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- 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.
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- 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.
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- 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.
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- 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.
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- 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).
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- ``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.
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- 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.
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- 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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