CRISPR-Cas is a defense system that evolved in single-celled organisms to target and destroy viruses. Researchers have harnessed the precision-targeting nature of this exquisitely sensitive system to edit specific sections of DNA and RNA, from any organism, in a wide range of applications.1 From eradicating malaria through gene drives in wild mosquitoes,2 to engineering food crops resistant to disease and climate change,3 the possibilities seem endless—especially in therapeutic medical applications to cure diseases.
It took decades of discoveries by may different scientists from across the globe to develop the CRISPR-Cas techniques that are applied today in biomedical research and that are FDA-approved as precision medicine therapies. It’s been a long but promising journey that began when CRISPR was first discovered in 1993 by Francisco Mojica.4 By 2012, two separate research teams had reported how to harness its power with Cas9-mediated cleavage to precisely edit mouse and human genomes.4 Many other teams contributed additional nuggets of understanding for how the CRISPR-Cas system works—and can be manipulated.4
CRISPR-Cas techniques have fundamentally changed how researchers envision cures tied to gene therapy. The genetic editing process is so precise, it’s been widely compared to using a pair of scissors. Its cost-effectiveness and speed also make it highly attractive. Medical therapies based on CRISPR-Cas techniques have already successfully cured people of certain kinds of diseases, with the promise of more on the horizon.
Existing and emerging therapies
Today, CRISPR-Cas-therapies are being developed for both inherited and non-inherited diseases.5 These techniques are being tested in clinical trials to cure specific blood diseases (sickle cell anemia and beta thalassemia)6 and eye diseases (Leber congenital amaurosis, retinitis pigmentosa) in some patients.5 A gene therapy that used CRISPR to cure retinal dystrophy that is specifically due to a biallelic mutation in gene RPE65 was approved by the Food and Drug Administration in 2017. Its longterm effects are still being evaluated.5
Current investigational applications include:5
- B-cell acute lymphoblastic leukemia
- Lung and esophageal cancers
- Multiple myeloma
- Cervical cancer
- Alzheimer’s disease
- Huntington’s disease
- Duchenne muscular dystrophy
Cures for non-genetic diseases
- Autism spectrum disorder
CRISPR has deep potential in medical applications. But there are a few limitations that researchers are tackling to unleash its full power to the broadest number of people.
Of particular interest is how a person’s immune system responds to aspects of this therapy. Some people’s immune system reacts to the viral capsid that functions as a cargo-delivery shell to usher the editing tools into a cell (eg, adeno-associated virus therapies).1 An immune limiting response is more common if a person needs to undergo the therapy more than once;1 but it can exclude some patients from being able to receive the treatment.7
An innovative approach that is underway to side step this problem is to engineer a different delivery system, such as biodegradable synthetic lipid nanoparticles.1,7 But sometimes a person’s immunity is also triggered by the Cas9 proteins themselves. This is spurring an innovative search for synthetic variants.7
Most CRISPR gene editing approaches to curing diseases have so far focused on conditions caused by single genes, which makes sense as the technique is being developed. But many diseases involve multiple genes and their interactions. In the future, it’s very likely that existing limitations will be engineered into the rear review mirror, and that tomorrow’s CRISPR therapies will target multiple genes and the epigenome.
Current research is already paving the way for this to become reality.
What the future holds
Diversification of the distinct classes of CRISPR-targeting systems is expanding.8 Researchers at ETH Zurich are working on an update to the CRISPR system that substitutes the Cas12a enzyme for the more commonly used Cas9 enzyme. This switch allows researchers to edit genes in 25 targeted sites simultaneously, delivered on a single plasmid. In the future, dozens or even hundreds of sites could be edited simultaneously using this approach. This would allow “a powerful platform to investigate and orchestrate the sophisticated genetic programs underlying complex cell behaviors,” the researchers report.9
Future CRISPR-based therapies may shift emphasis not just from single to multiple genes, but also to editing applied to the epigenome.7 Researchers at Duke University are experimenting with an approach that uses multiple proteins and is centered on a process called CRISPR-associated complex for antiviral defense (CASCADE). This approach allows for the activation and repression of targeted gene expression, and it offers a way around undesirable issues with a patient’s immunogenic response.1
But the potential of CRISPR-based therapies in precision medicine are hampered somewhat by the regulatory process. “Current regulatory models that require large numbers of patients to establish safety and efficacy are not applicable to curative technologies that address a mutation that is found in a single patient or very few patients,” according to Karen Bulaklak and Charles Gerbach, who recently wrote a commentary in Nature Communications on the future of gene therapy.8 For CRISPR-based therapies to become widely available, and for personalized gene-based therapies to become a reality, regulatory changes may be needed.
Demonstrating safety and efficacy is still important, even if a trial consists of 1 patient. But the approval process could be simplified by green-lighting “drugs that have an established platform . . . but with different underlying nucleic acid targets,” according to a team of researchers who recently summarized the approaches and challenges in translating CRISPR-Cas therapies.10
Several CRISPR-based drugs are working their way through the drug development pipeline, and we will likely be hearing news of their achievements and new applications for quite some time.
- Drug Target Review. How CRISPR will evolve in the future. Available at https://www.drugtargetreview.com/article/52485/how-will-crispr-evolve-in-the-future/. Accessed 9/7/2021.
- Bier E, Sober E. Gene editing and the war against malaria. American Scientist. Available at https://www.americanscientist.org/article/gene-editing-and-the-war-against-malaria. Accessed 9/7/2021
- Zaidi S SeA, Mahas A, Vanderschuren H. et al. Engineering crops of the future: CRISPR approaches to develop climate-resilient and disease-resistant plants. Genome Biol. 2020;21:289. https://doi.org/10.1186/s13059-020-02204-y
- Broad Institute. CRISPR Timeline. Available at https://www.broadinstitute.org/what-broad/areas-focus/project-spotlight/crispr-timeline. Accessed 9/7/2021.
- Prabhune M. Diseases CRISPR Could Cure. Synthego. 3/23/2021. Available at https://www.synthego.com/blog/crispr-cure-diseases. Accessed 9/7/2021.
- Frangoul H, Altshuler D, Cappellini MD, et al. CRISPR-Cas9 Gene Editing for Sickle Cell Disease and β-Thalassemia. N Engl J Med. 2021;384(3):252-260. doi:10.1056/NEJMoa2031054
- Pickar-Oliver A, Gersbach CA. The next generation of CRISPR-Cas technologies and applications. Nat Rev Mol Cell Biol. 2019;20(8):490-507. doi:10.1038/s41580-019-0131-5
- Bulaklak K, Gersbach CA. The once and future gene therapy. Nat Commun. 2020;11(1):5820. Published 2020 Nov 16. doi:10.1038/s41467-020-19505-2
- Campa CC, Weisbach NR, Santinha AJ, Incarnato D, Platt RJ. Multiplexed genome engineering by Cas12a and CRISPR arrays encoded on single transcripts. Nat Methods. 2019;16(9):887-893. doi:10.1038/s41592-019-0508-6
- Tay LS, Palmer N, Panwala R, Chew WL, Mali P. Translating CRISPR-Cas Therapeutics: Approaches and Challenges. CRISPR J. 2020;3(4):253-275. doi:10.1089/crispr.2020.0025