Germline epimutation of hMLH1 as a factor in HNPCC

描述(申请人提供)：本项目旨在研究DNA错配修复(MMR)基因hMLH1的表观遗传沉默(“表观突变”)在缺乏MMR的肿瘤的发展中所起的作用。遗传性非息肉病性结直肠癌(HNPCC)是一种由于MMR基因的基因突变而发展成多种癌症的家族综合征。但许多似乎患有HNPCC的患者并没有可识别的突变。突变可以通过使基因失活来模仿基因突变。突变可能存在于生殖细胞中，因此是可遗传的，但它不会以与DNA序列变化相同的方式传递。我们发现一些明显的HNPCC患者在hMLH1中存在这样的突变，这可以通过评估hMLH1启动子中的胞嘧啶甲基化来识别。我们建议通过筛查缺乏MMR基因突变的疑似HNPCC病例来识别更多此类病例。这一策略可能会确定 这解释了为什么在一些家庭中，癌症的易感性并不表现为孟德尔式的特征。我们还将询问hMLH1的表型是否更有可能发生在hMLH1基因座的某些基因类型上，这表明该基因座的遗传结构可能使其易于沉默。许多散发性(非家族性)癌症患者在MMR中表现出缺陷。我们假设，一些人患癌症的风险增加，因为他们只在一小部分细胞中携带hMLH1突变，即他们是嵌合体。我们将检测2000名正常献血者外周血中hMLH1突变的比例，从而建立一个正常范围。我们还将评估散发性MMR缺陷肿瘤患者中hMLH1缺失的比例。这些研究有可能发现证据表明，肿瘤抑制基因的突变是散发性癌症的潜在原因。他们还可能提供第一个明确的证据，表明表观遗传沉默可以遗传来产生家族性疾病，如HNPCC。

DESCRIPTION (provided by applicant): This project aims to investigate the role that epigenetic silencing ("epimutation") of the DNA mismatch repair (MMR) gene hMLH1 plays in the development of tumors that are deficient in MMR. Hereditary non-polyposis colorectal cancer (HNPCC) is a familial syndrome in which patients develop multiple cancers due to genetic mutation of a MMR gene. But many patients who appear to have HNPCC do not have an identifiable mutation. Epimutation can mimic genetic mutation by inactivating a gene. An epimutation may be present in germline cells, and so be heritable, but it is not passed on in the same way as a change in DNA sequence. We have found that some people with apparent HNPCC carry such an epimutation in hMLH1, which can be identified by assessing cytosine methylation in the hMLH1 promoter. We propose to identify more such cases by screening suspected HNPCC cases who lack mutation in an MMR gene. This strategy may identify kindreds in which the epimutation has been inherited, explaining why in some families cancer predisposition does not behave as a Mendelian trait. We will also ask if epimutation of hMLH1 is more likely to occur on certain genotypes at the hMLH1 locus, which would suggest that something about the genetic architecture of the locus can predispose it to silencing. Many sporadic (non-familial) cases of cancer exhibit deficiency in MMR. We hypothesize that some individuals have an increased risk of developing a cancer because they carry the hMLH1 epimutation in only a small proportion of their cells, i.e., they are mosaic. We will measure the proportion of the hMLH1 epimutation in peripheral blood of 2000 normal blood donors, thus establishing a normal range. We will also assess the proportion of hMLH1 epimutation in patients with sporadic MMRdeficient tumors. These studies have the potential to uncover evidence that epimutation of a tumor suppressor gene is an underlying cause of sporadic cancers. They may also produce the first clear evidence that epigenetic silencing can be inherited to produce a familial disorder such as HNPCC.