Hypothesis

We propose that progressive H3K27me3 drift during aging directly represses nuclear-encoded mitochondrial genes, causing mitochondrial dysfunction and increased ROS production. The resulting oxidative stress further destabilizes PRC2 targeting, amplifying H3K27me3 mislocalization and establishing a self‑reinforcing loop that drives cells into senescence.

Mechanistic Basis

• Primary event: In aged cells, H3K27me3 loses fidelity at canonical promoters and gains signal at inter‑peak regions (see epigenetic drift in Drosophila)[1]. This redistribution suppresses glycolytic genes and, we argue, also reduces expression of nuclear‑encoded mitochondrial transcripts such as NDUFS1, COX5B, and ATP5A.
• Consequence: Reduced mitochondrial gene expression lowers oxidative phosphorylation efficiency, elevates electron leak, and raises mitochondrial ROS.
• Feedback: ROS oxidizes cysteine residues in EZH2 and associated PRC2 subunits, altering their affinity for nucleosomes and promoting promiscuous binding to open chromatin[2]. This worsens H3K27me3 drift, deepening repression of metabolic genes.
• Amplification: Elevated ROS also triggers DNA damage signaling, which can recruit PRC2 to damage sites, further diverting the complex from its normal targets[3].

Predictions and Experiments

1. Gene‑specific repression: Aged fibroblasts will show decreased mRNA levels of a defined set of mitochondrial genes that correlate with local H3K27me3 gain at their promoters (ChIP‑seq + RNA‑seq).
2. ROS dependence: Treating old cells with a mitochondria‑targeted antioxidant (MitoQ) will reduce ROS, restore H3K27me3 fidelity at metabolic loci, and improve glycolytic flux.
3. PRC2 redox sensitivity: In vitro EZH2 activity assays will demonstrate inhibition by H2O2 at physiological concentrations, and mass spectrometry will detect oxidative modifications on EZH2 in senescent extracts.
4. Genetic test: Drosophila expressing a redox‑insensitive EZH2 mutant (Cys→Ser) will exhibit slower H3K27me3 drift, maintain higher mitochondrial gene expression, and live longer than controls.
5. Causality check: Overexpressing a mitochondrial transcription factor (TFAM) in aged human fibroblasts should rescue ROS levels despite persistent H3K27me3 drift, breaking the feedback loop and delaying senescence markers.

Potential Implications

If validated, this model positions mitochondrial ROS not merely as a downstream consequence of epigenetic drift but as an active driver that locks cells into a senescent state. Therapeutically, combining mild PRC2 inhibition with mitochondrial antioxidants could synergistically restore metabolic homeostasis and extend healthspan, offering a combinatorial strategy beyond single‑target approaches.