Insects transmit some of the world’s most debilitating pathogens, including those responsible for malaria, dengue fever, Chagas disease, and sleeping sickness. So, naturally, the best way to fight these life-threatening diseases is at their source. Historically, malaria and dengue control strategies have incorporated insect population control using insecticides, but in recent years, researchers have turned to genetic engineering. By developing mosquitoes that don’t carry such pathogens, researchers hope to stop disease spread in its tracks.
After many years of hopeful development, such genetically modified mosquitoes might finally be close to proving their worth. Field tests of genetically-sterilized mosquitoes, targeted at dengue-carrying species, are demonstrating encouraging suppression of mosquito populations, while a variety of genetically manipulated malaria- or dengue-resistant mosquitoes are nearing their chance at tackling mosquito-borne infections outside the laboratory.
Researchers creating genetically modified (GM) insects generally have one of two goals. “I call them the ‘bite, no-bite’ strategies,” said Anthony James, a molecular geneticist at the University of California, Irvine. “Bite” strategies modify the insects in such a way to prevent disease transmission to humans, whereas “no-bite” strategies aim to reduce or eliminate insect populations altogether, by, for example, rendering them incapable of producing viable offspring.
A similar strategy, known as the sterile insect technique (SIT), has been used to successfully shrink populations of tsetse flies, which carry the parasite that causes sleeping sickness. In SIT, male insects are sterilized through irradiation, then released into the wild, where they breed with wild females, but produce no offspring, thereby cutting the size of the next generation. By regularly releasing enough sterile males, officials can drastically reduce the number of disease-carrying insects.
Using genetic engineering could streamline the SIT strategy, said geneticist Luke Alphey of the University of Oxford. In 2002, Alphey developed a method he called RIDL—release of insects carrying a dominant lethal. He modified Aedes aegytpi, the primary carriers of dengue fever, to express a lethal toxin as larvae—but only when not exposed to the antibiotic tetracycline. A diet of tetracycline-rich food allows GM insects to develop normally in the lab, then released into the wild where there is no tetracycline, and progeny inheriting the toxin gene will be killed before adulthood.
The same year, Alphey formed the company Oxitec to implement his modified mosquitoes in the field. To date, Oxitec has collaborated with governments in the Cayman Islands, Malaysia, and Brazil to begin releasing his mosquitoes in dengue-plagued areas. Just last year, they reported 80 percent mosquito suppression in the Caymans, and the Brazilian trial is ongoing.
Alphey and his colleagues at the University of Oxford and Imperial College London are also developing a similar tetracycline-based “no-bite” strategy that renders females flightless. And another collaboration between researchers at the California Institute of Technology and Imperial College London is developing GM males, called “Semele,” which carry a toxin that kills wild females upon mating. These techniques have yet to be tested in the field.
Once a malaria parasite enters an Anopheles mosquito, it takes 2 weeks to complete its life cycle in its host, travel to the mosquito’s salivary glands, and become infectious to humans. This provides a golden window of opportunity for researchers hoping to develop strategies to make mosquitoes more resistant to the Plasmodium parasite—the so-called “bite” strategies, explained Marcelo Ramalho-Ortigão, an entomologist at Kansas State.
Some researchers are developing mosquitoes to express anti-malaria peptides and enzymes that inhibit parasite development, for example. Others, including James, are targeting even earlier stages of infection, engineering mosquitoes to express mouse-derived antibodies that block Plasmodium from ever invading a mosquito’s tissues.
Researchers at Johns Hopkins University (JHU) are trying a different tack—tweaking mosquitoes’ own immune systems. The few parasites resilient enough to evade a mosquito’s immune system are the ones that transmit disease, but JHU molecular biologist George Dimoupolos suspected that “if we boost [a mosquito’s] natural immune response, maybe we could achieve complete resistance.”
Dimopoulos’s group has already developed transgenic Anopheles mosquitoes that, in the lab, better resisted Plasmodium infection, with little cost to longevity and fecundity, and the researchers are currently working to devise similar strategies to combat dengue in A. aegypti mosquitoes as well.
GM versus wild
The success of both bite and no-bite strategies depends on the ability of the GM mosquitoes to spread through the wild population. When a GM mosquito mates with a wild mosquito, only some of the offspring will carry the new transgenic resistance genes. To ensure that transgenic genes are pushed into wild populations, scientists are developing “gene drive” strategies that guarantee the offspring of a GM-wild pairing will carry the new resistance genes.
The Semele strategy takes a particularly clever tack. While GM males carry a toxin that kills wild females, researchers have also developed GM females that carry an antidote. Thus, male GM mosquitoes only produce offspring with GM females—which are guaranteed to express the GM traits—while killing off wild females to reduce the wildtype population.
Another promising gene drive method, not yet tested in mosquitoes, uses a genetic element called Medea, a maternally-derived microRNA that silences expression of a protein important for embryo development, combined with a gene that rescues offspring. If transgenic female fruit flies mate with non-transgenic males, only those progeny that inherited Medea from their mother survive to adulthood. By pairing Medea with transgenic genes of interest, such as those that confer malaria resistance to mosquitoes, scientists hope to quickly propagate the transgenes in insect populations.
Yet another method under development relies on a homing endonuclease called I-SceI. Homing endonuclease genes (HEGs) are selfish genetic elements that can spread themselves to homologous chromosomes, converting heterozygote carriers to homozygotes that must pass on the HEG. Indeed, when researchers engineered Anopheles gambiae mosquitoes to carry I-Scel, they found that it spread quickly through caged populations. The next step is to embed transgenic genes into I-Scel, which the researchers hope will ensure their fast spread as well.
Though none of these methods have yet been field-tested, “I’m confident a gene drive system will be developed for mosquitoes,” said Dimopoulos. And with so many different disease-controlling GM mosquitoes in late-stage development, he predicts that the techniques will be put into field testing and possible practice “in the next 5 years or so.”