Comparison of the Methylation Patterns of the Two Rat Insulin Genes

We have investigated whether DNA methylation is involved in regulating the expression of the rat insulin genes. We studied cytosine methylation in and near the two insulin genes to examine the correlation of site-specific methylation with expression in different tissues and to compare the demethylation patterns of the two genes. For both genes, we found certain sites undermethylated only in insulin-producing tissues. However, the overall patterns of the two genes in expressing tissues are quite different. The insulin I gene is significantly less methylated than the insulin II gene in a tumor that makes equal amounts of the two insulin RNAs. In another tumor and cell line that make 5-fold higher levels of insulin I RNA than insulin II RNA, the changes in the methylation levels of the two genes do not correlate with the increased expression of insulin I. In nonexpressing tissues, the insulin I gene is completely methylated, while the insulin II gene is demethylated at a number of sites. Thus, the general level of methylation of the insulin genes does not correlate with their differential expression. Furthermore, the methylation at specific sites does not correlate either; some CG dinucleotides that appear to be important for one gene are not present in the other. Our results suggest that there is no specific control by methylation of the expression of the insulin genes in the rat.