Abstract LB-320: Identification of USP37 as a novel tumor suppressive p27-specific deubiquitylase

Medulloblastoma is a malignant pediatric brain tumor. Current treatments based on clinical features-centered stratification into standard- and high-risk groups, has vastly improved patient survival. However, a subset of patients with standard-risk features exhibit recurrence and leptomeningeal spread. Unfortunately, the outcome for such patients is dismal. This highlights the need for new stratification approaches and therapeutic decisions based on a better understanding of tumor biology. Since medulloblastomas are poorly differentiated tumors, we asked if the RE1 silencing transcription factor (REST), a repressor of neuronal differentiation genes, played a role in tumor etiology. Relative to normal cerebella, we observed an increase in REST levels in islands of tumor cells in over 70% of samples studied. Interestingly, patients with very high REST expression in their tumors exhibited poor overall and event free survival, identifying REST as a poor prognostic factor. The requirement for REST in tumor maintenance and progression was then established using knockdown and overexpression studies in mouse orthotopic models. To dissect the role of REST in medulloblastoma genesis, we knocked down its expression in cell lines and neurosphere cultures established from patient tissue. To our surprise, we observed a decrease in tumor cell proliferation following REST loss that was associated with destabilization of the cyclin-dependent kinase inhibitor, p27. Previous studies have shown a contribution of p27 loss to medulloblastoma development, however the underlying mechanisms were unclear. To investigate if REST contributed to a decline in p27 levels, we studied the expression of a panel of genes (a) whose expression was upregulated upon REST loss (b) that had the potential to be a direct REST target gene based on bioinformatics and (c) could potentially regulate p27 degradation. A gene encoding a deubiquitylase USP37 was further investigated and validated as a novel REST target, and its role in blocking p27 proteasomal degradation established by genetic and biochemical assays. Importantly, constitutive expression of wild type USP37 in tumor cells with high REST expression promoted p27 accumulation and blocked in vitro cell proliferation and tumor growth in the mouse brain. In contrast, a catalytically inactive mutant of USP37 failed to promote p27 stabilization and inhibit tumor growth in vivo. A significant negative correlation between REST and p27 and a positive correlation between USP37 and p27 was also seen in human tumors. Thus, our studies have identified REST as a novel prognostic factor in medulloblastoma. We have also provided evidence that its elevated expression blocks neuronal differentiation and contributes to sustained tumor cell proliferation through repression of USP37 expression and destabilization of p27. Our findings suggest that USP37 has a tumor suppressive role in medulloblastoma. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-320. doi:1538-7445.AM2012-LB-320