New ‘Gene Drive’ Can Wipe Out Disease-Carrying Mosquitoes

In some parts of the world, mosquitos are more than a nuisance. These insects can transmit deadly diseases like malaria, which kills 400,000 people every year. Public health agencies have worked to reduce mosquito populations, but researchers from Imperial College London have developed a genetic technique that could control mosquito populations with much greater effectiveness.

The study focused on the Anopheles essential mosquito, the primary carrier of malaria across most of Africa. The technique makes use of a so-called “gene drive” to push a desired piece of DNA into a broader population. Normally, the two copies of a gene carried by a mosquito (or any creature the reproduced sexually, for that matter) have a 50 percent chance of being passed on. So, you could add a gene to mosquitos, but it would end up diluted in the population unless it conferred a survivability advantage. That’s really the opposite of what you want with mosquitos.

Gene drives exist in nature, but the researchers managed to build one with the aid of the CRISPR/Cas9 genome editing technique. If you want to increase the heritability of your modified gene (i.e. create a gene drive), you need to replace the wild-type gene in an organism’s DNA before it reproduces. CRISPR allows scientists to make cuts at very specific locations in a DNA strand. In this case, CRISPR snips out the wild-type gene, encouraging the cell to repair the code using the modified gene as a template. As a result, you can push your modified trait to more than 50 percent of the mosquito’s offspring.

The team focused on a gene variant called doublesex that plays an important role in determining whether mosquitos are male or female. A male Anopheles gambiae with this mutation behaves normally and can mate with female mosquitoes to pass on the altered gene. Females with a single copy also behave normally to spread the mutation further.

With the aid of gene drives, the altered doublesex gene was transmitted to nearly 100 percent of these offspring. Female mosquitoes born with two copies of the modified doublesex gene have characteristics of both male and female insects. Like male mosquitoes, they can’t bite and transmit disease. They also cannot lay eggs. When the mutation reaches enough insects, the population crashes. In laboratory testing, it took seven to 11 generations for this to happen—about six months.

There’s a lot of work still to do before anyone will risk turning modified mosquitoes loose. While the insects are undeniably dangerous to humans, they’re also an essential part of the environment. We want to reduce the population, but not completely wipe them out.

Image credit: JJ Harrison

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