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The novel gammaretrovirus xenotropic murine leukemia virus-related virus (XMRV) was identified in human prostate cancer tissue in 2006, confirmed in 2009 and later linked to a second human condition chronic fatigue syndrome, CFS. These investigations, all carried out in the US, have not been reproduced in Europe or in China. We found no evidence for XMRV infection in CFS. Moreover, we failed to find evidence of XMRV infection in UK prostate cancer patients and in prostate cancer tissue taken from patients in India, Korea, Thailand and Japan, or in cancers other than that of the prostate. Our UK CFS patients were consistently XMRV-free. We did, however, generate false-positive results from prostate cancer patient tissue, despite the fact that the no-template controls in our PCR were consistently negative and the PCR for murine mitochondrial DNA was often also negative. Sources of this contamination will be discussed in our presentation.

Cancer gene therapy will benefit from vectors that are able to replicate in tumor tissue and cause a bystander effect. Replication-competent murine leukemia virus (MLV) has been described to have potential as cancer therapeutics, however, MLV infection does not cause a cytopathic effect in the infected cell and viral replication can only be studied by immunostaining or measurement of reverse transcriptase activity. We inserted the coding sequences for green fluorescent protein (GFP) into the proline-rich region (PRR) of the ecotropic envelope protein (Env) and were able to fluorescently label MLV. This allowed us to directly monitor viral replication and attachment to target cells by flow cytometry. We used this method to study viral replication of recombinant MLVs and split viral genomes, which were generated by replacement of the MLV env gene with the red fluorescent protein (RFP) and separately cloning GFP-Env into a retroviral vector. Co-transfection of both plasmids into target cells resulted in the generation of semi-replicative vectors, and the two color labeling allowed to determine the distribution of the individual genomes in the target cells and was indicative for the occurrence of recombination events. Fluorescently labeled MLVs are excellent tools for the study of factors that influence viral replication and can be used to optimize MLV-based replication-competent viruses or vectors for gene therapy.