大腸直腸癌之PCDH10基因後生遺傳的研究

大腸直腸癌 (colorectal cancer, CRC)是常見的癌症，為國人癌症十大死因的第三名，偶發性CRC發生過程大多會從腺瘤變成腺癌，且又依照不同的基因轉變分為LOH (loss of heterozygosity)及MSI (microsatellite instability)兩種路徑。除了基因序列變異的累積會造成癌細胞的發展，基因後生遺傳修飾 (epigenetic modification)也扮演重要角色。常見的基因後生遺傳修飾調控即藉由DNA methyltransferases (DNMTs)將基因啟動子 (promoter) CpG位點之cytosine第5號碳進行甲基化 (mC)。CpG甲基化會抑制啟動子和轉錄因子 (transcription factors, TFs)的結合，使基因無法轉錄。目前CRC後生遺傳修飾的研究上，已發現許多抑癌基因受到DNA甲基化抑制的調控。本研究室於尋找CRC相關抑癌基因的研究中，分析腫瘤在第四號染色體微衛星標記發生LOH的情形，結果於4q25-4q28.3發現三個LOH發生率高的區域，而細胞吸附分子protocadherin 10 (PCDH10)基因即位於其中一個區域上。近期有研究指出PCDH10可能是抑癌基因，在鼻咽癌、血癌或其他腫瘤中，此基因的表現似乎受到後生遺傳修飾的抑制。為了探討大腸直腸癌中PCDH10基因是否受到啟動子過度甲基化 (promoter hypermethylation)的調控，本論文利用一系列DNA甲基化分析方法，檢測大腸直腸癌細胞株及CRC病人檢體PCDH10甲基化情形，並進一步統計PCDH10甲基化與臨床資料及其他癌症相關基因甲基化的相關性。結果顯示，CRC腫瘤之PCDH10甲基化程度明顯高於大腸正常黏膜組織，並具有統計上顯著差異，而腫瘤之PCDH10甲基化於病患年齡、性別、腫瘤位置、腫瘤分化程度及CRC分期則無顯著差異。此外，藉由DNA去甲基化藥物5-aza-2'-deoxycytidine (5-Aza-CdR)處理CRC細胞株SW620，可使PCDH10基因啟動子去甲基化並恢復其mRNA的表現，進一步證明在大腸直腸癌中，PCDH10基因的表現確實受到DNA甲基化的調控。

關鍵字

大腸直腸癌； PCDH10 ；後生遺傳修飾； DNA甲基化

並列摘要

Colorectal cancer (CRC) is one of the most common malignancies and is the third leading cause of cancer death in Taiwan. Most sporadic CRCs are thought to develop from benign adenomas to carcinoma. The two major pathways in colorectal carcinogenesis, LOH (loss of heterozygosity) and MSI (microsatellite instability), accumulate different genetic abnormalities. Tumorigenesis is not only linked to genetic alteration as a contribution but also to epigenetic regulation as a major player in cancer development. DNA methylation remains the best-studied epigenetic mechanism. Methylation at the C-5 position of cytosine residues present in CpG dinucleotides by DNA methyltransferases (DNMTs). This methylation inhibits gene expression by directly blocking the binding of transcription factors (TFs) to the target gene promoter sequence. Tumor suppressor genes (TSGs) silencing by DNA methylation has been found in many cases of cancer. In our preliminary study of LOH frequency on chromosome 4 in cases of CRC, three LOH loci were frequently defined around 4q25-4q28.3. A cell adhesion molecule, protocadherin 10 (PCDH10), is located in one of these regions. In a recent study, PCDH10 was identified as a candidate TSG due to frequent silencing in nasopharyngeal, esophageal, lymphoma and multiple other carcinomas by DNA methylation. However, there is little known about PCDH10 methylation in CRC. Therefore, the purpose of this study was to deremine whether PCDH10 expression is regulated by DNA methylation. We analyzed the frequency of PCDH10 methylation in CRC cell lines and patient tissue samples. Our studies revealed that PCDH10 was highly methylated in both CRC cell lines and CRC tissues. Methylation of PCDH10 between tumor tissues of CRC patients and adjacent normal mucosa was significant, but was not necessarily correlated to age, sex, tumor location, tumor differentiation, or Dukes' stage. On the other hand, the colon cancer cell, SW620, once treated with an epigenetic demethylation drug, 5-aza-2'-deoxycytidine (5-Aza-CdR), can restore PCDH10 RNA expression. Thus, PCDH10 expression is regulated by epigenetic modification and is frequently methylated in CRC.

並列關鍵字

Colorectal cancer ； PCDH10 ； Epigenetic modification ； DNA methylation

參考文獻

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大腸直腸癌之PCDH10基因後生遺傳的研究

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