Specific methylation pattern at the 3' end of the human housekeeping gene for glucose 6-phosphate dehydrogenase

During detailed restriction enzyme mapping of the human X-linked gene Gd, specifying the enzyme glucose 6-phosphate dehydrogenase (G6PD), we have observed the presence, over a 14-kb DNA region spanning across the 3' end of the G6PD transcript, of a large number of methylatable sites. These include 60 HpaII sites, 13 SmaI sites, 22 AvaI sites and 46 HhaI sites. In male leukocyte DNA the majority of HpaII sites are resistant to digestion, indicating that they are in the Cm5CGG form. However, a few sites are found reproducibly unmethylated in 24 samples analyzed. By double and triple digestions we have mapped five unmethylated sites, four of which are within the gene transcript and one distal to the end of transcription. We have also identified a number of sites which are fully methylated, whereas for others the methylation status could not be positively assessed. Thus, in a housekeeping gene expressed in leukocytes, the 3' end is extensively methylated, but some specific sites are unmethylated. In female leukocyte DNA, we found that all sites methylated in males were also methylated. However, of the five sites that are unmethylated in males two are partly methylated in females. This additional site-specific methylation involves approximately 50% of the female leukocyte DNA, and we show evidence that it is associated with the inactive X-chromosome.