Advanced therapeutic approaches in sarcoglycanopathies.

Sarcoglycanopathies are rare autosomal recessive diseases belonging to the family of limb-girdle muscular dystrophies. They are caused by mutations in the genes coding for α-, ß-, γ-, and δ-sarcoglycan. The mutations impair the assembly of a key structural complex, which normally protects the sarcolemma of striated muscle from contraction-derived stress. Although heterogeneous, sarcoglycanopathies are characterized by progressive muscle degeneration, increased serum creatine kinase levels, loss of ambulation often during adolescence, and variable cardio-respiratory impairment. Genetic defects can impair sarcoglycan synthesis or produce a protein that is defective in folding. There is currently no effective treatment available; however, both gene replacement strategy and small molecule-based approaches show great promise and have entered or are starting to enter clinical trials.

Modeling Sarcoglycanopathy in Danio rerio.

Sarcoglycanopathies, also known as limb girdle muscular dystrophy 3-6, are rare muscular dystrophies characterized, although heterogeneous, by high disability, with patients often wheelchair-bound by late adolescence and frequently developing respiratory and cardiac problems. These diseases are currently incurable, emphasizing the importance of effective treatment strategies and the necessity of animal models for drug screening and therapeutic verification. Using the CRISPR/Cas9 genome editing technique, we generated and characterized δ-sarcoglycan and ß-sarcoglycan knockout zebrafish lines, which presented a progressive disease phenotype that worsened from a mild larval stage to distinct myopathic features in adulthood. By subjecting the knockout larvae to a viscous swimming medium, we were able to anticipate disease onset. The δ-SG knockout line was further exploited to demonstrate that a δ-SG missense mutant is a substrate for endoplasmic reticulum-associated degradation (ERAD), indicating premature degradation due to protein folding defects. In conclusion, our study underscores the utility of zebrafish in modeling sarcoglycanopathies through either gene knockout or future knock-in techniques. These novel zebrafish lines will not only enhance our understanding of the disease's pathogenic mechanisms, but will also serve as powerful tools for phenotype-based drug screening, ultimately contributing to the development of a cure for sarcoglycanopathies.

CFTR corrector C17 is effective in muscular dystrophy, in vivo proof of concept in LGMDR3.

Limb-girdle muscular dystrophy R3 (LGMDR3) is caused by mutations in the SGCA gene coding for α-sarcoglycan (SG). Together with ß- γ- and δ-SG, α-SG forms a tetramer embedded in the dystrophin associated protein complex crucial for protecting the sarcolemma from mechanical stresses elicited by muscle contraction. Most LGMDR3 cases are due to missense mutations, which result in non-properly folded, even though potentially functional α-SG. These mutants are prematurely discarded by the cell quality control. Lacking one subunit, the SG-complex is disrupted. The resulting loss of function leads to sarcolemma instability, muscle fiber damage and progressive limb muscle weakness. LGMDR3 is severely disabling and, unfortunately, still incurable. Here, we propose the use of small molecules, belonging to the class of cystic fibrosis transmembrane regulator (CFTR) correctors, for recovering mutants of α-SG defective in folding and trafficking. Specifically, CFTR corrector C17 successfully rerouted the SG-complex containing the human R98H-α-SG to the sarcolemma of hind-limb muscles of a novel LGMDR3 murine model. Notably, the muscle force of the treated model animals was fully recovered. To our knowledge, this is the first time that a compound designated for cystic fibrosis is successfully tested in a muscular dystrophy and may represent a novel paradigm of treatment for LGMDR3 as well as different other indications in which a potentially functional protein is prematurely discarded as folding-defective. Furthermore, the use of small molecules for recovering the endogenous mutated SG has an evident advantage over complex procedures such as gene or cell transfer.