GNE Myopathy: Etiology, Diagnosis, and Therapeutic Challenges

Abstract

GNE myopathy, previously known as hereditary inclusion body myopathy (HIBM), or Nonaka myopathy, is a rare autosomal recessive muscle disease characterized by progressive skeletal muscle atrophy. It has an estimated prevalence of 1 to 9:1,000,000. GNE myopathy is caused by mutations in the GNE gene which encodes the rate-limiting enzyme of sialic acid biosynthesis. The pathophysiology of the disease is not entirely understood, but hyposialylation of muscle glycans is thought to play an essential role. The typical presentation is bilateral foot drop caused by weakness of the anterior tibialis muscles with onset in early adulthood. The disease slowly progresses over the next decades to involve skeletal muscles throughout the body, with relative sparing of the quadriceps until late stages of the disease. The diagnosis of GNE myopathy should be considered in young adults presenting with bilateral foot drop. Histopathologic findings on muscle biopsies include fiber size variation, atrophic fibers, lack of inflammation, and the characteristic "rimmed" vacuoles on modified Gomori trichome staining. The diagnosis is confirmed by the presence of pathogenic (mostly missense) mutations in both alleles of the GNE gene. Although there is no approved therapy for this disease, preclinical and clinical studies of several potential therapies are underway, including substrate replacement and gene therapy-based strategies. However, developing therapies for GNE myopathy is complicated by several factors, including the rare incidence of disease, limited preclinical models, lack of reliable biomarkers, and slow disease progression.

Keywords: GNE myopathy; ManNAc; gene therapy; genetics; rare diseases; sialic acid..

Figures

Fig. 1

Sialic acid biosynthesis pathway. The biosynthesis of sialic acid (Neu5Ac) is an intracellular process depicted above with enzymes shown in yellow and substrates in white. The initial steps of this pathway occur in the cytoplasm, with the substrate, UDP-GlcNAc, which is derived from glucose. In the rate-limiting step of the pathway, UDP-GlcNAc is converted into ManNAc by UDP-GlcNAc 2-epimerase, encoded by the epimerase domain of GNE. ManNAc is phosphorylated by ManNAc kinase encoded by the kinase domain of GNE. Sialic acid becomes “activated” by CMP-sialic acid synthetase in the cell nucleus. CMP-sialic acid acts as a sialic acid donor to sialylate glycans on nascent glycoproteins (sia) and glycolipids in the Golgi; it also acts as a cytoplasmic feedback inhibitor of the UDP-GlcNAc 2-epimerase enzyme by binding to its allosteric site. Sialylation-increasing therapies (ManNAc and sialic acid), and sialylated glycoproteins (siallylactose and IVIG) are boxed in white. Adapted with permission from Xu et al. [1]

Fig. 2

Progressive muscle involvement in GNE myopathy. Representative lower extremity muscle MRI images of patients with GNE myopathy, with advancing disease progression from left to right. Coronal T1-weighted muscle MRI images (upper panels). Axial T1-weighted images of the mid-femoral thigh (middle panels). Axial T1-weighted images of the lower leg (lower panels). Progressive muscle atrophy of the lower extremities is noted initially in the anterior tibialis muscle (B lower panel), followed by involvement of muscles in the calves (C lower panel) and posterior thigh muscles (B, C middle panel), and finally involvement of the quadriceps in advanced stages of the disease (D middle panel)

Fig. 3

GNE gene and protein structure. (A) Exon (boxes)-intron (lines) structures of the two major human GNE mRNA transcripts, formed by alternative splicing of N-terminal exons. mRNA variant 1 (the longest splice form) encodes the hGNE2 protein, and mRNA variant 2 encodes the hGNE1 protein. Colored boxes represent the open reading frame and functional domains (colors correspond with translated protein domains in (B)). White boxes represent untranslated regions (UTRs) and white dotted lined boxes the skipped exons of each transcript. (B) hGNE2 protein structure. Gray UF, unknown function; dark red, UDP-GlcNAc 2-epimerase enzymatic activity encoding domain; vertical lined, putative nuclear export signal (NES); dotted, experimental allosteric region (AR) for CMP-sialic acid binding; green, ManNAc kinase enzymatic activity encoding domain. Amino acid numbering of both hGNE1 and hGNE2 protein isoforms is indicated below the structure. (C) Location on the GNE protein of frequent GNE mutations associated (see Table 1 for details)

Fig. 4

GNE Myopathy muscle histology and lectin histochemistry. (a) Stained cryosections of a muscle biopsy (biceps brachii) from a GNE myopathy patient. (1) H&E staining shows variation in muscle fiber size, atrophic fibers (arrowheads), and characteristic fibers with rimmed vacuoles (arrows). (2) Modified Gomori trichrome staining in which rimmed vacuoles appear as vacuoles rimmed by red granules. (3) Acid phosphatase staining showing intense lysosomal staining in regions with rimmed vacuoles, implying that these rimmed vacuolar areas maybe clusters of autophagic vacuoles. (4) Electron microscopy of a rimmed vacuole containing cytoplasmic region, confirming that structures are indeed composed of autophagic vacuoles and myeloid bodies, in addition to excessive cellular debris. Scale bar in (1) represents 50 μm in section images shown in (1, 2, 3). Scale bar in (4) denotes 1 nm. (b, c) Paraffin embedded skeletal muscle sections of control and GNE myopathy subjects (b) and mice (c) stained with the lectins (green) SNA (predominantly binding terminal α(2,6)-linked sialic acid on all glycans) or VVA (predominantly binding terminal GalNAc, without sialic acid attached, O-linked to serine or threonine residues of glycoproteins), costained with the nuclear dye DAPI (blue). GNE myopathy sections show hyposialylation, indicated by decreased staining of SNA and increased staining of VVA, compared to control sections. Muscle sections of GNE myopathy mutant mice (−/−) that received oral ManNAc therapy exhibited a sialylation status restored to the normal range. All confocal imaging was performed at the same microscope intensity settings per tissue and per lectin (with a 63× objective). All images are 1D projections of confocal Z-stacks. Images in (a): adapted with permission from Huizing et al., OMMBID [11]. Images in (b): adapted with permission from: Leoyklang et al. [65]. Images in (c): adapted with permission from Niethamer et al. [68]