Hypothesis

Mitochondrial DNA heteroplasmy beyond a threshold elevates reactive oxygen species (ROS) that directly oxidize and inhibit the ATP-dependent chromatin remodeler CHD4, causing loss of nucleosome sliding at enhancer regions and resulting in chromatin accessibility decline in aged hematopoietic stem cells (HSCs).

Mechanistic rationale

• ROS generated from damaged mtDNA can oxidize cysteine residues in the helicase domain of CHD4, impairing its ATPase activity 3.
• CHD4 is a core subunit of the NuRD complex that remodels nucleosomes at enhancers bound by lineage‑specifying transcription factors; its inhibition leads to stable nucleosome occupancy and reduced ATAC‑seq signal 6.
• This provides a direct biochemical bridge from mtDNA damage to the enhancer erosion observed in memory CD8+ T cells and neural stem cells 1,2.

Testable predictions

1. Increasing mtDNA heteroplasmy in primary human HSCs via mitoTALEN‑induced mtDNA breaks will raise mitochondrial ROS and decrease CHD4 ATPase activity measured in vitro.
2. Pharmacological ROS scavenging (e.g., MitoTEMPO) or expression of a ROS‑resistant CHD4 mutant (Cys→Ser) will restore chromatin accessibility at enhancers without altering mtDNA heteroplasmy levels.
3. Conversely, acute inhibition of CHD4 using siRNA will phenocopy the accessibility loss seen in high‑heteroplasmy HSCs, even when ROS are low.

Experimental approach

• Isolate CD34+ HSCs from young donors, introduce mitoTALENs targeting common mtDNA deletion sites to generate graded heteroplasmy (0‑80%).
• Measure mtDNA copy number, heteroplasmy by ddPCR, mitochondrial ROS by MitoSOX flow.
• Assess CHD4 activity via ATPase assay and chromatin immunoprecipitation for CHD4 at enhancers.
• Perform ATAC‑seq and RNA‑seq to correlate accessibility changes with gene expression.
• Rescue experiments with MitoTEMPO or CHD4‑Cys→Ser transgene.

Falsifiability

If ROS scavenging fails to rescue chromatin accessibility despite lowering mitochondrial ROS, or if CHD4 activity remains unaltered across heteroplasmy levels, the hypothesis would be refuted, suggesting that mtDNA influences chromatin through alternative pathways or that chromatin changes are upstream of mitochondrial dysfunction.