Desmin myopathy, a skeletal myopathy with cardiomyopathy caused by mutations in the desmin gene

Is Cardiac Transplantation Still a Contraindication in Patients with Muscular Dystrophy-Related End-Stage Dilated Cardiomyopathy? A Systematic Review

Inherited muscular diseases (MDs) are genetic degenerative disorders typically caused by mutations in a single gene that affect striated muscle and result in progressive weakness and wasting in affected individuals. Cardiac muscle can also be involved with some variability that depends on the genetic basis of the MD (Muscular Dystrophy) phenotype. Heart involvement can manifest with two main clinical pictures: left ventricular systolic dysfunction with evolution towards dilated cardiomyopathy and refractory heart failure, or the presence of conduction system defects and serious life-threatening ventricular arrhythmias. The two pictures can coexist. In these cases, heart transplantation (HTx) is considered the most appropriate option in patients who are not responders to the optimized standard therapeutic protocols. However, cardiac transplant is still considered a relative contraindication in patients with inherited muscle disorders and end-stage cardiomyopathies. High operative risk related to muscle impairment and potential graft involvement secondary to the underlying myopathy have been the two main reasons implicated in the generalized reluctance to consider cardiac transplant as a viable option. We report an overview of cardiac involvement in MDs and its possible association with the underlying molecular defect, as well as a systematic review of HTx outcomes in patients with MD-related end-stage dilated cardiomyopathy, published so far in the literature.

Pathophysiological mechanisms of cardiomyopathies induced by desmin gene variants located in the C-Terminus of segment 2B

Desmin, the most abundant intermediate filament in cardiomyocytes, plays a key role in maintaining cardiomyocyte structure by interconnecting intracellular organelles, and facilitating cardiomyocyte interactions with the extracellular matrix and neighboring cardiomyocytes. As a consequence, mutations in the desmin gene (DES) can lead to desminopathies, a group of diseases characterized by variable and often severe cardiomyopathies along with skeletal muscle disorders. The basic desmin intermediate filament structure is composed of four segments separated by linkers that further assemble into dimers, tetramers and eventually unit-length filaments that compact radially to give the final form of the filament. Each step in this process is critical for proper filament formation and allow specific interactions within the cell. Mutations within the desmin gene can disrupt filament formation, as seen by aggregate formation, and thus have severe cardiac and skeletal outcomes, depending on the locus of the mutation. The focus of this review is to outline the cardiac molecular consequences of mutations located in the C-terminal part of segment 2B. This region is crucial for ensuring proper desmin filament formation and is a known hotspot for mutations that significantly impact cardiac function.

Krüpple-like factors in cardiomyopathy: emerging player and therapeutic opportunities

Cardiomyopathy, a heterogeneous pathological condition characterized by changes in cardiac structure or function, represents a significant risk factor for the prevalence and mortality of cardiovascular disease (CVD). Research conducted over the years has led to the modification of definition and classification of cardiomyopathy. Herein, we reviewed seven of the most common types of cardiomyopathies, including Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC), diabetic cardiomyopathy, Dilated Cardiomyopathy (DCM), desmin-associated cardiomyopathy, Hypertrophic Cardiomyopathy (HCM), Ischemic Cardiomyopathy (ICM), and obesity cardiomyopathy, focusing on their definitions, epidemiology, and influencing factors. Cardiomyopathies manifest in various ways ranging from microscopic alterations in cardiomyocytes, to tissue hypoperfusion, cardiac failure, and arrhythmias caused by electrical conduction abnormalities. As pleiotropic Transcription Factors (TFs), the Krüppel-Like Factors (KLFs), a family of zinc finger proteins, are involved in regulating the setting and development of cardiomyopathies, and play critical roles in associated biological processes, including Oxidative Stress (OS), inflammatory reactions, myocardial hypertrophy and fibrosis, and cellular autophagy and apoptosis, particularly in diabetic cardiomyopathy. However, research into KLFs in cardiomyopathy is still in its early stages, and the pathophysiologic mechanisms of some KLF members in various types of cardiomyopathies remain unclear. This article reviews the roles and recent research advances in KLFs, specifically those targeting and regulating several cardiomyopathy-associated processes.

A case report of adolescent myofibrillar myopathy due to a de novo R406W pathogenic variant in desmin with symptoms of "hypertrophic cardiomyopathy"

Objective

Myofibrillar myopathies (MFM) are a group of sporadic and inherited progressive skeletal muscle disorders that can lead to physical disability and premature death. To date, pathogenic variants in different genes are associated with MFM. MFM induced by variants in the Desmin (DES) gene is the most common subtype of MFM.

Case presentation

A 15-year-old boy with MFM was described, whose symptoms first presented as cardiac symptoms. Enlarged right and left atria, thickened ventricular septal (IVS) and mild mitral (MR) and tricuspid regurgitation (TR) in the echocardiography were found. Atrial fibrillation, intermittent atrioventricular (AV) block, ST-T changes in the dynamic electrocardiogram (ECG) were shown. Mild myopathic changes in the electromyographic exam were detected. Ultrastructural analysis found slight Z-line changes and a few small myolysis lesions, but no abnormal inclusion bodies. Genetic testing detected a heterozygous missense variant (c.1216C > T) of DES, and 2 rare variants: TNNI3K (c.1102C > G) and PRDM16 (c.3074G > A). The patient's parents didn't show skeletal and cardiac muscle disorders. DNA sequencing analysis showed no variant of DES was carried by them. Thus, we detected a case of MFM caused by de novo DES variant c.1216C > T/p.Arg406Trp with predominantly myocardial alterations.

Desmin and its molecular chaperone, the αB-crystallin: How post-translational modifications modulate their functions in heart and skeletal muscles?

Maintenance of the highly organized striated muscle tissue requires a cell-wide dynamic network through protein-protein interactions providing an effective mechanochemical integrator of morphology and function. Through a continuous and complex trans-cytoplasmic network, desmin intermediate filaments ensure this essential role in heart and in skeletal muscle. Besides their role in the maintenance of cell shape and architecture (permitting contractile activity efficiency and conferring resistance towards mechanical stress), desmin intermediate filaments are also key actors of cell and tissue homeostasis. Desmin participates to several cellular processes such as differentiation, apoptosis, intracellular signalisation, mechanotransduction, vesicle trafficking, organelle biogenesis and/or positioning, calcium homeostasis, protein homeostasis, cell adhesion, metabolism and gene expression. Desmin intermediate filaments assembly requires αB-crystallin, a small heat shock protein. Over its chaperone activity, αB-crystallin is involved in several cellular functions such as cell integrity, cytoskeleton stabilization, apoptosis, autophagy, differentiation, mitochondria function or aggresome formation. Importantly, both proteins are known to be strongly associated to the aetiology of several cardiac and skeletal muscles pathologies related to desmin filaments disorganization and a strong disturbance of desmin interactome. Note that these key proteins of cytoskeleton architecture are extensively modified by post-translational modifications that could affect their functional properties. Therefore, we reviewed in the herein paper the impact of post-translational modifications on the modulation of cellular functions of desmin and its molecular chaperone, the αB-crystallin.

Mapping and Ablation of Ventricular Tachycardia in Inherited Left Ventricular Cardiomyopathies

Advances in the field of human genetics have led to an accumulating understanding of the genetic basis of distinct nonischemic cardiomyopathies associated with ventricular tachycardias (VTs) and sudden cardiac death. To date, there is an increasing proportion of patients with inherited cardiomyopathies requiring catheter ablation for VTs. This review provides an overview of disease-causing gene mutations frequently encountered and relevant for clinical electrophysiologists. Available data on VT ablation in patients with an inherited etiology and a phenotype of a nondilated left ventricular cardiomyopathy, dilated cardiomyopathy, or hypertrophic cardiomyopathy are summarized. VTs amenable to catheter ablation are related to nonischemic fibrosis. Recent insights into genotype-phenotype relations of subtype and location of fibrosis have important implications for treatment planning. Current strategies to delineate nonischemic fibrosis and related arrhythmogenic substrates using multimodal imaging, image integration, and electroanatomical mapping are provided. The ablation approach depends on substrate location and extension. Related procedural aspects including patient-tailored (enhanced) ablation strategies and outcomes are outlined. Challenging substrates for VT and the underlying inherited etiologies with a high risk for rapid progressive heart failure contribute to poor outcomes after catheter ablation. Electroanatomical data obtained during ablation may allow the identification of patients at particular risk who need to be considered for early work-up for left ventricular assist device implantation or heart transplantation.

The Myocardial Accumulation of Aggregated Desmin Protein in a Case of Desminopathy with a de novo DES p.R406W Mutation

Desminopathy is a cardiac and skeletal myopathy caused by disease-causing variants in the desmin (DES) gene and represents a subgroup of myofibrillar myopathies, where cytoplasmic desmin-postive immunoreactivity is the pathological hallmark. We herein report a 28-year-old Japanese man who was initially diagnosed with sporadic hypertrophic cardiomyopathy with atrioventricular block at 9 years old and developed weakness in the soft palate and extremities. The myocardial tissue dissected during implantation of the ventricular-assisted device showed a dilated phase of hypertrophic cardiomyopathy and intracellular accumulation of proteinase K-resistant desmin aggregates. Genetic testing confirmed a de novo mutation of DES, which has already been linked to desminopathy. As the molecular diagnosis of desminopathy is challenging, particularly if patients show predominantly cardiac signs and a routine skeletal muscle biopsy is unavailable, these characteristic pathological findings of endomyocardial proteinase K-resistant desmin aggregates might aid in clinical practice.

Genetic Abnormalities of the Sinoatrial Node and Atrioventricular Conduction

The peculiar electrophysiological properties of the sinoatrial node and the cardiac conduction system are key components of the normal physiology of cardiac impulse generation and propagation. Multiple genes and transcription factors and metabolic proteins are involved in their development and regulation. In this review, we have summarized the genetic underlying causes, key clinical findings, and the latest available clinical evidence. We will discuss clinical diagnosis and management of the genetic conditions associated with conduction disorders that are more prevalent in clinical practice, for this reason, very rare genetic diseases presenting sinus node or cardiac conduction system abnormalities are not discussed.

Deep Characterization of a Greek Patient with Desmin-Related Myofibrillar Myopathy and Cardiomyopathy

Desmin is a class III intermediate filament protein highly expressed in cardiac, smooth and striated muscle. Autosomal dominant or recessive mutations in the desmin gene (DES) result in a variety of diseases, including cardiomyopathies and myofibrillar myopathy, collectively called desminopathies. Here we describe the clinical, histological and radiological features of a Greek patient with a myofibrillar myopathy and cardiomyopathy linked to the c.734A>G,p.(Glu245Gly) heterozygous variant in the DES gene. Moreover, through ribonucleic acid sequencing analysis in skeletal muscle we show that this variant provokes a defect in exon 3 splicing and thus should be considered clearly pathogenic.

Desmin Knock-Out Cardiomyopathy: A Heart on the Verge of Metabolic Crisis

Desmin mutations cause familial and sporadic cardiomyopathies. In addition to perturbing the contractile apparatus, both desmin deficiency and mutated desmin negatively impact mitochondria. Impaired myocardial metabolism secondary to mitochondrial defects could conceivably exacerbate cardiac contractile dysfunction. We performed metabolic myocardial phenotyping in left ventricular cardiac muscle tissue in desmin knock-out mice. Our analyses revealed decreased mitochondrial number, ultrastructural mitochondrial defects, and impaired mitochondria-related metabolic pathways including fatty acid transport, activation, and catabolism. Glucose transporter 1 and hexokinase-1 expression and hexokinase activity were increased. While mitochondrial creatine kinase expression was reduced, fetal creatine kinase expression was increased. Proteomic analysis revealed reduced expression of proteins involved in electron transport mainly of complexes I and II, oxidative phosphorylation, citrate cycle, beta-oxidation including auxiliary pathways, amino acid catabolism, and redox reactions and oxidative stress. Thus, desmin deficiency elicits a secondary cardiac mitochondriopathy with severely impaired oxidative phosphorylation and fatty and amino acid metabolism. Increased glucose utilization and fetal creatine kinase upregulation likely portray attempts to maintain myocardial energy supply. It may be prudent to avoid medications worsening mitochondrial function and other metabolic stressors. Therapeutic interventions for mitochondriopathies might also improve the metabolic condition in desmin deficient hearts.

The Location of Disease-Causing DES Variants Determines the Severity of Phenotype and the Morphology of Sarcoplasmic Aggregates

Desmin (DES) is the main intermediate muscle filament that connects myofibrils individually and with the nucleus, sarcolemma, and organelles. Pathogenic variants of DES cause desminopathy, a disorder affecting the heart and skeletal muscles. We aimed to analyze the clinical features, morphology, and distribution of desmin aggregates in skeletal muscle biopsies of patients with desminopathy and to correlate these findings with the type and location of disease-causing DES variants. This retrospective study included 30 patients from 20 families with molecularly confirmed desminopathy from 2 neuromuscular referral centers. We identified 2 distinct patterns of desmin aggregates: well-demarcated subsarcolemmal aggregates and diffuse aggregates with poorly delimited borders. Pathogenic variants located in the 1B segment and the tail domain of the desmin molecule are more likely to present with early-onset cardiomyopathy compared to patients with variants in other segments. All patients with mutations in the 1B segment had well-demarcated subsarcolemmal aggregates, but none of the patients with variants in other desmin segments showed such histological features. We suggest that variants located in the 1B segment lead to well-shaped subsarcolemmal desmin aggregation and cause disease with more frequent cardiac manifestations. These findings will facilitate early identification of patients with potentially severe cardiac syndromes.

Global O-GlcNAcylation changes impact desmin phosphorylation and its partition toward cytoskeleton in C2C12 skeletal muscle cells differentiated into myotubes

Desmin is the guardian of striated muscle integrity, permitting the maintenance of muscle shape and the efficiency of contractile activity. It is also a key mediator of cell homeostasis and survival. To ensure the fine regulation of skeletal muscle processes, desmin is regulated by post-translational modifications (PTMs). It is more precisely phosphorylated by several kinases connecting desmin to intracellular processes. Desmin is also modified by O-GlcNAcylation, an atypical glycosylation. However, the functional consequence of O-GlcNAcylation on desmin is still unknown, nor its impact on desmin phosphorylation. In a model of C2C12 myotubes, we modulated the global O-GlcNAcylation level, and we determined whether the expression, the PTMs and the partition of desmin toward insoluble material or cytoskeleton were impacted or not. We have demonstrated in the herein paper that O-GlcNAcylation variations led to changes in desmin behaviour. In particular, our data clearly showed that O-GlcNAcylation increase led to a decrease of phosphorylation level on desmin that seems to involve CamKII correlated to a decrease of its partition toward cytoskeleton. Our data showed that phosphorylation/O-GlcNAcylation interplay is highly complex on desmin, supporting that a PTMs signature could occur on desmin to finely regulate its partition (i.e. distribution) with a spatio-temporal regulation.

Clinical Exome Sequencing Revealed a De Novo FLNC Mutation in a Child with Restrictive Cardiomyopathy

Restrictive cardiomyopathy (RCM) is a rare disease of the myocardium caused by mutations in several genes including TNNT2, DES, TNNI3, MYPN and FLNC. Individuals affected by RCM often develop heart failure at a young age, requiring early heart transplantation. A 7-year-old patient was referred for genetic testing following a diagnosis of restrictive cardiomyopathy. Clinical exome sequencing analysis identified a likely pathogenic mutation in the FLNC gene [(NM_001458.5 c.6527_6547dup p.(Arg2176_2182dup)]. Its clinical relevance was augmented by the fact that this variant was absent in the parents and was thus interpreted as de novo. Genetic testing is a powerful tool to clarify the diagnosis, guide intervention strategies and enable cascade testing in patients with pediatric-onset RCM.

Regulation of the evolutionarily conserved muscle myofibrillar matrix by cell type dependent and independent mechanisms

Skeletal muscles play a central role in human movement through forces transmitted by contraction of the sarcomere. We recently showed that mammalian sarcomeres are connected through frequent branches forming a singular, mesh-like myofibrillar matrix. However, the extent to which myofibrillar connectivity is evolutionarily conserved as well as mechanisms which regulate the specific architecture of sarcomere branching remain unclear. Here, we demonstrate the presence of a myofibrillar matrix in the tubular, but not indirect flight (IF) muscles within Drosophila melanogaster. Moreover, we find that loss of transcription factor H15 increases sarcomere branching frequency in the tubular jump muscles, and we show that sarcomere branching can be turned on in IF muscles by salm-mediated conversion to tubular muscles. Finally, we demonstrate that neurochondrin misexpression results in myofibrillar connectivity in IF muscles without conversion to tubular muscles. These data indicate an evolutionarily conserved myofibrillar matrix regulated by both cell-type dependent and independent mechanisms.

Multiomics Approach Reveals an Important Role of BNIP3 in Myocardial Remodeling and the Pathogenesis of Heart Failure with Reduced Ejection Fraction

Previous work showed a role of BNIP3 in myocardial remodeling and progression to HFrEF. We utilized a multiomics approach to unravel BNIP3-related molecular mechanisms in the pathogenesis of HFrEF. BNIP3 knockdown in HFrEF improved glycolysis, pyruvate metabolism, branched-chain amino acid catabolism, and oxidative phosphorylation, and restored endoplasmic reticulum (ER)–mitochondrial (mt) calcium and ion homeostasis. These effects of BNIP3 on cardiac metabolism were related to its interaction and downregulation, and/or phosphorylation, of specific mt-proteins involved in the aforementioned metabolic pathways, including the MICOS and SLC25A families of carrier proteins. BNIP3 affected ER–mt-calcium and ion homeostasis via its interaction-induced VDAC1 dimerization and modulation of VDAC1 phosphorylation at Ser104 and Ser241, and the downregulation of LETM1. At the ER level, BNIP3 interacted with the enzyme SERCA2a and the PKA signaling complex, leading to the downregulation of SERCA2a and PKA-mediated Ser16 phospholamban phosphorylation. Additionally, BNIP3 attenuated AMPK and PRKCE activity by modulating AMPK phosphorylation at Ser485/491 and Ser377 residues, and PRKCE phosphorylation at Thr521 and Thr710 residues. BNIP3 also interacted with sarcomeric, cytoskeletal, and cellular transcription and translation proteins, and affected their expression and/or phosphorylation. In conclusion, BNIP3 modulates multiple pathobiological processes and constitutes an attractive therapeutic target in HFrEF.

Diagnostic biomarkers of dilated cardiomyopathy

BACKGROUND

Dilated cardiomyopathy (DCM) is a condition involving dilation of cardiac chambers, which results in contraction impairment. Besides invasive and non-invasive diagnostic procedures, cardiac biomarkers are of great importance in both diagnosis and prognosis of the disease. These biomarkers are categorized into three groups based on their site; cardiomyocyte biomarkers, microenvironmental biomarkers and macroenvironmental biomarkers.

AIMS

In this review, an overview of characteristics, epidemiology, etiology and clinical manifestations of DCM is provided. In addition, the most important biomarkers, of all three categories, and their diagnostic and prognostic values are discussed.

CONCLUSION

Considering the association of DCM with conditions such as infections and autoimmunity, which are prevalent among the population, introducing efficient diagnostic tools is of high value for the early detection of DCM to prevent its severe complications. The three discussed classes of biomarkers are potential candidates for the detection of DCM. However, further studies are necessary in this regard.

Genetic Abnormalities of the Sinoatrial Node and Atrioventricular Conduction

The peculiar electrophysiological properties of the sinoatrial node and the cardiac conduction system are key components of the normal physiology of cardiac impulse generation and propagation. Multiple genes and transcription factors and metabolic proteins are involved in their development and regulation. In this review, we have summarized the genetic underlying causes, key clinical findings, and the latest available clinical evidence. We will discuss clinical diagnosis and management of the genetic conditions associated with conduction disorders that are more prevalent in clinical practice, for this reason, very rare genetic diseases presenting sinus node or cardiac conduction system abnormalities are not discussed.

Features of myocardial injury detected by cardiac magnetic resonance in a patient with desmin-related restrictive cardiomyopathy

Myocardial fibrosis detected by cardiac magnetic resonance (CMR) has been reported in patients with desmin‐related myopathy, although its characteristics remain unclear. Here, we describe a case of desmin‐related restrictive cardiomyopathy wherein CMR imaging revealed myocardial oedema, ischaemia, and fibrosis in the left ventricle; the different types and processes of myocardial injury were detected by CMR. Middle wall left ventricular enhancement may be a feature of late gadolinium enhancement, and the lateral wall is often involved in cases of myocardial injury. CMR is useful for the early detection of cardiac involvement and the prediction of prognosis in patients diagnosed with desmin‐related myopathy.

Biochemical and structural basis of the passive mechanical properties of whole skeletal muscle

Passive mechanical properties of whole skeletal muscle are not as well understood as active mechanical properties. Both the structural basis for passive mechanical properties and the properties themselves are challenging to determine because it is not clear which structures within skeletal muscle actually bear passive loads and there are not established standards by which to make mechanical measurements. Evidence suggests that titin bears the majority of the passive load within the single muscle cell. However, at larger scales, such as fascicles and muscles, there is emerging evidence that the extracellular matrix bears the major part of the load. Complicating the ability to quantify and compare across size scales, muscles and species, definitions of muscle passive properties such as stress, strain, modulus and stiffness can be made relative to many reference parameters. These uncertainties make a full understanding of whole muscle passive mechanical properties and modelling these properties very difficult. Future studies defining the specific load bearing structures and their composition and organization are required to fully understand passive mechanics of the whole muscle and develop therapies to treat disorders in which passive muscle properties are altered such as muscular dystrophy, traumatic laceration, and contracture due to upper motor neuron lesion as seen in spinal cord injury, stroke and cerebral palsy.

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.

Skeletal and Cardiac Muscle Disorders Caused by Mutations in Genes Encoding Intermediate Filament Proteins

Intermediate filaments are major components of the cytoskeleton. Desmin and synemin, cytoplasmic intermediate filament proteins and A-type lamins, nuclear intermediate filament proteins, play key roles in skeletal and cardiac muscle. Desmin, encoded by the DES gene (OMIM *125660) and A-type lamins by the LMNA gene (OMIM *150330), have been involved in striated muscle disorders. Diseases include desmin-related myopathy and cardiomyopathy (desminopathy), which can be manifested with dilated, restrictive, hypertrophic, arrhythmogenic, or even left ventricular non-compaction cardiomyopathy, Emery–Dreifuss Muscular Dystrophy (EDMD2 and EDMD3, due to LMNA mutations), LMNA-related congenital Muscular Dystrophy (L-CMD) and LMNA-linked dilated cardiomyopathy with conduction system defects (CMD1A). Recently, mutations in synemin (SYNM gene, OMIM *606087) have been linked to cardiomyopathy. This review will summarize clinical and molecular aspects of desmin-, lamin- and synemin-related striated muscle disorders with focus on LMNA and DES-associated clinical entities and will suggest pathogenetic hypotheses based on the interplay of desmin and lamin A/C. In healthy muscle, such interplay is responsible for the involvement of this network in mechanosignaling, nuclear positioning and mitochondrial homeostasis, while in disease it is disturbed, leading to myocyte death and activation of inflammation and the associated secretome alterations.

Functional characterization of novel alpha-helical rod domain desmin (DES) pathogenic variants associated with dilated cardiomyopathy, atrioventricular block and a risk for sudden cardiac death

BACKGROUND

Desmin is the major intermediate filament (IF) protein in human heart and skeletal muscle. So-called 'desminopathies' are disorders due to pathogenic variants in the DES gene and are associated with skeletal myopathies and/or various types of cardiomyopathies. So far, only a limited number of DES pathogenic variants have been identified and functionally characterized.

METHODS AND RESULTS

Using a Sanger- and next generation sequencing (NGS) approach in patients with various types of cardiomyopathies, we identified two novel, non-synonymous missense DES variants: p.(Ile402Thr) and p.(Glu410Lys). Mutation carriers developed dilated (DCM) or arrhythmogenic cardiomyopathy (ACM), and cardiac conduction disease, leading to spare out the exercise-induced polymorphic ventricular tachycardia; we moved this variant to data in brief. To investigate the functional impact of these four DES variants, transfection experiments using SW-13 and H9c2 cells with native and mutant desmin were performed and filament assembly was analyzed by confocal microscopy. The DES_p.(Ile402Thr) and DES_p.(Glu410Lys) cells showed filament assembly defects forming cytoplasmic desmin aggregates. Furthermore, immunohistochemical and ultrastructural analysis of myocardial tissue from mutation carriers with the DES_p.(Glu410Lys) pathogenic variant supported the in vitro results.

CONCLUSIONS

Our in vitro results supported the classification of DES_p.(Ile402Thr) and DES_p.(Glu410Lys) as novel pathogenic variants and demonstrated that the cardiac phenotypes associated with DES variants are diverse and cell culture experiments improve in silico analysis and genetic counseling because the pathogenicity of a variant can be clarified.

Dysfunctional Mitochondrial Dynamic and Oxidative Phosphorylation Precedes Cardiac Dysfunction in R120G-αB-Crystallin-Induced Desmin-Related Cardiomyopathy

Background The mutated α-B-Crystallin (CryABR120G) mouse model of desmin-related myopathy (DRM) shows an age-dependent onset of pathologic cardiac remodeling and progression of heart failure. CryABR120G expression in cardiomyocytes affects the mitochondrial spatial organization within the myofibrils, but the molecular perturbation within the mitochondria in the relation of the overall course of the proteotoxic disease remains unclear. Methods and Results CryABR120G mice show an accumulation of electron-dense aggregates and myofibrillar degeneration associated with the development of cardiac dysfunction. Though extensive studies demonstrated that these altered ultrastructural changes cause cardiac contractility impairment, the molecular mechanism of cardiomyocyte death remains elusive. Here, we explore early pathological processes within the mitochondria contributing to the contractile dysfunction and determine the pathogenic basis for the heart failure observed in the CryABR120G mice. In the present study, we report that the CryABR120G mice transgenic hearts undergo altered mitochondrial dynamics associated with increased level of dynamin-related protein 1 and decreased level of optic atrophy type 1 as well as mitofusin 1 over the disease process. In association with these changes, an altered level of the components of mitochondrial oxidative phosphorylation and pyruvate dehydrogenase complex regulatory proteins occurs before the manifestation of pathologic adverse remodeling in the CryABR120G hearts. Mitochondria isolated from CryABR120G transgenic hearts without visible pathology show decreased electron transport chain complex activities and mitochondrial respiration. Taken together, we demonstrated the involvement of mitochondria in the pathologic remodeling and progression of DRM-associated cellular dysfunction. Conclusions Mitochondrial dysfunction in the form of altered mitochondrial dynamics, oxidative phosphorylation and pyruvate dehydrogenase complex proteins level, abnormal electron transport chain complex activities, and mitochondrial respiration are evident on the CryABR120G hearts before the onset of detectable pathologies and development of cardiac contractile dysfunction.

Cardiomyopathies in children: classification, diagnosis and treatment

PURPOSE OF REVIEW

Cardiomyopathies are rare in the pediatric population, but significantly impact on morbidity and mortality. The present review aims to provide an overview of cardiomyopathies in children and some practical guidelines for their prognostic stratification and management.

RECENT FINDINGS

Pediatric cardiomyopathies may present as isolated cardiac muscle disease or in the context of complex clinical syndromes. The etiologic characterization represents an important step in the diagnosis and treatment of cardiomyopathies because of its impact on prognosis and on therapeutic measures. Indeed, replacement therapy is nowadays widely available and changes the natural history of the disease. More complex is the management of isolated cardiomyopathies, which lack specific therapies, mainly aimed at symptomatic relief. In this context, heart transplantation shows excellent outcomes in children, but wait-list mortality is still very high. Device therapy for sudden cardiac death prevention and the use of mechanical assist devices are becoming more common in the clinical practice and may help to reduce mortality.

SUMMARY

Providing insight into pediatric cardiomyopathies classification helps in the prognostication and management of such diseases. Recent years witnessed a significant improvement in mortality, but future research is still needed to improve quality of life and life expectations in the pediatric population.

Dual Functional States of R406W-Desmin Assembly Complexes Cause Cardiomyopathy With Severe Intercalated Disc Derangement in Humans and in Knock-In Mice

BACKGROUND

Mutations in the human desmin gene cause myopathies and cardiomyopathies. This study aimed to elucidate molecular mechanisms initiated by the heterozygous R406W-desmin mutation in the development of a severe and early-onset cardiac phenotype.

METHODS

We report an adolescent patient who underwent cardiac transplantation as a result of restrictive cardiomyopathy caused by a heterozygous R406W-desmin mutation. Sections of the explanted heart were analyzed with antibodies specific to 406W-desmin and to intercalated disc proteins. Effects of the R406W mutation on the molecular properties of desmin were addressed by cell transfection and in vitro assembly experiments. To prove the genuine deleterious effect of the mutation on heart tissue, we further generated and analyzed R405W-desmin knock-in mice harboring the orthologous form of the human R406W-desmin.

RESULTS

Microscopic analysis of the explanted heart revealed desmin aggregates and the absence of desmin filaments at intercalated discs. Structural changes within intercalated discs were revealed by the abnormal organization of desmoplakin, plectin, N-cadherin, and connexin-43. Next-generation sequencing confirmed the DES variant c.1216C>T (p.R406W) as the sole disease-causing mutation. Cell transfection studies disclosed a dual behavior of R406W-desmin with both its integration into the endogenous intermediate filament system and segregation into protein aggregates. In vitro, R406W-desmin formed unusually thick filaments that organized into complex filament aggregates and fibrillar sheets. In contrast, assembly of equimolar mixtures of mutant and wild-type desmin generated chimeric filaments of seemingly normal morphology but with occasional prominent irregularities. Heterozygous and homozygous R405W-desmin knock-in mice develop both a myopathy and a cardiomyopathy. In particular, the main histopathologic results from the patient are recapitulated in the hearts from R405W-desmin knock-in mice of both genotypes. Moreover, whereas heterozygous knock-in mice have a normal life span, homozygous animals die at 3 months of age because of a smooth muscle-related gastrointestinal phenotype.

CONCLUSIONS

We demonstrate that R406W-desmin provokes its severe cardiotoxic potential by a novel pathomechanism, where the concurrent dual functional states of mutant desmin assembly complexes underlie the uncoupling of desmin filaments from intercalated discs and their structural disorganization.

Molecular Perspectives of Mitochondrial Adaptations and Their Role in Cardiac Proteostasis

Mitochondria are the key to properly functioning energy generation in the metabolically demanding cardiomyocytes and thus essential to healthy heart contractility on a beat-to-beat basis. Mitochondria being the central organelle for cellular metabolism and signaling in the heart, its dysfunction leads to cardiovascular disease. The healthy mitochondrial functioning critical to maintaining cardiomyocyte viability and contractility is accomplished by adaptive changes in the dynamics, biogenesis, and degradation of the mitochondria to ensure cellular proteostasis. Recent compelling evidence suggests that the classical protein quality control system in cardiomyocytes is also under constant mitochondrial control, either directly or indirectly. Impairment of cytosolic protein quality control may affect the position of the mitochondria in relation to other organelles, as well as mitochondrial morphology and function, and could also activate mitochondrial proteostasis. Despite a growing interest in the mitochondrial quality control system, very little information is available about the molecular function of mitochondria in cardiac proteostasis. In this review, we bring together current understanding of the adaptations and role of the mitochondria in cardiac proteostasis and describe the adaptive/maladaptive changes observed in the mitochondrial network required to maintain proteomic integrity. We also highlight the key mitochondrial signaling pathways activated in response to proteotoxic stress as a cellular mechanism to protect the heart from proteotoxicity. A deeper understanding of the molecular mechanisms of mitochondrial adaptations and their role in cardiac proteostasis will help to develop future therapeutics to protect the heart from cardiovascular diseases.

The unified myofibrillar matrix for force generation in muscle

Human movement occurs through contraction of the basic unit of the muscle cell, the sarcomere. Sarcomeres have long been considered to be arranged end-to-end in series along the length of the muscle into tube-like myofibrils with many individual, parallel myofibrils comprising the bulk of the muscle cell volume. Here, we demonstrate that striated muscle cells form a continuous myofibrillar matrix linked together by frequently branching sarcomeres. We find that all muscle cells contain highly connected myofibrillar networks though the frequency of sarcomere branching goes down from early to late postnatal development and is higher in slow-twitch than fast-twitch mature muscles. Moreover, we show that the myofibrillar matrix is united across the entire width of the muscle cell both at birth and in mature muscle. We propose that striated muscle force is generated by a singular, mesh-like myofibrillar network rather than many individual, parallel myofibrils.

Skeletal muscle cells have long been considered to be made primarily of many individual, parallel myofibrils. Here, the authors show that the striated muscle contractile machinery forms a highly branched, mesh-like myofibrillar matrix connected across the entire length and width of the muscle cell.

Come Together: Protein Assemblies, Aggregates and the Sarcostat at the Heart of Cardiac Myocyte Homeostasis

Homeostasis in vertebrate systems is contingent on normal cardiac function. This, in turn, depends on intricate protein-based cellular machinery, both for contractile function, as well as, durability of cardiac myocytes. The cardiac small heat shock protein (csHsp) chaperone system, highlighted by αB-crystallin (CRYAB), a small heat shock protein (sHsp) that forms ∼3–5% of total cardiac mass, plays critical roles in maintaining proteostatic function via formation of self-assembled multimeric chaperones. In this work, we review these ancient proteins, from the evolutionarily preserved role of homologs in protists, fungi and invertebrate systems, as well as, the role of sHsps and chaperones in maintaining cardiac myocyte structure and function. We propose the concept of the “sarcostat” as a protein quality control mechanism in the sarcomere. The roles of the proteasomal and lysosomal proteostatic network, as well as, the roles of the aggresome, self-assembling protein complexes and protein aggregation are discussed in the context of cardiac myocyte homeostasis. Finally, we will review the potential for targeting the csHsp system as a novel therapeutic approach to prevent and treat cardiomyopathy and heart failure.

Desminopathy: Novel Desmin Variants, a New Cardiac Phenotype, and Further Evidence for Secondary Mitochondrial Dysfunction

Background: The pleomorphic clinical presentation makes the diagnosis of desminopathy difficult. We aimed to describe the prevalence, phenotypic expression, and mitochondrial function of individuals with putative disease-causing desmin (DES) variants identified in patients with an unexplained etiology of cardiomyopathy. Methods: A total of 327 Czech patients underwent whole exome sequencing and detailed phenotyping in probands harboring DES variants. Results: Rare, conserved, and possibly pathogenic DES variants were identified in six (1.8%) probands. Two DES variants previously classified as variants of uncertain significance (p.(K43E), p.(S57L)), one novel DES variant (p.(A210D)), and two known pathogenic DES variants (p.(R406W), p.(R454W)) were associated with characteristic desmin-immunoreactive aggregates in myocardial and/or skeletal biopsy samples. The individual with the novel DES variant p.(Q364H) had a decreased myocardial expression of desmin with absent desmin aggregates in myocardial/skeletal muscle biopsy and presented with familial left ventricular non-compaction cardiomyopathy (LVNC), a relatively novel phenotype associated with desminopathy. An assessment of the mitochondrial function in four probands heterozygous for a disease-causing DES variant confirmed a decreased metabolic capacity of mitochondrial respiratory chain complexes in myocardial/skeletal muscle specimens, which was in case of myocardial succinate respiration more profound than in other cardiomyopathies. Conclusions: The presence of desminopathy should also be considered in individuals with LVNC, and in the differential diagnosis of mitochondrial diseases.

Cardiomyopathy with lethal arrhythmias associated with inactivation of KLHL24

Hypertrophic cardiomyopathy (HCM) is the most common inherited cardiovascular disorder, yet the genetic cause of up to 50% of cases remains unknown. Here, we show that mutations in KLHL24 cause HCM in humans. Using genome-wide linkage analysis and exome sequencing, we identified homozygous mutations in KLHL24 in two consanguineous families with HCM. Of the 11 young affected adults identified, 3 died suddenly and 1 had a cardiac transplant due to heart failure. KLHL24 is a member of the Kelch-like protein family, which acts as substrate-specific adaptors to Cullin E3 ubiquitin ligases. Endomyocardial and skeletal muscle biopsies from affected individuals of both families demonstrated characteristic alterations, including accumulation of desmin intermediate filaments. Knock-down of the zebrafish homologue klhl24a results in heart defects similar to that described for other HCM-linked genes providing additional support for KLHL24 as a HCM-associated gene. Our findings reveal a crucial role for KLHL24 in cardiac development and function.

A different background of arrhythmia in siblings with a positive family history of sudden death at young age

We present two symptomatic sisters who had a positive family history of sudden death. None of them had structural heart disease. In the 25‐year‐old proband, complex ventricular arrhythmia, cardiac conduction system disease, and skeletal muscle weakness were found. Genetic examination showed a pathogenic intronic variant in the desmin gene in the proband only. In the elder sister with palpitations, complex ventricular arrhythmia (>46 000 ectopic beats) was removed by radiofrequency ablation. This family case shows that complex ventricular arrhythmia may have different background within one family, genetic examinations should be performed in a person with broadest spectrum of symptoms.

Genetics of dilated cardiomyopathy: practical implications for heart failure management

Given the global burden of heart failure, strategies to understand the underlying cause or to provide prognostic information are critical to reducing the morbidity and mortality associated with this highly prevalent disease. Cardiomyopathies often have a genetic cause, and the field of heart failure genetics is progressing rapidly. Through a deliberate investigation, evaluation for a familial component of cardiomyopathy can lead to increased identification of pathogenic genetic variants. Much research has also been focused on identifying markers of risk in patients with cardiomyopathy with the use of genetic testing. Advances in our understanding of genetic variants have been slightly offset by an increased recognition of the heterogeneity of disease expression. Greater breadth of genetic testing can increase the likelihood of identifying a variant of uncertain significance, which is resolved only rarely by cellular functional validation and segregation analysis. To increase the use of genetics in heart failure clinics, increased availability of genetic counsellors and other providers with experience in genetics is necessary. Ultimately, through ongoing research and increased clinical experience in cardiomyopathy genetics, an improved understanding of the disease processes will facilitate better clinical decision-making about the therapies offered, exemplifying the implementation of precision medicine.

2019 HRS expert consensus statement on evaluation, risk stratification, and management of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an arrhythmogenic disorder of the myocardium not secondary to ischemic, hypertensive, or valvular heart disease. ACM incorporates a broad spectrum of genetic, systemic, infectious, and inflammatory disorders. This designation includes, but is not limited to, arrhythmogenic right/left ventricular cardiomyopathy, cardiac amyloidosis, sarcoidosis, Chagas disease, and left ventricular noncompaction. The ACM phenotype overlaps with other cardiomyopathies, particularly dilated cardiomyopathy with arrhythmia presentation that may be associated with ventricular dilatation and/or impaired systolic function. This expert consensus statement provides the clinician with guidance on evaluation and management of ACM and includes clinically relevant information on genetics and disease mechanisms. PICO questions were utilized to evaluate contemporary evidence and provide clinical guidance related to exercise in arrhythmogenic right ventricular cardiomyopathy. Recommendations were developed and approved by an expert writing group, after a systematic literature search with evidence tables, and discussion of their own clinical experience, to present the current knowledge in the field. Each recommendation is presented using the Class of Recommendation and Level of Evidence system formulated by the American College of Cardiology and the American Heart Association and is accompanied by references and explanatory text to provide essential context. The ongoing recognition of the genetic basis of ACM provides the opportunity to examine the diverse triggers and potential common pathway for the development of disease and arrhythmia.

A novel phenotype with splicing mutation identified in a Chinese family with desminopathy

BACKGROUND

Desminopathy, a hereditary myofibrillar myopathy, mainly results from the desmin gene (DES) mutations. Desminopathy involves various phenotypes, mainly including different cardiomyopathies, skeletal myopathy, and arrhythmia. Combined with genotype, it helps us precisely diagnose and treat for desminopathy.

METHODS

Sanger sequencing was used to characterize DES variation, and then a minigene assay was used to verify the effect of splice-site mutation on pre-mRNA splicing. Phenotypes were analyzed based on clinical characteristics associated with desminopathy.

RESULTS

A splicing mutation (c.735+1G>T) in DES was detected in the proband. A minigene assay revealed skipping of the whole exon 3 and transcription of abnormal pre-mRNA lacking 32 codons. Another affected family member who carried the identical mutation, was identified with a novel phenotype of desminopathy, non-compaction of ventricular myocardium. There were 2 different phenotypes varied in cardiomyopathy and skeletal myopathy among the 2 patients, but no significant correlation between genotype and phenotype was identified.

CONCLUSIONS

We reported a novel phenotype with a splicing mutation in DES, enlarging the spectrum of phenotype in desminopathy. Molecular studies of desminopathy should promote our understanding of its pathogenesis and provide a precise molecular diagnosis of this disorder, facilitating clinical prevention and treatment at an early stage.

Slow-twitch skeletal muscle defects accompany cardiac dysfunction in transgenic mice with a mutation in the myosin regulatory light chain

Myosin light chain 2 ( MYL2) gene encodes the myosin regulatory light chain (RLC) simultaneously in heart ventricles and in slow-twitch skeletal muscle. Using transgenic mice with cardiac-specific expression of the human R58Q-RLC mutant, we sought to determine whether the hypertrophic cardiomyopathy phenotype observed in papillary muscles (PMs) of R58Q mice is also manifested in slow-twitch soleus (SOL) muscles. Skinned SOL muscles and ventricular PMs of R58Q animals exhibited lower contractile force that was not observed in the fast-twitch extensor digitorum longus muscles of R58Q vs. wild-type-RLC mice, but mutant animals did not display gross muscle weakness in vivo. Consistent with SOL muscle abnormalities in R58Q vs. wild-type mice, myosin ATPase staining revealed a decreased proportion of fiber type I/type II only in SOL muscles but not in the extensor digitorum longus muscles. The similarities between SOL muscles and PMs of R58Q mice were further supported by quantitative proteomics. Differential regulation of proteins involved in energy metabolism, cell-cell interactions, and protein-protein signaling was concurrently observed in the hearts and SOL muscles of R58Q mice. In summary, even though R58Q expression was restricted to the heart of mice, functional similarities were clearly observed between the hearts and slow-twitch skeletal muscle, suggesting that MYL2 mutated models of hypertrophic cardiomyopathy may be useful research tools to study the molecular, structural, and energetic mechanisms of cardioskeletal myopathy associated with myosin RLC.-Kazmierczak, K., Liang, J., Yuan, C.-C., Yadav, S., Sitbon, Y. H., Walz, K., Ma, W., Irving, T. C., Cheah, J. X., Gomes, A. V., Szczesna-Cordary, D. Slow-twitch skeletal muscle defects accompany cardiac dysfunction in transgenic mice with a mutation in the myosin regulatory light chain.

Site-specific phosphorylation and caspase cleavage of GFAP are new markers of Alexander disease severity

Alexander disease (AxD) is a fatal neurodegenerative disorder caused by mutations in glial fibrillary acidic protein (GFAP), which supports the structural integrity of astrocytes. Over 70 GFAP missense mutations cause AxD, but the mechanism linking different mutations to disease-relevant phenotypes remains unknown. We used AxD patient brain tissue and induced pluripotent stem cell (iPSC)-derived astrocytes to investigate the hypothesis that AxD-causing mutations perturb key post-translational modifications (PTMs) on GFAP. Our findings reveal selective phosphorylation of GFAP-Ser13 in patients who died young, independently of the mutation they carried. AxD iPSC-astrocytes accumulated pSer13-GFAP in cytoplasmic aggregates within deep nuclear invaginations, resembling the hallmark Rosenthal fibers observed in vivo. Ser13 phosphorylation facilitated GFAP aggregation and was associated with increased GFAP proteolysis by caspase-6. Furthermore, caspase-6 was selectively expressed in young AxD patients, and correlated with the presence of cleaved GFAP. We reveal a novel PTM signature linking different GFAP mutations in infantile AxD.

Drosophila Hsp67Bc hot-spot variants alter muscle structure and function

The Drosophila Hsp67Bc gene encodes a protein belonging to the small heat-shock protein (sHSP) family, identified as the nearest functional ortholog of human HSPB8. The most prominent activity of sHSPs is preventing the irreversible aggregation of various non-native polypeptides. Moreover, they are involved in processes such as development, aging, maintenance of the cytoskeletal architecture and autophagy. In larval muscles Hsp67Bc localizes to the Z- and A-bands, which suggests its role as part of the conserved chaperone complex required for Z-disk maintenance. In addition, Hsp67Bc is present at neuromuscular junctions (NMJs), which implies its involvement in the maintenance of NMJ structure. Here, we report the effects of muscle-target overexpression of Drosophila Hsp67Bc hot-spot variants Hsp67BcR126E and Hsp67BcR126N mimicking pathogenic variants of human HSPB8. Depending on the substitutions, we observed a different impact on muscle structure and performance. Expression of Hsp67BcR126E affects larval motility, which may be caused by impairment of mitochondrial respiratory function and/or by NMJ abnormalities manifested by a decrease in the number of synaptic boutons. In contrast, Hsp67BcR126N appears to be an aggregate-prone variant, as reflected in excessive accumulation of mutant proteins and the formation of large aggregates with a lesser impact on muscle structure and performance compared to the Hsp67BcR126E variant.

The unfolding spectrum of inherited distal myopathies

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

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.

Intermediate filaments in cardiomyopathy

Intermediate filament (IF) proteins are critical regulators in health and disease. The discovery of hundreds of mutations in IF genes and posttranslational modifications has been linked to a plethora of human diseases, including, among others, cardiomyopathies, muscular dystrophies, progeria, blistering diseases of the epidermis, and neurodegenerative diseases. The major IF proteins that have been linked to cardiomyopathies and heart failure are the muscle-specific cytoskeletal IF protein desmin and the nuclear IF protein lamin, as a subgroup of the known desminopathies and laminopathies, respectively. The studies so far, both with healthy and diseased heart, have demonstrated the importance of these IF protein networks in intracellular and intercellular integration of structure and function, mechanotransduction and gene activation, cardiomyocyte differentiation and survival, mitochondrial homeostasis, and regulation of metabolism. The high coordination of all these processes is obviously of great importance for the maintenance of proper, life-lasting, and continuous contraction of this highly organized cardiac striated muscle and consequently a healthy heart. In this review, we will cover most known information on the role of IFs in the above processes and how their deficiency or disruption leads to cardiomyopathy and heart failure.

Aberrant Mitochondrial Fission Is Maladaptive in Desmin Mutation-Induced Cardiac Proteotoxicity

Background

Desmin filament proteins interlink the contractile myofibrillar apparatus with mitochondria, nuclei and the sarcolemma. Mutations in the human desmin gene cause cardiac disease, remodeling, and heart failure but the pathophysiological mechanisms remain unknown.

Methods and Results

Cardiomyocyte‐specific overexpression of mutated desmin (a 7 amino acid deletion R172‐E178, D7‐Des Tg) causes accumulations of electron‐dense aggregates and myofibrillar degeneration associated with cardiac dysfunction. Though extensive studies demonstrated that these altered ultrastructural changes cause impairment of cardiac contractility, the molecular mechanism of cardiomyocyte death remains elusive. In the present study, we report that the D7‐Des Tg mouse hearts undergo aberrant mitochondrial fission associated with increased expression of mitochondrial fission regulatory proteins. Mitochondria isolated from D7‐Des Tg hearts showed decreased mitochondrial respiration and increased apoptotic cell death. Overexpression of mutant desmin by adenoviral infection in cultured cardiomyocytes led to increased mitochondrial fission, inhibition of mitochondrial respiration, and activation of cellular toxicity. Inhibition of mitochondrial fission by mitochondrial division inhibitor mdivi‐1 significantly improved mitochondrial respiration and inhibited cellular toxicity associated with D7‐Des overexpression in cardiomyocytes.

Conclusions

Aberrant mitochondrial fission results in mitochondrial respiratory defects and apoptotic cell death in D7‐Des Tg hearts. Inhibition of aberrant mitochondrial fission using mitochondrial division inhibitor significantly preserved mitochondrial function and decreased apoptotic cell death. Taken together, our study shows that maladaptive aberrant mitochondrial fission causes desminopathy‐associated cellular dysfunction.

Current and future circulating biomarkers for cardiac amyloidosis

Cardiac amyloidosis (CA) comprises a heterogeneous group of medical conditions affecting the myocardium. It presents with proteinaceous infiltration with variable degrees of severity, prevalence and evolution. Despite this heterogeneity, erroneous protein folding is the common pathophysiologic process, yielding the formation of a single misfolded protein (monomer) that progressively evolves and ultimately forms amyloid fibers. Additionally, by seeding out from the organs of origin, intermediates called oligomers metastasize and restart the process. Such self-echoing behavior makes the secondary affected organs as important as the primary ones. Unfortunately, CA can be clinically challenging and only suggestive in a late stage of its natural history, leaving a narrow therapeutic time window available. In light of the evolutionary nature of amyloidosis, here, we propose a new classification of the currently used biomarkers based on time stages with different specificity and applicability across CA subtypes. Early markers (free light chains, serum amyloid A, β2-microglobulin, osteopontin and osteoprotegerin) can be employed for disease detection. Intermediate markers [soluble suppression of tumorigenicity 2 (sST-2), midregional proadrenomedullin (MR-proADM), von Willebrand factor (vWF), hepatocyte growth factor (HGF), matrix metalloproteinases (MMPs) and tissue inhibitor metalloproteinases (TIMPs)] can provide information on the biological mechanisms of myocardial damage. As in heart failure, late-stage biomarkers (troponins and natriuretic peptides) can help clinicians with prognosis and therapeutic response evaluation in CA. Such findings have generated a remarkable foundation for our current knowledge on CA. Nevertheless, we envision a future class of biomarkers targeted at upstream events capable of detecting folding defects, which will ultimately expand the therapeutic window.

The role of heat shock proteins and co-chaperones in heart failure

The ongoing contractile and metabolic demands of the heart require a tight control over protein quality control, including the maintenance of protein folding, turnover and synthesis. In heart disease, increases in mechanical and oxidative stresses, post-translational modifications (e.g., phosphorylation), for example, decrease protein stability to favour misfolding in myocardial infarction, heart failure or ageing. These misfolded proteins are toxic to cardiomyocytes, directly contributing to the common accumulation found in human heart failure. One of the critical class of proteins involved in protecting the heart against these threats are molecular chaperones, including the heat shock protein70 (HSP70), HSP90 and co-chaperones CHIP (carboxy terminus of Hsp70-interacting protein, encoded by the Stub1 gene) and BAG-3 (BCL2-associated athanogene 3). Here, we review their emerging roles in the maintenance of cardiomyocytes in human and experimental models of heart failure, including their roles in facilitating the removal of misfolded and degraded proteins, inhibiting apoptosis and maintaining the structural integrity of the sarcomere and regulation of nuclear receptors. Furthermore, we discuss emerging evidence of increased expression of extracellular HSP70, HSP90 and BAG-3 in heart failure, with complementary independent roles from intracellular functions with important therapeutic and diagnostic considerations. While our understanding of these major HSPs in heart failure is incomplete, there is a clear potential role for therapeutic modulation of HSPs in heart failure with important contextual considerations to counteract the imbalance of protein damage and endogenous protein quality control systems.This article is part of the theme issue 'Heat shock proteins as modulators and therapeutic targets of chronic disease: an integrated perspective'.

Molecular insights into cardiomyopathies associated with desmin (DES) mutations

Increasing usage of next-generation sequencing techniques pushed during the last decade cardiogenetic diagnostics leading to the identification of a huge number of genetic variants in about 170 genes associated with cardiomyopathies, channelopathies, or syndromes with cardiac involvement. Because of the biochemical and cellular complexity, it is challenging to understand the clinical meaning or even the relevant pathomechanisms of the majority of genetic sequence variants. However, detailed knowledge about the associated molecular pathomechanism is essential for the development of efficient therapeutic strategies in future and genetic counseling. Mutations in DES, encoding the muscle-specific intermediate filament protein desmin, have been identified in different kinds of cardiac and skeletal myopathies. Here, we review the functions of desmin in health and disease with a focus on cardiomyopathies. In addition, we will summarize the genetic and clinical literature about DES mutations and will explain relevant cell and animal models. Moreover, we discuss upcoming perspectives and consequences of novel experimental approaches like genome editing technology, which might open a novel research field contributing to the development of efficient and mutation-specific treatment options.