Specific effects of 5-fluoropyrimidines and 5-azapyrimidines on modification of the 5 position of pyrimidines, in particular the synthesis of 5-methyluracil and 5-methylcytosine in nucleic acids.

5-Fluoropyrimidines and 5-azapyrimidines were found in our laboratory to be specific inhibitors of modification reactions taking place at the 5 position of pyrimidines in nucleic acids. Thus, 5-fluorouracil and 5-fluorouridine specifically inhibit the formation of 5-methyluracil, pseudouridine, and 5,6-dihydrouracil in tRNA. 5-Fluorocytidine, which is partially biotransformed to 5-fluorouracil derivatives in mammalian cells, inhibits the formation of 5-methyluracil, pseudouridine, 5,6-dihydrouracil, and 5-methylcytosine, and 5-azacytidine is a specific inhibitor of the formation of 5-methylcytosine in tRNA and DNA. Inhibitory effects on tRNA modifications require RNA synthesis, as shown by the observation that various inhibitors of RNA synthesis block the drug effects. An inhibitory low-molecular-weight (4-7S) RNA, consisting mainly of tRNA and pre-tRNA, was isolated from livers of mice after treatment with 5-azacytidine. This RNA, when added to an in vitro tRNA methyltransferase assay, specifically interfered with the formation of 5-methylcytosine in substrate tRNA. Similarly, a DNA inhibiting the synthesis of 5-methylcytosine in an in vitro DNA methylation assay was isolated from L1210 leukemic cells treated with a high dose of 5-azacytidine for a short time. Our data are consistent with the hypothesis that incorporation of 5-azacytosine into positions that are normally occupied by C residues destined to become methylated is required for the inhibition to occur, and a similar situation probably applies to the 5-fluoropyrimidine analogs. Analog base moieties occupying such sites are likely to bind strongly, perhaps irreversibly, to the active sites of the particular modifying enzymes. All our observations with the 5-fluoro- and 5-azapyrimidines are in accord with this hypothesis. It was also observed that administration of 5-azacytidine to mice led to strong inhibition of tRNA cytosine-5-methyltransferase, while at the same time the activities and capacities of purine-specific tRNA methyltransferases became strongly elevated after an initial lag period. We speculate that such increases may represent a response of the cell to the methylation defect induced by the drug. Undermodified tRNAs present in neoplastic cells may also trigger an increased synthesis of modifying enzymes. A scheme has been presented which explains increased tRNA turnover and increased activities of modifying enzymes in neoplastic cells as a consequence of a primary defect in tRNA modification.