肿瘤的表观遗传调控

肿瘤的表观遗传调控( Epigenetic Regulation of Cancer)1Chromatin packaging2表观遗传学概念w 表观遗传 (epigenetics)的概念是在 1942年由Waddington提出。指 DNA序列不发生变化但是基因表达却发生了可遗传的改变，也就是说基因型未变化而表型却发生了改变，这种变化是细胞内除了遗传信息以外的其他可遗传物质的改变，并且这种改变在发育和细胞增殖过程中能稳定地传递下去。3Cancer epigeneticsIn addition to having genetic causes, cancer can also be considered an epigenetic disease.Regulation by genetics involves a change in the DNA sequence, whereas epigenetic regulation involves alteration in chromatin structure and methylation of the promoter region45w表观遗传学的特点：l 可遗传的，即这类改变通过有丝分裂或减数分裂，能在细胞或个体世代间遗传；l 可逆性的基因表达调节，也有较少的学者描述为基因活性或功能的改变；l 没有 DNA序列的改变或不能用 DNA序列变化来解释。l 表观遗传修饰主要包括 DNA以及一些与 DNA密切相关的蛋白质 (例如组蛋白 )的化学修饰，另外某些非编码的 RNA也在表观遗传修饰中起着重要的作用。5Epigenetic Regulation of CancerSite SpecificHypermethylationGlobalHypomethylationHistone ModificationsRegulating FactorsDietaryHormonalGeneticVerma M, Srivastava S. Lancet Oncol. (2002) 3:755-63.DNA MethyltransferasesHistone MethyltransferasesHistone Acetylases/DeacetylasesEpigenetics Regulates:Cell Cycle ControlDNA DamageApoptosisInvasionX-Chromosome InactivationImprintingAging6表观遗传修饰从多个水平调控基因表达v DNA: DNA甲基化v蛋白质：组蛋白修饰v 染色质：染色质重塑v RNA：非编码 RNA7DNA甲基化w DNA甲基化 (DNA methylation)是研究得最清楚、 也是最重要的表观遗传修饰形式， 通过将S一腺苷甲硫氨酸作为甲基供体，并在 DNA甲基转移酶 (DNA methyltransferase， DNMT)的催化下， CpG二核苷酸中的胞嘧啶环上 5位置的氢被活性甲基所取代，从而转变成为 5甲基胞嘧啶 (5-methylcytosine， 5mC)。8Enzymatic methylation of the C5 position of cytosine residues can effect epigenetic inheritance by altering the expression of genes and by transmission of DNAmethylation patterns through cell division.Thus, in addition to its wellknown role in deaminationmutational hotspots in human DNA, DNA methylationmay contribute to gene inactivation in cancer.910w 哺乳动物基因组中 5mC占胞嘧啶总量的 2%-7%，约70%的 5mC存在于 CpG二连核苷。w 在结构基因的 5 端调控区域 , CpG二连核苷常常以成簇串联形式排列，这种富含 CpG二连核苷的区域称为 CpG岛 (CpG islands)，其大小为 500-1000bp，约 56%的编码基因含该结构。w 基因调控元件 (如启动子 )所含 CpG岛中的 5mC会阻碍转录因子复合体与 DNA的结合。l DNA甲基化一般与基因沉默相关联；l 非甲基化一般与基因的活化相关联 ；l 而去甲基化往往与一个沉默基因的重新激活相关联 。11l癌细胞的整个基因组水平处于低甲基化状态，比正常低 20% 60%，这种低甲基化大多发生于编码区和内含子区域，以及约占人类基因组20% 30%的重复序列区l抑癌基因启动子区域 CpG岛高度甲基化，且与DNA结合的组蛋白广泛去乙酰化 12Esteller, and J. G. Herman. Cancer as an epigenetic disease: DNAmethylation and chromatin alterations in human tumors. J Pathol, 2001.13Gene silencingDNA methylation is a powerful mechanism for thesuppression of gene activity.There is reciprocal relationship between the density of methylated cytosine residues and the transcriptional activity of a gene.The methyl groups do not affect base pairing but can influence proteinDNA interactions by protruding into the major groove.14The strong effect of 5methylcytosine (5mC) in mammalian promoter regions suggests that DNA methylation inhibits transcription by interfering with transcription initiation. DNA methylation reduces the binding affinity of sequencespecific transcription factors. Methylationdependent, sequencespecific DNAbindingproteins, such as MDBP may act as transcriptionalrepressors.15组蛋白修饰w 组蛋白修饰 (histone modification)是表观遗传研究的重要内容。w 组蛋白的 N端是不稳定的、无一定组织的亚单位 ，其延伸至核小体以外，会受到不同的化学修饰，这种修饰往往与基因的表达调控密切相关。w 被组蛋白覆盖的基因如果要表达，首先要改变组蛋白的修饰状态，使其与 DNA的结合由紧变松，这样靶基因才能与转录复合物相互作用。因此，组蛋白是重要的染色体结构维持单元和基因表达的负控制因子。 16Histone modifications including acetylation,methylation and phosphorylation are important in transcriptional regulation and many are stably maintained during cell division, although the mechanism for this epigenetic inheritance is not yet well understood.Proteins that mediate these modifications are often associated within the same complexes as those that regulate DNA methylation.17Covalent modification of histones: Acetylation of lysinesMethylation of lysines and argininesPhosphorylations of serines and threonines Histone Modifications18Histones5 types:H2A, H2B (slightly lys rich),H3,H4 (arg rich)H1 (lys rich). All relatively small proteins. Per 200 bp of DNA: 2 molecules each of H2A, H2B, H3, H4 and one molecule of H1.19Histone AcetylationAcetylation of the lysine residues at the N terminus of histone proteins removes positive charges, thereby reducing the affinity between Histones and DNA. This makes RNA polymerase and transcription factors easier to access the promoter region. Therefore, in most cases, histone acetylation enhances transcription while histone deacetylation represses transcription20Transcription process and its regulation by histone modification21Histone acetylation and cancerIn acute promyelocytic leukaemia:The oncoprotein produced by the fusion of the PML (promyelocytic leukaemia) gene and the retinoic acid receptor a gene appears to suppress the transcription of specific genes through the recruitment of HDACs.Thus the cancer cell is unable to undergo differentiation, leading to excessive proliferation.Similar phenomena:retinoic acid receptor aPLZF (promyelocytic leukaemia zinc finger protein)fusion, AML1 (acute myelocytic leukaemia protein 1)ETO fusion, and also in theMyc/Mad/Max signalling pathway involved in solid malignancies.22It is clear that HDAC enzymes seldom operate alone.Many proteins, with various functions such as recruitment, co-repression orchromatin remodelling, are involved in forming a complex that results in the repressor complex.23There are two protein families with HDAC activity: the recently discoveredSIR2 family of NAD+-dependent HDACs and the classical HDAC family.Members of the classical HDAC family fall into two different phylogeneticclasses - class I and class IIThe class I HDACs (HDAC1, 2, 3 and 8) are most closely related to the yeast(Saccharomyces cerevisiae) transcriptional regulator RPD3.Class II HDACs (HDAC4, 5, 6, 7, 9 and 10) share domains with similarity to HDA1, another deacetylase found in yeast .Recently a new member of the HDAC family has been identified, HDAC1124A wide variety of processes are associated with the inhibition of HDACs,such as apoptosis, necrosis, differentiation, inhibition of proliferation and cytostasis.Currently, many efforts are b