Autism spectrum disorder (ASD) refers to a range of conditions characterized by challenges with social skills, repetitive behaviors, speech and nonverbal communication. Autism affects an estimated 1 in 44 children in the United States today.
What Causes Autism?
The exact causes of ASD are not fully understood, but research suggests autism likely arises from a combination of genetic and environmental factors that influence early brain development. There are probably hundreds of autism-risk genes. In about 10% of cases, autism is associated with a known genetic condition, such as fragile X syndrome or tuberous sclerosis. In other cases, autism may result from spontaneous mutations in genes that occur during early fetal development.
Key Points on Autism Causes:
- Autism has a strong genetic component and tends to run in families.
- No single gene has been directly linked to autism. Most cases involve a complex combination of genetic risk factors.
- Other non-genetic factors, like advanced parental age, maternal illness during pregnancy, difficulties during birth, etc., may further influence risk.
Is Autism Hereditary?
Yes, autism has a strong hereditary component. Studies of twins have found if one identical twin has autism, the other twin will also have autism about 36-95% of the time. For non-identical (fraternal) twins, if one twin has autism, the other has about a 31% chance of also being affected. Comparatively, siblings of a child with autism have a 2 to 18% chance of also developing autism, which is still higher than the general population risk of around 1%.
Multiple gene variations are involved in autism. This makes autism inheritance more complex than a single-gene disorder like cystic fibrosis. In families with one autistic child, estimates suggest the recurrence risk for having a second child with autism ranges from about 3-18%, which is higher than the general population risk. Families who already have two children diagnosed with autism have an even higher chance >20% of having additional children with autism.
Autism Inheritance Pattern
Autism does not follow a clear Mendelian inheritance pattern like many other genetic conditions. The inheritance pattern is often described as:
- Multifactorial – meaning many genetic factors are involved, each making a small contribution to overall risk.
- Heterogeneous – meaning different combinations of genes may be involved in different families.
- Polygenic – meaning hundreds or more genes are likely involved, but each gene only mildly increases risk.
Gene variations inherited from parents add to risk, but are not fully deterministic of development of autism on their own. The environment further shapes gene expression and neurodevelopment. Overall this makes the inheritance pattern more complex than single-gene disorders.
Autism Risk Factors
Multiple factors beyond genetics can potentially contribute to autism risk, such as:
- Advanced parental age at time of conception (especially paternal age over 40 years old)
- Pregnancy complications (maternal bleeding, infection, preeclampsia, etc.)
- Premature birth
- Exposure to heavy metals and pollution
- Very low birth weight
However, the involvement of most non-genetic factors is debated and not conclusively proven. Many factors may correlate with higher autism rates without directly causing autism per se. More research is needed to better understand gene-environment interactions.
Genetic Testing for Autism
Currently there are no genetic tests that can diagnose autism or reliably predict autism risk. The exceptions are cases of known genetic syndromes like fragile X syndrome, which are associated with high rates of autism. Chromosomal microarray analysis and DNA sequencing can diagnose genetic syndromes associated with autism.
For idiopathic autism (no identifiable cause), genetic testing does not provide a diagnosis. However, research studies may analyze genes such as SHANK3, ARID1B, ADNP, SCN2A, CHD8 and other autism-risk genes to uncover new associations.
Who is Most at Risk for Having a Child with Autism?
While any family can have a child with autism, the following factors can increase risk:
- Having one child already diagnosed with autism
- Having a sibling, parent or close relative with autism
- Advanced parental age (over 30 years old, especially paternal age over 40)
- Carrying known genetic variants associated with autism
- Exposure to certain medications, toxins or infections during pregnancy
However, even in higher risk groups, there are no guarantees. Many families with multiple autism risk factors have children without autism. More research is needed to better predict individual risk.
Can Autism Be Prevented or Avoided?
Currently there are no evidence-based ways to prevent autism, since the causes are complex and not fully understood. Prenatal testing is unable to predict autism with certainty. Families with autism concerns should seek genetic counseling to discuss their personal risk factors and options. While autism cannot be cured, early intervention can greatly improve outcomes. Increased awareness, early screening and early intervention when delays are noticed can help children reach their full potential.
Conclusion
In summary, autism has a strong hereditary component and tends to run in families. However, there are no specific autism genes that account for most cases. Rather, autism arises from a complex combination of genetic risks and other environmental factors influencing early brain development. Recurrence risk is higher for families who already have a child with autism. While autism cannot be cured, early intervention provides the best opportunity to support healthy development and improve outcomes.