Age‑linked NAD⁺ depletion impairs mismatch repair through SIRT6 dysfunction, fostering epigenetic field cancerization in the colon

Hypothesis

Age‑related decline of nuclear NAD⁺ reduces SIRT6 activity, leading to hyperacetylation and instability of MSH2, which compromises mismatch repair (MMR) in histologically normal colonic epithelium. This MMR loss accelerates the accumulation of methylation‑driven silencing of tumor‑suppressor promoters (e.g., MGMT, SFRP2, WIF1) and promotes the expansion of epigenetically abnormal fields that precede colorectal cancer.

Mechanistic Model

1. NAD⁺ bioavailability falls with age due to increased CD38 activity and diminished salvage pathways. Low NAD⁺ limits the deacetylase function of SIRT6, a nuclear sirtuin that stabilizes MSH2 by removing acetyl groups from lysine residues.
2. Hyperacetylated MSH2 shows reduced affinity for DNA and increased proteasomal degradation, lowering overall MMR capacity.
3. Defective MMR fails to correct replication‑associated base mismatches, particularly at CpG sites, thereby increasing the probability of spontaneous deamination‑derived mutations and facilitating DNMT1‑mediated maintenance methylation.
4. The resulting rise in promoter methylation of MGMT, SFRP2, WIF1 creates patches of transcriptionally silenced tumor suppressors that behave as a field defect.
5. Because SIRT6 also deacetylates histones H3K9 and H3K56, its loss contributes to a more open chromatin state that predisposes to DNMT3A/B recruitment, amplifying the methylation wave across several centimeters of mucosa.

Testable Predictions

• Prediction 1: In colonic biopsies from age‑stratified donors without neoplastic lesions, nuclear NAD⁺ levels will inversely correlate with MSH2 acetylation and directly correlate with MMR activity (measured by microsatellite stability assay).
• Prediction 2: Pharmacological elevation of NAD⁺ (e.g., with nicotinamide riboside) in aged mice will decrease MSH2 acetylation, restore MMR efficiency, and reduce the incidence of MGMT/SFRP2/WIF1 promoter methylation in normal‑appearing colon.
• Prediction 3: Conditional knockout of SIRT6 in intestinal epithelial cells of young mice will recapitulate the age‑associated MMR decline and field‑like methylation patterns, whereas overexpression will protect aged mice from field expansion.

Experimental Design (brief)

1. Collect human colonic mucosal biopsies from donors aged 20‑40, 41‑60, 61‑80 (n=15 per group). Quantify NAD⁺ (LC‑MS/MS), SIRT6 activity (fluorometric deacetylase assay), MSH2 acetylation (immunoprecipitation‑Western), MMR function (fluorescence‑based mismatch repair assay), and methylation status of MGMT, SFRP2, WIF1 (bisulfite pyrosequencing).
2. Treat aged (18‑month) C57BL/6 mice with nicotinamide riboside (400 mg/kg/day) for 12 weeks. Compare to vehicle controls. Assess the same endpoints in colonic crypts isolated via laser capture microdissection.
3. Generate Villin‑CreERT2;Sirt6^fl/fl mice. Induce knockout at 2 months, analyze at 6 months for MMR metrics and promoter methylation. Parallel cohort with Villin‑CreERT2;Sirt6^OE (overexpression) aged to 18 months.

Potential Impact

If NAD⁺‑SIRT6‑MSH2 axis governs age‑related MMR loss in field cancerization, boosting NAD⁺ could serve as a chemopreventive strategy to halt epigenetic field expansion before malignant transformation. Moreover, NAD⁺ levels or SIRT6 activity might emerge as non‑invasive biomarkers for risk stratification in aging populations.

Supporting evidence links field cancerization to chromatin disorder and aberrant methylation in histologically normal mucosa [1], promoter hypermethylation of MGMT, SFRP2, WIF1 in >50 % of adjacent normal mucosa [2], and the prognostic value of epigenetic biomarkers [3] and [4].