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About one in every 40 Ashkenazi Jewish women carries a gene mutation that raises the risk that she will develop breast cancer to 82 percent, compared to 10 percent in the general population, according to a new study conducted by an American researcher.

The study, to be published today in the journal Science, also found that the mutation, which can occur in either the BRCA1 or the BRCA2 gene, significantly raises the risk of ovarian cancer - to 54 percent for BRCA1 mutations and 23 percent for BRCA2 mutations, compared to about 1.8 percent among the general population. Ovarian cancer, unlike breast cancer, is frequently fatal.

Dr. Mary-Claire King, the first author of the study and a professor of genome sciences at the University of Washington, Seattle, told Haaretz that in light of these findings, she "hopes that the Israeli health system will begin to fund routine testing for Ashkenazi women to determine whether they are in the at-risk group."

The researchers began by conducting genetic tests on 1,008 Ashkenazi Jewish women in the United States who had developed breast cancer, and later tested the women's relatives. The results revealed that the connection between the mutation and the cancer was much stronger than had been previously thought.

"The results are surprising, because most of the previous studies had estimated a lower risk level, except in those rare families where many women developed the disease," said Dr. Efrat Levy-Lahad, who wrote the editorial for the issue of Science in which King's study appears.

The study found a 20 percent chance that a woman in her forties who bears the mutation will have developed breast cancer. The risk rose to 55 percent for women in their sixties who carry the mutation, and 82 percent for women in their eighties.

Researchers also found that the mutation put even in women with no family history of breast cancer at a high risk - a finding that was attributed to the fact that many such families were either very small or contained a high proportion of men, thereby lowering the statistical probability of multiple cases of breast cancer. The fact that the risk remains high even in families with no history of the disease is apparently due to the ability of both the mother and the father to transmit the mutation.

Another surprising finding was that the risk factor can be reduced by healthy living - something that usually has little impact on genetically induced diseases. The study found that women who exercised actively when they were young - either dancing, in team sports or just walking a lot - and who maintained a healthy weight through the age of 21 were somewhat protected from breast cancer.

"If they carried the mutation, they still had a very high risk, but their age of diagnosis was pushed to later in life," King said, terming this finding "a source of hope."

One finding that bolstered the theory of lifestyle influence, said Levy-Lahad, was the marked difference in the frequency of the disease among women born before and after 1940. "From a genetic standpoint, there is no difference among the women," she said. "But there was a difference in lifestyle. [After 1940], women had fewer children and did so at a later age. There are also the issues of obesity and lack of physical activity. All of these affect the risk of developing breast or ovarian cancer."

But Levy-Lahad was less enthusiastic about King's recommendation that the Israeli health system immediately institute regular testing of Ashkenazi women to identify those with the mutation. King's study, she explained, was based on families in which at least one woman had already developed breast cancer. Thus what is needed now is a study that will determine whether the results also hold true for women who are selected at random.

Another question is whether women will actually want to know whether they bear the mutation - thereby forcing them to confront the question of whether to have their breasts or ovaries surgically removed as a preventive measure once they are finished bearing children.

King is convinced that the answer is yes. "If I were an Ashkenazi Israeli woman, I would want to know," she said.