During the 1960s, the pesticide DDT was in widespread use around the globe. Though the chemical is now banned from use in the US and many other countries due to health concerns, a new study finds women who were exposed to higher levels of the chemical in utero more than 50 years ago may be at almost fourfold increased risk of breast cancer than women exposed to lower levels.
Study co-author Barbara A. Cohn, of the Public Health Institute in Berkeley, CA, and colleagues publish their findings in the Journal of Clinical Endocrinology and Metabolism.
More than 5 decades ago, DDT (dichlorodiphenyltrichloroethane) was widely used to prevent insects from destroying agricultural crops or spreading diseases, such as malaria and typhus.
While regions such as Africa and Asia continue to use DDT to reduce the spread of malaria, the pesticide was banned from use in the US in 1972, as well as in many other countries during the years following.
DDT bans came into effect after the chemical was identified as an endocrine disruptor – that is, it interferes with the function of the hormone estrogen. Past research has also linked prenatal exposure to the pesticide with developmental problems in childhood, greater risk of birth defects, reduced fertility and increased risk of high blood pressure in women.
According to Cohn, there has long been suspicion that exposure to environmental chemicals such as DDT may be also linked to breast cancer. “But until now,” she adds, “there have been few human studies to support this idea.”
As such, Cohn and her colleagues assessed the risk of breast cancer among 9,300 women in the US born between 1959 and 1967 – a period when DDT use was common in the country.
The women were born to mothers who were part of the Child Health and Development Studies (CHDS), and they were tracked for 54 years from when they were in their mother’s womb.
To identify levels of DDT exposure among daughters in utero, they assessed stored blood samples that were taken from their mothers during pregnancy or within a few days after birth.
Using state records and health questionnaires completed by the daughters during the 54-year follow-up, the team was able to identify how many daughters were diagnosed with breast cancer.
Study provides ‘direct evidence’ of link between prenatal DDT exposure and breast cancer
Breast cancer was identified in 118 daughters during follow-up, The blood samples of their mothers were assessed for DDT levels and compared with blood samples from the mothers of 354 daughters who were not diagnosed with breast cancer.
The team found daughters of mothers who had higher levels of o,p’-DDT in their blood samples – a form of commercial DDT known to be one of the strongest endocrine disruptors – were 3.7 times more likely to develop breast cancer, compared with daughters of mothers who had lower levels of this DDT in their blood.
This finding remained even after accounting for mother’s history of breast cancer, according to the researchers.
The team notes that 83% of the breast cancers identified in the study were estrogen-receptor positive – in which the breast cancer cells contain estrogen receptors that may receive signals from the hormone, encouraging their growth.
In addition, the study revealed that exposure to higher levels of o,p’-DDT in utero was associated with a greater likelihood of being diagnosed with a more advanced form of breast cancer.
The team also found that women with greater exposure to o,p’-DDT were more likely to develop HER2-positive breast cancer – in which breast cancer cells contain a mutation that makes excess human epidermal growth factor receptor 2 (HER2) protein. The researchers note that past studies have found DDT can activate HER2.
Commenting on their findings, Cohn says:
“This 54-year study is the first to provide direct evidence that chemical exposures for pregnant women may have lifelong consequences for their daughters’ breast cancer risk. This study calls for a new emphasis on finding and controlling environmental causes of breast cancer that operate in the womb.”
Cohn says their findings should also encourage further studies that may lead to prevention, early detection and treatment strategies for the many women with DDT-related breast cancer who were exposed to the pesticide in utero.
“We also are continuing to research other chemicals to see which may impact breast cancer risk among our study participants,” she adds.