Cause of Autism and Does Inheritance play a role?

What causes autism?

Scientists aren’t certain about what causes ASD, but it’s likely that both genetics and environment play a role.  Researchers have identified a number of genes associated with the disorder.  Studies of people with ASD have found irregularities in several regions of the brain.  Other studies suggest that people with ASD have abnormal levels of serotonin or other neurotransmitters in the brain.  These abnormalities suggest that ASD could result from the disruption of normal brain development early in fetal development caused by defects in genes that control brain growth and that regulate how brain cells communicate with each other, possibly due to the influence of environmental factors on gene function.  While these findings are intriguing, they are preliminary and require further study.  The theory that parental practices are responsible for ASD has long been disproved.

What role does inheritance play?

Twin and family studies strongly suggest that some people have a genetic predisposition to autism.  Identical twin studies show that if one twin is affected, there is up to a 90 percent chance the other twin will be affected.  There are a number of studies in progress to determine the specific genetic factors associated with the development of ASD.  In families with one child with ASD, the risk of having a second child with the disorder is approximately 5 percent, or one in 20.  This is greater than the risk for the general population.  Researchers are looking for clues about which genes contribute to this increased susceptibility.  In some cases, parents and other relatives of a child with ASD show mild impairments in social and communicative skills or engage in repetitive behaviors.  Evidence also suggests that some emotional disorders, such as bipolar disorder, occur more frequently than average in the families of people with ASD.

What Causes Autism?

Causes

There is no known single cause for autism, but it is generally accepted that it is caused by abnormalities in brain structure or function. Brain scans show differences in the shape and structure of the brain in children with autism versus in neurotypical children. Researchers are investigating a number of theories, including the links among heredity, genetics and medical problems.

In many families, there appears to be a pattern of autism or related disabilities, further supporting the theory that the disorder has a genetic basis. While no one gene has been identified as causing autism, researchers are searching for irregular segments of genetic code that children with autism may have inherited. It also appears that some children are born with a susceptibility to autism, but researchers have not yet identified a single "trigger" that causes autism to develop.

Other researchers are investigating the possibility that under certain conditions, a cluster of unstable genes may interfere with brain development, resulting in autism. Still other researchers are investigating problems during pregnancy or delivery as well as environmental factors such as viral infections, metabolic imbalances and exposure to environmental chemicals.

Genetic Vulnerability

Autism tends to occur more frequently than expected among individuals who have certain medical conditions, including Fragile X syndrome, tuberous sclerosis, congenital rubella syndrome, and untreated phenylketonuria (PKU). Some harmful substances ingested during pregnancy also have been associated with an increased risk of autism. 

Environmental Factors

Research indicates other factors besides the genetic component are contributing to the rise in increasing occurrence of autism, such as environmental toxins (e.g. heavy metals such as mercury), which are more prevalent in our environment than in the past. Those with autism (or those at risk) may be especially vulnerable, as their ability to metabolize and detoxify these exposures can be compromised.