In vitro generation of novel pyrimethamine resistance mutations in the Toxoplasma gondii dihydrofolate reductase.
Pyrimethamine is a potent inhibitor of dihydrofolate reductase and is widely used in the treatment of opportunistic infections caused by the protozoan parasite Toxoplasma gondii. In order to assess the potential role of dhfr sequence polymorphisms in drug treatment failures, we examined the dhfr-ts genes of representative isolates for T. gondii virulence types I, II, and III. These strains exhibit differences in their sensitivities to pyrimethamine but no differences in predicted dhfr-ts protein sequences. To assess the potential for pyrimethamine-resistant dhfr mutants to emerge, three drug-sensitive variants of the T. gondii dhfr-ts gene (the wild-type T. gondii sequence and two mutants engineered to reflect polymorphisms observed in drug-sensitive Plasmodium falciparum) were subjected to random mutagenesis and transfected into either wild-type T. gondii parasites or dhfr-deficient Saccharomyces cerevisiae under pyrimethamine selection. Three resistance mutations were identified, at amino acid residues 25 (Trp-->Arg), 98 (Leu-->Ser), and 134 (Leu-->His).
Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
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D000483:Alleles; D000818:Animals; D030801:Animals, Genetically Modified; D000981:Antiprotozoal Agents / Q000494:pharmacology*; D002460:Cell Line; D004351:Drug Resistance; D005493:Folic Acid Antagonists / Q000494:pharmacology*; D017125:Genes, Protozoan; D016296:Mutagenesis; D021261:Parasitic Sensitivity Tests; D017354:Point Mutation; D011739:Pyrimethamine / Q000494:pharmacology*; D012441:Saccharomyces cerevisiae / Q000235:genetics; D013762:Tetrahydrofolate Dehydrogenase / Q000235:genetics*; D014122:Toxoplasma / Q000187:drug effects* / Q000201:enzymology* / Q000235:genetics; D014162:Transfection
D000981:Antiprotozoal Agents; D005493:Folic Acid Antagonists; D013762:Tetrahydrofolate Dehydrogenase; D011739:Pyrimethamine