HP1α-driven Phase Separation and Repair Pathway Choice in Response to Heterochromatin Damage

Double-strand breaks (DSBs) pose significant threat to genomic stability and need immediate attention from DNA Damage Response (DDR) machinery involved in Homologous Recombination (HR) or Non-homologous end joining (NHEJ). DDR in heterochromatin is challenging owing to the distinct chromatin organization. Heterochromatin Protein 1 (HP1) isoforms that contribute significantly to the organization of heterochromatin, have been shown to be involved in DDR. Mammalian HP1 has three isoforms, HP1, HP1{beta}, and HP1{gamma}, which possess significant homology and yet have distinct functions. HP1 is the only isoform known to undergo liquid-liquid phase separation. We show that the minute-scale dynamics of HP1 and HP1{beta} differ dramatically and they promote differential recruitment of HR vs. NHEJ factors at the sites of laser-induced clustered DSBs. Perturbing HP1 phase-separation abrogates both the recruitment of HR factors and readouts of HR. Our study provides a link between phase-separation and DDR-centric roles of HP1 and hints at spatial partitioning of repair pathways in response to damage in heterochromatin.