Environment may play a bigger role in autism than genetics. Photo courtesy of SeRVe Photography

Autism has long been considered among the most heritable of developmental brain disorders. But a new study of autistic twins indicates that shared environment has a larger impact on the development of autism and autism spectrum disorders than previously thought, and is in fact more important than genetics.

Researchers from Stanford and UC San Francisco examined the prevalence of autism among both identical and non-identical twin pairs. The frequency of identical twins both being diagnosed with autism was lower than would be predicted from statistical models of heritability. The scientists estimate that environmental factors common to twins explains 55% of susceptibility to autism, whereas genetics accounts for only 37%.

Another study published in the same issue of Archives of General Psychiatry suggests that the use of antidepressants by mothers doubles the risk of a child developing autism. Other environmental factors that could be important in autism risk include diet, pollution, parental age, birthweight and maternal infections during pregnancy.

Scientists studied twins like these to figure out that genetics accounts for about 37% of autism. Image courtesy of Jeff Balke.

Scientists have known that autism is a combination of genes and the environment for a long time. But the focus has been on genes because early twin studies suggested that autism was mostly genetic.

Scientists looked at sets of identical and fraternal twins to see how often both twins in a pair had autism. Remember, identical twins have the exact same DNA whereas fraternal twins only share as much DNA as any other siblings.

If autism were purely genetic, then both identical twins in a pair should either have it or not. It should be very rare for one twin in a pair to have autism and the other to not have autism. Fraternal twins should have it at about the same rate as any other siblings.

But if autism were purely environmental, then both twins in an identical or fraternal twin pair should get it at about the same rate. Depending on what part of the environment is causing the problem, this rate might be higher than that of siblings.

A study back in the 1970’s found that both twins in an identical pair had autism 72% of the time and that both fraternal twins never had it at the same time. This is where the 90% heritability for autism number came from.

The fact that fraternal twins never both had autism was weird from the start. Scientists knew that if one sibling had autism, the risk for the other siblings was anywhere from 3-14% which is higher than the general risk. Fraternal twins are siblings and so there should probably be some increased risk too.

In a new study, scientists did a more extensive study on 192 twin pairs and arrived at very different results. In this study, male identical twins both had autism 58% of the time and male fraternal twins both had autism 21% of the time. (Female numbers were similar.) These numbers suggest that genetics accounts for about 37% of autism. Still significant but nowhere near 90%!

If this study holds up, it means is that scientists can start looking at environmental effects. They’ve ruled out vaccines as a cause but there are lots of other possibilities. And many of these may happen before the child is even born.

For example, it may be that like Down syndrome or schizophrenia, parents’ age is a factor. Or it may be that diseases mom might have had or chemicals she might have been exposed to while pregnant could increase chances for autism. Or a host of other possibilities might be responsible.

What is important to keep in mind is that if scientists can identify an environmental cause, they can try to keep expectant mothers away. Or try to ameliorate the effects. In many cases, this will be much easier to deal with than genes.

For more, read

Sometimes autism that looks environmental can be genetic from Undestanding Genetics.

DNA magnified 850,000 times through a scanning electron
microscope

This should be a chance to pull DNA out of beef, strawberries, kumquats or even yourself and learn that you have around 100 billion miles* of DNA inside of you. In case you're interested, that'll reach from the Earth to Pluto and back when Pluto is farthest from Earth. And that is one person's DNA.

Add up everyone's DNA in the world and you get 125 million light years of DNA. (At least I think you do… these numbers are getting ridiculous!) That'll get us to the galaxy Andromeda and back 25 times. Add up all the DNA on Earth and… OK, that's probably enough of that.

There isn't just a lot of the stuff but it is amazing to me how similar all human DNA is. The latest estimates are that people are around 99.5% the same at the DNA level. That means that all those light years of DNA are mostly the same old thing just copied over and over.

Notice the mostly. With 6 billion letters of code in every person, a 0.5% difference means 30 million differences between you and me. It is these differences that make me look different than you. And to a varying degree, make me act differently than you.

This code doesn't work in a vacuum either. The environment can change how it works which is a big reason identical twins aren't really identical. And one of the reasons why it is so hard to figure out the genetics of complicated diseases like diabetes or heart disease.

Our DNA also changes with time. Things in the environment might damage it. Or our own cells can make mistakes when they make copies of themselves. What this means is that today's light years of human DNA will be different than the same stretched out DNA in 100 years.

This also means that you have some cells in your body have different DNA than the rest of your cells. And if a DNA change happens in sperm or egg cells, then they are passed on to the next generation. Which is where all the wonderful diversity around us originally came from.

As you can see, there is a lot about DNA to celebrate. It is huge and mysterious and we're just starting to get a good grasp on what it is all about.

I plan to spend the morning of DNA day at The Tech Museum in San Jose exciting kids (and hopefully some adults) about DNA by running five different hands on genetics programs all at once. It'll be a blast!

I have searched high and low for a list of DNA day activities here in the bay area but I haven't come across any. Does anyone know about other DNA day celebrations here in the bay area?

* Each cell has 6 feet of DNA and we are made up of around 50-100 trillion cells.

Dr. Barry Starr is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA.

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