Insecticides play a central role in efforts to counter the global impacts of mosquito-borne malaria and other diseases, which cause an estimated 750,000 deaths each year. These insect-specific chemicals, which cost more than $100 million to develop and bring to market, are also essential for controlling insect damage to crops that poses a challenge to food safety.
But in recent decades, many insects have genetically adapted to become less susceptible to the potency of insecticides. In Africa, where long-lasting insecticide-treated bed nets and indoor spraying are major weapons in the fight against malaria, many mosquito species across the continent have developed resistance to insecticides that reduces the effectiveness of these interventions. keys. In some regions, climate change is expected to exacerbate these problems.
University of California biologists have now developed a method that reverses insecticide resistance using CRISPR/Cas9 technology. A team including UC Santa Barbara researchers Craig Montell and Menglin Li, UC San Diego researchers Bhagyashree Kaduskar, Raja Kushwah, and Professor Ethan Bier of UCSD’s Tata Institute for Genetics and Society (TIGS) has used the gene-editing tool to replace an insecticide-resistant gene in fruit flies with the normal insecticide-susceptible form. Their realization, described in Nature Communication, could significantly reduce the amount of insecticides used.
“This strategy could be used to reverse resistance in mosquito disease vectors that spread devastating diseases that affect hundreds of millions of people each year,” said Craig Montell, professor of molecular, cellular and developmental biology at UC. Santa Barbara.
“This technology could also be used to increase the proportion of a naturally occurring genetic variant in mosquitoes that makes them refractory to malaria transmission or parasites,” said Bier, professor of cell and developmental biology at UCSD and main author of the article.
The researchers used a modified type of gene drive, a technology that uses CRISPR/Cas9 to cut genomes at targeted sites, to spread specific genes through a population. When a parent passes genetic elements to its offspring, the Cas9 protein cuts the other parent’s chromosome at the corresponding site and the genetic information is copied there so that all the offspring inherit the genetic trait. The new gene drive includes an add-on that Bier and his colleagues previously designed to bias the inheritance of single genetic variants (also called alleles) by also, at the same time, cutting out an unwanted genetic variant (e.g. insecticide resistant) and replacing it with the preferred variant (e.g. insecticide sensitive).
In the new study, the researchers used this “allelic drive” strategy to restore genetic susceptibility to insecticides, similar to insects in nature before they developed resistance. They focused on an insect protein known as the voltage-gated sodium channel (VGSC), which is the target of a widely used class of insecticides. Resistance to these insecticides, often called knockdown resistance, or “kdr,“results from mutations in the vgs gene that no longer allows the insecticide to bind to its target protein VGSC. The perpetrators replaced a resister kdr mutation with its normal natural counterpart which is sensitive to insecticides.
From a population consisting of 83% kdr (resistant) and 17% normal alleles (susceptible to insecticides), the allelic drive system reversed this proportion to 13% resistant and 87% wild-type in 10 generations. Bier also notes that adaptations conferring resistance to insecticides come at an evolutionary cost, making these insects less adapted in the Darwinian sense. Thus, combining the gene drive with the selective advantage of the most suitable wild-type genetic variant results in a highly efficient and cooperative system, he says.
Similar allelic drive systems could be developed in other insects, including mosquitoes. This proof-of-principle adds a new method to pest and vector control toolkits, as it could be used in combination with other strategies to enhance insecticide-based or pest reduction measures to reduce the spread of malaria.
“With these allelic replacement strategies, it should be possible to achieve the same degree of pest control with significantly less insecticide application,” Bier said. “It should also be possible to engineer self-eliminating versions of allelic drives programmed to act only transiently in a population to increase the relative frequency of a desired allele and then die out. Such allelic drives to local action could be reapplied if necessary to increase the abundance of a naturally occurring preferred trait, with the ultimate criterion being that there are no GMOs left in the environment.”
“An interesting possibility is to use allelic drives to introduce new versions of VGSC that are even more sensitive to insecticides than wild-type VGSCs,” Montell suggested. “This could potentially allow even lower levels of insecticides to be introduced into the environment to control pests and disease vectors.”
University of California – Santa Barbara