Fragile X prenatal analyses show full mutation females at increased risk for mosaic Turner syndrome: Loss of maternal X? C. Dobkin, G. Radu, X. Ding, W.T. Brown, S.L. Nolin. Dept Human Genetics, NYS Inst Basic Research, Staten Island, NY.
   Analysis of 475 prenatal samples for fragile X revealed 5 with 45,X/46,XX mosaicism for Turner syndrome among 86 females that carried the full mutation (>200 CGG repeats). In two cases 50% of the cells were 45,X and in the other three 14%. None of the other 160 female fetuses was found to be mosaic for the X chromosome. This highly significant association of Turner mosaicism and the fragile X full mutation is much more common than expected. Isolated cases of mosaic Turner syndrome in full mutation females have been reported previously but an increased prevalence was not apparent from these reports (Shapiro et al., AJMG 51:507, 1994; Tejada et al., J Med Genet 31:76, 1994; Wilkin et al., Prenat Diagn 20:851, 2000).
   We were interested to determine whether the maternal chromosome with the full mutation or the paternal chromosome was lost. Analysis of polymorphisms identified the parental origin of the lost chromosome in 3 of the 5 cases. Interestingly analysis of the two cases with 50% 45,X showed that the maternal X had been lost. Since approximately 75% of Turner syndrome cases show a loss of the paternal X, our results suggest that the presence of the fragile X mutation on the maternal chromosome may result in its loss.