Rett syndrome is a lifelong, genetic neurological disorder characterized by severe impairments including regression of speech, the appearance of seizures, and respiratory abnormalities. A hallmark of the disease is constant, repetitive hand movements.

A new study shows that RNA editing may repair the underlying cause of Rett Syndrome in a mouse model. The technology recoded enough RNA to restore half of the normal protein in three different kinds of neurons in the Rett mouse, demonstrating that programmable RNA editing can be utilized to repair mutations in mouse models of neurological disease.

The authors noted that mouse models of Rett syndrome “are ideally suited to test the efficacy of programmable RNA editing in vivo.” The team used a mouse model of a human MECP2 mutation. Rett syndrome is caused by de novo loss-of-function mutations in the gene MECP2, an X-linked transcriptional regulator.

The research is published in Cell Reports in a paper titled, “In Vivo Repair of a Protein Underlying a Neurological Disorder by Programmable RNA Editing.”

Hundreds of different mutations in the MECP2 gene have been found in people with Rett syndrome. The idea behind the strategy used in this new study is to produce a healthy MeCP2 protein by repairing the genetic error in the RNA.

“This was a proof of principle” that the technique works in the brain, said lead author John Sinnamon, PhD, a post-doctoral fellow in the lab of Gail Mandel, PhD, professor, at the Vollum Institute, Oregon Health and Science University.

In 2017, Sinnamon and Mandel reported their first success with the RNA approach, efficiently repairing the Rett mouse mutant RNA in developing neurons in culture.

In the new study, the team expanded on these results. They asked three questions: Is it possible to edit MeCP2 RNA in several different types of neurons in adult mice in vivo? If so, what types of neurons can be edited? Does editing restore MeCP2 protein function?

To address these questions, the researchers packaged a mouse Mecp2 RNA guide and human editing enzyme “editase” in a viral vector and introduced it directly into the hippocampus, the brain structure associated with learning and memory.

The injected editase repaired about half of the RNA produced by the mutant MeCP2 gene in each of three types of neurons located in different regions of the hippocampus and MeCP2 protein function was equally repaired in the neurons.

“It is encouraging that this RNA editing approach seems to be efficacious in different types of neurons in the brain,” said Mandel. A similar repair rate should be achievable throughout the brain, the researchers believe, if a vector can be delivered diffusely throughout the brain.

“In the next set of experiments,” Mandel said, “we will administer the virus by blood, so that the entire brain is subject to editing. This will allow us to monitor whether there is any amelioration of Rett symptoms in the mice.”

As a strategy to restore the normal function of MECP2, the single-base RNA editing approach of swapping out A and G could address about 40% of all known mutations that cause Rett syndrome, Sinnamon said.

The researchers reported that RNA of genes other than Mecp2 also were inadvertently edited, known as off-target effects. It is unknown what, if any, effect the off-target edits had in the mice. The experimental treatment appeared to cause no harm to the mice over the study time period.

As the RNA editing approach progresses through experimental steps, additional questions will need to be answered. “We need to know how much MECP2 RNA we need to repair in an individual cell and in how many cells in the nervous system,” Mandel said.

Her research group and others in the editing field also want to learn more about how to increase editing efficiency while diminishing off-target effects. “There are many bright and determined investigators working on these problems,” she said. “My hope is this paper will stimulate these creative minds even further. We are trying to take advantage of what’s happening in the field in real-time and apply the emerging optimizations to Rett and other neurological diseases.”

“The study is the first example of RNA editing in a mouse model of a neurological disease, and therefore a considerable step forward in the potential of RNA editing becoming a therapeutic for Rett syndrome,” said Monica Coenraads, executive director of Rett Syndrome Research Trust, which helped fund the study.

Graphical abstract for Sinnamon et al.
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