Single-stranded DNA oligomers stimulate error-prone alternative repair of DNA double-strand breaks through hijacking Ku protein.

In humans, DNA double-strand breaks (DSBs) are repaired by two mutually-exclusive mechanisms, homologous recombination or end-joining. Among end-joining mechanisms, the main process is classical non-homologous end-joining (C-NHEJ) which relies on Ku binding to DNA ends and DNA Ligase IV (Lig4)-mediated ligation. Mostly under Ku- or Lig4-defective conditions, an alternative end-joining process (A-EJ) can operate and exhibits a trend toward microhomology usage at the break junction. Homologous recombination relies on an initial MRN-dependent nucleolytic degradation of one strand at DNA ends. This process, named DNA resection generates 3' single-stranded tails necessary for homologous pairing with the sister chromatid. While it is believed from the current literature that the balance between joining and recombination processes at DSBs ends is mainly dependent on the initiation of resection, it has also been shown that MRN activity can generate short single-stranded DNA oligonucleotides (ssO) that may also be implicated in repair regulation. Here, we evaluate the effect of ssO on end-joining at DSB sites both in vitro and in cells. We report that under both conditions, ssO inhibit C-NHEJ through binding to Ku and favor repair by the Lig4-independent microhomology-mediated A-EJ process.This work was supported by grants from the Ligue Nationale Contre le Cancer (Equipe labellisée 2013) and Electricité de France. Y Yuan and S. Britton were funded by a doctoral grant from China Scholarship Council and a post-doctoral grant from Ligue Nationale Contre le Cancer, respectively. This work also benefited from the TRI Optical Imaging Platform at IPBS (Genotoul, Toulouse, France) supported by grants from the Région Midi-Pyrénées (CPER), the Grand Toulouse community, the Association pour la recherche contre le cancer (ARC Equipement No 8505), the CNRS and the EU through the FEDER program. Research in the S.P.J laboratory is funded by Cancer Research UK programme grant C6/A11224, the European Research Council, and the European Community Seventh Framework Programme grant agreement no. HEALTH-F2-2010-259893 (DDResponse). Core funding is provided by CRUK (C6946/A14492) and the Wellcome Trust (WT092096). S.P.J. receives his salary from the University of Cambridge, UK, supplemented by CRUK. P. Calsou is a scientist from INSERM, France.This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/nar/gkv89