Xenotrophic murine leukemia-related virus (XMRV) has been associated with Chronic Fatigue Syndrome (CFS), a debilitating condition of unknown etiology, and also prostate cancer. These associations have been hotly debated, with subsequent studies either supporting or questioning the connection.
However, most studies, whether they support or negate the link between XMRV and the two diseases, rely heavily on evidence provided by PCR testing. Therefore, when Retrovirology published four research articles in December 2010 that revealed underlying contamination issues associated with PCR analysis for XMRV, this impacted dramatically on the validity of previous research. All four studies demonstrate that murine genetic material can contaminate experiments leading to false positive findings.
Robinson et al show by PCR testing of 437 human tissue samples, that although XMRV gene sequences were detected in prostate cancer patients, other mouse gene sequences were also detected. Oakes and colleagues demonstrate that XMRV- positive blood samples from CFS patients tested positive for mouse mitochondrial DNA. Moreover, both studies show that samples from healthy patients were positive for both XMRV and mouse mitochondrial DNA. It is thought that the blood and tissue samples used in these studies were contaminated with mouse DNA prior to PCR analysis.
Contamination can occur during PCR too, as reported by Sato et al, who detected MLV- encoding nucleic acids in actual PCR reagents, namely from an RT-PCR kit provided by Invitrogen. This contamination would result in false detection of XMRV. Therefore, in their conclusion, the authors urge prudent evaluation of PCR kits prior to their use in studies to remove the risk of false positive results.
PCR testing for XMRV targets the 24-nt gag-leader deletion, but Hue et al in their research article, show that this deletion is not unique to XMRV and primers targeting this deletion amplify MLV sequences found in 12 different murine strains used in laboratories and 5 tumour cell lines. From their multi-pronged investigations, which included phylogenetic analysis, Hue et al conclude that XMRV is not a genuine human pathogen.
These recent publications cast serious doubt on previous findings that claim to prove a link between XMRV and CFS or prostate cancer, and stipulate that these sources of contamination are considered and accounted for in future investigations in order for new findings to be trusted.