A partial methylation profile for a CpG site is stably maintained in mammalian tissues and cultured cell lines

We wished to determine if a partial methylation profile for a specific CpG site was stably maintained in both mammalian tissues and cultured cell lines. To accomplish this, we identified a CpG site with a partial methylation profile located upstream of the mouse adenine phosphoribosyltransferase promoter region. This site was found to be methylated at a level of approximately 25% in mouse brain, kidney, lung, and skeletal muscle tissues, at a level close to 50% in liver, and at a level close to 0% in testis. These tissue-specific methylation profiles were not altered during aging. A methylation profile of approximately 25% at this CpG site was also observed in five mouse teratocarcinoma stem cell lines and one additional cultured cell line. This profile, however, was altered upon cellular differentiation, adenine phosphoribosyltransferase hemizygosity, and a loss of adenine phosphoribosyltransferase activity in some of the cultured cell lines. We conclude that partial methylation of a specific CpG site can be stably maintained both in vivo and in vitro and that a mechanism exists for its maintenance. The functional significance of a partial methylation profile remains to be determined.