A regulatory role for NBS1 in strand-specific mutagenesis during somatic hypermutation

• Published in: PloS one 2008-06, Vol.3 (6), p.e2482-e2482
• Main Authors: Du, Likun, Dunn-Walters, Deborah K, Chrzanowska, Krystyna H, Stankovic, Tanja, Kotnis, Ashwin, Li, Xin, Lu, Jiayi, Eggertsen, Gösta, Brittain, Claire, Popov, Sergey W, Gennery, Andrew R, Taylor, A Malcolm R, Pan-Hammarström, Qiang
• Format: Article
• Language: English
• Subjects: Activation-induced cytidine deaminase; Anemia; Ataxia; Ataxia telangiectasia; Ataxia Telangiectasia - genetics; B cells; Base excision repair; Base Sequence; Breakage; Cancer; Cell cycle; Cell Cycle Proteins - genetics; Cell Cycle Proteins - physiology; Cytidine deaminase; Deamination; Deoxyribonucleic acid; DNA; DNA glycosylase; DNA Primers; Endonuclease; Gene mutation; Genes; Humans; Immunoglobulins; Immunology; Immunology/Immune Response; Immunology/Leukocyte Development; In vivo methods and tests; Integrated software; Introns; Laboratories; Lymphocytes B; Medicin och hälsovetenskap; Medicine; Mismatch repair; MRE11 protein; Mutagenesis; Mutation; Nijmegen breakage syndrome; Nuclear Proteins - genetics; Nuclear Proteins - physiology; Nuclease; Nucleases; Patients; Polymerase Chain Reaction; Pyrimidines; Repair; RNA, Messenger - genetics; Somatic hypermutation; Studies; T cell receptors; Uracil; Uracil-DNA glycosidase
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0002482

Activation-induced cytidine deaminase (AID) is believed to initiate somatic hypermutation (SHM) by deamination of deoxycytidines to deoxyuridines within the immunoglobulin variable regions genes. The deaminated bases can subsequently be replicated over, processed by base excision repair or mismatch repair, leading to introduction of different types of point mutations (G/C transitions, G/C transversions and A/T mutations). It is evident that the base excision repair pathway is largely dependent on uracil-DNA glycosylase (UNG) through its uracil excision activity. It is not known, however, which endonuclease acts in the step immediately downstream of UNG, i.e. that cleaves at the abasic sites generated by the latter. Two candidates have been proposed, an apurinic/apyrimidinic endonuclease (APE) and the Mre11-Rad50-NBS1 complex. The latter is intriguing as this might explain how the mutagenic pathway is primed during SHM. We have investigated the latter possibility by studying the in vivo SHM pattern in B cells from ataxia-telangiectasia-like disorder (Mre11 deficient) and Nijmegen breakage syndrome (NBS1 deficient) patients. Our results show that, although the pattern of mutations in the variable heavy chain (V(H)) genes was altered in NBS1 deficient patients, with a significantly increased number of G (but not C) transversions occurring in the SHM and/or AID targeting hotspots, the general pattern of mutations in the V(H) genes in Mre11 deficient patients was only slightly altered, with an increased frequency of A to C transversions. The Mre11-Rad50-NBS1 complex is thus unlikely to be the major nuclease involved in cleavage of the abasic sites during SHM, whereas NBS1 might have a specific role in regulating the strand-biased repair during phase Ib mutagenesis.