PDTM-08. ROLE OF miR-212 AS A TUMOR SUPPRESSOR GENE IN NON-SHH/WNT MEDULLOBLASTOMA

Abstract

Medulloblastoma (MB), the most frequent malignant pediatric brain tumor is divided into four subgroups, i.e. wingless-type (WNT), sonic hedgehog (SHH), Group 3, and Group 4. Among them, haploinsufficiency of chromosome 17p13.3 is most commonly seen in Group 3 and 4 tumors, which are concurrently associated with the poorest prognosis. Recent studies have revealed the importance of microRNAs (miRNAs) in regulating posttranscriptional gene expression in MB tumorigenesis. In this study, we sought to identify the role of miR-212, which resides on chromosome 17p13.3, in the development of non-SHH/WNT medulloblastoma. RNA expression analysis showed significantly reduced expression of miR-212 in MB tumor cell lines and in ex vivo non-SHH/WNT MB tumor samples (n=16). To further elucidate its putative tumor suppressor role, miR-212 was over-expressed in non-SHH/WNT tumor cell line, HDMB03, followed by functional assays of tumor cell behavior. In these studies, significantly reduced cell proliferation, colony formation, migration and invasion were noted. Additionally, reduced levels of pAKT, a marker of cell proliferation, was confirmed in miR-212-overexpressing HDMB03 cells. Subsequent cell cycle analysis revealed increased G0/G1 cell cycle arrest with a concurrent reduction in the corresponding cell cycle regulatory proteins, i.e. CDK6 and cyclinD1. Myc amplification, which is facilitated by phosphorylation of serine-62 (p-c-Myc-S62), is a characteristic feature of Group 3 medulloblastoma and a marker of poor prognosis. Decreased levels of p-c-Myc-S62 (active form favoring proliferation) with a complimentary increase in p-c-Myc-T58 (inactive form favoring apoptosis) results in degradation of c-Myc protein and cellular apoptosis, a trend recapitulated in miR-212-overexpressing HDMB03 cells. In addition, the pro-apoptotic binding partners of c-Myc, i.e. Bin-1 and P19ARF, were noted to be upregulated in miR-212 over-expressed HDMB03 cells, further favoring apoptosis. That c-myc may also serve as a target of miR-212 is currently being studied. These results substantiate miR-212 as a tumor suppressor gene in non-SHH/WNT medulloblastoma.