Variation In Malaria Research Articles

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.

Antigenic Variation in Malaria: a 3′ Genomic Alteration Associated with the Expression of a P. knowlesi Variant Antigen

Antigenic variation of malaria parasites was discovered in P. knowlesi, using a schizont-infected cell agglutination (SICA) assay to detect variant antigens expressed at the surface of infected erythrocytes. Later studies utilizing stable clones, Pk1(A+) and its direct derivative, Pk1(B+)1+, showed that SICA[+] clones express distinct parasite-encoded antigens of ∼200 kDa. Here we identify a P. knowlesi variant antigen gene and cDNA and demonstrate that it encodes the 205 kDa variant antigen expressed by B+ parasites. This gene belongs to a multigene family, which we term SICAvar. Its ten-exon structure with seven cysteine-rich coding modules is unique compared to P. falciparum var genes. Further, we highlight a 3′ genomic alteration that we predict is related to SICAvar gene switching.

Antigenic variation in malaria: in situ switching, relaxed and mutually exclusive transcription of var genes during intra-erythrocytic development in Plasmodium falciparum.

Members of the Plasmodium falciparum var gene family encode clonally variant adhesins, which play an important role in the pathogenicity of tropical malaria. Here we employ a selective panning protocol to generate isogenic P.falciparum populations with defined adhesive phenotypes for CD36, ICAM-1 and CSA, expressing single and distinct var gene variants. This technique has established the framework for examining var gene expression, its regulation and switching. It was found that var gene switching occurs in situ. Ubiquitous transcription of all var gene variants appears to occur in early ring stages. However, var gene expression is tightly regulated in trophozoites and is exerted through a silencing mechanism. Transcriptional control is mutually exclusive in parasites that express defined adhesive phenotypes. In situ var gene switching is apparently mediated at the level of transcriptional initiation, as demonstrated by nuclear run-on analyses. Our results suggest that an epigenetic mechanism(s) is involved in var gene regulation.

Cloning the P. falciparum gene encoding PfEMP1, a malarial variant antigen and adherence receptor on the surface of parasitized human erythrocytes

Plasmodium falciparum-infected human erythrocytes evade host immunity by expression of a cell-surface variant antigen and receptors for adherence to endothelial cells. These properties have been ascribed to P. falciparum erythrocyte membrane protein 1(PfEMP1), an an igenically diverse malarial protein of 200–350 kDa on the surface of parasitized erythrocytes (PEs). We describe the cloning of two related PfEMP1 genes from the Malayan Camp (MC) parasite strain. Antibodies generated against recombinant protein fragments of the genes were specific for MC strain PfEMP1 protein. These antibodies reacted only with the surface of MC strain PEs and blocked adherence of these cells to CD36 but without effect on adherence to thrombospondin. Multiple forms of the PfEMP1 gene are apparent in MC parasites. The molecular basis for antigenic variation in malaria and adherence of infected erythrocytes to host cells can now be pursued.

Antigenic variation in malaria

Like most protozoan infections malaria is usually chronic. Even in the absence of persistent exoerythrocytic infection parasites may remain in the blood for months after antiplasmodial antibody is detectable in the serum.

Genes, vaccines and variation in malaria

lar biology in chemotherapeutic research. J. Hyde (University of Manchester Institute of Science and Technology) described his studies on lactate dehydrogenase (LDH) of P. falciparum as a model enzyme. Monoclonal antibodies have been used to isolate the LDH from total parasite protein preparations and to identify in-vitro RNA translation products. The P. falaparum LDH subunit has a molecular weight of 35 kDa. Compared to LDH from other sources, the parasite enzyme has an extremely biased amino acid composition, lysine and asparagine making up 23% of its content, reflecting the very high (79%) A 4T content of the DNA coding for this protein. Computer analysis using a program, DIAGON, which compares base sequences, further showed that the parasite enzyme showed a lower homology to B forms of LDH which predominate in vertebrate aerobic tissues than to the A form, and that the greatest homology lies in the N-terminal half of the molecule, the region of the enzyme associated with coenzyme binding. Although translation studies in vitro demonstrated that the gene coding for LDH was 1000 bases long, a 3.7 kb section was isolated from the total parasite DNA following cleavage with the restriction enzyme EcoRI. A 1.8 kb length of this DNA has been sequenced; it shows a 12% homology with the gene coding for chicken LDH, but is unusual in possessing no stop codon. J. Crampton (Liverpool School of Tropical Medicine) considered the genetic basis of resistance of dihydrofolate reductase (DHFR) inhibitors, such as pyrimethamine and proguanil, in P. falciparum. He reviewed strategies available for isolating the gene coding for this enzyme. DNA probes for the genes coding for DHFR in a range of other organisms did not identify the parasite gene, and he emphasized the need for a purified enzyme. An elegant analysis of the P. falciparum genome was described which involved the sequential cleavage of the parasite DNA with two different restriction enzymes, This procedure provides a sensitive method for analysing subtle changes in copy number in the genome and other changes in the DNA associated with the acquisition of drug resistance.

Immunological studies with simian malarias I. Antigenic variants of Plasmodium cynomolgi bastianellii

The concept of antigenic variation in malaria is supported by the present study on protective immunity. The immunity after infection is to some degree specific for each variant and falls rapidly after cure. Successive infections reinforce the immunity to subsequent challenge. The experiments to determine whether antigenic variation takes place in the mosquito were not conclusive, but suggested that it could occur.