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The genomes of people who have both autism and intellectual disability have more regions that may harbor recessive mutations than those of their unaffected siblings, according to a study published 11 July in the American Journal of Human Genetics1.
Recessive mutations lead to symptoms only when both copies of a gene are affected. That means 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 marriage between first cousins is common.
Many studies looking for autism-risk genes focus on de novo mutations, which arise spontaneously. The results of the new study suggest that inherited recessive mutations may also play a significant role in autism, especially when it’s accompanied by intellectual disability.
The researchers looked at 1,652 families from the Simons Simplex Collection (SSC), a database of families that have one child with autism and unaffected parents and siblings. The SSC is funded by the Simons Foundation, SFARI.org’s parent organization.
The researchers scanned the genomes of the children with autism and their siblings, looking for homozygous regions at least 2,500 kilobases (kb) long. Children who have autism and an intelligence quotient (IQ) of 70 or below have more of these regions than their unaffected siblings do, the study found. This is not the case for the children with autism who have an IQ above 70.
Individuals with autism who carry at least one 2,500-kb homozygous region also have lower IQ scores than do those with autism who do not have those regions. These two groups of individuals have equivalent symptoms otherwise, however. For example, 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.
1: Gamsiz E.D. et al. Am. J. Hum. Genet. 93, 103-109 (2013) PubMed