Role for Msh5 in the regulation of Ig class switch recombination

Abstract

Ig class switch recombination (CSR) and somatic hypermutation serve to diversify antibody responses and are orchestrated by the activity of activation-induced cytidine deaminase and many proteins involved in DNA repair and genome surveillance. Msh5, a gene encoded in the central MHC class III region, and its obligate heterodimerization partner Msh4 have a critical role in regulating meiotic homologous recombination and have not been implicated in CSR. Here, we show that MRL/lpr mice carrying a congenic H-2 b/b MHC interval exhibit several abnormalities regarding CSR, including a profound deficiency of IgG3 in most mice and long microhomologies at Ig switch (S) joints. We found that Msh5 is expressed at low levels on the H-2 b haplotype and, importantly, a similar long S joint microhomology phenotype was observed in both Msh5 and Msh4-null mice. We also present evidence that genetic variation in MSH5 is associated with IgA deficiency and common variable immune deficiency (CVID) in humans. One of the human MSH5 alleles identified contains two nonsynonymous polymorphisms, and the variant protein encoded by this allele shows impaired binding to MSH4. Similar to the mice, Ig S joints from CVID and IgA deficiency patients carrying disease-associated MSH5 alleles show increased donor/acceptor microhomology, involving pentameric DNA repeat sequences and lower mutation rates than controls. Our findings suggest that Msh4/5 heterodimers contribute to CSR and support a model whereby Msh4/5 promotes the resolution of DNA breaks with low or no terminal microhomology by a classical nonhomologous end-joining mechanism while possibly suppressing an alternative microhomology-mediated pathway.immunoglobulin subclass deficiency ͉ mismatch repair ͉ Msh4 A fter appropriate stimulation, B cells undergo class switch recombination (CSR), whereby the functionally rearranged V(D)J DNA segment is recombined with a downstream Ig constant region segment. The biochemistry of CSR is complex and involves the B cell-specific gene activation-induced cytidine deaminase, which initiates both CSR and somatic hypermutation (1). CSR also requires many ubiquitously expressed genes important for detecting DNA mismatches and breaks and regulating DNA repair (2). CSR occurs at specific DNA segments called switch (S) regions, which lie upstream of each constant region and contain hotspots for activation-induced cytidine deaminase-mediated cytosine deamination. The ligation of the S region with the downstream S regions is carried out by protein factors that comprise the nonhomologous end joining machinery for DNA repair (1, 2).Mismatch repair proteins play a critical role in safeguarding genetic stability. The key proteins for initiation of eukaryotic mismatch repair are homologues of bacterial MutS and MutL. In mammals, there are five MutS (Msh2, Msh3, Msh4, Msh5, and Msh6) and four MutL (Mlh1, Mlh3, Pms1, and Pms2) homologues. Each Mut homologue acts at the DNA repair or recombination site by forming heterodimers; Msh2-Msh6 (MutS␣), Msh2-Msh3 (MutS␤), M...

Alternative End-Joining and Classical Nonhomologous End-Joining Pathways Repair Different Types of Double-Strand Breaks during Class-Switch Recombination

“…In the mouse, this would be maximal for IgA and minimal for IgG1, with 14% identity between Sm and Sg1 and 33% identity between Sm and Sa (19,60). Indeed, when we analyzed Sm-Sg1 switch joins from Aicda +/2 mice, the increase in microhomology use was less evident (data not shown), in line with previous reports in human B cells (61)(62)(63). A recent report showing that CtIP does not affect class switching to IgG1 in mouse splenic B cells is also compatible with these conclusions (64).…”

Antibody deficiency diseases

“…Ig switch region joints from IgAD and CVID patients carrying the disease-associated MSH5 alleles show an increased donor/acceptor microhomology, involving pentameric DNA repeat sequences, and lower mutation rates than controls. These findings suggest that MSH4/5 heterodimers contribute to CSR and support a model whereby MSH4/5 promotes the resolution of DNA breaks with low or no terminal microhomology by a classical non-homologous end-joining mechanism and that defects in MSH5 may thus result in immunodeficiency [33]. These mutations are carried on the ancestral HLA B14, DR1 haplotype and although this haplotype has previously been shown to be associated with IgAD, it nevertheless accounts only for a small proportion of cases (1-2%).…”

Mapping of multiple susceptibility variants within the MHC region for 7 immune-mediated diseases

“…The peak primary association in the current study is to the HLA-DQB1*0201 allele, with a significant secondary association with HLA-DRB1*1501 that is protective, and other putative risk alleles in the class II and class III regions. Recently, mutations in MSH5, encoded within the class III region, affecting classswitch recombination and associated with the HLA-B14-DR1 haplotype, have been suggested to constitute a primary risk factor for IgAD (47). However, no mutations in MSH5 were identified on the HLA-B8-DR3 susceptibility haplotype, suggesting a more complex MHC etiology in this disease.…”