Wang P, Read M, Sims P F, Hyde J E
Department of Biochemistry and Applied Molecular Biology, University of Manchester Institute of Science and Technology (UMIST), UK.
Mol Microbiol. 1997 Mar;23(5):979-86. doi: 10.1046/j.1365-2958.1997.2821646.x.
Sulfadoxine/pyrimethamine (Fansidar) is widely used in Africa for treating chloroquine-resistant falciparum malaria. To clarify how parasite resistance to this combination arises, various lines of Plasmodium falciparum were used to investigate the role of naturally occurring mutations in the target enzyme, dihydropteroate synthetase (DHPS), in the parasite response to sulfadoxine inhibition. An improved drug assay was employed to identify a clear correlation between sulfadoxine-resistance levels and the number of DHPS mutations. Moreover, tight linkage was observed between DHPS mutations and high-level resistance in the 16 progeny of a genetic cross between sulfadoxine-sensitive (HB3) and sulfadoxine-resistant (Dd2) parents. However, we also demonstrate a profound influence of exogenous folate on IC50 values, which, under physiological conditions, may have a major role in determining resistance levels. Importantly, this phenotype does not segregate with dhps genotypes 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 effect correlates less well with DHFR sequence, indicating that the gene responsible may be closely linked to dhfr, rather than dhfr itself. These results have major implications for the acquisition of Fansidar resistance by malaria parasites.
周效磺胺/乙胺嘧啶（法西达）在非洲被广泛用于治疗对氯喹耐药的恶性疟。为阐明疟原虫对这种联合用药的耐药性是如何产生的，研究人员使用了多种恶性疟原虫株来探究目标酶二氢蝶酸合酶（DHPS）中自然发生的突变在疟原虫对周效磺胺抑制反应中的作用。采用了一种改进的药物检测方法来确定周效磺胺耐药水平与DHPS突变数量之间的明确关联。此外，在周效磺胺敏感（HB3）和耐药（Dd2）亲本之间的遗传杂交产生的16个后代中，观察到DHPS突变与高水平耐药之间存在紧密连锁。然而，我们还证明了外源性叶酸对半数抑制浓度（IC50）值有深远影响，在生理条件下，这可能在决定耐药水平方面起主要作用。重要的是，这种表型在杂交中并不与dhps基因型分离，而是与从亲本系遗传而来的二氢叶酸还原酶（dhfr）基因的两个等位基因完全连锁。然而，在不相关的株系中，这种叶酸效应与DHFR序列的相关性较差，这表明相关基因可能与dhfr紧密连锁，而不是dhfr本身。这些结果对疟原虫获得法西达耐药性具有重要意义。
