LINE-1 is preferentially hypomethylated within adenomatous polyps in the presence of synchronous colorectal cancer

Background: Conventional tubular adenomas are frequently detected in patients undergoing average risk screening colonoscopy and are over-represented in patients who will develop colorectal cancer (CRC). Whether features of adenomas could serve as predictors of synchronous CRC is not known. Here, we investigate whether global methylation markers, including LINE-1, differ within adenomas in patients with and without synchronous CRC. Methods: Colorectal tubular/tubulovillous adenomatous polyps in the absence (P group, n=45) and in the presence of synchronous CRC ( PC group, n=32) were identified. Global methylation and demethylation by ELISA for 5-methylcytosine (5-mC) and 5-hydroxymethyl cytosine (5-hmC), respectively, were assessed in polyps and adjacent normal non-neoplastic tissue. LINE-1 hypomethylation was assessed by pyrosequencing of bisulfite-converted DNA as well. Results: Global methylation (5-mC) showed no differences in overall methylation status in the adenomatous polyps in the two groups (5-mC relative to control %, PC group 0.117; P group 0.161, p=0.148). Global hydroxymethylation 5hmC was also not significantly different in adenomatous polyps of the PC group than in those of the P group (0.0059 vs 0.0097, p=0.681). Similarly, global 5-hmC was not different between normal tissues from patients without neoplasia in comparison to those from CRC patients (0.0461 +/- 0.080 vs 0.039 +/- 0.159, p=0.215). In contrast, adenomatous polyps of the PC group had lower levels of LINE-1 methylation compared to the adenomas in the P group (53.07 +/- 4.5 vs 59.95 +/- 5.4, p<0.001). LINE-1 methylation was also significantly lower in the normal tissue from cancer patients compared to that from patients without any neoplasia (58.07 +/- 3.78 vs 71.50 +/- 6.47, p<0.001). Conclusions: LINE-1 hypomethylation of precancerous adenomas correlates with the presence of synchronous CRC. Measurement of DNA hypomethylation levels of colorectal adenomas by LINE-1 could have future implications in approaches to defining CRC risk in screening programs.