Genes responsible for nonspecific mental retardation

Mental retardation (MR) is a group of heterogeneous clinical conditions. Th ere are more than 900 genetic disorders associated with MR and it affects a round 3% of the general population. MR can be subdivided into syndromic, if it is characterized by consistent and distinctive clinical findings, and n onspecific, if mental retardation is the only primary symptom among affecte d individuals. Many MR conditions described are syndromic, fragile X syndro me being the most common clinical entity among them. In the past years, kno wledge of the molecular basis of mental retardation has increased remarkabl y. Eight genes involved in nonspecific X-Linked MR have been identified so far, including FMR2 OPHN1, GDI1, PAK3, IL1RAPL, TM4SF2, VCX-A, and ARHGEF6. Two other genes also located on the X chromosome have been involved both i n syndromic and in MRX forms (RSK2 and XNP/ATR-X). New insights into the pa thogenesis of mental retardation are being provided by the discovery of the se genes involved in different cellular signaling pathways in the central n ervous system although many others remain to be identified. (C) 2001 Academ ic Press.