Clinical, Histopathologic, and Genetic Features of Patients With Myofibrillary and Distal Myopathies: Experience From the Italian Network.

BACKGROUND AND OBJECTIVES: The diagnostic process for myofibrillar myopathies (MFM) and distal myopathies (DM) is particularly complex because of the large number of causative genes, the existence of still molecularly undefined disease entities, and the overlapping features between the 2 categories. This study aimed to characterize a large cohort of patients affected by MFM and DM and identify the most important diagnostic and prognostic aspects of these diseases. METHODS: Patients with either a myopathological diagnosis of MFM or a clinical diagnosis of DM were included in this retrospective multicentric national study. Demographic, genetic, clinical, and histopathologic data of anonymized patients were collected from the neuromuscular centers of the Italian Association of Myology network. RESULTS: Data regarding 132 patients with MFM (mean age 57.0 ± 15.8 years, 49% female) and 298 patients with DM (mean age 50.7 ± 15.9 years, 40% female) were gathered from 20 neuromuscular centers. 69 patients fulfilled the criteria for both groups (distal myopathies with myofibrillar pathology, DM-MP). Molecular confirmation was achieved in 63% of the patients. Fifty-two percent of the patients with MFM carried pathogenic variants in either DES (n = 30), MYOT (n = 20), or DNAJB6 (n = 18), which were also the most frequent disease-causing genes in DM-MP, while GNE (n = 44) and MYH7 (n = 23) were the genes most commonly carrying pathogenic variants in DM. The mean age at onset varied from <25 years in patients with causative variants in MYH7 and DYSF to 59 years in patients with myotilinopathies. Cardiac involvement was reported in 29% of patients with MFM and 16% of patients with DM, with DES and MYH7 variants significantly associated with the development of cardiomyopathy. Respiratory impairment was more prevalent in patients with TTN and DES variants and rare in other disorders such as GNE myopathy and dysferlinopathies, which were instead associated, together with DNAJB6-related and PLIN4-related myopathies, with the risk of losing ambulation during the disease course. DISCUSSION: The Italian cohort of patients with MFM and DM recapitulates the phenotypic heterogeneity and the partial overlap between the 2 groups. However, in relative contrast to the encountered phenotypic variability, only 5 genes accounted for most of the molecular diagnoses. Specific genetic entities are associated with significantly increased risk of developing cardiorespiratory complications or loss of ambulation, which has relevant prognostic implications.

Functional characterization of GNE mutations prevalent in Asian subjects with GNE myopathy, an ultra-rare neuromuscular disorder.

UDP-GlcNAc 2-epimerase/ManNAc kinase (GNE) is a bifunctional enzyme (N-terminal epimerase and C-terminal Kinase domain) that catalyses the rate limiting step in sialic acid biosynthesis. More than 200 homozygous missense or compound heterozygous mutations in GNE have been reported worldwide to cause a rare neuromuscular disorder, GNE myopathy. It is characterized by a slowly progressive defect in proximal and distal skeletal muscles with patients becoming wheel-chair-bound. There are no current approved therapies available for GNE myopathy. ManNAc therapy is currently in advanced clinical trials and has shown signs of slowing the disease progression in a phase 2 trial. The present study aims to understand the effect of GNE mutation on its enzymatic activity and identification of potential small effector molecules. We characterized different GNE mutations (p.Asp207Val, p.Val603Leu, p.Val727Met, p.Ile618Thr and p.Arg193Cys) prevalent in Asian population that were cloned, expressed and purified from Escherichia coli as full-length recombinant proteins. Our study demonstrates that full length GNE can be expressed in E. coli in its active form and analysed for the functional activity. Each mutation showed variation in epimerase and kinase activity and responded to the small effector molecules (metformin, BGP-15 kaempferol, catechin, quercetin) in a differential manner. Our study opens an area for futuristic structural determination of full length GNE and identification of potential therapeutic molecules.

Extra-muscular manifestations in GNE myopathy patients: A nationwide repository questionnaire survey in Japan.

OBJECTIVE: GNE myopathy is a rare autosomal recessive adult-onset distal myopathy caused by biallelic pathogenic variants in GNE. Although some extra-muscular manifestations associated with GNE myopathy have been reported, little is known about whether they are disease-specific and how often they present. This study aimed to characterize extra-muscular manifestations of GNE myopathy. METHODS: We conducted a questionnaire survey of GNE myopathy patients registered in a national registry in Japan. The questionnaire requested information regarding idiopathic thrombocytopenia, cardiac involvement, respiratory involvement, sleep apnea syndrome (SAS), and psychiatric diseases. RESULTS: The response rate was 62.4% (126/198), yielding a total of 51 male and 75 female participants. Of the participants, 4.1% (5/123) had a diagnosis of idiopathic thrombocytopenia, and 16.3% (8/49) of males and 6.6% of females (5/76) had a diagnosis of SAS. In total, 0.8% (1/126) of participants had pervasive developmental disabilities and 14.7% (16/109) had a psychiatric disease. CONCLUSION: The frequencies of idiopathic thrombocytopenia and SAS among Japanese GNE myopathy patients were higher than those observed in the general Japanese population. Routine blood tests and evaluation of sleep-disordered breathing should be considered in order to better manage GNE myopathy patients.

Comprehensive genetic sequence and copy number analysis for Charcot-Marie-Tooth disease in a Canadian cohort of 2517 patients.

Charcot-Marie-Tooth disease (CMT) is one of the most common Mendelian disorders characterised by genetic heterogeneity, progressive distal muscle weakness and atrophy, foot deformities and distal sensory loss. In this report, we describe genetic testing data including comprehensive sequencing and copy number analysis of 34 CMT-related genes in a Canadian cohort of patients with suspected CMT. We have demonstrated a notable gender testing bias, with an overall diagnostic yield of 15% in males and 21% in females. We have identified a large number of novel pathogenic variants as well as variants of unknown clinical significance in CMT-related genes. In this largest to date analysis of gene CNVs in CMT, in addition to the common PMP22 deletion/duplication, we have described a significant contribution of pathogenic CNVs in several CMT-related genes. This study significantly expand the mutational spectrum of CMT genes, while demonstrating the clinical utility of a comprehensive sequence and copy number next-generation sequencing-based clinical genetic testing in patients with suspected diagnosis of CMT.

GNE myopathy in Chinese population: hotspot and novel mutations.

GNE myopathy is a rare autosomal recessive distal myopathy caused by mutations in UDP-N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase (GNE), the bi-functional enzyme critical for sialic acid biosynthesis. In this study, we summarized the clinical features, pathological characteristics, and genetic profiles of 46 GNE patients. The clinical and mutational profile of 54 previously reported Chinese patients were also reviewed. A total of 21 novel mutations, including a gross deletion spanning exon 1-2 and a retrotransposon insertion were found in our cohort, enlarging the spectrum of GNE mutations. The most frequent mutation in Chinese population was D207V, which accounts for 25.5% of total alleles (51/200). The age of onset was much later in the patients carrying D207V compared to other patients, indicated the less deleterious effect of D207V on enzyme activity. GNE myopathy may be overlooked in China with a relatively milder phenotype due to the common mutation D207V.

Phenotypic stratification and genotype-phenotype correlation in a heterogeneous, international cohort of GNE myopathy patients: First report from the GNE myopathy Disease Monitoring Program, registry portion.

GNE myopathy is a rare distal myopathy, caused by mutations in the GNE gene, affecting sialic acid synthesis. Clinical presentation varies from asymptomatic early stage patients to severely debilitating forms. This first report describes clinical presentations and severity of the disease, using data of 150 patients collected via the on-line, patient-reported registry component of the GNE Myopathy Disease Monitoring Program (GNEM-DMP). Disease progression was prospectively analysed, over a 2-year period, using the GNE myopathy functional activity scale (GNEM-FAS). The average annual rates of decline in function were estimated at -9.6% and -3.2% in ambulant and non-ambulant patients respectively. 4.3% of participants became non-ambulant within one year. The mean time from onset to required use of a wheelchair was 11.9 years. Mean delay of genetic diagnosis from symptom onset was 5.2 years. Mutation specific analysis demonstrated genotype-phenotype relationships; i.e. p.Ala662Val may be associated with a more severe phenotype, compared to p.Val727Met. Patients with compound heterozygous mutation in epimerase and kinase domain appeared to have a more severe phenotype compared to patients with both mutations located within one domain. Acknowledging the limitations of the study, these findings suggest that the severity of the GNE mutations affects disease severity. The GNEM-DMP is a useful data collection tool, prospectively measuring the progression of GNE myopathy, which could play an important role in translational and clinical research and further understanding of genotype-phenotype correlations.

[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.

Distal myopathies in Finnish patients.

Distal myopathies are a group of rare muscular dystrophies comprising more than 20 different genetic entities. The first distal myopathy in Finland, tibial muscular dystrophy, was identified more than 20 years ago. Muscle weakness predominantly affects the feet and hands, although variable weakness can be detected clinically and on muscle MRI in the proximal muscles in the later stages of the disease. Advanced molecular genetic techniques have enabled identification of several distinct distal myopathies in Finland. The clinical findings of different distal myopathies overlap, but there are also distinguishable differences that might help final genetic diagnostics.

[Sialic Acid Replacement Therapy for Distal Myopathy with Rimmed Vacuoles].

Distal myopathy with rimmed vacuoles or GNE myopathy, is an early adult-onset myopathy with slow progression that preferentially affects the tibialis anterior muscle. Severely affected patients show marked limb muscle atrophy together with respiratory dysfunction. The disease is caused by a mutation in the GNE gene that catalyzes two rate-limiting reactions in cytosolic sialic acid synthesis. Oral treatment with sialic acid metabolite prevents muscle atrophy and weakness in a mouse GNE myopathy model and a global Phase III study is currently underway. In addition, a global patient registry of neuromuscular cases is widely accepted as a useful tool to obtain epidemiological data and bolster patient recruitment for further development of this treatment strategy.

Mutational spectrum and clinical features in 35 unrelated mainland Chinese patients with GNE myopathy.

GNE myopathy is an autosomal recessive distal myopathy caused by biallelic mutation in the GNE gene. It shows great genetic heterogeneity among different ethnic groups. In this study, we summarized the mutational spectrum and clinical profiles in 35 unrelated GNE myopathy patients from mainland China. Molecular analysis revealed 16 novel (p.G47D, p.F66Y, p.E173A, p.Y186H, p.R246L, p.R263*, p.R306*, p.A366D, p.V512M, p.C520Y, p.G545R, p.G548S, p.V622G, p.A638P, IVS2+1G>A and c.2112delC) and 13 reported mutations. Notably, the p.D176V mutation was detected in 65.7% (23/35) of this patient cohort, giving an allele frequency of 34.3% (24/70). We estimated the carrier frequency of p.D176V to be 0.19% (1/520) in the normal population, although haplotype analysis indicated no founder effect in the patients carrying p.D176V mutation. Clinically, 29 patients presented with the classic phenotype of predominant distal weakness, while six patients presented with atypical phenotype. However, muscle magnetic resonance imaging showed that the vastus lateralis was spared in both subgroups. In conclusion, p.D176V mutation in the GNE gene, which was the second most common mutation in Japanese patients, was the most common mutation in this cohort of Chinese patients. Novel GNE mutations found in this study expanded the mutational spectrum associated with GNE myopathy. There is phenotypic heterogeneity among patients with GNE myopathy, but muscle magnetic resonance imaging can be useful for differential diagnosis.

GNE myopathy: current update and future therapy.

GNE myopathy is an autosomal recessive muscle disease caused by biallelic mutations in GNE, a gene encoding for a single protein with key enzymatic activities, UDP-N-acetylglucosamine 2-epimerase and N-acetylmannosamine kinase, in sialic acid biosynthetic pathway. The diagnosis should be considered primarily in patients presenting with distal weakness (foot drop) in early adulthood (other onset symptoms are possible too). The disease slowly progresses to involve other lower and upper extremities' muscles, with marked sparing of the quadriceps. Characteristic findings on biopsies of affected muscles include 'rimmed' (autophagic) vacuoles, aggregation of various proteins and fibre size variation. The diagnosis is confirmed by sequencing of the GNE gene. Note that we use a new mutation nomenclature based on the longest transcript (GenBank: NM_001128227), which encodes a 31-amino acid longer protein than the originally described one (GenBank: NM_005476), which has been used previously in most papers. Based upon the pathophysiology of the disease, recent clinical trials as well as early gene therapy trials have evaluated the use of sialic acid or N-acetylmannosamine (a precursor of sialic acid) in patients with GNE myopathy. Now that therapies are under investigation, it is critical that a timely and accurate diagnosis is made in patients with GNE myopathy.

Nationwide patient registry for GNE myopathy in Japan.

BACKGROUND: GNE myopathy is a slowly progressive autosomal recessive myopathy caused by mutations in the GNE (glucosamine (UDP-N-acetyl)-2-epimerase/N-acetylmannosamine kinase) gene. This study aimed to (1) develop a nationwide patient registry for GNE myopathy in order to facilitate the planning of clinical trials and recruitment of candidates, and (2) gain further insight into the disease for the purpose of improving therapy and care. METHODS: Medical records of genetically-confirmed patients with GNE myopathy at the National Center Hospital of the National Center of Neurology and Psychiatry (NCNP) were retrospectively reviewed in order to obtain data reflecting the severity and progression of the disease. We also referred to items in the datasheet of the nationwide registry of dystrophinopathy patients in the Registry of Muscular Dystrophies (Remudy). Items selected for the registration sheet included age, sex, age at onset, past history and complications, family history, body weight and height, pathological findings of muscle biopsy, grip power, walking ability, respiratory function, cardiac function, willingness to join upcoming clinical trials, and participation in patient associations. A copy of the original genetic analysis report was required of each patient. RESULTS: We successfully established the Remudy-GNE myopathy. Currently, 121 patients are registered nationwide, and 93 physicians from 73 hospitals collaborated to establish the registry. The mean age at onset was 27.7 ± 9.6 years, and 19.8% (24/121) of patients could walk without assistance. Mean presumed durations from onset to use of assistive devices (cane and/or braces) and a wheelchair, and loss of ambulation were 12.4, 15.2, and 21.1 years, respectively. Three patients had a past history and/or complication of idiopathic thrombocytopenia. To share the progress of this study with the community, newsletters were published on a regular basis, and included information regarding new phase I clinical trials for GNE myopathy. The newsletters also served as a medium to bring attention to the importance of respiratory evaluation and care for respiratory insufficiency. CONCLUSION: The Japanese Remudy-GNE myopathy is useful for clarifying the natural history of the disease and recruiting patients with genetically-confirmed GNE myopathy for clinical trials.

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.

Two recurrent mutations are associated with GNE myopathy in the North of Britain.

OBJECTIVE: GNE myopathy is a rare recessive myopathy associated with inclusion bodies on muscle biopsy. The clinical phenotype is associated with distal muscle weakness with quadriceps sparing. Most of the current information on GNE myopathy has been obtained through studies of Jewish and Japanese patient cohorts carrying founder mutations in the GNE gene. However, little is known about GNE myopathy in Europe where the prevalence is thought to be very low. METHODS: Patients were referred through the National Specialist Commissioning Team service for limb-girdle muscular dystrophies at Newcastle (UK). All patients harbouring mutations in the GNE gene were recruited for our study. Detailed clinical and genetic data as well as muscle MRIs and muscle biopsies were reviewed. RESULTS: We identified 26 patients harbouring mutations in the GNE gene. Two previously reported mutations (c.1985C>T, p.Ala662Val and c.1225G>T, p.Asp409Tyr) were prevalent in the Scottish, Northern Irish and Northern English populations; with 90% of these patients carrying at least one of the two mutations. Clinically, we confirmed the homogenous pattern of selective quadriceps sparing but noted additional features like asymmetry of weakness at disease onset. CONCLUSIONS: GNE myopathy is an important diagnosis to consider in patients presenting with distal leg muscle weakness. We report, for the first time, two common mutations in the north of Britain and highlight the broader spectrum of clinical phenotypes. We also propose that the prevalence of GNE myopathy may be underestimated due to the frequent absence of rimmed vacuoles in the muscle biopsy.

GNE myopathy in India.

BACKGROUND: GNE myopathy is a clinicopathologically distinct distal myopathy with autosomal-recessive inheritance. The GNE gene mutations are known to cause this form of distal myopathy MATERIALS AND METHODS: Over the last 6 years, a total of 54 patients from 48 families were diagnosed to have GNE myopathy based on the clinical and histopathological findings. We have reported on 23 cases earlier and from this cohort 12 patients from 11 families underwent genetic testing for GNE mutation. RESULTS: Nine patients belonging to eight families were confirmed as GNE myopathy by genetic analysis. There were six women and three men. Mean age of onset was 26.7 ± 5.47 years (20-36 years) and mean age at clinical examination was 32.3 ± 4.2 years (28-39 years). Mean duration of the illness was 5.7 ± 4.7 years (1-14 years). All had characteristic clinical features of progressive weakness and wasting of the anterior part of leg muscles, adductors of thighs and hamstrings with relative sparing of the quadriceps muscles. Biopsy from the tibialis anterior muscles revealed the presence of rimmed vacuoles. Mutation analysis of the GNE gene revealed that c. 2086G > A (p.Val696Met) change was common in our series like Thailand and six of eight families carried this mutation, heterozygously. CONCLUSION: These results show the presence of a common mutation in GNE gene in Southeast Asia.

Distal myopathies.

The distal myopathies are a heterogeneous group of genetic disorders defined by a predominant distal weakness at onset or throughout the evolution of the disease and by pathological data supporting a myopathic process. The number of genes associated with distal myopathies continues to increase. Fourteen distinct distal myopathies are currently defined by their gene and causative mutations, compared to just five entities delineated on clinical grounds two decades ago. The known proteins affected in the distal myopathies are of many types and include a significant number of sarcomeric proteins. The useful indicators for clinicians to direct towards a correct molecular diagnosis are the mode of inheritance, the age at onset, the pattern of muscle involvement, the serum creatine kinase level and the muscle pathology findings. This review gives an overview of the clinical and genetic characteristics of the currently identified distal myopathies with emphasis on some recent findings.

Prevalence of GNE p.M712T and hereditary inclusion body myopathy (HIBM) in Sangesar population of Northern Iran.

GNE myopathy or hereditary inclusion body myopathy (HIBM) is an ultra-rare severely disabling autosomal recessive adult onset muscle disease which affects roughly one to three individuals per million worldwide. Genetically, HIBM is caused by mutations in the glucosamine (UDP-N-acetyl)-2-epimerase/N-acetylmannosamine kinase gene (GNE), resulting in diminished enzyme function and reduced sialic acid biosynthesis. A founder variant GNE p.M712T was first described in patients of Iranian and Middle-Eastern descent living outside of Iran. Asymptomatic heterozygote or carrier frequency has been reported as high as 1 in 11 within the Persian-Jewish community residing in Los Angeles, CA. To investigate the prevalence of the p.M712T variant in Iran, we studied 792 samples collected from random individuals in Sangesar (Mahdishahr) in Northern Iran. DNA samples were obtained by buccal swab, and genotyping was performed by melting curve analysis. The results included 31 of 792 (3.91%) heterozygous carriers and 5 (0.31%) homozygotes for GNE p.M712T. All five homozygous individuals, age 30-64 years, were already symptomatic at the start of the study. Our findings suggest that the prevalence of GNE p.M712T is higher in the Sangesar population, comprised mostly of Muslim and Bahai descendants, compared with the general world population. Additional HIBM distribution studies are warranted within various subpopulations of Iran.

Respiratory dysfunction in patients severely affected by GNE myopathy (distal myopathy with rimmed vacuoles).

GNE myopathy is a rare and mildly progressive autosomal recessive myopathy caused by GNE mutations. Respiratory dysfunction has not been reported in GNE myopathy patients. In this study, we retrospectively reviewed the respiratory function of 39 severely affected GNE myopathy patients (13 men, 26 women) from medical records, and compared these parameters with various other patient characteristics (e.g., GNE mutations, age at onset, creatine kinase levels, and being wheelchair-bound) for correlations. The mean % forced vital capacity [FVC] was 92 (26) (range, 16-128). In 12/39 (31%) patients, %FVC was <80%. Of these 12 patients, 11 (92%) were entirely wheelchair-dependent. These patients exhibited significantly earlier onset (20 [4] vs. 30 [8] years, p<0.001) and lower creatine kinase levels (56 [71] vs. 279 [185] IU/L) than patients with normal respiratory function. Two patients exhibited severe respiratory failure and required non-invasive positive pressure ventilation. Patients with a homozygous mutation in the N-acetylmannosamine kinase domain exhibited lower %FVC, while only one compound heterozygous patient with separate mutations in the uridinediphosphate-N-acetylglucosamine 2-epimerase and the N-acetylmannosamine kinase domains had respiratory dysfunction. Our results collectively suggest that GNE myopathy can cause severe respiratory failure. Respiratory dysfunction should be carefully monitored in patients with advanced GNE myopathy characterized by early onset and homozygous homozygous mutations in the N-acetylmannosamine kinase domain.

Distal myopathies--new genetic entities expand diagnostic challenge.

Distal myopathies are a group of muscle diseases which share the clinical pattern of predominant weakness in the feet and/or hands. Rapid advance in the understanding of underlying gene defects have to date separated more than 20 distinct disorders and many are yet without genetic characterisation. No definite diagnosis can be made on other grounds than identification of the final molecular genetic defect. Besides usual investigations including EMG and muscle biopsy, muscle imaging is very important in defining the precise pattern of muscle involvement. Based on the combination of age at onset, mode of inheritance, pathology and muscle imaging, the number of underlying candidate genes for a certain disease can be significantly reduced, which is of help for the molecular genetic approach.