Abstract 155: Clinico-radiological and histomolecular analyses identify of a new subtype of diffuse midline glioma, H3 K27 and BRAF/FGFR1 co-altered

Abstract Diffuse midline gliomas (DMG) H3 K27-altered are incurable grade 4 gliomas and represent a major challenge in neuro-oncology. This tumor type is now classified in four subtypes by the 2021 edition of the WHO Classification of the Central Nervous System (CNS) tumors. However, the H3. 3-K27M subgroup still appears clinically and molecularly heterogenous. Recent publications reported that rare patients presenting a co-occurrence of H3. 3K27M with BRAF or FGFR1 alterations tended to have a better prognosis. To better study the role of these co-driver alterations, we assembled a large pediatric and adult cohort of 29 tumors H3K27-altered with co-occurring activating mutation in BRAF or FGFR1 as well as 31 previous cases from the literature. We performed a comprehensive histological, radiological, genomic, transcriptomic and DNA methylation analysis. Interestingly, unsupervised t-distributed Stochastic Neighbor Embedding (tSNE) analysis of DNA methylation profiles regrouped BRAFV600E and all but one FGFR1MUT DMG in a unique methylation cluster, distinct from the other DMG subgroups and also from ganglioglioma (GG) or high-grade astrocytoma with piloid features (HGAP). This new DMG subtype harbors atypical radiological and histopathological profiles with calcification and/or a solid tumor component both for BRAFV600E and FGFR1MUT cases. Contrary to other DMG, these tumors occur more frequently in the thalamus (70% for BRAFV600E and 58% for FGFR1MUT) and patients have a longer overall survival with a median above three years. The analyses of a H3. 3-K27M BRAFV600E tumor at diagnosis and corresponding in vitro cellular model showed that mutation in H3-3A was the first event in the oncogenesis, indicating a distinct oncogenesis from classical DMG despite an identical founder H3K27 alteration, and possible subclonal evolution. In conclusion, DMG, H3 K27 and BRAF/FGFR1 co-altered represent a new subtype of DMG with distinct genotype-phenotype characteristics, which deserve further attention with respect to trial interpretation and patient management. Our work also lays the foundations for therapeutic development highly-needed against this deadly disease. Citation Format: Lucie Auffret, Yassine Ajlil, Arnault Tauziede-Espariat, Thomas Kergrohen, Chloé Puiseux, Laurent Riffaud, Pascale Blouin, Anne-Isabelle Bertozzi, Pierre Leblond, Klas Blomgren, Sebastien Froelich, Alberto Picca, Mehdi Touat, Marc Sanson, Kevin Beccaria, Thomas Blauwblomme, Volodia Dangouloff-Ros, Nathalie Boddaert, Pascale Varlet, Marie-Anne Debily, Jacques Grill, David Castel. Clinico-radiological and histomolecular analyses identify of a new subtype of diffuse midline glioma, H3 K27 and BRAF/FGFR1 co-altered abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 155.