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Original Article
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Volume 347:886-894 September 19, 2002 Number 12
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Heritability of Mammographic Density, a Risk Factor for Breast Cancer
Norman F. Boyd, M.D., Gillian S. Dite, B.Sc., Jennifer Stone, M.Sc., Anoma Gunasekara, B.Sc., Dallas R. English, Ph.D., Margaret R.E. McCredie, Ph.D., Graham G. Giles, Ph.D., David Tritchler, D.Sc., Anna Chiarelli, Ph.D., Martin J. Yaffe, Ph.D., and John L. Hopper, Ph.D.

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ABSTRACT

Background Women with extensive dense breast tissue visible on a mammogram have a risk of breast cancer that is 1.8 to 6.0 times that of women of the same age with little or no density. Menopausal status, weight, and parity account for 20 to 30 percent of the age-adjusted variation in the percentage of dense tissue.

Methods We undertook two studies of twins to determine the proportion of the residual variation in the percentage of density measured by mammography that can be explained by unmeasured additive genetic factors (heritability). A total of 353 pairs of monozygotic twins and 246 pairs of dizygotic twins were recruited from the Australian Twin Registry, and 218 pairs of monozygotic twins and 134 pairs of dizygotic twins were recruited in Canada and the United States. Information on putative determinants of breast density was obtained by questionnaire. Mammograms were digitized, randomly ordered, and read by a blinded investigator.

Results After adjustment for age and measured covariates, the correlation coefficient for the percentage of dense tissue was 0.61 for monozygotic pairs in Australia, 0.67 for monozygotic pairs in North America, 0.25 for dizygotic pairs in Australia, and 0.27 for dizygotic pairs in North America. According to the classic twin model, heritability (the proportion of variants attributable to additive genetic factors) accounted for 60 percent of the variation in density (95 percent confidence interval, 54 to 66) in Australian twins, 67 percent (95 percent confidence interval, 59 to 75) in North American twins, and 63 percent (95 percent confidence interval, 59 to 67) in all twins studied.

Conclusions These results show that the population variation in the percentage of dense tissue on mammography at a given age has high heritability. Because mammographic density is associated with an increased risk of breast cancer, finding the genes responsible for this phenotype could be important for understanding the causes of the disease.


Source Information

From the Division of Epidemiology and Statistics, Ontario Cancer Institute (N.F.B., J.S., A.G., D.T.); Cancer Care Ontario (A.C.); and Imaging Research, Sunnybrook and Women's College Health Sciences Centre (M.J.Y.) — all in Toronto; the Centre for Genetic Epidemiology, University of Melbourne (G.S.D., J.L.H.); and the Centre for Cancer Epidemiology, Cancer Council Victoria (D.R.E., G.G.G.) — both in Melbourne, Australia; and the Department of Preventive and Social Medicine, University of Otago, Dunedin, New Zealand (M.R.E.M.).

Address reprint requests to Dr. Boyd at the Division of Epidemiology and Statistics, Ontario Cancer Institute, 610 University Ave., Toronto, ON M5G 2M9, Canada, or at boyd{at}uhnres.utoronto.ca.

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Related Letters:

Mammographic Density of the Breast
Vachon C. M., Sellers T. A., Pankratz V. S., Hall F. M., Boyd N. F., Hopper J. L., Thurfjell E.
Extract | Full Text | PDF  
N Engl J Med 2003; 348:174-175, Jan 9, 2003. Correspondence

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