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

Abstract

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 × 10−14 and 9.6 × 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 × 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. © 2008 Wiley‐Liss, Inc.