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

Abstract

Antigen receptor loci contain numerous gene segments that are recombined in response to antigen stimulation. The RAG1/2 complex makes the double-strand breaks that initiate recombination. The ends of these breaks are hairpins that can only be cleaved by the Artemis nuclease. Barry Sleckman and colleagues show that the specificity for Artemis is dictated by the histone variant H2AX, in cooperation with the repair protein MDC1. In the absence of H2AX, another nuclease, CtIP, can open the ends but they are not joined efficiently by classical non-homogeneous end-joining, and this leads to genomic instability. Antigen receptor loci contain numerous gene segments that are recombined in response to antigen stimulation. The RAG endonuclease makes the double-strand breaks that initiate recombination. The ends of these breaks are hairpins that can only be cleaved by the Artemis nuclease. Here, it is shown that the specificity for Artemis is dictated by the histone protein H2AX, in cooperation with the repair protein MDC-1. In the absence of H2AX, another nuclease, CtIP, can open the ends but they are not joined efficiently; this leads to genomic instability. DNA double-strand breaks (DSBs) are generated by the recombination activating gene (RAG) endonuclease in all developing lymphocytes as they assemble antigen receptor genes1. DNA cleavage by RAG occurs only at the G1 phase of the cell cycle and generates two hairpin-sealed DNA (coding) ends that require nucleolytic opening before their repair by classical non-homologous end-joining (NHEJ)1,2,3. Although there are several cellular nucleases that could perform this function, only the Artemis nuclease is able to do so efficiently2,3. 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 CtIP-mediated resection is inhibited by γ-H2AX and by MDC-1 (mediator of DNA damage checkpoint 1), which binds to γ-H2AX in chromatin flanking 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 repair4,5. 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 preserves the structural integrity of broken DNA ends in G1-phase lymphocytes, thereby preventing these DNA ends from accessing repair pathways that promote genomic instability.