H3F3B p.K27I-mutant diffuse midline glioma is a distinct subtype of H3K27-altered diffuse midline glioma

Area of Science:

• Neuro-oncology
• Cancer genomics
• Epigenetics

Background:

• Diffuse midline glioma (DMG) is a fatal brain tumor with distinct molecular subtypes.
• H3K27-altered DMGs include H3.3-mutant, H3.1/H3.2-mutant, EZHIP-overexpressing, and EGFR-mutant types.
• The role of H3F3B mutations in DMG pathogenesis and clinical outcomes remains unclear.

Purpose of the Study:

• To investigate the clinical and molecular characteristics of H3F3B-mutant diffuse midline gliomas.
• To determine the impact of H3F3B mutations on H3K27 trimethylation and patient prognosis.
• To delineate the unique molecular features and potential gliomagenesis mechanisms of H3F3B-mutant DMGs.

Main Methods:

• Retrospective collection of clinical and radiological data from 9 H3F3B-mutant DMG patients.
• DNA methylation profiling and next-generation sequencing of tumor specimens.
• Immunohistochemistry for H3K27me3 expression and unsupervised t-distributed stochastic neighbor embedding (t-SNE) analysis of methylation data.

Main Results:

• All tumors exhibited somatic H3F3B p.K27I mutation and loss of H3K27me3 expression.
• H3F3B-mutant DMGs formed a distinct methylation cluster, separate from other H3K27me3-loss gliomas.
• Frequent mutations in PPM1D and NF1 were observed; prognosis was poor, comparable to H3K27M-mutant DMGs.

Conclusions:

• H3F3B mutations are a distinct molecular driver in DMG, leading to H3K27 trimethylation loss and poor outcomes.
• H3F3B-mutant DMGs represent a unique subtype with characteristic DNA methylation and mutational profiles.
• These findings suggest divergent gliomagenesis pathways between H3F3B-mutant and canonical H3K27M-mutant DMGs.