Differences in p16 gene methylation and expression in benign and malignant ovarian tumors

Objective. The aim of this study was to test the hypothesis that DNA methylation is important for silencing the p16 tumor suppressor gene in ovarian epithelial tumors and to compare the prevalence of this mechanism among different ovarian epithelial tumor subtypes. Method. Methylation- specific PCR was used to analyze the p16 gene for DNA methylation in 20 ovarian cystadenomas, 15 low malignant potential (LMP) tumors, and 37 carcinomas, p16 expression was determined immunohistochemically in 58 of these tumors (16 cystadenomas, 13 LMP tumors, 29 carcinomas). Differences in methylation or expression rates between specific tumor subgroups were examined by Fisher's exact test. Results. Fragments from the distal promoter and beginning of the first exon of the p16 gene were both methylated in 5 of 15 (33%) LMP tumors compared to 2 of 37 (5%) carcinomas (P = 0.02). Those sites were also methylated in 5 of 20 (25%) cystadenomas. Lack of p16 expression was present in 7 of 16 cystadenomas, 4 of 13 LMP tumors, and 22 of 29 carcinomas (P [LMPs versus carcinomas] = 0.01) and correlated with methylation changes in LMP tumors (P = 0.05). p16 expression was correlated with mucinous differentiation in cystadenomas (P = 0.001). Conclusion. p16 silencing may be important for the development of ovarian carcinomas and a subset of LMP tumors. Changes in DNA methylation may be more important for inactivation of this gene (and perhaps other tumor suppressor genes) in LMP tumors, which lack many of the alternative mechanisms present in carcinomas, p16 expression is primarily related to mucinous differentiation in cystadenomas.