Abstract 648: Methylation levels of LINE-1 in primary lesion and matched metastatic lesions of colorectal cancer.

Background: Cancer cells exhibit two types of DNA methylation changes: global DNA hypomethylation and site-specific CpG island promoter hypermethylation. Genome-wide DNA hypomethylation plays an important role in genomic instability and carcinogenesis. As LINE-1 or L1 retrotransposon constitutes a substantial portion of the human genome, the level of L1 methylation is regarded to be a surrogate marker of global DNA methylation. In Colorectal cancer (CRC), L1 methylation in primary tumor has been shown to be highly variable, and L1 hypomethylation is strongly associated with poor prognosis. Nonetheless, no study has examined L1 methylation level in metastatic lesions [liver, lung, and lymph node (LN)] from CRCs, and assessed the heterogeneity of L1 methylation level within CRC tumor. Methods: A total of 65 patients with liver and/or lung metastases from CRC who were undergoing hepatic and/or lung resection at Kumamoto University Hospital were enrolled in this study. The bisulfite-pyrosequencing technology was utilized to quantify the L1 methylation level in 60 liver metastases, matched 26 primary tumors and matched 6 LN metastases, and 6 paired lung metastatic and primary tumors. The heterogeneity within primary tumor (superficial area vs. invasive area) was also evaluated (n=6). Results: The distribution of the L1 methylation level in liver metastases (n=60) was as follows: mean, 68.9; median, 71.0; SD, 10.8; range, 37.1-90.1 (all in 0-100 scale). L1 methylation level of liver metastases was not associated with sex, age at hepatic resection, tumor stage of primary lesion, or tumor size. L1 methylation level of liver metastases was significantly related with that of matched primary tumors (n=26; Pearson correlation coefficient r=0.71, p=0.0001). LN metastases showed similar L1 methylation level of matched primary tumors and liver metastases (n=6); the distribution of the L1 methylation level in matched 6 LN metastases was as follows: mean, 71.0; SD, 9.37; range, 59.3-81.9. In addition, lung metastases and matched primary tumors exhibited similar L1 methylation level; the distributions of the L1 methylation level in lung metastases and matched primary tumors (n=6) were as follows: mean, 65.6 and 71.3; range, 55.7-82.2 and 66.1-79.6, respectively. Interestingly, the heterogeneity of L1 methylation level was not observed; the difference between superficial area and invasive area was within 7 in all 6 cases. Conclusions: Liver, lung and LN metastases showed similar L1 methylation level with primary tumors. In addition, L1 methylation level of superficial area was roughly in accordance with that of invasive area; thus supporting the absence of heterogeneity of L1 methylation level within one tumor. These results may suggest that cancer cells acquire “global DNA hypomethylation” at the early stage of tumor development, and it is preserved through the process of cancer invasion and metastasis. Citation Format: Asuka Murata, Yoshifumi Baba, Masayuki Watanabe, Hironobu Shigaki, Shiro Iwagami, Yasuo Sakamoto, Yuji Miyamoto, Hideo Baba. Methylation levels of LINE-1 in primary lesion and matched metastatic lesions of colorectal cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 648. doi:10.1158/1538-7445.AM2013-648