SIRT6‑dependent deacetylation of TFAM limits mtDNA mutagenesis independently of NAD+‑driven mitochondrial biogenesis

Hypothesis

SIRT6 directly deacetylates the mitochondrial transcription factor TFAM, reducing its affinity for mtDNA and thereby lowering the rate of mtDNA point mutations and deletions. This activity protects the mitochondrial genome even when NAD+ levels are low, meaning that NMN supplementation extends lifespan primarily by preserving mtDNA integrity rather than by boosting mitochondrial biogenesis.

Mechanistic Reasoning

• SIRT6 is a nuclear NAD+‑dependent deacetylase that stabilizes chromatin and promotes DNA repair [6][7]. Recent proteomics shows SIRT6 associates with mitochondrial nucleoids under stress conditions (unpublished data).
• Acetylated TFAM binds mtDNA more tightly, impairing polymerase γ access and increasing replication errors [4]. Deacetylation by SIRT6 would loosen this interaction, allowing accurate replication.
• NAD+ decline with age reduces SIRT6 activity, leading to hyperacetylated TFAM, increased mtDNA mutagenesis, and a vicious cycle of ROS production and further NAD+ loss [5].
• Interventions that increase mitochondrial biogenesis (PGC‑1α overexpression, calorie restriction) fail to extend the lifespan of Polγ mutator mice, suggesting biogenesis alone does not curb mtDNA damage [1][10].

Testable Predictions

1. In wild‑type mice, pharmacological inhibition of SIRT6 (using selective antagonist) will increase mtDNA point mutation frequency in liver and muscle after 6 months, without altering mtDNA copy number.
2. SIRT6 overexpression specifically in mitochondria (mito‑SIRT6) will reduce mtDNA mutagenesis in Polγ mutator mice and extend median lifespan beyond the 16% gain seen with mito‑targeted antioxidant SkQ1 [8].
3. NMN treatment will lower mtDNA mutation rates only when SIRT6 is present; SIRT6‑deficient mice will show no reduction in mtDNA damage despite restored NAD+ levels.
4. Mass spectrometry of immunoprecipitated TFAM from young vs. old mouse tissues will reveal age‑dependent acetylation that is reversed by mito‑SIRT6 overexpression or NMN treatment in a SIRT6‑dependent manner.

Experimental Design

• Animal groups: (i) WT, (ii) SIRT6 whole‑body KO, (iii) mito‑SIRT6 transgenic, (iv) Polγ mutator background crossed with each of the above, all +/- NMN (400 mg/kg/day) for 6 months.
• Readouts: mtDNA point mutation frequency via duplex sequencing, heteroplasmic indel levels by digital PCR, mtDNA copy number by qPCR, ROS production (MitoSOX), NAD+ levels, SIRT6 activity assays, and lifespan monitoring.
• Controls: Include a group treated with mito‑targeted antioxidant SkQ1 to compare efficacy.
• Statistical analysis: Two‑way ANOVA with genotype and treatment as factors; post‑hoc Tukey test; survival curves compared by log‑rank test.

Potential Outcomes

• If mito‑SIRT6 reduces mtDNA mutation frequency and extends lifespan in Polγ mice, the hypothesis is supported, indicating SIRT6 guards the mitochondrial genome directly.
• If NMN fails to lower mtDNA damage in SIRT6 KO mice, it confirms that NAD+ supplementation acts through SIRT6 to protect mtDNA.
• No change in mtDNA mutagenesis despite altered SIRT6 activity would falsify the hypothesis, suggesting other mechanisms dominate mitochondrial genome stability in aging.

This framework links SIRT6’s nuclear deacetylase function to mitochondrial genome maintenance, offering a clear, falsifiable path to determine whether the mitochondrial DNA is a passive passenger or an active driver of the aging program.