Cancer Biology and Signal Transduction Involvement of microRNA-24 and DNA Methylation in Resistance of Nasopharyngeal Carcinoma to Ionizing Radiation

Nasopharyngeal carcinoma (NPC) is a malignant tumor originating in the epithelium. Radiotherapy is the standard therapy, but tumor resistance to this treatment reduces the 5-year patient survival rate dramatically. Studies are urgently needed to elucidate the mechanism of NPC radioresistance. Epigenetics—particularly microRNAs (miRNA) andDNAmethylation—plays an important role in carcinogenesis and oncotherapy.We used qRT-PCR analysis and identified an miRNA signature from differentially expressed miRNAs. Our objectives were to identify the role of miR24 in NPC tumorigenesis and radioresistance and to identify the mechanisms by which miR24 is regulated. We found that miR24 inhibited NPC cell growth, promoted cell apoptosis, and suppressed the growth of NPC xenografts. We showed that miR24 was significantly downregulated in recurrent NPC tissues. When combinedwith irradiation, miR24 acted as a radiosensitizer in NPC cells. One of themiR24 precursorswas embedded in aCpG island.AberrantDNAmethylationwas involved in NPC response to radiotherapy, which linked inactivation of miR24 through hypermethylation of its precursor promoter with NPC radioresistance. Treating NPC cells with the DNA-hypomethylating agent 5-aza-20deoxycytidine compensated for the reducedmiR24 expression. Together, our findings showed thatmiR24was negatively regulated by hypermethylation of its precursor promoter in NPC radioresistance. Our findings defined a central role for miR24 as a tumor-suppressive miRNA in NPC and suggested its use in novel strategies for treatment of this cancer. Mol Cancer Ther; 1–12. 2014 AACR.