Prenatal diagnosis of fragile X and Turner mosaicism in a 12‐week fetus

We report the prenatal diagnosis of fragile X in a Turner mosaic fetus, by cytogenetic and molecular means. To our knowledge this is only the fourth case of fragile X in a Turner syndrome (TS) female, and the ®rst to be diagnosed prenatally. This was the second pregnancy for the 29-year-old female fragile X carrier, with a premutation of approximately 80±85 repeats. Her previous pregnancy was terminated at 13 weeks, following the diagnosis of fragile X in the male fetus. Transabdominal chorionic villus sampling (CVS) was performed at 10 weeks' gestation, with tissue being analysed both by molecular and cytogenetic means. Chromosome analysis of 50 cells from two independent long-term CVS cultures revealed a mosaic karyotype of 45,X[19]/46,XX[31]. Molecular analysis revealed one X chromosome with a normal number of CGG repeats (<54) with the second X chromosome showing an unusually high repeat number (>1000) at the fragile X locus at Xq27.3 (Figure 1). The diagnosis of a mosaic TS fetus with a full fragile X mutation was made. The pregnancy was terminated at 13 weeks, with samples of skin, lung, muscle, amnion, cord and placenta cultured for con®rmation of the mosaicism and for cytogenetic expression of the fragile X, by FdU and thymidine induction using the methods of Rooney and Czepulkowski (1992). Mosaicism was con®rmed in skin, muscle, lung, placenta, and cord, with a frequency between 16.2% and 47.7% for the 45, X cell line. Cytogenetic analysis of fragile X expression was informative in only two tissues; 10 out of 72 cells from the muscle and lung were found to express the fragile site at Xq27.3. Four out of 24 cells containing 45 chromosomes expressed the fragile X, with only 1 cell out of 39 cells containing 46 chromosomes expressing the fragile X. Five of the remaining cells with less than 45 chromosomes contained an X chromosome expressing the fragile site. These results suggest the loss of the paternal X chromosome in the 45,X cell line, resulting in a signi®cant level of the full mutation-carrying maternal X chromosome to be active in the fetus. Fragile X syndrome (FXS), has been reported with aneuploidy, notably Klinefelters (KS) and Down syndrome. Fryns and Van den Berghe (1988), reported three FXS patients with KS in a total of 465 male FXS patients. The phenotypes of the patients varied from a typical Klinefelter syndrome phenotype in a mosaic XY/XXY patient to typical FXS phenotype with classical KS gonadal development; all were mentally retarded. A literature search revealed three previous fragile X±TS cases; all were investigated as part of family studies for fragile X. Shapiro et al. (1994) reported ®ndings on two fragile X±TS mosaic females. The ®rst patient, a 30-year-old female, had 5% of peripheral blood cells with monosomy X. Cytogenetic analysis of the fragile X from three cultures revealed 4 out of 150 cells positive for the Xq27.3 fragile site; two of these cells were also 45,X. Molecular analysis showed heterozygosity at FMR-1 with one X chromosome containing 300 copies of the triplet repeat CGG and the other a normal 38 copies. Physical examination did not reveal any Turner stigmata, and IQ was normal. The second patient was ascertained at 18 years of age. Peripheral blood chromosome analysis showed 74% of cells to be 45,X. None of her 45,X cells showed expression of the fragile site at Xq27.3, which was found in 27% of 46,XX cells (Saul et al., 1991). Physical examination showed short stature and mild dysmorphic facies; IQ was within normal range. Methylation studies on both patients showed methylation of only the abnormal allele in the ®rst patient and 90% methylation of the abnormal allele in the second patient. Both patients showed very skewed inactivation patterns, with the apparent selective inactivation of the mutant FMR-1 allele carrying X chromosome resulting in normal intellect. Freedenburg et al. (1999) reported a 19-year-old mosaic TS female with an abnormal isodicentric X chromosome and fragile X syndrome. Her karyo-