Tissue-specific levels of human glucose-6-phosphate dehydrogenase correlate with methylation of specific sites at the 3' end of the gene.

Glucose-6-phosphate dehydrogenase (G6PD) is a ubiquitous enzyme that supplies the cell with NADPH required for a variety of reductive reactions and biosynthetic processes. Therefore, the gene G6PD, located in mammals on the X chromosome, that specifies G6PD can be regarded as a typical housekeeping gene. We have investigated the expression of human G6PD in eight different fetal and adult tissues by determining the level of enzyme activity, the level of G6PD mRNA, and the methylation pattern of the 3' end of the gene, for which we have nucleic acid probes. By combining sequence information with results of Southern blot analysis of DNA samples digested with the methylation-sensitive restriction enzyme Hpa II, we have identified five specific sites that are unmethylated in all tissues examined, a number of sites that are uniformly methylated, and a number of sites that are sometimes methylated. A subset of Hpa II sites, designated on our restriction map as H37-H55, exhibit positive correlation between degree of methylation, level of mRNA, and level of G6PD activity. A comparison of these methylation patterns with those we previously have observed in the G6PD gene on the inactive X chromosome [Toniolo, D., D'Urso, M., Martini, G., Persico, M.G., Tufano, V., Battistuzzi, G. & Luzzatto, L. (1984) EMBO J. 3, 1987-1995] indicates that different sites are associated with X-inactivation and with the regulation of G6PD on the active X chromosome. We conclude that this housekeeping gene is subject to tissue-specific transcriptional regulation, which in turn correlates with methylation of specific sites located at and near the 3' end of the gene.