* Journal says validity of 2009 study "now in question"
* Antiviral treatments not recommended, researcher says
By Julie Steenhuysen
CHICAGO, May 31 (Reuters) - A mouse virus called XMRV, which has been fingered as a cause of chronic fatigue syndrome, is likely not responsible for the mysterious disease, according to two studies released on Tuesday.
The studies are the latest to refute a 2009 study that claimed to have found XMRV in the blood of patients with chronic fatigue syndrome, raising hope for a treatment for the debilitating condition that causes muscle pain, memory loss and overwhelming fatigue.
Science, which published the 2009 study, is running the two new reports along with an "editorial expression of concern," noting that the validity of the original chronic fatigue study by research teams Nevada and Maryland "is now in question."
The 2009 study raised hopes that patients with the baffling disease might benefit from a range of drugs designed to fight AIDS, cancer and inflammation.
XMRV was first reported in samples from a human prostate tumor in 2006, and has since been reported to be present in 6 to 27 percent of human prostate cancers.
Later research reported XMRV in the blood of 67 percent of people with chronic fatigue syndrome.
But several groups have challenged the claim that XMRV is circulating in people with studies that have failed to detect it in people with prostate cancer and chronic fatigue syndrome.
Last December, a team at University College London, the Wellcome Trust Sanger Institute and Oxford University said cell samples from patients in earlier studies were likely contaminated. [ID:nLDE71G0A0]
In one of the new studies, researchers the National Cancer Institute, one of the National Institutes of Health, suggest XMRV came about when two mouse leukemia viruses became mixed and contaminated lab experiments.
A second study by Konstance Knox and colleagues of the Wisconsin Virus Research Group in Milwaukee examined the blood of people whose tissue was used in the 2009 study and found no trace of the virus in any of the samples.
"Taken together, these results essentially close the door on XMRV as a cause of human disease," said John Coffin of Tufts University School of Medicine in Boston, who worked on one of the studies with Vinay Pathak of the National Cancer Institute.
To settle the debate, Pathak and colleagues studied human prostate cancer cells thought to contain XMRV, as well as tumors from these cells after they had been made to grow in mice, a common way to study new treatments that might not be safe to test in people.
A careful study of these showed that the while the initial prostate tumors grown in mice contained no XMRV, later tumors derived from this tissue did -- showing that the virus was not present in the original human tumor, as previously thought.
Instead, the virus appears to have infected tumor cells while they were in mice.
They also showed that the mice used in the study had two new viruses, and these viruses combined to form XMRV.
Pathak said the latest results are not what the team had expected, but they will likely allow researchers now to focus on the real causes of these diseases.
Meanwhile, Knox said people with chronic fatigue syndrome need to know that taking antiretroviral drugs such as those used to treat people with HIV/AIDS "will not benefit them, and may do them serious harm."
"Physicians should not be prescribing antiviral compounds used in the treatment of HIV/AIDS to patients on the basis of a chronic fatigue syndrome diagnosis or a XMRV test result," she said in a statement.
Chronic fatigue syndrome strikes an estimated 1 million to 4 million Americans.
(Editing by Cynthia Osterman)