PDTM-03. SURPRISING ROLE FOR WNT SIGNALING IN Trp53-MUTANT SHH MEDULLOBLASTOMA

The classification of specific cancers into distinct subgroups, based on emerging genomic technologies, has ushered in a new era of targeted therapeutics and companion diagnostics. This promise, however, is inconsistent with a model in which the growth of solid tumors, and their resistance to various chemotherapeutics, is driven by a small number of tumor-propagating cells whose transcriptome is unlikely to be captured by these technologies. We isolated such tumor propagating cells from a mouse model for SONIC HEDGEHOG (SHH)-subgroup medulloblastoma. Medulloblastoma sphere cultures were enriched for the medulloblastoma progenitor marker SOX2, resistant to SHH inhibitors, and formed tumors in vivo. Surprisingly, these cultures, and their ability to self-renew were WNT-dependent. We show that Trp53 loss in these sphere cultures activated canonical WNT signaling and a small molecule WNT inhibitor was able to reduce the propagation and growth of SHH-subgroup medulloblastoma in vivo, in an on-target manner, leading to increased survival. Our results suggest that the tumor propagating cells driving the growth of bulk SHH-dependent medulloblastoma are themselves WNT dependent. Further, these results suggest WNT pathway inhibition as a novel therapeutic strategy to treat TRP53 deficient SHH-subgroup medulloblastoma patients, for whom few therapeutic options exist.