It would be a challenge to find anybody in the scientific community today who is not familiar the words “CRISPR-Cas9”. Over the years, this revolutionary discovery has transformed the way we think of genes and what we can do to them. We can now cut them, delete them, and replace them—all with much higher precision than ever before. It is a technology that has ignited optimism, but at the same time, it is this significant power that sparks concerns regarding the ethics and safety of using such a mighty genetic tool.
A borrowed tool
What started off as a prokaryotic immune mechanism has become one of the most flexible tools in science. CRISPRs, which stands for “clustered regularly interspersed short palindromic repeats” are found in most species of bacteria as well as archaea, and normally act to confer protection against viral infections. In nature, invading viral DNA which has integrated into the bacterial genome is transcribed into CRISPR RNAs (crRNAs). These crRNAs hybridize with transactivating CRISPR RNA (tracrRNA) to make guide RNA (gRNA) that then binds to and forms a complex with the Cas9 nuclease. This RNA-Cas9 complex, using the RNA as a template, recognizes the complementary viral DNA within the bacterial genome and cleaves it.
Fast forward to the present. Scientists can now target almost any location in a genome by designing custom gRNA sequences. What’s more is that CRISPR is cheaper, faster, and more reliable than previous methods of gene-editing such as zinc finger nucleases and transcription activator-like effector nucleases (TALENS). The CRISPR system can not only be used to delete a gene but also allows for the insertion and modification of desired sequences as well. The possibilities are endless.
Should we be afraid?
Apprehension surrounding CRISPR is mainly found in the field of human genetics. Many worry that with the rise of this new technology, it will not be long before humans begin to exploit it. One of the most commonly touted fears is the concept of “designer babies”, choosing exactly how babies will be born—eye colour, lower risk of disease, enhanced athletic abilities etc.
In a recent TEDMED talk, Dr. Alta Charo, a Professor of Law and Bioethics at the University of Wisconsin, uses in vitro fertilization (IVF) as an example to highlight a time when an emerging biotechnology induced much fear in the general public. Made available in the early 1980s, Dr. Charo states that anxieties arose surrounding the idea that fertile parents would use IVF to select for “ideal children”. Instead, the vast majority of IVF users today either lack the ability to conceive naturally or seek to prevent a life-threatening disease from being inherited by their children. Dr. Charo stresses that the future clinical uses of CRISPR will mimic the responsible employment of IVF—“In action, the choices people make and the services physicians offered were responsible and measured, and did not necessarily doom us to some kind of eugenic destiny”.
As CRISPR technology progresses, it is also important to consider the potential dangers that can surface as it becomes more widely available. Companies, including The ODIN based in San Francisco, are producing DIY CRISPR Kits—complete with petri plates, buffers, Cas9 plasmids, gRNA plasmids, and template DNA—that offer buyers a chance to experiment with bacterial gene engineering right in their own homes. The kit provides everything one needs to alter the DNA of a non-pathogenic strain of E. coli that allows it to grow on media containing streptomycin. Since its release, it has become high in demand and multiple versions of the kit have emerged, including one with live genetically modifiable Green Tree Frogs. The CEO of ODIN, Dr. Josiah Zayner, refers to himself as a biohacker who hopes to expand research beyond the walls of the lab and encourages members of the public to try their hands in scientific experimentation. Interestingly, here in Canada, there are regulations that state gene therapy experiments involving the alteration of a germ line cannot be conducted, but other than that, there are few rules on biohacking. Experts are not concerned with the possibility of a genetically-modified organism wreaking havoc on the public yet, but if gene editing technologies continue to circulate at the current rate, it may be prudent for policymakers to consider how potential harms could be effectively controlled.
Driving mosquitoes to extinction?
While the use of CRISPR in humans is being cautiously explored, the technology is also being applied to other organisms. Researchers at the Imperial College London are using a “CRISPR-Cas9 gene drive” to eradicate controlled caged populations of Anopheles gambiae, a species of mosquito that contributes to the spread of malaria. In 2016, the World Health Organization reported that approximately 216 million individuals worldwide were affected by malaria, causing an estimated 445,000 deaths.
Gene drives target a specific gene and help boost the chances of it being inherited in offspring. As their progeny mate, the desired genotype frequency within the population continues to increase with the hopes that eventually the entire population will carry the sequence. Led by Dr. Andrea Crisanti, the study targeted a gene called doublesex (dsx) that is involved in the sex determination pathway of A. gambiae. Mosquitoes heterozygous for the drive were fertile, key for passing on the gene, while homozygous females became sterile. After 8 generations, no females remained, resulting in population breakdown.
Will the findings of the study be applied in the real world soon? According to Dr. Crisanti, “This breakthrough shows that gene drives can work”, but “there is still more work to be done”. But what would happen if a gene drive were to be set loose? Using a mathematical model, Dr. Kevin Esvelt and his colleagues from Harvard University discovered that a gene drive would be extremely aggressive in the wild and challenging to contain to a specific geographic location. It would be difficult to determine the specific effects of eradicating all mosquitoes, as this organism is part of the food chain across the world. Having said that, it has been suggested that the consequences of complete eradication would not be too serious, but in Dr. Crisanti’s opinion, “It will still be at least 5-10 years before we consider testing any gene drive in the wild.”
Still a long way to go…
It would be an understatement to say that CRISPR has rapidly expanded in the world of research and there is absolutely no question of the promise it holds for future scientific discovery. However, there is still much to learn on the applications and technological challenges that come with such a dynamic tool. Even though CRISPR is more efficient than earlier methods, it is still not perfect and when it comes to gene modifications in humans, there is no room for error.