Understanding pathophysiology of GNE myopathy and current progress towards drug development

GNE myopathy is a rare genetic neuromuscular disease that is caused due to mutations in the GNE gene responsible for sialic acid biosynthesis. Foot drop is the most common initial symptom observed in GNE myopathy patients. There is slow progressive muscle weakness in the lower and upper extremities while the quadriceps muscles are usually spared. The exact pathophysiology of the disease is unknown. Besides sialic acid biosynthesis, recent studies suggest either direct or indirect involvement of GNE in other cellular functions such as protein aggregation, apoptosis, ER stress, cell migration, HSP70 chaperone activity, autophagy, muscle atrophy, and myogenesis. Both animal and in vitro cell-based model systems are generated to elucidate the mechanism of GNE myopathy and evaluate the efficacy of therapies. The many therapeutic avenues explored include supplementation with sialic acid derivatives or precursors and gene therapy. Recent studies suggest other therapeutic options such as modulators of HSP70 chaperone (BGP-15), cofilin activator (CGA), and ligands like IGF-1 that may help to rescue cellular defects due to GNE dysfunction. This review provides an overview of the pathophysiology associated with GNE function in the cell and promising therapeutic leads to be explored for future drug development.

Hypertrophic Cardiomyopathy Complicated by Post-COVID-19 Myopericarditis in Patient with ANO5-Related Distal Myopathy

A 60-year-old male with hypertrophic cardiomyopathy, conduction disorders, post-COVID-19 myopericarditis and heart failure was admitted to the hospital's cardiology department. Blood tests revealed an increase in CPK activity, troponin T elevation and high titers of anticardiac antibodies. Whole exome sequencing showed the presence of the pathogenic variant NM_213599:c.2272C>T of the ANO5 gene. Results of the skeletal muscle biopsy excluded the diagnosis of systemic amyloidosis. Microscopy of the muscle fragment demonstrated sclerosis of the perimysium, moderate lymphoid infiltration, sclerosis of the microvessels, dystrophic changes and a lack of cross striations in the muscle fibers. Hypertrophy of the LV with a low contractile ability, atrial fibrillation, weakness of the distal skeletal muscles and increased plasma CPK activity and the results of the skeletal muscle biopsy suggested a diagnosis of a late form of distal myopathy (Miyoshi-like distal myopathy, MMD3). Post-COVID-19 myopericarditis, for which genetically modified myocardium could serve as a favorable background, caused heart failure decompensation.

Distal myopathy

Distal myopathies are a group of genetic, primary muscle diseases. Patients develop progressive weakness and atrophy of the muscles of forearm, hands, lower leg, or feet. Currently, over 20 different forms, presenting a variable age of onset, clinical presentation, disease progression, muscle involvement, and histological findings, are known. Some of them are dominant and some recessive. Different variants in the same gene are often associated with either dominant or recessive forms, although there is a lack of a comprehensive understanding of the genotype-phenotype correlations. This chapter provides a description of the clinicopathologic and genetic aspects of distal myopathies emphasizing known etiologic and pathophysiologic mechanisms.

Panorama of the distal myopathies

Distal myopathies are genetic primary muscle disorders with a prominent weakness at onset in hands and/or feet. The age of onset (from early childhood to adulthood), the distribution of muscle weakness (upper versus lower limbs) and the histological findings (ranging from nonspecific myopathic changes to myofibrillar disarrays and rimmed vacuoles) are extremely variable. However, despite being characterized by a wide clinical and genetic heterogeneity, the distal myopathies are a category of muscular dystrophies: genetic diseases with progressive loss of muscle fibers. Myopathic congenital arthrogryposis is also a form of distal myopathy usually caused by focal amyoplasia. Massive parallel sequencing has further expanded the long list of genes associated with a distal myopathy, and contributed identifying as distal myopathy-causative rare variants in genes more often related with other skeletal or cardiac muscle diseases. Currently, almost 20 genes (ACTN2, CAV3, CRYAB, DNAJB6, DNM2, FLNC, HNRNPA1, HSPB8, KHLH9, LDB3, MATR3, MB, MYOT, PLIN4, TIA1, VCP, NOTCH2NLC, LRP12, GIPS1) have been associated with an autosomal dominant form of distal myopathy. Pathogenic changes in four genes (ADSSL, ANO5, DYSF, GNE) cause an autosomal recessive form; and disease-causing variants in five genes (DES, MYH7, NEB, RYR1 and TTN) result either in a dominant or in a recessive distal myopathy. Finally, a digenic mechanism, underlying a Welander-like form of distal myopathy, has been recently elucidated. Rare pathogenic mutations in SQSTM1, previously identified with a bone disease (Paget disease), unexpectedly cause a distal myopathy when combined with a common polymorphism in TIA1. The present review aims at describing the genetic basis of distal myopathy and at summarizing the clinical features of the different forms described so far.

Sporadic Inclusion Body Myositis and Other Rimmed Vacuolar Myopathies

PURPOSE OF REVIEW

This article reviews the clinical, laboratory, and histopathologic features of sporadic inclusion body myositis (IBM) and explores its pathogenic overlap with inherited myopathies that have IBM-like pathology.

RECENT FINDINGS

Sporadic IBM is the most common acquired muscle disease in patients older than 50 years of age and is becoming more prevalent because of the increasing age of the population, the emerging development of more inclusive diagnostic criteria, and the advent of a diagnostic autoantibody. No effective therapy is known, and the pathogenic mechanism remains unclear. Some pathogenic insight can be gleaned from other myopathies with pathologic similarities or hereditary inclusion body myopathies. Although clinically distinct from sporadic IBM, preclinical models of hereditary inclusion body myopathy have offered an opportunity to move some therapies toward clinical development.

SUMMARY

Patients with sporadic IBM experience significant morbidity, and the disease is associated with a large unmet medical need. As therapies are developed, improved diagnosis will be essential. Early diagnosis relies on awareness, clinical history, physical examination, laboratory features, and appropriate muscle biopsy processing. Future research is needed to understand the natural history, identify genetic risk factors, and validate biomarkers to track disease progression. These steps are essential as we move toward therapeutic interventions.

[Therapeutic development for GNE myopathy.]

GNE myopathy is rare muscle disease which affect distal muscles. GNE gene, which encodes for a key enzyme in the sialic acid biosynthesis pathway, is mutated in the homozygote or compound heterozygote in the disease. The lack of sialic acid in skeletal muscle is the critical pathological process in GNE myopathy. GNE myopathy model mouse was established and supplementation of sialic acid improves the phenotype of model mouse. Phase Ⅰ clinical trial was conducted at Tohoku University Hospital using aceneuramic acid, followed by the trials using slow release product of sialic acid. Phase Ⅱ/Ⅲ study is ongoing.

Clinical and Electrophysiological Findings in Hereditary Inclusion Body Myopathy Compared With Sporadic Inclusion Body Myositis

OBJECTIVE

To compare the clinical and electrophysiological findings in hereditary inclusion body myopathy (hIBM) and sporadic inclusion body myositis (sIBM) patients.

METHODS

We retrospectively identified 8 genetically proven hIBM patients and 1 DNAJB6 myopathy with pathological features of hIBM, and compared their clinical, electromyographic, and serological data with a group of 51 pathologically proven sIBM patients.

RESULTS

hIBM patients had a younger mean age of onset (36 vs. 60 years, P = 0.0001). Diagnostic delay was shorter in sIBM (6 vs. 15 years, P = 0.0003). Wrist flexors (P = 0.02), digit flexors (P = 0.01), digit extensors (P = 0.02), and quadriceps (P = 0.008) muscles were more frequently affected in sIBM. Fibrillation potentials were more common in sIBM patients (P = 0.03). Electrical myotonia was found in 4 hIBM patients, not significantly different from sIBM patients (P = 0.45). Creatinine kinase was higher in sIBM patients (799 vs 232, P = 0.03).

CONCLUSIONS

sIBM and hIBM seem to have similar electromyographic changes. The combination of clinical, serological, and histopathological findings can guide genetic testing to the final diagnosis.

Distal myopathies

In this article, distal myopathy syndromes are discussed. A discussion of the more traditional distal myopathies is followed by discussion of the myofibrillar myopathies. Other clinically and genetically distinctive distal myopathy syndromes usually based on single or smaller family cohorts are reviewed. Other neuromuscular disorders that are important to recognize are also considered, because they show prominent distal limb weakness.

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.

Distal myopathy in multi-minicore disease

A 52-year-old man noted distal dominant slowly progressive muscle weakness at age 36 years. On muscle CT, the red muscles of the soleus, anterior tibial and paraspinal muscles, where type 1 fiber is known to predominate, were almost totally replaced by fat tissue while quadriceps femoris, gastrocnemius and upper extremity muscles were relatively spared. Quadriceps muscle biopsy revealed multi-minicores in addition to occasional larger cores, in about 70% of the type 1 fibers. A novel heterozygous nucleotide change c.5869T > A (p.S1957T) was identified in RYR1. Although pathogenicity was not confirmed, this nucleotide change was absent in 100 control DNA. We did not find a mutation in either multi-minicore disease-associated gene, SEPN1, or major distal myopathy-related genes, including GNE, ZASP, MYOT, exons 32-36 of MYH7, and the last exon of TTN. This is probably a unique form of distal myopathy characterized by the presence of multi-minicores with preferential involvement of type 1 fibers.

Heterozygous mutations affecting the epimerase domain of the GNE gene causing distal myopathy with rimmed vacuoles in a Taiwanese family

OBJECTIVES

Studies of distal myopathy with rimmed vacuoles (DMRV) revealed that most patients had mutations in the UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE) gene. However, the correlation between GNE mutations and clinical features was not fully understood.

PURPOSES

To report the correlation between the clinical features and genetic analysis of DMRV patients.

PATIENTS AND METHODS

The clinical presentations, histopathological findings, image studies, and genetic analyses of two patients with DMRV from a Taiwanese family were studied.

RESULTS

Two compound heterozygous mutations, Ile 241 Ser and Arg 246 Gln, located in the epimerase domain, were identified in both patients, who were of the same generation. In addition, the elder sister showed a progressive muscular dystrophy course with severe quadriceps and trunk muscle involvement.

CONCLUSION

The compound heterozygous mutations in the epimerase domain of the GNE gene are important in the severe phenotype of DMRV. However, the mechanisms leading to this phenotypic heterogeneity still remain to be elucidated.

Collagen VI related muscle disorders

Mutations in the genes encoding collagen VI (COL6A1, COL6A2, and COL6A3) cause Bethlem myopathy (BM) and Ullrich congenital muscular dystrophy (UCMD), two conditions which were previously believed to be completely separate entities. BM is a relatively mild dominantly inherited disorder characterised by proximal weakness and distal joint contractures. UCMD was originally described as an autosomal recessive condition causing severe muscle weakness with proximal joint contractures and distal hyperlaxity. Here we review the clinical phenotypes of BM and UCMD and their diagnosis and management, and provide an overview of the current knowledge of the pathogenesis of collagen VI related disorders.

Distal myopathy with rimmed vacuoles and cerebellar atrophy

Distal myopathies constitute a clinically and pathologically heterogeneous group of genetically determined neuromuscular disorders, where the distal muscles of the upper or lower limbs are affected. The disease of a 41-year-old male patient started with gait disturbances, when he was 25. The progression was slow, but after 16 years he became seriously disabled. Neurological examination showed moderate to severe weakness in distal muscles of all extremities, marked cerebellar sign and steppage gait. Muscle biopsy resulted in myopathic changes with rimmed vacuoles. Brain MRI scan showed cerebellar atrophy. This case demonstrates a rare association of distal myopathy and cerebellar atrophy.

Distal myopathies

PURPOSE OF REVIEW

The distal myopathies are a heterogeneous group of disorders that pose a challenge to both the clinician and geneticist. This article summarizes the findings of recent clinical, genetic and molecular studies and the current diagnostic approach to this group of patients.

RECENT FINDINGS

Publications over the past 5 years describe a number of new clinical phenotypes and genetic loci and further emphasize the overlap in clinical phenotype between a number of these disorders and between the distal and limb girdle myopathies and hereditary inclusion body myopathies. Recent studies have led to the identification of the genes and mutations responsible for early onset (Laing) myopathy and tibial (Udd) myopathy, and for distal myopathy with rimmed vacuoles (Nonaka), which has been shown to be allelic with quadriceps sparing hereditary inclusion body myopathy (IBM2), and have elucidated the underlying pathogenetic mechanisms in these conditions. New diagnostic approaches using magnetic resonance imaging, and a blood-based assay for dysferlin deficiency, have also been reported.

SUMMARY

These findings have important implications for future genetic linkage and gene expression studies and for the diagnostic approach to patients with a distal myopathy phenotype. They also hold promise for the eventual development of therapies for this group of disorders.

A new dominant distal myopathy affecting posterior leg and anterior upper limb muscles

OBJECTIVE

To report a dominant, slowly progressive early onset distal myopathy with sparing of the tibialis anterior.

METHODS

Twelve affected and two possibly affected members from an Australian kindred were examined and investigated by EMG, imaging studies, histopathology, and genetic analysis.

RESULTS

Affected patients had a slowly progressive condition with symmetric, distal weakness and wasting of the anterior upper and posterior lower limbs, with sparing of tibialis anterior, even in advanced disease. All patients remained ambulant and there was no evidence of cardiac or respiratory muscle involvement. Serum creatine kinase levels were either normal or mildly elevated. Imaging studies showed widespread involvement of the posterior and lateral leg compartments. Proximal muscles were radiologically abnormal only in advanced disease. Muscles that were mildly affected clinically appeared normal on imaging. EMG in nine patients showed widespread myopathic changes. Muscle histopathology in four patients showed either end stage muscle or nonspecific myopathic findings without inflammation or vacuoles. Microsatellite markers for distal myopathy loci were analyzed and all known distal myopathy phenotype genes and linkage regions were formally excluded by multipoint analysis.

CONCLUSIONS

The affected patients in this kindred display a clinically distinct myopathy, with selective involvement of posterior lower and anterior upper limb muscles. The genetic analysis suggests the existence of one more distal myopathy locus.

Laing early onset distal myopathy: slow myosin defect with variable abnormalities on muscle biopsy

BACKGROUND

Laing early onset distal myopathy (MPD1) is an autosomal dominant myopathy caused by mutations within the slow skeletal muscle fibre myosin heavy chain gene, MYH7. It is allelic with myosin storage myopathy, with the commonest form of familial hypertrophic cardiomyopathy, and with one form of dilated cardiomyopathy. However, the clinical picture of MPD1 is distinct from these three conditions.

OBJECTIVE

To collate and discuss the histological features reported in the muscle biopsies of MPD1 patients and to outline the clinical features.

RESULTS

The phenotype of MPD1 was consistent, with initial weakness of great toe/ankle dorsiflexion, and later development of weakness of finger extension and neck flexion. Age of onset was the only variable, being from birth up to the 20 s, but progression was always very slow. The pathological features were variable. In this retrospective series, there were no pathognomonic diagnostic features, although atrophic type I fibres were found in half the families. Rimmed vacuoles are consistently seen in all other distal myopathies with the exception of Myoshi distal myopathy. However, they were found in a minority of patients with MPD1, and were not prominent when present. Immunohistochemical staining for slow and fast myosin showed co-expression of slow and fast myosin in some type I fibres, possibly indicating a switch to type II status. This may be a useful aid to diagnosis.

CONCLUSIONS

The pathological findings in MPD1 are variable and appear to be affected by factors such as the specific muscle biopsied, the age of the patient at biopsy, and the duration of disease manifestations.

Early onset distal muscular dystrophy with normal dysferlin expression

A 7-year-old boy, who was noted to be a slow runner at the age of 2 years, had progressive muscle weakness and atrophy, preferentially affecting distal muscles. At 3 years of age, he had scoliosis and difficulty in standing on tip-toe. Serum creatine kinase was 1074IU/l. Muscle CT scan showed low-density areas in the lower legs and upper arms, but predominantly in the gastrocnemius and soleus muscles. Biopsy of the biceps brachii muscle showed moderate dystrophic changes with normal dysferlin expression on immunohistochemical and western blot analyses. Although muscle involvement mimicked that seen in Miyoshi myopathy (MM), the very early onset of the disease and scoliosis were quite unusual for MM. We, therefore, made the diagnosis of early onset dysferlin-positive distal muscular dystrophy, probably a new type of distal muscular dystrophy.

Tibial muscular dystrophy with late adult onset in a Spanish family

PURPOSE

We report autosomal dominant distal muscular dystrophy in 5 members of a Spanish family.

INTRODUCTION

This unusual muscular disorder has late adult onset and predominantly it affects the anterior compartment of the legs. This myopathy presented clinical and electromyographical characteristics, but unspecific histological findings. Early there have appeared genetical studies, the most frequently used is chromosome linkage, but it is not an absolute criterion for diagnosis, and it is not available in most hospitals. PATIENTS DESCRIPTIONS: In our cases walking difficulties appeared between the fourth and fifth decades, characterized by progressive and varied weakness with amyotrophy in the tibial anterior compartment. The electromyography confirmed the presence of a severe non-inflammatory myopathy, chronic and symmetric in the pretibial muscles and of less intensity in the calf muscles. The levels of creatine phosphokinase were normal and muscle biopsy identified a chronic, unspecific lesion with important fibrosis.

CONCLUSIONS

The findings, although with some phenotypical differences, were those commonly found in Markesbery-Griggs disease, tibial muscular dystrophy or late onset type 2 distal myopathy. We report a family affected by this muscular disorder, we describe the differential diagnosis and we discuss the review of the available literature.

[Cystinosis: an infrequent cause of distal myopathy]

INTRODUCTION

Cystinosis is a hereditary disease with clinical symptoms that are caused by the accumulation of cystine crystals in different tissues. Distal vacuolar myopathy has been reported as one of its later complications.

CASE REPORT

Here, we present the case of a 20-year-old male diagnosed with cystinosis at the age of 2 years, with severe renal involvement that required a transplant. The patient gradually developed weakness and atrophy of the muscles in his hands. Neurophysiological and histological studies enabled a diagnosis of distal vacuolar myopathy to be established, and electron microscopy revealed deposits of cystine crystals.

CONCLUSIONS

Cystinosis must be included within the differential diagnosis of distal myopathies. Timely treatment with cysteamine could prevent the development of this complication.