Epigenetic silencing of microRNA-34b/c plays an important role in the pathogenesis of malignant pleural mesothelioma

Purpose: Malignant pleural mesothelioma (MPM) is an aggressive tumor with a dismal prognosis. Unlike other malignancies, TP53 mutations are rare in MPM. Recent studies have showed that altered expression of microRNA (miRNA) is observed in human malignant tumors. In this study, we investigated the alterations of miR-34s, a direct transcriptional target of TP53, and the role of miR-34s on the pathogenesis of MPM. Experimental Design: Aberrant methylation and expression of miR-34s were examined in MPM cell lines and tumors. miR-34b/c was transfected to MPM cells to estimate the protein expression, cell proliferation, invasion, and cell cycle. Results: Aberrant methylation was present in 2 (33.3%) of 6 MPM cell lines and 13 (27.7%) of 47 tumors in miR-34a and in all 6 MPM cell lines (100%) and 40 (85.1%) of 47 tumors in miR-34b/c. Expression of miR-34a and 34b/c in all methylated cell lines was reduced and restored with 5-aza-2′-deoxycytidine treatment. Because epigenetic silencing was the major event in miR-34b/c, we investigated the functional role of miR-34b/c in MPM. miR-34b/c-transfected MPM cells with physiologic miR-34b/c expression exhibited antiproliferation with G ₁ cell cycle arrest and suppression of migration, invasion, and motility. The forced overexpression of miR-34b/c, but not p53, showed a significant antitumor effect with the induction of apoptosis in MPM cells. Conclusions: We show that the epigenetic silencing of miR-34b/c by methylation is a crucial alteration and plays an important role in the tumorigenesis of MPM, suggesting potential therapeutic options for MPM.