Pregnant women with high levels of DDE, a metabolite of the insecticide DDT, in their blood are more likely to have children who develop autism, NIEHS grantees reported in the American Journal of Psychiatry. In contrast, they found no association between mothers’ exposure to polychlorinated biphenyls (PCBs) and autism development in their children. Lead author Alan Brown, M.D., is from the Columbia University Medical Center and the New York State Psychiatric Institute.

The study is the first to use maternal biomarkers during pregnancy to connect exposure to an insecticide with the risk for a clinical diagnosis of autism,” said Cindy Lawler, Ph.D., chief of the Genes, Environment, and Health Branch at NIEHS. “Along with genetic susceptibility, our environment is important in the risk for developing autism.”

CDC estimates that 1 in 59 children will develop autism. Researchers are now working toward understanding the environmental factors that contribute to the risk of a clinical diagnosis of autism. (Photo courtesy of Shutterstock)

Chemicals persist in the body

Lead study author Brown is a professor of epidemiology and of psychiatry at Columbia University. (Photo courtesy of Columbia University)

According to estimates from the U.S. Centers for Disease Control and Prevention (CDC), 1 in 59 children in the U.S. will develop autism. Researchers have begun to untangle the link between genetics and autism, but the link between environmental factors and autism is less clear.

Although banned for more than 30 years in many countries due to suspected health effects, chemicals such as DDT and PCBs still exist in the environment due to their slow breakdown and the way they accumulate in plants and animals in the food chain. According to the CDC, most of the U.S. population has detectable levels of DDE.

"We think of these chemicals in the past tense, relegated to a long-gone era of dangerous 20th Century toxins," Brown said in a press release from the university. "Unfortunately, they are still present in the environment and are in our blood and tissues. In pregnant women, they are passed along to the developing fetus."

A wealth of data provides answers

With funding from NIEHS, an international team of researchers wanted to see if and how exposure to certain chemicals might affect childhood development. They used national health registry data and blood samples available from the Finnish Prenatal Study of Autism and Autism Spectrum Disorders (FIPS-A).

The team identified 778 cases of autism among children born during an 18-year period to women who participated in FIPS-A. Those mother-child pairs were matched — based on place and date of birth, sex, and residence — to mother-child pairs without autism, and blood-serum samples were analyzed for PCB and DDE levels.

DDT still exists in the environment and can be absorbed into the body. During pregnancy, this chemical can pass to the developing fetus at very high levels. (Photo courtesy of Shutterstock)

Mothers with the highest levels of DDE in their blood were one-third more likely than women with lower DDE levels to have children who developed childhood autism. When the investigators looked at the subgroup of individuals who had both autism and intellectual disability, the risk from high levels of DDE was more than doubled. However, the researchers found no link between maternal blood levels of PCBs and autism.

Lawler oversees research on autism and other studies of how environmental exposures combine with genetic susceptibility to influence risk of complex human diseases. (Photo courtesy of Steve McCaw)

“This is a very strong study design,” Lawler said. “The sample size was large, and the investigators were able to avoid biases that can occur in other studies, when children are enrolled after an autism diagnosis and researchers have to rely on indirect methods to estimate exposures during pregnancy. In this case, access to early- and mid-pregnancy biospecimens meant that they were able to measure the environmental chemicals collected prospectively in these mothers.”

Finland has a national health delivery system, so the researchers were able to avoid the problem of the people who enroll in a study being somehow different than the general population, she explained. “The finding has to be replicated,” she said, “but for a single study, this is solid.”

Next steps — unraveling mechanisms

As with many studies, these findings put forth more questions. For example, the mechanism for how DDT affects the risk of autism is still unknown. The study authors suggest two possibilities.

DDT has been linked with low birth weight and premature birth, which are known risk factors for autism. DDT is known to bind to proteins in the body called androgen receptors, which allow cells to respond to hormones. Through this, DDT might alter how sex hormones affect brain development.

Lawler reflected on how these findings can drive public health action. “When we find that environmental risk factors, such as DDT exposure, can impact autism risk, this can translate into how to avoid that exposure or reduce it,” she said. “Nongenetic risk factors, like chemical exposures, can be preventable, so that puts us in a different mindset of how to manage risk.”

Citation: Brown AS, Cheslack-Postava K, Rantakokko P, Kiviranta H, Hinkka-Yli-Salomaki S, McKeague IW, Surcel H, Sourander A. 2018. Association of maternal insecticide levels with autism in offspring from a national birth cohort. Am J Psychiatry; doi:10.1176/appi.ajp.2018.17101129 [online 16 August].

(Sheena Scruggs, Ph.D., is a Digital Outreach Coordinator in the NIEHS Office of Communications and Public Liaison.)