Diffuse midline gliomas with histone H3-K27M mutation: A rare case with PNET-like appearance and neuropil-like islands.

Diffuse midline glioma with histone H3-K27M mutation is a new tumor entity defined by the 2016 WHO Classification of Tumors of the Central Nervous System. A 51-year-old Chinese woman presented with neck pain for a month. Subsequent MRI revealed an intramedullary neoplasm extending from C5 to C7. Histologically, the cellular area of the tumor was composed of primitive, poorly differentiated, small cells with scant cytoplasm, nuclear molding, and brisk mitotic activity, exhibiting PNET-like appearance, while in the hypocellular area, oligodendroglioma-like cells were observed. More importantly, neuropil-like islands were observed in the cellular area. Microvascular proliferation was noted, with no necrosis. Besides histone H3K27M mutation, immunohistochemical staining also showed that the tumor cells were positive for oligodendrocyte lineage transcription factor 2 and ATRX. The neuropil-like areas were positive for synaptophysin, intermingled with scattered neuronal nuclear antigen positive cells. The Ki-67 proliferation index was about 30%, and tumor cells were highly immunopositive for p53. Sequencing for IDH1 codon 132 and IDH2 codon 172 gene mutations showed negative results. Furthermore, fluorescent analysis revealed 1p deletion in the lesion but no 19q deletion. Based on these findings, the tumor was diagnosed as diffuse midline gliomas with histone H3-K27M mutation in the spinal cord, corresponding to WHO grade IV. After 4 months of remission, the tumor recurred; 2 months later, the patient died. Herein, we report an extremely rare case of diffuse midline glioma with histone H3K27M mutation, which was morphologically characterized simultaneously by primitive neuroectodermal tumor-like appearance and neuropil-like islands.