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, HP1α, HP1β, and HP1γ, which possess significant homology and yet have distinct functions. HP1α is the only isoform known to undergo liquid-liquid phase separation. We show that the minute-scale dynamics of HP1α and HP1β differ dramatically and they promote differential recruitment of HR vs. NHEJ factors at the sites of laser-induced clustered DSBs. Perturbing HP1α 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 HP1α and hints at spatial partitioning of repair pathways in response to damage in heterochromatin.