Prevalence and penetrance of BRCA1 and BRCA2 mutations in a population-based series of breast cancer cases

Estimates of the contribution of BRCA1 and BRCA2 to breast cancer incidence in outbred populations have been based on studies that are either small or have selected for cases diagnosed at an early age. Only one of these has r eported an estimate of the breast cancer risk associated with a mutation in these genes, and there is no published ovarian cancer risk estimate derive d from a population-based case series. We screened a population-based serie s of breast cancer cases diagnosed before the age of 55 for mutations in BR CA I and BRCA2. Pedigree information from the mutation carriers was used to estimate penetrance and the proportion of familial risk of breast cancer d ue to BRCA I and BRCA2. We identified eight (0.7%) BRCA 1 and 16 (1.3%) BRC A2 mutation carriers in 1220 breast cancer cases tactual sample size 1435 a djusted for 15% polymerase chain reaction failure rate). Mutation prevalenc e was substantially higher in cases diagnosed before 35 years-of-age and wi th increasing number of relatives affected with breast or ovarian cancer. H owever, most mutation carriers were diagnosed in the older age groups, and a minority reported a first-degree relative with breast cancer. Breast canc er penetrance by age 80 was estimated to be 48% (95% CI 7-82%) for BRCA1 mu tation carriers and 74% (7-94%) for BRCA2 mutation carriers. Ovarian cancer penetrance for BRCA1 and BRCA2 combined was 22% (6-65%) by age 80. 17% of the familiar risk of breast cancer was attributable to BRCA1 and BRCA2. At birth, the estimated prevalence of BRCA1 mutation carriers was 0.07% or 0.0 9% depending on the penetrance function used for the calculation. For BRCA2 , the birth prevalence estimates were 0.14% and 0.22%. Mutations in the gen es BRCA I and BRCA2 are rare in the population and account for a small frac tion of all breast cancer in the UK. They account for less than one fifth o f the familial risk of breast cancer. Eligibility criteria for BRCA 1 and B RCA2 mutation testing based on family history and age of onset will identif y only a small proportion of mutation carriers. (C) 2000 Cancer Research Ca mpaign.