Epigenetic silencing of DKK2 and Wnt signal pathway components in human ovarian carcinoma

Abstract

Wnt/β-Catenin signaling dysregulation is involved in tumorigenesis. Furthermore, epigenetic modification of the Dickkopf (DKK) family (DKK1-DKK4) has been shown to be important in Wnt signaling regulation. In this study, the role of DKK2, a Wnt antagonist, in epithelial ovarian cancer (EOC) was evaluated by examining the expression and methylation of DKK2 in SKOV3 and ES-2 ovarian cancer cell lines and 78 tissues collected from patients (50 ovarian carcinoma, 20 benign tumor and 8 normal ovarian tissues). DKK2 is highly downregulated in EOCs; however, DKK2 expression levels are higher in both normal tissues and benign tumors. In most cases of ovarian carcinoma, DKK2 is methylated, compared with the more common unmethylated form present in benign tumors and normal ovarian tissues. Additionally, DKK2 may be epigenetically silenced by methylation in higher grades and stages of EOC. Functional analysis revealed that overexpression of DKK2 suppressed malignant cell growth and invasion in SKOV3 and ES-2 cell lines. The expression of the downstream genes of Wnt signaling, including β-catenin, c-Myc and cyclin D1, was decreased in DKK2-transfected cells compared with mock cells. The expression of matrix metalloproteinase-2 and focal adhesion kinase were also decreased in DKK2 transfectants, supporting findings indicating inhibition of cell migration and invasion. This report provides novel indications that DKK2 is a unique hypermethylated target gene in EOC and that DKK2 may contribute to tumorigenesis in EOC through the Wnt/β-catenin signaling mechanisms.