EPCO-30. UNCOVERING MECHANISMS OF RESISTANCE IN PEDIATRIC HIGH-GRADE GLIOMA (PHGG)

Abstract

Abstract Pediatric high-grade gliomas (pHGGs) are currently the leading cause of cancer-related deaths in children, remaining incurable in most cases. A molecular subgroup of pHGGs harbor G34R mutations in the histone H3.3, although the mechanisms of the H3.3-G34R/V are not well established. In most of the patients, the tumors resist radiation and relapse after standard of care (SOC). We aim to study the molecular mechanisms mediating resistance in these tumors. Knowing why certain tumors resist treatment allows us to implement targeted therapies. We performed single cell RNA sequencing on 3 non-treated (NT) glioma bearing mice, 3 mice resistant to treatment (R), and 3 mice susceptible to treatment (S). We isolated the tumor tissue (including tumor microenvironment) and processed the cells for single cell RNA sequencing. Using R, the single-cell RNA sequencing data was converted to Seurat datasets for analysis and subsequently combined and clustered using UMAP. We used the ScType and SingleR software tools to annotate cell types. After cell annotation, Gene Set Enrichment Analysis (GSEA) was performed between R and S tumors for each cell cluster as well as NT vs Treated. GSEA showed that susceptible tumor cells showed increased immune responses when compared to resistant cells. Additionally, resistant cells showed increased neuronal phenotypes as well as suppression of the ERK 1/2 cascade pathway in comparison to susceptible cells. There were also numerous differences in immune cell subpopulations, which may help uncover mechanisms of resistance. For example, certain immune cells show signs of increased epigenetic modifications in resistant vs. susceptible glioma cells. Our results indicate that the resistance of pHGG to treatment is associated with the establishment of immunosuppressive signals from tumor cells, and the suppression of tumor growth mediated by downregulation of the MAPK/ERK cascade.