Links Between Environmental Exposures and Autism Explored
PALO ALTO, California, July 5, 2011 (ENS) – The largest and most rigorous twin study of its kind to date has found that shared environment influences susceptibility to autism more than previously thought.
The study, supported by the National Institutes of Health and released today, found that shared environmental factors – experiences and exposures common to both twin individuals – accounted for 55 percent of strict autism and 58 percent of more broadly defined autism spectrum disorders.
Genetic heritability accounted for 37 percent of autism and 38 percent of autism spectrum disorders. Random environmental factors not shared among twins play a much smaller role.
Studies now are under way to determine if autism may be traceable, in part, to environmental exposures early during pregnancy.
Earlier twin studies had estimated the genetic heritability of autism to be as high as 90 percent. The new study found such concordance to be four to five times higher.
Twin boys (Photo by Faye)
“High fraternal twin concordance relative to identical twin concordance underscores the importance of both the environment and moderate genetic heritability in predisposing for autism,” explained Joachim Hallmayer, MD, of Stanford University, Palo Alto, California, a grantee of the NIH’s National Institute of Mental Health.
“Both types of twin pairs are more often concordant than what would be expected from the frequency of autism in the general population,” said Hallmayer. “However, the high concordance among individuals who share only half their genes relative to those who share all of their genes implies a bigger role for shared environmental factors.”
Hallmayer, senior co-investigator Neil Risch, Ph.D., of the University of California, San Francisco, and colleagues, report on findings of the California Autism Twins Study in the July 2011 issue of the journal “Archives of General Psychiatry.”
“These new findings are in line with other recent observations supporting both environmental and genetic contributions to autism spectrum disorders, with the environmental factors likely prenatal and the genetic factors highly complex and sometimes not inherited,” said National Institute of Mental Health Director Thomas Insel, MD.
The new study is the first to analyze a large sample of twins drawn from the general population; previous twin studies have been based on more limited samples, such as patients in treatment.
It is also the first to employ the latest standard in diagnosing autism, which requires structured clinical assessments based on interviews with the parents as well as direct observation of the child.
Drawing upon state records, the researchers initially identified 1,156 twin pairs, with at least one member affected by an autism spectrum disorder, born to California mothers between 1987 and 2004.
The children were all at least four years old, an age when autism can be reliably diagnosed. Ultimately, this group was winnowed to 192 twin pairs – 54 identical and 138 fraternal – for genetic analysis. Since autism disproportionately affects males, males outnumbered females by four to five times, with 80 of the pairs including both sexes.
Concordance for autism spectrum disorders was 77 percent among identical male pairs, and 31 percent among fraternal male pairs. In females, concordance for ASD was more closely spaced – 50 percent for identical and 36 percent for fraternal pairs.
By contrast, previous studies had found concordance rates for fraternal twins that were much lower, ranging only in the single digits.
“Spectrum disorders traditionally thought to have less genetic loading turn out to stem from a similar mix of environmental and genetic heritability as narrowly defined autism,” noted Thomas Lehner, PhD, chief of the NIMH Genomics Research Branch.
Yet, there can also be genetic influences that are not inherited from parents. New evidence emerged last month that rare, spontaneous mutations occur at abnormally high rates in autism.
Parents of autistic children and environmental health and autism experts called June 8 or reform of the outdated U.S. law regulating chemicals, the Toxic Substances Control Act of 1976.
They warned that the recent sharp rise in autism is likely due, in part, to the cocktail of toxic chemicals that pregnant women, fetuses, babies and young children encounter.
“Lead, mercury, and other neurotoxic chemicals have a profound effect on the developing brain at levels that were once thought to be safe. With some complex combination of insults, little brains reach a tipping point,” warned Donna Ferullo, director of program research at The Autism Society, told reporters on a conference call convened by the Safer Chemicals, Healthy Families Coalition.
The nationwide coalition represents more than 11 million people, including parents, health professionals, advocates for people with learning and developmental disabilities, reproductive health advocates, environmentalists and businesses.
Today in the United States, about one in every 110 children has autism, a disorder of neural development characterized by abnormalities of social interactions and communication, severely restricted interests and highly repetitive behavior. Boys are affected more than girls – one in every 70 boys will have autism.
Also participating in the research were investigators at: Autism Genetic Resource Exchange; California Department of Public Health; Kaiser Permanente; University of California, Davis. The research was also funded by Autism Speaks.