1. Is autism on the rise?
2. Nature and
nurture, but not vaccines
3. The system-making mind
up the brain
Despite widespread rumors of a link between
the MMR vaccine and rising autism rates, numerous studies have
failed to show any relationship between the two. Photo: Royal
Navy of Australia
Autism is a strongly genetic disorder.
Parents understand the trials of raising an autistic child. AP
One way to study the genetics of a disorder
like autism is to take high-quality photographs of whole chromosomes,
and look for abnormalities. Diagram: NIH
You've probably seen headlines fretting over the possibility that the combined measles, mumps and rubella (MMR) vaccine could be behind rising autism rates. The culprit, the stories go, is the preservative thimerosol, whose primary ingredient is mercury. Could mercury, infamous for damaging nerve cells, be at fault? Or could the live (but inactivated) virus in vaccines be the culprit? The scientific consensus -- which rang clear this year -- is a resounding no. To date, no study has shown a link between the MMR vaccine and autism (except the report that sparked the worry -- which was later retracted by the medical journal that published it and 10 of its 12 authors).
"We published a negative study in February on the relationship between the MMR vaccine and autism," says Yeargin-Allsopp at the CDC. "There was no relationship. No assocation." Still, measles vaccination rates have fallen in the U.S. and in the U.K. since the controversy flared, and measles has begun to creep up (see "Autism and pervasive developmental disorders" in the bibliography).
But to be sure, some environmental factors play a role. If one identical twin has an autism spectrum disorder, most of the time (estimates range from 60-90 percent) the other twin will, too. These numbers show that genes play a major role. But something else is happening ten percent of the time -- in the womb or after birth -- that allows one autistic twin and one non-autistic twin.
In a minority of people the cause of autism may be chance damage to the growing brain, says Rapin. But in others, whether an infant who inherited a genetic predisposition to autism does or does not develop autism depends on environmental influences that come early in life -- even before birth.
Rapin explains: "Suppose you inherit the tendency to have hay fever when you smell roses. But you live in Iceland, where roses don't grow. You're never going to have hay fever due to roses. It may be that some of these differences in severity in identical twins are due to non-genetic influences on brain development. The vaccine theory doesn't cut the mustard because identical twins get the same vaccine."
In some cases, doctors know what led to an autism birth. After the rubella epidemic of 1964, for example, there was a wave of autism in the U.S. -- some pregnant women who contracted rubella had children with the disorder. Exposure to the notorious morning sickness drug thalidomide has been linked with autism. Other evidence points to prenatal testosterone levels -- the more testosterone present in the womb, the more likely the child is to have an autism spectrum disorder.
Twin studies show that genetics plays a major role in autism. In siblings, including fraternal twins, that correlation is less than ten percent. And most autistic people are male (the male to female ratio is three to one).
But for sure, says Rapin, a single "autism gene" will never be found.
"What counts is not what made the brain abnormal, but what parts of the brain are abnormal," says Rapin. "No gene accounts for behavior. Genes account for proteins that make the brain. If the brain doesn't develop in the right way, the consequence could be autism."
Scientists employ three main approaches to study the genetics of autism:
Working forward. Taking populations of families
in which more than one person has an autism spectrum disorder, researchers
can look for "genetic markers," stretches
of DNA shared by many of the people. When they find these markers, more work
goes into finding specific genes -- and more to find the proteins the genes
Working backward. Scientists have described a number
of genes that affect brain development. They know certain parts of the brain
are affected in autism. Putting
the data together can reveal "candidate genes" -- genes that work in parts of
the brain that work differently in people with autism.
Working from a bird's eye. Disorders like Down's syndrome result from an extra
chromosome, but others involve smaller changes. Using microscopes, researchers
can inspect the chromosomes of people with autism for abnormalities. Doing
so can reveal abnormal areas of individual chromosomes, giving researchers
a target to inspect more closely. In 2002, researchers at Vanderbilt University discovered a cluster of abnormalities on chromosome 15 in some families of people with autism this way. Such anomalies probably account for a small proportion of autism cases.
So far, genetics research has confirmed what twin studies showed: that autism is a complex genetic disorder involving several, possibly dozens, of genes. There will probably never be a genetic test for all cases of autism and doctors will never be able to give children new brains, Rapin says. But the more that is known about the genes that control the brain's circuits, the better the foundation is for learning what causes -- and how to compensate for -- the deficits in autism.
Is this the extreme male brain?