H2AX prevents CtIP- mediated DNA end resection and aberrant repair in G1-phase lymphocytes

摘要

DNA double-strand breaks (DSBs) are generated by the recombina-tion activating gene (RAG) endonuclease in all developing lym-phocytes asthey assemble antigen receptor genes. DNA cleavage by RAG occurs only at the Gl phase of the cell cycle and generates two hairpin-sealed DNA (coding) ends that require nudeolytic opening before their repair by classical non-homologous end-joining (NHEJ). Although there are several cellular nucleuses that could perform this function, only the Artemis nudease is able to do so efficiently. Here, in vivo, we show that in murine cells the histone protein H2AX prevents nucleases other than Artemis from processing hairpin-sealed coding ends; in the absence of H2AX, CtIP can efficiently promote the hairpin opening and resection of DNA ends generated by RAG cleavage. This CtlP-mediated resec-tion is inhibited by γ-H2AX and by MDC-1 (mediator of DNA damage checkpoint 1), which binds to γ-H2AX in chromatin flank-ing DNA DSBs. Moreover, the ataxia telangiectasia mutated (ATM) kinase activates antagonistic pathways that modulate this resection. CtIP DNA end resection activity is normally limited to cells at post-replicative stages of the cell cycle, in which it is essential for homology-mediated repair. In G1-phase lymphocytes, DNA ends that are processed by CtIP are not efficiently joined by classical NHEJ and the joints that do form frequently use micro-homologies and show significant chromosomal deletions. Thus, H2AX pre-serves the structural integrity of broken DNA ends in Gl-phase lymphocytes, thereby preventing these DNA ends from accessing repair pathways that promote genomic instability.