Abstract
Non-homologous end-joining (NHEJ) and homologous recombination (HR) represent the two main pathways for repairing DNA double-strand breaks (DSBs). During the G2 phase of the mammalian cell cycle, both processes can operate and chromatin structure is one important factor which determines DSB repair pathway choice. ATM facilitates the repair of heterochromatic DSBs by phosphorylating and inactivating the heterochromatin building factor KAP-1, leading to local chromatin relaxation. Here, we show that ATM accumulation and activity is strongly diminished at DSBs undergoing end-resection during HR. Such DSBs remain unrepaired in cells devoid of the HR factors BRCA2, XRCC3 or RAD51. Strikingly, depletion of KAP-1 or expression of phospho-mimic KAP-1 allows repair of resected DSBs in the absence of BRCA2, XRCC3 or RAD51 by an erroneous PARP-dependent alt-NHEJ process. We suggest that DSBs in heterochromatin elicit initial local heterochromatin relaxation which is reversed during HR due to the release of ATM from resection break ends. The restored heterochromatic structure facilitates HR and prevents usage of error-prone alternative processes.
Highlights
DNA double-strand breaks (DSBs) are among the most deleterious cellular lesions since they threaten genomic integrity and cell viability
In contrast to c-Non-homologous end-joining (NHEJ), homologous recombination (HR) is restricted to the S and G2 phases of the cell cycle where break ends undergo extensive resection and homologous DNA sequences on the sister chromatid serve as a template for repair
We have previously shown that the structure of the chromatin is one important factor which determines the choice between these two pathways, such that DSBs localizing to highly condensed heterochromatic regions are mainly repaired by HR and breaks in more open euchromatic DNA undergo repair by NHEJ
Summary
DNA double-strand breaks (DSBs) are among the most deleterious cellular lesions since they threaten genomic integrity and cell viability. C-NHEJ repairs unresected break ends without the need for sequence homologies and can function throughout the cell cycle [8]. The repair process is completed by a complex of DNA ligase IV, XRCC4, and XLF/ Cernunnos [5]. In contrast to c-NHEJ, HR is restricted to the S and G2 phases of the cell cycle where break ends undergo extensive resection and homologous DNA sequences on the sister chromatid serve as a template for repair. In contrast to cNHEJ but similar to HR, alt-NHEJ involves CtIP-dependent resection. The resected break ends are subsequently rejoined by a process involving micro-homologies and various repair factors such as poly (ADP-ribose) polymerase (PARP), DNA ligase I or III, and XRCC1 [14,15,16,17]. Alt-NHEJ can efficiently operate in cells devoid of c-NHEJ factors, little is known about its ability to compensate for HR defects
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