DDDR-10. INHIBITING INSULIN SIGNALING REVERSES RESISTANCE TO PI3K-MTOR INHIBITORS IN AGGRESSIVE PEDIATRIC HIGH-GRADE GLIOMAS

Primary central nervous system (CNS) tumors are now the most common cause of childhood cancer–related deaths. Pediatric high-grade gliomas (pHGGs) are among the most lethal brain tumors with a 5-year survival rate of only 20%. MYCN pHGGs represent one subgroup with an unmet need for therapeutics. MYCN belongs to the family of MYC transcription factors that regulate numerous cancer hallmarks such as proliferation, apoptosis, and metabolism. While no direct inhibitors of MYCN are in clinical trial, current strategies focus on targeting the MYCN mediated transcriptional machinery or cell cycle regulators. Lack of relevant pHGG models for pre-clinical testing contribute to limited therapeutic efficacy. To address these knowledge gaps, we developed a novel mouse model of MYCN pHGG using the FLEx-Cre switch system, whereby neural stem cells are selectively delivered with MYCN cDNA and shRNA targeting the tumor suppressor genes p53 and Pten and form tumors in vivo. We identified that this model harbors hyper-activation of the PI3K/AKT/mTOR signaling pathway. We demonstrate that dual PI3K-mTOR blood brain barrier penetrant inhibitors are effective in reducing pHGG growth and MYCN protein levels. Because treatment-resistance is a fundamental feature of pHGGs, we developed a novel drug-resistance model of MYCN pHGG as a mechanistic tool to identify relevant resistance mechanisms. Using transcriptome analysis, we identified the insulin growth factor signaling pathway as our top mechanism of resistance. We hypothesized that MYCN is a critical driver of pHGG and can be effectively targeted via dual inhibition of PI3K-mTOR and IGF/Insulin signaling pathways. We tested next generation inhibitors of IGF and PI3K/mTOR pathways and performed genetic and pharmacological assays in our MYCN pHGG and human MYCN pHGG cells. Inhibition of both pathways in our MYCN pHGG model and human MYCN cells were synergistic, leading to significant decreases in cell growth and MYCN signaling.

TMET-09. LOSS OF MAT2A COMPROMISES METHIONINE METABOLISM AND REPRESENTS A VULNERABILITY IN H3K27M MUTANT GLIOMAS

H3K27-mutant diffuse midline gliomas (DMGs) are defined as grade IV tumors by the World Health Organization. DMGs are inoperable and resistant to chemo/radio therapies. Median survival ranges from 8-11 months, with 2% of patients surviving beyond 5 years. H3K27M mutations lead to global epigenetic and transcriptional reprogramming driven by global loss of negative transcriptional regulator H3K27 trimethylation (H3K27me3). Loss of H3K27me3 is an initiating event in gliomagenesis. This disease lacks appropriate models to predict disease biology and response to treatment. Therefore, we developed a novel syngeneic H3K27M mouse model. An unbiased integrated systems biology approach identified that H3K27M but not isogenic controls relied on the amino acid methionine and the enzyme Methionine Adenosyltransferase 2A (MAT2A). MAT2A is a central regulator of one-carbon metabolism by converting methionine to S-adenosylmethionine (SAM), the universal methyl-donor for protein and nucleotide methylation reactions. In complementary genetic approaches, we applied these findings to patient-derived cell lines with the H3K27M mutation. We hypothesize that MAT2A abrogation, genetic/pharmacological, would alter DMG viability by disrupting the methylome. The current MAT2A sensitivity paradigm is based on Methylthioadenosine Phosphorylase (MTAP) deletion through a synthetic lethal mechanism. We provide a novel mechanism whereby H3K27M cells are sensitive to MAT2A loss, independent of MTAP and through Adenosylmethionine Decarboxylase 1 (AMD1) overexpression disrupting MAT2A regulation. This results in H3K27M cells having lower MAT2A protein levels, conferring a sensitivity by inhibiting residual MAT2A. Genetic/pharmacological aberrations to MAT2A resulted in reduced proliferation. Parallel H3K36me3 ChIP and RNA-sequencing identified loss of oncogenic and developmental transcriptional programs associated with MAT2A loss. In vivo syngeneic and patient-derived xenograft models with both inducible MAT2A knockdown or methionine restricted diets showed extended survival. These results suggest novel interactions between methionine metabolism and the epigenome of H3K27M gliomas and provide evidence that MAT2A, presents exploitable therapeutic vulnerabilities in histone mutant gliomas.

DDEL-01. ENHANCING DRUG DELIVERY WITH MRgFUS FOR DIFFUSE INTRINSIC PONTINE GLIOMA MODEL

Diffuse intrinsic pontine glioma (DIPG) is a surgically unresectable and devasting tumor in children. To date, there have been no effective chemotherapeutics despite a myriad of clinical trials. The intact blood-brain barrier (BBB) in part is responsible for the limited clinical response to chemotherapy. MRI guided focused ultrasound (MRgFUS) is a promising non-invasive tissue ablative method for CNS tumors. Moreover, MRgFUS allows for the temporary disruption of BBB. Our first objective was to determine the feasibility and safety of temporary BBB disruption within the brainstem using MRgFUS following intravenous (IV) administration of microbubbles in vivo. Our second objective was to select effective chemotherapeutics against DIPG cell lines, and to examine their therapeutic effects with MRgFUS in a mouse model of DIPG which exhibits an intact BBB. The non-invasive opening of the BBB was determined in the brainstem of normal rodents using physiological monitoring and histological analysis. Doxorubicin was selected from a drug screen consisting of conventional chemotherapeutics using SU-DIPG4 and SU-DIPG17 cell lines. We established SU-DIPG17 xenografts which demonstrated diffusely infiltrative tumor growth similar to human DIPG. By LC-MS/MS analysis, MRgFUS led to a 4-fold increase in doxorubicin concentrations within the brainstem tumors following IV administration when compared to IV administration alone, We demonstrated feasibility and safety of MRgFUS in the rodent brainstem and have shown that MRgFUS increases doxorubicin uptake in the brainstem of a rodent model of DIPG. These preclinical data will be helpful in designing clinical trials of BBB disruption using MRgFUS for DIPG in children.

DIPG-47. HISTONE MUTATIONS ENHANCE RAS MEDIATED ERK5 GROWTH SIGNALING IN DIFFUSE MIDLINE GLIOMAS

Diffuse midline gliomas (DMGs) are incurable brain tumors with an aggressive onset. Apart from irradiation, there are currently no effective therapies available for patients with DMG, who have a median survival time of less than one year. Most DMG cells harbor mutations in genes encoding histone H3 (H3K27M) proteins, resulting in a global reduction of H3K27 trimethylation and activation of oncogenic signaling pathways. Here we show that the H3K27M mutations contribute to RAS pathway signaling, which is augmented by additional RAS activators including PDGFRA. H3K27M mutation led to increased expression of receptor tyrosine kinases (RTK). A RAS pathway functional screen identified ERK5, but not ERK1/2, as a RAS pathway effector important for DMG growth. Suppression of ERK5 decreased DMG cell proliferation and induced apoptosis in vitro and in vivo. In addition, depletion or inhibition of ERK5 significantly increased survival of mice intracranially engrafted with DMG cells. Mechanistically, ERK5 directly stabilized the proto-oncogene MYC at the protein level. Additionally, persistent ERK5 depletion does not result in complete growth inhibition and therefore we set out to determine potential adaptation or resistance mechanisms in response to ERK5 loss. Using RNA-sequencing and Immunoprecipitation (IP) mass spectrometry (IP-MS), we have identified several positive and negative feedbacks involved in ERK5 that are also targetable. These findings identify the H3K27M mutation as an enhancer of RAS activation in DMG with ERK5 and ERK5 regulated networks immediately actionable pathways.

ATRT-16. MODELLING ATRT THROUGH SWI/SNF COMPLEX DEFICIENCY IN GENETICALLY-ENGINEERED MOUSE MODELS

Atypical Teratoid/Rhabdoid Tumours (ATRT) are highly malignant neoplasms arising primarily in the CNS of children. They are defined by loss of function mutations in smarcb1, a gene serving a vital role in neurogenesis and differentiation. In order to recapitulate ATRT in the mouse, we used a Cre-Lox recombination system to conditionally knockout smarcb1 in specific cell compartments. Loss of smarcb1 in BLBP-expressing cells of the developing brain led to severe neurologic defects. Mice exhibited seizures, ataxia, and median 12-day survival. Histological analysis revealed severe thinning of the cerebral cortex and cerebellum. Temporally-targeted smarcb1 loss in BLBP/Nestin-expressing embryonic compartments did not result in tumour formation. Similarly, BLBP-expressing, smarcb1-deficient neural stem/progenitor cells (NSC/NPCs) were isolated and allografted but did not form tumours. These cells demonstrated decreased proliferation, higher apoptosis, and upregulation of p53, CDKN1A, and CDKN2A. In contrast, ubiquitous smarcb1 loss at the earlier embryonic day 6.5 produced widespread tumorigenicity in the forebrain, hindbrain, skullbase, and spine; each with unique phenotypes, survival, and morphology. We employed a clinically-relevant Nanostring gene-panel screen to stratify tumours into genetically distinct subgroups. Our findings indicate that smarcb1 plays an important role in CNS development. Loss of smarcb1 in NSC/NPCs is lethal, and its developmental context influences cell fate. Targeted smarcb1 loss likely plays a tumorigenic role at an earlier developmental stage than previously determined, in a diverse array of primitive stem cells. These data support the generation of a murine ATRT model capable of producing distinct tumour entities that recapitulate the human disease.

MRI-guided focused ultrasound enhances drug delivery in experimental diffuse intrinsic pontine glioma

Diffuse intrinsic pontine glioma (DIPG) is a surgically unresectable and devasting tumour in children. To date, there are no effective chemotherapeutics despite a myriad of clinical trials. The intact blood-brain barrier (BBB) is likely responsible for the limited clinical response to chemotherapy. MRI-guided focused ultrasound (MRgFUS) is a promising non-invasive method for treating CNS tumours. Moreover, MRgFUS allows for the temporary and repeated disruption of the BBB. Our group previously reported the feasibility of temporary BBB opening within the normal murine brainstem using MRgFUS following intravenous (IV) administration of microbubbles. In the current study, we set out to test the effectiveness of targeted chemotherapy when paired with MRgFUS in murine models of DIPG. Doxorubicin was selected from a drug screen consisting of conventional chemotherapeutics tested on patient-derived cell lines. We studied the RCAS/Tv-a model where RCAS-Cre, RCAS-PDGFB, and RCAS-H3.3K27M were used to drive tumourigenesis upon injection in the pons. We also used orthotopically injected SU-DIPG-6 and SU-DIPG-17 xenografts which demonstrated a diffusely infiltrative tumour growth pattern similar to human DIPG. In our study, SU-DIPG-17 xenografts were more representative of human DIPG with an intact BBB. Following IV administration of doxorubicin, MRgFUS-treated animals exhibited a 4-fold higher concentration of drug within the SU-DIPG-17 brainstem tumours compared to controls. Moreover, the volumetric tumour growth rate was significantly suppressed in MRgFUS-treated animals whose tumours also exhibited decreased Ki-67 expression. Herein, we provide evidence for the ability of MRgFUS to enhance drug delivery in a mouse model of DIPG. These data provide critical support for clinical trials investigating MRgFUS-mediated BBB opening, which may ameliorate DIPG chemotherapeutic approaches in children.

TMOD-18. TARGETING THE PI3K/AKT PATHWAY IN MYCN AMPLIFIED HIGH GRADE GLIOMAS

Pediatric glioblastoma (pGBM) are incurable brain tumors with overall poor prognosis and response to treatments due to molecular and epigenetic heterogeneity. In particular, the MYCN subtype of pGBM are a highly aggressive form of GBM with a dismal median survival of only 14 months. Furthermore, this subtype is enriched with loss of the tumor suppressor genes TP53 and PTEN, leading to aberrantly active PI3K-AKT signaling pathway and DNA-checkpoint abnormalities. Here, we report the generation of a novel syngeneic mouse model that recapitulates the features of the MYCN subtype of pGBM. We isolated Sox2-Cre neural stem cells from C57BL/6 mice and transduced inverted retroviral-cassettes of the murine Mycn oncogene simultaneously with shRNA targeting tumor suppressor genes p53 and Pten. Retroviral-cassettes are flanked by tandem LoxP sites arranged so that Cre recombinase expression inverts the cassettes in frame allowing for MYCN protein expression and loss of the P53/PTEN proteins. Transgene activation is accompanied with selectable cell surface markers and fluorescent tags enabling for fluorescent activated cell sorting (FACS) of the desired cell populations. Neural stem cells with MYCN protein expression and concurrent silencing of P53 and PTEN protein (NPP cells) result in significantly increased proliferation and activation of PI3K-AKT pathway as compared to control neural stem cells and have. Injection of NPP cells into the forebrain of immune competent C57BL/6 mice result in the formation of invasive high-grade gliomas with a lethal phenotype at ~50 days post injection. Using several next generation brain penetrant small molecule inhibitors of the PI3K-AKT pathway, we show inhibition of tumorigenesis in vitro. Moreover, we have identified several novel mechanisms of PI3KAKT treatment resistance and are currently identifying therapies that may overcome this resistance through RNA seq analysis. In summary, well defined genetic drivers of GBM can lead to informed mouse model generation to test promising therapies.

CBIO-23. PROXIMITY-DEPENDENT BIOTIN IDENTIFICATION (BIOID2) INDICATES MEMBRANE TRAFFICKING AND VESICLE TRANSPORT AS A POTENTIAL NOVEL FUNCTION OF EXTRACELLULAR SIGNAL-REGULATED KINASE 5 (ERK5)

INTRODUCTION

Pediatric High-Grade Gliomas (PHGG), which include Diffuse Midline Gliomas (DMG), are a leading cause of brain tumor death in children. Our recent work has identified extracellular signal-regulated kinase 5 (ERK5) as a critical mediator of cell survival in DMG, as ERK5 knockdown decreases cell proliferation and extends survival time in orthotopic xenograft mice. Further investigation into the structure of ERK5 shows that it has a kinase domain and, unlike other ERKs, a transactivation domain, and both are important for promoting cell proliferation.

HYPOTHESIS AND METHODS

We hypothesize that identifying interactors and substrates of ERK5 could identify clinically actionable proteins and provide a more mechanistic insight into ERK5 in the progression of PHGGS. To determine protein–protein associations (PPAs), we employed the proximity-dependent biotin identification (BioID2) method and generated inducible ERK5-BioID2 and ERK2-BioID2 constructs to overcome barriers of conventional screening methods for PPAs. ERK2, similar in structure but much more studied than ERK5, was used as a comparison. Using DIPG lines as proof of principle, we performed streptavidin pull down assays for putative biotinylated ERK PPA and followed with mass spectrometry to identify the ERK2 and ERK5 interactomes.

RESULTS

Using data-dependent acquisition (DDA), we identified several unique and common interactors of ERK5 compared to ERK2. Through STRING network and pathway analysis, we identified a novel function of ERK5 with respect to membrane trafficking and vesicle transport. Identification of interactors with ERK5 may lead to effective therapeutic combinations. Our current work is focused on validating these interactions and the function of ERK5 in these biological processes.

CONCLUSIONS

Currently, ERK5 is not as understood as ERK1 or ERK2. Identification of interactors and substrates of ERK5 will further our understanding of PHGG biology, and may lead to identifying druggable targets and pathways.

Targeting NAD+ Biosynthesis Overcomes Panobinostat and Bortezomib-Induced Malignant Glioma Resistance

To improve therapeutic responses in patients with glioma, new combination therapies that exploit a mechanistic understanding of the inevitable emergence of drug resistance are needed. Intratumoral heterogeneity enables a low barrier to resistance in individual patients with glioma. We reasoned that targeting two or more fundamental processes that gliomas are particularly dependent upon could result in pleiotropic effects that would reduce the diversity of resistant subpopulations allowing convergence to a more robust therapeutic strategy. In contrast to the cytostatic responses observed with each drug alone, the combination of the histone deacetylase inhibitor panobinostat and the proteasome inhibitor bortezomib synergistically induced apoptosis of adult and pediatric glioma cell lines at clinically achievable doses. Resistance that developed was examined using RNA-sequencing and pharmacologic screening of resistant versus drug-naïve cells. Quinolinic acid phosphoribosyltransferase (QPRT), the rate-determining enzyme for de novo synthesis of NAD⁺ from tryptophan, exhibited particularly high differential gene expression in resistant U87 cells and protein expression in all resistant lines tested. Reducing QPRT expression reversed resistance, suggesting that QPRT is a selective and targetable dependency for the panobinostat-bortezomib resistance phenotype. Pharmacologic inhibition of either NAD⁺ biosynthesis or processes such as DNA repair that consume NAD⁺ or their simultaneous inhibition with drug combinations, specifically enhanced apoptosis in treatment-resistant cells. Concomitantly, de novo vulnerabilities to known drugs were observed. IMPLICATIONS: These data provide new insights into mechanisms of treatment resistance in gliomas, hold promise for targeting recurrent disease, and provide a potential strategy for further exploration of next-generation inhibitors.

CSIG-31. ALTERNATIVE RECEPTOR TYROSINE KINASE SIGNALING AS A RESISTANCE MECHANISM TO ERK INHIBITION IN HIGH-GRADE GLIOMAS

Pediatric High-Grade Gliomas (PHGG), which include Diffuse Midline Gliomas (DMG), are a leading cause of brain tumor death in children. Our recent work has identified extracellular signal-regulated kinase 5 (ERK5) as a critical mediator of cell survival in PHGG. Suppression of ERK5 genetically or pharmacologically leads to decreased cell proliferation and increased apoptosis both in vitro and in vivo in multiple PHGG and H3K27M mutant DMG cell lines. Mechanistically, we show that ERK5 directly stabilizes the proto-oncogene MYC at the protein level, providing rationale to clinically target ERK5. ERK5 contains both a kinase domain (KD) and a transactivation domain (TAD), unlike all other ERKs. Unexpectedly, we found that our ERK5 depleted cells could be partially rescued by an ERK5 kinase domain dead (ERK5-KDD) but TAD intact construct. Additionally, persistent ERK5 depletion does not result in complete growth inhibition and therefore we set out to determine potential adaptation or resistance mechanisms in response to ERK5 loss. To address this, we performed RNA sequencing of DMG cells, comparing control cells to ERK5 knockdown cells, and performed gene-ontology (GO) pathway analysis to identify transcriptional changes that occur in response to ERK5 depletion. We identified 105 differentially expressed genes, and GO analysis identified alternative receptor tyrosine kinase (RTK) gene-expression as one of the top biological processes upregulated in response to ERK5 loss. We validated our top targets at the RNA and the protein level. Our top targets were Erb-B2 Receptor Tyrosine Kinase 4 (ERBB4) and Discoidin Domain Receptor Tyrosine Kinase 2 (DDR2), both clinically actionable targets. Our future work will focus on functional validation of these RTKs as potential resistance mechanisms to ERK5 loss. Identification of resistance mechanisms to ERK5 loss will have both biological and translational relevance and may lead to effective therapeutic combinations.

Effective and safe tumor inhibition using vinblastine in medulloblastoma

Most medulloblastoma protocols worldwide include vincristine during radiation and chemotherapy. A significant dose-limiting toxicity is peripheral neuropathy; however, there is a paucity of data to support the view that omission of vincristine does not impact survival. Herein we report two adolescent patients with Group 4 and SHH medulloblastoma, where vinblastine successfully replaced vincristine with resolution of their peripheral neuropathy. We furthermore show vinblastine is highly active in vitro and demonstrates equivalent antitumoral activity compared to vincristine. Substitution of vincristine with vinblastine in future studies should be considered for all patients with medulloblastoma, particularly those with hereditary neuropathy, severe vincristine toxicity, and adults.

Brainstem blood brain barrier disruption using focused ultrasound: A demonstration of feasibility and enhanced doxorubicin delivery

Magnetic Resonance Image-guided Focused Ultrasound (MRgFUS) has been used to achieve transient blood brain barrier (BBB) opening without tissue injury. Delivery of a targeted ultrasonic wave causes an interaction between administered microbubbles and the capillary bed resulting in enhanced vessel permeability. The use of MRgFUS in the brainstem has not previously been shown but could provide value in the treatment of tumours such as Diffuse Intrinsic Pontine Glioma (DIPG) where the intact BBB has contributed to the limited success of chemotherapy. Our primary objective was to determine whether the use of MRgFUS in this eloquent brain region could be performed without histological injury and functional deficits. Our secondary objective was to select an effective chemotherapeutic against patient derived DIPG cell lines and demonstrate enhanced brainstem delivery when combined with MRgFUS in vivo. Female Sprague Dawley rats were randomised to one of four groups: 1) Microbubble administration but no MRgFUS treatment; 2) MRgFUS only; 3) MRgFUS + microbubbles; and 4) MRgFUS + microbubbles + cisplatin. Physiological assessment was performed by monitoring of heart and respiratory rates. Motor function and co-ordination were evaluated by Rotarod and grip strength testing. Histological analysis for haemorrhage (H&E), neuronal nuclei (NeuN) and apoptosis (cleaved Caspase-3) was also performed. A drug screen of eight chemotherapy agents was conducted in three patient-derived DIPG cell lines (SU-DIPG IV, SU-DIPG XIII and SU-DIPG XVII). Doxorubicin was identified as an effective agent. NOD/SCID/GAMMA (NSG) mice were subsequently administered with 5 mg/kg of intravenous doxorubicin at the time of one of the following: 1) Microbubbles but no MRgFUS; 2) MRgFUS only; 3) MRgFUS + microbubbles and 4) no intervention. Brain specimens were extracted at 2 h and doxorubicin quantification was conducted using liquid chromatography mass spectrometry (LC/MS). BBB opening was confirmed by contrast enhancement on T1-weighted MR imaging and positive Evans blue staining of the brainstem. Normal cardiorespiratory parameters were preserved. Grip strength and Rotarod testing demonstrating no decline in performance across all groups. Histological analysis showed no evidence of haemorrhage, neuronal loss or increased apoptosis. Doxorubicin demonstrated cytotoxicity against all three cell lines and is known to have poor BBB permeability. Quantities measured in the brainstem of NSG mice were highest in the group receiving MRgFUS and microbubbles (431.5 ng/g). This was significantly higher than in mice who received no intervention (7.6 ng/g). Our data demonstrates both the preservation of histological and functional integrity of the brainstem following MRgFUS for BBB opening and the ability to significantly enhance drug delivery to the region, giving promise to the treatment of brainstem-specific conditions.

Integrated (epi)-Genomic Analyses Identify Subgroup-Specific Therapeutic Targets in CNS Rhabdoid Tumors

We recently reported that atypical teratoid rhabdoid tumors (ATRTs) comprise at least two transcriptional subtypes with different clinical outcomes; however, the mechanisms underlying therapeutic heterogeneity remained unclear. In this study, we analyzed 191 primary ATRTs and 10 ATRT cell lines to define the genomic and epigenomic landscape of ATRTs and identify subgroup-specific therapeutic targets. We found ATRTs segregated into three epigenetic subgroups with distinct genomic profiles, SMARCB1 genotypes, and chromatin landscape that correlated with differential cellular responses to a panel of signaling and epigenetic inhibitors. Significantly, we discovered that differential methylation of a PDGFRB-associated enhancer confers specific sensitivity of group 2 ATRT cells to dasatinib and nilotinib, and suggest that these are promising therapies for this highly lethal ATRT subtype.

Abstract LB-179: Integrated (epi)genomic analyses identify subgroup-specific therapeutic targets in CNS rhabdoid tumors

Atypical Teratoid Rhabdoid Tumors (ATRTs) are the most common malignant embryonal brain tumors arising in younger children that are distinctly lethal cancers for which effective therapies are lacking. Although ATRTs exhibit substantial clinical heterogeneity, exome studies reveal a relatively bland coding genome with only recurrent alterations of SMARCB1. Despite apparent genomic/genetic homogeneity, we recently reported that ATRTs comprise at least two transcriptional subclasses that correlate with different clinical features and treatment outcomes. However, the biological mechanisms and basis for molecular and therapeutic heterogeneity in ATRTs remained unclear. In this study, we integrated whole genome, exome, RNAseq as well as genome wide methylation and nucleosomal profiling analyses to comprehensively define the genomic and epigenomic landscape of ATRT sub-groups and identify sub-group specific therapeutic targets. Integration of multiplatform genomic analyses revealed novel recurrent genetic alterations in upto 20% of ATRTs. We observed predominantly structural coding events that targeted genes with functions in neural development and epigenetic regulation including BCR, MKL1 and EP300, thus suggesting greater complexity to the ATRT genome than previously appreciated. Global methylation (162) and gene expression analyses (90) of primary tumors indicated further segregation of ATRTs into three epigenetic sub-groups (group 1, 2A and 2B) that correlated with distinct lineage enriched gene expression profiles, global and SMARCB1 specific genotypes and different anatomic tumor locations and age at diagnosis. Group 1 ATRT exhibited enrichment of neurogenic/NOTCH signaling loci (ASCL1, FABP7, MYCN, C1ORF61, HES5/6, DLL1) and were predominantly supra-tentorial tumors arising in children at a median age of 24 months. In contrast Group 2A tumors arose predominantly in infra-tentorial locations in the youngest patients, while group 2B tumors were characteristically spinal in location. BMP signaling and mesenchymal differentiation genes (BMP4, BAMBI, PDGFRB) were commonly enriched in group 2A and B tumors; Group 2B tumors were additionally characterized by enrichment of MYCC, HOXB & C gene clusters. Remarkably, ATAC-seq analyses revealed distinct chromatin landscape associated with each ATRT sub-group, that correlated strikingly with sub-group specific therapeutic response in ATRT cell lines to a panel of signaling (NOTCH, BMP, Dasatinib) and epigenetic (EZH2, G9a, BRD4) inhibitors. Significantly, we discovered that differential methylation of a novel, PDGFRβ associated enhancer element confers robust sensitivity to tyrosine kinase inhibitors Dasatinib and Nilotinib in group 2 ATRTs, and suggest these as novel agents for this highly lethal ATRT sub-type.

Citation Format: Jonathon Torchia, Shengrui Feng, King Ching Ho, Louis Letourneau, Daniel Picard, Tiffany S. Chan, Alexandre Vasiljevic, Dong Anh Khuong Quang, Brian Golbourn, Dalia Barsyte-Lovejoy, Constanze Zeller, Patrick Sin-Chan, Natalia R. Agamez, Mei Lu, Lucie Lafay-Cousin, Joseph D. Norman, Maryam Fouladi, Lindsey M. Hoffman, Stefan Rutkowski, Torsten Pietsch, Alexander R. Judkins, Eric Bouffet, James T. Rutka, Cynthia E. Hawkins, Cheryl H. Arrowsmith, Daniel De Carvalho, Nada Jabado, Annie Huang. Integrated (epi)genomic analyses identify subgroup-specific therapeutic targets in CNS rhabdoid tumors. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-179.

PINK1 Is a Negative Regulator of Growth and the Warburg Effect in Glioblastoma

Proliferating cancer cells are characterized by high rates of glycolysis, lactate production, and altered mitochondrial metabolism. This metabolic reprogramming provides important metabolites for proliferation of tumor cells, including glioblastoma. These biological processes, however, generate oxidative stress that must be balanced through detoxification of reactive oxygen species (ROS). Using an unbiased retroviral loss-of-function screen in nontransformed human astrocytes, we demonstrate that mitochondrial PTEN-induced kinase 1 (PINK1) is a regulator of the Warburg effect and negative regulator of glioblastoma growth. We report that loss of PINK1 contributes to the Warburg effect through ROS-dependent stabilization of hypoxia-inducible factor-1A and reduced pyruvate kinase muscle isozyme 2 activity, both key regulators of aerobic glycolysis. Mechanistically, PINK1 suppresses ROS and tumor growth through FOXO3a, a master regulator of oxidative stress and superoxide dismutase 2. These findings highlight the importance of PINK1 and ROS balance in normal and tumor cells. PINK1 loss was observed in a significant number of human brain tumors including glioblastoma (n > 900) and correlated with poor patient survival. PINK1 overexpression attenuates in vivo glioblastoma growth in orthotopic mouse xenograft models and a transgenic glioblastoma model in Drosophila Cancer Res; 76(16); 4708-19. ©2016 AACR.

Mechanism of action and therapeutic efficacy of Aurora kinase B inhibition in MYC overexpressing medulloblastoma

Medulloblastoma comprises four molecular subgroups of which Group 3 medulloblastoma is characterized by MYC amplification and MYC overexpression. Lymphoma cells expressing high levels of MYC are susceptible to apoptosis following treatment with inhibitors of mitosis. One of the key regulatory kinases involved in multiple stages of mitosis is Aurora kinase B. We hypothesized that medulloblastoma cells that overexpress MYC would be uniquely sensitized to the apoptotic effects of Aurora B inhibition. The specific inhibition of Aurora kinase B was achieved in MYC-overexpressing medulloblastoma cells with AZD1152-HQPA. MYC overexpression sensitized medulloblastoma cells to cell death upon Aurora B inhibition. This process was found to be independent of endoreplication. Using both flank and intracranial cerebellar xenografts we demonstrate that tumors formed from MYC-overexpressing medulloblastoma cells show a response to Aurora B inhibition including growth impairment and apoptosis induction. Lastly, we show the distribution of AZD1152-HQPA within the mouse brain and the ability to inhibit intracranial tumor growth and prolong survival in mice bearing tumors formed from MYC-overexpressing medulloblastoma cells. Our results suggest the potential for therapeutic application of Aurora kinase B inhibitors in the treatment of Group 3 medulloblastoma.

Poly-ADP-Ribose Polymerase as a Therapeutic Target in Pediatric Diffuse Intrinsic Pontine Glioma and Pediatric High-Grade Astrocytoma

Pediatric high-grade astrocytomas (pHGA) and diffuse intrinsic pontine gliomas (DIPG) are devastating malignancies for which no effective therapies exist. We investigated the therapeutic potential of PARP1 inhibition in preclinical models of pHGA and DIPG. PARP1 levels were characterized in pHGA and DIPG patient samples and tumor-derived cell lines. The effects of PARP inhibitors veliparib, olaparib, and niraparib as monotherapy or as radiosensitizers on cell viability, DNA damage, and PARP1 activity were evaluated in a panel of pHGA and DIPG cell lines. Survival benefit of niraparib was examined in an orthotopic xenograft model of pHGA. About 85% of pHGAs and 76% of DIPG tissue microarray samples expressed PARP1. Six of 8 primary cell lines highly expressed PARP1. Interestingly, across multiple cell lines, some PARP1 protein expression was required for response to PARP inhibition; however, there was no correlation between protein level or PARP1 activity and sensitivity to PARP inhibitors. Niraparib was the most effective at reducing cell viability and proliferation (MTT and Ki67). Niraparib induced DNA damage (γH2AX foci) and induced growth arrest. Pretreatment of pHGA cells with a sublethal dose of niraparib (1 μmol/L) before 2 Gy of ionizing radiation (IR) decreased the rate of DNA damage repair, colony growth, and relative cell number. Niraparib (50 mg/kg) inhibited PARP1 activity in vivo and extended survival of mice with orthotopic pHGA xenografts, when administered before IR (20 Gy, fractionated), relative to control mice (40 vs. 25 days). Our data provide in vitro and in vivo evidence that niraparib may be an effective radiosensitizer for pHGA and DIPG.

Transcriptional profiling of GBM invasion genes identifies effective inhibitors of the LIM kinase-Cofilin pathway

Malignant gliomas are highly proliferative and invasive neoplasms where total surgical resection is often impossible and effective local radiation therapy difficult. Consequently, there is a need to develop a greater understanding of the molecular events driving invasion and to identify novel treatment targets. Using microarray analysis comparing normal brain samples and mesenchymal glioblastoma multiforme (GBM), we identified over 140 significant genes involved in cell migration and invasion. The cofilin (CFL) pathway, which disassembles actin filaments, was highly up-regulated compared to normal brain. Up-regulation of LIM domain kinase 1 and 2 (LIMK1/2), that phosphorylates and inactivates cofilin, was confirmed in an additional independent data set comparing normal brain to GBM. We identified and utilized two small molecule inhibitors BMS-5 and Cucurbitacin I directed against the cofilin regulating kinases, LIMK1 and LIMK2, to target this pathway. Significant decreases in cell viability were observed in glioma cells treated with BMS-5 and Cucurbitacin I, while no cytotoxic effects were seen in normal astrocytes that lack LIMK. BMS-5 and Cucurbitacin I promoted increased adhesion in GBM cells, and decreased migration and invasion. Collectively, these data suggest that use of LIMK inhibitors may provide a novel way to target the invasive machinery in GBM.

Telomerase inhibition abolishes the tumorigenicity of pediatric ependymoma tumor-initiating cells

Pediatric ependymomas are highly recurrent tumors resistant to conventional chemotherapy. Telomerase, a ribonucleoprotein critical in permitting limitless replication, has been found to be critically important for the maintenance of tumor-initiating cells (TICs). These TICs are chemoresistant, repopulate the tumor from which they are identified, and are drivers of recurrence in numerous cancers. In this study, telomerase enzymatic activity was directly measured and inhibited to assess the therapeutic potential of targeting telomerase. Telomerase repeat amplification protocol (TRAP) (n = 36) and C-circle assay/telomere FISH/ATRX staining (n = 76) were performed on primary ependymomas to determine the prevalence and prognostic potential of telomerase activity or alternative lengthening of telomeres (ALT) as telomere maintenance mechanisms, respectively. Imetelstat, a phase 2 telomerase inhibitor, was used to elucidate the effect of telomerase inhibition on proliferation and tumorigenicity in established cell lines (BXD-1425EPN, R254), a primary TIC line (E520) and xenograft models of pediatric ependymoma. Over 60 % of pediatric ependymomas were found to rely on telomerase activity to maintain telomeres, while no ependymomas showed evidence of ALT. Children with telomerase-active tumors had reduced 5-year progression-free survival (29 ± 11 vs 64 ± 18 %; p = 0.03) and overall survival (58 ± 12 vs 83 ± 15 %; p = 0.05) rates compared to those with tumors lacking telomerase activity. Imetelstat inhibited proliferation and self-renewal by shortening telomeres and inducing senescence in vitro. In vivo, Imetelstat significantly reduced subcutaneous xenograft growth by 40 % (p = 0.03) and completely abolished the tumorigenicity of pediatric ependymoma TICs in an orthotopic xenograft model. Telomerase inhibition represents a promising therapeutic approach for telomerase-active pediatric ependymomas found to characterize high-risk ependymomas.

ATM regulates 3-methylpurine-DNA glycosylase and promotes therapeutic resistance to alkylating agents

Alkylating agents are a first-line therapy for the treatment of several aggressive cancers, including pediatric glioblastoma, a lethal tumor in children. Unfortunately, many tumors are resistant to this therapy. We sought to identify ways of sensitizing tumor cells to alkylating agents while leaving normal cells unharmed, increasing therapeutic response while minimizing toxicity. Using an siRNA screen targeting over 240 DNA damage response genes, we identified novel sensitizers to alkylating agents. In particular, the base excision repair (BER) pathway, including 3-methylpurine-DNA glycosylase (MPG), as well as ataxia telangiectasia mutated (ATM), were identified in our screen. Interestingly, we identified MPG as a direct novel substrate of ATM. ATM-mediated phosphorylation of MPG was required for enhanced MPG function. Importantly, combined inhibition or loss of MPG and ATM resulted in increased alkylating agent-induced cytotoxicity in vitro and prolonged survival in vivo. The discovery of the ATM-MPG axis will lead to improved treatment of alkylating agent-resistant tumors.

SIGNIFICANCE

Inhibition of ATM and MPG-mediated BER cooperate to sensitize tumor cells to alkylating agents, impairing tumor growth in vitro and in vivo with no toxicity to normal cells, providing an ideal therapeutic window.

Role of the cofilin activity cycle in astrocytoma migration and invasion

The cofilin pathway plays a central role in the regulation of actin polymerization and the formation of cell membrane protrusions that are essential for cell migration. Overexpression of cofilin has been linked to the aggressiveness of a variety of different cancers. In these cancers, the phosphorylation of cofilin at Ser3 is a key regulatory mechanism modulating cofilin activity. The activation status of cofilin has been directly linked to tumor invasion. Accordingly, in this study, we examined the expression of cofilin and its activation status in astrocytoma cell lines and astrocytic tumors. We show that cofilin expression was increased and correlated with increasing grade malignant astrocytoma. In addition, both cofilin and LIMK had elevated expression in astrocytoma cell lines. Knockdown of cofilin by siRNA altered astrocytoma cell morphology and inhibited astrocytoma migration and invasion. Conversely, overexpression of a cofilin phosphorylation mutant in an in vivo intracranial xenograft model resulted in a more highly invasive phenotype than those xenographs expressing wild-type cofilin. Animals harboring astrocytomas stably expressing the cofilin phosphorylation mutant (cofilin-S3A) demonstrated marked local invasiveness and spread across the corpus callosum to the contralateral hemisphere in all animals. Taken together, these data indicate that the cofilin activity pathway may represent a novel therapeutic target to diminish the invasion of these highly malignant tumors.

ECT2 and RASAL2 mediate mesenchymal-amoeboid transition in human astrocytoma cells

Malignant astrocytomas are highly invasive brain tumors. The Rho family of cytoskeletal GTPases are key regulators of astrocytoma migration and invasion; expression of the guanine nucleotide exchange factor ECT2 is elevated in primary astrocytomas and predicts both survival and malignancy. Mice bearing orthotopically implanted astrocytoma cells with diminished ECT2 levels following ECT2 knockdown exhibit longer survival. Although ECT2 is normally expressed in the nucleus, we show that ECT2 is aberrantly localized to the cytoplasm in both astrocytoma cell lines and primary human astrocytomas, and colocalizes with RAC1 and CDC42 at the leading edge of migrating astrocytoma cells. Inhibition of ECT2 expression by RNA interference resulted in decreased RAC1 and CDC42 activity, but no change in RHO activity, suggesting that ECT2 is capable of activating these pro-migratory Rho family members. ECT2 overexpression in astrocytoma cells resulted in a transition to an amoeboid phenotype that was abolished with the ROCK inhibitor, Y-27632. Cytoplasmic fractionation of astrocytoma cells followed by ECT2 immunoprecipitation and mass spectrometry were used to identify protein-binding partners that modulate the activity of ECT2 toward RAC1 and RHO/ROCK. We identified RASAL2 as an ECT2-interacting protein that regulates RHO activity in astrocytoma cells. RASAL2 knockdown leads to a conversion to an amoeboid phenotype. Our studies reveal that ECT2 has a novel role in mesenchymal-amoeboid transition in human astrocytoma cells.

Abstract 1253: Fer is required for mammary tumorigenesis

Fer is a ubiquitously expressed cytoplasmic protein-tyrosine kinase (PTK) which is inducibly activated upon engagement of the epidermal and platelet-derived growth factor receptors (EGFR/PDGFR). Fer - and its homologous family member Fps/Fes - are structurally distinguished from other PTKs by a membrane-binding F-BAR domain implicated in regulating membrane-cytoskeletal dynamics during receptor endocytosis. We are investigating the role of Fer in EGFR internalization using both non-transformed mammary epithelial cells (MCF10A) and mammary tumor epithelial cells (MTEC) isolated from an ErbB2 mouse model of breast cancer. MCF10A Fer-knockdown cells (MCF10A-F2) and MTECs harboring a kinase-inactivating knock-in mutation at the fer locus (ferDR/DR) displayed elevated rates of EGF-induced endocytosis suggesting that Fer inhibits EGFR internalization. Elevated endocytic rates correlated with enhanced short-term MAPK signaling in both cell culture models and elevated steady-state Erk activity in ferDR/DR ErbB2 mammary tumors. Systems-level analyses using a physiochemical model of EGFR-MAPK signaling suggested that up-regulation of receptor endocytic rates could potentiate MAPK signal strength and sensitize this pathway to pharmacological inhibition at the receptor level. Inhibition of MAPK signaling with the EGFR/ErbB2 inhibitor, Lapatinib, confirmed this prediction, showing increased Erk signal-sensitivity to this drug in endocytosis-enhanced ferDR/DR MTECs. This correlated with a 10-fold reduction in the EC50 for Lapatinib cytotoxicity on ferDR/DR MTECs. Both the signaling and cytotoxicity phenoytypes were reversible by rescued expression of wild type Fer in ferDR/DR MTECs. These data reveal a novel mechanism by which potentiation of EGFR endocytosis enhances the sensitivity of downstream signaling to inhibition by ErbB inhibitors. In this respect, Fer is a unique kinase target because it provides a direct means of pharmacologically intervening with the mechanism of EGFR endocytosis. The therapeutic significance of potentiating receptor endocytosis by targeting Fer is highlighted by observations of: (1) delayed onset of ErbB2-mediated breast tumorigenesis in ferDR/DR genetic backgrounds in vivo; and; (2) impaired tumor growth of Fer-deficient MDA-MB-231 cells in mammary-fat pad transplantation models. We envisage that anti-Fer based treatments may sensitize breast - and potentially other carcinoma types - to anti-ErbB therapies. These observations provide biological proof-in-principle for developing small molecule inhibitor-based therapies against Fer in breast cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1253. doi:1538-7445.AM2012-1253