Smad4基因在台灣偶發型大腸直腸癌之突變分析

Abstract

大腸直腸癌是台灣常見的癌症之ㄧ，過去數十年間，其發生率與死亡率都不斷的攀升，因此，大腸直腸癌在公共衛生議題的重要性可見一斑。大腸直腸癌的發生是一個多重步驟的過程，許多基因變異的累積終致腫瘤的形成。而其中第十八條染色體長臂(18 q21)區域是大腸直腸癌經常發生基因缺失的位置。此區域有三個可能的抑癌基因(tumor suppressor gene)存在，它們分別是DCC、Smad2以及Smad4基因。由於目前DCC與Smad2基因的支持證據較為薄弱，部分研究結果顯示Smad4基因可能是參與大腸直腸癌癌化過程主要的抑癌基因。然而，Smad4基因在大腸直腸癌中所扮演的角色仍有待進一步釐清。 本研究重新確認台灣偶發型大腸直腸癌病人腫瘤中第十八條染色體長臂 18 q21位置缺失的比率和Smad4基因異常的情形。除了運用傳統突變偵測技術來偵測基因的突變外，我們更採用一種新技術(Multiplex ligation-dependent probe amplification; MLPA)來精確偵測特定區域的基因套數變化。在63個病人中，我們共發現19個新的突變，其中只有兩個是用傳統突變偵測方法找到的，由於MLPA技術的引入，Smad4基因突變的偵測比率大幅升高。相較於染色體18 q21區域缺失的比率(42.4%)，Smad4基因突變的比率(30.2%)雖略低，但這似乎意味著Smad4基因確實是參與在大腸直腸癌癌化過程中的抑癌基因。

Colorectal cancer is currently the third most common cancer in Taiwan. Over the past decade, the mortality rate of colorectal cancer has increased remarkably. Therefore, it has become an important public health issue these days. The initiation and progression of colorectal cancer is a multi-step process leading to the accumulation of genomic alterations occurring over the lifetime of a tumor. Chromosome 18q21 is one of the most frequently lost regions in colorectal cancer. There are three candidate tumor suppressor genes located in this region, including DCC, Smad2 and Smad4. Since accumulated evidence has failed to support DCC and Smad2 as tumor suppressor genes involved in colorectal carcinogenesis, Smad4 seems to be the major one. However, there is a gap existing between chromosome loss of 18q and mutations of Smad4. In our study, we reconfirmed the status of 18q and screened all the coding region of Smad4 gene for small mutations. Furthermore, we performed a novel method, multiplex ligation-dependent probe amplification (MLPA), to detect copy number changes of Smad4. Among 63 patients analyzed, 19 (30.2%) mutations were identified and only two were found by conventional mutation detection methods. By using MLPA, the frequency of Smad4 mutations was raised. Although the relative low frequency of Smad4 inactivation (30.2%) contrasts with the high frequency of LoH at 18q21 (42.4%), it may implicate that Smad4 is the major tumor suppressor gene contributing to colorectal carcinogenesis in 18q21 region.