肿瘤的表观遗传调控

1、1 肿瘤的表观遗传调控肿瘤的表观遗传调控 ( ( epigenetic regulation of cancer)2chromatin packaging3表观遗传学概念表观遗传学概念表观遗传表观遗传( (epigenetics) )的概念是在的概念是在19421942年由年由waddingtonwaddington提出。指提出。指dnadna序列不发生变化但是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细胞内除了遗传信息

2、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。能稳定地传递下去。4cancer epigenetics in 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 chrom

3、atin structure and methylation of the promoter region5表观遗传学的特点：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；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；为基因活性或功能的改变；l没有没有dnadna序列的改变或不能用序列的改变或不能用dnadna序列变化来解序列变化来解释。释。l表观遗传修饰主要包括表观遗传修饰主要包括dnadna以及一些与以及一些与dnad

4、na密切密切相关的蛋白质相关的蛋白质( (例如组蛋白例如组蛋白) )的化学修饰，另外的化学修饰，另外某些非编码的某些非编码的rnarna也在表观遗传修饰中起着重要也在表观遗传修饰中起着重要的作用。的作用。6epigenetic regulation of cancersite specifichypermethylationglobalhypomethylationhistone modificationsregulating factorsdietaryhormonalgeneticverma m, srivastava s. lancet oncol. (2002) 3:755-63.dn

5、a methyltransferaseshistone methyltransferaseshistone acetylases/deacetylasesepigenetics regulates:cell cycle controldna damageapoptosisinvasionx-chromosome inactivationimprintingaging7表观遗传修饰从多个水平调控基因表达表观遗传修饰从多个水平调控基因表达v dna: dna甲基化甲基化v蛋白质：组蛋白修饰蛋白质：组蛋白修饰v 染色质：染色质重塑染色质：染色质重塑v rna：非编码：非编码rna8dnadna甲基化

6、甲基化 dna甲基化甲基化(dna methylation)是研究得最清是研究得最清楚、楚、 也是最重要的表观遗传修饰形式，通过将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，并在一腺苷甲硫氨酸作为甲基供体，并在dna甲甲基转移酶基转移酶(dna methyltransferase，dnmt)的的催化下，催化下，cpg二核苷酸中的胞嘧啶环上二核苷酸中的胞嘧啶环上5位置位置的氢被活性甲基所取代，从而转变成为的氢被活性甲基所取代，从而转变成为5甲基甲基胞嘧啶胞嘧啶(5-methylcytosine，5mc)。9 enzymatic methylation of the c5 p

7、osition of cytosine residues can effect epigenetic inheritance by altering the expression of genes and by transmission of dna methylation patterns through cell division. thus, in addition to its wellknown role in deamination mutational hotspots in human dna, dna methylation may contribute to gene in

8、activation in cancer.1011哺乳动物基因组中哺乳动物基因组中5 5mcmc占胞嘧啶总量的占胞嘧啶总量的2%-7%2%-7%，约，约70%70%的的5 5mcmc存在于存在于cpgcpg二连核苷。二连核苷。在结构基因的在结构基因的55端调控区域端调控区域, , cpgcpg二连核苷常常二连核苷常常以成簇串联形式排列，这种富含以成簇串联形式排列，这种富含cpgcpg二连核苷的二连核苷的区域称为区域称为cpgcpg岛岛( (cpgcpg islands) islands)，其大小为，其大小为500-500-1000bp1000bp，约，约56%56%的编码基因含该结构。的编码基

9、因含该结构。基因调控元件基因调控元件( (如启动子如启动子) )所含所含cpgcpg岛中的岛中的5mc5mc会阻会阻碍转录因子复合体与碍转录因子复合体与dnadna的结合。的结合。ldnadna甲基化一般与基因沉默相关联；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而去甲基化往往与一个沉默基因的重新激活相关联。12l癌细胞的整个基因组水平处于低甲基化状态，癌细胞的整个基因组水平处于低甲基化状态，比正常低比正常低20%20%60%60%，这种低甲基化大多发生于，这种低甲基化大多发生于编码区和内含子区域，

10、以及约占人类基因组编码区和内含子区域，以及约占人类基因组20%20%30%30%的重复序列区的重复序列区l抑癌基因启动子区域抑癌基因启动子区域cpgcpg岛高度甲基化，且与岛高度甲基化，且与dnadna结合的组蛋白广泛去乙酰化结合的组蛋白广泛去乙酰化 13esteller, and j. g. herman. cancer as an epigenetic disease: dnamethylation and chromatin alterations in human tumors. j pathol, 2001.14gene silencingdna methylation is a p

11、owerful 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 gr

12、oove.15the 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, sequencespe

13、cific dnabinding proteins, such as mdbp may act as transcriptional repressors.16组蛋白修饰组蛋白修饰组蛋白修饰组蛋白修饰(histone(histone modification) modification)是表观遗传是表观遗传研究的重要内容。研究的重要内容。组蛋白的组蛋白的 n n端是不稳定的、无一定组织的亚单位端是不稳定的、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。种修饰往往与基因的表达调控密切相关。被组蛋白覆

14、盖的基因如果要表达，首先要改变组蛋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，使其与白的修饰状态，使其与dnadna的结合由紧变松，这样的结合由紧变松，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是重要的染色体结构维持单元和基因表达的负控制因子。因子。17 histone modifications including acetylation, methylation and phosphorylation are important in transcriptional regulati

15、on 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.18covalent modification of histone

16、s: acetylation of lysinesmethylation of lysines and argininesphosphorylations of serines and threonines histone modifications19histones5 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 on

17、e molecule of h1.20histone 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 mos

18、t cases, histone acetylation enhances transcription while histone deacetylation represses transcription21transcription process and its regulation by histone modification22histone acetylation and cancerin acute promyelocytic leukaemia: the oncoprotein produced by the fusion of the pml (promyelocytic

19、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

20、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.23it is clear that hdac enzymes seldom operate alone. many proteins, with various functions such as recruitment, co-repression or

21、chromatin remodelling, are involved in forming a complex that results in the repressor complex.24there 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 ph

22、ylogeneticclasses - class i and class ii the 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

23、a new member of the hdac family has been identified, hdac1125 a 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 being made to expand our knowledge of the hdacs

24、and to develop potent and stable hdaci. 26responsive genes by studying the effects of hdaci on the expression of various genes and their regulatory pathways, a more detailed picture will emerge of how the inhibition of hdacs, combined with the hdac expression profile of that cell, ultimately determi

25、nes the fate of the cell.27组蛋白修饰的种类组蛋白修饰的种类28染色质重塑染色质重塑染色质重塑（染色质重塑（chromatin remodelingchromatin remodeling）是一个）是一个重要的表观遗传学机制。重要的表观遗传学机制。染色质重塑是由染色质重塑复合物介导的一系染色质重塑是由染色质重塑复合物介导的一系列以染色质上核小体变化为基本特征的生物学列以染色质上核小体变化为基本特征的生物学过程。过程。组蛋白尾巴的化学修饰（乙酰化、甲基化及磷组蛋白尾巴的化学修饰（乙酰化、甲基化及磷酸化等）可以改变染色质结构，从而影响邻近酸化等）可以改变染色质结构，从而影

26、响邻近基因的活性。基因的活性。29johnstone 2002epigenetic mechanisms30chromatin-level regulation of gene expressiongarfinkel and ruden nutrition:20:56-62, 200431染色质修饰与重塑（共价修饰型与染色质修饰与重塑（共价修饰型与atpatp依赖型）依赖型）32micrornamicrornamirnamirna是近年来生命科学领域的研究热点，是一组生是近年来生命科学领域的研究热点，是一组生物体基因组编码的内源性非编码小物体基因组编码的内源性非编码小rnarna。mirnam

27、irna主要采用降解靶主要采用降解靶mrnamrna和抑制靶和抑制靶mrnamrna的翻译两种的翻译两种作用方式在转录后水平调控基因表达。作用方式在转录后水平调控基因表达。降解靶降解靶mrnamrna的方式与的方式与sirnasirna的作用方式相似，直接作的作用方式相似，直接作用于靶用于靶mrnamrna，直接导致，直接导致mrnamrna表达水平下降。表达水平下降。但绝大多数哺乳动物细胞中的但绝大多数哺乳动物细胞中的mirnasmirnas 并不导致靶并不导致靶mrnamrna的降解，而是通过与靶的降解，而是通过与靶mrnamrna的的33端非翻译区端非翻译区（utrutr）不完全匹配结合，抑制）不完全匹配结合，抑制mrnamrna翻译成蛋白质，翻译成蛋白质，使靶基因的蛋白质表达水平下降。使靶基因的蛋白质表达水平下降。33 a primary transcript (pri-mirna) is first processed into a stem-loop structure about 6080 nt (pre-mirna) by the rnase endonuclease drosha .pre-mirnas are exporte