CRISPR-Cas9, the Nobel Prize-winning gene-editing technology discovered by Jennifer Doudna and Emmanuelle Charpentier, is on the verge of entering clinical trials for Alzheimer’s disease. However, experts emphasize addressing the obstacles and wonder if scientists are rushing too hastily.
In July 2023, at the Alzheimer’s Association International Conference (AAIC), researchers from the University of California San Diego (UCSD) and Duke University School of Medicine presented two strategies to utilize CRISPR to prevent and treat Alzheimer’s in Amsterdam.
Brent Aulston’s team at UCSD targets APP.
Brent Aulston’s team at UCSD is utilizing CRISPR to alter the amyloid precursor protein (APP). AAP is a transmembrane protein expressed in numerous bodily tissues but mainly exists in the synapses of neurons. Aulston stated that human populations have shown that all hereditary factors contributing to Alzheimer’s impact how APP is processed. One or two enzymes physically cleave APP inside the cell. Alpha secretase is the first one of these enzymes. The breakdown of APP via alpha-secretase cleaves releases a protective molecule known as secreted APP-alpha. The bulk of APP in healthy individuals breaks down to create secreted APP-alpha. In Alzheimer’s patients, however, APP is driven into the opposite cleavage pathway, mediated by beta-secretase. As a result, it produces toxic fragments of amyloid-beta peptides. Aulston’s team is leveraging CRISPR technologies to modify the APP gene to get more alpha and fewer beta cuts. Aulston stressed that the goal is not to eliminate the APP protein entirely. He claimed that this protein plays a crucial role in the brain and performs various tasks. When removed from a mouse, the mouse shrinks, their brains shrink, there is increased neuroinflammation, and their cognitive function is impaired. Therefore, its expression should continue. So, to prevent the resultant protein from coming into contact with beta-secretase, the researchers devised their CRISPR to edit only a short piece of APP. Upon testing the strategy in mice, the researchers discovered that the animals had less amyloid-beta plaques and indicators of brain inflammation but higher levels of the neuroprotective APP. The mice also improved in behavioral and neurological symptoms.
Aulston believes their possible therapy technique is both safe and effective in mice.
Researchers from Duke University School of Medicine target APOE
Duke University School of Medicine researchers are employing CRISPR-Cas9 gene editing to reduce the expression of the APOE ε4 gene, which is associated with an elevated risk of Alzheimer’s. An estimated 15% to 25% of people have this variant, while up to 5% possess two copies. Ornit Chiba-Falek, professor of neurology at Duke University, division chief of translational brain sciences, and senior author of the study presented at AAIC, agreed with Aulston that the goal is to fine-tune the levels of the various APOE variants rather than completely eliminating the protein, which aids in transporting cholesterol and other types of fat in the bloodstream. For instance, APOE ε2 might offer some defense against Alzheimer’s, whereas APOE ε3, the most prevalent form, is thought to have no effect on the condition. The goal of the Duke team is to lower the synthesis of the APOE ε4-encoded protein.
According to the authors of the abstract presented at AAIC, the platform proved effective and precise in altering APOE ε4 and APOE expressions in humanized mouse models and miniature brains made from human induced pluripotent stem cells (hiPSC) from an Alzheimer’s patient. Additionally, they added, there has been no discernible editing of the ε3 allele in the isogenic hiPSC-derived neurons as a result of the precise targeting of the ε4 allele. Chiba-Falek and Boris Kantor, associate research professor of neurobiology at Duke and the study’s lead author, co-founded a biotech firm, CLAIRIgene, to expand and develop the platform, and Chiba-Falek said they are looking at potential partnerships with the pharmaceutical sector. Next are IND-enabling investigations, which, according to her, will concentrate on safety, administration route, and durability.
What are the challenges?
Aulston claimed that his team had reached the top of the first mountain after proving the notion of APP editing. They are currently at the base of the translational side of the other mountain. Treating a human brain is a little more challenging than treating a little mouse.
Professor Gerold Schmitt-Ulms, who studies Alzheimer’s, tauopathies, and prion illnesses at the University of Toronto, stated his reservations regarding the applicability of CRISPR to neurological disorders. According to him, the technology is “lagging behind” other gene therapy strategies in this field for two key reasons: immunogenicity and problems with delivery.
Schmitt-Ulms addressed the first concern by stating that the adeno-associated virus (AAV) capsids utilized in the delivery of the CRISPR machinery will cause an immune response that may reduce the efficacy of the treatments. According to him, around 70% of people have had exposure to natural AAVs, and as a result, we all have antibodies in our blood that would destroy AAV-based gene therapy vectors. CRISPR enzymes are also unknown to the body. They act as foreign agents; if expressed, your immune system will become more sensitive to them. Most current gene therapy clinical studies for neurodegenerative illnesses include the delivery of a specific overexpressed protein. The body is familiar with these proteins. For instance, the body is aware of APOE ε2 and won’t react negatively to it.
To date, no one has been able to deliver such gene therapies to every human brain cell. The best one can hope for right now is that the therapeutic virus only infects a small percentage of brain cells. Frequently, though, that is insufficient to bring about the desired result.
Tanzi stated that he thinks CRISPR should only be used in exceptional circumstances, such as for those who carry two copies of APOE ε4.
References
- McKenzie, H. CRISPR Shows Preclinical Promise in Treating Alzheimer’s, Challenges Persist. BioSpace. https://www.biospace.com/article/crispr-shows-preclinical-promise-in-treating-alzheimer-s-but-challenges-persist-/. Published Online: 21st Aug, 2023. Accessed: 12th Sep, 2023.
