Scientists have discovered the gene behind the most common type of inherited mental retardation, a significant advance in understanding not only a mental defect but also the genes that help fashion human intelligence.
In its mutant form, the gene causes a baffling kind of retardation, fragile X syndrome, that is proving to be more widespread the more adept experts have become at detecting and diagnosing it.
With the discovery of the gene, doctors should now be able to diagnose the disease unequivocally. That new skill will allow physicians to do prenatal diagnosis in families with a high incidence of fragile X syndrome.
The discovery will also permit researchers to identify syndrome patients whose retardation or behavioral difficulties had been attributed to things like autism or a lack of oxygen during birth.
Fragile X syndrome occurs in about 1 in 1,000 males and 1 in 2,500 females, but many more women may be silent carriers. As a cause of mental retardation, this is second only to Down's syndrome, which affects one in 600 babies but is not hereditary. The severity of fragile X syndrome ranges from mild learning
disabilities to retardation so severe that the afflicted can barely talk or function.
Although no treatments exist for the syndrome, scientists said that having identified the gene, they will be able to study how it operates in normal brain cells to influence intelligence, and why flaws in it cause retardation or related types of mental disorders.
"Fragile X has been a puzzling disorder, and there have been so many theories about the disease, all of which have been almost impossible to test," said Dr. Stephen Warren of Emory University School of Medicine in Atlanta, the main author of the new report, which is appearing in tomorrow's issue of the journal Cell.
"Now that we've got the gene, we can start asking a lot of exciting questions and doing a lot of straightforward research."
Warren also said he and his colleagues have a few clues to the shape and function of the protein produced by the gene. The protein has several properties unlike any yet observed in other human proteins, suggesting that it might react with neighboring proteins in brain cells in an entirely novel manner.
Parents told that their fetus has the syndrome may choose abortion, Warren said, but if they proceed with the pregnancy, they can be informed about the possible spectrum of symptoms. Adults found to be carrying the defect may choose not to risk having children in the first place.