A naturally occurring tyrosine to histidine replacement at residue 33 of human thymidylate synthase confers resistance to 5-fluoro-2'-deoxyuridine in mammalian and bacterial cells.

Structural changes in the macromolecular targets of pharmacological agents can result in alterations in the efficacy of these agents. In previous studies, we identified a variant structural form of thymidylate synthase (TS) that is associated with relative resistance to 5-fluoro-2'-deoxyuridine, in a human colonic tumor cell line. We now report on the use of DNA transfer techniques to examine directly the effects of each TS form on drug response. TS cDNA constructs, corresponding to the normal or variant TS mRNA, were expressed in Chinese hamster lung cells or in Escherichia coli, and response to 5-fluoro-2'-deoxyuridine was determined. We observed that expression of the variant TS, which differs from the normal form by a tyrosine to histidine substitution at residue 33, confers a 4-fold level of drug resistance in the mammalian cells, as well as in bacteria. The possible role of Tyr-33 in 5-fluoropyrimidine-mediated inhibition of TS is discussed.