Mutations in myotilin cause myofibrillar myopathy

Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, MN 55905, USA.
Neurology (Impact Factor: 8.29). 05/2004; 62(8):1363-71. DOI: 10.1212/01.WNL.0000123576.74801.75
Source: PubMed


The term myofibrillar myopathy (MFM) is a noncommittal term for a pathologic pattern of myofibrillar dissolution associated with accumulation of myofibrillar degradation products and ectopic expression of multiple proteins. Ultrastructural studies implicate the Z-disk as the site of the initial pathologic change, and mutations in two Z-disk-related proteins, desmin and alphaB-crystallin, have been identified in a minority of patients with MFM. The authors' objective was to determine whether mutations in myotilin, a key Z-disk component and the disease protein in limb-girdle muscular dystrophy (LGMD) 1A, are another cause of MFM.
The authors used histochemical, immunocytochemical, ultrastructural, and mutation analysis.
The authors detected four missense mutations in 6 of 57 patients with MFM in the serine-rich exon 2 of MYOT, where the two previously identified LGMD1A mutations are located. Three mutations were novel, and one had been previously identified in LGMD1A. Each patient had evidence for neuropathy, and at least three kinships had associated cardiomyopathy. Distal weakness greater than proximal weakness was present in three patients. Except for minor differences, the morphologic features were similar to those in other patients with MFM.
1) Mutations in myotilin cause MFM; 2) exon 2 of MYOT is a hotspot for mutations; 3) peripheral neuropathy, cardiomyopathy, and distal weakness greater than proximal weakness are part of the spectrum of myotilinopathy; 4) not all cases of myotilinopathy have a limb-girdle phenotype; and 5) the molecular basis of the majority of MFM cases remains to be discovered.

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    • "Myotilin is a z-disc protein that binds F-actin directly and bundles actin filaments in vitro [50]. However, despite the fact that mutations in the myotilin gene have been implicated in limb girdle muscular dystrophy 1A (LGMD1A) [51], myofibrillar myopathy (MFM) [52], and in a rare condition called spheroid body myopathy (SBM) [53], the function of myotilin in normal muscle physiology remains unclear. Annexins are structurally related calcium dependent phospholipid binding proteins with ability to promote contact between vesicle membranes [54]. "
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    ABSTRACT: The myotendinous junction is a specialized structure of the muscle fibre enriched in mechanosensing complexes, including costameric proteins and core elements of the z-disc. Here, laser capture microdissection was applied to purify membrane regions from the myotendinous junctions of mouse skeletal muscles, which were then processed for proteomic analysis. Sarcolemma sections from the longitudinal axis of the muscle fibre were used as control for the specificity of the junctional preparation. Gene ontology term analysis of the combined lists indicated a statistically significant enrichment in membrane-associated proteins. The myotendinous junction preparation contained previously uncharacterized proteins, a number of z-disc costameric ligands (e.g., actinins, capZ, αB cristallin, filamin C, cypher, calsarcin, desmin, FHL1, telethonin, nebulin, titin and an enigma-like protein) and other proposed players of sarcomeric stretch sensing and signalling, such as myotilin and the three myomesin homologs. A subset were confirmed by immunofluorescence analysis as enriched at the myotendinous junction, suggesting that laser capture microdissection from muscle sections is a valid approach to identify novel myotendinous junction players potentially involved in mechanotransduction pathways.
    Proteome Science 05/2014; 12(25). DOI:10.1186/1477-5956-12-25 · 1.73 Impact Factor
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    • "In addition to the limb muscle weakness that typifies all LGMDs, some patients with LGMD1A display unusual pharyngeal muscle involvement, causing dysarthric speech.2,4 Nevertheless, presentation can vary among individuals with MYOT mutations.1,2,3,4,5,6,7,8,9 Indeed, MYOT mutations have been identified as the most common underlying cause of myofibrillar myopathy (MFM; MFM3), which also affects limb muscles but has an inverse pattern of progression compared to LGMD1A.7,8,9 "
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    ABSTRACT: Recent progress suggests gene therapy may one day be an option for treating some forms of limb girdle muscular dystrophy (LGMD). Nevertheless, approaches targeting LGMD have so far focused on gene replacement strategies for recessive forms of the disease. In contrast, no attempts have been made to develop molecular therapies for any of the eight dominantly inherited forms of LGMD. Importantly, the emergence of RNA interference (RNAi) therapeutics in the last decade provided new tools to combat dominantly inherited LGMDs with molecular therapy. In this study, we describe the first RNAi-based, preclinical gene therapy approach for silencing a gene associated with dominant LGMD. To do this, we developed adeno-associated viral vectors (AAV6) carrying designed therapeutic microRNAs targeting mutant myotilin (MYOT), which is the underlying cause of LGMD type 1A (LGMD1A). Our best MYOT-targeted microRNA vector (called miMYOT) significantly reduced mutant myotilin mRNA and soluble protein expression in muscles of LGMD1A mice (the TgT57I model) both 3 and 9 months after delivery, demonstrating short- and long-term silencing effects. This MYOT gene silencing subsequently decreased deposition of MYOT-seeded intramuscular protein aggregates, which is the hallmark feature of LGMD1A. Histological improvements were accompanied by significant functional correction, as miMYOT-treated animals showed increased muscle weight and improved specific force in the gastrocnemius, which is one of the most severely affected muscles in TgT57I mice and patients with dominant myotilin mutations. These promising results in a preclinical model of LGMD1A support the further development of RNAi-based molecular therapy as a prospective treatment for LGMD1A. Furthermore, this study sets a foundation that may be refined and adapted to treat other dominant LGMD and related disorders.
    Molecular Therapy - Nucleic Acids 04/2014; 3(4):e160. DOI:10.1038/mtna.2014.13 · 4.51 Impact Factor
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    • "Myofibrillar myopathy (MFM) [OMIM:601419] refers to a group of genetically heterogeneous chronic neuromuscular disorders. MFMs are mainly caused by mutations in the desmin gene [1], while other forms of MFM are caused by mutations in alpha-B-crystallin [2], myotilin [3], ZASP [4], filamin C [5] or BAG3 [6] genes. Rigid spine syndrome caused by mutation in the SEPN1 gene is also considered as a MFM [7]. "
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    ABSTRACT: Background The clinical features of myofibrillar myopathies display a wide phenotypic heterogeneity. To this date, no studies have evaluated this parameter due to the absence of pertinent animal models. By studying two mutants of desmin, which induce subtle phenotypic differences in patients, we address this issue using an animal model based on the use of adeno-associated virus (AAV) vectors carrying mutated desmin cDNA. Methods After preparation of the vectors, they were injected directly into the tibialis anterior muscles of C57BL/6 mice to allow expression of wild-type (WT) or mutated (R406W or E413K) desmin. Measurements of maximal force were carried out on the muscle in situ and then the injected muscles were analyzed to determine the structural consequences of the desmin mutations on muscle structure (microscopic observations, histology and immunohistochemistry). Results Injection of AAV carrying WT desmin results in the expression of exogenous desmin in 98% of the muscle fibers without any pathological or functional perturbations. Exogenous WT and endogenous desmin are co-localized and no differences were observed compared to non-injected muscle. Expression of desmin mutants in mouse muscles induce morphological changes of muscle fibers (irregular shape and size) and the appearance of desmin accumulations around the nuclei (for R406W) or in subsarcolemmal regions of fibers (for E413K). These accumulations seem to occur and disrupt the Z-line, and a strong regeneration was observed in muscle expressing the R406W desmin, which is not the case for E413K. Moreover, both mutants of desmin studied here induce a decrease in muscle force generation capacity. Conclusions In this study we show that AAV-mediated expression of desmin mutants in mouse muscles recapitulate the aggregation features, the decrease in contractile function and the morphological changes observed in patients with myofibrillar myopathy. More importantly, our results suggest that the R406W desmin mutant induces a robust muscle regeneration, which is not the case for the E413K mutant. This difference could help to explain the phenotypic differences observed in patients. Our results highlight the heterogeneous pathogenic mechanisms between different desmin mutants and open the way for new advances in the study of myofibrillar myopathies.
    Skeletal Muscle 02/2013; 3(1):4. DOI:10.1186/2044-5040-3-4
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