Sirtuin 7 regulates dosage compensation and safeguards the female X-chromosome [ChIP_H2AX_gH2AX_TTF]

Sirtuins are deacetylases implicated in stress responses and longevity in mammals. Although their differential impact on the two sexes has been noted, underlying causes for sex biases in aging and cancer are not known. Here, using Sirtuin 7 (SIRT7) as a model in mice, we probe mechanisms leading to sex differences. We find that Sirt7-/- females have decreased fitness during embryogenesis and throughout life. Intriguingly, SIRT7 preferentially localizes to the X-chromosome, suggesting a potential defect in X-inactivation — the arm of dosage compensation that equalizes X-chromosome expression between XX females and XY males. Indeed, SIRT7 ablation causes increased Xist RNA levels, H3K27me3 enrichment, and gene repression on the inactive X (Xi) chromosome. Surprisingly, however, loss of SIRT7 exerts greatest effects on the active X (Xa). In Sirt7-/- females, the Xa demonstrates 3D structural disorganization, increased chromatin mixing, and acute propensity to break and form chromosome fusions in an Xa-specific manner. Disproportional to autosomes and Xi, the Xa becomes super-acetylated on H3K36 and super-activated in gene expression, resulting in an imbalance in the 2nd arm of dosage compensation — known as “Xa-hyperactivation” — which evolved to balance X-linked gene expression against the genome. These data reveal a crosstalk between sirtuins and dosage compensation and identify a novel regulator of Xa-hyperactivation.