Scientists at Tevard Biosciences recently published their new genetic treatment strategy to give hope to some patients with a fatal neuromuscular disease, Duchenne muscular dystrophy (DMD). Their tool, tested only in mice so far, uses a suppressor transfer RNA (tRNA) that can read through DNA mutations in a small subset of DMD patients to restore full-length, functional dystrophin protein.
Dystrophin is one of the largest proteins in the human body, and genetic mutations that alter its production cause DMD. Researchers have identified a variety of genetic mutations across DMD patients, and nonsense mutations account for 15% of all DMD cases.
Nonsense mutations are single DNA base changes that cause the protein translation machinery to read that section of DNA and its resulting RNA as a stop codon, telling the body to stop building the protein early and resulting in a smaller, or truncated, form of dystrophin. Truncated proteins are often completely non-functional, and the loss of dystrophin impairs locomotion, neurocognition, breathing, and cardiovascular function.
Dystrophin gene replacement therapies are hindered by the difficulty of packaging such a large gene into a small enough carrier to enter cells. The Tevard researchers overcame this challenge by engineering suppressor tRNAs that “can go anywhere that a stop codon is aberrantly placed throughout the gene”, according to Elisabeth Gardiner, chief scientific officer at Tevard. The group achieved dystrophin expression at 70% of healthy levels in a DMD mouse model by engineering suppressor tRNAs that bind the nonsense mutation, read it as an amino acid, and continue to build the rest of the dystrophin protein.
The new therapy improved dystrophin expression in muscle, heart, and diaphragm without showing off-target effects at other stop codons throughout the genome. Muscle strength and motor coordination improved without toxicity to other organs.
The suppressor tRNAs are packaged in adeno-associated virus (AAV) vectors for specific delivery to muscle cells. High AAV doses can be toxic in humans and have been responsible for multiple recent deaths in DMD patients treated with novel gene therapies. To reduce safety concerns, the scientists optimized the AAV vectors to reach muscle cells and deliver their cargo at lower doses than are used in other DMD gene therapies on the market or in clinical trials.
Scientists tested multiple suppressor tRNA and AAV combinations, finding that some worked better than others and that dystrophin expression increased in a dose-dependent way. Grip strength, an indicator of muscle function, improved in mice treated at some of the lowest doses, and only a 10-15% skeletal muscle dystrophin recovery was required to fully restore grip strength in mice.
The doses tested here were lower than in another mouse study with a different DMD gene therapy and provide some preliminary evidence that a lower dose could achieve functional benefit for human patients. The company plans to test its tRNA therapy in humans once it establishes safety and efficacy in animal models.
