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X-linked myopathy with excessive autophagy (XMEA) is a skeletal muscle disorder inherited in recessive fashion, affecting boys and sparing carrier females. Onset is in childhood with weakness of the proximal muscles of the lower extremities, progressing slowly to involve other muscle groups. Pathological analysis of skeletal muscle biopsies shows no inflammation, necrosis or apoptosis. Instead, forty to 80% of fibers exhibit giant autophagic vacuoles with heterogeneous degradative content. Numerous critical functions of all cells are compartmentalized in particular pH environments established by the intracellular transmembrane V-ATPase proton pump complex. Assembly of this complex, directed by the Vma21p chaperone, is well-studied in yeast but completely unknown in other organisms. The aim of my project was a better understanding of XMEA pathogenesis, with a focus on finding the disease-causing gene. In this thesis, I identify mutations in XMEA patients in a novel, previously uncharacterized gene, which we name VMA21. Most of the mutations are located in splicing-relevant positions and decrease splicing efficiency. After establishing that XMEA is caused by hypomorphic alleles of the VMA21 gene, I show that VMA21 is the diverged human orthologue of the yeast Vma21p protein, and that like Vma21p, it is an essential assembly chaperone of the V-ATPase. Decreased VMA21 reduces V-ATPase activity, resulting in altered lysosomal pH and a blockage at the degradative step of autophagy. Towards understanding disease pathogenesis, I show evidence of compensatory autophagy upregulation consecutive to the impaired clearance. Accumulated autolysosomes due to increased autophagy continue to face the degradative block and are slow to disappear. Instead, they merge to each other and form the characteristic giant XMEA vacuoles. These results uncover a novel mechanism of disease, namely macroautophagic overcompensation leading to cell vacuolation and tissue atrophy. This work describes the clinical outcome at the cusp of tolerable reduction in V-ATPase, with implications on common diseases like osteoporosis and cancer metastasis, where increased V-ATPase activity is an important component. Our XMEA patients show that the safety margin of reducing V-ATPase activity in humans is wide, increasing the potential to utilize chemical or biological V-ATPase inhibitors as possible therapies.

X-linked myopathy with excessive autophagy (XMEA) is a skeletal muscle disorder inherited in recessive fashion, affecting boys and sparing carrier females. Onset is in childhood with weakness of the proximal muscles of the lower extremities, progressing slowly to involve other muscle groups. Pathological analysis of skeletal muscle biopsies shows no inflammation, necrosis or apoptosis. Instead, forty to 80% of fibers exhibit giant autophagic vacuoles with heterogeneous degradative content. Numerous critical functions of all cells are compartmentalized in particular pH environments established by the intracellular transmembrane V-ATPase proton pump complex. Assembly of this complex, directed by the Vma21p chaperone, is well-studied in yeast but completely unknown in other organisms. The aim of my project was a better understanding of XMEA pathogenesis, with a focus on finding the disease-causing gene. In this thesis, I identify mutations in XMEA patients in a novel, previously uncharacterized gene, which we name VMA21. Most of the mutations are located in splicing-relevant positions and decrease splicing efficiency. After establishing that XMEA is caused by hypomorphic alleles of the VMA21 gene, I show that VMA21 is the diverged human orthologue of the yeast Vma21p protein, and that like Vma21p, it is an essential assembly chaperone of the V-ATPase. Decreased VMA21 reduces V-ATPase activity, resulting in altered lysosomal pH and a blockage at the degradative step of autophagy. Towards understanding disease pathogenesis, I show evidence of compensatory autophagy upregulation consecutive to the impaired clearance. Accumulated autolysosomes due to increased autophagy continue to face the degradative block and are slow to disappear. Instead, they merge to each other and form the characteristic giant XMEA vacuoles. These results uncover a novel mechanism of disease, namely macroautophagic overcompensation leading to cell vacuolation and tissue atrophy. This work describes the clinical outcome at the cusp of tolerable reduction in V-ATPase, with implications on common diseases like osteoporosis and cancer metastasis, where increased V-ATPase activity is an important component. Our XMEA patients show that the safety margin of reducing V-ATPase activity in humans is wide, increasing the potential to utilize chemical or biological V-ATPase inhibitors as possible therapies.