When female Anopheles mosquitoes take a blood meal from someone with malaria, a tiny Plasmodium parasite enters the mosquito’s digestive tract. That parasite can invade the mosquito’s salivary tissues, so when the insect takes another blood meal, the intruder can slip into the next human host and start a new malaria infection. Malaria is a life-threatening condition that infected 241 million people in 2020 and disproportionately affects vulnerable populations.

To combat the disease, researchers from the University of California, Irvine are developing genetically modified African malaria mosquitoes (Anopheles gambiae) that can’t transmit human malaria, alongside a gene-drive system that can quickly spread those genes and block the spread of the malaria parasite through the population. While this system usually operates with nearly 100 percent efficiency, a small number of mosquitoes will still wind up with mutant alleles that resist the gene drive. Could these mutant alleles sabotage the whole approach? In a paper published in GENETICS, Carballar-Lejarazú et al. looked at this phenomenon and found these mutations didn’t hamper the gene drive in their system.

Malaria-Resistant Mosquitoes

Contributing author Anthony James began exploring genetic methods for controlling vector-borne disease in the mid-1980s. Eventually he exploited mosquito genes that are only turned on in female mosquitoes after a blood meal and linked them with mouse antibodies that protect mice from human malaria parasites.

When these malaria-busting synthetic genes are inserted into mosquitoes, they can’t transmit malaria. And if it mates with a regular mosquito, the beneficial gene will be inherited like any other gene, gradually building up in the mosquito population. But what if that process could happen faster?

That’s when CRISPR gene editing technology hit the scene. “It seems like overnight when you work 10 or 15 years on something to make it work and then something new comes along and—in less than a year—you have it working,” says James.

Gene editing operates on the germline—the cells that will eventually become sperm or eggs—by snipping the normal chromosome and inserting the new sequence, in this case, the malaria fighting gene. In male mosquitoes, this works so well that each male passes on the new gene to nearly 100% of its offspring.

It’s a bit more complicated in female mosquitoes because egg cells are massive compared with sperm. When the system snips the normal chromosome and inserts the synthetic sequence, the second chromosome may be too far away to trigger the repair mechanism that sews the cut chromosome back up while including the system. That means there’s a chance the snipped chromosome will just stick itself back together—called nonhomologous end joining—possibly resulting in a mutant allele that resists the gene drive.

Exploring Gene Drive Mutations

To figure out if those mutant alleles could pose a problem for the gene drive system, the researchers linked the system to a somatic gene for eye color and marked it with a fluorescent protein. Then, the team performed various crosses to see how the genes passed on to future generations. Non-mutant progeny had black eyes (before adulthood) while those with the mutant allele had pink eyes. And all the progeny carrying the gene drive had eyes that fluoresced blue under light.

To make things a bit more complicated, some progeny were mosaics, with a mix of alleles and more complex eyes, but since the eye color gene isn’t part of the germline—it won’t pass on to the next generation—most of those mosaic mosquitoes still passed on the gene drive.

In the lab, about 25 percent of the first generation of progeny received the gene drive. By the fourth generation, the entire population had fluorescent blue eyes—meaning none of those mosquitoes could transmit malaria.

“Four generations is sufficient,” says James.  “That’s well short of one transmission season.”

James is quick to point out that this isn’t a magic bullet for malaria and there is more research to be done. There are also many thorny issues and debates for scientists and the broader community to work through before everyone is comfortable deploying gene drive mosquitoes in the wild. But James is hopeful the project to which he’s dedicated so many years may one day help ease the malaria crisis.

“There was a famous scientist who said a new idea doesn’t take hold because you change people’s minds; it takes hold because there’s a whole new generation of people that have grown up hearing about it,” says James. “I wish it was a little faster, but we’ll do our part, and hopefully people will take it up. It may not be me, but we have something to hand off.”

CITATION:

Cas9-mediated maternal effect and derived resistance alleles in a gene-drive strain of the African malaria vector mosquito, Anopheles gambiae

Rebeca Carballar-Lejarazú, Taylor Tushar, Thai Binh Pham, Anthony A James

GENETICS

2022: iyac055

https://doi.org/10.1093/genetics/iyac055

Melissa Mayer is a freelance science writer based in Portland, Oregon.

When geneticist Rob Unckless took his son to Lego Club at the local library, he was not expecting to start a new collaboration. The result is the striking piece of science-inspired art that graces the cover of the February issue of GENETICS.

Created by artist Kent Smith, “Attack of the 50 Foot Mosquito” was inspired by a paper by Unckless and his colleagues. The study examines a more subtle threat than a rampaging giant: the potential evolution of resistance to gene drives. Gene drives are a burgeoning new technology that use CRISPR-Cas9 genome editing to alter the genomes of an entire population. Cas9 is an enzyme that, when introduced into a cell, makes cuts in DNA that are then repaired through the endogenous homology-directed repair pathway. This repair process can be co-opted to change the final sequence; if a DNA sequence is introduced that carries the desired edit along with some homology to the cut DNA, it can serve as the template for repair. To turn this editing process into a gene drive, Cas9 and the guide RNA are also inserted permanently into the genome, so theoretically every heterozygote will automatically undergo this process. After the first transformation, the gene drive is self-propagating. This technology has been suggested as a way to control populations of disease vectors, like mosquitoes.

Though this technology is exciting and new, selfish genetic elements are definitely not. There are a multitude of natural examples of elements that manipulate their way into more than half of an individual’s offspring. And notably, many of these natural driving elements are associated with genetic suppressors that prevent their selfish activity and restore equal transmission of both alleles. In their paper, Unckless and colleagues discuss potential mechanisms of resistance to gene drive, including new mutations and accidental by-products of the repair process involved in gene drive. They use mathematical modeling to show the probability that resistance spreads through the population is very much dependent on the frequency at which resistant alleles arise in the first place. This study shows much care and study is needed to more effectively use gene drives in the wild.

This stunning cover was born when Unckless had just moved to Lawrence to begin an assistant professor position at the University of Kansas. He took his son to the Lego Club at the library; Kent Smith, a local artist who teaches in the School of Architecture and Design at KU and  specializes in science fiction-inspired artwork, was also there with his son. “I struck up a conversation with Kent,” says Unckless, “We talked about what I do and what he does, and decided it would be fun to collaborate on something.” When the manuscript was accepted by GENETICS, they decided to do a piece to submit as a potential cover.

“I loved being able to provide a fun visual for all of this amazing big brain science!” says Smith. “One of my favorite things about design and illustration is getting to research and learn about new subjects for each piece.” Unckless and Smith discussed what the paper showed about gene drives, and then Smith came up with a way to illustrate the evolution of resistance metaphorically. “Nature’s response to the gene drives comes in the form of a giant monster mosquito rampaging through a city,” he explains. The piece is a reference to the iconic poster for the 1958 science fiction movie Attack of the 50 Foot Woman. Unckless saw rough sketches and gave suggestions about things like mosquito anatomy and the mosquito-borne Zika, dengue, chikungunya, and yellow fever viruses swirling in the clouds of smoke and destruction. A tiny Andy Clark can also be seen fleeing destruction on his bike.

“I was lucky to have such a rich subject matter,” says Smith. “Collaboration and dialogue with Rob was very inspirational and allowed for some great brainstorming.”

The final product of this meeting between art and science is a tongue-in-cheek commentary on the potential perils of gene drive gone awry. Though rampaging giants are unlikely to result, scientists should carefully consider the consequences of this potentially powerful technology.

Unckless, R. L., Clark, A. G., & Messer, P. W. (2017). Evolution of resistance against CRISPR/Cas9 gene drive. GENETICS. 205(2):827-841. DOI: 10.1534/genetics.116.197285

http://www.genetics.org/content/205/2/827

More of Kent Smith’s artwork can be found here:

www.smittytown.com

https://www.facebook.com/KentSmithIllustration/

https://www.instagram.com/smittytownart/

https://society6.com/smittytown

Efforts to engineer genomes in wild populations have huge potential for good—but the real world is more complicated than the lab.

Until now, humans have never been able to seriously consider how to cheat evolution. But now that the CRISPR/Cas9 system has made genome editing easy and efficient, it might be possible to manipulate the genomes of entire wild populations—and even to force the spread of genes lethal to their carriers.

Although the risks and ethical concerns of this proposal have sparked debate, being able to control wild populations could have massive benefits, including eradicating devastating human diseases. But while researchers are able to create all types of drives in the lab, the limits on gene drive’s power in the wild haven’t received as much scientific and media attention. Fundamental questions remain unanswered. How will gene drives spread? How—and when—could resistance arise? Spurred by its potential to do good and to do harm, geneticists are racing to understand how gene drive might work—and how it might fail—in the wild.

Population Engineering

In principle, using CRISPR/Cas9 to drive a gene into a population is beautifully simple. Select a target site in the genome to be modified. Deliver to the host’s cells a template consisting of the DNA sequence to be inserted flanked by sequences homologous to the target site, along with a gene encoding the DNA-slicing enzyme Cas9 and a sequence coding for an RNA that guides Cas9 to the right DNA to cut. The Cas9 produced from this construct will chop the host’s genome at exactly the target site specified by the template.

When the cell tries to repair the cut, it often relies on DNA with a matching sequence from the homologous chromosome. But during this process, called homology-directed repair, the cell sometimes mistakes the gene drive template for the homologous chromosome. When the cut is repaired this way, the cell is tricked into including the entire gene drive sequence, cementing a permanent copy into the genome. Now the patched-up chromosome becomes a template itself: Cas9 can snip the normal copy of the chromosome and insert the drive there, too. Heterozygotes for the drive can then pass on the foreign sequence to their offspring as if they were homozygotes. Hypothetically, this would make it possible to engineer an entire wild population using a precise genome edit.

In this depiction of gene drive, a circular DNA molecule called a plasmid is inserted into cells. The plasmid contains a gene for the DNA-cutting enzyme Cas9, a sequence encoding a guide RNA (gRNA) that defines the sequence Cas9 will cut and a gene to be inserted at the cut site (payload gene). These sequences are flanked by homology arms (H1 and H2) that match up with DNA surrounding the cut sequence in the normal chromosome, allowing the whole DNA cassette on the plasmid (H1, Cas9, gRNA, payload gene, H2) to serve as a template for repairing the cut chromosome. Then, the repaired chromosome acts as a template itself when Cas9 cuts the homologous chromosome, so both copies of the chromosome in the cell contain the DNA cassette. Thus, the progeny will inherit the inserted sequence (almost) 100% of the time. Image by Thomas Julou (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons.

Moving Targets

Even in controlled lab environments, gene drive has inherent limitations. The trait to be modified must be genetically determined—preferably by a single gene. Gene drive isn’t practical in organisms with long generation times, such as humans. And for the drive to spread, drive carriers must be able to infiltrate the natural population, ensuring that the drive is passed on to the normal organisms’ progeny. This means gene drive can’t affect organisms that don’t reproduce sexually.

These constraints aren’t a problem for all applications. There’s considerable excitement about eradicating mosquito-borne illnesses like malaria—a disease that kills hundreds of thousands of people every year and sickens hundreds of millions more—by using lethal gene drives to control mosquito species that transmit the disease, or even push them to extinction.

Although the goal is ambitious, tantalizing evidence suggests it might be possible. In 2015, researchers at the University of California in San Diego reported a proof-of-concept experiment on a caged population of flies, which showed that heterozygotes for a drive could pass it on to 95-100% of their progeny, as opposed to the Mendelian 50%.

But while it’s promising that an allele can be driven to high frequency using this method, there are reasons to suspect that the situation in the wild may be very different. “For this drive to work, it needs to recognize the site in the genome where it’s going to insert itself,” says Philipp Messer, a researcher at Cornell University studying the evolution of resistance to gene drives. “In those lab populations, they were all the same strain, so they always had exactly the same sequence at that target site. And that’s why it worked in all of those flies.” In wild genomes, there’s much more natural variation, so susceptibility and resistance will be less consistent.

According to Messer’s recent work, this standing variation might not even be the biggest source of resistance alleles. Resistance can also arise through other mechanisms, such as mutations caused by the drive mechanism itself. When DNA is cut, the cell doesn’t always fix the break using homology-directed repair. Instead, it can simply stitch the ends of the DNA together—a technique called non-homologous end joining. This often results in mutations, which could leave the target site unrecognizable to the driver system.

Messer’s group is conducting experiments to better understand how these resistance alleles arise. They release flies with a driver construct into a caged population of normal lab flies, and then closely monitor the flies to see when the drive comes close to fixation—that is, when nearly every individual to carry the gene drive at both loci. Once this happens, they check the population for the resistance alleles they predict will evolve.

According to Messer, one strategy to avoid such resistance is to use multiple guide sequences or to target an essential gene, but whether those techniques will work in practice hasn’t been tested. Otherwise, he says, the only way resistance alleles like this won’t emerge is if the population is so small that there’s essentially no variation at the target site and the rate of nonhomologous end joining can somehow be reduced to near zero. “People now would say, ‘Well, we’re really happy if we can have a 99% conversion rate,’” Messer says. A 99% conversion rate means nonhomologous end joining occurs in a seemingly trivial 1% of cases. But, Messer says, “If you have a population of a billion insects, that still means in every generation non-homologous end joining happens in ten million insects. You have a tremendous supply of these resistance alleles.”

Inbreeding Escape

It’s possible that these resistance mechanisms won’t take effect rapidly enough to prevent the drive from spreading through a population. By the time resistance crops up, we may already have achieved our goal. But even if resistance is slow to develop, gene drives face other hurdles in natural populations. Another evasion method is inbreeding, a topic studied by Professor James Bull at the University of Texas at Austin. In normal circumstances, inbreeding can harm a species by reducing genetic diversity, making it more likely to succumb to threats. But when harmful gene drives spread through a population, inbreeders may have an advantage. This edge arises because inbreeding increases the frequency of homozygotes in the population.

“If the population doesn’t have any heterozygotes at all, then individuals either don’t carry the gene drive or they’re homozygous for it—in which case, they’re dead,” Bull says about lethal gene drive. According to Bull, whether a species will evade gene drive via inbreeding depends on the magnitude of the genetic costs of inbreeding. “The offspring that are inbred aren’t quite as fit as the wild type,” Bull says, and in some species, the fitness of inbred offspring may be well below that of the wild type. But if this reduction in fitness is less detrimental than the drive, it’s very likely that inbreeding will sidestep the drive.

To circumvent these resistance mechanisms, some have proposed introducing multiple drives at once, but even this may not be enough to defeat nature. Cellular molecules that interfere with the Cas9 endonuclease protein itself or its expression could be enough to suppress multiple drives at once. “We’re all guessing right now. To me, that makes it somewhat exciting,” Bull says.

According to Michael Wade, a Distinguished Professor of Biology at Indiana University Bloomington, drive mechanisms might involve only a certain type of gene, but the genes that oppose them could be anywhere in the genome. But despite many means for populations to evolve resistance to gene drives, several successful drives exist in nature.

The Drosophila genome, for instance, harbors several copies of DNA sequences called P-elements, selfish genes that splice duplicates of themselves into random sites in the genome. Fifty years ago, P-elements didn’t even exist; now, you can’t find a fruit fly without them. But one key difference is that P-elements can insert themselves anywhere in the genome; they don’t require a precise target site like the CRISPR/Cas9 system does. And aside from sometimes inserting in an important gene, P-elements aren’t harmful to the flies, unlike the many proposed gene drives intended to disable or even exterminate their host species.

All-Natural Drives

Genes drives much less benign than P-elements have also spread in the wild. In natural fruit fly populations, a complex of driven genes called Segregation Distorter interferes with sperm production in males that possess one copy of the drive, causing dysfunction of spermatids that lack the drive. As a result, Segregation Distorter has spread through the natural population even though it diminishes carriers’ reproductive fitness.

Another strikingly selfish gene, called the t-allele, has also been found in some mouse populations. Males homozygous for the t-allele are sterile, but heterozygotes disproportionately produce sperm bearing the t-allele. The allele can thus be driven to high frequency despite causing serious fitness impairments in male carriers. Promisingly for gene drive hopefuls, this implies that it might also be possible to use CRISPR/Cas9 to force a deleterious selfish gene of our own design to high frequency.

By default, the t-allele could never become fixed in a population: all males would be sterile, and the population would die out. But nature also has examples of pushback against drive that are less easy to explain. According to Wade, the selfish gene Medea (found in flour beetles) defies our expectations: “In the simple mathematics of the Medea model, Medea spreads to fixation, yet we find populations in nature where Medea exists at an intermediate frequency.”

No one knows what genetic variants are responsible for hampering Medea’s spread, but something in nature must be stopping it from becoming fixed—at least at the predicted rate. The interplay between drives and resistance appears more complex than we understand.

Inhospitable Hosts

Medea isn’t an exceptional case. According to Wade, most drives in nature have suppressors. If a drive weakens the organism that carries it, there will be selection for resistance against the drive. That’s why some have argued that rather than using drives to reduce or eliminate the population that carries it, gene drive might better be used to tamper with a vector so that it can no longer support a pathogen—for instance, by altering a gene in mosquitoes that the malaria parasite requires to grow or spread.

A gene drive that has no negative effect on its mosquito host might be able to spread through a population without meeting resistance mechanisms in the insects. But Bull questions how much we actually know about what gene to alter. And even if we know which mosquito gene to target, that gene probably exists in the host for a reason. “Knocking out a gene may often have consequences,” Bull says. “It may not kill the individual if it lacks it, but if we’re knocking fitness down 10 or 20%, it’s still going to be a pretty strong selective advantage for the insect to evolve resistance.”

Instead of altering a vector gene needed by the parasite, some have suggested inserting whole new parasite-fighting genes into vectors’ genomes—and proof-of-concept experiments in lab populations have shown promising results. But in larger wild populations, the parasite’s response is unpredictable, and a mutation or two might allow it to grow again.

Even if the parasite can’t evolve resistance, selection may still favor flies without the drive, preventing it from establishing itself in the population. Driver alleles cut chromosomes, and off-target effects could disrupt important parts of the genome. Although this hasn’t seemed to stop P-elements in flies from spreading, it may still be a hindrance for other drives, and different organisms may not have as much tolerance for slicing up chromosomes.

Cheating Evolution

Beyond these molecular obstacles, gene drive may face difficulties at an even larger scale. For many applications, the drive must be able to spread from one local population to another—and predicting how likely that will be and how rapidly it might occur requires understanding how far individuals migrate, how fast they move, and other factors. And even if individuals of the targeted population readily intermingle with those of neighboring populations, the drive may face new challenges due to genetic variance in the neighbors.

The barriers don’t end here: there are almost certainly other problems we haven’t even foreseen, and many are likely species-specific. But maybe that’s not such a bad thing. “Let’s say you don’t want to eradicate the population, but you just want to severely weaken it,” says Messer. “You could drive a really deleterious allele to a population, it would spread there, the population would decline, and eventually only the few individuals with resistance alleles would survive.” The suppressed population could then be subjected to other challenges, such as pesticides. In the case of malaria-carrying mosquitos, their numbers might be reduced enough to halt the spread of malaria before the population rebounds.

Others are also hopeful that despite gene drive’s flaws, there’s reason to be optimistic. “I think having a new tool in the arsenal, gene drive, is great,” Wade comments. “I just don’t think that’s going to become the only tool.”

Although many call for caution and further study, the potential for massive benefits has others clamoring to release CRISPR/Cas9 gene drives into the wild. Whether or not we’re ready, our predictions about gene drive’s effectiveness may soon undergo field tests. Although resistance would be a disappointment to many, the fact that it seems so likely to develop could actually serve as a natural safeguard against the out-of-control drives that has prompted concern about the technology. “I think if we know one thing for certain, it will be difficult in practice to actually cheat evolution,” Messer says. After all, evolution has spent a lot longer practicing this game than we have.

More on gene drives at Genes to Genomes:
Gene drive: More research, not more regulations
Modeling the promise and peril of gene drive

In October of this year, the National Academies of Science, Engineering, and Medicine held a public workshop to gather information regarding the safety and ethics of gene drive research.  GSA Public Policy Chair Allan Spradling sent the following comments to the committee for consideration. 

In the late 1980s I was one of the first scientists to propose the use of “gene drives” to alter the ability of insect vectors to transmit human pathogens based on Margaret Kidwell’s demonstration that the Drosophila P element had spread throughout wild populations world-wide in just a few decades.  At the World Health Organization in Geneva, I suggested that the ability of a problem population of malaria mosquito vectors to transmit human malaria might be altered by introducing transposons with the ability to spread throughout target populations of such vectors.

Allan Spradling

The introduced transposon(s) would be identified as novel transposons within populations of the same species that were naturally resistant to malaria transmission (if they existed, but this is not uncommon) in the hopes that the genomic alterations and physiological changes caused by introducing these elements might in themselves be sufficient to suppress disease transmission.  If such an approach was not feasible, one could add a gene to a highly active transposon with the ability to spread that laboratory research had demonstrated would interfere with malaria growth/transmission but exert minimal deleterious effects on the mosquito itself. In this case the introduction of the gene into the population would rely on the gene drive properties of the transposon itself and the possible selective advantage of a reduced malaria burden on the vector.  The idea of actually killing the vector species using a gene drive seemed unlikely to succeed, and was not recommended.  I believe that the gene drive-based approach to reducing the incidence of insect-borne diseases outlined above continues to have potential.  Today, I worry that recent concern over the safety of even studying gene drive systems in the laboratory threatens to derail this promising avenue of research.  In my view, such worries are not based on evidence of an actual danger sufficient to preclude the active pursuit of a research program under current recombinant DNA guidelines and as part of coordinated international development efforts that could bring immense benefit to humans around the world.

Akbari et al. call for stringent regulation of research using Drosophila melanogaster on “gene drives,” genetic constructs that at least in a laboratory setting can increase their inheritance above simple Mendelian expectation.  The new proposed regulations would include prior committee approval, restrictive laboratory design not readily available in most institutions, and time-consuming biological containment.  The motivation for new, stringent regulations is to absolutely preclude the inadvertent genetic modification of wild Drosophila populations.  Unfortunately, if adopted, these policies would make it considerably more difficult to carry out research on gene drives, despite their potential for beneficial applications and evolutionary interest.  We should ask ourselves whether imposing a new regime of regulations in the absence of any proven threat is a better course for society than continuing to study gene drives under the same controls as currently apply to other transgenic flies, while developing a rational framework for assessing hazards.

There are many reasons to suspect that laboratory-designed gene drives are a lot less dangerous than depicted in hypothetical scenarios.  Gene drives are natural and already widespread in wild populations.  While the particular variants of concern to Akbari et al. were built with CRISPR technology and are completely novel, even more potent gene drives, transposable elements, were built by evolution and are a ubiquitous feature of most or all genomes.  Drosophila research using active transposons such as PiggyBac that could theoretically spread in the wild has been carried out in laboratories around the world for decades under the current regulatory rules without problem.  There seems to be an assumption among proponents of new regulation that gene drives are guaranteed to move through wild populations.  Transposons are kept in check by a sophisticated immune system based on piRNAs not unlike CRISPR itself. Such piRNAs recognizing cas9 could be produced from a fragmented construct and spread in the population to inactivate gene drive activity.  Other means of inactivating the construct are also plausible.  Previous studies of biological elements such as transposons or viruses that can spread in the wild suggest that infectivity requires a high degree of evolutionary adaptation whose molecular basis is not well understood.  It seems questionable that a novel human-designed gene drive construct produced in the absence of such knowledge constitutes an imminent threat to a natural population.  Rather, there will likely be a significant learning curve before efficacious gene drives can be designed and employed in the manner that is hoped for beneficial purposes, if this is indeed even possible.  Premature regulation would lengthen that period and delay the possible benefits of this technology.

Consequently, I favor an alternative approach to the issue of gene drive research and if warranted, to its eventual application.  The properties of gene drives and their behavior in Drosophila research laboratories should continue to be studied under the current regimes of recombinant DNA regulation, which have been sufficient for the last 30 years to prevent the escape and spread in the wild of laboratory organisms and their natural gene drives.  New constructs that show promise in a laboratory population, should be tested in large outside cage environments containing wild Drosophila.  In addition to allowing an important line of research to continue, another major advantage of this approach is that Drosophila gene drive experiments will generate real data about which types of constructs represent a threat of dissemination in wild populations, under what circumstances and with what consequences for the affected animals, so that any new regime of regulation can be developed rationally and adopted as needed.  Truly successful gene drives, if they can be produced, should graduate to tests using target species, such as malaria vectors, in cages at supervised sites as part of international development efforts and under appropriate safety regimes as discussed in detail elsewhere.

The views expressed in guest posts are those of the author and are not necessarily endorsed by the Genetics Society of America.

Related Reading on Genes to Genomes:

Modeling the promise and peril of gene drives

WorkingWorking through the issues: Science, ethics and governance of gene drive research

What if we could eradicate malaria by engineering a mosquito population that doesn’t transmit the disease? What if we could control invasive species that outcompete natural populations? What if we could get rid of insecticide-resistant pests not by developing new chemical treatments, but instead by changing the population itself and driving it toward extinction?

Although scientists have long-imagined the potential of biological interventions to solve challenges like these, the ability of the CRISPR/Cas9 system to precisely edit genetic material, coupled with gene drive systems, offers a hope of success that’s within reach. But this potential gain also carries potential risks.

Much of the increasing attention to gene drive, including recent news coverage, has raised questions about whether these constructs can have their desired effect without simultaneously causing ecological harm.

A new report from Unckless et al. in the October issue of GENETICS builds on recent experimental work being carried out in the field by using mathematical models to estimate how quickly such gene replacement can spread through a population.

Modeling several scenarios using a Wright-Fisher model as their foundation, Unckless et al. demonstrate that adding modified genes through the mutagenic chain reaction (MCR) can have dramatic effects, with these genes fixing in populations after only a few generations, much more quickly that they would as a result of natural selection. Moreover, gene drive allows the potential of a particular allele to spread through a population, even if there is selection operating against it. The efficiency of these methods allow effects of this biological control strategy to appear very quickly, which may prove extraordinarily effective. That very speed and efficiency, which on one hand would be beneficial, also brings some level of risk for unintended consequences that are difficult if not impossible to control.

“We need to consider the population dynamics of gene drive in designing these types of strategies,” emphasizes study author Rob Unckless. “The outcome will vary considerably based on the strength of drive, fitness consequences, and dominance, and other factors. This means that we can’t expect to insert any old mutation into any old site in the genome and expect that within tens of generations, the population will be fixed for that mutation.” But this result may be possible in some instances.

“We need to try these techniques in very limited scales – first cages then enclosures – to assess how it spreads,” said Unckless.

Unckless also suggests that modeling should be done with multiple constructs, including beneficial mutations, deleterious mutations, different conversion efficiencies, and rates of non-homologous end joining, among others. In their paper, Unckless and colleagues explain how the math meets the applied: this modeling of MCR population dynamics can both put bounds on the frequency trajectories expected from the release of an MCR, but it may also identify possible choke points for controlling and preventing the expansion of an escaped or mutated MCR allele in a natural population.

Although Unckless suggests there’s more work to be done to assess the potential and peril of gene drives, early indications suggest the possibility of dramatic effect.

CITATION:
Unckless, R.L., Messer, P.W., Connallon, T., & A.G. Clark. 2015. Modeling the Manipulation of Natural Populations by Mutagenic Chain Reaction. GENETICS, 201:425-431 doi:http://doi.org/10.1534/genetics.115.177592