Researchers in San Francisco have discovered a new virus inside the tumors of some men with prostate cancer, raising the possibility that a viral infection may play a role in that disease as it does in a handful of other cancers.
The virus, a close relative of a microbe known to cause cancer in mice, was spotted by UCSF researchers using the same molecular screening equipment that helped find the SARS virus in 2003.
UCSF researcher Joseph DeRisi, who won a 2004 MacArthur Foundation "genius grant" after his work on the SARS investigation, said he did not expect to see a virus in any of the prostate tumors. "To my amazement, we found it," he said. "And it was a new virus, which was more amazing.''
Although the discovery may be an important break in the long effort to implicate a virus in prostate cancer, this new virus was found only in a small percentage of prostate tumors -- almost exclusively among men who are missing genes that normally help cells fight off viruses.
It will take more studies to determine whether the new virus, dubbed XMRV, plays any role at all in causing prostate cancer, or is merely a microbial bystander found at the tumor site.
As a result of the findings, researchers are developing a blood test to detect antibodies to the XMRV. The test would be used at first only in the laboratory so scientist can learn more about it.
The work was reported on Friday by Dr. Eric Klein, a prostate cancer specialist at the Glickman Urologic Institute of the Cleveland Clinic, during an American Society of Clinical Oncology symposium in San Francisco.
Klein has been hunting a prostate cancer-causing virus for nearly a decade, and 18 months ago began collaborating with DeRisi and his UCSF colleague, Dr. Don Ganem.
"Ten years ago, research suggested that certain genes predispose some men to prostate cancer,'' Klein said.
These men were missing a gene that produced a natural virus-fighting chemical. That suggested to Klein that prostate cancer might be caused by a virus that could exploit this weakness, and the hunt for a culprit was on.
Samples of prostate tumors were run in DeRisi's screening test, which can detect up to 2,000 known plant and animal viruses. After the test found evidence of a virus similar to one that infects mice, the UCSF researchers used additional lab work to fish out a copy of XMRV.
Some viruses have been clearly implicated in cancer. Hepatitis B increases the risk of liver cancer 100 fold. Cervical cancer is caused by infection with the human papilloma virus, which also causes genital warts.
Klein concedes that the evidence for a viral role in prostate cancer is not nearly as strong.
The XMRV virus was found in half of a small group of men with prostate cancer who also inherited from both parents the trait that disables the virus fighting enzyme. Only about 13 percent of males have such a genetic vulnerability.
Still, Klein would like to find out whether the virus infects a much larger portion of the population, and only lingers in those who lack the virus-fighting enzyme. Blood tests for exposure to the virus among prostate cancer sufferers may eventually show whether XMRV could have infected them long ago, possibly causing enough damage to start a slow-growing prostate tumor that shows up years after the virus has been cleared from their systems.
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