Merkel cell carcinoma (MCC) is a rare and aggressive skin cancer of elderly and immunosuppressed patients that is caused by the Merkel cell polyoma virus. Patients with MCC are 30 times more likely than the general population of developing CLL. A paper in Blood (2010; 116:5280-4) from Cologne reports on the clonal integration and truncating mutations of the large T antigen of Merkel cell polyoma virus in CLL patients. Normally this is only found in patients with MCC, but the German group found copies of the virus in 19/70 highly purified tumor cells from CLL patients. There were 3-4 logs fewer copy numbers compared to MCC tumors. In 6/19 cases there were mutations of the viral helicase gene, which indicates that this was not just a passenger virus.
One possible explanation for these findings is that the high incidence of MCC in CLL is not simply due to immunodeficiency, but that the MC polyoma virus is oncogenic (cancer causing) in B lymphocytes.