Abstract BACKGROUND H3K27M-altered diffuse midline gliomas (DMG) have poor outcomes with limited biomarkers to predict overall survival. Stratification based on chromosomal and molecular rearrangements represents a strategy to improve outcomes. METHODS We performed a retrospective analysis of 31 pediatric patients with biopsy-proven H3K27M-mutant DMG, who have undergone molecular next generation sequencing to define cytogenetics alterations that negatively impact overall survival. Median age at diagnosis in this cohort: 9.8 years (range 2.2-24.9 years). Among the cohort, 19 had H3F3A mutations confirmed by NGS, two had confirmed HIST1H3B mutations. Patients with intracranial masses received a standard fractionated radiation course with 54Gy or 59.4Gy XRT. RESULTS Pairwise comparisons demonstrate chromosomal losses and deletions are associated with poor overall survival (p=0.025, difference in Overall Survival [OS]: -19.9 months, 95%CI: -37.16 to -2.730) when compared to patients in the cohort without chromosomal deletions. Further subgroup analysis demonstrate worse outcomes in patients with deletions in chromosome 12 (p=0.003, difference in OS: -21.2 months, 95%CI: -34.74 to -7.702), deletions in chromosome 14 (p=0.007, difference in OS: -19.8 months, 95%CI: -33.72 to -5.892), or amplifications in chromosome 1 (p=0.003, difference in OS: -23.1 months, 95%CI: -37.53 to -8.613) relative to those without corresponding mutations. These data suggest that chromosomal alterations may serve as a potential biomarker to risk stratify patients. Likewise, genes within these deleted or amplified chromosomal segments may represent targets for therapeutic intervention.

NCOG-46. CHROMOSOMAL LOSSES CORRELATE WITH POOR OVERALL SURVIVAL IN H3K27M-ALTERED DIFFUSE MIDLINE GLIOMAS