The research involving Rett syndrome was reported in the October issue of the journal Nature Genetics. It could help doctors find out what goes wrong in the brain, which might lead to a treatment.
Rett syndrome almost invariably strikes girls. They develop normally until age 6 months to 18 months, then gradually lose the ability to speak, walk and control their hands. They withdraw from contact with other people and develop repetitive hand wringing and seizures.
The search for the gene has been hampered because Rett only rarely runs in families. The vast majority of the time, it strikes just once, apparently because of a onetime genetic mutation in an egg or sperm.
The discovery of the gene that becomes mutated was reported by Dr. Huda Zoghbi of the Baylor College of Medicine in Houston and the Howard Hughes Medical Institute, Dr. Uta Francke of Stanford University and the institute, and others.
Scientists do not know yet whether it is the only gene that causes the disorder. But it probably is at least the main gene, said Eric Hoffman of Children's National Medical Center in Washington.
Dr. Duane Alexander, director of the National Institute of Child Health and Human Development, which helped pay for the work, called the discovery "a definite turning point, which we hope will soon lead to better diagnosis and, ultimately, treatment for this disorder."
The gene, called MECP2, lies on the X chromosome. Males have only one copy of that chromosome per cell, and if it carries a defective MECP2 gene, they die before birth or shortly after. Girls have two X chromosomes, one inherited from each parent.
Zoghbi's study found mutations in MECP2 in five of 21 Rett patients, as well as in two affected half sisters. The remaining patients may have mutations in parts of the gene that have not been examined yet, she said.
Zoghbi said she sees no reason to screen all newborn girls for the genetic defect until an effective treatment is developed.