A novel desmin R355P mutation causes cardiac and skeletal myopathy

A Comprehensive Analysis of Non-Desmosomal Rare Genetic Variants in Arrhythmogenic Cardiomyopathy: Integrating in Padua Cohort Literature-Derived Data

Arrhythmogenic cardiomyopathy (ACM) is an inherited myocardial disease at risk of sudden death. Genetic testing impacts greatly in ACM diagnosis, but gene-disease associations have yet to be determined for the increasing number of genes included in clinical panels. Genetic variants evaluation was undertaken for the most relevant non-desmosomal disease genes. We retrospectively studied 320 unrelated Italian ACM patients, including 243 cases with predominant right-ventricular (ARVC) and 77 cases with predominant left-ventricular (ALVC) involvement, who did not carry pathogenic/likely pathogenic (P/LP) variants in desmosome-coding genes. The aim was to assess rare genetic variants in transmembrane protein 43 (TMEM43), desmin (DES), phospholamban (PLN), filamin c (FLNC), cadherin 2 (CDH2), and tight junction protein 1 (TJP1), based on current adjudication guidelines and reappraisal on reported literature data. Thirty-five rare genetic variants, including 23 (64%) P/LP, were identified in 39 patients (16/243 ARVC; 23/77 ALVC): 22 FLNC, 9 DES, 2 TMEM43, and 2 CDH2. No P/LP variants were found in PLN and TJP1 genes. Gene-based burden analysis, including P/LP variants reported in literature, showed significant enrichment for TMEM43 (3.79-fold), DES (10.31-fold), PLN (117.8-fold) and FLNC (107-fold). A non-desmosomal rare genetic variant is found in a minority of ARVC patients but in about one third of ALVC patients; as such, clinical decision-making should be driven by genes with robust evidence. More than two thirds of non-desmosomal P/LP variants occur in FLNC.

Immortalised murine R349P desmin knock-in myotubes exhibit a reduced proton leak and decreased ADP/ATP translocase levels in purified mitochondria

Desmin gene mutations cause myopathies and cardiomyopathies. Our previously characterised R349P desminopathy mice, which carry the ortholog of the common human desmin mutation R350P, showed marked alterations in mitochondrial morphology and function in muscle tissue. By isolating skeletal muscle myoblasts from offspring of R349P desminopathy and p53 knock-out mice, we established an immortalised cellular disease model. Heterozygous and homozygous R349P desmin knock-in and wild-type myoblasts could be well differentiated into multinucleated spontaneously contracting myotubes. The desminopathy myoblasts showed the characteristic disruption of the desmin cytoskeleton and desmin protein aggregation, and the desminopathy myotubes showed the characteristic myofibrillar irregularities. Long-term electrical pulse stimulation promoted myotube differentiation and markedly increased their spontaneous contraction rate. In both heterozygous and homozygous R349P desminopathy myotubes, this treatment restored a regular myofibrillar cross-striation pattern as seen in wild-type myotubes. High-resolution respirometry of mitochondria purified from myotubes by density gradient ultracentrifugation revealed normal oxidative phosphorylation capacity, but a significantly reduced proton leak in mitochondria from the homozygous R349P desmin knock-in cells. Consistent with a reduced proton flux across the inner mitochondrial membrane, our quantitative proteomic analysis of the purified mitochondria revealed significantly reduced levels of ADP/ATP translocases in the homozygous R349P desmin knock-in genotype. As this alteration was also detected in the soleus muscle of R349P desminopathy mice, which, in contrast to the mitochondria purified from cultured cells, showed a variety of other dysregulated mitochondrial proteins, we consider this finding to be an early step in the pathogenesis of secondary mitochondriopathy in desminopathy.

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.

Desmin Modulates Muscle Cell Adhesion and Migration

Cellular adhesion and migration are key functions that are disrupted in numerous diseases. We report that desmin, a type-III muscle-specific intermediate filament, is a novel cell adhesion regulator. Expression of p.R406W mutant desmin, identified in patients with desmin-related myopathy, modified focal adhesion area and expression of adhesion-signaling genes in myogenic C2C12 cells. Satellite cells extracted from desmin-knock-out (DesKO) and desmin-knock-in-p.R405W (DesKI-R405W) mice were less adhesive and migrated faster than those from wild-type mice. Moreover, we observed mislocalized and aggregated vinculin, a key component of cell adhesion, in DesKO and DesKI-R405W muscles. Vinculin expression was also increased in desmin-related myopathy patient muscles. Together, our results establish a novel role for desmin in cell-matrix adhesion, an essential process for strength transmission, satellite cell migration and muscle regeneration. Our study links the patho-physiological mechanisms of desminopathies to adhesion/migration defects, and may lead to new cellular targets for novel therapeutic approaches.

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 Muscle Mitochondria Dysfunction in Genetic Neuromuscular Disorders with Cardiac Phenotype

Mitochondrial dysfunction is considered the major contributor to skeletal muscle wasting in different conditions. Genetically determined neuromuscular disorders occur as a result of mutations in the structural proteins of striated muscle cells and therefore are often combined with cardiac phenotype, which most often manifests as a cardiomyopathy. The specific roles played by mitochondria and mitochondrial energetic metabolism in skeletal muscle under muscle-wasting conditions in cardiomyopathies have not yet been investigated in detail, and this aspect of genetic muscle diseases remains poorly characterized. This review will highlight dysregulation of mitochondrial representation and bioenergetics in specific skeletal muscle disorders caused by mutations that disrupt the structural and functional integrity of muscle cells.

Desmin mutations result in mitochondrial dysfunction regardless of their aggregation properties

Desmin, being a major intermediate filament of muscle cells, contributes to stabilization and positioning of mitochondria. Desmin mutations have been reported in conjunction with skeletal myopathies accompanied by mitochondrial dysfunction. Depending on the ability to promote intracellular aggregates formation, mutations can be considered aggregate-prone or non-aggregate-prone. The aim of the present study was to describe how expression of different desmin mutant isoforms effects mitochondria and contributes to the development of myocyte dysfunction. To achieve this goal, two non-aggregate-prone (Des S12F and Des A213V) and four aggregate-prone (Des L345P, Des A357P, Des L370P, Des D399Y) desmin mutations were expressed in skeletal muscle cells. We showed that all evaluated mutations affected the morphology of mitochondrial network, suppressed parameters of mitochondrial respiration, diminished mitochondrial membrane potential, increased ADP/ATP ratio, and enhanced mitochondrial DNA (mtDNA) release. mtDNA was partially secreted through exosomes as demonstrated by GW4869 treatment. Dysfunction of mitochondria was observed regardless the type of mutation: aggregate-prone or non-aggregate-prone. However, expression of aggregate-prone mutations resulted in more prominent phenotype. Thus, in this comparative study of six pathogenic desmin mutations that cause skeletal myopathy development, we confirmed a role of mitochondrial dysfunction and mtDNA release in the pathogenesis of desmin myopathies, regardless of the aggregation capacity of the mutated desmin.

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.

Mutant desmin substantially perturbs mitochondrial morphology, function and maintenance in skeletal muscle tissue

Secondary mitochondrial dysfunction is a feature in a wide variety of human protein aggregate diseases caused by mutations in different proteins, both in the central nervous system and in striated muscle. The functional relationship between the expression of a mutated protein and mitochondrial dysfunction is largely unknown. In particular, the mechanism how this dysfunction drives the disease process is still elusive. To address this issue for protein aggregate myopathies, we performed a comprehensive, multi-level analysis of mitochondrial pathology in skeletal muscles of human patients with mutations in the intermediate filament protein desmin and in muscles of hetero- and homozygous knock-in mice carrying the R349P desmin mutation. We demonstrate that the expression of mutant desmin causes disruption of the extrasarcomeric desmin cytoskeleton and extensive mitochondrial abnormalities regarding subcellular distribution, number and shape. At the molecular level, we uncovered changes in the abundancy and assembly of the respiratory chain complexes and supercomplexes. In addition, we revealed a marked reduction of mtDNA- and nuclear DNA-encoded mitochondrial proteins in parallel with large-scale deletions in mtDNA and reduced mtDNA copy numbers. Hence, our data demonstrate that the expression of mutant desmin causes multi-level damage of mitochondria already in early stages of desminopathies.

Two desmin gene mutations associated with myofibrillar myopathies in Polish families

Desmin is a muscle-specific intermediate filament protein which forms a network connecting the sarcomere, T tubules, sarcolemma, nuclear membrane, mitochondria and other organelles. Mutations in the gene coding for desmin (DES) cause skeletal myopathies often combined with cardiomyopathy, or isolated cardiomyopathies. The molecular pathomechanisms of the disease remain ambiguous. Here, we describe and comprehensively characterize two DES mutations found in Polish patients with a clinical diagnosis of desminopathy. The study group comprised 16 individuals representing three families. Two mutations were identified: a novel missense mutation (Q348P) and a small deletion of nine nucleotides (A357_E359del), previously described by us in the Polish population. A common ancestry of all the families bearing the A357_E359del mutation was confirmed. Both mutations were predicted to be pathogenic using a bioinformatics approach, including molecular dynamics simulations which helped to rationalize abnormal behavior at molecular level. To test the impact of the mutations on DES expression and the intracellular distribution of desmin muscle biopsies were investigated. Elevated desmin levels as well as its atypical localization in muscle fibers were observed. Additional staining for M-cadherin, α-actinin, and myosin heavy chains confirmed severe disruption of myofibrill organization. The abnormalities were more prominent in the Q348P muscle, where both small atrophic fibers as well large fibers with centrally localized nuclei were observed. We propose that the mutations affect desmin structure and cause its aberrant folding and subsequent aggregation, triggering disruption of myofibrils organization.

Significance of low desmin expression in cardiomyocytes in patients with idiopathic dilated cardiomyopathy

Desmin plays an essential role in maintaining cell cytoarchitecture, positioning and functioning of organelles, and the intercellular signaling pathway. It has been suggested that remodeling of desmin cytoskeleton might contribute to the progression of idiopathic dilated cardiomyopathy and might affect patients' long-term prognosis. We performed endomyocardial biopsy in 200 patients with idiopathic dilated cardiomyopathy. A total of 5 to 6 specimens were collected from the left ventricular (LV) wall. Desmin was detected with immunohistochemical staining and Western blotting. Immunohistochemistry revealed 4 types of desmin expression: I, normal staining at Z-lines and intercalated disks, giving a regular cross-section pattern; IIA, increased desmin staining at Z-lines and intercalated disks; IIB, increased desmin staining with irregular pattern of cross-striation and/or with presence of aggregates; and III, decreased or lack of desmin staining. Patients with type III had a greater New York Heart Association class and N-terminal pro-brain natriuretic peptide level, larger LV end-diastolic diameter, and lower LV ejection fraction than patients with type I (p <0.001). At the end of follow-up (mean duration 59 ± 33 months), 44 patients (22%) had died and 5 (2.5%) had undergone heart transplantation. Patients with type III had an increased risk of death or heart transplantation in univariate Cox proportional hazard regression models (adjusted hazard ratio 7.18, 95% confidence interval 2.96 to 17.40, p <0.001) and multivariate models (New York Heart Association class, LV end-diastolic diameter, LV ejection fraction, N-terminal pro-brain natriuretic peptide, gender, and age; hazard ratio 5.24, 95% confidence interval 1.58 to 17.38, p = 0.007). In conclusion, in patients with idiopathic dilated cardiomyopathy, a decrease or lack of desmin expression seems to be a strong, independent predictor of an unfavorable prognosis. Our outcomes support the relevance of exploring desmin expression as a potential target to treat heart failure progression.

Desminopathies: pathology and mechanisms

The intermediate filament protein desmin is an essential component of the extra-sarcomeric cytoskeleton in muscle cells. This three-dimensional filamentous framework exerts central roles in the structural and functional alignment and anchorage of myofibrils, the positioning of cell organelles and signaling events. Mutations of the human desmin gene on chromosome 2q35 cause autosomal dominant, autosomal recessive, and sporadic myopathies and/or cardiomyopathies with marked phenotypic variability. The disease onset ranges from childhood to late adulthood. The clinical course is progressive and no specific treatment is currently available for this severely disabling disease. The muscle pathology is characterized by desmin-positive protein aggregates and degenerative changes of the myofibrillar apparatus. The molecular pathophysiology of desminopathies is a complex, multilevel issue. In addition to direct effects on the formation and maintenance of the extra-sarcomeric intermediate filament network, mutant desmin affects essential protein interactions, cell signaling cascades, mitochondrial functions, and protein quality control mechanisms. This review summarizes the currently available data on the epidemiology, clinical phenotypes, myopathology, and genetics of desminopathies. In addition, this work provides an overview on the expression, filament formation processes, biomechanical properties, post-translational modifications, interaction partners, subcellular localization, and functions of wild-type and mutant desmin as well as desmin-related cell and animal models.

[Desmin filaments and their disorganization associated with myofibrillar myopathies]

Desmin, the muscle-specific intermediate filament protein, is one of the earliest markers expressed in all muscle tissues during development. It forms a three-dimensional scaffold around the myofibril Z-disc and connects the entire contractile apparatus to the subsarcolemmal cytoskeleton, the nuclei and other cytoplasmic organelles. Desmin is essential for tensile strength and muscle integrity. In humans, disorganization of the desmin network is associated with cardiac and/or skeletal myopathies characterized by accumulation of desmin-containing aggregates in the cells. Currently, 49 mutations have been identified in desmin gene. The majority of these mutations alter desmin filament assembly process through different molecular mechanisms and also its interaction with its protein partners. Here, we will give an overview of desmin network organization as well as the impact of desmin mutations on this process. Furthermore, we will discuss the different molecular mechanisms implicated in perturbation of the desmin filament assembly process.

Desmin-related myopathy

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

Molecular pathology of myofibrillar myopathies

Myofibrillar myopathies (MFMs) are clinically and genetically heterogeneous muscle disorders that are defined morphologically by the presence of foci of myofibril dissolution, accumulation of myofibrillar degradation products, and ectopic expression of multiple proteins. MFMs are the paradigm of conformational protein diseases of the skeletal (and cardiac) muscles characterised by intracellular protein accumulation in muscle cells. Understanding of this group of disorders has advanced in recent years through the identification of causative mutations in various genes, most of which encode proteins of the sarcomeric Z-disc, including desmin, alphaB-crystallin, myotilin, ZASP and filamin C. This review focuses on the MFMs arising from defects in these proteins, summarising genetic and clinical features of the disorders and then discussing emerging understanding of the molecular pathogenic mechanisms leading to muscle fibre degeneration. Defective extralysosomal degradation of proteins is now recognised as an important element in this process. Several factors--including mutant proteins, a defective ubiquitin-proteasome system, aggresome formation, mutant ubiquitin, p62, oxidative stress and abnormal regulation of some transcription factors--are thought to participate in the cascade of events occurring in muscle fibres in MFMs.

Mice expressing L345P mutant desmin exhibit morphological and functional changes of skeletal and cardiac mitochondria

Desmin mutations underlie inherited myopathies/cardiomyopathies with varying severity and involvement of the skeletal and cardiac muscles. We developed a transgenic mouse model expressing low level of the L345P desmin mutation (DESMUT mice) in order to uncover changes in skeletal and cardiac muscles caused by this mutation. The most striking ultrastructural changes in muscle from DESMUT mice were mitochondrial swelling and vacuolization. The mitochondrial Ca(2+) level was significantly increased in skeletal and cardiac myocytes from DESMUT mice compared to wild type cells during and after contractions. In isolated DESMUT soleus muscles, contractile function and recovery from fatigue were impaired. A SHIRPA screening test for neuromuscular performance demonstrated decreased motor function in DESMUT compared to WT mice. Echocardiographic changes in DESMUT mice included left ventricular wall hypertrophy and a decreased left ventricular chamber dimension. The results imply that low levels of L345P desmin acts, at least partially, by a dominant negative effect on mitochondria.

Intermediate filament diseases: desminopathy

Desminopathy is one of the most common intermediate filament human disorders associated with mutations in closely interacting proteins, desmin and alphaB-crystallin. The inheritance pattern in familial desminopathy is characterized as autosomal dominant or autosomal recessive, but many cases have no family history. At least some and likely most sporadic desminopathy cases are associated with de novo DES mutations. The age of disease onset and rate of progression may vary depending on the type of inheritance and location of the causative mutation. Typically, the illness presents with lower and later upper limb muscle weakness slowly spreading to involve truncal, neck-flexor, facial and bulbar muscles. Skeletal myopathy is often combined with cardiomyopathy manifested by conduction blocks, arrhythmias and chronic heart failure resulting in premature sudden death. Respiratory muscle weakness is a major complication in some patients. Sections of the affected skeletal and cardiac muscles show abnormal fibre areas containing chimeric aggregates consisting of desmin and other cytoskeletal proteins. Various DES gene mutations: point mutations, an insertion, small in-frame deletions and a larger exon-skipping deletion, have been identified in desminopathy patients. The majority of these mutations are located in conserved alpha-helical segments, but additional mutations have recently been identified in the tail domain. Filament and network assembly studies indicate that most but not all disease-causing mutations make desmin assembly-incompetent and able to disrupt a pre-existing filamentous network in dominant-negative fashion. AlphaB-crystallin serves as a chaperone for desmin preventing its aggregation under various forms of stress; mutant CRYAB causes cardiac and skeletal myopathies identical to those resulting from DES mutations.

Two related Dutch families with a clinically variable presentation of cardioskeletal myopathy caused by a novel S13F mutation in the desmin gene

Desmin-related myopathy is characterised by skeletal muscle weakness often combined with cardiac involvement. Mutations in the desmin gene have been described as a cause of desmin-related myopathy (OMIM 601419). We report here on two distantly related Dutch families with autosomal dominant inheritance of desmin-related myopathy affecting 15 family members. A highly heterogeneous clinical picture is apparent, varying from isolated dilated cardiomyopathy to a more generalised skeletal myopathy and mild respiratory problems. Morphological analysis of muscle biopsies revealed intracytoplasmic desmin aggregates (desmin and p62 staining). In both families we identified an identical novel pathogenic heterozygous missense mutation, S13F, in the 'head' domain of the desmin gene which cosegregates with the disease phenotype. This is the 5th reported missense mutation located at the 'head' domain of the desmin gene and the first reported Dutch family with desmin-related myopathy. This article illustrates the importance of analysing the desmin gene in patients with (familial) cardiac conduction disease, dilated cardiomyopathy and/or a progressive skeletal myopathy resembling limb-girdle muscular dystrophy.

Muscle intermediate filaments and their links to membranes and membranous organelles

Intermediate filaments (IFs) play a key role in the integration of structure and function of striated muscle, primarily by mediating mechanochemical links between the contractile apparatus and mitochondria, myonuclei, the sarcolemma and potentially the vesicle trafficking apparatus. Linkage of all these membranous structures to the contractile apparatus, mainly through the Z-disks, supports the integration and coordination of growth and energy demands of the working myocyte, not only with force transmission, but also with de novo gene expression, energy production and efficient protein and lipid trafficking and targeting. Desmin, the most abundant and intensively studied muscle intermediate filament protein, is linked to proper costamere organization, myoblast and stem cell fusion and differentiation, nuclear shape and positioning, as well as mitochondrial shape, structure, positioning and function. Similar links have been established for lysosomes and lysosome-related organelles, consistent with the presence of widespread links between IFs and membranous structures and the regulation of their fusion, morphology and stabilization necessary for cell survival.

Assembly defects of desmin disease mutants carrying deletions in the alpha-helical rod domain are rescued by wild type protein

Most mutations of desmin that cause severe autosomal dominant forms of myofibrillar myopathy are point mutations and locate in the central alpha-helical coiled-coil rod domain. Recently, two in-frame deletions of one and three amino acids, respectively, in the alpha-helix have been described and discussed to drastically interfere with the architecture of the desmin dimer and possibly also the formation of tetramers and higher order complexes [Kaminska, A., Strelkov, S.V., Goudeau, B., Olive, M., Dagvadorj, A., Fidzianska, A., Simon-Casteras, M., Shatunov, A., Dalakas, M.C., Ferrer, I., Kwiecinski, H., Vicart, P., Goldfarb, L.G., 2004. Small deletions disturb desmin architecture leading to breakdown of muscle cells and development of skeletal or cardioskeletal myopathy. Hum. Genet. 114, 306-313.]. Therefore, it was proposed that they may poison intermediate filament (IF) assembly. We have now recombinantly synthesized both mutant proteins and subjected them to comprehensive in vitro assembly experiments. While exhibiting assembly defects when analyzed on their own, both one-to-one mixtures of the respective mutant protein with wild type desmin facilitated proper filament formation. Transient transfection studies complemented this fundamental finding by demonstrating that wild type desmin is also rescuing these assembly defects in vivo. In summary, our findings strongly question the previous hypothesis that it is assembly incompetence due to molecular rearrangements caused by the mutations, which triggers the development of disease. As an alternative, we propose that these mutations cause subtle age-dependent structural alterations of desmin IFs that eventually lead to disease.