A Brief Overview and Outlook on Genetic Controls

Traveling along the highways and byways of the southern United States you will see acres of an ornamental vine, kudzu, creeping across the landscape and choking out native flora. First introduced into the US in the 1870s, kudzu has steadily been conquering untended landscapes ever since. For decades, combating kudzu has been a hot topic full of ideas and little delivery. As a result, I once had an idea for a sci-fi story based on the fact that kudzu, in actuality, was an intelligent 'alien' species with a geological era lifespan bent on world domination. Of course, the hero was a bright-eyed, idealistic graduate student who, while sequencing the genome, discovered kudzu's off-planet origins, and then raced against time to alter the genome and end the scourge.

From a historical perspective (Gould 2008), attempting to genetically control a 'pest' species to eradicate or suppress its population is not new. (I never said it was an original plot.) Amongst many reasons for the lack of completing said future sci-fi classic, though, is that I never could figure out an original, clever, and somewhat feasible way to exterminate the pest. Fortunately, Austin Burt had greater vision when he first developed the idea of engineered, site-specific 'homing' endonuclease gene drives in 2003 (Burt 2003). Now, with the advent of the CRISPR/Cas9 RNA-guided gene drives in the last several years, developing an evolutionary robust gene drive system than can help alter, suppress, or even eradicate whole pestiferous populations is closer to hand, albeit not without its challenges and concerns (Ysvelt et al 2014).

Attempting to manage pestiferous populations through genetic control dates back to the 1940s (Gould, 2008). The oldest and most practiced primary genetic control method is known as the Sterile Insect Technique (SIT). With SIT, males of a particular target species are harvested, irradiated or chemically treated to induce sterility, and then released in numbers that overwhelm the local population and, hopefully, eradicate the species. SIT has been applied to flies, beetles, and mosquitos with many successes and not a few failures. For the US, the most notable application of SIT was the successful eradication of the screw worm fly (which can cause billions of dollars in livestock losses) in the 1960s (Gould 2008). Current programs, primarily sponsored through the UN, include attempts to eradicate the tsetse fly in primarily African countries, such as Senegal. SIT approaches are a more brute force approach and have the greatest success when the habitat area is small, the population density is relatively low, the target species does not have a large migratory range and is it not adaptable to diverse habitats (Gould 2008).

Due to SIT's limitation, researchers recognized that more sophisticated genetic controls and methods would need to be developed (Gould 2008). As early as the 1940s, researchers were investigating the potential to eradicate pest populations by leveraging underdominance by introducing a second species that would overwhelm the target species, but later die off because they were not adapted to the habitat; or, through introducing a translocation in the target population that would eventually lead to eradication (Gould, 2008 & Thresher et al, 2014).

In nature, there are non-engineered genetic elements that can drive themselves through a population, increasing the likelihood they become fixed within a population (Ysvelt et al, 2014). When it was realized that SIT approaches would have limited effectiveness, research proceeded into leveraging natural potential drivers such as transposons through classical genetic breeding to create transgenic populations (Thresher et al, 2014). Yet, these approaches have met with limited success.

One interesting approach is experimenting with altering the microflora of mosquitos that carry dengue fever. Rather than alter the vector species, the researchers exploited the ability of a Wolbachia bacterial strain to rapidly invade mosquito populations thru cytoplasmic incompatibility (i.e. eggs and sperm are unable to form viable offspring). Once established in mosquitos, the Wolbachia interferes with disease transmission and lifespan (Hoffmann et al, 2011). Field tests in Australia have demonstrated that Wolbachia invasion can spread through a wild population and, currently, trials are being conducted in Brazil, Vietnam, and Malaysia (O'Neill, 2015). Determining the method's ability to actually reduce Dengue fever remains a work in progress..

Austin Burt's concept of engineering site-specific homing nucleases captured a lot of interest and work has proceeded in their development (Burt 2003, Thresher et al, 2014). To be effective, among other attributes, engineered gene drives should 1) to cut the target sequence highly efficiently, 2) cut with a high degree of specificity (i.e. no off target cuts), 3) repair the cut sequence by faithfully copying the desired gene(s) through homologous recombination rather than rather through non-homologous repair, and 4) be evolutionary stable (Ysvelt et al, 2014). The advent of more precise genome editing tools such as TALENS possess the ability to efficiently and precisely cut a target sequence, but do not provide the needed stability that leads to fixation in a population (Ysvelt et al, 2014).

CRISPR/Cas9 gene drives would potentially meet any of the criteria above (see video for more details on CRISPR/Cas9 system), plus provide the ability to engineer multiple loci, which would increase the likelihood of avoiding drive resistance and producing the desired outcome in the target population (if robustness could be maintained). To date, CRISPR/Cas9 is the most potentially powerful and applicable tool in creating efficacious gene drives, yet carries large concerns along with its large potential upside. In addition, to eradication of pestiferous and disease carrying insects, CRISPR/Cas9 gene drives bring greater potential to manage invasive species such as kudzu or lion fish, and to reduce resistance to synthetic and natural herbicides and pesticides.

In and of themselves, gene drives are not likely to be a magic bullet that will severely curtail malaria or invasions of zebra mussels. More likely CRISPR/Cas9 gene drives, will become another tool in the public health and ecosystem management tool box. For example, in the case of malaria, a localized and combined strategy such as utilizing gene drives where applicable in concert with informed application of pesticides such as DDT and the actual use of preventive netting and bed clothes could significantly reduce the more than 500,000 annual deaths due to malaria.

With great power comes great responsibility. Efforts to regulate and establish safety guidelines to prevent intentional and unintentional misuse of CRISPR/Cas9 are being put in place. But do we really need to worry about the potential of CRISPR/Cas9 gene drives to wreak untold havoc ecological havoc or severely alter populations outside of sci-fi horror stories? Of course the potential exists, but there are many practical and regulatory guidelines that prevent its likelihood. For one, gene drives take many generations before they spread sufficiently through a population. Therefore, they work best in species with relatively fast generation times. So, this not something that can alter human or livestock populations without unanimous stake holder acceptance or a massive and long term conspiracy.
 

Which reminds me of another plot for a potential sci-fi classic...