Inhibitors of dihydrofolate reductase (DHFR) have been a mainstay of chemotherapy of falciparum malaria for more than 50 years. Unfortunately, point mutations in DHFR are the major cause of resistance to drugs of this class, and mutations have rapidly diminished the clinical effectiveness of these drugs. We designed a simple yeast-based system to produce and analyze point mutations in the Plasmodium falciparum DHFR domain of the DHFR-thymidylate synthase gene that confer resistance to PM, the major antifolate currently used in malaria treatment, or to WR, an experimental antifolate. We used PCR mutagenesis, screened more than 1000 DHFR alleles that encoded functional enzymes and studied about 100 that were more resistant than a naturally occurring resistant allele (N51I and S108N). The IC₅₀ values for both drugs were determined for a subset of 44 alleles that carried only a single new mutation. Mutations that increased resistance to PM 10-100 fold (to > 10^-4) were identified in three regions of the DHFR domain: around amino acids 50, 188, and 213. In contrast, mutations to WR-resistance were far less common, and only conferred about 10-fold resistance (to about 10^-7 M). Even more interesting, only the mutations at 188 increased resistance to WR, and mutations in the 213 and other regions either had no effect, or actually increased sensitivity to WR. This collateral hypersensitivity raises the possibility that opposing selection for resistance/sensitivity to PM and WR might be used to slow selection of populations of P.falciparum resistant to antifolate treatment.

Mutation Inventory