Tissue and gene specific hypomethylation of the human parathyroid hormone gene: association with parathyroid hormone gene expression in parathyroid glands.

An association between decreased cytosine methylation at specific sites adjacent to or within eukaryotic genes and increased gene expression has been described. To determine if tissues secreting PTH show hypomethylation within the vicinity of the PTH gene, we compared the degree of cytosine methylation in DNA from parathyroid glands and control tissues (leukocytes, anterior pituitary, posterior pituitary, and placenta). We digested DNA with HpaII (which cleaves only unmethylated CCGG sequences) and MspI [which cleaves CCGG and C 5-methyl cytosine GG], hybridized the DNA fragments to a PTH complementary DNA probe, and scanned autoradiograms of Southern blots. After MspI digestion all tissues yielded equivalent amounts of a single hybridizing fragment of 6.7 kilobases. The degree of hypomethylation at sites within and flanking the PTH gene was determined as the ratio of the amount of hybridizing fragments obtained by methylation-sensitive digestion (HpaII) relative to methylation-insensitive digestion (MspI). DNA from all parathyroid glands showed significantly greater hypomethylation of the PTH gene than did DNA from control tissues that did not express the PTH gene. Despite variability in the levels of secretory activity of the different parathyroid glands, we found no significant differences in the degree of hypomethylation of the PTH gene. In contrast to the PTH gene studies, hypomethylation was not seen using GH probe on the same blots. Our findings thus suggest that tissue and gene specific hypomethylation of the PTH gene is associated with expression of the gene.