Variable effects of DNA-synthesis inhibitors upon DNA methylation in mammalian cells

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Abstract
Post-synthetic enzymatic hypermethylation of DNA was induced in hamster fibrosarcoma cells by the DNA synthesis inhibitors cytosine arabinoside, hydrosyurea and aphidicolin. This effect required direct inhibition of DNA polymerase a or reduction in deoxynucleotide pools and was not specific to a single cell type. At equivalently reduced levels of DNA synthesis, neither cyclobeximide, actinomycin D nor serum deprivation affected DNA methylation in this way. The topoisomerase inhibitors nalidixic acid and novobiocin caused significant hypomethylation indicating that increased 5-mCyt content was not a necessary consequence of DNA synthesis inhibition. The induced hypermethylation (1) occurred predominantly in that fraction of the DNA synthesized in the presence of inhibitor; (2) was stable in the absence of drug; (3) was most prominent in low molecular weight DNA representing sites of initiated but incomplete DNA synthesis; and (4) occurred primarily within CpG dinucleotides, although other dinucleotides were overmethylated as well. Drug-induced CpG hypermethylation may be capable of silencing genes, an effect which may be relevant to the aberrantly expressed genes characteristic of neoplastic cells.