Methylation profiling of adult astroblastoma

Aims and methods: astroblastoma is a rare glial brain tumor with singular morphology. Recurrent MN1-BEND2 fusions have been recently identified in most of pediatric cases. Adolescent and adult cases, however, remain molecularly poorly defined. Here, we performed clinical and molecular characterization of a retrospective cohort of 14 adult and one adolescent gliomas with astroblastic features. Results: strikingly, we found MN1 fusions a rare event in this age group (1/15). Using methylation profiling and targeted sequencing, most cases were reclassified as either pleomorphic xanthoastrocytomas (PXA) or high-grade glioma (HGG). PXA-like ABM show BRAF mutation (6/7 with V600E mutation and 1/7 with G466E mutation) and CD34 expression. Conversely, HGG-like ABM harbored specific mutations of diffuse midline glioma (2/5) or glioblastoma (3/5). These latter patients showed an unfavorable clinical course with significantly shorter overall survival (p = 0.027). MAPK pathway alterations (including FGFR fusion, BRAF and NF1 mutations) were present in 10 of 15 patients and overrepresented in the HGG group (3/5) compared to previously reported prevalence of these alterations in GBM and diffuse midline glioma. Conclusion: We suggest that astroblastoma comprises a variety of molecularly sharply defined entities. Adults’ astroblastomas harboring molecular features of PXA and HGG should be reclassified. CNS high-grade neuroepithelial tumors with MN1 alterations appears to be a truly pediatric entity and is uncommon in adult cases with a histology of astroblastoma. Astroblastic morphology in adults should thus prompt thorough molecular investigation aiming at a clear histomolecular diagnosis and identifying actionable drug targets, especially in MAPK pathway.