Zaspopathy in a large classic late-onset distal myopathy family

Distal muscular dystrophies

Distal muscular dystrophies are a group of inherited primary muscle disorders showing progressive weakness and atrophy preferentially in the hands, forearm, lower legs, or feet. Extensive progress in understanding the molecular genetic background has changed the classification and extended the list of confirmed entities to almost 20 different disorders, making the differential diagnostic procedure both easier and more difficult. Distal phenotypes first have to be differentiated from neurogenic disorders. The axonal form of Charcot-Marie-Tooth disease with late-onset distal weakness and distal forms of chronic spinal muscular atrophy may mimic those of the distal dystrophies. Increasing numbers of reports suggest increasing awareness of distal phenotypes in muscular dystrophy. Some disorders regularly progress eventually to involve proximal muscle, whereas others, such as tibial muscular dystrophy titinopathy (Udd), Welander distal myopathy, and distal myosinopathy (Laing), remain distal throughout the patient's lifetime. Pathologically there is a gradual degeneration and loss of muscle fibers with replacement by fibrous and fatty connective tissue, similar to the proximal forms of muscular dystrophy, frequently, but not always with rimmed vacuolar degenerative change. Strikingly, many of the genes involved in distal dystrophies code for sarcomeric proteins. However, the genetic programs leading to preferential involvement of distal muscles have remained unknown.

Clinical and myopathological evaluation of early- and late-onset subtypes of myofibrillar myopathy

Myofibrillar myopathies (MFM) are a group of disorders associated with mutations in DES, CRYAB, MYOT, ZASP, FLNC, or BAG3 genes and characterized by disintegration of myofibrils and accumulation of degradation products into intracellular inclusions. We retrospectively evaluated 53 MFM patients from 35 Spanish families. Studies included neurologic exam, muscle imaging, light and electron microscopic analysis of muscle biopsy, respiratory function testing and cardiologic work-up. Search for pathogenic mutations was accomplished by sequencing of coding regions of the six genes known to cause MFM. Mutations in MYOT were the predominant cause of MFM in Spain affecting 18 of 35 families, followed by DES in 11 and ZASP in 3; in 3 families the cause of MFM remains undetermined. Comparative analysis of DES, MYOT and ZASP associated phenotypes demonstrates substantial phenotypic distinctions that should be considered in studies of disease pathogenesis, for optimization of subtype-specific treatments and management, and directing molecular analysis.

Mutations in the N-terminal actin-binding domain of filamin C cause a distal myopathy

Linkage analysis of the dominant distal myopathy we previously identified in a large Australian family demonstrated one significant linkage region located on chromosome 7 and encompassing 18.6 Mbp and 151 genes. The strongest candidate gene was FLNC because filamin C, the encoded protein, is muscle-specific and associated with myofibrillar myopathy. Sequencing of FLNC cDNA identified a c.752T>C (p.Met251Thr) mutation in the N-terminal actin-binding domain (ABD); this mutation segregated with the disease and was absent in 200 controls. We identified an Italian family with the same phenotype and found a c.577G>A (p.Ala193Thr) filamin C ABD mutation that segregated with the disease. Filamin C ABD mutations have not been described, although filamin A and filamin B ABD mutations cause multiple musculoskeletal disorders. The distal myopathy phenotype and muscle pathology in the two families differ from myofibrillar myopathies caused by filamin C rod and dimerization domain mutations because of the distinct involvement of hand muscles and lack of pathological protein aggregation. Thus, like the position of FLNA and B mutations, the position of the FLNC mutation determines disease phenotype. The two filamin C ABD mutations increase actin-binding affinity in a manner similar to filamin A and filamin B ABD mutations. Cell-culture expression of the c.752T>C (p.Met251)Thr mutant filamin C ABD demonstrated reduced nuclear localization as did mutant filamin A and filamin B ABDs. Expression of both filamin C ABD mutants as full-length proteins induced increased aggregation of filamin. We conclude filamin C ABD mutations cause a recognizable distal myopathy, most likely through increased actin affinity, similar to the pathological mechanism of filamin A and filamin B ABD mutations.

The costamere bridges sarcomeres to the sarcolemma in striated muscle

Costameres are sub-membranous, Z-line associated structures found in striated muscle. They have been shown to have important roles in transmission of force from the sarcomere to the sarcolemma and extracellular matrix, maintaining mechanical integrity of the sarcolemma, and orchestrating mechanically related signaling. The costamere is akin to the more well-known focal adhesion complex present in most cells. The Z-line is a critical structural anchor for the sarcomere, but it is also a hot-spot for muscle cell signaling. Therefore functionally, the costamere represents a two-way signaling highway tethered between the Z-line and the extracellular matrix, relaying mechanical stress signals from outside the cell to intracellular signaling networks. In this role it can modulate myofibril growth and contraction. The major force generated by sarcomeres is transduced in the lateral direction from the sarcomere to the extracellular matrix through the costamere. Two major protein complexes have been described at the costamere: the dystrophin-glycoprotein complex and the integrin-vinculin-talin complex. The importance of these two protein complexes in striated muscle function has between demonstrated both in human disease and mouse models. Members of the dystrophin glycoprotein complex and integrins have both been reported to interact directly with filamin-C, thus linking costameric complexes with those present at the Z-line. Moreover, studies from our labs and others have shown that the Z-line proteins belonging to the PDZ-LIM domain protein family, enigma homolog (ENH) and cypher, may directly or indirectly be involved in this linkage. The following review will focus on the protein components of this linkage, their function in force transmission, and how the dysfunction or loss of proteins within these complexes contributes to muscular disease.

Selective deletion of long but not short Cypher isoforms leads to late-onset dilated cardiomyopathy

Cypher long (CypherL) and short (CypherS) isoforms are distinguished from each other by the presence and absence of three C-terminal LIM domains, respectively. Cypher isoforms are developmentally regulated, and mutations affecting both long and short isoforms are linked to muscle disease in humans. Given these data, we hypothesized that various Cypher isoforms play overlapping and unique roles in striated muscle. To determine the specific role of Cypher isoforms in striated muscle, we generated two mouse lines in which either CypherS or CypherL isoforms were specifically deleted. Mice specifically, deficient in CypherS isoforms had no detectable muscle phenotype. In contrast, selective loss of CypherL isoforms resulted in partial neonatal lethality. Surviving mutants exhibited growth retardation and late-onset dilated cardiomyopathy, which was associated with cardiac fibrosis and calcification, leading to premature adult mortality. At a young age, preceding development of cardiomyopathy, hearts from these mutants exhibited defects in both Z-line ultrastructure and specific aberrations in calcineurin-NFAT and protein kinase C pathways. Earlier onset of cardiac dilation relative to control wild-type mice was observed in young CypherL isoform knockout mice consequent to pressure overload, suggesting a greater susceptibility to the disease. In summary, we have identified unique roles for CypherL isoforms in maintaining Z-line ultrastructure and signaling that are distinct from the roles of CypherS isoforms, while highlighting the contribution of mutations in the long isoforms to the development of dilated cardiomyopathy.

Myofibrillar myopathies

Myofibrillar myopathies represent a group of muscular dystrophies with a similar morphologic phenotype. They are characterized by a distinct pathologic pattern of myofibrillar dissolution associated with disintegration of the Z-disk, accumulation of myofibrillar degradation products, and ectopic expression of multiple proteins and sometimes congophilic material. The clinical features of myofibrillar myopathies are more variable. These include progressive muscle weakness, that often involves or begins in distal muscles but limb-girdle or scapuloperoneal distributions can also occur. Cardiomyopathy and peripheral neuropathy are frequent associated features. EMG of the affected muscles reveals myopathic motor unit potentials and abnormal irritability often with myotonic discharges. Rarely, neurogenic motor unit potentials or slow nerve conductions are present. The generic diagnosis of myofibrillar myopathies is based on muscle biopsy findings in frozen sections. To date, all myofibrillar myopathy mutations have been traced to Z-disk-associated proteins, namely, desmin, αB-crystallin, myotilin, ZASP, filamin C and Bag3. However, in the majority of the myofibrillar myopathy patients the disease gene awaits discovery.

Myofibrillar myopathies

Myofibrillar myopathies (MFMs) represent a group of muscular dystrophies with a similar morphological phenotype. The diagnosis is established by muscle biopsy. The MFMs are characterized by a distinct pathological pattern of myofibrillar dissolution associated with disintegration of the Z-disk, accumulation of myofibrillar degradation products, and ectopic expression of multiple proteins that include desmin, αB-crystallin, dystrophin, and sometimes congophilic material. The clinical features of MFMs are more variable. These include progressive muscle weakness that often involves or begins in distal muscles, but limb-girdle or scapuloperoneal distributions can also occur. Cardiomyopathy and peripheral neuropathy are frequent associated features. Electromyography of the affected muscles reveals myopathic motor unit potentials and abnormal irritability, often with myotonic discharges. Rarely, neurogenic motor unit potentials or slowing of nerve conduction velocities are present. To date, all MFM mutations have appeared in Z-disk-associated proteins: namely, desmin, αB-crystallin, myotilin, ZASP, filamin C, and Bag3. However, in the majority of patients with MFM, the disease gene awaits discovery.

Markesbery disease: autosomal dominant late-onset distal myopathy: from phenotype to ZASP gene identification

In 1974, Markesbery et al. thoroughly characterized and reported a large kindred with distal muscle weakness of late adult-onset that was autosomal dominantly inherited. Clinical evidence supported myopathy rather than the usual neuropathy expected with distal weakness. Postmortem examination of two patients documented myopathy and excluded anterior horn cell disease or peripheral neuropathy as the cause. Distinctive morphologic changes were present in muscle. Widely accepted as a distinct disease entity, this disorder has recently been characterized as one of a group of myofibrillar myopathies resulting from mutations in several muscle proteins. Studies of members of the original family have now identified the molecular defect to be a mutation in ZASP, Z-band alternatively spliced PDZ-motif-containing protein. The specific mutation, A165V, was identified in all clinical affected family members by direct sequencing. Thus, Markesbery disease is a zaspopathy. Other families have been identified with the same mutation and a shared haplotype indicating a founder effect.

Kelch-like homologue 9 mutation is associated with an early onset autosomal dominant distal myopathy

Distal myopathies are a heterogeneous group of disorders characterized by progressive weakness and muscular atrophy, beginning in distal limb muscles and affecting proximal limb muscles at a later stage. We studied a large German kindred with 10 affected members. Weakness and atrophy of the anterior tibial muscles started between the ages of 8 and 16 years, followed by atrophy of intrinsic hand muscles. Progression was slow, and patients retained the ability to walk until the seventh decade. Serum creatinine kinase levels were increased in the range of 150–1400 U/l. Muscle biopsies showed myopathic changes, whereas immunohistochemistry showed normal expression of marker proteins for muscular dystrophies. Patients had reduced sensation with stocking-glove distribution in the distal limbs in later life. Nerve conduction studies revealed no evidence of neuropathy. Genome-wide linkage analysis in this family revealed a new locus for distal myopathy at 9p21.2-p22.3 (multipoint logarithm of the odds ratio = 4.21). By positional cloning we found a heterozygous mutation L95F in the Kelch-like homologue 9 gene, encoding a bric-a-brac Kelch protein. Molecular modelling indicated that the mutation may interfere with the interaction of the bric-a-brac domain with Cullin 3. Coimmunoprecipitation experiments confirmed that the mutation reduces association with Cullin 3 in the Kelch-like homologue 9-Cullin 3–E3 ubiquitin ligase complex, which is involved in ubiquitin-dependent protein degradation. We identified a unique form of early onset autosomal dominant distal myopathy which is associated with a Kelch-like homologue 9 mutation and interferes with normal skeletal muscle through a novel pathogenetic mechanism.

Loss of enigma homolog protein results in dilated cardiomyopathy

RATIONALE

The Z-line, alternatively termed the Z-band or Z-disc, is a highly ordered structure at the border between 2 sarcomeres. Enigma subfamily proteins (Enigma, Enigma homolog protein, and Cypher) of the PDZ-LIM domain protein family are Z-line proteins. Among the Enigma subfamily, Cypher has been demonstrated to play a pivotal role in the structure and function of striated muscle, whereas the role of Enigma homolog protein (ENH) in muscle remains largely unknown.

OBJECTIVE

We studied the role of Enigma homolog protein in the heart using global and cardiac-specific ENH knockout mouse models.

METHODS AND RESULTS

We identified new exons and splice isoforms for ENH in the mouse heart. Impaired cardiac contraction and dilated cardiomyopathy were observed in ENH null mice. Mice with cardiac specific ENH deletion developed a similar dilated cardiomyopathy. Like Cypher, ENH interacted with Calsarcin-1, another Z-line protein. Moreover, biochemical studies showed that ENH, Cypher short isoform and Calsarcin-1 are within the same protein complex at the Z-line. Cypher short isoform and Calsarcin-1 proteins are specifically downregulated in ENH null hearts.

CONCLUSIONS

We have identified an ENH-CypherS-Calsarcin protein complex at the Z-line. Ablation of ENH leads to destabilization of this protein complex and dilated cardiomyopathy.

Differences in aberrant expression and splicing of sarcomeric proteins in the myotonic dystrophies DM1 and DM2

Aberrant transcription and mRNA processing of multiple genes due to RNA-mediated toxic gain-of-function has been suggested to cause the complex phenotype in myotonic dystrophies type 1 and 2 (DM1 and DM2). However, the molecular basis of muscle weakness and wasting and the different pattern of muscle involvement in DM1 and DM2 are not well understood. We have analyzed the mRNA expression of genes encoding muscle-specific proteins and transcription factors by microarray profiling and studied selected genes for abnormal splicing. A subset of the abnormally regulated genes was further analyzed at the protein level. TNNT3 and LDB3 showed abnormal splicing with significant differences in proportions between DM2 and DM1. The differential abnormal splicing patterns for TNNT3 and LDB3 appeared more pronounced in DM2 relative to DM1 and are among the first molecular differences reported between the two diseases. In addition to these specific differences, the majority of the analyzed genes showed an overall increased expression at the mRNA level. In particular, there was a more global abnormality of all different myosin isoforms in both DM1 and DM2 with increased transcript levels and a differential pattern of protein expression. Atrophic fibers in DM2 patients expressed only the fast myosin isoform, while in DM1 patients they co-expressed fast and slow isoforms. However, there was no increase of total myosin protein levels, suggesting that aberrant protein translation and/or turnover may also be involved.

Myofibrillar myopathies: a clinical and myopathological guide

Myofibrillar myopathies (MFMs) are histopathologically characterized by desmin-positive protein aggregates and myofibrillar degeneration. Because of the marked phenotypic and pathomorphological variability, establishing the diagnosis of MFM can be a challenging task. While MFMs are partly caused by mutations in genes encoding for extramyofibrillar proteins (desmin, alphaB-crystallin, plectin) or myofibrillar proteins (myotilin, Z-band alternatively spliced PDZ-containing protein, filamin C, Bcl-2-associated athanogene-3, four-and-a-half LIM domain 1), a large number of these diseases are caused by still unresolved gene defects. Although recent years have brought new insight into the pathogenesis of MFMs, the precise molecular pathways and sequential steps that lead from an individual gene defect to progressive muscle damage are still unclear. This review focuses on the clinical and myopathological aspects of genetically defined MFMs, and shall provide a diagnostic guide for this numerically significant group of protein aggregate myopathies.

Post-transcriptional silencing of the Drosophila homolog of human ZASP: a molecular and functional analysis

In humans, mutations in ZASP (the gene for Z-band alternatively spliced PDZ-motif protein) are associated with dilated cardiomyopathy and left ventricular non-compaction. In particular, mutations in or around the Zasp motif seem to be related to myofibrillar myopathy. Thus, "zaspopathies" include symptoms such as Z-line disgregation, proximal and distal muscle weakness, cardiomyopathies, and peripheral neuropathies. In order to understand the role of ZASP in muscle structure and function, we have performed a molecular characterization of the Drosophila ortholog of human ZASP and a functional analysis following the post-transcriptional silencing of the Drosophila gene. Transcriptional analysis of dzasp has revealed six additional exons, with respect to the known 16, and multiple splice variants. We have produced transgenic lines harboring constructs that, through the use of the UAS/Gal4 binary system, have enabled us to drive dsRNA interference of dzasp in a tissue-specific manner. Knockdown individuals show locomotor defects associated with alterations of muscle structure and ultrastructure, consistent with a role of dzasp specifically in the maintenance of muscular integrity.

Lower limb radiology of distal myopathy due to the S60F myotilin mutation

Distal myopathies are a clinically and genetically heterogenous group of disorders in which the distal limb musculature is selectively or disproportionately affected. Precisely defining specific categories is a challenge because of overlapping clinical phenotypes, making it difficult to decide which of the many known causative genes to screen in individual cases. In this study we define the distinguishing magnetic resonance imaging findings in myotilin myopathy by studying 8 genealogically unrelated cases due to the same point mutation in TTID. Proximally, the vastii, biceps femoris and semimembranosus were involved with sparing of gracilis and sartorius. Distally, soleus, gastrocnemius, tibialis anterior, extensor hallicus and extensor digitorum were involved. This pattern contrasts with other distal myopathies and provides further support for the role of imaging in the clinical investigation of muscle disease.

Impaired binding of ZASP/Cypher with phosphoglucomutase 1 is associated with dilated cardiomyopathy

AIMS

Z-band alternatively spliced PDZ-motif protein (ZASP)/Cypher is a Z-disc component of which several dilated cardiomyopathy (DCM)-associated mutations have been reported. Most of the mutations were found in exons 4 and 10 of ZASP/Cypher gene LDB3 and both exons were expressed preferentially in the heart. The aim of this study was to investigate the functional alteration of ZASP/Cypher caused by the DCM-associated mutations.

METHODS AND RESULTS

The yeast-two-hybrid method was used to identify the protein bound to a domain encoded by exon 4 of LDB3. Interaction of ZASP/Cypher with the binding protein was investigated in relation to the functional alterations caused by LDB3 mutations. Localization of the ZASP/Cypher-binding protein was examined at the cellular level in rat cardiomyocytes. Phosphoglucomutase 1 (PGM1), a metabolic enzyme involved in glycolysis and gluconeogenesis, was identified as a protein interacting with ZASP/Cypher. PGM1 bound to ZASP/Cypher at the domains encoded by exons 4 and 10. Two LDB3 mutations in exon 4 (Ser189Leu and Thr206Ile) and another mutation in exon 10 (Ile345Met) reduced the binding to PGM1. PGM1 showed diffuse localization in the cytoplasm of rat cardiomyocytes under standard culture conditions, and distribution at the Z-discs was observed under stressed culture conditions. Binding of endogenous PGM1 and ZASP/Cypher was found to be enhanced by stress in rat cardiomyocytes.

CONCLUSION

ZASP/Cypher anchors PGM1 to Z-disc under conditions of stress. The impaired binding of PGM1 to ZASP/Cypher might be involved in the pathogenesis of DCM.

Differential involvement of sarcomeric proteins in myofibrillar myopathies: a morphological and immunohistochemical study

Myofibrillar myopathies (MFMs) are rare inherited or sporadic progressive neuromuscular disorders with considerable clinical and genetic heterogeneity. In the current study, we have analyzed histopathological and immunohistochemical characteristics in genetically identified MFMs. We performed a morphological and morphometrical study in a cohort of 24 genetically identified MFM patients (12 desmin, 6 alphaB-crystallin, 4 ZASP, 2 myotilin), and an extensive immunohistochemical study in 15 of these patients, using both well-known and novel antibodies directed against distinct compartments of the muscle fibers, including Z-disc and M-band proteins. Our morphological data revealed some significant differences between the distinct MFM subgroups: the consistent presence of 'rubbed-out' fibers in desminopathies and alphaB-crystallinopathies, an elevated frequency of vacuoles in ZASPopathies and myotilinopathies, and the presence of a few necrotic fibers in the two myotilinopathy patients. Immunohistochemistry showed that in MFM only a subset of Z-disc proteins, such as filamin C and its ligands myotilin and Xin, exhibited significant alterations in their localization, whereas other Z-disc proteins like alpha-actinin, myopodin and tritopodin, did not. In contrast, M-band proteins revealed no abnormalities in MFM. We conclude that the presence of 'rubbed-out' fibers are a suggestive feature for desminopathy or alphaB-crystallinopathy, and that MFM is not a general disease of the myofibril, but primarily affects a subgroup of stress-responsive Z-disc proteins.

Distal myopathy in multi-minicore disease

A 52-year-old man noted distal dominant slowly progressive muscle weakness at age 36 years. On muscle CT, the red muscles of the soleus, anterior tibial and paraspinal muscles, where type 1 fiber is known to predominate, were almost totally replaced by fat tissue while quadriceps femoris, gastrocnemius and upper extremity muscles were relatively spared. Quadriceps muscle biopsy revealed multi-minicores in addition to occasional larger cores, in about 70% of the type 1 fibers. A novel heterozygous nucleotide change c.5869T > A (p.S1957T) was identified in RYR1. Although pathogenicity was not confirmed, this nucleotide change was absent in 100 control DNA. We did not find a mutation in either multi-minicore disease-associated gene, SEPN1, or major distal myopathy-related genes, including GNE, ZASP, MYOT, exons 32-36 of MYH7, and the last exon of TTN. This is probably a unique form of distal myopathy characterized by the presence of multi-minicores with preferential involvement of type 1 fibers.

Mutation in BAG3 causes severe dominant childhood muscular dystrophy

OBJECTIVE

Myofibrillar myopathies (MFMs) are morphologically distinct but genetically heterogeneous muscular dystrophies in which disintegration of Z disks and then of myofibrils is followed by ectopic accumulation of multiple proteins. Cardiomyopathy, neuropathy, and dominant inheritance are frequent associated features. Mutations in alphaB-crystallin, desmin, myotilin, Zasp, or filamin-C can cause MFMs and were detected in 32 of 85 patients of the Mayo MFM cohort. Bag3, another Z-disk-associated protein, has antiapoptotic properties, and its targeted deletion in mice causes fulminant myopathy with early lethality. We therefore searched for mutations in BAG3 in 53 unrelated MFM patients.

METHODS

We searched for mutations in BAG3 by direct sequencing. We analyzed structural changes in muscle by histochemistry, immunocytochemistry, and electron microscopy, examined mobility of the mutant Bag3 by nondenaturing electrophoresis, and searched for abnormal aggregation of the mutant protein in COS-7 (SV-40 transformed monkey kidney fibroblast-7) cells.

RESULTS

We identified a heterozygous p.Pro209Leu mutation in three patients. All presented in childhood, had progressive limb and axial muscle weakness, and experienced development of cardiomyopathy and severe respiratory insufficiency in their teens; two had rigid spines, and one a peripheral neuropathy. Electron microscopy showed disintegration of Z disks, extensive accumulation of granular debris and larger inclusions, and apoptosis of 8% of the nuclei. On nondenaturing electrophoresis of muscle extracts, the Bag3 complex migrated faster in patient than control extracts, and expression of FLAG-labeled mutant and wild-type Bag3 in COS cells showed abnormal aggregation of the mutant protein.

INTERPRETATION

We conclude mutation in Bag3 defines a novel severe autosomal dominant childhood muscular dystrophy.

Genetics and pathogenesis of distal muscular dystrophies

Distal myopathies are distal muscular dystrophies because they are genetic disorders with progressive loss of muscle tissue. The true distal dystrophies not only show a distal onset; they also remain more distal than proximal throughout the course of the disease. Currently almost 20 different entities of distal muscular dystrophies have been genetically determined, compared to just five entities delineated on clinical grounds in the 1980s. Half of the genes underlying these disorders have been associated with distal phenotypes only, whereas the other genes can manifest also with other than distal phenotypes such as proximal, scapuloperoneal or generalized phenotypes. Interestingly, most of the genes causing distal muscular dystrophies code for protein components of the sarcomere, in contrast to the proximal dystrophies in which most of the genes cause defects in sarcolemmal proteins. The reason for why some gene defects predominantly affect distal muscles is not well understood. The fact that the majority of these defects are due to structural and functional components of the sarcomere is intriguing but so far it does not provide further clues for understanding or for therapeutic approaches. The highly selective involvement of muscles in many of the distal dystrophies is even less well understood.

Truncating mutations in C-terminal titin may cause more severe tibial muscular dystrophy (TMD)

Mutations in C-terminal titin cause autosomal dominant tibial muscular dystrophy (TMD) as reported previously. Samples from 25 new families and 25 sporadic new distal myopathy cases were screened for titin mutations. Three novel mutations were discovered in two families from Spain and two families from France. Two mutations, g.292998delT and g.293376delA lead to frameshift and premature stop codons in the second last and the last titin gene (TTN) exons, Mex5 and Mex6, respectively. The third was a nonsense mutation g.293379C>T (p.Q33396X) in Mex6. Patients with the upstream Mex5 mutation showed a more severe phenotype with earlier onset implying a genotype-phenotype correlation.

Myofibrillar myopathies

PURPOSE OF REVIEW

The aim of this communication is to provide an up-to-date overview of myofibrillar myopathies.

RECENT FINDINGS

The most important recent advance in the myofibrillar myopathies has been the discovery that mutations in Z band alternatively spliced PDZ-containing protein and filamin C, as well as in desmin, alphaB-crystallin and myotilin, result in similar pathologic alterations in skeletal muscle that are typical of myofibrillar myopathy. Despite the increasing genetic heterogeneity, the clinical and morphologic phenotypes are remarkably homogeneous. The typical clinical manifestation is slowly progressive proximal, distal or both proximal and distal limb muscle weakness. Cardiomyopathy can be associated and is sometimes the presenting finding. Peripheral neuropathy also occurs in some patients. In every myofibrillar myopathy, there is abnormal accumulation of an array of proteins at ectopic sites as well as accumulation of degraded myofibrillar proteins forming large aggregates. The key issue now is to analyze the molecular mechanisms underlying the cascade of events that destroy the myofibrillar architecture and trigger the aberrant expression of multiple proteins.

SUMMARY

Several disease genes have recently been recognized in myofibrillar myopathies. So far, the disease proteins identified are components of or chaperone for the Z-disk. In each case, the molecular defect leads to a stereotyped cascade of structural events in the muscle fiber.

Distinct muscle imaging patterns in myofibrillar myopathies

OBJECTIVE

To compare muscle imaging findings in different subtypes of myofibrillar myopathies (MFM) in order to identify characteristic patterns of muscle alterations that may be helpful to separate these genetic heterogeneous muscular disorders.

METHODS

Muscle imaging and clinical findings of 46 patients with MFM were evaluated (19 desminopathy, 12 myotilinopathy, 11 filaminopathy, 1 alphaB-crystallinopathy, and 3 ZASPopathy). The data were collected retrospectively in 43 patients and prospectively in 3 patients.

RESULTS

In patients with desminopathy, the semitendinosus was at least equally affected as the biceps femoris, and the peroneal muscles were never less involved than the tibialis anterior (sensitivity of these imaging criteria to detect desminopathy in our cohort 100%, specificity 95%). In most of the patients with myotilinopathy, the adductor magnus showed more alterations than the gracilis muscle, and the sartorius was at least equally affected as the semitendinosus (sensitivity 90%, specificity 93%). In filaminopathy, the biceps femoris and semitendinosus were at least equally affected as the sartorius muscle, and the medial gastrocnemius was more affected than the lateral gastrocnemius. The semimembranosus mostly showed more alterations than the adductor magnus (sensitivity 88%, specificity 96%). Early adult onset and cardiac involvement was most often associated with desminopathy. In patients with filaminopathy, muscle weakness typically beginning in the 5th decade of life was mostly pronounced proximally, while late adult onset (>50 years) with distal weakness was more often present in myotilinopathy.

CONCLUSIONS

Muscle imaging in combination with clinical data may be helpful for separation of distinct myofibrillar myopathy subtypes and in scheduling of genetic analysis.

Molecular pathology of myofibrillar myopathies

Myofibrillar myopathies (MFMs) are clinically and genetically heterogeneous muscle disorders that are defined morphologically by the presence of foci of myofibril dissolution, accumulation of myofibrillar degradation products, and ectopic expression of multiple proteins. MFMs are the paradigm of conformational protein diseases of the skeletal (and cardiac) muscles characterised by intracellular protein accumulation in muscle cells. Understanding of this group of disorders has advanced in recent years through the identification of causative mutations in various genes, most of which encode proteins of the sarcomeric Z-disc, including desmin, alphaB-crystallin, myotilin, ZASP and filamin C. This review focuses on the MFMs arising from defects in these proteins, summarising genetic and clinical features of the disorders and then discussing emerging understanding of the molecular pathogenic mechanisms leading to muscle fibre degeneration. Defective extralysosomal degradation of proteins is now recognised as an important element in this process. Several factors--including mutant proteins, a defective ubiquitin-proteasome system, aggresome formation, mutant ubiquitin, p62, oxidative stress and abnormal regulation of some transcription factors--are thought to participate in the cascade of events occurring in muscle fibres in MFMs.

Electron microscopy in myofibrillar myopathies reveals clues to the mutated gene

We studied the ultrastructural characteristics in patients with myofibrillar myopathy (MFM) and differentiated between MFM-subtypes using electron microscopic (EM) findings. The ultrastructural findings in 19 patients with different genetically proven MFMs (9 desmin, 5 alphaB-crystallin, 3 ZASP, 2 myotilin) were analyzed. In one ZASPopathy, we additionally performed an immunoEM study, using antibodies against desmin, alphaB-crystallin, ZASP and myotilin. The ultrastructural findings in desminopathies and alphaB-crystallinopathies were very similar and consisted of electrondense granulofilamentous accumulations and sandwich formations. They differed in the obvious presence of early apoptotic nuclear changes in alphaB-crystallinopathies. ZASPopathies were characterized by filamentous bundles (labeled with the myotilin antibody on immunoEM), and floccular accumulations of thin filamentous material. Tubulofilamentous inclusions in sarcoplasm and myonuclei in combination with filamentous bundles were characteristic for myotilinopathies. We conclude that MFMs ultrastructural findings can direct diagnostic efforts towards the causal gene mutated, and that EM should be included in the diagnostic workup of MFMs.

"Z"eroing in on the role of Cypher in striated muscle function, signaling, and human disease

The striated muscle Z line, a multiprotein complex at the boundary between sarcomeres, plays an integral role in maintaining striated muscle structure and function. Multiple Z-line-associated proteins have been identified and shown to play an increasingly important role in the pathogenesis of human muscle disease. Cypher/Z-band alternatively spliced PDZ-motif protein, a PDZ-LIM protein in the Z line, binds to alpha-actinin (via its PDZ domain) and has been suggested to function as a linker-strut to maintain cytoskeletal structural integrity during contraction. Cypher may also participate in signaling pathways by binding to protein kinase C via its LIM domains. Analysis of Cypher-deficient mice has revealed that Cypher plays an integral role in Z-line maintenance/integrity of striated muscles and the pathogenesis of congenital myopathies, including cardiomyopathy. These studies have led to the subsequent discovery of Cypher mutations in human patients with dilated cardiomyopathy, hypertrophic cardiomyopathy, as well as skeletal muscle myopathies, which have been recently termed zaspopathies. The recent discovery of various alternatively spliced isoforms of Cypher with potentially distinct structural and signaling roles brings a different level of complexity to the mechanisms underlying Cypher-based human myopathies. This review will focus on recent developments on the role of Cypher and its isoforms in striated muscle structure, signaling, and disease to provide insights into the mechanisms involved in the pathogenesis of Z-line-associated human myopathies.

The Z-disk diseases

Recent studies have identified disease-causing mutations in four genes that encode Z-disk proteins. Mutations in myotilin (MYOT), ZASP and filamin C (FLNC) encoding genes cause autosomal dominant myopathy that manifests in adulthood. The clinical features and morphological changes in myopathies caused by mutations in all three genes are highly similar. The disease typically manifests as distal myopathy, but may also affect proximal muscles and the heart. The morphological findings are typical of myofibrillar myopathy (MFM) and include Z-disk alterations and aggregation of dense filamentous material visible in trichrome staining. The disease mechanism is still unclear, but may involve structural alterations of the Z-disk caused by dysfunctional proteins or their abnormal accumulation due to defective degradation. Although the fourth gene product, telethonin, is also involved in the Z-disk organization, its mutations cause a different phenotype. Telethonin mutations result in recessive muscular dystrophy, which manifests in childhood as proximal weakness. The morphologic alterations caused by telethonin mutations are not well characterized, but may share common features of MFM. Future work aims at understanding the pathophysiology of Z-disk related disorders and identification of novel genetic defects in patients with morphological features of MFM.

Mononuclear muscle cells in Drosophila ovaries revealed by GFP protein traps

Genetic analysis of muscle specification, formation and function in model systems has provided valuable insight into human muscle physiology and disease. Studies in Drosophila have been particularly useful for discovering key genes involved in muscle specification, myoblast fusion, and sarcomere organization. The muscles of the Drosophila female reproductive system have received little attention despite extensive work on oogenesis. We have used newly available GFP protein trap lines to characterize of ovarian muscle morphology and sarcomere organization. The muscle cells surrounding the oviducts are multinuclear with highly organized sarcomeres typical of somatic muscles. In contrast, the two muscle layers of the ovary, which are derived from gonadal mesoderm, have a mesh-like morphology similar to gut visceral muscle. Protein traps in the Fasciclin 3 gene produced Fas3::GFP that localized in dots around the periphery of epithelial sheath cells, the muscle surrounding ovarioles. Surprisingly, the epithelial sheath cells each contain a single nucleus, indicating these cells do not undergo myoblast fusion during development. Consistent with this observation, we were able to use the Flp/FRT system to efficiently generate genetic mosaics in the epithelial sheath, suggesting these cells provide a new opportunity for clonal analysis of adult striated muscle.