PATH-17. NOVEL DUAL HISTONE 3 K27M AND G34R POINT MUTATIONS IN A MIDLINE GLIOMA

Abstract

Abstract BACKGROUND Histone H3 alterations due to mutations on H3F3A and HIST1H3B genes, have in recent years been associated with distinct entities and tumor locations within the context of infiltrative gliomas. H3K27M is associated with a midline location and is included in the WHO 2016 as the diffuse midline glioma H3K27M-mutant, a specific diagnostic entity. H3G34R, thought to be a mutually exclusive alteration, is less common but has been associated with a cerebral hemispheric location. We report the first case to our knowledge with both of these alterations in the same tumor. METHODS Clinical and pathologic records of the patient were reviewed and presented. RESULTS A 39-year-old man presented with acute right face, arm, and leg numbness and mild weakness; examination was notable for right lower motor neuron facial nerve palsy and numbness, along with numbness and subtle slowing of rapid alternating movements in the left arm and leg. A non-enhancing left thalamic mass was identified and stereotactically biopsied. Infiltrative glial neoplasm with moderately-increased cellularity was seen, with ovoid cells, enlarged nuclei, apoptotic bodies, and mitotic figures. No necrosis or microvascular proliferation seen. Immunostain was positive for H3K27M. O6-methylguanine-methyltransferase (MGMT) promoter was not methylated. Next-generation sequencing showed dual in cis H3 point mutations in K27M (HIST1H3B c.83A >T) and G34R (HIST1H3B c.103G >C). Additional alterations were noted in NF1, PIK3CA, ATRX, FGFR3, and NSD1. Isocitrate dehydrogenase (IDH1/2) mutations were not identified. CONCLUSION This case of a young man with a midline glioma is novel for carrying both H3K27M and H3G34R alterations and indicates these alterations are not mutually exclusive. The interaction seen here suggests H3K27M dominance for a midline phenotype.