8577 Background: MCC is an aggressive, chemoresistant skin cancer causing more deaths each year than chronic myelogenous leukemia. We discovered a new virus, Merkel cell polyomavirus (MCV), clonally integrated into ~80% of primary and metastatic MCC in 2008. To find therapeutic targets for this cancer, we examined cellular genes perturbed by MCV infection. Methods: Digital transcriptome subtraction was used to discover MCV and also to reveal survivin gene (BIRC5) upregulation in virus-positive tumors. MCV T antigen knockdown studies in seven MCC lines and large T (LT) transduction into BJ fibroblasts were used to confirm this. Drug screening was performed in vitro using Cell-Titer Glo assays in a two stage analysis. In vivo screening used an MKL-1 (MCV+) MCC NOD-SCIDg mouse xenograft model with a single three-week treatment round. Results: MCV large T oncoprotein induces survivin transcription through retinoblastoma protein sequestration by the LT LXCXE motif. MCV T antigen knockdown results in nonapoptotic MCC cell death and loss of survivin expression. YM155, a phase II survivin transcription inhibitor, causes MCV+ MCC cell necroptosis associated with autophagy at 1-12 nM EC50. Of 1359 other drugs from LOPAC and NCI Oncology Set II libraries, only bortezomib had in vitro potency comparable to YM155. In MKL-1 xenograft studies, mice were treated with saline, bortezomib or YM155 for three weeks using standard dosings. Bortezomib did not significantly improve mouse survival (33%) over saline (24%) during treatment. In contrast, all YM155-treated mice survived (100%, p<0.001) the 3 week treatment period. Tumors resumed growth once YM155 treatment was stopped suggesting that YM155 is cytostatic in vivo rather than cytotoxic. Conclusions: Survivin expression is induced by MCV LT and is critical to MCV+ MCC survival. A survivin inhibitor, YM155 was nontoxic to mice and cytostatic for MCV+ MCC xenografts. Using genomic technologies, in less than four years, the primary viral cause for most MCC was discovered, new diagnostic tests developed and a promising rational drug candidate identified. A cooperative group trial (E1611) for YM155 and bortezomib in MCC patients is currently planned.
Merkel Cell Polyomavirus Small T Antigen mRNA Level Is Increased following In Vivo UV-Radiation
