HGG-27.IDH- AND H3-WILDTYPE HIGH-GRADE GLIOMAS OCCURRING IN TEENAGERS AND YOUNG ADULT PATIENTS COMPRISE NOVEL MOLECULAR SUBGROUPS

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Abstract BACKGROUND High-grade gliomas (HGG) arise in any CNS location with a poor prognosis. HGGs in teenagers/young adults (TYA) are understudied; this project aimed to characterise these tumours and identify therapeutic targets. METHODS HGG samples (histone/IDH-wildtype, n=207, FFPE/FF, 13-30 years) were collected from national/international collaborators. DNA methylation profiling (Illumina EPIC BeadArrays, brain tumour classifier (MNPv12.5 R package)) classified cases against reference cohorts. Calibrated scores guided workflows to characterise mutational landscapes (RNA-based ArcherDx fusion panel (n=92), whole exome sequencing (n=107), histological review). RESULTS Of cases scoring >0.5, n=25 classified as PXA and n=8 as HGAP, differing from primary diagnoses. 53.4% (n=86) classified as paediatric-type subgroups ((pedHGG-RTK1A/B/C, 31.7%, n=51, associated with frequent PDGFRA, CDKN2A/B, SETD2, NF1 alterations), pedHGG-MYCN (8.1%, n=13, MYCN/ID2 amplifications), and pedHGG-RTK2A/B (7.5%, n=12)). 18.0% (n=29) classified as subgroups frequently seen in adults including GBM-MES (15.5%, n=25, enriched for RB1, PTEN, NF1 alterations) and GBM-RTK1/2 (2.5%, n=4, CDK4 amplifications). 16 cases were assigned to novel, poorly-characterised subgroups with distinct methylation profiles and molecular features including paediatric-specific pedHGG-A/B (n=10 6.2%) and HGG-E (n=6 3.7%) subgroups, and HGG-B (n=2 1.0%), and GBM-CBM (n=5 3.1%, frequent cerebellar location) subgroups, associated with variable histological morphology. 8 cases showed hypermutator phenotypes, enriched in HGG-E. Age-distribution/molecular profile comparisons using publicly available methylation and sequencing data for HGG-B (n=9), GBM-CBM (n=26) and GBM-MES-ATYP (n=53), irrespective of age, shows they are TYA-specific subgroups with the latter containing fewer chr7 gains and chr10 losses, and more CDKN2A/B deletions and MET amplifications, compared with adult-specific GBM-MES-TYP. Across the cohort, other frequent copy number changes included gains in chr1q (54%, frequent in pedHGG-RTK1B/C/MYCN, pedHGG-A/B), chr2 (22%, pedHGG-MYCN), and chr13 losses (64%, pedHGG-RTK1B/C). Focal amplifications included CDK6 (1.4%, n=3) and EGFR (1.0%, n=2). CONCLUSION TYA HGG comprise well-characterised, novel methylation subgroups with distinct methylation profiles and molecular characteristics, representing opportunities to refine treatment.

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