8 September 2009

Compelling new evidence for prostate cancer virus

by Kate Melville

A type of virus known to cause leukemia and sarcomas in animals has been found for the first time in malignant human prostate cancer cells. The finding could have important implications for the development of a vaccine to block infection by the virus and thus prevent the development of prostate cancer.

The researchers behind the study, published in the Proceedings of the National Academy of Sciences, say that if further investigation proves the virus (XMRV - Xenotropic murine leukemia virus-related virus) does cause prostate cancer in people, it would open opportunities for developing diagnostic tests, vaccines, and new therapies for treating the cancer.

"We found that XMRV was present in 27 percent of prostate cancers we examined and that it was associated with more aggressive tumors," said the University of Utah's Ila R. Singh, the study's senior author. "We still don't know that this virus causes cancer in people, but that is an important question we're going to investigate."

Along with providing the first proof that XMRV is present in malignant cells, the study also confirmed that XMRV is a gammaretrovirus, a simple retrovirus first isolated from prostate cancers in 2006. Gammaretroviruses are known to cause cancer in animals.

In the new study, Singh and her co-researchers examined more than 200 human prostate cancers, and compared them to more than 100 non-cancerous prostate tissues. They found 27 percent of the cancers contained XMRV, compared to only 6 percent of the benign tissues. The viral proteins were found almost exclusively in malignant prostatic cells, suggesting that XMRV infection may be directly linked to the formation of tumors.

Retroviruses insert a DNA copy of their genome into the chromosomes of the cells they infect. Such an insertion sometimes occurs adjacent to a gene that regulates cell growth, disrupting normal cell growth, resulting in more rapid proliferation of such a cell, which eventually develops into a cancer. This mechanism of carcinogenesis is followed by gammaretroviruses in general. Singh is currently examining if a similar mechanism might be involved with XMRV and prostate cancer.

Interestingly, Singh and her colleagues also showed that susceptibility to XMRV infection is not enhanced by a genetic mutation, as was previously believed. If XMRV were caused by the 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.

While the study answers important questions about XMRV, it also raises a number of other questions, such as whether the virus infects women, is sexually transmitted, how prevalent it is in the general population, and whether it causes cancers in tissues other than the prostate. "We have many questions right now," Singh concluded, "and we believe this merits further investigation."

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Source: University of Utah Health Sciences