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Background: The frequency and characteristics of disease in individuals who concomitantly harbor pathogenic mutations in both BRCA1 and BRCA2 genes are not established.Materials and methods: Data were collected from the database of Clalit Health Services National Familial Cancer Consultation Service. Probands referred to this clinical service and their family members are routinely tested for the three Jewish founder mutations (BRCA1:185delAG, 5382insC, BRCA2:6174delT). In addition, carriers identified in a population-based cohort of all cases diagnosed with breast cancer in Israel in 1987-1988 allowed the estimation of the population frequency of this phenomenon.Results: In the clinic-based series of 1191 carriers of mutations in BRCA1 or BRCA2 belonging to 567 families, 22 males and females (1.85%) from 17 different families (3.0%) were found to harbor two different mutations. These included 18 individuals (1.51%) who concomitantly carried the 185delAG BRCA1 and the 6174delT BRCA2 mutations and four individuals (0.34%) who carried the 5382insC BRCA1 and the 6174delT mutations. All individuals were heterozygote carriers and none had a double mutation of both founder mutations in the BRCA1 gene itself. Seven of the 16 double carrier women (46.7%) had a personal history of breast carcinoma, diagnosed at a mean age of 44.6, compared with 372/926 (40.2%) carriers of a single mutation diagnosed with a mean age at diagnosis of 48.1 [odds ratio (OR) = 1.3, 95% confidence interval (CI) 0.4-4.0]. One case (6.7%) had a personal history of ovarian carcinoma diagnosed at the age of 53 compared with 55/926 (5.9%) of the women with single mutation (OR = 1.1, CI = 0.2-7.6). The frequency of double mutations in the population-based national breast cancer cohort was 2.2% of all carriers, and 0.3% of all breast cancer cases in the Ashkenazi population in the cohort. The mean age at diagnosis of breast cancer was younger in the carriers of two mutations.Conclusion: Double carriers of mutations in the BRCA genes are rare and seem to be carrying a similar probability of developing breast and ovarian cancers as carriers of single mutations.

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