In models of DMD, gene editing allows exon skipping
The goal is to permanently change a cell's DNA to aid dystrophin production
A gene editing approach can restore dystrophin production in cell and animal models of Duchenne muscular dystrophy (DMD), according to its researchers.
The study, “Targeting Duchenne Muscular Dystrophy by Skipping DMD Exon 45 with Base Editors,” was published in Molecular Therapy Nucleic Acids.
DMD is caused by mutations in the gene DMD, which provides cells with instructions for making dystrophin. The protein normally functions like a shock absorber in muscle cells, helping to cushion them against damage. DMD-causing mutations prevent dystrophin from being made.
The DMD gene is composed of sections, called exons, which contain the genetic code needed to make dystrophin. The exons are interspersed with sections called introns, which don’t contain instructions for making protein, but help regulate the gene’s activity.
When DMD is “read” to make dystrophin, the entire genetic sequence, including exons and introns, is copied into a temporary molecule called messenger RNA (mRNA). The mRNA then undergoes a splicing process where the introns are removed and the exons are strung together to form the final protein-coding sequence, which is used to produce dystrophin.
The exons of the DMD gene normally fit together, somewhat like pieces of a puzzle, to form the mature protein-coding sequence. But most mutations that cause DMD lead to problems with this fit, so cells can’t produce a functional version of the protein.
‘Permanent modifications’ with gene editing to treat DMD
Exon skipping is a therapeutic approach that seeks to leave out one or more exons from the DMD gene’s mRNA, letting the remaining exons fit together better. This would allow cells to produce a shortened, but still functional, version of dystrophin.
Several exon-skipping therapies have been granted accelerated approval in the U.S. to treat DMD caused by specific mutations. A notable limitation of them is that they all require repeated administration over a person’s lifetime.
In this study, a research team led by scientists in the U.S. explored inducing exon skipping by using gene editing to permanently change a cell’s DNA so that exon skipping always happens when the DMD gene is “read” to make dystrophin.
“By directly modifying DNA, gene-editing systems can accomplish permanent modifications leading to long-lasting therapeutic benefit after a single administration,” wrote the scientists, who used a gene-editing technology called CRISPR to induce skipping of DMD exon 45. The approved therapy Amondys 45 (casimersen) works to skip this exon. “Exon 45 is a hotspot for mutations in the DMD gene that cause Duchenne muscular dystrophy, thus skipping exon 45 is a potential therapeutic approach which could potentially correct [approximately] 9% of Duchenne muscular dystrophy cases.”
Testing gene editing in cell, mouse models of DMD
In a series of tests in cell models, the researchers showed their gene-editing technique, which they dubbed CRISPR-SKIP, could effectively induce exon skipping and enable dystrophin production in muscle cells carrying amenable mutations.
In subsequent tests with a mouse model, the gene-editing platform was delivered to cells using an adeno-associated virus (AAV) vector. AAV is often used in gene therapies because it’s good at getting genetic material into human cells, but doesn’t cause serious disease.
This approach effectively led to exon skipping in the mice’s muscle cells, results showed, though there was variability from mouse to mouse. On average, about 23% of cells were correctly edited.
“It should be noted that these editing rates were accomplished through injection of an AAV dosage higher than the maximum dose recommended by the FDA [U.S. Food and Drug Administration] when adjusted to the estimated weight of the biceps femoris [thigh] muscle,” the scientists said.
Restoring dystrophin production by even 4% has shown promising effects in mouse models of DMD, they noted, adding more work is needed to optimize this approach. “The strategy described in this work with further optimization has potential for therapeutic impact,” they wrote.