Prevalence of Merkel cell polyomavirus in Merkel cell carcinoma

It has recently been shown that Merkel cell carcinoma, a rare and often lethal cutaneous malignancy, frequently harbors a novel clonally integrated polyomavirus aptly named Merkel cell polyomavirus. We aimed to study the prevalence of Merkel cell polyomavirus in cases of Merkel cell carcinoma, using specimens from formalin-fixed, paraffin-embedded tissue blocks. In our archives we identified 41 cases of Merkel cell carcinoma (from 29 different patients). Of these, 20 cases were primary cutaneous tumors, 4 were local recurrences, and 17 were metastases. PCR using two previously published primer sets, LT1 (440 bp amplicon) and LT3 (308 bp amplicon), as well as a novel primer set MCVPS1 (109 bp amplicon), was performed on all cases. Selected PCR products were sequenced to confirm amplicon identity. In addition, the MCVPS1 products were digested with BamH1, yielding an 83 bp product. Amplifiable DNA was recovered in all 41 study cases. The detection rate of Merkel cell polyomavirus for each of the three primer sets was 22 of 29 patients (76%) for MCVPS1, 12 of 29 (41%) for LT3, and 8 of 29 (28%) for LT1. The variation between primer set detection rates was largely due to poor DNA quality, as supported by poor amplification of the higher molecular weight markers in size control ladder products and the fact that all cases that were positive by LT1 and LT3 were positive by MCVPS1. Our findings provide further evidence to link Merkel cell polyomavirus with a possible role in the oncogenesis of Merkel cell carcinoma. On a more practical level, our paraffin-optimized primer set may be used as an ancillary test to confirm the diagnosis of Merkel cell carcinoma in the clinical setting or for screening other rare tumor types for the causative virus, especially those tumor types that are underrepresented in frozen tissue repositories.