121st ENMC International Workshop on Desmin and Protein Aggregate Myopathies. 7–9 November 2003, Naarden, The Netherlands

Pathophysiology of protein aggregation and extended phenotyping in filaminopathy

Mutations in FLNC cause two distinct types of myopathy. Disease associated with mutations in filamin C rod domain leading to expression of a toxic protein presents with progressive proximal muscle weakness and shows focal destructive lesions of polymorphous aggregates containing desmin, myotilin and other proteins in the affected myofibres; these features correspond to the profile of myofibrillar myopathy. The second variant associated with mutations in the actin-binding domain of filamin C is characterized by weakness of distal muscles and morphologically by non-specific myopathic features. A frameshift mutation in the filamin C rod domain causing haploinsufficiency was also found responsible for distal myopathy with some myofibrillar changes but no protein aggregation typical of myofibrillar myopathies. Controversial data accumulating in the literature require re-evaluation and comparative analysis of phenotypes associated with the position of the FLNC mutation and investigation of the underlying disease mechanisms. This is relevant and necessary for the refinement of diagnostic criteria and developing therapeutic approaches. We identified a p.W2710X mutation in families originating from ethnically diverse populations and re-evaluated a family with a p.V930_T933del mutation. Analysis of the expanded database allows us to refine clinical and myopathological characteristics of myofibrillar myopathy caused by mutations in the rod domain of filamin C. Biophysical and biochemical studies indicate that certain pathogenic mutations in FLNC cause protein misfolding, which triggers aggregation of the mutant filamin C protein and subsequently involves several other proteins. Immunofluorescence analyses using markers for the ubiquitin-proteasome system and autophagy reveal that the affected muscle fibres react to protein aggregate formation with a highly increased expression of chaperones and proteins involved in proteasomal protein degradation and autophagy. However, there is a noticeably diminished efficiency of both the ubiquitin-proteasome system and autophagy that impairs the muscle capacity to prevent the formation or mediate the degradation of aggregates. Transfection studies of cultured muscle cells imitate events observed in the patient's affected muscle and therefore provide a helpful model for testing future therapeutic strategies.

DNA sequencing errors in molecular diagnostics of filamin myopathy

BACKGROUND

Filamin myopathy is a neuromuscular disorder manifesting with predominantly limb-girdle muscle weakness and in many patients with diaphragm paralysis and cardiomyopathy, caused by mutations in the filamin C (FLNC) gene. Molecular diagnosis of filamin myopathy based on direct DNA sequencing of coding exons is compromised by the presence of a high homology pseudogene (pseFLNC) located approximately 53.6 kb downstream of the functional FLNC gene on chromosome 7q.

METHODS

Molecular cloning, RT-PCR and real-time PCR methods were used to detect sequence differences between the FLNC and pseFLNC that are implicated in known or potential molecular diagnostic errors. Overall, 50 patients with a phenotype resembling filamin myopathy have been screened for mutations in FLNC.

RESULTS

FLNC sequence inconsistencies caused by the interference from pseFLNC were identified and diagnostic errors involving, in particular, the detection of the most frequent disease-causing FLNC p.W2710X mutation resolved. Mismatches between the FLNC and pseFLNC sequences were tabulated for future use.

CONCLUSIONS

We devise a strategy that allows one to discern mutations occurring in the functional FLNC from those harbored in pseFLNC, thus preventing possible complications in future research and patient genetic testing.

Non-pathogenic protein aggregates in skeletal muscle in MLF1 transgenic mice

Protein aggregate formation in muscle is thought to be pathogenic and associated with clinical weakness. Over-expression of either wild type or a mutant form of myeloid leukemia factor 1 (MLF1) in transgenic mouse skeletal muscle and in cultured cells resulted in aggregate formation. Aggregates were detected in MLF1 transgenic mice at 6 weeks of age, and increased in size with age. However, histological examination of skeletal muscles of MLF1 transgenic mice revealed no pathological changes other than the aggregates, and RotaRod testing did not detect functional deficits. MLF1 has recently been identified as a protein that could neutralize the toxicity of intracellular protein aggregates in a Drosophila model of Huntington's disease (HD). We also demonstrate that MLF1 interacts with MRJ, a heat shock protein, which can independently neutralize the toxicity of intracellular protein aggregates in the Drosophila HD model. Our data suggest that over-expression of MLF1 has no significant impact on skeletal muscle function in mice; that progressive formation of protein aggregates in muscle are not necessarily pathogenic; and that MLF1 and MRJ may function together to ameliorate the toxic effects of polyglutamine or mutant proteins in myodegenerative diseases such as inclusion body myositis and oculopharyngeal muscular dystrophy, as well as neurodegenerative disease.

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

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

Variable pathogenic potentials of mutations located in the desmin alpha-helical domain

Mutations in the desmin gene have been recognized as a cause of desminopathy, a familial or sporadic disorder characterized by skeletal muscle weakness, often associated with cardiomyopathy or respiratory insufficiency. Distinctive histopathologic features include aberrant intracytoplasmic accumulation of desmin (DES). We present here comparative phenotypic, molecular, and functional characteristics of four novel and three previously reported, but not fully characterized, desmin mutations localized in desmin alpha-helical domain. The results indicate that the c.638C>T (p.A213V), c.1178A>T (p.N393I), and to some extent the c.1078G>C (p.A360P) mutations exhibit pathogenic potentials only if combined with other mutations in desmin or other genes and should therefore be considered conditionally pathogenic. The c.1009G>C (p.A337P), c.1013T>G (p.L338R), c.1195G>T (p.D399Y), and c.1201G>A (p.E401K) mutations make desmin filaments dysfunctional and are capable of causing disease. The pathogenic potentials of desmin mutations correlate with the type and location of the disease-associated mutations in the relatively large and structurally and functionally complex desmin molecule. Mutations within the highly conserved alpha-helical structures are especially damaging since the integrity of the alpha-helix is critical for desmin filament assembly and stability.

Congenital myopathy with abundant nemaline rods in a cat

Nemaline myopathy is associated with rod-shaped structures in muscle fibers. At least seven distinct clinical forms have been described in humans and mutations have been identified in five different thin-filament genes. Only a few cases of spontaneously occurring nemaline myopathy have been reported in animals and include an adult-onset form in a family of cats and an early-onset form in a dog. Here, we describe a 2-year-old male, neutered, domestic shorthaired cat that was referred to the Veterinary Medical Teaching Hospital, University of California-Davis, for evaluation of chronic, progressive weakness, and fine tremors. Neurologic deficits were restricted to the neuromuscular system. Electromyography showed mild to moderate diffuse spontaneous activity. Although rod bodies were prominent on light and electron microscopic evaluation of biopsies from several muscles, sarcoplasmic accumulations of dystrophin, desmin, and spectrin also were identified by immunohistochemistry. These findings may represent the occurrence of rod bodies in conjunction with a protein-aggregate myopathy.

Severe muscle disease-causing desmin mutations interfere with in vitro filament assembly at distinct stages

Desmin is the major intermediate filament (IF) protein of muscle. Recently, mutations of the desmin gene have been reported to cause familial or sporadic forms of human skeletal, as well as cardiac, myopathy, termed desmin-related myopathy (DRM). The impact of any of these mutations on filament assembly and integration into the cytoskeletal network of myocytes is currently not understood, despite the fact that all cause the same histopathological defect, i.e., desmin aggregation. To gain more insight into the molecular basis of this process, we investigated how mutations within the alpha-helical rod domain of desmin affect both the assembly of the recombinant protein in vitro as well as the filament-forming capacity in cDNA-transfected cells. Whereas 6 of 14 mutants assemble into seemingly normal IFs in the test tube, the other mutants interfere with the assembly process at distinct stages, i.e., tetramer formation, unit-length filament (ULF) formation, filament elongation, and IF maturation. Correspondingly, the mutants with in vitro assembly defects yield dot-like aggregates in transfected cells, whereas the mutants that form IFs constitute a seemingly normal IF cytoskeleton in the cellular context. At present, it is entirely unclear why the latter mutant proteins also lead to aggregate formation in myocytes. Hence, these findings may be a starting point to dissect the contribution of the individual subdomains for desmin pathology and, eventually, the development of therapeutic interventions.