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Environment may play key role in autism

5th July 2011

Experts tend to see autism spectrum disorders (ASDs) as being inherited, rather than depending on the environment a child grows up in.

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However, environment may play a much larger role than researchers previously thought, according to a recent US study.

In particular, environment may determine the way genes usually seen in connection with autism activate themselves.

The researchers studied a group of twins in which at least one child had an autism spectrum disorder.

Other researchers published a related study the same day, in which they found higher numbers of autistic children born to mothers who took antidepressants during pregnancy.

Lead author Joachim Hallmayer of Stanford University in Palo Alto, California, said that his was the first study to consider a large sample of twins drawn from a general population.

For the study, the researchers made use of a California registry of children who receive disability services, allowing them to identify 54 pairs of identical twins in which one or both of the children had an ASD.

The remaining 138 pairs of twins were all fraternal twins, with half of their genes in common.

In less than half of the identical twin pairs, both twins had an ASD.

The frequency of identical twin pairs in which both had ASD was almost exactly the same for both male and female pairs of twins, at 42.5% for male twins and 43% for female twins.

There was a bigger difference in the percentages for female-female and male-male fraternal twins, with 20% of all female-female fraternal twin pairs both having autism, and just under 13% for all male-male fraternal twin pairs.

Hallmayer said that the fact that identical twins shared the same genes made it much more likely that both children would have some form of autism.

At the same time, if having some form of autism completely depended on genetic factors, the researchers would have expected 100% of the identical twin pairs to both have it.

The researchers said that having a shared environment should explain the prevalence of ASDs in identical and fraternal twin pairs.

In other words, genes probably account for somewhere around 37% of the risk of severe autism and about 38% of the risk of ASDs in general.

As such, the researchers calculate that environmental factors probably account for about 55% of the risk of severe autism and 58% of the risk of having an ASD.

In earlier, smaller studied, genetics seemed to account for about 90% of the development of autism and related ASDs.

The authors of the present study explain that discrepancy based on the fact that they gave a clinical assessment to every child under study, whereas other findings relied on pre-existing diagnoses.

But some experts were not convinced of the usefulness of the finding.

Gary Goldstein, president and CEO of the Kennedy Krieger Institute in Baltimore, said he believed scientists needed to study the interaction of environmental and genetic factors in more detail.

He said there was also a wider-than-usual range of uncertainty (known as a confidence interval) in the researchers' statistical data.

Previous research has suggested that advanced maternal or paternal age, assisted reproductive technology and artificial insemination, maternal infections during pregnancy, and premature birth could all play a role in the development of ASD.

According to US government estimates, one in 100 children have some form of autism spectrum disorder.

However, a recent study of Korean children showed that the number of parents who require the help of an ASD specialist may not be the best indicator of the prevalence of ASD in populations.

In that study, the researchers found that about one third of all the children studied had some form of ASD.

 

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