MYC overexpression and SMARCA4 loss cooperate to drive medulloblastoma formation in mice

Group 3 medulloblastoma is one of the most aggressive types of childhood brain tumors. Roughly 30% of cases carry genetic alterations in MYC, SMARCA4, or both genes combined. While overexpression of MYC has previously been shown to drive medulloblastoma formation in mice, the functional significance of SMARCA4 mutations and their suitability as a therapeutic target remain largely unclear. To address this issue, we combined overexpression of MYC with a loss of SMARCA4 in granule cell precursors. Both alterations did not increase proliferation of granule cell precursors in vitro. However, combined MYC overexpression and SMARCA4 loss successfully induced tumor formation in vivo after orthotopic transplantation in recipient mice. Resulting tumors displayed anaplastic histology and exclusively consisted of SMARCA4 negative cells although a mixture of recombined and non-recombined cells was injected. These observations provide first evidence for a tumor-promoting role of a SMARCA4 deficiency in the development of medulloblastoma. In comparing the transcriptome of tumors to the cells of origin and an established Sonic Hedgehog medulloblastoma model, we gathered first hints on deregulated gene expression that could be specifically involved in SMARCA4/MYC driven tumorigenesis. Finally, an integration of RNA sequencing and DNA methylation data of murine tumors with human samples revealed a high resemblance to human Group 3 medulloblastoma on the molecular level. Altogether, the development of SMARCA4-deficient medulloblastomas in mice paves the way to deciphering the role of frequently occurring SMARCA4 alterations in Group 3 medulloblastoma with the perspective to explore targeted therapeutic options. We cultured granule cell precursors isolated from Math1-creERT2::Smarca4fl/fl mice at P7 after tamoxifen-induced loss of SMARCA4 (injection at P3) and performed transduction with a lentiviral construct to drive overexpression of MYC. These cells were transplanted into the cerebella of CD1nu/nu recipient mice, which successfully drove medulloblastoma formation. To analyze gene expression profiles of MYC/SMARCA4 tumors, we performed RNA sequencing of four tumors using RNA derived from FFPE punches. As a validation, we simultaneously sequenced three samples of the previously established SHH medulloblastoma model Math1-cre::Smofl/wt. Moreover, we included samples derived from Math1-creERT2::Smarca4fl/fl cerebella at P7 as a control. Tumors 1,3,4 in this series correspond to tumors 1,2,3 in DNA methylation analysis within the same project (GSE235924).