Sarcoglycanopathies: From clinical diagnosis to new promising therapies.

The sarcoglycanopathies are a severe form of limb girdle muscular dystrophy caused by mutations in the sarcoglycan genes SGCA, SGCB, SGCG, and SGCD, leading to reduced or absent expression of the alpha-, beta-, gamma-, and delta-sarcoglycan proteins respectively. Most patients develop a severe disease starting in the first decade of life that progresses quickly and eventually leads to a loss of ambulation before the age of 20. However, there is a marked heterogeneity in the prognosis, and several patients develop a milder phenotype. The factors correlating with disease progression are not completely known, but recent data suggest that remaining protein expression can be a key factor. The diagnosis is confirmed by genetic studies, which are sometimes not confirmative in the case of identifying variants of unknown significance or just one variant. New tools to understand the potential pathogenesis of missense variants have been developed; these are helping in the diagnosis of these diseases. Additionally, recent data on muscle MRI have revealed a characteristic pattern of involvement that can also support the diagnosis of the disease. In recent years, data coming from international collaborative studies have allowed an understanding of disease progression; however, this is only through retrospective data. There are no prospective studies collecting longitudinal data on skeletal and respiratory muscle function or cardiac structure and function progression over time which is hampering the development of new drugs in the field. Clinical trials with gene therapy are underway or are being designed in some of the subtypes of sarcoglycanopathies to advance therapeutic management.