Establishment and analysis of patient-specific Group 3 medulloblastoma mouse models

Medulloblastoma (MB) is the most common pediatric brain tumor with high morbidity and mortality. Among the four major MB groups, patients with Group3 MB currently have the worst outcome and nearly 50% are metastatic at the time of diagnosis. However, the molecular mechanisms underlying Group 3 MB are still unknown. Exploiting in vivo transfection of mouse cerebellar cells, we tested different patient- specific combinations of genes for their ability to induce Group 3 MB in mice. We identified Otx2 and cMYC as strong inducers of tumors faithfully recapitulating human Group 3 MB. To identify a druggable signaling pathway, we analyzed sequencing data of human patients harboring Otx2/cMYC overexpression. Among all the putative oncosuppressors, SMARCA4 is the most frequently mutated. Indeed, co-expression of SMARCA4 with Otx2 and cMYC is able to block tumorigenesis, through regulation of CDKN2B and CRABP1 expression. Interestingly, patient-specific SMARCA4T910M mutant is able to block wild- type SMARCA4 effects in a dominant-negative manner. Since SMARCA4 has an antagonistic relationship with histone methyltransferase EZH2, we used EZH2-inhibitors to recapitulate SMARCA4 antitumorigenic effects. Little is known about Group 3 MB developmental origin and several embryonic mouse cerebellar progenitors have been proposed as possible cell of origin. Here, we found that in postnatal mouse cerebellum, the S100b+ cells are competent to induce medulloblastoma while Math1, Sox2 and Ascl1 positive cells seem to be unresponsive to oncogenic insults. Taken together, our data suggest that the competency of different cells to develop MB could change during brain development, depending on their developmental stage and cellular identity.