A large majority of the Leishmania donovani isolates in the ISC are pre-adatapted to antimony

What is it about?

In a previous study we demonstrated the presence in the Indian subcontinent of two different genetic background: the Core Group (CG) in the Indo-Napalese low land and the ISC1 group in the Nepalese high land (Imamura et al. eLife). The particularity of all the 194 isolates from the CG is to have two amplifications inside their chromosomes. We wanted to understand how resistance to an anti-leishmanial drug, antimony, emerged so widely in the CG. The first striking fact was the natural higher antimony tolerance of all the strains from the CG. At the contrary all the isolates from the ISC1 group were extremely sensitive to the drug. In order to investigate the question we selected antimony resistant parasites using 2 strains from the CG and 1 strain from the ISC1 group. The first obvious outcome of this selection is the way the isolates from the different groups behaved. While the 2 isolates from the CG were easily selected in 4 replicates in only 20 weeks only 1 replicate from the ISC1 strain could be recover after 35 weeks. Then we performed the whole genome sequencing of all the selection intermediate and we couldn't find any significant modification of the genome for all the samples of the CG. For the antimony resistant isolate we discovered a chromosomal amplification at the level of the antimony sequestration pump bringing now that isolate to have the same level of gene copy number for the pump as the CG isolates. We hypothesised and verified that the amplification of the pump was responsible for the higher tolerance of the CG strains to antimonials. In fact antimonials have been in used in the ISC for more than 70 years before been withdrawn because more than 60% of the parasite population was resistant to it. In this paper we demonstrated for the first time a natural pre-adapation to resist to antimony since the 2 amplifications emerged before the massive use of antimonials. We linked the pre-adapation to the presence of 1 of the two amplifications, called the H-locus, carrying a pump to sequester antimonials making them inactive. We used whole genome sequencing and metabolomic approach to characterise the nature of the changes during a step wise drug exposure experiment. Looking at the genome We also hypothesised what made the parasite pre-adapted: (1) use of the active form of antimonials for nearly 10 years at the beginning of their use, (2) massive water contamination with arsenic, a chemical element close to antimony which has been linked to amplification of the H-locus and (3) before using antimonials and before Leishmania was discovered as being responsible for visceral leishmaniasis, it was considered as "quinine-resistant malaria" and thus treated with high doses of quinine which is known to lead to an amplification of the same transporter.

Featured Image

Why is it important?

So far this is the first description ever of such type of phenomenon in Leishmania. This would have a tremendous impact on our way to imagine drug resistance but also to conceive new drugs! If the parasite can be more tolerant because of external factors that are not related to human activities is it important to know it especially in a drug development strategy. For the moment drug development is carried using in general one or two well established laboratory strains for each Leishmania species. Based on the pre-adaptation discovery we strongly emphasize the need of using a large variety of recent clinical isolates coming from multiple endemic regions in the drug development pipeline. Furthermore it highlights the high susceptibility of the ISC1 isolates toward antimony and their biological differences compare to the CG isolates. In a recent these genomic and metabolomic differences were further described (Cuypers et al. Infection, Genetics and Evolution).

Perspectives