Rapid analysis of CGG repeat length in the FMR1 gene

The number of trinucleotide CGG repeats at the 5' untranslated region of the FMR1 gene is associated with the fragile X syndrome of mental retardation. We screened for the CGG repeat length in the FMR1 gene of the X-chromosomes from unrelated normal Chinese subjects recruited in Hong Kong and Dalian, a southern and a northern Chinese city respectively. These cities are about 3000 km apart and the residents have few historical interactions. Genomic DNA was analysed by PCR and detected by Southern hybridisation with a radiolabelled (CGG)5 probe for the CGG repeat number. A different distribution pattern of CGG allele size from the Caucasians is observed. It is a bimodal pattern with the most common CGG repeats allele at 29 against 30 in the Caucasians. Among the Hong Kong subjects, five alleles of more than 50 CGG repeats were detected, and four of those were in heterozygous females. There was no difference in the repeat patterns in subjects from the two cities, suggesting no genotypic variation in FMR1 between northern and southern Chinese.