237th ENMC International Workshop: GNE myopathy - current and future research Hoofddorp, The Netherlands, 14-16 September 2018

Distal myopathy

Distal myopathies are a group of genetic, primary muscle diseases. Patients develop progressive weakness and atrophy of the muscles of forearm, hands, lower leg, or feet. Currently, over 20 different forms, presenting a variable age of onset, clinical presentation, disease progression, muscle involvement, and histological findings, are known. Some of them are dominant and some recessive. Different variants in the same gene are often associated with either dominant or recessive forms, although there is a lack of a comprehensive understanding of the genotype-phenotype correlations. This chapter provides a description of the clinicopathologic and genetic aspects of distal myopathies emphasizing known etiologic and pathophysiologic mechanisms.

Assessment of thrombocytopenia, sleep apnea, and cardiac involvement in GNE myopathy patients

INTRODUCTION

We previously identified UDP-N-acetylglucosamine 2-epimerase (GNE) myopathy patients with sleep apnea and a past history of thrombocytopenia, but without disease-specific cardiac involvement. This study aimed to clarify the occurrence, severity, and serial changes of these complications.

METHODS

Thirty-three genetically confirmed GNE myopathy patients who participated in a 5-y longitudinal observational history study underwent platelet count and platelet-associated immunoglobulin G (PA-IgG) measurements, a sleep study, and electrocardiography (ECG), Holter ECG, and echocardiogram examinations.

RESULTS

Among the 33 patients, three had low platelet counts and 17 out of 26 were PA-IgG positive. No patient exhibited bleeding tendencies, and 3 out of 28 had low platelet counts. Muscle weakness was more pronounced, and summed MMT and grip power significantly lower, in PA-IgG-positive patients than in PA-IgG-negative patients. Of 19 patients, 7, 4, and 3 who underwent a sleep study had mild, moderate, and severe sleep apnea, respectively, and three started continuous positive airway pressure (CPAP). The respiratory disturbance index was not significantly correlated with physical evaluation items or forced vital capacity. All patients underwent ECG, 32 underwent cardiac ultrasound, and 25 underwent Holter ECG. No disease-specific cardiac involvement was noted, no serial changes during the follow-up period were observed for ECG and echocardiography, and none of the patients required therapy for cardiac abnormalities.

DISCUSSION

PA-IgG is a potential disease biomarker in GNE myopathy patients, although its significance needs to be clarified. While none of the patients in this study experienced cardiomyopathy or arrythmia due to myopathy, sleep apnea was identified as a frequent complication.

Panorama of the distal myopathies

Distal myopathies are genetic primary muscle disorders with a prominent weakness at onset in hands and/or feet. The age of onset (from early childhood to adulthood), the distribution of muscle weakness (upper versus lower limbs) and the histological findings (ranging from nonspecific myopathic changes to myofibrillar disarrays and rimmed vacuoles) are extremely variable. However, despite being characterized by a wide clinical and genetic heterogeneity, the distal myopathies are a category of muscular dystrophies: genetic diseases with progressive loss of muscle fibers. Myopathic congenital arthrogryposis is also a form of distal myopathy usually caused by focal amyoplasia. Massive parallel sequencing has further expanded the long list of genes associated with a distal myopathy, and contributed identifying as distal myopathy-causative rare variants in genes more often related with other skeletal or cardiac muscle diseases. Currently, almost 20 genes (ACTN2, CAV3, CRYAB, DNAJB6, DNM2, FLNC, HNRNPA1, HSPB8, KHLH9, LDB3, MATR3, MB, MYOT, PLIN4, TIA1, VCP, NOTCH2NLC, LRP12, GIPS1) have been associated with an autosomal dominant form of distal myopathy. Pathogenic changes in four genes (ADSSL, ANO5, DYSF, GNE) cause an autosomal recessive form; and disease-causing variants in five genes (DES, MYH7, NEB, RYR1 and TTN) result either in a dominant or in a recessive distal myopathy. Finally, a digenic mechanism, underlying a Welander-like form of distal myopathy, has been recently elucidated. Rare pathogenic mutations in SQSTM1, previously identified with a bone disease (Paget disease), unexpectedly cause a distal myopathy when combined with a common polymorphism in TIA1. The present review aims at describing the genetic basis of distal myopathy and at summarizing the clinical features of the different forms described so far.

Upregulation of Hallmark Muscle Genes Protects GneM743T/M743T Mutated Knock-In Mice From Kidney and Muscle Phenotype

Background:

Mutations in GNE cause a recessive, adult onset myopathy characterized by slowly progressive distal and proximal muscle weakness. Knock-in mice carrying the most frequent mutation in GNE myopathy patients, Gne^M743T/M743T, usually die few days after birth from severe renal failure, with no muscle phenotype. However, a spontaneous sub-colony remains healthy throughout a normal lifespan without any kidney or muscle pathology.

Objective:

We attempted to decipher the molecular mechanisms behind these phenotypic differences and to determine the mechanisms preventing the kidney and muscles from disease.

Methods:

We analyzed the transcriptome and proteome of kidneys and muscles of sick and healthy Gne^M743T/M743T mice.

Results:

The sick Gne^M743T/M743T kidney was characterized by up-regulation of extra-cellular matrix degradation related processes and by down-regulation of oxidative phosphorylation and respiratory electron chain pathway, that was also observed in the asymptomatic muscles. Surprisingly, the healthy kidneys of the Gne^M743T/M743T mice were characterized by up-regulation of hallmark muscle genes. In addition the asymptomatic muscles of the sick Gne^M743T/M743T mice showed upregulation of transcription and translation processes.

Conclusions:

Overexpression of muscle physiology genes in healthy Gne^M743T/M743T mice seems to define the protecting mechanism in these mice. Furthermore, the strong involvement of muscle related genes in kidney may bridge the apparent phenotypic gap between GNE myopathy and the knock-in Gne^M743T/M743T mouse model and provide new directions in the study of GNE function in health and disease.