Genetic and phenotypic characterisation of inherited myopathies in a tertiary neuromuscular centre.

Diagnosis of inherited myopathies can be a challenging and lengthy process due to broad genetic and phenotypic heterogeneity. In this study we applied focused exome sequencing to investigate a cohort of 100 complex adult myopathy cases who remained undiagnosed despite extensive investigation. We evaluated the frequency of genetic diagnoses, clinical and pathological factors most likely to be associated with a positive diagnosis, clinical pitfalls and new phenotypic insights that could help to guide future clinical practice. We identified pathogenic/likely pathogenic variants in 32/100 cases. TTN-related myopathy was the most common diagnosis (4/32 cases) but the majority of positive diagnoses related to a single gene each. Childhood onset of symptoms was more likely to be associated with a positive diagnosis. Atypical and new clinico-pathological phenotypes with diagnostic pitfalls were identified. These include the new emerging group of neuromyopathy genes (HSPB1, BICD2) and atypical biopsy findings: COL6A-related myopathy with mitochondrial features, DOK7 presenting as myopathy with minicores and DES-related myopathy without myofibrillar pathology. Our data demonstrates the diagnostic efficacy of broad NGS screening when combined with detailed clinico-pathological phenotyping in a complex neuromuscular cohort. Atypical clinico-pathological features may delay the diagnostic process if smaller targeted gene panels are used.

Mobility shift of beta-dystroglycan as a marker of GMPPB gene-related muscular dystrophy.

BACKGROUND: Defects in glycosylation of alpha-dystroglycan (α-DG) cause autosomal-recessive disorders with wide clinical and genetic heterogeneity, with phenotypes ranging from congenital muscular dystrophies to milder limb girdle muscular dystrophies. Patients show variable reduction of immunoreactivity to antibodies specific for glycoepitopes of α-DG on a muscle biopsy. Recessive mutations in 18 genes, including guanosine diphosphate mannose pyrophosphorylase B (GMPPB), have been reported to date. With no specific clinical and pathological handles, diagnosis requires parallel or sequential analysis of all known genes. METHODS: We describe clinical, genetic and biochemical findings of 21 patients with GMPPB-associated dystroglycanopathy. RESULTS: We report eight novel mutations and further expand current knowledge on clinical and muscle MRI features of this condition. In addition, we report a consistent shift in the mobility of beta-dystroglycan (ß-DG) on Western blot analysis of all patients analysed by this mean. This was only observed in patients with GMPPB in our large dystroglycanopathy cohort. We further demonstrate that this mobility shift in patients with GMPPB was due to abnormal N-linked glycosylation of ß-DG. CONCLUSIONS: Our data demonstrate that a change in ß-DG electrophoretic mobility in patients with dystroglycanopathy is a distinctive marker of the molecular defect in GMPPB.

Atypical periodic paralysis and myalgia: A novel RYR1 phenotype.

OBJECTIVE: To characterize the phenotype of patients with symptoms of periodic paralysis (PP) and ryanodine receptor (RYR1) gene mutations. METHODS: Cases with a possible diagnosis of PP but additional clinicopathologic findings previously associated with RYR1-related disorders were referred for a tertiary neuromuscular clinical assessment in which they underwent detailed clinical evaluation, including neurophysiologic assessment, muscle biopsy, and muscle MRI. Genetic analysis with next-generation sequencing and/or targeted Sanger sequencing was performed. RESULTS: Three cases with episodic muscle paralysis or weakness and additional findings compatible with a RYR1-related myopathy were identified. The McManis test, used in the diagnosis of PP, was positive in 2 of 3 cases. Genetic analysis of known PP genes was negative. RYR1 analysis confirmed likely pathogenic variants in all 3 cases. CONCLUSIONS: RYR1 mutations can cause late-onset atypical PP both with and without associated myopathy. Myalgia and cramps are prominent features. The McManis test may be a useful diagnostic tool to indicate RYR1-associated PP. We propose that clinicopathologic features suggestive of RYR1-related disorders should be sought in genetically undefined PP cases and that RYR1 gene testing be considered in those in whom mutations in SCN4A, CACNA1S, and KCNJ2 have already been excluded.

Clinical features of the myasthenic syndrome arising from mutations in GMPPB.

BACKGROUND: Congenital myasthenic syndrome (CMS) due to mutations in GMPPB has recently been reported confirming the importance of glycosylation for the integrity of neuromuscular transmission. METHODS: Review of case notes of patients with mutations in GMPPB to identify the associated clinical, neurophysiological, pathological and laboratory features. In addition, serum creatine kinase (CK) levels within the Oxford CMS cohort were retrospectively analysed to assess its usefulness in the differential diagnosis of this new entity. RESULTS: All patients had prominent limb-girdle weakness with minimal or absent craniobulbar manifestations. Presentation was delayed beyond infancy with proximal muscle weakness and most patients recall poor performance in sports during childhood. Neurophysiology showed abnormal neuromuscular transmission only in the affected muscles and myopathic changes. Muscle biopsy showed dystrophic features and reduced α-dystroglycan glycosylation. In addition, myopathic changes were present on muscle MRI. CK was significantly increased in serum compared to other CMS subtypes. Patients were responsive to pyridostigimine alone or combined with 3,4-diaminopyridine and/or salbutamol. CONCLUSIONS: Patients with GMPPB-CMS have phenotypic features aligned with CMS subtypes harbouring mutations within the early stages of the glycosylation pathway. Additional features shared with the dystroglycanopathies include myopathic features, raised CK levels and variable mild cognitive delay. This syndrome underlines that CMS can occur in the absence of classic myasthenic manifestations such as ptosis and ophthalmoplegia or facial weakness, and links myasthenic disorders with dystroglycanopathies. This report should facilitate the recognition of this disorder, which is likely to be underdiagnosed and can benefit from symptomatic treatment.

Targeting protein homeostasis in sporadic inclusion body myositis.

Sporadic inclusion body myositis (sIBM) is the commonest severe myopathy in patients more than 50 years of age. Previous therapeutic trials have targeted the inflammatory features of sIBM but all have failed. Because protein dyshomeostasis may also play a role in sIBM, we tested the effects of targeting this feature of the disease. Using rat myoblast cultures, we found that up-regulation of the heat shock response with arimoclomol reduced key pathological markers of sIBM in vitro. Furthermore, in mutant valosin-containing protein (VCP) mice, which develop an inclusion body myopathy, treatment with arimoclomol ameliorated disease pathology and improved muscle function. We therefore evaluated arimoclomol in an investigator-led, randomized, double-blind, placebo-controlled, proof-of-concept trial in sIBM patients and showed that arimoclomol was safe and well tolerated. Although arimoclomol improved some IBM-like pathology in the mutant VCP mouse, we did not see statistically significant evidence of efficacy in the proof-of-concept patient trial.

A novel MYH7 mutation occurring independently in French and Norwegian Laing distal myopathy families and de novo in one Finnish patient.

Laing early-onset distal myopathy is a rare autosomal dominant myopathy and caused by mutations in the MYH7 gene, encoding the slow beta myosin heavy chain. We report the first molecularly verified Laing distal myopathy in a French family caused by a novel p.Glu1508del mutation in the MYH7 gene. Interestingly, we identified the identical mutation in an unrelated Norwegian family and, as a de novo mutation, in one sporadic Finnish patient. Described in detail are the clinical and electrophysiological characteristics of 5 patients from the French family. The phenotype in the Finnish patient and the Norwegian patients is largely similar. This mutation causes a benign myopathy within the range of previously reported Laing myopathy phenotype variations. Onset of weakness in the tibialis anterior (TA) muscles occurred in early childhood in all patients. Finger extensor and neck flexor weakness together with Achilles tendon retractions were other frequent findings. The independent recurrence of the identical mutation without any founder background may reflect a mutational susceptibility of this residue, in accordance with some other MYH7 mutations previously reported. De novo mutations seem to be frequent in Laing distal myopathy. This is of clinical importance since a dominant family history is missing, which may confuse differential diagnostic efforts.