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Myology research highlights

Discover our weekly selection of scientific and medical publications in the field of myology and of neuromuscular diseases :  summary of each publication aimed at the general reader, highlighting the main points of the article and the authors’ conclusions
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30/04/2015 - Combined ACE inhibitor and beta blocker of no prophylactic value in mdx mice

Most patients with Duchenne muscular dystrophy (DMD) will develop cardiomyopathy; however, the evidence for prophylactic treatment of children with cardiac medications is limited. The authors have used the mdx mouse model of DMD to assess if early combination treatment with beta blocker (BB) and ACE inhibitor (AI) is superior to single treatment with either one of these drugs. Mice were assessed with cardiac MRI (ventricular structure and function, in vivo calcium influx (manganese-enhanced MRI)), pressure-volume loops, and histopathology. Combination treatment did not show benefits over treatment with AI or BB alone. Indeed, some beneficial aspects of BB and AI were lost when used in combination. None of the treatments impacted RV function. Combination treatment had no significant effect on sarcolemmal damage or histopathology. The study suggests that combined BB and AI may not confer an advantage at an early stage in DMD cardiomyopathy. However, limitations of the mdx model should be considered.

30/04/2015 - Clinical and genetic spectrum in limb-girdle muscular dystrophy type 2E

This study aimed to determine the clinical spectrum of limb-girdle muscular dystrophy 2E (LGMD2E) and to investigate whether genetic or biochemical features can predict the phenotype of the disease. A specific clinical protocol was created, including quantitative evaluation of motor, respiratory, and cardiac function. Phenotype was defined as severe or mild if the age at loss of ambulation occurred before or after 18 years. Molecular analysis of SGCB gene and biochemical features of muscle biopsies were reviewed. Thirty-two patients were included in this stud(16 male, 16 female; age 7-67 years; 15 severe, 12 mild, and 5 unknown). Neurologic examination showed proximal muscle weakness in all patients, but distal involvement was also observed in patients with severe disease early in the disease course. Cardiac involvement was observed in 20 patients (63%) even before overt muscle involvement. Six patients had restrictive respiratory insufficiency requiring assisted ventilation (19%). Seventeen different mutations were identified, and 3 were recurrent. The c.377_384dup (13 alleles) was associated with the severe form, the c.-22_10dup (10) with the milder form, and the c.341C>T (9) with both. The entire sarcoglycan complex was undetectable by muscle immunohistochemistry or Western blot in 9/10 severe cases and reduced in 7/7 mild cases. The residual amount of sarcoglycan in muscle resulted a predictor of age at loss of ambulation. This study expands the spectrum of phenotype in β-sarcoglycanopathy and provides strong evidence that severity of clinical involvement may be predicted by SGCB gene mutation and sarcoglycan protein expression.

30/04/2015 - Sensitive dynamometers to evaluate upper limbs in non-ambulant SMA patients

Assessment of the upper limb strength in non-ambulant neuromuscular patients remains challenging. Although potential outcome measures have been reported, longitudinal data demonstrating sensitivity to clinical evolution in spinal muscular atrophy patients are critically lacking. This study aimed to i) assess the feasibility and reliability of different strength (MyoGrip, MyoPinch) and functional (MoviPlate, MFM) measures in non-ambulant patients with SMA; ii) assess the sensitivity to change for the same measurements over one year; and iii) determine the sample size of non-ambulant patients with SMA needed in clinical trials to prove whether a given drug effectively stabilizes the disease. A total of 23 non-ambulant patients, 16 patients (males/females = 6/10; median age 15.4 years with a range from 10.7 to 31.1 years) with spinal muscular atrophy type II and 7 patients (males/females = 2/5; median age 19.9 years with a range from 8.3 to 29.9 years) with type III, were recruited for this study. The Brooke functional score was on median 3 with a range from 2 to 6. The average total vital capacity was 46%, and seven patients required non-invasive ventilation at night. Patients were assessed at baseline, 6 months, and 1 year using the Motor Function Measure and innovative devices MyoGrip, MyoPinch, and MoviPlate, which assess handgrip strength, key pinch strength, and hand/finger extension-flexion function, respectively. The study demonstrated the feasibility and reliability of these measures for all patients, and sensitivity to negative changes after the age of 14 years. The younger patients showed an increase of the distal force in the follow-up period. The distal force measurements and function were correlated to different functional scales. These data represent an important step in the process of validating these devices as potential outcome measures for future clinical trials.

30/04/2015 - Use of muscle MRI to diagnose X-linked myopathy with excessive autophagy

X-linked myopathy with excessive autophagy (XMEA) is an X-linked recessive myopathy due to recently reported mutations in the VMA21 gene. In this study, four men from 2 separate families were studied. The clinical presentation, genetic data, muscle biopsy, and muscle MRI were analyzed. A known VMA21 mutation c.163 + 4A>G and a new mutation c.163 + 3A>G, respectively were found in the 2 families. The clinical course was characterized by onset in childhood and progressive muscle weakness with a limb-girdle pattern. Muscle biopsy revealed a mild myopathy with an increased number of giant autophagic vacuoles. Whole body muscle MRI showed that pelvic girdle and proximal thighs were the most and earliest affected territories, with sparing of rectus femoris muscles. Muscle changes essentially consisted of degenerative fatty replacement. This study highlights the distinctive MRI pattern of muscle involvement, which can be helpful for diagnosis of XMEA even before muscle biopsy or genetic analysis is performed.

30/04/2015 - X-linked myopathy with excessive autophagy does not affect cardiac muscle

In X-linked myopathy with excessive autophagy (XMEA) progressive sarcoplasmic accumulation of autolysosomes filled with undegraded debris leads to atrophy and weakness of skeletal muscles. XMEA is caused by compromised acidification of lysosomes resulting from hypofunction of the proton pump vacuolar ATPase (V-ATPase), due to hypomorphic mutations in VMA21, whose protein product assembles V-ATPase. To what extent the cardiac muscle is affected is unknown. Therefore the authors of the present study performed a comprehensive cardiac evaluation in four male XMEA patients, and also examined pathology of one deceased patient's cardiac and skeletal muscle. None of the symptomatic men (aged 25-48 years) had history or symptoms of cardiomyopathy. Resting electrocardiograms and echocardiographies were normal. MRI showed normal left ventricle ejection fraction and myocardial mass. Myocardial late-gadolinium enhancement was not detected. The deceased patient's skeletal but not cardiac muscle showed characteristic accumulation of autophagic vacuoles. In conclusion, in classic XMEA the myocardium is structurally, electrically and clinically spared.

30/04/2015 - Oral fructose ingestion improves exercise tolerance in glycogen storage disease type IIIa

Glycogen storage disease type IIIa (GSDIIIa) is classically regarded as a glycogenosis with fixed weakness. However, the authors of the present study hypothesized that exercise intolerance in GSDIIIa is related to muscle energy failure and that oral fructose ingestion could improve exercise tolerance in this metabolic myopathy. They challenged metabolism with cycle-ergometer exercise and measured substrate turnover and oxidation rates using stable isotope methodology and indirect calorimetry in 3 patients and 6 age-matched controls on 1 day, and examined the effect of fructose ingestion on exercise tolerance in the patients on another day. Total fatty acid oxidation rates during exercise were higher in patients than controls, 32.1 (SE 1.2) vs 20.7 (SE 0.5; range 15.8-29.3) μmol/kg/min (p = 0.048), and oxidation of carbohydrates was lower in patients, 1.0 (SE 5.4) vs 38.4 (SE 8.0; range 23.0-77.1) μmol/kg/min (p = 0.024). Fructose ingestion improved exercise tolerance in the patients. Similar to patients with McArdle disease, in whom muscle glycogenolysis is also impaired, GSDIIIa is associated with a reduced skeletal muscle oxidation of carbohydrates and a compensatory increase in fatty acid oxidation, and fructose ingestion improves exercise tolerance. These results indicate that GSDIIIa should not only be viewed as glycogenosis with fixed skeletal muscle weakness, but should also be considered among the glycogenoses presenting with exercise-related dynamic symptoms caused by muscular energy deficiency. This study provides Class IV evidence that ingestion of fructose improves exercise tolerance in patients with GSDIIIa.

30/04/2015 - Understanding the role of mRNA regulation in oculopharyngeal muscular dystrophy pathogenesis

Oculopharyngeal muscular dystrophy (OPMD), a late-onset disorder characterized by progressive degeneration of specific muscles, results from the extension of a polyalanine tract in poly(A) binding protein nuclear 1 (PABPN1). While the roles of PABPN1 in nuclear polyadenylation and regulation of alternative poly(A) site choice are established, the molecular mechanisms behind OPMD remain undetermined. Here, we show, using Drosophila and mouse models, that OPMD pathogenesis depends on affected poly(A) tail lengths of specific mRNAs. We identify a set of mRNAs encoding mitochondrial proteins that are down-regulated starting at the earliest stages of OPMD progression. The down-regulation of these mRNAs correlates with their shortened poly(A) tails and partial rescue of their levels when deadenylation is genetically reduced improves muscle function. Genetic analysis of candidate genes encoding RNA binding proteins using the Drosophila OPMD model uncovers a potential role of a number of them. We focus on the deadenylation regulator Smaug and show that it is expressed in adult muscles and specifically binds to the down-regulated mRNAs. In addition, the first step of the cleavage and polyadenylation reaction, mRNA cleavage, is affected in muscles expressing alanine-expanded PABPN1. We propose that impaired cleavage during nuclear cleavage/polyadenylation is an early defect in OPMD. This defect followed by active deadenylation of specific mRNAs, involving Smaug and the CCR4-NOT deadenylation complex, leads to their destabilization and mitochondrial dysfunction. These results broaden our understanding of the role of mRNA regulation in pathologies and might help to understand the molecular mechanisms underlying neurodegenerative disorders that involve mitochondrial dysfunction.

30/04/2015 - Development of a functional composite score for SMA

The objective of this study was to construct a composite score of global function that will discriminate among people with Spinal Muscular Atrophy (SMA). Data were collected from 126 participants with SMA Types 2 and 3. Scores from the Hammersmith Functional Motor Scale-Expanded and Upper Limb Module were expressed as a percentage of the maximum score and Six-Minute Walk Test as percent predicted normal distance. A principal component analysis was performed on the correlation matrix for the 3 percentage scores. The first principal component yielded a composite score with approximately equal weighting of the 3 components and accounted for 82% of the total variability. The SMA Functional Composite score, an unweighted average of the 3 individual percentage scores, correlated almost perfectly with the first principal component. This combination of measures broadens the spectrum of ability that can be quantified in Type 2 and 3 SMA patients.