A novel FLNC frameshift and an OBSCN variant in a family with distal muscular dystrophy

OBSCN undergoes extensive alternative splicing during human cardiac and skeletal muscle development

BACKGROUND

Highly expressed in skeletal muscles, the gene Obscurin (i.e. OBSCN) has 121 non-overlapping exons and codes for some of the largest known mRNAs in the human genome. Furthermore, it plays an essential role in muscle development and function. Mutations in OBSCN are associated with several hypertrophic cardiomyopathies and muscular disorders. OBSCN undergoes extensive and complex alternative splicing, which is the main reason that its splicing regulation associated with skeletal and cardiac muscle development has not previously been thoroughly studied.

METHODS

We analyzed RNA-Seq data from skeletal and cardiac muscles extracted from 44 postnatal individuals and six fetuses. We applied the intron/exon level splicing analysis software IntEREst to study the splicing of OBSCN in the studied samples. The differential splicing analysis was adjusted for batch effects. Our comparisons revealed the splicing variations in OBSCN between the human skeletal and cardiac muscle, as well as between post-natal muscle (skeletal and cardiac) and the pre-natal equivalent muscle.

RESULTS

We detected several splicing regulations located in the 5'end, 3' end, and the middle of OBSCN that are associated with human cardiac or skeletal muscle development. Many of these alternative splicing events have not previously been reported. Our results also suggest that many of these muscle-development associated splicing events may be regulated by BUB3.

CONCLUSIONS

We conclude that the splicing of OBSCN is extensively regulated during the human skeletal/cardiac muscle development. We developed an interactive visualization tool that can be used by clinicians and researchers to study the inclusion of specific OBSCN exons in pre- and postnatal cardiac and skeletal muscles and access the statistics for the differential inclusion of the exons across the studied sample groups. The OBSCN exon inclusion map related to the human cardiac and skeletal muscle development is available at http://psivis.it.helsinki.fi:3838/OBSCN_PSIVIS/ . These findings are essential for an accurate pre- and postnatal clinical interpretation of the OBSCN exonic variants.

I told you to stop: obscurin's role in epithelial cell migration

The giant cytoskeletal protein obscurin contains multiple cell signaling domains that influence cell migration. Here, we follow each of these pathways, examine how these pathways modulate epithelial cell migration, and discuss the cross-talk between these pathways. Specifically, obscurin uses its PH domain to inhibit phosphoinositide-3-kinase (PI3K)-dependent migration and its RhoGEF domain to activate RhoA and slow cell migration. While obscurin's effect on the PI3K pathway agrees with the literature, obscurin's effect on the RhoA pathway runs counter to most other RhoA effectors, whose activation tends to lead to enhanced motility. Obscurin also phosphorylates cadherins, and this may also influence cell motility. When taken together, obscurin's ability to modulate three independent cell migration pathways is likely why obscurin knockout cells experience enhanced epithelial to mesenchymal transition, and why obscurin is a frequently mutated gene in several types of cancer.

Novel OBSCN variants associated with a risk to exercise-intolerance and rhabdomyolysis

Obscurin, encoded by the OBSCN gene, is a muscle protein consisting of three main splice isoforms, obscurin-A, obscurin-B, and obscurin kinase-only protein (also known as KIAA1639 or Obsc-kin). Obscurin is located at the M-band and Z-disks and interacts with titin and myomesin. It plays an important role in the stability and maintenance of the A- and M-bands and the subsarcolemmal organization of the microtubule network. Furthermore, obscurin is involved in Ca2+ regulation and sarcoplasmic reticulum function and is connected to several other muscle proteins. OBSCN gene variants have been reported to be relatively common in inherited cardiomyopathies. Here we reported two young patients with a history of cramps, myalgia, exercise intolerance, rhabdomyolysis, and myoglobinuria without any evidence of concomitant cardiomyopathy in association with novel OBSCN variants (c.24822C>A and c.2653+1G>C). Obscurin-deficient muscle fibers seem to have increased susceptibility to damage triggered by exercise that may lead to rhabdomyolysis. More studies are needed to clarify the diverse clinical phenotypes and the pathophysiology of OBSCN gene variants.

Application of the Nicoya OpenSPR to Studies of Biomolecular Binding: A Review of the Literature from 2016 to 2022

The Nicoya OpenSPR is a benchtop surface plasmon resonance (SPR) instrument. As with other optical biosensor instruments, it is suitable for the label-free interaction analysis of a diverse set of biomolecules, including proteins, peptides, antibodies, nucleic acids, lipids, viruses, and hormones/cytokines. Supported assays include affinity/kinetics characterization, concentration analysis, yes/no assessment of binding, competition studies, and epitope mapping. OpenSPR exploits localized SPR detection in a benchtop platform and can be connected with an autosampler (XT) to perform automated analysis over an extended time period. In this review article, we provide a comprehensive survey of the 200 peer-reviewed papers published between 2016 and 2022 that use the OpenSPR platform. We highlight the range of biomolecular analytes and interactions that have been investigated using the platform, provide an overview on the most common applications for the instrument, and point out some representative research that highlights the flexibility and utility of the instrument.

Novel Filamin C Myofibrillar Myopathy Variants Cause Different Pathomechanisms and Alterations in Protein Quality Systems

Myofibrillar myopathies (MFM) are a group of chronic muscle diseases pathophysiologically characterized by accumulation of protein aggregates and structural failure of muscle fibers. A subtype of MFM is caused by heterozygous mutations in the filamin C (FLNC) gene, exhibiting progressive muscle weakness, muscle structural alterations and intracellular protein accumulations. Here, we characterize in depth the pathogenicity of two novel truncating FLNc variants (p.Q1662X and p.Y2704X) and assess their distinct effect on FLNc stability and distribution as well as their impact on protein quality system (PQS) pathways. Both variants cause a slowly progressive myopathy with disease onset in adulthood, chronic myopathic alterations in muscle biopsy including the presence of intracellular protein aggregates. Our analyses revealed that p.Q1662X results in FLNc haploinsufficiency and p.Y2704X in a dominant-negative FLNc accumulation. Moreover, both protein-truncating variants cause different PQS alterations: p.Q1662X leads to an increase in expression of several genes involved in the ubiquitin-proteasome system (UPS) and the chaperone-assisted selective autophagy (CASA) system, whereas p.Y2704X results in increased abundance of proteins involved in UPS activation and autophagic buildup. We conclude that truncating FLNC variants might have different pathogenetic consequences and impair PQS function by diverse mechanisms and to varying extents. Further studies on a larger number of patients are necessary to confirm our observations.

The N-terminus of obscurin is flexible in solution

The N-terminal half of the giant cytoskeletal protein obscurin is comprised of more than 50 Ig-like domains, arranged in tandem. Domains 18-51 are connected to each other through short 5-residue linkers, and this arrangement has been previously shown to form a semi-flexible rod in solution. Domains 1-18 generally have slightly longer ~7 residue interdomain linkers, and the multidomain structure and motion conferred by this kind of linker is understudied. Here, we use NMR, SAXS, and MD to show that these longer linkers are associated with significantly more domain/domain flexibility, with the resulting multidomain structure being moderately compact. Further examination of the relationship between interdomain flexibility and linker length shows there is a 5 residue "sweet spot" linker length that results in dual-domain systems being extended, and conversely that both longer or shorter linkers result in a less extended structure. This detailed knowledge of the obscurin N terminus structure and flexibility allowed for mathematical modeling of domains 1-18, which suggests that this region likely forms tangles if left alone in solution. Given how infrequently protein tangles occur in nature, and given the pathological outcomes that occur when tangles do arise, our data suggest that obscurin is likely either significantly scaffolded or else externally extended in the cell.

Distal myopathy

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

Rare clinical phenotype of filaminopathy presenting as restrictive cardiomyopathy and myopathy in childhood

Background

FLNC is one of the few genes associated with all types of cardiomyopathies, but it also underlies neuromuscular phenotype. The combination of concomitant neuromuscular and cardiac involvement is not often observed in filaminopathies and the impact of this on the disease prognosis has hitherto not been analyzed.

Results

Here we provide a detailed clinical, genetic, and structural prediction analysis of distinct FLNC-associated phenotypes based on twelve pediatric cases. They include early-onset restrictive cardiomyopathy (RCM) in association with congenital myopathy. In all patients the initial diagnosis was established during the first year of life and in five out of twelve (41.7%) patients the first symptoms were observed at birth. RCM was present in all patients, often in combination with septal defects. No ventricular arrhythmias were noted in any of the patients presented here. Myopathy was confirmed by neurological examination, electromyography, and morphological studies. Arthrogryposes was diagnosed in six patients and remained clinically meaningful with increasing age in three of them. One patient underwent successful heart transplantation at the age of 18 years and two patients are currently included in the waiting list for heart transplantation. Two died due to congestive heart failure. One patient had ICD instally as primary prevention of SCD. In ten out of twelve patients the disease was associated with missense variants and only in two cases loss of function variants were detected. In half of the described cases, an amino acid substitution A1186V, altering the structure of IgFLNc10, was found.

Conclusions

The present description of twelve cases of early-onset restrictive cardiomyopathy with congenital myopathy and FLNC mutation, underlines a distinct unique phenotype that can be suggested as a separate clinical form of filaminopathies. Amino acid substitution A1186V, which was observed in half of the cases, defines a mutational hotspot for the reported combination of myopathy and cardiomyopathy. Several independent molecular mechanisms of FLNC mutations linked to filamin structure and function can explain the broad spectrum of FLNC-associated phenotypes. Early disease presentation and unfavorable prognosis of heart failure demanding heart transplantation make awareness of this clinical form of filaminopathy of great clinical importance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02477-5.

Case Reports: Novel Missense Variants in the Filamin C Actin Binding Domain Cause Variable Phenotypes

Filamin C is a large dimeric actin-binding protein, most prevalent in skeletal and cardiac muscle Z-discs, where it participates in sarcomere mechanical stabilization and intracellular signaling, interacting with numerous binding partners. Dominant heterozygous mutations of Filamin C gene cause several forms of myopathy and structural or arrhythmogenic cardiomyopathy. In this report we describe clinical and molecular findings of two Italian patients, in whom we identified two novel missense variants located within the Filamin C actin binding domain. Muscle imaging, histological and ultrastructural findings are also reported. Our results underline the extreme inter- and intrafamilial variability of clinical manifestations, hence the need to extend the investigation also to asymptomatic relatives, and the relevance of a broad diagnostic approach involving muscle electron microscopy, skeletal muscle magnetic resonance imaging and next generation sequencing techniques.

Impaired Intracellular Ca2+ Dynamics, M-Band and Sarcomere Fragility in Skeletal Muscles of Obscurin KO Mice

Obscurin is a giant sarcomeric protein expressed in striated muscles known to establish several interactions with other proteins of the sarcomere, but also with proteins of the sarcoplasmic reticulum and costameres. Here, we report experiments aiming to better understand the contribution of obscurin to skeletal muscle fibers, starting with a detailed characterization of the diaphragm muscle function, which we previously reported to be the most affected muscle in obscurin (Obscn) KO mice. Twitch and tetanus tension were not significantly different in the diaphragm of WT and Obscn KO mice, while the time to peak (TTP) and half relaxation time (HRT) were prolonged. Differences in force-frequency and force-velocity relationships and an enhanced fatigability are observed in an Obscn KO diaphragm with respect to WT controls. Voltage clamp experiments show that a sarcoplasmic reticulum's Ca2+ release and SERCA reuptake rates were decreased in muscle fibers from Obscn KO mice, suggesting that an impairment in intracellular Ca2+ dynamics could explain the observed differences in the TTP and HRT in the diaphragm. In partial contrast with previous observations, Obscn KO mice show a normal exercise tolerance, but fiber damage, the altered sarcomere ultrastructure and M-band disarray are still observed after intense exercise.

A Heterozygous Mutation in the Filamin C Gene Causes an Unusual Nemaline Myopathy With Ring Fibers

Autosomal dominant pathogenic variants in the filamin C gene (FLNC) have been associated with myofibrillar myopathies, distal myopathies, and isolated cardiomyopathies. Mutations in different functional domains of FLNC can cause various clinical phenotypes. A novel heterozygous missense variant c.608G>A, p.(Cys203Tyr) in the actin binding domain of FLCN was found to cause an upper limb distal myopathy (MIM #614065). The muscle MRI findings are similar to those observed in FLNC-myofibrillar myopathy (MIM #609524). However, the muscle biopsy revealed >20% of muscle fibers with nemaline bodies, in addition to numerous ring fibers and a predominance of type 1 fibers. Overall, this case shows some unique and rare aspects of FLNC-myopathy constituting a new morphologic phenotype of FLNC-related myopathies.

When is an obscurin variant pathogenic? The impact of Arg4344Gln and Arg4444Trp variants on protein-protein interactions and protein stability

Obscurin is a giant muscle protein that connects the sarcomere with the sarcoplasmic reticulum, and has poorly understood structural and signalling functions. Increasingly, obscurin variants are implicated in the pathophysiology of cardiovascular diseases. The Arg4344Gln variant (R4344Q) in obscurin domain Ig58, initially discovered in a patient with hypertrophic cardiomyopathy, has been reported to reduce binding to titin domains Z8-Z9, impairing obscurin’s Z-disc localization. An R4344Q knock-in mouse developed a cardiomyopathy-like phenotype with abnormal Ca²⁺-handling and arrhythmias, which were attributed to an enhanced affinity of a putative interaction between obscurin Ig58 and phospholamban (PLN) due to the R4344Q variant. However, the R4344Q variant is found in 15% of African Americans, arguing against its pathogenicity. To resolve this apparent paradox, we quantified the influence of the R4344Q variant (alongside another potentially pathogenic variant: Arg4444Trp (R4444W)) on binding to titin Z8-Z9, novex-3 and PLN using pull-down assays and microscale thermophoresis and characterized the influence on domain stability using differential scanning fluorimetry. We found no changes in titin binding and thermostability for both variants and modestly increased affinities of PLN for R4344Q and R4444W. While we could not confirm the novex-3/obscurin interaction, the PLN/obscurin interaction relies on the transmembrane region of PLN and is not reproducible in mammalian cells, suggesting it is an in vitro artefact. Without clear clinical evidence for disease involvement, we advise against classifying these obscurin variants as pathogenic.

Panorama of the distal myopathies

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

The Impact of Mitochondrial Deficiencies in Neuromuscular Diseases

Neuromuscular diseases (NMDs) are a heterogeneous group of acquired or inherited rare disorders caused by injury or dysfunction of the anterior horn cells of the spinal cord (lower motor neurons), peripheral nerves, neuromuscular junctions, or skeletal muscles leading to muscle weakness and waste. Unfortunately, most of them entail serious or even fatal consequences. The prevalence rates among NMDs range between 1 and 10 per 100,000 population, but their rarity and diversity pose difficulties for healthcare and research. Some molecular hallmarks are being explored to elucidate the mechanisms triggering disease, to set the path for further advances. In fact, in the present review we outline the metabolic alterations of NMDs, mainly focusing on the role of mitochondria. The aim of the review is to discuss the mechanisms underlying energy production, oxidative stress generation, cell signaling, autophagy, and inflammation triggered or conditioned by the mitochondria. Briefly, increased levels of inflammation have been linked to reactive oxygen species (ROS) accumulation, which is key in mitochondrial genomic instability and mitochondrial respiratory chain (MRC) dysfunction. ROS burst, impaired autophagy, and increased inflammation are observed in many NMDs. Increasing knowledge of the etiology of NMDs will help to develop better diagnosis and treatments, eventually reducing the health and economic burden of NMDs for patients and healthcare systems.

The importance of an integrated genotype-phenotype strategy to unravel the molecular bases of titinopathies

Next generation sequencing (NGS) has allowed the titin gene (TTN) to be identified as a major contributor to neuromuscular disorders, with high clinical heterogeneity. The mechanisms underlying the phenotypic variability and the dominant or recessive pattern of inheritance are unclear. Titin is involved in the formation and stability of the sarcomeres. The effects of the different TTN variants can be harmless or pathogenic (recessive or dominant) but the interpretation is tricky because the current bioinformatics tools can not predict their functional impact effectively. Moreover, TTN variants are very frequent in the general population. The combination of deep phenotyping associated with RNA molecular analyses, western blot (WB) and functional studies is often essential for the interpretation of genetic variants in patients suspected of titinopathy. In line with the current guidelines and suggestions, we implemented for patients with skeletal myopathy and with potentially disease causing TTN variant(s) an integrated genotype-transcripts-protein-phenotype approach, associated with phenotype and variants segregation studies in relatives and confrontation with published data on titinopathies to evaluate pathogenic effects of TTN variants (even truncating ones) on titin transcripts, amount, size and functionality. We illustrate this integrated approach in four patients with recessive congenital myopathy.

Expanding the Clinico-Genetic Spectrum of Myofibrillar Myopathy: Experience From a Chinese Neuromuscular Center

Background: Myofibrillar myopathy is a group of hereditary neuromuscular disorders characterized by dissolution of myofibrils and abnormal intracellular accumulation of Z disc-related proteins. We aimed to characterize the clinical, physiological, pathohistological, and genetic features of Chinese myofibrillar myopathy patients from a single neuromuscular center.

Methods: A total of 18 patients were enrolled. Demographic and clinical data were collected. Laboratory investigations, electromyography, and cardiac evaluation was performed. Routine and immunohistochemistry stainings against desmin, αB-crystallin, and BAG3 of muscle specimen were carried out. Finally, next-generation sequencing panel array for genes associated with hereditary neuromuscular disorders were performed.

Results: Twelve pathogenic variants in DES, BAG3, FLNC, FHL1, and TTN were identified, of which seven were novel mutations. The novel DES c.1256C>T substitution is a high frequency mutation. The combined recessively/dominantly transmitted c.19993G>T and c.107545delG mutations in TTN gene cause a limb girdle muscular dystrophy phenotype with the classical myofibrillar myopathy histological changes.

Conclusions: We report for the first time that hereditary myopathy with early respiratory failure patient can have peripheral nerve and severe spine involvement. The mutation in Ig-like domain 16 of FLNC is associated with the limb girdle type of filaminopathy, and the mutation in Ig-like domain 18 with distal myopathy type. These findings expand the phenotypic and genotypic correlation spectrum of myofibrillar myopathy.

Deletion of obscurin immunoglobulin domains Ig58/59 leads to age-dependent cardiac remodeling and arrhythmia

Obscurin comprises a family of giant modular proteins that play key structural and regulatory roles in striated muscles. Immunoglobulin domains 58/59 (Ig58/59) of obscurin mediate binding to essential modulators of muscle structure and function, including canonical titin, a smaller splice variant of titin, termed novex-3, and phospholamban (PLN). Importantly, missense mutations localized within the obscurin-Ig58/59 region that affect binding to titins and/or PLN have been linked to the development of myopathy in humans. To elucidate the pathophysiological role of this region, we generated a constitutive deletion mouse model, Obscn-ΔIg58/59, that expresses obscurin lacking Ig58/59, and determined the consequences of this manipulation on cardiac morphology and function under conditions of acute stress and through the physiological process of aging. Our studies show that young Obscn-ΔIg58/59 mice are susceptible to acute β-adrenergic stress. Moreover, sedentary Obscn-ΔIg58/59 mice develop left ventricular hypertrophy that progresses to dilation, contractile impairment, atrial enlargement, and arrhythmia as a function of aging with males being more affected than females. Experiments in ventricular cardiomyocytes revealed altered Ca2+ cycling associated with changes in the expression and/or phosphorylation levels of major Ca2+ cycling proteins, including PLN, SERCA2, and RyR2. Taken together, our work demonstrates that obscurin-Ig58/59 is an essential regulatory module in the heart and its deletion leads to age- and sex-dependent cardiac remodeling, ventricular dilation, and arrhythmia due to deregulated Ca2+ cycling.

First clinical and myopathological description of a myofibrillar myopathy with congenital onset and homozygous mutation in FLNC

Filamin C (encoded by the FLNC gene) is a large actin-cross-linking protein involved in shaping the actin cytoskeleton in response to signaling events both at the sarcolemma and at myofibrillar Z-discs of cross-striated muscle cells. Multiple mutations in FLNC are associated with myofibrillar myopathies of autosomal-dominant inheritance. Here, we describe for the first time a boy with congenital onset of generalized muscular hypotonia and muscular weakness, delayed motor development but no cardiac involvement associated with a homozygous FLNC mutation c.1325C>G (p.Pro442Arg). We performed ultramorphological, proteomic, and functional investigations as well as immunological studies of known marker proteins for dominant filaminopathies. We show that the mutant protein is expressed in similar quantities as the wild-type variant in control skeletal muscle fibers. The proteomic signature of quadriceps muscle is altered and ultrastructural perturbations are evident. Moreover, filaminopathy marker proteins are comparable both in our homozygous and a dominant control case (c.5161delG). Biochemical investigations demonstrate that the recombinant mutant protein is less stable and more prone to degradation by proteolytic enzymes than the wild-type variant. The unusual congenital presentation of the disease clearly demonstrates that homozygosity for mutations in FLNC severely aggravates the phenotype.

Structure and Function of Filamin C in the Muscle Z-Disc

Filamin C (FLNC) is one of three filamin proteins (Filamin A (FLNA), Filamin B (FLNB), and FLNC) that cross-link actin filaments and interact with numerous binding partners. FLNC consists of a N-terminal actin-binding domain followed by 24 immunoglobulin-like repeats with two intervening calpain-sensitive hinges separating R15 and R16 (hinge 1) and R23 and R24 (hinge-2). The FLNC subunit is dimerized through R24 and calpain cleaves off the dimerization domain to regulate mobility of the FLNC subunit. FLNC is localized in the Z-disc due to the unique insertion of 82 amino acid residues in repeat 20 and necessary for normal Z-disc formation that connect sarcomeres. Since phosphorylation of FLNC by PKC diminishes the calpain sensitivity, assembly, and disassembly of the Z-disc may be regulated by phosphorylation of FLNC. Mutations of FLNC result in cardiomyopathy and muscle weakness. Although this review will focus on the current understanding of FLNC structure and functions in muscle, we will also discuss other filamins because they share high sequence similarity and are better characterized. We will also discuss a possible role of FLNC as a mechanosensor during muscle contraction.

A mutation update for the FLNC gene in myopathies and cardiomyopathies

Filamin C (FLNC) variants are associated with cardiac and muscular phenotypes. Originally, FLNC variants were described in myofibrillar myopathy (MFM) patients. Later, high‐throughput screening in cardiomyopathy cohorts determined a prominent role for FLNC in isolated hypertrophic and dilated cardiomyopathies (HCM and DCM). FLNC variants are now among the more prevalent causes of genetic DCM. FLNC‐associated DCM is associated with a malignant clinical course and a high risk of sudden cardiac death. The clinical spectrum of FLNC suggests different pathomechanisms related to variant types and their location in the gene. The appropriate functioning of FLNC is crucial for structural integrity and cell signaling of the sarcomere. The secondary protein structure of FLNC is critical to ensure this function. Truncating variants with subsequent haploinsufficiency are associated with DCM and cardiac arrhythmias. Interference with the dimerization and folding of the protein leads to aggregate formation detrimental for muscle function, as found in HCM and MFM. Variants associated with HCM are predominantly missense variants, which cluster in the ROD2 domain. This domain is important for binding to the sarcomere and to ensure appropriate cell signaling. We here review FLNC genotype–phenotype correlations based on available evidence.

A mutation in the filamin c gene causes myofibrillar myopathy with lower motor neuron syndrome: a case report

Background

Myofibrillar myopathies (MFMs) are a genetically heterogeneous group of muscle disorders. Mutations in the filamin C gene (FLNC) have previously been identified in patients with MFM. The phenotypes of FLNC-related MFM are heterogeneous.

Case presentation

The patient was a 37-year-old male who first experienced weakness in the distal muscles of his hand, which eventually spread to the lower limbs and proximal muscles. Serum creatine kinase levels were moderately elevated. Obvious neuropathic changes in the electromyographic exam and edema changes in lower distal limb magnetic resonance imaging were observed. Histopathological examination revealed the presence of abnormal protein aggregates and angular atrophy in some muscle fibers. Ultrastructural analysis showed inordinate myofibrillar structures and dissolved myofilaments. DNA sequencing analysis detected a heterozygous missense mutation (c.7123G > A, p.V2375I) in the immunoglobulin (Ig)-like domain 21 of FLNC.

Conclusions

FLNC mutation c.7123G > A, p.V2375I in the immunoglobulin (Ig)-like domain 21 can be associated with distal myopathy with typical MFM features and lower motor neuron syndrome. Although electromyographic examination of our patient showed obvious neuropathic changes, MFM could not be excluded. Therefore, genetic testing is necessary to make an accurate diagnosis.

Electronic supplementary material

The online version of this article (10.1186/s12883-019-1410-7) contains supplementary material, which is available to authorized users.

Heterozygous variants in MYBPC1 are associated with an expanded neuromuscular phenotype beyond arthrogryposis

Encoding the slow skeletal muscle isoform of myosin binding protein-C, MYBPC1 is associated with autosomal dominant and recessive forms of arthrogryposis. The authors describe a novel association for MYBPC1 in four patients from three independent families with skeletal muscle weakness, myogenic tremors, and hypotonia with gradual clinical improvement. The patients carried one of two de novo heterozygous variants in MYBPC1, with the p.Leu263Arg variant seen in three individuals and the p.Leu259Pro variant in one individual. Both variants are absent from controls, well conserved across vertebrate species, predicted to be damaging, and located in the M-motif. Protein modeling studies suggested that the p.Leu263Arg variant affects the stability of the M-motif, whereas the p.Leu259Pro variant alters its structure. In vitro biochemical and kinetic studies demonstrated that the p.Leu263Arg variant results in decreased binding of the M-motif to myosin, which likely impairs the formation of actomyosin cross-bridges during muscle contraction. Collectively, our data substantiate that damaging variants in MYBPC1 are associated with a new form of an early-onset myopathy with tremor, which is a defining and consistent characteristic in all affected individuals, with no contractures. Recognition of this expanded myopathic phenotype can enable identification of individuals with MYBPC1 variants without arthrogryposis.

Molecular determinants of homo- and heteromeric interactions of Junctophilin-1 at triads in adult skeletal muscle fibers

In adult skeletal muscles, 2 junctophilin isoforms (JPH1 and JPH2) tether the sarcoplasmic reticulum (SR) to transverse tubule (T-tubule) membranes, generating stable membrane contact sites known as triads. JPHs are anchored to the membrane of the SR by a C-terminal transmembrane domain (TMD) and bind the T-tubule membrane through their cytosolic N-terminal region, which contains 8 lipid-binding (MORN) motifs. By combining expression of GFP-JPH1 deletion mutants in skeletal muscle fibers with in vitro biochemical experiments, we investigated the molecular determinants of JPH1 recruitment at triads in adult skeletal muscle fibers. We found that MORN motifs bind PI(4,5)P₂ in the sarcolemma, but do not mediate the selective localization of JPH1 at the T-tubule compartment of triads. On the contrary, fusion proteins containing only the TMD of JPH1 were able to localize at the junctional SR compartment of the triad. Bimolecular fluorescence complementation experiments indicated that the TMD of JPH1 can form dimers, suggesting that the observed localization at triads may result from dimerization with the TMDs of resident JPH1. A second domain, capable of mediating homo- and heterodimeric interactions between JPH1 and JPH2 was identified in the cytosolic region. FRAP experiments revealed that removal of either one of these 2 domains in JPH1 decreases the association of the resulting mutant proteins with triads. Altogether, these results suggest that the ability to establish homo- and heterodimeric interactions with resident JPHs may support the recruitment and stability of newly synthesized JPHs at triads in adult skeletal muscle fibers.

Calcium Homeostasis Is Modified in Skeletal Muscle Fibers of Small Ankyrin1 Knockout Mice

Small Ankyrins (sAnk1) are muscle-specific isoforms generated by the Ank1 gene that participate in the organization of the sarcoplasmic reticulum (SR) of striated muscles. Accordingly, the volume of SR tubules localized around the myofibrils is strongly reduced in skeletal muscle fibers of 4- and 10-month-old sAnk1 knockout (KO) mice, while additional structural alterations only develop with aging. To verify whether the lack of sAnk1 also alters intracellular Ca²⁺ handling, cytosolic Ca²⁺ levels were analyzed in stimulated skeletal muscle fibers from 4- and 10-month-old sAnk1 KO mice. The SR Ca²⁺ content was reduced in sAnk1 KO mice regardless of age. The amplitude of the Ca²⁺ transients induced by depolarizing pulses was decreased in myofibers of sAnk1 KO with respect to wild type (WT) fibers, while their voltage dependence was not affected. Furthermore, analysis of spontaneous Ca²⁺ release events (sparks) on saponin-permeabilized muscle fibers indicated that the frequency of sparks was significantly lower in fibers from 4-month-old KO mice compared to WT. Furthermore, both the amplitude and spatial spread of sparks were significantly smaller in muscle fibers from both 4- and 10-month-old KO mice compared to WT. These data suggest that the absence of sAnk1 results in an impairment of SR Ca²⁺ release, likely as a consequence of a decreased Ca²⁺ store due to the reduction of the SR volume in sAnk1 KO muscle fibers.

Obscurin is a semi-flexible molecule in solution

Obscurin, a giant modular cytoskeletal protein, is comprised mostly of tandem immunoglobulin-like (Ig-like) domains. This architecture allows obscurin to connect distal targets within the cell. The linkers connecting the Ig domains are usually short (3-4 residues). The physical effect arising from these short linkers is not known; such linkers may lead to a stiff elongated molecule or, conversely, may lead to a more compact and dynamic structure. In an effort to better understand how linkers affect obscurin flexibility, and to better understand the physical underpinnings of this flexibility, here we study the structure and dynamics of four representative sets of dual obscurin Ig domains using experimental and computational techniques. We find in all cases tested that tandem obscurin Ig domains interact at the poles of each domain and tend to stay relatively extended in solution. NMR, SAXS, and MD simulations reveal that while tandem domains are elongated, they also bend and flex significantly. By applying this behavior to a simplified model, it becomes apparent obscurin can link targets more than 200 nm away. However, as targets get further apart, obscurin begins acting as a spring and requires progressively more energy to further elongate.

Unraveling obscurins in heart disease

Obscurins, expressed from the single OBSCN gene, are a family of giant, modular, cytoskeletal proteins that play key structural and regulatory roles in striated muscles. They were first implicated in the development of heart disease in 2007 when two missense mutations were found in a patient diagnosed with hypertrophic cardiomyopathy (HCM). Since then, the discovery of over a dozen missense, frameshift, and splicing mutations that are linked to various forms of cardiomyopathy, including HCM, dilated cardiomyopathy (DCM), and left ventricular non-compaction (LVNC), has highlighted OBSCN as a potential disease-causing gene. At this time, the functional consequences of the identified mutations remain largely elusive, and much work has yet to be done to characterize the disease mechanisms of pathological OBSCN variants. Herein, we describe the OBSCN mutations known to date, discuss their potential impact on disease development, and provide future directions in order to better understand the involvement of obscurins in heart disease.

A case report: a heterozygous deletion (2791_2805 del) in exon 18 of the filamin C gene causing filamin C-related myofibrillar myopathies in a Chinese family

BACKGROUND

Filamin C-related myofibrillar myopathies (MFM) are progressive skeletal myopathies with an autosomal dominant inheritance pattern. The conditions are caused by mutations of the filamin C gene (FLNC) located in the chromosome 7q32-q35 region. Genetic variations in the FLNC gene result in various clinical phenotypes.

CASE PRESENTATION

We describe a 43-year-old woman who suffered filamin C-related MFM, with symptoms first presenting in the proximal muscles of the lower limbs and eventually spreading to the upper limbs and distal muscles. The patient's serum level of creatine kinase was mildly increased. Mildy myopathic changes in the electromyographic exam and moderate lipomatous alterations in lower limb MRI were found. Histopathological examination revealed increased muscle fiber size variability, disturbances in oxidative enzyme activity, and the presence of abnormal protein aggregates and vacuoles in some muscle fibers. Ultrastructural analysis showed inclusions composed of thin filaments and interspersed granular densities. DNA sequencing analysis detected a novel 15-nucleotide deletion (c.2791_2805del, p.931_935del) in the FLNC gene. The patient's father, sister, brother, three paternal aunts, one paternal uncle, and the uncle's son also had slowly progressive muscle weakness, and thus, we detected an autosomal dominant inheritance pattern of the disorder.

CONCLUSIONS

A novel heterogeneous 15-nucleotide deletion (c.2791_2805del, p.931_935del) in the Ig-like domain 7 of the FLNC gene was found to cause filamin C-related MFM. This deletion in the FLNC gene causes protein aggregation, abnormalities in muscle structure, and impairment in muscle fiber function, which leads to muscle weakness.

What Can be Learned from the Time Course of Changes in Low-Frequency Stimulated Muscle?

Not available.