Is there overlap between the genetic determinants of mammographic density and bone mineral density?

Mammographic density and bone mineral density, risk factors for breast cancer and osteoporotic fractures, respectively, are both thought to reflect cumulative exposure to estrogen and are highly heritable. We asked if there was overlap between the genes that explain their variances. We studied 63 monozygous and 71 dizygous female twin pairs ages 38 to 71 years (mean, 50 years). Absolute and percent mammographic densities were measured by a computer-assisted method, and bone mineral density was measured at the lumbar spine, femoral neck, and forearm by dual energy X-ray absorptiometry. After adjusting for age, height, and weight, the within-person and cross-trait cross-twin correlations between the mammographic density and bone mineral density measures were between -0.09 and 0.16 (SEs, 0.07-0.09) and independent of zygosity (all P > 0.05). We conclude that there is little, if any, overlap between the genetic or environmental determinants of disease risk associated with these traits.