DNA错配修复与肿瘤的发生及治疗

DNA错配修复系统能够识别和纠正错配的DNA碱基对,确保DNA复制过程的保真性.若是错配修复系统存在缺陷将导致基因突变或基因组不稳定性,最终会导致肿瘤的产生.错配修复系统不仅可以通过修复在DNA复制和重组过程中产生的碱基错配来维持基因组的稳定性,还可以通过识别DNA损伤介导细胞的凋亡,所以细胞的错配修复功能与化疗疗效也有密切相关性.错配修复系统对肿瘤诊断、治疗及预后的重要价值决定了它在肿瘤研究中的重要性.     

错配修复蛋白在DNA双链断裂修复中的作用

DNA双链断裂(DSBs)是最严重的DNA损伤之一,近年来受到人们广泛的关注.错配修复(MMR)系统广泛存在于生物体中,是细胞复制后的一种修复方式,通过矫正在DNA复制和重组过程中产生的碱基对错配和小的核苷酸插入或缺失而保持基因组的稳定性.研究发现MMR系统在DSBs修复中起着重要的作用,MMR蛋白通过与同源重组(HR)和非同源末端连接(NHEJ)修复相互作用参与DSBs修复.本文重点关注MMR通路几种关键蛋白在DSBs修复中的作用.     

DNA损伤修复基本方式的研究进展

DNA损伤修复基因可修复由不同原因导致的DNA损伤 ,从而保护遗传信息的完整性。DNA损伤修复有 3种基本形式 ,即碱基切除修复、核苷酸切除修复和错配修复。本文综述了DNA损伤修复 3种基本形式的研究进展情况并讨论了DNA链断裂重组和重接合修复及DNA聚合酶绕道修复DNA损伤     

错配修复研究新进展

错配修复是细胞复制后的一种修复机制,对维持细胞遗传稳定起重要作用。近年来,对错配修复的研究进展飞速。目前,已在人类细胞中发现若干与细菌、真菌错配修复基因高度同源的人类错配修复基因,并在许多肿瘤中证实有错配修复功能的缺陷。本文主要介绍近年来错配修复研究的一些新进展。     

错配修复基因hMSH2研究的新进展

错配修复基因是最近在遗传性非息肉性大肠癌 (HNPPC)中分离到一组遗传易感基因 ,该系统中任一基因突变 ,都会导致细胞错配修复功能缺陷 ,结果产生遗传不稳定 ,表现为复制错误或微卫星不稳定 ,因而容易发生肿瘤。近年来对错配修复基因的研究取得了很大的进展 ,其与 HNPPC之间的研究己有报道 ;但该家族中错配修复基因 h MSH2的研究则刚刚开展 ,本文就 h MSH2基因的分子生物学特性、作用机制、与肿瘤关系的研究及其存在的问题等方面的新进展作一简要综述     

胃癌组织微卫星DNA不稳定性的研究进展

由错配修复基因缺陷引起的微卫星DNA不稳定性是继发现癌基因激活、抑癌基因失活之后的又一癌变机制,微卫星不稳定性胃癌具有不同的临床病理生物学行为.胃癌组织微卫星不稳定性的深入研究将对揭示胃癌的发病机制、胃癌的诊断治疗和预后将产生积极的作用.     

胃癌细胞微卫生DNA不稳定性的研究进展

由错配修复基因缺陷引起的微卫星DNA不稳定性是继发现癌基因激活、抑癌基因失活之后的又一癌变机制,微卫星不稳定性胃癌具有不同的临床病理生物学行为.胃癌组织微卫星不稳定性的深入研究将对揭示胃癌的发病机制、胃癌的诊断治疗和预后将产生积极的作用.     

错配修复系统的生物学效应和机理

在ＤＮＡ正常代谢中，碱基错配、插入或缺失可导致基因组ＤＮＡ复制错误。已证明在细菌、酵母及高等真核细胞中，都存在错配修复系统。这方面的研究已取得了一些进展，包括Ｅ．ｃｏｌｉ．中的Ｍｕｔ ＨＬＳ修复系统，酵母中的错配修主其蛋白以及真核细胞中的错配修复基因及蛋白。这对消除ＤＮＡ生物合成错误，增加染色体复制的可信性，防止自由突出以及肿瘤的发生发展、诊断和治疗有着重要意义。     

微卫星DNA不稳定性及其在星形细胞肿瘤中的意义

星形细胞肿瘤中微卫星DNA不稳定性的出现及其频率取决于肿瘤的恶性程度.恶性程度高的星形细胞肿瘤常可检出MSI,且伴有错配修复基因的突变率也较高.某些MSI位点可作为间变性星形细胞瘤预后的一个指征.由于某些错配修复基因的缺陷导致微卫星DNA不稳定性,基因组不稳定性的影响在星形细胞肿瘤的发生、发展过程中具有重要意义.     

关于错配修复基因蛋白表达的检测对结肠癌患者预后价值的临床意义研究

目的 探讨错配修复基因1(human mutl homolog 1,hMLH1)与错配修复基因2(human mutl homolog 2,hMLH2)以及错配修复基因6(human mutl homolog 6,hMLH6)三种错配修复基因蛋白表达水平对直肠癌手术治疗后短期随访肿瘤淋巴转移预后的临床意义.方法 选取本院2015年1月至2016年12月确诊结肠癌患者60例,均接受手术治疗;采用免疫组织化学SP法检测患者肿瘤组织hMLH1、hMLH2、hMLH6蛋白的表达水平,并分析与术后随访半年肿瘤淋巴转移预后的关联情况.结果 研究中男性hMLH1与hMLH2以及hMLH6蛋白表达缺失率分别为45.00%,47.50%和52.50%;女性hMLH1与hMLH2以及hMLH6蛋白表达缺失率分别为50.00%,45.00%和65.00%.三种蛋白表达缺失率在不同分化程度肿瘤中的差别无统计学意义.hMLH1蛋白表达缺失在转移者中的比例(27.27%)小于无淋巴转移的患者(70.37%), P<0.05;hMLH2蛋白表达缺失率转移组患者低于无转移组,P<0.05;而hMLH6蛋白表达缺失率在转移与非转移患者中差别未见统计学意义.结论 hMLH1与hMLH2蛋白表达缺失患者淋巴转移比例低,预后相对良好.     

Genome instability and DNA damage accumulation in gene-targeted mice

Six major pathways for DNA repair have been identified. These include (1) DNA repair by direct reversal, (2) base excision repair, (3) mismatch repair, (4) nucleotide excision repair, (5) homologous recombination, and (6) non-homologous end-joining. In addition, several other cellular processes influence the response to DNA damage. The generation of gene-targeted organisms is crucial for assessing the relative contribution of single DNA repair proteins and DNA repair pathways in maintaining genome stability. In particular, the accumulation of DNA damage, mutations and cancer in unexposed gene-targeted animals illuminates the spontaneous load of a particular lesion and the relative significance of a single gene in a specific pathway. Strategies for the generation of gene-targeted mice have been available for 15 years and more than 100 different genes relevant to DNA repair have been targeted. This review describes some important progress made toward understanding spontaneous DNA damage and its repair, exemplified through one, or a few, gene-targeted mice from each major DNA repair pathway.     

Refining the Neuberger model: Uracil processing by activated B cells

During the immune response, B cells undergo a programed mutagenic cascade to promote increased affinity and expanded antibody function. The two processes, somatic hypermutation (SHM) and class switch recombination (CSR), are initiated by the protein activation‐induced deaminase (AID), which converts cytosine to uracil in the immunoglobulin loci. The presence of uracil in DNA promotes DNA mutagenesis though a subset of DNA repair proteins. Two distinct mechanisms have been proposed to control uracil processing. The first is through base removal by uracil DNA glycosylase (UNG), and the second is through detection by the mismatch repair (MMR) complex MSH2/6. In a study published in this issue of European Journal of Immunology, Dingler et al. [Eur. J. Immunol. 2014. 44: 1925‐1935] examine uracil processing in B cells in the absence of UNG and SMUG1 glycosylases. Similar to UNG, SMUG1 is an uracil glycosylase which can remove the uracil base. While Smug1^?/? mice show no clear deficiency in SHM or CSR, Ung^?/?Smug1^?/? mice display exacerbated phenotypes, suggesting a back‐up role for SMUG1 in antibody diversity. This new information expands the model of uracil processing in B cells and raises several interesting questions about the dynamic relationship between base excision repair and MMR.     

Role of DNA mismatch repair in apoptotic responses to therapeutic agents

Deficiencies in DNA mismatch repair (MMR) have been found in both hereditary cancer (i.e., hereditary nonpolyposis colorectal cancer) and sporadic cancers of various tissues. In addition to its primary roles in the correction of DNA replication errors and suppression of recombination, research in the last 10 years has shown that MMR is involved in many other processes, such as interaction with other DNA repair pathways, cell cycle checkpoint regulation, and apoptosis. Indeed, a cell's MMR status can influence its response to a wide variety of chemotherapeutic agents, such as temozolomide (and many other methylating agents), 6-thioguanine, cisplatin, ionizing radiation, etoposide, and 5-fluorouracil. For this reason, identification of a tumor's MMR deficiency (as indicated by the presence of microsatellite instability) is being utilized more and more as a prognostic indicator in the clinic. Here, we describe the basic mechanisms of MMR and apoptosis and investigate the literature examining the influence of MMR status on the apoptotic response following treatment with various therapeutic agents. Furthermore, using isogenic MMR-deficient (HCT116) and MMR-proficient (HCT116 3-6) cells, we demonstrate that there is no enhanced apoptosis in MMR-proficient cells following treatment with 5-fluoro-2'-deoxyuridine. In fact, apoptosis accounts for only a small portion of the induced cell death response.     

Mismatch repair in Gram-positive bacteria

DNA mismatch repair (MMR) is responsible for correcting errors formed during DNA replication. DNA polymerase errors include base mismatches and extra helical nucleotides referred to as insertion and deletion loops. In bacteria, MMR increases the fidelity of the chromosomal DNA replication pathway approximately 100-fold. MMR defects in bacteria reduce replication fidelity and have the potential to affect fitness. In mammals, MMR defects are characterized by an increase in mutation rate and by microsatellite instability. In this review, we discuss current advances in understanding how MMR functions in bacteria lacking the MutH and Dam methylase-dependent MMR pathway.     

Functional interrogation of Lynch syndrome‐associated MSH2 missense variants via CRISPR‐Cas9 gene editing in human embryonic stem cells

Lynch syndrome (LS) predisposes patients to cancer and is caused by germline mutations in the DNA mismatch repair (MMR) genes. Identifying the deleterious mutation, such as a frameshift or nonsense mutation, is important for confirming an LS diagnosis. However, discovery of a missense variant is often inconclusive. The effects of these variants of uncertain significance (VUS) on disease pathogenesis are unclear, though understanding their impact on protein function can help determine their significance. Laboratory functional studies performed to date have been limited by their artificial nature. We report here an in‐cellulo functional assay in which we engineered site‐specific MSH2 VUS using clustered regularly interspaced short palindromic repeats‐Cas9 gene editing in human embryonic stem cells. This approach introduces the variant into the endogenous MSH2 loci, while simultaneously eliminating the wild‐type gene. We characterized the impact of the variants on cellular MMR functions including DNA damage response signaling and the repair of DNA microsatellites. We classified the MMR functional capability of eight of 10 VUS providing valuable information for determining their likelihood of being bona fide pathogenic LS variants. This human cell‐based assay system for functional testing of MMR gene VUS will facilitate the identification of high‐risk LS patients.     

Functions of the MMR system and special roles of mutL in bacterial evolution

DNA mismatch repair guards the integrity of the genome of almost all organisms by correcting DNA biosynthetic errors and by ensuring the fidelity of homologous genetic recombination. MutL is one of the important proteins involved in mismatch repair system. It has been suggested to function as a master coordinator or molecular matchmaker because it interacts physically with MutS, the endonuclease MutH, and DNA helicase UvrD. It also binds to DNA and has an ATPase activity. MutL defective bacteria strains have elevated mutation rates and it has been reported recently that MutL defect may have an important impact on bacterial evolution.     

DNA polymerase delta in dna replication and genome maintenance

The eukaryotic genome is in a constant state of modification and repair. Faithful transmission of the genomic information from parent to daughter cells depends upon an extensive system of surveillance, signaling, and DNA repair, as well as accurate synthesis of DNA during replication. Often, replicative synthesis occurs over regions of DNA that have not yet been repaired, presenting further challenges to genomic stability. DNA polymerase δ (pol δ) occupies a central role in all of these processes: catalyzing the accurate replication of a majority of the genome, participating in several DNA repair synthetic pathways, and contributing structurally to the accurate bypass of problematic lesions during translesion synthesis. The concerted actions of pol δ on the lagging strand, pol ? on the leading strand, associated replicative factors, and the mismatch repair (MMR) proteins results in a mutation rate of less than one misincorporation per genome per replication cycle. This low mutation rate provides a high level of protection against genetic defects during development and may prevent the initiation of malignancies in somatic cells. This review explores the role of pol δ in replication fidelity and genome maintenance. Environ. Mol. Mutagen. 2012. © 2012 Wiley Periodicals, Inc.     

The utility of immunohistochemical detection of DNA mismatch repair gene proteins

Since the development of monoclonal antibodies against the MSH2 protein by Leach et al. in 1996, a series of investigations has been undertaken to determine the utility of immunohistochemical detection of DNA mismatch repair (MMR) gene proteins in the identification of hereditary or sporadic colorectal tumors with microsatellite instability. These studies, however, have been performed with different aims and on different patient populations. Interpretation of these immunohistochemical data relies on a thorough understanding of the biological and technical factors that affect the detection of MMR proteins. In this review, we analyze the data from the published research studies, pointing out the various factors affecting immunohistochemical detection of MMR proteins and projecting the utility of immunohistochemistry in different clinical settings.