CRISPR gene editing drives caged mosquitoes to extinction

Scientists are recruiting CRISPR gene editing to fight malaria, and at least in the lab, one group at Imperial College London has just devised a strategy that wipes out the disease’s carriers—mosquitoes—for good. In the experiment, researchers led by biologist Andrea Crisanti used CRISPR/Cas9 to generate a mutant gene that leaves female mosquitoes unable to reproduce, but allows males to continue spreading the female infertility mutation to offspring.

Crisanti’s team also made use of a genetic trick called a gene drive, which propagates the infertility mutation at unnaturally high rates. In two experiments, each with thousands of mosquitoes, the team showed that the gene drive causes the entire population of mosquitoes to completely crash after a dozen generations or less. “The power of the drive was unexpected and beyond our most optimistic expectations,” Crisanti says.

“Overall this is a remarkable study,” says Gaeten Burgio, a scientist that studies malaria at Australian National University. It’s the most successful execution of a gene drive in mosquitoes to date, and Crisanti’s design appears to circumvent the problem of resistance that has short-circuited previous gene drive studies, although it is still too early to know how well the technique would work in nature, Burgio adds.

The new study succeeded where others have failed due to the decision to target a gene called doublesex, which determines the sex of an insect. Unlike genes targeted in previous studies, mutations in doublesex rarely arise in nature because a properly functioning gene is critical for development. Although the gene is the same in both sexes, it is processed differently in females.

Crisanti’s team used CRISPR/Cas9 to break the female-specific region of doublesex in Anopheles gambiae, the mosquito species that spreads malaria. The male mosquitoes were unaffected by the edit. Female mosquitoes who only had one copy of doublesex disrupted were also fine, but insects with both copies of their gene broken displayed a slew of anomalies and intersex features, including underdeveloped reproductive organs and incorrectly formed male genitalia. The proboscises of these mutant mosquitoes were also malformed, rendering them unable to bite and sap blood. And most importantly, the mutants were infertile and couldn’t lay a single egg (Nat. Biotechnol. 2018, DOI: 10.1038/nbt.4245).

To turn the technique into a gene drive, the team next added the genetic instructions for making the CRISPR/Cas9 gene editor into the mosquito’s DNA, so that it copies itself and the doublesex mutation into nearly all offspring.

The group tested their gene drive in a population of 600 caged mosquitoes, starting with 300 normal female mosquitoes, 150 normal male mosquitoes, and 150 male mosquitoes that carried a single copy of the doublesex-disrupting CRISPR gene drive. In one run of the experiment, the gene drive had spread to every single mosquito by only the seventh generation. By the eighth generation, all mosquitoes carried the gene drive and mutant doublesex gene, meaning no eggs were laid, and the entire population died. In another run of the experiment, the drive was spread to all mosquitoes in just 11 generations.

“In principle, the drive can spread unchecked,” Crisanti says. “These results are a game changer and justify renewed efforts to further improve the technology.” In fact, since all insect species studied so far have a doublesex gene, Crisanti thinks that the gene could be an ”Achilles heel” for many disease-spreading insects and agricultural pests.

Another gene drive researcher, Valentino Gantz of University of California, San Diego, says that two cage studies are not enough to ensure that resistance to the gene drive doesn’t arise in the wild. If doublesex can mutate in a way that avoids being targeted by CRISPR, but still leaves the mosquitoes fertile, the gene drive would fail. “Their technology needs more testing, but it is extremely promising,” he says.

Crisanti’s project was backed by some big names who would like to see the technique put to use, including the Bill & Melinda Gates Foundation and the Defense Advanced Research Projects Agency (DARPA), part of the U.S. Department of Defense. Bill Gates himself is an outspoken supporter of gene editing technologies, and DARPA is one of the largest backers of gene drive research, thanks to its $65 million Safe Genes program supporting the development of gene editing tools and countermeasures.

The gene drive mosquitoes won’t be released into the wild anytime soon. The next step, now underway, is to test the gene drive in a larger population of mosquitoes caged under conditions that mimic the tropical environment favored by the insects in Africa. Nonetheless, the study’s preliminary success underscores how the gene drive field is overcoming scientific challenges more quickly than ethical and political ones.

The new gene drive “could save many lives, but might spread to every population of the target malarial mosquito throughout Africa,” explains Kevin Esvelt, a Massachusetts Institute of Technology scientist that pioneered the use of CRISPR gene drives. That means “the social and diplomatic challenges of securing international agreement are now arguably more formidable than the remaining technical hurdles.”