A large multigene family expressed during the erythrocytic schizogony of Plasmodium falciparum

The GP-45 Protein, a Highly Variable Antigen from Babesia bigemina, Contains Conserved B-Cell Epitopes in Geographically Distant Isolates

In B. bigemina, the 45 kilodaltons glycoprotein (GP-45) is the most studied. GP-45 is exposed on the surface of the B. bigemina merozoite, it is believed to play a role in the invasion of erythrocytes, and it is characterized by a high genetic and antigenic polymorphism. The objective of this study was to determine if GP-45 contains conserved B-cell epitopes, and if they would induce neutralizing antibodies. The comparative analysis of nucleotide and amino acids sequences revealed a high percentage of similarity between field isolates. Antibodies against peptides containing conserved B-cell epitopes of GP-45 were generated. Antibodies present in the sera of mice immunized with GP-45 peptides specifically recognize B. bigemina by the IFAT. More than 95% of cattle naturally infected with B. bigemina contained antibodies against conserved GP-45 peptides tested by ELISA. Finally, sera from rabbits immunized with GP-45 peptides were evaluated in vitro neutralization tests and it was shown that they reduced the percentage of parasitemia compared to sera from rabbits immunized with adjuvant. GP-45 from geographically distant isolates of B. bigemina contains conserved B-cell epitopes that induce neutralizing antibodies suggesting that this gene and its product play a critical role in the survival of the parasite under field conditions.

The flip-side of Cytoadherence: immune selection, antigenic variation and the var Genes of Plasmodium falciparum

In areas where Plasmodium falciparum is endemic, the natural immunity acquired by people exposed to frequent malaria infection is likely to have a differential selective impact upon different parasite genotypes. It has been suggested that the immune response directed against the variant antigen PfEMP1, which is expressed on the infected erythrocyte surface, is a crucial determinant of parasite population structure and favours the existence of distinct strains, or Varotypes. Here, Colin Sutherland summarizes current knowledge of the var multigene family, which encodes the PfEMP1 variants, and suggests that this information may allow certain predictions of the strain hypothesis to be tested directly.

Multi-character population study of the vir subtelomeric multigene superfamily of Plasmodium vivax, a major human malaria parasite

Plasmodium vivax, the most widely distributed human malaria parasite, contains the subtelomeric multigene vir superfamily corresponding to circa 10% of its coding genome. In this work, we used a multi-character strategy to study the vir gene repertoire circulating in natural parasite populations obtained directly from 32 human patients from endemic regions of Brazil and Sri Lanka. Cladistic analysis confirmed the existence of vir subfamilies, which varied in size and allele polymorphisms. Moreover, different motifs, protein domain, and secondary structures were predicted for each subfamily. Of importance, not all vir sequences possess a recognizable Pexel motif recently shown to be important, though not essential, signal for transportation to the cell membrane of infected red blood cells. Furthermore, subfamilies A and D display common structural features with the recently described P. falciparum SURFIN and Pfmc-2tm subtelomeric multigene families. These results suggest that VIR proteins can have different subcellular localizations and functions. This is the first study on a population level of the P. vivax vir subtelomeric multigene superfamily.

Clinical and molecular aspects of severe malaria

The erythrocytic cycle of Plasmodium falciparum presents a particularity in relation to other Plasmodium species that infect man. Mature trophozoites and schizonts are sequestered from the peripheral circulation due to adhesion of infected erythrocytes to host endothelial cells. Modifications in the surface of infected erythrocytes, termed knobs, seem to facilitate adhesion to endothelium and other erythrocytes. Adhesion provides better maturation in the microaerophilic venous atmosphere and allows the parasite to escape clearance by the spleen which recognizes the erythrocytes loss of deformability. Adhesion to the endothelium, or cytoadherence, has an important role in the pathogenicity of the disease, causing occlusion of small vessels and contributing to failure of many organs. Cytoadherence can also describe adhesion of infected erythrocytes to uninfected erythrocytes, a phenomenon widely known as rosetting. Clinical aspects of severe malaria, as well as the host receptors and parasite ligands involved in cytoadherence and rosetting, are reviewed here. The erythrocyte membrane protein 1 of P. falciparum (PfEMP1) appears to be the principal adhesive ligand of infected erythrocytes and will be discussed in more detail. Understanding the role of host receptors and parasite ligands in the development of different clinical syndromes is urgently needed to identify vaccination targets in order to decrease the mortality rates of this disease.

Antibodies raised against Bcvir15, an extrachromosomal double-stranded RNA-encoded protein from Babesia canis, inhibit the in vitro growth of the parasite

As part of a search for homologous members of the Plasmodium falciparum Pf60 multigene family in the intraerythrocytic protozoan parasite Babesia canis, we report here the characterization of a cDNA of 1,115 bp, which was designated Bcvir for its potential viral origin. The Bcvir cDNA contained two overlapping open reading frames (ORFs) (ORF1 from nucleotide [nt] 61 to 486 and ORF2 from nt 417 to 919), where Bcvir15, the deduced ORF1 peptide (M(1) to I(141)), is the main expressed product. The Bcvir cDNA was derived from an extrachromosomal dsRNA element of 1.2 kbp that was always found associated with a double-stranded RNA (dsRNA) of 2.8 kbp by hybridization, and no copy of this cDNA sequence was found in B. canis genomic DNA. Biochemical characterization of Bcvir15, by using polyclonal rabbit sera directed against recombinant proteins, indicated that it is a soluble protein which remained associated with the cytoplasm of the B. canis merozoite. Interestingly, purified immunoglobulins from the anti-glutathione S-transferase-Bcvir15 (at a concentration of 160 micro g/ml) induced 50% inhibition of the in vitro growth of B. canis, and the inhibitory effect was associated with morphological damage of the parasite. Our data suggest that the extrachromosomal dsRNA-encoded Bcvir15 protein might interfere with the intracellular growth of the parasite rather than with the process of invasion of the host cell by the merozoite. Epitope mapping of Bcvir15 identified three epitopes that might be essential for the function of the protein.

Efficacy and safety of dihydroartemisinin-piperaquine (Artekin) in Cambodian children and adults with uncomplicated falciparum malaria

The safety and efficacy of a novel combination of dihydroartemisinin (DHA) and piperaquine, Artekin (Holleykin Pharmaceuticals), were assessed in 106 patients (76 children and 30 adults) with uncomplicated falciparum malaria from 2 remote areas in Cambodia. Age-based doses were given at 0, 8, 24, and 32 h. Mean total DHA and piperaquine doses were 9.1 and 73.9 mg/kg, respectively, for children and 6.6 and 52.9 mg/kg for adults. All patients became aparasitemic within 72 h. Excluding the results for 1 child who died on day 4, there was a 96.9% 28-day cure rate (98.6% in children and 92.3% in adults). Patients who had recrudescent infection received low doses of Artekin. Side effects were reported by 22 patients (21%) but did not necessitate premature cessation of therapy. Although Artekin is a promising and inexpensive option for antimalarial therapy, further efficacy and pharmacokinetic studies are needed, especially for its use in children.

Three multigene families in Plasmodium parasites: facts and questions

Multigene families optimise fitness by providing a set of related genes with possibly different temporal and/or topological expression patterns. We analyse here the structural organisation and sequence diversity of the rDNA, sera and var C Plasmodium falciparum families, and discuss their consequences for parasite biology. The low rDNA copy number, which reduces reshuffling, is probably the corollary of the need for functionally distinct rRNAs in the insect and in the vertebrate host. The unusual intra-genome and population rDNA sequence diversity results in cells equipped with mosaic ribosome sets. The functional constraints are such that ribosome compatibility could influence parasite fitness and contribute to population structuring. Unlike the dispersed rDNA units, the sera family is arranged as a tandem gene cluster, with seven contiguous similar genes, and one more distantly related paralog. We address the question of the inclusion criteria in family definition. We discuss the results concerning the SERA proteins expression and function in the context of the long overlooked multigene family. The var C module is shared by var genes, 'orphan' var C and var C pseudogenes. Analysis of 125 var C deduced protein sequences highlights a well-conserved framework, including putative phosphorylation sites, consistent with the proposed function of mediating interaction with cytoskeletal proteins. The 5' and 3' flanking sequences of the var C pseudogenes are heterogeneous. In contrast, the flanking sequences of the uninterrupted var C modules show remarkable conservation. This is interesting in view of the silencing activity of the var intronic sequence on var expression. The 5' flanking sequence dichotomy reported for internal and sub-telomeric var genes extends to the 3' flanking sequences. This has profound implications for transcription regulation and generation of diversity. The var C family suggests a role for pseudogenes as a diversity reservoir and in genome dynamics by promoting ectopic recombination.

Antigenic variation at the infected red cell surface in malaria

Many pathogens that either rely on an insect vector to complete their life cycle (e.g., Trypanosoma spp. and Borrelia spp.) or exist in a unique ecological niche where transmission from host to host is sporadic (e.g., Neisseria spp.) have evolved strategies to maintain infection of their mammalian hosts for long periods of time in order to ensure their survival. Because they have to survive in the face of a fully functional immune system, a common feature of many of these organisms is their development of sophisticated strategies for immune evasion. For the above organisms and for malaria parasites of the genus Plasmodium, a common theme is the ability to undergo clonal antigenic variation. In all cases, surface molecules that are important targets of the humoral immune response are encoded in the genome as multicopy, nonallelic gene families. Antigenic variation is accomplished by the successive expression of members of these gene families that show little or no immunological cross-reactivity. In the case of malaria parasites, however, some of the molecules that undergo antigenic variation are also major virulence factors, adding an additional level of complication to the host-parasite interaction. In this review, we cover the history of antigenic variation in malaria and then summarize the more recent data with particular emphasis on Plasmodium falciparum, the etiological agent of the most severe form of human malaria.

Molecular technology and antigenic variation among intraerythrocytic hemoparasites: do we see reality?

Antigenic variation is one mechanism of immune evasion utilized by many microorganisms--encompassing such broad evolutionary groups as viruses, bacteria, and protozoa--to survive the onslaught of a specifically activated host immune system. Because of its importance to the survival of many infectious agents there is considerable interest in understanding this phenomenon. With knowledge of the molecular mechanisms by which these microbes deliberately manipulate their genomes, it may be possible to disrupt the molecular machinery of the responsible genetic mechanisms. Among intraerythrocytic parasites, genetic mechanisms that have been observed or postulated to control antigenic variation include segmental gene conversion, epigenetically controlled in situ transcriptional switching, alterations of chromosomal structure associated with transcriptional control, and recombination during sexual reproduction. Likely, more than one type of mechanism is used by all organisms that undergo antigenic variation. In this paper, both the observed mechanisms and some of the molecular technology used to detect these mechanisms are discussed. While often seemingly straightforward from a technical standpoint, sometimes subtle differences in the methods used to study this process may affect what is observed. Some examples of this phenomenon are discussed in the context of a small selection of intraerythrocytic parasites.

The apical organelles of malaria merozoites: host cell selection, invasion, host immunity and immune evasion

Malaria is caused by protozoan parasites belonging to the phylum Apicomplexa. These obligate intracellular parasites depend on the successful invasion of an appropriate host cell for their survival. This article is a broad overview of the molecular strategies employed by the merozoite, an invasive form of the malaria parasite, to successfully invade a suitable red blood cell.

Molecular aspects of severe malaria

Human infections with Plasmodium falciparum may result in severe forms of malaria. The widespread and rapid development of drug resistance in P. falciparum and the resistance of the disease-transmitting mosquitoes to insecticides make it urgent to understand the molecular background of the pathogenesis of malaria to enable the development of novel approaches to combat the disease. This review focuses on the molecular mechanisms of severe malaria caused by the P. falciparum parasite. The nature of severe malaria and the deleterious effects of parasite-derived toxins and host-induced cytokines are introduced. Sequestration, brought about by cytoadherence and rosetting, is linked to severe malaria and is mediated by multiple receptors on the endothelium and red blood cells. P. falciparum erythrocyte membrane protein 1 (PfEMP1) is the ligand responsible for a majority of binding interactions, and the multiply adhesive features of this sticky molecule are presented. Antigenic variation is also a major feature of PfEMP1 and of the surface of the P. falciparum-infected erythrocyte. Possible mechanisms of P. falciparum antigenic variation in asexual stages are further discussed. We conclude this review with a perspective and suggestions of important aspects for future investigations.

Molecular aspects of severe malaria

Human infections with Plasmodium falciparum may result in severe forms of malaria. The widespread and rapid development of drug resistance in P. falciparum and the resistance of the disease-transmitting mosquitoes to insecticides make it urgent to understand the molecular background of the pathogenesis of malaria to enable the development of novel approaches to combat the disease. This review focuses on the molecular mechanisms of severe malaria caused by the P. falciparum parasite. The nature of severe malaria and the deleterious effects of parasite-derived toxins and host-induced cytokines are introduced. Sequestration, brought about by cytoadherence and rosetting, is linked to severe malaria and is mediated by multiple receptors on the endothelium and red blood cells. P. falciparum erythrocyte membrane protein 1 (PfEMP1) is the ligand responsible for a majority of binding interactions, and the multiply adhesive features of this sticky molecule are presented. Antigenic variation is also a major feature of PfEMP1 and of the surface of the P. falciparum-infected erythrocyte. Possible mechanisms of P. falciparum antigenic variation in asexual stages are further discussed. We conclude this review with a perspective and suggestions of important aspects for future investigations.

A member of the Plasmodium falciparum Pf60 multigene family codes for a nuclear protein expressed by readthrough of an internal stop codon

Four large multigene families have been described in Plasmodium falciparum malaria parasites (var, rif, stevor and Pf60). var and rif genes code for erythrocyte surface proteins and undergo clonal antigenic variation. We report here the characterization of the first Pf60 gene. The 6.1 gene is constitutively expressed by all mature blood stages and codes for a protein located within the nucleus. It has a single copy, 7-exon, 5' domain, separated by an internal stop codon from a 3' domain that presents a high homology with var exon II. Double-site immunoassay and P. falciparum transient transfection using the reporter luciferase gene demonstrated translation through the internal ochre codon. The 6.1 N-terminal domain has no homology with any protein described to date. Sequence analysis identified a leucine zipper and a putative nuclear localization signal and showed a high probability for coiled coils. Evidence for N-terminal coiled coil-mediated protein interactions was obtained. This identifies the 6.1 protein as a novel nuclear protein. These data show that the Pf60 and var genes form a superfamily with a common 3' domain, possibly involved in regulating homo- or heteromeric interactions.

Malaria multigene families: the price of chronicity

In this article, Georges Snounou, William Jarra and Peter Preiser discuss the survival strategy of malaria parasites in the light of a novel mechanism of clonal phenotypic variation recently described for a multigene family of Plasmodium yoelii yoelii. The 235 kDa rhoptry proteins (Py235) encoded by these genes may be involved in the selection of red blood cells for invasion by merozoites. The new mechanism may explain the ability of individual parasites to adapt to natural variations in red blood cell subsets, while ensuring that sufficient merozoites escape immune attack, thus maintaining a chronic infection for extended periods. This counterpoints the antigenic variation exemplified by PfEMP1 proteins (a large family of proteins derived from P. falciparum), which operates at the population level. The possibility of manipulating the expression of functionally similar genes in other Plasmodium species could lead to therapies aimed at reducing clinical severity without compromising the acquisition and maintenance of immunity.

The genome of the malaria parasite

The genome of the human malaria parasite Plasmodium falciparum is being sequenced by an international consortium. Two of the parasite's 14 chromosomes have been completed and several other chromosomes are nearly finished. Even at this early stage of the project, analysis of the genome sequence has provided promising new leads for drug and vaccine development.

The complete nucleotide sequence of chromosome 3 of Plasmodium falciparum

Analysis of Plasmodium falciparum chromosome 3, and comparison with chromosome 2, highlights novel features of chromosome organization and gene structure. The sub-telomeric regions of chromosome 3 show a conserved order of features, including repetitive DNA sequences, members of multigene families involved in pathogenesis and antigenic variation, a number of conserved pseudogenes, and several genes of unknown function. A putative centromere has been identified that has a core region of about 2 kilobases with an extremely high (adenine + thymidine) composition and arrays of tandem repeats. We have predicted 215 protein-coding genes and two transfer RNA genes in the 1,060,106-base-pair chromosome sequence. The predicted protein-coding genes can be divided into three main classes: 52.6% are not spliced, 45.1% have a large exon with short additional 5' or 3' exons, and 2.3% have a multiple exon structure more typical of higher eukaryotes.

Rifins: a second family of clonally variant proteins expressed on the surface of red cells infected with Plasmodium falciparum

Many pathogens evade the host immune response or adapt to their environment by expressing surface proteins that undergo rapid switching. In the case of Plasmodium falciparum, products of a multigene family known as var are expressed on the surface of infected red cells, where they undergo clonal antigenic variation and contribute to malaria pathogenesis by mediating adhesion to a variety of host endothelial receptors and to uninfected red blood cells by forming rosettes. Herein we show that a second gene family, rif, which is associated with var at subtelomeric sites in the genome, encodes clonally variant proteins (rifins) that are expressed on the infected red cell surface. Their high copy number, sequence variability, and red cell surface location indicate an important role for rifins in malaria host-parasite interaction.

Antigenic variation in Plasmodium falciparum: mechanisms and consequences

In the past year, the major advances in malaria antigenic variation have been concerned with the transcription and switching of variant antigen genes, and the functional expression of regions of the major variant antigen. Also, new variant gene families have been discovered as a result of the Malaria Genome Project.

Chromosomal organisation of a gene family encoding rhoptry proteins in Plasmodium yoelii

The genomic organisation of the genes coding for a group of high molecular mass rhoptry proteins of the rodent malaria parasite Plasmodium yoelii YM was investigated using blotting, two dimensional gel electrophoresis and restriction fragment length analysis. The genes were found on chromosomes 1, 5, 6 and 10, with the possibility that related genes were also present on chromosomes 3 and 4. On chromosome 1 the genes were located close to one end, whereas they were present at both ends of chromosome 5, 6 and 10. Two genes, e3 and e8, that had been partially characterised previously were present on chromosomes 5 and 1, respectively. Based on an analysis of the 3' end of the genes, three subfamilies present on chromosomes 1, 5 and 6, and 10, respectively, were identified.

[Molecular analysis of Plasmodium falciparum infections in man]

Plasmodium falciparum diversity has been analysed in two Senegalese villages with different transmission conditions and distinct kinetics of immunity acquisition. A very large allelic polymorphism was observed in both villages, with a similar number of alleles but quite distinct allelic frequencies, indicating a substantial micro-geographical heterogeneity of malaria parasite populations. In addition, the molecular characteristics of the infections differed in both villages. As in most endemic areas, many infected subjects carry multiple parasite clones. In Dielmo, the number of distinct clones hosted decreases at the age of acquisition of an efficient immunity. There was no influence of age on the number of clones hosted in Ndiop where adults experience clinical attacks. This indicates that complexity reflects acquired immunity. The precise longitudinal follow-up of parasitaemia, clinical signs and parasite genetic characteristics showed a rapid turn over of parasite populations in the peripheral blood during the transmission season, suggesting that immunity does not prevent infection but restricts multiplication of numerous genotypes at the erythrocytic stage. Clinical malaria occurs after a rapid, apparently unrestricted growth of recently inoculated parasites. The successive clinical attacks experienced by children are associated with genotypes different for each attack and different from those that the child carried during preceding asymptomatic phases. These data indicate that parasite diversity contributes to the pathology of infection and that control of parasite density, which is at least in part strain-specific, is an essential element of protection against malaria clinical attacks.

stevor and rif are Plasmodium falciparum multicopy gene families which potentially encode variant antigens

Several multicopy gene families have been described in Plasmodium falciparum, including the var genes that code for the variant surface antigen PfEMP1, the stevor family of subtelomeric open reading frames and the rif interspersed repetitive elements. This report documents the chromosomal location of stevor genes, their transcription and characteristics of the deduced protein. On 14 chromosomes, 34 stevor copies were identified from the Dd2 parasite line. Most are in subtelomeric regions within 50 kb of the telomere. stevor genes are located close to var genes and rij. All stevor genes sequenced had two exons: a short exon 1 encoding a start codon and a transmembrane domain; exon 2 encoding for the remainder of the approximately 30 kDa protein and including two more transmembrane segments. A similar structure was found for copies of rif and its predicted protein. In both STEVOR and RIF proteins, a highly polymorphic region is predicted to be a loop on the outer side of the membrane. We propose that stevor and rif are members of a larger superfamily. The number of copies of stevor and rif, their location close to the var genes, their extreme polymorphism and the predicted structure of the proteins suggest that stevor and rif code for variant surface antigens.

Plasmodium falciparum asparagine and aspartate rich protein 2 is an evolutionary conserved protein whose repeats identify a new family of parasite antigens

We describe here a new Plasmodium falciparum antigen, asparagine and aspartate rich protein 2 (PfAARP2) of 150 kDa, which is encoded by a unique gene on chromosome 1. PfAARP2 is first expressed 12 h post-invasion and accumulates in trophozoites and schizonts. Immunofluorescence studies indicate that PfAARP2 is translocated into the red blood cell cytoplasm. The central region of Pfaarp2 contains blocks of repetitions encoding asparagine and aspartate residues, which define a new family of related genes dispersed on different chromosomes, and two members of this family have also been identified. Interestingly, the non-repeated N- and C-termini of PfAARP2 display significant similarity to two yeast and human predicted proteins, and its possible function is discussed.

Evidence for distinct prototype sequences within the Plasmodium falciparum Pf60 multigene family

Using oligonucleotides derived from Pf60.1, a member of the Plasmodium falciparum Pf60 multigene family, numerous fragments were amplified from genomic and cDNA from the 3D7 P. falciparum clone. DNA sequencing showed that the various fragments presented considerable diversity, indicating that the 3D7 repertoire contains at least 20 distinct versions of the region analysed. The various sequences aligned with either of two prototype sequences. Characteristic of the A-type was the presence of a 21 bp motif, present in variable copy number, as well as a sequence homologous to the Babesia sp. RAP-1 consensus. The B prototype sequence did not present such features and substantially differed from the A-type, due to accumulation of point mutations and numerous triplet deletions. Consistent with the marked differences between both sub-families, individual members from each sub-family did not cross-hybridise, produced distinct multiple band patterns on Southern blots and distinct chromosome profiles. Numerous hybrid sequences were observed. Interestingly, most var genes and var-related unspliced cDNAs described so far are of A/B hybrid type. These data suggest that the family has evolved by successive amplifications from two ancestral copies, with accumulation of mutations, as well as recombination and/or gene conversion events.

The chromosomal organization of the Plasmodium falciparum var gene family is conserved

The var gene family of Plasmodium falciparum encodes the protein PfEMP1 which is located on the surface of infected erythrocytes and is the receptor that mediates binding to ligands on endothelial cells. This family of proteins is responsible for antigenic variation and differences in binding phenotype to ligands such as CD36 and ICAM1. We have compared the organization of the var gene family in three in vitro cloned lines of P. falciparum and show that most var genes are located in the subtelomeric region of each chromosome closely linked to the repetitive sequence rep20. While most chromosomes possess var genes in the subtelomeric region, in each in vitro cloned line there are some chromosomes that have deleted subtelomeric repetitive regions which include var genes. Comparison of the location of var genes in a field isolate showed that it does not have any detectable subtelomeric deletions as all chromosomes contain var genes and rep20 sequences. We have detected three chromosomes (4, 7 and 12) that contain var gene loci in more stable central regions and the position of these genes on chromosome 4 in the cloned lines analysed is conserved. The location of most of the var gene family in the subtelomeric region of the genome of P. falciparum has important implications for the generation of antigenic diversity of the PfEMP1 protein.

Plasmodium falciparum: altered expressions of erythrocyte membrane-associated antigens during antigenic variation

The O and R antigenic variants of the Plasmodium falciparum Palo Alto strain present differences in the morphology of the infected red blood cell membrane, in their adhesion properties, surface immunofluorescence, and agglutination specificities and importantly, induce a variant-specific protection after a primary infection in Saimiri sciureus monkeys. To identify potential targets of variant-specific immunity, we have compared the antigenic makeup of both variants by immunoblot. O-specific monkey sera generated similar profiles on both parasite types, while R-specific sera showed a consistent difference on a high-molecular-mass undefined antigen. Distinct antibody specificities were eluted from the surface of O- or R-infected erythrocytes, generating variant-specific agglutination, surface immunofluorescence, and immunoblot profiles. An antiserum raised to Pf60.1, predicted to cross-react with the cytoplasmic domain of PfEMP1, reacted with specific, SDS-soluble antigens in both variants. Antigens associated with the membrane of the infected red blood cells were further investigated using several specific antisera. The 85-kDa HRP1 gene product was more abundant in O than in R parasites, while the reverse was observed for the PfEMP3 protein. These data indicate that O and R parasites differ in the expression of several antigens associated with the membrane of the infected red blood cell.

Disruption of a novel open reading frame of Plasmodium falciparum chromosome 9 by subtelomeric and internal deletions can lead to loss or maintenance of cytoadherence

Isolates of Plasmodium falciparum commonly undergo a large subtelomeric deletion of the right end of chromosome 9 during in vitro cultivation, accompanied by loss of ability to cytoadhere to melanoma cells and greatly lowered gametocyte production. ItG2, an isolate in which cytoadherence is stable, has undergone a subtelomeric deletion of intermediate length on chromosome 9. We show here that the deletions in all non-cytoadherent clones examined have breakpoints within or delete a novel open reading frame (the breakpoint open reading frame, BPORF) that is a unique sequence in the genome. Surprisingly, in ItG2 BPORF has been removed by an additional 15 kb internal deletion in chromosome 9. These results suggest mechanisms to account for the observed frequent deletion of the right arm of chromosome 9 and for the known stability of cytoadherence in ItG2. However, we were unable to detect var genes in this region of the ItG2 genome. We conclude that the product of a novel gene distinct from the var family is implicated in cytoadherence.

Extensive genetic diversity of Plasmodium falciparum isolates collected from patients with severe malaria in Dakar, Senegal

While some genetic host factors are known to protect against severe Plasmodium falciparum malaria, little is known about parasite virulence factors. We have compared the genetic characteristics of P. falciparum isolates collected from 56 severe malaria patients and from 30 mild malaria patients recruited in Hôpital Principal, Dakar, Senegal. All isolates were typed using polymerase chain reaction amplification of polymorphic genetic loci (MSP-1, MSP-2, HRP1, GLURP, CSP, RESA, and the multigene family Pf60). The complexity of infections was lower in severe than in mild malaria and the parasite genetic diversity in both groups was very large. No specific genetic make-up was associated with severity; there were, however, marked differences in allele frequencies in both groups, with a prevalence up to 60% of MSP-2 alleles specifically observed in the severe malaria isolates. In addition, the presence of MSP-1/RO33 alleles was significantly associated with a higher plasma level of tumour necrosis factor alpha receptor 1 (P < 0.05), a reported indicator of severity in human malaria. These results point to potential differences in the genetic characteristics of parasites inducing severe versus mild pathology.

Rhoptry organelles of the apicomplexa: Their role in host cell invasion and intracellular survival

Members of the phylum Apicomplexa are obligate intracellular parasites that invade erythrocytes, lymphocytes, macrophages or cells of the alimentary canal in various vertebrate species. Organelles within the apical complex of invasive stages facilitate host cell invasion. Parasites in this phylum cause some of the most debilitating diseases of medical and veterinary importance. These include malaria, toxoplasmosis, babesiosis, theileriosis (East Coast fever), and coccidiosis in poultry and livestock. In recent years, opportunistic infections caused by Cryptosporidium parvum, and recrudescent Toxoplasma gondii infections in AIDS patients have prompted intensified efforts in understanding the biology of these parasites. In this review, Tobili Sam-Yellowe examines the unifying and variant molecular features of rhoptry proteins, and addresses the role of multigene families in organelle function: the biogenesis of the rhoptries will also be examined, in an attempt to understand the sequence of events leading to successful packaging, modification and processing of proteins within the organelle.

Inhibition of Plasmodium falciparum malaria using antisense oligodeoxynucleotides

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.

The large diverse gene family var encodes proteins involved in cytoadherence and antigenic variation of Plasmodium falciparum-infected erythrocytes

The human malaria parasite Plasmodium falciparum evades host immunity by varying the antigenic and adhesive character of infected erythrocytes. We describe a large and extremely diverse family of P. falciparum genes (var) that encode 200-350 kDa proteins having the expected properties of antigenically variant adhesion molecules. Predicted amino acid sequences of var genes show a variable extracellular segment with domains having receptor-binding features, a transmembrane sequence, and a terminal segment that is a probable submembrane anchor. There are 50-150 var genes on multiple parasite chromosomes, and some are in clustered arrangements. var probes detect two classes of transcripts in steady-state RNA: 7-9 kb var transcripts, and an unusual family of 1.8-2.4 kb transcripts that may be involved in expression or rearrangements of var genes.