Distal myopathy with rimmed vacuoles and hereditary inclusion body myopathy

The Synthesis of UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine Kinase (GNE), α-dystroglycan, and β-galactoside α-2,3-sialyltransferase 6 (ST3Gal6) By Skeletal Muscle Cell As a Response To Infection with Trichinella Spiralis

The Nurse cell of the parasitic nematode Trichinella spiralis is a unique structure established after genetic, morphological and functional modification of a small portion of invaded skeletal muscle fiber. Even if the newly developed cytoplasm of the Nurse cell is no longer contractile, this structure remains well integrated within the surrounding healthy tissue. Our previous reports suggested that this process is accompanied by an increased local biosynthesis of sialylated glycoproteins. In this work we examined the expressions of three proteins, functionally associated with the process of sialylation. The enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) is a key initiator of the sialic acid biosynthetic pathway. The α-dystroglycan was the only identified sialylated glycoprotein in skeletal muscles by now, bearing sialyl-α-2,3-Gal-β-1,4-Gl-cNAc-β-1,2-Man-α-1-O-Ser/Thr glycan. The third protein of interest for this study was the enzyme β-galactoside α-2,3-sialyltransferase 6 (ST3Gal6), which transfers sialic acid preferably onto Gal-β-1,4-GlcNAc as an acceptor, and thus it was considered as a suitable candidate for the sialylation of the α-dystroglycan. The expressions of the three proteins were analyzed by real time-PCR and immunohistochemistry on modified methacarn fixed paraffin tissue sections of mouse skeletal muscle samples collected at days 0, 14 and 35 post infection. According to our findings, the up-regulation of GNE was a characteristic of the early and the late stage of the Nurse cell development. Additional features of this process were the elevated expressions of α-dystroglycan and the enzyme ST3Gal6. We provided strong evidence that an increased local synthesis of sialic acids is a trait of the Nurse cell of T. spiralis, and at least in part due to an overexpression of α-dystroglycan. In addition, circumstantially we suggest that the enzyme ST3Gal6 is engaged in the process of sialylation of the major oligosaccharide component of α-dystroglycan.

The role of amyloid β in the pathological mechanism of GNE myopathy

GNE myopathy is a hereditary muscle disorder characterized by muscle atrophy and weakness initially involving the lower distal extremities. The treatment of GNE myopathy mainly focuses on a sialic acid deficiency caused by a mutation in the GNE gene, but it has not achieved the expected effect. The main pathological features of GNE myopathy are myofiber atrophy and rimmed vacuoles, including accumulation of amyloid β, which is mainly found in atrophic muscle fibers. Although the role of amyloid β and other misfolded proteins on the nervous system has been widely recognized, the cause and process of the formation of amyloid β in the pathological process of GNE myopathy are unclear. In addition, amyloid β has been reported to be linked to quality control mechanisms of proteins, such as molecular chaperones, the ubiquitin–proteasome system, and the autophagy-lysosome system. Herein, we summarize the possible reasons for amyloid β deposition and illustrate amyloid β-mediated events in the cells and their role in muscle atrophy in GNE myopathy. This review represents an overview of amyloid β and GNE myopathy that could help identify a potential mechanism and thereby a plausible therapeutic for the disease.

Skeletal Muscle Magnetic Resonance Biomarkers in GNE Myopathy

OBJECTIVE

To characterize muscle involvement and evaluate disease severity in patients with GNE myopathy using skeletal muscle MRI and proton magnetic resonance spectroscopy (¹H-MRS).

METHODS

Skeletal muscle imaging of the lower extremities was performed in 31 patients with genetically confirmed GNE myopathy, including T1-weighted and short tau inversion recovery (STIR) images, T1 and T2 mapping, and ¹H-MRS. Measures evaluated included longitudinal relaxation time (T1), transverse relaxation time (T2), and ¹H-MRS fat fraction (FF). Thigh muscle volume was correlated with relevant measures of strength, function, and patient-reported outcomes.

RESULTS

The cohort was representative of a wide range of disease progression. Contractile thigh muscle volume ranged from 5.51% to 62.95% and correlated with thigh strength (r = 0.91), the 6-minute walk test (r = 0.82), the adult myopathy assessment tool (r = 0.83), the activities-specific balance confidence scale (r = 0.65), and the inclusion body myositis functional rating scale (r = 0.62). Four stages of muscle involvement were distinguished by qualitative (T1W and STIR images) and quantitative methods: stage I: unaffected muscle (T1 = 1,033 ± 74.2 ms, T2 = 40.0 ± 1.9 ms, FF = 7.4 ± 3.5%); stage II: STIR hyperintense muscle with minimal or no fat infiltration (T1 = 1,305 ± 147 ms, T2 = 50.2 ± 3.5 ms, FF = 27.6 ± 12.7%); stage III: fat infiltration and STIR hyperintensity (T1 = 1,209 ± 348 ms, T2 = 73.3 ± 12.6 ms, FF = 57.5 ± 10.6%); and stage IV: complete fat replacement (T1 = 318 ± 39.9 ms, T2 = 114 ± 21.2 ms, FF = 85.6 ± 4.2%). ¹H-MRS showed a significant decrease in intramyocellular lipid and trimethylamines between stage I and II, suggesting altered muscle metabolism at early stages.

CONCLUSION

MRI biomarkers can monitor muscle involvement and determine disease severity noninvasively in patients with GNE myopathy.

CLINICALTRIALSGOV IDENTIFIER

NCT01417533.

Relationship between markers of disease activity and progression in skeletal muscle of GNE myopathy patients using quantitative nuclear magnetic resonance imaging and 31P nuclear magnetic resonance spectroscopy

Background

Quantitative nuclear magnetic resonance imaging (NMRI) is an objective and precise outcome measure for evaluating disease progression in neuromuscular disorders. We aimed to investigate predictive 'disease activity' NMR indices, including water T₂ and ³¹P NMR spectroscopy (NMRS), and its relation to NMR markers of 'disease progression', such as the changes in fat fraction (ΔFat%) and contractile cross-sectional area (ΔcCSA), in GNE myopathy (GNEM) patients.

Methods

NMR was performed on a 3T clinical scanner, at baseline and at a 1-year interval, in 10 GNEM patients and 29 age-matched controls. Dixon-based fat-water imaging and water T₂ mapping were acquired in legs and thighs, and in the dominant forearm. ³¹P NMRS was performed at the level of quadriceps and hamstring. Water T₂ and ³¹P NMRS indices were determined for all muscle groups and visits. Correlations were performed with 'disease progression' indices ΔFat%, ΔcCSA and the muscle fat transformation rate (R_{muscle_transf}).

Results

In quadriceps, known to be relatively preserved in GNEM, water T₂ at baseline was significantly higher compared to controls, and correlated strongly with the one-year evolution of Fat% and cCSA and R_{muscle_transf}. Various ³¹P NMRS indices showed significant differences in quadriceps and hamstring compared to controls and correlations existed between these indices and ΔFat%, ΔcCSA and R_{muscle_transf}.

Conclusions

This study demonstrates that disease activity indices such as water T₂ and ³¹P NMRS may predict disease progression in skeletal muscles of GNEM patients, and suggests that these measures may be considered to be valuable surrogate endpoints in the assessment of GNEM disease progression.

Quantitative nuclear magnetic resonance imaging detects subclinical changes over 1 year in skeletal muscle of GNE myopathy

BACKGROUND AND OBJECTIVE

To identify the most responsive and sensitive clinical outcome measures in GNE myopathy.

METHODS

ClinBio-GNE is a natural history study in GNE myopathy. Patients were assessed prospectively by clinical, functional and quantitative nuclear magnetic resonance imaging (qNMRI) evaluations. Strength and functional tests included Myogrip, Myopinch, MoviPlate and Brooke assessments for upper limb and the 6-min walk distance for lower limb. qNMRI was performed for determining the degree of fatty infiltration and trophicity in leg, thigh, forearm and hand skeletal muscles. Ten GNE myopathy patients were included. Three patients were non-ambulant. Age and gender-matched healthy subjects were used as controls.

RESULTS

Fatty infiltration and contractile cross-sectional area changed inversely and significantly in lower distal limbs and in proximal lower and distal upper limbs over 1 year. qNMRI indices and functional assessment results were strongly correlated.

CONCLUSIONS

Even in a limited number of patients, qNMRI could detect a significant change over a 1-year period in GNE myopathy, which suggests that qNMRI could constitute a surrogate endpoint in this slowly progressive disease. Quantitative NMRI outcome measures can monitor intramuscular fat accumulation with high responsiveness. Longer follow-up should improve our understanding of GNE myopathy evolution and also lead to the identification of non-invasive outcome measures with the highest discriminant power for upcoming clinical trials.

A phase 3 randomized study evaluating sialic acid extended-release for GNE myopathy

Objective

To investigate the efficacy and safety of aceneuramic acid extended-release (Ace-ER), a treatment intended to replace deficient sialic acid, in patients with GNE myopathy.

Methods

UX001-CL301 was a phase 3, double-blind, placebo-controlled, randomized, international study evaluating the efficacy and safety of Ace-ER in patients with GNE myopathy. Participants who could walk ≥200 meters in a 6-minute walk test at screening were randomized 1:1, and stratified by sex, to receive Ace-ER 6 g/d or placebo for 48 weeks and assessed every 8 weeks. The primary endpoint was change in muscle strength over 48 weeks measured by upper extremity composite (UEC) score. Key secondary endpoints included change in lower extremity composite (LEC) score, knee extensor strength, and GNE myopathy–Functional Activity Scale (GNEM-FAS) mobility domain score. Safety assessments included adverse events (AEs), vital signs, and clinical laboratory results.

Results

Eighty-nine patients were randomized (Ace-ER n = 45; placebo n = 44). Change from baseline to week 48 for UEC score between treatments did not differ (least square mean [LSM] Ace-ER −2.25 kg vs placebo −2.99 kg; LSM difference confidence interval [CI] 0.74 [−1.61 to 3.09]; p = 0.5387). At week 48, there was no significant difference between treatments for the change in key secondary endpoints: LEC LSM difference (CI) −1.49 (−5.83 to 2.86); knee extension strength −0.40 (−2.38 to 1.58); and GNEM-FAS mobility domain score −0.72 (−2.01 to 0.57). Gastrointestinal events were the most common AEs.

Conclusions

Ace-ER was not superior to placebo in improving muscle strength and function in patients with GNE myopathy.

Classification of evidence

This study provides Class I evidence that for patients with GNE myopathy, Ace-ER does not improve muscle strength compared to placebo.

Fighting the Cause of Alzheimer's and GNE Myopathy

Age is the common risk factor for both neurodegenerative and neuromuscular diseases. Alzheimer disease (AD), a neurodegenerative disorder, causes dementia with age progression while GNE myopathy (GNEM), a neuromuscular disorder, causes muscle degeneration and loss of muscle motor movement with age. Individuals with mutations in presenilin or amyloid precursor protein (APP) gene develop AD while mutations in GNE (UDP N-acetylglucosamine 2 epimerase/N-acetyl Mannosamine kinase), key sialic acid biosynthesis enzyme, cause GNEM. Although GNEM is characterized with degeneration of muscle cells, it is shown to have similar disease hallmarks like aggregation of Aβ and accumulation of phosphorylated tau and other misfolded proteins in muscle cell similar to AD. Similar impairment in cellular functions have been reported in both disorders such as disruption of cytoskeletal network, changes in glycosylation pattern, mitochondrial dysfunction, oxidative stress, upregulation of chaperones, unfolded protein response in ER, autophagic vacuoles, cell death, and apoptosis. Interestingly, AD and GNEM are the two diseases with similar phenotypic condition affecting neuron and muscle, respectively, resulting in entirely different pathology. This review represents a comparative outlook of AD and GNEM that could lead to target common mechanism to find a plausible therapeutic for both the diseases.

Distal myopathy with rimmed vacuoles: Spectrum of GNE gene mutations in seven Chinese patients

Distal myopathy with rimmed vacuoles (DMRV) is a rare, autosomal, recessive inherited disease caused by mutations in the GNE gene. DMRV is an adult-onset disorder characterized by progressive muscle atrophy and weakness, which initially involves the distal muscles with quadriceps sparing. To date, >150 GNE mutations have been reported in different populations from around the world. The present study investigated the clinical, pathological and genetic characteristics of seven unrelated DMRV patients from China. Genetic analysis in these patients revealed three novel mutations (c.455_456insC, p.P421L, and p.A287T) and five previously reported mutations (p.D207V, p.C44S, p.G576R, p.A669P, and p.D218G). In addition, the literature on DMRV was reviewed to provide an overview of the disease and broaden the mutational spectrum of the GNE gene in China.

Aceneuramic Acid Extended Release Administration Maintains Upper Limb Muscle Strength in a 48-week Study of Subjects with GNE Myopathy: Results from a Phase 2, Randomized, Controlled Study

Background: GNE Myopathy (GNEM) is a progressive adult-onset myopathy likely caused by deficiency of sialic acid (SA) biosynthesis.

Objective: Evaluate the safety and efficacy of SA (delivered by aceneuramic acid extended-release [Ace-ER]) as treatment for GNEM.

Methods: A Phase 2, randomized, double-blind, placebo-controlled study evaluating Ace-ER 3 g/day or 6 g/day versus placebo was conducted in GNEM subjects (n = 47). After the first 24 weeks, placebo subjects crossed over to 3 g/day or 6 g/day for 24 additional weeks (dose pre-assigned during initial randomization). Assessments included serum SA, muscle strength by dynamometry, functional assessments, clinician- and patient-reported outcomes, and safety.

Results: Dose-dependent increases in serum SA levels were observed. Supplementation with Ace-ER resulted in maintenance of muscle strength in an upper extremity composite (UEC) score at 6 g/day compared with placebo at Week 24 (LS mean difference +2.33 kg, p = 0.040), and larger in a pre-specified subgroup able to walk ≥200 m at Screening (+3.10 kg, p = 0.040). After cross-over, a combined 6 g/day group showed significantly better UEC strength than a combined 3 g/day group (+3.46 kg, p = 0.0031). A similar dose-dependent response was demonstrated within the lower extremity composite score, but was not significant (+1.06 kg, p = 0.61). The GNEM-Functional Activity Scale demonstrated a trend improvement in UE function and mobility in a combined 6 g/day group compared with a combined 3 g/day group. Patients receiving Ace-ER tablets had predominantly mild-to-moderate AEs and no serious adverse events.

Conclusions: This is the first clinical study to provide evidence that supplementation with SA delivered by Ace-ER may stabilize muscle strength in individuals with GNEM and initiating treatment earlier in the disease course may lead to better outcomes.

Sialic acid deficiency is associated with oxidative stress leading to muscle atrophy and weakness in GNE myopathy

Sialic acids are monosaccharides found in terminal sugar chains of cell surfaces and proteins; they have various biological functions and have been implicated in health and disease. Genetic defects of the GNE gene which encodes a critical bifunctional enzyme for sialic acid biosynthesis, lead to GNE myopathy, a disease manifesting with progressive muscle atrophy and weakness. The likely mechanism of disease is a lack of sialic acids. There remains, however, an unexplained link between hyposialylation and the muscle atrophy and weakness. In this study, we found that muscle proteins were highly modified by S-nitrosylation, and that oxidative stress-responsive genes were significantly upregulated, in hyposialylated muscles from human GNE myopathy patients and model mice. In both in vitro and in vivo models, the production of reactive oxygen species (ROS) was elevated with cellular hyposialylation, and increasing overall sialylation by extrinsic sialic acid intake reduced ROS and protein S-nitrosylation. More importantly, the antioxidant, oral N-acetylcysteine led to amelioration of the muscle atrophy and weakness in Gne mutant mice. Our data provide evidence of additional important function of sialic acids as a ROS scavenger in skeletal muscles, expanding our understanding on how sialic acid deficiency contributes to disease pathology, and identify oxidative stress as a therapeutic target in GNE myopathy.

Characterization of Strength and Function in Ambulatory Adults With GNE Myopathy

Objective:

To characterize the pattern and extent of muscle weakness and impact on physical functioning in adults with GNEM.

Methods:

Strength and function were assessed in GNEM subjects (n = 47) using hand-held dynamometry, manual muscle testing, upper and lower extremity functional capacity tests, and the GNEM-Functional Activity Scale (GNEM-FAS).

Results:

Profound upper and lower muscle weakness was measured using hand-held dynamometry in a characteristic pattern, previously described. Functional tests and clinician-reported outcomes demonstrated the consequence of muscle weakness on physical functioning.

Conclusions:

The characteristic pattern of upper and lower muscle weakness associated with GNEM and the resulting functional limitations can be reliably measured using these clinical outcome assessments of muscle strength and function.

Hereditary inclusion body myopathy: A myopathy with unique topography of weakness, yet frequently misdiagnosed: Case series and review of literature

Background:

Hereditary inclusion body myopathy (HIBM) continues to be underrecognized clinically despite a characteristic topography of weakness with total sparing of quadriceps muscles and patient being wheelchair bound. We report seven patients of HIBM from four families in North India.

Methods and Results:

Seven patients from four different families were diagnosed to have HIBM. There was no consanguinity in any of the families. While one patient had two affected siblings, another had one affected siblings and the family history was noncontributory in two patients. Two of the siblings were available for examination and confirmed clinically to be suffering from HIBM. Among the seven patients, only one was still ambulatory at the time of diagnosis.

Discussion:

This is the first case report of occurrence of HIBM in North Indian population. Despite its unique clinical presentation, HIBM is frequently misdiagnosed resulting in unnecessary diagnostic and therapeutic interventions. A high index of suspicion of this rare myopathy along with proper clinical examination may go a long way in accurate prognostication and management of these patients.

Mutation in the caveolin-3 gene causes asymmetrical distal myopathy

Mutations in the gene encoding caveolin-3 (CAV3) can cause a broad spectrum of clinical phenotypes, including limb girdle muscular dystrophy, rippling muscle disease, distal myopathy (MD), idiopathic persistent elevation of serum creatine kinase and cardiomyopathy. MD is a relatively rare subtype of caveolinopathy. Here, we report a sporadic case of a middle-aged female Chinese patient with MD in which a CAV3 mutation was identical to that previously reported in cases of rippling muscle disease. T1-weighted enhanced skeletal muscle MRI of the lower limbs showed an abnormal signal in the distal and proximal muscles. A muscle biopsy revealed moderate dystrophic changes, and immunohistochemical staining showed reduced CAV-3 expression in the plasmalemma. Genetic analysis revealed a heterozygous c.136G > A (p.Ala46Thr) CAV3 mutation that appeared to be de novo because it was absent from the patient's parents. This study suggested that the CAV3 c.136G > A (p.Ala46Thr) mutation can cause MD as well as different phenotypes in different individuals, suggesting that additional unknown loci must affect the disease phenotypes.

Muscle histopathology in today's era of molecular genetics: Role and limitations

INTRODUCTION

Past few decades have seen an increasing application of techniques like electron microscopy, western blotting, and molecular genetics in the evaluation of muscle diseases. However, due to their limited availability, histopathological interpretation of muscle biopsies still remains an important component of diagnostic approach to muscle diseases. A systematic methodology is required in the evaluation and interpretation of muscle biopsies. This study was undertaken to analyze the histopathological spectrum of 164 muscle biopsies and to assess the diagnostic yield of basic histopathological procedures in the work up of muscle biopsy.

MATERIALS AND METHODS

Retrospective analysis was done for 164 cases of muscle biopsies. Step-wise approach was adopted to assess the efficacy of routine stains, enzyme histochemistry, and immunohistochemistry. Based on hematoxylin and rosin (H and E) appearance, biopsies were broadly categorized into destructive, nondestructive but myopathic, and inflammatory morphology. Role of special stains, enzyme, and immunohistochemical stains in each category was then evaluated.

RESULTS

On the basis of histopathological features, 164 muscle biopsies were broadly categorized into biopsies with abnormal histopathological features (115) and biopsies with normal histopathology (49). Abnormal muscle biopsies were further categorized into destructive morphology (56.5%), nondestructive but myopathic morphology (30.5%), and inflammatory pathology (13%). A near definitive diagnosis could be made in 115 cases out of 164 muscle biopsies on the basis of routine histopathology, enzyme histochemistry, and immunohistochemistry.

CONCLUSION

Though advanced techniques like electron microscopy, western blotting, and molecular genetics are essential for confirmatory diagnosis, a substantive diagnostic yield can be offered with the basic armamentarium of routine (frozen) stains, enzyme histochemistry, and immunohistochemistry.

Genetic defects in the hexosamine and sialic acid biosynthesis pathway

BACKGROUND

Congenital disorders of glycosylation are caused by defects in the glycosylation of proteins and lipids. Classically, gene defects with multisystem disease have been identified in the ubiquitously expressed glycosyltransferases required for protein N-glycosylation. An increasing number of defects are being described in sugar supply pathways for protein glycosylation with tissue-restricted clinical symptoms.

SCOPE OF REVIEW

In this review, we address the hexosamine and sialic acid biosynthesis pathways in sugar metabolism. GFPT1, PGM3 and GNE are essential for synthesis of nucleotide sugars uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and cytidine-5'-monophospho-N-acetylneuraminic acid (CMP-sialic acid) as precursors for various glycosylation pathways. Defects in these enzymes result in contrasting clinical phenotypes of congenital myasthenia, immunodeficiency or adult-onset myopathy, respectively. We therefore discuss the biochemical mechanisms of known genetic defects in the hexosamine and CMP-sialic acid synthesis pathway in relation to the clinical phenotypes.

MAJOR CONCLUSIONS

Both UDP-GlcNAc and CMP-sialic acid are important precursors for diverse protein glycosylation reactions and for conversion into other nucleotide-sugars. Defects in the synthesis of these nucleotide sugars might affect a wide range of protein glycosylation reactions. Involvement of multiple glycosylation pathways might contribute to disease phenotype, but the currently available biochemical information on sugar metabolism is insufficient to understand why defects in these pathways present with tissue-specific phenotypes.

GENERAL SIGNIFICANCE

Future research on the interplay between sugar metabolism and different glycosylation pathways in a tissue- and cell-specific manner will contribute to elucidation of disease mechanisms and will create new opportunities for therapeutic intervention. This article is part of a Special Issue entitled "Glycans in personalised medicine" Guest Editor: Professor Gordan Lauc.

Novel Mutation of the GNE Gene Presenting Atypical Mild Clinical Feature: A Korean Case Report

Glucosamine (UDP-N-acetyl)-2-epimerase/N-acetylmannosamine kinase (GNE) myopathy is caused by mutations in GNE, a key enzyme in sialic acid biosynthesis. Here, we reported a case of GNE that presented with atypical mild clinical feature and slow progression. A 48-year-old female had a complaint of left foot drop since the age of 46 years. Electromyography (EMG) and muscle biopsy from left tibialis anterior muscle were compatible with myopathy. Genetic analysis led to the identification of c.1714G>C/c.527A>T compound heterozygous mutation, which is the second most frequent mutation in Japan as far as we know. Previous research has revealed that c.1714G>C/c.527A>T compound heterozygous mutation is a mild mutation as the onset of the disease is much later than the usual age of onset of GNE myopathy and the clinical course is slowly progressive. This was the first case report in Korea of the clinicopathological characteristics of GNE myopathy with GNE (c.1714G>C/c.527A>T compound heterozygous) mutation.

Muscle biopsy and UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase gene mutation analysis in two Chinese patients with distal myopathy with rimmed vacuoles

Distal myopathy with rimmed vacuoles is an autosomal recessive genetic disease characterized by weakness of the anterior compartment of the lower limbs, sparing the quadriceps muscle, and rimmed vacuoles in muscle biopsies. The disease is caused by a mutation in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene located on chromosome 9p13.3. We present two cases of Chinese patients with progressive lower extremity weakness. Clinical presentation, laboratory evaluation, electrodiagnostic testing, muscle pathology, and genetic analysis are described. Patient 1 was found to have heterozygous missense mutations (p.C13S and p.G576R) in the GNE gene and patient 2 had a homozygous missense mutation (p.C13S). The mutation p.C13S has been reported previously in China, Japan, and South Korea; however, the mutation p.G576R has not been described previously.

Genetics of GNE myopathy in the non-Jewish Persian population

GNE myopathy is an autosomal recessive adult-onset disorder characterized by progressive muscle atrophy and weakness, initially involving the distal muscles, while often sparing the quadriceps. It is caused by variants in the GNE gene that encodes a key bifunctional enzyme in the sialic acid biosynthetic pathway. We investigated the clinical and molecular characteristics of 18 non-Jewish Persian patients from 11 unrelated GNE myopathy families. In addition, we reviewed the previously reported cases and suggest genotype-phenotype correlations for the identified variants. Comprehensive clinical and laboratory evaluations were carried out. Sequencing of the GNE gene was performed using genomic DNA from the patients. Screening of the identified variants was performed in all relevant family members. Molecular analyses identified three causative homozygous GNE variants in 11 families: c.2228T>C (p. M743T) in 7, c.830G>A (p.R277Q) in 2, and one novel variation (c.804G>A) in 2 families that results in a synonymous codon change (p.L268=) and likely creates a novel splice site affecting the protein function. This study confirms that c.2228T>C (p.M743T) is the most prevalent disease-causing variant in the non-Jewish Persian population, but other GNE variants can cause GNE myopathy in this population. The patients with all three different variants had similar ages of onset. The youngest patient was an 18-year-old girl in whom the c.830G>A (p.R277Q) variant was identified, whereas the oldest onset age (31 years) was seen in a male patient with c.804G>A (p.L268=). The results of this investigation expand our knowledge about the genotype-phenotype correlations in GNE myopathy and aid in clinical management and therapeutic interventions.

Mutation profile of the GNE gene in Japanese patients with distal myopathy with rimmed vacuoles (GNE myopathy)

BACKGROUND

GNE myopathy (also called distal myopathy with rimmed vacuoles or hereditary inclusion body myopathy) is an autosomal recessive myopathy characterised by skeletal muscle atrophy and weakness that preferentially involve the distal muscles. It is caused by mutations in the gene encoding a key enzyme in sialic acid biosynthesis, UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE).

METHODS

We analysed the GNE gene in 212 Japanese GNE myopathy patients. A retrospective medical record review was carried out to explore genotype-phenotype correlation.

RESULTS

Sixty-three different mutations including 25 novel mutations were identified: 50 missense mutations, 2 nonsense mutations, 1 insertion, 4 deletions, 5 intronic mutations and 1 single exon deletion. The most frequent mutation in the Japanese population is c.1714G>C (p.Val572Leu), which accounts for 48.3% of total alleles. Homozygosity for this mutation results in more severe phenotypes with earlier onset and faster progression of the disease. In contrast, the second most common mutation, c.527A>T (p.Asp176Val), seems to be a mild mutation as the onset of the disease is much later in the compound heterozygotes with this mutation and c.1714G>C than the patients homozygous for c.1714G>C. Although the allele frequency is 22.4%, there are only three homozygotes for c.527A>T, raising a possibility that a significant number of c.527A>T homozygotes may not develop an apparent disease.

CONCLUSIONS

Here, we report the mutation profile of the GNE gene in 212 Japanese GNE myopathy patients, which is the largest single-ethnic cohort for this ultra-orphan disease. We confirmed the clinical difference between mutation groups. However, we should note that the statistical summary cannot predict clinical course of every patient.

Sialyllactose ameliorates myopathic phenotypes in symptomatic GNE myopathy model mice

Patients with GNE myopathy, a progressive and debilitating disease caused by a genetic defect in sialic acid biosynthesis, rely on supportive care and eventually become wheelchair-bound. To elucidate whether GNE myopathy is treatable at a progressive stage of the disease, we examined the efficacy of sialic acid supplementation on symptomatic old GNE myopathy mice that have ongoing, active muscle degeneration. We examined the therapeutic effect of a less metabolized sialic acid compound (6'-sialyllactose) or free sialic acid (N-acetylneuraminic acid) by oral, continuous administration to 50-week-old GNE myopathy mice for 30 weeks. To evaluate effects on their motor performance in living mice, spontaneous locomotion activity on a running wheel was measured chronologically at 50, 65, 72 and 80 weeks of age. The size, force production, and pathology of isolated gastrocnemius muscle were analysed at the end point. Sialic acid level in skeletal muscle was also measured. Spontaneous locomotion activity was recovered in 6'-sialyllactose-treated mice, while NeuAc-treated mice slowed the disease progression. Treatment with 6'-sialyllactose led to marked restoration of hyposialylation in muscle and consequently to robust improvement in the muscle size, contractile parameters, and pathology as compared to NeuAc. This is due to the fact that 6'-sialyllactose is longer working as it is further metabolized to free sialic acid after initial absorption. 6'-sialyllactose ameliorated muscle atrophy and degeneration in symptomatic GNE myopathy mice. Our results provide evidence that GNE myopathy can be treated even at a progressive stage and 6'-sialyllactose has more remarkable advantage than free sialic acid, providing a conceptual proof for clinical use in patients.

A Novel Mutation of the GNE Gene in Distal Myopathy with Rimmed Vacuoles: A Case with Inflammation

Distal myopathy with rimmed vacuoles (DMRV) is an autosomal recessive or sporadic early adult-onset myopathy caused by mutations in the UDP-N-acetylglucosamine 2-epimerase and N-acetylmannosamine kinase (GNE) gene. Characteristic pathologic features of DMRV are rimmed vacuoles on muscle biopsy and tubulofilamentous inclusion in ultrastructural study. Presence of inflammation in DMRV is unusual. We report a sporadic case of DMRV in a 40-year-old Thai man who presented with slowly progressive distal muscle weakness. Gene analysis revealed a compound heterozygous mutation of the GNE gene including a novel mutation c.1057A>G (p.K353E) and a known mutation c.2086G>A (p.V696M). The latter is the most common mutation in Thai DMRV patients. The muscle pathology was compatible with DMRV except for focal inflammation.

Non-specific accumulation of glycosphingolipids in GNE myopathy

BACKGROUND

UDP-GlcNAc 2-epimerase/ManNAc 6-kinase (GNE) is a bifunctional enzyme responsible for the first committed steps in the synthesis of sialic acid, a common terminal monosaccharide in both protein and lipid glycosylation. GNE mutations are responsible for a rare autosomal recessive neuromuscular disorder, GNE myopathy (also called hereditary inclusion body myopathy). The connection between the impairment of sialic acid synthesis and muscle pathology in GNE myopathy remains poorly understood.

METHODS

Glycosphingolipid (GSL) analysis was performed by HPLC in multiple models of GNE myopathy, including patients' fibroblasts and plasma, control fibroblasts with inhibited GNE epimerase activity through a novel imino sugar, and tissues of Gne(M712T/M712T) knock-in mice.

RESULTS

Not only neutral GSLs, but also sialylated GSLs, were significantly increased compared to controls in all tested models of GNE myopathy. Treatment of GNE myopathy fibroblasts with N-acetylmannosamine (ManNAc), a sialic acid precursor downstream of GNE epimerase activity, ameliorated the increased total GSL concentrations.

CONCLUSION

GNE myopathy models have increased total GSL concentrations. ManNAc supplementation results in decrease of GSL levels, linking abnormal increase of total GSLs in GNE myopathy to defects in the sialic acid biosynthetic pathway. These data advocate for further exploring GSL concentrations as an informative biomarker, not only for GNE myopathy, but also for other disorders of sialic acid metabolism.

Gne depletion during zebrafish development impairs skeletal muscle structure and function

GNE Myopathy is a rare recessively inherited neuromuscular disorder caused by mutations in the GNE gene, which codes for the key enzyme in the metabolic pathway of sialic acid synthesis. The process by which GNE mutations lead to myopathy is not well understood. By in situ hybridization and gne promoter-driven fluorescent transgenic fish generation, we have characterized the spatiotemporal expression pattern of the zebrafish gne gene and have shown that it is highly conserved compared with the human ortholog. We also show the deposition of maternal gne mRNA and maternal GNE protein at the earliest embryonic stage, emphasizing the critical role of gne in embryonic development. Injection of morpholino (MO)-modified antisense oligonucleotides specifically designed to knockdown gne, into one-cell embryos lead to a variety of phenotypic severity. Characterization of the gne knockdown morphants showed a significantly reduced locomotor activity as well as distorted muscle integrity, including a reduction in the number of muscle myofibers, even in mild or intermediate phenotype morphants. These findings were further confirmed by electron microscopy studies, where large gaps between sarcolemmas were visualized, although normal sarcomeric structures were maintained. These results demonstrate a critical novel role for gne in embryonic development and particularly in myofiber development, muscle integrity and activity.

A novel MYH7 mutation with prominent paraspinal and proximal muscle involvement

Laing distal myopathy (LDM) is caused by mutations in the MYH7 gene, and known to have muscle weakness of distal limbs and neck flexors. Through whole exome sequencing, we identified a novel p.Ala1439Pro MYH7 mutation in a Korean LDM family. This missense mutation is located in more N-terminal than any reported rod domain LDM mutations. In the early stage of disease, the present patients showed similar clinical patterns to the previously described patients of LDM. However, in the later stage, fatty replacement and atrophy of paraspinal or proximal leg muscles was more severely marked than lower leg muscles, and asymmetric atrophies were observed in trapezius, subscapularis and adductor magnus muscles. Distal myopathy like LDM showed marked and predominant fatty infiltrations in paraspinal or proximal leg muscles with marked asymmetry. These observations expand the clinical spectrum of LDM with the MYH7 mutation.

Clinical characteristics and molecular genetic analysis of Korean patients with GNE myopathy

PURPOSE

Glucosamine (UDP-N-acetyl)-2-epimerase/N-acetylmannosamine kinase (GNE) myopathy is an autosomal recessive neuromuscular disorder characterized by early adult-onset weakness of the distal muscles of the lower limbs. The clinical spectrum of GNE myopathy varies, and it is not clear how the same GNE gene mutations can result in different phenotypes. Here, we present clinical, pathological and genetic characteristics of twenty-one Korean patients with GNE myopathy.

MATERIALS AND METHODS

Twenty-one GNE myopathy patients were included in this study, conducted from 2004 to 2011. Based on medical records, patients' gender, onset age, family history, clinical history, serum creatine kinase (CK) level, neurologic examination, findings of muscle biopsy, muscle imaging findings and electrophysiologic features were extensively reviewed. Mutation of the GNE gene (9p13.3) was confirmed by DNA direct sequencing analysis in all patients.

RESULTS

The mean onset age was 23.8±8.8 years (mean±SD). Patient serum CK levels were slightly to moderately elevated, ranging from 41 to 2610 IU. Among the patients, twelve patients were female and nine patients were male. Except for eight patients, all of the patients presented initially with only distal muscle weakness in the lower extremities. The most common mutation was V572L, followed by C13S.

CONCLUSION

The clinical manifestations of our patients with GNE mutations varied. Among twenty-one patients, thirteen patients showed the typical GNE myopathy phenotype. There was no relationship between clinical features and site of mutation. Therefore, we suggest that neither homozygous nor compound heterozygous models are correlated with disease phenotype or disease severity.

Cell stress molecules in the skeletal muscle of GNE myopathy

Background

Mutations of the UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine-kinase (GNE)-gene are causally related to GNE myopathy. Yet, underlying pathomechanisms of muscle fibre damage have remained elusive. In sporadic inclusion body myositis (sIBM), the pro-inflammatory cell-stress mediators αB-crystallin and inducible nitric oxide synthase (iNOS) are crucial markers of the disease pathology.

Methods

10 muscle biopsies from GNE myopathy patients were analyzed for mRNA-expression of markers of cell-stress, inflammation and β-amyloid and compared to non-myopathic controls. Using double-labeling immunohistochemistry, serial sections of skeletal muscle biopsies were stained for amyloid precursor protein (APP), major histocompatibility complex (MHC)-I, αB-crystallin, neural cell adhesion molecule (NCAM), interleukin (IL)-1β, β-amyloid, iNOS, and phosphorylated neurofilament (P-neurofilament) as well as hematoxylin/eosin histochemistry. Corresponding areas of all biopsies with a total of 2,817 muscle fibres were quantitatively assessed for all markers.

Results

mRNA-expression of APP, NCAM, iNOS, TNF-α and TGF-β was higher in GNE myopathy compared to controls, yet this was not statistically significant. The mRNA-expression of APP and αB-crystallin significantly correlated with the expression of several pro-inflammatory and cell-stress-associated markers as NCAM, IL-1β, TGF-β, CCL-3, and CCL4. By immunohistochemistry, αB-crystallin and iNOS were co-upregulated and the number of fibres positive for αB-crystallin, NCAM, MHC-I and iNOS significantly correlated with each other. A large fraction of fibres positive for αB-crystallin were double positive for iNOS and vice-versa. Moreover, several fibres with structural abnormalities were positive for αB-crystallin and iNOS. Notably, particularly normal appearing fibres displayed an overexpression of these molecules.

Conclusions

The cell-stress molecules αB-crystallin and iNOS are overexpressed in GNE myopathy muscle and may identify early disease mechanisms. The data help to better understand the pathology of GNE myopathy.

The first Italian patient with oculopharyngodistal myopathy: case report and considerations on differential diagnosis

Oculopharyngodistal myopathy is a clinicopathologically distinct muscular disease. The underlying genetic defect has not been identified. We report here a 43-year old woman with asymmetric bilateral ptosis, dysphonia, swallowing difficulties, and weakness of the distal leg muscles. Serum creatine kinase was moderately increased. Electromyography revealed myopathic changes and myotonic discharges. Both cardiologic and pneumologic evaluation did not reveal abnormalities. Muscle computed tomography images showed adipose tissue replacement of abdominis rectus, lateral vastus, adductor magnus, and both the posterior and anterior compartment muscles below the knee, with prevalent involvement of medial gastrocnemius muscle. Muscle biopsy uncovered changes in fiber size and the presence of atrophic fibers with rimmed vacuoles of varying diameter, and core-like structures in type I fibers. Diagnosis was performed according to clinical and histopathologic findings, which were fully consistent with the other reported patients, and on the genetic exclusion of similar conditions such as oculopharyngeal muscular dystrophy, myotonic dystrophy type 1 and multi-minicore disease associated to RYR1 mutations. Differential diagnosis with mitochondrial myopathies, facioscapulohumeral muscular dystrophy and distal myopathies was also considered. This is the first Italian case of oculopharyngodistal myopathy, further suggesting the worldwide distribution of this rare neuromuscular disorder.

Peracetylated N-acetylmannosamine, a synthetic sugar molecule, efficiently rescues muscle phenotype and biochemical defects in mouse model of sialic acid-deficient myopathy

Distal myopathy with rimmed vacuoles/hereditary inclusion body myopathy (DMRV/hIBM), characterized by progressive muscle atrophy, weakness, and degeneration, is due to mutations in GNE, a gene encoding a bifunctional enzyme critical in sialic acid biosynthesis. In the DMRV/hIBM mouse model, which exhibits hyposialylation in various tissues in addition to muscle atrophy, weakness, and degeneration, we recently have demonstrated that the myopathic phenotype was prevented by oral administration of N-acetylneuraminic acid, N-acetylmannosamine, and sialyllactose, underscoring the crucial role of hyposialylation in the disease pathomechanism. The choice for the preferred molecule, however, was limited probably by the complex pharmacokinetics of sialic acids and the lack of biomarkers that could clearly show dose response. To address these issues, we screened several synthetic sugar compounds that could increase sialylation more remarkably and allow demonstration of measurable effects in the DMRV/hIBM mice. In this study, we found that tetra-O-acetylated N-acetylmannosamine increased cell sialylation most efficiently, and in vivo evaluation in DMRV/hIBM mice revealed a more dramatic, measurable effect and improvement in muscle phenotype, enabling us to establish analysis of protein biomarkers that can be used for assessing response to treatment. Our results provide a proof of concept in sialic acid-related molecular therapy with synthetic monosaccharides.

Characterization of the Asian myopathy patients with VCP mutations

BACKGROUND AND PURPOSE

Mutations in the valosin-containing protein (VCP) gene are known to cause inclusion body myopathy with Paget's disease of bone and frontotemporal dementia (IBMPFD) and familial amyotrophic lateral sclerosis (ALS). Despite an increasing number of clinical reports, only one Asian family with IBMPFD has been described.

METHODS

To characterize patients with VCP mutations, we screened a total of 152 unrelated Asian families who were suspected to have rimmed vacuolar myopathy.

RESULTS

We identified VCP mutations in seven patients from six unrelated Asian families. Five different missense mutations were found, including a novel p.Ala439Pro substitution. All patients had adult-onset progressive muscle wasting with variable involvement of axial, proximal, and distal muscles. Two of seven patients were suggested to have mild brain involvement including cerebellar ataxia, and only one showed radiological findings indicating a change in bone. Findings from skeletal muscle indicated mixed neurogenic and myogenic changes, fibers with rimmed vacuoles, and the presence of cytoplasmic and nuclear inclusions. These inclusions were immunopositive for VCP, ubiquitin, transactivation response DNA-binding protein 43, and also histone deacetylase 6 (HDAC6), of which function is regulated by VCP. Evidence of early nuclear and mitochondrial damage was also characteristic.

CONCLUSIONS

  Valosin-containing protein mutations are not rare in Asian patients, and gene analysis should be considered for patients with adult-onset rimmed vacuolar myopathy with neurogenic changes. A wide variety of central and peripheral nervous system symptoms coupled with rare bone abnormalities may complicate diagnosis.

A preclinical trial of sialic acid metabolites on distal myopathy with rimmed vacuoles/hereditary inclusion body myopathy, a sugar-deficient myopathy: a review

Distal myopathy with rimmed vacuoles (DMRV), also called hereditary inclusion body myopathy (hIBM), is a moderately progressive hereditary muscle disorder affecting young adults. DMRV/hIBM is characterized clinically by muscle atrophy and weakness initially involving the distal muscles, and pathologically by the presence of small angular fibers, formation of rimmed vacuoles and deposition of various proteins in the muscle fibers. This disease is known to be caused by mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase gene, which encodes the essential enzyme in sialic acid biosynthesis, leading to a reduction of sialic acid levels in the serum and skeletal muscles of affected patients. As it is a metabolic disease, metabolite supplementation is theoretically one of the therapeutic options. In this review, recent animal models for DMRV/hIBM are briefly characterized followed by a focus on the administration of sialic acid metabolites as a reliable therapeutic option to DMRV/hIBM with the following points highlighted: the property of compounds, the pharmacokinetic metabolism in vivo, and the therapeutic effects on the DMRV/hIBM mouse model.

Involvement of NGF in the rat model of persistent muscle pain associated with taut band

Myofascial pain syndrome (MPS) is an important clinical condition characterized by chronic muscle pain and a myofascial trigger point (MTrP) located in a taut band (TB). However, its pathogenic mechanism is still unclear. We developed an animal model relevant to conditions of MPS, and analyzed the mechanism of the muscle pain in this model. We applied eccentric contraction (EC) to a rat's gastrocnemius muscle (GM) for 2 weeks, and examined the mechanical withdrawal thresholds, histological changes, and expressions and contents of nerve growth factor (NGF). The mechanical withdrawal threshold decreased significantly at the next day of first EC and continued up to 9 days after EC. TBs were palpable at 3 to 8 days after initiation of EC. In EC animals, necrotic and regenerating muscle cells were found significantly more than in control animals. In EC animals, NGF expressions in regenerating muscle cells and NGF contents of GM were significantly higher than control animals. Administration of NGF receptor (TrkA) inhibitor K252a showed significant suppression of mechanical hyperalgesia in EC animals. Repeated EC induced persistent mechanical muscle hyperalgesia associated with TB. NGF expressed in regenerating muscle cells may have an important role in persistent mechanical muscle hyperalgesia which might be relevant to pathogenesis of MPS.

PERSPECTIVE

The present study shows that NGF expressed in regenerating muscle cells is involved in persistent muscular mechanical hyperalgesia. NGF-TrkA signaling in primary muscle afferent neurons may be one of the most important and promising targets for MPS.

The proteomic profile of hereditary inclusion body myopathy

Hereditary inclusion body myopathy (HIBM) is an adult onset, slowly progressive distal and proximal myopathy. Although the causing gene, GNE, encodes for a key enzyme in the biosynthesis of sialic acid, its primary function in HIBM remains unknown. The goal of this study was to unravel new clues on the biological pathways leading to HIBM by proteomic comparison. Muscle cultures and biopsies were analyzed by two dimensional gel electrophoresis (2-DE) and the same biopsy extracts by isobaric tag for relative and absolute quantitation (iTRAQ). Proteins that were differentially expressed in all HIBM specimens versus all controls in each analysis were identified by mass spectrometry. The muscle cultures 2-DE analysis yielded 41 such proteins, while the biopsies 2-DE analysis showed 26 differentially expressed proteins. Out of the 400 proteins identified in biopsies by iTRAQ, 41 showed altered expression. In spite of the different nature of specimens (muscle primary cultures versus muscle biopsies) and of the different methods applied (2D gels versus iTRAQ) the differentially expressed proteins identified in each of the three analyses where related mainly to the same pathways, ubiquitination, stress response and mitochondrial processes, but the most robust cluster (30%) was assigned to cytoskeleton and sarcomere organization. Taken together, these findings indicate a possible novel function of GNE in the muscle filamentous apparatus that could be involved in the pathogenesis of HIBM.

Muscular dystrophies: an update on pathology and diagnosis

Muscular dystrophies are clinically, genetically, and molecularly a heterogeneous group of neuromuscular disorders. Considerable advances have been made in recent years in the identification of causative genes, the differentiation of the different forms and in broadening the understanding of pathogenesis. Muscle pathology has an important role in these aspects, but correlation of the pathology with clinical phenotype is essential. Immunohistochemistry has a major role in differential diagnosis, particularly in recessive forms where an absence or reduction in protein expression can be detected. Several muscular dystrophies are caused by defects in genes encoding sarcolemmal proteins, several of which are known to interact. Others are caused by defects in nuclear membrane proteins or enzymes. Assessment of both primary and secondary abnormalities in protein expression is useful, in particular the hypoglycosylation of alpha-dystroglycan. In dominantly inherited muscular dystrophies it is rarely possible to detect a change in the expression of the primary defective protein; an exception to this is caveolin-3.

Metabolic manipulation of glycosylation disorders in humans and animal models

In the last decade, over 40 inherited human glycosylation disorders were identified. Most patients have hypomorphic, rather than null alleles. The phenotypic spectrum is broad and most of the disorders affect embryonic and early post-natal development; a few appear in adult life. Some deficiencies can be treated with simple dietary sugar (monosaccharide) supplements. Here we focus on four glycosylation disorders that have been treated with supplements in patients or in model systems, primarily the mouse. Surprisingly, small differences in the amount of exogenous sugar have a major impact on the diseases in specific cells or organs while others are unaffected. The underlying mechanisms are mostly unknown, but changes in the contributions of the de novo, salvage and dietary pathways may contribute to the beneficial outcome. Clearly, the metabolic chart is not flat; all arrows are not equally robust at all points of time and space. This metabolic perspective may help explain some of these observations and guide the development of other vertebrate models of glycosylation disorders that can respond to dietary manipulation.

[Eludication of pathomechanism of and development of therapy for autophagic vacuolar myopathies]

Autophagic vacuolar myopathy (AVM) is an entity defined by the presence of autophagic vacuoles on muscle pathology. There are two emerging categories in AVM in addition to the best characterized Pompe disease. One is Danon disease and its related disorders, which are characterized by autophagic vacuoles with unique sarcolemmal features (AVSF). AVSF express virtually all sarcolemmal proteins, in addition to acetylcholinesterase, on their vacuolar membranes. Danon disease is caused by primary deficiency of a lysosomal membrane protein, LAMP-2. Interestingly, in this disease, the number of AVSF increases as the patients age. Other AVSF myopathies include X-linked myopathy with excessive autophagy which is now known to be caused by VMA21 mutations. The other AVM is typified by the presence of rimmed vacuoles, which are actually clusters of autophagic vacuoles on electron microscopy. One of the well known diseases in this group is distal myopathy with rimmed vacuoles (DMRV), also called hereditary inclusion body myopathy (HIBM). DMRV is caused by mutations in GNE gene that encode a rate-limiting enzyme in the sialic acid biosynthetic pathway. Interestingly, in DMRV model mice, sialic acid supplementation almost completely precluded the disease phenotype, indicating that decreased sialic acid is the cause of myopathic phenotype and sialic acid supplementation can prevent the disease process. Interestingly, both genetically diagnosable AVSF myopathies are primarily due to lysosomal dysfunctions. In contrast, rimmed vacuoles are secondarily caused by extra-lysosomal defects, such as hyposialylation in DMRV/HIBM, and are formed at later stages of the disease.

[Development of therapy for distal myopathy with rimmed vacuoles]

Distal myopathy with rimmed vacuoles (DMRV), also called hereditary inclusion body myopathy, is an autosomal recessive disorder caused by homozygous or compound heterozygous missense mutations in GNE which encodes a protein with two enzymatic activities in sialic acid biosynthesis: UDP-GlcNAc 2-epimerase and ManNAc kinase. The disease starts from 1540 years and is slowly progressive. DMRV preferentially affects tibialis anterior and hamstrings muscles, and has characteristic findings in muscle pathology which include rimmed vacuoles, tubulofilamentous inclusions, deposition of amyloid, and phosphorylated tau. We generated DMRV mice (Gne -/- hGNE D176V-Tg) by crossmating Gne knock-out heterozygous mouse and human GNE p.D176V transgenic mouse. This model mouse recapitulates DMRV clinically, pathologically, and biochemically by developing muscle weakness and atrophy from 21 weeks, amyloid deposition from 31 weeks, and rimmed vacuoles and phosphorylated tau from 41 weeks while having lifelong hyposialylation. We gave three types of GNE metabolites, ManNAc, NeuAc and sialyllactose, to DMRV mice orally from 15 weeks until 55 weeks of age. Sialic acid supplementation almost completely precluded the disease and virtually no sign of DMRV was seen even at 55 weeks of age, indicating that decreased sialic acid is the cause of myopathic phenotype and sialic acid supplementation can prevent the disease process.

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.

Prophylactic treatment with sialic acid metabolites precludes the development of the myopathic phenotype in the DMRV-hIBM mouse model

Distal myopathy with rimmed vacuoles (DMRV)-hereditary inclusion body myopathy (hIBM) is an adult-onset, moderately progressive autosomal recessive myopathy; eventually, affected individuals become wheelchair bound1. It is characterized clinically by skeletal muscle atrophy and weakness, and pathologically by rimmed vacuoles, which are actually accumulations of autophagic vacuoles2, 3, 4, scattered angular fibers and intracellular accumulation of amyloid and other proteins5. To date, no therapy is available for this debilitating myopathy, primarily because the disease pathomechanism has been enigmatic. It is known that the disease gene underlying DMRV-hIBM is GNE, encoding glucosamine (UDP-N-acetyl)-2-epimerase and N-acetylmannosamine kinase6, 7, 8--two essential enzymes in sialic acid biosynthesis9. It is still unclear, however, whether decreased sialic acid production causes muscle degeneration, as GNE has been proposed to have roles other than for sialic acid biosynthesis10, 11, 12. By showing that muscle atrophy and weakness are completely prevented in a mouse model of DMRV-hIBM after treatment with sialic acid metabolites orally, we provide evidence that hyposialylation is indeed one of the key factors in the pathomechanism of DMRV-hIBM. These results support the notion that DMRV-hIBM can potentially be treated simply by giving sialic acids, a strategy that could be applied in clinical trials in the near future.

Muscle weakness correlates with muscle atrophy and precedes the development of inclusion body or rimmed vacuoles in the mouse model of DMRV/hIBM

Distal myopathy with rimmed vacuoles (DMRV), also called hereditary inclusion body myopathy (hIBM), is characterized clinically by weakness and atrophy that initially involves the distal muscles and pathologically by the presence of rimmed vacuoles (RVs) or intracellular protein deposits in myofibers. It is caused by mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene that is important in sialic acid synthesis. Recently, we generated a mouse model (Gne(-/-)hGNED176VTg) that exhibits muscle weakness and pathological changes similar to DMRV patients. To gain better understanding of the pathomechanism of DMRV, we determined temporal changes in the overall motor performance of this model mouse for DMRV in correlation with the structure and function of isolated skeletal muscles and muscle pathology. These DMRV mice exhibited muscle weakness, decreased whole muscle mass and cross-sectional area (CSA), and reduced contractile power in an age-related manner. Single-fiber CSA further supported the finding of muscle atrophy that involved both type I and type II fibers. These results suggest that atrophy is highly correlated with reduced production of force at young age, both in vivo and ex vivo, thereby implicating the important role of atrophy in the pathomechanism of DMRV. In older age, and particularly in gastrocnemius muscles, RVs and intracellular inclusions were seen in type IIA fibers, further aggravating reduction of force and specific increase in twitch-tetanus ratio.

Validation of GNE:p.M712T identification by melting curve analysis

Hereditary inclusion body myopathy/distal myopathy with rimmed vacuoles is an adult onset autosomal recessive muscle-wasting disease common in people of Iranian-Jewish descent, due to the founder allelic variant GNE:p.M712T. High correlation of disease susceptibility with GNE:p.M712T allows its use as a molecular marker for diagnosis. In this study, we applied and validated the use of melting curve analysis using SimpleProbe technology for detection of this mutation using specimens obtained by mouthwash, buccal swab, and whole blood. The assay was then applied to 43 clinical specimens, and results were validated by additional methods. A probe spanning this mutation in exon 12 accurately discerns two Tm corresponding to its hybridization to wild-type and M712T-derived amplicons. A 10 degrees C divergence in Tm allowed rapid single-tube genotyping of reference and patient samples with 100% accuracy. Distal myopathy constitutes a large heterogeneous group of pathologies with similar physiological manifestations and little molecular markers for distinguishing subtypes. Application of SimpleProbes for detection of GNE:p.M712T on genomic DNA obtained from buccal epithelial cells allows accurate, rapid, and cost-effective identification of this allele in individuals at risk. This procedure is amenable to automated high-throughput applications and can be extended to both clinical and research applications.

Expression of caveolar components in primary desminopathy

Myofibrillar myopathies (MFM) involve accumulation of various proteins in the muscle cytoplasm. In myopathy with a heterozygous A337P mutation of the desmin gene, electron-micrographs showed aggregates of vesicular and tubular structures. Positive cytoplasmic reaction for caveolin-3 immunohistochemistry and cholera toxin B binding suggested that caveolae comprised some of the aggregates. As caveolae occur in the Golgi complex and are transported to the cell surface, the results suggest inhibition of their trafficking to the sarcolemma. Alternatively, they could be trapped during internalization. We hypothesize that the accumulation of multiple proteins in MFM could be partially due to inhibited intracellular trafficking.

Histone H1 is released from myonuclei and present in rimmed vacuoles with DNA in inclusion body myositis

To investigate myonuclear alterations in sporadic inclusion body myositis (s-IBM), we immuno-localized histones in muscles in 11 patients. The examination showed that vacuolar rims were frequently positive for histone H1. In triple-color fluorescence study, the H1-positive products were found on the inner side of an emerin-positive circle with DNA. Moreover, H1-positive materials appeared to be released into the cytoplasm in some vacuoles and myonuclei. The localization of H1 was different from phosphorylated Elk-1, which is a nuclear protein, but abnormally accumulated in the cytoplasm in s-IBM. The results strongly support the hypothesis that rimmed vacuoles are derived from the nucleus. The cytoplasmic H1-release suggests dysfunction of nuclear membranes in an early phase of the nuclear disintegration. We hypothesize that, in s-IBM muscles, compromised nuclear envelope may permit release of some nuclear components such as histone H1 and cannot facilitate the incorporation of others to the nucleus as in pElk-1.

Mutation analysis of the GNE gene in distal myopathy with rimmed vacuoles (DMRV) patients in Thailand

Distal myopathy with rimmed vacuoles (DMRV) is an early-adult-onset, distal myopathy caused by a mutation of the UDP-N-acetylglucosamine 2 epimerase/N-acetylmannosamine kinase (GNE) gene. We herein report four Thai patients with DMRV who carried compound heterozygous mutations of the GNE gene including three novel (p.G89R, p.P511T, and p.I656N) and two known mutations (p.A524V and p.V696M). All patients shared p.V696M in one allele. Our study demonstrates the mutation spectrum of the GNE gene in Thai patients with DMRV.

A Gne knockout mouse expressing human V572L mutation develops features similar to distal myopathy with rimmed vacuoles or hereditary inclusion body myopathy

Distal myopathy with rimmed vacuoles (DMRV) or hereditary inclusion myopathy (h-IBM) is an early adult-onset distal myopathy caused by mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene which encodes for a bifunctional enzyme involved in sialic acid biosynthesis. It is pathologically characterized by the presence of rimmed vacuoles especially in atrophic fibers, which also occasionally contain congophilic materials that are immunoreactive to beta-amyloid, lysosomal proteins, ubiquitin and tau proteins. To elucidate the pathomechanism of this myopathy and to explore the treatment options, we generated a mouse model of DMRV/h-IBM. We knocked out the Gne gene in the mouse, but this resulted in embryonic lethality. We therefore generated a transgenic mouse that expressed the human GNEV572L mutation, which is the most prevalent among Japanese DMRV patients, and crossed this with Gne((+/-)) mouse to obtain Gne((-/-))hGNEV572L-Tg. Interestingly, these mice exhibit marked hyposialylation in serum, muscle and other organs. Reduction in motor performance in these mice can only be seen from 30 weeks of age. A compelling finding is the development of beta-amyloid deposition in myofibers by 32 weeks, which clearly precedes rimmed vacuole formation at 42 weeks. These results show that the Gne((-/-)) hGNEV572L-Tg mouse mimics the clinical, histopathological and biochemical features of DMRV/h-IBM, making it useful for understanding the pathomechanism of this myopathy and for employing different strategies for therapy. Our findings underscore the notion that hyposialylation plays an important role in the pathomechanism of DMRV/h-IBM.