Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

---

Query: UMLS:C0024530 (malaria)
44,886 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

To facilitate genetic analysis of the protozoan parasite Toxoplasma gondii, sequences derived from the parasite's fused dihydrofolate reductase-thymidylate synthase (DHFR-TS) gene have been used to produce vectors suitable for stable molecular transformation. Mutations introduced into the DHFR coding region by analogy with pyrimethamine-resistant malaria confer drug resistance to Toxoplasma, providing useful information on the structure of fused DHFR-TS enzymes and a powerful selectable marker for molecular genetic studies. Depending on the particular drug-resistance allele employed and the conditions of selection, stable resistance can be generated either by single copy nonhomologous insertion into chromosomal DNA or by massively amplified transgenes. Frequencies of integration are independent of selection, and transgenes are stable without continued selection. Cointegration of a reporter gene adjacent to the selectable marker (under the control of an independent promoter) shows no loss of the cointegrated sequences over many parasite generations. By bringing the full power of molecular genetic analysis to bear on Toxoplasma, these studies should greatly facilitate the development of a model genetic system for Apicomplexan parasites.
...
PMID:Stable molecular transformation of Toxoplasma gondii: a selectable dihydrofolate reductase-thymidylate synthase marker based on drug-resistance mutations in malaria. 826 12

The activity of 51 synthetic cis-fused cyclopenteno-1,2,4-trioxanes has been examined against drug-sensitive and chloroquine-resistant malaria parasites in vivo. Some of them display high levels of blood schizontocidal activity when administered orally or subcutaneously. They retain their activity against lines of parasites that are resistant to widely differing antimalarials such as 4-aminoquinolines, aminoalcohols, dihydrofolate reductase inhibitors and artemisinin. The most potent compound of the present series is cis-(+/-)-4a,7a-dihydro-6,7a-di(p-fluorophenyl)spiro [cyclopentane-3,3'-7H-cyclopenta-1,2,4-trioxin], otherwise known as Fenozan-50F.
...
PMID:The chemotherapy of rodent malaria. XLIX. The activities of some synthetic 1,2,4-trioxanes against chloroquine-sensitive and chloroquine-resistant parasites. Part 2: Structure-activity studies on cis-fused cyclopenteno-1,2,4-trioxanes (fenozans) against drug-sensitive and drug-resistant lines of Plasmodium berghei and P. yoelii ssp. NS in vivo. 834 94

We studied inhibition of growth of the malaria parasite Plasmodium falciparum in in vitro culture using antisense (AS) oligodeoxynucleotides (ODNs) against different target genes. W2 and W2mef strains of drug-resistant parasites were exposed to AS ODNs over 48 hr, and growth was determined by microscopic examination and [3H]hypoxanthine incorporation. At ODN concentrations of 1 microM, phosphorothioate (PS) ODNs inhibited growth in a target-independent manner. However, between 0.5 and 0.005 microM, ODNs against dihydrofolate reductase, dihydropteroate synthetase, ribonucleotide reductase, the schizont multigene family, and erythrocyte binding antigen EBA175 significantly inhibited growth compared with a PS AS ODN against human immunodeficiency virus, two AS ODNs containing eight mismatches, or the sense strand controls (P < 0.0001). The IC50 was approximately 0.05 microM, whereas that for non-sequence-specific controls was 15-fold higher. PS AS ODNs against DNA polymerase alpha showed less activity than that for other targets, whereas a single AS ODN against triose-phosphate isomerase did not differ significantly from controls. We conclude that at concentrations below 0.5 microM, PS AS ODNs targeted against several malarial genes significantly inhibit growth of drug-resistant parasites in a nucleotide sequence-dependent manner. This technology represents an alternative method for identifying malarial genes as potential drug targets.
...
PMID:Inhibition of Plasmodium falciparum malaria using antisense oligodeoxynucleotides. 855 72

A rapid and simple method to detect pyrimethamine susceptibility of Plasmodium falciparum by analyzing DNA from whole blood is presented. Samples from cultured isolates and from patients infected with P. falciparum were spotted onto filter paper disks, dried, and stored for subsequent analyses. After extracting the P. falciparum DNA using Chelex-100 ion-chelating resin (Bio-Rad, Richmond, CA), the polymerase chain reaction (PCR) was used to amplify the dihydrofolate reductase (dhfr) gene. The PCR product of 727 basepairs was digested with the Alu I restriction endonuclease to detect whether the isolates were sensitive or resistant to the antimalarial drugs pyrimethamine and cycloguanil. This reaction endonuclease digests only DNA from pyrimethamine-sensitive parasites because the recognition locus of Alu I is changed by mutations giving rise to pyrimethamine and cycloguanil resistance. This method is simple and sensitive and could therefore bu used to study the epidemiology of pyrimethamine resistant in P. falciparum. The DHFR gene of pyrimethamine-resistance clones from Vietnamese patients showed three amino acid changes that were previously found in pyrimethamine-resistant isolates. Two other clones, T9/94 and T9/96, originally isolated from a single malaria patient from Thailand, had different DHFR gene sequences. The nucleotide sequence of the DHFR gene from T9/96 was identical to the wild-type DHFR sequence, whereas T9/94 possessed amino acid substitutions at positions 16 and 108 that have been described in cycloguanil-resistant parasites.
...
PMID:Rapid detection of pyrimethamine susceptibility of Plasmodium falciparum by restriction endonuclease digestion of the dihydrofolate reductase gene. 861 45

Plasmodium falciparum isolates from 24 Papua New Guinean patients with symptomatic malaria were tested for susceptibility to pyrimethamine and cycloguanil. Thirteen isolates were sensitive to both agents and the remainder exhibited varying degrees of resistance. No isolates were found to be resistant to one agent yet sensitive to the other and a positive correlation suggesting cross-resistance was found. Parasite DNA extracted from the patients' stained blood slides was amplified and sequenced to examine point mutations in the dihydrofolate reductase (DHFR) and dihydropteroate synthetase genes (DHPS) associated with antifolate resistance. All resistant isolates possessed mutations in the DHFR gene at codon 108, the majority changing from Ser to Asn, but one isolate from Ser to Thr, a change not previously reported in field isolates. A second mutation of the DHFR gene at Cys-59 to Arg was present in isolates with higher level resistance, but not exclusively so. Sequencing the DHPS gene, as a predictor of sulfadoxine resistance, revealed only one example that was different from DHPS alleles of sensitive isolates.
...
PMID:Point mutations in the dihydrofolate reductase and dihydropteroate synthetase genes and in vitro susceptibility to pyrimethamine and cycloguanil of Plasmodium falciparum isolates from Papua New Guinea. 878 Apr 62

Pyrimethamine and cycloguanil resistance of Plasmodium falciparum has been linked to mutations in the dihydrofolate reductase (dhfr) portion of the dhfr-ts gene. In this paper, the DNA sequence of the dhfr-ts gene of 50 isolates from Vietnam and 2 clones (T9/94 and T9/96) isolated from a malaria patient from Thailand have been analyzed. A comparison between these isolates and clones showed differential mutation patterns. Forty-eight isolates were found to consist of mutations associated with Pyr. A novel leucine mutation at position 140 was found in the isolate VP8 and in clone T9/94. The isolate VP8 and the clone T9/94 were found to also have the characteristic changes at positions 16 (Val) and 108 (Thr) that have been found in cycloguanil-resistant isolates. The isolate VP35 was shown to be resistant to both antifolates, while the clone T9/96 was found to be sensitive to both antifolates and to have a sequence identical to that of wild-type dhfr-ts. The two clones from a single patient showed the coexistence of resistant and sensitive clones in the absence of treatment by antifolates. Since cycloguanil resistance seems to be rare in Vietnam, cycloguanil alone or in combination with other antimalarial agents might be an alternative for treatment and prophylaxis, even in areas with high resistance to pyrimethamine.
...
PMID:Plasmodium falciparum: mutation pattern in the dihydrofolate reductase-thymidylate synthase genes of Vietnamese isolates, a novel mutation, and coexistence of two clones in a Thai patient. 888 32

Plasmodium falciparum dihydrofolate reductase-thymidylate synthase (DHFR-TS) is a well-known target for pyrimethamine and cycloguanil. The low amounts of enzyme obtainable from parasites or the currently available heterologous expression systems have thus far hindered studies of this enzyme. The 1912-base pair P. falciparum DHFR-TS gene was designed based on E. coli codon preference with unique restriction sites evenly placed throughout the coding sequence. The gene was designed and synthesized as three separated domains: the DHFR domain, the junctional sequence, and the TS domain. Each of these domains contained numerous unique restriction sites to facilitate mutagenesis. The three domains were assembled into a complete DHFR-TS gene which contained 30 unique restriction sites in the coding sequence. The bifunctional DHFR-TS was expressed from the synthetic gene as soluble enzyme in E. coli about 10-fold more efficiently than from the wild-type sequence. The DHFR-TS from the synthetic gene had kinetic properties similar to those of the wild-type enzyme and represents a convenient source of protein for further study. The unique restriction sites in the coding sequence permits easy mutagenesis of the gene which should facilitate further understanding of the molecular basis of antifolate resistance in malaria.
...
PMID:Chemical synthesis of the Plasmodium falciparum dihydrofolate reductase-thymidylate synthase gene. 901 Aug 45

Single and multiple mutations at residues 16, 51, 59, 108, and 164 of Plasmodium falciparum dihydrofolate reductase (pfDHFR) have been linked to antifolate resistance in malaria. We prepared and characterized all seven of the pfDHFR mutants found in nature, as well as six mutants not observed in nature. Mutations involving residues 51, 59, 108, or 164 conferred cross resistance to both the antifolates pyrimethamine and cycloguanil, whereas mutation of residue 16 specifically conferred resistance to cycloguanil. The antifolate resistance of enzyme mutants found in nature correlated with in vivo antifolate resistance; however, mutants not found in nature were either poorly resistant or had insufficient catalytic activity to support DNA synthesis. Thus, specific combinations of multiple mutations at target residues were selected in nature to optimize resistance. Further, the resistance of multiple mutants was more than the sum of the component single mutations, indicating that residues were selected for their synergistic as well as intrinsic effects on resistance. Pathways inferred for the evolution of pyrimethamine-resistant mutants suggested that all multiple mutants emerged from stepwise selection of the single mutant, S108N. Thus, we propose that drugs targeted to both the wild-type pfDHFR and S108N mutant would have a low propensity for developing resistance, and hence could provide effective antimalarial agents.
...
PMID:Antifolate-resistant mutants of Plasmodium falciparum dihydrofolate reductase. 903 17

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.
...
PMID:Sulfadoxine resistance in the human malaria parasite Plasmodium falciparum is determined by mutations in dihydropteroate synthetase and an additional factor associated with folate utilization. 907 34

Pyrimethamine and cycloguanil are competitive inhibitors of the Plasmodium enzyme dihydrofolate reductase (DHFR). They have been effective treatments for malaria, but rapid selection of populations of the parasite resistant to these drugs has compromised their effectiveness. Parasites resistant to either drug usually have point mutations in the dhfr gene, but the frequency of these mutations is unknown. To study drug resistance more effectively, we transferred the DHFR domain of the dhfr-thymidylate synthase gene from a drug-sensitive line of P. falciparum to a strain of the budding yeast, Saccharomyces cerevisiae, that lacks endogenous DHFR activity. Expression of the P. falciparum dhfr is controlled by the yeast dhfr 5' and 3' regulatory regions and the heterologous enzyme provided all of the functions of the yeast dhfr gene. These yeast were susceptible to pyrimethamine and cycloguanil at low concentrations that inhibit P. falciparum (IC50 about 10(-8) and 10(-7) M, respectively). Yeast expressing constructs with dhfr alleles from pyrimethamine-resistant strains were resistant to both pyrimethamine and cycloguanil (IC50 > 10(-6) M); resistance of the yeast depended on the dhfr allele they expressed. The experimental drug WR99210 efficiently killed all three yeast strains (IC50 about 10(-8) M) but the pyrR strains showed collateral hypersensitivity to drug. The yeast transformants carrying the drug-sensitive allele can now be screened quickly and quantitatively to identify new drugs or combinations of drugs and determine which drugs select resistant parasites least efficiently. Such compounds would be excellent candidates for development of treatments with a longer life in clinical practice.
...
PMID:Analysis in yeast of antimalaria drugs that target the dihydrofolate reductase of Plasmodium falciparum. 910 46


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>

---