Inhibition of USP7 destabilizes the noncanonical PRC1.1 complex and induces neuroblastoma differentiation

Pediatric cancers are frequently driven by genomic alterations that result in impaired differentiation during development. To identify complex-level dependencies required for differentiation in neuroblastoma, a pediatric cancer of the developing peripheral nervous system, we curated a list of protein complexes using the CORUM database and mined the Dependency Map (DepMap) using gene set enrichment analysis. This analysis identified the non-canonical PRC1.1 complex, which represses transcriptional activity through ubiquitination of histone 2A, lysine 119 (H2AK119Ub), as a selectively enriched dependency in neuroblastoma. Knockout of PRC1.1 subunits reduced neuroblastoma growth by inducing a neuronal differentiation program. While no known direct inhibitors of PRC1.1 exist, co-dependency analysis identified that the deubiquitinase USP7 strongly correlated with PRC1.1 dependency. Treatment with XL177A, a small molecule inhibitor of USP7, significantly reduced neuroblastoma growth in both cellular and animal models. Integrated RNA- and ChIP-sequencing showed that both PRC1.1 knockout and USP7 inhibition resulted in highly correlated transcriptional alterations and reduced H2AK119Ub deposition on chromatin, suggesting that USP7 inhibition reduced neuroblastoma growth through a PRC1.1-dependent mechanism. Mechanistically, global proteomics and ubiquitinomics revealed that USP7 inhibition disrupted non-canonical PRC1 complex assembly, resulting in destabilization of PRC1.1 and subsequent proteolysis. Our findings expand our understanding of the chromatin complexes required to maintain a de-differentiated state in neuroblastoma and suggest the therapeutic potential for USP7 inhibitors in the treatment of this disease. Implications: Our study reveals the potential for utilizing USP7 inhibitors to target epigenetic repression of differentiation programs in neuroblastoma by reducing PRC1 activity.