53BP1 promotes microhomology-mediated end-joining in G1-phase cells.

Xiahui Xiong,Chunhong Yan,Pan Fan,Henning Willers,Junran Zhang,Ying Wang,Zhihui Feng,Zhanwen Du

doi:10.1093/nar/gku1406

Abstract

Alternative non-homologous end joining (alt-NHEJ) was originally identified as a backup repair mechanism in the absence of classical NHEJ (c-NHEJ) factors but recent studies have demonstrated that alt-NHEJ is active even when c-NHEJ as well as homologous recombination is available. The functions of 53BP1 in NHEJ processes are not well understood. Here, we report that 53BP1 promotes DNA double-strand break (DSB) repair and genomic stability not only in c-NHEJ-proficient but also -deficient human G1-phase cells. Using an array of repair substrates we show that these effects of 53BP1 are correlated with a promotion of microhomology-mediated end-joining (MMEJ), a subtype of alt-NHEJ, in G1-phase. Consistent with a specific role in MMEJ we confirm that 53BP1 status does not affect c-NHEJ. 53BP1 supports sequence deletion during MMEJ consistent with a putative role in facilitating end-resection. Interestingly, promotion of MMEJ by 53BP1 in G1-phase cells is only observed in the presence of functional BRCA1. Depletion of both 53BP1 and BRCA1 increases repair needing microhomology usage and augments loss of DNA sequence, suggesting that MMEJ is a highly regulated DSB repair process. Together, these findings significantly expand our understanding of the cell-cycle-dependent roles of 53BP1 in DSB repair.

Highlights

DNA double-strand breaks (DSBs) may cause cell death or genomic instability if not properly repaired [1,2]
Homologous recombination (HR) and classical non-homologous end-joining (c-NHEJ) are the two major pathways for the repair of DSBs. c-NHEJ, which involves direct ligation of the broken DNA ends with or without limited endprocessing, is the main mechanism for DSB repair in the G1-phase of the cell cycle though it can occur in other cell cycle phases as well [3,4,5]. c-NHEJ is mediated by the DNA–PK complex, composed of a heterodimer of the Ku proteins, Ku70 and Ku80, and a catalytic sub-unit, DNA– PKcs
53BP1 is required for repairing irradiated with Gy (IR)-induced DSBs in the G1 phase of the cell cycle

Summary

Introduction

DNA double-strand breaks (DSBs) may cause cell death or genomic instability if not properly repaired [1,2]. AltNHEJ typically requires short patches of perfectly matched sequences known as microhomologies [27,28] This type of rejoining is commonly referred to as microhomologymediated end-joining (MMEJ) not all alt-NHEJ events require microhomology. Alt-NHEJ is associated with the generation of 3 single-strand overhangs which involves the MRE11/RAD50/NBS1 (MRN) complex and CtIP [8,16,18,29,30,31,32]. This repair process typically relies on more extensive processing and sequence deletion than seen with c-NHEJ though the mechanisms and factors involved remain largely unknown

Methods

Results

Conclusion