The utilization of the human phosphoglycerate kinase gene in the investigation of X-chromosome inactivation.

The X-linked human Pgk gene has been cloned and partially characterized, and some preliminary results have been obtained regarding active vs. inactive gene comparisons of chromatin structure and methylation patterns. As yet we can say nothing definitive about what role, if any, these differences may play in X inactivation. The studies showing that DNA from the inactive X chromosome in mature somatic cells does not function in transformation of the Hprt gene strongly imply modification of the inactive X chromosome at the DNA level. However, methylation studies with the Hprt, Gd Pgk genes have revealed a complexity of methylation patterns including hypermethylation of parts of the active X gene. Resolution of just what difference is critical in expression, differentiating between cause and effect, and extrapolating to the spreading and initiation aspects of X inactivation are still, unfortunately, long-range goals. The Pgk system may be of special value in unraveling some of these difficult questions. A unique autosomal Pgk locus exists and should allow an informative comparison between an X-linked housekeeping gene and an autosomal, tissue-specific gene encoding proteins of identical enzymatic function. The proximity of Pgk to the X-inactivation control center may be useful in identifying the starting point of this very important event in early mammalian development.