BRCA1 and sex ratio

Two recent papers suggest distorted sex and transmission ratios associated with BRCA1 mutations. If real, these would provide novel insights into the normal biological function of this gene and have implications for genetic epidemiologic methods used to estimate penetrance. We addressed these observations in two settings: offspring of 283 mutation carriers and 471 mutation negative subjects from BRCA1/2 mutation-positive families with multiple cases of breast and ovarian cancer (NCI families); and relatives of 115 BRCA1/2 mutation carriers from the Washington Ashkenazi Study (WAS). The male:female ratio was below one in both BRCA1 (0.85, 95% CI 0.7-1.1 in NCI families; 0.90, 95% CI 0.6-1.4 in WAS) and BRCA2 families (0.77, 95% CI 0.5-1.3 and 0.80, 95% CI 0.5-1.2, in the NCI and WAS study groups, respectively). None of the sex ratios deviated significantly from one, and there was no significant difference between BRCA1 and BRCA2 families. The reduced sex ratio was due largely to the offspring of males, a distortion that is probably an artifact of ascertainment biases. Among adult daughters without breast or ovarian cancer born to mutation carriers, as expected, fewer than 50% were mutation carriers (39% in BRCA1 families and 44% in BRCA2 families). It is difficult, due to ascertainment biases, to draw firm conclusions regarding sex ratios in studies of a sex-limited phenotype. Nonetheless, these observations do not support the idea that BRCA1 mutation carriers have a lower ratio of male offspring than BRCA2 mutation carriers.