Abstract B012: Defining the developmental origins and therapeutic vulnerabilities of pediatric high-grade glioma

Abstract Pediatric high-grade gliomas (pHGGs) are highly aggressive brain tumours and the leading cause of cancer-related mortality in children. Despite advances in molecular profiling, pHGGs have remained largely incurable. One major barrier to effective treatment is the profound intratumoural heterogeneity that characterizes end-stage disease, which obscures the early tumour-initiating events that arise during critical developmental windows of gliomagenesis. Increasing evidence suggests that this heterogeneity is driven by stem-like cell populations that drive tumour initiation, evolution, and ultimately give rise to therapy-resistant end-stage disease. We hypothesized that defining the earliest molecular and cellular states of gliomagenesis would reveal vulnerabilities that are lost during malignant progression. To test this, we developed a pediatric high-grade glioma model using Nestin-Cre;Trp53fl/fl mice combined with embryonic exposure to the alkylating agent N-ethyl-N-nitrosourea (ENU). This system reproducibly generates gliomas that closely resemble human high-grade disease, modeling genetic susceptibility in the context of environmental insult while enabling longitudinal analysis of discrete lesions across premalignant, early, and end-stage disease. Transcriptomic and histopathologic profiling revealed a progressive increase in genetic complexity, stemness features, and immune remodeling during tumour evolution, with early lesions displaying relative molecular homogeneity. Notably, recurrent activating Braf mutations emerged at the earliest detectable stages of transformation and were frequently retained in advanced tumours, mirroring the early emergence of BRAFV600E mutations reported in pediatric low- and high-grade gliomas. Consistent with this, RAS–RAF–MAPK pathway activation was evident prior to malignant progression. Functionally, cell lines derived from early-stage Braf-mutant lesions exhibited significantly enhanced sensitivity to BRAF inhibition compared to lines derived from end-stage tumours and patient-derived pHGG models, highlighting a developmentally restricted therapeutic window that may be missed by conventional treatment paradigms focused on end-stage tumours. Together, these findings define the temporal molecular evolution of pediatric gliomagenesis in vivo and identify early MAPK pathway activation as a developmentally constrained, targetable vulnerability with implications for therapeutic interception in pHGG. Citation Format: Anna Tymofyeyeva, Kinjal Desai, Margaret Javier, Garrett Bullivant, Jannine Forst, Nataliia Svergun, Michelle Kushida, Chunying Yu, Heather Whetstone, Peter Dirks. Defining the developmental origins and therapeutic vulnerabilities of pediatric high-grade glioma abstract. In: Proceedings of the AACR Special Conference in Cancer Research: Brain Cancer; 2026 Mar 23-25; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2026;86 (6Suppl): Abstract nr B012.