In a study of 150 men, the researchers identified the virus, called XMRV, and determined that it is 25 times more likely to be found in prostate cancer patients with a specific genetic mutation than men without the mutation.

"This is a virus that has never been seen in humans before," said co- author Eric Klein, M.D., Head of Urologic Oncology at the Glickman Urologic Institute of Cleveland Clinic. "This is consistent with previous epidemiologic and genetic research that has suggested that prostate cancer may result from chronic inflammation, perhaps as a response to infection."

Cleveland Clinic researcher, Robert H. Silverman, Ph.D., previously discovered a gene called RNaseL that fights viral infections. Men with mutations in this gene are at greater risk for prostate cancer. In their study, Cleveland Clinic and University of California researchers examined tissue samples of 86 prostate cancer patients whose prostates had been surgically removed.

In these samples, Cleveland Clinic researchers determined genetic variations in RNaseL and sent the findings to UCSF scientists Joe DeRisi, Ph.D., and Don Ganem, M.D. Using a DNA-hunting "virus chip" (ViroChip) developed by Dr. DeRisi, they discovered the new virus far more often in human prostate tumors shown to have the RNaseL mutation than not.

The ViroChip contains genetic sequences of more than 1,000 viruses. Using the chip and the patient samples from Cleveland Clinic, they found the XMRV virus in eight (40%) of the 20 men with two mutated copies of the RNaseL gene and only (1.5%) of the 66 men who had one copy or no copy of the mutated gene. Laboratory pathology at Cleveland Clinic confirmed the presence of the virus in prostate tissue.

While the genetics of prostate cancer are complex, one of the first genes implicated in the process was RNaseL, which serves as an important antiviral defense mechanism. Given the anti-viral role of this gene, some scientists have speculated that a virus could be involved in a subset of prostate cancer cases.

"While we can't state that this virus causes prostate cancer, these are remarkable findings because of the association of the virus with the mutation," said Dr. Robert Silverman, collaborating investigator in the study. "This project was possible only because of the willingness of physicians and scientists in different areas of expertise at the two institutions to work closely together towards a common goal, that of identifying a new infectious agent in prostate cancer."

Future research will examine patients' sexual history, personal and family medical history and viral infections as they relate to prostate cancer, Dr. Klein said.

A full report of the discovery, co-authored by the researchers, is in press in the journal PLoS Pathogens.

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