Heterochromatic Repeat Clustering Imposes a Physical Barrier on Homologous Recombination to Prevent Chromosomal Translocations

Abstract

Mouse pericentromeric DNA is composed of tandem major satellite repeats, which are heterochromatinized and cluster together to form chromocenters containing homologous sequences from different chromosomes. These clusters are protected from DNA damage and are refractory to DNA repair through homologous recombination (HR). Moreover, double stranded DNA breaks (DSBs) occurring within chromocenters relocate away from the core domain to be repaired. The mechanisms by which pericentric heterochromatin imposes a barrier on HR and the implications of repeat clustering are unknown. Here, we compare the spatial recruitment of HR factors upon DSBs induced specifically in human and mouse pericentromeric heterochromatin, which differ in their capacity to form clusters. We show that while DSBs in human pericentromeric heterochromatin increase the accessibility of the domain by disrupting HP1 α dimerization, mouse pericentromeric heterochromatin repeat clustering imposes a physical barrier that requires many layers of decompaction to be accessed. Our results are consistent with a model in which the 3D organization of the different heterochromatic regions dictates the spatial activation of DNA repair pathways and is key to prevent the activation of HR within clustered repeats and the onset of chromosomal translocations.