Hypothesis

We hypothesize that the age‑related decline of base excision repair (BER) in neurons stems from a combined loss of enzyme activity and NAD⁺‑dependent SIRT1‑mediated deacetylation, which together impair OGG1 and APE1 DNA binding and promote their proteasomal turnover. Restoring NAD⁺ levels while pharmacologically activating both OGG1 (mitochondrial and nuclear isoforms) and APE1 will synergistically reverse 8‑oxoguanine (8‑oxoG) accumulation, suppress cGAS‑STING‑driven neuroinflammation, and preserve cognitive function.

Mechanistic Basis

1. NAD⁺‑SIRT1 axis: SIRT1 deacetylates OGG1 and APE1, enhancing their affinity for damaged DNA and protecting them from ubiquitin‑mediated degradation [https://pmc.ncbi.nlm.nih.gov/articles/PMC3834072/]. NAD⁺ falls with age, reducing SIRT1 activity and leading to hyperacetylated, unstable repair proteins.
2. Lipid peroxidation blockade: 4‑hydroxynonenal (4‑HNE) adducts on OGG1/APE1 cysteines further inhibit catalysis [https://pmc.ncbi.nlm.nih.gov/articles/PMC5576886/]. SIRT1 activation can up‑regulate aldehyde‑detoxifying enzymes (e.g., ALDH2), lowering 4‑HNE burden.
3. Sequential bottleneck: OGG1 excises 8‑oxoG, creating an AP site that APE1 must incise. If either step is rate‑limiting, AP sites accumulate, triggering PARP1 overactivation and NAD⁺ depletion—a vicious loop.

Testable Predictions

• Prediction 1: In aged wild‑type mice, combined treatment with an NAD⁺ precursor (e.g., nicotinamide riboside) and a dual OGG1/APE1 activator will reduce hippocampal 8‑oxoG levels by >60 % compared with either monotherapy (<30 % reduction).
• Prediction 2: The combined treatment will lower cGAS‑STING signaling (phospho‑TBK1/IRF3) and downstream proinflammatory cytokines (IL‑1β, TNF‑α) to baseline young‑mouse levels, whereas single agents produce only partial suppression.
• Prediction 3: Behavioral assays (Morris water maze, novel object recognition) will show significant cognitive improvement only in the combination group, correlating with restored BER flux measured by ex vivo AP‑site incision assays.

Experimental Approach

• Animals: 18‑month‑old APP/PS1 mice and age‑matched controls; n=12 per group.
• Groups: Vehicle, NAD⁺ booster (NR 400 mg/kg/day), OGG1 activator (e.g., compound OGGi‑1), APE1 activator (e.g., compound APEi‑1), and the triple combination (NR + OGGi‑1 + APEi‑1).
• Readouts: (i) DNA‑oxidation (8‑oxoG immunofluorescence, slot‑blot), (ii) BER activity (glycosylase and incision assays in mitochondrial and nuclear extracts), (iii) SIRT1‑dependent deacetylation (acetyl‑OGG1/APE1 Western), (iv) 4‑HNE adducts (dot blot), (v) neuroinflammation (cGAS‑STING pathway, Iba1, cytokine ELISA), (vi) cognition (Morris water maze, novel object recognition).
• Statistical analysis: Two‑way ANOVA with post‑hoc Tukey; synergy assessed by Bliss independence model.

If the combination fails to outperform monotherapies, the hypothesis that NAD⁺‑SIRT1 coupling is essential for coordinated BER activation would be falsified, directing focus toward alternative regulatory layers (e.g., microRNA‑mediated translational control or mitochondrial import defects).