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iv58 NEURO-ONCOLOGY • JUNE 2017 entities. Promising candidates are currently being further characterized, e.g. Panobinostat showed activity in the majority of brain tumor models at low nanomolar concentrations. CONCLUSION: The growing importance of HDAC inhibitors is reflected by an increasing number of HDACi in clinical trials, with four HDACi already approved by the FDA for treating lymphomas and multiple myelomas. Our study supports the finding that HDACi are valid therapeutic agents and that selected inhibitors are promising candidates for future epigenetic therapy of primary brain tumors. TRTH-29. CONTEXT-SPECIFIC TUMOR SUPPRESSIVE FUNCTION OF THE CANONICAL WNT PATHWAY IN PEDIATRIC MEDULLOBLASTOMA HIGHLIGHTS A THERAPEUTIC STRATEGY FOR TREATMENT-REFRACTORY SUBGROUPS Branavan Manoranjan, Chitra Venugopal, Michelle Kameda-Smith, David Bakhshinyan, Minomi Subapanditha, Bradley Doble, and Sheila Singh; McMaster University, Hamilton, ON, Canada. Current molecular subgroups of childhood medulloblastoma (MB) recognize distinct disease entities of which activated Wnt signaling is associated with a distinct subgroup and the best overall outcome. In contrast, nonWnt MBs are characterized by metastatic disease, increased rate of recurrence, and poor overall survivorship. Given the excellent clinical outcome in Wnt-driven MB, we aimed to convert treatment-resistant MB subgroups 3 and 4 into an ostensibly benign tumor. Activated Wnt signaling by way of Wnt agonists decreased in vitro self-renewal of primary MB cells. Comparative RNA-sequencing of control and transgenic lines containing a stabilized beta-catenin mutant demonstrated a reduction in self-renewal genes following beta-catenin overexpression, including Sox2 and Bmi1. In order to validate the therapy-sensitive nature of Wnt-activated cells, we developed stable human Group 3 and 4 patient-derived lines containing a 7XTOPFlash reporter to determine the presence of endogenous Wnt signaling. Rare subclonal Wnt-active cells demonstrated a reduced self-renewal and tumor-initiating capacity through in vivo limiting dilution assays when compared to bulk Wnt-inactive cells from Group 3 and 4 MBs. The therapeutic relevance of these findings were demonstrated with an in vivo survival advantage in mice with orthotopic injections of cells containing stabilized beta-catenin overexpression or endogenous Wnt-active cells. Resulting xenograft tumors were smaller in size, maintained a lower rate of proliferation, and reduction in MB self-renewal genes. To develop a rationale clinical therapeutic, we used a novel substrate-competitive peptide inhibitor for GSK. Treatment with our peptide inhibitor showed a significant reduction in tumor burden and metastatic disease with a corresponding increase in survival of patientderived Group 3 and 4 tumors that were otherwise treatment-resistant. Our work establishes activated Wnt signaling as a novel treatment paradigm in childhood MB, identifies a rationale therapeutic approach for recurrent MB, and provides evidence for the context-specific tumor suppressive function of the canonical Wnt pathway. TRTH-30. PRELIMINARY EXPERIENCE WITH SERIAL WHOLE EXOME SEQUENCING OF PEDIATRIC CNS TUMORS AT DIAGNOSIS AND RECURRENCE. Luca Szalontay1, Danielle Pendrick1, Neil Feldstein1, Richard Anderson1,2, Eileen Stark1, Julia Glade Bender1,2, Jennifer Oberg1, Susan Hsiao1, Andrew Turk1, Anthony Sireci1, Mahesh Mansukhani1,2, and James Garvin1,2; 1Columbia University Medical Center, New York, NY, USA, 2Herbert Irving Comprehensive Cancer Center, New York, NY, USA. INTRODUCTION: Whole exome sequencing (WES) of newly diagnosed pediatric central nervous system (CNS) tumors is quickly becoming part of routine care. Through the Precision in Pediatric Sequencing (PiPseq) program at Columbia University, we have found potentially actionable mutations in more than 40% of evaluable CNS cases at diagnosis. More recently, we have integrated this approach into the management of patients undergoing surgery for CNS tumor recurrence. METHOD: After obtaining informed consent, tumor-normal WES with transcriptome analysis was performed in a CLIA-certified laboratory on fresh frozen or paraffin embedded CNS tumor samples and peripheral blood. RESULTS: 7 cases (5 male, 2 female; median age 5 years) with adequate diagnostic and recurrent tumor tissue were tested. No case had a somatic mutation of established clinical utility (tier 1). Among 3 embryonal tumors, a splice site variant in TSC1 (tier 2 mutation of potential utility) was detected in a medulloblastoma, but only at recurrence and not at initial diagnosis. FOXR2 overexpression was detected at diagnosis and confirmed at early progression of a temporal lobe tumor, prompting revision of the initial diagnosis of high grade glioma to CNS neuroblastoma subtype of PNET, and treated accordingly. In a third patient initially diagnosed with medulloblastoma, overexpression of PDGFRA, MDM4, CDKN2A, EGFR, OLIG2, and GFAP supported a change in diagnosis to glioblastoma. Two gliomas had tier 2 mutations detected at initial diagnosis and progression: SETD2 p.R2040* (optic nerve lesion, called pseudotumor initially but glioma at progression), and H3F3A p.K28M (thalamic low grade glioma). In one patient with ependymoma, copy number gain of 1q25 (associated with poor prognosis) was seen only in the recurrence specimen. CONCLUSION: Our preliminary experience suggests that in pediatric CNS tumor patients referred for reoperation at recurrence, repeat WES may reveal a previously unrecognized treatment option, at least in embryonal tumors. TRTH-31. EFFECTS OF TORC1/2 INHIBITOR MLN0128 ALONE AND IN COMBINATION WITH MEK INHIBITION IN BRAF MUTATED GLIOMA CELLS Sabine Mueller1, Rogier Dirk4, Xiaodong Yang1, Steve DuBois3, Angela Waanders2, Adam Resnick2, William Weiss1, and Daphne HaasKogan3; 1University of California, San Francisco, San Francisco, CA, USA, 2Children’s Hospital of Philadelphia, Philadelphia, PA, USA, 3Dana Farber Cancer Institute, Boston, MA, USA, 4VA University Medical Center, Amsterdam, The Netherlands. INTRODUCTION: MLN0128, a second-generation ATP-competitive pan-mTOR kinase inhibitor, acts on both mTORC1 and mTORC2. We investigated the effects of MLN0128 monotherapy and in combination with MEK and BRAFV600E inhibition in models of pediatric low-grade glioma (PLGG). METHODS: We used human glioma cell lines expressing BRAFV600E (AM38), wild-type BRAF (LN229, TN98, SF188) and isogenic systems of KIAA1549:BRAF-expressing NIH3T3 cells. Signaling inhibitors included MLN0128, everolimus, BRAFV600E specific inhibitor PLX4720, and MEK specific inhibitors AZD6244 and GSK1120212. Cell proliferation was determined using an ATP-based assay. Biochemical effects were assessed using western blot analysis. The DBTRG (BRAFV600E) xenograft mouse model was used to assess in vivo efficacy. RESULTS: MLN0128 monotherapy demonstrates more potent anti-proliferative effects compared to everolimus in all tested PLGG lines independent of BRAF status. As expected, treatment with MLN0128 leads to down-regulation of p-AKT, p-S6 and p-4EBP1, whereas treatment with everolimus only reduces expression of p-S6. MEK inhibition with AZD6244 effectively decreases p-ERK expression in all lines; however, the addition of MLN0128 to AZD6244 causes a rebound in p-ERK expression in BRAFWT gliomas but not in gliomas expression BRAFV600E or KIAA1549:BRAF alterations. In cells carrying either the BRAFV600E or KIAA1549:BRAF alteration, combined treatment with MLN0128 and GSK1120212 or AZD6244 leads to synergistic anti-proliferative effects but such synergy is not evident in BRAFWT gliomas. In vivo studies show superior efficacy of combinations of PLX4720 and MLN0128 compared to monotherapies (p<0.02) or combination of PLX4720 + everolimus (p=0.03). CONCLUSION: MLN0128 displays synergistic anti-neoplastic effects with MEK inhibition in gliomas bearing BRAFV600E or KIAA1549:BRAF. In BRAF wild-type cells such synergy is not present, potentially due to rebound in p-ERK when MLN0128 is combined with AZD6244. Combination therapy with MLN0128 + PLX4720 prolongs survival in a BRAFV600E mutant glioma model and is superior to monotherapy or everolimus + PLX4720 combination. TRTH-32. COMBINATION IMMUNOTHERAPY TO ACTIVATE THE INNATE IMMUNE MICROENVIRONMENT AGAINST PEDIATRIC BRAIN TUMORS Sharareh Gholamin, Rogelio Esparza, Samuel Cheshier, and Siddhartha Mitra; Stanford University School of Medicine, Stanford, CA, USA. Currently combinations of surgery, chemotherapy, and irradiation are utilized to treat pediatric malignant brain tumors; resulting in significant and permanent morbidity. Recently we published the effect of a humanized anti-CD47 antibody, Hu5F9-G4, on five aggressive and etiologically distinct pediatric brain tumors: Group 3 medulloblastoma (primary and metastatic), atypical teratoid rhabdoid tumor, primitive neuroectodermal tumor, pediatric glioblastoma, and diffuse intrinsic pontine glioma. Hu5F9-G4 demonstrated therapeutic efficacy in vitro and in vivo in patient-derived orthotopic xenograft models. Disabling the inhibitory signals transduced by SIRPa by blocking anti-CD47 antibodies we reduce the threshold for macrophage activation and promote phagocytic response driven by tumor-specific antibodies. Consequently, a number of approaches to augment macrophage responses to therapeutic antibodies are under investigation, including the exploration of new targets and development of antibodies with enhanced functions. We tested a unique paradigm to fully utilize the innate immune system by utilizing a three-pronged approach. a) Increase the infiltration of macrophages into the tumor. b) Block the anti-phagocytic signals and c) Stimulate pro-phagocytic signals. RESULTS: To determine the in vivo efficacy of combinatorial anti-CD47 and anti-CD40 therapies, Group 3 medulloblastoma xenografts were treated by, i.p. injections with either PBS (control), anti-CD47, anti-CD40 or a