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Two independent research teams have pinpointed the genetic glitch behind certain cases of autism, paving the way for the first DNA test to reliably predict who will develop the disorder.

Researchers with the Boston-based Autism Consortium have found that children who carry a missing or duplicated stretch of chromosome 16 have a 100-fold increased risk of developing autism. The mutation, confirmed through genetic scans of more than 1,400 people with autism in three separate study groups, appears most often to be the result of a random quirk not seen in either parent. Published in today's New England Journal of Medicine, the Boston paper estimates the mutation is linked to about 1 per cent of all autism cases.

In a separate study of 400 Canadian families, scientists at Toronto's Hospital for Sick Children have also identified this region of chromosome 16 as the culprit in 1 per cent of autism cases - and they plan to offer testing to patients as soon as possible, a breakthrough of great value to parents who have one child with autism and fear they face a higher risk of having another.

"This is the real thing," said Sick Kids senior scientist Steve Scherer, lead researcher on the Canadian paper to be published in the American Journal of Human Genetics next week.

"If you have this [genetic] change, chances are you are going to be on the spectrum of autism disorders."

The link between the mutation and autism is so strong that Dr. Scherer said it will herald "the first highly predictive DNA test for an autism disorder."

"We are transferring the technology [to perform the test] from our research lab to the diagnostic clinic as we speak."

Dr. Scherer said the test is relatively cheap and easy to perform because it involves a deleted or duplicated stretch of chromosome 16 that covers some 25 genes. The race is now on to understand those genes, but at least five are already known to be involved in brain function.

If the genetic code in this region is deleted - like a sentence missing several words - it cannot be read, said Dr. Scherer, and this seems to result in more severe cases of autism. If the code is duplicated, the disorder appears to be milder - like a sentence in which repeated words make it difficult, but not impossible, to read.

Autism is usually described as a spectrum of disorders, because its symptoms - usually characterized by impaired communication, social disabilities and repetitive behaviours - can vary widely in severity. It has in recent years been recognized as one of the most common developmental disorders of childhood, striking one in every 165 children. Roughly 3,000 new cases are identified in Canada each year.

Doctors generally rely on psychological tests to diagnose autism in children around age three. Last fall, the American Academy of Pediatrics recommended all babies be tested for autism twice by age two on the assumption that early therapy can counter the condition's worst effects. A DNA test, however, could identify those affected as infants, or even before they are born, to intervene as early as possible.

Wendy Roberts, co-director of Sick Kids' autism research unit and a co-author on the Canadian paper, said if a child tests positive for the chromosome 16 mutation, but neither parent carries it, the couple's risk of having another affected child would be no higher than that of the general population.

On Tuesday, for example, Dr. Roberts was able to tell one Southern Ontario mother that her teenaged son with autism does indeed carry this random mutation.

"She was so pleased to finally have some kind of explanation," Dr. Roberts said. While the mother is beyond the age of considering having more children, the test result will in the future be important to her daughter.

Mark Daly, lead author of the Boston paper and an assistant professor of medicine at Harvard Medical School, said in a statement that researchers are "still a long way from understanding how this chromosomal deletion or duplication increases the risk for autism, but this is a critical first step toward that knowledge."

The Autism Consortium, which includes 14 Boston-area hospitals and academic centres, identified the chromosome 16 mutation in two separate groups of more than 500 patients in the U.S. and another sample of 299 patients from Iceland. The team also checked the mutation in a control group of 18,834 Icelandic control subjects, in which two people, whose neurological profile is unknown, were carriers.

Children's Hospital Boston is now also offering a test for the chromosome 16 mutation.

Four unrelated people with autism were found to have the mutation among the 400 Canadian families. In only one case, however, did the child inherit it from one of the parents, Dr. Scherer said.

So far, the research suggests the mutation usually springs up randomly in the copy and shuffle of parents' DNA prior to conception.

Experts believe autism is generally the result of a complex interplay of multiple genes and environmental forces that seem to be different for different families. In 10 per cent of cases, for example, autism is the result of other known genetic conditions, such as Down syndrome or Fragile X. But little has been known about the genetic components in the other 90 per cent of cases.

Only in the past few years, with the technical ability to quickly and cheaply scan the genomes of hundreds of patients, have scientists begun to home in on the genetic links.

"That's why understanding 1 per cent for autism cases is a big deal," Dr. Scherer said.