Replication stress shapes a protective chromatin environment across fragile genomic regions

Recent integrative epigenome analyses highlight the importance of functionally distinct chromatin states for gene regulation, cellular differentiation and accurate cell function. How these states are established and maintained is a matter of intense investigation. Here, we present evidence for DNA damage as an unexpected means to shape a protective chromatin environment at genomic regions of recurrent replication stress. Upon aberrant fork stalling within these fragile regions, DNA damage signaling and concomitant H2AX phosphorylation coordinate the FACT histone chaperone -dependent deposition of macroH2A1.2, a histone H2A variant that promotes DNA repair by homologous recombination (HR). MacroH2A1.2, in turn, facilitates the accumulation of the tumor suppressor and HR effector BRCA1 at replication forks to protect from replication stress-induced DNA damage. Consequently, replicating primary cells steadily accrue macroH2A1.2 at fragile regions, whereas macroH2A1.2 loss in these cells triggers DNA damage signaling-dependent senescence, a hallmark of replication stress. Altogether, our findings demonstrate that recurrent DNA damage critically contributes to the chromatin landscape to ensure both the epigenetic and functional integrity of dividing cells. Detection of the histone protein macroH2A1.2 via ChIP-Seq, along with the DNA damage marker gH2Ax, in wild-type K562 and in conditions of macroH2A deficiency or ablation, before and after induction of replication stress