Merkel cell polyomavirus-positive Merkel cell carcinoma cells do not require expression of the viral small T antigen.

Increasing evidence suggests that Merkel cell carcinoma (MCC) is caused by the Merkel cell polyomavirus (MCV). The viral sequence encodes for two potential oncoproteins, i.e., the small T antigen (sT) and the large T antigen (LT). Indeed, sT has recently been shown to bear transforming activity. Here, we confirm this observation by demonstrating focus formation upon expression of MCV sT in NIH3T3 fibroblasts. On the other hand, however, we provide evidence that established MCC cells do not require sT for growth and survival. Silencing of sT protein expression by two different sT-specific short hairpin RNAs (shRNAs) leads to variable degrees of growth retardation in MCV-positive MCC cell lines. However, these effects are not sT specific, as proliferation of MCV-negative cell lines is similarly affected by these sT shRNAs. Furthermore, ectopic expression of shRNA-insensitive sT does not revert the growth inhibition implicated by sT silencing. Finally, the unambiguous and specific growth inhibition induced by means of an shRNA targeting both T antigens, can be completely rescued by ectopic expression of LT alone, thus demonstrating a dispensable role of sT. Altogether, our results indicate that MCV LT is more relevant in maintaining the proliferation and survival of established MCC cell lines.