Abstract A071 Mechanisms of radiation resistance in medulloblastoma

Abstract Medulloblastoma (MB) is the most prevalent malignant brain tumor in children that demonstrates clinical and genomic heterogeneity. Of the four major subgroups, Group 3 tumors (MYC-MB), and particularly the 3gsubtype, exhibit high levels of MYC, high rates of metastasis, and have the worst prognosis. Despite treatment with surgery, radiation and chemotherapy, Group 3 medulloblastoma patients are more likely to develop recurrent tumors which are far more aggressive resulting in a 5-year survival rate after recurrence of 12%. Thus, there is an imperative need to examine the underlying mechanisms mediating treatment resistance. Recent evidence has identified cellular plasticity, as a major driver of therapy resistance. To investigate whether epigenetic mediated cell plasticity contributes to Myc-MB therapy response we performed Single Nuclei Multiome ATAC + Gene Expression analysis of paired primary and relapsed tumors. We identified the emergence of novel progenitor populations in resistant tumors with stem-like gene expression networks and altered chromatin accessibility. These emergent popuoations demostrated new enhancer-promoer interactions and exhibited increased metabolic reprogramming with potential for theraputic targeting. Because radiation is one of the key treatment modalities for Myc-MB, we chose to model radiation resistance in further studies. Single-cell RNA sequencing of outgrown in vivo irradiated orthotopic xenograft MB tumors showed an overall upregulation of metabolic pathways with the enhancement of genes that mediate oxidative phosphorylation and glutamate metabolism centering on wild-type IDH1 (wtIDH1), consistent with our patient data. IR-resistant cells showed altered chromatin occupancy of H3K3me3 and H3K27Ac. Disruption of IDH1 activity in IR resistant cells in vitro altered the epigenomic landscape, facilitating reversion to the pre-irradiated state. Interestingly, we observed that pluripotent genes with bivalent regions (H3K4me3 and H3K27me3) were enhanced in radiation-resistant cells and suppressed by IDH1 inhibition. Metabolic mass spectrometry reveled that metabolites from the citric acid cycle and synthesis of unsaturated fatty acids were among the main metabolically elevated molecules in radiation-resistant cells consistnat with the observed epigenomic reprograming. IDH1 inhibition reversed the metabolic reprograming and resensitized MB cells to radiation. In total our data identify a novel mechanism of radiation resistance in medulloblastoma with potential theraputic implications. Citation Format: Bethany Veo, Dong Wang, John Desisto, Rajeev Vibhakar. Mechanisms of radiation resistance in medulloblastoma abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Pediatric Cancer Research; 2024 Sep 5-8; Toronto, Ontario, Canada. Philadelphia (PA): AACR; Cancer Res 2024;84(17 Suppl):Abstract nr A071.