Hypothesis

Age‑related decline across tissues stems from a progressive loss of PRC2‑mediated epigenetic fidelity at HOX loci, which destabilizes positional memory in mesenchymal stem cells (MSCs) and cascades into the canonical hallmarks of aging.

Mechanistic Model

PRC2 deposits H3K27me3 to maintain repressed chromatin states at developmental genes, especially HOX clusters. In young MSCs, this repression is precise, preserving lineage‑specific transcriptional programs. With age, stochastic erosion of H3K27me3—observed as increased cell‑to‑cell heterogeneity—allows aberrant HOX activation (e.g., Hoxa9) through compensatory gain of H3K4me3 and hydroxymethylation. Misexpressed HOX proteins reprogram MSCs toward a senescent, pro‑inflammatory phenotype, driving SASP secretion, impaired differentiation, and extracellular matrix remodeling. Because MSCs reside in niches that support multiple lineages (osteogenic, chondrogenic, adipogenic, fibroblastic), their epigenetic drift broadcasts deleterious signals systemically, manifesting as genomic instability, telomere attrition, mitochondrial dysfunction, and stem cell exhaustion across tissues.

Testable Predictions

1. Single‑cell multi‑omics of aged human bone marrow MSCs will show elevated transcriptomic variance at HOX loci correlated with loss of H3K27me3 signal, independent of global DNA methylation changes.
2. Restoring PRC2 activity (e.g., via EZH2 overexpression or inhibition of H3K27me3 demethylases) in aged MSCs will reduce Hoxa9/H3K4me3 enrichment, decrease SASP factor secretion, and improve multilineage differentiation capacity.
3. Transplanting PRC2‑rejuvenated MSCs into progeroid mice will attenuate tissue‑specific hallmarks (fibrosis, osteoporosis, impaired wound healing) more effectively than transplanting untreated aged MSCs.
4. Pharmacological induction of epigenetic noise (low‑dose DNMT inhibitors) in young MSCs will recapitulate age‑like HOX dysregulation and accelerate hallmark phenotypes in vivo.

Experimental Approach

• scCUT&Tag + scRNA‑seq on MSCs from young (3 mo) and aged (24 mo) mice to quantify H3K27me3/H3K4me3 heterogeneity at each HOX gene; compute epigenetic entropy scores and correlate with SASP gene expression.
• CRISPR‑a/EZH2 overexpression in aged MSCs followed by functional assays: CFU‑F, osteogenic/adipogenic differentiation, and secreted IL‑6/TNF‑α ELISA.
• In vivo rescue: inject syngeneic young, aged, and PRC2‑rescued MSCs into irradiated mice; monitor bone density (μCT), muscle grip strength, and circulating inflammatory cytokines over 8 weeks.
• Noise induction: treat young MSCs with 0.1 µM 5‑azacytidine for 48 h, then assess HOX entropy and SASP; compare to untreated aged MSCs.

Falsifiability

If aged MSCs show no increase in HOX‑locus epigenetic entropy, or if restoring PRC2 fidelity fails to mitigate SASP and functional decline, the hypothesis that PRC2‑driven epigenetic noise is the upstream controller of aging hallmarks would be refuted. Conversely, consistent support across these assays would substantiate a unified mechanistic origin for multisystem aging.