Characterisation of the dihydrofolate reductase-thymidylate synthetase gene from human malaria parasites highly resistant to pyrimethamine

To investigate the genetic basis of drug resistance in human malaria parasites, we have sequenced the entire dihydrofolate reductase thymidylate synthetase DHFR-TS bifunctional gene from the highly pyrimethamine-resistant K1 isolate of Plasmodium falciparum. The protein is predicted to consist of 607 amino acids (aa), (71,685 Da), with an N-terminal methionine encoded by the second start codon of the open reading frame. Compared to the sequence from drug-sensitive parasites, there are two nucleotide changes in the coding region which bring about a substitution of Arg for Cys at aa position 59 and Asn for Thr at aa position 108. Both changes occur in regions of the DHFR domain involved in inhibitor and cofactor binding and are hence strongly implicated in drug resistance. The gene is present as a single copy in both K1 and drug-sensitive FCR3 isolates, and is assigned to chromosome 4. Codon usage follows the pattern observed in that of malarial surface antigen genes, with the exception fo codons corresponding to Val and Pro. The Asn and Lys contents of the predicted protein are exceptionally high, these residues being particularly concentrated in the DHFR and junction domains.     

High-efficiency transfection and drug selection of genetically transformed blood stages of the rodent malaria parasite Plasmodium berghei

This protocol describes a method of genetic transformation for the rodent malaria parasite Plasmodium berghei with a high transfection efficiency of 10(-3)-10(-4). It provides methods for: (i) in vitro cultivation and purification of the schizont stage;(ii) transfection of DNA constructs containing drug-selectable markers into schizonts using the nonviral Nucleofector technology; and (iii) injection of transfected parasites into mice and subsequent selection of mutants by drug treatment in vivo. Drug selection is described for two (antimalarial) drugs, pyrimethamine and WR92210. The drug-selectable markers currently in use are the pyrimethamine-resistant dihydrofolate reductase (dhfr) gene of Plasmodium or Toxoplasma gondii and the DHFR gene of humans that confer resistance to pyrimethamine and WR92210, respectively. This protocol enables the generation of transformed parasites within 10-15 d. Genetic modification of P. berghei is widely used to investigate gene function in Plasmodium, and this protocol for high-efficiency transformation will enable the application of large-scale functional genomics approaches.     

Phylogenetic classification of protozoa based on the structure of the linker domain in the bifunctional enzyme, dihydrofolate reductase-thymidylate synthase

We have determined the crystal structure of dihydrofolate reductase-thymidylate synthase (DHFR-TS) from Cryptosporidium hominis, revealing a unique linker domain containing an 11-residue alpha-helix that has extensive interactions with the opposite DHFR-TS monomer of the homodimeric enzyme. Analysis of the structure of DHFR-TS from C. hominis and of previously solved structures of DHFR-TS from Plasmodium falciparum and Leishmania major reveals that the linker domain primarily controls the relative orientation of the DHFR and TS domains. Using the tertiary structure of the linker domains, we have been able to place a number of protozoa in two distinct and dissimilar structural families corresponding to two evolutionary families and provide the first structural evidence validating the use of DHFR-TS as a tool of phylogenetic classification. Furthermore, the structure of C. hominis DHFR-TS calls into question surface electrostatic channeling as the universal means of dihydrofolate transport between TS and DHFR in the bifunctional enzyme.     

Novel triazine JPC-2067-B inhibits Toxoplasma gondii in vitro and in vivo

BACKGROUND AND METHODOLOGY: Toxoplasma gondii causes substantial morbidity, mortality, and costs for healthcare in the developed and developing world. Current medicines are not well tolerated and cause hypersensitivity reactions. The dihydrotriazine JPC-2067-B (4, 6-diamino-1, 2-dihydro-2, 2-dimethyl-1-(3'(2-chloro-, 4-trifluoromethoxyphenoxy)propyloxy)-1, 3, 5-triazine), which inhibits dihydrofolate reductase (DHFR), is highly effective against Plasmodium falciparum, Plasmodium vivax, and apicomplexans related to T. gondii. JPC-2067-B is the primary metabolite of the orally active biguanide JPC-2056 1-(3'-(2-chloro-4-trifluoromethoxyphenyloxy)propyl oxy)- 5-isopropylbiguanide, which is being advanced to clinical trials for malaria. Efficacy of the prodrug JPC-2056 and the active metabolite JPC-2067-B against T. gondii and T. gondii DHFR as well as toxicity toward mammalian cells were tested. PRINCIPAL FINDINGS AND CONCLUSIONS: Herein, we found that JPC-2067-B is highly effective against T. gondii. We demonstrate that JPC-2067-B inhibits T. gondii growth in culture (IC50 20 nM), inhibits the purified enzyme (IC50 6.5 nM), is more efficacious than pyrimethamine, and is cidal in vitro. JPC-2067-B administered parenterally and the orally administered pro-drug (JPC-2056) are also effective against T. gondii tachyzoites in vivo. A molecular model of T. gondii DHFR-TS complexed with JPC-2067-B was developed. We found that the three main parasite clonal types and isolates from South and Central America, the United States, Canada, China, and Sri Lanka have the same amino acid sequences preserving key binding sites for the triazine. SIGNIFICANCE: JPC-2056/JPC-2067-B have potential to be more effective and possibly less toxic treatments for toxoplasmosis than currently available medicines.     

Selection against the dihydrofolate reductase-thymidylate synthase (DHFR-TS) locus as a probe of genetic alterations in Leishmania major.

The genome of the trypanosomatid protozoan genus Leishmania has been shown to undergo a number of changes relevant to drug resistance and virulence, such as gene amplification, chromosomal rearrangement, and variation in ploidy. Experimental approaches to the study of genomic changes have in some cases been limited by the fact that Leishmania cells are asexual diploids, as are some other trypanosomatids, pathogenic fungi, and cultured mammalian cells. Here we report upon a system which permits the measurement of several types of genomic change occurring at the dihydrofolate reductase-thymidylate synthase (DHFR-TS) locus. First, we show that DHFR-TS can function as a positive/negative marker. We used selection against DHFR-TS on a heterozygous line (+/HYG) to generate colonies exhibiting both loss of heterozygosity and structural mutations in DHFR-TS, permitting the first measurement of mutation frequencies in this parasite. Loss of heterozygosity occurred at a frequency ranging from 10(-4) to 10(-6) and was elevated 24-fold by treatment with gamma-irradiation, while the frequency of other events was less than 10(-6) and was increased more than 1,000-fold by nitrosoguanidine treatment. The frequency of loss of heterozygosity relative to other processes such as mutation and gene replacement has important implications for genetic variability in natural Leishmania populations and the generation of both targeted and random mutations. We also developed a protocol for null targeting of diploid cells, in which transfection of a DHFR-TS deletion construct into Leishmania cells followed by negative selection yielded parasites lacking DHFR-TS or foreign sequences. The null-targeting method can be applied to any diploid cell, at any locus for which a negative selection exists. Such marker-free auxotrophic Leishmania cells show potential as an attenuated vaccine, and the methods developed here provide a new approach for manipulating and characterizing the plasticity of the Leishmania genome.     

Hybridization arrest of cell-free translation of the malarial dihydrofolate reductase/thymidylate synthase mRNA by anti-sense oligodeoxyribonucleotides.

In order to inhibit the in vitro translation of Plasmodium falciparum mRNA coding for the bifunctional enzyme dihydrofolate reductase-thymidylate synthase (DHFR-TS), oligodeoxynucleotides (ODNs) were directed against the translation initiation site or a site in the TS-coding region. In both cases considerable hybridization arrest, i.e. greater than 50% inhibition, was only achieved if the lengths of the ODNs to the two regions were 30 and 39 nucleotides, respectively, or longer. The ODN with the highest efficiency was a 49-mer directed against the TS-coding region (OTS49); 45 microM was sufficient to inhibit the expression of DHFR-TS by almost 90%. In this case the synthesis of DHFR-TS was interrupted at the binding site of OTS49 by a RNase H-independent mechanism. The resulting polypeptide was smaller (55 kDa) than one subunit of the native protein (71 kDa) and lacked TS activity.     

Molecular epidemiology of drug resistance markers of Plasmodium falciparum in Yunnan Province, China

ABSTRACT: BACKGROUND: The mutations in Plasmodium falciparum chloroquine resistance transporter (pfcrt), multidrug resistance 1 (pfmdr1), dihydrofolate reductase (pfdhfr), dihydropteroate synthase (pfdhps) and ATPase (pfatp6) genes were associated with anti-malaria drug resistance. The aim of this study was to investigate the prevalence of polymorphisms in pfcrt, pfmdr1, pfdhfr, pfdhps and pfatp6 in Yunnan Province. Finger-prick blood samples were collected from malaria-positive patients from Yunnan Province in 2009-2010. Single-nucleotide polymorphisms (SNPs) in the resistance-related genes were analysed by various PCR-based methods. RESULTS: A total of 108 blood samples were collected. Although chloroquine has not been used to treat falciparum malaria for nearly 30 years, 95.3% of the parasites still carried the pfcrt K76T mutation, whereas the majority of isolates displayed the wild-type pfmdr1 N86 and D1246 sequences. The molecular level of sulphadoxine-pyrimethamine resistance in P. falciparum was high. The most prevalent mutation was pfdhfr C59R (95.9%), whereas the frequencies of the quadruple, triple and double mutants were 22.7% (N51I/C59R/S108N/I164L), 51.5% (N51I/C59R/S108N, N51I/C59R/I164L and C59R/S108N/ I164L) and 21.6% (N51I/ C59R, C59R/S108N and C59R/I164L), respectively. A437G (n=77) and K540E (n=71) were the most prevalent mutations in pfdhps, and 52.7% of the samples were double mutants, among which A437G/K540E was the most common double mutation (37/49). Quadruple mutants were found in 28.0% (26/93) of samples. A total of 8.6% of isolates (8/93) carried the S436A/A437G/A581G triple mutation. No mutations were found in pfatp6 codons 623 or 769, but another two mutations (N683K and R756K) were found in 4.6% (3/97) and 9.2% (6/97) of parasite isolates, respectively. CONCLUSIONS: This study identified a high frequency of mutations in pfcrt, pfdhfr and pfdhps associated with CQ and SP resistance in P. falciparum and no mutations linked to artemisinin resistance (pfatp6). Molecular epidemiology should be included in routine surveillance protocols and used to provide complementary information to assess the appropriateness of the current national anti-malarial drug policy.     

Essential protein-protein interactions between Plasmodium falciparum thymidylate synthase and dihydrofolate reductase domains

In Plasmodium falciparum, dihydrofolate reductase and thymidylate synthase activities are conferred by a single 70-kDa bifunctional polypeptide (DHFR-TS, dihydrofolate reductase-thymidylate synthase) which assembles into a functional 140-kDa homodimer. In mammals, the two enzymes are smaller distinct molecules encoded on different genes. A 27-kDa amino domain of malarial DHFR-TS is sufficient to provide DHFR activity, but the structural requirements for TS function have not been established. Although the 3'-end of DHFR-TS has high homology to TS sequences from other species, expression of this protein fragment failed to yield active TS enzyme, and it failed to complement TS(-) Escherichia coli. Unexpectedly, even partial 5'-deletion of full-length DHFR-TS gene abolished TS function on the 3'-end. Thus, it was hypothesized that the amino end of the bifunctional parasite protein plays an important role in TS function. When the 27-kDa amino domain (DHFR) was provided in trans, a previously inactive 40-kDa carboxyl-domain from malarial DHFR-TS regained its TS function. Physical characterization of the "split enzymes" revealed that the 27- and the 40-kDa fragments of DHFR-TS had reassembled into a 140-kDa hybrid complex. Thus, in malarial DHFR-TS, there are physical interactions between the DHFR domain and the TS domain, and these interactions are necessary to obtain a catalytically active TS. Interference with these essential protein-protein interactions could lead to new selective strategies to treat malaria resistant to traditional DHFR-TS inhibitors.     

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 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.     

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.     

A Population Genetic Model for the Initial Spread of Partially Resistant Malaria Parasites under Anti-Malarial Combination Therapy and Weak Intrahost Competition

To develop public-health policies that extend the lifespan of affordable anti-malarial drugs as effective treatment options, it is necessary to understand the evolutionary processes leading to the origin and spread of mutations conferring drug resistance in malarial parasites. We built a population-genetic model for the emergence of resistance under combination drug therapy. Reproductive cycles of parasites are specified by their absolute fitness determined by clinical parameters, thus coupling the evolutionary-genetic with population-dynamic processes. Initial mutations confer only partial drug-resistance. Therefore, mutant parasites rarely survive combination therapy and within-host competition is very weak among parasites. The model focuses on the early phase of such unsuccessful recurrent mutations. This ends in the rare event of mutants enriching in an infected individual from which the successful spread of resistance over the entire population is initiated. By computer simulations, the waiting time until the establishment of resistant parasites is analysed. Resistance spreads quickly following the first appearance of a host infected predominantly by mutant parasites. This occurs either through a rare transmission of a resistant parasite to an uninfected host or through a rare failure of drugs in removing “transient” mutant alleles. The emergence of resistance is delayed with lower mutation rate, earlier treatment, higher metabolic cost of resistance, longer duration of high drug dose, and higher drug efficacy causing a stronger reduction in the sensitive and resistant parasites’ fitnesses. Overall, contrary to other studies’ proposition, the current model based on absolute fitness suggests that aggressive drug treatment delays the emergence of drug resistance.     

Relative replicative fitness of human immunodeficiency virus type 1 mutants resistant to enfuvirtide (T-20)

Resistance to enfuvirtide (ENF; T-20), a fusion inhibitor of human immunodeficiency virus type 1 (HIV-1), is conferred by mutations in the first heptad repeat of the gp41 ectodomain. The replicative fitness of recombinant viruses carrying ENF resistance mutations was studied in growth competition assays. ENF resistance mutations, selected in vitro or in vivo, were introduced into the env gene of HIV-1(NL4-3) by site-directed mutagenesis and expressed in HIV-1 recombinants carrying sequence tags in nef. The doubling time of ENF-resistant viruses was highly correlated with decreasing ENF susceptibility (R(2) = 0.859; P < 0.001). Initial fitness experiments focused on mutants identified by in vitro selection in the presence of ENF (L. T. Rimsky, D. C. Shugars, and T. J. Matthews, J. Virol. 72:986-993, 1998). In the absence of drug, these mutants displayed reduced fitness compared to wild-type virus with a relative order of fitness of wild type > I37T > V38 M > D36S/V38 M; this order was reversed in the presence of ENF. Likewise, recombinant viruses carrying ENF resistance mutations selected in vivo displayed reduced fitness in the absence of ENF with a relative order of wild type > N42T > V38A > N42T/N43K approximately N42T/N43S > V38A/N42D approximately V38A/N42T. Fitness and ENF susceptibility were inversely correlated (r = -0.988; P < 0.001). Similar results were obtained with recombinants expressing molecularly cloned full-length env genes obtained from patient-derived HIV-1 isolates before and after ENF treatment. Further studies are needed to determine whether the reduced fitness of ENF-resistant viruses alters their pathogenicity in vivo.     

Synthesis and in vitro activities of ferrocenic aminohydroxynaphthoquinones against Toxoplasma gondii and Plasmodium falciparum

Fourteen ferrocenyl aminohydroxynaphthoquinones, analogues of atovaquone, were synthesized from the hydroxynaphthoquinone core. These novel atovaquone derivatives were tested for their in vitro activity against two apicomplexan parasites of medical importance, Toxoplasma gondii and Plasmodium falciparum, including resistant strains to atovaquone (T. gondii) and chloroquine (P. falciparum). Three of these ferrocenic atovaquone derivatives composed of the hydroxynaphthoquinone core plus an amino-ferrocenic group and an aliphatic chain with 6-8 carbon atoms were found to be significantly active against T. gondii. Moreover, these novel compounds were also effective against the atovaquone-resistant strain of T. gondii (Ato(R)).     

Compensatory evolution of pbp mutations restores the fitness cost imposed by β-lactam resistance in Streptococcus pneumoniae

The prevalence of antibiotic resistance genes in pathogenic bacteria is a major challenge to treating many infectious diseases. The spread of these genes is driven by the strong selection imposed by the use of antibacterial drugs. However, in the absence of drug selection, antibiotic resistance genes impose a fitness cost, which can be ameliorated by compensatory mutations. In Streptococcus pneumoniae, β-lactam resistance is caused by mutations in three penicillin-binding proteins, PBP1a, PBP2x, and PBP2b, all of which are implicated in cell wall synthesis and the cell division cycle. We found that the fitness cost and cell division defects conferred by pbp2b mutations (as determined by fitness competitive assays in vitro and in vivo and fluorescence microscopy) were fully compensated by the acquisition of pbp2x and pbp1a mutations, apparently by means of an increased stability and a consequent mislocalization of these protein mutants. Thus, these compensatory combinations of pbp mutant alleles resulted in an increase in the level and spectrum of β-lactam resistance. This report describes a direct correlation between antibiotic resistance increase and fitness cost compensation, both caused by the same gene mutations acquired by horizontal transfer. The clinical origin of the pbp mutations suggests that this intergenic compensatory process is involved in the persistence of β-lactam resistance among circulating strains. We propose that this compensatory mechanism is relevant for β-lactam resistance evolution in Streptococcus pneumoniae.     

Quantitative studies of the mutagenesis of Toxoplasma gondii.

The induction of mutants resistant to 5-fluorodeoxyuridine (FUDR) was used to measure the efficiency of various physical and chemical mutagens on extracellular and intracellular Toxoplasma gondii. The frequency of resistant mutant was measured by plaque assay in human fibroblast cultures in the presence and absence of FUDR. When considered as a function of lethality, the most efficient mutagenesis was obtained with nitrosoguanidine treatment of extracellular parasites and with ethylmethane sulfonate treatment of actively growing intracellular parasites. Each of these treatments increased the frequency of FUDR-resistant mutants from less than one to more than 200 per million parasites. Ultraviolet irradiation, X-rays, and the alkylating mustard ICR-191 also induced FUDR-resistant mutants in a dose-dependent fashion.     

Molecular genetic studies and delineation of the oculocutaneous albinism phenotype in the Pakistani population

ABSTRACT: BACKGROUND: Oculocutaneous albinism (OCA) is caused by a group of genetically heterogeneous inherited defects that result in the loss of pigmentation in the eyes, skin and hair. Mutations in the TYR, OCA2, TYRP1 and SLC45A2 genes have been shown to cause isolated OCA. No comprehensive analysis has been conducted to study the spectrum of OCA alleles prevailing in Pakistani albino populations. METHODS: We enrolled 40 large Pakistani families and screened them for OCA genes and a candidate gene, SLC24A5. Protein function effects were evaluated using in silico prediction algorithms and ex vivo studies in human melanocytes. The effects of splice-site mutations were determined using an exon-trapping assay. RESULTS: Screening of the TYR gene revealed four known (p.Arg299His, p.Pro406Leu, p.Gly419Arg, p.Arg278*) and three novel mutations (p.Pro21Leu, p.Cys35Arg, p.Tyr411His) in ten families. Ex vivo studies revealed the retention of an EGFP-tagged mutant (p.Pro21Leu, p.Cys35Arg or p.Tyr411His) tyrosinase in the endoplasmic reticulum (ER) at 37degreesC, but a significant fraction of p.Cys35Arg and p.Tyr411His left the ER in cells grown at a permissive temperature (31degreesC). Three novel (p.Asp486Tyr, p.Leu527Arg, c.1045-15T>G) and two known mutations (p.Pro743Leu, p.Ala787Thr) of OCA2 were found in fourteen families. Exon-trapping assays with a construct containing a novel c.1045-15T>G mutation revealed an error in splicing. No mutation in TYRP1, SLC45A2, and SLC24A5 was found in the remaining 16 families. Clinical evaluation of the families segregating either TYR or OCA2 mutations showed nystagmus, photophobia, and loss of pigmentation in the skin or hair follicles. Most of the affected individuals had grayish-blue colored eyes. CONCLUSIONS: Our results show that ten and fourteen families harbored mutations in the TYR and OCA2 genes, respectively. Our findings, along with the results of previous studies, indicate that the p.Cys35Arg, p.Arg278* and p.Gly419Arg alleles of TYR and the p.Asp486Tyr and c.1045-15T>G alleles of OCA2 are the most common causes of OCA in Pakistani families. To the best of our knowledge, this study represents the first documentation of OCA2 alleles in the Pakistani population. A significant proportion of our cohort did not have mutations in known OCA genes. Overall, our study contributes to the development of genetic testing protocols and genetic counseling for OCA in Pakistani families.     

Mutations in alpha-tubulin confer dinitroaniline resistance at a cost to microtubule function

Protozoan microtubules are sensitive to disruption by dinitroanilines, compounds that kill intracellular Toxoplasma gondii parasites without affecting microtubules in vertebrate host cells. We previously isolated a number of resistant Toxoplasma lines that harbor mutations to the alpha1-tubulin gene. Some of the mutations are localized in or near the M and N loops, domains that coordinate lateral interactions between protofilaments. Other resistance mutations map to a computationally identified binding site beneath the N loop. Allelic replacement of wild-type alpha1-tubulin with the individual mutations is sufficient to confer dinitroaniline resistance. Some mutations seem to increase microtubule length, suggesting that they increase subunit affinity. All mutations are associated with replication defects that decrease parasite viability. When parasites bearing the N loop mutation Phe52Tyr are grown without dinitroaniline selection, they spontaneously acquired secondary mutations in the M loop (Ala273Val) or in an alpha-tubulin-specific insert that stabilizes the M loop (Asp367Val). Parasites with the double mutations have both reduced resistance and diminished incidence of replication defects, suggesting that the secondary mutations decrease protofilament affinity to increase parasite fitness.     

Discovery of a novel Toxoplasma gondii conoid-associated protein important for parasite resistance to reactive nitrogen intermediates

Toxoplasma gondii modifies its host cell to suppress its ability to become activated in response to IFN-γ and TNF-α and to develop intracellular antimicrobial effectors, including NO. Mechanisms used by T. gondii to modulate activation of its infected host cell likely underlie its ability to hijack monocytes and dendritic cells during infection to disseminate to the brain and CNS where it converts to bradyzoites contained in tissue cysts to establish persistent infection. To identify T. gondii genes important for resistance to the effects of host cell activation, we developed an in vitro murine macrophage infection and activation model to identify parasite insertional mutants that have a fitness defect in infected macrophages following activation but normal invasion and replication in naive macrophages. We identified 14 independent T. gondii insertional mutants out of >8000 screened that share a defect in their ability to survive macrophage activation due to macrophage production of reactive nitrogen intermediates (RNIs). These mutants have been designated counter-immune mutants. We successfully used one of these mutants to identify a T. gondii cytoplasmic and conoid-associated protein important for parasite resistance to macrophage RNIs. Deletion of the entire gene or just the region encoding the protein in wild-type parasites recapitulated the RNI-resistance defect in the counter-immune mutant, confirming the role of the protein in resistance to macrophage RNIs.     

Amplification of a Cryptosporidium parvum gene fragment encoding thymidylate synthase.

Currently, there is no effective therapy for cryptosporidiosis and it is unclear why antifolate drugs which are effective treatments for infections caused by closely related parasites are not also effective against Cryptosporidium parvum. In protozoa, the target of these drugs, dihydrofolate reductase (DHFR), exists as a bifunctional enzyme also manifesting thymidylate synthase (TS) activity and is encoded by a fused DHFR-TS gene. In order to prepare a probe to isolate the C. parvum DHFR-TS gene we have used degenerate oligonucleotides whose sequences are based on strongly conserved regions of TS protein sequence to prime the polymerase chain reaction (PCR) with C. parvum DNA. The PCR amplified a 375-bp DNA fragment which was cloned and sequenced; the deduced amino acid sequence had significant identity with known TS sequences, including strict conservation of all phylogenetically invariant TS amino acid residues. The cloned PCR fragment was used as a probe to isolate a number of overlapping clones from a C. parvum genomic library which were definitively shown to be of cryptosporidial origin by genomic Southern and molecular karyotype analyses. The deduced protein sequence of C. parvum TS was most similar to the bifunctional TS enzymes of Plasmodium chabaudi and Plasmodium falciparum.     

pfmdr1 mutations associated with chloroquine resistance incur a fitness cost in Plasmodium falciparum

Efforts to control malaria worldwide have been hindered by the development and expansion of parasite populations resistant to many first-line antimalarial compounds. Two of the best-characterized determinants of drug resistance in the human malaria parasite Plasmodium falciparum are pfmdr1 and pfcrt, although the mechanisms by which resistance is mediated by these genes is still not clear. In order to determine whether mutations in pfmdr1 associated with chloroquine resistance affect the capacity of the parasite to persist when drug pressure is removed, we conducted competition experiments between P. falciparum strains in which the endogenous pfmdr1 locus was modified by allelic exchange. In the absence of selective pressure, the component of chloroquine resistance attributable to mutations at codons 1034, 1042 and 1246 in the pfmdr1 gene also gave rise to a substantial fitness cost in the intraerythrocytic asexual stage of the parasite. The loss of fitness incurred by these mutations was calculated to be 25% with respect to an otherwise genetically identical strain in which wild-type polymorphisms had been substituted at these three codons. At least part of the fitness loss may be attributed to a diminished merozoite viability. These in vitro results support recent in vivo observations that in several countries where chloroquine use has been suspended because of widespread resistance, sensitive strains are re-emerging.