In Brief
Researchers from UC San Diego's School of Medicine have tested a modified CRISPR-Cas9 technique designed to target RNA instead of DNA. Rcas9 could potentially improve the lives of patients with ALS, Huntington's disease, or myotonic dystrophy by delaying the progression of their disorders.

Editing RNA

The most efficient and effective gene-editing tool in use today is CRISPR-Cas9. Just this year, researchers have successfully used it for a wide variety of experiments, from modifying garden vegetables to encoding a GIF in bacterial DNA. Most recently, the tool was used to remove a genetic disease from a human embryo.

Although undeniably powerful, CRISPR-Cas9 does have its limitations; it can only target DNA. To extend its capabilities to include RNA editing, researchers from the University of California San Diego (UCSD) School of Medicine developed a modification of CRISPR, and they’re calling their tool RNA-targeting Cas9 (RCas9).

In a study published in Cell, the UCSD team tested their technique by correcting the kinds of molecular mistakes that cause people to develop microsatellite repeat expansion diseases, such as hereditary amyotrophic lateral sclerosis (ALS) and Huntington’s disease.

crispr-cas9 rcas9 als gene editing
Muscle cells before (L) and after (R) receiving RCas9 treatment. Image Credit: UCSD

During standard CRISPR-CAs9 gene editing, a “guide” RNA  is instructed to deliver a Cas9 enzyme to a specific DNA molecule. The researchers from UCSD instead instructed it to target an RNA molecule.

Tests conducted in the laboratory showed that RCas9 removed 95 percent of problem-causing RNA for myotonic dystrophy types 1 and 2, Huntington’s disease, and one type of ALS. The technique also reversed 93 percent of the dysfunctional RNA targets in the muscle cells of patients with myotonic dystrophy type 1, resulting in healthier cells.

“This is exciting because we’re not only targeting the root cause of diseases for which there are no current therapies to delay progression, but we’ve re-engineered the CRISPR-Cas9 system in a way that’s feasible to deliver it to specific tissues via a viral vector,” senior author Gene Yeo, a cellular and molecular medicine professor at UCSD School of Medicine, explained in a press release.

Improving Countless Lives

Across the globe, an estimated 450,000 patients are said to be living with ALS. Roughly 30,000 of those are from the U.S. where 5,600 people are diagnosed with the diseases every year. The exact number of Huntington’s disease cases, however, isn’t quite as easy to pin down. One estimate says that around 30,000 Americans display symptoms of it, while more than 200,000 are at risk.

Regardless of the exact numbers, these two neurological diseases clearly affect a significant number of people. This prevalence and the absence of a known cure makes the UCSD team’s research all the more relevant. Even more exciting is the fact that the same kinds of RNA mutations targeted by this study are known to cause more than 20 other genetic diseases.

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“Our ability to program the RCas9 system to target different repeats, combined with low risk of off-target effects, is its major strength,” co-first author of the study Ranjan Batra said in the UCSD press release.

However, the researchers do know that what they’ve accomplished is just a first step. While RCas9 works in a lab, they still have to figure out how it will fare when tested in actual patients.

“The main thing we don’t know yet is whether or not the viral vectors that deliver RCas9 to cells would elicit an immune response,” explained Yeo. “Before this could be tested in humans, we would need to test it in animal models, determine potential toxicities, and evaluate long-term exposure.”

Ultimately, while RCas9 couldn’t exactly deliver a cure, it could potentially extend patients’ healthy years. For disease like ALS and Huntington’s, that’s a good place to start.