P01.31.A THE YAP/TAZ-TEAD AXIS PROMOTES TUMOR STEM CELL FUNCTIONALITY AND CELLULAR HETEROGENEITY IN PEDIATRIC HIGH-GRADE GLIOMAS

Abstract BACKGROUND Pediatric high-grade gliomas (pHGGs) are diffusely invasive and rapidly growing brain malignancies and are among the leading causes of cancer-related deaths in children. However, current treatments for pHGGs are largely ineffective. Thus, alternative therapeutic approaches are urgently needed. Genetic mutations observed in these gliomas include H3 oncohistone mutations and alterations in receptor tyrosine kinases (RTKs), including amplification, mutation, and gene fusions. However, directly targeting RTK variants with tyrosine kinase inhibitors (TKIs) has shown only partial responses in patients, due in part to the molecular and cellular heterogeneity within these tumors. Therefore, we sought to develop new experimental model systems that recapitulate inter- and intra-tumor heterogeneity to enable therapeutic discovery. METHODS To explore new therapeutic approaches, we created human brain organoid-tumor models of pHGG, in which patient-derived tumor cells are engrafted into ex vivo human cortical brain organoids that mimic the neonatal human brain. These organoid models provide an improved system for examining how the neural CNS microenvironment shapes cellular and molecular heterogeneity among tumor cells over time. We tested these models for responses to small molecule inhibitors and targeted therapies, including approaches that target a range of developmental and oncogenic effector pathways downstream of RTKs. Candidate targets included the YAP and TAZ transcription factors, which cooperate with TEAD co-factors to promote target gene expression, orchestrate stem cell fate decisions, and drive proliferation during embryonic neural development. We examined responses in both tumor and non-tumor cells in the neural microenvironment following YAP/TAZ pharmacologic and genetic inhibition, using single-cell RNA sequencing and immunofluorescence to visualize cellular responses. RESULTS We observed that YAP-TEAD function is required for the growth and progression of pHGGs with oncogenic RTK mutations and H3 oncohistone mutations. Depletion of YAP reduced proliferation, inhibited pHGG stem cell self-renewal, and induced apoptosis in pHGG cells. Treatment with small molecule inhibitors targeting YAP/TAZ and TEAD mimicked the effects of genetic YAP inhibition. Further preclinical work in primary tumor cell cultures and mouse genetic models of pHGG correlated with our findings from organoid models, supporting the potential therapeutic relevance of YAP/TAZ-TEAD inhibition for pHGG treatment. CONCLUSIONS Our results show that YAP/TAZ-TEAD inhibition may represent a promising therapeutic avenue for pHGGs.