Sequential changes in DNA methylation patterns of the rat phosphoenolpyruvate carboxykinase gene during development

The cytosolic phosphoenolpyruvate carboxykinase [PEPCK; GTP:oxaloacetate carboxy-lase (transphosphorylating), EC 4.1.1.32] gene was isolated from a rat genomic library, and a map of the methylatable sites C-C-G-G and G-C-G-C has been constructed. The extent of methylation of 18 sites in the PEPCK gene in adult liver, kidney, spleen, and heart muscle and in fetal liver has been analyzed using the 5-methylcytosine sensitive enzymes Hpa II and Hha I. This analysis revealed extensive undermethylation of the PEPCK gene in the adult liver and kidney (PEPCK-expressing tissue), whereas the gene in adult spleen and heart muscle as well as in fetal liver (PEPCK-nonexpressing tissues) was heavily methylated. However, unlike the gene in the adult nonexpressing tissues, a region in the middle of the gene was found to be partially hypomethylated in fetal liver. This hypomethylation correlates with the competence of the fetal liver gene to be expressed. Treatment of fetuses by in utero injection of 5-azacytidine causes a hypomethylation-associated activation of the PEPCK gene. Taken together, the present findings suggest a sequential loss of methyl groups during development. When related to PEPCK gene expression, the sequential loss of methyl groups demonstrates an early stage prior to transcription characterized by hypomethylation of discrete sites and a later developmental hypomethylation of all sites associated with the mature active PEPCK gene around the time of birth.