Clinical Heterogeneity in Korean Patients with Nemaline Myopathy

Purpose

Nemaline myopathy (NM) is a clinical heterogeneous congenital myopathy characterized by the presence of subsarcolemmal or cytoplasmic rod-like structures that call nemaline bodies in the muscle fibers. The purpose of this study was to investigate the clinical diversity and pathological features of Korean patients with NM.

Materials and Methods

Eight patients underwent analyses of clinical manifestations by a structured protocol. Diagnoses were established by a muscle biopsy.

Results

Two patients had the typical congenital type, which exhibited neonatal hypotonia and delayed motor milestone, and five patients had the childhood onset type, which exhibited mild gait disturbance as a first symptom. One patient had the adult onset type, which showed acute respiratory failure. Limb weakness was proximal-dominant occurred in six patients. Hyporeflexia was observed in most patients. Elongated faces and high arched palates and feet were also observed. On light microscopy, the nemaline bodies were observed in type 1 and 2 fibers. All patients showed type 1 predominance and atrophy. In the two cases in which ultrastructural studies were performed, typical nemaline rods and disorganized myofibrillar apparatus were detected.

Conclusion

In conclusion, the eight Korean patients in this study with NM shared common clinical expressions such as proximal limb weakness, reduced deep tendon reflex, and dysmorphic features. This study, however, showed that clinical heterogeneity ranged from typical congenital, mildly affected childhood to the adult onset form with acute respiratory failure. The pathological findings in this study were in accordance with those of other previous reports.     

Thin filament proteins mutations associated with skeletal myopathies: Defective regulation of muscle contraction

In humans, more than 140 different mutations within seven genes (ACTA1, TPM2, TPM3, TNNI2, TNNT1, TNNT3, and NEB) that encode thin filament proteins (skeletal α-actin, β-tropomyosin, γ-tropomyosin, fast skeletal muscle troponin I, slow skeletal muscle troponin T, fast skeletal muscle troponin T, and nebulin, respectively) have been identified. These mutations have been linked to muscle weakness and various congenital skeletal myopathies including nemaline myopathy, distal arthrogryposis, cap disease, actin myopathy, congenital fiber type disproportion, rod-core myopathy, intranuclear rod myopathy, and distal myopathy, with a dramatic negative impact on the quality of life. In this review, we discuss studies that use various approaches such as patient biopsy specimen samples, tissue culture systems or transgenic animal models, and that demonstrate how thin filament proteins mutations alter muscle structure and contractile function. With an enhanced understanding of the cellular and molecular mechanisms underlying muscle weakness in patients carrying such mutations, better therapy strategies can be developed to improve the quality of life.     

Congenital myopathies: diseases of the actin cytoskeleton

Congenital myopathies are clinical and genetic heterogeneous disorders characterized by skeletal muscle weakness ranging in severity. Three major forms have been identified: actin myopathy, intranuclear rod myopathy, and nemaline myopathy. Nemaline myopathy is the most common of these myopathies and is further subdivided into seven groups according to severity, progressiveness, and age of onset. At present, five genes have been linked to congenital myopathies. These include alpha-actin (ACTA1), alpha- and beta-tropomyosin (TPM3 and TPM2), troponin T (TNNT1), and nebulin (NEB). Their protein products are all components of the thin filament of the sarcomere. The mutations identified within these genes have varying impacts on protein structure and give rise to different forms of congenital myopathies. Greater understanding of muscle formation and cause of disease can be established through the study of the effect of mutations on the functional proteins. However, a major limitation in the understanding of congenital myopathies is the lack of correlation between the degree of sarcomeric disruption and disease severity. Consequently, great difficulty may be encountered when diagnosing patients and predicting the progression of the disorders. There are no existing cures for congenital myopathies, although improvements can be made to both the standard of living and the life expectancy of the patient through various therapies.     

Mutations of tropomyosin 3 (TPM3) are common and associated with type 1 myofiber hypotrophy in congenital fiber type disproportion

Congenital fiber type disproportion (CFTD) is a rare congenital myopathy characterized by hypotonia and generalized muscle weakness. Pathologic diagnosis of CFTD is based on the presence of type 1 fiber hypotrophy of at least 12% in the absence of other notable pathological findings. Mutations of the ACTA1 and SEPN1 genes have been identified in a small percentage of CFTD cases. The muscle tropomyosin 3 gene, TPM3, is mutated in rare cases of nemaline myopathy that typically exhibit type 1 fiber hypotrophy with nemaline rods, and recently mutations in the TPM3 gene were also found to cause CFTD. We screened the TPM3 gene in patients with a clinical diagnosis of CFTD, nemaline myopathy, and with undefined congenital myopathies. Mutations in TPM3 were identified in 6 out of 13 patients with CFTD, as well as in one case of nemaline myopathy. Review of muscle biopsies from patients with diagnoses of CFTD revealed that patients with a TPM3 mutation all displayed marked disproportion of fiber size, without type 1 fiber predominance. Several mutation‐negative cases exhibited other abnormalities, such as central nuclei and central cores. These results support the utility of the CFTD diagnosis in directing the course of genetic testing. Hum Mutat 30:1–8, 2009. © 2009 Wiley‐Liss, Inc.     

Nemaline myopathy and distal arthrogryposis associated with an autosomal recessive TNNT3 splice variant

A male neonate presented with severe weakness, hypotonia, contractures and congenital scoliosis. Skeletal muscle specimens showed marked atrophy and degeneration of fast fibers with striking nemaline rods and hypertrophy of slow fibers that were ultrastructurally normal. A neuromuscular gene panel identified a homozygous essential splice variant in TNNT3 (chr11:1956150G > A, NM_006757.3:c.681+1G > A). TNNT3 encodes skeletal troponin‐T_fast and is associated with autosomal dominant distal arthrogryposis. TNNT3 has not previously been associated with nemaline myopathy (NM), a rare congenital myopathy linked to defects in proteins associated with thin filament structure and regulation. cDNA studies confirmed pathogenic consequences of the splice variant, eliciting exon‐skipping and intron retention events leading to a frameshift. Western blot showed deficiency of troponin‐T_fast protein with secondary loss of troponin‐I_fast. We establish a homozygous splice variant in TNNT3 as the likely cause of severe congenital NM with distal arthrogryposis, characterized by specific involvement of Type‐2 fibers and deficiency of troponin‐T_fast.     

Muscle weakness in a mouse model of nemaline myopathy can be reversed with exercise and reveals a novel myofiber repair mechanism

Patients with the inherited muscle disease nemaline myopathy experience prolonged muscle weakness following periods of immobility. We have examined endurance exercise as a means of improving recovery following muscle inactivity in our alpha-tropomyosin(slow)(Met9Arg)-transgenic mouse model of nemaline myopathy. Physical inactivity, mimicked using a hindlimb immobilization protocol, resulted in fiber atrophy and severe muscle weakness. Following immobilization, the nemaline mice (NM) were weaker than WT mice but regained whole-body strength with exercise training. The disuse-induced weakness and the regain of strength with exercise in NM were associated with the respective formation and resolution of nemaline rods, suggesting a role for rods in muscle weakness. Muscles from NM did not show the typical features of muscle repair during chronic stretch-immobilization of the soleus muscle (regeneration occurred with relative lack of centralized nuclei). This indicates that the normal process of regeneration may be altered in nemaline myopathy and may contribute to poor recovery. In conclusion, endurance exercise can alleviate disuse-induced weakness in NM. The altered myofiber repair process in the nemaline mice may be a response to primary myofibrillar damage that occurs in nemaline myopathy and is distinct from the classical repair in muscular dystrophy resulting from plasma membrane defects.     

Physiological consequences of tropomyosin mutations associated with cardiac and skeletal myopathies

Mutations have been identified in alpha-tropomyosin (Tm), a key regulatory protein in striated muscle cells, that are associated with a human cardiac myopathy, hypertrophic cardiomyopathy (FHC) and a human skeletal myopathy, nemaline myopathy (NM). In this review, we highlight experiments aimed at identifying the underlying mechanisms by which mutations in alpha-Tm cause inherited diseases of cardiac and skeletal muscle. Gene transfer of normal and mutant alpha-Tm to isolated adult cardiac myocytes was used to study the primary effects of mutant alpha-Tm proteins on the structure and contractile function of fully differentiated striated muscle cells. Both FHC and NM mutant alpha-Tm proteins incorporated normally into the adult muscle sarcomere, similar to normal Tm but exerted differential "dominant-negative" effects on the contractile function of the muscle cell. FHC mutant alpha-Tm proteins produced hypersensitivity of Ca2+-activated force production with a hierarchy that was related to the clinical severity of each mutation. Conversely, the NM mutant alpha-Tm produced a hyposensitivity of Ca2+-activated force production that may underlie, at least in part, the muscle weakness observed in NM. Taken together, the results suggest that the differential changes in the ability of the mutant Tm proteins to regulate muscle contraction in response to changing Ca2+ concentrations underlie the differential clinical presentation of the cardiac and skeletal muscle myopathies associated with mutations in alpha-Tm.     

Gene-related protein surplus myopathies

Numerous muscular dystrophies, such as dystrophinopathies, sarcoglycanopathies, and emerino- and laminopathies, are marked by the absence or reduction of mutant transsarcolemmal or nuclear proteins. In addition to these recently identified minus-proteinopathies, there are a growing number of plus-proteinopathies among neuromuscular disorders marked by a surplus or excess of endogenous proteins within muscle fibers of different, i.e., nontranssarcolemmal and nonnuclear types. These proteins are often filamentous; for example, desmin and actin accrue in respective desmin-related myopathies, among which are entities marked by mutant desmin, true desminopathies, and actinopathy, the latter often seen as a subgroup in nemaline myopathies. Desmin-related myopathies consist largely of those marked by desmin-containing inclusions and those characterized by desmin-containing granulofilamentous material. When mutations in the desmin gene can be identified, the mutant desmin is thought to form the major myopathological lesion. Together with desmin, other proteins often accumulate. The spectrum of these proteins is quite diverse and encompasses such proteins as dystrophin, nestin, vimentin, alphaB-crystallin, ubiquitin, amyloid precursor protein, and beta-amyloid epitopes, as well as gelsolin and alpha(1)-antichymotrypsin. Among these associated proteins, one, alphaB-crystallin, has been found mutant in one large family, justifying the term alphaB-crystallinopathy as a separate condition among the desmin-related myopathies. Other proteins accruing with desmin have not yet been identified as mutant in desmin-related myopathies. Mutations in the desmin gene entail missense mutations and small deletions. The formation of mutant actin may lead to aggregates of actin filaments which may or may not be associated with formation of sarcoplasmic and/or intranuclear nemaline bodies. A considerable number of missense mutations in the sarcomeric actin gene ACTA1 have been discovered in patients with nemaline myopathy and also in a few patients without myopathological evidence of nemaline bodies in biopsied skeletal muscle fibres. Apart from alphaB-crystallin, no other proteins coaggregating with actin in actin filament aggregates of actinopathy or the actin mutation type of nemaline myopathy have so far been identified. Two further candidates for protein surplus myopathies are hyaline body myopathy, which is marked by accumulation of granular nonfilamentous material within muscle fibers that is rich in myosin and adenosine triphosphatase activities, and hereditary inclusion body myopathies, which are marked by accumulation of tubulofilaments similar to the helical filaments of Alzheimer neurofibrillary tangles. These tubulofilaments consist of diverse proteins as well, though no mutant protein has yet been discovered. So far, no genes responsible for familial hyaline body and hereditary inclusion body myopathies have been identified. The discovery of mutant proteins, desmin, alphaB-crystallin, and actin, as components of surplus or excess proteins accumulating in muscle fibers in certain neuromuscular conditions is responsible for the recent emergence of this new concept of gene-related protein surplus myopathies.     

Evidence for a dominant-negative effect in ACTA1 nemaline myopathy caused by abnormal folding, aggregation and altered polymerization of mutant actin isoforms

We have studied a cohort of nemaline myopathy (NM) patients with mutations in the muscle alpha-skeletal actin gene (ACTA1). Immunoblot analysis of patient muscle demonstrates increased gamma-filamin, myotilin, desmin and alpha-actinin in many NM patients, consistent with accumulation of Z line-derived nemaline bodies. We demonstrate that nebulin can appear abnormal secondary to a primary defect in actin, and show by isoelectric focusing that mutant actin isoforms are present within insoluble actin filaments isolated from muscle from two ACTA1 NM patients. Transfection of C2C12 myoblasts with mutant actin(EGFP) constructs resulted in abnormal cytoplasmic and intranuclear actin aggregates. Intranuclear aggregates were observed with V163L-, V163M- and R183G-actin(EGFP) constructs, and modeling shows these residues to be adjacent to the nuclear export signal of actin. V163L and V163M actin mutants are known to cause intranuclear rod myopathy, however, intranuclear bodies were not reported in patient R183G. Transfection studies in C2C12 myoblasts showed significant alterations in the ability of V136L and R183G actin mutants to polymerize and contribute to insoluble actin filaments. Thus, we provide direct evidence for a dominant-negative effect of mutant actin in NM. In vitro studies suggest that abnormal folding, altered polymerization and aggregation of mutant actin isoforms are common properties of NM ACTA1 mutants. Some of these effects are mutation-specific, and likely result in variations in the severity of muscle weakness seen in individual patients. A combination of these effects contributes to the common pathological hallmarks of NM, namely intranuclear and cytoplasmic rod formation, accumulation of thin filaments and myofibrillar disorganization.     

Recessive ACTA1 variant causes congenital muscular dystrophy with rigid spine

Variants in ACTA1, which encodes α-skeletal actin, cause several congenital myopathies, most commonly nemaline myopathy. Autosomal recessive variants comprise approximately 10% of ACTA1 myopathy. All recessive variants reported to date have resulted in loss of skeletal α-actin expression from muscle and severe weakness from birth. Targeted next-generation sequencing in two brothers with congenital muscular dystrophy with rigid spine revealed homozygous missense variants in ACTA1. Skeletal α-actin expression was preserved in these patients. This report expands the clinical and histological phenotype of ACTA1 disease to include congenital muscular dystrophy with rigid spine and dystrophic features on muscle biopsy. This represents a new class of recessive ACTA1 variants, which do not abolish protein expression.     

Typical nemaline bodies presenting in a patient with polymyositis

A 66-year-old woman presented with a progressive myopathy affecting the proximal limbs and unusual pathological findings of nemaline bodies on muscle biopsy. Histological examination demonstrated that the bodies were mainly located in the subsarcolemmal region of atrophic fibers, exhibited strong immunoreactivity with antibodies to both alpha-actinin and m-actin, and had a typical lattice-like appearance at higher magnification on electron microscopy. These findings were the same as those for nemaline myopathy. The patient responded to steroid therapy, but relapse occurred after steroid was discontinued. Given the clinical criteria of polymyositis, we believe that the occurrence of nemaline bodies in our patient should be interpreted primarily as an epiphenomenon of primary myopathy.     

A recurrent pathogenic variant in TPM2 reveals further phenotypic and genetic heterogeneity in multiple pterygium syndrome-related disorders

Multiple pterygium syndrome (MPS) disorders are a phenotypically and genetically heterogeneous group of conditions characterized by multiple joint contractures (arthrogryposis), pterygia (joint webbing) and other developmental defects. MPS is most frequently inherited in an autosomal recessive fashion but X-linked and autosomal dominant forms also occur. Advances in genomic technologies have identified many genetic causes of MPS-related disorders and genetic diagnosis requires large targeted next generation sequencing gene panels or genome-wide sequencing approaches. Using the Illumina TruSightOne clinical exome assay, we identified a recurrent heterozygous missense substitution in TPM2 (encoding beta tropomyosin) in three unrelated individuals. This was confirmed to have arisen as a de novo event in the two patients with parental samples. TPM2 mutations have previously been described in association with a variety of dominantly inherited neuromuscular phenotypes including nemaline myopathy, congenital fibre-type disproportion, distal arthrogryposis and trismus pseudocamptodactyly, and in a patient with autosomal recessive Escobar syndrome and a nemaline myopathy. The three cases reported here had overlapping but variable features. Our findings expand the range of TMP2-related phenotypes and indicate that de novo TMP2 mutations should be considered in isolated cases of MPS-related conditions.     

Identification of a founder mutation in TPM3 in nemaline myopathy patients of Turkish origin

To date, six genes are known to cause nemaline (rod) myopathy (NM), a rare congenital neuromuscular disorder. In an attempt to find a seventh gene, we performed linkage and subsequent sequence analyses in 12 Turkish families with recessive NM. We found homozygosity in two of the families at 1q12-21.2, a region encompassing the gamma-tropomyosin gene (TPM3) encoding slow skeletal muscle alpha-tropomyosin, a known NM gene. Sequencing revealed homozygous deletion of the first nucleotide of the last exon, c.913delA of TPM3 in both families. The mutation removes the last nucleotide before the stop codon, causing a frameshift and readthrough across the termination signal. The encoded alphaTm(slow) protein is predicted to be 73 amino acids longer than normal, and the extension to the protein is hypothesised to be unable to form a coiled coil. The resulting tropomyosin protein may therefore be non-functional. The affected children in both families were homozygous for the mutation, while the healthy parents were mutation carriers. Both of the patients in Family 1 had the severe form of NM, and also an unusual chest deformity. The affected children in Family 2 had the intermediate form of NM. Muscle biopsies showed type 1 (slow) fibres to be markedly smaller than type 2 (fast) fibres. Previously, there had been five reports, only, of NM caused by mutations in TPM3. The mutation reported here is the first deletion to be identified in TPM3, and it is likely to be a founder mutation in the Turkish population.European Journal of Human Genetics (2008) 16, 1055-1061; doi:10.1038/ejhg.2008.60; published online 2 April 2008.     

An autosomal dominant congenital myopathy with cores and rods is associated with a neomutation in the RYR1 gene encoding the skeletal muscle ryanodine receptor

Central core disease (CCD) and nemaline myopathy (NM) are congenital myopathies for which differential diagnosis is often based on the presence either of cores or rods. Missense mutations in the skeletal muscle ryanodine receptor gene (RYR1) have been identified in some families with CCD. Mutations in the alpha-tropomyosin and alpha-actin genes have been associated with most dominant forms of NM. Analysis of the RYR1 cDNA in a French family identified a novel Y4796C mutation that lies in the C-terminal channel-forming domain of the RyR1 protein. This mutation was linked not only to a severe and penetrant form of CCD, but also to the presence of rods in the muscle fibres and to the malignant hyperthermia susceptibility (MHS) phenotype. The Y4796C mutation was introduced into a rabbit RYR1 cDNA and expressed in HEK-293 cells. Expression of the mutant RYR1 cDNA produced channels with increased caffeine sensitivity and a significantly reduced maximal level of Ca(2+) release. Single-cell Ca(2+) analysis showed that the resting cytoplasmic level was increased by 60% in cells expressing the mutant channel. These data support the view that the rate of Ca(2+) leakage is increased in the mutant channel. The resulting chronic elevation in myoplasmic concentration is likely to be responsible for the severe expression of the disease. Haplotyping analysis indicated that the mutation arose as a neomutation in the proband. This first report of a neomutation in the RYR1 gene has strong implications for genetic linkage studies of MHS or CCD, two diseases characterized by a genetic heterogeneity.     

Nebulin (NEB) mutations in a childhood onset distal myopathy with rods and cores uncovered by next generation sequencing

Recessive nebulin (NEB) mutations are a common cause of nemaline myopathy (NM), typically characterized by generalized weakness of early-onset and nemaline rods on muscle biopsy. Exceptional adult cases with additional cores and an isolated distal weakness have been reported. The large NEB gene with 183 exons has been an obstacle for the genetic work-up. Here we report a childhood-onset case with distal weakness and a core-rod myopathy, associated with recessive NEB mutations identified by next generation sequencing (NGS). This 6-year-old boy presented with a history of gross-motor difficulties following a normal early development. He had distal leg weakness with bilateral foot drop, as well as axial muscle weakness, scoliosis and spinal rigidity; additionally he required nocturnal respiratory support. Muscle magnetic resonance (MR) imaging showed distal involvement in the medial and anterior compartment of the lower leg. A muscle biopsy featured both rods and cores. Initial targeted testing identified a heterozygous Nebulin exon 55 deletion. Further analysis using NGS revealed a frameshifting 4 bp duplication, c.24372_24375dup (P.Val8126fs), on the opposite allele. This case illustrates that NEB mutations can cause childhood onset distal NM, with additional cores on muscle biopsy and proves the diagnostic utility of NGS for myopathies, particularly when large genes are implicated.     

Fatal neonatal nemaline myopathy

Nemaline myopathy was first reported in 1963 and has been considered to be a congenital, non-progressive myopathy with weakness since birth. However, severe forms leading to death in infancy have been rarely reported. Recently we necropsied a female infant with fatal neonatal nemaline myopathy who required mechanical ventilatory support immediately following delivery. She had suffered from recurrent pneumonia and died at five months of age. Light and electron microscopic studies on both muscle biopsy and autopsy specimens were diagnostic of nemaline myopathy. Characteristic rod-like structures were demonstrated within skeletal muscles, and accumulations of thin filaments were seen in numerous muscle fibers. The origin of these structures in the Z-disks and other morphologic features were the same as those of the rods occurring in congenital cases of rod myopathy. No involvement was observed in the cardiac muscle or in the smooth muscle of the gastrointestinal tract. It is likely that involvement of the skeletal muscles of the pharyngeal areas, intercostal spaces, and diaphragm might have contributed to the difficulty in swallowing and respiratory distress.     

Congenital nemaline myopathy with dilated cardiomyopathy: an autopsy study

A 3-year-old boy with congenital nemaline myopathy had generalized muscle weakness and hypotonia since birth. He developed cardiac symptoms at 2 years of age and died from congestive heart failure. At autopsy, the heart was markedly dilated, involving both ventricles. Rod bodies were recognized not only in skeletal muscles but in cardiac muscles on light and electron microscopy. Desmin and alpha-actinin, which constitute Z-line protein, were shown to localize in the rod structures in both skeletal and myocardial cells by immunohistochemistry. Seven cases of nemaline myopathy with cardiomyopathy have been reported in the literature. All of these patients were over 20 years of age, and the condition appeared mostly in the adult onset and the asymptomatic forms. This is the first infantile case of congenital nemaline myopathy which showed dilated cardiomyopathy with a fatal outcome.     

FATAL NEONATAL NEMALINE MYOPATHY

Nemaline myopathy was first reported in 1963 and has been considered to be a congenital, non-progressive myopathy with weakness since birth. However, severe forms leading to death in infancy have been rarely reported. Recently we necropsied a female infant with fatal neonatal nemaline myopathy who required mechanical ventilatory support immediately following delivery. She had suffered from recurrent penumonia and died at five months of age. Light and electron microscopic studies on both muscle biopsy and autopsy specimens were diagnostic of nemaline myopathy. Characteristic rodlike structures were demonstrated within skeletal muscles, and accumulations of thin filaments were seen in numerous muscle fibers. The origin of these structures in the Z-disks and other morphologic features were the same as those of the rods occurring in congenital cases of rod myopathy. No involvement was observed in the cardiac muscle or in the smooth muscle of the gastrointestinal tract. It is likely that involvement of the skeletal muscles of the pharyngeal areas, intercostal spaces, and diaphragm might have contributed to the difficulty in swallowing and respiratory distress.