Six Genes Are Preferentially Transcribed by the Circulating and Sequestered Forms of Plasmodium falciparum Parasites That Infect Pregnant Women

Abstract

In areas of stable malaria transmission, susceptibility to Plasmodium falciparum malaria increases during first pregnancy. Women become resistant to pregnancy malaria over successive pregnancies as they acquire antibodies against the parasite forms that sequester in the placenta, suggesting that a vaccine is feasible. Placental parasites are antigenically distinct and bind receptors, like chondroitin sulfate A (CSA), that are not commonly bound by other parasites. We used whole-genome-expression analysis to find transcripts that distinguish parasites of pregnant women from other parasites and employed a novel approach to define and adjust for cell cycle timing of parasites. Transcription of six genes was substantially higher in both placental parasites and peripheral parasites from pregnant women, and each gene encodes a protein with a putative export sequence and/or transmembrane domain. This cohort of genes includes var2csa, a member of the variant PfEMP1 gene family previously implicated in pregnancy malaria, as well as five conserved genes of unknown functions. Women in East Africa acquire antibodies over successive pregnancies against a protein encoded by one of these genes, PFD1140w, and this protein shows seroreactivity similar to that of VAR2CSA domains. These findings suggest that a suite of genes may be important for the genesis of the placental binding phenotype of P. falciparum and may provide novel targets for therapeutic intervention.

Binding affinity of Plasmodium falciparum-infected erythrocytes from infected placentas and laboratory selected strains to chondroitin 4-sulfate

“…A scenario wherein novel proteins unrelated to PfEMP1 playing roles in the adhesive process cannot be ruled out. Consistent with these predictions, recent studies implicate novel proteins in IRBC adherence [13][14][15][16]. Thus, the development of a vaccine for placental malaria remains challenging.…”

NSR-seq transcriptional profiling enables identification of a gene signature of Plasmodium falciparum parasites infecting children

“…Two recent DNA microarray studies failed to identify parasite genes whose expression was related to disease severity (13,14). In contrast, microarray and proteomic studies of pregnancy malaria have identified sets of genes and proteins, including VAR2CSA, that are more abundant in parasites isolated from pregnant women than in those from children (15,16) or that are more abundant in placental parasites than in a laboratory-adapted reference strain (17).…”

“…We show that this technique excludes the majority of human globin and rRNA and enables the identification of parasite genes whose transcription levels vary among field isolates. In addition, NSR-seq detects most genes previously discovered by microarray analysis as upregulated in parasites infecting pregnant women, including the pregnancy malaria vaccine candidate var2csa (15), demonstrating that this technique is suitable for the identification of differentially regulated parasite genes. More importantly, NSR-seq identifies 4 genes that are expressed preferentially by children's parasites (glutamic acid-rich protein [garp/PFA0620c]; mature-parasite-infected erythrocyte surface antigen [MESA/PfEMP2/PFE0040c]; glycophorin-binding protein 2 [Gbph2/PF13_0010]; and probable protein PF10_0350]), which might have roles of particular importance in the pathogenesis of childhood malaria.…”

Towards a vaccine against pregnancy-associated malaria

“…Although these findings do not exclude the role of VAR2CSA in the binding of iE to CSA, they suggest that parasite binding to CSA might involve multiple binding ligands or a multiprotein complex comprising VAR2CSA PfEMP1 and other proteins, the identification of which remains an important goal. Microarray approach in transcriptome analysis of field isolates has found specific transcription of novel genes encoding potential surface antigens in fresh placental isolates (Francis et al, 2007;Tuikue Ndam et al, 2008). However, molecular, structural and functional data are still needed to the understanding the biological relevance of those PAM-specific antigens and thus aid in defining critical constructs as vaccine candidates.…”