Down-Regulation of the miR-25 and miR-30d Contributes to the Development of Anaplastic Thyroid Carcinoma Targeting the Polycomb Protein EZH2

CONTEXT: We have previously demonstrated that a set of micro-RNA (miRNA) is significantly down-regulated in anaplastic thyroid carcinomas with respect to normal thyroid tissues and to differentiated thyroid carcinomas. OBJECTIVE: The objective was to evaluate the role of two of these down-regulated miRNA, miR-25 and miR-30d, in thyroid carcinogenesis. DESIGN: miR-25 and miR-30d expression was restored in the ACT-1, 8505c, and FRO anaplastic thyroid cell lines, and their effects on cell proliferation, migration, and target expression were evaluated. RESULTS: We report that miR-25 and miR-30d target the polycomb protein enhancer of zeste 2 (EZH2) that has oncogenic activity and is drastically up-regulated in anaplastic thyroid carcinomas but not in the differentiated ones. Ectopic expression of miR-25 and miR-30d inhibited proliferation and colony formation of anaplastic thyroid carcinoma cells by inducing G2/M-phase cell-cycle arrest. Finally, we found an inverse correlation between the expression of these miRNA and the EZH2 protein levels in anaplastic thyroid carcinomas, suggesting a critical role of these miRNA in regulating EZH2 expression also in vivo. CONCLUSION: The down-regulation of miR-25 and miR-30d could contribute to the process of thyroid cancer progression, leading to the development of anaplastic carcinomas targeting EZH2 mRNA.