Distinct Histomorphology in Molecular Subgroups of Glioblastomas in Young Patients

Glioblastomas (GBMs) are malignant brain tumors that can be divided into different molecular subtypes based on genetics, global gene expression, and methylation patterns. Among these subgroups, "IDH" GBMs carry mutations within IDH1 or IDH2 The "K27" and "G34" subgroups are characterized by distinct mutations within Histone 3 (H3). These subtypes can be identified by sequencing methods and are particularly found in younger patients. To determine whether the molecular subtypes correlate with distinct histological features among the diverse histologic patterns of GBM, we performed a blinded assessment of the histology of GBMs of 77 patients ≤30 years old at the time of biopsy. The tumors were of the following molecular subtypes: IDH (n = 12), H3 K27M (n = 25), H3 G34R (n = 12), or no IDH/H3 mutations (n = 28). Of IDH-mutated cases, 75% had microcystic features or gemistocytic tumor cells. K27 GBMs had higher cell densities and pronounced nuclear pleomorphism, with 28% harboring tumor giant cells. All G34 GBMs had variable extents of a poorly differentiated/primitive neuroectodermal tumor-like morphology. GBMs without IDH/H3 mutations had foci of epitheliod-appearing cells. Thus, molecular GBM subgroups are associated with distinct histological patterns, suggesting that morphological features reflect the specific underlying molecular genetic abnormalities.