Novel therapeutic perspectives for sarcoglycanopathy by assisting protein folding

Sarcoglycanopathy, the collective name of four forms of Limb Girdle Muscular Dystrophy (LGMD 2C-2F), is a rare genetic disorder affecting mainly the proximal musculature. Defects in any one of the genes coding for α-, β-, γ - or δ -sarcoglycan (SG), forming a key structural tetramer in the sarcolemma of striated muscles, strongly affect SG-complex formation/stability. Disease severity is strictly related to the residual level of sarcoglycans in the sarcolemma, with the most severe forms characterized by the almost complete loss of the proteins. Most of the sarcoglycan defects are missense mutations producing a full length but folding defective protein. We have proven that the primary pathological event in sarcoglycanopathy occurs in the Endoplasmic Reticulum (ER), where the quality control system, by proof-reading newly synthesized sarcoglycans, recognizes and directs to the proteasomal degradation the folding-defective mutants. This event causes the secondary loss of the wild-type partners. We have also demonstrated that many missense mutants retain their function and that the entire complex can be properly rescued by blocking the degradation of these mutants. These findings opened new perspectives for the therapy of this neglected disease allowing to design small molecule-based approaches aimed not only to merely inhibit sarcoglycan mutants degradation, but particularly to help their folding so that, structurally stabilized, these mutants can skip disposal and traffic at the proper site of action. To this intent, we have tested several small molecules, known as protein folding correctors screened for the treatment of cystic fibrosis, in both cell models expressing folding defective forms of α-SG and primary myogenic cells isolated from a patient suffering of LGMD2D*. We have observed, by western blot and immunofluorescence analyses, that treatments with these compounds lead to the accumulation of different α-SG mutants that are competent to assemble with the wild type partners and traffic to the cell membrane. Although the mechanism of action of CFTR correctors on sarcoglycans is still unknown and needs to be clarify, these data represent the proof of principle of a “protein repair strategy” that can be developed to treat LGMD2D utilizing well-known and available small molecules correcting mutant folding. * Human samples have been provided by the Neuromuscular Bank of Tissues and DNA samples of the Italian Telethon Foundation.