"The hunt is now on to find the genes responsible for breast density," says Professor John Hopper, Director of the University of Melbourne's Centre for Genetic Epidemiology, which houses the Australian Twin Registry.
"Once found, the genes may help define new sub-types of breast cancer, and enable targeted prevention strategies and treatment. It might also explain a greater amount of the genetic effect on breast cancer than do the recently discovered genes BRCA1 and BRCA2," he says.
Australian breast cancer researchers, in collaboration with Canadian colleagues, published the study results in the latest edition of New England Journal of Medicine. The study involved 607 twin pairs from Melbourne, Sydney and Perth, recruited through the Australian Twin Registry, and 355 North American twin pairs.
The research found that in both the Australian and North American twins, genetically-identical twin pairs had similar breast densities. Non-identical pairs, who share half their genes, had breast densities about half as similar.
Breast density is determined by the amount of connective and epithelial tissue - which appears light on a mammogram - compared with amount of fat tissue - which appears dark. It is not related to firmness and cannot be felt in the breast. It is only detectable by having a mammogram.
Women with breast density in the top 20 percent have a five-fold increased risk of breast cancer than women in the bottom 20 percent for their age.
"Although there is evidence that age and some lifestyle factors, such as having children, can change breast density, there is still a large spread in the distribution of breast density across women of the same age," says Hopper.
"This results show that genetic factors play the major role in explaining why women of the same age have different breast density. It also helps explain why having a family history of breast cancer is a risk factor for the disease," says Hopper.
"There may be many genes involved with breast density, and for an individual woman it will be the combined effect of these genes, rather than a single mutation in just one "high-risk" gene like BRCA1 or BRCA2, that determines her risk," he says.
Finding the breast density genes and identifying their mechanisms will help understand the causes of breast cancer.
The Australian research was funded by the National Breast Cancer Foundation and involved researchers from The University of Melbourne, The Cancer Council Victoria, NSW Cancer and St Vincents BreastScreen, Melbourne.
Journal
New England Journal of Medicine