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Genetic factors may play a large role in explaining variation in the mammographic areas of both dense and nondense tissue in the breast
Percent mammographic density (PMD),
defined by the radiological appearance of epithelial and stromal tissue in
a mammogram, is an established and strong risk factor for breast cancer.
Our previous twin studies comprised of 571 monozygous and 380 dizygous
twin pairs recruited from Australia and North America showed that the
heritability of PMD was 63%. The
computer assisted method that we have used to measure PMD generates a
continuous quantitative measure of the area of the breast and the area of
dense tissue. This allows separate examination of the factors associated
with the absolute areas of dense and non-dense areas (total area minus
dense area) in the mammogram. These associations are of interest because
the association of PMD with risk of breast cancer must be explained by
associations of one or both of the dense and non-dense areas with risk.
Here we found that after adjusting for measured determinants,
heritability was estimated to be 65% (95% confidence interval (CI) 60-70%) for
dense area and 66% (95% CI 61-71%) for non-dense area.
That is, differences in the amount of mammographically
dense and non-dense (i.e.fatty) tissue in the breast across the population
is likely due to genetic factors. We also found
that dense area and non-dense area are negatively correlated.
About
two-thirds of the negative correlation
between dense and non-dense area is explained by the same genetic factors
influencing both traits, but in opposite directions. These findings have implications for studies aimed at finding
the genes involved in mammographic densities, and hence risk of breast
cancer.
Bibliographical reference:
Jennifer Stone Centre for Molecular Environmental Genetic and Analytic Epidemiology, The University of Melbourne, Carlton, Victoria, Australia
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