53BP1 mediates G1- dominant, backup-repair pathway for DNA double-strand breaks

Abstract

To investigate the role of 53BP1 in repair of DNA double-strand breaks in vertebrate cells, we established a 53BP1 knockout cell line by using hyper-recombinogenic chicken B cell line DT40. When asynchronous cells were irradiated, no significant difference was observed in X-ray sensitivity between wild type and 53BP1-null cell lines. When cells were synchronized, 53BP1-null cells showed elevated X-ray sensitivity in G1 phase, whereas Ligase IV-null cells showed elevated X-ray sensitivity in G1- early S phase, suggesting that the repair system defect in 53BP1-null cells dominantly functions in the G1 phase of the cell cycle, and differ from NHEJ pathway. Furthermore, X-ray sensitivity analysis with or without wortmannin in G1-arrested cells revealed that 53BP1-dependent repair pathway was resistant, whereas NHEJ pathway was sensitive to wortmannin. These data suggest that there is the third repair pathway for DNA double-strand breaks, which is 53BP1-dependent, wortmannin resistant and functions dominantly in G1 phase of the cell cycle.