Mosaic Pattern of H3 K27M-Mutant Protein Expression in a Diffuse Midline Glioma-A Diagnostic Dilemma for the Pathologist

Diffuse midline glioma, H3 K27M-mutant, is a World Health Organization (WHO) grade IV glioma arising in pons, thalamus, and spinal cord. They show mutations resulting in replacement of lysine at position 27 by methionine (K27M) of histone genes, H3F3A, HIST1H3B, and HIST1H3C The H3 K27M mutant protein is identified in tumor tissue by immunohistochemistry. As these mutations are clonal and homogeneous, the mutant protein is normally identified in all tumor cells. Here we report a case of diffuse midline glioma with mosaic pattern of expression of H3 K27M mutant protein and discuss the diagnostic and therapeutic implications of this unusual pattern.