title: Repair of radiation induced DNA double strand breaks by backup NHEJ is enhanced in G2.
abstract: In higher eukaryotes DNA double strand breaks (DSBs) are repaired by homologous recombination (HRR) or non-homologous end joining (NHEJ). In addition to the DNA-PK dependent pathway of NHEJ (D-NHEJ), cells employ a backup pathway (B-NHEJ) utilizing Ligase III and PARP-1. The cell cycle dependence and coordination of these pathways is being actively investigated. We examine DSB repair in unperturbed G1 and G2 phase cells using mouse embryo fibroblast (MEF) mutants defective in D-NHEJ and/or HRR. WT and Rad54(-/-) MEFs repair DSBs with similar efficiency in G1 and G2 phase. LIG4(-/-), DNA-PKcs(-/-), and Ku70(-/-) MEFs show more pronounced repair defects in G1 than in G2. LIG4(-/-)/Rad54(-/-) MEFs repair DSBs as efficiently as LIG4(-/-) MEFs suggesting that the increased repair efficiency in G2 relies on enhanced function of B-NHEJ rather than HRR. In vivo and in vitro plasmid end joining assays confirm an enhanced function of B-NHEJ in G2. The results show a new and potentially important cell cycle regulation of B-NHEJ and generate a framework to investigate the mechanistic basis of HRR contribution to DSB repair.
authors.affiliation: Institute of Medical Radiation Biology, University of Duisburg-Essen Medical School, Hufelandstr 55, Essen, Germany.
authors.forename: Wenqi
authors.initials: W
authors.lastname: Wu
authors.forename: Minli
authors.initials: M
authors.lastname: Wang
authors.forename: Weizhong
authors.initials: W
authors.lastname: Wu
authors.forename: Satyendra K
authors.initials: SK
authors.lastname: Singh
authors.forename: Tamara
authors.initials: T
authors.lastname: Mussfeldt
authors.forename: George
authors.initials: G
authors.lastname: Iliakis
mesh_terms: D000818:Animals; D034961:Antigens, Nuclear / Q000235:genetics; D016466:CHO Cells; D002460:Cell Line; D006224:Cricetinae; D003412:Cricetulus; D053903:DNA Breaks, Double-Stranded; D004265:DNA Helicases; D000072481:DNA Ligase ATP; D011088:DNA Ligases / Q000235:genetics; D004260:DNA Repair / Q000235:genetics / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000235:genetics; D033041:Embryo Research; D005434:Flow Cytometry; D016195:G2 Phase / Q000502:physiology*; D000072200:Ku Autoantigen; D051379:Mice; D018345:Mice, Knockout; D009687:Nuclear Proteins / Q000235:genetics
chemical_list: D034961:Antigens, Nuclear; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D004265:DNA Helicases; C103534:Rad54l protein, mouse; C000606510:Xrcc6 protein, mouse; D000072200:Ku Autoantigen; D011088:DNA Ligases; D000072481:DNA Ligase ATP