A new study tightens the suspected link between a virus and prostate cancer, and raises the possibility that infection with the virus could be an indicator of aggressive tumors that require swift treatment.
"We're not making any causal association at this moment," stressed Dr. Ila R. Singh, an associate professor of pathology at the University of Utah, lead author of a report on the virus, known as xenotropic murine leukemia virus-related virus (XMRV).
"There probably are multiple causes of prostate cancer, but for the first time we have analyzed prostate cancer and normal prostate tissue and found cancers are much more likely to have [the virus]," Singh said.
The research is published in this week's issue of Proceedings of the National Academy of Sciences.
A link between XMRV and prostate cancer was first reported two years ago by researchers at the Cleveland Clinic and the University of California, San Francisco. They found the virus in cells around tumors.
The new study, involving more than 300 prostate cancer specimens, found that 27 percent of them carried the virus.
"It was also more likely to be present in more aggressive tumors," Singh said. "We found it in 20 percent of the least aggressive tumors and over 45 percent of the most aggressive tumors."
So, a test for presence of XMRV could be at least a partial solution to the major problem facing doctors who treat prostate cancer: distinguishing the minority of virulent, life-threatening cancers from the majority of tumors which grow so slowly that "watchful waiting" may be enough.
Prostate cancers now are diagnosed by a test for prostate-specific antigen (PSA), a protein produced by the gland. However, PSA tests cannot single out aggressive cancers, and there is a major debate on whether widespread use of these tests leads to overdiagnosis and overtreatment.
"I don't know yet if this is the better test, but it might be linked to more aggressive prostate cancers," Singh speculated.
Robert A. Silverman, a professor of cancer biology at the Cleveland Clinic, was one of the researchers who first reported the association of XMRV with prostate cancer. He called the new study "very exciting."
"Finding it in cancer cells makes it easier to reconcile with the idea of a cancer-causing virus than in our prior study," Silverman said. "The prior study found the virus in cells surrounding the cancer. We can't say with certainty that XMRV is a cause of cancer, but it still is a candidate for a cancer-causing virus."
Even if causation is not proved, "XMRV could be a marker for aggressive tumors," Silverman said.
His group has continued research on the virus, one of which indicates that human semen promotes the activity of XMRV, Silverman said.
The new study also overturns a previously reported association between XMRV infection and a genetic variation carried by a small percentage of men. "We don't find any such association," Singh said. So, the new research appears to expands the population at risk from the virus to all men -- whether they carry the genetic variant or not.
Singh and her colleagues are expanding their research on XMRV. "We don't know if women have the virus, and we are looking at cervical cells from Pap smears," she said. "We are looking at seminal fluid from men and also at tissues other than the prostate. We have two large series of autopsies of male and female organs. We are also looking for antibodies to the virus in serum as a way to detect infection."