Autophagic vacuolar pathology in desminopathies

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.

X-linked Myopathy with Excessive Autophagy - A Rare Cause of Vacuolar Myopathy in Children

X-linked myopathy with excessive autophagy (XMEA) is a rare, recently characterized type of autophagic vacuolar myopathy caused by mutations in the VMA21 gene. It is characterized by slowly progressive weakness restricted to proximal limb muscles and generally has a favorable outcome. The characteristic histological and ultrastructural features distinguish this entity from other mimics, notably Danon disease. XMEA is an under recognized disease and should be considered in the differentials of slowly progressive myopathy in children. Awareness of this rare entity is also important for the pathologists in order to distinguish it from other causes of vacuolar myopathy in view of its favourable prognosis. We report the first genetically confirmed case of XMEA from India in an 8-year-old boy which was diagnosed based on the characteristic light microscopic and ultrastructural findings on muscle biopsy and subsequently confirmed by mutation analysis. The differential diagnostic considerations are also discussed.

Desminopathy presenting as late onset bilateral facial weakness, with diagnosis supported by lower limb MRI

A 63 year old male presented with a 20 year history of facial weakness and several years of nasal regurgitation and dysphonia. Examination revealed bilateral facial weakness with nasal speech. Serum creatine kinase was 918 U/L. Neurophysiological studies suggested a myopathy and biopsy of the left vastus lateralis showed serpentine basophilic inclusions in the sarcoplasm and strong oxidative enzyme activity suggesting mitochondria accumulation. The muscle MRI showed selective fatty replacement within semitendinosus, gastrocnemius and soleus indicative of a desminopathy. A heterozygous missense variant c.17C>G (p.Ser6Trp) was identified within DES, predicted to be pathogenic in silico and previously described in a family with distal limb weakness. There are no previous case reports of desminopathy presenting with facial weakness, to our knowledge. Diagnosis was suggested following myoimaging of clinically unaffected muscles. Our study highlights the importance of muscle MRI in the diagnostic evaluation of muscle disease and further expands the known phenotypic heterogeneity of desminopathies.

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.

Differential diagnosis of vacuolar myopathies in the NGS era

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

Autophagy Defects in Skeletal Myopathies

Autophagy is an evolutionarily conserved catabolic process that targets different types of cytoplasmic cargo (such as bulk cytoplasm, damaged cellular organelles, and misfolded protein aggregates) for lysosomal degradation. Autophagy is activated in response to biological stress and also plays a critical role in the maintenance of normal cellular homeostasis; the latter function is particularly important for the integrity of postmitotic, metabolically active tissues, such as skeletal muscle. Through impairment of muscle homeostasis, autophagy dysfunction contributes to the pathogenesis of many different skeletal myopathies; the observed autophagy defects differ from disease to disease but have been shown to involve all steps of the autophagic cascade (from induction to lysosomal cargo degradation) and to impair both bulk and selective autophagy. To highlight the molecular and cellular mechanisms that are shared among different myopathies with deficient autophagy, these disorders are discussed based on the nature of the underlying autophagic defect rather than etiology or clinical presentation.

Sequestosome-1 (p62) expression reveals chaperone-assisted selective autophagy in immune-mediated necrotizing myopathies

Diffuse myofiber necrosis in the context of inflammatory myopathy is the hallmark of immune‐mediated necrotizing myopathy (IMNM). We have previously shown that skeletal muscle fibers of IMNM patients may display nonrimmed vacuoles and sarcoplasmic irregularities. The dysfunctional chaperone activity has been linked to the defective assembly of skeletal muscle proteins and their degradation via lysosomes, autophagy and the proteasomal machinery. This study was undertaken to highlight a chaperone‐assisted selective autophagy (CASA) pathway, functionally involved in protein homeostasis, cell stress and the immune response in skeletal muscle of IMNM patients. Skeletal muscle biopsies from 54 IMNM patients were analyzed by immunostaining, as well as by qPCR. Eight biopsies of sIBM patients served as pathological controls, and eight biopsies of nondisease control subjects were included. Alteration of autophagy was detectable in all IMNM biopsy samples highlighted via a diffuse sarcoplasmic staining pattern by p62 and LC3 independent of vacuoles. This pattern was at variance with the coarse focal staining pattern mostly confined to rimmed vacuoles in sIBM. Colocalization of p62 with the chaperone proteins HSP70 and αB‐crystalline points to the specific targeting of misfolded proteins to the CASA machinery. Bcl2‐associated athanogene 3 (BAG3) positivity of these fibers emphasizes the selectivity of autophagy processes and these fibers also express MHC class I sarcolemma. Expression of genes involved in autophagy and endoplasmic reticulum (ER) stress pathways studied here is significantly upregulated in IMNM. We highlight that vacuoles without sarcolemmal features may arise in IMNM muscle biopsies, and they must not be confounded with sIBM‐specific vacuoles. Further, we show the activation of selective autophagy and emphasize the role of chaperones in this context. CASA occurs in IMNM muscle, and specific molecular pathways of autophagy differ from the ones in sIBM, with p62 as a unique identifier of this process.

Desmin forms toxic, seeding-competent amyloid aggregates that persist in muscle fibers

Protein aggregation and the deposition of amyloid is a common feature in neurodegeneration, but can also be seen in degenerative muscle diseases known as myofibrillar myopathies (MFMs). Hallmark pathology in MFM patient muscle is myofibrillar disarray, aggregation of the muscle-specific intermediate filament, desmin, and amyloid. In some cases, a missense mutation in desmin leads to its destabilization and aggregation. The present study demonstrates that similar to neurodegenerative proteins, desmin can form amyloid and template the amyloidogenic conversion of unaggregated desmin protein. This desmin-derived amyloid is toxic to myocytes and persists when introduced into skeletal muscle, in contrast to unaggregated desmin. These data demonstrate that desmin itself can form amyloid and expand the mechanism of proteinopathies to skeletal muscle.

Desmin-associated myofibrillar myopathy (MFM) has pathologic similarities to neurodegeneration-associated protein aggregate diseases. Desmin is an abundant muscle-specific intermediate filament, and disease mutations lead to its aggregation in cells, animals, and patients. We reasoned that similar to neurodegeneration-associated proteins, desmin itself may form amyloid. Desmin peptides corresponding to putative amyloidogenic regions formed seeding-competent amyloid fibrils. Amyloid formation was increased when disease-associated mutations were made within the peptide, and this conversion was inhibited by the anti-amyloid compound epigallocatechin-gallate. Moreover, a purified desmin fragment (aa 117 to 348) containing both amyloidogenic regions formed amyloid fibrils under physiologic conditions. Desmin fragment-derived amyloid coaggregated with full-length desmin and was able to template its conversion into fibrils in vitro. Desmin amyloids were cytotoxic to myotubes and disrupted their myofibril organization compared with desmin monomer or other nondesmin amyloids. Finally, desmin fragment amyloid persisted when introduced into mouse skeletal muscle. These data suggest that desmin forms seeding-competent amyloid that is toxic to myofibers. Moreover, small molecules known to interfere with amyloid formation and propagation may have therapeutic potential in MFM.

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.

Modulation of autophagy in human diseases strategies to foster strengths and circumvent weaknesses

Autophagy is central to the maintenance of intracellular homeostasis across species. Accordingly, autophagy disorders are linked to a variety of diseases from the embryonic stage until death, and the role of autophagy as a therapeutic target has been widely recognized. However, autophagy-associated therapy for human diseases is still in its infancy and is supported by limited evidence. In this review, we summarize the landscape of autophagy-associated diseases and current autophagy modulators. Furthermore, we investigate the existing autophagy-associated clinical trials, analyze the obstacles that limit their progress, offer tactics that may allow barriers to be overcome along the way and then discuss the therapeutic potential of autophagy modulators in clinical applications.

Imbalances in protein homeostasis caused by mutant desmin

AIMS

We investigated newly generated immortalized heterozygous and homozygous R349P desmin knock-in myoblasts in conjunction with the corresponding desminopathy mice as models for desminopathies to analyse major protein quality control processes in response to the presence of R349P mutant desmin.

METHODS

We used hetero- and homozygous R349P desmin knock-in mice for analyses and for crossbreeding with p53 knock-out mice to generate immortalized R349P desmin knock-in skeletal muscle myoblasts and myotubes. Skeletal muscle sections and cultured muscle cells were investigated by indirect immunofluorescence microscopy, proteasomal activity measurements and immunoblotting addressing autophagy rate, chaperone-assisted selective autophagy and heat shock protein levels. Muscle sections were further analysed by transmission and immunogold electron microscopy.

RESULTS

We demonstrate that mutant desmin (i) increases proteasomal activity, (ii) stimulates macroautophagy, (iii) dysregulates the chaperone assisted selective autophagy and (iv) elevates the protein levels of αB-crystallin and Hsp27. Both αB-crystallin and Hsp27 as well as Hsp90 displayed translocation patterns from Z-discs as well as Z-I junctions, respectively, to the level of sarcomeric I-bands in dominant and recessive desminopathies.

CONCLUSIONS

Our findings demonstrate that the presence of R349P mutant desmin causes a general imbalance in skeletal muscle protein homeostasis via aberrant activity of all major protein quality control systems. The augmented activity of these systems and the subcellular shift of essential heat shock proteins may deleteriously contribute to the previously observed increased turnover of desmin itself and desmin-binding partners, which triggers progressive dysfunction of the extrasarcomeric cytoskeleton and the myofibrillar apparatus in the course of the development of desminopathies.

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.

"Get the Balance Right": Pathological Significance of Autophagy Perturbation in Neuromuscular Disorders

Recent research has revealed that autophagy, a major catabolic process in cells, is dysregulated in several neuromuscular diseases and contributes to the muscle wasting caused by non-muscle disorders (e.g. cancer cachexia) or during aging (i.e. sarcopenia). From there, the idea arose to interfere with autophagy or manipulate its regulatory signalling to help restore muscle homeostasis and attenuate disease progression. The major difficulty for the development of therapeutic strategies is to restore a balanced autophagic flux, due to the dynamic nature of autophagy. Thus, it is essential to better understand the mechanisms and identify the signalling pathways at play in the control of autophagy in skeletal muscle. A comprehensive analysis of the autophagic flux and of the causes of its dysregulation is required to assess the pathogenic role of autophagy in diseased muscle. Furthermore, it is essential that experiments distinguish between primary dysregulation of autophagy (prior to disease onset) and impairments as a consequence of the pathology. Of note, in most muscle disorders, autophagy perturbation is not caused by genetic modification of an autophagy-related protein, but rather through indirect alteration of regulatory signalling or lysosomal function. In this review, we will present the mechanisms involved in autophagy, and those ensuring its tight regulation in skeletal muscle. We will then discuss as to how autophagy dysregulation contributes to the pathogenesis of neuromuscular disorders and possible ways to interfere with this process to limit disease progression.

The Limb-Girdle Muscular Dystrophies and the Dystrophinopathies

PURPOSE OF REVIEW

The classic approach to identifying and accurately diagnosing limb-girdle muscular dystrophies (LGMDs) relied heavily on phenotypic characterization and ancillary studies including muscle biopsy. Because of rapid advances in genetic sequencing methodologies, several additional LGMDs have been molecularly characterized, and the diagnostic approach to these disorders has been simplified. This article summarizes the epidemiology, clinical features, and genetic defects underlying the LGMDs.

RECENT FINDINGS

In recent years, the advent of next-generation sequencing has heralded an era of molecular diagnosis in conjunction with physical characterization. Inadvertently, this process has also led to the "next-generation aftermath," whereby variants of unknown significance are identified in most patients. Similar to the published diagnostic and treatment guidelines for Duchenne muscular dystrophy, diagnostic and treatment guidelines have recently been published for LGMDs. In addition, the first medication (based on the exon-skipping strategy) for treatment of patients with a subset of Duchenne muscular dystrophy has been recently approved by the US Food and Drug Administration (FDA).

SUMMARY

The LGMDs are a heterogeneous group of hereditary, progressive, and degenerative neuromuscular disorders that present with primary symptoms of shoulder girdle and pelvic girdle weakness. Although a combination of clinical and molecular genetic evaluations may be sufficient for accurate diagnosis of LGMDs in many cases, the contribution of imaging and histopathologic correlations still remains a critical, if not a necessary, component of evaluation in some cases.

Proteomics of rimmed vacuoles define new risk allele in inclusion body myositis

OBJECTIVE

Sporadic inclusion body myositis (sIBM) pathogenesis is unknown; however, rimmed vacuoles (RVs) are a constant feature. We propose to identify proteins that accumulate within RVs.

METHODS

RVs and intact myofibers were laser microdissected from skeletal muscle of 18 sIBM patients and analyzed by a sensitive mass spectrometry approach using label-free spectral count-based relative protein quantification. Whole exome sequencing was performed on 62 sIBM patients. Immunofluorescence was performed on patient and mouse skeletal muscle.

RESULTS

A total of 213 proteins were enriched by >1.5 -fold in RVs compared to controls and included proteins previously reported to accumulate in sIBM tissue or when mutated cause myopathies with RVs. Proteins associated with protein folding and autophagy were the largest group represented. One autophagic adaptor protein not previously identified in sIBM was FYCO1. Rare missense coding FYCO1 variants were present in 11.3% of sIBM patients compared with 2.6% of controls (p = 0.003). FYCO1 colocalized at RVs with autophagic proteins such as MAP1LC3 and SQSTM1 in sIBM and other RV myopathies. One FYCO1 variant protein had reduced colocalization with MAP1LC3 when expressed in mouse muscle.

INTERPRETATION

This study used an unbiased proteomic approach to identify RV proteins in sIBM that included a novel protein involved in sIBM pathogenesis. FYCO1 accumulates at RVs, and rare missense variants in FYCO1 are overrepresented in sIBM patients. These FYCO1 variants may impair autophagic function, leading to RV formation in sIBM patient muscle. FYCO1 functionally connects autophagic and endocytic pathways, supporting the hypothesis that impaired endolysosomal degradation underlies the pathogenesis of sIBM. Ann Neurol 2017;81:227-239.