Hypothesis

Rapamycin extends lifespan chiefly by lowering the rate of new oxidative DNA damage through enhanced mitophagy and reduced metabolic flux, while simultaneously dampening the transcription of base‑excision repair (BER) genes via the mTORC1‑S6K1‑p300 axis. The net effect is a shift from damage removal to damage prevention. In contexts where the existing lesion load is high—such as aged neurons or DNA‑repair‑deficient backgrounds—this shift becomes detrimental, limiting or abolishing the longevity benefit.

Mechanistic Rationale

1. Damage prevention arm – mTORC1 inhibition activates ULK1‑dependent autophagy and TFEB‑driven lysosomal biogenesis, leading to selective removal of damaged mitochondria (mitophagy). Lower mitochondrial ROS production decreases the formation of 8‑oxoguanine (8‑oxoG) lesions, as shown by reduced γ‑H2AX and 8‑oxoG immunoreactivity in rapamycin‑treated immune cells [5].
2. Repair suppression arm – Active mTORC1 phosphorylates S6K1, which in turn phosphorylates the histone acetyltransferase p300/CBP, promoting acetylation of H3K27 at promoters of BER genes (OGG1, APE1, POLβ). Inhibition of mTORC1 reduces S6K1 activity, decreasing p300‑mediated acetylation, resulting in a more compact chromatin state and lowered transcription of these repair enzymes. This aligns with the observation that rapamycin does not upregulate OGG1 or APE1 despite lowering new damage [1] and actively suppresses other repair pathways [2][3][4].
3. Trade‑off outcome – Cells experience fewer new lesions but retain a diminished capacity to excise existing ones. In post‑mitotic neurons, where lesion dilution via cell division is absent, the persistence of 8‑oxoG can impair transcriptional fidelity and promote senescence, counteracting the genoprotective effect of reduced ROS.

Testable Predictions

• Prediction 1: In wild‑type mice, chronic rapamycin treatment will decrease mitochondrial ROS and 8‑oxoG formation rates (measured by mitoSOX and 8‑oxoG ELISA) while reducing OGG1 mRNA and protein levels and decreasing H3K27ac at the Ogg1 promoter (ChIP‑qPCR).
• Prediction 2: Neuron‑specific mTORC1 knockout (or rapamycin delivery) will show the same ROS reduction but a pronounced accumulation of 8‑oxoG in hippocampal DNA compared with whole‑body treatment, correlating with impaired spatial memory.
• Prediction 3: In DNA‑repair‑deficient models (e.g., Ogg1^−/− or Ercc1^−/− mice), rapamycin will fail to extend lifespan, whereas co‑administration of an NAD⁺ booster (which activates SIRT1 and can enhance p300 activity) or neuronal overexpression of OGG1 will restore the longevity benefit.
• Prediction 4: Pharmacologic activation of S6K1 (e.g., via a membrane‑permeable peptide mimetic) in the presence of rapamycin will rescue p300‑dependent H3K27ac at BER promoters and OGG1 expression without abolishing autophagy, thereby uncoupling the two arms.

Potential Confounds & Controls

• Control for changes in food intake; pair‑fed groups should be included to isolate drug‑specific effects.
• Monitor autophagic flux (LC3‑II turnover with bafilomycin A1) to ensure that observed ROS changes are not secondary to altered lysosomal function.
• Use cell‑type‑specific reporters (e.g., Ogg1‑GFP) to distinguish transcriptional versus post‑translational regulation.

By directly linking mTORC1‑S6K1‑p300 chromatin signaling to the transcriptional output of BER, this hypothesis extends the current view of rapamycin as a mere stress mimetic and offers a clear, falsifiable framework for dissecting when nutrient‑signaling inhibition truly promotes healthspan versus when it merely postpones damage accumulation at the expense of repair capacity.