Role of MBD4 in double strand break formation during class switch recombination

MBD4 在类别转换重组过程中双链断裂形成中的作用

• 批准号: 8702378
• 负责人: Amy L Kenter
• 金额: $ 23.97万
• 依托单位: UNIVERSITY OF ILLINOIS AT CHICAGO
• 依托单位国家: 美国
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-08-01 至 2016-07-31
• 项目状态: 已结题
• 来源: https://reporter.nih.gov/project-details/8702378
• 关键词: 3' Untranslated Regions; APEXL2 Gene; Alternative Splicing; Antigens; B-Cell Activation; B-Lymphocytes; Base Excision Repairs; Binding; Biological Assay; C-terminal; Cell physiology; Cells; DNA; DNA Damage; DNA Repair; DNA Repair Pathway; DNA lesion; Distal; Distant; Exons; Gene Rearrangement; Genes; Genetic; Genetic Recombination; Genome; Genome Stability; Health; Homologous Gene; Human; IgE; IgG1; IgG3; Immune system; Immunoglobulin A; Immunoglobulin Class Switching; Immunoglobulin M; Immunoglobulin Somatic Hypermutation; Immunoglobulin Switch Recombination; Knock-out; Knockout Mice; Laboratories; Large Intestine Carcinoma; Left; Link; MLH1 gene; MSH2 gene; MSH6 gene; Malignant Neoplasms; Mature B-Lymphocyte; Messenger RNA; Mismatch Repair; Mus; Mutate; Open Reading Frames; PMS2 gene; Pathway interactions; Phenotype; Physiological; Play; Polymerase; Post-Translational Protein Processing; Process; Production; Protein Binding Domain; Protein Isoforms; Proteins; Reagent; Recruitment Activity; Role; Series; Site; System; TimeLine; Transcript; Untranslated Regions; Uracil; Variant; Zebrafish; activation-induced cytidine deaminase; base; demethylation; design; endonuclease; follow-up; homologous recombination; human APEX1 protein; protein complex; response; transition mutation; transversion mutation; tumorigenesis; uracil-DNA glycosylase

DESCRIPTION (provided by applicant): Activation induced deaminase (AID) is essential for both Ig somatic hypermutation (SHM) and class switch recombination (CSR) in mature B cells. AID deaminates dC to dU. There are four uracil DNA glycosylases, UNG, SMUG1, TDG, and methyl binding domain 4 (MBD4), that are capable of recognizing and removing dU in U:G mismatches. Using the base excision repair (BER) pathway, many (but not all) AID induced dU bases can be excised by a uracil DNA glycosylase (UNG) leaving an abasic site that can then be replicated over by error prone polymerase to produce both transition and transversion mutations. Genetic studies show that UNG deficiency in mice and humans leads to loss of class switch recombination (CSR) and impaired somatic hypermutation (SHM). Evidence indicates that TDG cannot substitute for UNG and SMUG1 plays little natural role in these processes since it is poorly expressed in activated B cells. Other U:G mismatches could be substrates for mismatch repair (MMR) MSH2/MSH6 binding which in turn recruit PMS2-MLH1. PMS2 nicks the DNA and contributes to the induction of DSBs in S regions. MBD4 protein was originally discovered by virtue of its interaction with MLH1, a constituent of the MMR protein complex and MMR is deeply involved in CSR and SHM. We were intrigued by the functional association of MBD4 and AID in the context of active DNA demethylation in zebrafish and have sought to explore this potential interaction in mature B cells engaged in CSR. Previous studies focused on Mbd4 knockout mice indicated no phenotype with regard to CSR and SHM. However, we noticed that there are alternative splice variants of Mbd4 mRNA with open reading frames, which would enable expression of the C-terminal end of the protein even when exons 2-5 are deleted. We constructed another knockout in which Mbd4 exons 6-8 and the 3'UTR were deleted in CH12 cells that are normally capable of inducible CSR. Strikingly we found that in Mbd4 deficient CH12 cells, CSR was significantly impaired even while all other criteria for CSR remain intact. Based on these intriguing new studies we propose to more fully examine MBD4 for functional isoforms and to construct a new mouse in which Mbd4 exons 6-8 and the 3'UTR have been deleted by targeted homologous recombination. Follow-up studies will characterize these mice with respect to CSR and SHM. Long term, this mouse will also be used to investigate the involvement of Mbd4 in mismatch repair and genome stability, keys to understanding oncogenesis.

描述（申请人提供）：激活诱导脱氨酶（AID）是成熟B细胞中Ig体细胞超突变（SHM）和类开关重组（CSR）所必需的。AID将dC分解成dU。尿嘧啶DNA糖基酶UNG、SMUG1、TDG和甲基结合结构域4 （MBD4）能够识别和去除U:G错配中的dU。使用碱基切除修复（BER）途径，许多（但不是全部）AID诱导的dU碱基可以被尿嘧啶DNA糖基化酶（UNG）切除，留下一个基本位点，然后可以被容易出错的聚合酶复制，产生转移和翻转突变。遗传学研究表明，小鼠和人类的UNG缺乏会导致类开关重组（CSR）缺失和体细胞超突变（SHM）受损。有证据表明，TDG不能替代UNG， SMUG1在这些过程中发挥的自然作用很小，因为它在活化的B细胞中表达不足。其他U:G错配可能是错配修复（MMR） MSH2/MSH6结合的底物，从而招募PMS2-MLH1。PMS2切割DNA，并有助于S区dsb的诱导。MBD4蛋白最初是通过与MLH1相互作用而被发现的，MLH1是MMR蛋白复合物的一个组成部分，MMR深度参与CSR和SHM。我们对MBD4和AID在斑马鱼活跃DNA去甲基化背景下的功能关联很感兴趣，并试图在参与CSR的成熟B细胞中探索这种潜在的相互作用。先前的研究主要集中在Mbd4敲除小鼠上，没有发现CSR和SHM相关的表型。然而，我们注意到Mbd4 mRNA有一些具有开放阅读框的剪接变体，即使删除了外显子2-5，也能表达该蛋白的c端。我们构建了另一个基因敲除，其中Mbd4外显子6-8和3'UTR在通常能够诱导CSR的CH12细胞中被删除。引人注目的是，我们发现在Mbd4缺陷的CH12细胞中，即使CSR的所有其他标准保持不变，CSR也明显受损。基于这些有趣的新研究，我们建议更全面地研究MBD4的功能异构体，并构建一种新的小鼠，其中MBD4外显子6-8和3'UTR被靶向同源重组删除。后续研究将描述这些小鼠在CSR和SHM方面的特征。从长远来看，这只小鼠还将用于研究Mbd4在错配修复和基因组稳定性中的作用，这是理解肿瘤发生的关键。