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Although it has been known for quite some time that genomic methylation is significantly altered in aging and neoplastic tissues and cells, the underlying mechanisms responsible for these alterations are not yet known. Since DNA methylation affects many different cellular processes including, most significantly, gene expression, elucidation of the basis for aberrations in DNA methylation in aging and cancer is of high priority. To address this problem, we sought to analyze changes in gene expression, protein production and enzyme activity of the three major DNA methyltransferases (Dnmtl, 3a, and 3b) in aging and neoplastically-transformed WI-38 human fetal lung fibroblasts. We have found that the gene expression of each of the three Dnmts parallels changes in protein production and enzyme activity of the Dnmts not only in aging cells, but also in WI-38 fibroblasts induced to undergo neoplastic transformation using defined genetic elements. These findings strongly implicate change in gene expression as an underlying mechanism in the altered genomic methylation of these cells. Striking changes in the gene expression of the Dnmts were observed in aging cells with the mRNA of Dnmtl becoming reduced while the mRNA of Dnmt3b increased steadily in aging cells consistent with our observations in protein production and activity of these enzymes. Surprisingly, Dnmt3a actually decreased in gene expression in aging cells. We therefore propose that the transcriptional control of Dnmt1, the predominant maintenance methyltransferase, is significantly suppressed in aging cells and contributes to the reduced genomic methylation of these cells. The paradoxical sporadic gene hypermethylation in aging cells appears to be related to transcriptional up-regulation of the Dnmt3b gene. In addition, we sought to explore the coordinated changes in gene expression, protein production, and enzyme activity of these Dnmts in early cellular transformation. In these cells, the gene expression of all the three major Dnmts were up-regulated followed by marked increases in Dnmt protein and enzyme activity. These results therefore collectively indicate that changes in transcriptional control of the Dnmts are the likely cause for the known alterations in DNA methylation in aging cells and in cells undergoing tumorigenesis. They also show that changes in transcription of Dnmtl and Dnmt3b are probably most important in affecting the generalized hypomethylation and specific hypermethylation seen in aging cells while gene expression of all the Dnmts is significantly increased in cancer cells. These findings should have broad implications in elucidating the underlying causes of changes in DNA methylation in aging and tumorigenesis and point to variations in gene expression of the individual Dnmts as a likely mechanism involved in these processes.