Sekine Hideharu, Ferreira Ricardo C, Pan-Hammarström Qiang, Graham Robert R, Ziemba Beth, de Vries Sandra S, Liu Jiabin, Hippen Keli, Koeuth Thearith, Ortmann Ward, Iwahori Akiko, Elliott Margaret K, Offer Steven, Skon Cara, Du Likun, Novitzke Jill, Lee Annette T, Zhao Nianxi, Tompkins Joshua D, Altshuler David, Gregersen Peter K, Cunningham-Rundles Charlotte, Harris Reuben S, Her Chengtao, Nelson David L, Hammarström Lennart, Gilkeson Gary S, Behrens Timothy W
Medical University of South Carolina, Charleston, SC 29425, USA.
Proc Natl Acad Sci U S A. 2007 Apr 24;104(17):7193-8. doi: 10.1073/pnas.0700815104. Epub 2007 Apr 4.
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.
免疫球蛋白类别转换重组（CSR）和体细胞超突变有助于使抗体反应多样化，并由激活诱导的胞苷脱氨酶以及许多参与DNA修复和基因组监测的蛋白质的活性来协调。Msh5是位于主要组织相容性复合体III类区域中心编码的一个基因，其专一的异源二聚体伴侣Msh4在调节减数分裂同源重组中起关键作用，且尚未发现与CSR有关。在此，我们表明携带同基因H-2(b/b)主要组织相容性复合体区间的MRL/lpr小鼠在CSR方面表现出几种异常，包括大多数小鼠中IgG3严重缺乏以及Ig转换（S）连接处存在长微同源性。我们发现Msh5在H-2(b)单倍型上低水平表达，重要的是，在Msh5和Msh4基因敲除小鼠中均观察到类似的长S连接处微同源性表型。我们还提供证据表明MSH5的基因变异与人类IgA缺乏和常见变异免疫缺陷（CVID）相关。鉴定出的一个人类MSH5等位基因包含两个非同义多态性，该等位基因编码的变异蛋白与Msh4的结合受损。与小鼠类似，携带与疾病相关的MSH5等位基因的CVID和IgA缺乏患者的Ig S连接处显示供体/受体微同源性增加，涉及五聚体DNA重复序列，且突变率低于对照组。我们的研究结果表明，Msh4/5异源二聚体有助于CSR，并支持一种模型，即Msh4/5通过经典的非同源末端连接机制促进低或无末端微同源性DNA断裂的修复，同时可能抑制另一种微同源性介导的途径。
