Pediatric Brain Cancer Research Articles

EPID-38. GLOBAL DISPARITIES AND PROJECTIONS IN PEDIATRIC BRAIN AND CNS CANCERS: A COMPREHENSIVE ANALYSIS THROUGH 2050

Abstract BACKGROUND Brain and other central nervous system (CNS) cancers are the most prevalent solid tumors and the leading cause of death among children worldwide. This study investigates trends in the incidence, mortality, and global distribution of pediatric brain and other CNS cancers projected through 2050. METHODS Data on pediatric brain and other CNS cancer incidence and mortality were extracted from Globocan 2022 for children aged 0 to 14 years. Age-standardized incidence and mortality rates were calculated by continent, country, and Human Development Index (HDI) categories. RESULTS The Globocan dataset revealed 24,677 new cases and 12,249 deaths, corresponding to age-standardized incidence rates (ASIRs) of 1.3 and age-standardized mortality rates (ASMRs) of 0.62 per 100,000 population. Significant geographic disparities were observed: North America reported the highest incidence rate at 3.6, followed by Europe (2.2), Latin America (1.7), Oceania (1.4), Asia (1.3), and Africa (0.67). Europe recorded the highest mortality rate at 0.89, followed by Latin America (0.87), North America (0.70), Asia (0.66), and Africa (0.40). A country-specific analysis revealed that the highest incidence rates were in Montenegro (3.7), Estonia (3.4), and Austria (3.2), whereas the highest mortality rates were in Lithuania (2.1), Angola (2.0), and Honduras (1.7). Additionally, 37.82% of new cases and 40.71% of deaths occurred in high-HDI countries. Projections: The annual increase in incidence and mortality rates for brain and other CNS cancers is projected at 1.3% through 2050, with new cases expected to rise to 25,004 and deaths to 12,404. CONCLUSIONS The findings indicate a 23% decrease in brain and other CNS cancer cases in high-HDI countries by 2050, contrasted with a 45.3% increase in low-HDI countries. These disparities underscore the critical need for global health strategies that promote equitable healthcare access and advanced treatments to reduce the global burden of these cancers and address inequities in health outcomes.

STEM-08. LIPID SATURATION HOMEOSTASIS PRESENTS A TARGETABLE VULNERABILITY IN MYC-AMPLIFIED GROUP 3 MEDULLOBLASTOMA

Abstract Medulloblastoma (MB) is the most common malignant pediatric brain cancer, originating from the cerebellum. Despite advancements in standard of care (SoC), MB remains fatal for 30% of patients. Tumor relapse, spinal metastasis and treatment resistance are the most prevalent in MYC-driven Group 3 MB (G3-MB). Patients surviving SoC are faced with life-long neurocognitive and neurodevelopmental deficits. These issues highlight the urgent need for improved treatment modalities. Reprogramming of cellular lipid metabolism is an emerging hallmark of cancer and may yield novel cancer-specific therapeutic options. Here we explore the lipidome of both G3-MB and its proposed cell of origin, human neural stem cells (hNSCs) by comparing untargeted lipidomics using Liquid-Chromatography-Mass Spectrometry (LC-MS). Comparative analyses revealed a differential abundance of distinct lipid species in G3-MB, with an overall reduced saturation level of fatty acids (FAs). These findings implicate the de novo lipid synthesis (DNL) pathway. We identified the enzymes involved in DNL to be essential for MB survival in our genome-wide CRISPR KO screen. Additionally, mRNA expression of the DNL enzymes increases at relapse in our SoC-adapted murine patient-derived xenograft (PDX) model. Pharmacological and genetic targeting of the DNL enzyme Stearoyl-CoA Desaturase 1 (SCD1) selectively targets G3-MB. Furthermore, small molecule treatment of SCD1 demonstrates efficacy against G3-MB PDX models in vivo. We identified SCD expression as a prognostic marker in Group 3 and 4 MB patients and identified possible pathways for MB treatment. Overall, these findings indicate that SCD1 is a potent target for the treatment of G3-MB.

PATH-60. AUTOMATED QUANTIFICATION OF KI-67 PROLIFERATION INDEX BASED ON WHOLE SLIDE IMAGES IN PEDIATRIC BRAIN TUMORS

Abstract The use of AI methods with histopathological images (pathomics) has strong potential to aid pathologists in pediatric brain cancer diagnosis and prognosis given the morphological and molecular heterogeneity of these tumors. However, use of pathomics in this context has been relatively limited. One example is the IHC-based tumor proliferation index (percentage of Ki-67 positive cells amongst all malignant cells) which is often routinely reviewed by pathologists clinically but remains to be objectively quantified or reported in a standardized manner. As a result, analysis of this marker across patients at-scale and in data-driven methods remains largely hindered. Here, we explore the extension of an automated deep learning AI tool (DeepLIIF), originally trained and validated with adult breast/prostate/liver data, for quantification of Ki-67 index from digitized whole slide images of a cohort of pediatric brain tumors from the Children’s Brain Tumor Network (N=834). We find that derived Ki-67 scores largely agreed with neuropathologist-reported Ki-67 activity in original clinical notes and could be used to successfully stratify glioma/astrocytoma patients (N=200) into low- and high-grade groups (LGG: M=11.6; SEM=1.2; HGG: M=18.7; SEM=2; p=0.001). Ki-67 scores were also related to the survival status of the patients (Alive: M=11.8; SEM=1.1; Deceased: M=18.8; SEM=2.2; p=0.003). Summary statistics across tumor types (glioma, astrocytoma, medulloblastoma, ATRT, ependymoma, etc.) and molecularly defined subtypes are also compared, providing direct insights into distributional properties between these groups that has not previously been feasible. Our results show the ability of AI-powered methods to quantify routine histopathology metrics with little human input, allowing objective comparisons between patients and opening the door for precision medicine approaches with predictive analytics in clinical contexts.

EXTH-02. MODIFYING CAR T CELL EPIGENETIC PROGRAMS TO IMPROVE THEIR PERSISTENCE AGAINST GROUP 3 MEDULLOBLASTOMAS

Abstract BACKGROUND Safe, effective therapies are desperately needed to improve outcomes for patients with medulloblastoma (MB), a malignant pediatric brain cancer. Chimeric antigen receptor (CAR)-T cells are a promising therapeutic option as they target T-cell cytotoxicity exclusively to tumor associated antigens. B7 homolog 3 (B7-H3) is an ideal target for CAR-T cell therapy as all MB subtypes express B7-H3 but healthy tissues do not. However, a major roadblock to tumor clearance and control is the inability of CAR-T cells to persist, suggesting that improving persistence will improve therapeutic efficacy. The goal of this study was to compare head-to-head the impact of knocking out TET2 and DNMT3A negative epigenetic regulators of CAR-T cell persistence against group 3 MB (G3MB), the most malignant MB subtype. METHODS We generated B7-H3 CAR-T cells with CD28ζ signaling domain and performed CRISPR/Cas9 knockout (KO) of TET2, DNMT3A, and AAVS1 as a control. We compared resulting CAR-T cell persistence via repeat stimulation assays in vitro against G3MB cell lines with varying B7-H3 antigen density (D341low, HDMB03mid, and D425high). We further compared KO CAR-T cells in vivo against D341low, HDMB03mid, and D425high orthotopic xenografts. RESULTS Our data shows that DNMT3A KO, but not TET2 KO, consistently improves the persistence and expansion of CAR-T cells against G3MB in vitro. In vivo, KO of epigenetic regulators, improves CAR-T cell therapy against highly aggressive D341low, HDMB03mid, and D425high but is insufficient to elicit curative responses. CONCLUSIONS Our study demonstrates that genetic KO of negative epigenetic regulators of CAR-T cell persistence can improve anti-tumor function against G3MB. We conclude that for the B7-H3.CD28ζ CAR, DNMT3A KO is superior for improving persistence over TET2 KO, but that DNMT3A KO is insufficient in vivo.

"If you build it, they will come": the convergence of funding, research and collaboration in paediatric brain cancer clinical trials.

"If you build it, they will come": the convergence of funding, research and collaboration in paediatric brain cancer clinical trials.

EPIGENETIC MODULATION OF STEMNESS AND DIFFERENTIATION BY VALPROIC ACID IN MEDULLOBLASTOMA

Abstract AIMS Changes in epigenetic processes, including histone acetylation, are proposed as key events affecting tumor cell function and the initiation and progression of pediatric brain cancers. Valproic acid (VPA) is an antiepileptic drug that acts partially by inhibiting histone deacetylases (HDACs) and could be repurposed as an epigenetic anticancer therapy. Here, we report VPA-induced alterations in features related to stemness and differentiation in medulloblastoma (MB) cells. METHOD Human MB cells were treated with different concentrations of VPA and cell viability was measured with a trypan exclusion assay. Cell cycle was evaluated with flow cytometry. Expansion of MB cancer stem cells (CSCs) was evaluated by a neurosphere formation assay. Reverse transcriptase polymerase chain reaction (RT-qPCR) was used to analyze mRNA expression and Western Blot to analyze protein levels. H3K9 histone acetylation (H3K9ac) was measured with immunofluorescence, and chromatin Immunoprecipitation (ChIP) was used to evaluate the occupancy of gene promoter regions by H3K9ac. RESULTS VPA reduced MB cell viability and expansion of MB CSCs, induced morphological changes consistent with neuronal differentiation, and increased expression of differentiation markers tubulin beta 3 class III (TUBB3) and enolase 2 (ENO2). Expression of stemness markers SOX2 and Nestin was differentially affected by VPA in cells representative of SHH or Group 3/4 MB molecular subgroups. VPA increased H3K9ac and its occupancy of promoter regions of TP53 and CDKN1A genes. CONCLUSION Our results indicate that VPA may exert antitumor effects in MB by influencing histone acetylation, which may result in modulation of stemness and stimulation of neuronal differentiation.

Characterization of aberrant splicing in pediatric central nervous system tumors reveals CLK1 as a candidate oncogenic dependency.

Pediatric brain cancer is the leading cause of disease-related mortality in children, and many aggressive tumors still lack effective treatment strategies. We characterized aberrant alternative splicing across pediatric brain tumors, identifying pediatric high-grade gliomas (HGGs) among the most heterogeneous. Annotating these events with UniProt, we identified 11,940 splice events in 5,368 genes leading to potential protein function changes. We discovered CDC-like kinase 1 (CLK1) is aberrantly spliced to include exon 4, resulting in a gain of two phosphorylation sites and subsequent activation. Inhibition of CLK1 with Cirtuvivint significantly decreased both cell viability and proliferation in the pediatric HGG KNS-42 cell line. Morpholino-mediated depletion of CLK1 exon 4 splicing reduced RNA expression, protein abundance, and cell viability with concurrent differential expression of 78 cancer genes and differential splicing at functional sites in 193 cancer genes. Our findings highlight a dependency of pediatric HGGs on CLK1 and represent a promising therapeutic strategy.

Unlocking the Door for Precision Medicine in Rare Conditions: Structural and Functional Consequences of Missense ACVR1 Variants.

Rare diseases and conditions have thus far received relatively less attention in the field of precision/personalized medicine than common chronic diseases. There is a dire need for orphan drug discovery and therapeutics in ways that are informed by the precision/personalized medicine scholarship. Moreover, people with rare conditions, when considered collectively across diseases worldwide, impact many communities. In this overarching context, Activin A Receptor Type 1 (ACVR1) is a transmembrane kinase from the transforming growth factor-β superfamily and plays a critical role in modulating the bone morphogenetic protein signaling. Missense variants of the ACVR1 gene result in modifications in structure and function and, by extension, abnormalities and have been predominantly linked with two rare conditions: fibrodysplasia ossificans progressiva and diffuse intrinsic pontine glioma. We report here an extensive bioinformatic analyses assessing the pool of 50,951 variants and forecast seven highly destabilizing mutations (R206H, G356D, R258S, G328W, G328E, R375P, and R202I) that can significantly alter the structure and function of the native protein. Protein-protein interaction and ConSurf analyses revealed the crucial interactions and localization of highly deleterious mutations in highly conserved domains that may impact the binding and functioning of the protein. cBioPortal, CanSAR Black, and existing literature affirmed the association of these destabilizing mutations with posterior fossa ependymoma, uterine corpus carcinoma, and pediatric brain cancer. The current findings suggest these deleterious nonsynonymous single nucleotide polymorphisms as potential candidates for future functional annotations and validations associated with rare conditions, further aiding the development of precision medicine in rare diseases.

Synthesis and Antitumour Evaluation of Tricyclic Indole-2-Carboxamides against Paediatric Brain Cancer Cells.

Antitumour properties of some cannabinoids (CB) have been reported in the literature as early as 1970s, however there is no clear consensus to date on the exact mechanisms leading to cancer cell death. The indole-based WIN 55,212-2 and SDB-001 are both known as potent agonists at both CB1 and CB2 receptors, yet we demonstrate herein that only the former can exert in vitro antitumour effects when tested against a paediatric brain cancer cell line KNS42. In this report, we describe the synthesis of novel 3,4-fused tricyclic indoles and evaluate their functional potencies at both cannabinoid receptors, as well as their abilities to inhibit the growth or proliferation of KNS42 cells. Compared to our previously reported indole-2-carboxamides, these 3,4-fused tricyclic indoles had either completely lost activities, or, showed moderate-to-weak antagonism at both CB1 and CB2 receptors. Compound 23 displayed the most potent antitumour properties among the series. Our results further support the involvement of non-CB pathways for the observed antitumour activities of amidoalkylindole-based cannabinoids, in line with our previous findings. Transcriptomic analysis comparing cells treated or non-treated with compound 23 suggested the observed antitumour effects of 23 are likely to result mainly from disruption of the FOXM1-regulated cell cycle pathways.

AB005. Development of tumour specific therapy for treatment of diffuse intrinsic pontine glioma (DIPG).

Diffuse intrinsic pontine glioma (DIPG), is an aggressive form of paediatric high-grade glioma (pHGG) that affects children below the age of 10 months. The survival period for a child suffering from DIPG has not changed in decades (approximately 10 months). This pattern is similar for most pHGG; even though the survival period is more extended, tumour recurrence and death are almost inevitable. This is primarily due to the presence of the blood-brain barrier (BBB), which blocks the entry of most therapeutics into the brain, and also due to tumour heterogeneity associated with central nervous system (CNS) tumours that blunt the efficacy of targeted therapy. The development of a meaningful cure for paediatric brain cancer hinges on discovering chemotherapy agents that (I) can cross the BBB; (II) accumulates explicitly in tumour tissues; and (III) can block pathways leading to the escape of cancer stem cells, promoting recurrence. This project aims to develop therapeutics that can cross the BBB, a significant hindrance to delivering medicines across the brain, and specifically target cancer cells without affecting normal brain cells. We will accomplish this by attaching novel dyes possessing tumour specificity to various classes of chemotherapy agents. The compounds will be tested on patient-derived paediatric brain cancer cell lines and the most potent compounds will be progressed to an animal model of DIPG. Several drug-dye conjugates were designed and synthesized to target various aberrant pathways involved in disease initiation and progression of DIPG. These were tested first in patient-derived DIPG cell lines. Several of these drug-dye conjugates showed potent antiproliferative effect in various DIPG cell lines. One of these conjugates is currently undergoing maximum tolerated dose study in an animal model of DIPG. The present work details an effort to develop BBB crossing tumour specific therapeutic agents for the treatment of DIPG. The work has resulted in several promising drug-dye conjugates showing antiproliferative activity in various patient-derived DIPG cell lines, enabling the progression of such conjugates into animal models of DIPG. Such studies will inform the utility of such drug-dye conjugates for application in difficult to treat pHGGs such as DIPG.

The Role of the RNA Helicase DDX3X in Medulloblastoma Progression.

Medulloblastoma is the most common pediatric brain cancer, with about five cases per million in the pediatric population. Current treatment strategies have a 5-year survival rate of 70% or more but frequently lead to long-term neurocognitive defects, and recurrence is relatively high. Genomic sequencing of medulloblastoma patients has shown that DDX3X, which encodes an RNA helicase involved in the process of translation initiation, is among the most commonly mutated genes in medulloblastoma. The identified mutations are 42 single-point amino acid substitutions and are mostly not complete loss-of-function mutations. The pathological mechanism of DDX3X mutations in the causation of medulloblastoma is poorly understood, but several studies have examined their role in promoting cancer progression. This review first discusses the known roles of DDX3X and its yeast ortholog Ded1 in translation initiation, cellular stress responses, viral replication, innate immunity, inflammatory programmed cell death, Wnt signaling, and brain development. It then examines our current understanding of the oncogenic mechanism of the DDX3X mutations in medulloblastoma, including the effect of these DDX3X mutations on growth, biochemical functions, translation, and stress responses. Further research on DDX3X's mechanism and targets is required to therapeutically target DDX3X and/or its downstream effects in medulloblastoma progression.

Diagnosis of pediatric central nervous system tumors using methylation profiling of cfDNA from cerebrospinal fluid

Pediatric central nervous system tumors remain challenging to diagnose. Imaging approaches do not provide sufficient detail to discriminate between different tumor types, while the histopathological examination of tumor tissue shows high inter-observer variability. Recent studies have demonstrated the accurate classification of central nervous system tumors based on the DNA methylation profile of a tumor biopsy. However, a brain biopsy holds significant risk of bleeding and damaging the surrounding tissues. Liquid biopsy approaches analyzing circulating tumor DNA show high potential as an alternative and less invasive tool to study the DNA methylation pattern of tumors. Here, we explore the potential of classifying pediatric brain tumors based on methylation profiling of the circulating cell-free DNA (cfDNA) in cerebrospinal fluid (CSF). For this proof-of-concept study, we collected cerebrospinal fluid samples from 19 pediatric brain cancer patients via a ventricular drain placed for reasons of increased intracranial pressure. Analyses on the cfDNA showed high variability of cfDNA quantities across patients ranging from levels below the limit of quantification to 40 ng cfDNA per milliliter of CSF. Classification based on methylation profiling of cfDNA from CSF was correct for 7 out of 20 samples in our cohort. Accurate results were mostly observed in samples of high quality, more specifically those with limited high molecular weight DNA contamination. Interestingly, we show that centrifugation of the CSF prior to processing increases the fraction of fragmented cfDNA to high molecular weight DNA. In addition, classification was mostly correct for samples with high tumoral cfDNA fraction as estimated by computational deconvolution (> 40%). In summary, analysis of cfDNA in the CSF shows potential as a tool for diagnosing pediatric nervous system tumors especially in patients with high levels of tumoral cfDNA in the CSF. Further optimization of the collection procedure, experimental workflow and bioinformatic approach is required to also allow classification for patients with low tumoral fractions in the CSF.

NURS-04. MUSIC THERAPY WITH CAREGIVERS FOR THOSE WITH PEDIATRIC BRAIN CANCER AND BRAIN TUMORS: A CRITICAL REVIEW OF THE LITERATURE AND RECOMMENDATIONS FOR FUTURE PROGRAMS

Abstract BACKGROUND Caregivers for those with pediatric brain cancer and brain tumors face significant challenges. During the course of treatment, pediatric brain cancer and brain tumor patients face not just the physical consequences of treatments, but the neurological effects as well. METHODS This critical review of the literature examines the effects of Music Therapy on Caregivers, Caregivers for cancer patients, and Caregivers for pediatric cancer patients. A literature review was specifically chosen with the intention of making recommendations for future program planning. Subjects explored include the psychological, physical, and financial impacts of caregiving. Additional concepts include personality traits of Caregivers and multicultural considerations in caregiving. The topics of brain cancer and the use of Music Therapy with patients are also explored. Special attention is given to Music Therapy for Caregivers for those with pediatric brain cancer and brain tumors. Main points under this headline also include Caregiver stressors, similarities and differences in multicultural caregiving, and the limited research available to Caregivers. RESULTS At the time the literature review, no research had been conducted in the area of Music Therapy and Caregivers for pediatric brain cancer and primary brain tumors. Music Therapy was proven, however, to be a significant non-pharmaceutical intervention for several areas, including anxiety and pain management. CONCLUSION The conclusion drawn was that more research is needed with Music Therapy and this particular population. Additionally, a basic framework for a Music Therapy program for Caregivers for those with pediatric brain cancer and brain tumors has been provided with recommendations on the expansion of the program.

MODL-04. ZEBRAFISH: THE FUTURE OF PEDIATRIC BRAIN CANCER RESEARCH

Abstract Pediatric brain cancers cause the highest rate of mortality amongst all childhood cancers. Most high-grade gliomas are difficult to treat mainly due to impermeability across the blood-brain barrier, or systemic toxicity. Here, we explore the benefits of zebrafish to assess drug toxicity, orthotopic pediatric brain cancer model development and drug screening. We assessed the toxicity of eight anti-cancer drugs with diverse mechanisms of action. In addition to the survival assay (LC50), we analyzed other teratogenic characteristics to demonstrate the detrimental developmental toxicity, which was critical in determining drug safety for future human usage. To develop fast and reproducible orthotopically xenografted pediatric brain cancer models, we injected approximately 80-100 mCherry expressing JHH-NF1-PA1 and TM-31 cells into the midbrain of 2 days post-fertilization (dpf) zebrafish embryos and imaged daily for next 4 days to monitor the survival, progression, and invasion of the injected cells. Our results show that both cell types were engrafted successfully and multiplied inside the developing zebrafish brain. As compared to JHH-NF1-PA1 cells, which mostly migrated caudally within the developing spinal cord, TM-31 migrated in multiple directions and metastasized five times more rapidly. To check the efficiency of VAL-083 and AZD-1775 in high-grade glioma, 2 dpf zebrafish embryos were injected with mCherry expressing HGG cells, SF8628. At one day post-injection (dpi), the xenografted embryos were exposed to different combinations of Val-083 and AZD-1775 for 72 hours. After 4 dpi, tumor cell invasion and migration in zebrafish were quantified. We found that the combination group had the lowest tumor burden with significantly lower fluorescence intensity, total area of migration, and reduced number of metastatic cells (p<0.001) compared to single or no drug treatment groups. Our study showed that zebrafish is a valuable, rapid, and cost-effective in-vivo pediatric brain cancer pre-clinical model to assess tumor proliferation, metastasis, and rapid drug screening.

HGG-53. MULTI-DIMENSIONAL INTEGRATIVE PROFILING IDENTIFIES BCL2L1 METHYLATION AS A PREDICTIVE BIOMARKER FOR MCL-1 ACTIVITY IN PEDIATRIC HIGH-GRADE GLIOMAS

Abstract BACKGROUND Pediatric high-grade gliomas (pHGGs) pose a significant challenge as the most aggressive central nervous system tumors in children. The lack of effective treatment and poor survival rates emphasize the critical need for innovative therapies to improve the prognosis of pediatric patients with pHGG. METHODS We executed CRISPR-cas9 knockout (KO) screenings in 65 pediatric and 10 adult high-grade glioma (HGG) cell lines to explore unique functional dependencies associated with pHGGs. Drug assays were carried out to assess the targetability of a gene dependency of interest in pHGG cell lines. Subsequently, Random Forest machine learning algorithm was employed on ‘omics’ datasets to identify biomarkers indicative of drug response. Finally, biomarkers were confirmed through sequencing-based methods across various pediatric cancers. RESULTS CRISPR-cas9 KO screens revealed 8 crucial genes essential for pHGG growth, namely MCL-1, ATIC, DHFR, EED, HDAC2, PARP1, PDK1, PIK3CA and TYMS. Among these, Myeloid Cell Leukemia (MCL-1) was further characterized as it emerged as the top differential genetic dependency in pHGGs. Consistent with this, MCL1 inhibitors exhibited potent anti-cancer activity on 43% of pHGG cell lines. Multi-feature prediction analysis identified 140 features correlated to response. Strikingly, a previously undescribed methylation site within the BCL2L1 locus, cg00300298, ranked in the top 5% of features. Tissue analysis revealed a wide range of other pediatric brain cancers that harbor the BCL2L1 cg00300298 methylation mark as compared to non-malignant brain tissue. Lastly, we validated the utility of BCL2L1 methylation as a predictive biomarker of MCL1 inhibitor sensitivity across a large panel of pediatric brain cancer cell lines. CONCLUSION Using an integrated genomic approach, we identify MCL1 as a distinct therapeutic target in BCL2L1-methylated pediatric brain cancers. This offers a rational approach for stratifying patients who may benefit from MCL1 inhibitors.

DIPG-94. PROTEOMIC AND BIOINFORMATICS ANALYSIS OF THE SURFACEOME AND EXTRACELLULAR MATRIX OF DIFFUSE MIDLINE GLIOMAS

Abstract INTRODUCTION Diffuse midline glioma (DMG) is a universally fatal pediatric brain cancer with a median overall survival of 9-11-months. The extracellular matrix (ECM) plays a significant role in regulating bidirectional communication between glioma cells and the niches of the tumor immune microenvironment (TIME); influencing expression of immunosuppressive cytokines and regulating immune cell infiltrations, all of which contribute to poor outcomes. DMG immunosuppression remains poorly understood but has implications for emerging immunotherapies such as CAR T cells. Using a sucrose gradient plasma membrane purification method coupled with quantitative high-resolution proteomics we have discovered a novel cancer-associated protein termed ‘Protein-J 4 (PJ-4)’. PJ-4 is an important component of the DMG-ECM that we believe contributes to immunosuppression. METHODS Expression of PJ-4 was validated by Immunoblotting, flow cytometry and immunofluorescences microscopy. CRISPR/dCas9 gene deletion and amplification of PJ-4 and its transcription factor are current being established using DMG patient-derived cell lines, and syngeneic DIPG models to help elucidate the biological significance of PJ-4 expression. RESULTS Immunofluorescences microscopy showed PJ-4’s translocation from the Golgi apparatus to the cell membrane is dependent neurosphere formation, hence the formation of an ECM. Further, PJ-4 demonstrated high affinity with pro-TGF-β and may serve to regulate its bioavailability in the TIME, suggestive of an immunomodulatory role. Immunohistochemistry analysis of our PJ-4, TGF-β and CD45 is currently underway using a constructed tissue microarray of healthy brain tissue samples and over 30 pediatric brain cancers to determine tumor specificity and correlation with immune cell infiltration. CONCLUSIONS Resistance to conventional therapies and a poor understanding of the TIME and ECM remains a significant challenge to the development of effective DMG therapies. The identification of PJ-4 as a cancer-associated constituent of the DMG-ECM may help us to develop strategies to that increase DMG response to standard-of-care radiation therapy, novel target agents and immunotherapies.

ATRT-13. NOVEL ACYLFULVENE LP-184 SYNERGIZES WITH RUCAPARIB TO KILL AT/RT CELLS

Abstract BACKGROUND Atypical Teratoid Rhabdoid Tumor (AT/RT) is a pediatric brain cancer with survival rates of less than 40%. Illudins are a novel class of DNA-damaging alkylating agents. LP-184, an investigational synthetic acylfulvene analog currently in a first-in-human Phase 1 trial, exhibits enhanced ability to penetrate the brain and damage DNA in tumor cells. We hypothesized that LP-184 would synergize with PARP inhibitors such as rucaparib which inhibit DNA repair. The combination of these therapies could be a promising treatment option for AT/RT. METHODS We tested LP-184 against the AT/RT cell lines BT37, CHLA06, CHLA05, and CHLA266. Cell viability assays were conducted by flow cytometry. Immunofluorescence was performed to visualize CC3 and BrdU. Western blots were performed to determine c-PARP and p-RB protein levels. Synergy experiments were performed using cell growth inhibition as the readout and quantified using the SynergyFinder online computational tool. RESULTS LP-184 suppressed AT/RT proliferation at nanomolar concentrations as measured by BrdU immunofluorescence and pRB western blots (BrdU BT37 p=0.0001, CHLA06 p=0.0001, CHLA05 p=0.033 50 nM concentration of LP-184 compared to DMSO control). Furthermore, LP-184 showed strong synergy with the PARP inhibitor rucaparib. Viable cell count was reduced by as much as 80% in combination compared to DMSO control (BT37 p=0.0001, CHLA06 p=0.0002, CHLA05 p=0.0077). Combination therapy decreased proliferation as measured by immunofluorescence and confirmed by pRB western blots (BrdU BT37 p=0.0004, CHLA06 p=0.0033, CHLA05 p=0.0154 compared to DMSO control). LP-184 and rucaparib induced apoptosis as measured by cleaved caspase 3 (CC3) and cPARP western blots (BT37 p=0.0002, CHLA06 p=0.0117, CHLA05 p=0.0001 %CC3+ compared to control). Survival studies in vivo are underway to test single agent and combination therapies of LP-184 and rucaparib in AT/RT. CONCLUSION LP-184 and rucaparib together could provide a compelling clinical option for patients with this poor-prognosis brain tumor.

IMMU-02. MODIFYING CAR T CELL EPIGENETIC PROGRAMS TO IMPROVE THEIR PERSISTENCE AGAINST GROUP 3 MEDULLOBLASTOMAS

Abstract BACKGROUND Safe, effective therapies are desperately needed to improve outcomes for patients with medulloblastoma (MB), a malignant pediatric brain cancer. Chimeric antigen receptor (CAR)-T cells are a promising therapeutic option as they target T-cell cytotoxicity exclusively to tumor associated antigens. B7-homolog 3 (B7-H3) is an ideal target for CAR-T cell therapy as all MB subtypes express B7-H3 and healthy tissues do not. However, a roadblock to tumor clearance is the inability of B7-H3 CAR-T cells to persist, suggesting that improving persistence will benefit anti-tumor efficacy. The goal of this study was to compare head-to-head knocking out TET2 or DNMT3A negative epigenetic regulators of CAR-T cell persistence against group 3 MB (G3MB), the most malignant MB subtype. METHODS We generated B7-H3 CAR-T cells with CD28ζ signaling domain and performed CRISPR/Cas9 knockout (KO) of TET2, DNMT3A, and AAVS1 as a control. We compared resulting CAR-T cell persistence via repeat stimulation assays in vitro against G3MB cell lines with varying B7-H3 antigen density (D341low, HDMB03mid, and D425high). We further compared KO CAR-T cells in vivo against D341low, HDMB03mid, and D425high orthotopic xenografts. RESULTS Our data shows that DNMT3A KO, but not TET2 KO, consistently improves the persistence and expansion of CAR-T cells against G3MB in vitro. In vivo, KO of epigenetic regulators moderately improves CAR-T cell therapy against highly aggressive D341low, HDMB03mid, and D425high but is insufficient to elicit curative responses. CONCLUSIONS Our study demonstrates that genetic KO of negative epigenetic regulators of CAR-T cell persistence can improve anti-tumor function against G3MB. We conclude that for the B7-H3.CD28ζ CAR, DNMT3A KO is superior for improving persistence over TET2 KO, but that DNMT3A KO is insufficient in vivo. We believe this is due to the aggressive nature of G3MB and we are optimizing in vivo studies for better representation of CAR-T cell persistence.

DIPG-14. METABOLIC REPROGRAMMING AS A RESISTANCE MECHANISM IN DIFFUSE MIDLINE GLIOMA

Abstract BACKGROUND Diffuse midline gliomas (DMG) are uniformly fatal pediatric brain cancers, refractory to current standards of care. Based on genomic and transcriptomic profiling data, we previously proposed that platelet-derived growth factor receptor alpha (PDGFRA) is crucial to the tumorigenic potential and maintenance of stem-like DMG cells and that targeting PDGFRA is an important therapeutic avenue in DMG. In comprehensive pre-clinical studies, we identified avapritinib, an FDA-approved PDGFRA inhibitor, to be highly effective in patient-derived xenograft (PDX) DMG models. We further reported the first clinical experience using avapritinib in recurrent DMG and demonstrated preliminary safety, tolerability, and efficacy of the drug in this population. While a subset of patients experienced a strong clinical response from avapritinib, tumor progression was eventually observed in all patients. METHODS Here, we investigated the molecular mechanisms that DMG cells upregulate following avapritinib treatment, with the aim to exploit them therapeutically. To uncover these mechanisms, we performed transcriptomic, metabolic and functional assays along with a combinatorial drug screening. RESULTS Transcriptional analysis of patient-derived DMG cell lines revealed an upregulation of genes associated with fatty acid metabolism and oxidative phosphorylation (OXPHOS) following treatment with avapritinib. Functional assays confirmed elevated OXPHOS in avapritinib-treated cells with significant (i) increase in mitochondrial energy transduction, (ii) increase in palmitate- (a product of fatty acid metabolism) driven oxygen consumption rate and (iii) greater incorporation of palmitate-derived carbons into the tricarboxylic acid cycle. To determine which therapies could target this dependency on OXPHOS and fatty acid metabolism, we performed a metabolic drug screening in patient-derived DMG cell lines and identified two metabolic drugs to have synergistic cytotoxic effects with avapritinib. CONCLUSION We revealed distinct metabolic reprogramming in avapritinib-treated DMG cells and showed that targeting these metabolic vulnerabilities might further increase the clinical benefit of PDGFRA inhibitor avapritinib.

MDB-61. STARVING TUMORS OF POLYAMINES: A NOVEL APPROACH IN TREATING AGGRESSIVE PEDIATRIC MEDULLOBLASTOMA

Abstract BACKGROUND Polyamines, essential intracellular polycations, govern fundamental cell processes and drive oncogenesis. Our research unveiled the dependency of pediatric Diffuse Midline Gliomas (DMG) on the polyamine pathway (Khan et al.,2021). Disruption of this pathway with the synthesis inhibitor DFMO, and the transport inhibitor AMXT1501, potently inhibited DMG tumors, yielding unprecedented survival in-vivo. We hypothesized that dual polyamine pathway targeting could similarly suppress growth in other aggressive pediatric brain tumors. METHODS/RESULTS We analyzed the expression of polyamine pathway regulators in high-risk pediatric brain cancers ZERO precision medicine platform and found elevated expression in medulloblastoma. ODC1, a polyamine synthesis driver, was found to have significant correlation in high-grade gliomas with four key functional tumor states—stemness, cell cycle, hypoxia, and angiogenesis. Medulloblastoma cells were sensitive to treatment with DFMO but showed a compensatory increase in polyamine transporter expression. Conversely, treatment with AMXT1501 increased ODC1 expression. Dual targeting of the polyamine pathway potently and synergistically inhibited cell proliferation and clonogenic potential in Group-3 medulloblastoma cells. Flow cytometric analysis of Annexin V-stained cells showed that treatment with DFMO and AMXT1501 in combination led to induction of apoptosis. Critically, dual polyamine inhibition potently sensitized medulloblastoma cells to the topoisomerase-I inhibitor, SN-38, leading to complete eradication of medulloblastoma cell proliferation and colony formation at nanomolar chemotherapeutic concentrations, with the effect most pronounced in Group-3 medulloblastoma. In triple combination-treated cells, over-representation analysis revealed heightened mitotic DNA damage, macroautophagy, and apoptosis, coupled with diminished neuron/synapse development, glycosylation, and cellular signaling pathways. We are currently testing this treatment strategy in aggressive orthotopic medulloblastoma models. CONCLUSION These findings suggest targeting the polyamine pathway as a promising strategy to sensitize aggressive pediatric medulloblastoma tumors to chemotherapy. DFMO/AMXT1501 is in Phase I evaluation for adults (NCT05500508) and advancing to a Phase I/II trial for children through the CONNECT consortium.