H2AX Prevents DNA Breaks from Progressing to Chromosome Breaks and Translocations

Abstract

Histone H2AX promotes DNA double-strand break (DSB) repair and immunoglobulin heavy chain (IgH) class switch recombination (CSR) in B-lymphocytes. CSR requires activation-induced cytidine deaminase (AID) and involves joining of DSB intermediates by end joining. We find that AID-dependent IgH locus chromosome breaks occur at high frequency in primary H2AX-deficient B cells activated for CSR and that a substantial proportion of these breaks participate in chromosomal translocations. Moreover, activated B cells deficient for ATM, 53BP1, or MDC1, which interact with H2AX during the DSB response, show similarly increased IgH locus breaks and translocations. Thus, our findings implicate a general role for these factors in promoting end joining and thereby preventing DSBs from progressing into chromosomal breaks and translocations. As cellular p53 status does not markedly influence the frequency of such events, our results also have implications for how p53 and the DSB response machinery cooperate to suppress generation of lymphomas with oncogenic translocations.

AID and RAG1 do not contribute to lymphomagenesis in Eμ c-myc transgenic mice

“…Our results combined with previous results (Ramiro et al, 2004(Ramiro et al, , 2006Franco et al, 2006b;Dorsett et al, 2007) suggest that the primary oncogenic role of AID is to induce c-myc/IgH translocations. Another possibility is that AID does not have an impact on lymphoma development when c-myc/IgH translocation events precede AID expression.…”

“…AID is specifically expressed in germinal center B cells (Muramatsu et al, 2000) and initiates somatic hypermutation and class switch recombination in Ig genes by deaminating cytidines within Ig genes (reviewed in (Di Noia and Neuberger, 2007)). Numerous reports show that c-myc/IgH or bcl-2/IgH translocations are mediated by the RAG proteins (Korsmeyer, 1992;Raghavan et al, 2004) and by the AID enzyme (Ramiro et al, 2004(Ramiro et al, , 2006Franco et al, 2006b;Dorsett et al, 2007), and indeed, these enzymes have been found to mutate or cause DNA breaks outside of Ig genes (Korsmeyer, 1992;Shen et al, 1998;Muschen et al, 2000;Pasqualucci et al, 2001;Franco et al, 2006a) reinforcing the possibility that many other genes are susceptible to AID-and RAG-mediated mutations. Although RAG proteins and AID mediate chromosomal translocations, it has yet to be established whether these enzymes play additional roles in subsequent transformation events.…”

Minding the gap: The underground functions of BRCA1 and BRCA2 at stalled replication forks

“…Phosphorylation of H2AX ("γ-H2AX") is a rapid, evolutionarily conserved event in the DSB response of eukaryotes and marks chromatin for hundreds of kilobases flanking the DSB [129]. H2AX contributes to DSBR, including SCR, by mechanisms that may involve cohesin complexes [46][47][48]130,131]. This "double life" of BRCA1 on ssDNA and on chromatin further underscores the diversity of functions of this protein, which include HR control, transcriptional regulation/gene silencing and DNA damage checkpoint control.…”

Non-homologous end-joining, a sticky affair

“…As the ATM protein kinase is the major H2AX kinase after induction of DSBs, the main function of ATM could be creation of a chromatin environment that facilitates MRN accumulation in nuclear foci. This notion is supported by the observation, that cells deficient in all these repair factors have similar repair defects, most notably increased levels of residual DSBs after ionizing radiation and chromosomal instability (Ward et al, 2003;Franco et al, 2006;Lou et al, 2006;Lavin and Kozlov, 2007).…”