DNA错配修复、染色体不稳定和肿瘤的关系

DNA错配修复系统可以识别并纠正DNA复制过程中出现的错误.保证基因组的稳定性和完整性.错配修复系统缺陷可能导致遗传物质发生突变,引发恶性肿瘤.肿瘤患者经常表现出染色体不稳定,具体表现为微卫星不稳定性和杂合性缺失.本文就DNA错配修复、染色体不稳定和肿瘤之间的联系予以综述.     

一组多功能的错配修复蛋白

错配修复蛋白是DNA错配修复系统中主要功能蛋白质,主要参与DNA复制过程中对错配碱基的识别和修复.近年来研究表明错配修复蛋白还参与DNA损伤信号的传递、细胞周期的调控、减数分裂和有丝分裂等.错配修复蛋白缺陷会增加患肿瘤的危险性或者直接导致肿瘤;由于错配修复蛋白参与了DNA损伤信号传递、周期调控,错配修复蛋白缺陷还会导致细胞对相关抗癌药物产生耐受.     

错配修复基因与涎腺肿瘤的关系

错配修复(mismatch repair,MMR)是DNA复制后的一种修复机制,对维持基因组稳定起重要作用.错配修复基因功能缺陷是继癌基因激活、抑癌基因失活之后又一肿瘤的发生、发展机制,错配修复基因的异常表达与全身多种肿瘤相关.涎腺肿瘤为口腔颌面部常见肿瘤之一,具有与其他系统肿瘤相似的组织学类型,多来源于肌上皮.近年来,有关涎腺肿瘤与错配修复基因的关系正逐步成为研究热点,本文就错配修复基因的组成、作用机制以及与涎腺肿瘤发生、发展的关系作一综述.     

DNA错配修复与肺癌

肺癌是目前世界上最常见的恶性肿瘤之一,虽然近年来对其研究较多,但其发生发展的确切机制仍不清楚。DNA错配修复作为一种重要的复制后修复系统,在确保DNA复制保真性、控制基因突变和维持基因组稳定等方面具有重要作用。近年研究表明,DNA错配修复系统与肺癌的发生、治疗及预后判断有着密切关系。本文主要对DNA错配修复系统在肺癌中的研究进展作一简要综述。     

错配修复蛋白MutS的新功能位点

MutS蛋白是DNA错配修复系统的关键成份,其突变会使细胞失去正常的错配修复功能,导致基因组不稳定和细胞异常.本研究利用易错PCR随机突变和利福平筛选,建立了研究MutS蛋白的新方法,发现影响MutS错配修复功能的新位点,并利用表面等离子共振、分子筛、farwestern等方法对错配修复功能缺陷的突变体进行了活性测定和分析;通过揭示MutS与错配修复功能相关的新信息,为MutS同源物多态性的研究及人源MutS同源物突变与癌症相关的研究提供新的线索.     

序言

<正>基因组不稳定是人体衰老、细胞癌变及其他一些疾病发生的根本原因之一。DNA代谢异常和DNA损伤应答缺陷是导致基因组不稳定的两个主要原因。DNA代谢包括DNA复制、重组和修复,它们是细胞最基本的生命活动。同时,通过亿万年的进化,细胞已进化出一套严格的DNA复制应急及损伤应答调控系统,以维持DNA序列及基因组信息的完整性。因此,阐明DNA代谢与DNA损伤应答的分子机制,不仅有重大的理论意义,也有     

GADD45α参与维持基因组稳定性并发挥肿瘤抑制效应的多样性分子机制

哺乳动物细胞在遭受应激损伤因素刺激时会启动一系列信号传导通路,从而引发细胞周期阻滞、DNA修复或细胞凋亡等效应,这些机制的异常与肿瘤的发生发展密切相关。GADD45α作为生长阻滞及DNA损伤诱导基因编码家族的一员,参与维持基因组稳定性、调控细胞周期行进、DNA损伤修复、细胞衰老及细胞凋亡等多种生物学过程,在肿瘤发生发展和肿瘤抑制反应中具有重要作用。我们简要综述了GADD45α参与维持基因组稳定性并发挥肿瘤抑制效应的分子机制。     

抑癌基因p53与肿瘤研究的最新进展

p53基因是迄今为止已发现的与人类肿瘤发生相关性最高的抑癌基因,其主要生物学功能是通过调控DNA修复、细胞周期停滞和诱导细胞凋亡,维持基因组和细胞稳定,抑制肿瘤生长;肿瘤血管再生、微小RNA（microRNA,miRNA）及肿瘤干细胞是近几年来肿瘤发生机理研究领域的热点,本文综述了p53基因在肿瘤血管再生、miRNA、肿瘤干细胞中作用的最新研究进展及其在肿瘤治疗中的应用。     

53BP1与DNA双链断裂损伤修复

DNA的精确复制和遗传对维持基因组稳定性有重要作用。DNA双链断裂损伤可能诱导细胞凋亡和染色质重排,在肿瘤的发生发展过程中发挥作用。53BP1是DNA双链断裂修复中的重要调节蛋白质之一,对调控损伤修复平衡和维持基因组稳定性起着重要作用。本文主要对53BP1的结构、生物学功能、信号通路、分子机制和翻译后修饰做一浅显的总结和展望,希望能为53BP1的深入研究提供一些理论基础。     

聚腺苷二磷酸-核糖聚合酶1功能与其抑制剂的作用及耐药机制

聚腺苷二磷酸-核糖聚合酶1(poly ADP-ribose polymerase-1,PARP1)是细胞中重要的修饰酶,其最广为人知的作用是通过自身PAR修饰,募集以XRCC1为首的多种DNA损伤修复效应蛋白质,参与DNA单、双链损伤修复。PARP1还能通过促进复制叉停滞与核小体解聚,为DNA损伤修复提供有利条件,维持基因组稳定性。近年来,除DNA损伤修复方面的作用,还发现PARP1能影响细胞凋亡、自噬与炎症通路,与神经退行性疾病的发生发展密切相关。而PARP抑制剂(PARP inhibitor,PARPi)是一种靶向PARP1,与细胞同源重组(homologous recombination,HR)缺陷表型共同作用,产生合成致死效应的抗肿瘤药物。该药物可捕获PARP1并抑制其活性,一方面直接干扰PARP1参与的DNA损伤修复通路,另一方面也抑制了PARP1介导的DNA损伤修复通路选择和复制叉停滞,使细胞基因组不稳定。然而,在临床治疗中常发现肿瘤细胞对PARPi不敏感。肿瘤细胞对PARPi耐药与自身基因突变高度相关,这些基因分别作用于细胞HR修复途径、PARP1循环途径、复制叉稳定性和药物主动外排等方面,在耐药肿瘤患者中确定具体的突变位点,将为临床治疗提供帮助。本文旨在对PARP1的功能作一综述,并重点介绍PARPi的作用机制和与肿瘤耐药相关的突变基因及其耐药机制,以期加深对细胞中PARP1介导的DNA损伤修复通路的认识,并为将来的临床治疗提供新思路。     

The role of the human DNA mismatch repair gene hMSH2 in DNA repair, cell cycle control and apoptosis: implications for pathogenesis, progression and therapy of cancer

The cellular DNA mismatch repair (MMR) pathway, involving the DNA mismatch repair genes MLH1 and MSH2, detects and repairs DNA replication errors. Defects in MSH2 and MLH1 account for most cases of hereditary non-polyposis colorectal cancer as well as for sporadic colorectal tumors. Additionally, increased expression of MSH2 RNA and/or protein has been reported in various malignancies. Loss of DNA MMR in mammalian cells has been linked to resistance to certain DNA damaging agents including clinically important cytotoxic chemotherapeutics. Due to other functions besides its role in DNA repair, that include regulation of cell proliferation and apoptosis, MSH2 has recently been shown to be of importance for pathogenesis and progression of cancer. This review summarizes our present understanding of the function of MSH2 for DNA repair, cell cycle control, and apoptosis and discusses its importance for pathogenesis, progression and therapy of cancer.     

Mechanisms and functions of DNA mismatch repair

DNA mismatch repair （MMR） is a highly conserved biological pathway that plays a key role in maintaining genomic stability. The specificity of MMR is primarily for base-base mismatches and insertion/deletion mispairs generated during DNA replication and recombination. MMR also suppresses homeologous recombination and was recently shown to play a role in DNA damage signaling in eukaryotic cells. Escherichia coli MutS and MutL and their eukaryotic homologs, MutSα and MutLα, respectively, are key players in MMR-associated genome maintenance. Many other protein components that participate in various DNA metabolic pathways, such as PCNA and RPA, are also essential for MMR. Defects in MMR are associated with genome-wide instability, predisposition to certain types of cancer including hereditary non-polyposis colorectal cancer, resistance to certain chemotherapeutic agents, and abnormalities in meiosis and sterility in mammalian systems.     

Human Pluripotent Stem Cells Have a Novel Mismatch Repair-dependent Damage Response

Human pluripotent stem cells (PSCs) are presumed to have robust DNA repair pathways to ensure genome stability. PSCs likely need to protect against mutations that would otherwise be propagated throughout all tissues of the developing embryo. How these cells respond to genotoxic stress has only recently begun to be investigated. Although PSCs appear to respond to certain forms of damage more efficiently than somatic cells, some DNA damage response pathways such as the replication stress response may be lacking. Not all DNA repair pathways, including the DNA mismatch repair (MMR) pathway, have been well characterized in PSCs to date. MMR maintains genomic stability by repairing DNA polymerase errors. MMR is also involved in the induction of cell cycle arrest and apoptosis in response to certain exogenous DNA-damaging agents. Here, we examined MMR function in PSCs. We have demonstrated that PSCs contain a robust MMR pathway and are highly sensitive to DNA alkylation damage in an MMR-dependent manner. Interestingly, the nature of this alkylation response differs from that previously reported in somatic cell types. In somatic cells, a permanent G₂/M cell cycle arrest is induced in the second cell cycle after DNA damage. The PSCs, however, directly undergo apoptosis in the first cell cycle. This response reveals that PSCs rely on apoptotic cell death as an important defense to avoid mutation accumulation. Our results also suggest an alternative molecular mechanism by which the MMR pathway can induce a response to DNA damage that may have implications for tumorigenesis.     

The multifaceted mismatch-repair system

By removing biosynthetic errors from newly synthesized DNA, mismatch repair (MMR) improves the fidelity of DNA replication by several orders of magnitude. Loss of MMR brings about a mutator phenotype, which causes a predisposition to cancer. But MMR status also affects meiotic and mitotic recombination, DNA-damage signalling, apoptosis and cell-type-specific processes such as class-switch recombination, somatic hypermutation and triplet-repeat expansion. This article reviews our current understanding of this multifaceted DNA-repair system in human cells.     

Role of excision mechanisms of DNA repair in induction of apoptosis

DNA repair and apoptosis lead to principally different final results: the first mechanism removes damages from DNA, restoring genome integrity; the second mechanism eliminates potentially dangerous cells harboring DNA lesions. The cells deficient in mismatch repair (MMR) demonstrate inceased resistance (viability) to DNA-damaging agents due to decreased ability to undergo apoptosis. This means that mechanism of MMR both restores structure of DNA and generates a signal for apoptosis. DNA breaks and single strand gaps, which are temporarily produced by excison mechanism during DNA repair, are suggested to be the initial signals for apoptosis. However pathway involved in such signaling at least partially is independent of p53 function.     

Mammalian DNA mismatch repair protects cells from UVB-induced DNA damage by facilitating apoptosis and p53 activation

DNA mismatch repair (MMR) is integral to the maintenance of genomic stability and more recently has been demonstrated to affect apoptosis and cell cycle arrest in response to a variety of adducts induced by exogenous agents. Comparing Msh2-null and wildtype mouse embryonic fibroblasts (MEFs), both primary and transformed, we show that Msh2 deficiency results in increased survival post-UVB, and that UVB-induced apoptosis is significantly reduced in Msh2-deficient cells. Furthermore, p53 phosphorylation at serine 15 is delayed or diminished in Msh2-deficient cells, suggesting that Msh2 may act upstream of p53 in a post-UVB apoptosis or growth arrest response pathway. Taken together, these data suggest that MMR heterodimers containing Msh2 may function as a sensor of UVB-induced DNA damage and influence the initiation of UVB-induced apoptosis, thus implicating MMR in protecting against UV-induced tumorigenesis.     

Mismatch repair in correction of replication errors and processing of DNA damage

The primary role of mismatch repair (MMR) is to maintain genomic stability by removing replication errors from DNA. This repair pathway was originally implicated in human cancer through an association between microsatellite instability in colorectal tumors in hereditary nonpolyposis colon cancer (HNPCC) kindreds. Microsatellites are short repetitive sequences which are often copied incorrectly by DNA polymerases because the template and daughter strands in these regions are particularly prone to misalignment. These replication-dependent events create loops of extrahelical bases which would produce frameshift mutations unless reversed by MMR. One consequence of MMR loss is a widespread expansion and contraction of these repeated sequences that affects the whole genome. Defective MMR is therefore associated with a mutator phenotype. Since the same pathway is also responsible for repairing base:base mismatches, defective cells also experience large increases in the frequency of spontaneous transition and transversion mutations. Three different approaches have been used to investigate the function of individual components of the MMR pathway. The first is based on the biochemical characterization of the purified protein complexes using synthetic DNA substrates containing loops or single mismatches. In the second, the biological consequences of MMR loss are inferred from the phenotype of cell lines established from repair-deficient human tumors, from tolerant cells or from mice defective in single MMR genes. In particular, molecular analysis of the mutations in endogenous or reporter genes helped to identify the DNA substrates for MMR. Finally, mice bearing single inactive MMR genes have helped to define the involvement of MMR in cancer prevention.     

DnaN clamp zones provide a platform for spatiotemporal coupling of mismatch detection to DNA replication

Mismatch repair (MMR) increases the fidelity of DNA replication by identifying and correcting replication errors. Processivity clamps are vital components of DNA replication and MMR, yet the mechanism and extent to which they participate in MMR remains unclear. We investigated the role of the Bacillus subtilis processivity clamp DnaN, and found that it serves as a platform for mismatch detection and coupling of repair to DNA replication. By visualizing functional MutS fluorescent fusions in vivo, we find that MutS forms foci independent of mismatch detection at sites of replication (i.e. the replisome). These MutS foci are directed to the replisome by DnaN clamp zones that aid mismatch detection by targeting the search to nascent DNA. Following mismatch detection, MutS disengages from the replisome, facilitating repair. We tested the functional importance of DnaN‐mediated mismatch detection for MMR, and found that it accounts for 90% of repair. This high dependence on DnaN can be bypassed by increasing MutS concentration within the cell, indicating a secondary mode of detection in vivo whereby MutS directly finds mismatches without associating with the replisome. Overall, our results provide new insight into the mechanism by which DnaN couples mismatch recognition to DNA replication in living cells.