Differential var gene transcription in Plasmodium falciparum isolates from patients with cerebral malaria compared to hyperparasitaemia

Abstract

The Plasmodium falciparum variant erythrocyte surface antigens known as PfEMP1, encoded by the var gene family, are thought to play a crucial role in malaria pathogenesis because they mediate adhesion to host cells and immuno-modulation. Var genes have been divided into three major groups (A, B and C) and two intermediate groups (B/A and B/C) on the basis of their genomic location and upstream sequence. We analysed expressed sequence tags of the var gene DBLα domain to investigate var gene transcription in relation to disease severity in Malian children. We found that P. falciparum isolates from children with cerebral malaria (unrousable coma) predominantly transcribe var genes with DBLα1-like domains that are characteristic of Group A or B/A var genes. In contrast, isolates from children with equally high parasite burdens but no symptoms or signs of severe malaria (hyperparasitaemia patients) predominantly transcribe var genes with DBLα0-like domains that are characteristic of the B and C-related var gene groups. These results suggest that var genes with DBLα1-like domains (Group A or B/A) may be implicated in the pathogenesis of cerebral malaria, while var genes with DBLα0-like domains promote less virulent malaria infections.

Plasmodium falciparum var gene expression is modified by host immunity

“…However, this would not explain the actual mechanism of disease onset, because many children with high cys2 levels did not have severe malaria. One possibility, suggested previously (19) is that many children with nonsevere malaria and high cys2 var expression would go on to get severe malaria if not treated. An alternative possibility is that high cys2 var expression merely increases the probability of an event that triggers the onset of severe malaria.…”

“…In a small pilot study of 12 isolates, children with poorly developed immune responses tended to express cys2 var genes with MFK motifs (16). More recently, Kyriacou et al showed that cys2 var genes were the dominant sequence type expressed by parasites isolated from children with cerebral malaria in Mali (19). These studies support the usefulness of a sequencing approach to comparing PfEMP1 expression between clinical parasite isolates.…”

“…The immense architectural diversity of var genes, together with their capacity to undergo recombination (15), yields limited positions for PCR amplification and sequence sampling. Therefore we (16) and others (17)(18)(19)(20)(21) have relied on analysis of short, Ϸ350 nucleotide, expressed sequence tags amplified from a region corresponding to a domain that is present in most PfEMP1 variants, DBL␣. To estimate PfEMP1 expression levels, reverse transcriptase PCR products are subcloned into Escherichia coli, bacterial colonies picked and sequenced, and the sequence tags classified and counted.…”

“…To estimate PfEMP1 expression levels, reverse transcriptase PCR products are subcloned into Escherichia coli, bacterial colonies picked and sequenced, and the sequence tags classified and counted. Previous studies have varied in their depth of sequencing from each parasite isolate, the number of patients in different clinical categories, time of RNA sampling within the parasite asexual cycle, method of sequence analysis, and whether the emphasis is placed more on the dominantly expressed sequence(s) in each patient (19,20) or on the overall percentage representation within each patient of different groups of sequence [supporting information (SI) Table S1] (16,22).…”

Pathogenic mechanisms in Plasmodium falciparum malaria

“…Se ha observado un incremento significativo en la expresión de transcriptos del grupo B en niños con malaria sintomática (moderada y grave) en Papúa, Nueva Guinea (26,29), Tanzania (28) y Mali (27), mientras los transcriptos de genes C se expresan de forma predominante en los casos de malaria asintomática e hiperparasitemias no asociadas con el desarrollo de manifestaciones graves (26,27,29). Se sugiere la participación de proteínas codificadas por genes de tipo A en el desarrollo de paludismo complicado; dominios DBL-1 del grupo A se han asociado con enfermedad grave y falla multiorgánica en pacientes de Brasil (30 (27,29). Sin embargo, en Papúa no se encontró relación entre enfermedad grave y expresión de genes var A, ni con la formación de rosetas (26,29).…”