大腸直腸癌中GALNT2或C1GALT1的過度表現與T抗原之增加有關

Abstract

在台灣地區，惡性腫瘤多年蟬聯十大死因榜首，而大腸直腸癌是排名第三的癌症死因。台灣地區的大腸直腸癌發生率仍不斷上升中。腫瘤相關醣抗原Tn和T抗原是O-聚醣的根部結構，在正常的上皮組織中，Tn和T抗原會經由不同醣類轉移酶轉換成更複雜的結構。Tn抗原的表現和大腸直腸癌的腫瘤組織分化程度有關；T抗原陽性的病患伴隨更高的癌症肝轉移率。至今腫瘤組織中大量表現Tn和T抗原的原因仍不明確。我們試圖從合成Tn、T抗原的轉移酶：GalNAc-transferase (GALNT family)、Core 1 ß-3-galactosyltransferase (C1GALT1)，找出癌細胞大量表現Tn、T抗原的原因。我們選定四個候選酶分別是：GALNT1、GALNT2、GALNT10和C1GALT1。我們收集61個大腸直腸癌病人非腫瘤和腫瘤組織，並利用RT-PCR、西方點墨法、免疫組織化學染色法檢視這四個轉移酶在腫瘤組織的表現量，並且找出它們和臨床資料的關聯性。再以免疫組織化學染色法染Tn和T抗原，利用Chi square統計出Tn、T抗原和這四個轉移酶之間的相關性。免疫組織化學染色結果顯示，55%之病人腫瘤組織的GALNT2表現量上升，並且和Tn (p<0.001)和T (p = 0.002)抗原的表現相關。和正常組織相比，過度表現GALNT2的病人有較差的存活率(p = 0.004)。綜合RT-PCR、西方點墨法、免疫組織化學染色法的結果，C1GALT1在67-72%的病人腫瘤中的表現量上升，而且和T抗原表現相關(p = 0.004)，而腫瘤中過度表現C1GALT1的病人在癌症完全緩解後，更容易產生復發的情形(p=0.0052)。西方點墨法的結果顯示，41%之病人腫瘤的GALNT1表現量下降(p = 0.009)，並且和Tn (p = 0.301)、T (p = 0.592)抗原的表現無關；76%之病人腫瘤的GALNT10表現量下降(p<0.001)，並且和Tn (p = 0.294)、T (p = 1.0)抗原的表現無關。根據這些結果我們推測，O-聚醣合成的初始階段中GALNT2和C1GALT1在大腸直腸癌細胞的表現量上升，可能是造成Tn和T抗原過度累積的原因之一，進而增加癌細胞惡性的能力。未來需要更多研究進一步證明我們的假設，並探討GALNT2和C1GALT1作為標靶治療的潛力。

Malignant tumors have been the top cause of death for many years. Colorectal cancer is the third leading cancer death in Taiwan and its incidence is still rising. Tumor associated carbohydrate antigens, Tn and T , are the short O-glycans. Expression of Tn antigen has been associated with differentiation status of colorectal cancer. Patients with T-positive tumors have significantly higher risk to develop liver metastases. The reason for overexpression of Tn and T antigens in tumor tissues remains unclear. To uncover the reason for Tn and T antigen overexpression in colorectal cancer, we selected 3 candidate enzymes from the 20 members of GALNT family, which catalyzed a GalNAc structure to Ser/Thr residue in forming Tn antigen, namely GALNT1, GALNT2, GALNT10, and the core 1 ß-3-galactosyl- transferase (C1GALT1). C1GALT1 is the only enzyme that catalyzes the formation of T antigen from the Tn antigen. We collected 61 samples of paired tumor and non-tumor colorectal tissues. We analyzed the expression levels of the four selected enzymes by RT-PCR, Western blot, and immunohistochemistry and correlated the results with clinicopathologic features. Tn or T antigens were stained by immunohistochemistry, and Chi square was used for statistical analysis. Immunohistochemical results showed that 55% of colorectal cancer patients displayed higher GALNT2 expression and was associated with lower survival rate (p = 0.004). GALNT2 expression was correlated with Tn and T antigen expression. Approximately 67 to 72% patients showed overexpression of C1GALT1 and they tend to recur after complete remission (p = 0.0052). C1GALT1 expression was associated with T antigen expression. Results from Western blotting showed that GALNT1 and GALNT2 were downregulated in 41% (p = 0.009) and 72% (p<0.001) of patients, respectively. Neither GALNT1 nor GALNT10 expression correlated with Tn or T antigen expression. According to these results, we speculate that overexpression of GALNT2 and C1GALT1 might enhance the malignant ability of colorectal cancer through accumulation of Tn and T antigen in cancer. Further study is needed to prove our hypothesis and to investigate GALNT2 and C1GALT1 as potential therapeutic targets for colorectal cancer.