Discovering How Environment Contributes To Breast Cancer

The trend toward increasing breast cancer incidence in the United States is indeed disturbing ― even though increased screening, early detection and better treatment have at the same time produced a trend toward fewer breast cancer deaths.

Some breast cancer risk is due to genetics. For instance, about 5 percent is due to rare mutations in just two genes called BRCA1 and BRCA2. But the variability of breast cancer incidence internationally and its overall increase over time suggest that environmental factors are primary causes. Among immigrant populations to the United States, for example, breast cancer incidence has increased markedly in just one or two generations. The gene pool does not change so quickly.

Environmental Exposures Are Not Limited to Pollutants
The public and breast cancer advocates may think of environmental toxins when they think of carcinogens. But to scientists, environmental influences include not only toxicants, but also diet and lifestyle.

Delayed childbearing is a lifestyle choice and a growing trend. Putting off having children ― or never bearing children ― is considered a “reproductive” risk factor. Reproductive risks increase a woman’s exposure to her own estrogen. Estrogen clearly is an important, beneficial hormone, but it also helps foster breast cancer. Breastfeeding infants helps lower the risk. Other reproductive risks that increase estrogen effects are early age of first menstruation and late menopause ― not regarded as modifiable.

In addition to genes, breast cancer risks a woman can’t control include increasing age and a family history of breast cancer.

Other known, modifiable breast cancer risks include being overweight after menopause and taking hormone replacement therapy. Alcohol consumption ― more than one drink per day ― also is a known risk factor for breast cancer. A woman can lower her risk for breast cancer by exercising. These modifiable risks all are considered environmental factors.

Nationwide Collaborators Will Push Forward
Still, known risk factors only account for perhaps half of breast cancers. UCSF and three other research institutions nationwide now are leading a new research consortium to identify additional environmental exposures that may increase breast cancer risk.

The Breast Cancer and Environment Research Centers (BCERCs) are jointly supported by the National Institute of Environmental Health Sciences and the National Cancer Institute. These centers are unique in joining breast cancer advocates with academic and other researchers. The other three BCERCs are led by the Fox Chase Cancer Center in Philadelphia, Michigan State University and the University of Cincinnati.

Epidemiological and laboratory research projects are coordinated and procedures are standardized across sites, so that the data may be more easily pooled or compared. Advocates play a key role at all sites and are led by the Marin County Breast Cancer Watch at the Bay Area BCERC. Advocates have been involved in designing the studies, and they ensure that communication ― input and feedback ― flows between researchers and members of the communities where the research is being conducted.

Girls Are a Focus in Search for Cancer Causes
The epidemiologic focus of the research is on girls who have not yet sexually matured. Mounting evidence indicates that there is a window of vulnerability for breast cancer in adolescence. Environmental exposures at the time of breast development can increase the risk of breast cancer decades later. This has been observed in women who received radiation therapy during childhood, in Japanese women who were exposed to radiation from an atomic bomb blast as girls and in laboratory mice exposed to chemical carcinogens prior to sexual maturation.

In the Bay Area, 400 7-year-old girls will be participating in the study. They are members of the Kaiser Permanente Medical Care Program whose Division of Research in Oakland is a major collaborator in the Bay Area BCERC. Their birth and past growth records are available from medical charts. The girls will receive regular checkups and be surveyed periodically about diet, exercise and other factors that may affect exposure to breast cancer risks.

Lab studies of mice, carried out by UCSF researchers and their collaborators at the Lawrence Berkeley National Laboratory, will help researchers explore biological mechanisms that might link environmental exposures to cancer, including exposures identified as being of concern by community members and advocates.

A large part of the search effort will focus on chemical exposures. Blood samples obtained from participating girls at the four BCERCs are being evaluated for industrial contaminants encountered in outdoor air, indoor air, water, food, food containers and cooking utensils. Researchers are also tracking hormone levels. This “biomonitoring” mirrors efforts begun in recent years by the Centers for Disease Control and Prevention, which aims to track chemical exposures in a representative sample of the US adult population.

“The BCERCs were created as a result of continuing public concern and lack of scientific evidence to convincingly confirm or rule out a role for environmental chemicals in breast cancer,” according to Robert Hiatt, MD, PhD, principal investigator for the Bay Area BCERC and director of population sciences for the UCSF Comprehensive Cancer Center. “The BCERCs are likely to contribute new discoveries because of their unique focus on the determinants of pubertal maturation, the interaction of laboratory and population science, and the role of community advocates in the scientific process.”