MDB-79. THE DEUBIQUITINATING ENZYME USP7 COOPERATES WITH SONIC HEDGEHOG SIGNALING TO PROMOTE MEDULLOBLASTOMA

Abstract BACKGROUND Constitutive activation of the Sonic Hedgehog (SHH) signaling pathway during cerebellar development may lead to medulloblastoma (MB), one of the most common malignant pediatric brain tumor. Clinically, SHH-MB with mutations downstream of SMO or TP53-mutated are a real challenge and alternative targeted therapies are sorely needed for this group of patients. In SHH-MB, the transcription factor Atoh1 is constitutively overexpressed and essential for tumor initiation and progression. Previous work demonstrated that proteasomal-mediated degradation of Atoh1 induces tumor regression in vitro and in vivo, therefore deciphering mechanisms controlling Atoh1 stabilization should provide new avenues for drug development in SHH-MB. METHODS Using a proteomic approach, we identified the deubiquitinating enzyme Usp7 as a positive regulator of Atoh1 in MB cells. We then applied a SILAC-based quantitative ubiquitinomics to investigate the Usp7 target landscape in SHH-MB. We also employed several biochemical approaches to demonstrate the functional interaction between Atoh1 and Usp7 in SHH-MB. In parallel, we validated the impact of Usp7 depletion in vitro and in vivo using relevant SHH-MB models. Last, we investigated large transcriptomic and proteomic datasets to unveil the prognostic value of Usp7 in SHH-MB human tumors. RESULTS We demonstrate that Usp7 binds and accumulates Atoh1 protein by counteracting its ubiquitylation. Post-natal electroporation in vivo directly into the developing cerebella also indicates that overexpression of Usp7 promotes proliferation, whereas in the absence of Atoh1 Usp7 was no longer able to spread cell growth, demonstrating a functional link between the Usp7-Atoh1 axis. In vivo, disruption of Usp7 expression in SHH-MB blocks tumor proliferation. Pharmacological inhibition of Usp7 triggers tumor growth in a syngeneic SHH MB mouse model. Lastly, SHH-MB cells with either TP53 or SMO downstream mutations are sensitive to Usp7 inhibition. CONCLUSIONS Overall, our data pinpoint Usp7 as a promising target against Atoh1, a master regulator of SHH-MB.