Defective signal joint recombination in fanconi anemia fibroblasts reveals a role for Rad50 in V(D)J recombination

Primary immunodeficiencies associated with DNA-repair disorders

DNA-repair pathways recognise and repair DNA damaged by exogenous and endogenous agents to maintain genomic integrity. Defects in these pathways lead to replication errors, loss or rearrangement of genomic material and eventually cell death or carcinogenesis. The creation of diverse lymphocyte receptors to identify potential pathogens requires breaking and randomly resorting gene segments encoding antigen receptors. Subsequent repair of the gene segments utilises ubiquitous DNA-repair proteins. Individuals with defective repair pathways are found to be immunodeficient and many are radiosensitive. The role of repair proteins in the development of adaptive immunity by VDJ recombination, antibody isotype class switching and affinity maturation by somatic hypermutation has become clearer over the past few years, partly because of identification of the genes involved in human disease. We describe the mechanisms involved in the development of adaptive immunity relating to DNA repair, and the clinical consequences and treatment of the primary immunodeficiency resulting from such defects.

Cellular and molecular consequences of defective Fanconi anemia proteins in replication-coupled DNA repair: mechanistic insights

The Fanconi anemia (FA) molecular network consists of 15 "FANC" proteins, of which 13 are associated with mutations in patients with this cancer-prone chromosome instability disorder. Whereas historically the common phenotype associated with FA mutations is marked sensitivity to DNA interstrand crosslinking agents, the literature supports a more global role for FANC proteins in coping with diverse stresses encountered by replicative polymerases. We have attempted to reconcile and integrate numerous observations into a model in which FANC proteins coordinate the following physiological events during DNA crosslink repair: (a) activating a FANCM-ATR-dependent S-phase checkpoint, (b) mediating enzymatic replication-fork breakage and crosslink unhooking, (c) filling the resulting gap by translesion synthesis (TLS) by error-prone polymerase(s), and (d) restoring the resulting one-ended double-strand break by homologous recombination repair (HRR). The FANC core subcomplex (FANCA, B, C, E, F, G, L, FAAP100) promotes TLS for both crosslink and non-crosslink damage such as spontaneous oxidative base damage, UV-C photoproducts, and alkylated bases. TLS likely helps prevent stalled replication forks from breaking, thereby maintaining chromosome continuity. Diverse DNA damages and replication inhibitors result in monoubiquitination of the FANCD2-FANCI complex by the FANCL ubiquitin ligase activity of the core subcomplex upon its recruitment to chromatin by the FANCM-FAAP24 heterodimeric translocase. We speculate that this translocase activity acts as the primary damage sensor and helps remodel blocked replication forks to facilitate checkpoint activation and repair. Monoubiquitination of FANCD2-FANCI is needed for promoting HRR, in which the FANCD1/BRCA2 and FANCN/PALB2 proteins act at an early step. We conclude that the core subcomplex is required for both TLS and HRR occurring separately for non-crosslink damages and for both events during crosslink repair. The FANCJ/BRIP1/BACH1 helicase functions in association with BRCA1 and may remove structural barriers to replication, such as guanine quadruplex structures, and/or assist in crosslink unhooking.

Roles for NBS1 in alternative nonhomologous end-joining of V(D)J recombination intermediates

Recent work has highlighted the importance of alternative, error-prone mechanisms for joining DNA double-strand breaks (DSBs) in mammalian cells. These noncanonical, nonhomologous end-joining (NHEJ) pathways threaten genomic stability but remain poorly characterized. The RAG postcleavage complex normally prevents V(D)J recombination-associated DSBs from accessing alternative NHEJ. Because the MRE11/RAD50/NBS1 complex localizes to RAG-mediated DSBs and possesses DNA end tethering, processing, and joining activities, we asked whether it plays a role in the mechanism of alternative NHEJ or participates in regulating access of DSBs to alternative repair pathways. We find that NBS1 is required for alternative NHEJ of hairpin coding ends, suppresses alternative NHEJ of signal ends, and promotes proper resolution of inversional recombination intermediates. These data demonstrate that the MRE11 complex functions at two distinct levels, regulating repair pathway choice (likely through enhancing the stability of DNA end complexes) and participating in alternative NHEJ of coding ends.

MRN complex function in the repair of chromosomal Rag-mediated DNA double-strand breaks

The Mre11–Rad50–Nbs1 (MRN) complex functions in the repair of DNA double-strand breaks (DSBs) by homologous recombination (HR) at postreplicative stages of the cell cycle. During HR, the MRN complex functions directly in the repair of DNA DSBs and in the initiation of DSB responses through activation of the ataxia telangiectasia-mutated (ATM) serine-threonine kinase. Whether MRN functions in DNA damage responses before DNA replication in G0/G1 phase cells has been less clear. In developing G1-phase lymphocytes, DNA DSBs are generated by the Rag endonuclease and repaired during the assembly of antigen receptor genes by the process of V(D)J recombination. Mice and humans deficient in MRN function exhibit lymphoid phenotypes that are suggestive of defects in V(D)J recombination. We show that during V(D)J recombination, MRN deficiency leads to the aberrant joining of Rag DSBs and to the accumulation of unrepaired coding ends, thus establishing a functional role for MRN in the repair of Rag-mediated DNA DSBs. Moreover, these defects in V(D)J recombination are remarkably similar to those observed in ATM-deficient lymphocytes, suggesting that ATM and MRN function in the same DNA DSB response pathways during lymphocyte antigen receptor gene assembly.