Mosquitoes Genetically Modified as Malaria-Blocking Insect Model

Scientists used CRISPR gene editing to create a parasite-resistant strain of Anopheles mosquito capable of rapidly introgressing malaria-blocking genes into a mosquito population through its progeny, ultimately eliminating the insects’ ability to transmit the Plasmodium falciparum parasite to humans. This new model represents a notable advance in the effort to establish an anti-malarial mosquito population, which with further development could help eradicate a disease that sickens millions worldwide each year.

To create this breed, researchers at the University of California at Irvine and San Diego (CA, USA) inserted a DNA element into the germ-line of Anopheles stephensi (a leading malaria vector in Asia) that resulted in the gene, for preventing parasite transmission, being passed on to an astonishing 99.5% of offspring, resulting in near-Mendelian inheritance ratios of the transgene.

“This opens up the real promise that this technique can be adapted for eliminating malaria,” said Anthony James, professor at UCI. For nearly 20 years, the James lab has focused on engineering anti-disease mosquitoes. His anti-dengue fever models have been tested in cage trials in Mexico and in 2012 he helped show that antibodies adapted from immune system of mice can be introduced into mosquitoes to impair pathogen biology. This trait, though, was inherited by only about half of the progeny.

UCSD biologists Ethan Bier and Valentino Gantz working with Drosophila recently announced the development of a new CRISPR-based mutagenic chain-reaction method for generating mutations in both copies of a gene, allowing for germ-line transmission of mutations with 95% inheritance rate. The two groups collaborated to apply Bier and Gantz’s method to James’ mosquitoes. Prof. Gantz packaged antimalaria genes using the DNA-cutting Cas9 enzyme and a guide-RNA to create a genetic cassette that, when injected into a mosquito embryo, targeted a highly specific spot in the germ line DNA to insert the antimalaria antibody genes. To ensure that the element carrying the malaria-blocking antibodies had reached the desired DNA site, the researchers included in the cassette a protein that gave the progeny red fluorescence in the eyes. Amazingly, almost all offspring—99.5%—exhibited this trait.

Further testing will be needed to confirm the efficacy of the antibodies and could eventually lead to field studies. “This is a significant first step,” said James, “We know the gene works. The mosquitoes we created are not the final brand, but we know this technology allows us to efficiently create large populations.” Prof. Bier added, “The ability of this system to carry large genetic payloads should have broad applications to the future use of related Crispr-based ‘active genetic’ systems.”

The study, by Gantz VM et al., was published online ahead of print November 23, 2015, in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).

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