Diffuse hemispheric glioma with H3 p.K28M (K27M) mutation: Unusual non-midline presentation of diffuse midline glioma, H3 K27M-altered?

Diffuse midline glioma, H3 K27-altered (DMG-H3 K27) is an aggressive group of diffuse gliomas that predominantly occurs in pediatric patients, involves midline structures, and displays loss of H3 p.K28me3 (K27me3) expression by immunohistochemistry and characteristic genetic/epigenetic profile. Rare examples of a diffuse glioma with an H3 p.K28M (K27M) mutation and without involvement of the midline structures, so-called "diffuse hemispheric glioma with H3 p.K28M (K27M) mutation" (DHG-H3 K27), have been reported. Herein, we describe 2 additional cases of radiologically confirmed DHG-H3 K27 and summarize previously reported cases. We performed histological, immunohistochemical, molecular, and DNA methylation analysis and provided clinical follow-up in both cases. Overall, DHG-H3 K27 is an unusual group of diffuse gliomas that shows similar clinical, histopathological, genomic, and epigenetic features to DMG-H3 K27 as well as enrichment for activating alterations in MAPK pathway genes. These findings suggest that DHG-H3 K27 is closely related to DMG-H3 K27 and may represent an unusual presentation of DMG-H3 K27 without apparent midline involvement and with frequent MAPK pathway activation. Detailed reports of additional cases with clinical follow-up will be important to expand our understanding of this unusual group of diffuse gliomas and to better define the clinical outcome and how to classify DHG-H3 K27.