Morphological diversity in H3G34-mutant high-grade gliomas: Ganglionic and epithelioid features

Although increasing numbers of central nervous system (CNS) tumors with stereotypic morphological, molecular, and/or site-specific features have been recently reported, morphological diversity is often recognized within a tumor category as more cases are encountered. Such was the case with diffuse midline gliomas, H3K27M-mutant. Therefore, it is not surprising that two cases of H3G34-mutant. CNS tumors with advanced ganglionic differentiation were recently published, especially given the posited role of this mutation in neuronal differentiation. We have encountered, in a 17-year-old female, a third example with advanced neoplastic ganglion cell differentiation that mimicked anaplastic ganglioglioma, with the ganglionic elements further confirmed as neoplastic by H3G34 immunohistochemistry (IHC). We therefore sought to review our experience with H3G34-mutant tumors, assessing for morphological diversity, supplemented by IHC. Six cases (ages 17 - 33 years), all confirmed on mutational analyses, were identified that were further negative for BRAFV600E or other major oncogenic mutations/fusions. The index anaplastic ganglioglioma-like case manifested multifocal large dysmorphic ganglion cells IHC+ for synaptophysin, chromogranin, and neurofilament, but no CD34 immunopositivity. A tumor from a 33-year-old male contained rare neuronal-like cells, with subtler enlargement, that were synaptophysin and neurofilament protein IHC+ and exceeded the size expected in "primitive neuroectodermal like (PNET)" tumors. A third example with morphological diversity was a glioblastoma with prominent epithelioid cells. We conclude that a spectrum of morphological differentiation does occur beyond the well-known glioblastoma or PNET-like morphology in H3G34-mutant tumors, adding to the literature one more example with advanced ganglionic differentiation and one with epithelioid features.