LaminA/C regulates epigenetic and chromatin architecture changes upon aging of hematopoietic stem cells

The decline of hematopoietic stem cell (HSC) function upon aging contributes to the senescent immune remodeling and to leukemia pathogenesis. Aged HSCs show changes in their epigenome, like alterations in the global and local DNA/histone methylation and histone acetylation landscape. Previously, we showed a correlation between high Cdc42 activity and the loss of intra-nuclear epigenetic polarity (epipolarity), as indicated by the specific location of histone H4 lysine 16 acetylation (H4K16ac). Here, we show that not all histone modifications display a polar localization and that loss of H4K16ac amount and epipolarity is specific to aged HSCs. Increased levels of H4K16ac are insufficient to restore polarity in aged HSCs and for the restoration of HSC function. Changes in H4K16ac upon aging and rejuvenation of HSCs are correlated to a shift of chromosome 11 architecture and nuclear volume and shape. Surprisingly, by taking advantage of knock-out mouse models we demonstrate that increased Cdc42 activity levels correlate with the repression of LaminA/C expression, which control chromosome 11 distribution, H4K16ac polarity and the nuclear volume and shape of aged HSCs. These chromatin and epigenetic architecture changes are targeted by altering the activity of the small RhoGTPase Cdc42, that regulates LaminA/C. Collectively, our data show that chromatin architecture changes in stem cells are reversible by changing levels of Cdc42 activity, revealing an unanticipated way to pharmacologically target LaminA/C expression and revert alterations of the epigenetic architecture in aged HSCs.