Researchers at the University of Minnesota Cancer Center have found
evidence that mamographic breast density, a measure of the relative proportions
of fat, connective tissue, and glandular epithelial tissue in the breast
that is a strong, independent predictor of breast cancer risk, is genetically
influenced. The study, published in the April 16th Journal of the National
Cancer Institute, also suggests that 12% of the population has a genetic
mutation that results in a breast density twice that of women without the
"It isn't known how high breast density increases the risk of breast
cancer," said lead investigator Thomas Sellers, associate professor
of epidemiology and associate director of the Cancer Center. "But
the risk is considered second only to the risk incurred by mutations in
the BRCA1 and BRCA2 genes."
The researchers obtained routine mammograms and a variety of health-related
data from 1,370 women from 258 different Minnesota families. The study
population included 65 mother-daughter pairs and 275 sister-sister pairs.
All of the women were related, either genetically or by marriage, to an
original group of 544 breast cancer patients seen at the University of
Minnesota between 1944 and 1952. Sellers and colleagues are studying these
families as part of the Breast Cancer Family Cohort study.
Breast Density Partly Modifiable
Sellers says that the study is significant because the researchers were
able to separate out other risk factors, such as waist-to-hip ratio, physical
activity, hormone replacement therapy, and number of live births, to detect
a Mendelian dominant inheritance pattern (a pattern where only one copy
of the mutated gene is necessary to transfer the trait to offspring). They
were unable, however, to completely eliminate the possibility of a recessive
pattern, which would require more than one copy of a gene to produce a
trait. However, if breast density is, indeed, controlled by a dominant
gene, about 12% of the population would be expected to carry a form of
the gene that results in a breast density about twice that of the rest
of the population. "The good news is that while breast density risk
factor is partly genetic, it's also shaped by the environment, and so can
be modified," Sellers said.
In previous studies, Sellers and colleagues have reported on potential
risk factors for breast cancer, including waist-to-hip ratio, fat in the
diet, and the connection between breast cancer in women and prostate cancer
in men. These finding stem from the Iowa Women's Study, an ongoing health
study of more than 40,000 women in Iowa.