ATM stabilizes DNA double-strand-break complexes during V(D)J recombination

Abstract

The ATM (ataxia-telangiectasia mutated) protein kinase mediates early cellular responses to DNA double-strand breaks (DSBs) generated during metabolic processes or by DNA-damaging agents. ATM deficiency leads to ataxia-telangiectasia, a disease marked by lymphopenia, genomic instability and an increased predisposition to lymphoid malignancies with chromosomal translocations involving lymphocyte antigen receptor loci. ATM activates cell-cycle checkpoints and can induce apoptosis in response to DNA DSBs. However, defects in these pathways of the DNA damage response cannot fully account for the phenotypes of ATM deficiency. Here, we show that ATM also functions directly in the repair of chromosomal DNA DSBs by maintaining DNA ends in repair complexes generated during lymphocyte antigen receptor gene assembly. When coupled with the cell-cycle checkpoint and pro-apoptotic activities of ATM, these findings provide a molecular explanation for the increase in lymphoid tumours with translocations involving antigen receptor loci associated with ataxia-telangiectasia.

Breaking down cell cycle checkpoints and DNA repair during antigen receptor gene assembly

“…Adaptation mutants in yeast eventually die after 10 divisions due to loss of genetic material (Sandell and Zakian, 1993). Exonucleolytic processing of V(D)J recombination coding ends has been observed in ATM À/À lymphocytes (Bredemeyer et al, 2006), and we also noted a striking degradation of chromosomes in approximately 2/3 of the aberrant metaphases (Callen et al, 2007). This was evident by the loss of hybridization of a probe designed to be at least 100 kb downstream of the initial RAG-dependent break sites.…”

“…Consistent with this, RAG is essential for the rapid generation of thymomas in ATM À/À mice that carry TCRa translocations (Petiniot et al, 2000). It has been proposed that ATM activity facilitates NHEJ repair of V(D)J recombination associated breaks by stabilizing DNA ends in the RAG post-synaptic cleavage complex (Bredemeyer et al, 2006;Huang et al, 2007) (Figure 1). …”

“…Foci formation at antigen-receptor loci requires RAG-mediated cleavage in developing lymphocytes and is dependent on AID in cells undergoing CSR (Chen et al, 2000;Petersen et al, 2001). Loss of ATM does not abrogate V(D)J recombination, but significantly impairs the ability to repair coding ends, resulting in the loss of chromosome integrity (Bredemeyer et al, 2006;Matei et al, 2006;Huang et al, 2007;Vacchio et al, 2007). Approximately 10% of primary ATM À/À T cells harbor chromosomal aberrations associated with TCRa loci (Liyanage et al, 2000;Callen et al, 2007;Huang et al, 2007), and these lesions are dependent on RAG-mediated cleavage (Callen et al, 2007).…”

Programmed DNA breaks in lymphoid cells: repair mechanisms and consequences in human disease

“…Hence, ATM maintains DNA ends in repair complexes generated during V(D)J recombination (Fig. 1e) and therefore ATM deficiency leads to instability of post‐cleavage complexes and loss of coding ends from these complexes 14. The MRN complex associates with ATM after induction of DNA damage and is required for ATM activation and recruitment of ATM to DSBs 15.…”

DNA double-strand break repair and development

“…Furthermore, in a main system affected in A-T, the nervous system, ATM primarily appears to function in regulating apoptosis to eliminate cells with DNA damage (Herzog et al, 1998), as also appears to be the case in the immune system Callen et al, 2007). However, recently ATM-dependent phosphorylation of the NHEJ factor Artemis suggests a direct role in repair of some DSBs (Riballo et al, 2004), and V(D)J recombination requires a repair function of ATM (Bredemeyer et al, 2006;Callen et al, 2007), although ATM also has an important apoptotic role in this system (Callen et al, 2007).…”