RARE Daily

Simplifying RNA Editing for Treating Genetic Diseases

February 11, 2022

Bioengineers at the University of California San Diego have found a way to repair disease-causing mutations in RNA without compromising precision or efficiency by engineering a new kind of guide RNA that recruit the cell’s own mechanism to make edits at a precise target RNA region.

The new RNA editing technology holds promise for treating genetic diseases. In a proof of concept, UC San Diego researchers showed that the technology can treat a mouse model of Hurler syndrome, a rare genetic disease, by correcting its disease-causing mutation in RNA. The findings appeared in Nature Biotechnology.

The technology makes efficient use of RNA editing enzymes that naturally occur in the body’s cells. These enzymes are called adenosine deaminases acting on RNA (ADARs). They bind to RNA and convert some of the adenosine (A) bases to inosine (I), which is read by the cell’s translation machinery as guanosine (G).

Researchers have been exploring RNA editing approaches with ADARs to correct the G-to-A mutation behind genetic disorders, such as cystic fibrosis, Rett syndrome, and Hurler syndrome. A big advantage of RNA editing—over DNA editing, for example—is that changes to RNA are only temporary, since RNA has a short lifespan. So even if off-target edits occur, they would be short lived.

To make a targeted A-to-I (or essentially, an A-to-G) edit on RNA using ADARs, a short accessory strand of RNA—called a guide RNA—is needed to guide ADARs to the target and make the desired change there. Traditional guide RNAs are not efficient at using native ADARs in the cell, so they require external ADARs to be brought into the cell to work. But that complicates delivery and makes it more susceptible to off-target effects.

Prashant Mali, a bioengineering professor at the UC San Diego Jacobs School of Engineering, and his colleagues engineered a new kind of guide RNA that is extremely effective at recruiting the cell’s own ADARs to make edits at a precise target RNA region.

“We can simply deliver just a small piece of RNA inside the cell and repair mutations in vivo,” he said. “We don’t have to provide any extra enzymes.”

The team designed the guide RNAs to target the single G-to-A mutation that causes Hurler syndrome. This mutation prevents the body from producing an enzyme that is necessary for breaking down complex sugars. Buildup of these sugars causes severe tissue damage, skeletal abnormalities, cognitive impairment, and other serious health problems. Systemic injection of the guide RNAs into diseased mice resulted in correction of 7 percent to 17 percent of the mutant RNAs after two weeks, as well as a 33 percent decrease in the buildup of complex sugars.

One aspect that makes the new guide RNAs effective is that they are longer than traditional guide RNAs.

“This basically makes them stickier for ADARs already present in the cell to come and bind to them,” said Mali.

Other unique design features make them more stable and precise than traditional guide RNAs. They can last for days and stay on the target RNA region for longer periods of time, whereas RNA in general gets quickly destroyed by the cell. That’s because these guide RNAs are built as circular rather than linear molecules; being circular makes them resistant to the cell’s RNA-degrading enzymes. In terms of precision, these guide RNAs only allow changes at the target A and not at any other nearby As. They do this by folding into loop structures at predetermined spots along the target RNA region, which prevents off-target As from getting edited.

Shape Therapeutics, a Seattle-based biotechnology startup co-founded by Mali, is working to translate this and several other RNA editing technologies developed in Mali’s lab into the clinic.

Author: Rare Daily Staff

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