Differential expression of var gene groups is associated with morbidity caused by Plasmodium falciparum infection in Tanzanian children

Abstract

The var gene family of Plasmodium falciparum encodes the variant surface antigen Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1). PfEMP1 is considered an important pathogenicity factor in P. falciparum infection because it mediates cytoadherence to host cell endothelial receptors. var genes can be grouped into three major groups, A, B, and C, and the conserved var genes, var1-4, according to sequence similarities in coding and noncoding upstream regions. Using real-time quantitative PCR in a study conducted in Tanzania, the var transcript abundances of the different var gene groups were compared among patients with severe, uncomplicated, and asymptomatic malaria. Transcripts of var group A and B genes were more abundant in patients with severe malaria than in patients with uncomplicated malaria. In general, the transcript abundances of var group A and B genes were higher for children with clinical malaria than for children with asymptomatic infections. The var group C and var1-like transcript abundances were similar between the three sample groups. A transcript abundance pattern similar to that for var group A was observed for var2csa and var3-like genes. These results suggest that substantial and systematic differences in var gene expression exist between different clinical presentations.

FIG. 1.

Differences in var transcript abundance (x-fold changes) between 3D7_UM and antibody-selected 3D7_SM parasites (15). Transcript abundance was measured by using primers targeting var gene groups (white) or primers targeting single var genes (black) and are summarized corresponding to var gene groups. A twofold change in var transcript abundance (dashed lines) was arbitrarily defined as the cutoff for biologically significant changes in var transcript abundance.

FIG. 2.

Transcript abundances of var gene groups in parasites from children with AM and from children suffering from UM or SM. Transcript abundances are shown relative to the average abundances in uncomplicated cases (ΔC_T values). Panel A shows var group A transcript abundances measured with primers A1-3 in quantitative PCR and with upsA-probe in the MGB assay. Similarly, panels B and C show the transcript abundances of group B and C var genes, respectively. Panel D shows the transcript abundances measured with primers BC1 and BC2, targeting group B and C genes. Panel E shows transcript abundances measured with primers targeting the conserved var genes var1-3. Boxes outline 25th to 75th percentiles, with medians indicated as a line inside each box and whiskers illustrating the 5th and 95th percentiles. Horizontal lines with asterisks below the plots indicate statistically significant differences in transcript abundance between groups (one-way analysis of variance; P < 0.05 after Bonferroni correction).