Differential methylation of hypoxanthine phosphoribosyltransferase genes on active and inactive human X chromosomes.

Previous theoretical considerations and some experimental data have suggested a role for DNA methylation in the maintenance of mammalian X chromosome inactivation. The isolation of specific X-encoded sequences makes it possible to investigate this hypothesis directly. We have used cloned fragments of the human hypoxanthine phosphoribosyltransferase (HPRT) gene and methylation-sensitive restriction enzymes to study methylation patterns in genomic DNA of individuals with different numbers of X chromosomes and in somatic cell hybrid lines containing human X chromosomes that are either active or inactive or have been reactivated by treatment with 5-azacytidine. The results of these analyses show that there is hypomethylation of active X chromosomes relative to inactive X chromosomes in the 5' region of this gene. In the middle region of the gene, however, a site that is consistently undermethylated on inactive X chromosomes was identified. Taken together, the data suggest that the overall pattern of methylation, rather than methylation of specific sites, plays a role in the maintenance of X chromosome inactivation.