Molecular insights into mental retardation: multiple functions for the Fragile X mental retardation protein?

Mental retardation is a frequent cause of intellectual and physical impairment. Several genes associated with mental retardation have been mapped to the X chromosome, among them, there is FMR1. The absence of or mutation in the Fragile Mental Retardation Protein, FMRP, is responsible for the Fragile X syndrome. FMRP is an RNA binding protein that shuttles between the nucleus and the cytoplasm. FMRP binds to several mRNAs including its own mRNA at a sequence region containing a G quartet structure. Some of the candidate downstream genes recently identified encode for synaptic proteins. Neuronal studies indicate that FMRP is located at synapses and loss of FMRP affects synaptic plasticity. At the synapses, FMRP acts as a translational repressor and in particular regulates translation of specific dendritic mRNAs, some of which encode cytoskeletal proteins and signal transduction molecules. This action occurs via a ribonucleoprotein complex that includes a small dendritic non-coding neuronal RNA that determines the specificity of FMRP function via a novel mechanism of translational repression. Since local protein synthesis is required for synaptic development and function, this role of FMRP likely underlies some of the behavioural and developmental symptoms of FRAXA patients. Finally we review recent work on the Drosophila system that connects cytoskeleton remodelling and FMRP function.