Comparison between the cytogenetic test for fragile X and the molecular analysis of the FMR-1 gene in Japanese mentally retarded individuals.

The prevalence of the fragile X syndrome has been estimated by the results of population studies in which the disease was mostly diagnosed by cytogenetic examinations. To examine the reliability of the cytogenetic analysis for the estimation of the prevalence of the fragile X syndrome, the CGG repeat in the FMR-1 gene was assayed by Southern blot hybridization and polymerase chain reaction (PCR) in an institutionalized group of mentally retarded individuals consisting of 305 males and 129 females. The data thus obtained were compared with the cytogenetic data. The DNA analysis detected 7 full mutations among the alleles of the 296 unrelated males and 2 full mutations among the alleles of the 129 unrelated females. These findings were consistent with the cytogenetic data. No premutation was found in 554 X chromosomes in the unrelated mentally retarded patients nor 826 X chromosomes in unrelated control individuals. The distribution of the CGG repeat number in the normal range was not significantly different between the mentally retarded individuals and the control individuals. These data suggest that the estimates of the prevalence of the fragile X syndrome based on cytogenetic data in the population studies are almost reliable. Based on the finding that no premutations were found in this study, a small difference in the prevalence of the fragile X syndrome between Caucasians and Japanese is suggested.