Recent Advances in Medical Gene-Editing

By: Simeon Swaby, Biology & Society ‘26

In a world where new technological advancements often generate immense hype, only to be met with subsequent considerations of limitations and drawbacks, it is common to approach emerging technologies with a critical eye. Around 2012-2014, CRISPR Cas9 was hailed as a brilliant new form of gene-editing that would revolutionize the way diseases are treated by directly removing errant genes [1]. However, despite the initial excitement, CRISPR has since receded from the spotlight, with media attention shifting towards other emerging technologies like AI. Despite its lack of fanfare recently, CRISPR and gene-editing as a whole have experienced a renaissance in technological advancement in the decade since it became widely discussed. I wanted to catalog the recent breakthroughs in gene-editing to see how close we are to its widespread adoption in clinical settings. 

It is important to first lay out each of the major gene-editing technologies. There are currently three major technologies in widespread use: CRISPR-Associated Protein-9 (Cas9), Transcription-Activator-Like-Effector Nucleases (TALEN), and Zinc-Finger-Nucleases (ZFN)[2]. Each technology varies in how they go about editing genomic sequences, but for now, we will be focusing on CRISPR since it is the technology with the most potential to be used clinically. 

CRISPR utilizes a bacteria’s immune system response to carefully cut away portions of the genome [3]. In short, when a bacteria is infected with a virus, the virus’ DNA is integrated within the cell’s for later recognition. When the same virus reappears, the cell uses this previous genomic record to locate and attack the virus. CRISPR works by using a synthetic RNA with a guide sequence, allowing it to attach to the cell’s genome in a similar way to a virus’ sequence. This sequence is then given to a Cas9 enzyme, which identifies the target sequence and cuts it from the genome. Researchers are then able to use the cell’s repair function to edit the cut-off region how they wish.

It is crucial to understand that the repair phase in gene editing, particularly with CRISPR technology, is not as straightforward as simply selecting specific pieces to add to the new sequence. After a sequence is removed, it is typically repaired through one of three main mechanisms: homology-directed repair (HDR), non-homologous end joining (NHEJ), and microhomology-mediated end joining (MMEJ) [4]. Among these mechanisms, NHEJ is the most prevalent, yet it is also the most error-prone, often resulting in unintended mutations during the repair process.These mutations can potentially be minimized by introducing DNA templates into the cell. These templates serve to guide the repair process by specifying the new sequence to replace the removed portion of the original sequence.

Overall, this process has shown potential in treating a variety of genetic diseases, most prominently, sickle-cell anemia. However, there are still a variety of challenges in the technology’s use. The most prominent issue has been improving the specificity of the technology. It remains quite common for the guide sequence to unintentionally attach to the wrong sequence, running the risk of removing sequences which could cause dangerous disruptions to normal gene function [5]. To reduce these occurrences, researchers have developed new techniques to enhance targeting of desired genes. 

These methods are separated into nonrational, rational, and combined approaches. Non-rational methods involve the use of mutagenesis and throughput screening to create mutated proteins like SpartaCas that show higher levels of accuracy in their targets. Rational methods involve the use of structural information and computational modeling to develop new variants of Cas9 with enhanced targeting. Finally, researchers usually use a combination of both methods to achieve desired results. 

Enhancing the specificity of Cas9 will be extremely important going forward as CRISPR is integrated into medicine, but what about replacing the genes that have been removed? DNA templates may be able to lightly influence what sequence is replaced, but is there a way to directly insert the desired genes into a missing sequence? 

 One such technique, base editing, is being pioneered by the biotechnology company Verve Therapeutics. Base editing involves directly swapping individual bases in genes. Instead of excising base C and relying on the cell to replace it with T, researchers can directly exchange base C with T. This therapy is currently undergoing small clinical trials, focusing on a gene associated with the protein PCSK9 and its role in high cholesterol. The adoption of this technology could mark a breakthrough in gene-based treatments [6]. 

Back in December, the US and the UK approved the first CRISPR-based therapy to treat sickle-cell anemia. This therapy reduces the amount of hemoglobin that converts into sickle-shaped cells, representing a significant milestone. Gene editing has made remarkable progress since its initial trials in the 1970s and 1980s [7]. 

So far the biggest roadblock to the clinical use of gene-editing technologies is their prohibitive cost. At the time of writing, most non-CRISPR gene therapies cost between $450,00 to $2 million per treatment, an impossible price to pay for someone from the lower to middle class [8]. This does not even include recurring costs from medication and repeated returns for treatment. This is a problem that will sadly limit treatments to only the highest income earners for the foreseeable future. Hopefully, as time goes on, researchers will be able to find ways to reduce the cost of these treatments so that all can benefit.   

References

1. Pollack, A. (2014, March 3). A Powerful New Way to Edit DNA. The New York Times. https://www.nytimes.com/2014/03/04/health/a-powerful-new-way-to-edit-dna.html

2. Gaj, T., Sirk, S. J., Shui, S., & Liu, J. (2016). Genome-Editing Technologies: Principles and Applications. Cold Spring Harbor Perspectives in Biology, 8(12), a023754. https://doi.org/10.1101/cshperspect.a023754

3. Li, Z.-H., Wang, J., Xu, J.-P., Wang, J., & Yang, X. (2023). Recent advances in CRISPR-based genome editing technology and its applications in cardiovascular research. Military Medical Research, 10(1), 12. https://doi.org/10.1186/s40779-023-00447-x

4. Song, B., Yang, S., Hwang, G.-H., Yu, J., & Bae, S. (2021). Analysis of NHEJ-Based DNA Repair after CRISPR-Mediated DNA Cleavage. International Journal of Molecular Sciences, 22(12), 6397. https://doi.org/10.3390/ijms22126397

5. Huang, X., Yang, D., Zhang, J., Xu, J., & Chen, Y. E. (2022). Recent Advances in Improving Gene-Editing Specificity through CRISPR–Cas9 Nuclease Engineering. Cells, 11(14), 2186. https://doi.org/10.3390/cells11142186

6. Gene editing had a banner year in 2023. (n.d.). MIT Technology Review. Retrieved February 29, 2024, from https://www.technologyreview.com/2023/12/22/1085809/gene-editing-had-a-banner-year-in-2023/

7. Carroll, D. (2017). Genome Editing: Past, Present, and Future. The Yale Journal of Biology and Medicine, 90(4), 653–659.

8. Subica A. M. (2023). CRISPR in Public Health: The Health Equity Implications and Role of Community in Gene-Editing Research and Applications. American journal of public health, 113(8), 874–882. https://doi.org/10.2105/AJPH.2023.307315