Despite their diminutive size, mosquitoes are the deadliest animal in the world, carrying and spreading diseases that kill millions of humans every year. But scientists are working out how to use mosquitoes’ biology against them, and now trials are underway to use genetically modified mosquitoes to combat yellow fever, dengue and Zika. In July, the first open trial of genetically sterilised mosquitoes launched in Burkina Faso, in an attempt to curb the spread of malaria.
But the promise of eradicating the world’s deadliest animal also raises ethical and ecological concerns – what if tampering with the DNA of living organisms goes wrong?
Oxitec, a UK-based biotechnology company, has been testing whether genetically modified (GM) mosquitoes can suppress a population of non-modified mosquitoes since 2009. The strategy: deploy (non-biting) sterile male Aedes aegypti mosquitoes bearing a lethal gene that is passed onto the offspring and, as a result, will see the population plummet. Oxitec’s mosquitoes were bred using wild mosquitoes from Cuba and Mexico, then released in a Brazilian town.
This week, an independent analysis of an early trial of Oxitec’s technology was published in Nature Scientific Reports suggesting that the ostensibly sterile GM mosquitoes have interbred with local mosquitoes and produced offspring that made it to sexual maturity. The company’s head of regulatory science, Nathan Rose, isn’t surprised by the findings. “We’ve always been very clear that a certain percentage of the mosquitoes survive and that this survival is harmless,” he says. “And that survival could allow some of the natural genes present in the mosquito to be passed onto the wild mosquitoes.”
Oxitec’s genetically-modified OX513A mosquitoes were designed to be “self limiting”, meaning that when they mate with wild females, their offspring inherit a copy of the gene that will prevent them from surviving to adulthood. As an insect with a short life cycle of around a month, the company’s approach requires GM mosquitoes to be released continuously to drive the numbers of the local population down – that’s because the genetic modification will decline gradually with every new generation until it disappears completely.
Mass-rearing and mass-releasing sterile male mosquitoes may be able to suppress a target population but the method requires releasing frequent batches of altered organisms. In a similar effort to battle the Mediterranean fruit fly, a damaging pest to citrus fruits, millions of sterile flies are released in infected parts of the US and Dominican Republic area every week.
“Sterile males are very localised and very reversible, so they only really have an effect where you release them. You need to release a lot of them and pretty frequently,” says Luke Alphey, head of arthropod genetics at the Pirbright Institute in the UK. As soon as the sterile insects die out, their non-sterile replacements move in and start breeding again as normal. As the co-founder of Oxitec, Alphey previously worked on sterile insect techniques but, since moving onto the new role, has shifted his focus on finding more cost-effective methods where fewer GM mosquitoes could be released into an area with a more sustained effect.
Enter gene drives, a controversial genetic technology that breaks the laws of inheritance. A gene drive can copy-and-paste a specific DNA sequence from the chromosome carrying it to the other chromosome, ensuring it is always passed to the offspring. Over multiple generations, the gene rapidly spreads through a population.
Using the gene-editing system CRISPR, researchers at Imperial College London altered a gene in Anopheles gambiae mosquitoes – one of the primary carriers of malaria in sub-Saharan Africa – which made females sterile and, at least in the lab, caused mosquito populations to collapse within six months.
In July, Target Malaria – a non-profit research consortium led by Imperial College London and backed by the Bill & Melinda Gates Foundation – released a test batch of genetically-engineered mosquitoes in Burkina Faso, although these were not yet equipped with gene drives. The innovative technology could have the potential to knock out entire A. gambiae populations so that they cannot transmit the disease – but such experiments may have unforeseen effects that cannot be reversed and have therefore been strictly confined to laboratories.
Scientific and technical advancements on gene drives are moving fast, while individual countries are still trying to figure out how to best regulate them. Such experiments will always have to weigh the environmental risks against the benefits for public health or food security, says Michael Bonsall, a professor of mathematical biology at the University of Oxford, who has worked with the World Health Organisation to produce a framework for testing GM mosquitoes. “If you put something out there that’s driving through the population and is carrying this gene through, and the performance of those insects is not having a detriment on the spread of this trait. But it turns out to have some unintended consequences, then what do you do?” he asks, clarifying that scrutiny is crucial in this field but the regulatory environment shouldn’t be too strict to allow for innovation.
Even if there was a way to reverse or overwrite a gene drive that has already gone wrong, Bonsall believes it would be difficult to convince society to do another experiment. “However, there’s a lot of attention on thinking about how can you limit the spread of a drive,” he says. Essentially, scientists are working to build gene drive systems with built-in controls that would stop them from going global. Funding most of these efforts is the US Defense Advanced Research Projects Agency, which has invested some $65 (£52) million into gene drive research as part of its Safe Genes programme – the agency also awarded a $2.6 (£2) million grant to the research led by Pirbright Institute’s Luke Alphey.
Creating a “daisy chain” is one way MIT researchers have proposed to put brakes on gene drives. This works by splitting the genetically-edited component into three parts – A, B and C – all of which are needed for the edit to take effect. For B to be put in place, C must first appear in the genome. And for A to be put in place – completing the edit – both B and C must be present. A batch of modified mosquitoes would carry all three elements, passing on element A and B to all offspring – but only half will inherit element C, which appears on only one chromosome. While element A and B will spread more rapidly with each generation, element C will gradually be lost, which will then lead to a gradual fading out of all of the elements from subsequent generation’s genomes.
The “daisy-chain” gene drive technology can be seen as a middle ground between the well-established sterile insect techniques and more controversial proposals. “What we focus on is trying to make locally acting gene drives that will allow a genetic trait to persist and to maybe increase in frequency in a target population,” says Alphey. “They won’t inevitably spread through the whole species, but will rather be relatively localised to places where you release them.”
The single-component gene drive versions that are developed by the Target Malaria group still seem to be a step ahead. “Our current plan is to have a dossier (or application) for field testing ready to submit in 2025,” says Austin Burt, professor of evolutionary genetics at Imperial College. In the meantime, the team still has a lot of technical work and tests to do – from modelling impacts to designing field testing protocols to doing risk and impact assessments. One question that commonly arises concerns the ecological impacts of reducing the numbers of malaria mosquitoes – a question that the team is investigating in the field. “What is known in the literature suggests that Anopheles gambiae is not a so-called “keystone” species, and reducing its numbers is unlikely to have large cascading effects,” says Burt.
“There are concerns about this, without a doubt, and nothing is risk free,” says Bonsall. “But if we believe that controlling malaria, dengue or pest is something that society wants to improve public health risks or save crops, then developing tools for this has to be the right thing to do.”
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