Facing the genetic heterogeneity in neuromuscular disorders: Linkage analysis as an economic diagnostic approach towards the molecular diagnosis

The variability of SMCHD1 gene in FSHD patients: evidence of new mutations

In this study, we investigated the sequence of (Structural Maintenance of Chromosomes flexible Hinge Domain containing 1) SMCHD1 gene in a cohort of clinically defined FSHD (facioscapulohumeral muscular dystrophy) patients in order to assess the distribution of SMCHD1 variants, considering the D4Z4 fragment size in terms of repeated units (RUs; short fragment: 1–7 RU, borderline: 8-10RU and normal fragment: >11RU). The analysis of SMCHD1 revealed the presence of 82 variants scattered throughout the introns, exons and 3’untranslated region (3′UTR) of the gene. Among them, 64 were classified as benign polymorphisms and 6 as VUS (variants of uncertain significance). Interestingly, seven pathogenic/likely pathogenic variants were identified in patients carrying a borderline or normal D4Z4 fragment size, namely c.182_183dupGT (p.Q62Vfs^*48), c.2129dupC (p.A711Cfs^*11), c.3469G>T (p.G1157^*), c.5150_5151delAA (p.K1717Rfs^*16) and c.1131+2_1131+5delTAAG, c.3010A>T (p.K1004^*), c.853G>C (p.G285R). All of them were predicted to disrupt the structure and conformation of SMCHD1, resulting in the loss of GHKL-ATPase and SMC hinge essential domains. These results are consistent with the FSHD symptomatology and the Clinical Severity Score (CSS) of patients. In addition, five variants (c.^*1376A>C, rs7238459; c.^*1579G>A, rs559994; c.^*1397A>G, rs150573037; c.^*1631C>T, rs193227855; c.^*1889G>C, rs149259359) were identified in the 3′UTR region of SMCHD1, suggesting a possible miRNA-dependent regulatory effect on FSHD-related pathways. The present study highlights the clinical utility of next-generation sequencing (NGS) platforms for the molecular diagnosis of FSHD and the importance of integrating molecular findings and clinical data in order to improve the accuracy of genotype–phenotype correlations.

Dysferlinopathy in Switzerland: clinical phenotypes and potential founder effects

Background

Dysferlin is reduced in patients with limb girdle muscular dystrophy type 2B, Miyoshi myopathy, distal anterior compartment myopathy, and in certain Ethnic clusters.

Methods

We evaluated clinical and genetic patient data from three different Swiss Neuromuscular Centers.

Results

Thirteen patients from 6 non-related families were included. Age of onset was 18.8 ± 4.3 years. In all patients, diallelic disease-causing mutations were identified in the DYSF gene. Nine patients from 3 non-related families from Central Switzerland carried the identical homozygous mutation, c.3031 + 2T>C. A possible founder effect was confirmed by haplotype analysis. Three patients from two different families carried the heterozygous mutation, c.1064_1065delAA. Two novel mutations were identified (c.2869C>T (p.Gln957Stop), c.5928G>A (p.Trp1976Stop)).

Conclusions

Our study confirms the phenotypic heterogeneity associated with DYSF mutations. Two mutations (c.3031 + 2T>C, c.1064_1065delAA) appear common in Switzerland. Haplotype analysis performed on one case (c. 3031 + 2T>C) suggested a possible founder effect.

Two novel nebulin variants in an adult patient with congenital nemaline myopathy

Congenital myopathies are clinically and genetically heterogeneous disorders, which often remain genetically undiagnosed for many years. Here we present a 40-year old patient with an almost lifelong history of a congenital myopathy of unknown cause. Muscle biopsy in childhood revealed mild myopathic features and rods. Clinical examination on presentation at the age of 40 revealed a facial weakness, atrophy and weakness of the arm muscles and distal leg muscles with mild contractures of the foot flexors and the right elbow. Subsequently, the nebulin gene was identified as a putative candidate gene by linkage analyses, but sequence analysis only revealed one heterozygous splice site mutation in intron 73 (c.10872+1G>T). Therefore, "Next Generation Sequencing" was performed, which revealed a second pathogenic variant in exon 145 (c.21622A>C). Compound-heterozygous carrier status was confirmed via sequence analysis of the index patient's parents. Whole body muscle MRI showed a muscle involvement as previously described in nebulin-associated myopathies. Based on biopsy material, genetic analyses and muscle MRI, we identified two novel, compound-heterozygous variants in the nebulin gene after a 30 year clinical history, which cause a classical childhood type of nemaline myopathy.

Epigenetic alterations and an increased frequency of micronuclei in women with fibromyalgia

Fibromyalgia (FM), characterized by chronic widespread pain, fatigue, and cognitive/mood disturbances, leads to reduced workplace productivity and increased healthcare expenses. To determine if acquired epigenetic/genetic changes are associated with FM, we compared the frequency of spontaneously occurring micronuclei (MN) and genome-wide methylation patterns in women with FM (n = 10) to those seen in comparably aged healthy controls (n = 42 (MN); n = 8 (methylation)). The mean (sd) MN frequency of women with FM (51.4 (21.9)) was significantly higher than that of controls (15.8 (8.5)) (χ² = 45.552; df = 1; P = 1.49 × 10⁻¹¹). Significant differences (n = 69 sites) in methylation patterns were observed between cases and controls considering a 5% false discovery rate. The majority of differentially methylated (DM) sites (91%) were attributable to increased values in the women with FM. The DM sites included significant biological clusters involved in neuron differentiation/nervous system development, skeletal/organ system development, and chromatin compaction. Genes associated with DM sites whose function has particular relevance to FM included BDNF, NAT15, HDAC4, PRKCA, RTN1, and PRKG1. Results support the need for future research to further examine the potential role of epigenetic and acquired chromosomal alterations as a possible biological mechanism underlying FM.

Salbutamol-responsive limb-girdle congenital myasthenic syndrome due to a novel missense mutation and heteroallelic deletion in MUSK

Congenital myasthenic syndromes (CMS) are clinically and genetically heterogeneous disorders characterized by a neuromuscular transmission defect. In recent years, causative mutations have been identified in atleast 15 genes encoding proteins of the neuromuscular junction. Mutations in MUSK are known as a very rare genetic cause of CMS and have been described in only three families, world-wide. Consequently, the knowledge about efficient drug therapy is very limited. We identified a novel missense mutation (p.Asp38Glu) heteroallelic to a genomic deletion affecting exons 2–3 of MUSK as cause of a limb-girdle CMS in two brothers of Turkish origin. Clinical symptoms included fatigable limb weakness from early childhood on. Upon diagnosis of a MUSK-related CMS at the age of 16 and 13 years, respectively, treatment with salbutamol was initiated leading to an impressive improvement of clinical symptoms, while treatment with esterase inhibitors did not show any benefit. Our findings highlight the importance of a molecular diagnosis in CMS and demonstrate considerable similarities between patients with MUSK and DOK7-related CMS in terms of clinical phenotype and treatment options.

Genetics of neuromuscular disorders

Neuromuscular disorders affect the peripheral nervous system and muscle. The principle effect of neuromuscular disorders is therefore on the ability to perform voluntary movements. Neuromuscular disorders cause significant incapacity, including, at the most extreme, almost complete paralysis. Neuromuscular diseases include some of the most devastating disorders that afflict mankind, for example motor neuron disease. Neuromuscular diseases have onset any time from in utero until old age. They are most often genetic. The last 25 years has been the golden age of genetics, with the disease genes responsible for many genetic neuromuscular disorders now identified. Neuromuscular disorders may be inherited as autosomal dominant, autosomal recessive, or X-linked traits. They may also result from mutations in mitochondrial DNA or from de novo mutations not present in the peripheral blood DNA of either parent. The high incidence of de novo mutation has been one of the surprises of the recent increase in information about the genetics of neuromuscular disorders. The disease burden imposed on families is enormous including decision making in relation to presymptomatic diagnosis for late onset neurodegenerative disorders and reproductive choices. Diagnostic molecular neurogenetics laboratories have been faced with an ever-increasing range of disease genes that could be tested for and usually a finite budget with which to perform the possible testing. Neurogenetics has moved from one known disease gene, the Duchenne muscular dystrophy gene in July 1987, to hundreds of disease genes in 2011. It can be anticipated that with the advent of next generation sequencing (NGS), most, if not all, causative genes will be identified in the next few years. Any type of mutation possible in human DNA has been shown to cause genetic neuromuscular disorders, including point mutations, small insertions and deletions, large deletions and duplications, repeat expansions or contraction and somatic mosaicism. The diagnostic laboratory therefore has to be capable of a large number of techniques in order to identify the different mutation types and requires highly skilled staff. Mutations causing neuromuscular disorders affect the largest human proteins for example titin and nebulin. Successful molecular diagnosis can make invasive and expensive diagnostic procedures such as muscle biopsy unnecessary. Molecular diagnosis is currently largely based on Sanger sequencing, which at most can sequence a small number of exons in one gene at a time. NGS techniques will facilitate molecular diagnostics, but not for all types of mutations. For example, NGS is not good at identifying repeat expansions or copy number variations. Currently, diagnostic molecular neurogenetics is focused on identifying the causative mutation(s) in a patient. In the future, the focus might move to prevention, by identifying carriers of recessive diseases before they have affected children. The pathobiology of many of the diseases remains obscure, as do factors affecting disease severity. The aim of this review is to describe molecular diagnosis of genetic neuromuscular disorders in the past, the present and speculate on the future.

Hexosamine biosynthetic pathway mutations cause neuromuscular transmission defect

Neuromuscular junctions (NMJs) are synapses that transmit impulses from motor neurons to skeletal muscle fibers leading to muscle contraction. Study of hereditary disorders of neuromuscular transmission, termed congenital myasthenic syndromes (CMS), has helped elucidate fundamental processes influencing development and function of the nerve-muscle synapse. Using genetic linkage, we find 18 different biallelic mutations in the gene encoding glutamine-fructose-6-phosphate transaminase 1 (GFPT1) in 13 unrelated families with an autosomal recessive CMS. Consistent with these data, downregulation of the GFPT1 ortholog gfpt1 in zebrafish embryos altered muscle fiber morphology and impaired neuromuscular junction development. GFPT1 is the key enzyme of the hexosamine pathway yielding the amino sugar UDP-N-acetylglucosamine, an essential substrate for protein glycosylation. Our findings provide further impetus to study the glycobiology of NMJ and synapses in general.

Late-onset autosomal dominant limb girdle muscular dystrophy and Paget's disease of bone unlinked to the VCP gene locus

The broadwide spectrum of differential diagnoses of autosomal dominant muscular dystrophies in adults can be specified by additional features. The combination of late-onset muscular dystrophy, rimmed vacuoles and inclusion bodies in the muscle biopsy, and Paget's disease of bone suggests a mutation in the Valosin-containing protein gene (VCP, p97 or CDC48) even without dementia. We report on a German family with late-onset autosomal dominant muscular dystrophy starting in the pelvic girdle about age 40years, a subsequent rapidly-progressing course, high alkaline phosphatase and Paget's disease of bone. Clinical examination revealed no cognitive impairment. Histology showed myopathic changes with rimmed vacuoles and inclusion bodies on muscle biopsy. Mutations in VCP, filamin C, desmin, alphaB-crystallin, ZASP and myosin heavy chains 2 and 7 as well as the genes for facioscapulohumeral muscular dystrophy, Myotonic Dystrophy I and II, and LGMD1A-G were excluded by a combination of linkage analysis and direct sequencing. The family presented here suggests that a yet-unknown genetic defect can give rise to an autosomal dominant myopathy with Paget's disease but without dementia.

Klinik und Genetik der Gliedergürteldystrophien

Gliedergürtelmuskeldystrophien („limb girdle muscle dystrophies“, LGMD) sind eine klinisch sowie genetisch heterogene Gruppe von Muskelkrankheiten, von denen bis heute 7 dominante (LGMD1A–G) und 15 rezessive Formen (LGMD2A–O) beschrieben sind. Viele davon beginnen im Jugendlichenalter und führen in der Regel über die folgenden 2–4 Jahrzehnte zur Gehunfähigkeit. Die Symptomatik beginnt häufig im Beckengürtel, die Muskeln des Schultergürtels folgen in unterschiedlichem Abstand. Allele Formen der vorgestellten LGMD können auch einen distalen Prädilektionstyp aufweisen, wie die Miyoshi-Myopathie durch Mutationen im Dysferlingen. Die häufigsten Formen aus der Gruppe der rezessiven LGMD sind Calpainopathien (LGMD2A), Dystrophien durch Mutationen im FKRP-Gen (FKRP: „Fukutin-related protein“, LGMD2I) sowie Dysferlinopathien (LGMD2B). Es folgen in der Häufigkeit die Sarkoglykanopathien, die oft bereits im Kindesalter beginnen. Bei vielen Formen führt der Gendefekt zur Störung eines sarkolemmalen Proteins. Wegen der großen Heterogenität folgt die molekulargenetische Analyse in der Regel der Muskelbiopsie mit immunhistologischer Aufarbeitung. Eine spezifische Therapie ist bislang nicht verfügbar. Die Behandlung von Kontrakturen und die Überwachung evtl. begleitender Kardiomyopathien stehen neben der humangenetischen Beratung im Vordergrund.

Progressive muscle atrophy with hypokalemic periodic paralysis and calcium channel mutation

A family with hypokalemic periodic paralysis (HypoPP) and motor neuron degeneration is reported. In conjunction with HypoPP, the index patient developed progressive muscle atrophy. The calcium channel gene CACNA1S showed a mutation encoding p.R528H, which has been related previously to HypoPP. We propose that CACNA1S mutations may comprise a previously unrecognized genetic risk factor in a greater spectrum of motor unit disorders including amyotrophic lateral sclerosis.

The childhood limb-girdle muscular dystrophies

The heterogeneous childhood limb-girdle muscular dystrophies have originally been defined as a group of autosomal recessive and dominant diseases with progressive weakness and wasting of shoulder and pelvic-girdle muscles. Over the last 12 years, the underlying genetic defects for many of the diseases have been identified and insight into pathomechanisms of disease has been gained. At the same time, improved diagnostic techniques have allowed to extend the phenotypic spectrum for many of these devastating conditions, which showed that clinical symptoms and pathological findings are not restricted to skeletal muscles. Childhood limb-girdle muscular dystrophies are systemic diseases that often affect the musculoskeletal, respiratory, and cardiovascular system and that can go along with central nervous system involvement and gastrointestinal symptoms. The systemic nature of the diseases requires adequate management strategies that improve symptoms, longevity, and quality of life of the patients. As we are entering an era of translational research the need for precise molecular diagnoses, a thorough understanding of the natural history of the diseases and guidelines for standardized assessments of the patients become even more relevant. In this review, the best characterized childhood limb-girdle muscular dystrophies are discussed and their management aspects highlighted.