A large, thorough hunt for a mouse retrovirus known as XMRV in people who have chronic fatigue syndrome (CFS)—including in patients who tested positive for the virus in other labs—has come up empty-handed, further deflating the hope that a cause for this baffling disease has been found. "I'd urge people to move on rather than to keep their hopes hanging on the link between XMRV and CFS," says Ila Singh, a virologist at the University of Utah in Salt Lake City who led the new study.
The link between XMRV, which stands for the cumbersomely named xenotropic murine leukemia virus-related virus, and CFS has sparked debate since it was first reported in a study published online 8 October 2009 issue of Science. Singh's study is the latest of several that have failed to find XMRV in CFS patients. It is also the first to analyze samples from patients who were part of the original study. As Singh and her colleagues report online in the 4 May Journal of Virology, they analyzed blood samples from 100 CFS patients—including 14 who tested positive in the Science report—and 200 healthy controls. The team looked for evidence of XMRV in several ways, including fishing for viral sequences with the ultrasensitive PCR assay, trying to grow infectious virus in cell cultures, and scouring the blood for antibodies to viral proteins. (XMRV earlier had been tied to prostate cancer; the current Singh study does not address that link.)
"Singh bent over backward to try to use the same assays as published, allowing her to knock down what I consider to be a real straw man but that nonetheless was out there," says retrovirologist John Coffin of Tufts University in Medford, Massachusetts. Coffin initially supported theScience report but subsequently concluded, with many others, that XMRV is a contaminant. In a recent study he provided evidence that the virus accidentally originated during mouse lab experiments.
Singh says she and her co-workers also had problems with contamination. When they ran one PCR assay that had been used in the original Science report, they found 5% of both CFS and control samples tested positive for XMRV. "It was very confusing until we figured out it was contamination," says Singh. Specifically, they fingered a PCR reagent, the Taq polymerase enzyme, as the source of the mouse sequences they detected. They further found that one of the machines they used to test samples also had been contaminated with XMRV in studies they had done months before the current analysis. "Everyone who works with mice has mouse retrovirus contamination in their lab," says Coffin. "I probably have it in my home swimming pool."
The lead author of the contested Science paper, retrovirologist Judy Mikovits of the Whittemore Peterson Institute for Neuro-Immune Disease in Reno, Nevada, isn't persuaded by the Singh group's inability to detect XMRV in anyone. "I was astounded when I leaned that Ila [Singh] didn't find it," says Mikovits. "These are good scientists."
Mikovits says she remains confident that the 14 CFS patients she selected for Singh's group have XMRV in their bodies. "These people are infected," says Mikovits. "This study says nothing. We have complete confidence in every bit of the results in the Science paper. We don't think any of it is wrong. There is no evidence of contamination in our lab, and we have controlled for that all along."
Mikovits notes that Singh's group did not use the identical protocols for every analysis, and stresses that discrepancies between their labs may also reflect her own finding that XMRV levels vary in patients day to day. Singh counters that although some protocol differences exist, they worked closely with Mikovits' team to replicate the original work. Singh says the fact that they didn't find XMRV in any of these patients is significant. "She [Mikovits] pointed us toward patients that she had repeatedly tested positive," says Singh. "We should have found at least one that was positive. Not all of them would have gone negative on the day when a phlebotomist met with them."
Mikovits cautions that her Science report did not assert that XMRV causes CFS but only claimed to have detected XMRV in CFS patients. But the large community of CFS patients, who often find themselves confronting a medical establishment that questions the very existence of their disease, pounced on this finding, and some even started taking antiretroviral medicine to treat their supposed XMRV infections.
Singh's lab earlier reported that antiretroviral drugs do work against XMRV in test tube studies. But she now cautions CFS patients that taking them is unwarranted and even dangerous.
The XMRV saga is far from over. Unlike Coffin and many other skeptics, Singh contends that a virus similar to XMRV does infect humans, and her own work supports the prostate cancer connection. "There is still considerable data supporting the link to prostate cancer that cannot be easily explained by contamination," she says. "More work needs to be done before that question can be settled."
The debate about whether XMRV infects humans and is linked to disease promises to come to a head later this year, when two different studies sponsored by the U.S. National Institutes of Health are completed. The studies both involve Mikovits and several other independent labs testing the same samples. Mikovits says if all the samples in these studies test negative, including in her own lab, the day could come when she changes her mind. "But I don't expect to get to that day," she says.