HGG-27. THE IMPACT OF MISMATCH REPAIR DEFICIENCY ON GLIOMAS IN CHILDREN, ADOLESCENTS, AND YOUNG ADULTS; A REPORT FROM THE IRRDC AND THE GLIOMA TASK FORCE

Mismatch repair deficiency (MMRD) is a pan-cancer mechanism resulting in universal hypermutation. MMRD-gliomas are resistant to chemoradiation but respond to immunotherapy. MMRD can occur somatically or be inherited as part of Lynch Syndrome (LS) or Constitutional Mismatch Repair Deficiency (CMMRD). However, the prevalence of MMRD in gliomas of children, adolescents, and young adults (CAYA), and impact of germline inheritance is unknown. We utilized functional genomic tools on population-based and validation cohorts from 3 large databases (Toronto, St-Jude and CBTN) to determine the prevalence, subgroup, and impact of germline mutations on MMRD-gliomas. Ongoing data on 998 pediatric-gliomas from Toronto reveals that MMRD is extremely rare in pediatric low-grade gliomas (PLGG) but common in pediatric high-grade gliomas (PHGG) (9%, p<0.0001). Similarly, data from St-Jude and CBTN (n=374) reveals that MMRD exists only in PHGG (8%). In both cohorts, MMRD-PHGG are enriched for RAS/MAPK, IDH, and TP53 mutations while pediatric-type fusions, BRAF-V600E and histone mutations are absent. In AYA-HGG (n=884), the most common groups include glioblastomas without chromosome-7/10 alterations (9%) and IDH1-astrocytomas, while MMRD is not present in IDH-oligodendrogliomas. All but 1 CAYA-MMRD-gliomas harbored germline MMR mutations. Data from the IRRDC on 174 patients reveal that LS is more common than CMMRD in gliomas, with the median age of onset 11 and 29 years in CMMRD and LS, respectively (p<0.001). Furthermore, CMMRD gliomas are commonly caused by germline PMS2 and MSH6 mutations, enriched for secondary polymerase mutations (60%, p<0.001), and exhibit ultra-hypermutation. In contrast, LS-gliomas are caused by MSH2 and MLH1 mutations, and enriched for IDH1 mutations (32%, p<0.025) with a lower mutational burden. Our data reveals high prevalence of MMRD in CAYA-HGG with alarming impact of germline predisposition. Specifically, unrecognized LS patients with AYA MMRD-HGG. These findings support universal screening for MMRD in HGG to identify patients for immunotherapy and surveillance.