R. Quinlivan

Ataluren treatment of patients with nonsense mutation dystrophinopathy

Introduction: Dystrophinopathy is a rare, severe muscle disorder, and nonsense mutations are found in 13% of cases. Ataluren was developed to enable ribosomal readthrough of premature stop codons in nonsense mutation (nm) genetic disorders. Methods: Randomized, double‐blind, placebo‐controlled study; males ≥5 years with nm‐dystrophinopathy received study drug orally 3 times daily, ataluren 10, 10, 20 mg/kg (N = 57); ataluren 20, 20, 40 mg/kg (N = 60); or placebo (N = 57) for 48 weeks. The primary endpoint was change in 6‐Minute Walk Distance (6MWD) at Week 48. Results: Ataluren was generally well tolerated. The primary endpoint favored ataluren 10, 10, 20 mg/kg versus placebo; the week 48 6MWD Δ = 31.3 meters, post hoc P = 0.056. Secondary endpoints (timed function tests) showed meaningful differences between ataluren 10, 10, 20 mg/kg, and placebo. Conclusions: As the first investigational new drug targeting the underlying cause of nm‐dystrophinopathy, ataluren offers promise as a treatment for this orphan genetic disorder with high unmet medical need. Muscle Nerve 50: 477–487, 2014

RYR1 mutations are a common cause of congenital myopathies with central nuclei.

Centronuclear myopathy (CNM) is a rare congenital myopathy characterized by prominence of central nuclei on muscle biopsy. CNM has been associated with mutations in MTM1, DNM2, and BIN1 but many cases remain genetically unresolved. RYR1 encodes the principal sarcoplasmic reticulum calcium release channel and has been implicated in various congenital myopathies. We investigated whether RYR1 mutations cause CNM.

Long-term benefits and adverse effects of intermittent versus daily glucocorticoids in boys with Duchenne muscular dystrophy

Objective To assess the current use of glucocorticoids (GCs) in Duchenne muscular dystrophy in the UK, and compare the benefits and the adverse events of daily versus intermittent prednisolone regimens. Design A prospective longitudinal observational study across 17 neuromuscular centres in the UK of 360 boys aged 3–15 years with confirmed Duchenne muscular dystrophy who were treated with daily or intermittent (10 days on/10 days off) prednisolone for a mean duration of treatment of 4 years. Results The median loss of ambulation was 12 years in intermittent and 14.5 years in daily treatment; the HR for intermittent treatment was 1.57 (95% CI 0.87 to 2.82). A fitted multilevel model comparing the intermittent and daily regiments for the NorthStar Ambulatory Assessment demonstrated a divergence after 7 years of age, with boys on an intermittent regimen declining faster (p<0.001). Moderate to severe side effects were more commonly reported and observed in the daily regimen, including Cushingoid features, adverse behavioural events and hypertension. Body mass index mean z score was higher in the daily regimen (1.99, 95% CI 1.79 to 2.19) than in the intermittent regimen (1.51, 95% CI 1.27 to 1.75). Height restriction was more severe in the daily regimen (mean z score −1.77, 95% CI −1.79 to −2.19) than in the intermittent regimen (mean z score −0.70, 95% CI −0.90 to −0.49). Conclusions Our study provides a framework for providing information to patients with Duchenne muscular dystrophy and their families when introducing GC therapy. The study also highlights the importance of collecting longitudinal natural history data on patients treated according to standardised protocols, and clearly identifies the benefits and the side-effect profile of two treatment regimens, which will help with informed choices and implementation of targeted surveillance.

Mutations in GDP-Mannose Pyrophosphorylase B Cause Congenital and Limb-Girdle Muscular Dystrophies Associated with Hypoglycosylation of α-Dystroglycan

Congenital muscular dystrophies with hypoglycosylation of α-dystroglycan (α-DG) are a heterogeneous group of disorders often associated with brain and eye defects in addition to muscular dystrophy. Causative variants in 14 genes thought to be involved in the glycosylation of α-DG have been identified thus far. Allelic mutations in these genes might also cause milder limb-girdle muscular dystrophy phenotypes. Using a combination of exome and Sanger sequencing in eight unrelated individuals, we present evidence that mutations in guanosine diphosphate mannose (GDP-mannose) pyrophosphorylase B (GMPPB) can result in muscular dystrophy variants with hypoglycosylated α-DG. GMPPB catalyzes the formation of GDP-mannose from GTP and mannose-1-phosphate. GDP-mannose is required for O-mannosylation of proteins, including α-DG, and it is the substrate of cytosolic mannosyltransferases. We found reduced α-DG glycosylation in the muscle biopsies of affected individuals and in available fibroblasts. Overexpression of wild-type GMPPB in fibroblasts from an affected individual partially restored glycosylation of α-DG. Whereas wild-type GMPPB localized to the cytoplasm, five of the identified missense mutations caused formation of aggregates in the cytoplasm or near membrane protrusions. Additionally, knockdown of the GMPPB ortholog in zebrafish caused structural muscle defects with decreased motility, eye abnormalities, and reduced glycosylation of α-DG. Together, these data indicate that GMPPB mutations are responsible for congenital and limb-girdle muscular dystrophies with hypoglycosylation of α-DG.

The spectrum of pathology in central core disease

Central core disease is a congenital myopathy with muscle weakness defined pathologically by the presence of extensive areas in muscle fibres that are devoid of oxidative enzyme activity. The gene responsible has been shown to be the ryanodine receptor 1 on chromosome 19q13 and mutations have now been identified in several patients. Some cases with the morphological defect remain molecularly undefined, particularly those studied before molecular studies were available. We have studied three families with congenital onset, each with a dominantly inherited mutation in a C-terminal exon of the ryanodine receptor 1. They illustrate the spectrum of pathology that can be observed in patients with the myopathic features of central core disease. We show that extensive fibrosis and fat may be present, type 1 fibre uniformity may occur in the absence of cores; cores may be central or peripheral, single or multiple; and that an appearance of multiple focal minicores might cause a diagnostic pathological dilemma. In addition, we show the value of immunocytochemistry in identifying cores, in particular the use of antibodies to desmin and gamma-filamin.

Brain involvement in muscular dystrophies with defective dystroglycan glycosylation

To assess the range and severity of brain involvement, as assessed by magnetic resonance imaging, in 27 patients with mutations in POMT1 (4), POMT2 (9), POMGnT1 (7), Fukutin (4), or LARGE (3), responsible for muscular dystrophies with abnormal glycosylation of dystroglycan (dystroglycanopathies).

A comparative study of α-dystroglycan glycosylation in dystroglycanopathies suggests that the hypoglycosylation of α-dystroglycan does not consistently correlate with clinical severity

Hypoglycosylation of α‐dystroglycan underpins a subgroup of muscular dystrophies ranging from congenital onset of weakness, severe brain malformations and death in the perinatal period to mild weakness in adulthood without brain involvement. Mutations in six genes have been identified in a proportion of patients. POMT1, POMT2 and POMGnT1 encode for glycosyltransferases involved in the mannosylation of α‐dystroglycan but the function of fukutin, FKRP and LARGE is less clear. The pathological hallmark is reduced immunolabeling of skeletal muscle with antibodies recognizing glycosylated epitopes on α‐dystroglycan. If the common pathway of these conditions is the hypoglycosyation of α‐dystroglycan, one would expect a correlation between clinical severity and the extent of hypoglycosylation. By studying 24 patients with mutations in these genes, we found a good correlation between reduced α‐dystroglycan staining and clinical course in patients with mutations in POMT1, POMT2 and POMGnT1. However, this was not always the case in patients with defects in fukutin and FKRP, as we identified patients with mild limb–girdle phenotypes without brain involvement with profound depletion of α‐dystroglycan. These data indicate that it is not always possible to correlate clinical course and α‐dystroglycan labeling and suggest that there might be differences in α‐dystroglycan processing in these disorders.

Muscle histology vs MRI in Duchenne muscular dystrophy

Objective: There are currently no effective treatments to halt the muscle breakdown in Duchenne muscular dystrophy (DMD), although genetic-based clinical trials are being piloted. Most of these trials have as an endpoint the restoration of dystrophin in muscle fibers, hence requiring sufficiently well-preserved muscle of recruited patients. The choice of the muscles to be studied and the role of noninvasive methods to assess muscle preservation therefore require further evaluation. Methods: We studied the degree of muscle involvement in the lower leg muscles of 34 patients with DMD >8 years, using muscle MRI. In a subgroup of 15 patients we correlated the muscle MRI findings with the histology of open extensor digitorum brevis (EDB) muscle biopsies. Muscle MRI involvement was assigned using a scale 0–4 (normal–severe). Results: In all patients we documented a gradient of involvement of the lower leg muscles: the posterior compartment (gastrocnemius > soleus) was most severely affected; the anterior compartment (tibialis anterior/posterior, popliteus, extensor digitorum longus) least affected. Muscle MRI showed EDB involvement that correlated with the patients age (p = 0.055). We show a correlation between the MRI and EDB histopathologic changes, with MRI 3–4 grades associated with a more severe fibro-adipose tissue replacement. The EDB was sufficiently preserved for bulk and signal intensity in 18/22 wheelchair users aged 10–16.6 years. Conclusion: This study provides a detailed correlation between muscle histology and MRI changes in DMD and demonstrates the value of this imaging technique as a reliable tool for the selection of muscles in patients recruited into clinical trials.

McArdle disease: a clinical review.

Methods The clinical phenotype of 45 genetically confirmed McArdle patients is described. Results In the majority of patients (84%), the onset of symptoms was from early childhood but diagnosis was frequently delayed until after 30 years of age. Not all patients could recognise a second wind although it was always seen with exercise assessment. A history of myoglobinuria was not universal and episodes of acute renal failure had occurred in a minority (11%). The condition does not appear to adversely affect pregnancy and childbirth. Clinical examination was normal in most patients, muscle hypertrophy was present in 24% and mild muscle wasting and weakness were seen only in patients over 40 years of age and was limited to shoulder girdle and axial muscles. The serum creatine kinase was elevated in all but one pregnant patient. Screening for the mutations pArg50X (R50X) and pGly205Ser (G205S) showed at least one mutated allele in 96% of Caucasian British patients, with an allele frequency of 77% for pArg50X in this population. A 12 min walking test to evaluate patients is described. Conclusion The results demonstrated a wide spectrum of severity with the range of distance walked (195–1980 m); the mean distance walked was 512 m, suggesting significant functional impairment in most patients.

A novel trial design to study the effect of intravenous immunoglobulin in chronic inflammatory demyelinating polyradiculoneuropathy

Using a novel trial design, we prospectively examined the effect of intravenous immunoglobulin in seven patients with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) in a doubleblind, placebo-controlled cross-over study. We suggest that the commonly used manual muscle testing and Rankin scale are not sufficiently sensitive to measure changes in CIDP and should not be used as isolated outcome measures. We propose a timed 10-m walk, the Nine-Hole Peg Test, the Hammersmith Motor Ability Score, and myometry as alternative measures which are valid, reliable and sensitive. Our trial design permitted the measurement of a treatment response in three responders despite different patterns of disability typical of the broad clinical picture seen in CIDP.