Too many connections in the frontal lobe of the brain may help explain some of the learning problems experienced by people with autism, U.S. researchers said.
They said autistic children with a common autism risk gene appear to have a lot of brain connections clustered in the frontal lobe, a part of the brain important for learning.
But they had fewer connections to other parts of the brain, a finding that may help explain how this gene variant rewires the brain.
"This is a key piece of the puzzle we've been searching for," said Dr. Daniel Geschwind of the University of California, Los Angeles, who worked on the study published in Science Translational Medicine.
"Now we can begin to unravel the mystery of how genes rearrange the brain's circuitry not only in autism, but in many related neurological disorders."
Autism is a complex and mysterious brain disorder usually diagnosed in early childhood. It is characterized by difficulties in social interaction and communication, ranging from mild to profound impairment.
Geschwind and colleagues used functional magnetic resonance imaging to scan the brains of 32 children as they did learning-related tasks. Half of the children had autism, and half did not.
The team measured the strength of brain connections as the children worked.
They found children with the version of the gene linked with autism called contactin associated protein-like 2 or CNTNAP2 had strong brain connections within the frontal lobe, but weaker connections to the rest of the brain.
"In children who carry the risk gene, the front of the brain appears to talk mostly with itself," Ashley Scott-Van Zeeland, now at Scripps Translational Science Institute, said in a statement.
"It doesn't communicate as much with other parts of the brain and lacks long-range connections to the back of the brain," she said.
Because this version of the gene is common in healthy people as well, the team looked to see how it affected 40 children who did not have autism.
"We actually saw the same pattern," Scott-Van Zeeland said in a telephone interview.
"Kids who carried the risk version of the gene had the same extra connections in the frontal lobe as the first group. Of course, they didn't have autism," she said.
Scott-Van Zeeland said researchers know that autism is caused by more than one gene, but the CNTNAP2 gene helps explain part of the picture.
Autism spectrum disorders are diagnosed in one in 110 children in the United States and affect four times as many boys as girls.
"One third of the population carries this variant in its DNA," Geschwind cautioned. "It's important to remember that the gene variant alone doesn't cause autism -- it just increases risk."