A retrovirus known to cause leukemia and cancerous tumors in animals has for the first time been found in malignant prostate cells -- raising the intriguing question of its role in prostate cancer.
The retrovirus -- called Xenotropic murine leukemia virus-related virus, or XMRV -- was found in 27 percent of prostate cancers, compared to 6 percent of benign tissues examined by University of Utah and Columbia University researchers.
It also was associated with more aggressive kinds of tumors.
"We're not saying this virus causes cancer," said Ila R. Singh, associate professor of pathology at the U. and the senior author of a study published online Monday in the Proceedings of the National Academy of Sciences. "The best we can say is it is associated with prostate cancer."
Prostate cancer is the second most common kind of cancer in men. Discovering XMRV to be its cause could lead to new diagnostic tests and perhaps a vaccine, though Singh said the latter could be difficult. Researchers so far have been unsuccessful in developing a vaccine for HIV, another retrovirus.
XMRV first was isolated by researchers at the University of California, San Francisco and the Cleveland Clinic. Singh, who during the past decade had been studying a related virus that causes leukemia in mice, was curious.
Viruses, she said, now are known to cause several kinds of cancer. The human papilloma virus, or HPV, has been linked to cervical cancer. Epstein-Barr, a common herpesvirus, has been strongly associated with nasopharyngeal carcinoma; and chronic infection with hepatitis C virus and/or hepatitis B virus is believed to cause the majority of liver cancer cases.
Still, researchers haven't exhaustively explored every kind of cancerous tumor for the presence of a virus, partly because they don't always know what they are looking for, she said.
The UCSF and Cleveland Clinic's isolation of XMRV from prostate cancer tissues in 2006 was her first clue. Researchers there had not looked at nonmalignant prostate tissues, so were unable to link XMRV to prostate cancer.
Singh said retroviruses work by getting inside cells and inserting a copy of their DNA into the cell's chromosome, which then becomes part of all the cell's progeny.
This "integration" is random and can happen anywhere in the chromosome. If it happens next to the gene responsible for cell growth, however, the cell is programmed to reproduce itself, leading to the formation of tumors.
In another important finding, Singh and her colleagues also showed that susceptibility to the retrovirus is not enhanced by a specific genetic mutation, as was previously reported.
If XMRV were caused by that mutation, only the 10 percent of the population who carry the mutated gene would be at risk for infection with virus. But Singh found no connection between XMRV and the mutation, meaning the risk for infection may extend to the population at large.
Singh said she is developing a test to detect XMRV antibodies in blood to gauge how prevalent the retrovirus may be in the general population.
And she is trying to find out how it gets around.
The U. researcher's team has collected hundreds of cervical tissue samples from women, and seminal fluid samples from men, to explore whether it is sexually transmitted.
Yet other avenues for exploration are whether XMRV is found in different organs -- Singh's team will study randomly collected tissue samples taken during hundreds of autopsies of men and women -- and whether it plays any role in causing breast cancer. The retrovirus has a "hormonal-responsive element" in its genome, she said, which is significant since some kinds of breast cancer are hormone responsive.