Diving the ocean of variants in pursuit of a muscular dystrophies differential diagnosis

Abstract

Muscular dystrophies (MD) are a group of rare inherited diseases that cause weakness and progressive degeneration of skeletal muscle. They are caused by mutations in genes encoding structural skeletal muscle proteins or proteins necessary for the stability and proper functioning of muscle fibers. However, the clinical symptoms of these pathologies overlap, hindering differential diagnosis, which is of paramount importance to establish the standard of care. Therefore, it is important to carry out molecular studies to be able to differentiate between each type of MD. Here, we focus on the case of Limb-Girdle MD, which are frequently misdiagnosed as Dystrophinopathies, the most frequent type of MD and caused by mutations in the DMD gene. The present work aims to detect molecular alterations in MD genes in patients with a presumptive dystrophinopathy clinical diagnosis but no DMD mutation identified. A cohort of 106 Dystrophinopathy suspected males, with no alteration detected in DMD by MLPA, was referred to our laboratory for WES analysis. In a subset of 21, no small mutation in the DMD gene was detected. Therefore, we deepened the screening to all the MD genes included in the Gene Table of Neuromuscular Disorders. For recessive MD disorders, when only one mutation was identified, MLPA (SGCA, SGCB, SGCD, SGCG and FKRP) was implemented for deletion/duplication screening. Dystrophinopathy mutations were detected in the DMD gene in 81.1 % of patients. Further analysis of the WES results of the remaining individuals, allowed us to identify possibly pathogenic molecular alterations, in other MD associated genes, in 11 of them (10.4 %). Thus, reaching a WES detection rate of 91.5 %. We found 2 large deletions in SGCA and SGCD by MLPA. Finally, our work highlights the importance of extending the mutation screening to all the MD associated genes in patients without alterations in DMD, given that a misdiagnosis could lead to an error in the selection of the standard of care.