Prenatal diagnosis of a hypermethylated full fragile X mutation in chorionic villi of a male fetus.

Fragile X syndrome, one of the most common human genetic diseases, is characterised by a unique genetic mechanism which involves dynamic mutation because of a heritable unstable DNA sequence and abnormal DNA methylation. Direct detection of the dynamic mutation and its methylation status at the DNA level would facilitate reliable tests for prenatal and postnatal diagnosis of the disease and for carrier detection. However, it has been suggested that DNA methylation can not be used as the basis for prenatal diagnosis as the CpG island is not always methylated in chorionic villus DNA. We report here a male fetus exhibiting both extensive somatic heterogeneity and abnormal hypermethylation of the full fragile X mutation in chorionic villus DNA as well as in fetal tissue DNA. Our results indicate that both somatic heterogeneity and hypermethylation of the full fragile X mutation are events that are clearly detectable in the 11th to 12th week of pregnancy.