WEDNESDAY, Sept. 18, 2002 (HealthDayNews) -- Genetics appear to be a driving force behind whether a woman has dense breast tissue, a major risk factor for breast cancer.
According to a study in tomorrow's issue of The New England Journal of Medicine, 60 percent to 67 percent of Western women's breast density is determined by genes.
More than a dozen studies have shown that women who have extensive dense breast tissue have a risk of breast cancer that is 1.8 to six times higher than that of women of the same age with little or no density.
There are several possible explanations for this, says Dr. Norman Boyd, co-lead investigator of the study and head of the division of epidemiology and statistics at the Ontario Cancer Institute in Canada.
One may be that women with dense breasts simply have more cells that are at risk of undergoing the genetic changes that can give rise to breast cancer. These cells may also be dividing more rapidly, or it may be that the relative immaturity of dense tissue makes it more susceptible to developing a malignancy. A woman's breast tissue usually becomes less dense as she ages.
"It may reflect many things, including hormonal levels," adds Dr. Alan Stolier, medical director of the Lieselotte Tansey Breast Center at the Ochsner Clinic Foundation in New Orleans.
Dense breast tissue is visible on a mammogram and is not equivalent with firmness. It is determined by the amount of connective and epithelial tissue in the breast -- as opposed to fat tissue.
A woman with density in 75 percent of her breast is five times more likely to develop breast cancer than someone with little or no breast density. About one-third of breast cancer cases can be attributed to having dense tissue in more than half of the breast, researchers say.
About 5 percent to 10 percent of breast cancer cases can be traced to the BRCA1 and BRCA 2 genes, scientists believe.
However, the new findings suggest that even more genes may be involved.
In the latest study, Boyd, along with co-lead investigator John Hopper of the University of Melbourne in Australia, studied a total of 571 pairs of identical twins and 380 pairs of fraternal twins in the United States, Canada and Australia.
All the study participants were between the ages of 40 and 70 and did not have breast cancer. Although many of the women lived in different cities or even continents than their twins, they were all predominantly of European ancestry.
Heredity accounted for 60 percent of the variation in density in Australia twins, 67 percent in American twins and 63 percent in all twins.
The researchers don't yet know how many genes are involved with breast density or what they might do.
"We postulate that there must be genes that are causing density, and we think that at least some of the genes must also influence susceptibility to cancer. So, we're going to try to find out what genes they are and find out what they're doing physiologically," Boyd says.
One promising hypothesis is that some of these genes affect hormones.
"What that inherited quality is you don't know. But you might take a guess that it has to do with production and metabolism of female hormones," Stolier says. "It would not surprise me to learn that twins who share the same genes also metabolize estrogen and progesterone the same way, and I would think that that may have something to do with the density of breasts."
Boyd agrees. "It's very likely that [the genes are] regulating hormones. We know that specific hormones and growth factors are associated with density."
What To Do
For more information on breast density and the risk of breast cancer, visit the University of Toronto. For more on breast cancer, check the American Cancer Society.
