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T E Wellems

Publications and source records attributed to T E Wellems.

At least 19 recordsLinked to original sources
PubMed2001

Malaria. Cooperative silencing elements in var genes.

Each Plasmodium falciparum malaria parasite carries about 50 var genes from a diverse family that encode variable adhesion proteins on the infected red blood cells of the host, but individual parasites single out just one var gene for expression and silence all the others. Here we show that this silencing is established during the DNA-synthesis phase (S phase) of the cell cycle and that it depends on the cooperative interaction between two elements in separate control regions of each var gene (the 5'-flanking region and the intron). This finding should help to clarify the mechanisms by which parasites coordinate the silencing and activation of var genes that are responsible for antigenic variation in malaria.

Animals↗
PubMed2001

Chloroquine-resistant malaria.

The development of chloroquine as an antimalarial drug and the subsequent evolution of drug-resistant Plasmodium strains had major impacts on global public health in the 20th century. In P. falciparum, the cause of the most lethal human malaria, chloroquine resistance is linked to multiple mutations in PfCRT, a protein that likely functions as a transporter in the parasite's digestive vacuole membrane. Rapid diagnostic assays for PfCRT mutations are already employed as surveillance tools for drug resistance. Here, we review recent field studies that support the central role of PfCRT mutations in chloroquine resistance. These studies suggest chloroquine resistance arose in > or = 4 distinct geographic foci and substantiate an important role of immunity in the outcomes of resistant infections after chloroquine treatment. P. vivax, which also causes human malaria, appears to differ from P. falciparum in its mechanism of chloroquine resistance. Investigation of the resistance mechanisms and of the role of immunity in therapeutic outcomes will support new approaches to drugs that can take the place of chloroquine or augment its efficiency.

Animals↗
PubMed2001

Evidence for different mechanisms of chloroquine resistance in 2 Plasmodium species that cause human malaria.

Chloroquine (CQ)-resistant Plasmodium vivax malaria was first reported 12 years ago, nearly 30 years after the recognition of CQ-resistant P. falciparum. Loss of CQ efficacy now poses a severe problem for the prevention and treatment of both diseases. Mutations in a digestive vacuole protein encoded by a 13-exon gene, pfcrt, were shown recently to have a central role in the CQ resistance (CQR) of P. falciparum. Whether mutations in pfcrt orthologues of other Plasmodium species are involved in CQR remains an open question. This report describes pfcrt homologues from P. vivax, P. knowlesi, P. berghei, and Dictyostelium discoideum. Synteny between the P. falciparum and P. vivax genes is demonstrated. However, a survey of patient isolates and monkey-adapted lines has shown no association between in vivo CQR and codon mutations in the P. vivax gene. This is evidence that the molecular events underlying P. vivax CQR differ from those in P. falciparum.

Amino Acid Sequence↗
PubMed2001

A molecular marker for chloroquine-resistant falciparum malaria.

BACKGROUND: Chloroquine-resistant Plasmodium falciparum malaria is a major health problem, particularly in sub-Saharan Africa. Chloroquine resistance has been associated in vitro with point mutations in two genes, pfcrt and pfmdr 1, which encode the P. falciparum digestive-vacuole transmembrane proteins PfCRT and Pgh1, respectively. METHODS: To assess the value of these mutations as markers for clinical chloroquine resistance, we measured the association between the mutations and the response to chloroquine treatment in patients with uncomplicated falciparum malaria in Mali. The frequencies of the mutations in patients before and after treatment were compared for evidence of selection of resistance factors as a result of exposure to chloroquine. RESULTS: The pfcrt mutation resulting in the substitution of threonine (T76) for lysine at position 76 was present in all 60 samples from patients with chloroquine-resistant infections (those that persisted or recurred after treatment), as compared with a base-line prevalence of 41 percent in samples obtained before treatment from 116 randomly selected patients (P<0.001), indicating absolute selection for this mutation. The pfmdr 1 mutation resulting in the substitution of tyrosine for asparagine at position 86 was also selected for, since it was present in 48 of 56 post-treatment samples from patients with chloroquine-resistant infections (86 percent), as compared with a base-line prevalence of 50 percent in 115 samples obtained before treatment (P<0.001). The presence of pfcrt T76 was more strongly associated with the development of chloroquine resistance (odds ratio, 18.8; 95 percent confidence interval, 6.5 to 58.3) than was the presence of pfmdr 1 Y86 (odds ratio, 3.2; 95 percent confidence interval, 1.5 to 6.8) or the presence of both mutations (odds ratio, 9.8; 95 percent confidence interval, 4.4 to 22.1). CONCLUSIONS: This study shows an association between the pfcrt T76 mutation in P. falciparum and the development of chloroquine resistance during the treatment of malaria. This mutation can be used as a marker in surveillance for chloroquine-resistant falciparum malaria.

Adult↗
PubMed2001

Rapid recombination among transfected plasmids, chimeric episome formation and trans gene expression in Plasmodium falciparum.

Although recombination is known to be important to generating diversity in the human malaria parasite P. falciparum, the low efficiencies of transfection and the fact that integration of transfected DNA into chromosomes is observed only after long periods (typically 12 weeks or more) have made it difficult to genetically manipulate the blood stages of this major human pathogen. Here we show that co-transfection of a P. falciparum line with two plasmids, one expressing a green fluorescent protein (gfp) reporter and the other expressing a drug resistance marker (Tgdhfr-ts M23), allowed selection of a population in which about approximately 30% of the parasites produce GFP. In these GFP-producing parasites, the transfected plasmids had recombined into chimeric episomes as large as 20 kb and could be maintained under drug pressure for at least 16 weeks. Our data suggest that chimera formation occurs early (detected by 7--14 days) and that it involves homologous recombination favored by presence of the same P. falciparum 5'hrp3 UTR promoting transcription from each plasmid. This indicates the presence of high levels of homologous recombination activity in blood stage parasites that can be used to drive rapid recombination of newly introduced DNA, study mechanisms of recombination, and introduce genes for trans expression in P. falciparum.

Animals↗
PubMed2001

Conservation of a novel vacuolar transporter in Plasmodium species and its central role in chloroquine resistance of P. falciparum.

Chloroquine resistance in Plasmodium falciparum has recently been shown to result from mutations in the novel vacuolar transporter, PfCRT. Field studies have demonstrated the importance of these mutations in clinical resistance. Although a pfcrt ortholog has been identified in Plasmodiumvivax, there is no association between in vivo chloroquine resistance and codon mutations in the P. vivax gene. This is consistent with lines of evidence that suggest alternative mechanisms of chloroquine resistance among various malaria parasite species.

Animals↗
PubMed2000

Frequent ectopic recombination of virulence factor genes in telomeric chromosome clusters of P. falciparum.

Persistent and recurrent infections by Plasmodium falciparum malaria parasites result from the ability of the parasite to undergo antigenic variation and evade host immune attack. P. falciparum parasites generate high levels of variability in gene families that comprise virulence determinants of cytoadherence and antigenic variation, such as the var genes. These genes encode the major variable parasite protein (PfEMP-1), and are expressed in a mutually exclusive manner at the surface of the erythrocyte infected by P. falciparum. Here we identify a mechanism by which var gene sequences undergo recombination at frequencies much higher than those expected from homologous crossover events alone. These recombination events occur between subtelomeric regions of heterologous chromosomes, which associate in clusters near the nuclear periphery in asexual blood-stage parasites or in bouquet-like configurations near one pole of the elongated nuclei in sexual parasite forms. We propose that the alignment of var genes in heterologous chromosomes facilitates gene conversion and promotes the diversity of antigenic and adhesive phenotypes. The association of virulence factors with a specific nuclear subcompartment may also have implications for variation during mitotic recombination in asexual blood stages.

Animals↗
PubMed2000

Hemoglobin C associated with protection from severe malaria in the Dogon of Mali, a West African population with a low prevalence of hemoglobin S.

The malaria hypothesis proposes a survival advantage for individuals with hemoglobin variants in areas of endemic Plasmodium falciparum malaria. Hemoglobin C (HbC) is a possible example in West Africa, where this hemoglobin has a centric distribution with high frequencies among certain populations including the Dogon ethnic group. To test whether HbC is associated with protection from malaria, we performed a case-control study in the Dogon of Bandiagara, Mali. HbC was present in 68 of 391 (17.4%) of uncomplicated malaria control cases, whereas it was detected in only 3 of 67 cases (4.5%) of severe malaria (odds ratio [OR], 0.22; P =. 01). Further, HbC was present in only 1 of 34 cases (2.9%) with cerebral manifestations, the most common presentation of severe malaria in this population (OR, 0.14; P =.03). Episodes of uncomplicated malaria and parasitemias (4800-205 050/microL) were identified in cases of homozygous HbC (HbCC), which indicates that P falciparum parasites are able to efficiently replicate within HbCC erythrocytes in vivo. These findings suggest that HbC does not protect against infection or uncomplicated malaria but can protect against severe malaria in the Dogon population of Bandiagara, Mali. The data also suggest that the protective effect associated with HbC may be greater than that of HbS in this population.

Anemia, Sickle Cell↗
PubMed2000

EBL-1, a putative erythrocyte binding protein of Plasmodium falciparum, maps within a favored linkage group in two genetic crosses.

The Duffy binding-like (DBL) superfamily of Plasmodium falciparum encompasses genes which encode ligands for host cell receptors. This superfamily includes two distinct groups of genes, the var genes which encode antigenically variant cytoadherence proteins (PfEMP1), and the eba-175 gene which encodes a glycophorin A binding protein involved in erythrocyte invasion. Here we describe another DBL superfamily member related to eba-175, the ebl-1 gene. Like the eba-175 gene, ebl-1 is a single copy gene encoding DBL domains that have sequences and an overall arrangement distinct from var genes. The inheritance of ebl-1 was found to be strongly favored in two genetic crosses in which one parental clone lacked a chromosome segment carrying the gene. A proliferation phenotype has been previously linked to the same chromosome segment in the first genetic cross. These results suggest that ebl-1 and eba-175 are related members of a multigene family involved in the invasion of erythrocytes by P. falciparum.

Amino Acid Sequence↗
PubMed2000

Allelic modifications of the cg2 and cg1 genes do not alter the chloroquine response of drug-resistant Plasmodium falciparum.

The determinant of chloroquine resistance (CQR) in a Plasmodium falciparum cross was previously mapped by linkage analysis to a 36 kb segment of chromosome 7. Candidate genes within this segment have been previously shown to include two genes, cg2 and cg1, that have complex polymorphisms linked to the CQR phenotype. Using DNA transfection and allelic exchange, we have replaced these polymorphisms in CQR parasites with cg2 and cg1 sequences from chloroquine sensitive parasites. Drug assays of the allelically-modified lines show no change in the degree of CQR, providing evidence against the hypothesis that these polymorphisms are important to the CQR phenotype. Similarly, no change was found in the degree to which verapamil or other chloroquine sensitizers reverse CQR in the transformants. These results and the high though not complete degree of association of CQR with cg2 and cg1 polymorphisms in field isolates suggest involvement of another nearby gene in the P. falciparum CQR mechanism.

Alleles↗
PubMed2000

Mutations in the P. falciparum digestive vacuole transmembrane protein PfCRT and evidence for their role in chloroquine resistance.

The determinant of verapamil-reversible chloroquine resistance (CQR) in a Plasmodium falciparum genetic cross maps to a 36 kb segment of chromosome 7. This segment harbors a 13-exon gene, pfcrt, having point mutations that associate completely with CQR in parasite lines from Asia, Africa, and South America. These data, transfection results, and selection of a CQR line harboring a novel K761 mutation point to a central role for the PfCRT protein in CQR. This transmembrane protein localizes to the parasite digestive vacuole (DV), the site of CQ action, where increased compartment acidification associates with PfCRT point mutations. Mutations in PfCRT may result in altered chloroquine flux or reduced drug binding to hematin through an effect on DV pH.

Amino Acid Sequence↗
PubMed1999

A genetic map and recombination parameters of the human malaria parasite Plasmodium falciparum.

Genetic investigations of malaria require a genome-wide, high-resolution linkage map of Plasmodium falciparum. A genetic cross was used to construct such a map from 901 markers that fall into 14 inferred linkage groups corresponding to the 14 nuclear chromosomes. Meiotic crossover activity in the genome proved high (17 kilobases per centimorgan) and notably uniform over chromosome length. Gene conversion events and spontaneous microsatellite length changes were evident in the inheritance data. The markers, map, and recombination parameters are facilitating genome sequence assembly, localization of determinants for such traits as virulence and drug resistance, and genetic studies of parasite field populations.

Animals↗
PubMed1999

Intra-cluster recombination and var transcription switches in the antigenic variation of Plasmodium falciparum.

Antigenic variation and immune evasion by Plasmodium falciparum parasitized erythrocytes are mediated by expression switches among members of the multicopy var gene family. Here we describe a cluster of var genes on chromosome 12 that showed spontaneous recombination and switches in the transcription of individual genes. The transcription switches were not associated with sequence changes in promoter regions. Transfected episomes containing a luciferase reporter under control of a var promoter were expressed regardless of the transcriptional status of the endogenous promoter. The results suggest epigenetic regulation of P. falciparum var gene transcription that depends upon the local structure of chromatin and its associated proteins.

Animals↗
PubMed1999

Genome projects, genetic analysis, and the changing landscape of malaria research.

Genome analysis of the Plasmodium falciparum malaria parasite already is identifying genes relevant to therapeutic- and vaccine-related research. The genetic blueprint of P. falciparum will ultimately need to be understood at multiple levels of an integrated system and will provide a detailed account of the life processes of the parasite and of the devastating disease it causes.

Animals↗