Histone chaperones and modifiers cooperate to maintain heterochromatin integrity following DNA damage.

Heterochromatin is a critical chromatin compartment, whose integrity governs genome stability and cell fate transitions. How heterochromatin features, including higher-order chromatin folding and histone modifications associated with transcriptional silencing, are maintained following a genotoxic stress challenge is unknown. Here, we establish a system for targeting UV damage to pericentric heterochromatin in mammalian cells and for tracking the heterochromatin response to UV in real time. We uncover profound heterochromatin compaction changes during repair, orchestrated by the UV damage sensor DDB2. Importantly, the restoration of heterochromatin folding is uncoupled from the maintenance of heterochromatin-specific histone modifications. We also unveil a central role for the methyltransferase SETDB1 in the maintenance of heterochromatic histone marks after UV, SETDB1 coordinating H3 methylation with new histone deposition by histone H3 chaperones in damaged heterochromatin. Our data thus shed light on fundamental molecular mechanisms safeguarding higher-order chromatin integrity following DNA damage.