VMA21 deficiency leads to autophagic dysregulation and altered vesicle trafficking in X-linked myopathy with excessive autophagy.

X-Linked myopathy with excessive autophagy (XMEA) is a rare vacuolar myopathy caused by mutations in Vma21, an assembly chaperone required for vacuolar H⁺-ATPase (V-ATPase) function. However, the mechanisms linking Vma21 deficiency to progressive muscle pathology remain poorly understood, in part due to the lack of suitable animal models. To address this gap, we generated conditional Vma21 knockout mouse models to investigate the consequences of Vma21 loss in striated muscle. Combined deletion of Vma21 in skeletal and cardiac muscle resulted in early lethality driven by severe cardiomyopathy associated with autophagic dysregulation, preceding the development of skeletal muscle pathology. In contrast, inducible skeletal muscle-specific deletion of Vma21 produced progressive muscle weakness and myopathy characterized by centralized nuclei, fiber splitting, and increased fiber size variability. Affected skeletal muscle also recapitulated defining pathological hallmarks of XMEA, including basal lamina reduplication and autophagic vacuoles with sarcolemmal features (AVSFs). Ultrastructural analysis revealed membrane-bound vacuoles containing partially undegraded material that frequently accumulated at the subsarcolemmal region, together with clusters of vesicular structures. Notably, mutant muscle exhibited increased staining for the late endosomal/exosomal marker CD63, which strongly colocalized with the complement membrane attack complex C5b-9. A similar increase in CD63 staining and its colocalization with C5b-9 were observed in skeletal muscle biopsies from patients with XMEA. Together, these models faithfully recapitulate key pathological features of XMEA and identify the accumulation of CD63-positive structures and their colocalization with C5b-9 as previously unrecognized features of Vma21-deficient skeletal muscle, implicating altered vesicle trafficking in XMEA pathogenesis.