[Unstable DNA sequence and methylation in fragile X syndrome]

Fragile X syndrome is characterized as an inherited unstable DNA sequence: the increasement of (CGG)n copy number. It suffered from the inactivation of FMR-1 (fragile X mental retardation 1), which is inhibited by amplification of (CGG)n and methylation of CpG island. The (CGG)n repeat variation in normal Chinese population was detected by PCR with sequencing gel analysis. We also analysed the amplification of (CGG)n and methylation of CpG island at Xq27.3 from 15 individuals in 6 families by Southern hybridization. These results indicate that abnormal methylation of CpG island always occurs in Fra (X) patients together with large amplification of (CGG)n. The (CGG)n could be either stable or amplified in springs of carrier females. These suggest that there would be a new genetic mechanism dynamic mutation.