The multifunctional Mre11-Rad50-Nbs1 (MRN) protein complex recruits ATM/Tel1 checkpoint kinase and CtIP/Ctp1 homologous recombination (HR) repair factor to double-strand breaks (DSBs). HR repair commences with the 5′-to-3′ resection of DNA ends, generating 3′ single-strand DNA (ssDNA) overhangs that bind Replication Protein A (RPA) complex, followed by Rad51 recombinase. In Saccharomyces cerevisiae, the Mre11-Rad50-Xrs2 (MRX) complex is critical for DSB resection, although the enigmatic ssDNA endonuclease activity of Mre11 and the DNA-end processing factor Sae2 (CtIP/Ctp1 ortholog) are largely unnecessary unless the resection activities of Exo1 and Sgs1-Dna2 are also eliminated. Mre11 nuclease activity and Ctp1/CtIP are essential for DSB repair in Schizosaccharomyces pombe and mammals. To investigate DNA end resection in Schizo. pombe, we adapted an assay that directly measures ssDNA formation at a defined DSB. We found that Mre11 and Ctp1 are essential for the efficient initiation of resection, consistent with their equally crucial roles in DSB repair. Exo1 is largely responsible for extended resection up to 3.1 kb from a DSB, with an activity dependent on Rqh1 (Sgs1) DNA helicase having a minor role. Despite its critical function in DSB repair, Mre11 nuclease activity is not required for resection in fission yeast. However, Mre11 nuclease and Ctp1 are required to disassociate the MRN complex and the Ku70-Ku80 nonhomologous end-joining (NHEJ) complex from DSBs, which is required for efficient RPA localization. Eliminating Ku makes Mre11 nuclease activity dispensable for MRN disassociation and RPA localization, while improving repair of a one-ended DSB formed by replication fork collapse. From these data we propose that release of the MRN complex and Ku from DNA ends by Mre11 nuclease activity and Ctp1 is a critical step required to expose ssDNA for RPA localization and ensuing HR repair.
Somatic hypermutation of Ig genes enables B cells of the germinal center to generate high-affinity immunoglobulin variants. Key intermediates in somatic hypermutation are deoxyuridine lesions, introduced by activation-induced cytidine deaminase. These lesions can be processed further to abasic sites by uracil DNA glycosylase. Mutagenic replication of deoxyuridine, or of its abasic derivative, by translesion synthesis polymerases is hypothesized to underlie somatic hypermutation.Rev1 is a translesion synthesis polymerase that in vitro incorporates uniquely deoxycytidine opposite deoxyuridine and abasic residues. To investigate a role of Rev1 in mammalian somatic hypermutation we have generated mice deficient for Rev1. Although Rev1−/− mice display transient growth retardation, proliferation of Rev1−/− LPS-stimulated B cells is indistinguishable from wild-type cells. In mutated Ig genes from Rev1−/− mice, C to G transversions were virtually absent in the nontranscribed (coding) strand and reduced in the transcribed strand. This defect is associated with an increase of A to T, C to A, and T to C substitutions. These results indicate that Rev1 incorporates deoxycytidine residues, most likely opposite abasic nucleotides, during somatic hypermutation. In addition, loss of Rev1 causes compensatory increase in mutagenesis by other translesion synthesis polymerases.
B cells use translesion DNA synthesis (TLS) to introduce somatic mutations around genetic lesions caused by activation-induced cytidine deaminase. Monoubiquitination at lysine164 of proliferating cell nuclear antigen (PCNAK164) stimulates TLS. To determine the role of PCNAK164 modifications in somatic hypermutation, PCNAK164R knock-in mice were generated. PCNAK164R/K164R mutants are born at a sub-Mendelian frequency. Although PCNAK164R/K164R B cells proliferate and class switch normally, the mutation spectrum of hypermutated immunoglobulin (Ig) genes alters dramatically. A strong reduction of mutations at template A/T is associated with a compensatory increase at G/C, which is a phenotype similar to polymerase η (Polη) and mismatch repair–deficient B cells. Mismatch recognition, monoubiquitinated PCNA, and Polη likely cooperate in establishing mutations at template A/T during replication of Ig genes.
Studies on human B cells have featured CD27 as a marker and mediator of the B cell response. We have studied CD27 expression and function on B cells in the mouse. We find that B cells acquire CD27 at the centroblast stage and lose it progressively upon further differentiation. It is not a marker for somatically mutated B cells and is present at very low frequency on memory B cells. Enrichment of CD27 among centroblasts and the presence of its ligand CD70 on occasional T and B cells in or near germinal centers (GCs) suggested a role for CD27/CD70 interactions in clonal B cell expansion. Accordingly, GC formation in response to influenza virus infection was delayed in CD27 knockout mice. CD27 deficiency did not affect somatic hypermutation or serum levels of virus-specific IgM, IgG, and IgA attained in primary and recall responses. Adoptive transfer of T and B cells into CD27/CD28−/− mice revealed that CD27 promotes GC formation and consequent IgG production by two distinct mechanisms. Stimulation of CD27 on B cells by CD28+ Th cells accelerates GC formation, most likely by promoting centroblast expansion. In addition, CD27 on T cells can partially substitute for CD28 in supporting GC formation.
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