Characterization of Strength and Function in Ambulatory Adults With GNE Myopathy

Role of HSP70 chaperone in protein aggregate phenomenon of GNE mutant cells: Therapeutic lead for GNE Myopathy

Limited treatment options and research in understanding the pathomechanisms of rare diseases has raised concerns about their therapeutic development. One such poorly understood ultra-rare neuromuscular disorder is GNE Myopathy (GNEM) which is caused due to mutation in key sialic acid biosynthetic enzyme, GNE. Treatment with sialic acid or its derivatives/precursors slows the disease progression, but curative strategies need to be explored further. Pathologically, muscle biopsy samples of GNEM patients reveal rimmed vacuole formation due to aggregation of β-amyloid, Tau, presenilin proteins with unknown mechanism. The present study aims to understand the mechanism of protein aggregate formation in GNE mutant cells to decipher role of chaperones in disease phenotype. The pathologically relevant GNE mutations expressed as recombinant proteins in HEK cells was used as a model system for GNEM to estimate extent of protein aggregation. We identified HSP70, a chaperone, as binding partner of GNE. Downregulation of HSP70 with altered BAG3, JNK, BAX expression levels was observed in GNE mutant cells. The cell apoptosis was observed in GNE mutation specific manner. An activator of HSP70 chaperone, BGP-15, rescued the phenotypic defects due to GNE mutation, thereby, reducing protein aggregation significantly. The results were further validated in rat skeletal muscle cell lines carrying single Gne allele. Our study suggests that HSP70 activators can be a promising therapeutic target in the treatment of ultra-rare GNE Myopathy disease.

Effect of GNE Mutations on Cytoskeletal Network Proteins: Potential Gateway to Understand Pathomechanism of GNEM

GNE myopathy is an inherited neuromuscular disorder caused by mutations in GNE (UDP-N-acetylglucosamine 2-epimerase/N-acetyl mannosamine kinase) gene catalyzing the sialic acid biosynthesis pathway. The characteristic features include muscle weakness in upper and lower extremities, skeletal muscle wasting, and rimmed vacuole formation. More than 200 GNE mutations in either epimerase or kinase domain have been reported worldwide. In Indian subcontinent, several GNE mutations have been recently identified with unknown functional correlation. Alternate role of GNE in various cellular processes such as cell adhesion, migration, apoptosis, protein aggregation, and cytoskeletal organization have been proposed in recent studies. We aim to understand and compare the effect of various GNE mutations from Indian origin on regulation of the cytoskeletal network. In particular, F-actin dynamics was determined quantitatively by determining F/G-actin ratios in immunoblots for specific proteins. The extent of F-actin polymerization was visualized by immunostaining with Phalloidin using confocal microscopy. The proteins regulating F-actin dynamics such as RhoA, cofilin, Arp2, and alpha-actinin were studied in various GNE mutants. The altered level of cytoskeletal organization network proteins affected cell migration of GNE mutant proteins as measured by wound healing assay. The functional comparison of GNE mutations will help in better understanding of the genotypic severity of the disease in the Indian population. Our study offers a potential for identification of therapeutic molecules regulating actin dynamics in GNE specific mutations.

Results from a 3-year Non-interventional, Observational Disease Monitoring Program in Adults with GNE Myopathy

BACKGROUND:

GNE myopathy is a rare, autosomal recessive, muscle disease caused by mutations in GNE and is characterized by rimmed vacuoles on muscle biopsy and progressive distal to proximal muscle weakness.

OBJECTIVE:

Investigate the clinical presentation and progression of GNE myopathy.

METHODS:

The GNE Myopathy Disease Monitoring Program was an international, prospective, observational study in subjects with GNE myopathy. Muscle strength was assessed with hand-held dynamometry (HHD), with upper extremity (UE) and lower extremity (LE) composite scores reflecting upper and lower extremity muscle groups, respectively. The GNE myopathy–Functional Activity Scale (GNEM-FAS) was used to further assess impairment in mobility, upper extremity function, and self-care.

RESULTS:

Eighty-seven of 101 enrolled subjects completed the trial until study closure by the sponsor; 60 completed 36 months. Mean (SD) HHD UE composite score decreased from 34.3 kg (32.0) at baseline to 29.4 kg (32.6) kg at month 36 (LS mean change [95%CI]: –3.8 kg [–5.9, –1.7]; P = 0.0005). Mean (SD) HHD LE composite score decreased from 32.0 kg (34.1) at baseline to 25.5 kg (31.2) at month 36 (LS mean change [95%CI]: –4.9 [–7.7, –2.2]; P = 0.0005). GNEM-FAS scores were more severe at baseline in subjects who walked <200 meters versus ≥200 meters in 6 minutes; in both groups, GNEM-FAS total, mobility, UE, and self-care scores decreased from baseline through month 36.

CONCLUSIONS:

These findings demonstrate progressive decline in muscle strength in GNE myopathy and provide insight into the appropriate tools to detect clinically meaningful changes in future GNE myopathy interventional trials.

Early and consistent pattern of proximal weakness in GNE myopathy

BACKGROUND

GNE myopathy is widely regarded as a distal myopathy. Involvement of proximal musculature in this condition has not been systematically studied.

METHODS

The phenotype of genetically confirmed patients with GNE myopathy was analyzed. Fourteen groups of muscles were evaluated with Medical Research Council (MRC) grading and the average muscle scores (AMS:1-10) were calculated.

RESULTS

Fully documented AMS data was available in 31 of 65 patients. It showed a consistent pattern of severe weakness of hip adductors, hip flexors, knee flexors, and foot dorsiflexors, with milder weakness of the hip extensors and abductors. The knee extensors were largely unaffected. The proximal weakness appeared early in the course of the disease. Proximal muscle weakness was also present in the remaining 34 patients in whom the data were limited. A variant in exon 13 (c.2179G > A) was very common (81.5%).

CONCLUSIONS

The GNE phenotype in this Indian cohort exhibited mixed proximal and distal involvement. Weakness of adductors and flexors of the hip formed an integral part of the phenotype.

Limitations of 6-minute walk test reference values for spinal muscular atrophy

INTRODUCTION

The 6-minute walk test (6MWT) is a well-established clinical assessment of functional endurance, validated as a measure of walking ability in spinal muscular atrophy (SMA). The current availability of disease-modifying therapies for SMA indicates a growing need for normative reference data to compare SMA patients with healthy controls.

METHODS

The literature was searched in two scientific databases. Studies were evaluated and selected based on adherence to American Thoracic Society guidelines for administering the 6MWT. Reference equations from the selected studies were applied to 6MWT data collected from SMA patients to calculate and compare % predicted values.

RESULTS

Three pediatric and six adult studies were selected for comparison. The % predicted values using the pediatric and adult equations ranged from 47.7 ± 18.2% to 67.6 ± 26.2% and 43.0 ± 17.9% to 59.5 ± 26.2%, respectively, and were significantly different (P < 0.001).

DISCUSSION

Results suggest significant variability between % predicted values derived from published reference equations in children and adults, despite adherence to 6MWT standardization.

GNE myopathy - A cross-sectional study on spatio-temporal gait characteristics

GNE myopathy is a rare, predominantly distal myopathy, involving mainly the lower limbs and presenting with gait disturbances. In this cross-sectional study gait evaluation of 23 (14 men) genetically confirmed GNE myopathy patients was done using Instrumented walkway analysis (GAITRite®) along with video gait capture. We recorded the topographical pattern of muscles involvement in lower limbs and correlated Functional Ambulation Profile-FAP and Medical Research council-MRC grading of lower limb scores with duration of illness. Early foot flat, foot drop gait with wider out-toed stance and higher perturbations with increased pressure at heel and decreased arm swing were noted. Muscle topography showed predominant weakness in ankle dorsi-flexors, flexor hallucis longus, extensor hallucis longus, hip adductors and knee flexors with stark sparing of quadriceps and relative sparing of hip- abductors, extensors, flexors and ankle plantar-flexors. Gait parameters in women were significantly more affected than men (p < 0.05) for the same duration of illness. FAP score and MRC grading of lower limb scores correlated significantly with duration of illness (p < 0.05). We observed that ankle dorsiflexors were affected earliest with sparing of quadriceps muscles in these patients.

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.