错配修复 (MMR) 是维持基因组稳定性的重要机制，其功能的丧失导致结直肠癌。MMR丧失所致癌的共同特征是表.现微卫星不稳定 (MSI)。然而约20% 的结直肠癌虽有MSI，却无MMR缺陷。 新近研究表明，H3K36me3 虽不参与MM.R 反应, 但与MMR识别蛋白MutSα相互作用从而招募MutSα到染色质上. 故推测H3K36me3的缺失和影响其与MutS.α相互作用的突变可能是产生MSI的原因。为证实这一假说，我们将对中国和以色列MSI与MMR不匹配的结直肠癌.进行如下研究：1) 检测H3K36三甲基酶SETD2的突变；2）免疫组化分析H3K36me3的水平；3）研究MMR基因突变（.包括阻断H3K36me3-MutSα 相互作用的突变）对MMR功能的影响。本研究将不仅解开MMR与MSI在癌中不匹配的科.学难题，而且将为结直肠癌早期诊断和有效治疗提供新的生物标记物。

DNA mismatch repair (MMR) maintains genome stability by correcting mismatches generated during DNA.replication. Defects in MMR genes are associated with certain types of cancers, including.hereditary non-polyposis colorectal cancer (HNPCC, also called Lynch syndrome) and a subset of.sporadic colorectal cancers. A characteristic of these MMR-deficient cancers is their ability to.display frequent alterations in simple repetitive DNA sequences, a phenomenon called.microsatellite instability (MSI). However, subsequent studies revealed that at least 20% of.MSI-positive colorectal cancers (including Chinese and Israeli HNPCC families) do not carry a.detectable mutation in MMR genes. The molecular mechanism underlying MSI in these cases has been a.long-standing puzzle in the field. Recent studies have shown that an epigenetic histone mark,.histone H3 Lys36 trimethylation (H3K36me3), is not required for MMR in vitro, but is essential for.MMR in vivo as H3K36me3 interacts with the PWWP domain of mismatch recognition protein MutSα to.recruit the protein chromatin for MMR. Defects in H3K36 trimethyltransferase SETD2 confer an.MMR-deficient phenotype. We therefore hypothesize that lack of H3K36me3 or mutations in the MutSα.PWWP domain that disrupt the H3K36me3-MutSα interaction is responsible for some of the.MSI-positive colorectal cancer without MMR gene mutations. To test this hypothesis, we will target.Chinese and Israeli HNPCC families without MMR genes mutations for the following experiments: 1).determining SETD2 mutations by DNA sequencing analysis; 2) measuring H3K36me3 levels using.immunohistochemistry analysis; 3) determining the impact of MutSα PWWP germline mutations on.MMR function; 4) determining MMR activity of key germline mutations of MMR genes. A successful.completion of this study will not only provide definitive clues to the long-standing puzzle that.MSI-positive cancers contain no mutations in MMR genes, but also novel biomarkers for colorectal.cancer detection and potential targets for treatment.

本项目旨在研究DNA错配修复 (MMR) 机制是如何维持基因组稳定性以及如何与癌症的发生发展相关联。MMR功能的缺失会导致结直肠癌的发生，MMR丧失所导致的癌的共同特征之一是表现微卫星不稳定 (MSI)。然而，研究表明约20%的结直肠癌虽具有MSI表征，却无MMR缺陷。新近研究表明，组蛋白修饰H3K36me3 虽不参与MMR反应，但可与MMR识别蛋白MutSα相互作用从而将MutSα招募到染色质上。因此我们推测，H3K36me3的缺失以及能影响其与MutSα相互作用的基因突变可能是产生MSI表征的原因。为证实这一假说，我们将对中国和以色列MSI与MMR不匹配的结直肠癌进行免疫组化分析H3K36me3水平及检测其三甲基酶SETD2基因的突变。本研究将不仅解开MMR与MSI在癌中不匹配的科学难题，还将为结直肠癌的早期诊断和有效治疗提供新的生物标记物。