BY BENJAMIN LEVY
Which animal is statistically most likely to kill you? While your mind may have conjured images of snarling lions, grizzly bears, or venomous snakes, any public health researcher would tell you unequivocally that the deadliest animal in the world is the mosquito. The females of dozens of species of mosquito serve as vectors (transmitters) for several infectious diseases – among them malaria, dengue, and yellow fever. Malaria, for example, is caused by parasites in the Plasmodium family that are transmitted by Anopheles mosquitoes, with P. falciparum being the most deadly. This disease kills 2.7 million people each year. While bed nets, drugs, and insecticides – the norm for malaria prevention and treatment – have been successful thus far, this could soon change. Recent years have seen a severe rise in insecticide and drug resistance, which is potentially catastrophic for even the most widespread vector and parasite control strategies.
Using biotechnology to control mosquitoes has been previously covered by Juxtaposition (see Sudipta Saha’s article on page 10). In this article, I look more closely at one of the strategies Sudipta mentions: CRISPR/Cas9 gene drives, which allow mosquitoes to be genetically modified in a way that decreases pathogen transmission (called “vector competence”).
CRISPR, or “clustered regularly interspaced palindromic repeats”, has been revolutionizing genomic research since its discovery by a team at the University of California. Researchers can now use CRISPR to hone in on any gene, cut that gene out, and introduce a new sequence of DNA that the cell then copies into that region of the chromosome.
Not your grandfather’s inheritance
Arguably, CRISPR’s most exciting and controversial application has been the gene drive. This technique involves combining the CRISPR enzyme, Cas9, with an introduced transgene, from another organism, that scientists want to spread through a given population. Typically, when researchers introduce a new gene into a population, the spread of that gene is limited, especially if that gene is deleterious. However, the Cas9 enzyme is able to convert the transgene from heterozygous (only on one chromosome) to homozygous (on both chromosomes) in progeny, facilitating the spread of the gene. Additionally, recent studies have shown that, when introduced into a lab population of mosquitoes, a single male could pass a gene onto nearly 100% of offspring after several generations. This means that researchers have the ability to genetically modify not just individuals, but entire populations. One study, conducted by the University of California, was able to drive a gene for an anti-Plasmodium antibody through a lab population of Anopheles stephensi.
Could malaria’s days be numbered?
With the ability to drive a gene into a population of mosquitoes, it is possible to see a future in which malaria (and in fact, all vector-borne diseases) is no longer a significant threat to global health. In addition, unlike bed nets and drugs – which must be manufactured, distributed, and used by individuals to prevent infection – once the mosquitoes are released, they would automatically begin spreading the anti-Plasmodium genes.
Of course, in the case of gene drives, there are still several risks to consider. For example, P. falciparum has shown signs of resistance to commonly prescribed anti-malarial drugs. It is highly likely, then, that a mosquito engineered to produce antibodies against Plasmodium would cause Plasmodium to develop resistance. One way of working around this issue would be to release successive gene drives before Plasmodium begins to show signs of resistance and before malaria rates increase. Ideally, the released drive would also cut out the original drive from the chromosome, so as to avoid producing high levels of Cas9 protein that may be toxic to mosquitoes.
Genetically modified mosquito populations could also result in unforeseen ecological problems. It would be nearly impossible to create a comprehensive model of all the ecological interactions that could be altered if a gene drive were implemented in native mosquitoes like Anopheles gambiae. Releasing a gene drive could alter mosquito behaviour or parasite life cycles in unexpected ways, especially in the case of so-called “suppression drives” that work by reducing the population by passing on genes to cause sterility. Additionally, no one has tested whether the gene drive components are toxic to predators. Should this be the case, a gene drive could reduce the predator population, causing a subsequent explosion of the mosquito population.
An ethical dilemma
If the gene drive is successful, significant changes to mosquito populations could ensue. Namely, nearly all of the mosquitoes in an area endemic for a specific mosquito-borne disease would contain a gene introduced by foreign researchers. Relating to this issue, Sudipta’s article covered opposition to Oxitec’s initial field trials of transgenic sterile mosquitoes. Residents in the Florida Keys, where a trial was being conducted, protested against Oxitec’s release of thousands of genetically modified mosquitoes, fearing the consequences of being bitten by experimental mosquitoes. It is possible that similar opposition could occur due to trials involving mosquitoes with gene drives.
Obtaining informed consent for such a trial would pose another issue, especially since initiating a gene drive could affect everybody in a particular community and beyond. Would it be necessary for researchers to receive consent from the entire community? Or, would it simply be enough to obtain agreement from the community leaders, whether they are elected politicians or village elders?
Furthermore, it is essential that researchers engaged in field trials of gene drives do so with as much cultural sensitivity as possible. The bulk of research thus far has been done in Western nations like the United States and the United Kingdom, but presumably this technology would be most frequently used in malaria-prone areas like Sub-Saharan Africa and Bangladesh. Before implementation, researchers would need to integrate the needs and ideas of the community into each trial. The gene drive technology is different from bed nets, medication, and vaccines because there is no ongoing individual opt-in. Once mosquitoes with gene drives are released, individuals can no longer choose whether or not they want transgenic mosquitoes in their ecosystem. Other recent public health campaigns, such as India’s incredibly successful campaign against Polio, relied on a top-down approach. The government of India launched a concerted effort, employing mass communication and teams of health workers to immunize children and keep effective records, resulting in polio’s virtual eradication from one of the largest countries in the world. Yet in Mozambique, Public Health officials have struggled to persuade locals to allow the indoor spraying of insecticides. One key difference between these campaigns is how the government responded to resistance: while in India, children were offered toys in return for being vaccinated, the Mozambique government considered making it a law that residents must consent to indoor spraying or face consequences. It is clear that the success of a campaign is also highly dependent on the community’s willingness of participate.
Researchers need to understand the historical and social context in which they are working, where paternalistic attitudes of Western exceptionalism may in some cases still subtly guide their perceptions of local communities. It is tempting to view Western solutions as inherently desirable, but as can be seen in the toilet crisis and the recent Ebola crisis, cultural disparities can be as difficult to overcome as the disease itself. Effectively and fairly engaging local communities will be one of the most difficult parts of implementing gene drives.
A future for gene drives in the fight against malaria
The eradication of malaria, along with other major diseases, was one of the UN’s Millennium Development Goals. With all of the risks associated with malaria, we cannot risk halting research on such a promising new tool that could help slow the spread of this disease. It is unlikely that gene drives will replace traditional methods of malaria control. Instead, this technology should be implemented in tandem with bed nets, drugs, and new vaccines for malaria.
Currently, there are four pillars to the WHO’s malaria strategy:
- Long-lasting insecticidal nets (LLINs)
- Indoor residual spraying of insecticides (IRS)
- Rapid diagnostic tests (RDTs)
- Artemisinin combination therapies (ACTs), i.e. drug therapy
Aside from gene drives, the newest tool to be added to this arsenal is a new malaria vaccine. While there are scores of potential new vaccines, one vaccine, called RTS,S/AS01, created by a partnership between GlaxoSmithKline and the Malaria Vaccine Initiative, has received approval. Although the WHO has not yet recommended its implementation, citing uncertainty about dosing and cost effectiveness, this is the first time we have an effective vaccine against malaria.
Where does the gene drive fit into all of this? It is still too early to tell. The WHO and the Bill and Melinda Gates foundation, two of the largest organizations leading the fight against malaria, do not currently fund any research on gene drives, likely because the technology is still in its infancy. However, the potential to use gene drives against other vector-borne diseases has not escaped notice. Researchers are looking into gene drives to fight the Zika virus, which is spread by Aedes aegypti, an invasive species of mosquito in Latin America. It is possible that in several years, gene drive-carrying mosquitoes will be a readily accessible tool whenever a disease like Zika suddenly flares up as it did this year.
The potential for gene drives to treat and even eradicate malaria and other vector-borne diseases is truly exciting and at the same time unsettling. The former, because of the obvious health benefits of drastically reducing the prevalence of the disease, and the latter, because of the importance of implementing the gene drive correctly, with the ideas and concerns of all community stakeholders taken into account. How this field shapes up in the next few years is likely to determine the face of vector-borne disease fighting well into the future.
Benjamin Levy is a first year life sciences student in the Stowe-Gullen stream of the Vic One program. He hopes to enrol in a Global Health specialist next year. He is researching mosquito gene drives for a project in Vic One.