Hypomethylation of retrotransposable elements correlates with genomic instability in non-small cell lung cancer

LINE-1 and Alu elements are non-LTR retrotransposons, constituting together over 30% of the human genome and they are frequently hypomethylated in human tumors. A relationship between global hypomethylation and genomic instability has been shown, however, there is little evidence to suggest active role for hypomethylation-mediated reactivation of retroelements in human cancer. In our study, we examined by Pyrosequencing the methylation levels of LINE-1 and Alu sequences in 48 primary nonsmall cell carcinomas and their paired adjacent tissues. We demonstrate a significant reduction of the methylation levels of both elements (p = 7.7 x 10(-14) and 9.6 x 10(-7), respectively). The methylation indices of the 2 elements correlated (p = 0.006), suggesting a possible common mechanism for their methylation maintenance. Genomic instability was measured utilizing 11 fluorescent microsatellite markers located on lung cancer hot-spot regions such as 3p, 5q 9p, 13q and 17p. Hypomethylation of both transposable elements was associated with increased genomic instability (LINE, p = 7.1 x 10(-5); Alu, p = 0.008). The reduction of the methylation index of LINE-1 and Alu following treatment of 3 lung cell lines with 5-aza-2'-deoxycitidine, consistently resulted in increased expression of both elements. Our study demonstrates the strong link between hypomethylation of transposable elements with genomic instability in non-small cell lung cancer and provides early evidence for a potential active role of these elements in lung neoplasia. As demethylating agents are now entering lung cancer trials, it is imperative to gain a greater insight into the potential reactivation of silent retrotransposons in order to advance for the clinical utilization of epigenetics in cancer therapy.