HeeJin Cheon is a senior at Cornell University studying Biological Sciences.
Currently there is a flood of news articles on CRISPR gene-editing technology, as well as possible achievements and failures that scientists have seen so far in treating certain diseases such as HIV. Just what exactly is this astounding technology, and can it really leave a dent in human history?
CRISPR (which stands for clustered regularly interspaced palindromic repeats) is a DNA editing system that enables sensitive gene editing in a variety of organisms, including human embryos[2,3]. It is paired with a protein called Cas9 that serves as an endonuclease, an enzyme that cuts DNA at specific sites. The ability to cut DNA at precise locations is important because it allows scientists to insert or delete genes, allowing for the “manipulation of genes” that we all talk about. The CRISPR-Cas9 system comes from a recently-discovered yet ancient defense system used by bacteria and archaea against invading viruses (think of it as a bacterial immune system). Since its original discovery, several scientists have recognized that we can harness CRISPR’s original immune function to cut and edit whatever DNA we want at precise locations. The original system has been modified to increase its effectiveness not only in genome editing, but also to function as a complex that can bind DNA at precise locations to regulate other relevant functions, such as activation or inhibition of ‘reading’ DNA to make more or less protein of interest.
Other gene-editing technologies, such as zinc finger nucleases (ZFN) and transcription activator-like effector nucleases (TALEN), have been used in the past and work quite differently, but the CRISPR-Cas9 system allows for modification of two or more target genes, and it generally is a cheaper and a faster way for scientists to induce site-specific DNA cuts. However, the CRISPR-Cas9 system is not without its caveats. It can still induce off-target DNA cleavage - that is, it can cut DNA in unwanted areas - which might have tremendous consequences with respect to human embryo editing. Additionally, not all targeted cells will contain the desired DNA modifications, requiring a screen process for the cells that contain the DNA modifications that scientists have induced. This poses a problem for in vivo applications such as human embryo editing, since it essentially produces a genetic mosaic.
There have been several attempts to cure HIV-1 using CRISPR-Cas9, but recent findings suggest that HIV-1 is not an easy enemy to subdue with this technology. The original approach was to equip T cells, which play an important role in our immune system, with CRISPR-Cas9 in order to target specific sequences of the virus so that it could find and cripple the invader. However, mutations arose in the virus itself that allowed it to evade identification by the CRISPR-Cas9 system – a lot like the way viruses and bacteria evade our own immune systems. This demonstrates that CRISPR-Cas9 is perhaps not the panacea that many people think it is.
Was there any success using CRISPR-Cas9 to treat other diseases? In brief, yes. Although the research with HIV does not seem promising, there are a couple of leads in other disease sectors, such as Huntington’s disease, a neurodegenerative disease where patients experience decline in cognition and impaired coordination. Nonetheless, there are several illnesses that have been successfully reversed using alternate gene editing technologies. For example, scientists have successfully corrected muscular dystrophy in human cells using ZFN, and treated a one-year-old girl with leukemia using TALEN[9,10].
As with other genome editing techniques, CRISPR-Cas9 is under scrutiny by bioethicists and scientists concerned about its potential for abuse. As mentioned earlier, human embryo editing is a major concern and unfortunately, rapid advancements in CRISPR-Cas9 technology have preceded substantial ethical discussion. Nonetheless, we should be careful about entertaining any “slippery slope” arguments regarding the creation of so-called “designer babies” and selection for desired genes in humans. This is largely due to the fact that the limits of this technology are just now beginning to emerge, such as the fact that the efficiency of targeting all cells for desired genetic modification is low.
CRISPR-Cas9 is an important tool with distinct benefits compared to other genome editing techniques. Because of its ease of application, not only will it be used for laboratory experiment purposes or treating diseases, but it also has potential in material and food production. For instance, one way is by manipulating biology circuits, as we have done with insulin. However, we should be wary of CRISPR’s limits as well instead of jumping to conclusions about its potential applications to the extent that we inadvertently discourage thorough exploration of the technology. Every biological tool has its caveats (nothing in life is a guaranteed success!), and none can solve every problem in the world. For this reason, identifying and understanding both the benefits and limitations of CRISPR-Cas9 will serve us well in the long term.
1. Callaway, E. HIV Fights Off CRISPR Gene-Editing Attack. Nature (2016).
2. Gilbert, L. A. et al. CRISPR-mediated modular RNA-guided regulation of transcription in eukaryotes. Cell 154, 442–451 (2013).
3. Kang, X. et al. Introducing precise genetic modifications into human 3PN embryos by CRISPR/Cas-mediated genome editing. J. Assist. Reprod. Genet. (2016). doi:10.1007/s10815-016-0710-8
4. Hsu, P. D., Lander, E. S. & Zhang, F. Development and applications of CRISPR-Cas9 for genome engineering. Cell 157, 1262–1278 (2014).
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9. Ousterout, D. G. et al. Correction of dystrophin expression in cells from Duchenne muscular dystrophy patients through genomic excision of exon 51 by zinc finger nucleases. Mol. Ther. 23, 523–32 (2015).
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11. Doudna, J. My whirlwind year with CRISPR. Nature 8–10 (2015). doi:10.1038/528469a