We hypothesize that the higher basal nuclear acetyl-CoA concentration observed in XX cells stems from dosage-sensitive expression of X-linked escapee genes that directly generate or regulate acetyl-CoA for histone acetylation. It's clear that two strong candidates are ACSS2, which converts acetate to acetyl-CoA in the nucleus, and the histone demethylase KDM6A/UTX, which escapes X-inactivation and can transcriptionally activate ACLY and ACSS2. In XX cells, two active copies of these genes raise acetyl-CoA production, increase HAT activity, and promote acetylation of lysines on histones H3K27ac and H4K16ac at promoters of longevity-associated genes such as TFEB, SIRT1 and FOXO3. XY cells, with only one copy, generate less nuclear acetyl-CoA, show lower histone acetylation, and consequently exhibit an accelerated epigenetic aging clock. This mechanism would explain why female mammals consistently outlive males independent of gonadal hormones.

Predictions

• XX fibroblasts will display ~20-30% higher nuclear acetyl-CoA levels than XY counterparts, a difference that can't be detected after siRNA knock-down of ACSS2 or KDM6A.
• CRISPR-mediated deletion of one allele of ACSS2 or KDM6A in XX cells will reduce nuclear acetyl-CoA and H3K27ac to XY levels, while duplication of the X-linked allele in XY cells will raise them to XX levels.
• In mouse models, XX animals carrying a heterozygous knockout of Acss2 or Kdm6a will lose their survival advantage relative to wild-type XX littermates, whereas XY mice transgenic for an extra copy of Acss2 or Kdm6a will show extended median lifespan.
• Pharmacological elevation of nuclear acetyl-CoA (e.g., acetate supplementation) will rescue the shortened lifespan of XY; Acss2-/-; Kdm6a-/- double mutants to wild-type XX levels, whereas inhibition of ACLY with SB-204990 will erase the XX benefit.

Falsifiability If nuclear acetyl-CoA concentrations, histone acetylation marks at longevity gene promoters, or lifespan don't correlate with X-linked dosage of ACSS2 and KDM6A as described, the hypothesis is refuted. Conversely, a consistent dose-dependent effect across cell types, tissues and species would support the idea that the X chromosome functions as a longevity chromosome via metabolic-epigenetic coupling.

References [1] https://pubmed.ncbi.nlm.nih.gov/33917812/ [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC8068152/ [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC6699407/ [4] https://pmc.ncbi.nlm.nih.gov/articles/PMC3625512/