Genetic Role Overstated, Breast Cancer Study Says
In a finding that may lend some reassurance to women with a family history of breast cancer, Harvard researchers report that genetics appear to play a smaller role in development of the disease than had been thought.
Although previous studies have suggested that women with a mother and sister who have breast cancer have six to 14 times the normal risk of developing breast tumors, a major new study reported today in the Journal of the American Medical Assn. says that their risk is only about 2 1/2 times the normal risk. The chances are only about twice normal if just the mother or the sister has the disease, also lower than previous estimates.
Breast cancer is the most common form of cancer in women. It will strike an estimated 175,000 in the United States this year, killing 45,000.
The new study also shows that except in a small number of families in which many women are stricken with the disease, genetics plays only a small role in the onset of breast cancer. The survey of 121,700 nurses who have been monitored since 1976 indicates that only about 2.5% of breast tumors are genetic in origin.
If women have an affected mother or sister, “they need not feel they will inevitably develop breast cancer,” said Dr. Graham Colditz, an epidemiologist at the Harvard School of Public Health and lead author of the report.
“The most important feature of the study is that only a small proportion of breast cancer can be attributed to family history,” said epidemiologist Louise Brinton of the National Cancer Institute. “There is a lot of misconception . . . among the public that the risk (of breast cancer) is determined solely by whether or not they have relatives with it. That is not true.”
In another report based on the nurses study to be published this week in the New England Journal of Medicine, the same team found that dietary supplementation with Vitamins C and E provided no protection against breast cancer but that normal levels of Vitamin A in the diet may act as a protective factor.
The incidence of breast cancer has been increasing dramatically in the United States over the last three decades. In 1960, only 63,000 cases were diagnosed, and researchers estimated that one in every 14 women would develop breast cancer. Today, it is estimated that one in every nine women will develop breast cancer, which is the leading cause of cancer death among women ages 15 to 54. Researchers attribute part of the increase to better detection, but they are mystified about the bulk of the increase. The average five-year survival rate is 75%.
The new information is the most recent from the Nurses Health Study, which is conducted through Harvard’s Brigham and Women’s Hospital. The study was established in 1976 to examine how diet and lifestyle affect the risk of disease. The 121,700 nurses, ages 30 to 55, were carefully examined at the beginning of the study and at two-year intervals. They also periodically filled out extensive questionnaires about their diet, dietary supplements and other risk factors.
Excluding women who had already had some form of cancer at the beginning of the study, 117,988 women participated. Of those, 2,389 had developed breast cancer through 1988.
The researchers broke them down into groups based on their family history. The vast majority of nurses, 93.2%, had neither a mother nor a sister with breast cancer. Women in this group had a 7% risk of developing breast cancer.
Of the nurses whose mothers had breast cancer, about 11% were at risk of developing the cancer. The relative risk of developing breast cancer varied, however, with the age of onset of the mother’s cancer. If the mother was younger than 40 when the cancer occurred, the daughters had about four times the normal risk, but if the mother was over 70 at the time of onset, the risk was only 1.5 times normal.
Of those who had sisters with breast cancer, about 13% were at risk. In a small number of cases, 14, both the mother and a sister had developed breast cancer. For these women, the risk was 17%, or about 2.5 times higher than the risk in the general population.
For all women whose mothers had breast cancer, the risk was 1.8 times normal. This was somewhat smaller than previous estimates, which ranged as high as four times normal.
The researchers did not look at families in which more than two close relatives had developed breast cancer. Previous studies, however, have shown that the risk for women in these rare families is as high as 80% and that the breast cancer is almost completely genetic in origin in such cases.
In contrast, the nurses study indicates that most cases of breast cancer are not genetic in origin. “That lends doubt as to how useful identifying a gene (for breast cancer) will be in preventing most cases of breast cancer,” Brinton said.
“This doesn’t change the message we want to send out,” said epidemiologist Eugenia Calle of the American Cancer Society. “If you have a family history, you have a modestly increased risk. But all women should be conscientious about screening for breast cancer, whether they have a family history or not.”
The results with vitamins were not particularly unexpected, said Dr. David J. Hunter of Harvard, who found that below normal levels of Vitamin A increase the risk of breast cancer by about 20%. The main conclusion, he said, is that women who do not get enough Vitamin A in their diet should increase their consumption of vegetables and other foods rich in the vitamin.
“If a woman feels that she can’t increase her intake (of such foods) and wants to take a supplement, the amount in multivitamins is adequate,” Hunter said. “There is no need to take Vitamin A supplements which, in large doses, can have adverse effects.”
Previously reported results from the nurses study show that Vitamin E reduces the risk of heart disease.
