Researchers have found that people who have both autism and intellectual disability have more regions in their brain that harbor recessive mutations than their unaffected siblings.
Recessive mutations only lead to symptoms when both copies of a gene are affected, which means that that regions of DNA that are identical, or homozygous, on both copies of the chromosome may contain harmful mutations associated with autism.
These homozygous regions arise as a result of shared ancestry, and so are prevalent in places where inbreeding occurs.
The results of the new study suggests that inherited recessive mutations may also play a significant role in autism, especially when it’s accompanied by intellectual disability. The researchers checked 1,652 families from the Simons Simplex Collection (a database of families that has children with autism and unaffected relatives).
The researchers scanned the genomes of both the autistic children and their siblings, looking for homozygous regions that were at least 2,5000 kilobases long. They found that children who have both autism and an IQ of 70 or below have more of these regions than their unaffected siblings do—this was not the case for children with autism who had an IQ above 70.
Individuals with autism who carry at least one 2,5000-kb homozygous region tend to have lower IQ scores than do those with autism who lack those regions. The two groups of individuals have equivalent symptoms otherwise.
Girls with autism who have at least one of these regions have an average IQ of 74, whereas the average IQ for all individuals with autism in the study is about 82…