Partial loss of desmin expression due to a leaky splice site variant in the human DES gene is associated with neuromuscular transmission defects

Recessive desminopathies are rare and often present as severe early-onset myopathy. Here we report a milder phenotype in three unrelated patients from southern India (2 M, 1F) aged 16, 21, and 22 years, who presented with childhood-onset, gradually progressive, fatigable limb-girdle weakness, ptosis, speech and swallowing difficulties, without cardiac involvement. Serum creatine kinase was elevated, and repetitive nerve stimulation showed decrement in all. Clinical improvement was noted with pyridostigmine and salbutamol in two patients. All three patients had a homozygous substitution in intron 5: DES(NM_001927.4):c.1023+5G>A, predicted to cause a donor splice site defect. Muscle biopsy with ultrastructural analysis suggested myopathy with myofibrillar disarray, and immunohistochemistry showed partial loss of desmin with some residual staining, while western blot analysis showed reduced desmin. RT-PCR of patient muscle RNA revealed two transcripts: a reduced normal desmin transcript and a larger abnormal transcript suggesting leaky splicing at the intron 5 donor site. Sequencing of the PCR products confirmed the inclusion of intron 5 in the longer transcript, predicted to cause a premature stop codon. Thus, we provide evidence for a leaky splice site causing partial loss of desmin associated with a unique phenotypic presentation of a milder form of desmin-related recessive myopathy overlapping with congenital myasthenic syndrome.

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.

Critical contribution of mitochondria in the development of cardiomyopathy linked to desmin mutation

BACKGROUND

Beyond the observed alterations in cellular structure and mitochondria, the mechanisms linking rare genetic mutations to the development of heart failure in patients affected by desmin mutations remain unclear due in part, to the lack of relevant human cardiomyocyte models.

METHODS

To shed light on the role of mitochondria in these mechanisms, we investigated cardiomyocytes derived from human induced pluripotent stem cells carrying the heterozygous DESE439K mutation that were either isolated from a patient or generated by gene editing. To increase physiological relevance, cardiomyocytes were either cultured on an anisotropic micropatterned surface to obtain elongated and aligned cardiomyocytes, or as a cardiac spheroid to create a micro-tissue. Moreover, when applicable, results from cardiomyocytes were confirmed with heart biopsies of suddenly died patient of the same family harboring DESE439K mutation, and post-mortem heart samples from five control healthy donors.

RESULTS

The heterozygous DESE439K mutation leads to dramatic changes in the overall cytoarchitecture of cardiomyocytes, including cell size and morphology. Most importantly, mutant cardiomyocytes display altered mitochondrial architecture, mitochondrial respiratory capacity and metabolic activity reminiscent of defects observed in patient's heart tissue. Finally, to challenge the pathological mechanism, we transferred normal mitochondria inside the mutant cardiomyocytes and demonstrated that this treatment was able to restore mitochondrial and contractile functions of cardiomyocytes.

CONCLUSIONS

This work highlights the deleterious effects of DESE439K mutation, demonstrates the crucial role of mitochondrial abnormalities in the pathophysiology of desmin-related cardiomyopathy, and opens up new potential therapeutic perspectives for this disease.

Loss of function variants in DNAJB4 cause a myopathy with early respiratory failure

DNAJ/HSP40 co-chaperones are integral to the chaperone network, bind client proteins and recruit them to HSP70 for folding. We performed exome sequencing on patients with a presumed hereditary muscle disease and no genetic diagnosis. This identified four individuals from three unrelated families carrying an unreported homozygous stop gain (c.856A > T; p.Lys286Ter), or homozygous missense variants (c.74G > A; p.Arg25Gln and c.785 T > C; p.Leu262Ser) in DNAJB4. Affected patients presented with axial rigidity and early respiratory failure requiring ventilator support between the 1st and 4th decade of life. Selective involvement of the semitendinosus and biceps femoris muscles was seen on MRI scans of the thigh. On biopsy, muscle was myopathic with angular fibers, protein inclusions and occasional rimmed vacuoles. DNAJB4 normally localizes to the Z-disc and was absent from muscle and fibroblasts of affected patients supporting a loss of function. Functional studies confirmed that the p.Lys286Ter and p.Leu262Ser mutant proteins are rapidly degraded in cells. In contrast, the p.Arg25Gln mutant protein is stable but failed to complement for DNAJB function in yeast, disaggregate client proteins or protect from heat shock-induced cell death consistent with its loss of function. DNAJB4 knockout mice had muscle weakness and fiber atrophy with prominent diaphragm involvement and kyphosis. DNAJB4 knockout muscle and myotubes had myofibrillar disorganization and accumulated Z-disc proteins and protein chaperones. These data demonstrate a novel chaperonopathy associated with DNAJB4 causing a myopathy with early respiratory failure. DNAJB4 loss of function variants may lead to the accumulation of DNAJB4 client proteins resulting in muscle dysfunction and degeneration.

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.

Clinical trajectory of a patient with filaminopathy who developed arrhythmogenic cardiomyopathy, myofibrillar myopathy, and multiorgan tumors

We report a case of a patient presenting with arrhythmogenic cardiomyopathy, myofibrillar myopathy, and multiorgan tumors. A 41-year-old woman with a history of hypertrophic cardiomyopathy, diagnosed at 6 years of age, developed scoliosis after puberty. Following spinal surgery to address the scoliosis, she developed recurrent severe arrhythmia and heart failure. She developed hypoventilation at age 29 years. Proximal dominant weakness and mild elevation of serum creatine kinase indicated possible myopathy. Myofibrillar myopathy was diagnosed by muscle biopsy at age 30 year. Acute abdomen was repeatedly reported from age 33 years, eventually leading to a diagnosis of gastric polyp and erosive ulcer. A urinary bladder tumor was found at age 35 years, and breast cancer was diagnosed at age 40 years. Whole exome sequencing detected a heterozygous missense mutation in Filamin C. Recent evidences suggest that filamins are associated with tumors, and this case further highlights the clinical spectrum of filaminopathy.

Integrated proteomic and transcriptomic profiling identifies aberrant gene and protein expression in the sarcomere, mitochondrial complex I, and the extracellular matrix in Warmblood horses with myofibrillar myopathy

Background

Myofibrillar myopathy in humans causes protein aggregation, degeneration, and weakness of skeletal muscle. In horses, myofibrillar myopathy is a late-onset disease of unknown origin characterized by poor performance, atrophy, myofibrillar disarray, and desmin aggregation in skeletal muscle. This study evaluated molecular and ultrastructural signatures of myofibrillar myopathy in Warmblood horses through gluteal muscle tandem-mass-tag quantitative proteomics (5 affected, 4 control), mRNA-sequencing (8 affected, 8 control), amalgamated gene ontology analyses, and immunofluorescent and electron microscopy.

Results

We identified 93/1533 proteins and 47/27,690 genes that were significantly differentially expressed. The top significantly differentially expressed protein CSRP3 and three other differentially expressed proteins, including, PDLIM3, SYNPO2, and SYNPOL2, are integrally involved in Z-disc signaling, gene transcription and subsequently sarcomere integrity. Through immunofluorescent staining, both desmin aggregates and CSRP3 were localized to type 2A fibers. The highest differentially expressed gene CHAC1, whose protein product degrades glutathione, is associated with oxidative stress and apoptosis. Amalgamated transcriptomic and proteomic gene ontology analyses identified 3 enriched cellular locations; the sarcomere (Z-disc & I-band), mitochondrial complex I and the extracellular matrix which corresponded to ultrastructural Z-disc disruption and mitochondrial cristae alterations found with electron microscopy.

Conclusions

A combined proteomic and transcriptomic analysis highlighted three enriched cellular locations that correspond with MFM ultrastructural pathology in Warmblood horses. Aberrant Z-disc mechano-signaling, impaired Z-disc stability, decreased mitochondrial complex I expression, and a pro-oxidative cellular environment are hypothesized to contribute to the development of myofibrillar myopathy in Warmblood horses. These molecular signatures may provide further insight into diagnostic biomarkers, treatments, and the underlying pathophysiology of MFM.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07758-0.

c.3G>A mutation in the CRYAB gene that causes fatal infantile hypertonic myofibrillar myopathy in the Chinese population

Infantile hypertonic myofibrillar myopathy is characterized by the rapid development of rigid muscles and respiratory insufficiency soon after birth, with very high mortality. It is extremely rare, and only a few cases having been reported until now. Here we report four Chinese infants with fatal neuromuscular disorders characterized by abdominal and trunk skeletal muscle stiffness and rapid respiratory insufficiency progression. Electromyograms showed increased insertion activities and profuse fibrillation potentials with complex repetitive discharges. Immunohistochemistry staining of muscle biopsies showed accumulations of desmin in the myocytes. Powdery Z-bands with dense granules across sarcomeres were observed in muscle fibers using electron microscopy. All patients carry a homozygous c.3G>A mutation in the CRYAB gene, which resulted in the loss of the initiating methionine and the absence of protein. This study's findings help further understand the disease and highlight a founder mutation in the Chinese population.

The p.Ala2430Val mutation in filamin C causes a "hypertrophic myofibrillar cardiomyopathy"

Hypertrophic cardiomyopathy (HCM) often leads to heart failure. Mutations in sarcomeric proteins are most frequently the cause of HCM but in many patients the gene defect is not known. Here we report on a young man who was diagnosed with HCM shortly after birth. Whole exome sequencing revealed a mutation in the FLNC gene (c.7289C > T; p.Ala2430Val) that was previously shown to cause aggregation of the mutant protein in transfected cells. Myocardial tissue from patients with this mutation has not been analyzed before and thus, the underlying etiology is not well understood. Myocardial tissue of our patient obtained during myectomy at the age of 23 years was analyzed in detail by histochemistry, immunofluorescence staining, electron microscopy and western blot analysis. Cardiac histology showed a pathology typical for myofibrillar myopathy with myofibril disarray and abnormal protein aggregates containing BAG3, desmin, HSPB5 and filamin C. Analysis of sarcomeric and intercalated disc proteins showed focally reduced expression of the gap junction protein connexin43 and Xin-positive sarcomeric lesions in the cardiomyocytes of our patient. In addition, autophagy pathways were altered with upregulation of LC3-II, WIPI1 and HSPB5, 6, 7 and 8. We conclude that the p.Ala2430Val mutation in FLNC most probably is associated with HCM characterized by abnormal intercalated discs, disarray of myofibrils and aggregates containing Z-disc proteins similar to myofibrillar myopathy, which supports the pathological effect of the mutation.

A novel recessive mutation affecting DNAJB6a causes myofibrillar myopathy

Mutations in the DNAJB6 gene have been identified as rare causes of myofibrillar myopathies. However, the underlying pathophysiologica mechanisms remain elusive. DNAJB6 has two known isoforms, including the nuclear isoform DNAJB6a and the cytoplasmic isoform DNAJB6b, which was thought to be the pathogenic isoform. Here, we report a novel recessive mutation c.695_699del (p. Val 232 Gly fs*7) in the DNAJB6 gene, associated with an apparently recessively inherited late onset distal myofibrillar myopathy in a Chinese family. Notably, the novel mutation localizes to exon 9 and uniquely encodes DNAJB6a. We further identified that this mutation decreases the mRNA and protein levels of DNAJB6a and results in an age-dependent recessive toxic effect on skeletal muscle in knock-in mice. Moreover, the mutant DNAJB6a showed a dose-dependent anti-aggregation effect on polyglutamine-containing proteins in vitro. Taking together, these findings reveal the pathogenic role of DNAJB6a insufficiency in myofibrillar myopathies and expand upon the molecular spectrum of DNAJB6 mutations.

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.

Homozygous expression of the myofibrillar myopathy-associated p.W2710X filamin C variant reveals major pathomechanisms of sarcomeric lesion formation

Filamin C (FLNc) is mainly expressed in striated muscle cells where it localizes to Z-discs, myotendinous junctions and intercalated discs. Recent studies have revealed numerous mutations in the FLNC gene causing familial and sporadic myopathies and cardiomyopathies with marked clinical variability. The most frequent myopathic mutation, p.W2710X, which is associated with myofibrillar myopathy, deletes the carboxy-terminal 16 amino acids from FLNc and abolishes the dimerization property of Ig-like domain 24. We previously characterized "knock-in" mice heterozygous for this mutation (p.W2711X), and have now investigated homozygous mice using protein and mRNA expression analyses, mass spectrometry, and extensive immunolocalization and ultrastructural studies. Although the latter mice display a relatively mild myopathy under normal conditions, our analyses identified major mechanisms causing the pathophysiology of this disease: in comparison to wildtype animals (i) the expression level of FLNc protein is drastically reduced; (ii) mutant FLNc is relocalized from Z-discs to particularly mechanically strained parts of muscle cells, i.e. myotendinous junctions and myofibrillar lesions; (iii) the number of lesions is greatly increased and these lesions lack Bcl2-associated athanogene 3 (BAG3) protein; (iv) the expression of heat shock protein beta-7 (HSPB7) is almost completely abolished. These findings indicate grave disturbances of BAG3-dependent and -independent autophagy pathways that are required for efficient lesion repair. In addition, our studies reveal general mechanisms of lesion formation and demonstrate that defective FLNc dimerization via its carboxy-terminal domain does not disturb assembly and basic function of myofibrils. An alternative, more amino-terminally located dimerization site might compensate for that loss. Since filamins function as stress sensors, our data further substantiate that FLNc is important for mechanosensing in the context of Z-disc stabilization and maintenance.

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

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

Myofibrillar Myopathy Mimicking Polyneuropathy

A 76-year-old man with a 5-year history of gait difficulties was suspected to have length-dependent sensorimotor polyneuropathy. Electrodiagnostic results pointed to a foot drop of neurogenic etiology, except for the prominence of myotonic discharges on needle EMG. Tests for acquired and genetic causes of polyneuropathy were unrevealing. The patient's first-degree cousin, with a much different clinical phenotype had been diagnosed with myofibrillar myopathy. Our patient was eventually found to carry the same myotilin c.179C>T p.Ser60Phe mutation. Muscle MRI was helpful in delineating clinically unsuspected involvement of paraspinal and pelvi-femoral muscles, as well as showing marked myopathic fatty infiltration of distal leg muscles. The association of neuropathy and myopathy is a recognized feature of myofibrillar myopathy. In some patients with unexplained foot drop, whole-body muscle MRI and a dedicated genetic mutation testing strategy may help reveal a diagnosis of genetic myopathy.

A novel heterozygous mutation in the C-terminal region of HSPB8 leads to limb-girdle rimmed vacuolar myopathy

Mutations in heat shock protein B8 were initially identified in inherited neuropathies and were more recently found to cause a predominantly distal myopathy with myofibrillar pathology and rimmed vacuoles. Rare patients also had proximal weakness. Only very few pathogenic variants have been identified in HSPB8. Disruption of the chaperone activity of heat shock protein B8 impairs chaperone-assisted selective autophagy and results in protein aggregation. We report a 23-year-old patient who presented with a 4-year history of predominantly proximal lower limb weakness due to a novel variant in HSPB8. The creatine kinase level was mildly elevated. Electrodiagnostic studies demonstrated a proximal-predominant myopathy without evidence of neuropathy, and muscle histopathology revealed rimmed vacuoles and myofibrillar protein aggregates. Whole exome sequencing identified a de novo frameshift variant in the C-terminal region of HSPB8 (c.577_580dupGTCA, p.Thr194Serfs*23). This case demonstrates that HSPB8-related disorders can present with early onset limb-girdle myopathy without associated neuropathy.

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

Background

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

Case presentation

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

Conclusions

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

Electronic supplementary material

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

Myofibrillar myopathy caused by a novel FHL1 mutation presenting a mild myopathy with ankle contracture

FHL1-related myopathies are clinically heterogeneous, involving skeletal and cardiac muscles. Overlapping clinical features include joint contractures, rigid spine, scapuloperoneal weakness and cardiac diseases. Histopathologically, reducing bodies are the most characteristic finding, but not present in all FHL1-related cases. Non-specific dystrophic pathology without reducing body is usual in the forms of X-linked myopathy with postural muscle atrophy, Emery-Dreifuss muscular dystrophy and isolated hypertrophic cardiomyopathy. Here, we describe a patient with mild weakness with ankle contracture. We finally concluded he has a FHL1-related myopathy at an extreme end of phenotypic spectrum of FHL1 myopathy, which one might miss to recognize as a form of myopathy. The genetic variant was detected by whole exome sequencing, and its pathogenicity was clearly confirmed with pathological and biochemical studies. This is the first FHL1 case with a mildest phenotype backed by biochemical/genetic evidence. This report will help clinicians hesitating to further evaluate mild cases to better correlate the genotype to the phenotype.

A novel mutation in the N-terminal acting-binding domain of Filamin C protein causing a distal myofibrillar myopathy

Variants in Filamin C (FLNC) gene may cause either cardiomyopathies or different myopathies. We describe a family affected by a distal myopathy with autosomal dominant inheritance. The onset of the disease was in the third decade with gait impairment due to distal leg weakness. Subsequently, the disease progressed with an involvement of proximal lower limbs and hand muscles. Muscle biopsy, performed in one subject,identified relevant myofibrillar abnormalities. We performed a target gene panel testing for myofibrillar myopathies by NGS approach which identified a novel mutation in exon 3 of FLNC gene (c.A664G:p.M222V), within the N-terminal actin-binding (ABD) domain. This variant has been identified in all affected members of the family, thus supporting its pathogenic role. Differently from previously identified variants, our family showed a predominant leg involvement and myofibrillar aggregates, thus further expanding the spectrum of Filamin C related myopathies.

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.

BAG3 mutation in a patient with atypical phenotypes of myofibrillar myopathy and Charcot-Marie-Tooth disease

Bcl2-associated athanogene 3 (BAG3) mutations have been reported to cause the myofibrillar myopathy (MFM) which shows progressive limb muscle weakness, respiratory failure, and cardiomyopathy. Myopathy patients with BAG3 mutation are very rare. We described a patient showing atypical phenotypes. We aimed to find the genetic cause of Korean patients with sensory motor polyneuropathy, myopathy and rigid spine. We performed whole exome sequencing (WES) with 423 patients with sensory motor polyneuropathy. We found BAG3 mutation in one patient with neuropathy, myopathy and rigid spine syndrome, and performed electrophysiological study, whole body MRI and muscle biopsy on the patient. A de novo heterozygous p.Pro209Leu (c.626C>T) mutation in BAG3 was identified in a female myopathy. She first noticed a gait disturbance and spinal rigidity at the age of 11, and serum creatine kinase levels were elevated ninefolds than normal. She showed an axonal sensory-motor polyneuropathy like Charcot-Marie-Tooth disease (CMT), myopathy, rigid spine and respiratory dysfunction; however, she did not show any cardiomyopathy, which is a common symptom in BAG3 mutation. Lower limb MRI and whole spine MRI showed bilateral symmetric fatty atrophy of muscles at the lower limb and paraspinal muscles. When we track traceable MRI 1 year later, the muscle damage progressed slowly. As far as our knowledge, this is the first Korean patient with BAG3 mutation. We described a BAG3 mutation patient with atypical phenotype of CMT and myopathy, and those are expected to broaden the clinical spectrum of the disease and help to diagnose it.

Dysregulated autophagy in restrictive cardiomyopathy due to Pro209Leu mutation in BAG3

Myofibrillary myopathies (MFM) are hereditary myopathies histologically characterized by degeneration of myofibrils and aggregation of proteins in striated muscle. Cardiomyopathy is common in MFM but the pathophysiological mechanisms are not well understood. The BAG3-Pro209Leu mutation is associated with early onset MFM and severe restrictive cardiomyopathy (RCM), often necessitating heart transplantation during childhood. We report on a young male patient with a BAG3-Pro209Leu mutation who underwent heart transplantation at eight years of age. Detailed morphological analyses of the explanted heart tissue showed intracytoplasmic inclusions, aggregation of BAG3 and desmin, disintegration of myofibers and Z-disk alterations. The presence of undegraded autophagosomes, seen by electron microscopy, as well as increased levels of p62, LC3-I and WIPI1, detected by immunohistochemistry and western blot analyses, indicated a dysregulation of autophagy. Parkin and PINK1, proteins involved in mitophagy, were slightly increased whereas mitochondrial OXPHOS activities were not altered. These findings indicate that altered autophagy plays a role in the pathogenesis and rapid progression of RCM in MFM caused by the BAG3-Pro209Leu mutation, which could have implications for future therapeutic strategies.

The desmin network is a determinant of the cytoplasmic stiffness of myoblasts

BACKGROUND INFORMATION

The mechanical properties of cells are essential to maintain their proper functions, and mainly rely on their cytoskeleton. A lot of attention has been paid to actin filaments, demonstrating their central role in the cells mechanical properties, but much less is known about the participation of intermediate filament (IF) networks. Indeed the contribution of IFs, such as vimentin, keratins and lamins, to cell mechanics has only been assessed recently. We study here the involvement of desmin, an IF specifically expressed in muscle cells, in the rheology of immature muscle cells. Desmin can carry mutations responsible for a class of muscle pathologies named desminopathies.

RESULTS

In this study, using three types of cell rheometers, we assess the consequences of expressing wild-type (WT) or mutated desmin on the rheological properties of single myoblasts. We find that the mechanical properties of the cell cortex are not correlated to the quantity, nor the quality of desmin expressed. On the contrary, the overall cell stiffness increases when the amount of WT or mutated desmin polymerised in cytoplasmic networks increases. However, myoblasts become softer when the desmin network is partially depleted by the formation of aggregates induced by the expression of a desmin mutant.

CONCLUSIONS

We demonstrate that desmin plays a negligible role in the mechanical properties of the cell cortex but is a determinant of the overall cell stiffness. More particularly, desmin participates to the cytoplasm viscoelasticity.

SIGNIFICANCE

Desminopathies are associated with muscular weaknesses attributed to a disorganisation of the structure of striated muscle that impairs the active force generation. The present study evidences for the first time the key role of desmin in the rheological properties of myoblasts, raising the hypothesis that desmin mutations could also alter the passive mechanical properties of muscles, thus participating to the lack of force build up in muscle tissue.

A knock-in/knock-out mouse model of HSPB8-associated distal hereditary motor neuropathy and myopathy reveals toxic gain-of-function of mutant Hspb8

Mutations in the small heat shock protein B8 gene (HSPB8/HSP22) have been associated with distal hereditary motor neuropathy, Charcot-Marie-Tooth disease, and recently distal myopathy. It is so far not clear how mutant HSPB8 induces the neuronal and muscular phenotypes and if a common pathogenesis lies behind these diseases. Growing evidence points towards a role of HSPB8 in chaperone-associated autophagy, which has been shown to be a determinant for the clearance of poly-glutamine aggregates in neurodegenerative diseases but also for the maintenance of skeletal muscle myofibrils. To test this hypothesis and better dissect the pathomechanism of mutant HSPB8, we generated a new transgenic mouse model leading to the expression of the mutant protein (knock-in lines) or the loss-of-function (functional knock-out lines) of the endogenous protein Hspb8. While the homozygous knock-in mice developed motor deficits associated with degeneration of peripheral nerves and severe muscle atrophy corroborating patient data, homozygous knock-out mice had locomotor performances equivalent to those of wild-type animals. The distal skeletal muscles of the post-symptomatic homozygous knock-in displayed Z-disk disorganisation, granulofilamentous material accumulation along with Hspb8, αB-crystallin (HSPB5/CRYAB), and desmin aggregates. The presence of the aggregates correlated with reduced markers of effective autophagy. The sciatic nerve of the homozygous knock-in mice was characterized by low autophagy potential in pre-symptomatic and Hspb8 aggregates in post-symptomatic animals. On the other hand, the sciatic nerve of the homozygous knock-out mice presented a normal morphology and their distal muscle displayed accumulation of abnormal mitochondria but intact myofiber and Z-line organisation. Our data, therefore, suggest that toxic gain-of-function of mutant Hspb8 aggregates is a major contributor to the peripheral neuropathy and the myopathy. In addition, mutant Hspb8 induces impairments in autophagy that may aggravate the phenotype.

Translocation of molecular chaperones to the titin springs is common in skeletal myopathy patients and affects sarcomere function

Myopathies encompass a wide variety of acquired and hereditary disorders. The pathomechanisms include structural and functional changes affecting, e.g., myofiber metabolism and contractile properties. In this study, we observed increased passive tension (PT) of skinned myofibers from patients with myofibrillar myopathy (MFM) caused by FLNC mutations (MFM-filaminopathy) and limb-girdle muscular dystrophy type-2A due to CAPN3 mutations (LGMD2A), compared to healthy control myofibers. Because the giant protein titin determines myofiber PT, we measured its molecular size and the titin-to-myosin ratio, but found no differences between myopathies and controls. All-titin phosphorylation and site-specific phosphorylation in the PEVK region were reduced in myopathy, which would be predicted to lower PT. Electron microscopy revealed extensive ultrastructural changes in myofibers of various hereditary myopathies and also suggested massive binding of proteins to the sarcomeric I-band region, presumably heat shock proteins (HSPs), which can translocate to elastic titin under stress conditions. Correlative immunofluorescence and immunoelectron microscopy showed that two small HSPs (HSP27 and αB-crystallin) and the ATP-dependent chaperone HSP90 translocated to the titin springs in myopathy. The small HSPs, but not HSP90, were upregulated in myopathic versus control muscles. The titin-binding pattern of chaperones was regularly observed in Duchenne muscular dystrophy (DMD), LGMD2A, MFM-filaminopathy, MFM-myotilinopathy, titinopathy, and inclusion body myopathy due to mutations in valosin-containing protein, but not in acquired sporadic inclusion body myositis. The three HSPs also associated with elastic titin in mouse models of DMD and MFM-filaminopathy. Mechanical measurements on skinned human myofibers incubated with exogenous small HSPs suggested that the elevated PT seen in myopathy is caused, in part, by chaperone-binding to the titin springs. Whereas this interaction may be protective in that it prevents sarcomeric protein aggregation, it also has detrimental effects on sarcomere function. Thus, we identified a novel pathological phenomenon common to many hereditary muscle disorders, which involves sarcomeric alterations.

A novel dominant D109A CRYAB mutation in a family with myofibrillar myopathy affects αB-crystallin structure

Myofibrillar myopathy (MFM) is a group of inherited muscular disorders characterized by myofibrils dissolution and abnormal accumulation of degradation products. So far causative mutations have been identified in nine genes encoding Z-disk proteins, including αB-crystallin (CRYAB), a small heat shock protein (also called HSPB5). Here, we report a case study of a 63-year-old Polish female with a progressive lower limb weakness and muscle biopsy suggesting a myofibrillar myopathy, and extra-muscular multisystemic involvement, including cataract and cardiomiopathy. Five members of the proband's family presented similar symptoms. Whole exome sequencing followed by bioinformatic analysis revealed a novel D109A mutation in CRYAB associated with the disease. Molecular modeling in accordance with muscle biopsy microscopic analyses predicted that D109A mutation influence both structure and function of CRYAB due to decreased stability of oligomers leading to aggregate formation. In consequence disrupted sarcomere cytoskeleton organization might lead to muscle pathology. We also suggest that mutated RQDE sequence of CRYAB could impair CRYAB chaperone-like activity and promote aggregation of lens crystallins.

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.

Mitochondrial dysfunction in myofibrillar myopathy

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Clonally expanded mtDNA deletions were found in a small number of patient fibres.

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Complex I and IV deficiency is higher than in control muscle.

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Mitochondrial mass is significantly reduced in patients relative to controls.

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No relationship between MFM protein aggregates and reduced mitochondrial mass.

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Negative correlations was detected between mitochondrial mass and muscle fibre area.

Myofibrillar myopathies (MFM) are characterised by focal myofibrillar destruction and accumulation of myofibrillar elements as protein aggregates. They are caused by mutations in the DES, MYOT, CRYAB, FLNC, BAG3, DNAJB6 and ZASP genes as well as other as yet unidentified genes. Previous studies have reported changes in mitochondrial morphology and cellular positioning, as well as clonally-expanded, large-scale mitochondrial DNA (mtDNA) deletions and focal respiratory chain deficiency in muscle of MFM patients. Here we examine skeletal muscle from patients with desmin (n = 6), ZASP (n = 1) and myotilin (n = 2) mutations and MFM protein aggregates, to understand how mitochondrial dysfunction may contribute to the underlying mechanisms causing disease pathology. We have used a validated quantitative immunofluorescent assay to study respiratory chain protein levels, together with oxidative enzyme histochemistry and single cell mitochondrial DNA analysis, to examine mitochondrial changes. Results demonstrate a small number of clonally-expanded mitochondrial DNA deletions, which we conclude are due to both ageing and disease pathology. Further to this we report higher levels of respiratory chain complex I and IV deficiency compared to age matched controls, although overall levels of respiratory deficient muscle fibres in patient biopsies are low. More strikingly, a significantly higher percentage of myofibrillar myopathy patient muscle fibres have a low mitochondrial mass compared to controls. We concluded this is mechanistically unrelated to desmin and myotilin protein aggregates; however, correlation between mitochondrial mass and muscle fibre area is found. We suggest this may be due to reduced mitochondrial biogenesis in combination with muscle fibre hypertrophy.

In vivo characterization of human myofibrillar myopathy genes in zebrafish

Myofibrillar myopathies (MFM) are progressive diseases of human heart and skeletal muscle with a severe impact on life quality and expectancy of affected patients. Although recently several disease genes for myofibrillar myopathies could be identified, today most genetic causes and particularly the associated mechanisms and signaling events that lead from the mutation to the disease phenotype are still mostly unknown. To assess whether the zebrafish is a suitable model system to validate MFM candidate genes using targeted antisense-mediated knock-down strategies, we here specifically inactivated known human MFM disease genes and evaluated the resulting muscular and cardiac phenotypes functionally and structurally. Consistently, targeted ablation of MFM genes in zebrafish led to compromised skeletal muscle function mostly due to myofibrillar degeneration as well as severe heart failure. Similar to what was shown in MFM patients, MFM gene-deficient zebrafish showed pronounced gene-specific phenotypic and structural differences. In summary, our results indicate that the zebrafish is a suitable model to functionally and structurally evaluate novel MFM disease genes in vivo.

BAG3 myofibrillar myopathy presenting with cardiomyopathy

Myofibrillar myopathies (MFMs) are a heterogeneous group of neuromuscular disorders distinguished by the pathological hallmark of myofibrillar dissolution. Most patients present in adulthood, but mutations in several genes including BCL2-associated athanogene 3 (BAG3) cause predominantly childhood-onset disease. BAG3-related MFM is particularly severe, featuring weakness, cardiomyopathy, neuropathy, and early lethality. While prior cases reported either neuromuscular weakness or concurrent weakness and cardiomyopathy at onset, we describe the first case in which cardiomyopathy and cardiac transplantation (age eight) preceded neuromuscular weakness by several years (age 12). The phenotype comprised distal weakness and severe sensorimotor neuropathy. Nerve biopsy was primarily axonal with secondary demyelinating/remyelinating changes without "giant axons." Muscle biopsy showed extensive neuropathic changes that made myopathic changes difficult to interpret. Similar to previous cases, a p.Pro209Leu mutation in exon 3 of BAG3 was found. This case underlines the importance of evaluating for MFMs in patients with combined neuromuscular weakness and cardiomyopathy.

Targeted sequencing and identification of genetic variants in sporadic inclusion body myositis

Sporadic inclusion body myositis (sIBM) has clinical, pathologic and pathomechanistic overlap with some inherited muscle and neurodegenerative disorders. In this study, DNA from 79 patients with sIBM was collected and the sequencing of 38 genes associated with hereditary inclusion body myopathy (IBM), myofibrillar myopathy, Emery-Dreifuss muscular dystrophy, distal myopathy, amyotrophic lateral sclerosis and dementia along with C9orf72 hexanucleotide repeat analysis was performed. No C9orf72 repeat expansions were identified, but; 27 rare (minor allele frequency <1%) missense coding variants in several other genes were identified. One patient carried a p.R95C missense mutation in VCP and another carried a previously reported p.I27V missense mutation in VCP. Mutations in VCP cause IBM associated with Paget's disease of the bone (PDB) and fronto-temporal dementia (IBMPFD). Neither patient had a family history of weakness or manifested other symptoms reported with VCP mutations such as PDB or dementia. In vitro analysis of these VCP variants found that they both disrupted autophagy similar to other pathogenic mutations. Although no clear genetic etiology has been implicated in sIBM pathogenesis, our study suggests that genetic evaluation in sIBM may be clinically meaningful and lend insight into its pathomechanism.

Autophagic vacuolar pathology in desminopathies

Autophagic vacuolar myopathies are an emerging group of muscle diseases with common pathologic features. These include autophagic vacuoles containing both lysosomal and autophagosomal proteins sometimes lined with sarcolemmal proteins such as dystrophin. These features have been most clearly described in patients with Danon's disease due to LAMP2 deficiency and X-linked myopathy with excessive autophagy (XMEA) due to mutations in VMA21. Disruptions of these proteins lead to lysosomal dysfunction and subsequent autophagic vacuolar pathology. We performed whole exome sequencing on two families with autosomal dominantly inherited myopathies with autophagic vacuolar pathology and surprisingly identified a p.R454W tail domain mutation and a novel p.S6W head domain mutation in desmin, DES. In addition, re-evaluation of muscle tissue from another family with a novel p.I402N missense DES mutation also identified autophagic vacuoles. We suggest that autophagic vacuoles may be an underappreciated pathology present in desminopathy patient muscle. Moreover, autophagic vacuolar pathology can be due to genetic etiologies unrelated to primary defects in the lysosomes or autophagic machinery. Specifically, cytoskeletal derangement and the accumulation of aggregated proteins such as desmin may activate the autophagic system leading to the pathologic features of an autophagic vacuolar myopathy.

Distal myopathies

In this article, distal myopathy syndromes are discussed. A discussion of the more traditional distal myopathies is followed by discussion of the myofibrillar myopathies. Other clinically and genetically distinctive distal myopathy syndromes usually based on single or smaller family cohorts are reviewed. Other neuromuscular disorders that are important to recognize are also considered, because they show prominent distal limb weakness.

A new disease allele for the p.C30071R mutation in titin causing hereditary myopathy with early respiratory failure

Hereditary myopathy with early respiratory failure is an autosomal dominant myopathy caused by mutations in the 119th fibronectin-3 domain of titin. To date all reported patients with the most common mutation in this domain (p.C30071R) appear to share ancestral disease alleles. We undertook this study of two families with the p.C30071R mutation to determine whether they share the same haplotype as previously reported British families or whether the mutation arose as a de novo event. We sequenced the 119th fibronectin-3 domain in these two probands and flanking polymorphisms associated with the British haplotype in hereditary myopathy with early respiratory failure. A family of Indian descent had a haplotype that was not compatible with the British shared haplotype. Cloning of the 119th fibronectin-3 domain in this patient demonstrated polymorphisms rs191484894 and novel noncoding variant c.90225C>T on the same allele as the mutation, which is distinct from previously reported British families. This proves that the p.C30071R mutation itself (rather than the haplotype containing this mutation) causes hereditary myopathy with early respiratory failure and suggests its independent origin in different ethnic groups.

Differential proteomic analysis of abnormal intramyoplasmic aggregates in desminopathy

Desminopathy is a subtype of myofibrillar myopathy caused by desmin mutations and characterized by protein aggregates accumulating in muscle fibers. The aim of this study was to assess the protein composition of these aggregates. Aggregates and intact myofiber sections were obtained from skeletal muscle biopsies of five desminopathy patients by laser microdissection and analyzed by a label-free spectral count-based proteomic approach. We identified 397 proteins with 22 showing significantly higher spectral indices in aggregates (ratio >1.8, p<0.05). Fifteen of these proteins not previously reported as specific aggregate components provide new insights regarding pathomechanisms of desminopathy. Results of proteomic analysis were supported by immunolocalization studies and parallel reaction monitoring. Three mutant desmin variants were detected directly on the protein level as components of the aggregates, suggesting their direct involvement in aggregate-formation and demonstrating for the first time that proteomic analysis can be used for direct identification of a disease-causing mutation in myofibrillar myopathy. Comparison of the proteomic results in desminopathy with our previous analysis of aggregate composition in filaminopathy, another myofibrillar myopathy subtype, allows to determine subtype-specific proteomic profile that facilitates identification of the specific disorder.

BIOLOGICAL SIGNIFICANCE

Our proteomic analysis provides essential new insights in the composition of pathological protein aggregates in skeletal muscle fibers of desminopathy patients. The results contribute to a better understanding of pathomechanisms in myofibrillar myopathies and provide the basis for hypothesis-driven studies. The detection of specific proteomic profiles in different myofibrillar myopathy subtypes indicates that proteomic analysis may become a useful tool in differential diagnosis of protein aggregate myopathies.