Hypothesis

We hypothesize that the age‑dependent loss of active enhancers in stem cells stems from a failure of replication‑coupled chromatin opening at YY1‑bound super‑enhancers, and that this failure is modulated by RhoA‑driven nuclear actin dynamics.

Mechanistic Rationale

• Recent repli‑ATAC‑seq shows that de novo chromatin opening occurs specifically in the replicated fraction during identity changes [6].
• YY1 is required for super‑enhancer–promoter looping and its loss makes genes five‑fold more likely to decline with age [2].
• RhoA inhibition can partially reverse the aberrant accessibility of aged HSCs [7], suggesting that actin‑linked signaling influences chromatin states.

We propose that YY1 serves as a scaffold that brings the replication‑coupled remodeler complex (CAF‑1/HIRA) to super‑enhancers during S‑phase. In young cells, active RhoA promotes nuclear actin polymerization, which facilitates remodeler access and enables proper nucleosome deposition after DNA replication, preserving enhancer openness. With age, RhoA activity declines (or becomes dysregulated), reducing nuclear actin and impairing recruiter recruitment. Consequently, replicated super‑enhancers fail to re‑establish an open chromatin state, leading to progressive erosion of H3K27ac‑marked enhancers at tumor‑suppressor loci and downstream transcriptional silencing.

This model explains why distal regulatory elements show greater age‑related changes than promoters [3], why the chromatin barrier is cell‑intrinsic [5], and why enhancer loss is most pronounced at active (H3K27ac) sites [1]—they depend most on replication‑coupled renewal.

Testable Predictions

1. In young stem cells, super‑enhancer regions will exhibit early‑S‑phase replication timing and coincident YY1 and CAF‑1 occupancy; aged cells will show delayed replication timing and reduced YY1/CAF‑1 co‑binding.
2. Acute RhoA inhibition in aged HSCs will restore nuclear actin levels, increase YY1‑dependent recruitment of CAF‑1 to super‑enhancers during S‑phase, and rescue repli‑ATAC‑seq signal at those loci.
3. Preventing replication (e.g., with low‑dose aphidicolin) will abolish the rescuing effect of RhoA inhibition, confirming that the effect is replication‑coupled.

Experimental Approach

• Perform repli‑ATAC‑seq and Repli‑Seq on sorted young and aged HSCs to map replication timing of YY1‑bound super‑enhancers (use YY1 ChIP‑seq data from [2]).
• Conduct sequential ChIP‑reChIP for YY1 and CAF‑1 (or HIRA) in fractionated S‑phase cells.
• Treat aged HSCs ex vivo with a RhoA inhibitor (e.g., Rhosin) or activator (CN03) and assess nuclear actin by Phalloidin staining, YY1/CAF‑1 co‑occupancy, and repli‑ATAC‑seq signal.
• Include a replication block (aphidicolin) arm to test dependence on S‑phase.
• Measure functional readouts: competitive transplantation and leukemic transformation assays to link chromatin rescue to phenotype.

If predictions hold, the data will support a model where replication‑coupled, YY1‑mediated chromatin opening—gated by RhoA‑actin signaling—is a central, cell‑intrinsic mechanism driving enhancer erosion during stem‑cell aging.

Key citations: repli‑ATAC‑seq [6]; YY1‑looping [2]; RhoA reversal [7]; enhancer erosion [1]; distal element changes [3]; transplantation barrier [5]