Mutations in GDP-Mannose Pyrophosphorylase B Cause Congenital and Limb-Girdle Muscular Dystrophies Associated with Hypoglycosylation of α-Dystroglycan

Keren Carss ,Lucy Feng,Sonia Messina,Akanchha Kesari,Francesco Muntoni,Silvia Torelli,Moniek Riemersma,Shu Yau,Enrico Bertini,Tobias Willer,Alexander Hoischen,Jaya Punetha,Elizabeth Stevens,Lina Brodd,Elizabeth Wraige,Leigh B Waddell ,Eric P Hoffman ,Carla Grosmann ,Dirk Lefeber ,Caroline A Sewry ,Helen Young ,Monique Van Scherpenzeel ,Hans Van Bokhoven ,R Quinlivan ,Daniel G Macarthur ,Derek L Stemple ,Matthew E Hurles ,Steven A Moore ,Kevin P Campbell ,Carsten G Bönnemann ,Sebahattin Çırak ,A Reghan Foley ,José E Abdenur ,Yung Yao Lin ,Klaus North

doi:10.1016/j.ajhg.2013.05.009

Abstract

Congenital muscular dystrophies with hypoglycosylation of α-dystroglycan (α-DG) are a heterogeneous group of disorders often associated with brain and eye defects in addition to muscular dystrophy. Causative variants in 14 genes thought to be involved in the glycosylation of α-DG have been identified thus far. Allelic mutations in these genes might also cause milder limb-girdle muscular dystrophy phenotypes. Using a combination of exome and Sanger sequencing in eight unrelated individuals, we present evidence that mutations in guanosine diphosphate mannose (GDP-mannose) pyrophosphorylase B (GMPPB) can result in muscular dystrophy variants with hypoglycosylated α-DG. GMPPB catalyzes the formation of GDP-mannose from GTP and mannose-1-phosphate. GDP-mannose is required for O-mannosylation of proteins, including α-DG, and it is the substrate of cytosolic mannosyltransferases. We found reduced α-DG glycosylation in the muscle biopsies of affected individuals and in available fibroblasts. Overexpression of wild-type GMPPB in fibroblasts from an affected individual partially restored glycosylation of α-DG. Whereas wild-type GMPPB localized to the cytoplasm, five of the identified missense mutations caused formation of aggregates in the cytoplasm or near membrane protrusions. Additionally, knockdown of the GMPPB ortholog in zebrafish caused structural muscle defects with decreased motility, eye abnormalities, and reduced glycosylation of α-DG. Together, these data indicate that GMPPB mutations are responsible for congenital and limb-girdle muscular dystrophies with hypoglycosylation of α-DG.

Highlights

Congenital muscular dystrophy (CMD) represents a clinically and genetically heterogeneous group of neuromuscular disorders characterized by the onset of muscle weakness, often associated with limb contractures, at birth or within the first few months of life
The spectrum of severity observed in these cases ranges from a child with a classical CMD presentation characterized by muscle weakness at birth and motor and cognitive developmental delays including ataxia, absent speech development, and inability to walk unsupported (P1) to children presenting in the first few months of life with hypotonia, muscle weakness, delayed acquisition of independent mobility, and mild intellectual disability but normal brain structure (P3 and P4) to children presenting in the first few years of life with mild limb-girdle weakness and mild intellectual disability (P2 and P8) to a child with normal cognitive function following a limb-girdle muscular dystrophy (LGMD) disease course (P7)
Through an international collaborative effort, we identified recessive GMPPB mutations in seven further dystroglycanopathy cases (P2–P8) with phenotypes ranging from severe CMD with the inability to walk to severe intellectual disability and epilepsy to later-onset phenotypes resembling LGMD with only mild intellectual disability or no evidence of brain involvement

Summary

Introduction

Congenital muscular dystrophy (CMD) represents a clinically and genetically heterogeneous group of neuromuscular disorders characterized by the onset of muscle weakness, often associated with limb contractures, at birth or within the first few months of life. Dystroglycan is translated from a single mRNA DAG1 (MIM 128239) and cleaved into a and b subunits.[1,2] Extracellular a-DG noncovalently binds to the transmembrane protein b-dystroglycan (b-DG), which in turn associates with intracellular proteins involved in force and signal transduction. A-DG binds a number of extracellular ligands, including laminins, perlecan, agrin, neurexin, pikachurin, and slit.[2,3,4,5] Extensive and tissue-specific glycosylation is essential for the binding of a-DG to these extracellular-matrix ligands.[6,7,8,9,10] Recessive mutations in DAG1 have been identified in a single case of mild muscular dystrophy, suggesting the existence of a primary subset of dystroglycanopathies.[11]

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