Gene Drive Research Takes Aim at Malaria

Posted on December 1st, 2015 by Dr. Francis Collins

Malaria has afflicted humans for millennia. Even today, the mosquito-borne, parasitic disease claims more than a half-million lives annually [1]. Now, in a study that has raised both hope and concern, researchers have taken aim at this ancient scourge by using one of modern science’s most powerful new technologies—the CRISPR/Cas9 gene-editing tool—to turn mosquitoes from dangerous malaria vectors into allies against infection [2].

The secret behind this new strategy is the “gene drive,” which involves engineering an organism’s genome in a way that intentionally spreads, or drives, a trait through its population much faster than is possible by normal Mendelian inheritance. The concept of gene drive has been around since the late 1960s [3]; but until the recent arrival of highly precise gene editing tools like CRISPR/Cas9, the approach was largely theoretical. In the new work, researchers inserted into a precise location in the mosquito chromosome, a recombinant DNA segment designed to block transmission of malaria parasites. Importantly, this segment also contained a gene drive designed to ensure the trait was inherited with extreme efficiency. And efficient it was! When the gene-drive engineered mosquitoes were mated with normal mosquitoes in the lab, they passed on the malaria-blocking trait to 99.5 percent of their offspring (as opposed to 50 percent for Mendelian inheritance).

This inheritance pattern persisted through three generations of gene-drive engineered male mosquitoes mated with normal females. With such highly efficient transmission, the researchers estimate that the malaria-blocking trait could be spread throughout an entire local population of mosquitoes in just a single breeding season by releasing one gene-drive engineered mosquito for every 100 mosquitoes in the wild.

This pioneering work stems from a unique collaboration that builds upon the separate strengths of two California labs, both of which receive NIH support. The first is led by Ethan Bier of the University of California, San Diego, who uses fruit flies as a model to study the genetics of development and human disease processes. The second is led by Anthony James of the University of California, Irvine, who studies the biology of insects that serve as the vectors for transmitting malaria, dengue, and other diseases.

For more than 25 years, James has been exploring ways to combat malaria by modifying mosquitoes. A few years ago in lab experiments, his group genetically engineered a malaria vector mosquito, Anopheles stephensi, to make two mouse antibodies that successfully blocked the mosquito’s ability to transmit Plasmodium falciparum, a parasitic protozoa that causes malaria [4]. Still, many obstacles remained to using such mosquitoes to fight malaria in real-world conditions—including the fact that their anti-malaria genes would spread through the wild population relatively slowly because they’d still be passed along through normal, Mendelian inheritance.

Then, in January 2015, James received an unexpected message from the Bier lab, reporting success in using CRISPR/Cas9 technology to develop a highly efficient method to spread traits among fruit fly populations [5]. His reaction: “Holy mackerel!” James knew this could be the gene drive that he and other malaria researchers had long been seeking. So, with great enthusiasm, he and the San Diego group decided to collaborate on developing a similar system for A. stephensi—a system that paired his anti-malaria genes with their CRISPR/cas9 gene drive, which enabled those genes to be edited automatically into a precise location in the genome of all of the mosquito’s future offspring.

Among the many challenges facing the researchers was the relatively large size of the recombinant DNA segment,or cassette, which made it more difficult to insert into the genome than smaller pieces. Containing the two anti-malaria genes, the genes for the CRISPR RNA guides and cas9 cutting enzyme, and a gene for a telltale red fluorescent marker, the cassette was nearly 17,000 DNA bases long. Using a system that scanned the insects’ eyes for the glowing red marker, researchers had to screen more than 25,000 mosquito larvae to identify two males that carried the gene drive. But those two males proved to be enough. When bred with normal, or wild-type, females, these gene-drive males spread their malaria-blocking genes to nearly all of their offspring and did so in each of three generations tested.

While the California researchers are optimistic about future applications of their work, they note that gene drive technology is unlikely to have the power to eradicate malaria all by itself. Rather, they envision the release of gene-drive, anti-malaria mosquitoes to breed with wild populations as a means of reducing the odds of infection, thereby complementing other research aimed at reducing and, ultimately, wiping out the disease through vaccines, drugs, and/or alternate vector control strategies. Also, the gene-drive mosquitoes may guard against the re-introduction of malaria-transmitting mosquitoes in communities or regions after they’ve been rendered malaria-free, allowing resources to be focused on new sites. Still, a potential downstream concern would be development of resistance because the DNA cassette works by forcing the mosquito to express a pair of antibodies against the malaria parasite, and changes in the parasite’s genome might render it no longer susceptible to these antibodies.

Besides their relevance for malaria, these findings serve to underscore how fast the field of gene drive research is moving—and how close science may be to having the ability to alter rapidly the gene pool of an entire population of organisms, leading to health or environmental consequences that may be difficult to reverse. While all of us would celebrate the eradication of malaria, such major changes in an entire ecosystem need careful study. Serious discussions on the science, oversight, governance, and ethics of gene drive research are needed.

To respond to this, NIH, along with the Foundation for the NIH, the Defense Advanced Research Projects Agency, and the Bill and Melinda Gates Foundation, recently asked the National Academy of Sciences, Engineering, and Medicine to convene an ad hoc committee of experts to examine the field and make recommendations for responsible conduct of gene drive research and its practical use in non-human organisms [6]. A series of public meetings, workshops, and webinars is currently underway, with a final report expected next year.

Emphasizing the stringent precautions that they’ve taken to prevent the unintentional release of genetically modified organisms in their own work, the authors of the malaria study support the need for such proactive discussions, noting that “significant advances in regulatory structures and ethical models of community engagement are as important as the further scientific development of these technologies” [2]. Others in this exciting new field would be well advised to follow their thoughtful lead.

References:

[1] Fact sheet on the World Malaria Report. World Health Organization. 2014 December.

[2] Highly efficient Cas9-mediated gene drive for population modification of the malaria vector mosquito Anopheles stephensi. Gantz VM, Jasinskiene N, Tatarenkova O, Fazekas A, Macias VM, Bier E, James AA. Proc Natl Acad Sci U S A.  2015 November 23. [Epub ahead of print]

[3] Possible use of translocations to fix desirable genes in insect pest populations. Curtis CF. Nature. 1968 27 April;218, 368-369.

[4] Transgenic Anopheles stephensi coexpressing single-chain antibodies resist Plasmodium falciparum development. Isaacs AT, Jasinskiene N, Tretiakov M, Thiery I, Zettor A, Bourgouin C, James AA. Proc Natl Acad Sci U S A. 2012 Jul 10;109(28):E1922-1930.

[5] Genome editing. The mutagenic chain reaction: a method for converting heterozygous to homozygous mutations. Gantz VM, Bier E. Science. 2015 Apr 24;348(6233):442-444.

[6] Gene drive research in non-human organisms: recommendations for responsible conduct. The National Academies of Sciences, Engineering, and Medicine, 2015.

Links:

Guidance framework for testing genetically modified mosquitoes, WHO/TDR and FNIH, June 2014.

Ethan Bier (University of California, San Diego)

Anthony James (University of California, Irvine)

 NIH Support: National Institute of Allergy and Infectious Diseases; National Institute of Neurological Disorders and Stroke