A gene-edited mouse model of limb-girdle muscular dystrophy 2C for testing exon skipping

Limb-girdle muscular dystrophy type 2C is caused by autosomal recessive mutations in the gamma-sarcoglycan (SGCG) gene. The most common SGCG mutation is a single nucleotide deletion from a stretch of five thymine residues in SGCG exon 6 (521 Delta T). This founder mutation disrupts the transcript reading frame, abolishing protein expression. An antisense oligonucleotide exon-skipping method to reframe the human 521 Delta T transcript requires skipping four exons to generate a functional, internally truncated protein. In vivo evaluation of this multi-exon skipping, antisense-mediated therapy requires a genetically appropriate mouse model. The human and mouse gamma-sarcoglycan genes are highly homologous in sequence and gene structure, including the exon 6 region harboring the founder mutation. Herein, we describe a new mouse model of this form of limb-girdle muscular dystrophy generated using CRISPR/Cas9-mediated gene editing to introduce a single thymine deletion in murine exon 6, recreating the 521 Delta T point mutation in Sgcg. These mice express the 521 Delta T transcript, lack gamma-sarcoglycan protein and exhibit a severe dystrophic phenotype. Phenotypic characterization demonstrated reduced muscle mass, increased sarcolemmal leak and fragility, and decreased muscle function, consistent with the human pathological findings. Furthermore, we showed that intramuscular administration of a murine-specific multiple exon-directed antisense oligonucleotide cocktail effectively corrected the 521 Delta T reading frame. These data demonstrate a molecularly and pathologically suitable model for in vivo testing of a multi-exon skipping strategy to advance preclinical development of this genetic correction approach.