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Chromosome Abnormalities Raise Risk for Autism

Findings affects 1% of all cases of developmental disorder, researchers say

WEDNESDAY, Jan. 9 (HealthDay News) -- Abnormalities on chromosome 16 appear to raise children's risk for developing autism, a new study suggests.

The discovery, made by a consortium of autism researchers, pinpoints one of the causes of a disorder that is turning out to be as complex in its origins as it is in its symptoms. Not only is autism complicated, its incidence has grown rapidly in recent years, with an estimated one in every 150 children in the United States now struck by the neurological disorder.

"This has given us another piece of the puzzle of the genetics of autism," said study leader Mark Daly, a member of the Autism Consortium with the Center for Human Genetic Research at Massachusetts General Hospital Center in Boston. "Autism is very complex, and we have only a few pieces in hand. We're trying to gain an understanding of the biological mechanisms underlying it. This is an opportunity to understand that."

According to the study, published Jan. 9 in the online edition of the New England Journal of Medicine, a section of chromosome 16 is deleted or duplicated in about 1 percent of people with autism spectrum disorders (ASDs). About 15 percent of autism cases have known genetic causes.

"We can only explain a fraction of all the kids with autism, and we know most have some genetic contribution," added study co-author Dr. David Miller, assistant director of the Genetics Diagnostic Laboratory at Children's Hospital Boston. "If we were making a top 10 list of the most important genetic factors contributing to autism that we know about, this would certainly be on that list."

However, some experts questioned the far-reaching value of the findings.

"These types of studies are association studies. They have to go many further steps to make a causal link," said Rajesh Miranda, an associate professor of neuroscience and experimental therapeutics at Texas A&M Health Science Center College of Medicine, in College Station. "This is also a very small proportion of autism patients. What it probably indicates is that autism is quite heterogeneous, and what we think of as one disease may be hundreds. The upside is this identifies a new way of looking at things."

One advocate felt the discovery was only a drop in a larger sea of unknowns.

"Autism is growing at epidemic proportions, and so is the money being dedicated to genetic studies like this. It is an absolute fact that you cannot have a genetic epidemic. Yet, hundreds of millions of dollars continue to be allocated to genetics while environmental factors research continues to be underfunded at the National Institutes of Health," said Laura Bono, a board member of the National Autism Association (NAA). "So, while NAA is happy to know that 1 percent of the autism puzzle has been found, according to this group, our nation must ramp up our environmental research efforts to determine what chemicals/substances in the environment are triggering healthy children to regress into autism."

This latest paper follows closely on the heels of another study, which was published in the Dec. 21 online issue of Human Molecular Genetics, that found the exact same deletion was significantly associated with autism. That study also found duplications but, said corresponding author Susan Christian, an associate professor of human genetics at the University of Chicago Medical Center, "our data [on the duplications] was not that clear-cut." That may be because the Chicago researchers used a smaller sample size, she added.

Several groups, including the authors of these two papers, have been working on the same abnormality, she noted. "The work was done in parallel. Neither one of us knew the other one was working on it until we met at the American Society of Human Genetics meeting," Christian said. "The work was completed by both groups at the same time."

The causes of autism remain cloaked in mystery, although epidemiological studies suggest that 90 percent of these cases have a genetic component. Most of the specific genes involved, however, have not been identified.

According to an accompanying editorial, some 60 different genetic, metabolic and neurological disorders have been associated with autism, involving about 10 percent of ASD patients.

The authors of the current study conducted a complete genome scan of more than 3,000 children and families whose DNA was held at the Autism Genome Research Exchange. All the families had at least one child with autism or a related disorder.

The region in question contains about 25 genes that do not yet have a clear connection to the development of autism.

In most cases, the abnormality on the chromosome was not inherited from a parent but occurred de novo, happening during embryonic development. This means the chances of having another child with autism may be closer to 5 percent, as opposed to 50 percent if the abnormality was inherited, the researchers explained.

The study also dealt with another dimension: testing actual patients at Children's Hospital Boston. "We had found several cases with a chromosome 16 change, and when we originally started to see it, we weren't able to say exactly what it meant," said Miller. "As a result of this collaborative effort, we were able to go back to the families where we had told them we found something, but we didn't know what it meant, and now we know."

That knowledge can result in practical benefits for patients and families.

"One of the biggest impacts of doing genetic testing is that you can diagnose this genetic susceptibility even before you make a full diagnosis," Miller said. "In the U.S., the average age for clinical diagnosis of autism is 5 years old, but we know that kids who are much younger can have a tremendous amount of benefit from getting better services. You can do this test on a kid 1 year old who you are starting to be worried about and find out if they have that risk."

Such genetic screening, according to Miranda, is the wave of the future and, indeed, is already being done. "It's like picking out a needle in a haystack, which we couldn't do before," he said.

More information

Find out more about the fight against autism at Autism Speaks.

SOURCES: Mark J. Daly, Ph.D., Autism Consortium member, assistant professor, medicine, Massachusetts General Hospital Center and Harvard Medical School, Boston; David Miller, M.D., Ph.D., assistant director, Genetics Diagnostic Laboratory, Children's Hospital Boston; Rajesh Miranda, Ph.D., associate professor, neuroscience and experimental therapeutics, Texas A&M Health Science Center College of Medicine, College Station; Susan Christian, Ph.D., associate professor, human genetics, University of Chicago Medical Center; Laura Bono, board member, National Autism Association; Jan. 9, 2008, New England Journal of Medicine online

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