A New Gene Therapy Shows Promise for Treating Duchenne Muscular Dystrophy

Gene replacement using adeno-associated virus (AAV) vectors is a promising approach to treat many diseases. However, the packaging capacity of AAV (about 4.7 kilobases) poses a challenge to this treatment modality, limiting its application in diseases associated with larger protein coding sequences (such as the 14 kilobases of mRNA in Duchenne muscular dystrophy).

In a new study, researchers from the University of Washington developed a new gene therapy that uses a split intein-mediated protein trans-splicing mechanism to express larger dystrophins to replace the defective DMD gene in muscles affected by Duchenne muscular dystrophy. They found several pairs of split inteins that can effectively connect two or three protein fragments to produce a medium-sized dystrophin or full-length dystrophin. The relevant research results were recently published online in the journal Nature, with the title of the paper "Split intein-mediated protein trans-splicing to express large dystrophins".

Their study showed that delivering two or three AAVs to mice with muscular dystrophy resulted in robust expression of larger dystrophin proteins, resulting in significant improvements in their physiological condition. Furthermore, using the potent myotropic viral vector AAVMYO, they demonstrated that a low total dose (2X10¹³ viral genomes per kg) was sufficient to express larger dystrophin proteins throughout the body's skeletal muscle and significantly improve the physiological condition of mice with muscular dystrophy. Their data showed that larger dystrophin proteins have clear functional advantages over the minute amounts of dystrophin being tested in clinical trials. This approach could benefit many patients with Duchenne or Becker muscular dystrophy, regardless of genotype, and could also be applicable to many other diseases caused by mutations in larger genes.

These findings suggest that the new gene therapy they developed to treat Duchenne muscular dystrophy shows promise not only in halting the deterioration of muscles in patients with this genetic disease, but also in the future in repairing these damaged muscles.

Currently, there is no cure for Duchenne muscular dystrophy, and existing treatments and drugs can only slow the disease. All patients with Duchenne muscular dystrophy are male. The gene is on the X chromosome, and they begin to show symptoms around age 4, usually dying in their twenties or thirties.

Figure 1. Illustration of splitting sites tested with triple AAV vector strategy to express the full-length dystrophin. (Tasfaout H, et al., 2024)

Chamberlain noted that what has troubled scientists in the past is that the gene that needs to be repaired is the largest gene in nature. Until now, there has been no way to get enough of the protein into the muscle. "It's like delivering a king-size bed, but you can't get it into the house," he said.

The new gene therapy, which has been successful in mouse models, uses a series of adeno-associated viral vectors (AAVs), tiny shuttles that originate from viruses and are currently being used to deliver gene therapies to human cells. Instead of one AAV, the gene therapy uses a series of AAVs, each of which carries a portion of the therapeutic dystrophin protein into the muscle and embeds the instructions. Once inside the body, it begins to assemble in order to make the necessary genetic repair.

Going back to the metaphor of the bed, not only did its parts arrive piece by piece, but the deliveryman began assembling the bed as soon as he entered the house. "The next step for this gene therapy is human trials, which will begin in about two years," Chamberlain said.

In the lab, the gene therapy not only stopped the disease from progressing, but also reversed most of the pathological changes associated with dystrophin. Ultimately, Chamberlain and Hichem Tasfaout, first author of the paper, hope this approach can reverse muscle atrophy and restore normal health to muscle tissue.

"This new gene therapy also uses a novel AAV vector, AAVMYO, which allows for lower doses and therefore may reduce or eliminate some of the side effects of previous approaches," Chamberlain said.

Reference

Tasfaout H, et al. Split intein-mediated protein trans-splicing to express large dystrophins. Nature, 2024: 1-9.