P.1.6 Investigating collagen VI biosynthesis and assembly in the context of ALG2 impairment in Ullrich/CMS-like family

Abstract

Ullrich Congenital Muscular Dystrophy (UCMD) and Bethlem Myopathy (BM) are muscle disorders within the spectrum of Collagen VI-related myopathies. These disorders are associated with mutations in the Collagen VI genes (COL6A1, COL6A2 and COL6A3), which are components of extracellular matrix (ECM). Several UCMD and BM phenotypes have been reported with no mutations in Collagen VIA genes, indicating genetic heterogeneity in these disorders. We investigated a consanguineous Saudi Arabian family with a UCMD-like phenotype but without mutations in the COL6A genes. By a combined approach of autozygosity analysis, exome sequencing and sanger sequencing we were able to identify an indel in exon 1 of the ALG2 (Asparagine-linked glycosylation 2) gene; c.283–296delGGGGACTGGCTGCc.283–293insAGTCCCCGGC p.73–76delGDWLinsSPR. ALG2 encodes mannosyltransferase of the N-glycosylation pathway. Due to the recently discovered role of N-glycosylation pathway genes in congenital myasthenia syndrome (CMS) (1) two patients from the family underwent electromyography repetitive nerve stimulation, which revealed a potential neuromuscular junction defect. Due to the fact that the phenotype of the family showed a mix of UCMD and CMS-like features, and that proper N-glycosylation is essential for secretion of collagens to extracellular matrix (ECM) (3), we aimed to investigate collagen VI assembly and secretion in the context of deficient Alg2. Collagen VI was assayed in ALG2 mutant patient dermal fibroblast cells and shRNA knock down cells by immunofluorescence and western blotting to determine whether the pathomechanism of ALG2 mutation was dependent or independent of collagen VI which in turn would allow for insight into whether these neuromuscular phenotypes could form part of the wider spectrum of congenital disorders of glycosylation due to impaired N-linked glycosylation.