Hypothesis

A bistable PRC2‑dependent epigenetic switch drives the coordinated onset of aging hallmarks.

We propose that aging is not a collection of independent deteriorations but the result of a single upstream controller: a bistable epigenetic landscape centered on Polycomb Repressive Complex 2 (PRC2)–mediated H3K27me3 domains. In youthful cells, PRC2 activity is restrained at specific developmental promoters, maintaining an open chromatin configuration that supports robust proteostasis, low senescence, and restrained inflammation. With age, stochastic fluctuations or metabolic shifts push the system past a threshold, causing PRC2 to spread and lock these promoters into a repressive state. This switch simultaneously alters DNA methylation patterns, reduces transcriptional fidelity, and triggers downstream hallmarks—epigenetic drift, cellular senescence, chronic inflammation, and proteostasis collapse—because those processes are directly coupled to the PRC2‑defined chromatin state {Yamanaka factors reverse multiple hallmarks; PRC2 epigenomic signature defines age-associated DNA methylation; Developmental drift hypotheses frame aging as continued post-developmental epigenetic shifts}.

Partial reprogramming with OSK transiently reduces PRC2 activity, pushing cells back toward the youthful attractor, but the aged state is reinforced by positive feedback loops: H3K27me3 recruits DNA methyltransferases, which further stabilize repression, and senescence‑associated secretory phenotype (SASP) signaling reinforces PRC2 expression via NF‑κB. Consequently, once OSK withdrawal occurs, the system relaxes back to the aged attractor, explaining the transient nature of rejuvenation {Partial reprogramming's rejuvenation effects are transient; Combination therapies targeting different hallmarks produce additive or synergistic lifespan effects}.

Testable predictions

1. Pharmacological or genetic stabilization of the youthful PRC2 state will sustain hallmark reversal after OSK withdrawal.

   • Use CRISPRi to knock down EZH2 or overexpress the H3K27me3 demethylase KDM6B in aged mice during a short OSK pulse.
   • Measure epigenetic clocks, senescence markers (p16^INK4a^), inflammatory cytokines, and proteasome activity weeks after OSK cessation.
   • Expected outcome: maintained youthful epigenetic profile and delayed hallmark recurrence compared with OSK‑only controls.

2. Single‑cell multi‑omics will reveal two discrete epigenetic attractors rather than a continuous drift.

   • Perform scATAC‑seq and scRNA‑seq on liver cells from young, middle‑aged, and old mice before and after OSK treatment.
   • Apply trajectory inference (e.g., Palantir) and clustering.
   • Expected outcome: distinct clusters corresponding to youthful and aged states, with OSK shifting cell proportions toward the youthful cluster; aged cells will show bimodal H3K27me3 occupancy at PRC2 target loci {Single‑cell epigenetic aging appears driven by widespread small-amplitude slow changes}.

3. Disrupting the feedback loop between PRC2 and SASP will blunt the aged attractor’s stability.

   • Block IL‑6 signaling with neutralizing antibodies in aged mice undergoing OSK treatment.
   • Assess whether the duration of hallmark reversal is extended.
   • Expected outcome: prolonged rejuvenation, indicating SASP‑PRC2 feedback contributes to attractor stability.

Falsifiability If stabilizing PRC2 activity fails to prolong hallmark reversal, or if single‑cell analyses show only gradual shifts without distinct attractors, the bistable switch model would be refuted. Conversely, confirmation would support the idea that a single upstream epigenetic controller orchestrates the aging phenotype, reframing interventions toward locking the youthful state rather than repeatedly resetting hallmarks.