GMPPA defects cause a neuromuscular disorder with [alpha]-dystroglycan hyperglycosylation

Introduction Cell surface proteins are important for the attachment of a cell to the extracellular matrix (ECM). They are modified by post-translational covalent attachment of branched carbohydrate oligomers. N-linked oligosaccharides [...]
GDP-mannose-pyrophosphorylase-B (GMPPB) facilitates the generation of GDP-mannose, a sugar donor required for glycosylation. GMPPB defects cause muscle disease due to hypoglycosylation of [alpha]-dystroglycan ([alpha]-DG). AlphaDG is part of a protein complex, which links the extracellular matrix with the cytoskeleton, thus stabilizing myofibers. Mutations of the catalytically inactive homolog GMPPA cause alacrima, achalasia, and mental retardation syndrome (AAMR syndrome), which also involves muscle weakness. Here, we showed that Gmppa-KO mice recapitulated cognitive and motor deficits. As structural correlates, we found cortical layering defects, progressive neuron loss, and myopathic alterations. Increased GDP-mannose levels in skeletal muscle and in vitro assays identified GMPPA as an allosteric feedback inhibitor of GMPPB. Thus, its disruption enhanced mannose incorporation into glycoproteins, including [alpha]-DG in mice and humans. This increased [alpha]-DG turnover and thereby lowered [alpha]-DG abundance. In mice, dietary mannose restriction beginning after weaning corrected [alpha]-DG hyperglycosylation and abundance, normalized skeletal muscle morphology, and prevented neuron degeneration and the development of motor deficits. Cortical layering and cognitive performance, however, were not improved. We thus identified GMPPA defects as the first congenital disorder of glycosylation characterized by [alpha]-DG hyperglycosylation, to our knowledge, and we have unraveled underlying disease mechanisms and identified potential dietary treatment options.