GE-05 * STUDY OF EPIGENETIC DEREGULATION OF A NOVEL DEUBIQUITYLASE IN MEDULLOBLASTOMA

A major goal of our work is to study medulloblastoma biology to facilitate the development of targeted therapy. Here, we have dissected the contribution of epigenetics to tumor development. Epigenetic perturbations are pharmacologically reversible and may have therapeutic potential. The RE1-Silencing Transcription Factor (REST) is an epigenetic modulator, and a repressor of neuronal differentiation genes. Its expression is significantly elevated in human medulloblastomas, and is associated with poor prognosis. REST is a driver of medulloblastoma in mouse orthotopic models. To understand REST's contribution to tumor development, we created a novel genetically engineered mouse model in which REST transgene is conditionally elevated in the cerebellar granule progenitors (GNPs), the cells of origin of some medulloblastoma. Transgene induction caused GNP hyperproliferation, accompanied by a failure to express the cell cycle regulator-p27. Using genetic and biochemical approaches, we attributed the lack of p27 expression to direct REST-mediated repression of a novel deubiquitylase, USP37. We also identified a role for USP37 in p27 protein stabilization, and terminal cell cycle exit. USP37 and p27 levels were significantly correlated in patient samples, and their loss was associated with poor overall patient survival. Importantly, constitutive USP37 expression blocked the tumorigenic potential of high-REST human medulloblastomas, suggesting a tumor suppressive function for USP37. REST binding to the USP37 promoter caused increased histone H3 lysine (K)-9 tri-methylation, an epigenetic mark associated with gene silencing. The histone methyl-transferase G9a, which promotes this modification, is a known REST co-repressor. Importantly, genetic ablation of G9a or pharmacological inhibition of its activity upregulated USP37 expression in a REST-dependent manner, and blocked tumor growth in mice. Our work has provided the first link between epigenetic deregulation of the proteasome and medulloblastoma development, and supports the manipulation of the REST-epigenome for future therapeutic application.