title: Genomic instability, endoreduplication, and diminished Ig class-switch recombination in B cells lacking Nbs1.
abstract: Mre11, Rad50, and Nbs1 form an evolutionarily conserved protein complex (Mre11-Rad50-Nbs1, MRN) that has been proposed to function as a DNA damage sensor. Hypomorphic mutations in Mre11 and Nbs1 result in the human ataxia-telangiectasia-like disorder and Nijmegen breakage syndrome (NBS), respectively. In contrast, complete inactivation of Mre11, Rad50, or Nbs1 leads to early embryonic lethality, suggesting that the hypomorphic mutations may fail to reveal some of the essential functions of MRN. Here, we use Cre-loxP-mediated recombination to restrict Nbs1 deletion to B lymphocytes. We find that disruption of Nbs1 results in the accumulation of high levels of spontaneous DNA damage, impaired proliferation, and chromosomal endoreduplication. Moreover, we show that Ig class-switch recombination (CSR) is diminished in Nbs1-deficient B cells. The CSR defect is B cell-intrinsic, independent of switch-region transcription, and a consequence of inefficient recombination at the DNA level. Our findings reveal that Nbs1 is critical for efficient Ig CSR and maintenance of the integrity of chromosomal structure and number.
authors.affiliation: Laboratory of Molecular Immunology and Howard Hughes Medical Institute, The Rockefeller University, New York, NY 10021, USA.
authors.forename: Bernardo
authors.initials: B
authors.lastname: Reina-San-Martin
authors.forename: Michel C
authors.initials: MC
authors.lastname: Nussenzweig
authors.forename: André
authors.initials: A
authors.lastname: Nussenzweig
authors.forename: Simone
authors.initials: S
authors.lastname: Difilippantonio
mesh_terms: D000818:Animals; D001402:B-Lymphocytes / Q000166:cytology / Q000276:immunology*; D018929:Cell Culture Techniques; D018797:Cell Cycle Proteins / Q000235:genetics* / Q000276:immunology; D002874:Chromosome Mapping; D004268:DNA-Binding Proteins; D005434:Flow Cytometry; D042822:Genomic Instability / Q000276:immunology*; D007134:Immunoglobulin Switch Region / Q000235:genetics*; D051379:Mice; D018345:Mice, Knockout; D009687:Nuclear Proteins / Q000172:deficiency* / Q000235:genetics* / Q000276:immunology; D016133:Polymerase Chain Reaction; D020133:Reverse Transcriptase Polymerase Chain Reaction; D016196:S Phase
chemical_list: D018797:Cell Cycle Proteins; D004268:DNA-Binding Proteins; C466108:Nijmegen breakage syndrome 1 protein, mouse; D009687:Nuclear Proteins