic acid and deoxycholic acid, which inhibit vagal firing via enterochromaffin cell serotonin release (see [6]), thereby weakening the cholinergic brake on NF‑κB–driven inflammation. NF‑κB signaling represses MLH1/MSH2 promoters, compounding the repair defect.

Testable predictions:

1. In aged mice (≥20 months), vagotomy will cause a significant drop in colonic MLH1/MSH2 protein levels and an increase in microsatellite instability markers compared with sham‑operated controls.
2. Chronic optogenetic or pharmacological vagal stimulation (e.g., via clozapine‑N‑oxide‑activated DREADDs in cholinergic neurons) in aged mice will restore MutSα expression, reduce γH2AX foci in stem cells, and lower tumor multiplicity in an azoxymethane/DSS model.
3. Germ‑free aged mice will exhibit preserved vagal tone (measured by heart‑rate variability) and higher colonic MLH1/MSH2 despite chronological age, whereas colonization with a microbiota enriched in bile‑acid‑degrading strains will replicate the vagal suppression and MMR loss seen in conventionally raised aged mice.
4. Single‑cell RNA‑sequencing of colonic crypts from aged human donors will reveal an inverse correlation between vagal marker (CHAT, VAChT) expression in the myenteric plexus and MutSα gene signatures in LGR5+ stem cells, with this relationship strengthened in subjects exhibiting high fecal secondary bile‑acid concentrations.

Falsification: If vagal stimulation fails to rescue MutSα levels or if germ‑free status does not protect against age‑related MMR decline, the hypothesis would be refuted, indicating that neural regulation of DNA repair is either absent or overridden by cell‑intrinsic senescence mechanisms.

This framework directly links the understudied gut‑brain axis to the observed anatomical and age‑dependent patterns of MMR deficiency in colorectal cancer, offering a concrete, experimentally tractable pathway to explain why microbial aging may outpace cellular aging in driving tumorigenesis.