Hereditary inclusion body myopathy with a novel mutation in the GNE gene associated with proximal leg weakness and necrotizing myopathy

Mutation update for GNE gene variants associated with GNE myopathy

The GNE gene encodes the rate-limiting, bifunctional enzyme of sialic acid biosynthesis, uridine diphosphate-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE). Biallelic GNE mutations underlie GNE myopathy, an adult-onset progressive myopathy. GNE myopathy-associated GNE mutations are predominantly missense, resulting in reduced, but not absent, GNE enzyme activities. The exact pathomechanism of GNE myopathy remains unknown, but likely involves aberrant (muscle) sialylation. Here, we summarize 154 reported and novel GNE variants associated with GNE myopathy, including 122 missense, 11 nonsense, 14 insertion/deletions, and seven intronic variants. All variants were deposited in the online GNE variation database (http://www.dmd.nl/nmdb2/home.php?select_db=GNE). We report the predicted effects on protein function of all variants well as the predicted effects on epimerase and/or kinase enzymatic activities of selected variants. By analyzing exome sequence databases, we identified three frequently occurring, unreported GNE missense variants/polymorphisms, important for future sequence interpretations. Based on allele frequencies, we estimate the world-wide prevalence of GNE myopathy to be ∼4-21/1,000,000. This previously unrecognized high prevalence confirms suspicions that many patients may escape diagnosis. Awareness among physicians for GNE myopathy is essential for the identification of new patients, which is required for better understanding of the disorder's pathomechanism and for the success of ongoing treatment trials.

UDP-GlcNAc 2-Epimerase/ManNAc Kinase (GNE): A Master Regulator of Sialic Acid Synthesis

UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase is the key enzyme of sialic acid biosynthesis in vertebrates. It catalyzes the first two steps of the cytosolic formation of CMP-N-acetylneuraminic acid from UDP-N-acetylglucosamine. In this review we give an overview of structure, biochemistry, and genetics of the bifunctional enzyme and its complex regulation. Furthermore, we will focus on diseases related to UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase.

Distal myopathy with rimmed vacuoles: clinical and muscle morphological characteristics and spectrum of GNE gene mutations in 53 Chinese patients

OBJECTIVES

Distal myopathy with rimmed vacuoles (DMRV) is a typical autosomal recessive hereditary inclusion body myopathy, characterized by slowly progressive distal muscle weakness with relative sparing of the quadriceps. This study aimed to investigate the variability of clinical and morphological presentation and the spectrum of Glucosamine (UDP-N-acetyl)-2-epimerase/N-acetylmannosamine kinase (GNE) mutations in Chinese DMRV patients.

METHODS

We retrospectively reviewed the medical records of 37 patients with DMRV in PLA General Hospital from 1986 to 2011, and further conducted a review of 16 reported Chinese DMRV patients from other hospitals. We systematically analyzed the clinical, muscle morphological features and GNE gene mutation status of all DMRV patients.

RESULTS

A total of 53 DMRV patients were studied. Fourteen cases had family history and other 39 cases were sporadic. Fifteen cases showed atypical pathological presentation as mononuclear cell invasion into necrotic or non-necrotic muscle fibers. Rare initial symptom, earlier age of onset and more dysmorphic presentations were shown in sporadic patients. Eighteen mutations in GNE gene were identified. c.317T>C (p.I106T) was a novel GNE gene mutation. c.1892C>T (p.A631V), c.527A>T (p.D176V) and c.1523T>C (p.L508S) were the common GNE mutations in Chinese DMRV patients.

DISCUSSION

The clinical, pathological and genetic characteristics of DMRV are distinct in Chinese patients.

Limb-girdle phenotype is frequent in patients with myopathy associated with GNE mutations

The gene GNE encodes a bifunctional enzyme, UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase. Its mutations are found in distal myopathy with rimmed vacuoles (DMRV) and hereditary inclusion body myopathy (HIBM). Those disorders are characterized clinically by predominant anterior tibial muscle weakness and atrophy, and pathologically by rimmed vacuoles on muscle biopsy. We analyzed 11 Korean patients with GNE mutations. The mutations showed ethnic similarity to those of Japanese patients, showing the highest frequency with V572L. Another mutation of C13S was also found recurring in our patient group. Interestingly, about half of the patients showed limb-girdle myopathy rather than distal myopathy. This was further represented by limb muscle CT scans revealing atrophic hamstring and relatively spared anterior tibial muscle. However, quadriceps muscles were consistently spared both in distal and limb-girdle phenotypes. In conclusion, this study demonstrates a phenotypic diversity associated with GNE mutations. Recognizing a wider clinical spectrum of GNE mutations will help benefit more patients with imminent new therapy.

A novel autosomal dominant inclusion body myopathy linked to 7q22.1-31.1

We describe a novel autosomal dominant hereditary inclusion body myopathy (HIBM) that clinically mimics limb girdle muscular dystrophy in a Chinese family. We performed a detailed clinical assessment of 36 individuals spanning four generations. The age of onset ranged from the 30s to the 50s. Hip girdle, neck flexion and axial muscle weakness were involved at an early stage. This disease progressed slowly, and a shoulder girdle weakness appeared later in the disease course. Muscle biopsies showed necrotic, regenerating, and rimmed vacuolated fibers as well as congophilic inclusions in some of the fibers. Electron micrograph revealed cytoplasmic inclusions of 15–21 nm filaments. A genomewide scan and haplotype analyses were performed using an Illumina Linkage-12 DNA Analysis Kit (average spacing 0.58 cM), which traced the disease to a new locus on chromosome 7q22.1–31.1 with a maximum multi-point LOD score of 3.65. The critical locus for this unique disorder, which is currently referred to as hereditary inclusion body myopathy 4 (HIBM4), spans 8.78 Mb and contains 65 genes. This localization raises the possibility that one of the genes clustered within this region may be involved in this disorder.

Muscle imaging findings in GNE myopathy

GNE myopathy (MIM 600737) is an autosomal recessive muscle disease caused by mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene. Besides the typical phenotype, characterized by the initial involvement of the distal leg muscles that eventually spreads proximally with sparing of the quadriceps, uncommon presentations with a non-canonical clinical phenotype, unusual muscle biopsy findings or both are increasingly recognized. The aim of our study was to characterize the imaging pattern of pelvic and lower limb muscles in GNE myopathy, thus providing additional diagnostic clues useful in the identification of patients with atypical features. We retrospectively evaluated muscle MRI and CT scans of a cohort of 13 patients heterogeneous for GNE mutations and degree of clinical severity. We found that severe involvement of the biceps femoris short head and, to a lesser extent, of the gluteus minimus, tibialis anterior, extensor hallucis and digitorum longus, soleus and gastrocnemius medialis was consistently present even in patients with early or atypical disease. The vastus lateralis, not the entire quadriceps, was the only muscle spared in advanced stages, while the rectus femoris, vastus intermedius and medialis showed variable signs of fatty replacement. Younger patients showed hyperintensities on T2-weighted sequences in muscles with a normal or, more often, abnormal T1-weighted signal. Our results define a pattern of muscle involvement that appears peculiar to GNE myopathy. Although these findings need to be further validated in a larger cohort, we believe that the recognition of this pattern may be instrumental in the initial clinical assessment of patients with possible GNE myopathy.

Novel GNE mutations in two phenotypically distinct HIBM2 patients

Homozygous mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene cause hereditary inclusion body myopathy type 2 (HIBM2). We describe two unrelated American patients with novel GNE mutations. While one patient followed a typical disease course for HIBM2 with an onset at age 25 and rimmed vacuole pathology on muscle biopsy, the second patient had several features atypical for HIBM2. This patient's onset was at age 55, included distal weakness, quadriceps sparing and respiratory insufficiency. His muscle biopsy showed prominent necrosis without rimmed vacuoles. This study expands the phenotype and illustrates the clinical spectrum of HIBM2 identified in a U.S. based neuromuscular clinic.

Molecular modeling of the bifunctional enzyme UDP-GlcNAc 2-epimerase/ManNAc kinase and predictions of structural effects of mutations associated with HIBM and sialuria

The bifunctional enzyme UDP-GlcNAc 2-epimerase/ ManNAc kinase (GNE/MNK), encoded by the GNE gene, catalyzes the first two committed, rate-limiting steps in the biosynthesis of N-acetylneuraminic acid (sialic acid). GNE/MNK is feedback inhibited by binding of the downstream product, CMP-sialic acid in its allosteric site. GNE mutations can result in two human disorders, hereditary inclusion body myopathy (HIBM) or sialuria. So far, no active site geometry predictions or conformational transitions involved with function are available for mammalian GNE/MNK. The N-terminal GNE domain is homologous to various prokaryotic 2-epimerases, some of which have solved crystallographic structures. The C-terminal MNK domain belongs to the sugar kinases superfamily; its crystallographic structure is solved at 2.84 A and three-dimensional structures have also been reported for several other kinases. In this work, we employed available structural data of GNE/MNK homologs to model the active sites of human GNE/MNK and identify critical amino acid residues responsible for interactions with substrates. In addition, we modeled effects of GNE/MNK missense mutations associated with HIBM or sialuria on helix arrangement, substrate binding, and enzyme action. We found that all reported mutations are associated with the active sites or secondary structure interfaces of GNE/MNK. The Persian-Jewish HIBM founder mutation p.M712T is located at the interface alpha4alpha10 and likely affects GlcNAc, Mg2+, and ATP binding. This work contributes to further understanding of GNE/MNK function and ligand binding, which may assist future studies for therapeutic options that target misfolded GNE/MNK in HIBM and/or sialuria.

Hereditary inclusion body myopathy: a decade of progress

Hereditary Inclusion Body Myopathy (HIBM) is an autosomal recessive, quadriceps sparing type commonly referred to as HIBM but also termed h-IBM or Inclusion Body Myopathy 2 (IBM2). The clinical manifestations begin with muscle weakness progressing over the next 10-20 years uniquely sparing the quadriceps until the most advanced stage of the disease. Histopathology of an HIBM muscle biopsy shows rimmed vacuoles on Gomori's trichrome stain, small fibers in groups and tubulofilaments without evidence of inflammation. In affected individuals distinct mutations have been identified in the GNE gene, which encodes the bifunctional enzyme uridine diphospho-N-acetylglucosamine (UDP-GlcNAc) 2-epimerase/N-acetyl-mannosamine (ManNAc) kinase (GNE/MNK). GNE/MNK catalyzes the first two committed steps in the biosynthesis of acetylneuraminic acid (Neu5Ac), an abundant and functionally important sugar. The generation of HIBM animal models has led to novel insights into both the disease and the role of GNE/MNK in pathophysiology. Recent advances in therapeutic approaches for HIBM, including administration of N-acetyl-mannosamine (ManNAc), a precursor of Neu5Ac will be discussed.