Myosinopathies: pathology and mechanisms
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The myosin heavy chain (MyHC) is the molecular motor of muscle and forms the backbone of the sarcomere thick filaments. Different MyHC isoforms are of importance for the physiological properties of different muscle fiber types. Hereditary myosin myopathies have emerged as an important group of diseases with variable clinical and morphological expression depending on the mutated isoform and type and location of the mutation. Dominant mutations in developmental MyHC isoform genes (MYH3 and MYH8) are associated with distal arthrogryposis syndromes. Dominant or recessive mutations affecting the type IIa MyHC (MYH2) are associated with early-onset myopathies with variable muscle weakness and ophthalmoplegia as a consistent finding. Myopathies with scapuloperoneal, distal or limb-girdle muscle weakness including entities, such as myosin storage myopathy and Laing distal myopathy are the result of usually dominant mutations in the gene for slow/β cardiac MyHC (MYH7). Protein aggregation is part of the features in some of these myopathies. In myosin storage myopathy protein aggregates are formed by accumulation of myosin beneath the sarcolemma and between myofibrils. In vitro studies on the effects of different mutations associated with myosin storage myopathy and Laing distal myopathy indicate altered biochemical and biophysical properties of the light meromyosin, which is essential for thick filament assembly. Protein aggregates in the form of tubulofilamentous inclusions in association with vacuolated muscle fibers are present at late stage of dominant myosin IIa myopathy and sometimes in Laing distal myopathy. These protein aggregates exhibit features indicating defective degradation of misfolded proteins. In addition to protein aggregation and muscle fiber degeneration some of the myosin mutations cause functional impairment of the molecular motor adding to the pathogenesis of myosinopathies.
KeywordsMyopathy Myosin Myosin heavy chain Mutation Myosin storage myopathy Laing distal myopathy Protein aggregate
Myosin heavy chain isoforms
Myosin heavy chain isoforms expressed in human muscle
Muscle fiber type
Smooth muscle MyHC
In addition to the common MyHC isoforms expressed in fibers of adult human limb muscles, there are special MyHC isoforms expressed in specific muscles. Muscles of the head and neck such as the masseter muscle show a more diverse expression of MyHC isoforms than limb muscles  and very fast contracting fibers found in extraocular muscles express extraocular MyHC isoform (MYH13) .
The non-muscle conventional class II MyHC genes MYH9 and MYH10 encode the non-muscle myosins IIA (MYHIIA) and IIB (MYHIIB), whereas MYH11 encodes smooth muscle MyHC.
Myosin heavy chain diseases
The first striated muscle MyHC isoform associated with disease in humans was slow/β cardiac MyHC (MYH7) . More than 200 different dominant mutations in MYH7 have been associated with hypertrophic and dilated cardiomyopathy. Mutations in slow/α-cardiac MyHC (MYH6) have been reported to cause hypertrophic and dilated cardiomyopathy and atrial septal defect [14, 16].
MYH9-related disorders are autosomal dominant syndromes, variably affecting platelet formation, hearing, and kidney function, and result from mutations in the human nonmuscle myosin IIA heavy chain gene (MYH9). They have previously been described as distinct disorders including Sebastian, Fechtner and Epstein syndromes and May-Hegglin anomaly [7, 27].
Myopathies associated with mutations in skeletal muscle myosin heavy chains
Major clinical characteristics
Skeletal muscle pathology
Autosomal dominant myopathy with congenital joint contractures, ophthalmoplegia and rimmed vacuoles
Congenital, reversible joint contractures. Ophthalmoplegia.
Mild proximal muscle weakness in childhood.
Progressive course in some adults affecting ambulation
Rimmed vacuoles with protein aggregates, composed of 15-20 nm tubulofilaments, in adults with progressive course and dystrophic muscle changes. Structural alterations with minicores in type 2 fibers in childhood and in mildly affected muscles of adults. Reduced number and small type 2 fibers in some cases.
Autosomal recessive myopathy with ophthalmoplegia
Mild to moderate muscle weakness, usually mild facial involvement. Ophthalmoplegia
Complete absence of type 2A muscle fibers. Variable, unspecific myopathic changes with fatty infiltration. Type 2B fibers may be lacking.
DA1 OMIM #108120
Freeman-Sheldon syndrome, DA2A,
DA2B, OMIM #601680
Multiple congenital joint contractures with predominant distal involvement. No muscle weakness
Minor unspecific changes
Familial hypertrophic/dilated cardiomyopathy,
Cardiac failure, arrhythmia,
sudden cardiac arrest
Irregular structure with cores in type 1 muscle fibers in some patients
Myosin storage myopathy
Onset from childhood to middle age. Weakness of limb girdle, scapuloperoneal or distal muscles. Mild weakness or severe weakness affecting ambulation
Subsarcolemmal protein aggregates in type 1 fibers that reacts with antibodies to myosin but not to desmin. Granular and partly filamentous structure on EM. Myofibrillar disarray.
Laing early-onset distal myopathy,
Usually onset of distal muscle weakness in childhood, but may be much later. Slowly progressive course with initial weakness of ankle dorsiflexion and “hanging big toe” sign
Fiber size variability, internalized nuclei, frequently small type 1 fibers. Dystrophic changes may occur. Rimmed vacuoles and protein aggregates in rare cases, either as inclusions of 15-20 nm tubulo-filaments or as cytoplasmic bodies. Minicores may be frequent.
Scapuloperoneal and limb girdle syndromes
Scapuloperoneal or limb girdle muscle weakness without morphological features of myosin storage
Unspecific changes including fiber type disproportion
Trismus and pseudocamtodactyly syndrome, DA7
Congenital contractures of hands, feet and jaws with trismus and hand and foot deformities with pseudocamptodactyly
Loss of thick filaments and myosin, but preserved thin filaments can be seen as an unspecific alteration in various conditions such as dermatomyositis but more typically in critical illness myopathy . This loss of myosin and other proteins associated with the thick filaments is frequently triggered by systemic corticosteroid hormone treatment, postsynaptic block of neuromuscular transmission and prolonged mechanical ventilation. The loss of myosin can be identified by electron microscopy showing a characteristic disappearance of thick filaments or by measuring the ratio of myosin to actin after separation of muscle proteins by gel electrophoresis.
Myopathies associated with developmental MyHC isoforms, MYH3 and MYH8
The essential roles of embryonic and fetal MyHC isoforms for normal fetal development has been highlighted by the identification of dominant MYH3 and MYH8 mutations associated with distal arthrogryposis (DA) syndromes. In addition to common DA manifestations such as clubfeet and clenched fists these syndromes frequently include also decreased movement of proximal joints, facial dysmorphism and other manifestations [8, 26].
At least 18 mutations in the globular head and rod domains of MYH3 have been associated with DA1, DA2A (Freeman–Sheldon syndrome) and DA2B (Sheldon–Hall syndrome) [3, 71, 79]. The amino acid at position 672 (R672) is the most frequently mutated residue associated with Freeman–Sheldon syndrome indicating a mutational hotspot of this residue . One mutation has been identified in MYH8 in several unrelated families with the trismus-pseudocamptodactyly syndrome [78, 82]. Interestingly, this mutation in MYH8 affects the residue R764 (R764Q), which is paralogous to R672 in MYH3. The embryonic and fetal MyHCs are expressed during embryonic and fetal development [24, 31] and it has been suggested that mutations in MYH3 and MYH8 cause a developmental myopathy resulting in reduced fetal movement and joint contractures [71, 79]. There are few reports on muscle pathology in distal arthrogryposis associated with MyHC mutations. These reports are limited to muscle biopsies from children and adults, which show minor unspecific changes .
Myopathies associated with MyHC IIa, MYH2
Dominant as well as recessive mutations in MYH2 that cause myopathy have been identified.
Autosomal dominant MyHC IIa myopathy, which has also been referred to as “autosomal dominant myopathy with congenital joint contractures, ophthalmoplegia and rimmed vacuoles”, was originally identified as a muscle disorder in western Sweden . The disease was mapped to chromosome 17p13.1  and later demonstrated to be caused by a heterozygous missense mutation in MYH2 encoding MyHC IIa . The mutation changes the highly conserved and negatively charged glutamate at position 706 to the positively charged lysine (E706K). The mutated residue is located in the SH1 helix in the core of motor domain, which is highly conserved through evolution.
A prenatal onset of the disease was indicated by multiple joint contractures that were present at birth in the majority of the patients. The joint contractures preferentially involved the fingers and/or hips and generally resolved early in childhood. Hypotonia was not a prominent feature and the early development was normal. External ophthalmoplegia was present in all patients, ranging from a slight impairment of upward gaze in affected children to a generalized ophthalmoparesis in some adult cases. The muscle weakness and atrophy predominantly involved proximal muscles of the shoulder and pelvic girdles, and also back and hand muscles. Atrophy of the quadriceps muscles was prominent in the more severely affected adults. The muscle weakness was remarkably variable. Most children and adolescents and some adults were mildly affected, whereas some adults had experienced progressive muscle weakness affecting ambulation from age 30 to 50 years. While mildly affected cases had normal serum creatine kinase (s-CK) levels, it was slightly elevated in the family members showing a progressive course. A fine action hand tremor has been noticed in several cases.
The severity of the disease was apparently related to the amount of expressed MyHC IIa protein in muscle . Severely affected individuals and muscles showed a large proportion of type 2A muscle fibers, and the fibers with rimmed vacuoles expressed MyHC IIa. Studies on the effects of endurance training have been performed [69, 74]. An 8-week endurance training program on a stationary exercise bicycle had no adverse effects. The peak watt and walking speed increased, but there was no significant increase in muscle strength or isometric endurance . The training program resulted in a shift in expression from fast to slow MyHC isoforms. There was no significant change in the MyHC IIa expression, but frequent hybrid fibers expressing more than one MyHC isoform were identified .
Patients with recessive MyHC IIa mutations have recently been identified. The so far published cases are homozygous or compound heterozygous for truncating mutations in MYH2, accompanied by complete loss of MyHC IIa protein and absence of type 2A muscle fibers . The clinical picture was surprisingly mild with minor or moderate generalized muscle weakness including facial muscle weakness. A consistent finding was external ophthalmoplegia, which was only occasionally associated with ptosis. Magnetic resonance imaging (MRI) of lower limb muscles in two cases demonstrated selective involvement of certain muscles with fatty infiltration. There was predominant involvement of medial gastrocnemius in the lower legs, combined with predominant involvement of the semitendinosus, gracilis and vastus lateralis muscles in the thigh.
Muscle biopsy demonstrated in addition to complete loss of MyHC IIa protein, unspecific myopathic changes with fiber size variability, internalized nuclei and interstitial fatty infiltration . Some muscle biopsy samples demonstrated type 1 fiber uniformity. Unlike the dominant MyHC IIa myopathy, no rimmed vacuoles or protein aggregates have been identified.
Myopathies associated with slow/beta MyHC, MYH7
Overall, mutations in MYH7 are predominantly missense mutations located in the globular myosin head, potentially affecting the binding sites for actin or nucleotides but there are also numerous mutations in the rod region. More than 200 different mutations have been associated with either hypertrophic or dilated cardiomyopathy, but there are also pure skeletal myopathies and combination of myopathy and cardiomyopathy.
Myosin storage myopathy
A dominant missense mutation changing the highly conserved and positively charged arginine at position 1845 to the uncharged, aromatic tryptophan (R1845W) was the first mutation identified in myosin storage myopathy. This mutation has been reported in several unrelated cases indicating that the C nucleotide at position 5533 is a mutational hotspot [32, 34, 55, 65, 76]. Two additional dominant missense mutations, H1901L  and L1793P , have later been reported to cause myosin storage myopathy. Recently, a heterozygous A to G transition in exon 40 of MYH7 was reported in several individuals of a five-generation family . The mutation is predicted to change the terminal stop codon (TAG) to a tryptophan (W), resulting in an elongation of the C-terminal tail region (X1936WfsX32). Another dominant in frame deletion of the amino acid lysine at position 1784 (K1784del) in exon 37 of MYH7 has recently been reported in a case with myosin storage myopathy . Myosin storage myopathy combined with cardiomyopathy was reported in three siblings homozygous for a missense mutation (E1883K), but with apparently unaffected parents . Mutations in the distal rod region of slow/β cardiac MyHC associated with myosin storage myopathy may thus be either dominant or recessive.
Myosin storage myopathy was first reported as a familial myopathy with probable lysis of myofibrils in type 1 fibers by Cancilla et al. . The inclusions were described as hyaline bodies because of their unstructured appearance at light microscopy , and the term hyaline body myopathy was used in some subsequent case reports. After the discovery that the storage material consists of mainly myosin heavy chain immunoreactive material and that the mutated gene is the slow/β cardiac MyHC gene, MYH7, the term myosin storage myopathy was introduced  and has been adopted and used in most subsequent reports.
The clinical manifestations are highly variable ranging from no weakness to severe impairment of ambulation [9, 11, 13, 15, 34, 42, 55, 59, 65, 68, 76, 81]. This variability can also be present within a family [11, 55, 81]. Onset is usually in childhood but may be much later. Presenting signs and symptoms are often delayed motor milestones, difficulties in climbing stairs or running and a waddling gate. The distribution of muscle weakness is usually proximal in upper extremities with difficulties in lifting the arms above the shoulder level and scapular winging has been described in many cases. In the lower extremities a distal involvement with foot drop is common and pseudo-hypertrophy of the calves is frequently encountered. Apart from a scapulo-peroneal distribution of weakness, a predominant limb-girdle weakness can be seen in some cases [13, 15]. The course is usually slowly progressive and scoliosis sometimes supervenes [11, 68] and some patients need assisted ventilation.
Evidence of cardiomyopathy associated with the myopathy is usually not found. However, in one woman with muscle weakness since age 30, hypertrophic cardiomyopathy was diagnosed at age 51 . Her 24-year-old daughter presented with cardiac failure already at age 3 months and was diagnosed with signs of left ventricular non-compaction. Three severely affected siblings homozygous for apparently recessive mutations had myosin storage myopathy and hypertrophic cardiomyopathy with cardiac failure that was lethal in two cases .
Magnetic resonance imaging (MRI) of muscle in myosin storage myopathy in ten individuals from one family demonstrated in the lower limbs early involvement of the biceps femoris and semimembranosus muscles and relative sparing of the semitendinosus in the posterior compartment of the thigh . In the distal muscles of the legs there was a predominant involvement of the medial gastrocnemius, tibialis anterior, extensor hallucis longus and extensor digitorum longus muscles. In the upper limbs there was a predominant involvement of the deltoid muscle. There was no asymmetry.
Laing (Gowers–Laing) distal myopathy
Laing distal myopathy was first linked to chromosome 14q11 [35, 83], and it was later demonstrated to be caused by mutations in MYH7 . Most cases have presented early in childhood and hence the term “early-onset” distal myopathy has been used in several reports. Onset is, however, not always in childhood but can occur with a range from congenital to 50 years [35, 36, 41, 44, 77, 83]. The clinical course in typical cases starts with early-onset weakness of ankle dorsiflexors and a “hanging big toe” sign. Calf hypertrophy may be present. During the slowly progressive course finger extensor and neck flexion weakness and later also proximal limb and facial muscle weakness supervenes. Patients usually remain ambulant. Serum CK levels are normal or moderately elevated. Cardiac involvement is usually not a feature of Laing distal myopathy but there are a few exceptions with diseases associated with MYH7 mutations and presenting with distal myopathy as well as cardiomyopathy [20, 29, 53, 80].
Muscle MRI has demonstrated early changes in the anterior compartments of the lower leg especially the extensor hallucis longus and tibialis anterior muscles [21, 44, 77]. Later in the course and in more severely affected patients additional muscles in the thigh may be involved.
Other skeletal muscle phenotypes associated with MYH7 mutations
Although the most important skeletal muscle manifestations of MYH7 mutations are referred to as either myosin storage myopathy or Laing distal myopathy several cases cannot be classified into these groups because they either do not display distal muscle weakness or do not exhibit hyaline bodies. Muscle biopsy of some patients with mutations that are known to cause myosin storage myopathy may not reveal the pathognomonic changes, but instead unspecific changes  or alterations diagnostic for congenital fiber type disproportion . A few patients with MYH7 mutations that otherwise cause distal myopathy could be classified as having limb-girdle syndrome or scapuloperoneal myopathy thus overlapping with the clinical features of myosin storage myopathy . In some but not all patients with cardiomyopathy due to MYH7 mutations, but without overt muscle weakness, cores may be identified in skeletal muscle fibers .
MYH3 mutations and distal arthrogryposis
A correct understanding of the cause of distal arthrogryposis due to MYH3 mutations would require examination of muscle pathology during the developmental period when MYH3 is expressed. Since MYH3 mutations associated with distal arthtrogryposis are dominant, it may be hypothesized that they affect muscle function during early development through either haploinsufficiency with insufficient dosage of a functional embryonic MyHC or a dominant negative effect of the mutated allele. Most of the MYH3 mutations are missense, but it is not known whether the mutant allele is expressed. Although the hemizygous loss of MyHC IIa expression is tolerated well in individuals with heterozygous MYH2 null mutations , the situation may be different with embryonic MyHC, since it is a predominant MyHC isoform expressed during an early period of development. Another possibility would be that MYH3 mutations impose dominant negative effects by functional or structural alterations as seen in dominant mutations affecting the adult MyHC isoforms. However, studies on disease models are needed to pinpoint the pathogenesis of developmental myopathy caused by MYH3 mutations.
The MyHC IIa E706K mutation
The E706K mutation associated with the autosomal dominant MyHC IIa myopathy has been studied at the molecular level in different systems. The mutated residue is situated in the SH1 helix in the core of myosin motor domain, which is strongly conserved throughout the myosin class II family. The SH1 helix plays a central role in the conformational changes of the myosin head during the ATP cycle.
Several studies have suggested impaired functional properties as the primary molecular defect of the E706K mutated MyHC protein. In vitro motility studies on myosin IIa isolated from single muscle fibers of patients carrying the E706K mutation clearly showed a marked reduction of speed . Furthermore, studies on the motor function of E683K mutant myosin of Dictyostelium discoideum, which is equivalent to the human MyHC IIa E706K mutation, demonstrated a threefold reduction of the ATP hydrolysis step followed by the slower acto-myosin dissociation . Consequently, these effects lead to a reduced velocity of contraction. A transgenic C. elegans was generated to simulate MyHC IIa myopathy in order to assess the functional or structural effects of the E706K mutation . Worms that were null mutant for UNC-54 (encoding the major C. elegans myosin heavy chain expressed in body wall muscle) were partly rescued by transfection with wild type UNC-54. Transfection with a construct encoding UNC-54 with an E710K mutation corresponding to the human MyHC IIa E706K mutation resulted in restored thick filaments but completely paralyzed worms . In addition, mutations in SH1 helix of nonmuscle class II MYHIIA, encoded by MYH9 (R702C and R705H), have been associated with a group of inherited giant-platelet disorders in several families . Association of diseases with mutations in SH1 helix of myosin in both muscle and non-muscle MyHC illustrates the crucial role of this thiol region in the head of myosin for proper function of the protein. These different studies all point to a severe defect of the molecular motor function caused by the E706K mutation.
Structural changes in the muscle fibers and muscle fiber loss are likely to be an additional mechanism causing muscle weakness. Morphological studies on muscle biopsy specimens have demonstrated a clear correlation between the expression of the mutated MyHC IIa and dystrophic changes in muscle . The high expression of mutant protein combined with aging may cause defective elimination of misfolded proteins, resulting in accumulation of protein aggregates and secondary muscle fiber degeneration. This hypothesis is supported by the accumulation of ubiquitinated and p62-labeled protein aggregates in vacuolated muscle fibers. This mechanism could be similar to what is seen in inclusion body myositis although the upstream events are different [6, 46, 47]. The same mechanism could also be involved in the muscle degeneration and protein aggregation seen in advanced cases of Laing distal myopathy.
In a recent investigation of the molecular background of the mouse mutant ariel a recessive missense mutation, L342Q, in the motor domain of Myh4 was found to cause a myofibrillar myopathy-like phenotype with protein aggregates in fast MyHC IIb fibers . The mutant protein formed also aggregates in COS7 cells and did not incorporate into thick filaments of C2C12 cells. In ariel heterozygotes, which showed no weakness or aggregates, the level of mutant protein was only 7 %, indicating efficient degradation of the defective protein prohibiting protein aggregation. MYH4 is not expressed in human limb muscle. However, mRNA but not protein has been identified in human masseter muscle . A functional equivalent of mouse MyHC IIb is MyHC IIx (MYH1) in humans. So far no disease has been associated with human MYH1 mutations. Protein aggregation with features similar to those in myofibrillar myopathy and in the ariel mouse was identified in one patient with a mutation in the rod region of MyHC I (MYH7) mutation .
MyHC IIa null mutations
The selective muscle involvement of certain muscles with fatty infiltration and loss of type 2A muscle fibers with interstitial fatty infiltration in patients with MyHC IIa null myopathy indicate that complete loss of one adult MyHC cannot be compensated for by other MyHC isoforms . This is further supported by studies on strains of mice null for different MyHC isoforms indicating that different isoforms of MyHC are functionally unique and cannot substitute for one another [1, 2, 60]. Mice MyHC IIb or IId null strains are viable but exhibit growth and muscle defects with significant decreases in body mass and mean muscle mass. Although both strains showed evidence of skeletal muscle pathology, the extent and the pattern of affected muscles did not correlate with the abundance and distribution of the two MyHC isoforms in normal mice. Together these observations indicate that MyHC isoforms impose unique structural and functional roles and support the hypothesis that MyHC isoforms are unable to replace one another.
Mutations in the LMM region of slow/β-cardiac MyHC
Mutations in the LMM region can affect the ability of the protein to form stable and functional thick filaments, based on the amino acid change, the position in the heptad repeat motif and the location in the LMM. Missense mutations associated with myosin storage myopathy (L1793P, R1845W, E1883K and H1901L) are located within or closed to the 29-residue assembly competence domain, in the C-terminus coiled-coil rod region of MyHC, which is known to be critical for the proper assembly of sarcomeric myosin rod filaments . These mutated residues are located either in the outer b or f position, where the side chains are available to interact with other myosin dimers or other proteins, or in the d position that stabilize the structure of the protein. Mutations at outer positions may cause improper filament formation through disturbed interaction with other myosin dimers and thereby perturb thick filament assembly.
A disturbed interaction with other sarcomeric proteins such as titin, myosin-binding proteins (MyBP) C and H, M-protein or myomesin 1, may also be considered. In this context it is of interest to note that one patient with a mutation in the rod region of MyHC I showed cytoplasmic bodies , a type of protein aggregates that are otherwise typically associated with A-band titin (TTN) mutations .
The aberrant accumulations of myosin in the muscle fibers of affected individuals with myosin storage myopathy might be due to improper incorporation of mutated myosin into thick filaments or disassembly of the filaments resulting in accumulation of mutant protein between the myofibrils and beneath the sarcolemma. The observation that some of the accumulated myosin appears to be ubiquitinated indicates that it is marked to be degraded through the proteasomal pathway . The accumulation may be due to insufficient degradation. It is noteworthy that the inclusion bodies are not labeled by p62, which is otherwise common in protein aggregate diseases .
Several in vitro studies have been performed to investigate molecular mechanisms involved in the pathogenesis of diseases caused by mutations in the rod region of muscle MyHC. Biochemical and biophysical characterization of the effects of the myosin storage myopathy mutations in the LMM region have suggested adverse effects of the mutations in the ability of the protein to form stable and functional thick filaments . However, analyses of different mutations demonstrated that each mutation has a unique effect on the biochemical and biophysical properties of the LMM . The pathogenic mechanisms of the R1500P and L1706P causing Laing early-onset distal myopathy have been investigated by biochemical assays and in different cellular systems [5, 12]. By co-expression of wild-type and mutant proteins in non-muscle COS-7 cells, in the C2C12 mouse muscle cell line and in neonatal rat ventricular myocytes, dominant adverse effects could be demonstrated including formation of myosin aggregates . Generation of transgenic C. elegans indicated that the proline mutations did not affect the thick filament formation or muscle force generation but myosin aggregates were identified in the R1500P mutants .
It is still unclear why Laing distal myopathy and myosin storage myopathy are associated with different muscle pathologies. Myosin storage myopathy mutations are located in exons 37–40 in the LMM region of slow/β-cardiac MyHC within or close to the assembly competence domain, which is crucial for proper filament assembly. Consequently, mutations in this region may cause defective integration of dimers into the thick filament leading to accumulation of unassembled MyHC. In contrast, mutations associated with Laing distal myopathy that are situated at distance from the assembly competence domain might cause other effects on the thick filament structure and function leading to different pathology.
Further work is needed to get insight into the mechanisms governing the cardiac and/or skeletal muscle involvement as well as the pathogenesis leading to protein aggregation in some of the myosin myopathies. For this purpose and for classification of the diseases, which may show clinical overlap, it is essential to perform morphological investigations. Based on correlations between genotype and muscle phenotype in humans, the use of animal models in combination with in vitro studies will allow investigation of the disease mechanisms and clarify the functional impact of mutations affecting different isoforms and different domains of the MyHC.
This work was supported by the Swedish Research Council Proj. No. 07122 and 20628.
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