Scientists find way to supercharge gene therapy for Pompe disease
Genetic 'boosters' helped restore muscle function in mice for more than a year
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A single dose of a specialized gene therapy has successfully normalized muscle function for more than a year in a mouse model of Pompe disease. By using “data-mined” genetic sequences designed to supercharge enzyme production specifically in muscle cells, researchers achieved protein levels up to 30 times higher than those achieved with previous methods.
The study, “Long-Term Functional Correction of Pompe Disease and Increased α-Glucosidase Expression after Gene Therapy with Novel Combinations of Muscle-Targeted Transcriptional Cis-Regulatory Elements,” was published in Human Gene Therapy.Â
Pompe disease is caused by mutations in the GAA gene, which provides instructions to make an enzyme needed to break down glycogen, a complex sugar molecule. In people with Pompe disease, the enzyme doesn’t function correctly, so glycogen builds up to toxic levels in cells, particularly muscle cells. The resulting cell damage leads to disease symptoms such as muscle weakness.
Delivering a functional GAA gene
The idea behind gene therapy for Pompe disease is straightforward: deliver a healthy version of the GAA gene to muscle cells, enabling them to produce a functional enzyme that clears toxic glycogen. In recent years, scientists have developed several gene therapies for muscle diseases using viral vectors — essentially, viruses that are modified to deliver a therapeutic gene rather than cause an infection. A specific virus called adeno-associated virus (AAV) is commonly used for this purpose because it’s easy to work with in a lab and doesn’t typically cause illness in people.
But getting a healthy version of the GAA gene into muscle cells is only half the battle — for gene therapy to be effective, the gene needs to be turned on inside the cell, allowing production of a functional enzyme at high enough levels. This is traditionally done using genetic sequences called promoters. A promoter is where relevant proteins bind to initiate transcription of that gene into RNA, which is later translated into a protein.
In this study, scientists used data-mining approaches to identify a novel set of promoters, known as cis-regulatory elements (CREs), that are particularly effective at activating genes in muscle cells. They then constructed an AAV-based gene therapy that delivers a healthy version of the GAA gene to muscle cells, with muscle-specific CRE promoters at the start.
In cell studies, the researchers found that AAV vectors using CRE promoters to read a luciferase reporter gene (one that produces a bioluminescent signal that can be measured) led to up to 30 times higher protein expression in muscle cells than standard promoters used in previous efforts at muscle-directed gene therapy.
The scientists then tested the gene therapy in a mouse model of Pompe disease. Results showed a significant decrease in glycogen accumulation and long-term normalization of skeletal muscle function, with performance comparable to that of healthy mice in grip and hanging strength tests. The gene therapy also showed benefits for the heart and diaphragm (the main muscle in the chest used for breathing).
“The use of [CREs] resulted in relatively robust expression in the skeletal muscle, diaphragm, and heart, leading to phenotypic correction of these affected tissues in the treated [Pompe] mice,” the researchers concluded, noting that “sustained therapeutic effects could be achieved that lasted for more than 1 year.”
Although this gene therapy approach showed promise, it is not without limitations. In particular, the scientists noted that this approach does not correct Pompe manifestations in the brain and spinal cord. “Ideally, novel AAV capsids [outer shells] that enable efficient targeted transduction [delivery of genetic material to cells] of skeletal muscle, diaphragm, heart and brain would be necessary to effectively treat all clinical manifestations of Pompe disease,”
“The current preclinical study strongly supports the use of CRE arrays to enhance the performance of gene therapy vectors for treating Pompe patients, justifying new gene therapy clinical trials,” the team concluded.
