July 10, 2016

Gene drives: Application of CRISPR-Cas9 genome editing to combat insect-borne diseases

by David Warmflash

Lions and tigers and bears may be dangerous, but when it comes to killing humans the record goes to a much smaller animal. It’s the mosquito and it’s got public health officials pretty worried this summer as athletes, their families, and friends (plus tens of thousands of fans) gear up for the Olympic Games in Rio de Janeiro, Brazil. Specifically, the problem is the Aedes aegypti mosquito. This biting insect carries the Zika virus. The Zika virus leads to a disease that causes birth defects and other complications, reaching pandemic levels in Central and South America in late 2015. Aedes aegypti is also the insect that transmits dengue fever, yellow fever, and chikungunya.

Aedes aegypti mosquito

Aedes aegypti, taking a bloodmeal. Courtesy of CDC.

One possible way to combat the Aedes aegypti problem is to release high numbers of genetically modified (GM) male mosquitoes. Because male mosquitoes don’t bite humans, millions could be released. The GM males are engineered to mate with females to produce offspring that will die before reaching adulthood. If you can release enough of these special Aedes males, the population will go down resulting in fewer female mosquitoes to infect people with Zika and the other diseases.

This may sound like a neat trick, but there’s also another strategy in the works. It’s potentially much more powerful and it’s being studied for use on populations of the most deadly mosquito of all — Anopheles. That’s the mosquito that carries Plasmodium, the parasite that causes malaria, which killed an estimated 438,000 people around the planet in 2015, many of them children.

Life Cycle of the Malaria Parasite

The life cycle of the malaria parasite, showing the cycle of human and mosquito infection. Courtesy of NIH.

If you think that the GM mosquito sounds powerful, then meet the GE mosquito. GE stands for “genetically edited.” We say “edited” instead of “modified,” because these potential malaria-fighting mosquitoes are being produced with a new genome editing technology called CRISPR-Cas9 (CRISPR for short). CRISPR systems have many possible applications, some of which we discussed in the last post. By applying the editing system to Anopheles mosquitoes, scientists at Harvard University and the University of California and San Diego and Irvine have demonstrated that they can achieve a gene drive.

Instead of inundating a mosquito population with males that are eager to mate but cannot have grandchildren and thus would merely suppress the population for a season or so, a gene drive could force a new trait into the mosquito population. CRISPR is a programmable system that can be used to cut out any genetic sequence as well as insert new ones. To fight malaria, you could use CRISPR to slip into the population a gene that kills the next generation, or better, a gene that makes the mosquitoes resist becoming infected with the malaria parasite. The second option is thought to be more prudent because eliminating members of a particular species could have undesirable consequences, like disrupting the food chain for other organisms.

The key feature of a gene drive is that the gene is more than dominant. As you may recall from introductory genetics, a dominant gene expresses itself even if an offspring receives it from just one parent. In Mendelian genetics, the chances of a parent transmitting such a gene to offspring is 50 percent, but in a gene drive the chances are higher than 50 percent. In the GE Anopheles mosquitoes that the researchers have developed, the chances are nearly 100 percent. How does this work? The male mosquitoes engineered to introduce the malaria resistance have a CRISPR system on one of their chromosomes containing a few components. One component is the sequence for a gene that makes antibodies against the malaria parasite. Another component recognizes a certain sequence on the chromosome that’s homologous to the chromosome containing the CRISPR-Cas9 system (the other member of the chromosome pair). When that sequence is recognized, the Cas9, which is a special protein, makes a cut and the entire system copies itself and splices the copy into place where the cut was made. Thus, the engineered males mate with native females, producing offspring which carry the anti-malaria gene, not just on a chromosome from the father but also on the homologous chromosome received from the mother!

Molecular mechanism of gene drive

The molecular mechanism of gene drive. ©Thomas Julou

Now, you may be thinking that this all sounds wonderful, but what if they make a mistake? What if there are consequences in the ecology that nobody predicted, or what if the gene drive works its way through a country and then the neighboring country protests that it does not want the gene drive crossing the border? A potential solution is to stop the drive via a new CRISPR system carrying an “eraser.” The eraser can be delivered with male mosquitoes and be programmed to recognize the same sequence that was used as a site for cutting the DNA for insertion of the anti-malaria gene. But in the eraser CRISPR you simply don’t include the anti-malaria gene or you can substitute a sequence that produces nothing.

To make sure that they can really do this safely, researchers are still studying the system under laboratory conditions, while the international community debates the potential risks and benefits of the system. But gene drives will probably be used sooner or later, because of their potential to eliminate malaria, Zika, and other insect borne diseases with a power greater than all of the lions, tigers, and bears that you could round up.

David Warmflash

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David is an astrobiologist and science writer. He received his M.D. from Tel Aviv University Sackler School of Medicine, and has done post doctoral work at Brandeis University, the University of Pennsylvania, and the Johnson Space Center, where he was part of the NASA's first cohort of astrobiology training fellows. He has been involved in science outreach for more than a decade and since 2002 has collaborated with The Planetary Society on studying the effects of the space environment on small organisms.