Prime editing, which does not induce double-stranded breaks in DNA, can correct AHC mutations in the brains of mice.

For babies born with alternative hemiplegia of childhood (AHC), an extremely rare and severe neurodevelopmental disorder, there may be no obvious symptoms for several months. Then the attacks begin: recurrent bouts of paralysis on one side of the body (hemiplegia), involuntary muscle contractions, and seizures.¹ With no disease-modifying treatments available, patients experience muscle loss and hypomobility, developmental delays, and cognitive impairments.²

In a new study published in Cell, researchers from the Broad Institute and the Jackson Laboratory used prime editing technology to correct the underlying genetic causes of AHC in vivo in a mouse model of the disease.³ With further development, the authors believe this approach could offer real hope for a one-time AHC therapy, with broader implications in the treatment of other neurodevelopmental disorders. “This level of editing efficiency in the brain is really quite remarkable,” coauthor Cathleen Lutz of the Jackson Laboratory said in a press release.

Led by base and prime editing pioneer David Liu of the Broad Institute, the team targeted five different mutations in the ATP1A3 gene, which encodes a transmembrane ion pump crucial for brain development and function. Together, these mutations account for up to 70 percent of AHC cases.

The authors focused on prime editing, a precision genetic engineering technique derived from CRISPR-Cas9 technology. By combining a DNA nickase, a guide RNA, and a reverse transcriptase enzyme, scientists can use prime editing to create almost any type of substitution, deletion, or insertion at a target region of the genome. Also, because it does not induce double-stranded breaks in the genome, this technique is theoretically safer for therapeutic applications.³

The team first tested both base editing and prime editing techniques to correct the mutations in induced pluripotent stem cells (iPSC) derived from AHC patients, then focused on optimizing the latter strategy. Once they had shown correction was possible in vitro, they needed to answer an important question: Assuming they could safely deliver the therapy in vivo to the brain, would it provide an effective treatment for AHC, or was the damage done before birth?

To find out, they put their prime editing approach to the test in two AHC mouse models—one for each of the two most common human AHC mutations. The team packaged the prime editing system in a dual adeno-associated viral vector (AAV)—specifically the AAV9 capsid, which is approved by the FDA as a delivery vehicle for therapeutic applications and can efficiently transduce nerve tissue.

After a single injection of the therapy into the brains of postnatal mice, the team compared the treated group to the untreated AHC animals. They observed high editing efficiencies, with up to 85 percent correction in one model and 46 percent in the other. In both models, the treated mice displayed significant improvements in survival, reductions in the severity of AHC attacks and the frequency of convulsions, and improved recovery from attacks compared to the untreated animals. They also had rescued motor coordination, comparable to wild-type mice. “Before this study, we didn’t even know if we could intervene in AHC after birth in an animal,” said coauthor Alexander Sousa of the Broad Institute in a press release. “Now we know you can.”

The authors suggest that with further research and less invasive delivery methods, their approach could provide a much-needed therapeutic avenue for the treatment of AHC in humans. Further, they say these results could inform the treatment of other diseases caused by ATP1A3 mutations, as well as other neurodevelopmental disorders. “This study is an important milestone for prime editing and one of the most exciting examples of therapeutic gene editing to come from our team,” said Liu in the press release. “It opens the door to one day repairing the underlying genetic causes of many neurological disorders that have long been considered untreatable.”