Base Editing Brings Scientists Closer to Treating Mitochondrial Disease
Summary
- Dr. Jose Barrera-Paez published a study in Science Translation Medicine discussing a base editing technique to treat mitochondrial disease.
- The study found that base editing remains imprecise and at higher doses it can introduce detrimental errors.
- There are no cures for people diagnosed with mitochondrial diseases.
Dr. Jose Barrera-Paez’s story with mitochondrial genetics is one of love at first read. As an undergraduate student studying in Spain, he stumbled across some papers about mitochondrial genetics—the genes specific to tiny organelles that produce energy inside our cells. He felt an instant passion.
“I knew I wanted to do something related to mitochondrial genetic engineering,” he said.
That passion led him to the lab of Carlos T. Moraes, Ph.D., a professor of neurology at the University of Miami Miller School of Medicine, where Dr. Barrera-Paez spent the last six years completing his Ph.D. He and his team use a base editing technique to treat mitochondrial disease in an animal model, but they found that base editing remains imprecise and, at higher doses, it can introduce detrimental errors.
The results were published in Science Translational Medicine.
“The field of mitochondrial gene editing is in its infancy still,” said Dr. Moraes. “It was a major breakthrough that we could change bases in mitochondrial DNA, because we couldn’t do that before. But the way we do it is not absolutely precise yet.”
Genetic Mitochondrial Diseases
Genetic diseases typically refer to diseases caused by problems in an individual’s nuclear DNA. These genes influence eye color, height, personality traits and disease risk. But that’s not the only DNA in a person’s body. Cells contain a network of interconnected mitochondria and approximately 1,000 copies of mitochondrial DNA per cell. Mutations in these genes can also cause a variety of illnesses that lead to devastating muscle weakness, neurological problems, heart disease and more.
“As of today, there are no cures available to patients diagnosed with a mitochondrial disease,” said Dr. Barrera-Paez.
Over the past decade or so, researchers have made major progress in editing nuclear DNA. Those strategies don’t always work as well for editing mitochondrial DNA. CRISPR, for example, is the gold standard for nuclear gene editing, but it does not work in mitochondria.
One DNA-editing technique used by the Moraes Lab takes advantage of nucleases to root out and cut sequences of disease-causing mitochondrial DNA. Once it is cut, cellular machinery repairs the remaining nuclear DNA. But because there are so many mitochondrial DNA molecules within each cell, there’s no machinery to repair them. Instead, a broken DNA molecule will disintegrate. The remaining healthy mitochondrial DNA will repopulate the missing mitochondria.
“Our cells don’t have a good system to repair breaks on mitochondrial DNA,” said Dr. Moraes.
For some people, disease is caused by mutations in only some of the mitochondrial DNA. If this is the case, then this type of gene editing may theoretically help them. Mitochondria with damaged DNA would disintegrate, leaving mostly healthy mitochondrial DNA. But for disease caused by mutations present in all mitochondrial DNA, this strategy would leave no healthy mitochondrial DNA.
Enter base editing. With base editing, scientists don’t just cleave out the damaged DNA. They fix it to restore the correct sequence.
The Next Step in Editing Mitochondrial DNA
“In 2020, the first mitochondrial base editor was reported,” said Dr. Barrera-Paez. Instead of just removing dysfunctional mitochondrial DNA, “we can try to directly correct it with the base editor. As soon as we got our hands on that, of course, we had a field day with the idea.”
Dr. Barrera-Paez, Dr. Moraes and their team used a mouse model that had a pathological mitochondrial DNA mutation. They were unable to directly edit the problematic mutation. But they were able to target a different section of mitochondrial DNA with AAV9-DdCBE that could compensate for the damage.
Untreated mice had elevated levels of lactate in their hearts. These levels dropped once the mice had been treated with mitochondrial DNA base editing.
But the system wasn’t perfect. As the researchers raised the amount of base editing in hopes of increasing its therapeutic effect, they started seeing more and more off-target edits. At the highest doses, these edits caused severe, adverse effects which, in some cases, were fatal.
“There were some issues we wanted to highlight in this paper,” said Dr. Barrera-Paez. “The more editing that you generate with the system, the more likely you are to also generate toxic, off-target editing.”
The field is excited about the potential of base editing mitochondrial DNA to treat disease, but Dr. Barrera-Paez and Dr. Moraes emphasized that the system needs more precision before it’s used in humans.
“Every study takes us a little closer to where we have to be,” said Dr. Moraes.
Tags: CRISPR gene editing, Dr. Carlos Moraes, gene editing, mitochondrial diseases, student research