BIOL-23. LOSS OF MICRORNAS COOPERATES WITH TUMOR SUPPRESSORS TO INITIATE MYC-DEPENDENT METASTATIC MEDULLOBLASTOMA

To determine the role of microRNA regulation in brain tumor development, we incorporated a conditional allele of the microRNA processing enzyme Dicer to a glioma mouse model based on inactivation of the tumor suppressors Nf1, Trp53, and Pten using the Nestin-creERT(2) transgene. Loss of Dicer and tumor suppressors at adult ages led to glioma development; however, mutant mice tamoxifen induced at early postnatal ages developed medulloblastoma (MB) instead of glioma. The switch in tumor spectrum occurred with 100% penetrance and the cerebellar tumors were histologically indistinguishable from human MB. The minimum genetic mutations required for MB formation were Dicer1 and Trp53, while additional loss of Pten produced more invasive tumors with leptomeningeal metastasis. Analysis of tumor transcriptome and MB subtype-specific markers show that Dicer tumors most resemble human Sonic Hedgehog (SHH) MB. The presence of leptomeningeal metastasis, Trp53 loss and upregulation of Mycn and Gli2 in the setting of activated Shh signaling suggests that this model phenocopies extremely high risk p53-mutated SHH MB, which carries poor prognosis and is associated with the majority of treatment failures in SHH MB. Analysis of pre-symptomatic mutant mice showed proliferative defects and retained cells in the external granule cell layer (EGL), suggesting that tumors may arise from cerebellar granule neuron precursors (CGNPs). However, targeting CGNPs using Math1-creERT(2) did not lead to MB development, suggesting that an earlier EGL precursor may be required for tumorigenesis. Dicer tumors exhibit reduced microRNAs and RNA-induced silencing complexes consistent with loss of Dicer. One of these downregulated microRNAs, miR-101, is a regulator of Mycn and its overexpression leads to Mycn downregulation and inhibition of MB growth. Likewise, genetic and pharmacologic inhibition of Myc leads to inhibition of MB growth. These studies demonstrate a role for microRNAs in Myc-dependent MB development and show an alternative pathway for SHH MB tumorigenesis.