Sometimes chunks of DNA rearrange themselves, making them genetically unstable and prone to error.

When egg and sperm unite, the resulting embryo gets one set of chromosomes from each parent; the genes on these chromosomes “recombine,” swapping pieces of DNA to ensure that the offspring is unique. Sometimes, however, these chunks of DNA rearrange within themselves, making them genetically unstable and prone to error. In the September Nature Genetics, three independent studies have turned up one such site that may underlie mental retardation. In one report, Evan Eichler of the University of Washington School of Medicine and colleagues developed a technique to catalog genetic “hot spots” and compare them among normal subjects and children with mental retardation. About 6 percent of the children showed deletions, or missing sections, in these unstable regions. In four individuals, a mutation appeared in the same area, a large stretch of DNA encompassing about six genes—including a gene called tau, which plays a role in Alzheimer’s disease, Parkinson’s disease, and a disorder called frontotemporal dementia. Previously, no link between these conditions and retardation had been suspected. “The new findings provide a starting point to identify genes involved in normal mental functioning as well as some brain disorders,” Eichler says. He adds that in the future, mental retardation may be at least partly treatable. Already, researchers working with fruit flies have reduced abnormal pathways and partially reversed brain damage analogous to Fragile X syndrome, another form of retardation. A hunt is on for medications that may mimic this effect. One day, a similar approach might help children with other mental handicaps.