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Pyrimethamine resistance in the malaria parasite Plasmodium falciparum is characterized by specific point mutations in the dihydrofolate reductase (DHFR) domain of the bifunctional dihydrofolate reductase-thymidylate synthase (DHFR-TS) gene. We have previously explored the effect of these mutations by engineering homologous alleles of Toxoplasma gondii DHFR-TS, which can readily be expressed as recombinant protein for enzymatic studies, or as allelic replacements in transgenic parasites. In order to directly assess the costs of pyrimethamine-resistance in vivo, we have carried out competition studies between mixtures of T. gondii tachyzoites harbouring wild-type or mutant DHFR-TS alleles, both in tissue culture and in mice. Arg59+Asn108 mutants (using the P. falciparum numbering system) exhibit no significant fitness defects in vitro, but a fitness defect of 1.8% per generation in mice. Arg59+Ser223 mutants exhibit fitness defects of >2.8% per generation both in vitro and in vivo, which may explain why this highly pyrimethamine-resistant allele has not been observed in the field. It is important to note that long-term propagation of parasites in vitro or in vivo can produce adaptations affecting fitness by >3.7% per generation, necessitating careful attention to background in head-to-head competition studies. A sensitive PCR-based assay permits different growth rates to be assessed even in the absence of a drug resistance marker that can be scored by plaque assay.


Leah M Fohl, David S Roos. Fitness effects of DHFR-TS mutations associated with pyrimethamine resistance in apicomplexan parasites. Molecular microbiology. 2003 Nov;50(4):1319-27

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PMID: 14622418

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