Altered in vitro muscle differentiation in X-linked myopathy with excessive autophagy (XMEA)

Abstract

X-linked myopathy with excessive autophagy (XMEA) is a muscle genetic disease associated to weakness of proximal muscles. It is caused by mutations in the VMA21 gene, coding for a chaperone that functions in the vacuolar ATPase (v-ATPase) assembly. Mutations associated to lower content of assembled v-ATPases lead to an increase in lysosomal pH, culminating in a partial blockage of macroautophagy, with accumulation of vacuoles of undigested content. Here, we studied a five-year-old boy affected by XMEA, caused by a small indel in the VMA21 gene. Detection of sarcoplasmic Lc3-positive vacuoles in his muscle biopsy confirmed an autophagy defect. To understand how autophagy is regulated in XMEA myogenesis, we used patient-derived muscle cells to evaluate autophagy during in vitro muscle differentiation. An increase in lysosomal pH was observed in patient's cells, compatible with predicted functional defect of his mutation. Additionally, there was an increase in autophagic flux in XMEA myotubes. Interestingly, we observed that differentiation of XMEA myoblasts is altered, with increased myotube formation observed through higher fusion index, which was not dependent on lysosomal acidification. Moreover, no variation in the expression of myogenic factors nor the presence of regenerating fibers in the patient's muscle were observed. Since myoblast fusion is a tightly regulated process, the uncontrolled fusion of XMEA myoblasts might generate cells that are not as functional as normal muscle cells. Our data bring new evidences for the reason of a predominant muscle involvement, in the context of the XMEA phenotype.