SULFADOXINE RESISTANCE IN THE HUMAN MALARIA PARASITE PLASMODIUM-FALCIPARUM IS DETERMINED BY MUTATIONS IN DIHYDROPTEROATE SYNTHETASE AND AN ADDITIONAL FACTOR ASSOCIATED WITH FOLATE UTILIZATION

Sulfadoxine/pyrimethamine (Fansidar) is widely used in Africa for trea ting chloroquine-resistant falciparum malaria, To clarify how parasite resistance to this combination arises, various lines of Plasmodium fa lciparum were used to investigate the role of naturally occurring muta tions in the target enzyme, dihydropteroate synthetase (DHPS), in the parasite response to sulfadoxine inhibition. An improved drug assay wa s employed to identify a clear correlation between sulfadoxine-resista nce levels and the number of DHPS mutations, Moreover, tight linkage w as observed between DHPS mutations and high-level resistance in the 16 progeny of a genetic cross between sulfadoxine-sensitive (HB3) and su lfadoxine-resistant (Dd2) parents, However, we also demonstrate a prof ound influence of exogenous folate on IC50 values, which, under physio logical conditions, may have a major role in determining resistance le vels, Importantly, this phenotype does not segregate with dhps genotyp es in the cross, but shows complete linkage to the two alleles of the dihydrofolate reductase (dhfr) gene inherited from the parental lines, However, in unrelated lines, this folate affect correlates less well with DHFR sequence, indicating that the gene responsible may be closel y linked to dhfr, rather than dhfr itself. These results have major im plications for the acquisition of Fansidar resistance by malaria paras ites.