Merkel Cell Polyomavirus Small T Antigen Initiates Merkel Cell Carcinoma-like Tumor Development in Mice.

Merkel cell carcinoma (MCC) tumor cells express several markers detected in normal Merkel cells, a nonproliferative population of neuroendocrine cells that arise from epidermis. MCCs frequently contain Merkel cell polyomavirus (MCPyV) DNA and express viral transforming antigens, sT and tLT, but the role of these putative oncogenes in MCC development, and this tumor's cell of origin, are unknown. Using a panel of preterm transgenic mice, we show that epidermis-targeted coexpression of sT and the cell fate-determinant atonal bHLH transcription factor 1 (ATOH1) leads to development of widespread cellular aggregates, with histology and marker expression mimicking that of human intraepidermal MCC. The MCC-like tumor phenotype was dependent on the FBXW7-binding domain of sT, but not the sT-PP2A binding domain. Coexpression of MCPyV tLT did not appreciably alter the phenotype driven by either sT or sT combined with ATOH1. MCPyV sT, when coexpressed with ATOH1, is thus sufficient to initiate development of epidermis-derived MCC-like tumors in mice. Cancer Res; 77(12); 3151-7. ©2017 AACR.

Invasive mouse gastric adenocarcinomas arising from Lgr5+ stem cells are dependent on crosstalk between the Hedgehog/GLI2 and mTOR pathways.

Gastric adenocarcinoma is the third most common cause of cancer-related death worldwide. Here we report a novel, highly-penetrant mouse model of invasive gastric cancer arising from deregulated Hedgehog/Gli2 signaling targeted to Lgr5-expressing stem cells in adult stomach. Tumor development progressed rapidly: three weeks after inducing the Hh pathway oncogene GLI2A, 65% of mice harbored in situ gastric cancer, and an additional 23% of mice had locally invasive tumors. Advanced mouse gastric tumors had multiple features in common with human gastric adenocarcinomas, including characteristic histological changes, expression of RNA and protein markers, and the presence of major inflammatory and stromal cell populations. A subset of tumor cells underwent epithelial-mesenchymal transition, likely mediated by focal activation of canonical Wnt signaling and Snail1 induction. Strikingly, mTOR pathway activation, based on pS6 expression, was robustly activated in mouse gastric adenocarcinomas from the earliest stages of tumor development, and treatment with rapamycin impaired tumor growth. GLI2A-expressing epithelial cells were detected transiently in intestine, which also contains Lgr5+ stem cells, but they did not give rise to epithelial tumors in this organ. These findings establish that deregulated activation of Hedgehog/Gli2 signaling in Lgr5-expressing stem cells is sufficient to drive gastric adenocarcinoma development in mice, identify a critical requirement for mTOR signaling in the pathogenesis of these tumors, and underscore the importance of tissue context in defining stem cell responsiveness to oncogenic stimuli.

Merkel cell polyomavirus small T antigen is oncogenic in transgenic mice.

Merkel cell carcinoma (MCC) is a rare and deadly neuroendocrine skin tumor frequently associated with clonal integration of a polyomavirus, Merkel cell polyomavirus (MCPyV), and MCC tumor cells express putative polyomavirus oncoprotein small T antigen (sTAg) and truncated large T antigen. Here, we show robust transforming activity of sTAg in vivo in a panel of transgenic mouse models. Epithelia of preterm sTAg-expressing embryos exhibited hyperplasia, impaired differentiation, increased proliferation, and apoptosis, and activation of a DNA damage response. Epithelial transformation did not require sTAg interaction with the protein phosphatase 2A protein complex, a tumor suppressor in some other polyomavirus transformation models, but was strictly dependent on a recently described sTAg domain that binds Fbxw7, the substrate-binding component of the Skp1/Cullin1/F-box protein ubiquitin ligase complex. Postnatal induction of sTAg using a Cre-inducible transgene also led to epithelial transformation with development of lesions resembling squamous cell carcinoma in situ and elevated expression of Fbxw7 target proteins. Our data establish that expression of MCPyV sTAg alone is sufficient for rapid neoplastic transformation in vivo, implicating sTAg as an oncogenic driver in MCC and perhaps other human malignancies. Moreover, the loss of transforming activity following mutation of the sTAg Fbxw7 binding domain identifies this domain as crucial for in vivo transformation.