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Segregation Analysis of 231 Ashkenazi Jewish Families for Evidence of Additional Breast Cancer Susceptibility Genes¹

Laboratory of Population Genetics, National Cancer Institute, NIH, Bethesda, Maryland 20892 [D. J. K., J. P. S.], and Department of Epidemiology, Johns Hopkins University School of Public Health, Baltimore, Maryland [T. H. B.]

Between 5 and 10% of breast cancer is attributable to inherited cancer susceptibility genes. Mutations in the genes BRCA1 and BRCA2 account for two-thirds of hereditary breast cancer cases. Using segregation analysis, families of cases without BRCA1/2 mutations were studied for statistical evidence of another major breast cancer gene in a community-based sample of Jewish probands tested previously for the presence of three BRCA founder mutations. A total of 231 probands with breast cancer, who do not carry a founder mutation, reported complete data on 602 female first-degree relatives of probands over age 20; 78 of these relatives had breast cancer.

Segregation analysis was used to evaluate the likelihood of various genetic and nongenetic models. Sporadic, environmental, and general Mendelian genetic models fit the family data poorly and were rejected. A Mendelian recessive model fit better than dominant and codominant models, although none of these could be rejected. Cumulative incidence curves predicted by the recessive and codominant models fit observed incidence among first-degree relatives well. The assumption of Mendelian transmission of a major recessive gene(s) is compatible with the data.

The recessive model predicts that 4% of women would carry the high-risk genotype, with 85% of them developing breast cancer by age 70. There was significant heterogeneity between these families and the 114 BRCA1/2 mutation-positive families from the same study population, implying that this apparent recessive effect is not because of undetected BRCA1/2 mutations. The study adds support for a major autosomal recessive component to breast cancer susceptibility.