NOVEL CELL MODELS OF CNS TUMORS WITH BCOR FUSION OR INTERNAL TANDEM DUPLICATION SUGGEST FGFR AND PDGFR AS PROMISING THERAPY TARGETS

Abstract Central nervous system (CNS) tumors with BCOR internal tandem duplications (CNS-BCOR ITD) are aggressive malignancies recently included in the 2021 WHO Classification of CNS tumors. This entity is characterized by ITDs within the PUFD domain of BCOR, potentially interfering with protein-protein interactions and preventing non-canonical polycomb repressive complex 1.1 (ncPRC1.1) complex formation. Additionally, other BCOR alterations like frame shift mutations and gene fusions have been described. However, the underlying molecular mechanisms promoting tumor aggressiveness remain unknown. We established cell models from one patient harboring a BCOR frameshift mutation and another one with a concomitant BCORL1-fusion. Two additional models were derived from a patient with a CNS-BCOR ITD tumor. Multidrug screening uncovered high sensitivity against defined receptor tyrosine kinase (RTK) inhibitors (TKIs). In detail, ponatinib, nintedanib, and dovitinib reduced cell viability at half maximal inhibitory concentrations (IC50) in the low micro-molar range (<2.5 µM). Expression analyses of the respective TKI targets suggested fibroblast growth factor receptor 3 (FGFR3) and platelet derived growth factor receptor A (PDGFRA) as central players in this response. RTK inhibition resulted in strongly impaired downstream MAPK and Pi3K/AKT signaling. Vice versa, exposure to the RTK ligands bFGF and PDGFAA increased S6, Erk and Akt phosphorylation. Next, we treated two patients – one with a BCOR frame shift mutation/BCORL1-gene fusion and one with an ITD with nintedanib – within a multimodal treatment approach and achieving complete remission and disease stabilization, respectively. Ultimately, we analyzed respective RTK ligands in patient cerebral spinal fluid (CSF) and found FGF18 and PDGFA to correlate with tumor treatment response and progression. Summarizing, we uncover a central role of defined RTK signaling modules in the malignant phenotype of CNS-BCOR-ITD and tumors harboring BCOR alterations and elucidate their potential as therapeutic targets. Currently, we aim to dissect the interconnection between BCOR/BCORL1 alterations and RTK hyperactivation.