The study zeros in on the genes that may lead to the marked extroverted behavior seen in children with Williams syndrome, demonstrating that "hyper-sociability" - especially the drive to greet and interact with strangers -- follows a unique developmental path.
The path is not only different from typical children but also from children with other developmental disorders of the nervous system. The study appears in the online version of the American Journal of Medical Genetics.
Teresa Doyle and Ursula Bellugi of the Salk Institute, along with Julie Korenberg and John Graham of UCLA and Cedars-Sinai Medical Center, Los Angeles, found that children with Williams syndrome scored significantly higher on tests measuring behavior in social situations, including their ability to remember names and faces, eagerness to please others, empathy with others' emotions and tendency to approach strangers.
The authors also performed genetic screening on a young girl to look for genetic underpinnings of this pronounced social behavior.
"We've known for many years that children with Williams syndrome are markedly more social than other children, in spite of the moderate mental retardation and physical problems that also are associated with the disorder," said Doyle. "Here we not only have shown hyper-social behavior as a hallmark symptom that follows a characteristic developmental course in Williams syndrome, but we may be closer to identifying the genes involved in regulating that behavior."
Williams syndrome is rare, occurring in only one in every 20,000 people. It arises from the deletion of no more than 20 genes from one chromosome of the seventh chromosome pair.
Virtually everyone with Williams syndrome has exactly the same set of genes missing. People with the disorder have characteristic facial and physical features, certain cardiovascular problems and mild to moderate mental retardation.
However, in addition to their very extroverted natures, as adults Williams patients also possess unusually adept language skills given their level of general cognitive abilities. Bellugi and her laboratory have been studying the syndrome for years in an effort to understand how language is processed in the brain and what genes may be involved.
Very rarely, an individual with Williams syndrome has a slightly smaller or larger gene deletion than is typical for the syndrome. The researchers analyzed the genetic deletion of a two-year-old girl with Williams syndrome who did not exhibit this especially outgoing behavior.
She was shy around strangers, and scored significantly lower on sociability measures, much more resembling 2-year-old children without Williams syndrome. It was found that this girl retained at least one gene that most people with the disorder have missing. This indicated that the gene (or genes) she retained might be altering the hyper-sociability usually observed among children with Williams syndrome.
"We don't know at this point whether these genes are involved in regulating social behavior in the general population, or whether their involvement is specific to Williams syndrome," said Bellugi. "We will need to conduct tests in more patients who do not have all the gene deletions seen in typical Williams syndrome, and compare and contrast genetics and behavior."
The research was supported by grants from the National Institutes of Health, the James S. McDonnell Foundation, the Oak Tree Philanthropic Foundation, and the U.S. Department of Energy.
The Salk Institute for Biological Studies, located in La Jolla, Calif., is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and conditions, and the training of future generations of researchers. Jonas Salk, M.D., founded the institute in 1960 with a gift of land from the City of San Diego and the financial support of the March of Dimes Birth Defects Foundation.