Hereditary inclusion-body myopathy associated with cardiomyopathy: report of two siblings

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.

Different electrophysiology patterns in GNE myopathy

Background

GNE myopathy is a rare distal myopathy caused by mutations of the GNE gene. A few cases of GNE myopathy accompanied by neurogenic features of electrophysiology mimicking hereditary motor neuropathy were reported recently. We confirmed this feature and described the clinical phenotype and mutations of GNE myopathy in these rare cases.

Results

The absence of lower limb tendon reflexes, decreased compound muscle action potentials in lower leg motor nerves, and neurogenic pattern of electromyography suggested neuropathy in four patients. However, muscle pathology revealed a predominantly myogenic pattern. The follow-up electroneurography results implied that the compound motor action potential amplitudes deteriorated over time. Next-generation sequencing identified three novel variants of the GNE gene, c.2054T > C (p.Val685Ala), c.424G > A (p.Gly142Arg) and c.944T > C (p.Phe315Ser), as well as two hotspot mutations, c.115C > T(p.Arg39*) and c.620A > T(p.Asp207Val), in these patients. These novel mutations cosegregated with disease in the family.

Conclusions

These rare cases supported the existence of neurogenic features of electrophysiology different from the typical myopathic pattern of GNE myopathy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02355-0.

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.

Expanding the clinicopathological-genetic spectrum of GNE myopathy by a Chinese neuromuscular centre

GNE myopathy is a heterogeneous group of ultrarare neuromuscular disorders caused by mutations in the GNE gene. An estimated prevalence of 1~21/1,000,000 leads to a deficiency of data and a lack of availability of samples to conduct clinical research on this neuromuscular disorder. Although GNE, which is the mutated gene responsible for the disease, is well known as the key enzyme in the biosynthesis pathway of sialic acid, the clinicopathological-genetic spectrum of GNE mutant patients is still unclear and expanding. This study presents ten unrelated patients with GNE myopathy, discovering five novel missense mutations. Clinical, electrophysiological, imaging, pathological and genetic data are presented in a retrospective manner. Interestingly, several patients in the cohort were found to have peripheral neuropathy and inflammatory cell infiltration in muscle biopsies, which have seldom been reported. This study, conducted by a neuromuscular centre in China, is the first attempt to highlight these abnormal clinicopathological features and associate them with genetic mutations in GNE myopathy.

GNE Myopathy: Etiology, Diagnosis, and Therapeutic Challenges

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

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

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Patient registry is a valuable tool in international GNE myopathy research.

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The registry expands the knowledge of GNE myopathy genetics and epidemiology.

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The registry allows monitoring of the disease progression and discovering diversity.

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The data suggest possible genotype–phenotype correlation in GNE myopathy.

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.

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.

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.

Mutation update for GNE gene variants associated with GNE myopathy

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