Progress In Mental Retardation Research

Mental retardation, defined as IQ under 70, affects two to three percent of the population, either as part of a complex syndrome (e.g., Down syndrome) or by itself (non-specific MR). Many different gene defects can cause non-specific MR, including a variety of mutations on the X chromosome that mostly affect males. Because sufferers with different mutations can have similar symptoms, pinpointing individual genes for MR is difficult. In this review, Gécz and Mulley describe how sequence data and resources derived from the Human Genome Project have come together with traditional genetic studies to jumpstart research on non-specific mental retardation.

As a result, scientists have recently identified seven X-linked genes whose mutations cause non-specific MR, among an estimated 20-100 such genes on the X chromosome and possibly hundreds altogether on the human genome. Interestingly, most of the identified genes participate in intracellular signaling and are highly active in the hippocampus, a brain region involved in human memory. With the advent of new genomic technologies such as DNA arrays, say Gécz and Mulley, we might realistically hope to identify all genes involved in mental retardation. Such an achievement would illuminate not only the causes of mental retardation but also the workings of the healthy human brain.