EPIG-08DOWNREGULATION OF miR-379/miR-656 CLUSTER (C14MC) IN OLIGODENDROGLIOMAS WITH POSSIBLE MECHANISTIC AND CLINICOPATHOLOGICAL IMPLICATIONS

MicroRNAs(miRNAs) are often grouped into clusters in the genome, and dysregulation of miRNA clusters has been shown in various cancers. miR-379/miR-656 cluster(C14MC) is located within an imprinted chromosomal region, Dlk1-Dio3 locus, which also harbours the non-coding, maternally expressed gene MEG3. MEF2 and JUN have been reported as key transcription regulators for C14MC. Downregulation of C14MC has been observed in several CNS tumors, including GBM. However, the status and clinicopathological significance of C14MC in Oligodendrogliomas(ODGs) are unexplored. Therefore, expression of 47 miRNAs from C14MC was analyzed in 45 ODGs(24 Grade II, 21 Grade III) and 10 controls using customized TaqMan low density array microfluidic assay(TLDA). Further, its expression was correlated with clinicopathological data. Expression of MEF-2, JUN and MEG3 were examined by RT-PCR. Significant downregulation of C14MC was observed in 90% of ODGs. 19 miRNAs were differentially expressed between both the grades, indicating a diagnostic role. Methylation data analysis showed hyper-methylation of the genomic region encompassing the C14MC(beta value-0.9; p < 0.05), which was further substantiated by significant downregulation of MEG3 expression. Treatment of HS683 cells(anaplastic ODG cell line) with DNA-demethylating agent 5-azacytidine resulted in up-regulation of three(miR-485-5p, -369-3p, -382; p < 0.05) out of six randomly selected miRNAs. These results suggest that C14MC locus is possibly under epigenetic regulation in ODGs. In addition, downregulation of MEF2 and JUN, suggest that deregulation of C14MC in ODGs may be attributed to repression of these transcription factors. Further, two miRNAs(miR-656, -487b; p < 0.04) correlated with better progression-free survival, indicating their prognostic significance. This is the first report demonstrating the silencing of the second largest microRNA cluster in ODGs, implicating its tumor-suppressive role in their pathogenesis, along with diagnostic and prognostic significance. The hypermethylation of C14MC along with downregulation of MEG3, MEF2 and JUN could be the possible mechanisms for C14MC downregulation in ODG, independently or in synergy.