In a major advance, researchers have successfully corrected genetic mutations in neurons with a single injection into the brains of a mouse model. This technique did not completely reverse the disease caused by the mutation, but it did reduce symptoms and extend the lifespans of these mice, which were modeling a disorder known as alternating hemiplegia in childhood (AHC). Mice carrying the mutations that cause AHC are typically at risk of sudden death. The work has been reported in Cell.
"Five years ago, people would have thought that going into the brain of a living organism and correcting DNA was science fiction. Today, we know this is doable," said study co-author Markus Terrey, a JAX neuroscientist. "Doing this directly in the brain of a living organism is scientifically fascinating. You can go into the brain, fix the mutation, and have the cells corrected for the rest of their life."
This effort aimed to correct two mutations that are known to cause AHC in patients; they occur in a gene called ATP1A3, and the mutations are called D801N and E815K. Mice that carry these mutations have symptoms that are similar to those in patients.
In humans, these mutations can start to cause problems in infancy, and may lead to developmental problems; sudden paralysis that can last a few minutes or sometimes days; eye movement issues; and muscle stiffness. Treatments are limited and don't do much to relieve patients of symptoms.
The challenge with using gene therapy for many other disorders, is that it's hard to correct the mutations before symptoms start. It can be difficult to identify people who need to the treatment before they start to have health problems. Then once the problems have started, the treatment is less likely to fix all of the problems.
In this work, the researchers are using a disorder with obvious, early symptoms to create a treatment strategy that might then be adapted for other diseases.
The investigators used prime editing to change the incorrect letters, or bases, in the DNA sequence, rather than trying to insert a totally new sequence to repair the mutation. They utilized an infectious but nonpathogenic virus to deliver the gene editing reagents to brain cells. The process was performed shortly after the mice were born.
In this approach, prime editing was able to alter 85% of mutations in brain cells to reflect the normal sequence, and create the normal protein from that gene. This led to better motor skills, fewer seizure-like episodes, and longer lifespans in the mouse model. The study also showed that there were few undesired edits to brain cells, suggesting that this method is safe.
Now the scientists want to identify the best age for delivering this treatment in the mouse model, in order to relieve or stop the disease.
"We haven't necessarily reversed the disease, but we've shown we can ameliorate symptoms when treatment was given very early on when the animals were born," Lutz said. "The money shot, which we're working on now, is testing whether we can treat the disease after symptoms appear, when the mice are already showing signs like dystonia and epilepsy. If we can show benefit then, that's a whole new level. That would be a major step forward."
Sources: The Jackson Laboratory, Cell
