Myosin storage myopathy associated with a heterozygous missense mutation in MYH7

MYH7 in cardiomyopathy and skeletal muscle myopathy

Myosin heavy chain gene 7 (MYH7), a sarcomeric gene encoding the myosin heavy chain (myosin-7), has attracted considerable interest as a result of its fundamental functions in cardiac and skeletal muscle contraction and numerous nucleotide variations of MYH7 are closely related to cardiomyopathy and skeletal muscle myopathy. These disorders display significantly inter- and intra-familial variability, sometimes developing complex phenotypes, including both cardiomyopathy and skeletal myopathy. Here, we review the current understanding on MYH7 with the aim to better clarify how mutations in MYH7 affect the structure and physiologic function of sarcomere, thus resulting in cardiomyopathy and skeletal muscle myopathy. Importantly, the latest advances on diagnosis, research models in vivo and in vitro and therapy for precise clinical application have made great progress and have epoch-making significance. All the great advance is discussed here.

A novel heterozygous missense MYH7 mutation potentially causes an autosomal dominant form of myosin storage myopathy with dilated cardiomyopathy

BACKGROUND

The MYH7 gene, which encodes the slow/ß-cardiac myosin heavy chain, is mutated in myosin storage myopathy (MSM). The clinical spectrum of MSM is quite heterogeneous in that it ranges from cardiomyopathies to skeletal myopathies or a combination of both, depending on the affected region. In this study, we performed clinical and molecular examinations of the proband of an Iranian family with MSM in an autosomal dominant condition exhibiting proximal muscle weakness and dilated cardiomyopathy.

METHODS

Following thorough clinical and paraclinical examinations, whole-exome sequencing `was performed on the proband (II-5). Pathogenicity prediction of the candidate variant was performed through in-silico analysis. Co-segregation analysis of the WES data among the family members was carried out by PCR-based Sanger sequencing.

RESULTS

A novel heterozygous missense variant, MYH7 (NM_000257): c.C1888A: p.Pro630Thr, was found in the DNA of the proband and his children and confirmed by Sanger sequencing. The in-silico analysis revealed that p.Pro630Thr substitution was deleterious. The novel sequence variant fell within a highly conserved region of the head domain. Our findings expand the spectrum of MYH7 mutations.

CONCLUSIONS

This finding could improve genetic counseling and prenatal diagnosis in families with clinical manifestations associated with MYH7-related myopathy.

Mutations in proteins involved in E-C coupling and SOCE and congenital myopathies

In skeletal muscle, Ca²⁺ necessary for muscle contraction is stored and released from the sarcoplasmic reticulum (SR), a specialized form of endoplasmic reticulum through the mechanism known as excitation–contraction (E-C) coupling. Following activation of skeletal muscle contraction by the E-C coupling mechanism, replenishment of intracellular stores requires reuptake of cytosolic Ca²⁺ into the SR by the activity of SR Ca²⁺-ATPases, but also Ca²⁺ entry from the extracellular space, through a mechanism called store-operated calcium entry (SOCE). The fine orchestration of these processes requires several proteins, including Ca²⁺ channels, Ca²⁺ sensors, and Ca²⁺ buffers, as well as the active involvement of mitochondria. Mutations in genes coding for proteins participating in E-C coupling and SOCE are causative of several myopathies characterized by a wide spectrum of clinical phenotypes, a variety of histological features, and alterations in intracellular Ca²⁺ balance. This review summarizes current knowledge on these myopathies and discusses available knowledge on the pathogenic mechanisms of disease.

Understanding Musculoskeletal Disorders Through Next-Generation Sequencing

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An insight into musculoskeletal disorders through advancements in next-generation sequencing (NGS) promises to maximize benefits and improve outcomes through improved genetic diagnosis.

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The primary use of whole exome sequencing (WES) for musculoskeletal disorders is to identify functionally relevant variants.

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The current evidence has shown the superiority of NGS over conventional genotyping for identifying novel and rare genetic variants in patients with musculoskeletal disorders, due to its high throughput and low cost.

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Genes identified in patients with scoliosis, osteoporosis, osteoarthritis, and osteogenesis imperfecta using NGS technologies are listed for further reference.

Molecular and cellular basis of genetically inherited skeletal muscle disorders

Neuromuscular disorders comprise a diverse group of human inborn diseases that arise from defects in the structure and/or function of the muscle tissue - encompassing the muscle cells (myofibres) themselves and their extracellular matrix - or muscle fibre innervation. Since the identification in 1987 of the first genetic lesion associated with a neuromuscular disorder - mutations in dystrophin as an underlying cause of Duchenne muscular dystrophy - the field has made tremendous progress in understanding the genetic basis of these diseases, with pathogenic variants in more than 500 genes now identified as underlying causes of neuromuscular disorders. The subset of neuromuscular disorders that affect skeletal muscle are referred to as myopathies or muscular dystrophies, and are due to variants in genes encoding muscle proteins. Many of these proteins provide structural stability to the myofibres or function in regulating sarcolemmal integrity, whereas others are involved in protein turnover, intracellular trafficking, calcium handling and electrical excitability - processes that ensure myofibre resistance to stress and their primary activity in muscle contraction. In this Review, we discuss how defects in muscle proteins give rise to muscle dysfunction, and ultimately to disease, with a focus on pathologies that are most common, best understood and that provide the most insight into muscle biology.

Dysregulation of NRAP degradation by KLHL41 contributes to pathophysiology in nemaline myopathy

Nemaline myopathy (NM) is the most common form of congenital myopathy that results in hypotonia and muscle weakness. This disease is clinically and genetically heterogeneous, but three recently discovered genes in NM encode for members of the Kelch family of proteins. Kelch proteins act as substrate-specific adaptors for Cullin 3 (CUL3) E3 ubiquitin ligase to regulate protein turnover through the ubiquitin-proteasome machinery. Defects in thin filament formation and/or stability are key molecular processes that underlie the disease pathology in NM; however, the role of Kelch proteins in these processes in normal and diseases conditions remains elusive. Here, we describe a role of NM causing Kelch protein, KLHL41, in premyofibil-myofibil transition during skeletal muscle development through a regulation of the thin filament chaperone, nebulin-related anchoring protein (NRAP). KLHL41 binds to the thin filament chaperone NRAP and promotes ubiquitination and subsequent degradation of NRAP, a process that is critical for the formation of mature myofibrils. KLHL41 deficiency results in abnormal accumulation of NRAP in muscle cells. NRAP overexpression in transgenic zebrafish resulted in a severe myopathic phenotype and absence of mature myofibrils demonstrating a role in disease pathology. Reducing Nrap levels in KLHL41 deficient zebrafish rescues the structural and function defects associated with disease pathology. We conclude that defects in KLHL41-mediated ubiquitination of sarcomeric proteins contribute to structural and functional deficits in skeletal muscle. These findings further our understanding of how the sarcomere assembly is regulated by disease-causing factors in vivo, which will be imperative for developing mechanism-based specific therapeutic interventions.

Multi-system neurological disorder associated with a CRYAB variant

We report a multiplex family with extended multisystem neurological phenotype associated with a CRYAB variant. Two affected siblings were evaluated with whole exome sequencing, muscle biopsy, laser microdissection, and mass spectrometry-based proteomic analysis. Both patients and their mother manifested a combination of early-onset cataracts, cardiomyopathy, cerebellar ataxia, optic atrophy, cognitive impairment, and myopathy. Whole exome sequencing identified a heterozygous c.458C>T variant mapped to the C-terminal extension domain of the Alpha-crystallin B chain, disrupting its function as a molecular chaperone and its ability to suppress protein aggregation. In accordance with the molecular findings, muscle biopsies revealed subsarcolemmal deposits that appeared dark with H&E and trichrome staining were negative for the other routine histochemical staining and for amyloid with the Congo-red stain. Electron microscopy demonstrated that the deposits were composed of numerous parallel fibrils. Laser microdissection and mass spectrometry-based proteomic analysis revealed that the inclusions are almost exclusively composed of crystallized chaperones/heat shock proteins. Moreover,  a structural model suggests that Ser153 could be involved in monomer stabilization, dimer association, and possible binding of partner proteins. We propose that our report potentially expands the complex phenotypic spectrum of alpha B-crystallinopathies with possible effect of a CRYAB variant on the central nervous system.

Impaired muscle morphology in a Drosophila model of myosin storage myopathy was supressed by overexpression of an E3 ubiquitin ligase

Myosin is vital for body movement and heart contractility. Mutations in MYH7, encoding slow/β-cardiac myosin heavy chain, are an important cause of hypertrophic and dilated cardiomyopathy, as well as skeletal muscle disease. A dominant missense mutation (R1845W) in MYH7 has been reported in several unrelated cases of myosin storage myopathy. We have developed a Drosophila model for a myosin storage myopathy in order to investigate the dose-dependent mechanisms underlying the pathological roles of the R1845W mutation. This study shows that a higher expression level of the mutated allele is concomitant with severe impairment of muscle function and progressively disrupted muscle morphology. The impaired muscle morphology associated with the mutant allele was suppressed by expression of Thin (herein referred to as Abba), an E3 ubiquitin ligase. This Drosophila model recapitulates pathological features seen in myopathy patients with the R1845W mutation and severe ultrastructural abnormalities, including extensive loss of thick filaments with selective A-band loss, and preservation of I-band and Z-disks were observed in indirect flight muscles of flies with exclusive expression of mutant myosin. Furthermore, the impaired muscle morphology associated with the mutant allele was suppressed by expression of Abba. These findings suggest that modification of the ubiquitin proteasome system may be beneficial in myosin storage myopathy by reducing the impact of MYH7 mutation in patients.

Myosins and Disease

Myosins constitute a superfamily of actin-based molecular motor proteins that mediates a variety of cellular activities including muscle contraction, cell migration, intracellular transport, the formation of membrane projections, cell adhesion, and cell signaling. The 12 myosin classes that are expressed in humans share sequence similarities especially in the N-terminal motor domain; however, their enzymatic activities, regulation, ability to dimerize, binding partners, and cellular functions differ. It is becoming increasingly apparent that defects in myosins are associated with diseases including cardiomyopathies, colitis, glomerulosclerosis, neurological defects, cancer, blindness, and deafness. Here, we review the current state of knowledge regarding myosins and disease.

Determinants of Base Editing Outcomes from Target Library Analysis and Machine Learning

Although base editors are widely used to install targeted point mutations, the factors that determine base editing outcomes are not well understood. We characterized sequence-activity relationships of 11 cytosine and adenine base editors (CBEs and ABEs) on 38,538 genomically integrated targets in mammalian cells and used the resulting outcomes to train BE-Hive, a machine learning model that accurately predicts base editing genotypic outcomes (R ≈ 0.9) and efficiency (R ≈ 0.7). We corrected 3,388 disease-associated SNVs with ≥90% precision, including 675 alleles with bystander nucleotides that BE-Hive correctly predicted would not be edited. We discovered determinants of previously unpredictable C-to-G, or C-to-A editing and used these discoveries to correct coding sequences of 174 pathogenic transversion SNVs with ≥90% precision. Finally, we used insights from BE-Hive to engineer novel CBE variants that modulate editing outcomes. These discoveries illuminate base editing, enable editing at previously intractable targets, and provide new base editors with improved editing capabilities.

Differential diagnosis of vacuolar myopathies in the NGS era

Altered autophagy accompanied by abnormal autophagic (rimmed) vacuoles detectable by light and electron microscopy is a common denominator of many familial and sporadic non-inflammatory muscle diseases. Even in the era of next generation sequencing (NGS), late-onset vacuolar myopathies remain a diagnostic challenge. We identified 32 adult vacuolar myopathy patients from 30 unrelated families, studied their clinical, histopathological and ultrastructural characteristics and performed genetic testing in index patients and relatives using Sanger sequencing and NGS including whole exome sequencing (WES). We established a molecular genetic diagnosis in 17 patients. Pathogenic mutations were found in genes typically linked to vacuolar myopathy (GNE, LDB3/ZASP, MYOT, DES and GAA), but also in genes not regularly associated with severely altered autophagy (FKRP, DYSF, CAV3, COL6A2, GYG1 and TRIM32) and in the digenic facioscapulohumeral muscular dystrophy 2. Characteristic histopathological features including distinct patterns of myofibrillar disarray and evidence of exocytosis proved to be helpful to distinguish causes of vacuolar myopathies. Biopsy validated the pathogenicity of the novel mutations p.(Phe55*) and p.(Arg216*) in GYG1 and of the p.(Leu156Pro) TRIM32 mutation combined with compound heterozygous deletion of exon 2 of TRIM32 and expanded the phenotype of Ala93Thr-caveolinopathy and of limb-girdle muscular dystrophy 2i caused by FKRP mutation. In 15 patients no causal variants were detected by Sanger sequencing and NGS panel analysis. In 12 of these cases, WES was performed, but did not yield any definite mutation or likely candidate gene. In one of these patients with a family history of muscle weakness, the vacuolar myopathy was eventually linked to chloroquine therapy. Our study illustrates the wide phenotypic and genotypic heterogeneity of vacuolar myopathies and validates the role of histopathology in assessing the pathogenicity of novel mutations detected by NGS. In a sizable portion of vacuolar myopathy cases, it remains to be shown whether the cause is hereditary or degenerative.

Mutations in the tail domain of MYH3 contributes to atrial septal defect

Atrial septal defect (ASD) is one of the most common congenital heart defects diagnosed in children. Sarcomeric genes has been attributed to ASD and knockdown of MYH3 functionally homologues gene in chick models indicated abnormal atrial septal development. Here, we report for the first time, a case-control study investigating the role of MYH3 among non-syndromic ASD patients in contributing to septal development. Four amplicons which will amplifies the 40 kb MYH3 were designed and amplified using long range-PCR. The amplicons were then sequenced using indexed paired-end libraries on the MiSeq platform. The STREGA guidelines were applied for planning and reporting. The non-synonymous c. 3574G>A (p.Ala1192Thr) [p = 0.001, OR = 2.30 (1.36-3.87)] located within the tail domain indicated a highly conserved protein region. The mutant model of c. 3574G>A (p.Ala1192Thr) showed high root mean square deviation (RMSD) values compared to the wild model. To our knowledge, this is the first study to provide compelling evidence on the pathogenesis of MYH3 variants towards ASD hence, suggesting the crucial role of non-synonymous variants in the tail domain of MYH3 towards atrial septal development. It is hoped that this gene can be used as panel for diagnosis of ASD in future.

Cautionary note on the use of Caenorhabditis elegans to study muscle phenotypes caused by mutations in the human MYH7 gene

Mutations in the human MYH7 gene, encoding a slow skeletal muscle/β-cardiac myosin heavy chain, cause different types of myopathies. The nematode model Caenorhabditis elegans has frequently been employed to study the molecular and physiological consequences of MYH7 mutations in muscle function by introducing mutations into the unc-54 gene, the worm MYH7 ortholog. We report here that the C. elegans model is not appropriate for such studies if they involve expression of the UNC-54 protein (wild-type or fused to green fluorescent protein) above endogenous levels.

Epidemiological study and genetic characterization of inherited muscle diseases in a northern Spanish region

Background

Inherited muscle diseases are a group of rare heterogeneous muscle conditions with great impact on quality of life, for which variable prevalence has previously been reported, probably due to case selection bias. The aim of this study is to estimate the overall and selective prevalence rates of inherited muscle diseases in a northern Spanish region and to describe their demographic and genetic features. Retrospective identification of patients with inherited muscle diseases between 2000 and 2015 from multiple data sources. Demographic and molecular data were registered.

Results

On January 1, 2016, the overall prevalence of inherited muscle diseases was 59.00/ 100,000 inhabitants (CI 95%; 53.35–65.26). Prevalence was significantly greater in men (67.33/100,000) in comparison to women (50.80/100,000) (p = 0.006). The highest value was seen in the age range between 45 and 54 (91.32/100,000) years. Myotonic dystrophy type 1 was the most common condition (35.90/100,000), followed by facioscapulohumeral muscular dystrophy (5.15/100,000) and limb-girdle muscular dystrophy type 2A (2.5/100,000).

Conclusions

Prevalence of inherited muscle diseases in Navarre is high in comparison with the data reported for other geographical regions. Standard procedures and analyses of multiple data sources are needed for epidemiological studies of this heterogeneous group of diseases.

The unfolding spectrum of inherited distal myopathies

Distal myopathies are a group of rare muscle diseases characterized by distal weakness at onset. Although acquired myopathies can occasionally present with distal weakness, the majority of distal myopathies have a genetic etiology. Their age of onset varies from early-childhood to late-adulthood while the predominant muscle weakness can affect calf, ankle dorsiflexor, or distal upper limb muscles. A spectrum of muscle pathological changes, varying from nonspecific myopathic changes to rimmed vacuoles to myofibrillar pathology to nuclei centralization, have been noted. Likewise, the underlying molecular defect is heterogeneous. In addition, there is emerging evidence that distal myopathies can result from defective proteins encoded by genes causative of neurogenic disorders, be manifestation of multisystem proteinopathies or the result of the altered interplay between different genes. In this review, we provide an overview on the clinical, electrophysiological, pathological, and molecular aspects of distal myopathies, focusing on the most recent developments in the field. Muscle Nerve 59:283-294, 2019.

Myosin storage myopathy mutations yield defective myosin filament assembly in vitro and disrupted myofibrillar structure and function in vivo

Myosin storage myopathy (MSM) is a congenital skeletal muscle disorder caused by missense mutations in the β-cardiac/slow skeletal muscle myosin heavy chain rod. It is characterized by subsarcolemmal accumulations of myosin that have a hyaline appearance. MSM mutations map near or within the assembly competence domain known to be crucial for thick filament formation. Drosophila MSM models were generated for comprehensive physiological, structural, and biochemical assessment of the mutations' consequences on muscle and myosin structure and function. L1793P, R1845W, and E1883K MSM mutant myosins were expressed in an indirect flight (IFM) and jump muscle myosin null background to study the effects of these variants without confounding influences from wild-type myosin. Mutant animals displayed highly compromised jump and flight ability, disrupted muscle proteostasis, and severely perturbed IFM structure. Electron microscopy revealed myofibrillar disarray and degeneration with hyaline-like inclusions. In vitro assembly assays demonstrated a decreased ability of mutant myosin to polymerize, with L1793P filaments exhibiting shorter lengths. In addition, limited proteolysis experiments showed a reduced stability of L1793P and E1883K filaments. We conclude that the disrupted hydropathy or charge of residues in the heptad repeat of the mutant myosin rods likely alters interactions that stabilize coiled-coil dimers and thick filaments, causing disruption in ordered myofibrillogenesis and/or myofibrillar integrity, and the consequent myosin aggregation. Our Drosophila models are the first to recapitulate the human MSM phenotype with ultrastructural inclusions, suggesting that the diminished ability of the mutant myosin to form stable thick filaments contributes to the dystrophic phenotype observed in afflicted subjects.

Muscular MRI-based algorithm to differentiate inherited myopathies presenting with spinal rigidity

OBJECTIVES

Inherited myopathies are major causes of muscle atrophy and are often characterized by rigid spine syndrome, a clinical feature designating patients with early spinal contractures. We aim to present a decision algorithm based on muscular whole body magnetic resonance imaging (mWB-MRI) as a unique tool to orientate the diagnosis of each inherited myopathy long before the genetically confirmed diagnosis.

METHODS

This multicentre retrospective study enrolled 79 patients from referral centres in France, Brazil and Chile. The patients underwent 1.5-T or 3-T mWB-MRI. The protocol comprised STIR and T1 sequences in axial and coronal planes, from head to toe. All images were analyzed manually by multiple raters. Fatty muscle replacement was evaluated on mWB-MRI using both the Mercuri scale and statistical comparison based on the percentage of affected muscle.

RESULTS

Between February 2005 and December 2015, 76 patients with genetically confirmed inherited myopathy were included. They were affected by Pompe disease or harbored mutations in RYR1, Collagen VI, LMNA, SEPN1, LAMA2 and MYH7 genes. Each myopathy had a specific pattern of affected muscles recognizable on mWB-MRI. This allowed us to create a novel decision algorithm for patients with rigid spine syndrome by segregating these signs. This algorithm was validated by five external evaluators on a cohort of seven patients with a diagnostic accuracy of 94.3% compared with the genetic diagnosis.

CONCLUSION

We provide a novel decision algorithm based on muscle fat replacement graded on mWB-MRI that allows diagnosis and differentiation of inherited myopathies presenting with spinal rigidity.

KEY POINTS

• Inherited myopathies are rare, diagnosis is challenging and genetic tests require specialized centres and often take years. • Inherited myopathies are often characterized by spinal rigidity. • Whole body magnetic resonance imaging is a unique tool to orientate the diagnosis of each inherited myopathy presenting with spinal rigidity. • Each inherited myopathy in this study has a specific pattern of affected muscles that orientate diagnosis. • A novel MRI-based algorithm, usable by every radiologist, can help the early diagnosis of these myopathies.

MYH7 mutation associated with two phenotypes of myopathy

The mutations of MYH7 (slow skeletal/β-cardiac myosin heavy chain) are commonly found in familial hypertrophic/dilated cardiomyopathy, and also can cause Laing early-onset distal myopathy (LDM), myosin storage myopathy (MSM), and congenital myopathy with fiber-type disproportion (CFTD). Here we report two cases whose diagnosis was hereditary myopathy according to clinical feature and muscle pathology analysis. High-throughput genomic sequencing (next generation sequencing) was performed to validate the diagnosis. Two MYH7 mutations, p.R1845W and p.E1687del, were identified. p.R1845W was found in a male patient showing weakness of both terminal lower legs without foot drop. Muscle pathology stainings characteristically showed the hyaline body in the intracytoplasmic location. The novel mutation p.E1687del was found in a family with seven patients. The proband showed foot drop, scoliosis, and winged scapula, while his mother only showed mild foot drop and winged scapula. Muscle pathology analysis showed congenital centronucleus myopathy. Both cases only showed muscular disorder and had no cardiomyopathy. This study, for the first time, reports the MYH7 mutations associated with centronucleus myopathy.

Congenital myopathies: clinical phenotypes and new diagnostic tools

Congenital myopathies are a group of genetic muscle disorders characterized clinically by hypotonia and weakness, usually from birth, and a static or slowly progressive clinical course. Historically, congenital myopathies have been classified on the basis of major morphological features seen on muscle biopsy. However, different genes have now been identified as associated with the various phenotypic and histological expressions of these disorders, and in recent years, because of their unexpectedly wide genetic and clinical heterogeneity, next-generation sequencing has increasingly been used for their diagnosis. We reviewed clinical and genetic forms of congenital myopathy and defined possible strategies to improve cost-effectiveness in histological and imaging diagnosis.

Novel phenotypic variant in the MYH7 spectrum due to a stop-loss mutation in the C-terminal region: a case report

Background

Defects of the slow myosin heavy chain isoform coding MYH7 gene primarily cause skeletal myopathies including Laing Distal Myopathy, Myosin Storage Myopathy and are also responsible for cardiomyopathies. Scapuloperoneal and limb-girdle muscle weakness, congenital fiber type disproportion, multi-minicore disease were also reported in connection of MYH7. Pathogeneses of the defects in the head and proximal rod region of the protein are well described. However, the C-terminal mutations of the MYH7 gene are less known. Moreover, only two articles describe the phenotypic impact of the elongated mature protein product caused by termination signal loss.

Case presentation

Here we present a male patient with an unusual phenotypic variant of early-onset and predominant involvement of neck muscles with muscle biopsy indicating myopathy and sarcoplasmic storage material. Cardiomyopathic involvements could not be observed. Sequencing of MYH7 gene revealed a stop-loss mutation on the 3-prime end of the rod region, which causes the elongation of the mature protein.

Conclusions

The elongated protein likely disrupts the functions of the sarcomere by multiple functional abnormalities. This elongation could also affect the thick filament degradation leading to protein deposition and accumulation in the sarcomere, resulting in the severe myopathy of certain axial muscles. The phenotypic expression of the detected novel MYH7 genotype could strengthen and further expand our knowledge about mutations affecting the structure of MyHCI by termination signal loss in the MYH7 gene.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Hypertrophic Cardiomyopathy Cardiac Troponin C Mutations Differentially Affect Slow Skeletal and Cardiac Muscle Regulation

Mutations in TNNC1—the gene encoding cardiac troponin C (cTnC)—that have been associated with hypertrophic cardiomyopathy (HCM) and cardiac dysfunction may also affect Ca²⁺-regulation and function of slow skeletal muscle since the same gene is expressed in both cardiac and slow skeletal muscle. Therefore, we reconstituted rabbit soleus fibers and bovine masseter myofibrils with mutant cTnCs (A8V, C84Y, E134D, and D145E) associated with HCM to investigate their effects on contractile force and ATPase rates, respectively. Previously, we showed that these HCM cTnC mutants, except for E134D, increased the Ca²⁺ sensitivity of force development in cardiac preparations. In the current study, an increase in Ca²⁺ sensitivity of isometric force was only observed for the C84Y mutant when reconstituted in soleus fibers. Incorporation of cTnC C84Y in bovine masseter myofibrils reduced the ATPase activity at saturating [Ca²⁺], whereas, incorporation of cTnC D145E increased the ATPase activity at inhibiting and saturating [Ca²⁺]. We also tested whether reconstitution of cardiac fibers with troponin complexes containing the cTnC mutants and slow skeletal troponin I (ssTnI) could emulate the slow skeletal functional phenotype. Reconstitution of cardiac fibers with troponin complexes containing ssTnI attenuated the Ca²⁺ sensitization of isometric force when cTnC A8V and D145E were present; however, it was enhanced for C84Y. In summary, although the A8V and D145E mutants are present in both muscle types, their functional phenotype is more prominent in cardiac muscle than in slow skeletal muscle, which has implications for the protein-protein interactions within the troponin complex. The C84Y mutant warrants further investigation since it drastically alters the properties of both muscle types and may account for the earlier clinical onset in the proband.

Myosin Storage Myopathy in C. elegans and Human Cultured Muscle Cells

Myosin storage myopathy is a protein aggregate myopathy associated with the characteristic subsarcolemmal accumulation of myosin heavy chain in muscle fibers. Despite similar histological findings, the clinical severity and age of onset are highly variable, ranging from no weakness to severe impairment of ambulation, and usually childhood-onset to onset later in life. Mutations located in the distal end of the tail of slow/ß-cardiac myosin heavy chain are associated with myosin storage myopathy. Four missense mutations (L1793P, R1845W, E1883K and H1901L), two of which have been reported in several unrelated families, are located within or closed to the assembly competence domain. This location is critical for the proper assembly of sarcomeric myosin rod filaments. To assess the mechanisms leading to protein aggregation in myosin storage myopathy and to evaluate the impact of these mutations on myosin assembly and muscle function, we expressed mutated myosin proteins in cultured human muscle cells and in the nematode Caenorhabditis elegans. While L1793P mutant myosin protein efficiently incorporated into the sarcomeric thick filaments, R1845W and H1901L mutants were prone to formation of myosin aggregates without assembly into striated sarcomeric thick filaments in cultured muscle cells. In C. elegans, mutant alleles of the myosin heavy chain gene unc-54 corresponding to R1845W, E1883K and H1901L, were as effective as the wild-type myosin gene in rescuing the null mutant worms, indicating that they retain functionality. Taken together, our results suggest that the basis for the pathogenic effect of the R1845W and H1901L mutations are primarily structural rather than functional. Further analyses are needed to identify the primary trigger for the histological changes seen in muscle biopsies of patients with L1793P and E1883K mutations.

Detection of clinically relevant copy-number variants by exome sequencing in a large cohort of genetic disorders

Purpose:

Copy-number variation is a common source of genomic variation and an important genetic cause of disease. Microarray-based analysis of copy-number variants (CNVs) has become a first-tier diagnostic test for patients with neurodevelopmental disorders, with a diagnostic yield of 10–20%. However, for most other genetic disorders, the role of CNVs is less clear and most diagnostic genetic studies are generally limited to the study of single-nucleotide variants (SNVs) and other small variants. With the introduction of exome and genome sequencing, it is now possible to detect both SNVs and CNVs using an exome- or genome-wide approach with a single test.

Methods:

We performed exome-based read-depth CNV screening on data from 2,603 patients affected by a range of genetic disorders for which exome sequencing was performed in a diagnostic setting.

Results:

In total, 123 clinically relevant CNVs ranging in size from 727 bp to 15.3 Mb were detected, which resulted in 51 conclusive diagnoses and an overall increase in diagnostic yield of ~2% (ranging from 0 to –5.8% per disorder).

Conclusions:

This study shows that CNVs play an important role in a broad range of genetic disorders and that detection via exome-based CNV profiling results in an increase in the diagnostic yield without additional testing, bringing us closer to single-test genomics.

Genet Med advance online publication 27 October 2016

MYH7-related myopathies: clinical, histopathological and imaging findings in a cohort of Italian patients

BACKGROUND

Myosin heavy chain 7 (MYH7)-related myopathies are emerging as an important group of muscle diseases of childhood and adulthood, with variable clinical and histopathological expression depending on the type and location of the mutation. Mutations in the head and neck domains are a well-established cause of hypertrophic cardiomyopathy whereas mutation in the distal regions have been associated with a range of skeletal myopathies with or without cardiac involvement, including Laing distal myopathy and Myosin storage myopathy. Recently the spectrum of clinical phenotypes associated with mutations in MYH7 has increased, blurring this scheme and adding further phenotypes to the list. A broader disease spectrum could lead to misdiagnosis of different congenital myopathies, neurogenic atrophy and other neuromuscular conditions.

RESULTS

As a result of a multicenter Italian study we collected clinical, histopathological and imaging data from a population of 21 cases from 15 families, carrying reported or novel mutations in MYH7. Patients displayed a variable phenotype including atypical pictures, as dropped head and bent spine, which cannot be classified in previously described groups. Half of the patients showed congenital or early infantile weakness with predominant distal weakness. Conversely, patients with later onset present prevalent proximal weakness. Seven patients were also affected by cardiomyopathy mostly in the form of non-compacted left ventricle. Muscle biopsy was consistent with minicores myopathy in numerous cases. Muscle MRI was meaningful in delineating a shared pattern of selective involvement of tibialis anterior muscles, with relative sparing of quadriceps.

CONCLUSION

This work adds to the genotype-phenotype correlation of MYH7-relatedmyopathies confirming the complexity of the disorder.

Prenatal diagnosis of congenital myopathies and muscular dystrophies

Congenital myopathies and muscular dystrophies constitute a genetically and phenotypically heterogeneous group of rare inherited diseases characterized by muscle weakness and atrophy, motor delay and respiratory insufficiency. To date, curative care is not available for these diseases, which may severely affect both life-span and quality of life. We discuss prenatal diagnosis and genetic counseling for families at risk, as well as diagnostic possibilities in sporadic cases.

De novo exonic mutation in MYH7 gene leading to exon skipping in a patient with early onset muscular weakness and fiber-type disproportion

Here we report on a case of MYH7-related myopathy in a boy with early onset of muscular weakness and delayed motor development in infancy. His most affected muscles were neck extensors showing a dropped head sign, proximal muscles of lower limbs with positive Gower's sign, and trunk muscles. Brain and spinal cord MRI scans, echocardiography, and laboratory analyses including creatine kinase and lactate did not reveal any abnormalities. Muscle histopathology showed fiber-type disproportion. Whole exome sequencing of the parents-offspring trio revealed a novel de novo c.5655G>A p.(Ala1885=) synonymous substitution of the last nucleotide in exon 38 of the MYH7 gene. Further RNA investigations proved the skipping of exon 38 (p.1854_1885del). This is a first report of an exon-skipping mutation in the MYH7 gene causing myopathy. This report broadens both the phenotypic and genotypic spectra of MYH7-related myopathies.

A de novo mutation of the MYH7 gene in a large Chinese family with autosomal dominant myopathy

Laing distal myopathy (LDM) is an autosomal dominant myopathy that is caused by mutations in the slow/beta cardiac myosin heavy-chain (MYH7) gene. It has been recently reported that LDM presents with a wide range of clinical manifestations. We herein report a large Chinese family with autosomal dominant myopathy. The affected individuals in the family presented with foot drop in early childhood, along with progressive distal and proximal limb weakness. Their characteristic symptoms include scapular winging and scoliosis in the early disease phase and impairment of ambulation in the advanced phase. Although limb-girdle muscle dystrophy (LGMD) was suspected initially, a definite diagnosis could not be reached. As such, we performed linkage analysis and detected four linkage regions, namely 1q23.2-24.1, 14q11.2-12, 15q26.2-26.3 and 17q24.3. Through subsequent whole exome sequencing, we found a de novo p.K1617del causative mutation in the MYH7 gene and diagnosed the disease as LDM. This is the first LDM case in China. Our patients have severe clinical manifestations that mimic LGMD in comparison with the patients with the same mutation reported elsewhere.

The sarcomeric M-region: a molecular command center for diverse cellular processes

The sarcomeric M-region anchors thick filaments and withstands the mechanical stress of contractions by deformation, thus enabling distribution of physiological forces along the length of thick filaments. While the role of the M-region in supporting myofibrillar structure and contractility is well established, its role in mediating additional cellular processes has only recently started to emerge. As such, M-region is the hub of key protein players contributing to cytoskeletal remodeling, signal transduction, mechanosensing, metabolism, and proteasomal degradation. Mutations in genes encoding M-region related proteins lead to development of severe and lethal cardiac and skeletal myopathies affecting mankind. Herein, we describe the main cellular processes taking place at the M-region, other than thick filament assembly, and discuss human myopathies associated with mutant or truncated M-region proteins.

New cardiac and skeletal protein aggregate myopathy associated with combined MuRF1 and MuRF3 mutations

Protein aggregate myopathies (PAMs) define muscle disorders characterized by protein accumulation in muscle fibres. We describe a new PAM in a patient with proximal muscle weakness and hypertrophic cardiomyopathy, whose muscle fibres contained inclusions containing myosin and myosin-associated proteins, and aberrant distribution of microtubules. These lesions appear as intact A- and M-bands lacking thin filaments and Z-discs. These features differ from inclusions in myosin storage myopathy (MSM), but are highly similar to those in mice deficient for the muscle-specific RING finger proteins MuRF1 and MuRF3. Sanger sequencing excluded mutations in the MSM-associated gene MYH7 but identified mutations in TRIM63 and TRIM54, encoding MuRF1 and MuRF3, respectively. No mutations in other potentially disease-causing genes were identified by Sanger and whole exome sequencing. Analysis of seven family members revealed that both mutations segregated in the family but only the homozygous TRIM63 null mutation in combination with the heterozygous TRIM54 mutation found in the proband caused the disease phenotype. Both MuRFs are microtubule-associated proteins localizing to sarcomeric M-bands and Z-discs. They are E3 ubiquitin ligases that play a role in degradation of sarcomeric proteins, stabilization of microtubules and myogenesis. Lack of ubiquitin and the 20S proteasome subunit in the inclusions found in the patient suggested impaired turnover of thick filament proteins. Disruption of microtubules in cultured myotubes was rescued by transient expression of wild-type MuRF1. The unique features of this novel myopathy point to defects in homeostasis of A-band proteins in combination with instability of microtubules as cause of the disease.

Homozygous MYH7 R1820W mutation results in recessive myosin storage myopathy: scapuloperoneal and respiratory weakness with dilated cardiomyopathy

Myosin storage myopathy (MSM) is a protein aggregate myopathy caused by the accumulation of myosin in muscle fibres and results from MYH7 mutation. Although MYH7 mutation is also an established cause of variable cardiomyopathy with or without skeletal myopathy, cardiomyopathy with MSM is a rare combination. Here, we update the clinical findings in the two brothers that we previously reported as having recessively inherited MSM characterized by scapuloperoneal distribution of weakness and typical hyaline-like bodies in type 1 muscle fibres. One of the patients, weak from childhood but not severely symptomatic until 28 years of age, had an unusual combination of MSM, severe dilated cardiomyopathy, and respiratory impairment at the age of 44 years. We identified homozygous missense mutation c.5458C>T (p.R1820W) in exon 37 in these patients as the second recessive MYH7 mutation reported to date.

A rare mutation in MYH7 gene occurs with overlapping phenotype

Mutations in the beta-myosin heavy chain gene (MYH7) cause different muscle disorders. The specific molecular pathobiological processes that cause these different phenotypes remains unexplained. We describe three members of a family with an autosomal dominant mutation in the distal rod of MYH7 [c.5401G> A (p.Glu1801Lys)] displaying a complex phenotype characterized by Laing Distal Myopathy like phenotype, left ventricular non compaction cardiomyopathy and Fiber Type Disproportion picture at muscle biopsy. We suggest that this overlapping presentation confirm the phenotypic variability of MYH7 myopathy and may be helpful to improve the genotype phenotype correlation.

Hyaline Body Myopathy: Adulthood Manifestations

Background:

Hyaline body myopathy (HBM) is a rare chronic nonprogressive congenital myopathy, with variable patterns of inheritance.

Methods:

We describe a patient with congenital HBM with progression of weakness and increasing muscle pain in adulthood. Three muscle biopsies, done at various times in her life, are reported.

Results:

Symptoms started during childhood; however, as an adult, following a period of stability with no progression of the disease, the patient became symptomatic with worsening proximal limb weakness, severe aching pain and hypertrophy of calves. Moderate elevations of serum creatine kinase and myopathic features were noted on electrophysiologic testing. Muscle pathology showed significant fatty infiltration of skeletal muscle and increased number of fibers with internal nuclei. Histology demonstrated the presence of subsarcolemmal, well-delineated hyaline areas, which on histochemical studies was shown to be limited to type1 fibers. The hyaline bodies were dark with pH 4.2 ATPase and with immunohistochemical studies reacted only with myosin heavy chain slow. Electron microscopy showed the hyaline bodies to be composed of nonmembrane bound, fairly even sized granular material, which merged with the adjacent myofibrils. Earlier muscle biopsies, done during childhood, also revealed presence of similar subsarcolemmal hyaline deposits.

Conclusion:

There appears to be a pattern of presentation with adulthood progression in HBM, which has not been described before. Further case studies are required to understand the clinical progression in HBM.

Laing distal myopathy pathologically resembling inclusion body myositis

Mutations in MYH7 cause autosomal dominant Laing distal myopathy. We present a family with a previously reported deletion (c.5186_5188delAGA, p.K1729del). Muscle pathology in one family member was characterized by an inflammatory myopathy with rimmed vacuoles, increased MHC Class I expression, and perivascular and endomysial muscle inflammation comprising CD3⁺, CD4⁺, CD8⁺, and CD68⁺ inflammatory cells. Interestingly, this biopsy specimen contained TDP-43, p62, and SMI-31-positive protein aggregates typical of inclusion body myositis. These findings should alert physicians to the possibility that patients with MYH7 mutations may have muscle biopsies showing pathologic findings similar to inclusion body myositis.

Distal myosin heavy chain-7 myopathy due to the novel transition c.5566G>A (p.E1856K) with high interfamilial cardiac variability and putative anticipation

Myosin-heavy-chain 7 (MYH7)-myopathy manifests clinically with a distal, scapuloperoneal, limb-girdle (proximal), or axial distribution and may involve the respiratory muscles. Cardiac involvement is frequent, ranging from relaxation impairment to severe dilative cardiomyopathy. Progression and earlier onset of cardiac disease in successive generations with MYH7-myopathy is unreported. In a five-generation family MYH7-myopathy due to the novel c.5566G > A (p.E1856K) mutation manifested with late-onset, distal > proximal myopathy and variable degree of cardiac involvement. The index patient developed distal myopathy since age 49 y and anginal chest pain. Her mother had distal myopathy and impaired myocardial relaxation. The daughter of the index patient had discrete myopathy but left ventricular hypertrabeculation/noncompaction and ventricular arrhythmias requiring an implantable cardioverter defibrillator. The granddaughter of the index patient had infantile dilated cardiomyopathy without overt myopathy. Cardiac involvement may be present in MYH7-myopathy and may be progressive between the generations, ranging from relaxation abnormality to noncompaction, ventricular arrhythmias, and dilated cardiomyopathy.

Novel mutations widen the phenotypic spectrum of slow skeletal/β-cardiac myosin (MYH7) distal myopathy

Laing early onset distal myopathy and myosin storage myopathy are caused by mutations of slow skeletal/β-cardiac myosin heavy chain encoded by the gene MYH7, as is a common form of familial hypertrophic/dilated cardiomyopathy. The mechanisms by which different phenotypes are produced by mutations in MYH7, even in the same region of the gene, are not known. To explore the clinical spectrum and pathobiology, we screened the MYH7 gene in 88 patients from 21 previously unpublished families presenting with distal or generalized skeletal muscle weakness, with or without cardiac involvement. Twelve novel mutations have been identified in thirteen families. In one of these families, the father of the proband was found to be a mosaic for the MYH7 mutation. In eight cases, de novo mutation appeared to have occurred, which was proven in four. The presenting complaint was footdrop, sometimes leading to delayed walking or tripping, in members of 17 families (81%), with other presentations including cardiomyopathy in infancy, generalized floppiness, and scoliosis. Cardiac involvement as well as skeletal muscle weakness was identified in nine of 21 families. Spinal involvement such as scoliosis or rigidity was identified in 12 (57%). This report widens the clinical and pathological phenotypes, and the genetics of MYH7 mutations leading to skeletal muscle diseases.

Approach to the diagnosis of congenital myopathies

Over the past decade there have been major advances in defining the genetic basis of the majority of congenital myopathy subtypes. However the relationship between each congenital myopathy, defined on histological grounds, and the genetic cause is complex. Many of the congenital myopathies are due to mutations in more than one gene, and mutations in the same gene can cause different muscle pathologies. The International Standard of Care Committee for Congenital Myopathies performed a literature review and consulted a group of experts in the field to develop a summary of (1) the key features common to all forms of congenital myopathy and (2) the specific features that help to discriminate between the different genetic subtypes. The consensus statement was refined by two rounds of on-line survey, and a three-day workshop. This consensus statement provides guidelines to the physician assessing the infant or child with hypotonia and weakness. We summarise the clinical features that are most suggestive of a congenital myopathy, the major differential diagnoses and the features on clinical examination, investigations, muscle pathology and muscle imaging that are suggestive of a specific genetic diagnosis to assist in prioritisation of genetic testing of known genes. As next generation sequencing becomes increasingly used as a diagnostic tool in clinical practise, these guidelines will assist in determining which sequence variations are likely to be pathogenic.

Recessive myosin myopathy with external ophthalmoplegia associated with MYH2 mutations

Myosin myopathies comprise a group of inherited diseases caused by mutations in myosin heavy chain (MyHC) genes. Homozygous or compound heterozygous truncating MYH2 mutations have been demonstrated to cause recessive myopathy with ophthalmoplegia, mild-to-moderate muscle weakness and complete lack of type 2A muscle fibers. In this study, we describe for the first time the clinical and morphological characteristics of recessive myosin IIa myopathy associated with MYH2 missense mutations. Seven patients of five different families with a myopathy characterized by ophthalmoplegia and mild-to-moderate muscle weakness were investigated. Muscle biopsy was performed to study morphological changes and MyHC isoform expression. Five of the patients were homozygous for MYH2 missense mutations, one patient was compound heterozygous for a missense and a nonsense mutation and one patient was homozygous for a frame-shift MYH2 mutation. Muscle biopsy demonstrated small or absent type 2A muscle fibers and reduced or absent expression of the corresponding MyHC IIa transcript and protein. We conclude that mild muscle weakness and ophthalmoplegia in combination with muscle biopsy demonstrating small or absent type 2A muscle fibers are the hallmark of recessive myopathy associated with MYH2 mutations.

Three-dimensional structure of the human myosin thick filament: clinical implications

High resolution information about the three-dimensional (3D) structure of myosin filaments has always been hard to obtain. Solving the 3D structure of myosin filaments is very important because mutations in human cardiac muscle myosin and its associated proteins (e.g. titin and myosin binding protein C) are known to be associated with a number of familial human cardiomyopathies (e.g. hypertrophic cardiomyopathy and dilated cardiomyopathy). In order to understand how normal heart muscle works and how it fails, as well as the effects of the known mutations on muscle contractility, it is essential to properly understand myosin filament 3D structure and properties in both healthy and diseased hearts.

The aim of this review is firstly to provide a general overview of the 3D structure of myosin thick filaments, as studied so far in both vertebrates and invertebrate striated muscles. Knowledge of this 3D structure is the starting point from which myosin filaments isolated from human cardiomyopathic samples, with known mutations in either myosin or its associated proteins (titin or C-protein), can be studied in detail. This should, in turn, enable us to relate the structure of myosin thick filament to its function and to understanding the disease process. A long term objective of this research would be to assist the design of possible therapeutic solutions to genetic myosin-related human cardiomyopathies.

Recessive MYL2 mutations cause infantile type I muscle fibre disease and cardiomyopathy

A cardioskeletal myopathy with onset and death in infancy, morphological features of muscle type I hypotrophy with myofibrillar disorganization and dilated cardiomyopathy was previously reported in three Dutch families. Here we report the genetic cause of this disorder. Multipoint parametric linkage analysis of six Dutch patients identified a homozygous region of 2.1 Mb on chromosome 12, which was shared between all Dutch patients, with a log of odds score of 10.82. Sequence analysis of the entire linkage region resulted in the identification of a homozygous mutation in the last acceptor splice site of the myosin regulatory light chain 2 gene (MYL2) as the genetic cause. MYL2 encodes a myosin regulatory light chain (MLC-2V). The myosin regulatory light chains bind, together with the essential light chains, to the flexible neck region of the myosin heavy chain in the hexameric myosin complex and have a structural and regulatory role in muscle contraction. The MYL2 mutation results in use of a cryptic splice site upstream of the last exon causing a frameshift and replacement of the last 32 codons by 20 different codons. Whole exome sequencing of an Italian patient with similar clinical features showed compound heterozygosity for two other mutations affecting the same exon of MYL2, also resulting in mutant proteins with altered C-terminal tails. As a consequence of these mutations, the second EF-hand domain is disrupted. EF-hands, assumed to function as calcium sensors, can undergo a conformational change upon binding of calcium that is critical for interactions with downstream targets. Immunohistochemical staining of skeletal muscle tissue of the Dutch patients showed a diffuse and weak expression of the mutant protein without clear fibre specificity, while normal protein was absent. Heterozygous missense mutations in MYL2 are known to cause dominant hypertrophic cardiomyopathy; however, none of the parents showed signs of cardiomyopathy. In conclusion, the mutations in the last exon of MYL2 are responsible for a novel autosomal recessive lethal myosinopathy due to defects changing the C-terminal tail of the ventricular form of the myosin regulatory light chain. We propose 'light chain myopathy' as a name for this MYL2-associated myopathy.

A novel mutation expands the genetic and clinical spectrum of MYH7-related myopathies

MYH7 mutations are an established cause of Laing distal myopathy, myosin storage myopathy, and cardiomyopathy, as well as additional myopathy subtypes. We report a novel MYH7 mutation (p.Leu1597Arg) that arose de novo in two unrelated probands. Proband 1 has a myopathy characterized by distal weakness and prominent contractures and histopathology typical of multi-minicore disease. Proband 2 has an axial myopathy and histopathology consistent with congenital fiber type disproportion. These cases highlight the broad spectrum of clinical and histological patterns associated with MYH7 mutations, and provide further evidence that MYH7 is likely responsible for a greater proportion of congenital myopathies than currently appreciated.

Myosinopathies: pathology and mechanisms

The myosin heavy chain (MyHC) is the molecular motor of muscle and forms the backbone of the sarcomere thick filaments. Different MyHC isoforms are of importance for the physiological properties of different muscle fiber types. Hereditary myosin myopathies have emerged as an important group of diseases with variable clinical and morphological expression depending on the mutated isoform and type and location of the mutation. Dominant mutations in developmental MyHC isoform genes (MYH3 and MYH8) are associated with distal arthrogryposis syndromes. Dominant or recessive mutations affecting the type IIa MyHC (MYH2) are associated with early-onset myopathies with variable muscle weakness and ophthalmoplegia as a consistent finding. Myopathies with scapuloperoneal, distal or limb-girdle muscle weakness including entities, such as myosin storage myopathy and Laing distal myopathy are the result of usually dominant mutations in the gene for slow/β cardiac MyHC (MYH7). Protein aggregation is part of the features in some of these myopathies. In myosin storage myopathy protein aggregates are formed by accumulation of myosin beneath the sarcolemma and between myofibrils. In vitro studies on the effects of different mutations associated with myosin storage myopathy and Laing distal myopathy indicate altered biochemical and biophysical properties of the light meromyosin, which is essential for thick filament assembly. Protein aggregates in the form of tubulofilamentous inclusions in association with vacuolated muscle fibers are present at late stage of dominant myosin IIa myopathy and sometimes in Laing distal myopathy. These protein aggregates exhibit features indicating defective degradation of misfolded proteins. In addition to protein aggregation and muscle fiber degeneration some of the myosin mutations cause functional impairment of the molecular motor adding to the pathogenesis of myosinopathies.

Congenital myopathies

Congenital myopathies are a heterogeneous group of inherited muscle disorders, characterized by the predominance of particular histopathological features on muscle biopsy, such as cores (central core disease) or rods (nemaline myopathy). Clinically, early onset of the disease, stable or slowly progressive muscle weakness, hypotonia and delayed motor development are common in most forms. As a result, the diagnosis of a subtype of congenital myopathy is largely based on the presence of specific structural abnormalities in the skeletal muscle detected by enzyme-histochemistry and electron microscopy studies. During the last decades there have been significant advances in the identification of the genetic basis of most congenital myopathies. However, there is significant genetic heterogeneity within the main groups of congenital myopathies, and mutations in one particular gene may also cause diverse clinical and morphological phenotypes. Thus, the nosography and nosology in this field is still evolving.

Novel FLNC mutation in a patient with myofibrillar myopathy in combination with late-onset cerebellar ataxia

INTRODUCTION

Mutations in the gene that encodes filamin C, FLNC, represent a rare cause of a distinctive type of myofibrillar myopathy (MFM).

METHODS

We investigated an Italian patient by means of muscle biopsy, muscle and brain imaging and molecular analysis of MFM genes.

RESULTS

The patient harbored a novel 7256C>T, p.Thr2419Met mutation in exon 44 of FLNC. Clinical, pathological and muscle MRI findings were similar to the previously described filaminopathy cases. This patient had, in addition, cerebellar ataxia with atrophy of cerebellum and vermis evident on brain MRI scan. Extensive screening failed to establish a cause of cerebellar atrophy.

CONCLUSIONS

We report an Italian filaminopathy patient, with a novel mutation in a highly conserved region. This case raises the possibility that the disease spectrum caused by FLNC may include cerebellar dysfunction.

Decreased DNA fragmentation and apoptotic signaling in soleus muscle of hypertensive rats following 6 weeks of treadmill training

Cardiovascular diseases such as hypertension are associated with a generalized skeletal myopathy including a proapoptotic phenotype. Current evidence suggests that exercise may alter apoptosis-related signaling in skeletal muscle; however, the effect of exercise on skeletal muscle DNA fragmentation and apoptotic signaling is unclear in hypertensive animals. Male normotensive Wistar Kyoto (WKY; n = 24) and spontaneously hypertensive rats (SHR; n = 24) were assigned to a sedentary (SED) condition or exercise (EX) consisting of progressive treadmill running 5 days/wk for 6 wks. Consistent with our previous work we found that soleus muscle of hypertensive animals had significantly higher DNA fragmentation (a hallmark of apoptosis), elevated proapoptotic factors (Bax, caspase-3 activity), and lower antiapoptotic proteins (apoptosis repressor with caspase recruitment domain, Bcl-2, X-linked inhibitor of apoptosis protein) compared with normotensive rats. In addition, soleus muscle of hypertensive animals displayed myosin accumulation and fragmentation, had elevated cytosolic cytochrome c, second mitochondrial-derived activator of caspase (Smac), apoptosis inducing factor (AIF), and endonuclease G protein levels, higher nuclear AIF content, and greater muscle reactive oxygen species generation compared with normotensive animals. Interestingly, exercise training significantly lowered DNA fragmentation and myosin accumulation/fragmentation in soleus muscle of hypertensive rats. Furthermore, exercise training significantly reduced cytosolic levels of cytochrome c as well as cytosolic and nuclear AIF in soleus muscle of hypertensive animals. This beneficial response is likely due to exercise-mediated elevations in Bcl-2, heat shock protein 70, and manganese superoxide dismutase protein content, as well as reductions in Bax protein levels and the Bax-to-Bcl-2 ratio. These results suggest that regular exercise training provides protection against skeletal muscle apoptosis by altering a number of apoptosis regulatory proteins and by influencing mitochondrial-mediated apoptotic signaling mechanisms.

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.

Mutations in MYH7 cause Multi-minicore Disease (MmD) with variable cardiac involvement

Central Core Disease (CCD) and Multi-minicore Disease (MmD) (the "core myopathies") have been mainly associated with mutations in the skeletal muscle ryanodine receptor (RYR1) and the selenoprotein N (SEPN1) gene. A proportion of cases remain unresolved. Mutations in MYH7 encoding the beta myosin heavy chain protein have been implicated in cardiac and, less frequently, skeletal muscle disorders. Here we report four patients from two families with a histopathological diagnosis of MmD, presenting in childhood with slowly progressive muscle weakness, more proximal in Family 1 and more distal in Family 2, and variable degrees of cardiorespiratory impairment evolving later in life. There was also a strong family history of sudden death in the first family. Muscle biopsies obtained in early childhood showed multiple minicores as the most prominent feature. Sequencing of the MYH7 gene revealed heterozygous missense mutations, c.4399C>G; p.Leu1467Val (exon 32) in Family 1 and c.4763G>C; p.Arg1588Pro (exon 34) in Family 2. These findings suggest MYH7 mutations as another cause of a myopathy with multiple cores, in particular if associated with dominant inheritance and cardiac involvement. However, clinical features previously associated with this genetic background, namely a more distal distribution of weakness and an associated cardiomyopathy, may only evolve over time.