Human models of Merkel cell carcinoma oncogenesis

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Merkel cell carcinoma (MCC) is a very aggressive skin tumor in which - due to divergent etiology - two subtypes can be distinguished. While in about 80% of cases MCC is caused by Merkel cell polyomavirus (MCPyV) (virus-positive; VP-MCC), the virus-negative (VN)-MCC result from UV-induced mutations and are thus characterized by high mutational load. From which cells of the skin MCCs arise and whether the two subtypes have the same origin is still controversial. A number of cells such as Merkel cells, epidermal stem cells, fibroblasts or B cell lineage cells are discussed as possible candidates. During the first funding period, we investigated combined tumors consisting of a squamous cell carcinoma (SCC) and a VN-MCC part. Using genetic analyses, we demonstrated that the MCCs developed by transdifferentiation from the SCCs. Thus, these MCCs represent epithelial tumors, as do two VP-MCCs, for which we were able to demonstrate origination from trichoblastomas based on genetic similarities. Because trichoblastomas derive from KRT17+ cells of the hair follicle and show Merkel cell differentiation potential, our hypothesis is that KRT17+ Merkel cell progenitor cells localized in the hair follicle are cells of origin for VP-MCCs. This hypothesis of a hair follicle origin is also supported by a recently published MCC mouse model and, because of the location of these cells, may furthermore explain the generally low mutation load of VP-MCCs. In contrast, the origin of VN-MCC would be sun-exposed precursor cells of the interfollicular epidermis or, not infrequently, SCC cells, since VN-MCC is found associated with SCC in up to 25% of cases. In the proposed project, the transdifferentiation from SCC to MCC will be retraced by appropriate genetic manipulation in vitro with the required vectors already been generated in the first funding period. In addition, the MCPyV oncoproteins (small T and large T antigen) will be expressed in human skin organoids to generate a complex human Merkel cell carcinoma model. In the first funding period, the generation of hair-bearing skin organoids from pluripotent stem cells was successfully established in Würzburg. In addition, different vector systems were generated and tested, allowing individual or combinational expression of T antigens either constitutively, Dox-inducible or cell type-specific (e.g. in KRT17+ cells) in skin organoids. Thus, all the tools are available to investigate the question of VP-MCC origin in a complex human model system.