Myofibrillar myopathies

Expanding Spectrum of Desmin-Related Myopathy, Long-term Follow-up, and Cardiac Transplantation

OBJECTIVE

We aimed to determine the genetic and clinical phenotypes of desmin-related myopathy patients and long-term outcomes after cardiac transplant.

METHODS

Retrospective review of cardiac and neurological manifestations of genetically confirmed desmin-related myopathy patients (Jan 1^st, 1999-Jan 1^st, 2020).

RESULTS

Twenty-five patients in 20 different families were recognized. Median age at onset of symptoms was 20 years (range: 4-50), median follow-up time of 36 months (range: 1-156). Twelve patients initially presented with skeletal muscle involvement and 13 with cardiac disease. Sixteen patients had both cardiac and skeletal muscle involvement. Clinically muscle weakness distribution was distal (n=11), proximal (n=4) or both (n=7) of 22 patients. Skeletal muscle biopsy from patients with missense and splice site variants (n=12) showed abnormal fibers containing amorphous material in Gomori trichrome stained sections. Patients with cardiac involvement had atrioventricular conduction abnormalities or cardiomyopathy. The most common ECG abnormality was complete AV block in 11 patients all of whom required a permanent pacemaker at a median age of 25 years (range: 16-48). Sudden cardiac death resulting in implantable cardioverter defibrillator (ICD) shocks or resuscitation were reported in 3 patients, a total of 5 patients had ICDs. Orthotopic cardiac transplantation was performed in 3 patients at 20, 35 and 39 years of age.

CONCLUSIONS

Pathogenic variants in desmin can lead to varied neurological and cardiac phenotypes beginning at a young age. Two-thirds of the patients have both neurologic and cardiac symptoms usually starting in the third decade. Heart transplant was tolerated with improved cardiac function and quality of life.

A family with adult-onset myofibrillar myopathy with BAG3 mutation (P470S) presenting with axonal polyneuropathy

We report a family with adult-onset myofibrillar myopathy with BAG3 mutation who presented peroneal weakness and axonal polyneuropathy, mimicking axonal Charcot-Marie-Tooth disease. The male proband noticed difficulty in tiptoeing at age 34. At age 42, the examination showed muscle weakness and atrophy in distal lower extremities with diminished patellar and Achilles tendon reflexes. Thermal and vibration sensations were also impaired in both feet. The serum CK level was 659 U/L. On muscle imaging, predominant semitendinosus muscle atrophy coexisted with atrophies in the quadriceps, gastrocnemius and lumbar paraspinal muscles. The muscle biopsy showed myofibrillar myopathy with fiber type grouping. His 68-year-old mother also had suffered from distal leg weakness and sensory impairment since her forties. A heterozygous mutation in BAG3 (P470S) was identified in both patients. Clinical features of myofibrillar myopathy with axonal polyneuropathy were consistent with BAG3-related myopathy. Our patients showed remarkably mild presentations without cardiomyopathy, unlike the majorities of previously reported cases.

Myofibril diameter is set by a finely tuned mechanism of protein oligomerization in Drosophila

Myofibrils are huge cytoskeletal assemblies embedded in the cytosol of muscle cells. They consist of arrays of sarcomeres, the smallest contractile unit of muscles. Within a muscle type, myofibril diameter is highly invariant and contributes to its physiological properties, yet little is known about the underlying mechanisms setting myofibril diameter. Here we show that the PDZ and LIM domain protein Zasp, a structural component of Z-discs, mediates Z-disc and thereby myofibril growth through protein oligomerization. Oligomerization is induced by an interaction of its ZM domain with LIM domains. Oligomerization is terminated upon upregulation of shorter Zasp isoforms which lack LIM domains at later developmental stages. The balance between these two isoforms, which we call growing and blocking isoforms sets the stereotyped diameter of myofibrils. If blocking isoforms dominate, myofibrils become smaller. If growing isoforms dominate, myofibrils and Z-discs enlarge, eventually resulting in large pathological aggregates that disrupt muscle function.

Prevalence, pathological mechanisms, and genetic basis of limb-girdle muscular dystrophies: A review

Limb-girdle muscular dystrophies (LGMDs) are a highly heterogeneous group of neuromuscular disorders that are associated with weakness and wasting of muscles in legs and arms. Signs and symptoms may begin at any age and usually worsen by time. LGMDs are autosomal disorders with different types and their prevalence is not the same in different areas. New technologies such as next-generation sequencing can accelerate their diagnosis. Several important pathological mechanisms that are involved in the pathology of the LGMD include abnormalities in dystrophin-glycoprotein complex, the sarcomere, glycosylation of dystroglycan, vesicle and molecular trafficking, signal transduction pathways, and nuclear functions. Here, we provide a comprehensive review that integrates LGMD clinical manifestations, prevalence, and some pathological mechanisms involved in LGMDs.

Identification of a novel nonsense mutation in kyphoscoliosis peptidase gene in an Iranian patient with myofibrillar myopathy

Myofibrillar myopathies (MFMs) are rare genetic and slowly progressive neuromuscular disorders. Several pathogenic mutations have been reported in MFM-related genes including DES, CRYAB, MYOT, LDB3 or ZASP, FLNC, BAG3, FHL1 and DNAJB6. Although MFMs is commonly inherited in an autosomal dominant manner, the inheritance pattern and novel mutated genes are not thoroughly elucidated in some cases. Here, we report discovery of a novel nonsense mutation in a 29-year-old Iranian male patient with motor disorders and deformity in his lower limbs. His parents are second cousins. Hereditary Motor Sensory Neuropathy as initial genetic diagnosis was ruled out. Whole exome sequencing using NGS on Illumina HiSeq4000 platform was performed to identify the disease and possible mutated gene(s). Our data analysis identified a homozygous nonsense unreported c.C415T (p.R139X) variant on kyphoscoliosis peptidase (KY) gene (NM_178554: exon4). Sanger sequencing of this mutation has been performed for his other related family members. Sequencing and segregation analysis was confirmed the NGS results and autosomal recessive inheritance pattern of the disease.

Cardiac arrhythmia and late-onset muscle weakness caused by a myofibrillar myopathy with unusual histopathological features due to a novel missense mutation in FLNC

Myofibrillar myopathies (MFM) are mostly adult-onset diseases characterized by progressive morphological alterations of the muscle fibers beginning in the Z-disk and the presence of protein aggregates in the sarcoplasm. They are mostly caused by mutations in different genes that encode Z-disk proteins, including DES, CRYAB, LDB3, MYOT, FLNC and BAG3. A large family of French origin, presenting an autosomal dominant pattern, characterized by cardiac arrhythmia associated to late-onset muscle weakness, was evaluated to clarify clinical, morphological and genetic diagnosis. Muscle weakness began during adult life (over 30 years of age), and had a proximal distribution. Histology showed clear signs of a myofibrillar myopathy, but with unusual, large inclusions. Subsequently, genetic testing was performed in MFM genes available for screening at the time of clinical/histological diagnosis, and desmin (DES), αB-crystallin (CRYAB), myotilin (MYOT) and ZASP (LDB3), were excluded. LMNA gene screening found the p.R296C variant which did not co-segregate with the disease. Genome wide scan revealed linkage to 7q.32, containing the FLNC gene. FLNC direct sequencing revealed a heterozygous c.3646T>A p.Tyr1216Asn change, co-segregating with the disease, in a highly conserved amino acid of the protein. Normal filamin C levels were detected by Western-blot analysis in patient muscle biopsies and expression of the mutant protein in NIH3T3 showed filamin C aggregates. This is an original FLNC mutation in a MFM family with an atypical clinical and histopathological presentation, given the presence of significantly focal lesions and prominent sarcoplasmic masses in muscle biopsies and the constant heart involvement preceding significantly the onset of the myopathy. Though a rare etiology, FLNC gene should not be excluded in early-onset arrhythmia, even in the absence of myopathy, which occurs later in the disease course.

How does α-actinin-3 deficiency alter muscle function? Mechanistic insights into ACTN3, the 'gene for speed'

An estimated 1.5 billion people worldwide are deficient in the skeletal muscle protein α-actinin-3 due to homozygosity for the common ACTN3 R577X polymorphism. α-Actinin-3 deficiency influences muscle performance in elite athletes and the general population. The sarcomeric α-actinins were originally characterised as scaffold proteins at the muscle Z-line. Through studying the Actn3 knockout mouse and α-actinin-3 deficient humans, significant progress has been made in understanding how ACTN3 genotype alters muscle function, leading to an appreciation of the diverse roles that α-actinins play in muscle. The α-actinins interact with a number of partner proteins, which broadly fall into three biological pathways-structural, metabolic and signalling. Differences in functioning of these pathways have been identified in α-actinin-3 deficient muscle that together contributes to altered muscle performance in mice and humans. Here we discuss new insights that have been made in understanding the molecular mechanisms that underlie the consequences of α-actinin-3 deficiency.

The role of ubiquitin ligases in cardiac disease

Rigorous surveillance of protein quality control is essential for the maintenance of normal cardiac function, while the dysregulation of protein turnover is present in a diverse array of common cardiac diseases. Central to the protein quality control found in all cells is the ubiquitin proteasome system (UPS). The UPS plays a critical role in protein trafficking, cellular signaling, and most prominently, protein degradation. As ubiquitin ligases (E3s) control the specificity of the UPS, their description in the cardiomyocyte has highlighted how ubiquitin ligases are critical to the turnover and function of the sarcomere complex, responsible for the heart's required continuous contraction. In this review, we provide an overview of the UPS, highlighting a comprehensive overview of the cardiac ubiquitin ligases identified to date. We then focus on recent studies of new cardiac ubiquitin ligases outlining their novel roles in protein turnover, cellular signaling, and the regulation of mitochondrial dynamics and receptor turnover in the pathophysiology of cardiac hypertrophy, cardiac atrophy, myocardial infarction, and heart failure. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy".

Myofibrillar myopathies

Myofibrillar myopathies (MFMs) are rare, inherited or sporadic, progressive neuromuscular disorders with considerable clinical and genetic heterogeneity. MFMs are defined morphologically by foci of myofibril dissolution that begins at the Z-disk, accumulation of myofibrillar degradation products, and ectopic expression of a large number of proteins including desmin. To date, mutations in six genes are known to cause MFMs, accounting for approximately half of the MFM patients identified. The causative genes encode mainly sarcomeric Z-disk(-related) proteins: desmin, αB-crystallin, myotilin, Z-band alternatively spliced PDZ motif containing protein (ZASP), filamin C and the antiapoptotic BCL2-associated athanogene 3 (Bag3). Although in most MFM patients the disease presents in adulthood and evolves slowly, some patients with desminopathy, αB-crystallinopathy or Bag3opathies have an infantile or juvenile disease onset. Cardiac involvement is very common in desminopathies and can sometimes be the initial or only symptom of the disease. Respiratory symptoms are noted during childhood in αB-crystallinopathies. Early severe cardiac and respiratory involvement is seen in Bag3opathies. Optical microscopic and immunohistochemical features are similar in MFMs; however, ultrastructural findings can be useful to differentiate between the distinct MFM subtypes. No curative treatment for MFMs is currently available. Careful follow-up, especially of cardiac and respiratory function, is important.

Filamin C-related myopathies: pathology and mechanisms

The term filaminopathy was introduced after a truncating mutation in the dimerization domain of filamin C (FLNc) was shown to be responsible for a devastating muscle disease. Subsequently, the same mutation was found in patients from diverse ethnical origins, indicating that this specific alteration is a mutational hot spot. Patients initially present with proximal muscle weakness, while distal and respiratory muscles become affected with disease progression. Muscle biopsies of these patients show typical signs of myofibrillar myopathy, including disintegration of myofibrils and aggregation of several proteins into distinct intracellular deposits. Highly similar phenotypes were observed in patients with other mutations in Ig-like domains of FLNc that result in expression of a noxious protein. Biochemical and biophysical studies showed that the mutated domains acquire an abnormal structure causing decreased stability and eventually becoming a seed for abnormal aggregation with other proteins. The disease usually presents only after the fourth decade of life possibly as a result of ageing-related impairments in the machinery that is responsible for disposal of damaged proteins. This is confirmed by mutations in components of this machinery that cause a highly similar phenotype. Transfection studies of cultured muscle cells reflect the events observed in patient muscles and, therefore, may provide a helpful model for testing future dedicated therapeutic strategies. More recently, FLNC mutations were also found in families with a distal myopathy phenotype, caused either by mutations in the actin-binding domain of FLNc that result in increased actin-binding and non-specific myopathic abnormalities without myofibrillar myopathy pathology, or a nonsense mutation in the rod domain that leads to RNA instability, haploinsufficiency with decreased expression levels of FLNc in the muscle fibers and myofibrillar abnormalities, but not to the formation of desmin-positive protein aggregates required for the diagnosis of myofibrillar myopathy.

Pathophysiology of protein aggregation and extended phenotyping in filaminopathy

Mutations in FLNC cause two distinct types of myopathy. Disease associated with mutations in filamin C rod domain leading to expression of a toxic protein presents with progressive proximal muscle weakness and shows focal destructive lesions of polymorphous aggregates containing desmin, myotilin and other proteins in the affected myofibres; these features correspond to the profile of myofibrillar myopathy. The second variant associated with mutations in the actin-binding domain of filamin C is characterized by weakness of distal muscles and morphologically by non-specific myopathic features. A frameshift mutation in the filamin C rod domain causing haploinsufficiency was also found responsible for distal myopathy with some myofibrillar changes but no protein aggregation typical of myofibrillar myopathies. Controversial data accumulating in the literature require re-evaluation and comparative analysis of phenotypes associated with the position of the FLNC mutation and investigation of the underlying disease mechanisms. This is relevant and necessary for the refinement of diagnostic criteria and developing therapeutic approaches. We identified a p.W2710X mutation in families originating from ethnically diverse populations and re-evaluated a family with a p.V930_T933del mutation. Analysis of the expanded database allows us to refine clinical and myopathological characteristics of myofibrillar myopathy caused by mutations in the rod domain of filamin C. Biophysical and biochemical studies indicate that certain pathogenic mutations in FLNC cause protein misfolding, which triggers aggregation of the mutant filamin C protein and subsequently involves several other proteins. Immunofluorescence analyses using markers for the ubiquitin-proteasome system and autophagy reveal that the affected muscle fibres react to protein aggregate formation with a highly increased expression of chaperones and proteins involved in proteasomal protein degradation and autophagy. However, there is a noticeably diminished efficiency of both the ubiquitin-proteasome system and autophagy that impairs the muscle capacity to prevent the formation or mediate the degradation of aggregates. Transfection studies of cultured muscle cells imitate events observed in the patient's affected muscle and therefore provide a helpful model for testing future therapeutic strategies.

Clinical and myopathological characteristics of desminopathy caused by a mutation in desmin tail domain

BACKGROUND

Most of the previously described pathogenic mutations in desmin are located in highly conserved α-helical domains that play an important role in intermediate filament assembly. The role of the C-terminus non-α-helical 'tail' domain is much less investigated and until recently mutations in this domain have been implicated in only a few patients. The majority of reported desminopathy cases caused by the tail mutations were sporadic, creating a representation bias regarding the disease frequency and phenotypic characteristics.

METHODS

We performed detailed genotype-phenotype analysis of autosomal dominant desminopathy associated with tail domain mutations in a four-generation autosomal dominant family with 16 members affected by a progressive cardiac and/or skeletal myopathy caused by a c.1346A>C (p.Lys449Thr) mutation located in the tail domain of desmin.

RESULTS

Phenotypic features in patients with tail domain mutations are similar to those in patients with mutations localized in the 1B and 2B α-helical domains.

CONCLUSION

We recommend that the tail domain is searched for mutations as intensely as desmin coil domains which until recently were considered to be more 'functional'.

In vivo characterization of mutant myotilins

Myofibrillar myopathy (MFM) is a group of disorders that are pathologically defined by the disorganization of the myofibrillar alignment associated with the intracellular accumulation of Z-disk-associated proteins. MFM is caused by mutations in genes encoding Z-disk-associated proteins, including myotilin. Although a number of MFM mutations have been identified, it has been difficult to elucidate the precise roles of the mutant proteins. Here, we present a useful method for the characterization of mutant proteins associated with MFM. Expression of mutant myotilins in mouse tibialis anterior muscle by in vivo electroporation recapitulated both the pathological changes and the biochemical characteristics observed in patients with myotilinopathy. In mutant myotilin-expressing muscle fibers, myotilin aggregates and is costained with polyubiquitin, and Z-disk-associated proteins and myofibrillar disorganization were commonly seen. In addition, the expressed S60C mutant myotilin protein displayed marked detergent insolubility in electroporated mouse muscle, similar to that observed in human MFM muscle with the same mutation. Thus, in vivo electroporation can be a useful method for evaluating the pathogenicity of mutations identified in MFM.

Myofibrillar myopathy caused by a mutation in the motor domain of mouse MyHC IIb

Ariel is a mouse mutant that suffers from skeletal muscle myofibrillar degeneration due to the rapid accumulation of large intracellular protein aggregates. This fulminant disease is caused by an ENU-induced recessive mutation resulting in an L342Q change within the motor domain of the skeletal muscle myosin protein MYH4 (MyHC IIb). Although normal at birth, homozygous mice develop hindlimb paralysis from Day 13, consistent with the timing of the switch from developmental to adult myosin isoforms in mice. The mutated myosin (MYH4(L342Q)) is an aggregate-prone protein. Notwithstanding the speed of the process, biochemical analysis of purified aggregates showed the presence of proteins typically found in human myofibrillar myopathies, suggesting that the genesis of ariel aggregates follows a pathogenic pathway shared with other conformational protein diseases of skeletal muscle. In contrast, heterozygous mice are overtly and histologically indistinguishable from control mice. MYH4(L342Q) is present in muscles from heterozygous mice at only 7% of the levels of the wild-type protein, resulting in a small but significant increase in force production in isolated single fibres and indicating that elimination of the mutant protein in heterozygotes prevents the pathological changes observed in homozygotes. Recapitulation of the L342Q change in the functional equivalent of mouse MYH4 in human muscles, MYH1, results in a more aggregate-prone protein.

Filamin C plays an essential role in the maintenance of the structural integrity of cardiac and skeletal muscles, revealed by the medaka mutant zacro

Filamin C is an actin-crosslinking protein that is specifically expressed in cardiac and skeletal muscles. Although mutations in the filamin C gene cause human myopathy with cardiac involvement, the function of filamin C in vivo is not yet fully understood. Here we report a medaka mutant, zacro (zac), that displayed an enlarged heart, caused by rupture of the myocardiac wall, and progressive skeletal muscle degeneration in late embryonic stages. We identified zac to be a homozygous nonsense mutation in the filamin C (flnc) gene. The medaka filamin C protein was found to be localized at myotendinous junctions, sarcolemma, and Z-disks in skeletal muscle, and at intercalated disks in the heart. zac embryos showed prominent myofibrillar degeneration at myotendinous junctions, detachment of myofibrils from sarcolemma and intercalated disks, and focal Z-disk destruction. Importantly, the expression of γ-actin, which we observed to have a strong subcellular localization at myotendinous junctions, was specifically reduced in zac mutant myotomes. Inhibition of muscle contraction by anesthesia alleviated muscle degeneration in the zac mutant. These results suggest that filamin C plays an indispensable role in the maintenance of the structural integrity of cardiac and skeletal muscles for support against mechanical stress.

A novel CRYAB mutation resulting in multisystemic disease

Mutations in the CRYAB gene, encoding alpha-B crystallin, cause distinct clinical phenotypes including isolated posterior polar cataract, myofibrillar myopathy, cardiomyopathy, or a multisystemic disorder combining all these features. Genotype/phenotype correlations are still unclear. To date, multisystemic involvement has been reported only in kindred harboring the R120G substitution. We report a novel CRYAB mutation, D109H, associated with posterior polar cataract, myofibrillar myopathy and cardiomyopathy in a two-generation family with five affected individuals. Age of onset, clinical presentation, and muscle abnormalities were very similar to those described in the R120G family. Alpha-B crystallin may form dimers and acts as a chaperone for a number of proteins. It has been suggested that the phenotypic diversity could be related to the various interactions between target proteins of individual mutant residues. Molecular modeling indicates that residues D109 and R120 interact with each other during dimerization of alpha-B crystallin; interestingly, the two substitutions affecting these residues (D109H and R120G) are associated with the same clinical phenotype, thus suggesting a similar pathogenic mechanism. We propose that impairment of alpha-B crystallin dimerization may also be relevant to the pathogenesis of these disorders.

AAV6-mediated systemic shRNA delivery reverses disease in a mouse model of facioscapulohumeral muscular dystrophy

Treatment of dominantly inherited muscle disorders remains a difficult task considering the need to eliminate the pathogenic gene product in a body-wide fashion. We show here that it is possible to reverse dominant muscle disease in a mouse model of facioscapulohumeral muscular dystrophy (FSHD). FSHD is a common form of muscular dystrophy associated with a complex cascade of epigenetic events following reduction in copy number of D4Z4 macrosatellite repeats located on chromosome 4q35. Several 4q35 genes have been examined for their role in disease, including FRG1. Overexpression of FRG1 causes features related to FSHD in transgenic mice and the FRG1 mouse is currently the only available mouse model of FSHD. Here we show that systemic delivery of RNA interference expression cassettes in the FRG1 mouse, after the onset of disease, led to a dose-dependent long-term FRG1 knockdown without signs of toxicity. Histological features including centrally nucleated fibers, fiber size reduction, fibrosis, adipocyte accumulation, and inflammation were all significantly improved. FRG1 mRNA knockdown resulted in a dramatic restoration of muscle function. Through RNA interference (RNAi) expression cassette redesign, our method is amenable to targeting any pathogenic gene offering a viable option for long-term, body-wide treatment of dominant muscle disease in humans.

Facioscapulohumeral muscular dystrophy (FSHD) region gene 1 (FRG1) is a dynamic nuclear and sarcomeric protein

Facioscapulohumeral muscular dystrophy (FSHD) region gene 1 (FRG1) is a candidate gene for mediating FSHD pathophysiology, however, very little is known about the endogenous FRG1 protein. This study uses immunocytochemistry (ICC) and histology to provide insight into FRG1's role in vertebrate muscle development and address its potential involvement in FSHD pathophysiology. In cell culture, primary myoblast/myotube cultures, and mouse and human muscle sections, FRG1 showed distinct nuclear and cytoplasmic localizations and nuclear shuttling assays indicated the subcellular pools of FRG1 are linked. During myoblast differentiation, FRG1's subcellular distribution changed dramatically with FRG1 eventually associating with the matured Z-discs. This Z-disc localization was confirmed using isolated mouse myofibers and found to be maintained in adult human skeletal muscle biopsies. Thus, FRG1 is not likely involved in the initial assembly and alignment of the Z-disc but may be involved in sarcomere maintenance or signaling. Further analysis of human tissue showed FRG1 is strongly expressed in arteries, veins, and capillaries, the other prominently affected tissue in FSHD. Overall, we show that in mammalian cells, FRG1 is a dynamic nuclear and cytoplasmic protein, however in muscle, FRG1 is also a developmentally regulated sarcomeric protein suggesting FRG1 may perform a muscle-specific function. Thus, FRG1 is the only FSHD candidate protein linked to the muscle contractile machinery and may address why the musculature and vasculature are specifically susceptible in FSHD.

Deficiency of α-actinin-3 is associated with increased susceptibility to contraction-induced damage and skeletal muscle remodeling

Sarcomeric α-actinins (α-actinin-2 and -3) are a major component of the Z-disk in skeletal muscle, where they crosslink actin and other structural proteins to maintain an ordered myofibrillar array. Homozygosity for the common null polymorphism (R577X) in ACTN3 results in the absence of fast fiber-specific α-actinin-3 in ∼20% of the general population. α-Actinin-3 deficiency is associated with decreased force generation and is detrimental to sprint and power performance in elite athletes, suggesting that α-actinin-3 is necessary for optimal forceful repetitive muscle contractions. Since Z-disks are the structures most vulnerable to eccentric damage, we sought to examine the effects of α-actinin-3 deficiency on sarcomeric integrity. Actn3 knockout mouse muscle showed significantly increased force deficits following eccentric contraction at 30% stretch, suggesting that α-actinin-3 deficiency results in an increased susceptibility to muscle damage at the extremes of muscle performance. Microarray analyses demonstrated an increase in muscle remodeling genes, which we confirmed at the protein level. The loss of α-actinin-3 and up-regulation of α-actinin-2 resulted in no significant changes to the total pool of sarcomeric α-actinins, suggesting that alterations in fast fiber Z-disk properties may be related to differences in functional protein interactions between α-actinin-2 and α-actinin-3. In support of this, we demonstrated that the Z-disk proteins, ZASP, titin and vinculin preferentially bind to α-actinin-2. Thus, the loss of α-actinin-3 changes the overall protein composition of fast fiber Z-disks and alters their elastic properties, providing a mechanistic explanation for the loss of force generation and increased susceptibility to eccentric damage in α-actinin-3-deficient individuals.

BAG3 directly interacts with mutated alphaB-crystallin to suppress its aggregation and toxicity

A homozygous disruption or genetic mutation of the bag3 gene causes progressive myofibrillar myopathy in mouse and human skeletal and cardiac muscle disorder while mutations in the small heat shock protein αB-crystallin gene (CRYAB) are reported to be responsible for myofibrillar myopathy. Here, we demonstrate that BAG3 directly binds to wild-type αB-crystallin and the αB-crystallin mutant R120G, via the intermediate domain of BAG3. Peptides that inhibit this interaction in an in vitro binding assay indicate that two conserved Ile-Pro-Val regions of BAG3 are involved in the interaction with αB-crystallin, which is similar to results showing BAG3 binding to HspB8 and HspB6. BAG3 overexpression increased αB-crystallin R120G solubility and inhibited its intracellular aggregation in HEK293 cells. BAG3 suppressed cell death induced by αB-crystallin R120G overexpression in differentiating C2C12 mouse myoblast cells. Our findings indicate a novel function for BAG3 in inhibiting protein aggregation caused by the genetic mutation of CRYAB responsible for human myofibrillar myopathy.

Infantile onset myofibrillar myopathy due to recessive CRYAB mutations

Mutations in the αB-crystallin (CRYAB) gene, encoding a small heat shock protein with chaperone function, are a rare cause of myofibrillar myopathy with autosomal-dominant inheritance, late-onset and moderate severity. We report a female infant presenting from 4 months with profound muscle stiffness, persistent creatine kinase elevation and electromyography characterized by spontaneous electrical activity and pseudomyotonic discharges. Muscle biopsy suggested a myofibrillar myopathy and genetic testing revealed homozygosity for the CRYAB mutation c.343delT (p.Ser115ProfsX14). These findings suggest a severe, recessively inherited form of CRYAB-related myofibrillar myopathy. Profound muscle stiffness as the main presenting feature indicates αB-crystallin as a potent modifier of muscle contractility.

Neuromuscular imaging in inherited muscle diseases

Driven by increasing numbers of newly identified genetic defects and new insights into the field of inherited muscle diseases, neuromuscular imaging in general and magnetic resonance imaging (MRI) in particular are increasingly being used to characterise the severity and pattern of muscle involvement. Although muscle biopsy is still the gold standard for the establishment of the definitive diagnosis, muscular imaging is an important diagnostic tool for the detection and quantification of dystrophic changes during the clinical workup of patients with hereditary muscle diseases. MRI is frequently used to describe muscle involvement patterns, which aids in narrowing of the differential diagnosis and distinguishing between dystrophic and non-dystrophic diseases. Recent work has demonstrated the usefulness of muscle imaging for the detection of specific congenital myopathies, mainly for the identification of the underlying genetic defect in core and centronuclear myopathies. Muscle imaging demonstrates characteristic patterns, which can be helpful for the differentiation of individual limb girdle muscular dystrophies. The aim of this review is to give a comprehensive overview of current methods and applications as well as future perspectives in the field of neuromuscular imaging in inherited muscle diseases. We also provide diagnostic algorithms that might guide us through the differential diagnosis in hereditary myopathies.

BAG3 and Hsc70 interact with actin capping protein CapZ to maintain myofibrillar integrity under mechanical stress

RATIONALE

A homozygous disruption or genetic mutation of the bag3 gene, a member of the Bcl-2-associated athanogene (BAG) family proteins, causes cardiomyopathy and myofibrillar myopathy that is characterized by myofibril and Z-disc disruption. However, the detailed disease mechanism is not yet fully understood.

OBJECTIVE

bag3(-/-) mice exhibit differences in the extent of muscle degeneration between muscle groups with muscles experiencing the most usage degenerating at an accelerated rate. Usage-dependent muscle degeneration suggests a role for BAG3 in supporting cytoskeletal connections between the Z-disc and myofibrils under mechanical stress. The mechanism by which myofibrillar structure is maintained under mechanical stress remains unclear. The purpose of the study is to clarify the detailed molecular mechanism of BAG3-mediated muscle maintenance under mechanical stress.

METHODS AND RESULTS

To address the question of whether bag3 gene knockdown induces myofibrillar disorganization caused by mechanical stress, in vitro mechanical stretch experiments using rat neonatal cardiomyocytes and a short hairpin RNA-mediated gene knockdown system of the bag3 gene were performed. As expected, mechanical stretch rapidly disrupts myofibril structures in bag3 knockdown cardiomyocytes. BAG3 regulates the structural stability of F-actin through the actin capping protein, CapZβ1, by promoting association between Hsc70 and CapZβ1. BAG3 facilitates the distribution of CapZβ1 to the proper location, and dysfunction of BAG3 induces CapZ ubiquitin-proteasome-mediated degradation. Inhibition of CapZβ1 function by overexpressing CapZβ2 increased myofibril vulnerability and fragmentation under mechanical stress. On the other hand, overexpression of CapZβ1 inhibits myofibrillar disruption in bag3 knockdown cells under mechanical stress. As a result, heart muscle isolated from bag3(-/-) mice exhibited myofibrillar degeneration and lost contractile activity after caffeine contraction.

CONCLUSIONS

These results suggest novel roles for BAG3 and Hsc70 in stabilizing myofibril structure and inhibiting myofibrillar degeneration in response to mechanical stress. These proteins are possible targets for further research to identify therapies for myofibrillar myopathy or other degenerative diseases.

An Italian case of hereditary myopathy with early respiratory failure (HMERF) not associated with the titin kinase domain R279W mutation

Hereditary myopathy with early respiratory failure (HMERF) is a rare disorder characterized by severe respiratory involvement at onset, muscle weakness starting in the early adulthood, and cytoplasmic bodies with peculiar immunohistochemical reactivity on muscle biopsy. Here we describe a patient who presented with hypercapnic coma at age 32. A detailed light and electron microscopy analysis on muscle biopsy was performed and, together with clinical data, led to the diagnosis. The R279W mutation in the TTN gene was excluded. This report expands the geographical region of incidence and encourages additional studies to clarify the genetic heterogeneity of the condition.

Desmin-related myopathy

Desmin-related myopathy (DRM) is an autosomally inherited skeletal and cardiac myopathy, mainly caused by dominant mutations in the desmin gene (DES). We provide (i) a literature review on DRM, including clinical manifestations, inheritance, molecular genetics, myopathology and management and (ii) a meta-analysis of reported DES mutation carriers, focusing on their clinical characteristics and potential genotype-phenotype correlations. Meta-analysis: DES mutation carriers (n = 159) with 40 different mutations were included. Neurological signs were present in 74% and cardiological signs in 74% of carriers (both neurological and cardiological signs in 49%, isolated neurological signs in 22%, and isolated cardiological signs in 22%). More than 70% of carriers exhibited myopathy or muscular weakness, with normal creatine kinase levels present in one third of them. Up to 50% of carriers had cardiomyopathy and around 60% had cardiac conduction disease or arrhythmias, with atrioventricular block as an important hallmark. Symptoms generally started during the 30s; a quarter of carriers died at a mean age of 49 years. Sudden cardiac death occurred in two patients with a pacemaker, suggesting a ventricular tachyarrhythmia as cause of death. The majority of DES mutations were missense mutations, mostly located in the 2B domain. Mutations in the 2B domain were predominant in patients with an isolated neurological phenotype, whereas head and tail domain mutations were predominant in patients with an isolated cardiological phenotype.

DNA sequencing errors in molecular diagnostics of filamin myopathy

BACKGROUND

Filamin myopathy is a neuromuscular disorder manifesting with predominantly limb-girdle muscle weakness and in many patients with diaphragm paralysis and cardiomyopathy, caused by mutations in the filamin C (FLNC) gene. Molecular diagnosis of filamin myopathy based on direct DNA sequencing of coding exons is compromised by the presence of a high homology pseudogene (pseFLNC) located approximately 53.6 kb downstream of the functional FLNC gene on chromosome 7q.

METHODS

Molecular cloning, RT-PCR and real-time PCR methods were used to detect sequence differences between the FLNC and pseFLNC that are implicated in known or potential molecular diagnostic errors. Overall, 50 patients with a phenotype resembling filamin myopathy have been screened for mutations in FLNC.

RESULTS

FLNC sequence inconsistencies caused by the interference from pseFLNC were identified and diagnostic errors involving, in particular, the detection of the most frequent disease-causing FLNC p.W2710X mutation resolved. Mismatches between the FLNC and pseFLNC sequences were tabulated for future use.

CONCLUSIONS

We devise a strategy that allows one to discern mutations occurring in the functional FLNC from those harbored in pseFLNC, thus preventing possible complications in future research and patient genetic testing.

The role of muscle biopsy in the age of genetic testing

PURPOSE OF REVIEW

The purpose of this review is to discuss the role of muscle biopsy in the current age of genetic testing.

RECENT FINDINGS

The diagnostic approach to patients with suspected genetically determined myopathies has been altered by recent advances in molecular diagnostic technologies and by the increased number of conditions for which the genetic alterations have been identified. Myopathological aspects can narrow down the differential diagnosis when the clinical phenotype is not informative enough and can help guide the molecular investigation.

SUMMARY

Here, we review genetic and myopathological aspects of selected genetically determined myopathies.

Sarcomeric α-actinins and their role in human muscle disease

In skeletal muscle, the sarcomeric α-actinins (α-actinin-2 and -3) are a major component of the Z-line and crosslink actin thin filaments to maintain the structure of the sarcomere. Based on their known protein binding partners, the sarcomeric α-actinins are likely to have a number of structural, signaling and metabolic roles in skeletal muscle. In addition, the α-actinins interact with many proteins responsible for inherited muscle disorders. In this paper, we explore the role of the sarcomeric α-actinins in normal skeletal muscle and in the pathogenesis of a range of neuromuscular disorders.

Prevalence of genetic muscle disease in Northern England: in-depth analysis of a muscle clinic population

We have performed a detailed population study of patients with genetic muscle disease in the northern region of England. Our current clinic population comprises over 1100 patients in whom we have molecularly characterized 31 separate muscle disease entities. Diagnostic clarity achieved through careful delineation of clinical features supported by histological, immunological and genetic analysis has allowed us to reach a definitive diagnosis in 75.7% of our patients. We have compared our case profile with that from Walton and Nattrass' seminal study from 1954, also of the northern region, together with data from other more recent studies from around the world. Point prevalence figures for each of the five major disease categories are comparable with those from other recent studies. Myotonic dystrophies are the most common, comprising 28.6% of our clinic population with a point prevalence of 10.6/100,000. Next most frequent are the dystrophinopathies and facioscapulohumeral muscular dystrophy making up 22.9% (8.46/100,000) and 10.7% (3.95/100,000) of the clinic population, respectively. Spinal muscular atrophy patients account for 5.1% or 1.87/100,000 patients. Limb girdle muscular dystrophy, which was described for the first time in the paper by Walton and Nattrass (1954) and comprised 17% of their clinic population, comprises 6.2% of our clinic population at a combined prevalence of 2.27/100,000. The clinic population included patients with 12 other muscle disorders. These disorders ranged from a point prevalence of 0.89/100 000 for the group of congenital muscular dystrophies to conditions with only two affected individuals in a population of three million. For the first time our study provides epidemiological information for X-linked Emery-Dreifuss muscular dystrophy and the collagen VI disorders. Each of the X-linked form of Emery-Dreifuss muscular dystrophy and Ullrich muscular dystrophy has a prevalence of 0.13/100,000, making both very rare. Bethlem myopathy was relatively more common with a prevalence of 0.77/100,000. Overall our study provides comprehensive epidemiological information on individually rare inherited neuromuscular conditions in Northern England. Despite the deliberate exclusion of relatively common groups such as hereditary motor and sensory neuropathy (40/100,000) and mitochondrial disorders (9.2/100,000), the combined prevalence is 37.0/100,000, demonstrating that these disorders, taken as a group, encompass a significant proportion of patients with chronic disease. The study also illustrates the immense diagnostic progress since the first regional survey over 50 years ago by Walton and Nattrass.

Defective myotilin homodimerization caused by a novel mutation in MYOT exon 9 in the first Japanese limb girdle muscular dystrophy 1A patient

Myotilin is a muscle-specific Z disk protein. Several missense mutations in the myotilin gene (MYOT) have been identified in limb girdle muscular dystrophy (LGMD), myofibrillar myopathy, and distal myopathy patients. All previously reported pathogenic MYOT mutations have been identified only in Exon 2. We sequenced MYOT in 138 patients diagnosed as having LGMD, myofibrillar myopathy, or distal myopathy, and identified a novel MYOT mutation in Exon 9 encoding the second immunoglobulin-like domain in 1 patient with clinically typical LGMD. By light microscopy, there were scattered fibers with rimmed vacuoles and myofibrillary disorganization in the patient's muscle biopsy; accumulation of Z disk proteins was observed by immunohistochemistry. Immunoblot analysis demonstrated that the amount of myotilin monomer was increased in the patient muscle, but that the myotilin homodimeric band was decreased. Functional analysis of the myotilin mutation using a yeast 2-hybrid system revealed defective homodimerization of the mutant myotilin and decreased interaction between mutant myotilin and alpha-actinin. The homodimerization defect was further demonstrated by immunoprecipitation. This is the first MYOT mutation outside of Exon 2 in an LGMD type 1A patient and the first MYOT mutation identified in the Japanese population. This mutation in the second immunoglobulin-like domain impairs myotilin dimerization and alters the binding between myotilin and alpha-actinin, which is known to be important for actin bundling.

Severe infantile-onset cardiomyopathy associated with a homozygous deletion in desmin

Desminopathy is a genetically heterogeneous disorder with autosomal dominant pattern of inheritance in most affected families; the age of disease onset is on average 30 years. We studied a patient with a history of recurrent episodes of syncope from infancy who later developed second-degree AV block and restrictive cardiomyopathy; she subsequently suffered several episodes of ventricular tachyarrhythmia requiring implantation of bicameral defibrillator. Neurological examination revealed rapidly progressive bilateral facial weakness, winging of the scapulae, symmetric weakness and atrophy of the trunk muscles, shoulder girdle and distal muscles of both upper and lower extremities. Muscle biopsy demonstrated signs of myofibrillar myopathy with prominent subsarcolemmal desmin-reactive aggregates. Molecular analysis identified a homozygous deletion in DES resulting in a predicted in-frame obliteration of seven amino acids (p.R173_E179del) in the 1B domain of desmin. We describe the youngest known desminopathy patient with severe cardiomyopathy and aggressive course leading to the devastation of cardiac, skeletal and smooth musculature at an early age.

Interplay between the ubiquitin-proteasome system and autophagy in proteinopathies

Proteinopathies are a family of human disease caused by toxic aggregation-prone proteins and featured by the presence of protein aggregates in the affected cells. The ubiquitin-proteasome system (UPS) and autophagy are two major intracellular protein degradation pathways. The UPS mediates the targeted degradation of most normal proteins after performing their normal functions as well as the removal of abnormal, soluble proteins. Autophagy is mainly responsible for degradation of defective organelles and the bulk degradation of cytoplasm during starvation. The collaboration between the UPS and autophagy appears to be essential to protein quality control in the cell. UPS proteolytic function often becomes inadequate in proteinopathies which may lead to activation of autophagy, striving to remove abnormal proteins especially the aggregated forms. HADC6, p62, and FoxO3 may play an important role in mobilizing this proteolytic consortium. Benign measures to enhance proteasome function are currently lacking; however, enhancement of autophagy via pharmacological intervention and/or lifestyle change has shown great promise in alleviating bona fide proteinopathies in the cell and animal models. These pharmacological interventions are expected to be applied clinically to treat human proteinopathies in the near future.

Differential involvement of sarcomeric proteins in myofibrillar myopathies: a morphological and immunohistochemical study

Myofibrillar myopathies (MFMs) are rare inherited or sporadic progressive neuromuscular disorders with considerable clinical and genetic heterogeneity. In the current study, we have analyzed histopathological and immunohistochemical characteristics in genetically identified MFMs. We performed a morphological and morphometrical study in a cohort of 24 genetically identified MFM patients (12 desmin, 6 alphaB-crystallin, 4 ZASP, 2 myotilin), and an extensive immunohistochemical study in 15 of these patients, using both well-known and novel antibodies directed against distinct compartments of the muscle fibers, including Z-disc and M-band proteins. Our morphological data revealed some significant differences between the distinct MFM subgroups: the consistent presence of 'rubbed-out' fibers in desminopathies and alphaB-crystallinopathies, an elevated frequency of vacuoles in ZASPopathies and myotilinopathies, and the presence of a few necrotic fibers in the two myotilinopathy patients. Immunohistochemistry showed that in MFM only a subset of Z-disc proteins, such as filamin C and its ligands myotilin and Xin, exhibited significant alterations in their localization, whereas other Z-disc proteins like alpha-actinin, myopodin and tritopodin, did not. In contrast, M-band proteins revealed no abnormalities in MFM. We conclude that the presence of 'rubbed-out' fibers are a suggestive feature for desminopathy or alphaB-crystallinopathy, and that MFM is not a general disease of the myofibril, but primarily affects a subgroup of stress-responsive Z-disc proteins.

Clinical, histological and genetic characterization of reducing body myopathy caused by mutations in FHL1

We recently identified the X-chromosomal four and a half LIM domain gene FHL1 as the causative gene for reducing body myopathy, a disorder characterized by progressive weakness and intracytoplasmic aggregates in muscle that exert reducing activity on menadione nitro-blue-tetrazolium (NBT). The mutations detected in FHL1 affected highly conserved zinc coordinating residues within the second LIM domain and lead to the formation of aggregates when transfected into cells. Our aim was to define the clinical and morphological phenotype of this myopathy and to assess the mutational spectrum of FHL1 mutations in reducing body myopathy in a larger cohort of patients. Patients were ascertained via the detection of reducing bodies in muscle biopsy sections stained with menadione-NBT followed by clinical, histological, ultrastructural and molecular genetic analysis. A total of 11 patients from nine families were included in this study, including seven sporadic patients with early childhood onset disease and four familial cases with later onset. Weakness in all patients was progressive, sometimes rapidly so. Respiratory failure was common and scoliosis and spinal rigidity were significant in some of the patients. Analysis of muscle biopsies confirmed the presence of aggregates of FHL1 positive material in all biopsies. In two patients in whom sequential biopsies were available the aggregate load in muscle sections appeared to increase over time. Ultrastructural analysis revealed that cytoplasmic bodies were regularly seen in conjunction with the reducing bodies. The mutations detected were exclusive to the second LIM domain of FHL1 and were found in both sporadic as well as familial cases of reducing body myopathy. Six of the nine mutations affected the crucial zinc coordinating residue histidine 123. All mutations in this residue were de novo and were associated with a severe clinical course, in particular in one male patient (H123Q). Mutations in the zinc coordinating residue cysteine 153 were associated with a milder phenotype and were seen in the familial cases in which the boys were still more severely affected compared to their mothers. We expect the mild end of the spectrum to significantly expand in the future. On the severe end of the spectrum we define reducing body myopathy as a progressive disease with early, but not necessarily congenital onset, distinguishing this condition from the classic essentially non-progressive congenital myopathies.

Maintenance of muscle mass, fiber size, and contractile function in mice lacking the Z-disc protein myotilin

BACKGROUND

Myofibrillar myopathies constitute a rare group of congenital neuromuscular disorders, frequently associated with mutations in Z-disc proteins such as myotilin. Myotilin location and interactions with other Z-disc proteins are clearly defined, but its role in the regulation of muscle structure and function remains unknown. The present study aims at investigating this specific role of myotilin.

METHODS

Skeletal and cardiac muscles were collected from adult mice with a targeted deletion of myotilin (myo(-/-)) and wild-type animals (myo(+/+)).

RESULTS AND CONCLUSION

Similar skeletal and cardiac muscle weights were observed in myo(-/-) and myo(+/+) mice. At the muscle cell level, the size and force production of single membrane permeabilized fibers were identical between myo(-/-) and myo(+/+) rodents. Thus, myotilin does not have a significant influence on muscle mass, muscle fiber size, or regulation of muscle contraction. Alternatively, compensatory over-expressions of other elements including proteins from the same subfamily, or Z-disc proteins such as telethonin, or intermediate filaments may compensate for the lack of myotilin.