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

Abstract

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.

Comparative Genomics of Transcriptional Control in the Human Malaria Parasite Plasmodium falciparum

“…This may alter the properties of the ribosomes and, through altered translation rates, modify patterns of cell growth and development. Secondly, in situ var gene switching is mediated at the level of transcriptional inititiation (Scherf et al 1998); this switching mechanism results in only one var gene being actively transcribed in a single parasite, whereas the remaining copies remain silenced, and plays an important role in Plasmodium survival and virulence. Finally, two P. falciparum promoters have been identifed whose activation marks the developmental switches executed during the sexual differentiation process (Dechering et al 1999).…”

Variable var transition rates underlie antigenic variation in malaria

“…The transcription of var genes appears to be tightly regulated such that only a single full-length copy is transcribed in one parasite at any one time (15,16). This process results in the mutually exclusive expression of PfEMP1 variants at the IE surface.…”

“…This process results in the mutually exclusive expression of PfEMP1 variants at the IE surface. Transcriptional switching occurs in situ and, in contrast to similar processes in other organisms, does not require specific gene rearrangements or DNA deletion events (16). No changes in the pattern of DNA methylation or DNase I hypersensitivity have been associated with the expressed variant, nor does expression rely on the presence of variant specific promoter sequences (1).…”

Variability and Exclusion in Host and Parasite: Epigenetic Regulation of Ig and var Expression

“…However, while termination of the rearrangement process depends on negative feedback generated by signaling of pre-and B cell membrane receptors, the mutually exclusive manner of expression at the var gene family relies entirely on noncoding elements of the P. falciparum genome. Moreover, while DNA-demethylation precedes Ig rearrangement and is essential for the rearrangement to take place (1,2,14), methylation patterns are largely indistinguishable between active and inactive var genes, although subtle differences may still exist (6). This is in agreement with the highly reversible nature of the var decision, which precludes long-term modifications such as DNA demethylation.…”

“…During the proliferation phase of the parasite infection, expression of PfEMP1 switches from one var gene to another, giving rise to parasite antigenic variation and thus facilitating host immune evasion (6). The var switching process is regulated at the level of transcription and follows the rule that only one gene is expressed at a time in a single parasite (6).…”