CRISPR for Treating ALS - A Long and Bumpy Road Ahead
One of the most remarkable cases of amyotrophic lateral sclerosis (ALS) was Stephen Hawking, a renowned physicist for his research on black holes. He lived for more than 50 years with ALS because it is believed that he possessed a variant of the disease.1 Unfortunately, the vast majority of patients diagnosed with ALS will not live past three years and seldom five years.2 Over time, affected individuals succumb to respiratory failure.1 4,000 people in Canada alone are living with ALS and the disease is prevalent in 2 per 100,000 people.3 With the introduction of the Ice Bucket Challenge, a social media-based charity campaign that helped raise awareness and funding for research, the acronym ALS became a household word. The challenge was a much-needed call to action because there are no cure or treatment options to stop the progression of ALS.
ALS can also occur in people with or without a family background of the disease. In fact, only 10% of all cases are familial, and the remaining 90% are sporadic.4 Since the majority of cases are sporadic in nature, it amplifies the challenge to find a cure because it must be flexible enough to target random defects. That is why drugs, which can vary in degree of effect and have a specific mode of action, are not very useful. Nevertheless, the United States Food and Drug Administration (FDA) has approved two drugs for ALS, Rilutek, which extends the lifespan of ALS patients by months, and Radicava, which slows deterioration.4 Unfortunately, the two drugs are not curative. Nearly 150 years have passed since the first documented case of ALS and over 70 years since baseball player Lou Gehrig, who ALS was eponymously named after, was diagnosed.5 After all this time, have we come closer to finding a cure?
At the end of 2017, researchers at the University of California, Berkeley took the first step in developing a cure for ALS that meets the specificity requirement using CRISPR.6 Working on mice and targeting the SOD1 gene shown to be related to familial ALS, the researchers managed to increase the survival of ALS mice, delayed onset of disease, improved motor function, and decreased muscle atrophy.6 If this therapy translates to humans, it would greatly improve the quality of life for ALS patients. Unfortunately, the solution to one problem is another problem.
CRISPR is not safe for human use yet. Although CRISPR is the talk of the scientific community because of its potential to treat various genetic diseases, there are possible off-site effects.7 Off-site effects occur when Cas9 cuts at a wrong part of the DNA, which can cause unpredictable mutations (Figure 1). This becomes a concern in clinical applications where an off-site mutation can potentially knockout a gene responsible for the regulation or production of a necessary protein. Current research is trying to work around this problem by altering the Cas9 protein to make single-stranded cuts (nickase) rather than a double-stranded break (Figure 2).7 Two different Cas9 proteins with different sgRNAs that cut on opposite sides of a DNA duplex are required to make the change to the DNA.7 For an off-site mutation to occur, both Cas9 proteins must cut incorrectly at opposite sides of the DNA. If only one of the Cas9 cuts incorrectly, the single-stranded break would be resealed, and no change would occur.
Researchers are attempting to further increase specificity by working on reprogramming the Cas9 nuclease to target a new and more specific PAM sequence.8 The PAM sequence is a short three-nucleotide sequence that is needed for Cas9 to recognize the target site.7 This can be a limitation if the target gene does not have a PAM region. However, one group was able to modify the normal Cas9 protein to recognize other PAM sites as well as improve specificity in human cells.8
Finally, immunogenicity is also an obstacle to getting CRISPR from benchtop to bedside. The bacterial origin of the CRISPR-Cas9 system means that the structural proteins will likely activate human immune responses.9 The same applies to the delivery mechanism of the CRISPR-Cas9 system into cells.9 Viral vectors are often used as delivery systems into cells because viruses naturally transfer their genetic material into host cells while avoiding the immune system. However, if the viral vector is detected as foreign, it will compromise the effectiveness of the therapy. One way to circumvent this is by the addition of a compound called polyethylene glycol (PEG). PEGylation is known to prevent antigens on viral vector surfaces from being detected by the immune system.10
The use of CRISPR to cure ALS is promising since no cure currently exists. Research should now focus on the shortcomings of CRISPR to prevent off-site mutations and immunogenicity. Overcoming these obstacles will make CRISPR a safer and more reliable method for clinical applications on ALS patients. It is also important to continue searching for genes associated with the disease. The finding of these genes will provide more potential targets for CRISPR therapy.11 Although still in its infancy, CRISPR will likely play an important role in future ALS research.
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4. Amyotrophic Lateral Sclerosis (ALS) Fact Sheet. National Institute of Neurological Disorders and Stroke. https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Fact-Sheets/Amyotrophic-Lateral-Sclerosis-ALS-Fact-Sheet.
5. What is ALS. The ALS Association. http://www.alsa.org/about-als/what-is-als.html.
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11. Reuters Staff. Ice Bucket Challenge Credited with ALS Breakthrough. Scientific American. https://www.scientificamerican.com/article/ice-bucket-challenge-credited-with-als-breakthrough/.
12. O’Geen H, Yu AS, Segal DJ. How specific is CRISPR/Cas9 really? Curr Opin Chem Biol. 2015; 29, 72-78.
13. Peng RX, Lin GG, Li JM. Potential pitfalls of CRISPR/Cas9-mediated genome editing. FEBS J. 2016; 283, 1218-1231.
Cite This Article:
Chan D., Chan G., Palczewski K., Lewis K., Ho J. CRISPR for Treating ALS - A Long and Bumpy Road Ahead. Illustrated by G. Gupta. Rare Disease Review. February 2019. DOI:10.13140/RG.2.2.12995.25120.