Hypothesis: Aged‑donor somatic cells retain repressive H3K27me3 marks at core circadian gene promoters during reprogramming to iPSCs; these marks are not fully erased, persist in the pluripotent state, and are reinstated upon redifferentiation, thereby compromising circadian amplitude and accelerating aging phenotypes.

Mechanistic Rationale: The circadian network couples transcriptional feedback loops to chromatin dynamics via CLOCK‑SIRT1‑mediated histone acetylation and PER/CRY‑directed recruitment of repressive complexes. In aged cells, chronic NAD+ decline diminishes SIRT1 activity, tipping the balance toward histone methylation. PRC2 (EZH2‑containing) deposits H3K27me3 at CpG‑rich promoters of BMAL1, CLOCK, and PER1, a modification that is known to survive reprogramming at certain loci[6]. Because clock genes are hubs that regulate NAD+ biosynthesis (through NAMPT) and mitochondrial function, their silencing creates a feed‑forward loop: reduced NAD+ further weakens SIRT1, stabilizing H3K27me3 and dampening circadian output. This loop could explain why iPSCs derived from old donors show delayed or blunted oscillation emergence upon differentiation[2] and why redifferentiated cells retain epigenetic age signatures despite pluripotency.

Testable Predictions:

1. ChIP‑qPCR for H3K27me3 will show significantly higher enrichment at BMAL1, CLOCK, PER1, and CRY1 promoters in iPSCs from donors >65 years compared with iPSCs from donors <30 years, despite comparable pluripotency marker expression.
2. Upon redifferentiation into fibroblasts or neurons, the aged‑donor derived lineages will exhibit lower bioluminescence amplitude (using a Bmal1‑Luc reporter) and longer period variability than young‑donor counterparts.
3. Pharmacological inhibition of EZH2 (e.g., GSK126) during the iPSC phase will reduce H3K27me3 at circadian loci, rescue NAD+ levels via increased NAMPT expression, and restore robust circadian rhythms in the resulting somatic cells.
4. Conversely, CRISPR‑dCas9‑KRAB targeted to the same promoters in young‑donor iPSCs will recapitulate the aged phenotype: diminished oscillation strength and accelerated accumulation of senescence‑associated β‑galactosidase after differentiation.

Experimental Design:

• Obtain peripheral blood mononuclear cells from young (20‑30 y) and old (65‑80 y) donors.
• Generate iPSCs using non‑integrating episomal vectors; validate pluripotency.
• Perform ChIP‑seq for H3K27me3 and RNA‑seq on iPSCs and day‑14 differentiated fibroblasts/neurons.
• Measure circadian dynamics with real‑time luciferase reporting (Bmal1‑Luc) over 5‑7 days under serum‑shock conditions.
• Treat parallel iPSC cultures with EZH2 inhibitor or DMSO control during maintenance; assess rescue of H3K27me3, NAD+ levels (LC‑MS), and clock amplitude.
• Use CRISPR‑dCas9‑KRAB to epigenetically silence circadian promoters in young‑donor iPSCs as a gain‑of‑function control.

Falsifiability: If H3K27me3 levels at circadian promoters are equivalent between young and old iPSC lines, or if EZH2 inhibition fails to improve circadian amplitude in aged‑donor derivatives, the hypothesis would be refuted. Likewise, if artificial repression of these promoters in young cells does not phenocopy aging markers, the proposed causal link would be weakened.

Implications: Demonstrating a persistent epigenetic memory at circadian regulators would position the clock not merely as a downstream read‑out of aging but as a gatekeeper of reprogramming fidelity. It would suggest that rejuvenation strategies must include epigenetic resetting of the timing system—potentially via transient PRC2 inhibition or NAD+ boosting—to achieve true cellular age reversal.