CHICAGO, Dec. 29 (Xinhua) -- Researchers at Washington University School of Medicine in St. Louis have shown in a study that Chiari 1 malformation, a common brain disorder, can be caused by variations in two genes involved in brain development.

To identify genes that cause Chiari 1 malformation, the researchers sequenced all the genes of 668 people with the condition, as well as 232 of their relatives. Of these relatives, 76 also had Chiari 1 malformation and 156 were unaffected.

Sequencing revealed that people with Chiari 1 malformation were significantly more likely to carry mutations in a family of genes known as chromodomain genes. Several of the mutations were de novo, meaning the mutation had occurred in the affected person during fetal development and was not present in his or her relatives. In particular, the chromodomain genes CHD3 and CHD8 included numerous variants associated with the malformation.

Further experiments in tiny, transparent zebrafish showed that the gene CHD8 is involved in regulating brain size. When the researchers inactivated one copy of the fish's CHK8 gene, the animals developed unusually large brains, with no change in their overall body size.

Chromodomain genes help control access to long stretches of DNA, thereby regulating expression of whole sets of genes. Since appropriate gene expression is crucial for normal brain development, variations in chromodomain genes have been linked to neurodevelopmental conditions such as autism spectrum disorders, developmental delays, and unusually large or small heads.

The association between chromodomain genes and head size inspired the researchers to measure the heads of children with Chiari malformations, comparing them to age-matched controls and to population averages provided by the Centers for Disease Control and Prevention. Children with Chiari tended to have larger than average heads. Those children with the largest heads, bigger than 95 percent of children of the same age, were four times more likely to be diagnosed with the malformation.

The findings suggest that children with larger heads or people with other neurodevelopmental disorders linked to chromodomain genes may benefit from screening for Chiari malformation.

"A lot of kids that have autism or developmental disorders associated with chromodomain genes may have undiscovered Chiari malformations," said senior author Gabriel Haller, an assistant professor of neurosurgery, of neurology and of genetics. "The only treatment right now is surgery. Discovering the condition early would allow us to watch, knowing the potential for serious symptoms is there, and perform that surgery as soon as it's necessary."

About one in 100 children has a common brain disorder called Chiari 1 malformation, but most of the time such children grow up normally and no one suspects a problem. But in about one in 10 of those children, the condition causes headaches, neck pain, hearing, vision and balance disturbances, or other neurological symptoms.

The findings, posted on the university's website on Monday, have been published in the American Journal of Human Genetics.