Hypothesis\nCircadian fluctuations in NAD+ levels directly regulate SIRT1 activity, which deacetylates both core clock components and DNA methyltransferase 1 (DNMT1). This keeps the MLH1 promoter unmethylated, preserves mismatch repair (MMR) fidelity in intestinal stem cells, and prevents field cancerization. When NAD+ rhythms are blunted, SIRT1 activity drops, DNMT1 remains acetylated and hyperactive, leading to aberrant MLH1 promoter methylation, MMR deficiency, and increased tumor risk.\n\n### Mechanistic Rationale\nThe circadian clock drives rhythmic expression of NAMPT, the rate‑limiting enzyme in the NAD+ salvage pathway [1]. NAD+ is an essential cofactor for the deacetylase SIRT1, which modulates the acetylation state of CLOCK:BMAL1 heterodimers, influencing their DNA‑binding affinity and transcriptional output [2]. SIRT1 also deacetylates DNMT1, reducing its enzymatic activity and preventing promoter hypermethylation of tumor‑suppressor genes such as MLH1 [3]. In the intestinal crypt, BMAL1 already restrains Hippo‑Yap signaling to keep stem‑cell proliferation in check [4]. We propose a second layer of protection: circadian NAD+ peaks reinforce SIRT1‑mediated DNMT1 deacetylation, keeping the MLH1 promoter unmethylated and MMR proficient. Loss of this rhythm (e.g., by chronic light‑at‑night or aging‑related NAD+ decline) tilts the balance toward DNMT1 hyperactivity, MLH1 silencing, and genomic instability.\n\n### Experimental Design\n1. Model – Use Axin2‑Lgr5‑CreERT2; Rosa26‑tdTomato mice to label intestinal stem cells. Induce circadian disruption via constant light (LL) for 8 weeks.\n2. Intervention – Provide timed NAD+ precursor (nicotinamide riboside, NR) either ad libitum or restricted to the subjective night (when NAD+ normally peaks).\n3. Readouts – \n - Measure hepatic and colonic NAD+ levels by LC‑MS at 4‑hour intervals.\n - Assess SIRT1 activity (deacetylation of p53) and DNMT1 acetylation status by immunoprecipitation‑Western.\n - Quantify MLH1 promoter methylation via bisulfite sequencing in sorted tdTomato+ stem cells.\n - Evaluate MMR function using a microsatellite instability assay.\n - Count aberrant crypt foci and tumors after azoxymethane (AOM) challenge.\n4. Controls – LL mice receiving vehicle, and normal light/dark (LD) mice with/without NR.\n\n### Predictions\n- LL will dampen colonic NAD+ amplitude, reduce SIRT1 activity, increase DNMT1 acetylation, and elevate MLH1 promoter methylation in stem cells, leading to MMR deficiency and higher tumor burden.\n- Night‑restricted NR will restore NAD+ peaks, rescue SIRT1‑mediated DNMT1 deacetylation, keep MLH1 unmethylated, preserve MMR proficiency, and suppress tumorigenesis despite LL.\n- Ad libitum NR will partially improve outcomes but be less effective because mistimed NAD+ fails to synchronize with the clock.\n\n### Falsifiability\nIf night‑restricted NR fails to normalize DNMT1 acetylation or MLH1 methylation, or if tumor incidence remains unchanged, the hypothesis that circadian NAD+‑SIRT1 gates epigenetic fidelity of MMR is refuted. Conversely, successful rescue would support the idea that the circadian firewall operates, at least in part, through a metabolic‑epigenetic checkpoint that can be pharmacologically reinforced.\n\n### References\n[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC3934468/\n[2] https://doi.org/10.1038/s41422-020-0385-7\n[3] https://pubmed.ncbi.nlm.nih.gov/23102497/\n[4] https://pmc.ncbi.nlm.nih.gov/articles/PMC12839077/