Scientists Use CRISPR-Cas9 as ‘DNA Scissors’ to Cut Out Alzheimer’s Gene in Mice, Study Shows
Using the gene editing technology CRISPR-Cas9 like a pair of “DNA scissors,” researchers were able to essentially cut out the beta-secretase 1 (BACE1) gene in a mouse model of Alzheimer’s disease, a study shows.
Deleting this gene — which drives the production of amyloid-β proteins in the brain that can accumulate on the outsides of neurons — resulted in better cognitive function and lower levels of amyloid-beta plaques, a key hallmark of the disease, the researchers said.
Genetics, or the DNA in cells that make up a person’s genes, are known to be a key factor that contribute toward the development of Alzheimer’s. Several genes linked to the neurodegenerative disease have been identified.
New technologies such as CRISPR are allowing scientists to treat diseases with a genetic background directly at the source of the problem: the mutated DNA.
CRISPR/Cas9 is a unique technology that enables researchers to edit parts of the genome — an organism’s complete set of DNA, including all of its genes — by removing, adding, or altering sections of the DNA sequence. Much like a tailor-made suit, genes can be “designed” to adequately and specifically serve the researchers’ needs.
In this study, CRISPR-Cas9 was used like a pair of “DNA scissors” to cut out the BACE1 gene — known to be linked to Alzheimer’s — from nerve cells, or neurons, in the brains of adult mice. Expression of this gene results in the production of an enzyme called BACE1 that breaks down the amyloid precursor protein (APP) into amyloid-beta.
When there is too much amyloid-beta in the brain, the protein sticks together on the outside of neurons to form plaques. These plaques stop brain cells from working properly and eventually result in cell death, contributing to the underlying symptoms of Alzheimer’s disease.
There are some treatments in development, called BACE1 inhibitors, that aim to target this enzyme. These molecules intend to block BACE1 from functioning, and consequently, stop excessive amyloid-beta production and aggregation, or clumping together.
The advantage of using gene editing technologies, like CRISPR-Cas9, is that the gene at the root of the problem — in this case BACE1 — can be “muted” from the cells where it is causing damage.
“We aimed to see whether CRISPR-Cas9, one of the latest developments in biotechnology, can open up a new direction for treating dementia, which is, at present, considered an incurable disease,” Jongpil Kim, PhD, professor at Dongguk University in Korea, and one of the study’s authors, said in a press release.
The researchers used two different mouse models for their experiments: one representing familial Alzheimer’s disease, with five mutations, and a second that expressed a mutated version of the amyloid precursor protein. APP mutations are responsible for one type of early onset Alzheimer’s. They result in a version of APP that doesn’t get processed correctly and leads to the production of more of the plaque-forming amyloid-beta. The team noted that the APP mice “may represent a more relevant model of human Alzheimer’s disease.”
All of the mice received an injection of the BACE1-targeting CRISPR-Cas9 therapy directly into the brain.
Just four weeks after treatment, there was a 70% reduction in BACE1 expression. This resulted in significantly lower amounts of amyloid-beta plaque accumulation and in decreased secretion of amyloid-beta42 — a small beta-amyloid protein fragment known to make up amyloid plaques.
BACE1 expression and amyloid-beta plaque accumulation was even lower when the mice were given two doses of the therapy, the results showed.
Importantly, the researchers noted that “Bace1 expression was more efficiently inhibited by Cas9 … than chemical Bace1 inhibitors.”
The mice were monitored over time and the effects of the treatment were still observed eight and 12 weeks after injection.
Compared with control mice that did not receive the therapy, the treated mice had better cognition, learning capacity, and memory. These results were gathered from the performance of behavioral tests, such as the Morris water maze — used to study spatial learning and memory — and fear conditioning.
The researchers concluded that gene editing technologies certainly have potential when it comes to treating diseases with an underlying genetic cause. However, since gene editing is not reversible, the scientists note that it is essential that there are no “off-target” effects. That means that more research is necessary to ensure that only the gene of interest is being deleted — and that there are no other changes anywhere else in the DNA that could lead to unwanted mutations in essential genes.
To evaluate potential off-target effects, the researchers analyzed all of the DNA in the mice to check for unwanted changes. The team did detect some alterations, but these were in line with previous studies and their numbers were very low.
“These results suggest that Cas9 … activity does not elevate mutation rates or cause gross genomic alterations on a genome-wide scale in vivo [in the lab],” the researchers said. They indicated that any side effects should be minimal.
“Our proof-of-principle studies on in vivo gene editing [in living mice] using non-viral Cas9 nanocomplexes broaden the potential application of CRISPR– Cas9 systems to a number of neurodegenerative diseases,” the scientists said. They noted that they hope to use this strategy to develop treatments for other forms of dementia, such as Parkinson’s disease.
Despite these promising results, the investigators note that much more research is needed before this kind of treatment reaches the clinic.
“More extensive characterization of the timing, dosing, and long-term outcomes of Cas9 treatment is necessary; further safety studies examining the potential off-target and long-term effects in the brains of higher primates are required,” they concluded.