Mouse Virus May Be
Linked To Human
Breast Cancer
By Suzanne Rostler

NEW YORK (Reuters Health) -- In a provocative new theory, a team of researchers from the US and Canada suggests that a cancer-causing virus carried by mice may be the cause of some cases of breast cancer in humans.
According to the theory, a species of mouse that inhabits Western Europe, North and South America, Australia, New Zealand, and Hawaii may transmit a mammary tumor virus to women. The team notes that the highest number of breast cancer cases occur in countries where a particular breed of domestic mouse is common.
Previous research has shown DNA that is nearly identical to mouse mammary tumor virus (MMTV) is found in 40% of human breast tumors, the study in the January 19th issue of the British Journal of Cancer, notes.
However, it is not yet clear how the virus could be transmitted to so many women and whether certain factors such as diet prevent the virus from leading to breast cancer, explained study co-author Richard Sage of the University of California, Santa Barbara, in an interview.
If correct, the hypothesis could help researchers to develop a vaccine to prevent a large percentage of breast cancers. The disease kills more than 40,000 women in the US a year, according to the American Cancer Society (ACS).
It would also increase the number of known causes for breast cancer. So far, the only known cause is a genetic mutation that accounts for 5% of all cases.
Causes for all other cases of breast cancer remain unknown, although the ACS reports that risk factors include age (77% of women with breast cancer are over 50 years); race (white women are slightly more likely to be diagnosed with breast cancer than black women, although black women are more likely to die of the disease); obesity; and lifetime exposure to estrogen.
The investigators looked at international rates of breast cancer, and found that higher rates occur in areas where a species of mouse known as Mus domesticus is found, compared with Eastern Europe, Japan and China -- countries where the mouse is not found.
People moving to areas where this mouse species is common were also found to be at greater risk of developing breast cancer. Studies have shown that Japanese women moving to the US, Soviet Jews moving to Israel, and South Asians moving to the UK all show increased breast cancer incidence, Sage and colleagues write.
The authors also cite a study in which researchers who worked with mice with MMTV in a laboratory developed an immune response to the virus, suggesting that they had become infected.
``This shows that people handling the virus who are tested for the virus can develop a positive reaction. But the test itself does not distinguish between virus particles or immunological response in the form of human antibodies,'' Sage explained.
He stressed that more research is needed to identify the relationship between Mus domesticus and rates of breast cancer, and to clarify how it is transmitted from mice to humans.
Still, ``we are claiming that the virus is a direct causal factor for some percentages of breast cancer,'' he said. SOURCE: British Journal of Cancer January 19, 2000.


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