cientists have used a revolutionary genetic tool to create mosquitoes unable to spread malaria, raising the possibility that lab-engineered insects could be released into the wild to stop a scourge that kills more than half a million people a year.
A team from the University of California reported Monday that they inserted genes into mosquitoes designed to block the parasite that carries malaria, and that within a few generations virtually all the insects’ descendants had inherited the antimalaria DNA.
If the technique works in nature as it did in the lab, releasing just a few thousand of the genetically modified mosquitoes to mate with regular mosquitoes could, within months, produce an entire population unable to transmit the disease to people.
The technology used to engineer the insects has stirred controversy, however, because of fears it could alter ecosystems in unpredictable ways, and the National Academy of Sciences is studying it in order to come up with proposed regulations.
The experiment, described in Proceedings of the National Academy of Sciences, represents another achievement for CRISPR-Cas9, a powerful genome-editing tool that allows scientists to alter an organism’s DNA more quickly than anything before, and for “gene drive,” a technique that rapidly spreads a trait through more of a population than the conventional rules of inheritance allow.
“This is a major advance,” said biologist Kevin Esvelt of Harvard’s Wyss Institute for Biologically Inspired Engineering, who has done pioneering work in gene drive and led efforts to conduct such research safely. “It shows that gene drive interventions will likely be effective” against mosquito-borne diseases.
If so, it’s an advance that may set a speed record in biology.
Last December, biologists Ethan Bier and Valentino Gantz of the University of California, San Diego, got a gene drive to work in fruit flies, the first time anyone had accomplished that in insects. (Esvelt and his colleagues did it in yeast a few months before.) Rather than half of fruit flies inheriting yellow coloring when only one parent had it, as standard genetics predicts, 97 percent of offspring did.
The UCSD duo called it a “mutagenic chain reaction” and applied for a patent on it. Early this year they began collaborating with Anthony James of the University of California, Irvine, who for nearly 20 years has sought genetic techniques to eradicate malaria.
James’ change-the-mosquitoes strategy stems from the failure of an alternative approach to eradicate malaria — killing mosquitoes, as with DDT — and takes a page from spycraft: Rather than destroying the enemy, turn mosquitoes into biological double agents that block the malaria parasite instead of transmitting it.
“The real enemy is the parasite, not the mosquito,” said James, who led the new research. With this breakthrough “we can recruit the mosquito to help us out.”
Throwing a bag over its head
For the new study, Gantz used CRISPR-Cas9 to insert a package of new genes into 680 embryos of Anopheles stephensi mosquitoes, the main carrier of malaria in Asia.
Two of the genes make antibodies that attack the malaria parasite, Plasmodium. When James slipped such genes into mosquitoes in a 2012 study, in the days before CRISPR and gene drive, the resulting antibodies were so effective that the mosquitoes had no Plasmodium in their salivary glands, and so couldn’t transmit the disease if they bit people.
“The antibodies interact with molecules on the surface of the parasite in a way that’s like throwing a bag over its head,” James said. “The parasite can’t ‘see’ the insect tissue, so there’s no parasite in the salivary glands.”
Ordinarily, as genetically engineered mosquitoes mate with regular ones, traits like antimalaria antibodies are inherited by only half the offspring. The new genes eventually get washed out.
The California team, therefore, also inserted the genome-editing CRISPR system into the mosquitoes’ genomes. This was the gene drive. They also dropped in a gene that makes a glowing red pigment, allowing the scientists to tell at a glance whether the gene drive was working: red meant success.
“We’re a hop, skip, and jump away from actual gene drive candidates for eventual release.”
Gene drive works like an embedded Sorcerer’s Apprentice, making copy after copy of the inserted genes. When an engineered mosquito mates with a regular one, their offspring inherit the whole gene-editing package (including antimalaria genes and red-coloring genes) from the engineered parent. They inherit regular DNA from the other parent. But the gene drive cuts the regular DNA; in response, the genome repairs itself with the antimalaria and other inserted genes. Result: offspring with two copies of these genes. Like Mickey Mouse’s brooms, one becomes two over and over again.
When such insects mate with regular ones, the same process of making two copies of the edited genes is repeated, and eventually all the progeny carry double doses of antimalaria genes.
The scientists had their doubts that it would work. “We did a tremendous amount of guessing” about the right genes to insert, James said. “No one had ever shown that some of these genes would function as we hoped.” There were also concerns that the Cas9 portion of the CRISPR gene-editing system was toxic to mosquitoes.
It wasn’t. Hundreds of the engineered mosquitoes survived to adulthood. The UC scientists mated a few with regular mosquitoes, and by the third generation, 99 percent of offspring were glowing red: nearly all had inherited the antimalaria genes.
The next steps are to confirm that the antimalaria antibodies produced by the inserted genes really do neutralize the malaria parasite. James had shown that in earlier experiments using standard genetic engineering, but in the gene-drive study the scientists didn’t test for it.
Researchers not involved in the study nevertheless expressed high hopes for it. “It is quite possible that this technology would become an important tool in the control of malaria,” said geneticist Peter Atkinson of the University of California, Riverside, whose research involves genetic approaches to controlling insect pests. “It would constitute a very, very significant advance in the field.”
In July, the UC scientists and two dozen other researchers released a public letter committing themselves to conducting gene-drive experiments only in secure facilities, to ensure engineered organisms didn’t escape. “This experiment is intrinsically much less likely than fruit flies to accidentally spread, since [the Asian malaria mosquito] does not breed in California,” said Harvard biologist George Church, who signed the letter and, with Esvelt, has applied for a patent on gene drive. But Esvelt said he would have liked the California team to develop a “reversal drive,” which could undo the genetic changes their gene drive produced, “just in case something did go wrong.”
More needs to be done before scientists release malaria mosquitoes with gene drive even into large enclosures, let alone into the environment. They need to show that antimalaria gene drive works in the diverse populations of mosquitoes in nature, not just laboratory breeds, and that the malaria-blocking trait really works to keep Plasmodium out of the insects’ salivary glands.
The Californians will have company making that happen. Church’s lab, collaborating with several Harvard colleagues, has made “progress on a similar CRISPR approach in the dominant African malaria vector, Anopheles gambiae,” he told STAT.
The biggest hurdle to field trials might have nothing to do with science, but with convincing the public, especially in countries where malaria is endemic, that gene drive is safe.
Experts trying to develop guidelines for use of gene drive technology had better hurry. The field is barreling ahead, and the mosquito experiment, said Esvelt, “suggests that we’re a hop, skip, and jump away from actual gene drive candidates for eventual release.”