The role of p53 in remodeling DNA methylation in cancer

描述（由申请人提供）：基因组DNA的甲基化模式在肿瘤发生过程中发生显著改变。在人类肿瘤中，dna -胞嘧啶甲基化的总体水平降低，但CpG岛通常变得高甲基化，导致肿瘤抑制基因频繁的表观遗传失活。最常见的观察之一是肿瘤抑制基因的一个等位基因突变或丢失，而另一个等位基因因超甲基化而沉默。引发这种异常从头甲基化的事件知之甚少，也不知道三种特征DNA甲基转移酶中的哪一种是负责任的。然而，众所周知，甲基化机制在发育过程中必须受到严格的调控，并且在肿瘤发生过程中，参与这种调控的因素的平衡似乎被破坏了。在所有类型的人类肿瘤中，超过50%的肿瘤抑制基因p53发生突变或缺失，在许多病例中，p53似乎是肿瘤发生的早期事件。我们最近发现，在没有刺激激活p53的情况下，p53与DNA甲基转移酶1 （Dnmt1）启动子结合，p53的激活减少了这种结合，p53功能的丧失诱导Dnmt1的上调。这些数据表明，这种重要的癌症基因的改变或缺失可能会促进异常的基因组甲基化。我们现在提议确定p53介导的DNA甲基转移酶1控制的潜在机制，它与相关DNA甲基转移酶的普遍性，以及它在致癌过程中肿瘤抑制基因功能丧失中的作用。我们建议在体外和体内研究野生型和突变型p53与Dnmt基因座启动子区域的相互作用，利用染色质免疫沉淀，利用siRNA技术调节p53和Dnmt的水平，并利用串联质谱技术研究Dnmt基因启动子上涉及p53及其突变形式的蛋白复合物的组成。

DESCRIPTION (provided by applicant): The pattern of methylation of genomic DNA becomes significantly altered during oncogenesis. In human tumors, the overall level of DNA-cytosine methylation is decreased, but CpG islands generally become hypermethylated, resulting in frequent epigenetic inactivation of tumor suppressor genes. One of the most common observations is that one allele of a tumor suppressor gene becomes mutated or lost, and the other allele becomes silenced by hypermethylation. Little is known of the events that trigger this aberrant de novo methylation, nor of is it known which of the three characterized DNA methyltransferases is responsible. It is however, well recognized that the methylation machinery must be under strict regulatory control during development, and that the balance of factors involved in this regulation appears to be disrupted during tumorigenesis. The tumor suppressor gene, p53 is mutated or lost in more than 50% of all types of human tumors, and appears to be an early event in tumorigenesis in many cases. We have recently found that p53 binds the DNA methyltransferase 1 (Dnmt1) promoter in the absence of stimuli that activate p53, that activation of p53 reduces this binding, and that loss of p53 function induces upregulation of Dnmt1. These data suggest that aberrant genomic methylation might be promoted by alteration or loss of this important cancer gene. We now propose to determine the underlying mechanism of p53-mediated control of DNA methyltransferase1, its generality to the related DNA methyltransferases, and its role in the loss of tumor suppressor gene function during carcinogenesis. We propose to study the interaction of wild type and mutant p53s with the promoter regions of the Dnmt loci both in vitro and in vivo, using chromatin immunoprecipitation, to modulate the levels of p53 and Dnmts using siRNA technology, and to study the composition of protein complexes involving p53 and its mutant forms resident on the promoters of the Dnmt genes using tandem mass spectrometry. These studies will lead to an understanding of the early events in tumor progression that result in disruption of the control of DNA methylation. The inappropriate tumor suppressor gene silencing that follows this loss of control appears to be critical in promoting oncogenesis.