The active genes in the body have to be copied, or transcribed into RNA molecules, which are then processed in a variety of ways before being translated into proteins. One processing mechanism involves removing portions of genes that do not code for protein. The sequences of genes in the genome include both exons, which are the parts that can code for protein, and introns, which are removed, or spliced out by the cell before the sequence is translated into protein.
In recent years, scientists have found ways to force the cell skip over exons that may include mutations that cause disease. This method is known as SPLICER, and it could be applied in situations where toxic proteins are produced by the cell, which causes disease. It may be useful as a treatment in cases of Duchenne's muscular dystrophy or Huntington's disease, for example. Scientists have also now shown that it could potentially be applied in Alzheimer's disease. The work has been reported in Nature Communications.
SPLICER is a modification of the well known gene-editing technique known as CRISPR, in which a guide RNA takes a DNA-cutting enzyme called Cas9 to a specific portion of the genome. But the guide RNA has to be designed in very specific ways. In SPLICER, a modified version of Cas9 is applied, which is not limited by the same constraints as the previous enzyme. This allows it to target many different sequences. One of those sequences is in a gene that is related to Alzheimer's.
"Exon skipping only works if the resulting protein is still functional, so it can't treat every disease with a genetic basis. That's the overall limitation of the approach," noted senior study author Pablo Perez-Pinera, a professor at the University of Illinois Urbana-Champaign (U. of I.).
"But for diseases like Alzheimer's, Parkinson's, Huntington's, or Duchenne's muscular dystrophy, this approach holds a lot of potential. The immediate next step is to look at the safety of removing the targeted exons in these diseases, and make sure we aren't creating a new protein that is toxic or missing a key function. We would also need to do longer term animal studies and see if the disease progresses over time."
In this study, researchers improved upon SPLICER so that it would skip over extremely specific regions of DNA.
"With current exon-skipping techniques, sometimes not all of the exon gets skipped, so there's still part of the sequence we don't want expressed," noted first study author Angelo Miskalis, a graduate student at U. of I. "We wanted to prevent that."
The investigators engineered SPLICER so that it would target both the start and end of a specific sequence, instead of previous approaches that have aimed for either the start or end.
Since the amyloid precursor protein (APP) gene has been very well characterized because of its association with Alzheimer's disease, the scientists targeted a particular region or exon in this gene. This exon is the area where a cleavage event takes place, leading to the formation of aberrant proteins in Alzheimer's.
When this approach was tested in neurons growing in culture, the amyloid plaques that are a hallmark of Alzheimer's stopped forming. In a mouse model, the treatment reduced plaque formation by about 25 percent compared to untreated mice. While much more research will be needed, this is a promising first step in a new appraoch to treating Alzheimer's disease.
Sources: University of Illinois Urbana-Champaign, Nature Communications
