背景5hmC- 哺乳动物DNA中的“第六碱基” 早在上世纪70年代,Penn等人首次发现在哺乳动物DNA中存在5-羟甲基化胞嘧啶(5hmC)(Penn et al., 1972)。然而该发现并未得到反复证实
睾丸特异性基因启动子中5hmC的分布
在大脑DNA中,睾丸特异性基因的启动子区域一般都是高甲基化的。事实上,正是因为启动子区域发生高甲基化,所以睾丸特异性基因和其他生殖细胞特异表达的基因在体细胞通常是表达受抑制的 (De Smet et al., 1999),但是与高丰度的5mC不同的是,这些生殖相关基因中的5hmC含量几乎可以忽略不计。
胚胎干细胞和分化过程中的5hmC动态调控
由于染色质均衡态的构象及基因的组织特异性表达都与5hmC存在特定的联系,因此研究人员猜测5hmC的存在使一些特定的基因座能够快速响应适当的生物学信号刺激从而被激活。例如组织特异性基因的激活可能通过5mC去甲基化途径(5mC-5hmC-C)或者募集特异识别5hmC的转录调控因子来响应分化信号(Pastor et al., 2011)。又如,胚胎干细胞分化为拟胚体(EBs)过程中,多能性相关基因的启动子区的5hmC丰度下降,而这些位置的5mC丰度却在增加(Ficz et al., 2011)。此外,有人提出在产生诱导干细胞过程中,5hmC可以清除干性相关基因启动子区域的甲基化修饰 (Meissner et al., 2008)同时也能大规模地清除原生殖细胞的甲基化修饰(Iqbal et al., 2011; Ito et al., 2010)。
氧气感知与调控
Song等人发现基因中5hmC富集程度与低氧环境下的血管生成密切相关。Tet蛋白催化5mC变成5hmC的过程需要氧分子(Ito et al., 2010; Tahiliani et al., 2009)。HIF蛋白是哺乳动物系统中一个著名的氧气感知器,主要参与了缺氧诱导的血管生成等生理过程,而且研究表明HIF蛋白与Tet蛋白催化结构域均属于单铁双加氧超家族。因此,有人猜测在哺乳在动物细胞中,Tet蛋白氧化5mC成5hmC可能组成了一个新的氧气感受和调节通路。
受精卵细胞的表观重编程
哺乳动物的早期胚胎获得发育全能性时候就在表观基因组水平发生了大规模的重编程。而研究表明哺乳动物受精卵中的5hmC与表观重编程有密切联系。据报道,受精卵的雄原核会在基因组范围内发生DNA去甲基化事件,这种去甲基化又明显依赖于DNA的复制。Wosido等人发现伴随着5mC水平的下降,雄原核5hmC的含量却在逐渐地积累。这些雄原核的5hmC可能是由氧化酶Tet3氧化5mC产生的。
然而目前对5hmC在雄原核中的作用了解的还是不很清楚。5mC氧化生成5hmC的一个直接效应可能就是中和5mC对基因表达的抑制。我们知道,小鼠胚胎基因组的活化发生在两细胞时期,而那些在精子发生过程被DNA甲基化所抑制的许多基因如Oct4和Nanog都需要重新激活才能使得发育继续进行。5mC氧化生成5hmC后,5mC抑制蛋白不能够识别并结合该DNA序列,从而解除了5mC的抑制作用。(Jin et al., 2010; Valinluck et al., 2004)。另一种可能就是,5hmC替换5mC后就不再作为维持性甲基化酶DNMT1的底物(Valinluck and Sowers, 2007),这就意味着在早期胚胎的DNA复制过程中,即使DNMT酶存在活性,5hmC的形成也能够稀释DNA中甲基化CpG的含量。
神经元的发育和成熟
Song等人实验观察到小鼠小脑中的5hmC含量随发育的时期而增加。与出生7天后的小鼠小脑相比,成年小鼠的5hmC水平显著提高,这表明5hmC有可能直接参与了神经元的发育和成熟。
5hmC在疾病中的作用
5hmC与神经退行性疾病
研究人员通过对衰老过程中含有5hmC的5425个基因进行GO(Gene Ontology)富集分析和信号通路(Pathway)富集分析发现,显著富集的信号通路都是与神经退行性失调,血管生成和缺氧应答过程密切相关。而且对富集基因列表的分析发现成年小鼠大约有15/23的基因发生了5羟甲基化修饰,而这些基因都是以前鉴定可能会引起小鼠和人的Purkinje细胞退化的(Lim et al., 2006)。总之,5hmC修饰经常发生在一些参与神经退行性失调的基因上面,因此5hmC可能对人神经失调疾病的发生有促进作用。
5hmC与癌症
实验表明,5hmC在结肠癌组织中的含量显著下降,甚至在结肠癌细胞株当中几乎无法检测到。多样本(38例结肠癌组织和8例正常结肠组织)的检测进一步证实结肠癌组织中的5hmC含量远低于正常组织。这些结果表明至少在结肠组织中,5hmC对肿瘤形成和发生可能具有负调控作用。其中可能的机制是DNA通过5hmC影响5mC的总量,所以结肠癌组织5hmC含量减少后将导致受到高度甲基化所抑制的肿瘤抑制基因和凋亡相关基因无法被重新激活,从而使这些肿瘤细胞免受凋亡基因或抑制基因的调控(Li and Liu, 2011)。
5hmC还可能直接参与了白血病的发生。TET2位于染色体的4q24,这个区域在脊髓发育不全病人中经常发生染色体缺失或重排(Viguie et al., 2005)。Tet2错义突变主要发生C-端包含2OGFeDO的结构域,进而干扰了Tet2催化5mC变成5hmC的活性(Ko et al., 2010)。与正常样本相比,TET2突变的急性髓性白血病人的骨髓样本中5hmC含量非常低(Ko et al., 2010),所有这些结果表明5hmC可能参与急性髓性白血病的发生。
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5羟甲基化的研究背景及与疾病的关系价格
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