Invasion of human erythrocytes is essential for Plasmodium falciparum parasite survival and pathogenesis, and is also a complex phenotype. While some later steps in invasion appear to be invariant and essential, the earlier steps of recognition are controlled by a series of redundant, and only partially understood, receptor-ligand interactions. Reverse genetic analysis of laboratory adapted strains has identified multiple genes that when deleted can alter invasion, but how the relative contributions of each gene translate to the phenotypes of clinical isolates is far from clear. We used a forward genetic approach to identify genes responsible for variable erythrocyte invasion by phenotyping the parents and progeny of previously generated experimental genetic crosses. Linkage analysis using whole genome sequencing data revealed a single major locus was responsible for the majority of phenotypic variation in two invasion pathways. This locus contained the PfRh2a and PfRh2b genes, members of one of the major invasion ligand gene families, but not widely thought to play such a prominent role in specifying invasion phenotypes. Variation in invasion pathways was linked to significant differences in PfRh2a and PfRh2b expression between parasite lines, and their role in specifying alternative invasion was confirmed by CRISPR-Cas9-mediated genome editing. Expansion of the analysis to a large set of clinical P. falciparum isolates revealed common deletions, suggesting that variation at this locus is a major cause of invasion phenotypic variation in the endemic setting. This work has implications for blood-stage vaccine development and will help inform the design and location of future large-scale studies of invasion in clinical isolates.
Bibliographical noteFunding Information:
This work was supported by the Wellcome Trust (090851) and the Medical Research Council UK (Grant no. MR/M01360X/1 to TGC and SC). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The authors would like to acknowledge advice and input from Mehdi Ghorbal and Ellen Bushell in the production of the genome editing constructs. The parental and progeny parasite lines were kindly provided by Dr. Karen Hayton, Professor Thomas Wellems and Dr Mike Ferdig.
© 2018 Campino et al. http://creativecommons.org/licenses/by/4.0/.