Repair of DNA double strand breaks (DSBs) must be properly orchestrated within the diverse nuclear chromatin regions in order to maintain genome stability. The choice between the two main DSB repair pathways: non-homologous end joining (NHEJ) and homologous recombination (HR), is dictated by the cell cycle as well as chromatin context. Constitutive heterochromatin, which forms a dense, generally transcriptionally silent structure near centromeres and telomeres, is essential for maintaining genome stability and is enriched for repetitive DNA sequences. The compact nature of this chromatin structure and the presence of many repetitive sequences make heterochromatin potentially vulnerable to erroneous DSB repair.
Several studies have revealed that heterochromatic regions display specialized temporal and spatial responses to DNA damage that differ from repair in euchromatin. Irradiation (IR)-induced damage has been shown to result in relaxation of the heterochromatin domain and relocalization of resected DSBs to outside the heterochromatin domain, where HR is thought to be the main mode of repair.
Here, we have developed an in vivo single DSB system specifically targeted to heterochromatic as well as euchromatic loci. We find, using live imaging of larval discs, that single DSBs can recapitulate the heterochromatin-specific spatial and temporal DSB dynamics previously seen with IR-induced breaks in cell culture.
Importantly, we identify a prominent role for both the NHEJ and HR pathways in heterochromatic DSB repair and find that the homologous chromosome can be used as a template for HR repair in both eu- and heterochromatin.
These data suggest that, although DSBs in heterochromatin and euchromatin are subject to diverse spatiotemporal dynamics, repair pathway choice is ultimately highly similar in these two distinct chromatin regions.