Researchers identify a molecular marker for parasites resistant to malaria treatment

An international collaboration led by researchers from the Institut Pasteur in Cambodia, and the Institut Pasteur and CNRS in France, has recently identified in the malaria parasite, Plasmodium falciparum, genetic changes that are found in parasites resistant to artemisinin treatment in Cambodia.

These seminal findings were published recently in the journal Nature.

Chloroquine-resistant P. falciparum parasites first emerged in Western Cambodia in the 1950s. Since then, chloroquine resistance spread to nearly every corner of the world. Highly effective antimalarial therapies are currently all based on artemisinins and since they were introduced in the early 2000s, deaths from malaria have decreased markedly[i]. Unfortunately, antimalarial resistance has emerged again in the same area of Southeast Asia, but this time, it is resistance to artemisinins[ii]. This emerging resistance threatens the global health community’s efforts to control and eliminate malaria.

In recent years, malaria researchers have worked assiduously to identify genetic changes that correlate with artemisinin resistance in P. falciparum parasites. Two earlier papers identified regions of the parasite genome that appeared to harbor genetic determinants of artemisinin resistance, but these studies stopped short of pinpointing a specific gene responsible for the resistance ^{iii, iv}.

The collaborative work of researchers at the Institut Pasteur in Cambodia, the Institut Pasteur and CNRS  in France, and the National Institute of Allergy and Infectious Diseases in the USA, used a combination of different scientific approaches, including genomics, molecular biology, clinical studies and epidemiology to identify the precise genetic changes that mark these resistant parasites.

Frédéric Ariey, Head of Research at the Institut Pasteur in Paris, explains their approach: “First of all we started to use genomic sequencing of a strain of Plasmodium falciparum made resistant to high levels of artemisinin in the laboratory; we then compared this genome with the non-resistant parent strain and identified the differences. This comparison enabled us to identify a mutation that marked the resistant strain, a change in a specific gene called K13-propeller.”

Genetic changes in the K13 gene were then also identified in artemisinin-resistant parasite strains that have become increasingly prevalent in Cambodia. This enabled the researchers to establish a strong correlation between the presence of mutations in a particular part of the K13 gene and in parasites that showed reduced susceptibility to artemisinin both in a newly developed laboratory test^v and in patients treated for malaria in six regions of Cambodia.

Didier Ménard from the Institut Pasteur in Cambodia confirms: “We were able to usesome large sets of patient blood samples that were gathered over the last 10 years to show the invasion of K13-mutant alleles in areas in Cambodia where artemisinin resistance has been observed.”

This work represents a significant step in the race to identify the key factors that cause artemisinin resistance. The results will enable researchers to review their data and validate the correlation of the marker with parasite sensitivity to artemisinins. Once validated, the marker can then be used as a tool to map quickly the extent of the resistance and to design actions to control its spread or emergence in other endemic areas, most notably across the Greater Mekong Subregion, South Asia and sub-Saharan Africa.

Going forward, the authors also suggest that more work is required to determine how changes in the K13-propeller cause the poor response to artemisinin, and to identify any additional genetic loci related to even higher levels of artemisinin resistance (Nature 2013 page 4).

University of Maryland Professor Chris Plowe summarises the importance of this publication^vi, “This new marker gives us a tool that will make it possible to map the distribution of artemisinin resistance very quickly. There are a number of important research questions that need to be answered, but in the meantime knowing the distribution of K13 resistant genotypes will be very useful in planning malaria elimination efforts in the Greater Mekong Subregion.”

View the paper “A molecular marker of artemisinin-resistant Plasmodium falciparum malaria” (Nature 505, 50–55; 02 January 2014; doi: 10.1038/nature12876).

References

i Source: The World Health Organisation’s (WHO) World Malaria Report 2013 (December 2013)

iⁱ Dondorp AM, Nosten F, Yi P, et al. Artemisinin Resistance in Plasmodium falciparum Malaria. New England Journal of Medicine 2009; 361:455-67

Amaratunga C, Sreng S, Suon S et al. Artemisinin-resistant Plasmodium falciparum in Pursat province, western Cambodia: a parasite clearance rate study. Lancet Infectious Diseases 2012; Volume 12, Issue 11, November 2012, Pages 851–858.

iⁱⁱ Cheeseman IH, Miller BA, Nair S, et al. A major genome region underlying artemisinin resistance in malaria. Science 2012; 336:79-82.

^iv Takala-Harrison S, Clark TG, Jacob CG, et al. Genetic loci associated with delayed clearance of Plasmodium falciparum following artemisinin treatment in Southeast Asia. Proc Natl Acad Sci USA 2012; doi: 10.1073/pnas.1211205110

^v  Witkowski B, Amaratunga C, Khim N, et al. Novel phenotypic assays for the detection of artemisinin-resistant Plasmodium falciparum malaria in Cambodia: in-vitro and ex-vivo drug-response studies. The Lancet Infectious Diseases 2013; 13:1043-9.

^vi Christopher V. Plowe. Malaria: Resistance nailed. Nature 2013 doi:10.1038/nature12845 (published online December 18)