CHARACTERIZATION AND LOCALIZATION OF THE FMR-1 GENE-PRODUCT ASSOCIATED WITH FRAGILE-X SYNDROME

THE fragile X syndrome is the most frequent form of inherited mental r etardation after Down's syndrome, having an incidence of one in 1,250 males1,2. The fragile X syndrome results from amplification of the CGG repeat found in the FMR-1 gene3-6. This CGG repeat shows length varia tion in normal individuals and is increased significantly in both carr iers and patients3-6; it is located 250 base pairs distal to a CpG isl and6 which is hypermethylated in fragile X patients4-7. The methylatio n probably results in downregulation of FMR-1 gene expression8. No inf ormation can be deduced about the function of the FMR-1 protein from i ts predicted sequence. Here we investigate the nature and function of the protein encoded by the FMR-1 gene using polyclonal antibodies rais ed against the predicted amino-acid sequences. Four different protein products, possibly resulting from alternative splicing, have been iden tified by immunoblotting in lymphoblastoid cell lines of healthy indiv iduals. All these proteins were missing in cell lines from patients no t expressing FMR-1 messenger RNA. The intracellular localization of th e FMR-1 gene products was investigated by transient expression in COS- 1 cells and found to be cytoplasmic. Localization was also predominant ly cytoplasmic in the epithelium of the oesophagus, but in some cells was obviously nuclear.