Studies have shown that autism is a genetic disease. Several genes have been associated with autism because they appear repeatedly in families with high rates of autism. But this does not explain the widespread occurrence of sporadic autism – cases in which one family member has the disorder.
Over the last few years, groups like The Simons Foundation have gathered evidence to suggest that sporadic autism is caused by new mutations unique to the affected individual. The Simons Foundation has worked with academic researchers to obtain DNA and cell samples from thousands of unaffected parents and their autistic children; unaffected siblings are also included. This has allowed researchers to screen for changes in the DNA that are unique to the affected individual. The researchers looked for copy number variations, also called CNVs, which occur when a chromosome has either ended up with an extra copy of part of the genome, or seen a part of the genome deleted.
A general trend has emerged: new CNVs (deletions or duplications that are not present in either parent) are much more common in autistic children than they are in their unaffected siblings. One study found that they showed up 3.4 times more often; the other showed 3.9 times. In either case, there were many more changes in the dose of genes among autistic individuals.
These findings definitely support the idea that new mutations cause sporadic autism. But the studies also show that the mutations do not occur at a high enough rate to be the cause of all cases. Some other causes may include mutations that cannot be detected by checking for CNVs, or environmental factors that cause DNA damage.
Over all, it has been difficult to identify the genes because many times they are completely unique to an affected individual. The researchers involved are already screening more individuals, and preparing to start sequencing DNA from the same collection, so we should have an even clearer picture in the near future.