Researchers said they created a new Crispr-based system to edit RNA instead of DNA in human cells, offering a way around some of the ethical and scientific challenges associated with editing the genome and helping advance a new avenue to potentially treat diseases.
Scientists at the Broad Institute of MIT and Harvard repurposed the Cas13 enzyme in the Crispr system to target and correct disease-causing mutations in RNA in cells. The new RNA-editing system, which the scientists have dubbed Repair, allows the editing of individual RNA letters, correcting a common mutation known to play a role in a number of diseases. They described their research in a paper published in Science Wednesday.
Crispr, which stands for clustered regularly interspaced short palindromic repeats, serves as the immune system of bacteria and is used to defend against viral attacks.
RNA, or ribonucleic acid, is a molecule and is found in all cells. It serves as a messenger, carrying the DNA’s instructions to the cells for making the proteins that are essential for life. Scientists have long believed RNA to be a good therapeutic target, but initial work using the Crispr system has focused on another enzyme, Cas9, and on making edits in DNA, which cause permanent changes to a person’s genome.
Unlike with DNA, cells constantly produce more RNA. As a result, RNA editing carries potentially fewer scientific and ethical risks. For one thing, if “off-target” changes occur, a key area of concern in editing DNA, the RNA edits are potentially reversible. Understanding of the biology of many diseases is also rapidly changing, and RNA editing allows scientists to more easily revise the therapy as more research is done.
“Crispr editing of RNA creates more opportunities for things we can do therapeutically,” says Elizabeth McNally, director of the Center for Genetic Medicine at Northwestern University’s Feinberg School of Medicine, who wasn’t involved in the study but is working on efforts to use the Crispr system to treat forms of muscular dystrophy and other conditions.
Scientists have been looking for ways to modulate RNA in order to treat diseases. Last month, Alnylam Pharmaceuticals , which said it spent 15 years developing its therapy, reported positive results in a trial of a so-called RNA-interference drug, patisiran, which stops production of a disease-causing protein to treat a rare nerve disorder, familial amyloid polyneuropathy. If approved by the Food and Drug Administration, patisiran would be the first commercial product based on RNA interference.
Crispr offers another approach.
In nature, there are many different Cas13 enzymes that bacteria use to target and cut RNA as part of its defense system. Omar Abudayyeh, one of the co-first authors of the Science paper, said the researchers spent more than 18 months studying different Cas13 enzymes to find the most effective to adapt for RNA editing in human cells.
From the Archives
The scientists eventually chose a Cas13 enzyme from the Prevotella bacteria, then combined it with a protein called ADAR2. The Cas13 enzyme targets the desired spot on the RNA, and the ADAR2 protein corrects the disease-causing mutation by changing individual RNA letters, in this case adenosines to inosines.
No cutting is done, avoiding the problem of figuring out how to get the cell to make a proper repair, among the challenges faced when using Crispr systems to cut DNA.
To test whether Repair works, in the lab the researchers created the mutations that lead to two diseases, Fanconi anemia and X-linked nephrogenic diabetes insipidus, introduced the mutations in human cells, and used the Crispr-based Repair system to correct the RNA. The scientists say they are working to adapt the system to do other letter corrections.
Feng Zhang, the senior author on the paper, says he believes that Cas9 and Cas13 are “only the tip of the iceberg. There are likely many more Crispr systems to be discovered that will have therapeutic applications.”
Gene Yeo of the University of California San Diego, who led a team that published a paper earlier this year adapting the Cas9 Crispr enzyme to edit RNA and set up a company to try to bring RNA editing into clinical trials, says researchers still need to overcome a number of hurdles before RNA editing is used in people. For one they must ensure that RNA edits don’t trigger an adverse response by the body’s immune system.
But he says when it comes to developing Crispr therapeutics to treat human diseases, “editing RNA may be more therapeutically amenable than editing DNA.”