By Maggie Fox, Health and Science Editor
WASHINGTON, July 10 (Reuters) - Researchers studying more than 100 families prone to autism said they had identified at least six new genes that appear to underlie the disorder -- and said they suggest it may be possible to treat it sometimes.
Their study, published in the journal Science, reinforces the common wisdom that autism is not just a single disease but can be caused by a range of genetic and environmental factors.
It also showed that, at least in many of the families studied, autism appears to be caused by the combination of faulty DNA and something in the physical or social environment of an affected child after birth.
The mutations appear to disrupt genes important to the developing brain, and which are turned on and off by activity of brain cells known as neurons that are stimulated by early childhood experiences.
"There appear to be many separate mutations involved, with each family having a different genetic cause," Dr. Thomas Insel, director of the National Institute of Mental Health, which helped pay for the study, said in a statement.
"The one unifying observation from this new report is that all of the relevant mutations could disrupt the formation of vital neural connections during a critical period when experience is shaping the developing brain."
Dr. Christopher Walsh and Dr. Eric Morrow of Harvard Medical School in Boston and colleagues studied 104 families in the United States, Pakistan, Turkey, Jordan, Saudi Arabia and Kuwait. All had family members with autism and in 88 of the families, first-cousin marriages were common.
Such consanguineous marriages can make recessive disorders -- those that require two faulty copies of a gene -- more common. "Marriage between first cousins increases the prevalence of neurological birth defects by about 100 percent," the researchers wrote.
"Autism symptoms emerge at an age when the developing brain is refining the connections between neurons in response to a child’s experience," Walsh said.
"Whether or not certain important genes turn on is thus dependent on experience-triggered neural activity. Disruption of this refinement process may be a common mechanism of autism-associated mutations."
One finding that offered some hope -- many of the mutations did not result in missing or damaged genes, but simply turned them off.
"This means that we would not need to replace the gene, if we could only figure out how to reactivate it, perhaps with medications," said Morrow.
Walsh, who is also a Howard Hughes Medical Institute investigator, noted that studies have shown that enriched learning environments can help some children with autism.
Such extra training may help activate pathways in the brain that bypass the broken on/off switches, he said.
Autism, which is marked by impaired social interaction and communication, or a related disorder like Asperger’s syndrome, affects an estimated one out of every 150 U.S. children, the U.S. Centers for Disease Control and Prevention estimates. Asperger’s is marked by social awkwardness.
Autism can cause mental retardation in up to 70 percent and seizures in 20 to 25 percent of cases.
"At the moment, we understand the genetic causes of 15 to 20 percent of autism," Walsh said. "The remaining 80 percent remain unexplained."
(Editing by Cynthia Osterman)