Merkel cell polyomavirus T antigens and cancer

描述（由申请方提供）：默克尔细胞癌（MCC）是一种罕见的神经内分泌皮肤肿瘤，由于无法获得有效治疗，晚期疾病预后不良。大多数MCC携带来自新型多瘤病毒默克尔细胞多瘤病毒（MCPyV）的序列，并表达两种推定的癌蛋白：MCPyV小T抗原（stAg）和肿瘤特异性截短的大T抗原（tLTAg）。与其他病毒癌蛋白一样，MCPyV转化抗原靶向发挥肿瘤抑制因子功能的内源性蛋白：tLTAg靶向RB 1，而stAg靶向PP 2A和Fbw 7（SCF泛素连接酶复合物的一种组分）。功能丧失研究指出tLTAg和stAg在MCC中的重要作用。另一方面，功能获得性研究表明，stAg在培养细胞中具有转化潜力，但tLTAg和全长LTAg都没有。这种stAg驱动的转化涉及4 E-BP 1磷酸化和由此产生的帽依赖性翻译的去抑制，并且依赖于结合Fbw 7的新型stAg功能结构域。目前尚不清楚这些体外结果与MCC肿瘤发生、扩展和进展的相关性。更具体地，MCPyV T抗原作为体内肿瘤发展的致癌驱动因素的作用尚未得到解决。在这个探索性的拨款申请中，我们计划开发和表征小鼠模型，使用常规和Cre诱导的转基因小鼠测试皮肤细胞对MCPyV st和LTAgs表达的体内反应。拟议的研究非常重要，因为它们将通过定义完整动物中MCPyV标签的生物活性来帮助填补我们知识中的关键空白，从而为研究促进MCC发展和维持的因素提供了一套急需的工具。此外，这些探索性研究将为鉴定MCPyV TAg细胞靶点奠定基础，这些靶点通过非病毒机制的失调可能更普遍地促进癌症发展。拟议的工作可能与MCC患者有直接的翻译相关性，因为它可能导致识别新的治疗靶点，并产生急需的功能测定和临床前试验的小鼠模型。

DESCRIPTION (provided by applicant): Merkel cell carcinoma (MCC) is a rare neuroendocrine skin tumor with a poor prognosis at advanced disease stages due to the unavailability of effective treatments. Most MCCs carry sequences from a novel polyomavirus, Merkel cell polyomavirus (MCPyV), and express two putative oncoproteins: MCPyV small t antigen (stAg) and tumor-specific truncated large T antigen (tLTAg). Like other viral oncoproteins, MCPyV transforming antigens target endogenous proteins that function as tumor suppressors: tLTAg targets RB1 while stAg targets PP2A and Fbw7, a component of the SCF ubiquitin ligase complex. Loss-of-function studies point to an important role for both tLTAg and stAg in MCC. Gain-of-function studies, on the other hand, show that stAg, but neither tLTAg nor full-length LTAg, has transforming potential in cultured cells. This stAg-driven transformation involves 4E-BP1 phosphorylation and resultant de-repression of cap-dependent translation, and is dependent on a novel stAg functional domain that binds Fbw7. The relevance of these in vitro findings to MCC tumor initiation, expansion, and progression, is currently unknown. More specifically, the role of MCPyV T antigens as oncogenic drivers of tumor development in vivo has not been addressed. In this exploratory grant application, we plan to develop and characterize mouse models testing the in vivo response of skin cells to expression of MCPyV st and LTAgs using both conventional and Cre-inducible transgenic mice. The proposed studies are highly significant since they will help fill a critical gap in our knowledge by defining the biological activity of MCPyV TAgs in intact animals, thus providing a much-needed set of tools for studying factors contributing to the development and maintenance of MCC. In addition, these exploratory studies will set the stage for identification of MCPyV TAg cellular targets whose deregulation via non-viral mechanisms may contribute more generally to cancer development. The proposed work is likely to have direct translational relevance to MCC patients as it may lead to the identification of new therapeutic targets and yield much-needed mouse models for functional assays and preclinical trials.