Muscular dystrophies (MDs) are genetic disorders that are characterized by progressive striated muscle degeneration. Deletions within the dystrophin gene account for the majority of cases, and loss-of-function mutations in SGCG, which encodes the γ- sarcoglycan transmembrane subunit account for a variety of limb girdle MDs (LGMDs). Antisense oligonucleotide (AON) therapies to promote exon skipping and the generation of truncated, partially functional proteins have been proposed for the treatment of MDs. Quan Gao and colleagues at the University of Chicago developed an exon skipping strategy that uses AONs designed to generate a truncated γ-sarcoglycan protein (termed Mini-Gamma) that retains integral functionality. Similar to full-length γ- sarcoglycan, Mini-Gamma co-localized with β- and δ-sarcoglycan subunits at the plasma membrane when expressed in cultured cells. In both an established Drosophila model of muscular dystrophy and γ-sarcoglycan-deficient mice, Mini-Gamma also localized to the plasma membrane. In the fly model, Mini-Gamma improved motility and restored heart morphology. Expression of Mini-Gamma in Sgcg null mice ameliorated skeletal muscle defects, reduced thickening of the diaphragm muscle, reduced fibrosis, and improved heart function. Importantly, treatment of human LGMD cells with Mini-Gamma produced the properly truncated SGCG transcript. Together, the results of this study demonstrate that AON-mediated exon skipping can rescue MD-associated phenotypes and suggest Mini-Gamma as a promising therapy for individuals with SGCG mutations. The accompanying image shows the expression of Mini-Gamma (green) in the heart tube of a γ-sarcoglycan-deficient fly. Note plasma membrane-associated staining within the heart tube structure.
Exon skipping uses antisense oligonucleotides as a treatment for genetic diseases. The antisense oligonucleotides used for exon skipping are designed to bypass premature stop codons in the target RNA and restore reading frame disruption. Exon skipping is currently being tested in humans with dystrophin gene mutations who have Duchenne muscular dystrophy. For Duchenne muscular dystrophy, the rationale for exon skipping derived from observations in patients with naturally occurring dystrophin gene mutations that generated internally deleted but partially functional dystrophin proteins. We have now expanded the potential for exon skipping by testing whether an internal, in-frame truncation of a transmembrane protein γ-sarcoglycan is functional. We generated an internally truncated γ-sarcoglycan protein that we have termed Mini-Gamma by deleting a large portion of the extracellular domain. Mini-Gamma provided functional and pathological benefits to correct the loss of γ-sarcoglycan in a
Quan Q. Gao, Eugene Wyatt, Jeff A. Goldstein, Peter LoPresti, Lisa M. Castillo, Alec Gazda, Natalie Petrossian, Judy U. Earley, Michele Hadhazy, David Y. Barefield, Alexis R. Demonbreun, Carsten Bönnemann, Matthew Wolf, Elizabeth M. McNally