Radical Eradication: A (Population) Crash Course in Genetic Engineering

Malaria is a life-threatening blood disease that plagues nearly two-thirds of the world’s population. The disease in manifested by parasites of the Plasmodium genus and transmitted to humans through the bite of female Anopheles mosquitoes, which serve as the primary disease vectors. Roughly 200 million people per year are infected with malaria, and approximately 400,000 deaths are reported annually, with children under the age of five comprising the majority of victims.

Africa disproportionately bears the global brunt of this devastating illness, with approximately 92% of all reported cases, as well as 93% of all reported deaths, originating from the continent. This can be partially attributed to the fact that the conditions for transmission are essentially ideal there: the principal vector species Anopheles gambiae are abundant in this region, and not only do they prefer to source their blood from humans over animals, but the mosquitoes also tend to have a longer lifespan, which allows the most common and deadly malaria parasite, Plasmodium falciparum, to complete its life cycle, which contributes to higher disease transmission efficacy.

Though malaria is a preventable disease, often the areas affected most lack access or resources, or are politically unstable, all factors that can contribute to the absence of consistent, functional malaria control programs. Though malaria is also a curable disease, it has long been debated whether eradication was even within the realm of possibility. There are four species of Plasmodium parasites responsible for the pathogenesis of malaria and each exhibit different forms of drug resistance and each responds differently to different medications. This alone makes the prospect of developing a single overarching vaccine for all strains of malaria an improbable achievement and the idea of eradication practically impossible.

A CRISP[E]R APPROACH

In a study recently published in Nature Biotechnology, a team of scientists were able to effectively implement a new, though indubitably controversial, type of genetic modification. The team was able to weaponize mosquitoes to take out…other mosquitoes! They were able to engineer male mosquitoes to rapidly pass down a fatal mutation through generations of their own species, effectively sterilizing all female offspring, eliminating the possibility of successful reproduction and resulting in a population crash.

Utilizing the dynamic technology known as CRISPR, the London-based team was able to genetically modify male mosquitoes of the malaria-transmitting species A. gambiae  with gene drive. The concept of gene drives and the idea to engineer them as a means of controlling insect populations that spread disease is not novel, however, up to this point in recent history, the technology to bring these ideas into reality has simply not existed.

In typical sexual reproduction, there are two possible versions of any given gene that the offspring can inherit, one from each parent. According to the traditional rules of genetics, there is then a 50% chance of the offspring inheriting either version of the gene. Gene drives completely negate these rules, and result in the rapid transmission of the desired engineered modification to almost all of the offspring.

The gene drive in this particular study selected a specific region of what is known in insects as the “doublesex” gene, which controls sexual determination and development. The region that they targeted would specifically impact female phenotypic sexual characteristics.

Female mosquitoes who initially have only one copy of the mutation would still present as healthy and normal, in terms of looks and behavior, and as a result were still reproductively able to continue spreading the mutation. Females born with two copies of the mutation would be phenotypically impacted, resulting in male-mouthed females that had lacked typical reproductive organs. These altered characteristics would render them sterile, unable to bite and by extension, unable to spread the malaria parasite.

For this research, two cage trials were run concurrently, utilizing 300 wild-type females, 150 wild-type males and 150 genetically-modified males. The aim of this experiment was for the gene drive to push the inheritance of this modified doublesex gene through multiple generations until the mutation reaches 100% prevalence in female offspring, leading to a population crash due to the inability to reproduce.

In the span of about six months, the team was able to achieve population crashes in both cage trials, as the populations reached 100% mutation frequency at generation 7 and generation 11, respectively.

GOING AU NATURALE 

Building upon the success of the initial study, a new phase of study, launched in Terni, Italy at the beginning of February, will get to test the model on a larger scale and under conditions designed to replicate as close to a natural environment as possible. Through this environmental imitation, the idea is for the surroundings to encourage natural behavior, thus providing more accurate insight into what the results of a gene drive release might look like if it were unfolding in their natural hot, humid habitats.

To set the stage for this experiment in the lab, the team of scientists are utilizing six huge cages that will each contain individual populations of A. gambiae mosquitoes. Every wall of each cage is lined floor to ceiling with mosquito netting, and each cage is supplied with stacks of hollow cylinders comprised of moist clay for the mosquitoes to utilize as shelter and heated canisters of cow’s blood, to help the specimens simulate drawing blood from a live animal.

Photo Credit: Firkin

Each chamber is additionally equipped with large black boxes with white backgrounds, and computer-regulated lighting systems. The boxes and their backgrounds play an important role in this experiment, as they are meant to mimic swarming, a key part of the mosquito mating ritual, and the controlled lighting is critical in simulating day and night cycles, particularly sunset as that is the typical time mating occurs.

To initiate the study, lab technicians methodically introduced small glass dishes to each mosquito-infested chamber. These glass dishes contained dozens of pupal-stage modified mosquitoes, which will quickly develop and assimilate into the already-thriving populations of the hundreds of normal mosquitoes that inhabit each cage. Two of the cages received an amount of modified insects equivalent to 25% of the normal population, another two received an amount of modified mosquitoes equivalent to approximately 50% of the normal population and the final two received no modified mosquito pupals and will serve as control populations.

Scientists will be collecting thousands of eggs from the chambers every week to monitor the spread of the lethal sterilizing mutation throughout each model population, and hoping that after about six months to a year, they will be able to see the effects of the engineered mutation play out.

EXAMINING EXPERIMENTATION EFFORTS

At least one thing has been made clear from these studies. The gravity of this situation – with just how controversial this technology and the implication of its successes in the laboratory is – is emphatically not lost on any of the scientists involved with these projects, which is a good indicator that this technology is in the best possible hands

As with most new and groundbreaking technologies, CRISPR carries the weight of great power but those who use it must also bear the baggage of great responsibility. Part of that responsibility is the thorough examination of the pros and cons and meticulous analysis of every “what if”.

When it comes to the decision to move this experimentation out of the lab and into real world applications, there is obviously some rightful pushback. This is not a topic to be taken lightly, and there is a very real possibility that we could see severe consequences, should anyone decide to continue full-speed ahead without evaluating all the possibilities.

It’s almost too easy to build a case against the advancement of these genetic engineering mechanisms simply by looking at the magnitude of all the unknown factors. Because there really is no precedent for this applied technology, there seem to be innumerable potential risks no matter which way you slice it.

From a con point of view, let’s say we were to press on with this particular experimentation in real-world environments. The potential eradication of the A. gambiae mosquitoes using this technology could present a variety of plausible undesirable effects. Delicate ecosystems could be thrown out of whack. The technology could stretch beyond the desired species and inadvertently lead to the destruction of non-disease-transmitting mosquitoes, or even the destruction of important pollinators.

Furthermore, what if the population crash of A. gambiae leaves an empty niche in an ecosystem that could be filled by another more problematic disease-peddling species?  And on an even broader scale, what if this gene-editing technology falls into the wrong hands? What if those wrong hands belong to terrorists who use the technology to develop biological warfare agents?

Photo Credit: GDJ

Though all these unknowns are valid questions that undeniably should be posed and thoroughly discussed, its equally important to not get so absorbed in what could go wrong, that we can no longer see what could go right. What if the large scale study is wildly successful? What if we can then apply the study to real life populations, and we eliminate huge populations of malaria-causing A. gambiae, effectively eliminating, if not entirely eradicating, the disease? We could then turn our focus to ending other insect-borne diseases plaguing humanity, such as Zika, Lyme Disease, West Nile, sleeping sickness, and yellow fever. What if we are able to spare millions of people from agonizing, unnecessary suffering?

What if we start to apply the gene drive model to other categories of global-scale issues? We could help endangered ecosystems thrive again, by targeting invasive rodents, fungi and plants. We could clean up harmful oil spills in our oceans utilizing genetically modified bacteria. We could engineer more efficient crops to aide the global hunger crisis, and minimize the need for harmful pesticides and herbicides by modifying bugs to stop consuming crops in the first place.

Depending on your perspective, everywhere you look you could either see potential problems or see potential solutions. Don’t disregard the problems as they arise–welcome them! Sit with them, make them a cup of tea, get to know them better. Just try to keep your eyes on the solutions–the advances, the unveiling of previous mysteries, the cures–as those are what make the pursuit of science worthwhile.

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Natalie is a Science Writer at Promega. She earned her B.S. in Microbiology from the University of Wisconsin-Madison, and her Associate's Degree in Science from Cottey College. In her spare time, she can be found playing volleyball, making music, chipping away at her never-ending stack of craft projects, and volunteering with animals.

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