MDB-12. TRANS-SPECIES ANALYSIS OF REPLICATION-REPAIR DEFICIENT (RRD) MEDULLOBLASTOMA AND RESPONSE TO IMMUNE-CHECKPOINT INHIBITION: AN IRRDC REPORT

BACKGROUND: Replication-repair deficiency (RRD) stemming from mismatch repair and polymerase-proofreading gene defects (MMRD/PPD) lead to hypermutant childhood cancers, most frequently brain tumors. While medulloblastoma is reported, the clinical, genomic and immune landscape remains unknown. METHODS: We analysed the genome, methylome, transcriptome (bulk, single-nuclei) and immune-microenvironment of the largest cohort of RRD-medulloblastoma patients enrolled by the International RRD Consortium, and correlated these to clinical outcomes. RRD mice-models were used to preclinically assess response to immunotherapy. RESULTS: RRD-medulloblastoma (n=40) were enriched for anaplasia (55%) and localised disease (85%). Methylation/nano-string-based subgrouping failed to classify 40%, while the remaining clustered with the SHH-subgroup. Copy-number changes were notably infrequent (<20%). All tumors harboured hypermutation and microsatellite instability in contrast to non-RRD controls (p<0.0001). Pathogenic variants were frequent in POLE/POLD1 (80%), TP53 (48%) and SHH-pathway genes (PTCH1, SUFU, SMO: 56%). Interestingly, some tumors additionally had glioma-driver alterations (ATRX, NF1: ~50%), similar to RRD cerebellar glioblastoma, but distinct from both non-RRD medulloblastoma (n=791) and hemispheric glioblastoma (n=733) controls. Moreover, although bulk-transcriptome matched non-RRD SHH-medulloblastoma, single-nuclei analyses suggested an earlier cell-of-origin potentially explaining the heterogenous phenotypes resulting from the driver mutations. Uniquely, immune analyses (gene expression and immunohistochemistry) demonstrated high intra-tumoral CD8 T-cell infiltration. Three-year progression-free survival was 60%. Outcome was worse for RRD-medulloblastoma harbouring TP53-mutation (p=0.04) and failing subgroup-classification (p=0.004). Nestin/Cre MSH2-/- POLE;p.S459F mice developing medulloblastoma recapitulated human disease and demonstrated response to anti-PD1 monotherapy. Recurrent/progressive human tumors including those harbouring TP53-mutations exhibited radiological responses to anti-PD1 monotherapy, which was associated with prolonged ongoing survival in comparison to those not treated with checkpoint-inhibitors (p=0.02). CONCLUSIONS: Hypermutant RRD-medulloblastoma reveal heterogenous phenotypes due to unique spectrum of driver mutations and an early cell of origin. Their immune-hot microenvironment allows successful salvage treatment with checkpoint-inhibitors for those failing chemo-radiation, including the high-risk SHH/TP53-mutant and unclassified subgroups.