NEW YORK – By defining essential genes in the primate malaria-causing parasite Plasmodium knowlesi, independent research teams have gotten a look at potential contributors to antimalarial drug resistance, while highlighting metabolic and other adaptations in Plasmodium parasites specialized for infecting different hosts.
For the first of the studies, published in Science, investigators in the US, Brazil, and Portugal presented findings from essential genome analyses done using transposon mutagenesis screening in the P. knowlesi parasite — a zoonotic malaria parasite in the same Plasmodium sub-genus as the human malaria parasite species P. vivax — in the presence or absence of the antimalaria drug artemisinin.
"[W]e harnessed the development of high-efficiency molecular genetics in P. knowlesi to elicit genome-wide transposon mutagenesis to provide the most complete determination of gene essentiality for the blood-stage infection in any Plasmodium [species]," co-senior and co-corresponding authors Manoj Duraisingh, an immunology and infectious diseases researcher at the Harvard T.H. Chan School of Public Health, and Kourosh Zarringhalam, with the University of Massachusetts Boston, and their colleagues wrote.
Using a modified quantitative insertion-site sequencing (QIseq) strategy, the team tracked the consequences of more than 1.4 million piggyBac transposon insertions across the P. knowlesi genome in haploid, "blood-stage" parasites grown in rhesus macaque red blood cells, identifying 2,037 essential genes and 2,124 dispensable genes, along with sequences linked to altered parasite fitness and genes with yet-to-be-determined essentiality.
In the presence of the active metabolite in the antimalarial drug artemisinin, meanwhile, the investigators saw similarities between essential genes found in P. knowlesi and those previously reported in other malaria parasite species such as P. falciparum. Even so, their analyses suggested that P. knowlesi is particularly reliant on genes from the energy-generating mitochondrial metabolism pathways.
"Our findings aid prioritization of drug and vaccine targets for the Plasmodium vivax clade and reveal mechanisms of resistance that can inform therapeutic development," the authors suggested, calling the new resource "the most complete determination of gene essentiality for the blood-stage infection in any Plasmodium [species]."
In another paper published in Science on Thursday, investigators at the University of South Florida, the University of Cambridge, and other centers in the UK, France, and Thailand presented findings from their own supersaturation mutagenesis screening study of P. knowlesi.
Based on data for more than 175,000 piggyBac transposon insertions mapped with QIseq sequencing in blood-stage parasites grown in human red blood cell cultures, the team unearthed genes with differing essentiality between malaria-causing parasites with distinct host specializations, despite overall conservation found in P. knowlesi and P. falciparum, as well as the rodent-infecting P. berghei species.
"This 'supersaturation' level of mutagenesis, with an average transposon insertion frequency of every 138 base pairs, allowed us to score essentiality for 98 percent of genes," co-senior and co-corresponding authors John Adams, with the University of South Florida and Mahidol University in Thailand, and Julian Rayner, at the University of Cambridge, and their colleagues reported, noting that the "domain-level resolution of essentiality provides a completely new level of Plasmodium genome annotation."
By comparing their P. knowlesi mutagenesis map with published data for the human P. falciparum parasite and the rodent parasite P. berghei, for example, members of that team highlighted metabolic network shifts in Plasmodium species that have become adept at infecting distinct host species.
"Our data indicate that as malaria parasites evolved and adapted to new hosts and their intracellular microenvironments, they have undergone metabolic rewiring, and that P. knowlesi in particular has a high level of metabolic plasticity that evidence suggests extends to other vivax-clade malaria parasites," the authors of that study concluded. "Even though the conservation of orthologs is high, the functional essentiality of genes has changed to adapt to the particular physiology of the different hosts."