Brief Overview
CRISPR is a highly specific and efficient genome editing technology. CRISPR facilitates the addition, deletion, or alteration of an organism’s DNA (NIH). If you would like a more specific explanation of CRISPR, feel free to watch the 1:39 video from the Mayo Clinic below.
As one can imagine, this is exciting and simultaneously, terrifying technology. It holds incredible potential for countless rare, genetic diseases. The concept of curing a disease by removing a faulty gene, is not new, but CRISPR is definitely one of the more cost-effective and reliable ways to do so. Thus far, CRISPR technology has been tested on many rare disease areas, including cancers, sickle cell anemia, Huntington’s Disease, Alzheimer’s Disease, and ALS. In 2018, Target ALS provided CRISPR Therapeutics with a 2-year grant to study ALS and Frontotemporal Dementia (FTD). We are excitedly awaiting those results.
Current CRISPR Challenges
The success of CRISPR technology depends on the structural validity of in vivo disease models, meaning how well the animal model of the disease actually portrays the disease in humans. The ALS field has historically struggled with this issue. CRISPR success also depends on the capabilities of high throughput genome mapping technology, meaning, how much knowledge we have about the genome we will be editing. As we observe these two key elements (animal model validity and genome mapping technology) becoming more efficient and reliable, we will begin to see very obvious therapeutic results from CRISPR.
Another roadblock to CRISPR’s usage in ALS is the fact that sporadic ALS is a messy disease. Messy meaning there are often a number of genes that contribute to the disease manifestation. Not only that, but these genes are not all ‘bad’ genes, but they are mutated in some way. For example, the protein C9orF72 is indicated in both familial and sporadic ALS. The normal function of this gene is to “influence the production of RNA from genes, the production of proteins from RNA, and the transport of RNA within the cell” (NIH). In ALS, C9orF72 is expressed as a hexanucleotide repeat expansion. If you program CRISPR technology to eliminate C9orF72, you may eliminate ALS, but you will also eliminate all the normal function of the protein, which could be deadly.
CRISPR Breakthroughs
Treatment of a Mouse Model of ALS by In Vivo Base Editing
In this study done at the University of Illinois, ALS disease progression was slowed in mice when CRISPR was applied. Researchers utilized cytidine base editors (CBEs) to remove SOD1 gene expression. “CBEs are a type of CRISPR single-base editors that change specific nucleotide bases in a DNA sequence” (ALSNewsToday). The result of using this type of CRISPR technology is that it does not delete the SOD1 gene, but rather it induces a ‘stop’ signal early in the gene expression so that it cannot progress to be a mutant SOD1 gene. Very cool.
The most common CRISPR headline you will see in relation to ALS is this one:
CRISPR reveals possible ALS drug target
In this particular study, a research team at Stanford used CRISPR to effectively ‘knockout’ or cut random genes out of the genome so that they observe “genes that protect neurons against the toxic effects of protein aggregates by being inactivated” (FierceBiotech). Their study revealed that “knocking out a gene called TMX2 prevented cell health in mouse neurons.” In the ALS world right now, finding possible therapeutic targets is a huge deal and most researchers are hard at work knocking out genes to do so.
The final, and arguably the most exciting, CRISPR breakthrough is highlighted in these two articles:
CRISPR-based method allows for reversible RNA editing
RNA-targeting CRISPR could yield treatment for Huntington’s and ALS.
The methodology behind this concept is mind-blowing. This article is from 2017, but they are still fine-tuning the science today. Researchers at the Broad Institute have dubbed the technology: REPAIR (RNA Editing for Programmable A to I Replacement). How does it work? To start, one has to understand the basics of transcription and translation. Here is a crude summary: DNA is transcribed to RNA which is then translated into proteins. This technology does not cut DNA, but instead, it binds to RNA and effectively changes one nucleoside in the RNA helix. Therefore, when RNA goes to translate into a protein, it will encode for a properly functioning protein instead of a mutant protein involved in disease progression. Amazing.
Woof, that was a lot of science. Thank you reading if you made it to the end. To conclude: CRISPR Technology holds incredible potential for ALS and other rare diseases. Researchers just need to perfect it in a lab so that when it is applied to humans, no one comes out missing a limb :).