FRAXE fragile site associated mental retardation remains unique among X-linked mental retardation phenotypes due to its very mild to borderline nature (50 < IQ < 85). It is the most prevalent form of non-specific X-linked mental retardation so far delineated, with an estimated incidence of at least 1/50-100,000 males, and with more than 50 families known worldwide. The FRAXE site is within, or immediately adjacent to, the 5' untranslated region of the FMR2 gene. Hyperexpansion of the FRAXE CCG repeat silences transcription of the gene. The structure of FMR2 has been characterized, but its function remains unknown. Gene localizations for numerous (> 75) large families with non-specific X-linked mental retardation (MRX) have been determined so far. Recently the molecular basis for some of them has been unravelled by identification of the responsible genes, which participate in complex common signalling pathways. This review summarises the new data on FRAXE associated mental retardation and the FMR2 gene in the light of the recent discoveries of new genes responsible for other forms of non-specific X-linked mental retardation.
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