A new three-part gene therapy method restored full-length dystrophin expression and improved muscle and heart health in mice with Duchenne muscular dystrophy (DMD), offering a promising strategy for treating DMD and other genetic diseases caused by large genes, according to a study published recently in The Journal of Clinical Investigation.
What is dystrophin?
Dystrophin is found in small quantities in muscle cells, where its role is to protect the cell from daily damage. When it mutates, it causes muscle cell degeneration and inflammation, leading to damage in the tissue of the bones, lungs and heart.
DMD is caused by mutations in the DMD gene, which produces a large 11.2-kilobase mRNA — a size too large to fit in the adeno-associated virus (AAV) vectors commonly used in gene therapies. Researchers overcame this size limitation by splitting the gene into three fragments and rejoining them inside the body using special protein-splicing sequences called inteins.
“Our results demonstrate the feasibility of expressing a therapeutic protein from multiple fragments deliverable by AAV vectors, which could be relevant to many genetic diseases caused by loss-of-function mutations in extra-large genes, such as nemaline or RYR1-related myopathies,” said this study’s authors. “Nevertheless, the development of multi-AAV-vector therapies could be challenging for human applications.”
Scientists tested this approach in mdx4cv mice, a well-established model for DMD. They used a muscle-targeting AAVMYO1 vector to carry the dystrophin fragments and delivered either low or medium total doses of the therapy. Three months after injection, treated mice showed significant production of full-length dystrophin in leg, heart and diaphragm muscles. Dystrophin appeared in up to 55% of leg muscle fibers and 68% of heart cells, with expression in diaphragm tissue as well.
Read more about DMD therapies
The therapy led to notable physical improvements. Muscle fibers were healthier, larger and more tightly packed. Markers of muscle damage decreased, and key proteins that had been lost in untreated mice reappeared. Importantly, muscle strength and resistance to damage from overuse were restored to near-normal levels. Even at low doses, treated mice regained specific muscle force and showed less injury during eccentric contraction tests.
Older mice with severe disease also benefited. At 17 months, these mice typically show weakened muscles, scar tissue and heart problems. Seven months after receiving the therapy, their leg muscle strength returned to levels seen in healthy mice, and they resisted mechanical stress better than untreated animals. Though diaphragm improvements were milder, the therapy still boosted function.
Heart performance also improved. In high-stress tests, hearts from treated mice pumped more efficiently, matching wild-type controls. Full-length dystrophin was found in 40% of heart cells, and heart tissue looked healthier under the microscope.
For patients, this strategy may expand the reach of gene therapy to larger genetic targets and could bring new hope for treating DMD and other diseases that were previously out of reach.
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