WASHINGTON (Reuters) - Using a new method that separates patients by their symptoms, U.S. researchers said on Friday they found a new genetic link to autism and suggested the approach might be used to pinpoint the genetic causes of other diseases as well.

The research also suggests that several different causes may be behind autism, a disturbing and increasingly common behavioral disorder that baffles parents and doctors alike.

The new study, published in the American Journal of Human Genetics, links certain types of autism to a place on chromosome 15 linked with several other disorders.

It had been suspected in autism for some time, but researchers had been unable to show that people with certain versions of genes on chromosome 15 were more likely to have autism.

The team at Duke University Medical Center in North Carolina and at the University of South Carolina decided to separate out autistic children by their actual behaviors.

Autism, which affects one in 1,000 children in the United States, is defined by a wide range of symptoms, many of which are just an exaggeration of normal childhood behavior.

"All kids with autism by definition have some form of repetitive behavior," Duke child psychologist Michael Cuccaro, who helped lead the study, said in a telephone interview.

"One kind of a classical autism feature ... may be a child doing something with his hands and arms. He might be flapping them, might be waving them in front of the eyes."

Cuccaro's team focused on children with other, more prominent symptoms. "These are kids who if you changed the furniture in the room, they become extremely upset and have difficulty with that. If they normally went to bed at 7:30 and before they did that they took a bath and put pajamas on, if you changed that they would have great difficulty with that."

SIMILAR MUTATIONS

When those particular children and teen-agers were separated out, the researchers were able to find a series of mutations on chromosome 15 that seemed to be similar, said Dr. Margaret Pericak-Vance, a Duke geneticist who led the study.

They used a statistical trick, but they double-checked and the association was clear, Pericak-Vance said.

Both she and Cuccaro say that supports what many researchers have said -- that autism is a complex disease and may not be caused by the same thing in every patient.

"It's like any complex disease -- there are a number of underlying causes for it and they manifest similarly," she said. "The next thing is to look at possible interactions between the genes in this region. This region seems to be involved in a lot of different disorders."

Pericak-Vance said her team was already trying the new approach to separate out different kinds of Alzheimer's disease. Alzheimer's has different forms -- some are seen earlier in life than others, and Pericak-Vance hopes the method might find a genetic difference among them.

The gene on chromosome 15 that seems to be affected in the autism patients controls a neurotransmitter called GABA. That message-carrying chemical acts to turn off brain cells.

As the behavior seen in these children seems to be an extreme version of what every child does at one time or another, it could be that these particular symptoms are caused by the brain's failure to turn off a signal. In other words, it does not know when to stop -- thus the obsessive behavior.

The research did little to answer questions about whether environmental causes may be behind autism, Cuccaro said. One group of parents believes childhood vaccines may be a cause, although several studies aimed at finding out if that is true have shown no link.

"What we are coming to find about vaccines now is that there is not a lot of support for a link between the vaccine and autism," said Cuccaro.

The study may help scientists find a way to treat autism, which is now incurable. If a precise genetic cause of one behavior is found, it might be possible to design a drug that will correct it.

That would not be a cure -- autism is too complex for that -- but it may be possible to moderate some of the symptoms, Cuccaro said.