Hypothesis

Rapamycin extends lifespan by inducing a reversible, diapause‑like epigenetic state that mimics ancestral famine‑survival programs. Continuous mTORC1 inhibition lowers intracellular S-adenosylmethionine (SAM) and shifts histone methylation toward repressive marks (e.g., ↑H3K27me3) at growth‑promoting loci while permitting acetylation of stress‑resistance promoters. This chromatin configuration is maintained only while the drug is present; upon withdrawal, SAM pools rebound and the repressive marks are erased, abolishing the longevity benefit 1. We propose that a brief, co‑administered histone deacetylase inhibitor (HDACi) can lock‑in the diapause epigenome by increasing histone acetylation at stress‑response genes, making the state resistant to SAM‑driven demethylation after rapamycin cessation. Consequently, mice receiving rapamycin followed by a short HDACi pulse should retain extended lifespan and healthspan after rapamycin withdrawal, whereas rapamycin alone will not.

Testable Predictions

1. Epigenetic lock‑in – Mice treated with rapamycin (14 mg/kg diet) for 8 weeks followed by a 3‑day pulse of the HDACi valproic acid (300 mg/kg i.p.) will show persistent ↑H3K27me3 at mTOR‑target genes and ↑H3K9ac at FOXO‑target loci after rapamycin withdrawal, whereas rapamycin‑only mice will revert to baseline within 2 weeks 2.
2. Durable longevity – In a survival cohort, the rapamycin + HDACi group will exhibit a significant lifespan extension (≥12% median increase) after rapamycin is stopped at 20 months, while the rapamycin‑only group will lose the benefit and survive like controls 3.
3. Healthspan preservation – Gripping strength, glucose tolerance, and macrophage phagocytosis will remain improved in the combined group for at least 6 weeks post‑withdrawal, correlating with the retained epigenetic signature 4.
4. Specificity – Using an inactive HDACi analog or a genetic HDAC knockout will abolish the durable effect, confirming that acetylation, not off‑target toxicity, is required 5.

Experimental Design

• Animals: C57BL/6J mice, both sexes, n=30 per group.
• Groups: (1) Control diet, (2) Rapamycin continuous, (3) Rapamycin → withdrawal, (4) Rapamycin → withdrawal + HDACi pulse, (5) HDACi pulse alone.
• Timeline: Start interventions at 20 months; rapamycin for 8 weeks; HDACi pulse on days 55‑57; withdraw rapamycin at week 8; monitor survival and health markers for up to 12 months.
• Readouts: Survival curves (Kaplan‑Meier, log‑rank test), western blot / ChIP‑seq for H3K27me3 and H3K9ac in liver and muscle, metabolomics for SAM/SAH ratio, frailty index.

Falsifiability

If the rapamycin + HDACi cohort fails to show either sustained epigenetic marks or a lifespan advantage after rapamycin cessation compared with rapamycin‑only, the hypothesis is refuted. Conversely, a durable benefit would support the idea that rapamycin’s effect is a reversible hormetic state that can be stabilized by epigenetic locking.

Relevance

This framework connects nutrient‑sensing pathways to epigenetic memory, suggesting that geroprotectors may need to be paired with chromatin‑modifying agents to convert transient stress responses into lasting rejuvenation.