High grade astrocytomas are aggressive brain tumors that are associated with a dismal prognosis and are considered incurable with a mean survival of less than one year despite intensive multimodality therapy. The limits of current treatment modalities indicate a need for innovative therapies targeting grade III and grade IV (glioblastoma multiforme, GBM). Recent studies have shown that post translational covalent modification of proteins and epigenetic regulation of chromatin play a central role in the control of cell growth, differentiation, and proliferation. Chromatin remodeling enzymes like histone deacetylase (HDAC), DNA methyltransferase and protein arginine methyltransferase 5 (PRMT5) are involved in silencing pro inflammatory and tumor suppressor gene (TSG) expression and contribute towards cellular transformation. The PRMT5 enzyme contributes towards transcriptional silencing of several important regulatory genes by methylating arginine residues on histone proteins (at histone 4 arginine residue 3 (H4R3) and H3R8). Our laboratory has demonstrated that epigenetic processes driven by PRMT5 over expression are relevant in regulation of key oncogenic pathways that are operable in high grade astrocytomas. Patient-derived GBM cell lines and primary GBM tumors over express abundant levels of PRMT5 protein. Normal brain tissue and low or intermediate grade astrocytomas do not over express PRMT5 suggesting that overexpression may selectively occur in high grade, more aggressive gliomas. The degree of PRMT5 over expression inversely correlated with survival of GBM patients and with proliferation of GBM cell lines. We developed small inhibitory RNA molecules (SiRNA) to inhibit PRMT5 expression leading to demethylation of target histone tail arginine residues and transcriptional de-repression and rapid translation of tumor suppressor and immune modulatory proteins. Inhibition of PRMT5 over expression led GBM cells to undergo cell cycle arrest, apoptosis, complete inhibition of cell migration and sensitized cells to the toxic effects of temazolomide, a drug considered standard of care in management of patients with GBM. We believe our work has shown that PRMT5 is both an important prognostic factor and an attractive therapeutic target for GBM. We have developed computational modeling systems utilizing crystallographic structure of homologous PRMT enzymes allowing for construction of a molecular model of PRMT5 and rapid screening of over 10,000 small molecule compounds. This method has led to the discovery of several small molecules that inhibit PRMT5 activity. Here we present a multidisciplinary plan utilizing novel strategies to generate more potent and selective small molecule inhibitors of PRMT5 activity. Promising compounds will be rigorously evaluated in both in vitro and in vivo development platforms and will enhance our ability to discover a new class of drugs that selectively target a promising therapeutic target in GBM.
High grade astrocytomas are the most common brain tumors afflicting both pediatric and adult populations and are associated with a dismal prognosis with most patients surviving less than 1 year despite aggressive multimodal therapy with surgery, radiation and chemotherapy. We have shown that the PRMT5 protein is selectively over expressed in high grade astrocytomas, enhances growth and survival, serves as an adverse prognostic factor in glioma patients and is an attractive therapeutic target. Here we propose methods to promote the rapid discovery of small molecules capable of selectively inhibiting PRMT5 activity and present a preclinical development plan to generate novel drugs inhibiting a new target that is selectively over expressed in high grade astrocytomas.
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