Merkel Cell Polyomavirus–Positive Merkel Cell Carcinoma Requires Viral Small T-Antigen for Cell Proliferation

Abstract

To the Editor Angermeyer et al. (2013) claim that “Merkel Cell Polyomavirus-Positive Merkel Cell Carcinoma Cells Do Not Require Expression of the Viral Small T Antigen” (Angermeyer et al., 2013). This controversial conclusion is based on their inability to detect Merkel cell polyomavirus (MCV) small T (sT) protein expression and to inhibit cell growth by putative sT knockdown in MCV-MCC cells. These findings contradict existing evidence showing MCV sT protein expression in MCV-MCC cancer tissues (Shuda et al., 2011) and cell lines (see Figure. 2, Houben et al., 2010) (Guastafierro et al., 2013). To investigate this discrepancy, we tried replicating Angermeyer et al's results using the same antibodies to detect MCV sT (CM8E6 (Kwunet al., 2009), CM5E1 (Shuda et al., 2011) and 2T2 ((Wang et al., 2012), kindly provided by C. Buck) on a panel of MCV-MCC cell lines (Figure 1). MCV sT and large T (LT) are alternatively-spliced viral oncoproteins sharing a common N-terminus but having different C-termini, thus CM8E6 and 2T2 detects all isoforms of T antigens, while CM5E1 detects only sT and CM2B4 detects only LT and related isoforms. Differences in protein expression levels between MCV LT and sT are likely dependent on either premRNA or post-transcriptional protein processing. For positive and negative controls, we used UISO cells transiently transfected with the MCV T antigen locus ({type:entrez-nucleotide,attrs:{text:JN038578,term_id:354683949,term_text:JN038578}}JN038578) or with corresponding empty vector. UISO, commonly described as being from MCC origin (Houben et al., 2007), is negative for MCV and miRNA ontology studies show it clusters with cell lines of breast cancer origin (Renwick et al., 2013). In contrast to Angermeyer et al., the 19 kD MCV sT band is readily detected in all MCV-MCC cell lines (open arrows) but not in UISO cells. Figure 1 Detection of Merkel cell polyomavirus (MCV) small T (sT) antigen expression by multiple MCV T antigen antibodies. Figure 2 Merkel cell polyomavirus (MCV) small T (sT) antigen knockdown inhibits MCV-positive Merkel cell carcinoma (MCC) cell proliferation.