Abstract 933: The expression of PRMT5 methyltransferase mediates cell survival and metastatic phenotype in malignant melanoma

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL Post-translational modification of proteins is involved at all levels of cellular regulation. The PRMT5 enzyme is a type II arginine methyltransferase that catalyzes the transfer of a methyl group to two of three guanidino nitrogen atoms within the arginine molecule. This enzyme has been shown to methylate histone H3 at arginine 8 (H3R8) and H4R3 to trigger silencing of tumor suppressor genes. In addition, PRMT5 has been shown to interact with p53, TRAIL receptor, and the CDK4 complex to regulate the cell cycle or apoptosis. Although prior studies have provided insight into these mechanistic features of the PRMT enzymes, most data are derived from a limited panel of cell lines. PRMT5 over-expression has recently been shown to influence progression of leukemia and lymphoma. However, few, if any publications exist which document the role of PRMT enzymes in melanoma. We hypothesized that PRMT5 expression promotes metastasis and contributes to reduced immunologic recognition of melanoma cells. Immunoblot analysis and confocal microscopy revealed that PRMT5 is expressed in a panel of melanoma cell lines regardless of the mutational status of B-Raf, NRas, or p53. siRNA-mediated inhibition of PRMT5 led to apoptosis and restored CXCL10 chemokine expression. PRMT5 expression was next determined by immunohistochemical (IHC) analysis of formalin-fixed samples from patients with melanoma (n=56 primary; n=20 metastases) or benign nevi (n=24). The specimens were obtained as a commercially-available tissue microarray, or procured as de-identified primary patient samples (IRB #20100071; P.I. Lesinski; OSU IRB # 2002H0089; co-PI: Peters). PRMT5 expression was significantly increased in the nucleus of melanoma cells as compared to normal epidermis or benign nevi (p=0.001). The level of cytoplasmic PRMT5 expression was also elevated in melanoma lesions, however its distribution in benign tissues was bimodal, and expressed in a subset of benign samples. Notably, nuclear PRMT5 expression increased as melanoma cells invaded the dermis or in melanoma lesions exhibiting pagetoid spread. Finally, we have identified a specific inhibitor of PRMT5 arginine methyltransferase activity from a small molecule library. This lead compound (BLL-1) blocked the dimethylation of arginine 3 on histone H4 (H4R3me2s). A series of titration experiments revealed reduction of both H4R3me2s and proliferation rate. Treatment of multiple melanoma cell lines with BLL-1 (24 hr) led to significantly increased apoptosis. These data represent the first report of PRMT5 and its inhibition in melanoma. Together, this suggests PRMT5 inhibition as a potential therapeutic strategy by virtue of its ability to promote apoptosis and restore immunomodulatory gene expression. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 933. doi:10.1158/1538-7445.AM2011-933