Gene Drives: Using CRISPR-Cas9 in Disease Control
Despite recent advancements in vaccine development and disease control, vector-borne diseases (namely malaria, Yellow fever, sleeping sickness and Chagas’ disease) still cause over one million deaths annually. Mosquitoes are perhaps the most notorious of such vectors, but other arthropod species may also carry the causative parasites. Controlling the transmittance of these diseases is a formidable task: draining wetlands to reduce the number of breeding sites is costly and causes widespread damage to ecosystems whilst insecticides will never specifically target the vector and pose risks to human health if they enter the food chain. Additionally, emerging resistance to insecticides is an increasingly prevalent problem that limits their long-term use.
Using CRISPR-Cas9 technology to manipulate vector biology could dramatically increase the efficacy of disease-control programmes. This has been extensively researched in malaria control strategies, though has the potential to be applied to any vector-borne disease. The causative agents of malaria (Plasmodium spp.) are transmitted to humans when a female Anophelesmosquito takes a blood meal. The malarial parasite then reproduces in the mosquito’s gut and migrates to the salivary glands, ready to be injected into the next host.
Selfish genetic elements are segments of an organism’s genome that are transmitted to the next generation at a higher frequency than other components of the same genome (Werren, Nur & Wu, 1988). Gene drives utilise this property of selfish genetic elements to introduce genetic changes into a host genome that subsequently spread more quickly in the population than a normal mutation would (DiCarlo, 2015).
The “selfish” nature of the alleles means that they do not necessarily have to confer a fitness benefit to the host to be spread, and in the case of gene drives, the alleles are usually deleterious as they reduce fecundity. One such method of reducing reproductive success is using an “X-shredded” encoded by the Y chromosome, which cleaves the X chromosome at many locations and thus renders only Y-carrying gametes viable (Champer et al., 2016).
Alternatively, it was proposed that a subset of these selfish genetic elements, termed Homing Endonuclease Genes (HEGs), could be used in gene drives to confer a sterility phenotype to either male or female mosquitos. These cause a double-stranded break at a specific recognition sequence, which is then repaired by homology-dependent repair (Burt, 2003). Consequently, when HEGs insert into a heterozygous individual, they convert it to a homozygote (hence “homing”).
Already, the CRISPR-Cas9 system has been used disrupt AGAP005958, AGAP011377 and AGAP007280, haplosufficient female fertility genes in Anophles gambiae. This was achieved using an engineered CRISPR homing allele during gametogenesis (Hammond et al, 2016). The result was a very high frequency of knock-outs and a transmission rate to the next generation of up to 99.6% (Hammond et al, 2015). Additionally, homozygous progeny (achieved in the F2) were almost completely sterile, whilst any heterozygotes had a severely reduced reproductive capacity.
The above is an example of using CRISPR-Cas9 to have a “population suppression” effect, meaning that it would lead to the eventual extinction of the local population of mosquitos (Champer, Buchman and Akbari, 2016). However, this deliberate extinction of another species evokes some ethical quandaries. Opponents of the use of gene drives – particularly when they involve extinction – often invoke the “sanctity of life” principle (Pugh, 2016), essentially stating that we do not have the “right” to cause the deliberate extinction of another species. However, Pugh argues that to protect the mosquito from eradication would bestow it with a moral status that outweighs the reasons we have to eliminate them: protecting human health.
Nevertheless, if the issues posed by population modification prove intractable, CRISPR-Cas9 may still be used to reduce malarial transmission. In “population modification”, traits that enable to mosquito to carry and transmit malaria are modified, resulting in a population of mosquitos with a reduced capacity to carry the parasite (Gantz et al., 2015). One such modification would be to alter the vector’s immune system such that it is now susceptible to the effects of the parasite, resulting in death upon infection.
An alternate use of gene drives has been proposed: biological control of invasive species. The New Zealand government has also expressed interest in this application, intending to use it as part of their plan to be “predator-free” by 2050 (Regalado, 2017). As they are quite different issues, both have their own set of ethical and regulatory issues that accompany them, but there is one stark commonality: they force us to examine our relationship with another species.
Before gene drives can be enacted there are many issues that need to be overcome. As with any new technology, some consequences can be anticipated but many others remain unknown. Gene drives have the potential to eradicate a species and as such the ability to irrevocable alter the earth’s ecosystems. The magnitude of these effects is unknown: ecosystems may be flexible enough such that the loss of mosquitos would be of little consequence. Conversely, some species may be so specialised that after the loss of mosquitoes prey-switching is unlikely and further, unintentional extinctions may occur (Fang, 2010). Some efforts have been made to predict the consequences of mosquito eradication (e.g. Poulin, Lefebvre and Paz, 2010), but they also show different effects depending upon the local environment.
Other, political factors may also hinder the use of gene drive technology, with established regulations proving to be one of the largest hurdles. The 2003 Cartagena Protocol on Biosafety – which has been ratified by 170 countries – prohibits the movement of live, modified organisms across international borders (Oye et al, 2014). Modified mosquitos would prove impossible to control, and consequently countries that have signed the treaty may be unable to use gene drives.
The focus has been on malaria, but there are many other diseases against which this can be used: dengue, malaria, chikungunga etc.. Despite decades of effort and research, there are yet no vaccines to combat them (Papathanos, 2009; Pugh, 2016). If we are to meet the United Nation’s Sustainable Development Goal of ending the epidemic of malaria by 2030 (U.N. General Assembly, 2015), we may need to employ gene drives despite some unease.
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