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 res ults of population studies in which the disease was mostly diagnosed b y cytogenetic examinations. To examine the reliability of the cytogene tic analysis for the estimation of the prevalence of the fragile X syn drome, the CGG: repeat in the FMR-1 gene was assayed by Southern blot hybridization and polymerase chain reaction (PCR) in an institutionali zed group of mentally retarded individuals consisting of 305 males and 129 females. The data thus obtained were compared with the cytogeneti c data. The DNA analysis detected 7 full mutations among the alleles o f the 296 unrelated males and 2 full mutations among the alleles of th e 129 unrelated females. These findings were consistent with the cytog enetic data. No premutation was found in 554 X chromosomes in the unre lated mentally retarded patients nor 826 X chromosomes in unrelated co ntrol individuals. The distribution of the CGG repeat number in the no rmal range was not significantly different between the mentally retard ed individuals and the control individuals. These data suggest that th e estimates of the prevalence of the fragile X syndrome based on cytog enetic data in the population studies are almost reliable. Based on th e finding that no premutations were found in this study, a small diffe rence in the prevalence of the fragile X syndrome between Caucasians a nd Japanese is suggested. (c) 1994 Wiley-Liss,Inc.