Conducted by an international team of researchers, the analysis, published in eLIFE in March 2016, studied more than 200 strains of L. donovani taken from patients in Bangladesh, India and Nepal in 2002— 2011.
The parasite, transmitted by sandflies, infects 300,000 and kills 20,000 of the world’s poorest people every year, primarily in South Asia.
“Drug resistance has been a major problem for kala-azar (leishmaniasis) treatment in the Indian subcontinent,” says Jean-Claude Dujardin, lead author of the paper. “Thanks to whole-genome analysis, we were able to identify subtle differences between strains, and even distinguish resistant strains from sensitive ones.”
Antimony-based drugs were the standard treatment through the last century until reports of drug-resistant strains in the Indian state of Bihar in the 1980s led to their abandonment. Antimonials were replaced by the oral cancer drug miltefosine in 2005, but this has been showing increasing treatment failure in India and Nepal. To understand how the parasite develops drug resistance, Dujardin and his colleagues used advanced genetic sequencing and tracked L. donovani to the mid-19th century when British physicians in colonial India confused the disease with malaria.
The DNA of the core group began to differentiate in the 1960s, coinciding with the end of anti-malarial DDT spraying campaigns. “This is when resistance to antimonials exploded,” says Dujardin. Sifting through the sequenced genomes, a two-nucleobase difference was found in a drug-resistant group of parasites. “It was frightening to think that we could have missed it — I can’t say whether we found it by chance or due to rigorous analysis,” says Dujardin. The L. donovani genome has an average of 30 million base pairs.
The mutation was found in a gene that encodes the protein aquaglyceroporin-1 responsible for pumping antimonial drugs into L. donovani, thus making it inactive. The researchers also found other genetic adaptations, including multiple gene copies of another protein that pumps out antimonials already in the parasite. “Leishmania can, in answer to the environment, play with the copy numbers of its chromosomes and genes,” explains Dujardin.
“The study confirms a long-drawn hypothesis,” says Rita Mukhopadhyay, associate professor at Florida International University, who identified, in 2004, the role of aquaglyceroporin-1 in antimonial resistance. “I strongly believe that aquaglyceroporin-1 has a much greater role to play in the physiology of Leishmania and its transmission from the fly to humans,” she says.
The researchers hope to sequence the whole genome of parasites isolated directly from clinical samples, as opposed to growing them in culture, eventually introducing near-real-time monitoring of the parasite’s genetic evolution. “This will allow us to see quickly if any new clones emerge,” says Dujardin.
This piece was produced by SciDev.Net’s South Asia desk.