Advances in X-linked mental retardation

Mutations in genes on the X chromosome rival chromosome aberrations as a cause of mental retardation. Progress in the clinical and molecular delineation of X-linked mental retardation has outpaced progress in understanding autosomal mental retardation. This is a result in large part of the identification of large families in which mental retardation has segregated in an X-linked pattern and the greater ease with which molecular technologies can be applied to hemizygosity in males.

About one-third of the estimated 165 genes associated with syndromal mutations of genes on the X chromosome and one-fourth of the estimated 100 genes associated with nonsyndromal mutations of genes on the X chromosome have been identified. In a number of instances, the same gene is responsible for syndromal and nonsyndromal mutations of genes on the X chromosome. The molecular delineation of mutations of genes on the X chromosome has allowed certain conditions to be lumped together on the basis of allelism and has caused others that appear clinical similar to remain separate.

The clinical and molecular advances have allowed X-linked mental retardation to be more clearly delineated, have provided the means of confirmatory laboratory testing, and have ushered in an era of carrier testing, prenatal diagnosis, and prevention strategies.