CRISPR/ CAS9

Genetic modification methods have been around for few decades, but the CRISPR/CAS9 system has still caused quite a stir among scientists and society over its potential and the ethics of using the system for some of the potentials. Our old methods of GM didn’t have a lot of power. We could add nucleic material to the cell, but we didn’t have great control over modifying the genome. There are several reasons genetic modification is so pursued: drug design (by allowing good models of disease to be created)1, protein studies (by using CRISPR to modify proteins in the cell so that they can be easily monitored), and gene therapy (by deleting harmfully mutated genes, replacing with good copies, inserting genes that act positively, etc.) This video from McGovern Institute for Brain Research at MIT has a simple (and very prettily animated) introduction to GM and CRISPR/CAS9.2 

Interestingly, the CRISPR/CAS9 system was discovered in bacteria in the 1980s, but only in the last few years has the potential been recognized. With recognition also came concern. So I’ve summarized the arguments for and against using CRISPR/CAS9. As always, this is just a summary covering some of the pros and cons. Each person can use their own experiences to interpret the arguments, and may lean one way or find a middle ground.

(For background information on how genes make proteins, genomes, etc. go to the Basics and Background part of the blog.)

FOR:
Drug development:

When studying cancer, one approach is to use mouse cells grown in dishes. CRISPR has been used to mutate genes that code for proteins thought to be involved in cancer. When we can mutate these proteins and see cancer cells stop dividing and growing, we can predict that these proteins are important for the growth and spread of that cancer. This lets us develop drugs that can target the proteins. One recent example used CRISPR mutations in mouse myeloid leukemia cells to find 19 potential drug targets.3

Model development: One problem we have with studying disease is that we don’t always have good models. Obviously, studying human disease has ethical implications. We don’t want to prod a person’s arm just to study a protein. So we use animal cells grown in a dish (cultured cells), or live animals such as mice. Often, these cells don’t naturally have the same diseases we want to study. So we can use CRISPR to make the mutations to make the diseases. This lets us use these new models to find ways to cure the diseases.4

Genome editing: CRISPR’s potential to edit human genomes is probably the one that causes the most alertness. Already, scientists have been using CRISPR to edit harmfully mutated genes that cause diseases.5 For transparency’s sake, this study did produce a successfully edited embryo with completely transformed cells, and it was published before a moratorium by the general science community was called on using CRISPR for human genome editing. If CRISPR could be used to edit pluripotent cells we can replace damaged or cancerous cells in patients.  Studies testing CRISPR’s effectiveness in pluripotent cells (cells that can become any kind of cell, such as blood cells, nerve cells, etc.) have had moderate success, but are yet to be effectively tested for disease therapy in patients. Early in 2016 a Chinese research team reported that clinical studies have started to use CRISPR to transform a patient’s own immune cells to better target cancer cells.6

AGAINST:

Off-target effects: A few studies have shown CRISPR/CAS9 having effects on DNA sequences that weren’t intended to be modified.7,8 This can occur because humans have large genomes with multiple genes that are highly similar to each other in sequence. Current research seeks to reduce off target effects.

Passing mutations to future generations: If an off-target mutation is made, there’s the possibility, since it’s in the genome, the mutation could be passed on to future generations, which could have harmful effects.7,9 One current example is the attempt to modify mosquitos so they no longer carry vector-born viruses, such as the Zika virus. These mosquitoes are modified using CRISPR/CAS9 to carry genes for lethal proteins and then released into the wild to mate with female mosquitoes (the carriers of Zika), so that the chances of the offspring surviving and then infecting other organisms with Zika dramatically reduces. We don’t know what will happen once these mosquitoes are released. While designed to essentially die off quickly, just living long enough to lethal genes to new mosquito generations, we don’t know that there won’t be other outcomes. It could occur that the modified mosquitoes undergo spontaneous mutations that cause them to not produce the lethal protein. Other effects are just speculation. We really just don’t know what’s possible because this is the first time this has been done. Once these mosquitoes are released, every person around them becomes part of the experiment (willingly or unwillingly), and that poses its own ethical dilemma.

Regulation: The USDA recently announced that it won’t be regulating GMO mushrooms created using CRISPR/Cas9. Because CRISPR was used to delete a gene that causes mushrooms to turn brown, rather than introducing genetic material, the USDS stated that regulation isn’t necessary.10 This is supported by the USDA’s regulatory guidelines, which state that whenever new genetic material is introduced into an organism, it becomes necessary to test its safety and effect on the environment.11 There is no mention of testing organisms modified by deletion. According to Business Insider’s Erin Brodwin, USDA policy focuses on studying organisms with introduced materials.  It is still unclear how the US government intends to regulate CRISPR/Cas9, and the matter is complicated by the broad utility of the technology. While it is used to create GMOs, it is also used to study proteins, to create new drugs, and as a potential source of gene therapy. As a technology with so much potential for good or for bad, CRISPR must obviously have some regulation, but will likely be regulated by coordination of several agencies.

Patenting: Biotech companies have already attempted to patent the utility of CRISPR for therapeutics. This feeds into the larger issue with patents’ potential to be more helpful for corporate profits than for human benefits. However, patenting can also help with regulation as it lets the patent owner(s) either be the sole utilizers of a technology of manufacturers of a product or control and regulate others doing the same.

Genome editing: For the first time, we have a tool to edit the human genome. This can be a pro and a con of CRISPR. One hurdle to overcome is actually getting CRISPR to work effectively as an editing tool, especially in human embryos. So far no study has successfully used CRISPR for human embryo genome editing. One of the latest studies to use CRISPR in human embryo genome editing had similar results to the first study with human embryos, which had a 4.9% editing efficiency.12 This 4.9% of embryos were also mosaics, containing both edited cells and non-edited cells, which means that the mutated gene hadn’t been completely fixed throughout the embryo.  Typically it takes quite a few years of modifying a method before it may ever be a successful therapy, but with embryo research, every study has to be as successful as possible because of the ethical concerns of embryo research. The number of embryos used has to be extremely limited. It’s a sensitive subject and embryo research is never approached lightly. This brings up the ethical issue of using human embryos for, first, figuring out if CRISPR can be successfully used to edit embryos, and two, whether it should be. So far no scientists have reported using CRISPR in viable embryos. The embryos used previously have been 3PN embryos, which have three copies of the human genome, instead of two, and so would normally be inviable. These embryos were reported to be donated from fertility clinics. Typically, when trying in vitro fertilization, 3PN embryos are discovered before implantation and then not implanted as the pregnancy will not be successful. Also, while the system could be used to prevent a genetic disease, it could also be used for unnecessary enhancement, or even harm, such as to control a population by controlling their genetics. There is also the possibility that CRISPR could offer more privileged members of society another advantage over the less privileged/ unprivileged. If human enhancement by CRISPR becomes allowed, it isn’t hard to imagine that biotech companies might have exorbitant prices for embryo editing, which only the richest could afford. This could lead to widening of the economic gap.

References

  1. Wrigley, J.D.; Maresca, M.; Birmingham, K.; Boohlooly-Y, M.; Mayr, M.; Precise genome editing: the key to a CRISPR drug discovery pipeline? Drug Discovery World. Spring 2015. http://www.ddw-online.com/enabling-technologies/p303319-precise-genome-editing:-the-key-to-a-crispr-drug-discovery-pipeline-spring-15.html (accessed Nov 10, 2016).
  2. McGovern Institute for Brain Research at MIT. Genome Editing with CRISPR-Cas9. November 5, 2014. https://www.youtube.com/watch?v=2pp17E4E-O8 (accessed Nov 10, 2016).
  3. Shi, J.; Wang, E.; Milazzo, J.; Wang, Z.; Kinney, J.; Vakoc, C. Discovery of cancer drug targets by CRISPR-Cas9 screening of protein domains. Nature Biotechnology20156 (33).
  4. Yang, W.; Tu, Z.; Sun, Q.; Li, X. CRISPR/Cas9: Implications for Modeling and Therapy for Neurodegenerative Diseases. Frontiers in Molecular Science20169 (30).
  5. Liang, P; Xu, Y; Zhang, X; Ding, C; Huang, R; Zhang, Z. Crispr/Cas9-mediated gene editing in human tripronuclear zygotes. Protein Cell20156 (5).
  6. Rodriguez, E. Ethical Issues in Using Crispr/Cas9 System. J Clin Res Bioeth20167(2).
  7. Cryanoski, D. Chinese Scientists to Pioneer First Human CRISPR Trial. Nature News. 21 July 2016. Accessed 26 January 2016. http://www.nature.com/news/chinese-scientists-to-pioneer-first-human-crispr-trial-1.20302
  8. Fu, Y.; Foden, J.; Khayter, C.; Maeder, M.; Reyon, D.; Joung, J.; Sander, J. High-frequency off-target mutagenesis induced by CRISPR-Cas nucleases in human cells. Nature Biotechnology201331(9), 822-827.
  9. Sifferline, A. Tale from the Front Lines. Time. February 3, 2016. http://time.com/the-war-against-mosquito/ (accessed Nov 10, 2016).
  10. Brodwin, E. Everything you think you know about genetically modified food is about to change. Business April 14, 2016. http://www.businessinsider.com/the-us-government-says-crop-edited-with-crispr-wont-be-regulated-2016-4
  11. Biotechnology Frequently Asked Questions (FAQs). USDAhttp://www.usda.gov/wps/portal/usda/usdahome?navid=AGRICULTURE&contentid=BiotechnologyFAQs.xml
  12. Kang, X; He, W; Huang, Y; Yu, Q; Chen, Y; Xingcheng, G; Sun, X; Fan, Y. Introducing precise genetic modifications into human 3PN embryos by CRISPR/ Cas-mediated genome editing.

 

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