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...     

基因甲基化研究概述

表观遗传学(epigenetics)反映的主要是基因型和表型的关系。研究范围包括:DNA甲基化、组蛋白修饰、染色体重塑、遗传印记、X染色体失活、RNA调控、转座元件、副突变、位置效应斑、等位反式互补等[1～5]。近年来研究较多的是胞嘧啶DNA甲基化、RNA干扰(RNAi)调控、组蛋白修饰3种,尤其是DNA甲基化。2003年欧洲HEC宣布实施人类表观基因组计划(HEP),以指导和系统地研究DNA基因甲基化在人类表观遗传、胚胎发育、基因印记、等位基因失活及肿瘤发生中的作用[6]。本文就现阶段对DNA甲基化研究的基本现状作一概述,以获得1个整体认识,从而在此基础上对其作进一步研究。1CpG岛DNA甲基化多发生于DNA链上胞嘧啶第5位碳原子,其与甲基共价结合,胞嘧啶由此被修饰为5甲基胞嘧啶(5mC),哺乳动物基因组中5mC占胞嘧啶总量的2%～7%,70%～90%的5mC存在于CG二联核苷。CG在DNA中呈回文结构,通常称为CpG(p表示磷酸基),中文为磷酸胞苷酰[7]。哺乳动物中,CpG序列在基因组中出现的频率仅有1%,远低于基因组中的其它双核苷酸序列。但在基因组的某些区域中,CpG序列密度很高,可以达均值的5倍以上...     

DNA甲基化与骨髓增生异常综合征研究进展

随着肿瘤表观遗传学的深入研究,DNA甲基化异常在血液肿瘤的发生和转化中起着重要作用。近来研究显示p15基因、降钙素基因、紧密连接蛋白-1基因等高度甲基化与骨髓增生异常综合征的发生发展具有重要关系。5-氮杂胞苷和5-2′-脱氧胞嘧啶等在骨髓增生异常综合征患者的治疗中已经取得一定效果。去甲基化治疗有望成为骨髓增生异常综合征治疗的有效方案之一。     

DNA甲基化异常与肿瘤

 DNA甲基化是真核生物基因表达调控的一种方式。在哺乳动物,DNA甲基化发生在二核苷酸胞嘧啶(CPG)的第5位碳原子上,即5-甲基胞嘧啶(5-mc)[1]。     

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.     

RNA m5C甲基化修饰在肿瘤中的研究进展

5-甲基胞嘧啶（5-methylcytosine,m5C）是RNA中重要的甲基化修饰之一,也是近年来的研究热点。随着甲基化测序技术的发展,在编码RNA及非编码RNA中均证实存在大量的m5C甲基化修饰。RNA m5C甲基化修饰受m5C甲基转移酶、去甲基化酶及m5C甲基化结合蛋白的调控。m5C甲基化修饰调节RNA的稳定性、转运、翻译和压力应激等,并参与调节肿瘤的发生发展、侵袭转移、肿瘤耐药等进程。本文对RNA m5C甲基化修饰的调控机制、功能以及在肿瘤中的研究进展进行综述,以期为肿瘤诊治研究提供新思路。      

DNA羟甲基化酶基因TET1在肿瘤发生中的作用

TET1 （ten-eleven-translocation 1）是一种羟甲基化酶基因,该酶能够催化5甲基胞嘧啶（5-methyl-cytosine,5mC）形成5 羟甲基胞嘧啶（5-hydroxymethyl-cytosine,5hmC）,在DNA甲基化调控中发挥重要作用。最近研究表明, TET1 的低表达与肿瘤发 生有关,可作为癌症治疗潜在标志物,表明 TET1 可作为肿瘤抑制基因。此外,除了它的双氧酶活性外, TET1 还可以诱导上皮细 胞间质转变,并充当调节基因转录的辅助活化因子,如癌症的低氧应答基因的调节因子。这表明 TET1 也可做为癌基因促进肿瘤 的发生。因此,在癌症和发育生物学中,TET1的调控机制是十分复杂的, TET1 基因突变也已有报道。本文就TET1在肿瘤发生中 的不同作用进行了综述,深入阐述TET1的作用对拓展DNA去甲基化机制的认识及发现肿瘤治疗新靶标具有重要价值。     

微小RNA与DNA甲基化相互调控在胃癌中的作用

表观遗传修饰在胃癌发生发展及预后中起重要作用.微小RNA( miRNA)和DNA甲基化是表观遗传学的两个重要调控机制.miRNA抑制靶基因信使RNA的翻译,参与基因转录后水平调控.基因启动子区CpG岛的异常甲基化与肿瘤的发生关系密切,可导致癌基因、抑癌基因或其他肿瘤相关基因的表达异常.胃癌中存在miRNA和DNA甲基化...     

DNA甲基化与大肠癌发生的研究进展

DNA甲基化由于影响基因突变、基因表达调控、基因组稳定性等方面 ,因此在肿瘤的发生和演进过程中扮演着一定的角色。近来 ,DNA甲基化机制及其与大肠癌发生关系的研究进展很快 ,现对最近 3年这方面的进展作一简要介绍。1　 DNA甲基化的概念DNA甲基化主要指在胞嘧啶的 5位碳上加上一个甲基基团 ,该反应由 S-腺苷蛋氨酸 (SAM)提供甲基 ,由 DNA甲基转移酶 (DMT)催化 ,形成 5甲基胞嘧啶 (5 m C)。 5 m C是真核细胞中唯一天然存在的修饰碱基 ,约占整个胞嘧啶的 3% ,而 90 %的 5 m C存在于 Cp G序列中。哺乳动物 DNA中 5 0 %～ 90 %的…     

DNA甲基化转移酶及DNA甲基化转移酶抑制剂在肿瘤研究中的新视角

表观遗传改变例如DNA甲基化涉及多种癌症的发生和进展。DNA甲基化包括可逆的甲基基团添加至CpG二核苷酸中5′位胞嘧啶上。而DNA甲基化转移酶(DNA methyltransferases,DNMT)是负责甲基基团添加至CpG二核苷酸的酶,与组蛋白修饰一起,是发生于转录抑制所必须的起始事件。已经证明在多种人恶性肿瘤中DNMT的表达增高,并且通过DNMT介导的基因失活促进肿瘤进展。DNMT在衰老与凋亡过程中都起到一定作用。表观遗传改变是潜在可逆的,这刺激DNA甲基化抑制剂药理学的发展,为肿瘤的治疗提供了一个新的途径。     

Role of TET2 Mutations in Myeloproliferative Neoplasms

Recently, 5-hydroxymethylcytosine (5-hmC), the 6th base of DNA, was discovered as the product of the hydroxylation of 5-methylcytosine (5-mC) by the ten-eleven translocation (TET) oncogene family members. One of them, TET oncogene family member 2 (TET2), is mutated in a variety of myeloid malignancies, including in 15% of myeloproliferative neoplasms (MPNs). Recent studies tried to go further into the biological and epigenetic function of TET2 protein and 5-hmC marks in the pathogenesis of myeloid malignancies. Although its precise function remains partially unknown, TET2 appears to be an important regulator of hematopoietic stem cell biology. In both mouse and human cells, its inactivation leads to a dramatic deregulation of hematopoiesis that ultimately triggers blood malignancies. Understanding this leukemogenic process will provide tools to develop new epigenetic therapies against blood cancers.     

5-hydroxymethylcytosine: A new insight into epigenetics in cancer

DNA methylation at the 5 position of cytosine (5-mC) has emerged as a key epigenetic marker that plays essential roles in various biological and pathological processes. 5-mC can be converted to 5-hydroxymethylcytosine (5-hmC) by the ten–eleven translocation (TET) family proteins, which is now widely recognized as the “sixth base” in the mammalian genome, following 5-mC, the “fifth base”. 5-hmC is detected to be abundant in brain and embryonic stem cells, and is also distributed in many different human tissues. Emerging evidence has shown that 5-hmC and TET family might serve unique biological roles in many biological processes such as gene control mechanisms, DNA methylation regulation, and involved in many diseases, especially cancers. In this paper we provide an overview of the role of 5-hmC as a new sight of epigenetics in human cancer.     

Ten-eleven translocation 1 (TET1) gene is a potential target of miR-21-5p in human colorectal cancer

DNA 5-methylcytosine (5-mC) methylation, a key epigenetic mark, is critical for biological and pathological processes. Aberrant DNA methylation occurs in all tumor types and correlates with tumor suppressor gene silencing. DNA methylation is thought to be very stable, and active DNA demethylation remains a long-standing enigma. Recently, the ten-eleven translocation (TET) family of oxygenases are found to oxidize 5-mC to 5-hydroxymethylcytosine (5-hmC), which is prerequisite for active DNA demethylation. Both TET1 expression and global 5-hmC content are significantly reduced in colorectal cancer (CRC), the top leading cause of cancer-related death in the world. However, the involving molecular mechanisms are still unclear. The oncogenic microRNA (miRNA) miR-21-5p has recently identified as a diagnostic and prognostic biomarker in CRC. In this study, TET1 was predicted as a novel target of miR-21-5p by using a web-based predictive software starBase v2.0. We found that the 3′-UTR region of TET1 gene contains a miR-21-5p-binding site. Examination of tumor tissues from CRC patients found that loss of TET1 was associated with the progression of CRC to advance stages. In addition, negative correlation of miR-21-5p and TET1 expression was also observed. Transfection of the synthetic miR-21-5p mimic or inhibitor into the colorectal cancer cells could inhibit or increase the TET1-3′-UTR luciferase activity, respectively. Our results demonstrate that TET1 is a potential target of miR-21-5p in CRC.     

TET family proteins and 5-hydroxymethylcytosine in esophageal squamous cell carcinoma

Mammalian DNA is epigenetically marked by 5′-cytosine methylation (5-methylcytosine [5-mC]). The Ten-eleven translocation (TET) enzymes (TET1, TET2, and TET3) are implicated in DNA demethylation, through dioxygenase activity that converts 5-mC to 5-hydroxymethylcytosine (5-hmC). Although decreased TET is reportedly associated with decreased 5-hmC levels in various cancers, functions of 5-hmC and TET expression in esophageal squamous cell carcinoma (ESCC) are unclear. We used ELISA and immunohistochemistry tests to analyze 5-hmC status in ESCC tissues, RT-qPCR to analyze TET family mRNA expression in normal and tumor tissues, and pyrosequencing to quantify LINE-1 (i.e., global DNA methylation) levels. ELISA and immunohistochemical testing showed 5-hmC levels were significantly lower in ESCC than in paired normal tissues (P < 0.0001). TET2 expression was significantly lower in ESCCs than paired normal tissues (P < 0.0001), and significantly associated with 5-hmC levels in ESCCs (P = 0.003, r = 0.33). 5-hmC levels were also significantly associated with LINE-1 methylation level (P = 0.0002, r = 0.39). Patients with low 5-hmC levels had shorter overall survival than those with higher levels, although not significantly so (P = 0.084). In conclusion, 5-hmC expression was decreased in ESCC tissues, and was associated with TET2 expression level. TET2 reduction and subsequent 5-hmC loss might affect ESCC development.     

Loss of the epigenetic mark, 5-Hydroxymethylcytosine,correlates with small cell/nevoid subpopulations and assists in microstaging of human melanoma

Melanomas in the vertical growth phase (VGP) not infrequently demonstrate cellular heterogeneity. One commonly encountered subpopulation displays small cell/nevoid morphology. Although its significance remains unknown, such subpopulations may pose diagnostic issues when faced with differentiating such changes from associated nevus or mistaking such regions for nevic maturation (pseudomaturation). That ‘loss’ of the epigenetic biomarker, 5-hydroxymethylcytosine (5-hmC), is a hallmark for melanoma and correlates with virulence prompted us to explore the diagnostic utility and biological implications of 5-hmC immunohistochemistry (IHC) in melanomas with small cell/nevoid subpopulations. Fifty-two cases were included in this study, including melanomas with small cell/nevoid subpopulations (MSCN) or melanomas with pre-existing nevus (MPEN). Semiquantitative and computer-validated immunohistochemical analyses revealed invariable, uniform loss of 5-hmC in the conventional melanoma component. By contrast, the nevic components in MPEN cases demonstrated strong nuclear immunopositivity. In MSCN cases, there was partial to complete loss of 5-hmC restricted to these nevoid areas. Based on recent data supporting tight correlation between 5-hmC loss and malignancy, our findings indicate a potential ‘intermediate’ biological nature for small cell/nevoid subpopulations. Because 5-hmC assisted in differentiating such regions from associated nevus, the use of 5-hmC as an adjunct to microstaging in difficult cases showing VGP heterogeneity should be further explored.     

High expression of 5-hydroxymethylcytosine and isocitrate dehydrogenase 2 is associated with favorable prognosis after curative resection of hepatocellular carcinoma

Background

The expression of 5-hydroxymethylcytosine (5-hmC) and isocitrate dehydrogenase 2 (IDH2) is frequently downregulated in numerous cancers. 5-hmC and IDH2 expression in hepatocellular carcinoma (HCC) has yet to be determined.

Methods

The immunohistochemical expression of 5-hmC and IDH2 were analyzed in tissue microarrays containing samples from 646 patients who had undergone hepatectomy for histologically proven HCC. The prognostic value of 5-hmC and IDH2 were evaluated by Cox regression and Kaplan-Meier analyses.

Results

We discovered that low 5-hmC and IDH2 expression was associated with malignant behaviors. Low 5-hmC or IDH2 expression alone and combined 5-hmC and IDH2 expression were associated with lower overall survival (OS) rates and higher cumulative recurrence rates. Multivariate analysis indicated that 5-hmC or IDH2 and 5-hmC/IDH2 were independent prognostic indicators for OS and time to recurrence (TTR), which was confirmed in an independent validation cohort.

Conclusions

5-hmC and IDH2 correlate with less aggressive tumor behavior in HCC. When 5-hmC and IDH2 are considered together, they serve as a prognostic marker in patients with surgically resected HCCs.     

Loss of 5-hydroxymethylcytosine is accompanied with malignant cellular transformation

Dysregulated DNA methylation followed by abnormal gene expression is an epigenetic hallmark in cancer. DNA methylation is catalyzed by DNA methyltransferases, and the aberrant expression or mutations of DNA methyltransferase genes are found in human neoplasm. The enzymes for demethylating 5-methylcytosine were recently identified, and the biological significance of DNA demethylation is a current focus of scientific attention in various research fields. Ten-eleven translocation (TET) proteins have an enzymatic activity for the conversion from 5-methylcytosine to 5-hydroxymethylcytosine (5-hmC), which is an intermediate of DNA demethylation. The loss-of-function mutations of TET2 gene were reported in myeloid malignancies, suggesting that impaired TET-mediated DNA demethylation could play a crucial role in tumorigenesis. It is still unknown, however, whether DNA demethylation is involved in biological properties in solid cancers. Here, we show the loss of 5-hmC in a broad spectrum of solid tumors: for example, a significant reduction of 5-hmC was found in 72.7% of colorectal cancers (CRCs) and 75% of gastric cancers compared to background tissues. TET1 expression was decreased in half of CRCs, and a large part of them was followed by the loss of 5-hmC. These findings suggest that the amount of 5-hmC in tumors is often reduced via various mechanisms, including the downregulation of TET1. Consistently, in the in vitro experiments, the downregulation of TET1 was clearly induced by oncogene-dependent cellular transformation, and loss of 5-hmC was seen in the transformed cells. These results suggest the critical roles of aberrant DNA demethylation for oncogenic processes in solid tissues.     

软组织肉瘤表观遗传学研究进展

软组织肉瘤是一组起源于间叶组织的恶性肿瘤，在成人恶性肿瘤中约占1%，但是其亚类繁多并且部分软组织肉瘤恶性程度较高，5年生存率为50%~60%。在不断探索软组织肉瘤发病机制的过程中，研究发现除了基因组变化可以致瘤，表观遗传学的改变与肿瘤的发生、发展也有着密切的联系。与遗传学不同，表观遗传学不涉及基因核苷酸序列的变化，主要通过DNA甲基化、组蛋白修饰、非编码RNA调控及染色质重塑等方式发生可遗传的基因表达。软组织肉瘤发生、发展过程中涉及的表观遗传学机制可以为临床诊断和治疗用药提供更多新思路。本文将对软组织肉瘤的表观遗传学研究进展进行综述。      

表观遗传调控因子SNF5和EZH2在肿瘤发生发展中的研究进展

表观遗传调控在基因表达、DNA复制、修复和重组等重要过程中起着关键作用,表观遗传调控异常会引发多种疾病尤其是癌症。SNF5是SWI-SNF染色体重塑复合物的核心成分之一,具有强效抑癌作用。果蝇zeste基因增强子同源物2（EZH2）是目前与肿瘤关联密切且复杂的组蛋白甲基化酶,具有促癌功能。已有研究报道了表观遗传调控因子SNF5和EZH2在不同肿瘤发生发展中的作用及二者之间的相互关系。     

Invasion and metastasis in pancreatic cancer

Pancreatic cancer remains a challenging disease with an overall cumulative 5-year survival rate below 1%. The process of cancer initiation, progression and metastasis is still not understood well. Invasion and tumor metastasis are closely related and both occur within a tumour-host microecology, where stroma and tumour cells exchange enzymes and cytokines that modify the local extracellular matrix, stimulate cell migration, and promote cell proliferation and tumor cell survival. During the last decade considerable progress has been made in understanding genetic alterations of genes involved in local and systemic tumor growth. The most important changes occur in genes which regulate cell cycle progression, extracellular matrix homeostasis and cell migration. Furthermore, there is growing evidence that epigenetic factors including angiogenesis and lymphangiogenesis may participate in the formation of tumor metastasis. In this review we highlight the most important genetic alterations involved in tumor invasion and metastasis and further outline the role of tumor angiogenesis and lymphangiogenesis in systemic tumor dissemination.