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What are the 3 main causes of autism?

Autism spectrum disorder (ASD) refers to a range of conditions characterized by challenges with social skills, repetitive behaviors, speech and nonverbal communication. The exact causes of ASD are still being researched, but many experts agree that there are likely multiple factors that contribute to its development. The three main categories of potential causes are:

Genetic Factors

Research strongly supports that genes are one of the leading risk factors for ASD. While no single gene has been identified as causing autism, multiple gene mutations and variants have been associated with the disorder:

  • ASD tends to run in families. Parents who have a child with ASD have a 2%–18% chance of having a second child who is also affected.
  • Identical twin studies show that if one twin has ASD, the other will be affected about 36–95% of the time. For non-identical twins, if one child has ASD, the other is affected about 0–31% of the time.
  • At least 100 genes have been associated with ASD, many of which are involved in brain development and function.

However, the genetics of autism are complex. In the majority of cases, it is not caused by a single genetic mutation. Instead, multiple common gene variants combine to increase the risk for ASD. Other factors like environmental influences also play an important role.

Examples of Genetic Syndromes Linked to Autism

While autism is not caused by any one genetic syndrome, researchers have identified some genetic conditions that have high rates of co-occurring ASD. Some examples include:

  • Fragile X syndrome – Caused by mutations in the FMR1 gene. About 25-50% of people with fragile X also meet the criteria for ASD.
  • Tuberous sclerosis – Caused by mutations in the TSC1 or TSC2 genes. Around 25-60% of people with tuberous sclerosis have ASD.
  • Angelman syndrome – Characterized by defects in the UBE3A gene. Up to 80% of children with Angelman syndrome exhibit symptoms of ASD.

However, it’s important to note that while these genetic disorders are associated with an increased risk, the majority of autism cases have no known single genetic cause. Research is ongoing to better understand the complex interplay between genetic and non-genetic factors that contribute to ASD.

Environmental Influences

While autism has a strong genetic basis, researchers have also identified several environmental risk factors that may play a role by influencing early brain development:

  • Advanced parental age – Children born to older parents are at a higher risk of ASD, likely due to increased chances for genetic mutations.
  • Prenatal infections – Exposure to infections like rubella or CMV during pregnancy may increase autism risk.
  • Medications/toxins – Some medications like valproic acid and pesticide exposure have been linked to higher ASD rates.
  • Air pollution – High air pollution exposure, especially during pregnancy and first year of life, may modestly increase autism risk.

However, most environmental influences do not directly cause autism on their own. More research is needed to understand potential gene-environment interactions.

Prenatal Brain Development

Many of the external factors associated with increased autism risk can influence very early brain growth and development during pregnancy. As autism likely originates prenatally, this suggests potential windows where the developing brain may be most vulnerable to environmental influences.

Some evidence for early brain developmental differences in autism includes:

  • Enlarged brain size and overgrowth of neurons observed in some children with ASD as early as 2-3 years old.
  • Changes in brain signaling pathways and proteins that regulate brain cell number and connectivity.

Ongoing research aims to explain how genetic and non-genetic factors may converge to affect early brain development and lead to the onset of autism symptoms.

Neurodevelopmental Differences

In addition to genetics and environment, another influence that likely plays an important role in ASD is differences in how the brain develops and functions. Some examples include:

  • Imbalance between excitatory and inhibitory signaling – Too much excitatory signaling and not enough inhibitory signals may lead to excess neuron activity associated with autism behaviors.
  • Disrupted neural networks and connectivity – Imaging studies show atypical connectivity between key brain regions may underlie social-communication challenges.
  • Impaired processing and integration of sensory information – Atypical sensory reactivity and symptoms are very common features of autism starting early in life.

Ongoing neurobiology research continues to uncover differences in brain structure, connectivity and information processing that appear unique to ASD. While the reasons are unclear, many believe an early developmental imbalance in the brain leads to a “cascade” of effects on brain growth and functional connectivity over time that results in autism behaviors.

Key Brain Differences Associated with Autism

Though there is large individual variability, some of the most replicated brain differences found in MRI and postmortem studies of autism include:

  • Enlarged brain volume starting around 2-3 years old.
  • Increased brain weight and neuron number.
  • Altered structure of the amygdala, cerebellum and cortex.
  • Differences in neurotransmitters like serotonin, GABA and glutamate.
  • Reduced functional connectivity between key brain regions.

However, there are few brain differences specific or unique to autism. Ongoing research aims to better distinguish general markers of atypical neurodevelopment from brain changes most directly related to core autism features.

Conclusion

In summary, while no single cause has been identified, researchers agree that autism most likely develops due to a combination of genetic, environmental and neurological factors that impact early brain development. Genetic vulnerability appears to be the strongest risk factor, with multiple genes involved that affect brain growth and functioning. Non-genetic influences like parental age, prenatal infections and exposure to medications can further interact with these genes to affect development. Ultimately these converging factors alter early developmental trajectories to produce differences in brain connectivity and information processing that manifest as the symptoms of autism.

While significant progress has been made, there is still much unknown about the complex causes of autism. With further research, scientists hope to uncover more specifics about the intricate interplay between nature and nurture that leads to autism spectrum disorder in certain individuals. This knowledge will help guide the development of more targeted treatments and prevention approaches.