Merkel cell polyomavirus T antigens and cancer

描述(申请人提供)：默克尔细胞癌(MCC)是一种罕见的神经内分泌皮肤肿瘤，由于缺乏有效的治疗方法，在疾病晚期预后较差。大多数MCC携带一种新的多瘤病毒--默克尔细胞多瘤病毒(MCPyV)的序列，并表达两种可能的癌蛋白：MCPyV小T抗原(STAg)和肿瘤特异性截短大T抗原(TLTAg)。与其他病毒癌蛋白一样，MCPyV转化抗原以发挥肿瘤抑制作用的内源性蛋白为靶点：TLTAg靶向RB1，而STAG靶向PP2A和Fbw7，后者是SCF泛素连接酶复合体的组成部分。功能丧失研究表明tLTAg和STAg在MCC中都起着重要作用。另一方面，功能获得研究表明，STAG，但不是tLTAg或全长LTAg，在培养细胞中具有转化潜力。这种STAG驱动的转化涉及4E-BP1的磷酸化和由此产生的帽依赖翻译的降阻，并依赖于一个新的STAG功能结构域，该结构域与Fbw7结合。这些体外研究结果与MCC肿瘤的发生、扩展和进展的相关性目前尚不清楚。更具体地说，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。在这一探索性拨款申请中，我们计划使用传统和Cre诱导的转基因小鼠来建立和表征小鼠模型，以测试皮肤细胞对MCPyV st和LTAgs表达的体内反应。拟议的研究具有非常重要的意义，因为它们将通过定义MCPyV标签在完整动物中的生物学活性来帮助填补我们知识中的一个关键空白，从而为研究导致MCC发生和维持的因素提供一套亟需的工具。此外，这些探索性研究将为识别MCPyV标签细胞靶标奠定基础，这些靶标通过非病毒机制解除调控可能更广泛地促进癌症的发展。这项拟议的工作可能对MCC患者具有直接的翻译相关性，因为它可能导致识别新的治疗靶点，并产生急需的小鼠模型，用于功能分析和临床前试验。

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.