There are many achievements in science and technology that deserve celebration from 2015, and also provide a hopeful and optimistic outlook for 2016. Some of these achievements include the successful landing of SpaceX’s Falcon 9 rocket after being launched into space, progress toward developing an Ebola vaccine, and progress in developing protocols to use stem cells to produce blood that can be used in transfusions. One of these innovations that has had a great deal of media coverage is a gene-editing technology called CRISPR (pronounced crisper).
CRISPR is an acronym that stands for clustering regularly interspaced short palindromic repeats, and is a part of the adaptive immune system of bacteria. It is composed of RNA and an enzyme that slices up viruses. There are areas in the bacterial genome that have repeated palindromic DNA and between those sequences are what are referred to as spacers that contain a portion of the pathogen’s DNA. If there are pathogens present that have DNA that matches one of these spacer sequences, then the pathogen’s DNA will be sliced. When a new pathogen is encountered, more of these spacer regions are created in the bacteria’s genome.
CRISPR can be used to edit the genome of any organism by inactivating genes, modifying genes, or inserting genes into a genome. Some possible uses include fixing mutations that cause genetic diseases (this has been done in the lab for cystic fibrosis), more easily genetically engineering lab animals, and creating new traits in agricultural crops to, for example, better withstand drought or pests. Using genetics to accomplish these tasks is not a new idea, but using CRISPR is relatively new and is faster and more precise and efficient than previously developed methods.
Since its development in 2012, CRISPR has been used to genetically engineer organisms in the lab such as rhesus macaques, mice, zebrafish, fruit flies, yeast, and some plants. There is one way in which CRISPR is being used that is of particular interest to me: manipulating vector genomes to help decrease transmission and prevalence of pathogens in vector populations. Malaria is the system this method has been investigated in most recently.
CRISPR is being used in conjunction with a methodology called gene drive systems to genetically alter mosquitoes. Gene drive systems are a way to propel a gene of choice throughout a population. It is accomplished by transferring the gene of interest to almost all of a carrier’s offspring through breaking the typical inheritance rules. What CRISPR provides is a method to more accurately and efficiently insert genes and/or manipulate genes in the genome.
There are two approaches that have recently been published that show promise. One that disrupts three genes for female fertility, and when a female inherits a copy from both parents the female will be infertile. This would lead to smaller mosquito populations, to the point that malaria transmission cannot be supported. The second approach is to give mosquitoes genes that make them resistant to malaria. These genes will then be passed on in the germline leading to populations of mosquitoes that are resistant to malaria.
Both of these approaches show promise, but there is much more testing that would need to be accomplished before releasing any mosquitoes into the wild. First of all, CRISPR is still a new method and, although extremely promising, there are still some aspects that need to be considered. Researchers have found that there is the possibility of off target cutting, meaning that areas in the genome that don’t exactly match the sequence of interest can, and sometimes do, become recognized and cut. Also, there is some worry that lab animals that have these genetic alterations will escape the lab, and researchers are not sure how this will affect wild populations.
In general, the ecological effects of using the CRISPR system with the gene drive systems is not known. Mosquitoes interact with other species and do play an ecological role in the ecosystems in which they occur, so especially in using the technique that will reduce population sizes of mosquitoes, more testing needs to occur in order to determine possible affects to mosquito populations. This will be true for any vector species this methodology is developed for in the future. As 2016 progresses, more research will occur to better understand the affects of these methodologies, but how close we will be to testing these in wild populations toward the end of 2016 is not known.
Gantz, Valentio M., Nijole Jasinskiene, Olga Tatarenkova, Aniko Fazekas, Vanessa M. Macias, Ethan Bier, and Anthony A. James. 2015. Highly efficient Cas9-mediated gene drive for population modification of the malaria vector mosquito Anopheles stephensi. PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1521077112
Hammond, Andrew, Roberto Galizi, Kyros Kyrou, Alekos Simoni, Carla Siniscalchi, Dimitris Katsanos, Matthew Gribble, Dean Baker, Eric Marois, Steven Russell, Austin Burt, Nikolai Windbichler, Andrea Crisanti, and Tony Nolan. 2015. A CRISPR-Cas9 gene drive system targeting female reproduction in the malaria mosquito vector Anopheles gambiae. Nature Biotechnology, doi:10.1038/nbt.3439