Methylation of HpaII site at the human DXS16 locus on Xp22 as an assay forabnormal patterns of X inactivation

The highly polymorphic human DXS16 locus on Xp22 contains a BglII restricti on fragment length polymorphism with 33% heterozygosity. We report that met hylation of the HpaII site, 3.1 kb away from this restriction fragment leng th polymorphism, correlates with X-inactivation. The BglII polymorphism dis tinguishes between the maternal and paternal alleles, and HpaII digestion i dentifies their methylation status. The accuracy of this assay was tested o n more than 30 control females and some patients with known patterns of X-i nactivation. The data obtained from this assay agree substantially with tho se obtained using the androgen receptor assay, which is widely used for det ecting patterns of X-inactivation. This is the first marker on Xp22 found t o be suitable for clonal analysis. Of additional significance is this marke r's proximity to the pseudoautosomal boundary on the X chromosome and its p otential use in identifying rare events occurring in this region, which lea d to escape from normal X-inactivation, Am. J. Med. Genet. 98:64-69. (C) 20 01 Wiley-Liss, Inc.