表观遗传学与结直肠癌的关系研究进展

表观遗传学(Epigenetics)与遗传学(Genetics)是相对应的概念。遗传学改变是指基于基因序列改变所致基因表达水平变化,如基因突变、基因杂合丢失和微卫星不稳定等;而表观遗传学改变则是指基于非基因序列改变所致基因表达水平变化,如DNA甲基化、组蛋白修饰和基因组印迹等。研究表明遗传和表观遗传异常都参与肿瘤的发生。结直肠癌是常见的恶性肿瘤之一,全世界每年大约有120万新发病例,同时至少60万患者死亡[1]。结直肠癌有着极为复杂的发生发展机制,传统观点认为肿瘤的主要机制是由于致癌因素造成DNA序列变异而导致细胞生长、分化失控。近年来,随着研究的深入,发现DNA序列以外的调控机制异常在肿瘤的发生、发展过程中更为普遍[2],这种不依赖于DNA序列变化的可遗传的调控机制即为表观遗传学机制,这也为结直肠癌的诊断和治疗开辟了新的途径。1表观遗传学分子机制与结直肠癌1.1DNA甲基化与结直肠癌DNA甲基化是指由DNA甲基转移酶(DN-MT)催化,把S-腺苷甲硫氨酸(SAM)的甲基转移到胞嘧啶5位碳原子上,生成5-甲基胞嘧啶(5-mC)引起基因表达异常的过程。DNA甲基化包括整体基因组的低甲基化和启动子特殊区域CpG岛...     

结直肠癌发生的分子病理学基础

结直肠癌发生是遗传学和表观遗传学改变累积的结果.基因组不稳定导致基因突变和DNA甲基化模式改变是结直肠癌发生的主要分子事件.鉴定不同患者中导致结直肠癌发生的关键基因突变及甲基化表型,对结直肠癌进行分子病理分型和诊断,是进行个体化治疗和靶向治疗的前提.     

DNA甲基化在肺癌中的研究进展

表观遗传学主要是研究基因表达的变化,而这种变化能够通过减数分裂遗传,但不涉及有关基因碱基序列的改变.随着肿瘤发病机制研究在分子生物学方面的逐步深入,发现DNA甲基化、非编码RNA表达调控和组蛋白的修饰等表观遗传学(epigenetics)改变引起的调控机制异常与人体多种恶性肿瘤的发生发展过程有着密切联系.研究表明抑癌基因启动子区高度DNA甲基化状态在恶性肿瘤的发生发展和侵袭转移中发挥着重要作用.在肺癌发生的早期,出现很多DNA甲基化状态的改变.因此,甲基化检测可能对肺癌患者的早期诊断、治疗及预后评估具有重要意义.     

结直肠癌相关基因甲基化研究现状和进展

目的：回顾和总结国内外对结直肠癌相关基因甲基化异常改变的研究及其进展。方法：应用Medline及CNKI期刊全文数据库检索系统，以“结直肠癌、甲基化以及结直肠癌相关基因（如K-ras、p53、c-myc、APC、p16、RASSF1A、hMLH1和ERβ）”等为关键词，检索197901～2008—01结直肠癌相关基因甲基化研究的相关文献。纳入标准：1）结直肠癌与甲基化异常改变；2）结直肠癌相关基因。最后纳入35篇文献。结果：在结直肠癌组织中，K-ras和p53基因的异常改变以点突变为主；c-myc基因局部低甲基化是结直肠癌发生中的早期事件，与细胞增殖密切；APC基因除了杂合性丢失（LOH）致失活外，启动子异常高甲基化致失活也很常见；结直肠癌组织呈现一个不定的甲基化分布密度（所有CpG岛中甲基化CV-G占的比例）则可能是p16基因启动子甲基化模式在体内的特征；RASSF1A基因主要因启动子甲基化而失活；hMLH1失活引起MSI（＋）表型可能与右侧散发性结肠癌的分期有密切的关系；ERβ甲基化模式的分析对判断肿瘤预后有价值。结论：因不同基因而异，除了DNA点突变、缺失、插入和重排等遗传学改变外，甲基化等表观遗传学改变在结直肠癌发生中也占重要地位；甲基化研究对结直肠癌的防治将起到积极的探索意义。     

从分子分型演变看结直肠癌分子生物学行为

结直肠癌是一种异质性很强的肿瘤,肿瘤的异质性是肿瘤在发生发展过程中由基因突变、特征性遗传和表观遗传学改变等众多因素逐渐累积而成。根据肿瘤发生部位、染色体不稳定性、微卫星不稳定性、锯齿状通路改变以及DNA甲基化等因素对结直肠癌进行分子分型,以判断预后及指导治疗。本文回顾了近年来与结直肠肿瘤演进过程相关的分子分型的发展,结直肠癌有不同的分子亚群,而这些分子亚群通过不同的机制决定肿瘤的发生及发展,并与患者的预后密切相关。从分子分型的发展折射出对结直肠癌分子生物学行为的认知逐步深入,为结直肠癌的精准治疗奠定了基础。     

表观遗传学在结直肠癌中的应用进展

结直肠癌（CRC）的发生、发展伴随着许多基因的表达变化,但其具体调控机制至今仍未完全阐明。近年来对CRC表观遗传学尤其是微小RNA（miRNA）、异常DNA甲基化及组蛋白修饰状态等方面的研究受到广泛关注。研究证实,CRC进展过程中均存在异常的甲基化基因和miRNA的表达变化。与癌症基因组的基因突变一样,这类遗传学的分子改变在CRC中扮演着重要角色。表观遗传学的特异性改变可作为CRC诊断、治疗和预后的临床生物学标记物,对表观遗传学进行深入研究对CRC的防治具有重要指导意义。     

B细胞淋巴瘤表观遗传学研究进展

表观遗传学是功能基因组学的重要组成部分,是研究DNA序列不发生变化但基因表达发生遗传性改变的一门遗传学分支学科,主要包括DNA甲基化、组蛋白修饰、染色体重塑和微小RNA(miRNA)等基因表达调节机制.表观遗传学调控机制在B细胞淋巴瘤的发展中具有重要作用,有研究表明在B细胞淋巴瘤中常见表观遗传学模式的不稳定性和分子突变.文章从DNA甲基化、组蛋白修饰、miRNA等方面对B细胞淋巴瘤的发生、发展、治疗及预后进行综述.     

散发性结直肠癌PTEN基因突变与微卫星改变间的关系

背景与目的：研究散发性结直肠癌(Sporadic colorectal cancer,SCRC) 组织中抑癌基因PTEN突变高发区外显子5、7和8的突变与人类结肠癌基因不稳定性间的关系,以进一步研究结肠癌的分子遗传学发病机制。材料与方法：采用聚合酶链反应（PCR）、PCR结合单链构象多态性（PCR-SSCP）技术对68例散发性结直肠癌患者的癌组织及其癌旁组织的PTEN基因第5、7 和第8外显子突变情况和结直肠癌高度敏感的微卫星检测位点Bat25、Bat40及D18S21的改变情况进行检测分析。结果：微卫星改变阳性个体组中PTEN基因突变发生率为28.57%(4/ 14),与微卫星改变阴性个体组中PTEN基因突变发生率相比较,差异有统计学意义（χ2=8.06,P <0.01）。 结论：PTEN基因的突变在微卫星不稳定性结肠癌的发生发展中产生一定的作用。     

MicroRNA与DNA甲基化相互调节机制在肝细胞癌中的研究进展

肝细胞癌是原发性肝癌的主要类型,也是人类恶性程度较高的肿瘤之一,其发病机制至今尚未完全阐明.表观遗传学机制在肿瘤的发生、发展中起重要作用,DNA甲基化和微小RNA (microRNA,miRNA)的调控机制属于表观遗传学的研究范畴.研究表明,DNA甲基化及miRNA在肝细胞癌的形成中分别或协同发挥着重要作用,miRNA是一类在转录后水平调节基因表达的非编码短链RNA.研究表明,DNA甲基化和组蛋白修饰不仅可以调节蛋白编码基因的表达,而且可以调节miRNA的表达.在肝细胞癌中,一些异常表达的miRNAs(如miR-125b、miR-1-1、miR-124、miR-203和miR-191)是通过表观遗传学机制调控的.另外,在肝细胞癌中还发现了一类miRNAs通过调控表观遗传学通路中关键分子来改变整个基因组的表观遗传学状态.本文就DNA甲基化和miRNA之间复杂的相互调节机制在肝细胞癌发生和发展中的研究进展进行综述.     

错配修复基因异常改变与大肠癌的关系

研究表明，大肠癌(colorectal cancer，CRC)的发生是1个多因素多步骤的过程，是机体的内因与环境的外因相互作用的结果。其中错配修复基因的改变与遗传性和散发性结直肠癌的基因不稳定性有关。本文就错配修复基因异常导致大肠癌的机制及其诊断与治疗的最新研究进展做一综述。     

Focus on genetic and epigenetic events of colorectal cancer pathogenesis: implications for molecular diagnosis

Originally, colorectal cancer (CRC) tumorigenesis was understood as a multistep process that involved accumulation of tumor suppressor genes and oncogenes mutations, such as APC, TP53 and KRAS. However, this assumption proposed a relatively limited repertoire of genetic alterations. In the last decade, there have been major advances in knowledge of multiple molecular pathways involved in CRC pathogenesis, particularly regarding cytogenetic and epigenetic events. Microsatellite instability, chromosomal instability and CpG island methylator phenotype are the most analyzed cytogenetic changes, while DNA methylation, modifications in histone proteins and microRNAs (miRNAs) were analyzed in the field of epigenetic alterations. Therefore, CRC development results from interactions at many levels between genetic and epigenetic amendments. Furthermore, hereditary cancer syndrome and individual or environmental risk factors should not be ignored. The difficulties in this setting are addressed to understand the molecular basis of individual susceptibility to CRC and to determine the roles of genetic and epigenetic alterations, in order to yield more effective prevention strategies in CRC patients and directing their treatment. This review summarizes the most investigated biomolecular pathways involved in CRC pathogenesis, their role as biomarkers for early CRC diagnosis and their possible use to stratify susceptible patients into appropriate screening or surveillance programs.     

Effect of classification based on combination of mutation and methylation in colorectal cancer prognosis

Colorectal cancer (CRC) is caused by an accumulation of genetic alterations and epigenetic alterations. The molecular classification of CRCs based on genetic alterations and epigenetic alterations is evolving. Here, we examined mutations and methylation status in CRCs to determine if the combination of genetic and epigenetic alterations predicts prognosis. We examined 134 sporadic CRCs. We used the direct sequencing method to identify mutations in BRAF and AKT1, which are downstream of KRAS and PIK3CA, respectively, in the EGFR pathway. We used the Methylight method to determine the methylation status of hMLH1, p16, MINT1, MINT2 and MINT31. Both BRAF and AKT1 mutations were found in only one case (0.75%). Aberrant methylation of hMLH1, p16, MINT1, MINT2 and MINT31 was detected in 22.4, 35.1, 32.8, 59.7 and 41.0% of cases, respectively. The clinicopathological factors were not significantly correlated to mutation or methylation. Among the patients who had no mutation in the EGFR pathway, the overall survival was significantly shorter in the patients with methylation compared to the patients with no methylation in hMLH1 and p16 (p=0.0318). Methylation could play a key role in the prognosis of patients without mutations in the EGFR pathway. The combination of genetic and epigenetic alterations may be a good marker for the prognosis of CRC patients.     

BRAF mutation, CpG island methylator phenotype and microsatellite instability occur more frequently and concordantly in mucinous than non-mucinous colorectal cancer

Mucinous colorectal cancer (CRC) has been reported to have distinct clinicopathological and genetic characteristics. However, the incidence and the relationship among microsatellite instability (MSI), CpG island methylator phenotype (CIMP) and BRAF and KRAS mutations in mucinous and non-mucinous CRC are not known. Activating mutations of BRAF and KRAS and their relationship with MSI and CIMP were examined in 83 sporadic CRC specimens (26 mucinous and 57 non-mucinous CRC). MSI, CIMP, BRAF and KRAS mutation were observed in 17, 24, 25 and 36% of the tumors, respectively. BRAF mutation was highly correlated with MSI (p < 0.001) and CIMP (p < 0.001). A higher incidence of MSI (27% vs. 12%), CIMP (38% vs. 18%, p < 0.05) and BRAF mutation (46% vs. 16%, p < 0.01) was observed in mucinous CRC. KRAS mutation (27% vs. 40%) was observed more frequently in non-mucinous CRC. Significantly higher percentages of mucinous CRC (54%, p < 0.05) had MSI or CIMP or BRAF mutations. Concordant occurrence of 2 or more of these alterations was observed in 39% of mucinous CRC and only 11% of non-mucinous CRC (p < 0.01). The more frequent occurrence and closer association among MSI, CIMP and BRAF mutation in mucinous CRC observed in our study further supports the idea that its pathogenesis may involve distinct genetic and epigenetic changes.     

Environmental Exposures,Tumor Heterogeneity,and Colorectal Cancer Outcomes

Considerable progress has been made in colorectal cancer (CRC) over the past two decades but it still remains the third commonest and third most deadly cancer in the USA. Epidemiologic and experimental studies have continued to describe links between environmental risk factors for developing nonhereditary CRC. However, the most significant recent advances have come through the understanding of CRC tumor heterogeneity and its impact on the variability of treatment efficacy. Using the KRAS mutation as a predictive biomarker for anti-epidermal growth factor receptor therapy is an example of how the genetic diversity of tumors can lead to a more individual or so-called personalized medicine with the goal of providing more benefits to patients, without unnecessary adverse side effects. Most recent reviews on this topic have focused on the key cancer pathways for which targeted therapies for CRC already exist. Less attention has recently been given to the three major genetic subtypes of CRC. Chromosomal instability, microsatellite instability, and CpG island methylator phenotype are thought to be the three major genetic and epigenetic alterations that drive tumorigenesis. In the past, these genetic alterations were used as a prognostic indicator, but recent data have demonstrated their correlation with treatment response.     

Transposon mutagenesis identifies candidate genes that cooperate with loss of transforming growth factor‐beta signaling in mouse intestinal neoplasms

Colorectal cancer (CRC) results from the accumulation of gene mutations and epigenetic alterations in colon epithelial cells, which promotes CRC formation through deregulating signaling pathways. One of the most commonly deregulated signaling pathways in CRC is the transforming growth factor β (TGF‐β) pathway. Importantly, the effects of TGF‐β signaling inactivation in CRC are modified by concurrent mutations in the tumor cell, and these concurrent mutations determine the ultimate biological effects of impaired TGF‐β signaling in the tumor. However, many of the mutations that cooperate with the deregulated TGF‐β signaling pathway in CRC remain unknown. Therefore, we sought to identify candidate driver genes that promote the formation of CRC in the setting of TGF‐β signaling inactivation. We performed a forward genetic screen in mice carrying conditionally inactivated alleles of the TGF‐β receptor, type II (Tgfbr2) using Sleeping Beauty (SB) transposon mediated mutagenesis. We used TAPDANCE and Gene‐centric statistical methods to identify common insertion sites (CIS) and, thus, candidate tumor suppressor genes and oncogenes within the tumor genome. CIS analysis of multiple neoplasms from these mice identified many candidate Tgfbr2 cooperating genes and the Wnt/β‐catenin, Hippo and MAPK pathways as the most commonly affected pathways. Importantly, the majority of candidate genes were also found to be mutated in human CRC. The SB transposon system provides an unbiased method to identify Tgfbr2 cooperating genes in mouse CRC that are functionally relevant and that may provide further insight into the pathogenesis of human CRC.     

Predictive and Prognostic Markers in Colorectal Cancer

Colorectal cancer (CRC) has a complex pathogenesis involving multiple sequential steps with accumulation of genetic alterations including mutations, gene amplification, and epigenetic changes. Treatment of CRC has undergone a paradigm shift over the past decade due in part to a better understanding of the biology of the disease and development of newer drugs including biologic agents. In the era of personalized medicine, it is attractive to investigate the molecular pathways leading to colorectal cancer tumorigenesis, thus raising the possibility of identifying novel therapeutic targets. It has intuitive appeal to hypothesize that biomarkers that have prognostic and/or predictive value are those that are intimately connected to the pathogenesis of CRC. In this article, we focus on prognostic and predictive markers in CRC that have a substantial body of data in support of their potential role in routine clinical practice.     

Poorly differentiated colorectal adenocarcinomas show higher rates of microsatellite instability and promoter methylation of p16 and hMLH1: a study matched for T classification and tumor location

BACKGROUND: Extensive genetic and epigenetic analysis of poorly differentiated colorectal adenocarcinomas (Por) has been difficult, as the number of cases is too small. METHODS: We investigated genetic and epigenetic alterations of 53 cases of Por and 53 cases of well-differentiated colorectal adenocarcinomas (WD) to clarify their differences. The cases of WD were matched with the cases of Por for T classification and tumor location, which influence genetic and epigenetic alterations. We evaluated microsatellite instability (MSI) status and loss of heterozygosity (LOH) of four loci (2p, 5q, 17p, 18q), and defined "MSI tumors" as those that showed MSI-H, and "chromosomal instability (CIN) tumors" as those that showed LOH but not MSI-H. Further, we evaluated the methylation status of the hMLH1 and p16 promoter region. RESULTS: MSI tumors were significantly more frequent in Por (22.6%) than in WD (3.8%; P = 0.0041). CIN tumors were significantly less frequent in Por (64.2%) than in WD (83.0%; P = 0.046). Further, methylation of the p16 and hMLH1 promoter region was significantly more frequent in Por than in WD (P = 0.037, P = 0.047, respectively). CONCLUSIONS: Our results indicate that Por tumorigenesis strongly correlates with MSI and methylation of the p16 and hMLH1 promoter region.     

DNA tumor viruses and colorectal cancer

Colorectal cancers (CRCs) are characterized by forms of genomic or epigenetic instability that generate a large number of genetic variations, some of which provide the growth and survival advantages necessary for neoplastic growth. The mechanistic basis of these fundamental processes is currently unknown. Herein, we review evidence that links the human polyomavirus JC virus mechanistically with these processes in CRC. We propose a model in which JC virus is ubiquitously present in the gastrointestinal tracts of healthy people and speculate that through a process that remains to be discovered, the virus becomes activated, expresses a potent oncogene, and triggers chromosomal instability and the methylator phenotype resulting in CRC.     

The role of epigenetic therapies in colorectal cancer

Although developments in the diagnosis and therapy of colorectal cancer (CRC) have been made in the last decade, much work remains to be done as it remains the second leading cause of cancer death. It is now well established that epigenetic events, together with genetic alterations, are key events in initiation and progression of CRC. Epigenetics refers to heritable alterations in gene expression that do not involve changes in the DNA sequence. These alterations include DNA methylation, histone alterations, chromatin remodelers, and noncoding RNAs. In CRC, aberrations in epigenome may also involve in the development of drug resistance to conventional drugs such as 5-fluorouracil, oxaliplatin, and irinotecan. Thus, it has been suggested that combined therapies with epigenetic agents may reverse drug resistance. In this regard, DNA methyltransferase inhibitors and histone deacetylase inhibitors have been extensively investigated in CRC. The aim of this review is to provide a brief overview of the preclinical data that represent a proof of principle for the employment of epigenetic agents in CRC with a focus on the advantages of combinatorial therapy over single-drug treatment. We will also critically discuss the results and limitations of initial clinical experiences of epigenetic-based therapy in CRC and summarize ongoing clinical trials. Nevertheless, since recent translational research suggest that epigenetic modulators play a key role in augmenting immunogenicity of the tumor microenvironment and in restoring immune recognition, we will also highlight the recent developments of combinations strategies of immunotherapies and epigenetic therapies in CRC, summarizing preclinical, and clinical data to signify this evolving and promising field for CRC treatment.