Incorporating Genetic Sequences into a Mosquito Genome to Prevent Malaria: A Case Study in Oryzias latipes

Modelling suggests that these genetic modifications could break malaria transmission in sub-Saharan Africa, but no gene-drive mosquitoes have yet been released in the wild.

“It shouldn’t be acceptable that children still die of malaria, that expectant mothers still get complications due to malaria,” says Jonathan Kayondo, a vector biologist at the Uganda Virus Research Institute in Entebbe.

Gene drive was first proposed decades ago, in the context of naturally occurring selfish genetic elements — portions of DNA that enhance their own transmission. Transposable elements are the same as the sequences of DNA. These are present in a wide range of organisms, including humans. In order for transposable elements to integrate throughout an organisms genome, they need to be replicated on their own and passed on to offspring at higher rates than 50%.

The idea of using these portions of DNA to spread genes throughout a population of disease vectors, such as mosquitoes, was first proposed in the early 1990s2. However, researchers hit a problem: transposable elements would sometimes land in unfortunate places in an organism’s genome. The consequences of interrupting an existing genetic sequence by adding transposable elements can vary from the benign — changes in pigmentation in the medaka fish (Oryzias latipes)3, for example — to the decidedly harmful. In humans, transposable elements that insert themselves into certain genes are linked to the blood cancer acute lymphoblastic leukaemia, as well as the bone-marrow condition Fanconi anaemia4. The problem for researchers working on mosquito control was that the transposable elements would land somewhere in the insects’ genome that was immediately lethal, preventing the modification from being passed on. “We were working with that and we were just getting clobbered,” recalls molecular geneticist Anthony A. James at the University of California, Irvine.

The advent of CRISPR-Cas9 genome editing in 2012 finally gave scientists the tool they needed after years of trying to improve genes that are selfish. These molecular scissors can be used to precisely place genetic sequences into the genome. According to researchers, if the technology was incorporated directly into the genes, it would be possible to keep a single gene from one generation to the next.

A study of population replacement and suppression in Africa by A. gambiae, frogs, salamanders, and Colin Carlson

James, however, thinks that population replacement could be sufficient. He says that if the population was suppressed it could always come back. In an ideal scenario, James sees replacement and suppression being used in tandem.

That could become particularly complicated if researchers in one country introduce mosquitoes with a gene drive into another. Local people would be dependent on the US team for hundreds of years, if a gene drive was released in sub-Saharan Africa. He says that it sounds like colonialization.

She is investigating whether the suppression of A. gambiae will increase other competing insect populations, or if the mosquitoes play a part in pollination. The male and female insects do eat flower nectar, but the data so far shows that the mosquitoes do not really interact with it. She says that they are basically nectar robbers.

Implications for Malaria-control efforts could arise from the results. In particular, identifying the areas that mosquitoes inhabit during aestivation could provide an opportunity to reduce mosquito populations.

The team, led by Colin Carlson, analysed the distribution of 22 species of mosquitoes in sub-Saharan Africa between 1898 and 2016 to assess the shifts in mosquito ranges that are already under way. The mosquitoes are moving south at a rate of over 5 kilometres per year and are travelling at an average rate of over six metres per year. The current projections for land-bound species is 1.7 kilometres south per year, which is much quicker than the previous estimates.

An international team led by Michael Springborn at the University of California, Davis, suggests that a collapse in the populations of amphibians such as frogs and salamanders drove an increase in malaria incidence in Central America. This is the first causality proof of an amphibian-species loss affecting human health in a natural setting.

The findings are consistent with temperature change in the region, supporting the idea that the warming climate is already changing the distribution of these vectors of infectious diseases. However, the authors warn that further work needs to be done to assess the extent to which climate change can explain these observations.

In vitro creation of sporozoites to control Plasmodium falciparum by the use of cytoplasmic isoleucyl transfer RNA synthetase

A class of compound known as thienopyrimidines have been shown to kill Plasmodium falciparum parasites at multiple stages of their life cycle. The compounds clear both blood- and liver-stage parasites by targeting an enzyme known as cytoplasmic isoleucyl transfer RNA synthetase (cIRS). Some parasites are resistant to artemisinin, so these could be an alternative to it.

Other compounds that target the class of enzyme to which cIRS belongs are already available, but their use is limited, partly owing to drug toxicity. The compounds the team analysed showed low toxicity compared to human cell lines. The compounds binding to a different part of the cIRS makes it difficult for it to work. With the multistage effects of these compounds and the nature of the resistance-conferring genes found during testing, there is little risk that substantial resistance will emerge.

The researchers demonstrated that the in vitro sporozoites produced could infect human liver cells in culture and infect mice with humanized livers. The life cycle of parasites can be accomplished without mosquitoes by taking the blood stage from the mice and using it to generate fresh sporozoites.

The Sanaria team processed gametocytes-laden blood in wells containing cells from the fruit fly, to solve the problem. These feeder cells aided the development of oocytes, from which millions of sporozoites emerged.