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Study: Identical Twins of Breast Cancer Patients Face High Risk for the Disease (dateline June 25, 2002)

Identical twins of women with breast cancer are three times more likely to develop the disease themselves compared to other women, according to a newly released analysis by a British researcher. The finding suggests that the majority of breast cancers occur in women who inherit a high risk for the disease. The study also found that identical twins whose sisters develop breast cancer at a young age have a higher-than-average risk of being diagnosed at an early age too. In addition to increasing scientific understanding about the genetics of breast cancer, the finding will help physicians more effectively monitor and manage identical twins of women with breast cancer.

The study involved analyzing 1,300 pairs of identical twins and 1,000 pairs of non-identical twins, of which one twin had already developed breast cancer. The analysis was conducted by Professor Julian Peto of the British charity Cancer Research UK, and data were collected by Dr Thomas Mack of the USC/Norris Comprehensive Cancer Center in Los Angeles, California. The finding was presented at the May 2002 Oncogenomics conference in Dublin, Ireland.

Dr. Peto found that one-third of women whose identical twin sisters developed breast cancer would also develop the disease at some point in their lives, compared to one-ninth of the general population of women. Non-identical twins of women with breast cancer were at a significantly lower risk of developing breast cancer, which indicates that inheriting specific genes (or combinations of genes) plays a large role in breast cancer risk. (Identical twins have the same genetic make-up).

"If a woman’s identical twin has breast cancer, her shared genes will mean she has a high risk of the disease too," said Dr. Peto, in a Cancer Research UK news release. "We now think that many – possibly the majority – of breast cancers occur in a minority of women with an inherited risk. Identifying and monitoring these susceptible women is going to be an important challenge."

Dr. Peto also discovered that 25% of identical twins of women diagnosed with breast cancer before age 40 would develop breast cancer themselves by age 60, compared to only 4% of the general female population. However, identical twins of women diagnosed with breast cancer before age 40 were at no higher risk of breast cancer in later life than identical twins of women diagnosed after age 50, which means they were only at risk of developing the disease at a young age.

According to Dr. Peto, this finding suggests that some genes may determine when a women is at an increased risk of breast cancer while other genes may be responsible for how high the risk will be. "It's like a radio alarm clock, with some genes behaving as the timer and others as the volume control," said Dr. Peto.

The study adds to a growing number of studies on breast cancer genetics, which are important for increasing knowledge of the disease and tailoring treatments. Many experts believe the new "targeted therapies" that affect certain genetic functions of breast cancer cells are the future of cancer treatment. For example, the drug Herceptin (generic name, trastuzumab) has been successful in treating advanced breast cancer patients who carry too many copies of a gene called HER2 that can affect cancer aggressiveness and prognosis.

According to Dr. Peto, the study also provides physicians with more information about hereditary breast cancer, which will be useful for monitoring identical twins of women who have been diagnosed with the disease.

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