Myositis non-inflammatory mechanisms: An up-dated review.

Idiopathic inflammatory myopathies (IIM) represent a heterogeneous group of rare muscular diseases, with no clearly known causes. IIM frequently have an incomplete response to treatment due to the difficulty in distinguishing between IIM forms, and due to neglect their non-inflammatory causes. Important data concerning non-immune mechanisms in IIM pathology have been recently accumulated. There is a correlation between inflammatory and non-inflammatory mechanisms, but their involvement in IIM pathogenesis is still unknown. Here we review some of the most important data regarding the non-immune IIM pathology, highlighting possible future therapeutic targets: endoplasmic reticulum stress, ATP metabolism, ROS generation, autophagy, and microRNAs disturbances.

Human NCL Neuropathology.

The neuronal ceroid lipofuscinoses (NCL) currently encompass fourteen genetically different forms, CLN1 to CLN14, but are all morphologically marked by loss of nerve cells, particularly in the cerebral and cerebellar cortices, and the cerebral and extracerebral formation of lipopigments. These lipopigments show distinct ultrastructural patterns, i.e., granular, curvilinear/rectilinear and fingerprint profiles. They contain-although to a different degree among the different CLN forms-subunit C of ATP synthase, saposins A and D, and beta-amyloid proteins. Extracerebral pathology, apart from lipopigment formation, which provides diagnostic information, is scant or non-existent. The retina undergoes atrophy in all childhood forms. While many new data and findings have been obtained by immunohistochemistry in mouse and other animal models, similar findings in human NCL are largely missing, thus recommending respective studies of archived brain tissues. The newly described NCL forms, i.e., CLN 10 to CLN 14, also require further studies to provide complete neuropathology. This article is part of a Special Issue entitled: "Current Research on the Neuronal Ceroid Lipofuscinoses (Batten Disease)".

De novo prion aggregates trigger autophagy in skeletal muscle.

In certain sporadic, familial, and infectious prion diseases, the prion protein misfolds and aggregates in skeletal muscle in addition to the brain and spinal cord. In myocytes, prion aggregates accumulate intracellularly, yet little is known about clearance pathways. Here we investigated the clearance of prion aggregates in muscle of transgenic mice that develop prion disease de novo. In addition to neurodegeneration, aged mice developed a degenerative myopathy, with scattered myocytes containing ubiquitinated, intracellular prion inclusions that were adjacent to myocytes lacking inclusions. Myocytes also showed elevated levels of the endoplasmic reticulum chaperone Grp78/BiP, suggestive of impaired protein degradation and endoplasmic reticulum stress. Additionally, autophagy was induced, as indicated by increased levels of beclin-1 and LC3-II. In C2C12 myoblasts, inhibition of autophagosome maturation or lysosomal degradation led to enhanced prion aggregation, consistent with a role for autophagy in prion aggregate clearance. Taken together, these findings suggest that the induction of autophagy may be a central strategy for prion aggregate clearance in myocytes. IMPORTANCE In prion diseases, the prion protein misfolds and aggregates in the central nervous system and sometimes in other organs, including muscle, yet the cellular pathways of prion aggregate clearance are unclear. Here we investigated the clearance of prion aggregates in the muscle of a transgenic mouse model that develops profound muscle degeneration. We found that endoplasmic reticulum stress pathways were activated and that autophagy was induced. Blocking of autophagic degradation in cell culture models led to an accumulation of aggregated prion protein. Collectively, these findings suggest that autophagy has an instrumental role in prion protein clearance.

Th2-M2 immunity in lesions of muscular sarcoidosis and macrophagic myofasciitis.

OBJECTIVE: To analyse the paradox of a lack of giant cell formation and fibrosis in chronic lesions of macrophagic myofasciitis (MMF) in comparison with muscular sarcoidosis (MuS). METHODS: Inflammatory lesions and contiguous muscle regions from biopsy samples of 10 patients with MuS and 10 patients with MMF were cut out by laser microdissection. Mediators of the T helper cell (Th)1 inducing classical macrophage activation (e.g. STAT1, IFNγ and CXCR3), and Th2 inducing alternative activation of macrophages (e.g. CD206/MRC1, STAT6, SOCS1), molecules involved in development of fibrosis (e.g. TGFß) and giant cells (e.g. TYROBP), were assessed by immunohistochemistry and real-time polymerase chain reaction (PCR). RESULTS: STAT6-induced Th2 immunity was associated with up-regulated gene expression of MRC1, SOCS1 and TGFB in inflammatory foci, in comparison with adjacent tissue. TYROBP and TREM2, genes regulating giant cell formation, were more strongly expressed in lesions of MuS patients than in those of MMF. TGFß co-localized with CD206(+) macrophages in MuS but not in MMF. Conversely, Th1 immunity was illustrated by STAT1 staining both in macrophages and myofibres in MuS, but not in MMF. Also, STAT1-induced IFNG and CXCR3 expression in lesions and the surrounding tissue was elevated compared with normal controls, but without statistically significant differences. CONCLUSION: Giant cell and typical granuloma formations, including fibrogenesis, is dependent on two main mechanisms, both involving specific macrophage activation: a strong Th2-M2 polarization and a significant expression of TYROBP and TGFß in macrophages. The low-grade alternative activation of macrophages in MMF lesions and poor TYROBP and TGFßco-expression are obviously insufficient to produce giant cells.

The neuronal ceroid-lipofuscinoses: a historical introduction.

The neuronal ceroid-lipofuscinoses (Batten disease) collectively constitute one of the most common groups of inherited childhood onset neurodegenerative disorders, and have also been identified in many domestic and laboratory animals. The group of human neuronal ceroid-lipofuscinoses currently comprises 14 genetically distinct disorders, mostly characterised by progressive mental, motor and visual deterioration with onset in childhood or adolescence. Abnormal autofluorescent, electron-dense granules accumulate in the cytoplasm of nerve cells, and this storage process is associated with selective destruction and loss of neurons in the brain and retina. The present paper outlines nearly 200 years of clinical, neuropathological, biochemical and molecular genetic research, gradually leading, since 1995, to the identification of 13 different genes and over 360 mutations that underlie these devastating brain disorders and form the basis of a new classification system. These genes are evidently of vital importance for the normal development and maintenance of cerebral neurons. Elucidation of their functions and interactions in health and disease is a prerequisite for the identification of possible therapeutic targets, but may also further our understanding of the basic mechanisms of neurodegeneration and ageing. An account is also given of the development of international cooperation and free access electronic resources facilitating NCL research. This article is part of a Special Issue entitled: The Neuronal Ceroid Lipofuscinoses or Batten Disease.

Human pathology in NCL.

In childhood the neuronal ceroid lipofuscinoses (NCL) are the most frequent lysosomal diseases and the most frequent neurodegenerative diseases but, in adulthood, they represent a small fraction among the neurodegenerative diseases. Their morphology is marked by: (i) loss of neurons, foremost in the cerebral and cerebellar cortices resulting in cerebral and cerebellar atrophy; (ii) an almost ubiquitous accumulation of lipopigments in nerve cells, but also in extracerebral tissues. Loss of cortical neurons is selective, indiscriminate depletion in early childhood forms occurring only at an advanced stage, whereas loss of neurons in subcortical grey-matter regions has not been quantitatively documented. Among the fourteen different forms of NCL described to date, CLN1 and CLN10 are marked by granular lipopigments, CLN2 by curvilinear profiles (CVPs), CLN3 by fingerprint profiles (FPPs), and other forms by a combination of these features. Among extracerebral tissues, lymphocytes, skin, rectum, skeletal muscle and, occasionally, conjunctiva are possible guiding targets for diagnostic identification, the precise type of NCL then requiring molecular analysis within the clinical and morphological context. Autosomal-recessive adult NCL has been linked molecularly to different childhood forms, i.e. CLN1, CLN5, and CLN6, whilst autosomal-dominant adult NCL, now designated as CLN4, is caused by a newly identified separate gene, DNAJC5. This article is part of a Special Issue entitled: The Neuronal Ceroid Lipofuscinoses or Batten Disease.

SIL1 mutations and clinical spectrum in patients with Marinesco-Sjogren syndrome.

Marinesco-Sjögren syndrome is a rare autosomal recessive multisystem disorder featuring cerebellar ataxia, early-onset cataracts, chronic myopathy, variable intellectual disability and delayed motor development. More recently, mutations in the SIL1 gene, which encodes an endoplasmic reticulum resident co-chaperone, were identified as the main cause of Marinesco-Sjögren syndrome. Here we describe the results of SIL1 mutation analysis in 62 patients presenting with early-onset ataxia, cataracts and myopathy or combinations of at least two of these. We obtained a mutation detection rate of 60% (15/25) among patients with the characteristic Marinesco-Sjögren syndrome triad (ataxia, cataracts, myopathy) whereas the detection rate in the group of patients with more variable phenotypic presentation was below 3% (1/37). We report 16 unrelated families with a total of 19 different SIL1 mutations. Among these mutations are 15 previously unreported changes, including single- and multi-exon deletions. Based on data from our screening cohort and data compiled from the literature we found that SIL1 mutations are invariably associated with the combination of a cerebellar syndrome and chronic myopathy. Cataracts were observed in all patients beyond the age of 7 years, but might be missing in infants. Six patients with SIL1 mutations had no intellectual disability, extending the known wide range of cognitive capabilities in Marinesco-Sjögren syndrome to include normal intelligence. Modestly constant features were somatic growth retardation, skeletal abnormalities and pyramidal tract signs. Examination of mutant SIL1 expression in cultured patient lymphoblasts suggested that SIL1 mutations result in severely reduced SIL1 protein levels irrespective of the type and position of mutations. Our data broaden the SIL1 mutation spectrum and confirm that SIL1 is the major Marinesco-Sjögren syndrome gene. SIL1 patients usually present with the characteristic triad but cataracts might be missing in young children. As cognitive impairment is not obligatory, patients without intellectual disability but a Marinesco-Sjögren syndrome-compatible phenotype should receive SIL1 mutation analysis. Despite allelic heterogeneity and many families with private mutations, the phenotype related to SIL1 mutations is relatively homogenous. Based on SIL1 expression studies we speculate that this may arise from a uniform effect of different mutations on protein expression.

Immune-mediated necrotizing myopathy is characterized by a specific Th1-M1 polarized immune profile.

Immune-mediated necrotizing myopathy (IMNM) is considered one of the idiopathic inflammatory myopathies, comprising dermatomyositis, polymyositis, and inclusion body myositis. The heterogeneous group of necrotizing myopathies shows a varying amount of necrotic muscle fibers, myophagocytosis, and a sparse inflammatory infiltrate. The underlying immune response in necrotizing myopathy has not yet been addressed in detail. Affected muscle tissue, obtained from 16 patients with IMNM, was analyzed compared with eight non-IMNM (nIMNM) tissues. Inflammatory cells were characterized by IHC, and immune mediators were assessed by quantitative real-time PCR. We demonstrate that immune- and non-immune-mediated disease can be distinguished by a specific immune profile with significantly more prominent major histocompatibility complex class I expression and complement deposition and a conspicuous inflammatory infiltrate. In addition, patients with IMNM exhibit a strong type 1 helper T cell (T1)/classically activated macrophage M1 response, with detection of elevated interferon-γ, tumor necrosis factor-α, IL-12, and STAT1 levels in the muscle tissue, which may serve as biomarkers and aid in diagnostic decisions. Furthermore, B cells and high expression of the chemoattractant CXCL13 were identified in a subgroup of patients with defined autoantibodies. Taken together, we propose a diagnostic armamentarium that allows for clear differentiation between IMNM and nIMNM. In addition, we have characterized a Th1-driven, M1-mediated immune response in most of the autoimmune necrotizing myopathies, which may guide therapeutic options in the future.

Practical application of electron microscopy to neuromuscular diseases.

Concerning individual neuromuscular conditions, electron microscopy may be considered "essential," "helpful," or "wasteful." "Essential" electron microscopy should provide a clear diagnosis, because of the disease specificity of the ultrastructural findings, in particular as to inclusions within muscle fibers, such as cylindrical spirals and reducing bodies. Electron microscopy may be "helpful" in detecting ultrastructural features preceding typical light microscopic findings, for instance, undulating tubules in endothelial cells. Congenital, metabolic, and inflammatory myopathies may often be more easily and more reliably diagnosed by means of the electron microscope. Diagnostically "wasteful" electron microscopy may pertain to muscular dystrophies, neurogenic atrophy, and myotonic diseases.

Storage diseases: diagnostic position.

Storage diseases are metabolic multiorgan conditions, which may be divided into lysosomal and nonlysosomal diseases. Disorders of the lysosomal type require electron microscopy for morphological diagnosis. It is the metabolic substrate that determines involvement of the cell type or organ in the individual storage disease, allowing extracerebral biopsies, for instance, in the neuronal ceroid-lipofuscinoses (NCL). A hierarchy of tissues biopsied for diagnosis can be based on easy accessibility: blood lymphocytes, skin, conjunctiva, rectum, skeletal muscle. Lysosomal diseases are divided into vacuolar and nonvacuolar ones. NCL display variegated ultrastructural patterns. Drugs may induce lysosomal storage. Finally, polyglucosan body diseases require attention.

Ultrastructural myopathology in the molecular era.

Electron microscopy is an essential component of myopathology, both in diagnostics and research of neuromuscular diseases. Although recently reduced in the diagnostic armamentarium, it has greatly been expanded to mouse models in research. Mostly it is descriptive, but a few additional techniques in combination with transmission electron microscopy have been employed. Foremost among them is immunoelectron microscopy, which assists in guiding molecular analysis in hereditary conditions, but may be vital in diagnostics of certain acquired entities, e.g., undulating tubules in dermatomyositis and in those congenital myopathies where genes and mutations remain to be identified, as in cylindrical spirals myopathy and hexagonal crystalloid-body myopathy.

A brief history of the congenital myopathies - the myopathological perspective.

Congenital myopathies are defined by early clinical onset, slow progression, hereditary nature and disease-specific myopathological lesions - however, with exceptions - demanding special techniques in regard to morphological diagnostic and research work-up. To identify an index disease in a family requires a muscle biopsy - and no congenital myopathy has ever been first described at autopsy. The nosographic history commenced when - in addition to special histopathological techniques in the earliest classical triad of central core disease, 1956, nemaline myopathy, 1963, and centronuclear myopathy, 1966/67, within a decade - electron microscopy and enzyme histochemistry were applied to unfixed frozen muscle tissue and, thus, revolutionized diagnostic and research myopathology. During the following years, the list of structure-defined congenital myopathies grew to some 40 conditions. Then, the introduction of immunohistochemistry allowed myopathological documentation of proteins and their abnormalities in individual congenital myopathies. Together with the diagnostic evolution of molecular genetics, many more congenital myopathies were described, without new disease-specific lesions or only already known ones. These were nosographically defined by individual mutations in hitherto congenital myopathies-unrelated genes. This latter development may also affect the nomenclature of congenital myopathies in that the mutant gene needs to be attached to the individually identified congenital myopathies with or without the disease-specific lesion, such as CCD-RYR1 or CM-RYR1. This principle is similar to that of the nomenclature of Congenital Disorders of Glycosylation. Retroactive molecular characterization of originally and first described congenital myopathies has only rarely been achieved.

Pathogenic variants in three families with distal muscle involvement.

Three families suspected of distal hereditary motor neuropathy underwent genetic screening with the aim to identify the molecular defect underlying the disease. The description of the identification reflects the shift in molecular diagnostics that was made during the last decades. Our candidate gene approach yielded a known pathogenic variant in BSCL2 (p.Asn88Ser) in one family, and via a CMT-capture, in HSPB1 (p.Arg127Trp), in addition to five other variations in Charcot-Marie-Tooth-related genes in the proband of the second family. In the third family, using whole exome sequencing, followed by linkage-by-location, a three base pair deletion in exon 33 of MYH7 (p.Glu1508del) was found, a reported pathogenic allele albeit for a myopathy. After identification of the causative molecular defect, cardiac examination was performed for patients of the third family and this demonstrated abnormalities in three out of five affected family members. Heterogeneity and expansion of clinical phenotypes beyond known characteristics requires a wider set of genes to be screened. Whole exome/genome analysis with limited prior clinical information may therefore be used to precede a detailed clinical evaluation in cases of large families, preventing screening of a too narrow set of genes, and enabling the identification of novel disease-associated genes. In our cases, the variants had been reported, and co-segregation analysis confirmed the molecular diagnosis.

M2 polarized macrophages and giant cells contribute to myofibrosis in neuromuscular sarcoidosis.

The etiopathogenesis of sarcoidosis, a systemic granulomatous disease, still remains obscure. A multitude of organs have been described to be affected in systemic sarcoidosis. Skeletal muscles may also be affected, leading to myalgia and weakness. A workup of the specific immune response with emphasis on the macrophage response is provided herein. Affected muscle tissue from seven patients with systemic sarcoidosis was analyzed and compared with that from seven patients with other myopathies containing macrophagocytic infiltration. Monocytes/macrophages and giant cells in granulomas of muscle tissue from patients with sarcoidosis show a status of alternative activation (M2) based on their expression of CD206, CD301, arginase-1, and suppressor of cytokine signaling-1 as a consequence of a functionally type 2 helper T cell (Th2)-biased cytokine profile. Significant fibrosis and up-regulation of CCL18 were associated with the M2 phenotype of macrophages. Conversely, up-regulated Th1 cytokines did not result in significant classical activation of macrophages (M1), with poor inducible nitric oxide synthase and cyclooxygenase-2 production. Giant cell formation was further associated with up-regulated expression of DNAX-activating protein of 12 kDa (DAP12; gene symbol TYROBP). Functionally, alternative activation of macrophages on the basis of a Th2-biased immune response may induce clinical symptoms and chronic evolution of neuromuscular sarcoidosis. This is the first characterization of Th2-mediated immune mechanisms in neuromuscular sarcoidosis suggesting that a specific macrophage activation status leading to myofibrosis may be a key event in the pathogenesis of this disease.

Myofibrillar disorganization characterizes myopathy of camptocormia in Parkinson's disease.

Camptocormia is a highly disabling syndrome that occurs in various diseases but is particularly associated with Parkinson's disease (PD). Although first described nearly 200 years ago, the morphological changes associated with camptocormia are still under debate and the pathophysiology is unknown. We analyzed paraspinal muscle biopsies of 14 PD patients with camptocormia and compared the findings to sex-matched postmortem controls of comparable age to exclude biopsy site-specific changes. Camptocormia in PD showed a consistent lesion pattern composed of myopathic changes with type-1 fiber hypertrophy, loss of type-2 fibers, loss of oxidative enzyme activity, and acid phosphatase reactivity of lesions. Ultrastructurally, myofibrillar disorganization and Z-band streaming up to electron-dense patches/plaques were seen in the lesions. No aberrant protein aggregation, signs of myositis or mitochondriopathy were found, but the mitochondrial content of paraspinal muscles in patients and controls was markedly higher than known from limb biopsies. Additionally, we were able to demonstrate a link between the severity of the clinical syndrome and the degree of the myopathic changes. Because of the consistent lesion pattern, we propose criteria for the diagnosis of camptocormia in PD from muscle biopsies. The morphological changes show obvious parallels to the muscle pathology of experimental tenotomy reported in the 1970s, which depend on an intact innervation and do not occur after interruption of the myotactic reflexes. A dysregulation of the proprioception could be part of the pathogenesis of camptocormia in Parkinson's disease, particularly in view of the clinical symptoms of rigidity and loss of muscle strength.

Congenital myopathies: The current status.

Within the history of neuromuscular diseases (NMD), congenital myopathies (CM) represent a relatively new category introduced in the mid-nineteen hundreds upon advent and subsequent application of enzyme histochemistry and electron microscopy by establishing the three major CM, central core disease, nemaline myopathy, and centronuclear myopathy which later pluralized each when the molecular era began at the end of last century. Quickly, during the following 5 decades, many new CM entities were described, based on muscle biopsies and their CM-characteristic myopathology, the former a prerequisite to recognizing an individual CM, the latter of the nosological hallmark of the individual CM. When the molecular era ushered in immunohistochemistry the spectrum and nosography of CM altered in that some CM became allelic to other cohorts of NMD, e.g., congenital muscular dystrophies, other muscular dystrophies, distal myopathies based on different or identical mutations in the same gene. The nosological spectrum of a defective gene also enlarged by recognizing several entities with mutations in the same gene, and same or similar nosological conditions originated from mutations in different genes. Lately, however, CM were reported which lacked any individual myopathological hallmarks, but were clearly based on molecular defects, a fair number of them being newly identified ones. Few CM still remain without any molecular clarification. This nosographic development rendered the original definition of such new CM questionable and brought uncertainty to their classification and nomenclature.

Morphologic and Molecular Patterns of Polymyositis With Mitochondrial Pathology and Inclusion Body Myositis.

BACKGROUND AND OBJECTIVE: To characterize morphological and molecular underpinnings of polymyositis with mitochondrial pathology (PM-Mito) in comparison with sporadic inclusion body myositis (IBM) and to define common and distinct pathophysiologic features with a focus on interferon (IFN)-associated inflammation and T-cell response. METHODS: In this cross-sectional study, skeletal muscle biopsy samples and clinical and laboratory data from patients with PM-Mito and IBM were analyzed at Charité university hospital in Berlin, Germany. All available PM-Mito biopsy samples, an equal number of randomly selected IBM biopsy samples, and randomly selected nondiseased controls (NDCs) were included in the study. Biopsy samples were studied by histopathology, immunohistochemistry, and quantitative PCR (qPCR) and compared with biopsies derived from NDCs. Primary outcomes included cell counts for immunohistochemistry and gene expression (fold-change values compared with those in NDCs) for qPCR. RESULTS: Twenty-five skeletal muscle biopsy samples of patients with PM-Mito and IBM were included in the study and compared with 5 biopsy samples from NDCs. PM-Mito and IBM qualitatively harbored a strikingly similar molecular signature and shared important histopathologic features. Expression of IFN-induced guanylate-binding protein (GBP)6 and T-cell function-related KLRG1 distinguished IBM from PM-Mito biopsies with IBM patients showing significantly higher expression of GBP6 and KLRG1. Cryptic exon expression was detected in both patient groups with IBM patients showing higher expression levels. Skeletal muscle biopsies from IBM patients showed significantly more GBP6+ cells and KLRG1+ lymphocytes in comparison with biopsies from patients with PM-Mito. CD45+, CD68+, CD57+, PD1+, and CD8+ cytotoxic T cells were also significantly more abundant in patients with IBM. Clinically, patients with PM-Mito presented with a spectrum of muscle-related symptoms including myalgia, proximal paraparesis, proximal tetraparesis, and incomplete IBM-like patterns. Thirteen of 14 (93%) patients with PM-Mito for whom clinical follow-up was available later developed clinically defined IBM. Notably, 2 follow-up biopsies obtained 5 and 7 years after the first ones were available in this cohort, both showing histopathologic progress to net IBM including GBP6 and KLRG1 upregulation. DISCUSSION: Our combined data suggest that specific IFN-mediated inflammation plays a key role in both IBM and PM-Mito. GBP6 was identified as a new molecule of type II IFN-induced inflammation distinguishing IBM from PM-Mito. Skeletal muscles from both groups harbor dysfunctional T cells of similar type, albeit in different quantity. T-cell senescence exemplified by KLRG1 positivity does not play a significant role in PM-Mito. Based on these findings, we propose to include PM-Mito in the spectrum of IBM (IBM-spectrum disease [IBM-SD]) as a possible early form of this disease. The establishment of IBM-SD as a larger entity could potentially have a significant effect on the design of trials and therapeutic interventions.

Congenital myopathy and epidermolysis bullosa due to PLEC variant.

We report on an adult Turkish patient with mild myopathy with a fiber-type disproportion and mitochondrial disorganization caused by genetic variants in the plectin gene (PLEC). Molecular genetic panel testing revealed two homozygous variants in PLEC (NM_000445.4): c.8306C>G (p.Pro2769Arg) and c.7506 + 5C>G (p. ?) that were classified as variants of unknown significance (class 3) following ACMG guidelines for variant classification in genetic diagnostics. A thorough reassessment of the patient revealed mild skin blistering (epidermolysis bullosa simplex, EBS). This illustrates the importance of deep phenotyping of neuromuscular patients.

A novel mutation in NEB causing foetal nemaline myopathy with arthrogryposis during early gestation.

Nemaline myopathies are a clinically and genetically heterogeneous group of congenital myopathies, mainly characterized by muscle weakness, hypotonia and respiratory insufficiency. Here, we report a male foetus of consanguineous parents with a severe congenital syndrome characterized by arthrogryposis detected at 13 weeks of gestation. We describe severe complex dysmorphic facial and musculoskeletal features by post mortem fetal examination confirming the prenatal diagnosis. Histomorphological and ultrastructural studies of skeletal muscle reveal mini-rods in myotubes caused by a novel homozygous splice-site mutation in NEB (NM_001164508, chr2:g.152,417,623C>A GRCh37.p11 | c.19,102-1G>T ENST00000397345.3). No rods were seen in the myocardium. We discuss the relevance of this mutation in the context of nemaline myopathies associated with early developmental musculoskeletal disorders.

How much mutant protein is needed to cause a protein aggregate myopathy in vivo? Lessons from an exceptional desminopathy.

Myofibrillar myopathies are caused by mutations in desmin, alphaB-crystallin, myotilin, ZASP, and filamin C genes. Since the vast majority of myofibrillar myopathy causing mutations are heterozygous single amino acid substitutions or small in-frame deletions, the pathogenic role of mutant versus wild-type protein cannot be assessed in human skeletal muscle by standard immunodetection techniques. We report on an exceptional desminopathy due to a heterozygous c.735G>C mutation. Immunoblotting detected full-length 53 kDa desmin and a truncated 50 kDa variant in skeletal muscle from three affected patients of two different families. RT-PCR identified three desmin mRNA species encoding for wild-type and two mutant proteins, p.Glu245Asp and p.Asp214_Glu245del. Since previous functional studies on the p.Glu245Asp mutant showed biological properties identical to wild-type desmin, the truncated p.Asp214_Glu245del desmin is the disease-causing mutant. Semiquantitative RT-PCR established a fraction of the truncated desmin mRNA species in a range from 24% to 37%. Initial quantification of corresponding desmin proteins in the muscle biopsy of the index patient of one family indicated a fraction of only 10% of the truncated species. However, serial analyses of different sections from each muscle biopsy revealed a high intra- and interindividual variability of the truncated desmin protein level within a range from 5% to 43%. Desmin assembly studies in vitro have established clear-cut pathogenic ratios of mutant versus wild-type proteins. However, our findings point out a far more complex situation in human skeletal muscle. The heterogeneously distributed mutation load within and between individual specimens, which reflects local differences in the expression and/or turnover of the mutant protein in different areas containing multiple myonuclear domains, renders it impossible to define an exact pathogenic threshold of a specific mutant in vivo.