Merkel cell carcinoma (MCC) is a very rare, but aggressive, neuroendocrine skin cancer. MCC accounts for forty-times less skin cancer than melanoma but it is three-times as lethal, with frequent recurrence and metastasis two years after diagnosis. Although MCC was first characterized in the early 1970’s, it wasn’t until 2008 that a previously unidentified polyomavirus, later termed Merkel cell polyomavirus (MCPyV), was found to be integrated into 80% of MCC tumors, making it the etiological agent of 80% of MCC cases. In MCC, the virus encodes two main oncoproteins: a truncated form of the Large Tumor antigen (LT-t) and Small Tumor antigen (ST). Although LT-t participates in tumor maintenance through its ability to bind and inhibit the cell cycle regulator Retinoblastoma (Rb), ST has been found to be the dominant transforming protein. Studies by the Chang and Moore lab at the University of Pittsburg found that ST can bind and sequester ubiquitin ligases, enzymes responsible for the post-translational addition of ubiquitin chains on proteins destined to proteosomal degradation. They showed that sequesteriation of Fbw7, a component of the Skp1-Cul1-Fbox protein (SCF) ubiquitin ligase complex, by MCPyV ST prevented Fbw7 from degrading its targets such as c-myc or LT-t, thereby decreasing their turnover. This model has been accepted as the mechanism by which MCPyV ST transform cells, as the overaccumulation of both viral and cellular oncoproteins could lead to aberrant proliferation, and ultimately transformation and tumorigenesis.
However, this model was difficult for the Galloway lab to understand, explains Kristine Dye, PhD student and first author of the work published in PLoS Pathogens early this year: “This mechanism never sat well with us because none of the MCPyV T antigens contain the canonical Fbw7 degron sequence.” Indeed the degron sequence is a phosphorylated region common of all binding partners of Fbw7. “This led to us investigate how MCPyV ST was binding to Fbw7 in the absence of a canonical Fbw7 degron.”
The authors decided first to hunt down the interaction of Fbw7 with MCPyV T antigens as previously reported. MCPyV LT, LT-t and ST antigens were individually overexpressed in Flag-tagged Fbw7 expressing cells and and a Flag immunoprecipitation was performed followed by an T antigen immunoblotting. Neither of the MCPyV T antigens bound Fbw7, whereas the positive control, SV40 LT, was able to co-immunoprecipitate, demonstrating an inability of the MCPyV T antigens to bind Fbw7, consistent with them not containing the canonical Fbw7 degron. Further more, although Dye and colleagues were able to confirm that MCPyV ST stabilized the LT protein, they also demonstrated that this happened independently of Fbw7, contrarily to what had been previously described. Similar results were obtained with another ubiquitin ligase reported to bind MCPyV T antigens.
Although the authors have not yet elucidated the mechanism of action underlying the stabilization of the MCPyV LT antigen by the ST antigen, their findings stongly question the currently accepted model. Dye emphasizes: “Ultimately we found that MCPyV ST does not bind Fbw7, and that this mechanism of MCPyV ST transformation and tumorigenesis is false.” Their work is a good example of the importance of publishing so-called negative data. “The ultimate goal is to find a cure or therapy for MCC based off of the research in the field,” reminds Dye. “If the research findings are false, and never corrected, this can severely hamper the development of efficacious therapies. Furthermore, these publications made it seem that the mechanism by which MCPyV ST transforms cells was resolved, making further investigation unnecessary. Therefore, our paper disproving this mechanism reopens the door for further investigation to find the true mechanism by which MCPyV ST can lead to the development and maintenance of MCC, and ultimately facilitate the development of therapies.”
This work was supported by the National Institute of Health.
Fred Hutch/UW Cancer Consortium member Drs. Denise Galloway and Bruce Clurman contributed to this research.
Dye KN, Welcker M, Clurman BE, Roman A, Galloway DA. Merkel cell polyomavirus Tumor antigens expressed in Merkel cell carcinoma function independently of the ubiquitin ligases Fbw7 and β-TrCP. PLoS Pathog. 2019. 15(1):e1007543. https://doi.org/10.1371/journal.