Parental origin of normal X chromosomes in Turner syndrome patients with various karyotypes: implications for the mechanism leading to generation of a 45,X karyotype.

The parental origin of the X chromosome of 45,X females has been the subject of many studies, and most of them have shown that the majority (60-80%) of the X chromosomes are maternal in origin. However, studies on the parental origin of normal X chromosomes are relatively limited for Turner syndrome (TS) females with sex chromosome aberrations. In this study, we used PCR-based typing of highly polymorphic markers and an assay of methylation status of the androgen receptor gene to determine the parental origin of normal X chromosomes in 50 unbiased TS females with a variety of karyotypes. Our results showed a higher paternal meiotic error rate leading to the generation of abnormal sex chromosomes, especially in the case of del(Xp) and abnormal Y chromosomes. Isochromosome Xq and ring/marker X chromosomes, on the other hand, were equally likely the result of both maternal and paternal meiotic errors. A thorough review of previous results, together with our data suggests, that the majority of TS karyotype are caused by paternal meiotic errors that generate abnormal sex chromosomes, and that most 45,X cells are generated by mitotic loss of these abnormal sex chromosomes, resulting in maternal X dominance in these cells.