Three new studies have uncovered extremely rare genetic mutations that shed new light on the potential environmental and biological roots of autism, a brain disorder that causes social and developmental delays in children, beginning at a young age. Scientists say the DNA glitches found in a small subset of autistic children were not inherited by them, but occurred spontaneously at their conception, increasing their risk for developing the disorder.
One study found that having the rare genetic mutations could increase by 5 to 20 times a child's risk of developing autism spectrum disorders. These disorders range from mild cognitive delays and developmental impairments such as Asperger's syndrome to profound social dysfunctions and repetitive behaviors. Autism is being diagnosed, on average, in one of every 88 children in the United States, according to a recent government estimate.
Another study turned up biological evidence to support previous observations that the mutations are four times more likely to originate in male DNA than in the female DNA, and are more likely to appear in children of middle-aged and older fathers than in those of fathers younger than 35. Researchers speculate that the frequent turnover in a male's sperm cells increases the chance for errors in the genetic copying process. When a parent transmits such a transcription error to his offspring, the result can be a genetic mutation in the child that can cause autism. But researchers stress the risk of getting one of these badly-copied genes is extremely small.
The mutations, also called "de novo" mutations, are spontaneous abnormalities that scientists say are distributed widely across the genome of affected children. They account for a very small percentage of diagnosed cases of autism, a diverse family of disorders with a variety of suspected genetic and environmental causes.
Mark Daly of the Center for Human Genetics at the Broad [BROH-de] Institute at Harvard University and the Massachusetts Institute of Technology, led one of the studies. Daly says the the findings give autism researchers a starting point to better understand the biology of the disorder.
"So it doesn't explain all of autism and, in fact, more than half of the cases of autism don't have these types of mutations," said Daly. "But because they are very rare, they allow us to pinpoint when we see multiple kids with autism with mutations in the same gene."
Scientists say there could be hundreds of mutated autism genes that code for proteins responsible for brain development. In the studies, all 549 children had the de novo mutations and their parents did not, allowing researchers to compare the genes of their children to that of their mothers and fathers, leading to the discovery of the DNA glitches.
Thomas Lehner is head of genomics at the U.S. National Institute of Mental Health, which funded one of the studies. Lehner says the latest findings point researchers toward understanding the biological architecture of autism.
"And even better, we can design experiments that will help us get to point where we say, 'We are now able to understand a large proportion of the genetic variants - what we call a genetic liability - to autism,'" said Lehner.
The latest studies give researchers new drug targets for treating autism. Mark Daly of the Broad Institute says scientists have seen promising results in animal studies.
"It's possible to reverse some of these symptoms in mice, even after the brain is developed," he said. "So it underscores that anything we think we know, we really are not sure of, as it pertains to diseases and disorders of the brain."
Scientists hope to identify many more autism genes within a few years.
The three studies on the discovery of de novo genetic mutations in autism are published jointly in the journal Nature.