Could Crispr Flip the Switch on Insects’ Resistance to Pesticides?

2022-02-02

WHILE THE COVID-19 pandemic raged across the world in 2020, another disease was quietly infecting more than 220 million people on the continent of Africa: malaria. That year, the disease led to more than 600,000 deaths, most of them children. Caused by the parasite Plasmodium, the illness is spread through the bites of infected female Anopheles mosquitoes.

Insecticide-treated bed nets and indoor spraying have long been some of the most effective strategies for combating the disease. But decades of using these chemicals has lessened their potency.

It happens like this: Insecticides kill off most of the mosquitoes in an area. But a small number may survive because something about their genetic makeup makes them unaffected by the pesticide. Mosquitoes within that small population mate with each other and pass on their genes to their offspring, breeding more resistant mosquitoes. In some cases, resistance has built up just a few years after the introduction of an insecticide. It makes fighting deadly mosquitoes a constant game of whack-a-mole.

Insecticides remain the frontline in fighting malaria, because interventions like building mosquito-resistant housing are still experimental, and the effort to develop a vaccine has taken decades. Last summer the World Health Organization recommended Mosquirix, the first anti-parasitic vaccine, for African children under age 5, but it is only 30 percent effective at preventing serious disease, and will take many years to achieve approval and distribution among individual nations.

Researchers at UC San Diego and the Tata Institute for Genetics and Society in India have developed a potential way to fight back: Using Crispr gene editing, they replaced an insecticide-resistant gene in fruit flies with the normal form of the gene and propagated the change through insects in the lab. The approach, known as a gene drive, is described in a January 12 paper in Nature Communications, and the team believes it can be translated into mosquitoes.

“This technology I think offers a solution to the conundrum we’re facing now, which is that there hasn’t been a new category of insecticides developed for over 30 years,” says Ethan Bier, professor of cell and developmental biology at UC San Diego and senior author of the paper. “If you can go on using the ones you’ve got by re-sensitizing the mosquitoes to those, I think that would be an enormous benefit.”

A gene drive is a type of technology that overrules the laws of heredity to spread a trait through a population more quickly than it would happen naturally, forcing that gene into a population’s offspring. In this case, the change essentially reboots the gene pool to what it was before the insects evolved resistance to a particular pesticide.

The group’s gene drive uses a molecule called a guide RNA that directs the Crispr system to remove the undesired variant of a gene—in this case, an insecticide-resistant mutation called kdr. When one parent transmits its genetic information to their offspring, a protein called Cas9 binds to the guide RNA, cuts out the mutated gene, and replaces it with the normal variant from the other parent. The normal variant is then copied and all the offspring inherit it.

The team first tried the process on fruit flies because they have a similar maturation time as mosquitoes, plus the researchers had already built gene-editing tools specific to fruit flies for previous experiments. They started with a population of flies in which 83 percent had the resistant variant and 17 percent had the normal version. In 10 generations, their gene drive flipped that ratio so that 17 percent were resistant and 83 percent were not. Fruit flies and mosquitoes each have a life cycle of about two weeks, so it would take several months to re-sensitize an entire insect population to pesticides.

Bier’s team thinks the strategy could achieve a high degree of pest control while using far less insecticide. Other scientists working on gene drives want to use the technology to eliminate the use of pesticides altogether. One tack has been to genetically engineer the mosquitoes to kill the malaria parasite that they host. Another has focused on eradicating mosquitoes themselves: By using a gene drive to render males or females infertile, you could conceivably crash an entire population of mosquitoes.

Lab tests of gene drives have shown that it’s possible to spread a desired genetic trait through several generations. But studies have also found that resistance to gene drives can emerge because some mosquitoes don’t inherit the desired trait. In the wild, resistance is almost certain to occur, meaning that gene drives would probably still leave behind some mosquitoes that could bite humans and transmit disease.

Fredros Okumu, a parasitologist and entomologist who serves as director of science at the Ifakara Health Institute in Tanzania, says the type of gene drive tested by Bier’s team could be used as a followup to one of these other approaches by making the leftover population easier to target with pesticides. Using both types of gene drives could “counter any weaknesses of either method alone,” he says.

But insecticide resistance in the wild is complex. It can arise from dozens of genetic mutations. Okumu says that, for this strategy to work, scientists would have to know the precise genetic mutation that’s causing resistance in a population of insects. Across Africa, many Anopheles mosquitoes are resistant to a class of insecticides called pyrethroids, which includes DDT.

“A system like this would be best only in areas where certain individual gene mutations are directly linked to observable resistance features,” he says. “Still, I am personally very excited to see this.”

As history has shown, mosquitoes are not easy to control in the wild. Take the Aedes aegypti mosquito, which transmits dengue, chikungunya, yellow fever, and Zika viruses. The pest is widespread throughout the western hemisphere, ranging from the mid-Atlantic region of the United States all the way to South America. But it wasn’t always so pervasive. It arrived in the New World some 500 years ago on European slave ships that brought the insect from its native West Africa.

By the 1950s and 1960s, Aedes aegypti was virtually wiped out in Latin America after aggressive spraying of DDT. The campaigns were so successful that mosquito control efforts dwindled. But eventually, Aedes aegypti reappeared.

Bier and other scientists agree that one application of a gene drive is unlikely to work for the long term. Even if you could wipe out mosquitoes in one area, Aedes aegypti’s journey shows us that the pest can travel halfway around the world, pop up in a new place, and establish a new population. A gene drive like the one Bier’s team developed might need to be applied seasonally, especially if multiple resistant genes are present within a population or new ones arise.

“This is no silver bullet,” Bier says. “You never win when you try to play the evolutionary game with insects.” His team is now working on translating the fruit fly gene drive into lab mosquitoes.

George Annas, a professor of health law and ethics at Boston University, says any gene drive—whether it’s the traditional kill-all version or Bier’s resistance-reversing approach—will need broad public support from people living in that area before it can be tested outside a laboratory. And convincing the public to release genetically modified mosquitoes just to keep using insecticides, which come with a host of negative health and environmental effects, could be a hard sell.

“A lot of people think we shouldn’t use insecticides at all,” Annas says. “The idea of using heavy-duty genetic editing so that we can continue using insecticides isn’t going to appeal to everyone.”

Ethicists have long raised other concerns about the potential ecological effects of releasing gene drive technology into the wild, including worries about resistance boomeranging back again. Annas, who authored a code of ethics for gene drive research, wants to see researchers develop a mechanism to recall or stop a gene drive if something unexpected happens once it’s released. “I’m not saying we’re going to develop a super mosquito, but that’s not out of the realm of possibility,” he says. “A gene drive might make things worse and you certainly don’t want to do that.”

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