The abbreviations stand for defects involving Breast Cancer Gene 1 and Breast Cancer Gene 2 that are strongly linked with breast and ovarian cancers.
The new findings show tamoxifen can reduce breast cancer risk by 62 percent in women with the BRCA2 mutations. They also underscore how rare the mutations are present in only about 7 percent of women studied and help clarify conflicting results in previous studies on tamoxifen's effectiveness.
Tamoxifen inhibits growth of cancer cells that are sensitive to the hormone estrogen. Evidence suggests that most BRCA2-linked tumors are estrogen-positive and that most BRCA1 tumors are estrogen-negative, or not fueled by the hormone. This explains the new findings, said University of Washington geneticist Mary-Claire King, the lead researcher.
She emphasized that the study did not address treatment of existing breast cancer with tamoxifen, which has been shown to help reduce a recurrence in women with estrogen-positive tumors who have BRCA1 or BRCA2 mutations.
The findings appear in Wednesday's Journal of the American Medical Association.
King and colleagues analyzed blood samples from 288 women aged 35 or older who took tamoxifen or a placebo for five years in a national breast cancer prevention study that started in April 1992.
Only 19 women had BRCA1 or BRCA2 mutations.
Three women with BRCA2 mutations who used tamoxifen developed breast cancer during the study, compared with eight taking the dummy drug. Among BRCA1 women, breast cancer developed in five tamoxifen patients and in three placebo patients.
King said the results will help high-risk women who decide to undergo genetic testing "make an informed decision about what kind of preventive medicine they ought to follow."
The study is important because it helps solidify previous evidence that tamoxifen is preventive and a "viable option" for some women, said Dr. Lynn Hartmann, a Mayo Clinic breast cancer researcher.
Some women who learn they have the BRCA mutations decide to have their breasts removed in hopes of preventing the disease.
Hartmann was lead author of a study published last week that showed removing both breasts can help high-risk women reduce their chances to near zero.
But she said the new study does not answer whether tamoxifen, which increases the risk for endometrial cancer and blood clots, is a better choice than radical surgery.
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