Research Article

Uncovering the essential genes of the human malaria parasite Plasmodium falciparum by saturation mutagenesis

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Science  04 May 2018:
Vol. 360, Issue 6388, eaap7847
DOI: 10.1126/science.aap7847

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Saturating malaria mutagenesis

Malaria is caused by eukaryotic Plasmodium spp. parasites that classically infect red blood cells. These are difficult organisms to investigate genetically because of their AT-rich genomes. Zhang et al. have exploited this peculiarity by using piggyBac transposon insertion sites to achieve saturation-level mutagenesis for identifying and ranking essential genes and drug targets (see the Perspective by White and Rathod). Genes that are current candidates for drug targets were identified as essential, in contrast to many vaccine target genes. Notably, the proteasome degradation pathway was confirmed as a target for developing therapeutic interventions because of the several essential genes involved and the link to the mechanism of action of the current frontline drug, artemisinin.

Science, this issue p. eaap7847; see also p. 490

Structured Abstract

INTRODUCTION

Malaria remains a devastating global parasitic disease, with the majority of malaria deaths caused by the highly virulent Plasmodium falciparum. The extreme AT-bias of the P. falciparum genome has hampered genetic studies through targeted approaches such as homologous recombination or CRISPR-Cas9, and only a few hundred P. falciparum mutants have been experimentally generated in the past decades. In this study, we have used high-throughput piggyBac transposon insertional mutagenesis and quantitative insertion site sequencing (QIseq) to reach saturation-level mutagenesis of this parasite.

RATIONALE

Our study exploits the AT-richness of the P. falciparum genome, which provides numerous piggyBac transposon insertion targets within both gene coding and noncoding flanking sequences, to generate more than 38,000 P. falciparum mutants. At this level of mutagenesis, we could distinguish essential genes as nonmutable and dispensable genes as mutable. Subsequently, we identified 2680 genes essential for in vitro asexual blood-stage growth.

RESULTS

We calculated mutagenesis index scores (MISs) and mutagenesis fitness scores (MFSs) in order to functionally define the relative fitness cost of disruption for 5399 genes. A competitive growth phenotype screen confirmed that MIS and MFS were predictive of the fitness cost for in vitro asexual growth. Genes predicted to be essential included genes implicated in drug resistance—such as the “K13” Kelch propeller, mdr, and dhfr-ts—as well as targets considered to be high value for drugs development, such as pkg and cdpk5. The screen revealed essential genes that are specific to human Plasmodium parasites but absent from rodent-infective species, such as lipid metabolic genes that may be crucial to transmission commitment in human infections. MIS and MFS profiling provides a clear ranking of the relative essentiality of gene ontology (GO) functions in P. falciparum. GO pathways associated with translation, RNA metabolism, and cell cycle control are more essential, whereas genes associated with protein phosphorylation, virulence factors, and transcription are more likely to be dispensable. Last, we confirm that the proteasome-degradation pathway is a high-value druggable target on the basis of its high ratio of essential to dispensable genes, and by functionally confirming its link to the mode of action of artemisinin, the current front-line antimalarial.

CONCLUSION

Saturation-scale mutagenesis allows prioritization of intervention targets in the genome of the most important cause of malaria. The identification of more than 2680 essential genes, including ~1000 Plasmodium-conserved essential genes, will be valuable for antimalarial therapeutic research.

Saturation-scale mutagenesis of P. falciparum reveals genes essential and dispensable for asexual blood-stage development.

(Top) A high-resolution map of a ~50-kb region of chromosome 13 depicts an essential gene cluster, including K13, that lacks insertions in the coding DNA sequence (CDS) but is flanked by dispensable genes with multiple CDS-disrupting insertions. (Left) The MIS rates the potential mutability of P. falciparum genes based on the number of recovered CDS insertions relative to the potential number that could be recovered through large-scale mutagenesis. (Right) The MFS rates the relative fitness of P. falciparum genes based on QIseq scores of transposon insertion sites in each gene.

Abstract

Severe malaria is caused by the apicomplexan parasite Plasmodium falciparum. Despite decades of research, the distinct biology of these parasites has made it challenging to establish high-throughput genetic approaches to identify and prioritize therapeutic targets. Using transposon mutagenesis of P. falciparum in an approach that exploited its AT-rich genome, we generated more than 38,000 mutants, saturating the genome and defining mutability and fitness costs for over 87% of genes. Of 5399 genes, our study defined 2680 genes as essential for optimal growth of asexual blood stages in vitro. These essential genes are associated with drug resistance, represent leading vaccine candidates, and include approximately 1000 Plasmodium-conserved genes of unknown function. We validated this approach by testing proteasome pathways for individual mutants associated with artemisinin sensitivity.

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