Comparative studies on dihydrofolate reductases from Plasmodium falciparum and Aotus trivirgatus

Plasmodium sensitivity to artemisinins: magic bullets hit elusive targets

Artemisinins are efficacious antimalarial drugs widely employed as first-line treatment in endemic countries under the form of combined therapies. Different molecular modes of action have been postulated to explain the parasiticidal effect of these compounds; however, none has been unequivocally accepted, and their physiological relevance is still questioned. Similarly, no definite genetic determinant of Plasmodium sensitivity to artemisinins has been identified so far. A better understanding of the mode of action of artemisinins and the genetic basis of laboratory-induced or field-observed altered susceptibility is crucial for malaria control. In this review different models of artemisinins' molecular action are briefly presented, focusing on recent advances, and the evidence of potential association between various gene polymorphisms and artemisinin resistance is comprehensively reviewed.

Mechanisms of resistance of malaria parasites to antifolates

Antifolate antimalarial drugs interfere with folate metabolism, a pathway essential to malaria parasite survival. This class of drugs includes effective causal prophylactic and therapeutic agents, some of which act synergistically when used in combination. Unfortunately, the antifolates have proven susceptible to resistance in the malaria parasite. Resistance is caused by point mutations in dihydrofolate reductase and dihydropteroate synthase, the two key enzymes in the folate biosynthetic pathway that are targeted by the antifolates. Resistance to these drugs arises relatively rapidly in response to drug pressure and is now common worldwide. Nevertheless, antifolate drugs remain first-line agents in several sub-Saharan African countries where chloroquine resistance is widespread, at least partially because they remain the only affordable, effective alternative. New antifolate combinations that are more effective against resistant parasites are being developed and in one case, recently introduced into use. Combining these antifolates with drugs that act on different targets in the parasite should greatly enhance their effectiveness as well as deter the development of resistance. Molecular epidemiological techniques for monitoring parasite drug resistance may contribute to development of strategies for prolonging the useful therapeutic life of this important class of drugs.

Resistance to levamisole resolved at the single-channel level

Levamisole is commonly used to treat nematode parasite infections but therapy is limited by resistance. The purpose of this study was to determine the mechanism of resistance to this selective nicotinic drug. Levamisole receptor channel currents in muscle patches from levamisole-sensitive and levamisole-resistant isolates of the parasitic nematode Oesophagostomum dentatum were compared. The number of channels present in patches of sensitive and resistant isolates was similar at 10 microM levamisole, but at 30 microM and 100 microM the resistant isolate contained fewer active patches, suggesting desensitization. Mean Po and open times were reduced in resistant isolates. The distribution of conductances of channels in the sensitive isolate revealed a heterogeneous receptor population and the presence of G25, G35, G40, and G45 subtypes. A G35 subtype was missing in the resistant isolate. Resistance to levamisole was produced by changes in the averaged properties of the levamisole receptor population, with some receptors from sensitive and resistant isolates having indistinguishable characteristics.

Dihydrofolate reductase and antifolate resistance in malaria

The dihydrofolate reductase (DHFR, EC 1.5.1.3) domain of Plasmodium falciparum bifunctional dihydrofolate reductase-thymidylate synthase (DHFR-TS) is an attractive target of two important antifolate antimalarials: pyrimethamine (Pyr) and cycloguanil (Cyc). Over recent years, knowledge of malarial DHFR and mechanism(s) of antifolate resistance have increased substantially. These observations have provided an important framework for better understanding the molecular basis of antifolate resistance in malaria. This article provides a brief review and update on molecular aspects relevant to antifolate resistance in malaria.

In vivo and in vitro derived Palo Alto lines of Plasmodium falciparum are genetically unrelated

The Uganda Palo Alto strain of Plasmodium falciparum (FUP) is routinely used as a reference isolate in a number of laboratories. It is one of the few P. falciparum strains that can both be propagated in vivo in monkeys and maintained in culture. The Palo Alto parasites have been characterized for several biochemical and molecular markers, but many of the data reported so far are contradictory. We have analyzed and compared by Southern blotting, PCR and DNA sequencing, several DNA preparations obtained from different FUP lines and from the FCR3 strain. We show here that FUP lines propagated in Saimiri monkeys (FUP/S) and those maintained in culture (FUP/C) for many years in our laboratory differ in the various genetic markers investigated (P190, MSA2, S-Ag, KAHRP, 96 tR, pPFPA rep 20 and pPF 11.1). Therefore, at the present, two genetically unrelated strains of P. falciparum widely distributed over numerous laboratories are designated FUP/Palo Alto. When the Saimiri-propagated FUP/S line was used to initiate an in vitro culture in human red blood cells, no evidence of instability or genetic drift was obtained. The growth rate and genomic characteristics remained constant for several months. Likewise, the FUP/C line was found unchanged after three transfers in Saimiri monkeys. FUP/CP parasites were shown to be genetically closely related to FCR3. In addition, a subline of FUP/C strain selected by repeated flotation on gelatin was found to differ in several characters such as KHARP, P190 and S-antigen genes, which are known to be located on different chromosomes.

Molecular genetics of drug resistance in Plasmodium faiciparum malaria

Resistance to dihydro folate reductase inhibitors and resistance to chloroquine have been mapped to single genetic loci in Plasmodium falciparum. Specific point mutations in the dihydro folate reductase gene confer different degrees of resistance to two dihydro folate inhibitors, cycloguanil and pyrimethamine, depending on the positions of the mutations and the residues involved. The chloroquine resistance locus has been mapped to a 400 kilobase (kb) segment of chromosome 7 in a P. falciparum cross. Identification and characterization of genes within this segment should lead to an understanding of the rapid drug efflux mechanism responsible for chloroquine resistance.

The dihydrofolate reductase-thymidylate synthetase gene in the drug resistance of malaria parasites

Resistance to antifolate drugs such as pyrimethamine is widespread among malaria parasites of the most pathogenic species Plasmodium falciparum. These drugs inhibit the dihydrofolate reductase activity of the dihydrofolate reductase-thymidylate synthetase (DHFR-TS) bifunctional enzyme. This review examines work done to characterize the enzyme, the cloning of plasmodial DHFR-TS genes, chromosomal mapping studies of these genes by pulsed-field gel electrophoresis, and the structural insights into the mechanism of drug resistance that have been gained by comparing genes from drug-sensitive parasites with those from drug-resistant strains that have arisen in the field or after experimental induction.

The prevention of antimalarial drug resistance

The deployment of antiprotozoal drugs on a large scale for prophylaxis or monotherapy inevitably results in the selection of drug-resistance. The use of appropriately selected drug combinations may impede this process. Point mutations underlie resistance to dihydrofolate reductase inhibitors such as pyrimethamine. Potentiating combinations of such compounds with sulfonamides or sulfones have effectively delayed resistance to them. The use of triple combinations may be of value in protecting such compounds as chloroquine and mefloquine, resistance to which is associated in some cases with gene amplification. It is essential to seek partner compounds for any new antimalarials, e.g. artemisinin. Past experience with existing compounds is discussed and the need to make use of all available means of interrupting malaria transmission is stressed, rather than depending entirely on drugs.

Point mutations in the dihydrofolate reductase-thymidylate synthase gene as the molecular basis for pyrimethamine resistance in Plasmodium falciparum

The dihydrofolate reductase-thymidylate synthase (DHFR-TS) bifunctional complex from pyrimethamine-sensitive (3D7) and drug-resistant (HB3 and 7G8) clones from Plasmodium falciparum was purified to homogeneity. A modified sequence of purification steps with a 10-formylfolate affinity column at its center, allows the isolation of the enzyme complex with a 10-fold higher yield than previously reported, irrespective of the pyrimethamine resistance of the parasites. Titration of the homogenous DHFR-TS complex with the inhibitor revealed a 500-fold lower affinity of the enzyme from clone 7G8 for the drug than found with the enzyme from clone 3D7. Direct comparison of the homogenous enzyme preparations on SDS-PAGE revealed no difference in the molecular mass of the DHFR-TS from the 3 clones, nor could a reproducible difference be detected in the peptide patterns obtained after digesting the DHFR-TS complex with various proteases. The amplification of segments from the DHFR-TS coding region of the 3 clones and 7 isolates of P. falciparum by polymerase chain reaction resulted in fragments of the predicted length without any size heterogeneity. The DNA sequence of the DHFR coding region from FCR-3, 3D7, HB3 and 7G8 differs in a total of 4 nucleotides. One point mutation changes amino acid residue 108 from threonine (FCR-3) or serine (3D7) to asparagine (HB3 and 7G8). The presence of asparagine-108 appears to be the molecular basis of pyrimethamine resistance of HB3 and 7G8. The degree of resistance is associated with a point mutation affecting the codon for amino acid 51 in 7G8.

De novo and salvage biosynthesis of pteroylpentaglutamates in the human malaria parasite, Plasmodium falciparum

Plasmodium falciparum was shown to synthesize pteroylpolyglutamate de novo from guanosine 5'-triphosphate (GTP), p-aminobenzoate (PABA), and L-glutamate (L-Glu). The parasite also had the capacity to synthesize pteroylpolyglutamate from both intact and degradation moieties (p-aminobenzoylglutamate and pterin-aldehyde) of exogenous folate added into the growth medium. The major product was identified as 5-methyl-tetrahydroteroylpentaglutamate following exposure to pteroylpolyglutamate hydrolase and oxidative degradation of the C9-N10 bond in the molecule and identification of products by reversed-phase high performance liquid chromatography. Inhibition of pteroylpentaglutamate synthesis from the radiolabelled metabolic precursors (GTP, PABA, L-Glu) and folate by the antifolate antimalarials, pyrimethamine and sulfadoxine at therapeutic concentrations, may suggest the existence of a unique biosynthetic pathway in the malaria parasite.

Amino acid changes linked to pyrimethamine resistance in the dihydrofolate reductase-thymidylate synthase gene of Plasmodium falciparum

We describe the isolation and the sequence of the gene for the bifunctional enzyme dihydrofolate reductase-thymidylate synthase (DHFR-TS; EC 1.5.1.3 and EC 2.1.1.45, respectively) from two pyrimethamine-resistant clones of Plasmodium falciparum, HB3 and 7G8. We have also derived the sequence of the DHFR portion of the gene, by amplification using polymerase chain reaction, for the pyrimethamine-sensitive clone 3D7 and the pyrimethamine-resistant strains V-1, K-1, Csl-2, and Palo-alto. The deduced protein sequence of the resistant DHFR portion of the enzyme from HB3 contained a single amino acid difference from the pyrimethamine-sensitive clone 3D7. It is highly likely that this difference is involved in the mechanism of drug resistance in HB3. The sequence of the DHFR gene from other pyrimethamine-resistant strains contains the same amino acid difference from the sensitive clone 3D7. However, they all differ at one other site that may influence pyrimethamine resistance. The DHFR-TS gene is present as a single copy on chromosome 4 in all pyrimethamine-sensitive and pyrimethamine-resistant isolates tested. Therefore, the molecular basis of pyrimethamine resistance in the parasites tested is not amplification of the DHFR-TS gene.

Evidence that a point mutation in dihydrofolate reductase-thymidylate synthase confers resistance to pyrimethamine in falciparum malaria

Analysis of a genetic cross of Plasmodium falciparum and of independent parasite isolates from Southeast Asia, Africa, and South America indicates that resistance to pyrimethamine, an antifolate used in the treatment of malaria, results from point mutations in the gene encoding dihydrofolate reductase-thymidylate synthase (EC 1.5.1.3 and EC 2.1.1.45, respectively). Parasites having a mutation from Thr-108/Ser-108 to Asn-108 in DHFR-TS are resistant to the drug. The Asn-108 mutation occurs in a region analogous to the C alpha-helix bordering the active site cavity of bacterial, avian, and mammalian enzymes. Additional point mutations (Asn-51 to Ile-51 and Cys-59 to Arg-59) are associated with increased pyrimethamine resistance and also occur at sites expected to border the active site cavity. Analogies with known inhibitor/enzyme structures from other organisms suggest that the point mutations occur where pyrimethamine contacts the enzyme and may act by inhibiting binding of the drug.

The changing response of Plasmodium falciparum to antimalarial drugs in east Africa

For the past 20 years, chloroquine chemotherapy has been the single most effective malaria control measure in East Africa. The advent of chloroquine-resistant Plasmodium falciparum has reduced the clinical effectiveness of chloroquine and this trend is likely to continue. Combinations of antifol drugs are at present effective inhibitors of most P. falciparum infections in the region, in spite of widespread resistance to pyrimethamine. The development of (i) sensitive methods for monitoring changes in sensitivity to antifol combinations, (ii) more effective and less costly alternatives to commercially available combinations, and (iii) investigation of host and parasite factors leading to drug treatment failure in P. falciparum infections has been the primary goal of the Wellcome Trust Research Laboratories in Kenya (directed by Dr W.M. Watkins) within the malaria programme of the Kenya Medical Research Institute, and collaborating laboratories at the School of Tropical Medicine and the University of Liverpool.

Pyrimethamine resistant Plasmodium falciparum: overproduction of dihydrofolate reductase by a gene duplication

The accumulation of [3H]pyrimethamine by pyrimethamine-resistant (Pyrr) mutants of the Plasmodium falciparum strain FCR3 was examined by measuring the accumulation of drug in infected red blood cells. [3H]Pyrimethamine was stage specifically accumulated in trophozoites and schizont infected red blood cells. The mutant parasites accumulated drug as efficiently as FCR3. Pyrimethamine was associated with a high molecular weight protein that eluted from a Sephadex G200 column exactly as [3H]fluorodeoxyuridinemonophosphate (FdUMP) labeled parasite dihydrofolate reductase-thymidylate synthetase (DHFR-TS) enzyme. These results suggested that the pyrimethamine resistance was not associated with decreased drug permeability of the membrane. DHFR-TS-[3H]FdUMP enzyme complex of all the Pyrr mutants and FCR3 had a monomer of 70 kDa as measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. One highly resistant mutant, FCR3-D7, exhibited a 5-10 fold higher uptake of pyrimethamine and a proportionately higher amount of DHFR-TS protein than FCR3 but only a normal level of DHFR activity. The genomic DNA of FCR3-D7 was shown to contain at least twice as much DHFR-TS specific DNA than either FCR3-D8, another Pyrr mutant, or FCR3. Preliminary results suggested some of the DHFR-TS genetic material in FCR3-D7 is associated with a gene duplication.

The mechanism of pyrimethamine resistance in Plasmodium falciparum

The uptake of radioactive pyrimethamine by a sensitive and a resistant strain of Plasmodium falciparum, the metabolic fate of pyrimethamine inside these parasites and the kinetic properties of dihydrofolate reductase (DHFR) from both strains have been studied. Uptake of the drug was identical in both strains and no metabolite of pyrimethamine was found in either strain. DHFR from the resistant strain was 300 times less sensitive to inhibition by pyrimethamine than the enzyme from the sensitive strain, while the Michaelis constant for dihydrofolate remained unchanged and inhibition was competitive in both cases. Altered properties of plasmodial DHFR are apparently the only mechanism responsible for pyrimethamine resistance in the strain of Plasmodium falciparum studied.

Plasmodium falciparum: induction, selection, and characterization of pyrimethamine-resistant mutants

We have selected eight pyrimethamine resistant mutants of a cloned, drug sensitive, Plasmodium falciparum malaria parasite, strain FCR3. The mutants exhibited resistance to between 10 and 200 times higher concentrations of drug than the wild type parasite. The mutants were selected from cultured parasites that were either unmutagenized or N-methyl-N'-nitro-N-nitrosoguanidine mutagenized. One mutant was shown to contain a mutant dihydrofolate reductase enzyme in parasite extracts that exhibited (1) a five- to ninefold reduction in its binding of methotrexate, (2) an undetectable enzyme activity based on the spectrophotometric conversion of dihydrofolate to tetrahydrofolate, and (3) essentially normal amounts of the parasite's bifunctional thymidylate synthetase-dihydrofolate reductase enzyme. Other mutants exhibited both normal dihydrofolate reductase specific activity and normal enzyme sensitivity to the inhibitory activity of the drug.

Altered dihydrofolate reductase in pyrimethamine-resistant Plasmodium falciparum

Dihydrofolate reductase (EC 1.5.1.3, tetrahydrofolate dehydrogenase), the target enzyme for the chemotherapeutic attack by pyrimethamine, has been studied in drug-sensitive and resistant strains of Plasmodium falciparum. No evidence was found for overproduction of this enzyme in drug-resistant strains. Results presented here indicate that pyrimethamine resistance of P. falciparum depends on a modified dihydrofolate reductase, which shows less affinity for pyrimethamine and dihydrofolate. The inhibition constants for pyrimethamine increased from 0.19 nM for the drug-sensitive strain FCH-5 to 4.1 and 21.6 nM for the drug-resistant strains FVOR and K 1, respectively. In addition, the Km-values for dihydrofolate increased from 2.5 microM to 21 and 28 microM, respectively. The type of inhibition by pyrimethamine changed from competitive with respect to dihydrofolate in drug-sensitive strain to non-competitive in drug-resistant strains of P. falciparum.

In vitro activities of and mechanisms of resistance to antifol antimalarial drugs

Certain drugs that interfere with folate metabolism (sulfones, sulfonamides, and inhibitors of dihydrofolate reductase) play an important role in the chemotherapy and prophylaxis of malaria. The activities and mechanisms of action of these drugs are regarded as similar in most respects to their activities against procaryotic microorganisms. Believed incapable of utilizing intact exogenous folates, plasmodia have been regarded as dependent on de novo synthesis of required folate cofactors. The present investigation, conducted in pursuit of a method for testing the in vitro susceptibility of Plasmodium falciparum to antifol antimalarial drugs, produced evidence that earlier assumptions about the folate metabolism of this organism are not correct. Three of four isolates of P. falciparum were successfully maintained in a culture medium depleted of folic acid and p-aminobenzoic acid. The antimalarial activities of sulfonamides and dihydrofolate reductase inhibitors were, furthermore, variably antagonized by the presence of folic acid and p-aminobenzoic acid in the culture medium. Optimum conditions for assessment of antifol antimalarial activity in vitro therefore require precise control of these factors in the culture medium. Our results suggest that resistance to antifol antimalarial drugs involves a complex of factors related to both the de novo synthesis of active folate cofactors and the ability to utilize exogenous intact folates in various forms.

Mechanism of pyrimethamine resistance in recent isolates of Plasmodium falciparum

Clones of Plasmodium falciparum prepared from recent isolates of infected blood were studied to determine the molecular mechanism of naturally occurring pyrimethamine resistance. Total DNA, as well as thymidylate synthetase and dihydrofolate reductase activities, were characterized from these lines. Restriction analysis of DNA from pyrimethamine-susceptible and -resistant lines of the parasite showed no obvious amplification of any DNA fragment. Further, analysis of DNA from resistant and susceptible lines by centrifugation in cesium chloride-ethidium bromide revealed no extrachromosomal amplification in the resistant line. Comparison of the dihydrofolate reductase enzyme activity in the two lines revealed similar KmS for substrate but a large difference in the inhibition constant for pyrimethamine. Additionally, the enzyme from the resistant line was considerably more stable in vitro than the corresponding enzyme from the susceptible line. The thymidylate synthetase activity in the two lines was similar and unaffected by pyrimethamine. The mechanism of drug resistance in this isolate involves altered properties of the dihydrofolate reductase conferring both a different affinity for the drug and increased stability.