By guest blogger and Rodale.com advisor Philip J. Landrigan, MD
Dr. Landrigan has been a member of the faculty of Mount Sinai School of Medicine in New York City since 1985, and Chair of the Department of Preventive Medicine since 1990. He was named Dean for Global Health in 2010. Dr. Landrigan is also the director of the Children’s Environmental Health Center.
Each year, biologically based disorders of brain development—autism, attention deficient/hyperactivity disorder (ADHD), mental retardation, dyslexia, and subclinical neurodevelopmental disabilities—affect between 200,000 and 400,000 of the 4 million babies born in the United States. That’s between 5 and 10 percent of all American children.
There is great concern about the rising rates of autism spectrum disorder (ASD). The Centers for Disease Control and Prevention (CDC) report that the prevalence of ASD increased by 57 percent between 2002 and 2006, and is now affecting one of every 110 babies born in the U.S.
Beyond Genetic Research
Until recently, most research into the causes of ASD had focused on genetic factors. This elegant research has linked a series of genetic factors to autism—including specific gene mutations, recurrent chromosomal microduplications, microdeletions, and copy number variations.
However, none of these genetic factors accounts for more than 2 to 3 percent of cases of autism. And taken together, the identified genetic causes account for no more than 20 to 25 percent of cases. Moreover, genetic inheritance alone fails to account for important clinical and epidemiological features of autism. For example, some families have children who exhibit fully developed cases of autism, side by side with other family members who manifest only certain “autistic traits.” Sporadic cases of autism also occur in families who have no prior history of ASD. Ultimately, a purely genetic theory has difficulty explaining discordant development of autism in identical twins, variations in expression within families, and the rise in incidence of autism.
The shortcomings of genetic research—coupled with emerging research on the developing brain’s vulnerability to external exposures—suggest that environmental factors also contribute to autism and learning disabilities. In many instances, it is quite possible that environmental exposures interact with individually inherited genetic susceptibilities. In fact, the U.S. National Academy of Sciences has reported that 28 percent of developmental disabilities in children may be caused by environmental exposures, concluding that environmental factors are solely responsible for causation in 3 percent of cases, and that they act in concert with genetic factors in 25 percent of cases.
The Case for an Environmental Contribution
We know that the brains of infants and children are extremely sensitive to toxic exposures in the environment. New research has identified “critical windows of vulnerability” in fetal life and early childhood when exposures to toxic chemicals can cause devastating injury to the brain and nervous system. Research has also found that toxic chemicals can damage the developing brains of infants and children at extremely low doses. Thus, there are no safe exposure “thresholds” in early brain development, as even low-level chemical exposures have been shown to cause lost intelligence, shortened attention, and disrupted behavior.
We are now learning that these chemical exposures can damage the brains of infants and children to cause autism and learning disabilities. For example, lead, methylmercury, PCBs, manganese, organophosphate pesticides, DDT, and ethyl alcohol have all been identified as causes of childhood brain damage.
Of grave concern is that this list of chemicals may be only the tip of a much larger problem. Currently, 200 synthetic chemicals are identified as neurotoxic in adult humans. Another 1,000 chemicals have been identified as neurotoxic in experimental models. Likelihood is high that among these 1,200 neurotoxic chemicals, there are some with the potential to cause childhood brain injury that could result in autism and learning disabilities. As these diseases continue to rise, there is a dire need to investigate which chemicals contribute to the causes of autism and learning disabilities. Now is the time to take action in discovering the environmental contribution.
Exploring the Environmental Causes of Autism and Learning Disabilities
To address these research questions, the Children’s Environmental Health Center (CEHC), which I direct at the Mount Sinai School of Medicine, convened at an all-day workshop on December 8, 2010. This meeting, titled Exploring the Environmental Causes of Autism and Learning Disabilities, was cosponsored by advocacy leaders Autism Speaks, and was held at the New York Academy of Medicine in New York City.
By bringing together international autism-research leaders, the workshop allowed us to review current research and create a dialogue about the environmental contribution to autism and learning disabilities. Ultimately, the conclusions from this workshop will help refine our future research strategy, guiding our understanding of which chemicals to investigate further as potential environmental causes of autism and learning disabilities.
Currently, we believe that autism may result from a combination of genetic susceptibility and external exposure: “the wrong chemical at the wrong time.” At the workshop, we identified several environmental exposures that have been linked to ASD:
- Thalidomide: A medication taken by women in early pregnancy during the 1950s.
- Misoprostol: A medication used in other parts of the world to induce abortion in the first trimester.
- Prenatal rubella infection: Timing of the infection during pregnancy determines the severity of neurodevelopmental effects.
- Valproic acid: A medication used to control epilepsy.
- Organophosphate insecticide chlorpyrifos: A pesticide once widely used in the U.S., it is now banned and used only in agricultural settings.
While these chemicals don’t account for the current rises in autism, as none—except the organophosphate pesticides—are commonly used in the United States, their proven links to autism establish proof of principle for an environmental contribution.
Read more about the workshop, and the research presented there, in part 2 of Dr. Landrigan’s post, coming soon.