VARIATION OF THE CGG REPEAT IN FMR-1 GENE IN NORMAL AND FRAGILE-X CHINESE SUBJECTS

The fragile X syndrome is believed to be caused by an expansion of a C GG trinucleotide repeat segment in the FMR-I gene on the fragile X sit e of the long arm of the X-chromosome. To understand the variation of the CGG repeat in the FMR-1 gene in southern Chinese from the Hong Kon g and Guangzhou area, we undertook the present study. A total of 83 no rmal end three fragile X subjects were examined. In the normal group, 16 distinct alleles, ranging in size from 272 bp to 332 bp with 17 to 37 CGG repeats were detected. A repeat size of 29 was the most frequen t. Compared with data collected in the USA, the repeat size observed i n this population was somewhat smaller. Whether this discrepancy is du e to ethnic difference remains to be determined. The three fragile X p atients examined in this study did not have a greatly expanded CGG seg ment. One of them may be a mosaic with one full and one premutation al lele. The other two patients, although having clinical and cytological features of fragile X syndrome, had a CGG repeat size within normal r ange. To explain this, we infer that the mutation in these patients ma y be caused by other mechanisms, such as other types of FMR-I mutation or mutation in another site. It is possible that the expansion of the CGG repeats may not be as frequent a cause of fragile X syndrome in s outhern Chinese as in other ethnic groups.