A new muscle glycogen storage disease associated with glycogenin-1 deficiency

Abstract

We describe a slowly progressive myopathy in 7 unrelated adult patients with storage of polyglucosan in muscle fibers. Genetic investigation revealed homozygous or compound heterozygous deleterious variants in the glycogenin-1 gene (GYG1). Most patients showed depletion of glycogenin-1 in skeletal muscle, whereas 1 showed presence of glycogenin-1 lacking the C-terminal that normally binds glycogen synthase. Our results indicate that either depletion of glycogenin-1 or impaired interaction with glycogen synthase underlies this new form of glycogen storage disease that differs from a previously reported patient with GYG1 mutations who showed profound glycogen depletion in skeletal muscle and accumulation of glycogenin-1.

© 2014 The Authors Annals of Neurology published by Wiley Periodicals, Inc. on behalf of American Neurological Association.

Figures

FIGURE 1

Characteristic morphological alterations of skeletal muscle. (A) Hematoxylin–eosin (HE) staining showing bluish inclusions in muscle fibers (arrows). (B) The inclusions contain periodic acid–Schiff (PAS)-positive material (arrows), and some fibers lack normal intermyofibrillar glycogen (asterisk). (C) The storage material is partially resistant to alpha-amylase treatment (arrows). (D–F) The storage material (arrows) is stained with antibodies for desmin, P62/SQSTM1, and ubiquitin. Scale bars in A–F correspond to 5μm. (G, H) Electron microscopy demonstrating abnormal glycogen presenting with ovoid structures composed of partly filamentous material surrounded by a rim of more dense glycogen granules and mitochondria.

FIGURE 2

Genetic and protein analyses. (A) Schematic illustration of the coding and noncoding region of the gene GYG1 (NM_004130). Detected variants are marked in different colors for each patient (P1, light blue; P2, green; P3, orange; P4, red; P5, yellow; P6, purple; P7, dark blue). (B) Reverse transcriptase polymerase chain reaction products covering exons 1 to 4. Lane 1: size ladder; lane 2: control sample with wild-type sequence; lane 3: patient homozygous for the GYG1 c.14313G>C variant resulting in transcripts with skipping of exon 2; lane 4: patient heterozygous for the GYG1 c.14313G>C variant, showing 2 bands, 1 of normal size and 1 with aberrant splicing with exon 2 skipping (lower band). (C) Schematic illustration of the consequences of incorrect splicing. The GYG1 c.14313G>C variant results in skipping of exon 2. (D) Normally spliced transcript. (E) Aberrantly spliced transcript with exon 2 skipping. (F) Results of Western blot analysis of glycogenin-1 in skeletal muscle from a normal control (WT) and patients (P1, P4, P5, P6, and P7) performed with (1) and without (2) alpha-amylase treatment. After alpha-amylase treatment, glycogenin-1 was detected in P1, P5, and P7, demonstrating that these patients produce some residual glycogenin-1. Without alpha-amylase treatment, free glycogenin-1 was detected in P7. (G) Western blot analysis of a normal control, P7, and a patient with glycogen synthase deficiency. The glycogenin-1 detected in P7 as well as in the glycogen synthase– deficient patient revealed a gel shift corresponding to approximately 1kDa, indicating that it was autoglucosylated. Glycogenin-1 in P7 is reduced in size due to the truncating variant. (H) Western blot analyses of glycogen synthase (GYS1) and branching enzyme (GBE1) in P1, P4, P5, P6, P7, 1 patient with GYS1 deficiency, and 1 with RBCK1 deficiency. No obvious upregulation or downregulation of these enzymes was evident in patients with pathogenic GYG1 variants. (I) In vitro autoglucosylation demonstrated that glycogenin-1 in P3 and P4 was unable to autoglucosylate when uridine diphosphate (UDP) glucose was added (1). The truncated glycogenin-1 in P7 increased in size when UDP glucose was added, showing that it was autoglucosylated.