The study offers compelling evidence that spontaneous, or de novo, mutations contribute to autism in at least 27 percent of families in which the parents and siblings are unaffected.
The new research has also established conclusively that 'higher-IQ' autism, which mostly affects boys, has a different genetic basis from 'lower-IQ' autism, which commonly affects both boys and girls.
The researchers sequenced the whole exomes, the protein-coding regions of the genome of 2,515 families from the Simons Simplex Collection, a large repository of genetic, biological and phenotypic data from 'simplex' families, which consist of one child with autism, unaffected parents and usually at least one unaffected sibling.
The study pinpointed seven genes that have mutations in three or more children with autism, implicating these genes in the disorder with near certainty.
It also identified another 20 genes with mutations in two children. Each of these genes has more than a 90 percent chance of being a true autism gene, the researchers said.
"We have a set of genes for which now, if people see a likely gene-disrupting mutation when sequencing a young child, there's a high risk of the child developing autism, and that, to my mind, is pretty powerful stuff," said Evan Eichler of the University of Washington, a Simons Foundation Autism Research Initiative (SFARI) Investigator.
"Recognizing this early on may allow for earlier interventions, such as behavioural therapies, improving outcomes in children," said Eichler.
"High-IQ autism is almost entirely a male disease, which has been a bit of a puzzle," Michael Wigler of Cold Spring Harbor Laboratory in New York said.     

"The paper clearly indicates that the genetic causality differs from that of girls and lower-IQ boys. That had been speculated, but now it's clear, and it's very important," said Wigler.
The exome sequencing also showed that girls with autism are particularly likely to have mutations in genes expressed during early embryonic development.
This finding, combined with the fact that autism is less prevalent in girls than boys, suggests that girls may somehow be protected against mutations in autism genes that are expressed in later stages of development, but not those expressed in early embryonic development.
The study identified about 300 new autism candidate genes, about half of which are likely to be true autism genes, researchers said.
The research was published in the journal Nature.

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