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DURHAM, N.C. -- A combination of two malfunctioning genes increases the risk of autism among African Americans, researchers at Duke University Medical Center have found. This same gene combination also increases the risk of autism in Caucasians, the Duke team reported in a 2005 study.

The researchers believe their new study is the first to examine the genetics of autism in African Americans, despite the disorder's equal prevalence across ethnic groups.

The finding will be critical to developing new treatments for both African Americans and Caucasians, the researchers said.

"It is essential that we define which genes play a role in particular ethnic groups so we can devise treatments that target the specific malfunctioning genes," said Margaret Pericak-Vance, Ph.D., director of the Duke Center for Human Genetics and senior author of the study.

The research is published in the online issue of Neurogenetics and will appear in the July, 2006 print edition of the journal. The research was supported by the National Institutes of Health, the National Alliance of Autism Research, the Hussman Foundation and the Autism Genetic Resource Exchange.

Autism is characterized by repetitive behaviors and severe impairment in social interaction and communication. Up to 1.5 million people in the United States have autism, and it is the nation's fastest growing developmental disability.

In their study, the Duke researchers analyzed the genes of 54 African American families and 557 Caucasian families in which a member had autism. They searched along chromosome 15, which had previously been linked to autism, for genes that regulate a brain chemical, or neurotransmitter, called GABA.

GABA inhibits nerve cells from firing once their message has been transmitted. In this way, the neurotransmitter acts as an information filter that prevents the brain from becoming over-stimulated.

If the GABA system malfunctions, the brain can be flooded with sensory information that overwhelms the brain's processing capabilities, leading to some of the behaviors that characterize autism, said Michael Cuccaro, Ph.D., a Duke clinical psychologist and study co-author.

The researchers found that in African Americans and Caucasians, the interaction of two malfunctioning GABA receptor genes -- GABRB1 and GABRA4 -- can increase the risk of autism. GABA receptors are docking sites on the surface of brain cell neurons. GABA binds to these docking sites and inhibits the neurons from firing.

"We found that GABRA4 increases of the risk of autism, and its interaction with GABRB1 further increases the risk of autism," Pericak-Vance said.

Existing autism medications, including the sedative diazepam and certain antiepileptic drugs, already target the GABA system in a broad manner. But understanding the precise gene-gene interaction may enable scientists to develop new drugs that more precisely target the specific genes involved, the researchers said.

Identifying genes that confer autism risk has been difficult precisely because multiple genes must interact to confer risk, Pericak-Vance said. In fact, scientists hypothesize that as many as 100 genes may be involved in autism.

"Multiple genes that by themselves have small effects can interact with one another to produce a large effect," Pericak-Vance said.

Further complicating the search for autism genes, she said, is that each ethnic group possesses unique genes that can interact with autism-associated genes to slightly alter the course of the disease.

For example, certain symptoms associated with autism, such as delayed language development and problems handling daily life tasks, are more severe in African American individuals with autism than in Caucasians, Cuccaro said. Such differences make it important to understand the range of underlying genes that contribute to the disorder in various population groups.

"Many diseases, such as sickle cell and cystic fibrosis, have different genetic origins in African Americans versus other racial groups," Cuccaro said. "The most effective drugs are those which target the specific genes that are malfunctioning, so we must define which genes play a role for each ethnic group."