What's the Link Between Genetics and Autism?

At least 83% of autism caused by inherited genes

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Researchers have always believed that genetics plays an important role in autism, but many were convinced that a huge increase in autism diagnoses was caused by environmental issues. Recent research suggests that genetics may be responsible for as many as 90 percent of cases of autism, with environmental issues playing a much smaller role.

What Do Autism Researchers Mean By 'Genetics'?

According to the National Institutes of Health: "A gene is the basic physical and functional unit of heredity.

Genes, which are made up of DNA, act as instructions to make molecules called proteins. In humans, genes vary in size from a few hundred DNA bases to more than two million bases. The Human Genome Project has estimated that humans have between 20,000 and 25,000 genes." Human genes are almost identical from person to person. In fact, only about 1 percent of our DNA defines how one person differs from another. 

Genes have a profound impact on our physical and mental status. But while genes are inherited from our parents, not all genetic differences are heritable. That's because genetic changes (called mutations) may occur in a single individual, having nothing to do with inheritance. Mutations can occur spontaneously (without any known cause) or as a result of an environmental exposure. 

When autism researchers look at genetics, they may be exploring one of several different questions. Among them:

  • To what degree is autism inherited from parents?
  • To what degree is autism caused by spontaneous changes in genes that are not inherited?
  • Which specific genes or sets of genes determine whether a person is autistic?
  • What kinds of changes to individual genes would suggest autism?
  • How is autism related to known genetic disorders such as Fragile X disease?
  • Are different genes responsible for different types of autism?
  • Are there environmental impacts that cause genetic changes that lead to autism?

What Do We Know About Autism and Genetics?

With very few exceptions, researchers have been unable to answer questions about autism and genetics with any certainty. We don't know, for example, exactly which combinations of genetic changes are likely to cause autism. We don't know whether different genetic changes lead to high or low functioning autism. We don't know whether it is possible to change the likelihood of inheriting autism. We don't know whether genetic therapy could have a positive impact on people with autism.

Here, however, is some of what we do know, according to the NIH:

  • ASD has a tendency to run in families, but the inheritance pattern is usually unknown. People with gene changes associated with ASD generally inherit an increased risk of developing the condition, rather than the condition itself. 
  • Changes in over 1,000 genes have been reported to be associated with ASD, but a large number of these associations have not been confirmed. Most of the gene variations have only a small effect, and variations in many genes can combine with environmental risk factors, such as parental age, birth complications, and others that have not been identified, to determine an individual's risk of developing this complex condition. 
  • In about 2 to 4 percent of people with ASD, rare gene mutations or chromosome abnormalities are thought to be the cause of the condition, often as a feature of syndromes that also involve additional signs and symptoms affecting various parts of the body.
  • According to a recent study, as many as 2500 different genes could be associated with autism. This enormous number was uncovered through new technology, and means that studying autism has become increasingly complex.

Genetics and the Environment 

There is no doubt that environmental factors interact with genetics to cause various types of autism.

But recent studies make it clear that the environmental factors are, in general, both subtle and complex. According to the National Institutes of Environmental Health Sciences, certain environmental exposures may increase the risk of autism, but they are not known to actually cause autism. They include:

  • Advanced parental age at time of conception
  • Prenatal exposure to air pollution
  • Maternal obesity or diabetes
  • Extreme prematurity and very low birth weight
  • Any birth difficulty leading to periods of prenatal oxygen deprivation to the baby’s brain
  • Prenatal exposure to certain pesticides
  • Prenatal exposure to Valproate or Thalidomide
  • Lack of prenatal nutrition

How might any of these exposures impact genetics? The answers are not yet known, though research is ongoing. We do know that none of these exposures is a "recipe" for autism; many children are born of older parents, or premature, or in polluted areas who are not autistic. This suggests that some children who are at genetic risk of autism developed the disorder after a particular environmental exposure.

What's More Important: Genetics or Environment?

2017 studies explored the question of whether inherited genetics or environment are more significant causes of autism. Overwhelming, the evidence points to genetics. In fact, according to one study:

Studies have found that autism spectrum disorder (ASD) aggregates in families, and twin studies estimate the proportion of the phenotype variance due to genetic factors (heritability) to be about 90 percent.

In a previous study, ASD heritability was estimated to be 0.50, and shared familial environmental influences to be 0.04. To define presence or absence of ASD, the study used a data set created to take into account time-to-event effects in the data, which may have reduced the heritability estimates.

Another study which reanalyzed a group of children in Sweden from 1982 through 2006 including twins, siblings, and half-siblings found that "the incidence of ‘inherited’ autism was about 83 percent, whereas the non-shared environmental influence was estimated at 17 percent."

In other words, if these studies are correct, the vast majority of autism is inherited. This finding has significant implications for families with multiple autistic individuals, and may be important in discovering therapies likely to prevent or treat autism.

A Word From Verywell

What does the research mean for parents? While it doesn't provide a great deal of actionable information, it does make it clear that environmental factors play a minor role in autism. That means that parents need not worry that ordinary life choices or behaviors were responsible for their child's disorder. And that means that parents can be emotionally free to focus, not on their child's prenatal past, but rather on their future.

Sources:

Brooks, Megan. Genetic factors account for the majority of autism risk. Medscape. September 27, 2017. https://www.medscape.com/viewarticle/886250

Krishnan, A. et al, Genome-wide prediction and functional characterization of the genetic basis of autism spectrum disorder. Nature Neuroscience, 2016; DOI: 10.1038/nn.4353National Institute of Environmental Health Sciences. Autism. Web, 2017. https://www.niehs.nih.gov/health/topics/conditions/autism/index.cfm

Sandin S, Lichtenstein P, Kuja-Halkola R, Hultman C, Larsson H, Reichenberg A. The heritability of autism spectrum disorder. JAMA. 2017;318(12):1182–1184. doi:10.1001/jama.2017.12141

Science News. Autism genes identified using new approach. August 1, 2016. https://www.sciencedaily.com/releases/2016/08/160801113827.htm

U.S. National Library of Medicine. Autism. Web, 2017. https://ghr.nlm.nih.gov/condition/autism-spectrum-disorder#diagnosis

Zayed​, A. . Confirmed: Heredity Is The Main Cause Of Autism. Consumer Health Digest. Web. 2017. https://www.consumerhealthdigest.com/health-news/genetics-increase-autism-risk.html​

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