Histological features in diffuse midline glioma, H3 K27M-mutant, WHO grade IV

Introduction
Diffuse midline glioma H3K27M-mutant (DMGH3K27M) is a newly recognized entity which typically arises in the pons, thalamus or spinal cord of children and young adults. The mutation can be detected using a H3K27M-mutation-specific antibody. ATRX-mutations occur in 10-15%. DMGH3K27M patients have a grim prognosis, and tumors are automatically classified as WHO-grade IV. Histological features of malignancy are not required for the diagnosis. We wanted to analyze histological features, ATRX-mutations, patient age and tumor location in DMGH3K27M-mutated gliomas operated at Rigshospitalet between 01.01.15 and 31.12.17.
Methods
We retrieved all DMGH3K27M-mutated gliomas (12 cases, stereotactic biopsies). One case was excluded because of sparse material. Histological features of malignancy were recorded. We performed immunohistochemical analyses for H3K27M-mutation, IDH1-mutation and ATRX.
Results and discussion
Median age: 16 years (4-44 years). Seven were located in pons, one in brainstem NOS, and three in thalamus. Based solely on histologic features, six cases would have been graded as WHO-grade IV, three as WHO-grade III and two as WHO-grade II. Five of ten cases had an ATRX-mutation. IDH-mutations were not detected.
DMGH3K27M occurred in one older patient. We detected a higher frequency of ATRX-mutations than reported in the literature, but our series was small. The presence of ATRX-mutation in absence of an IDH-mutation should provoke a search for histone-3-mutation. Some tumors lack histological signs of malignancy, and can be undergraded unless a H3K27M-mutation is investigated for.