When CASGEVY, the first CRISPR-based therapy, was approved by the FDA in December, it gave CRISPR technology the validation it needed. Any promise it had held before that moment was simply investigational. In this case, the approval of CASGEVY was granted for the treatment of two blood disorders – sickle cell disease (SCD) and beta-thalassemia. But what if CRISPR could also become a vital tool in treating neurodegenerative diseases?
Neurodegenerative diseases are tremendously difficult to treat. As Cem Zorlular, chief executive officer (CEO) of Er-Kim Pharmaceuticals pointed out, the brain has “billions of neurons and trillions of connections.” Ultimately, it has an extremely complex structure that scientists are yet to fully understand.
Zorlular continued: “This complexity makes it challenging to target specific regions of the brain with treatments. Additionally, neurodegenerative diseases often have a long latent period, meaning symptoms may not manifest for many years after the onset of the disease. This situation complicates the development of treatments that can slow down or prevent the progression of the disease.”
Despite this, a gene editing technique such as CRISPR could be a promising approach. “It’s particularly relevant for neurodegenerative diseases like Alzheimer’s and Parkinson’s, characterized by ongoing loss of neurons and nerve networks, leading to movement disorders, cognitive issues, and speech and breathing difficulties,” explained Zorlular. “Alzheimer’s, for instance, is linked to mutations in specific genes like APP, PSEN1, and PSEN2. CRISPR can edit these genes, potentially addressing the underlying causes of these diseases”.
At this moment in time, the research around using CRISPR for neurodegenerative diseases is still very much in the early stages. In this article, we explore several studies evaluating how CRISPR could be used to treat the two most common neurodegenerative diseases, Alzheimer’s disease and Parkinson’s disease, as well as the rare, inherited condition known as Huntington’s disease.
Two CRISPR-based approaches for Alzheimer’s disease unveiled at Alzheimer’s Association International Conference
At the Alzheimer’s Association International Conference (AAIC) in Amsterdam last year, researchers unveiled two methods of utilizing CRISPR for Alzheimer’s disease, both of which looked at ways genes can increase the risk of developing Alzheimer’s, and how editing those genes could cut the risk of developing the disease or protect the brain from the build-up of amyloid, which is believed to be a unique cause of the disease.
The first study came from researchers at the University of San Diego. They developed a gene-editing strategy using CRISPR that targets the amyloid precursor protein (APP) – a protein that is responsible for an overproduction of beta-amyloid in the brain, which eventually leads to the plaque build-ups that are a hallmark of Alzheimer’s. The APP gene produces different products, some of which are protective, while others are associated with pathology, like beta-amyloid. In their approach, the scientists aimed to decrease beta amyloid production while simultaneously increasing neuroprotective actions.
The researchers conducted experiments on mice with Alzheimer’s disease in order to test their strategy. Here, they found that their CRISPR treatment resulted in a reduction in beta-amyloid plaques and a decrease in brain inflammation markers. They also saw an increase in neuroprotective APP products, as well as improvements in behavioral and nervous system function. On top of this, the gene editing did not cause any undesirable side effects in healthy mice.
Meanwhile, in the second study, Duke University researchers developed another CRISPR approach, this time targeting a genetic risk factor for the neurodegenerative disease called APOE-e4. Inheriting this gene increases the likelihood that someone will develop Alzheimer’s. Although the presence of the gene is not a guarantee that a person will get the disease, people who have one copy have a two- to three-fold higher risk of getting it, while two copies increase the risk eight- to twelvefold.
In this study, researchers used an epigenome therapy platform based on the CRISPR/dCas9-editing strategy to lower the levels of APOE-e4. The lead candidate from this platform was found to reduce APOE-e4 levels in human induced pluripotent stem cell-derived miniature brains from an Alzheimer’s patient, as well as in humanized mouse models. Furthermore, the approach did not affect the levels of other APOE variants believed to have a neutral or protective effect.
Of course, these studies are still in their very early stages. But with recently approved Alzheimer’s treatments causing potentially serious side effects, finding more innovative approaches to treat the neurodegenerative disease, like the ones in these CRISPR studies, is vital.
CRISPR for Parkinson’s disease
Parkinson’s affects a type of neuron called dopaminergic neurons, which are found in the substantia nigra region of the midbrain and are essential for voluntary movement and behavioral processes. The vast majority of Parkinson’s cases occur sporadically; only 10% are genetically inherited.
Aggregates of misfolded ɑ-synuclein protein, also known as Lewy bodies, are thought to be involved in the underlying pathophysiology of Parkinson’s, and ɑ-synuclein is abundant in dopaminergic neurons. The expression of α-synuclein is also closely linked to SNCA gene, which is one of the most important predictive locations for sporadic PD. The mutation called Ala53Thr (A53T) in SNCA is recognized as one of the most prominent risk factors for early-onset PD.
Relating to this, a study was conducted in 2022, which showed that using the CRISPR-Cas9 system to delete the A53T-SNCA gene significantly improved conditions related to Parkinson’s disease, such as the overproduction of α-synuclein, reactive microgliosis, dopaminergic neurodegeneration, and motor symptoms associated with Parkinson’s.
For the most part, though, genetic variants linked to most sporadic Parkinson’s cases are unclear, and molecular mechanisms of Parkinson’s progression are yet to be completely understood. The challenge with studying the underlying mechanisms of Parkinson’s is the complexity of multiple genetic mutations that may be involved. But this is also an area where CRISPR can be well-utilized. It can help to screen genetic variants in Parkinson’s to determine its cause and can be effectively used to develop cellular and whole-organism research models to study the Parkinson’s phenotype.
For example, a study conducted at the University of Pittsburgh leveraged Synthego’s (who provide engineered cells and CRISPR kits) genome editing potential to generate NADPH oxidase (NOX1, NOX2, and NOX4) knockout cell models for Parkinson’s. The study demonstrated the role of the Nox2 enzyme in oxidative stress-related degeneration, including ɑ-synuclein accumulation, protein import impairment in mitochondria, and the activation of leucine-rich repeat kinase 2 (LRRK2).
Ultimately, at the minute, most of CRISPR’s potential for Parkinson’s lies in genetic screening, but this could change once scientists begin to better understand the underlying mechanisms of the neurodegenerative disease.
Huntington’s disease: a promising candidate for gene therapy
Because Huntington’s is an inherited neurodegenerative disorder caused by a single mutation and the presence of an abnormal protein, it makes it the perfect candidate for CRISPR gene editing. Huntington’s is caused by trinucleotide (CAG) expansions in the Huntingtin (HTT) gene resulting in long stretches of the amino acid glutamine in the huntingtin protein. The hyperexpansion of CAG repeats (40 or higher) leads to the onset of Huntington’s symptoms.
One of the most promising and most recent CRISPR studies – conducted last year – for Huntington’s disease, came from researchers at Jinan University. They were able to demonstrate that CRISPR-Cas9 editing can be used to correct the mutation in HTT, replacing the hyperexpansion with a normal CAG repeat. The gene therapy was packaged into an AAV vector, and was delivered to pigs by either intracranial or intravenous injection. A single treatment resulted in the depletion of mutant huntingtin protein and a reduction in neurotoxicity and related symptoms. This preclinical research is very promising and could hopefully progress into clinical trials.
Another recent proof-of-concept study, which used CRISPR-based RNA editing, from researchers at Johns Hopkins and UC San Diego, demonstrated a significant depletion of mutant transcripts in three different induced pluripotent stem cells (iPSC) lines derived from Huntington’s disease patients, each carrying a different number of repeats. Subsequent in vivo experiments in a mouse model of Huntington’s disease demonstrated that treated mice had significantly improved motor function – an effect that continued for up to eight months in the mice, suggesting lasting therapeutic benefits.
The reason scientists used a CRISPR-based RNA editing approach in this particular study is because the mutant messenger RNA (mRNA) transcripts that are produced in Huntington’s disease have been found to significantly contribute to disease pathogenesis, which makes them a good target for treatment.
Hurdles to overcome
As previously mentioned, neurodegenerative diseases are tricky to treat and, although CRISPR is showing promise for some of these diseases, there is still a long way to go.
“Scientists still do not fully understand the mechanisms behind most neurodegenerative diseases,” commented Buse Baran, commercial operations analyst and gene therapy expert at Er-Kim Pharmaceuticals. “This lack of understanding makes it difficult to develop targeted treatments addressing the root causes of the diseases. Moreover, neurodegenerative diseases typically affect a wide range of cells in the brain, making it challenging to target treatments exclusively to the affected cells.”
She also added that the delivery of treatments to the brain is another significant challenge because the blood-brain barrier hinders the passage of most substances into the brain, making it difficult to deliver treatments effectively.
CRISPR is also still a relatively new technology; one which has only just proven its potential to the world. Baran said that, although it holds great potential, it still requires further development, and challenges remain around the risk of off-target effects, ethical concerns, unknown long-term safety, and delivery hurdles.
However, Baran also noted that the approval of CASGEVY and the demonstrated success of a CRISPR-based treatment in clinical trials – in terms of both efficacy and safety – will provide a positive foundation for other potential CRISPR-based therapies. “CRISPR is a versatile technology that is becoming increasingly efficient and precise, capable of being utilized in the treatment of a wide range of diseases…As the efficacy and safety data of CRISPR technology continues to improve, the likelihood of obtaining approvals will also increase.”
So, as more successful studies continue to be conducted testing CRISPR in the treatment of neurodegenerative diseases, it is very possible that we may see a number of CRISPR-based treatments enter the clinic for diseases like Alzheimer’s, Parkinson’s, and Huntington’s, in the near future.