As detailed in chapter 7, family history of breast cancer is associated with increased risk. This is especially so if the history includes a first-degree family member who was affected at an early age or had bilateral disease.
Much attention is given to "hereditary" breast cancer when, in fact, those cases that can be attributed to a single-gene mutation are very rare. Epidemiological evidence suggests that BRCA1 and BRCA2 together account for only about 5% of all breast cancers.110 Germline p53 mutations and perhaps the ATM gene account for an even smaller proportion of breast cancers.111-113
Although BRCA1 and BRCA2 are often described together, there are important epi-demiological distinctions between them. The BRCA1 gene has been associated with increased risk of ovarian cancer, while BRCA2 has been suggested to play a role in male breast cancer and possibly other cancers, such as pancreatic and prostate. The BRCA1 gene is associated with an early age at onset, but this is less clear for tumors associated with BRCA2. The histology of BRCA1- and BRCA2-associated tumors differs from sporadic cases and from each other.
Lifetime risk estimates for women who carry a mutation in either BRCA gene are changing with increased study. The lifetime risk estimate for breast cancer from BRCA1 from family-based studies is 80%-90% and 64% for ovarian cancer by age 70.114 However, non-family-based estimates are much lower. In one study of Jewish women, risks for breast cancer were 56% and 16% for ovarian cancer among women with a BRCA1 or BRCA2 mutation, respectively.115 In a study of women referred to breast clinics, only 16% of breast cancer patients with a positive family history carried a BRCA1 mutation.116 In a hospital-based sample of ovarian cancer patients, 3% had a mutant BRCA1 gene.117 As additional population-based estimates are reported, these risk estimates will continue to be refined. An important challenge remains to find the genetic and environmental co-factors that interact with BRCA1 and BRCA2 and lead to variations in gene penetrance and expression.
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