X chromosome inactivation patterns in 45,X/46,XX mosaics

To investigate X chromosome inactivation (XCI) patterns in 45,X/46,XX mosai cs, genomic DNA was extracted from peripheral blood samples of 15 female su bjects who showed different proportions of 45,X cell clones. XCI patterns w ere analyzed using two assays. The first assay was the BstXI restriction en donuclease detection of an X-linked phosphoglycerate kinase (PGK) gene poly morphism following digestion of the DNA with methylation-sensitive HpaII, o r with methylation-insensitive AfaI as a control. The second assay was the detection of a CAG triplet repeat polymorphism in the X-linked androgen rec eptor (AR) gene after sodium bisulfite treatment. Of the 15 subjects, 11 we re informative due to heterozygosity for at least one of the polymorphisms (6 were heterozygous for the PGK polymorphism and 9 were heterozygous for t he AR polymorphism). Four of the 11 informative subjects (36%) showed extre mely skewed XCI for at least one of the polymorphisms, which was a much hig her incidence than previously reported for normal females. Moreover, 3 of t hese 4 women had proportions of 45,X cell clones greater than 20%. Although our results may be due to several possible cytogenetic or molecular mechan isms, the most likely explanation is that cases of 45,X/46,XX that contain relatively high levels of 45,X cell clones probably arose due to structural aberrations of the X chromosome undetectable by conventional karyotyping.