P10.21.B DIFFUSE MIDLINE GLIOMA, H3 K27 AND BRAF/FGFR1 CO-ALTERED: DISTINCT BIOLOGY AND THERAPEUTIC OPPORTUNITIES

Abstract BACKGROUND Diffuse midline gliomas (DMG) H3 K27-altered are incurable WHO grade 4 gliomas that pose a significant therapeutic challenge in neuro-oncology. The 2021 WHO Classification of CNS Tumors recognizes four subtypes of DMG, yet clinical and molecular diversity suggests additional subgroups may exist. We recently identified a novel subtype of DMG characterized by concurrent BRAF or FGFR1 mutations alongside H3K27M mutations or EZHIP overexpression, suggesting a unique clinical entity within this category. MATERIAL AND METHODS A retrospective cohort of 60 adult and pediatric DMG cases was assembled, all exhibiting both MAPK pathway alterations (BRAFV600E or FGFR1 mutations) and either H3K27M mutations or EZHIP overexpression. These cases underwent comprehensive clinical, radiological, histopathological, and genomic analysis. DNA methylation profiling was performed to assess epigenetic distinctions, and transcriptomic analyses were used to explore pathway activation and gene expression signatures. Additionally, patient-derived models were developed to study therapeutic responses, and specific clinical response in patient are reported. RESULTS This newly characterized DMG subtype shows distinct features: a higher incidence in the thalamus (27% overall; 70% for BRAFV600E and 58% for FGFR1MUT cases), and greater prevalence in adults for FGFR1MUT tumors. These tumors are associated with improved overall survival, with a median exceeding three years. Unsupervised methylation analysis revealed a unique cluster, distinct from both classical DMG H3K27-altered and lower-grade midline gliomas with MAPK alterations. Radiologically and histologically, these tumors often exhibit calcifications and a solid component—features uncommon in canonical DMG. Transcriptomic analysis revealed a senescence-associated expression profile involving the TP53/CDKN1A axis and marked PI3K/AKT pathway upregulation. Additionally, preliminary clinical data indicate enhanced sensitivity to everolimus in this subgroup. CONCLUSION DMG H3 K27 and BRAF/FGFR1 co-altered represent a novel and distinct molecular subtype of DMG with unique radiological, histopathological, and prognostic characteristics. Their distinct biology, improved prognosis, and differential drug response profile support their recognition as a separate entity, warranting specific stratification in future clinical trials and personalized therapeutic approaches.