Abstract B2: Epigenetic deregulation of a novel deubiquitylase promotes medulloblastoma genesis

Medulloblastoma is a poorly differentiated and hyperproliferative malignant pediatric brain tumor. It frequently arises in the cerebellum. Current therapies while efficacious against primary tumors not only cause severe neurologic deficits in children with brain tumors, but are also ineffective against recurrences. The overall goal of our work is to gain a better understanding of tumor biology so that therapies that target tumor specific events can be developed. This approach would also spare the normal brain from therapy-related toxicity. The RE1 Silencing Transcription Factor (REST) is an epigenetic modulator of gene expression and a repressor of neuronal differentiation. While normal cerebellar neurons did not express REST, its levels were significantly elevated in human medulloblastoma samples, and was associated with poor prognosis for patients. To examine the feasibility of developing REST as a therapeutic target, we performed genetic and pharmacological experiments to explore its role in driving medulloblastoma formation. We observed that REST loss in human tumor cells abrogates their tumorigenic potential in mouse intracranial models. Conversely, REST elevation in neural progenitors promoted tumor formation in the context of Myc overexpression. REST-expressing tumor cells failed to undergo differentiation and had sustained proliferation. REST is a known repressor of genes required for terminal neurogenesis. Interestingly, we made a novel observation that REST also controlled tumor cell proliferation through direct transcriptional repression of a gene encoding a novel deubiquitylase called USP37. Using purified USP37 protein, we demonstrated that it deubiquitylated the cell cycle regulator p27, a critical mediator of normal cell cycle exit during neurogenesis. In patient samples, we noted a significant correlation between loss of USP37, decreased p27 protein levels and poor prognosis. Importantly, constitutive expression of wildtype USP37, but not mutant USP37, promoted a decrease in tumor cell proliferation in vitro and a loss of tumorigenic potential in mouse orthotopic models. Since REST-mediated transcriptional silencing is effected by its associated histone deacetylase (HDAC) and histone methyltransferase (HMT)-G9a chromatin remodeling complexes, we performed genetic and pharmacological assays to identify the specific epigenetic activity promoting USP37 down-regulation in medulloblastoma cells. These studies revealed that G9a loss or inhibition of its activity upregulated USP37 expression and blocked tumor cell proliferation in vitro and tumor growth in vivo. Together, these findings suggest that USP37 is a novel tumor suppressor gene in medulloblastoma and that its epigenetic modulation in REST-high tumors may have potential therapeutic application. Citation Format: Rashieda Jonine Hatcher, Vrushali Datar, Chandra M. Das, Jason Fangusaro, Stewart Goldman, Rishi Lulla, Veena Rajaram, Vidya Gopalakrishnan. Epigenetic deregulation of a novel deubiquitylase promotes medulloblastoma genesis. [abstract]. In: Proceedings of the AACR Special Conference on Pediatric Cancer at the Crossroads: Translating Discovery into Improved Outcomes; Nov 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;74(20 Suppl):Abstract nr B2.