On any computer, you can edit a word document with ease. Simply hover your mouse over the misspelled word, click, and replace any incorrect characters. CRISPR-Cas9 is the genetic variation of word processing, allowing humans to edit their DNA (the molecule stored in an organism’s genome, where information is held that instructs the organism how to function) [1, 2]. CRISPR-Cas9 is a protein used by bacteria and archaea (extremely small, single-celled organisms) to cut the DNA of an invading pathogen [1]. Cas-9 uses a record of previously encountered diseases (this record is called CRISPR) to identify the pathogen and then begin its attack [1]. CRISPR-Cas9 can now be engineered to recognize and alter sections of DNA in humans, allowing scientists to fix unwanted mutations in the genome [1]. However, using CRISPR technology to fix human genes is fraught with challenges, pushing what many hope to be an upcoming health aid years into the future [3, 4, 5, 6, 7].
The first issue with CRISPR is damage done to the gene away from the desired cut site, or the section of DNA that needs fixing [3]. This can happen if CRISPR-Cas9 mistakes a section of DNA for the cut site due to similar, but not identical, DNA sequences [4]. These undesired changes in DNA may lead to pathogens such as tumors when done in a large number of cells [4]. While different engineering modifications can help reduce off-site cutting, these changes can restrict what sections of DNA are able to be targeted for gene editing and can increase the size of CRISPR-Cas9, making it more difficult to deliver CRISPR into a cell and correct the genome [5].
Thus another obstacle arises in successfully implementing CRISPR-Cas9: delivering the protein into cells so that it can begin changing the genome [4, 5]. To date, a universal delivery system for CRISPR-Cas9 has yet to be discovered [5]. Instead, the CRISPR-Cas9 delivery methods are highly dependent on the situation, with trade-offs occurring when one method is chosen over another [5]. Microinjection, where needles are used to deliver CRISPR into cells, is highly efficient, controllable and independent of the weight of CRISPR-Cas9 [4, 5]. However, microinjection is not possible in living organisms (in vivo), due to the need of a microscope to guide the needle, greatly reducing its potential use [4]. Potentially viable in vivo delivery methods, such as modified viruses (viral vectors) that can carry CRISPR-Cas9 into a cell also face serious issues. Certain viral vectors can potentially trigger tumors, cause an immune response that removes the virus from the cell or are highly dependent on the weight of CRISPR-Cas9, making it difficult to use in many situations [4]. Nonviral delivery mechanisms, such as lipid nanoparticles (LNPs), which are spheres of lipids that can carry CRISPR-Cas9 into the cell, are not nearly as dangerous as viral vector delivery methods [4]. However, LNPs are inefficient (meaning larger than desired portions of the genome go unedited) and are also affected by the size and weight of CRISPR-Cas9, limiting the environments in which they can be used [4, 5].
Perhaps the largest obstacle in making CRISPR useful is that scientists still do not understand much of the human genome [6, 7]. Imagine trying to edit a massive book without knowing the rules of grammar or how to spell; similarly, editing the human genome is only possible if scientists know how the genome works. Yet, there is still debate even over the number of genes that exist [6]. Furthermore, research has neglected large swaths of the human genome, focusing instead on only a few thousand genes out of the tens of thousands of genes that humans possess [7]. Shockingly, five percent of protein-coding genes generate half of the studies, and based on current trends, it will take at least fifty years before all genes are studied in detail [7]. By not studying these other genes, important therapies are potentially prevented from discovery [7].
Thus, in order for CRISPR to aid humanity in its fight against pathogens, much needs to be improved. Prioritizing the engineering of CRISPR-Cas9, making it more safe and deliverable into the cell, alongside more intensive study of the human genome are required should scientists and doctors want to utilize CRISPR as a large-scale health tool. Until these improvements are made, CRISPR will forever be a tool of tomorrow and humanity will greatly suffer from missing out on such a technology [8].
References:
- Vidyasagar, Aparna. (2018, April 20). What is CRISPR? Livescience. https://www.livescience.com/58790-crispr-explained.html
- Nature Publishing Group. (n.d.). genome. Nature news. https://www.nature.com/scitable/definition/genome-43/.
- Kosicki, M., Tomberg, K. & Bradley, A. (2018). Repair of double-strand breaks induced by CRISPR–Cas9 leads to large deletions and complex rearrangements. Nature Biotechnology, 36, 765–771. https://doi.org/10.1038/nbt.4192
- Yip, B. H. (2020). Recent Advances in CRISPR/Cas9 Delivery Strategies. Biomolecules, 10(6), 839. https://doi.org/10.3390/biom10060839
- Lino, Christopher A., Harper, Jason C., Carney, James P. & Timlin, Jerilyn A. (2018). Delivering CRISPR: a review of the challenges and approaches. Drug Delivery, 25:1, 1234-1257. DOI: 10.1080/10717544.2018.1474964
- Salzberg, S.L. (2018). Open questions: How many genes do we have?. BMC Biol, 16, 94. https://doi.org/10.1186/s12915-018-0564-x
- Stoeger T, Gerlach M, Morimoto RI & Nunes Amaral LA. (2018). Large-scale investigation of the reasons why potentially important genes are ignored. PLOS Biology 16(9): e2006643. https://doi.org/10.1371/journal.pbio.2006643
- Fernandez, Clara Rodriguez. (2021, April 13). Eight Diseases That CRISPR Technology Could Cure. Labiotech. https://www.labiotech.eu/best-biotech/crispr-technology-cure-disease/