大腸直腸癌之基因與環境交互作用

Abstract

大腸直腸癌的致癌過程牽涉到「基因與環境的交互作用」。本研究室以早發性大腸直 腸癌（發病年齡小於40 歲者）為模型來進一步闡明「基因與環境」交互作用的機轉。 我們先前的研究指出發病年齡小於40 歲與大於60 歲者相較，其有較高的機會出現MSI-H （high-frequency microsatellite instability），其比例分別為29.4﹪與6.3﹪（p＜0.001）。 然而進一步追蹤這二組病患的家族史卻顯示二組並無差別，分別是17.4﹪與14.3﹪ （p=0.859）。此結果意味著早發性大腸直腸癌的產生是由於環境的作用，而非來自遺 傳。在本研究中我們共收集54 位年齡小於40 歲的早發性大腸直腸癌且表現MSI-H 的病 患作分析。實驗步驟包括核酸誤配基因（mismatch repair genes）hMSH2 與hMLH1 的西 方點墨法檢驗，dHPLC 與直接定序方法分析hMLH1 的基因突變，以及分析hMLH1 promoter region 的過度甲基化情況（hypermethy lation）。 我們發現西方點墨法中hMLH1 不表現的機率63.0﹪（n=34），而hMSH2 不表現的機率為3.7﹪（n=2）。以dHPLC 及直接定序的分法分析hMLH1 基因（n=34）發現基因突變率為5.6﹪（n=3），其餘31 位中我們發現hMLH1 基因之promoter region 的過度甲基化現象在28 位病患（51.9﹪） 出現，而其餘3 位則未知其突變情況。此一研究結果顯示台灣地區早發性大腸直腸癌的 產生是透過MSI 的路徑，而hMLH1 promoter region 的過度甲基化是造成MSI 的主要機 轉，此意味著早發性大腸直腸癌的產生主要仍是環境的變動造成。

The colorectal tumorigenesis involves the interaction between environmental and genetic alterations. In this study, we further clarify this mechanism, i.e., the gene-environment interaction, based on the previous study that early onset colorectal cancers (age of disease onset less than 40 years old) tended to present with high fuquency microsatellite instability (MSI-H), as compared with late onset counterparts (age of disease onset more than 60 years old), with 29.4% and 6.3%, respectively (p＜0.001). However, further tracing family history showed that there was no significant difference between these 2 groups of patients, with the incidence of a positive family history of cancer 17.4% and 14.3%, respectively (p=0.0859). This result implied that the early-onset colorectal cancers in Taiwan result predominantly from environmental rather than familial germline alteration. In the present study, we further analyze early onset colorectal cancers with MSI-H. The experimental methods included Western blotting of mismatch repair genes hMSH2 and hMLH1, and dHPLC analysis, direct sequencing, and hypermethylation analysis of hMLH1 gene. We found that hMLH1protein was negative in 63.0% (n=34), and hMSH2protein was negative in 3.7% (n=2), during the Western blotting analysis. dHPLC and direct gene sequencing showed that genetic mutation of hMLH1 gene was present in 3 patients (5.6%), 28 (51.9%) of the remaining 31 patients presented with hypermethylation on the promoter region of hMLH1, whereas the mechanism for altmations of hMLH1 in the other 3 patients was unknown. This result implied that a subset of early onset of colorectal cancer was through the carcinogenic pathway of MSI, and the hypermethylation of hMLH1 promoter represented a major mechanism for the presentation of MSI-H in the early-onset colorectal cancers of Taiwan. Therefore, we concluded that environmental factors might be more important than genetic factors for the tumorigenesis of early onset colorectal cancer in Taiwan.