The world's largest genetic scan of people with autism in their families has found that many patients have their own unique pattern of genetic mutations, not necessarily inherited.
The findings published in the journal Nature help confirm the strong role that genes play in autism, and also suggest that small genetic disruptions may begin in the parents' eggs and sperm.
"Our research strongly suggests that this type of rare genetic variation is important and accounts for a significant portion of the genetic basis of autism," said Tony Monaco of the Wellcome Trust Center for Human Genetics at Oxford University, who helped lead the study.
"By identifying the genetic causes of autism, we hope in the future to be able to improve the diagnosis and treatment of this condition which can affect children and their families so severely," he told reporters on a telephone briefing.
Stephen Scherer of the Hospital for Sick Children in Toronto, Canada, said the results would "lead to a paradigm shift when it comes to understanding the root causes of autism."
Scientists from 60 research institutions in 12 countries worked for three years on the Autism Genome Project.
Autism is a complex and mysterious brain disorder usually first diagnosed in early childhood. It is characterized by difficulties in social interaction, communication, and understanding other people's emotions and behavior.
Autism disorders are diagnosed in one in 110 children in the United States and affect four times as many boys as girls. In Britain, an estimated 133,500 children have autism, according to the National Autistic Society.
The Autism Genome Project studied the genes of 996 people with autism and 1,287 without, all with European ancestry.
The team found that people with autism tended to have more losses and duplications of whole chunks of DNA. These deletions or insertions are called copy number variants and can disrupt how genes work.
UNIQUE MUTATIONS
People with autism had on average 19 percent of these genetic changes than those without the condition. The team also found that each autistic case had a different set of disturbances, although some of these affected genes with similar functions.
"Here's where it gets tricky — every child showed a different disturbance in a different gene," said Dr. Stanley Nelson from the University of California Los Angeles.
Daniel Geschwind, also of UCLA, said the findings suggested that "tiny genetic errors may occur during formation of the parents' eggs and sperm," and these variations are copied into the child's DNA.
"The autistic child is the first in their family to carry that variant. The parents do not have it," he said.
The findings support an emerging consensus within the scientific community that autism is caused in part by many "rare variants," or genetic changes, which are found in less than one percent of the population as a whole.
While each of these variants may only account for a small fraction of autism cases, together they are starting to account for a greater percentage of people with autism, the scientists said, and may give clues for possible future treatments.
What the study does not answer is how the genetic changes occur. Genes can be altered by the environment and the age of the parents has been linked with autism in some studies.
Some advocacy groups maintain that vaccines can cause autism, although many studies have ruled this out and a British researcher whose work gave the vaccine movement its impetus has had his study and medical license withdrawn.
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