Prenatal diagnosis of fragile X syndrome by direct detection of the dynamic mutation due to an unstable DNA sequence.

The fragile X syndrome is the most common familial form of mental retardation. The mutation causing the syndrome is dynamic mutation due to an unstable DNA (CCG)n repeat localized at Xq27.3. We have previously reported a PCR procedure to prepare a diagnostic probe, pPCRfx1, which can be used to determine the genotype of fragile X mutation individuals by Southern blot analysis. In the present study, pPCRfx1 was applied to the prenatal diagnosis, using chorionic villus cells, of a fetus which was at risk of having fragile X syndrome. In the PstI assay, the Southern blot showed the typical pattern of a female carrier with the full mutation. Analysis of the DNA methylation patterns by EcoRI + EagI assay showed that the EagI restriction site was not methylated on the mutated X chromosome of chorionic villi, but the sites were totally methylated in the brain and other tissues of the fetus. Thus the fetus was diagnosed to be a heterozygous female carrier of the dynamic mutation involved in the fragile X syndrome.