MBRS-12. A TRANSPOSON MUTAGENESIS SCREEN IDENTIFIES Rreb1 AS A DRIVER FOR GROUP 3 MEDULLOBLASTOMA

Medulloblastoma (MB) is the most common malignant childhood brain tumor. MB can be divided into four major subgroups – WNT, Sonic hedgehog (SHH), Group 3 (G3), and Group 4 (G4) – that exhibit distinct genetic alterations, gene expression profiles, and clinical outcomes. Patients with G3-MB have the worst prognosis, and a deeper understanding of this disease is critical for development of new therapies. Most G3-MBs express high levels of the MYC oncogene, suggesting that MYC plays an important role in tumorigenesis. To identify genes that cooperate with MYC to promote formation of G3-MB, we performed an in vivo mutagenesis screen using mice expressing the Sleeping Beauty (SB) transposon. Cerebellar stem cells from transposon/transposase-expressing mice were infected with viruses encoding Myc, and transplanted into the cerebellum of adult hosts. The resulting tumors were sequenced to identify transposon-targeted genes, and these genes were functionally analyzed to determine whether they could cooperate with Myc to drive G3-MB. These studies identified the transcription factor Ras-responsive element binding protein 1 (Rreb1) as a potent Myc-cooperating gene. Tumors driven by Myc and Rreb1 resemble G3-MB at a histological and molecular level. Moreover, RREB1 is overexpressed in human G3-MB, and knockdown of RREB1 impairs growth of G3-MB cell lines and patient-derived xenografts. Ongoing studies are aimed at identifying the mechanisms by which Rreb1 contributes to tumor growth. Our studies demonstrate an important role for RREB1 in G3-MB, and provide a new model that can be used to identify therapeutic targets and develop more effective therapies for medulloblastoma.

TMOD-30. IDENTIFYING NEW DRIVERS OF GROUP 3 MEDULLOBLASTOMA

Medulloblastoma (MB) is the most common malignant childhood brain tumor. MB can be divided into four major subgroups – WNT, Sonic hedgehog (SHH), Group 3 (G3-MB), and Group 4 (G4-MB) – that exhibit distinct genetic alterations, gene expression profiles, and clinical outcomes. Patients with G3-MB have the worst prognosis, and a deeper understanding of this form of the disease is critical for development of new therapies. Most G3-MBs express high levels of the MYC oncogene, suggesting that MYC plays an important role in tumorigenesis. However, MYC overexpression is not sufficient to drive tumor formation. To identify genes that cooperate with MYC to promote development of G3-MB, we performed an in vivo mutagenesis screen using mice expressing the Sleeping Beauty (SB) transposon. Cerebellar stem cells isolated from transposon/transposase-expressing transgenic mice were infected with viruses encoding Myc, and these cells were transplanted into the cerebellum of adult hosts. Tumors that arose were subjected to DNA and RNA sequencing to identify candidate genes, and these genes were subjected to functional analysis to determine whether they could cooperate with Myc to drive G3-MB. These studies identified the transcription factor Ras-responsive element binding protein 1 (Rreb1) as a potent Myc-cooperating gene. Tumors driven by Myc and Rreb1 (MR tumors) resemble G3-MB at a histological and molecular level. Moreover, RREB1 is overexpressed in human G3-MB, and knockdown of RREB1 expression impairs growth of G3-MB cell lines and patient-derived xenografts. Ongoing studies are aimed at identifying the molecular mechanisms by which Rreb1 contributes to tumor growth. Our studies demonstrate an important role for RREB1 in G3-MB, and provide a new model that can be used to identify therapeutic targets and develop more effective and less toxic therapies for this devastating pediatric brain tumor.

Tumor necrosis factor overcomes immune evasion in p53-mutant medulloblastoma

Many immunotherapies act by enhancing the ability of cytotoxic T cells to kill tumor cells. Killing depends on T cell recognition of antigens presented by class I major histocompatibility complex (MHC-I) proteins on tumor cells. In this study, we showed that medulloblastomas lacking the p53 tumor suppressor do not express surface MHC-I and are therefore resistant to immune rejection. Mechanistically, this is because p53 regulates expression of the peptide transporter Tap1 and the aminopeptidase Erap1, which are required for MHC-I trafficking to the cell surface. In vitro, tumor necrosis factor (TNF) or lymphotoxin-β receptor agonist can rescue expression of Erap1, Tap1 and MHC-I on p53-mutant tumor cells. In vivo, low doses of TNF prolong survival and synergize with immune checkpoint inhibitors to promote tumor rejection. These studies identified p53 as a key regulator of immune evasion and suggest that TNF could be used to enhance sensitivity of tumors to immunotherapy.

TNF superfamily cytokines overcome immune evasion in medulloblastoma

Many immunotherapies act by enhancing T cell killing of tumor cells. CD8+ cytotoxic T cells recognize antigens presented by class I major histocompatibility complex (MHC-I) proteins on tumor cells. Here we show that medulloblastomas lacking the p53 tumor suppressor do not express surface MHC-I and are therefore resistant to immune rejection. Mechanistically, this is because p53 regulates expression of the peptide transporter Tap1 and the aminopeptidase Erap1, which are required for MHC-I trafficking to the cell surface. Treatment with tumor necrosis factor (TNF) or lymphotoxin beta receptor agonist (LTβRag) rescues expression of Erap1, Tap1 and MHC-I on p53-mutant tumor cells. In vivo, TNF treatment prolongs survival and markedly augments the efficacy of the immune checkpoint inhibitor anti-PD-1. These studies identify p53 as a key regulator of immune evasion in vivo, and suggest that TNF could be used to enhance sensitivity of p53-mutant tumors to immunotherapy.