Differences in LINE-1 Methylation Between Endometriotic Ovarian Cyst and Endometriosis-Associated Ovarian Cancer

Background Endometriosis in endometriosis-associated ovarian cancer (EAOC) refers to lesions that can derive from endometriotic ovarian cysts (ECs) that form in the ovarian endometrium with the potential to transform into full-blown ovarian cancer. Hypomethylation of long interspersed element-1 (LINE-1 or L1) is a common epigenomic event in several cancers and is strongly associated with ovarian cancer progression. Objectives To evaluated alterations in LINE-1 methylation between EC, ovarian endometrioid adenocarcinoma (OEA), EAOC, and ovarian clear cell carcinoma (OCC). Methods/ Materials First, LINE-1 methylation status in 19 normal endometrium, 29 EC, 35 OCC, and 22 OEA tissues from unrelated samples were compared. Then, specific areas of eutopic endometrium, contiguous endometriosis, and cancer arising from 16 EAOCs were collected by microdissection and analyzed for LINE-1 methylation status. Results The total LINE-1 methylation levels were significantly different among the endometrium, endometriosis, and ovarian cancer (P < 0.001). A stepwise decrease in LINE-1 methylation was observed in the following order: normal endometrium, EC, OEA, and OCC. Interestingly, endometriosis in EAOC of both OEA (P = 0.016) and OCC (P = 0.003) possessed a higher percentage of LINE-1 unmethylated loci than EC. Conclusion Our data implicate that LINE-1 hypomethylation is an early molecular event involved in OEA and OCC malignant transformation. Precise measurements of LINE-1 methylation may help to distinguish EC and endometriosis in EAOC.