MBRS-54. POOR SURVIVAL IN REPLICATION REPAIR DEFICIENT HYPERMUTANT MEDULLOBLASTOMA AND CNS EMBRYONAL TUMORS: A REPORT FROM THE INTERNATIONAL RRD CONSORTIUM

BACKGROUND: Mutations in mismatch repair (MMR) and DNA-polymerase (POL) genes lead to DNA replication repair deficiency (RRD), resulting in a growing group of previously under-recognized childhood brain tumors. Medulloblastoma and embryonal tumors are rarely reported in RRD. Their biological and clinical significance is unknown. METHODS: We analyzed the clinical and genomic data of embryonal tumors registered in the International RRD Consortium. RESULTS: Twenty-six tumors were centrally reviewed to confirm medulloblastoma (n=18), embryonal-tumor, NOS (n=5), and three glioblastoma (excluded). Embryonal tumors were observed at a young age (median: 7-years, IQR: 5;11), and all but one exhibited clinical cues (café-au-lait macules/ family history) of germline RRD. Medulloblastomas with RRD exhibited high-risk features, including anaplastic histology (50%), and SHH-subgroup with TP53-mutation (50%). Importantly, 68% harbored POLE/POLD1 mutations, resulting in median tumor mutation burden of 164 mut/mb. POL-mutated tumors were significantly ultra-hypermutated (>100 mut/mb) than tumors with MMR-deficiency alone (p=0.015). Synchronous and metachronous tumors were observed in 40%. However 90% of the deaths were related to the diagnosis of embryonal CNS tumor. Median survival for the entire cohort was 17-months (95% CI: 10 to 23). Predicted 3-year survival was 37% for medulloblastoma, with no survivors among other embryonal tumors. CONCLUSIONS: This is the largest cohort of replication repair deficient medulloblastoma reported till date. The tumors are hypermutated, harbor somatic mutations in TP53 and/or POLE/POLD1, and have very poor survival with current chemo-irradiation based approaches. These biologically unique tumors expand the spectrum of high-risk TP53-mutant SHH-medulloblastoma, and need novel strategies for treatment.