Genetic epidemiology of breast cancer: Segregation analysis of 389 Icelandic pedigrees

A genetic epidemiologic investigation of breast cancer involving 389 breast cancer pedigrees including information on 14,721 individuals from the Icelandic population-based cancer registry is presented. Probands were women born in or after 1920 and reported to have breast cancer in the cancer registry. The average age of the 389 probands was 45.5 years (SD 8.92). Segregation analyses was performed evaluating residual maternal effects, a dichotomous cohort effect, and assuming the age at diagnosis followed a logistic distribution after log-transformation. Familial aggregation could be best explained by the inheritance of a high-risk allele leading to early onset breast cancer among the homozygotes, which represent approximately 2.6% of the population. A Mendelian codominant model was selected as the best fitting model, with an estimated age at diagnosis of 51.8 years among these high-risk homozygotes, 64.0 years for heterozygotes and 76.3 years for the low-risk genotype. The predicted cumulative risk for homozygote carriers of the high-risk allele is 32.2% by age 60, compared to 16.4% for heterozygotes and 5.0% for non-carriers of the same age. These predicted age profiles in the current study complement recent reports from Iceland of a majority of BRCA2 mutation carriers being diagnosed with breast cancer below the age of 50 years, and 60 years being the mean age at diagnosis for non-carriers. This model also predicted a high background risk of breast cancer for women in this population (estimated susceptibility γ = 0.44 ± 0.08). This implies that if carriers and non-carriers did not die of competing causes, the estimated risk of being diagnosed with breast cancer by age 80 years irrespective of carrier status is 11.4%.