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Thursday, December 15, 2011

Autism Science Update: Genes Versus Environment

By Roy Benaroch, MD

Autism is now thought to affect between 1 in 100 to in 1 in 150 children. There’s a broad spectrum of manifestations—in the most severe form, there is no verbal communication and very limited social interactions. The majority of cases, though, are in the mild to moderate range, and those kids gave a good prognosis when therapy is started early. Though there is no single “cure,” we do have effective therapy that can really help.

It’s now clear that there isn’t a single cause of autism—just like there isn’t a single cause of cancer or heart disease. As with many complex, real-world disorders, both environmental and genetic causes play a role. Some recent studies are helping us understand the complex interrelationship between these causes.

A study of 192 twin pairs published in November, 2011 tried to isolate environmental versus genetic causes by comparing identical to fraternal twins. Identical twins have the same genes; fraternal twins have the same parents, but are not any more similar genetically than any other pair of siblings. Using fancy math, these researchers confirmed that there are both genetic and environmental causes—and that the influence of both was about in the 50-50% to 60-40% range. However, twin pairs share identical environments before they’re born, which makes it difficult for studies like this to separate genetic from environmental factors that occur before birth.

Which brings us to another study, this one based on autopsies that studied the brains of children with autism versus matched neurotypical kids. Researchers looked closely at brain structure and found that children with autism had far more neurons packed into certain areas of the brain than children who are unaffected. This difference in fundamental brain architecture had to arise very early, probably within the first one or two trimesters of pregnancy—that’s when neuronal development and cell growth takes place. If confirmed, this study supports the idea that very, very early influences are critical, and that children who are destined to have autism may well already have the disorder at birth, even if manifestations aren’t seen yet.

There are many potential influences on brain development that occur early in pregnancy, and many factors to consider: stress, infections, nutrition, genetics, and other exposures. An example of one such environmental trigger was revealed in another recent study, which showed an increase in autism among children whose mothers took antidepressant medications early during their pregnancies. While medication exposures couldn’t account for all cases, they could certainly be one of many potential exposures that may interact with genetic factors to cause autism.

There is still a lot to learn, and many parents are understandably frustrated by the slow pace of science. A “magic bullet”—one cure that works!—would be wonderful, but it’s seeming very unlikely that there is going to be a single cause or a single cure. Meanwhile, parents and doctors should continue to concentrate their efforts on the most effective current strategy: getting these kids identified and into effective therapy early.

Posted by: Roy Benaroch, MD, FAAP at 10:57 am

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