Double-Strand Breaks in Heterochromatin Move Outside of a Dynamic HP1a Domain to Complete Recombinational Repair

Abstract

SUMMARY Double-strand breaks (DSBs) in heterochromatic repetitive DNAs pose significant threats to genome integrity, but information about how such lesions are processed and repaired is sparse. We observe dramatic expansion and dynamic protrusions of the heterochromatin domain in response to ionizing radiation (IR) in Drosophila cells. We also find that heterochromatic DSBs are repaired by homologous recombination (HR) but with striking differences from euchromatin. Proteins involved in early HR events (resection) are rapidly recruited to DSBs within heterochromatin. In contrast, Rad51, which mediates strand invasion, only associates with DSBs that relocalize outside of the domain. Hetero-chromatin expansion and relocalization of foci require checkpoint and resection proteins. Finally, the Smc5/6 complex is enriched in heterochromatin and is required to exclude Rad51 from the domain and prevent abnormal recombination. We propose that the spatial and temporal control of DSB repair in heterochromatin safeguards genome stability by preventing aberrant exchanges between repeats.

Differential contribution of HP1 proteins to DNA end resection and homology-directed repair

“…17 In contrast, knockdown of the HP1 Drosophila ortholog HP1a does not impair RAD51 recruitment. 18 Intriguingly as well, knocking-out of C. elegans HP1 orthologs (hpl-1 and hpl-2) leads to opposite outcomes on cell survival after ionizing radiation (IR): hpl-1 KO shows increased resistance to IR, while hpl-2 KO is highly sensitive. 14 Therefore, it is necessary to sort out the particular contribution of each HP1 paralog in the DDR and the molecular mechanisms at stake.…”

Regulation of Recombination and Genomic Maintenance

“…It appears unlikely that HR can occur on such highly condensed substrates. However, the discovery the HR may be the predominant DSB-repair pathway for heterochromatic DSBs (Goodarzi et al 2010;Chiolo et al 2011) suggests that specific mechanisms may exist for HR to act on highly condensed substrates. Although chromosome condensation may provide an intuitive barrier to HR, in turn, HR has been shown to affect chromosome condensation.…”

“…First, centromeres do participate in HR but centromeric HR is specifically restricted to gene conversion (Shi et al 2010). Second, it was shown that DSB repair in heterochromatic regions was specifically channeled to HR-mediated repair (Chiolo et al 2011). This seminal counterintuitive finding discovered that the broken chromosome (and presumably its unbroken sister chromatid template) is extruded from the heterochromatic subcompartment to insulate the repair process from other repeat templates to avoid genomic rearrangements.…”

Perinuclear tethers license telomeric DSBs for a broad kinesin- and NPC-dependent DNA repair process

“…DSBs within rDNA repeats were found to transiently exit the Rad52/recombination-repressive environment of the nucleolus in which they typically reside to undergo repair without compromising genome integrity 8,9 . In flies, heterochromatic DSBs exit their Rad51/recombinationrepressive subnuclear domain to undergo repair 10 . Similarly, heterochromatic DSBs were found to relocate outside their domain for repair in cultured human cells 11 .…”