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Twin Study Shows New Breast Cancer Risk

Hormonal Surges in Adolescence Put Some Women at Risk

WebMD Health News

June 4, 2003 -- Early puberty may increase the risk of breast cancer later in life for some women who have a family history of breast cancer. A new study shows that the hormonal surge that occurs during puberty may play a more important role in the development of breast cancer in these genetically susceptible women than other hormone-related factors traditionally associated with breast cancer risk.

Researchers say breast cancer is thought to result from excessive exposure to ovarian hormones throughout a woman's lifetime. But their findings suggest that there may be another way in which hormones interact with a woman's genes to increase the risk of breast cancer.

In particular, certain women may have an unusual genetic sensitivity to the hormonal surge at the start of menstruation that affects their risk of developing breast cancer more than their lifetime hormonal exposure.

"We don't know all the causes of breast cancer, and this study provides some insights into another pathway that could lead to the discovery of additional genes that might help explain the causes of hereditary breast cancer," says researcher Ann S. Hamilton, PhD, assistant professor of preventive medicine at the University of Southern California, in a news release. "All breast cancer might not be due to hormone exposure over a lifetime."

A New Breast Cancer Risk

The study, published in the June 5 issue of TheNew England Journal of Medicine, involved 1,811 sets of identical and nonidentical female twins in which one or both of the twins had breast cancer. To see what factors might affect the women's breast cancer risk, researchers asked the women questions about the age of their first menstrual period, when they had their first child, how many children they had, and when they began menopause.

They found only one factor stood out among identical twins who both were diagnosed with breast cancer. The twin who began menstruating earlier was more than five times as likely than the other twin to develop breast cancer at an earlier age. And women who started menstruating before age 12 were especially susceptible to getting breast cancer first.

"Within these pairs, the factors usually found to be the strongest predictors -- age at first full-term pregnancy, parity, and age at menopause -- were completely unrelated to the sequence of diagnoses," write the researchers.

However, among identical twin pairs in which only one twin had breast cancer, a later first pregnancy, fewer children, and later menopause were associated with an increased risk of breast cancer, but age of puberty was not.

Researchers say that because identical twins share the same genes, those twins who both develop breast cancer are thought to have a hereditary form of breast cancer. But when only one twin of a pair gets breast cancer, the disease is thought to be sporadic and influenced less by genetic factors.

Therefore, researchers say the study suggests that most cases of hereditary breast cancer are not related to total hormone exposure during a woman's lifetime, but to an unusual susceptibility to the hormonal surge of puberty.

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