WASHINGTON, DC -- A seven year study of breast cancer clusters on Long Island has found no link between the disease and exposure to chemical pollutants known as organochlorine compounds. But a separate study found that high exposure to pollutants called polycyclic aromatic hydrocarbons was linked to a modest increase in risk of developing breast cancer.
The results were announced today in three separate papers stemming from the Long Island Breast Cancer Study Project, one of the largest and most comprehensive environmental epidemiologic studies ever undertaken to explore the environmental factors that may trigger breast cancer.
Exhaust from diesel buses and other vehicles contains polycyclic aromatic hydrocarbons (PAHs), which some studies have linked to increased risk of certain cancers. Photo courtesy EPA The researchers found no increased rate of breast cancer among area women who might have been exposed to organochlorine compounds such as the pesticide DDT. However, high levels of exposure to polycyclic aromatic hydrocarbons (PAHs), found in cigarette smoke, vehicle exhaust and certain foods, appears to elevate women's risk of breast cancer by 50 percent in New York's Suffolk and Nassau counties.
"The goal of this population based, case control study was to determine whether breast cancer incidence in women in these two counties was associated with exposures to environmental contaminants," said principal investigator Dr. Marilie Gammon, associate professor of epidemiology at the University of North Carolina at Chapel Hill School of Public Health. "What we observed did not support that possibility strongly."
Teasing Out Cancer Causes
The Long Island study was ordered by Congress in 1993 in response to reports of elevated breast cancer deaths in a number of northeastern states. Though many of the cases can be traced back to known risk factors, such as a family history of breast cancer, or having a first child at a later age, the reasons for the remaining cases are unknown.
Congress ordered the National Cancer Institute to examine "potential environmental and other risks contributing to the incidence of breast cancer" in Nassau, Suffolk and Schoharie counties in New York and in Tolland County, Connecticut. The project now includes more than 10 studies exploring different environmental factors, along with laboratory research designed to help explain the development of breast cancer.
Among the possible triggers for breast cancer under study are exposure to contaminated drinking water, sources of indoor and outdoor air pollution, including emissions from aircraft, electromagnetic fields, pesticides and other toxic chemicals, and hazardous and municipal wastes.
High PAH Exposure Increases Cancer Risk
PAHs were examined because these compounds are known to cause breast cancer in rodents, and a few small human studies have suggested a possible association between these chemicals and increased risk of breast cancer. Women can be exposed to PAHs by breathing in cigarette smoke, and exhaust from cars, trucks and planes, and by eating grilled and smoked foods.
Starting in 1996, Gammon and her colleagues collected blood samples from 1,508 women in Nassau and Suffolk counties who were newly diagnosed with breast cancer, and a similar number of women who did not have cancer. The blood samples were checked for direct evidence of exposure to organochlorine compounds and PAHs, both of which leave traces in the body for months or years.
Study participants also completed a questionnaire, and at some of the women's homes, researchers collected samples of house dust, tap water and yard soil to test for organochlorine compounds and PAHs.
Exposure to PAHs causes the chemicals attach to DNA, creating compounds known as adducts. Women with the highest levels of PAH adducts in their blood had a 50 percent greater chance of developing breast cancer, the study found.
Compared to other known risk factors for cancer, a 50 percent increase in risk is considered modest, the researchers note. For example, smoking increases the risk of developing lung cancer by 900 percent to 1,000 percent, while a family history of breast cancer increases risk by 100 percent to 200 percent.
In addition, only women with the highest adduct levels showed an increased risk of cancer - lower levels, even levels higher than average, were not associated with higher risk.
"These ambiguous findings shed some doubt on a clear cause and effect association," Gammon said. "The study team is continuing to investigate this issue on Long Island through additional research focused on the possibility of individual responses to environmental exposures."
For example, some women may have stronger responses to PAHs because of their higher levels of the hormone estrogen. Estrogen boosts cell turnover, making it harder for the body to repair damaged cells before they replicate and pass the damage to new cells. Breast cancer victims are known to have more estrogen in their blood on average than other women.
"We know too, for example, that if a woman's ovaries, which produce estrogen, are removed before she is 35, her risk of breast cancer drops by half," Gammon said. "Men, who have little estrogen, rarely get breast cancer."
The increase in breast cancer risk associated with PAH was restricted to women with breast tumors that were either estrogen receptor and progesterone receptor positive or who were negative for both, she said. This means the link was stronger for women with cancers that are considered hormone sensitive, but the reason for the link is unclear.
"Our findings with polycyclic aromatic hydrocarbons suggest that women's individual responses to similar PAH exposures might be more relevant to breast cancer development than the absolute amount of PAH exposure," Gammon said. "A lot more work needs to be done to sort out exactly what and how environmental exposures may promote breast cancer."
No Link Between Organochlorines and Breast Cancer
The study focused on organochlorine compounds because smaller studies had suggested that the pesticide DDT - an organochlorine compound that has been banned in the United States since 1972 - might be associated with increased risk for breast cancer.
The researchers looked at DDT and its metabolite DDE, chlordane, dieldrin and polychlorinated biphenyls (PCBs). PCBs are a family of chemicals used in transformers, capacitators and other electrical equipment. They are no longer used in new equipment, but because they break down so slowly, many areas still have high levels of PCBs in the environment.
The study found no evidence that organochlorine compounds are associated with the elevated rates of breast cancer on Long Island. However, the researchers say that it is possible that breast cancer risk in some individuals may be associated with organochlorine exposures because of individual differences in metabolism and ability to repair DNA damage.
"Recent research by other investigators suggests that organochlorine compounds may be related to the type of breast cancer that has clinical characteristics that are associated with worse survival. This is an important issue that we are continuing to investigate among the women in our study," Gammon said.
Two previous, hospital based studies conducted through the Long Island Breast Cancer Study Project also found no link between exposure to organochlorine compounds and increased risk of breast cancer.
More Work Ahead
While the current study did not identify specific environmental factors as a cause for breast cancer on Long Island, it does provide clues to help direct future studies. For example, Dr. Habibul Ahsan of Columbia University in New York and colleagues are now examining whether certain common DNA variations, involved in the biosynthesis and metabolism of estrogen and environmental carcinogens, are associated with risk for breast cancer.
Gammon and Dr. Regina Santella of Columbia University are examining the role of gene variants that influence the oxidative stress of environmental contaminants. Oxidative stress is a term used to describe a type of cellular damage caused by the metabolism of oxygen.
Meanwhile, Gammon and her colleagues are continuing to follow the women who participated in the current study to determine whether organochlorine compounds, PAHs and lifestyle factors influence the survival of Long Island women diagnosed with breast cancer. Other studies in the Long Island Breast Cancer Study Project are looking at possible links between breast cancer and electromagnetic fields, hazardous waste sites, chemicals in drinking water and other potential risk factors.
The current research, reported today in two papers in the journal "Cancer Epidemiology, Biomarkers & Prevention," is also available at: http://cancer.gov/cancerinfo/LIBCSP
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