HGG-15. HIGH-GRADE GLIOMAS OCCURRING IN TEENAGERS AND YOUNG ADULT PATIENTS COMPRISE NOVEL MOLECULAR SUBGROUPS

Abstract

Abstract High-grade gliomas (HGG) arise in any central nervous system location, in adults and children, with a poor prognosis. HGGs in teenagers and young adults (TYA) are understudied; this project aimed to characterise these tumours and identify potential therapeutic targets. HGG samples (n=195, FFPE/FF, 13-30 years) were collected, excluding well-characterised entities (histone/IDH-mutant). DNA methylation profiling (Illumina EPIC BeadArrays, brain tumour classifier (MNP v12.5 R package), n=195) classified cases against a reference cohort. Calibrated scores guided workflows to characterise mutational landscapes including RNA-based ArcherDx fusion panel (n=112), whole exome sequencing (n=84), and histological reviews in parallel. Well-characterised entities (n=50, including IDH-mutant tumours (n=18), PXA (n=12)) were excluded. Of the cases scoring >0.6, 64% classified as paediatric-type subgroups (pedHGG-RTK1A/B/C (30%), pedHGG-MYCN (10%), pedHGG-A/B (7%), pedHGG-RTK2A (6%)). 29% classified as subgroups more frequently seen in adults (GBM-MES (12%), GBM-RTK1/2 (3%)). 54% classified with a score <0.6. 38% cases were assigned to novel, recently identified, poorly-characterised subgroups with distinct methylation profiles and molecular features (HGG-B, HGG-E, ANTCON, GBM-CBM). Frequency comparisons to publicly available methylation data showed GBM-MES-ATYP tumours are TYA age-specific, in contrast to adult age-specific GBM-MES-TYP. Copy number profiling identified frequent changes across the cohort, including chromosomal gains (chr1q (54%), chr2 (22%), chr7 (41%)) and losses (chr10 (40%), chr13 (64%)). Focal amplifications included PDGFRA (12%), CDK4 (7%), MYCN/ID2 (3%), MYC (2%) and EGFR (0.7%) and the most common focal deletion was CDKN2A/p16 (12%). The most frequent glioma-associated somatic variants included TP53, PDGFRA, EGFR, and NF1. BCOR alterations were enriched compared to adult and paediatric reference cohorts. 10% cases showed a hypermutator phenotype, enriched in HGG-E. CDK4 amplifications were recurrent in GBM-RTK1 and GBM-CBM. Histology showed variable cytological and architectural features within novel subgroups. TYA HGG comprise novel subgroups with distinct methylation profiles and molecular characteristics, representing opportunities to refine future treatment.