PATH-34. CLINICOPATHOLOGIC CHARACTERIZATION OF DIFFUSE PEDIATRIC-TYPE HIGH-GRADE GLIOMA, H3-WILDTYPE AND IDH-WILDTYPE: AN AGGRESSIVE AND GENOMICALLY COMPLEX GROUP OF TUMORS AFFECTING CHILDREN AND ADULTS

Abstract

Abstract Diffuse pediatric-type high-grade glioma, H3-wildtype and IDH-wildtype (pHGG) was newly introduced in the 2021 WHO CNS5. pHGG is further divided into distinct methylation subtypes (MYCN, A/B, RTK1, RTK2). Compared to adult-type diffuse gliomas, relatively little is known about pHGG, especially in the adult population and we therefore aimed to more fully understand the clinicopathological spectrum of pHGG. Our cohort of methylation-profiling-confirmed pHGGs (n=299) included pediatric (n=211) and adult (n=88) populations (age range=0-79 years). The mean age was highest in the A/B subtypes (27.7 years) and lowest in the MYCN subtype (13.5 years). Median survival (18 months for the entire cohort), MGMT promoter methylation status, and history of prior radiation did not differ between the pediatric and adult populations. MGMT promoter methylation was highest in RTK1C (51%) and lowest in subtype A (0%). All those with history of prior radiation (n=37) were classified as RTK1, with subclass distribution differing between patients with history of prior radiation (RTK1A=7.3%, RTK1B=41.8%, RTK1C=50.9%) versus those without (RTK1A=41.8%, RTK1B=21.5%, RTK1C=36.7%). Interestingly, the genomic somatic copy number alteration (SCNA) load did not differ for RTK1 based on prior radiation history status. However, adult pHGG did harbor a higher SCNA load than pediatric pHGG (p< 0.01). The higher SCNA load was driven by genomic copy number losses rather than amplifications, including adult pHGG harboring more CDKN2A/B and RB1 deletions. Amplifications of MET, CDK4, PDGFRA, MYC, and KIT were present across all subtypes, but were enriched in RTK1. MYCN amplification was enriched in the MYCN subtype. EGFR, MDM4, and CDK6 amplifications did not have subtype predilection. Overall, while the pediatric and adult pHGG have similar outcomes, there are differences in their genomic structures, most notably in adult pHGG harboring a higher SCNA load. Overall, pHGG represent a biologically diverse set of clinically aggressive tumors that warrant further intensive study.