Mutations of the slow muscle alpha-tropomyosin gene, TPM3, are a rare cause of nemaline myopathy

Tropomyosin 3 (TPM3) function in skeletal muscle and in myopathy

The tropomyosin genes (TPM1-4) contribute to the functional diversity of skeletal muscle fibers. Since its discovery in 1988, the TPM3 gene has been recognized as an indispensable regulator of muscle contraction in slow muscle fibers. Recent advances suggest that TPM3 isoforms hold more extensive functions during skeletal muscle development and in postnatal muscle. Additionally, mutations in the TPM3 gene have been associated with the features of congenital myopathies. The use of different in vitro and in vivo model systems has leveraged the discovery of several disease mechanisms associated with TPM3-related myopathy. Yet, the precise mechanisms by which TPM3 mutations lead to muscle dysfunction remain unclear. This review consolidates over three decades of research about the role of TPM3 in skeletal muscle. Overall, the progress made has led to a better understanding of the phenotypic spectrum in patients affected by mutations in this gene. The comprehensive body of work generated over these decades has also laid robust groundwork for capturing the multiple functions this protein plays in muscle fibers.

Novel Compound Heterozygous Splice-Site Variants in TPM3 Revealed by RNA Sequencing in a Patient with an Unusual Form of Nemaline Myopathy: A Case Report

BACKGROUND

Pathogenic variants in the TPM3 gene, encoding slow skeletal muscle α-tropomyosin account for less than 5% of nemaline myopathy cases. Dominantly inherited or de novo missense variants in TPM3 are more common than recessive loss-of-function variants. The recessive variants reported to date seem to affect either the 5' or the 3' end of the skeletal muscle-specific TPM3 transcript.

OBJECTIVES

The aim of the study was to identify the disease-causing gene and variants in a Finnish patient with an unusual form of nemaline myopathy.

METHODS

The genetic analyses included Sanger sequencing, whole-exome sequencing, targeted array-CGH, and linked-read whole genome sequencing. RNA sequencing was done on total RNA extracted from cultured myoblasts and myotubes of the patient and controls. TPM3 protein expression was assessed by Western blot analysis. The diagnostic muscle biopsy was analyzed by routine histopathological methods.

RESULTS

The patient had poor head control and failure to thrive, but no hypomimia, and his upper limbs were clearly weaker than his lower limbs, features which in combination with the histopathology suggested TPM3-caused nemaline myopathy. Muscle histopathology showed increased fiber size variation and numerous nemaline bodies predominantly in small type 1 fibers. The patient was found to be compound heterozygous for two splice-site variants in intron 1a of TPM3: NM_152263.4:c.117+2_5delTAGG, deleting the donor splice site of intron 1a, and NM_152263.4:c.117 + 164 C>T, which activates an acceptor splice site preceding a non-coding exon in intron 1a. RNA sequencing revealed inclusion of intron 1a and the non-coding exon in the transcripts, resulting in early premature stop codons. Western blot using patient myoblasts revealed markedly reduced levels of the TPM3 protein.

CONCLUSIONS

Novel biallelic splice-site variants were shown to markedly reduce TPM3 protein expression. The effects of the variants on splicing were readily revealed by RNA sequencing, demonstrating the power of the method.

Novel autosomal dominant TPM3 mutation causes a combined congenital fibre type disproportion-cap disease histological pattern

Tropomyosin 3 (TPM3) gene mutations associate with autosomal dominant and recessive nemaline myopathy 1 (NEM1), congenital fiber type disproportion myopathy (CFTD) and cap myopathy (CAPM1), and a combination of caps and nemaline bodies. We report on a 47-year-old man with polyglobulia, restricted vital capacity and mild apnea hypopnea syndrome, requiring noninvasive ventilation. Physical assessment revealed bilateral ptosis and facial paresis, with high arched palate and retrognathia; global hypotonia and diffuse axial weakness, including neck and upper and lower limb girdle and foot dorsiflexion weakness. Whole body MRI showed a diffuse fatty replacement with an unspecific pattern. A 122 gene NGS neuromuscular disorders panel revealed the heterozygous VUS c.709G>A (p.Glu237Lys) on exon 8 of TMP3. A deltoid muscle biopsy showed a novel histological pattern combining fiber type disproportion and caps. Our findings support the pathogenicity of the novel TPM3 variant and widen the phenotypic gamut of TMP3-related congenital myopathy.

Transcript-Based Diagnosis and Expanded Phenotype of an Intronic Mutation in TPM3 Myopathy

INTRODUCTION

Congenital myopathies are a broad group of inborn muscle disorders caused by a multitude of genetic factors, often characterized by muscle atrophy and hypotonia.

METHODS

Clinical studies, imaging, histology, whole-exome sequencing (WES) and muscle tissue RNA studies.

RESULTS

We describe a severe congenital myopathy manifesting at birth with bilateral clubfeet, delayed motor development and hypotonia, becoming evident by 4 months of age. At 3 years of age, the patient had tongue fasciculations, was bedridden, and was chronically ventilated via tracheostomy. Imaging studies demonstrated severe muscle atrophy and, surprisingly, cerebral atrophy; electromyography demonstrated a myasthenic pattern and histological evaluation did not facilitate a definitive diagnosis. Trio WES did not identify a causative variant, except for a non-canonical intronic TPM3 c.118-12G>A variant of uncertain significance. Transcript analysis of muscle tissue from the patient proved the pathogenicity of this homozygous variant, with a 97% reduction in the muscle-specific TPM3.12 transcript.

DISCUSSION

This study broadens the phenotypic spectrum of recessive TPM3 disease, highlighting tongue fasciculations and bilateral clubfoot, as well as possibly-related cerebral atrophy. It also shows the importance of a broad approach to genetic analysis and the utility of RNA-based studies, demonstrating efficacy of early genome and transcriptome queries in facilitating rapid and cost-effective diagnosis of congenital myopathies.

Clinico-pathological features and mutational spectrum of 16 nemaline myopathy patients from a Chinese neuromuscular center

Nemaline myopathy (NM) is a congenital myopathy of great heterogeneity, characterized by the presence of rods in the cytoplasm of muscle fibers. The samples of 16 nemaline myopathy patients diagnosed by characteristically pathological features went through whole exon sequencing. Clinico-pathological and genetic features of the cases were systematically analyzed. According to the classification of nemaline myopathy by ENMC, 8 cases are typical congenital subtype, 6 cases are childhood/juvenile onset subtype and 2 case are adult onset subtype. In histological findings, characteristic purple-colored rods are discovered under modified gömöri trichrome staining (MGT). Electron microscopy revealed the presence of high electron-dense nemaline bodies around the submucosa and the nucleus nine patients (9/16 56.3%) were detected pathogenic causative mutations, among whom mutations in the NEB gene were the most frequent (6 patients, 66.7%). KBTBD13 gene mutation was discovered in two patients and ACTA1 gene mutation was discovered in 1 patient. Nemaline myopathy is a congenital myopathy with highly clinico-pathological and genetic heterogeneity. NEB gene mutation is the most common mutation, in which splicing change c.21522 +3A > G is hotspot mutation in Chinese NM patients.

Knockin mouse model of the human CFL2 p.A35T mutation results in a unique splicing defect and severe myopathy phenotype

Cofilin-2 is an actin-binding protein that is predominantly expressed in skeletal and cardiac muscles and belongs to the AC group of proteins, which includes cofilin-1 and destrin. In humans, cofilin-2 (CFL2) mutations have been associated with congenital myopathies that include nemaline and myofibrillar myopathy. To understand the pathogenicity of the human CFL2 mutation, p.A35T, that first linked cofilin-2 with the human disease, we created a knock-in mouse model. The Cfl2A35T/A35T (KI) mice were indistinguishable from their wild-type littermates at birth, but they rapidly worsened and died by postnatal day 9. The phenotypic, histopathologic and molecular findings mimicked the constitutive Cfl2-knockout (KO) mice described previously, including sarcomeric disruption and actin accumulations in skeletal muscles and negligible amounts of cofilin-2 protein. In addition, KI mice demonstrated a marked reduction in Cfl2 mRNA levels in various tissues including skeletal muscles. Further investigation revealed evidence of alternative splicing with the presence of two alternate transcripts of smaller size. These alternate transcripts were expressed at very low levels in the wild-type mice and were significantly upregulated in the mutant mice, indicating that pre-translational splicing defects may be a critical component of the disease mechanism associated with the mutation. Evidence of reduced expression of the full-length CFL2 transcript was also observed in the muscle biopsy sample of the patient with p.A35T mutation.

A Cross-Sectional Study of Nemaline Myopathy

Objective

Nemaline myopathy (NM) is a rare neuromuscular condition with clinical and genetic heterogeneity. To establish disease natural history, we performed a cross-sectional study of NM, complemented by longitudinal assessment and exploration of pilot outcome measures.

Methods

Fifty-seven individuals with NM were recruited at 2 family workshops, including 16 examined at both time points. Participants were evaluated by clinical history and physical examination. Functional outcome measures included the Motor Function Measure (MFM), pulmonary function tests (PFTs), myometry, goniometry, and bulbar assessments.

Results

The most common clinical classification was typical congenital (54%), whereas 42% had more severe presentations. Fifty-eight percent of individuals needed mechanical support, with 26% requiring wheelchair, tracheostomy, and feeding tube. The MFM scale was performed in 44 of 57 participants and showed reduced scores in most with little floor/ceiling effect. Of the 27 individuals completing PFTs, abnormal values were observed in 65%. Last, bulbar function was abnormal in all patients examined, as determined with a novel outcome measure. Genotypes included mutations in ACTA1 (18), NEB (20), and TPM2 (2). Seventeen individuals were genetically unresolved. Patients with pathogenic ACTA1 and NEB variants were largely similar in clinical phenotype. Patients without genetic resolution had more severe disease.

Conclusion

We present a comprehensive cross-sectional study of NM. Our data identify significant disabilities and support a relatively stable disease course. We identify a need for further diagnostic investigation for the genetically unresolved group. MFM, PFTs, and the slurp test were identified as promising outcome measures for future clinical trials.

Whole-exome analyses of congenital muscular dystrophy and congenital myopathy patients from India reveal a wide spectrum of known and novel mutations

BACKGROUND AND PURPOSE

Congenital muscular dystrophies (CMDs) and congenital myopathies (CMs) are a group of genetically and clinically heterogeneous degenerative primary muscle disorders with onset at birth or during infancy. Due to vast heterogeneity, clinical examination and protein-based analyses often fail to identify the genetic causes of these diseases. The aim of this study was to genetically diagnose a cohort of 36 difficult-to-diagnose CMD and CM cases of Indian origin using next-generation sequencing methods.

METHODS

Whole-exome sequencing (WES) was performed to identify pathogenic mutations in previously reported CMD and CM-related genes using variant calling and stringent variant filtration process. Subsequently, in silico homology modelling and molecular dynamics simulations (MDS) studies were undertaken for a number of novel and missense variants.

RESULTS

A total of 33 and 21 rare and deleterious mutations were identified in 28 genes previously reported in CMD and CM based on OMIM, ClinVar and Orphanet, respectively. We could accurately diagnose 54% patients (n = 12/22) in the CMD group and 35% patients (n = 5/14) in the CM group. Furthermore, MDS studies for mutations located in LMNA, LAMA2 and RYR1 suggest that the wild-type proteins are more stable than their mutant counterparts, implying a potential mechanism of pathogenesis.

CONCLUSION

The WES findings led us to identify reported as well as novel variants for the first time in Indian patients with CMD and CM. This allowed us to achieve an accurate genetic diagnosis, which was difficult using conventional diagnostic tools. Transferring these WES findings to clinical practice will help guide clinical care of the affected patients and inform genetic counselling.

Myostatin Inhibition Using ActRIIB-mFc Does Not Produce Weight Gain or Strength in the Nebulin Conditional KO Mouse

Mutations in at least 12 genes are responsible for a group of congenital skeletal muscle diseases known as nemaline myopathies (NMs). NMs are associated with a range of clinical symptoms and pathological changes often including the presence of cytoplasmic rod-like structures (nemaline bodies) and myofiber hypotrophy. Our recent work has identified a variable degree of behavioral benefit when treating 2 NM mouse models due to mutations in Acta1 with myostatin inhibition. This study is focused on the effects of delivering ActRIIB-mFc (Acceleron; a myostatin inhibitor) to the nebulin conditional knockout KO (Neb cKO) mouse model of NM. Treatment of Neb cKO mice with ActRIIB-mFc did not produce increases in weight gain, strength, myofiber size, or hypertrophic pathway signaling. Overall, our studies demonstrate a lack of response in Neb cKO mice to myostatin inhibition, which differs from the response observed when treating other NM models.

Congenital fiber type disproportion caused by TPM3 mutation: A report of two atypical cases

Congenital fiber type disproportion (CFTD) is a rare congenital myopathy subtype defined by slow type 1 hypotrophy in the absence of any other major structural findings such as rods, central nuclei or cores. Dominant missense changes in slow alpha-tropomyosin coded by TPM3 gene are the main cause of the CFTD. There are only a few reports of recessive loss-of-function mutations in TPM3 causing severe Nemaline Myopathy and CFTD. We present two patients harboring TPM3 mutations. The first is a novel homozygous missense variant with a mild CFTD clinical phenotype inherited in a recessive fashion. The second is a previously reported heterozygous mutation presenting within pronounced early axial involvement and dropped head. This report expands the genotype-phenotype correlation in the TPM3 myopathy showing a recessive mutation causing a mild clinical phenotype and also shows that TPM3 mutations should be part of the investigation in patients with dropped head.

Update on the Genetics of Congenital Myopathies

The congenital myopathies form a large clinically and genetically heterogeneous group of disorders. Currently mutations in at least 27 different genes have been reported to cause a congenital myopathy, but the number is expected to increase due to the accelerated use of next-generation sequencing methods. There is substantial overlap between the causative genes and the clinical and histopathologic features of the congenital myopathies. The mode of inheritance can be autosomal recessive, autosomal dominant or X-linked. Both dominant and recessive mutations in the same gene can cause a similar disease phenotype, and the same clinical phenotype can also be caused by mutations in different genes. Clear genotype-phenotype correlations are few and far between.

Myostatin inhibition using mRK35 produces skeletal muscle growth and tubular aggregate formation in wild type and TgACTA1D286G nemaline myopathy mice

Nemaline myopathy (NM) is a heterogeneous congenital skeletal muscle disease with cytoplasmic rod-like structures (nemaline bodies) in muscle tissue. While weakness in NM is related to contractile abnormalities, myofiber smallness is an additional abnormality in NM that may be treatable. We evaluated the effects of mRK35 (a myostatin inhibitor developed by Pfizer) treatment in the TgACTA1D286G mouse model of NM. mRK35 induced skeletal muscle growth that led to significant increases in animal bodyweight, forelimb grip strength and muscle fiber force, although it should be noted that animal weight and forelimb grip strength in untreated TgACTA1D286G mice was not different from controls. Treatment was also associated with an increase in the number of tubular aggregates found in skeletal muscle. These findings suggest that myostatin inhibition may be useful in promoting muscle growth and strength in Acta1-mutant muscle, while also further establishing the relationship between low levels of myostatin and tubular aggregate formation.

Tropomyosin 3.5 protects the F-actin networks required for tissue biomechanical properties

Tropomyosins (Tpms) stabilize F-actin and regulate interactions with other actin-binding proteins. The eye lens changes shape in order to focus light to transmit a clear image, and thus lens organ function is tied to its biomechanical properties, presenting an opportunity to study Tpm functions in tissue mechanics. Mouse lenses contain Tpm3.5 (also known as TM5NM5), a previously unstudied isoform encoded by Tpm3, which is associated with F-actin on lens fiber cell membranes. Decreased levels of Tpm3.5 lead to softer and less mechanically resilient lenses that are unable to resume their original shape after compression. While cell organization and morphology appear unaffected, Tmod1 dissociates from the membrane in Tpm3.5-deficient lens fiber cells resulting in reorganization of the spectrin-F-actin and α-actinin-F-actin networks at the membrane. These rearranged F-actin networks appear to be less able to support mechanical load and resilience, leading to an overall change in tissue mechanical properties. This is the first in vivo evidence that a Tpm protein is essential for cell biomechanical stability in a load-bearing non-muscle tissue, and indicates that Tpm3.5 protects mechanically stable, load-bearing F-actin in vivoThis article has an associated First Person interview with the first author of the paper.

1q21.3 deletion involving GATAD2B: An emerging recurrent microdeletion syndrome

GATAD2B gene is involved in chromatin modification and transcription activity. Loss-of-function mutations of GATAD2B have recently been defined to cause a recognizable syndrome with intellectual disability (ID). Human TPM3 gene encoding thin filament protein is associated with myopathies. Both genes are located on chromosome 1q21.3. We herein report an infant with feeding difficulty, developmental delay, hypotonia, and dysmorphic features including small palpebral fissures, telecanthus, sparse hair and eyebrow, cup-shaped ears, and clinodactyly. Karyotype was normal. Single nucleotide polymorphism array revealed a 1.06 Mb deletion of chromosome 1q21.3, which was confirmed to be de novo. The deleted region encompassed 35 genes, including three known disease-associated genes, namely GATAD2B, TPM3, and HAX1. We further identify and summarize seven additional patients with 1q21.3 microdeletion from literature review and clinical databases (DECIPHER, ISCA/ClinGen). Genomic location analysis of all eight patients revealed different breakpoints and no segmental duplication, indicating that non-homologous end joining is a likely mechanism underlying this particular microdeletion. This data suggests that 1q21.3 microdeletion is a recurrent microdeletion syndrome with distinguishable phenotypes, and loss of function of GATAD2B is the major contributor of the characteristic facies and ID. Additionally, the deletion of TPM3 warrants a risk of concomitant muscle disease in our patient. © 2017 Wiley Periodicals, Inc.

KLHL41 stabilizes skeletal muscle sarcomeres by nonproteolytic ubiquitination

Maintenance of muscle function requires assembly of contractile proteins into highly organized sarcomeres. Mutations in Kelch-like protein 41 (KLHL41) cause nemaline myopathy, a fatal muscle disorder associated with sarcomere disarray. We generated KLHL41 mutant mice, which display lethal disruption of sarcomeres and aberrant expression of muscle structural and contractile proteins, mimicking the hallmarks of the human disease. We show that KLHL41 is poly-ubiquitinated and acts, at least in part, by preventing aggregation and degradation of Nebulin, an essential component of the sarcomere. Furthermore, inhibition of KLHL41 poly-ubiquitination prevents its stabilization of nebulin, suggesting a unique role for ubiquitination in protein stabilization. These findings provide new insights into the molecular etiology of nemaline myopathy and reveal a mechanism whereby KLHL41 stabilizes sarcomeres and maintains muscle function by acting as a molecular chaperone. Similar mechanisms for protein stabilization likely contribute to the actions of other Kelch proteins.

Treatment with ActRIIB-mFc Produces Myofiber Growth and Improves Lifespan in the Acta1 H40Y Murine Model of Nemaline Myopathy

Nemaline myopathies (NMs) are a group of congenital muscle diseases caused by mutations in at least 10 genes and associated with a range of clinical symptoms. NM is defined on muscle biopsy by the presence of cytoplasmic rod-like structures (nemaline rods) composed of cytoskeletal material. Myofiber smallness is also found in many cases of NM and may represent a cause of weakness that can be counteracted by treatment. We have used i.p. injection of activin type IIB receptor (ActRIIB)-mFc (an inhibitor of myostatin signaling) to promote hypertrophy and increase strength in our prior murine work; we therefore tested whether ActRIIB-mFc could improve weakness in NM mice through myofiber hypertrophy. We report a study of ActRIIB-mFc treatment in the Acta1 H40Y mouse model of NM. Treatment of Acta1 H40Y mice produced significant increases in body mass, muscle mass, quadriceps myofiber size, and survival, but other measurements of strength (forelimb grip strength, ex vivo measurements of contractile function) did not improve. Our studies also identified that the complications of urethral obstruction are associated with mortality in male hemizygote Acta1 H40Y mice. The incidence of urethral obstruction and histologic evidence of chronic obstruction (inflammation) were significantly lower in Acta1 H40Y mice that had been treated with ActRIIB-mFc. ActRIIB-mFc treatment produces a mild benefit to the disease phenotype in Acta1 H40Y mice.

TPM3 deletions cause a hypercontractile congenital muscle stiffness phenotype

OBJECTIVE

Mutations in TPM3, encoding Tpm3.12, cause a clinically and histopathologically diverse group of myopathies characterized by muscle weakness. We report two patients with novel de novo Tpm3.12 single glutamic acid deletions at positions ΔE218 and ΔE224, resulting in a significant hypercontractile phenotype with congenital muscle stiffness, rather than weakness, and respiratory failure in one patient.

METHODS

The effect of the Tpm3.12 deletions on the contractile properties in dissected patient myofibers was measured. We used quantitative in vitro motility assay to measure Ca(2+) sensitivity of thin filaments reconstituted with recombinant Tpm3.12 ΔE218 and ΔE224.

RESULTS

Contractility studies on permeabilized myofibers demonstrated reduced maximal active tension from both patients with increased Ca(2+) sensitivity and altered cross-bridge cycling kinetics in ΔE224 fibers. In vitro motility studies showed a two-fold increase in Ca(2+) sensitivity of the fraction of filaments motile and the filament sliding velocity concentrations for both mutations.

INTERPRETATION

These data indicate that Tpm3.12 deletions ΔE218 and ΔE224 result in increased Ca(2+) sensitivity of the troponin-tropomyosin complex, resulting in abnormally active interaction of the actin and myosin complex. Both mutations are located in the charged motifs of the actin-binding residues of tropomyosin 3, thus disrupting the electrostatic interactions that facilitate accurate tropomyosin binding with actin necessary to prevent the on-state. The mutations destabilize the off-state and result in excessively sensitized excitation-contraction coupling of the contractile apparatus. This work expands the phenotypic spectrum of TPM3-related disease and provides insights into the pathophysiological mechanisms of the actin-tropomyosin complex.

A Large Deletion Affecting TPM3, Causing Severe Nemaline Myopathy

Background and Objectives:

Nemaline myopathy may be caused by pathogenic variants in the TPM3 gene and is then called NEM1. All previously identified disease-causing variants are point mutations including missense, nonsense and splice-site variants. The aim of the study was to identify the disease-causing gene in this patient and verify the NM diagnosis.

Methods:

Mutation analysis methods include our self-designed nemaline myopathy array, The Nemaline Myopathy Comparative Genomic Hybridisation Array (NM-CGH array), whole-genome array-CGH, dHPLC, Sanger sequencing and whole-exome sequencing. The diagnostic muscle biopsy was investigated further by routine histopathological methods.

Results:

We present here the first large (17–21 kb) aberration in the α-tropomyosin_slow gene (TPM3), identified using the NM-CGH array. This homozygous deletion removes the exons 1a and 2b as well as the promoter of the TPM3 isoform encoding Tpm3.12st. The severe phenotype included paucity of movement, proximal and axial weakness and feeding difficulties requiring nasogastric tube feeding. The infant died at the age of 17.5 months. Muscle biopsy showed variation in fibre size and rods in a population of hypotrophic muscle fibres expressing slow myosin, often with internal nuclei, and abnormal immunolabelling revealing many hybrid fibres.

Conclusions:

This is the only copy number variation we have identified in any NM gene other than nebulin (NEB), suggesting that large deletions or duplications in these genes are very rare, yet possible, causes of NM.

Muscle weakness in TPM3-myopathy is due to reduced Ca2+-sensitivity and impaired acto-myosin cross-bridge cycling in slow fibres

Dominant mutations in TPM3, encoding α-tropomyosinslow, cause a congenital myopathy characterized by generalized muscle weakness. Here, we used a multidisciplinary approach to investigate the mechanism of muscle dysfunction in 12 TPM3-myopathy patients. We confirm that slow myofibre hypotrophy is a diagnostic hallmark of TPM3-myopathy, and is commonly accompanied by skewing of fibre-type ratios (either slow or fast fibre predominance). Patient muscle contained normal ratios of the three tropomyosin isoforms and normal fibre-type expression of myosins and troponins. Using 2D-PAGE, we demonstrate that mutant α-tropomyosinslow was expressed, suggesting muscle dysfunction is due to a dominant-negative effect of mutant protein on muscle contraction. Molecular modelling suggested mutant α-tropomyosinslow likely impacts actin-tropomyosin interactions and, indeed, co-sedimentation assays showed reduced binding of mutant α-tropomyosinslow (R168C) to filamentous actin. Single fibre contractility studies of patient myofibres revealed marked slow myofibre specific abnormalities. At saturating [Ca(2+)] (pCa 4.5), patient slow fibres produced only 63% of the contractile force produced in control slow fibres and had reduced acto-myosin cross-bridge cycling kinetics. Importantly, due to reduced Ca(2+)-sensitivity, at sub-saturating [Ca(2+)] (pCa 6, levels typically released during in vivo contraction) patient slow fibres produced only 26% of the force generated by control slow fibres. Thus, weakness in TPM3-myopathy patients can be directly attributed to reduced slow fibre force at physiological [Ca(2+)], and impaired acto-myosin cross-bridge cycling kinetics. Fast myofibres are spared; however, they appear to be unable to compensate for slow fibre dysfunction. Abnormal Ca(2+)-sensitivity in TPM3-myopathy patients suggests Ca(2+)-sensitizing drugs may represent a useful treatment for this condition.

Skeletal muscle microRNA and messenger RNA profiling in cofilin-2 deficient mice reveals cell cycle dysregulation hindering muscle regeneration

Congenital myopathies are rare skeletal muscle diseases presenting in early age with hypotonia and weakness often linked to a genetic defect. Mutations in the gene for cofilin-2 (CFL2) have been identified in several families as a cause of congenital myopathy with nemaline bodies and cores. Here we explore the global messenger and microRNA expression patterns in quadriceps muscle samples from cofillin-2-null mice and compare them with sibling-matched wild-type mice to determine the molecular pathways and mechanisms involved. Cell cycle processes are markedly dysregulated, with altered expression of genes involved in mitotic spindle formation, and evidence of loss of cell cycle checkpoint regulation. Importantly, alterations in cell cycle, apoptosis and proliferation pathways are present in both mRNA and miRNA expression patterns. Specifically, p21 transcript levels were increased, and the expression of p21 targets, such as cyclin D and cyclin E, was decreased. We therefore hypothesize that deficiency of cofilin-2 is associated with interruption of the cell cycle at several checkpoints, hindering muscle regeneration. Identification of these pathways is an important step towards developing appropriate therapies against various congenital myopathies.

Expanding the phenotype associated with the NEFL mutation: neuromuscular disease in a family with overlapping myopathic and neurogenic findings

IMPORTANCE

Newer sequencing technologies in combination with traditional gene mapping techniques, such as linkage analysis, can help identify the genetic basis of disease for patients with rare disorders of uncertain etiology. This approach may expand the phenotypic spectrum of disease associated with those genetic mutations.

OBJECTIVE

To elucidate the molecular cause of a neuromuscular disease among a family in which 4 members, a mother and her 3 sons, were affected.

DESIGN, SETTING, AND PARTICIPANTS

Two of 4 affected members manifested nemaline myopathy, a common subtype of congenital myopathy, while the other 2 had a nonspecific myopathy. Single-nucleotide polymorphism-based linkage analysis was performed on DNA samples from the 4 affected family members, and whole-genome sequencing was performed in the proband. Real-time quantitative reverse transcription-polymerase chain reaction, immunofluorescence, and Western blot analysis were performed on muscle biopsy specimens.

MAIN OUTCOMES AND MEASURES

Whole-genome sequencing and linkage analysis identified a variant in a gene that explains the phenotype.

RESULTS

We identified a novel neurofilament light polypeptide (NEFL) nonsense mutation in all affected members. NEFL mutations have been previously linked to Charcot-Marie-Tooth disease in humans. This led us to reevaluate the diagnosis, and we recognized that several of the findings, especially those related to the muscle biopsy specimens and electromyography, were consistent with a neurogenic disease.

CONCLUSIONS AND RELEVANCE

NEFL mutations are known to cause Charcot-Marie-Tooth disease in humans and motor neuron disease in mice. We report the identification of an NEFL mutation in a family clinically manifesting congenital myopathy. We also describe potential overlap between myopathic and neurogenic findings in this family. These findings expand the phenotypic spectrum of diseases associated with NEFL mutations. This study is an example of the power of genomic approaches to identify potentially pathogenic mutations in unsuspected genes responsible for heterogeneous neuromuscular diseases.

Clinical and pathological features of patients with nemaline myopathy

Nemaline myopathy (NM) is a rare congenital myopathy of great heterogeneity, characterized by the presence of rods in the cytoplasm of muscle fibers. This study aimed to summarize and analyze retrospectively the clinicopathological features of 28 patients with NM. Among the 28 patients, 15 were classified as of the typical congenital type, manifested as lower- or four-limb weakness as the first symptom and slowly progressive course. Six patients were classified as of childhood onset type, with lower-limb weakness and progressive course. Seven patients were classified as of the adult onset type, with rapidly progressive course and obvious muscle atrophy. Patient's 1, 16 and 23 had rapid clinical progression. On follow up, the three patients showed respiratory failure. Limb weakness in all patients was proximal‑dominant. Hypotonia was observed in most patients. High arched feet were also observed as dysmorfic features. In all patients, the creatine kinase (CK) level was normal or mildly elevated, and electromyography revealed myogenic changes. Nemaline bodies were observed under a light microscope in more than half of the patients' muscle fibers, and especially in type I fibers. All patients showed fiber type I predominance and atrophy. Modified Gömöri trichrome staining showed characteristic purple‑colored rods. Muscle electron microscopy revealed the presence of high electron‑dense nemaline bodies around the nucleus, and of a disorganized myofibrillar apparatus, with broken myofilaments and irregular myofibrils and Z lines. The 28 patients with NM shared a number of clinical features, such as proximal limb weakness, reduced deep tendon reflex and dysmorfic features. Differences were also observed between the three types of patients, with regards to course progression, disease severity and respiratory failure. In conclusion, patients with NM showed great clinical heterogeneity. The diagnosis of NM was mainly based on the muscle biopsy.

Differential muscle hypertrophy is associated with satellite cell numbers and Akt pathway activation following activin type IIB receptor inhibition in Mtm1 p.R69C mice

X-linked myotubular myopathy is a congenital myopathy caused by deficiency of myotubularin. Patients often present with severe perinatal weakness, requiring mechanical ventilation to prevent death from respiratory failure. We recently reported that an activin receptor type IIB inhibitor produced hypertrophy of type 2b myofibers and modest increases of strength and life span in the severely myopathic Mtm1δ4 mouse model of X-linked myotubular myopathy. We have now performed a similar study in the less severely symptomatic Mtm1 p.R69C mouse in hopes of finding greater treatment efficacy. Activin receptor type IIB inhibitor treatment of Mtm1 p.R69C animals produced behavioral and histological evidence of hypertrophy in gastrocnemius muscles but not in quadriceps or triceps. The ability of the muscles to respond to activin receptor type IIB inhibitor treatment correlated with treatment-induced increases in satellite cell number and several muscle-specific abnormalities of hypertrophic signaling. Treatment-responsive Mtm1 p.R69C gastrocnemius muscles displayed lower levels of phosphorylated ribosomal protein S6 and higher levels of phosphorylated eukaryotic elongation factor 2 kinase than were observed in Mtm1 p.R69C quadriceps muscle or in muscles from wild-type littermates. Hypertrophy in the Mtm1 p.R69C gastrocnemius muscle was associated with increased levels of phosphorylated ribosomal protein S6. Our findings indicate that muscle-, fiber type-, and mutation-specific factors affect the response to hypertrophic therapies that will be important to assess in future therapeutic trials.

Severe protein aggregate myopathy in a knockout mouse model points to an essential role of cofilin2 in sarcomeric actin exchange and muscle maintenance

Mutations in the human actin depolymerizing factor cofilin2 result in an autosomal dominant form of nemaline myopathy. Here, we report on the targeted ablation of murine cofilin2, which leads to a severe skeletal muscle specific phenotype within the first two weeks after birth. Apart from skeletal muscle, cofilin2 is also expressed in heart and CNS, however the pathology was restricted to skeletal muscle. The two close family members of cofilin2 - ADF and cofilin1 - were co-expressed in muscle, but unable to compensate for the loss of cofilin2. While primary myofibril assembly and muscle development were unaffected in cofilin2 mutant mice, progressive muscle degeneration was observed between postnatal days 3 and 7. Muscle pathology was characterized by sarcoplasmic protein aggregates, fiber size disproportion, mitochondrial abnormalities and internal nuclei. The observed muscle pathology differed from nemaline myopathy, but showed combined features of actin-associated myopathy and myofibrillar myopathy. In cofilin2 mutant mice, the postnatal expression pattern and turnover of sarcomeric α-actin isoforms were altered. Levels of smooth muscle α-actin were increased and remained high in developing muscles, suggesting that cofilin2 plays a crucial role during the exchange of α-actin isoforms during the early postnatal remodeling of the sarcomere.

Phenotypes of myopathy-related beta-tropomyosin mutants in human and mouse tissue cultures

Mutations in TPM2 result in a variety of myopathies characterised by variable clinical and morphological features. We used human and mouse cultured cells to study the effects of β-TM mutants. The mutants induced a range of phenotypes in human myoblasts, which generally changed upon differentiation to myotubes. Human myotubes transfected with the E41K-β-TM(EGFP) mutant showed perinuclear aggregates. The G53ins-β-TM(EGFP) mutant tended to accumulate in myoblasts but was incorporated into filamentous structures of myotubes. The K49del-β-TM(EGFP) and E122K-β-TM(EGFP) mutants induced the formation of rod-like structures in human cells. The N202K-β-TM(EGFP) mutant failed to integrate into thin filaments and formed accumulations in myotubes. The accumulation of mutant β-TM(EGFP) in the perinuclear and peripheral areas of the cells was the striking feature in C2C12. We demonstrated that human tissue culture is a suitable system for studying the early stages of altered myofibrilogenesis and morphological changes linked to myopathy-related β-TM mutants. In addition, the histopathological phenotype associated with expression of the various mutant proteins depends on the cell type and varies with the maturation of the muscle cell. Further, the phenotype is a combinatorial effect of the specific amino acid change and the temporal expression of the mutant protein.

In vivo and in vitro investigations of heterozygous nebulin knock-out mice disclose a mild skeletal muscle phenotype

Nemaline myopathy is the most common congenital skeletal muscle disease, and mutations in the nebulin gene account for 50% of all cases. Recent studies suggest that the disease severity might be related to the nebulin expression levels. Considering that mutations in the nebulin gene are typically recessive, one would expect that a single functional nebulin allele would maintain nebulin protein expression which would result in preserved skeletal muscle function. We investigated skeletal muscle function of heterozygous nebulin knock-out (i.e., nebulin(+/-)) mice using a multidisciplinary approach including protein and gene expression analysis and combined in vivo and in vitro force measurements. Skeletal muscle anatomy and energy metabolism were studied strictly non-invasively using magnetic resonance imaging and 31P-magnetic resonance spectroscopy. Maximal force production was reduced by around 16% in isolated muscle of nebulin(+/-) mice while in vivo force generating capacity was preserved. Muscle weakness was associated with a shift toward a slower proteomic phenotype, but was not related to nebulin protein deficiency or to an impaired energy metabolism. Further studies would be warranted in order to determine the mechanisms leading to a mild skeletal muscle phenotype resulting from the expression of a single nebulin allele.

Congenital myopathies

Congenital myopathies are a heterogeneous group of inherited muscle disorders, characterized by the predominance of particular histopathological features on muscle biopsy, such as cores (central core disease) or rods (nemaline myopathy). Clinically, early onset of the disease, stable or slowly progressive muscle weakness, hypotonia and delayed motor development are common in most forms. As a result, the diagnosis of a subtype of congenital myopathy is largely based on the presence of specific structural abnormalities in the skeletal muscle detected by enzyme-histochemistry and electron microscopy studies. During the last decades there have been significant advances in the identification of the genetic basis of most congenital myopathies. However, there is significant genetic heterogeneity within the main groups of congenital myopathies, and mutations in one particular gene may also cause diverse clinical and morphological phenotypes. Thus, the nosography and nosology in this field is still evolving.

Myotubularin-deficient myoblasts display increased apoptosis, delayed proliferation, and poor cell engraftment

X-linked myotubular myopathy is a severe congenital myopathy caused by deficiency of the lipid phosphatase, myotubularin. Recent studies of human tissue and animal models have discovered structural and physiological abnormalities in myotubularin-deficient muscle, but the impact of myotubularin deficiency on myogenic stem cells within muscles is unclear. In the present study, we evaluated the viability, proliferative capacity, and in vivo engraftment of myogenic cells obtained from severely symptomatic (Mtm1δ4) myotubularin-deficient mice. Mtm1δ4 muscle contains fewer myogenic cells than wild-type (WT) littermates, and the number of myogenic cells decreases with age. The behavior of Mtm1δ4 myoblasts is also abnormal, because they engraft poorly into C57BL/6/Rag1null/mdx5cv mice and display decreased proliferation and increased apoptosis compared with WT myoblasts. Evaluation of Mtm1δ4 animals at 21 and 42 days of life detected fewer satellite cells in Mtm1δ4 muscle compared with WT littermates, and the decrease in satellite cells correlated with progression of disease. In addition, analysis of WT and Mtm1δ4 regeneration after injury detected similar abnormalities of satellite cell function, with fewer satellite cells, fewer dividing cells, and increased apoptotic cells in Mtm1δ4 muscle. These studies demonstrate specific abnormalities in myogenic cell number and behavior that may relate to the progression of disease in myotubularin deficiency, and may also be used to develop in vitro assays by which novel treatment strategies can be assessed.

Normal myofibrillar development followed by progressive sarcomeric disruption with actin accumulations in a mouse Cfl2 knockout demonstrates requirement of cofilin-2 for muscle maintenance

Cofilin-2, a small actin-binding protein and member of the AC protein family that includes cofilin-1 and destrin, is predominantly expressed at sarcomeres in skeletal and cardiac muscles. The role of cofilin-2 in muscle development and function is unclear. In humans, recessive cofilin-2 mutations have been associated with nemaline myopathy with minicores. To investigate the functional role of cofilin-2 in vivo, we generated constitutive and muscle-specific cofilin-2-deficient mice using a cre-loxP strategy. Cofilin-2-deficient mice were similar to their wild-type (WT) littermates at birth, but died by day 8. They were significantly smaller, severely weak and had very little milk in their stomachs. The sarcomeric structure was intact at birth, but by Day 7, skeletal muscles showed severe sarcomeric disruptions starting at the Z-line, along with filamentous actin accumulations consistent with a lack of actin depolymerization activity. Cofilin-2-deficient muscles contained elevated numbers of slow fibers and exhibited upregulation of slow fiber-specific genes. Increased amounts of other sarcomeric proteins including α-actinin-2, α-sarcomeric actin and tropomyosin were also present. While destrin was not expressed in either WT or cofilin-2-deficient muscles, cofilin-1 was similarly expressed in developing myofibers of both genotypes. There was no evidence for compensatory changes in expression of either family member in cofilin-2-deficient tissues. The onset of pathology and weakness in cofilin-2-deficient muscles correlated with normal developmental loss of cofilin-1 expression within myofibers, suggesting that cofilin-1 serves as an early developmental sarcomeric isoform. Overall, cofilin-2, although not critical for muscle development, is essential for muscle maintenance.

Inhibition of activin receptor type IIB increases strength and lifespan in myotubularin-deficient mice

X-linked myotubular myopathy (XLMTM) is a congenital disorder caused by deficiency of the lipid phosphatase, myotubularin. Patients with XLMTM often have severe perinatal weakness that requires mechanical ventilation to prevent death from respiratory failure. Muscle biopsy specimens from patients with XLMTM exhibit small myofibers with central nuclei and central aggregations of organelles in many cells. It was postulated that therapeutically increasing muscle fiber size would cause symptomatic improvement in myotubularin deficiency. Recent studies have elucidated an important role for the activin-receptor type IIB (ActRIIB) in regulation of muscle growth and have demonstrated that ActRIIB inhibition results in significant muscle hypertrophy. To evaluate whether promoting muscle hypertrophy can attenuate symptoms resulting from myotubularin deficiency, the effect of ActRIIB-mFC treatment was determined in myotubularin-deficient (Mtm1δ4) mice. Compared with wild-type mice, untreated Mtm1δ4 mice have decreased body weight, skeletal muscle hypotrophy, and reduced survival. Treatment of Mtm1δ4 mice with ActRIIB-mFC produced a 17% extension of lifespan, with transient increases in weight, forelimb grip strength, and myofiber size. Pathologic analysis of Mtm1δ4 mice during treatment revealed that ActRIIB-mFC produced marked hypertrophy restricted to type 2b myofibers, which suggests that oxidative fibers in Mtm1δ4 animals are incapable of a hypertrophic response in this setting. These results support ActRIIB-mFC as an effective treatment for the weakness observed in myotubularin deficiency.

Changes in cross-bridge cycling underlie muscle weakness in patients with tropomyosin 3-based myopathy

Nemaline myopathy, the most common non-dystrophic congenital myopathy, is caused by mutations in six genes, all of which encode thin-filament proteins, including NEB (nebulin) and TPM3 (α tropomyosin). In contrast to the mechanisms underlying weakness in NEB-based myopathy, which are related to loss of thin-filament functions normally exerted by nebulin, the pathogenesis of muscle weakness in patients with TPM3 mutations remains largely unknown. Here, we tested the hypothesis that the contractile phenotype of TPM3-based myopathy is different from that of NEB-based myopathy and that this phenotype is a direct consequence of the loss of the specific functions normally exerted by tropomyosin. To test this hypothesis, we used a multidisciplinary approach, including muscle fiber mechanics and confocal and electron microscopy to characterize the structural and functional phenotype of muscle fibers from five patients with TPM3-based myopathy and compared this with that of unaffected control subjects. Our findings demonstrate that patients with TPM3-based myopathy display a contractile phenotype that is very distinct from that of patients with NEB-based myopathy. Whereas both show severe myofilament-based muscle weakness, the contractile dysfunction in TPM3-based myopathy is largely explained by changes in cross-bridge cycling kinetics, but not by the dysregulation of sarcomeric thin-filament length that plays a prominent role in NEB-based myopathy. Interestingly, the loss of force-generating capacity in TPM3-based myopathy appears to be compensated by enhanced thin-filament activation. These findings provide a scientific basis for differential therapeutics aimed at restoring contractile performance in patients with TPM3-based versus NEB-based myopathy.

Congenital fibre type disproportion associated with mutations in the tropomyosin 3 (TPM3) gene mimicking congenital myasthenia

Congenital myopathy with fibre type disproportion (CFTD) has been associated with mutations in ACTA1, SEPN1, RYR1 and TPM3 genes. We report the clinico-pathological and electrophysiological features of 2 unrelated cases with heterozygous TPM3 mutation. Case 1 is a 19-year-old lady who presented with motor delay in infancy, respiratory failure in early teens requiring non-invasive ventilation despite being ambulant, ptosis, axial more than proximal weakness and scoliosis. Case 2 is a 7-year-old boy with hypotonia, feeding difficulties, motor delay and scoliosis, also requiring non-invasive ventilation while ambulant. Muscle biopsies in both cases showed fibre type disproportion. Muscle MRI (Case 1) showed mild uniformly increased interstitial tissue in and around the muscles. Sequencing of TPM3 in case 1 revealed a previously described heterozygous c.503G > A(pArg168His) missense variant in exon 5 and a novel heterozygous missense mutation c.521A > C(pGlu174Ala), also in exon 5, in case 2. A mild abnormality in the single fibre EMG was documented on electrophysiology in both cases. These cases highlight the neuromuscular transmission defect in CFTD secondary to TPM3 mutations.

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.

Autosomal dominant nemaline myopathy caused by a novel alpha-tropomyosin 3 mutation

Nemaline myopathy (NM) is a genetically and clinically heterogenous muscle disorder, which is myopathologically characterized by nemaline bodies. Mutations in six genes have been reported to cause NM: Nebulin (NEB Pelin 1999), alpha-skeletal muscle actin (ACTA1 Nowak 1999), alpha-slow tropomyosin (TPM3 Laing 1995), beta-tropomyosin (TPM2 Donner 2002), slow troponin T (TNNT1 Johnston 2000) and cofilin 2 (CFL2 Agrawal 2007). The majority of cases are due to mutation in NEB and ACTA1. We report on the clinical, myopathological and muscle MRI findings in a German family with autosomal dominant NM due to a novel pathogenic TPM3 mutation (p.Ala156Thr).

Actin isoform expression patterns during mammalian development and in pathology: insights from mouse models

The dynamic actin cytoskeleton, consisting of six actin isoforms in mammals and a variety of actin binding proteins is essential for all developmental processes and for the viability of the adult organism. Actin isoform specific functions have been proposed for muscle contraction, cell migration, endo- and exocytosis and maintaining cell shape. However, these specific functions for each of the actin isoforms during development are not well understood. Based on transgenic mouse models, we will discuss the expression patterns of the six conventional actin isoforms in mammals during development and adult life. Ablation of actin genes usually leads to lethality and affects expression of other actin isoforms at the cell or tissue level. A good knowledge of their expression and functions will contribute to fully understand severe phenotypes or diseases caused by mutations in actin isoforms.

Mutations and polymorphisms of the skeletal muscle alpha-actin gene (ACTA1)

The ACTA1 gene encodes skeletal muscle alpha-actin, which is the predominant actin isoform in the sarcomeric thin filaments of adult skeletal muscle, and essential, along with myosin, for muscle contraction. ACTA1 disease-causing mutations were first described in 1999, when a total of 15 mutations were known. In this article we describe 177 different disease-causing ACTA1 mutations, including 85 that have not been described before. ACTA1 mutations result in five overlapping congenital myopathies: nemaline myopathy; intranuclear rod myopathy; actin filament aggregate myopathy; congenital fiber type disproportion; and myopathy with core-like areas. Mixtures of these histopathological phenotypes may be seen in a single biopsy from one patient. Irrespective of the histopathology, the disease is frequently clinically severe, with many patients dying within the first year of life. Most mutations are dominant and most patients have de novo mutations not present in the peripheral blood DNA of either parent. Only 10% of mutations are recessive and they are genetic or functional null mutations. To aid molecular diagnosis and establishing genotype-phenotype correlations, we have developed a locus-specific database for ACTA1 variations (http://waimr.uwa.edu.au).

Disease severity and thin filament regulation in M9R TPM3 nemaline myopathy

The mechanism of muscle weakness was investigated in an Australian family with an M9R mutation in TPM3 (alpha-tropomyosin(slow)). Detailed protein analyses of 5 muscle samples from 2 patients showed that nemaline bodies are restricted to atrophied Type 1 (slow) fibers in which the TPM3 gene is expressed. Developmental expression studies showed that alpha-tropomyosin(slow) is not expressed at significant levels until after birth, thereby likely explaining the childhood (rather than congenital) disease onset in TPM3 nemaline myopathy. Isoelectric focusing demonstrated that alpha-tropomyosin(slow) dimers, composed of equal ratios of wild-type and M9R-alpha-tropomyosin(slow), are the dominant tropomyosin species in 3 separate muscle groups from an affected patient. These findings suggest that myopathy-related slow fiber predominance likely contributes to the severity of weakness in TPM3 nemaline myopathy because of increased proportions of fibers that express the mutant protein. Using recombinant proteins and far Western blot, we demonstrated a higher affinity of tropomodulin for alpha-tropomyosin(slow) compared with beta-tropomyosin; the M9R substitution within alpha-tropomyosin(slow) greatly reduced this interaction. Finally, transfection of the M9R mutated and wild-type alpha-tropomyosin(slow) into myoblasts revealed reduced incorporation into stress fibers and disruption of the filamentous actin network by the mutant protein. Collectively, these results provide insights into the clinical features and pathogenesis of M9R-TPM3 nemaline myopathy.

Thick and thin filament gene mutations in striated muscle diseases

The sarcomere is the fundamental unit of cardiac and skeletal muscle contraction. During the last ten years, there has been growing awareness of the etiology of skeletal and cardiac muscle diseases originating in the sarcomere, an important evolving field. Many sarcomeric diseases affect newborn children, i. e. are congenital myopathies. The discovery and characterization of several myopathies caused by mutations in myosin heavy chain genes, coding for the major component of skeletal muscle thick filaments, has led to the introduction of a new entity in the field of neuromuscular disorders: myosin myopathies. Recently, mutations in genes coding for skeletal muscle thin filaments, associated with various clinical features, have been identified. These mutations evoke distinct structural changes within the sarcomeric thin filament. Current knowledge regarding contractile protein dysfunction as it relates to disease pathogenesis has failed to decipher the mechanistic links between mutations identified in sarcomeric proteins and skeletal myopathies, which will no doubt require an integrated physiological approach. The discovery of additional genes associated with myopathies and the elucidation of the molecular mechanisms of pathogenesis will lead to improved and more accurate diagnosis, including prenatally, and to enhanced potential for prognosis, genetic counseling and developing possible treatments for these diseases. The goal of this review is to present recent progress in the identification of gene mutations from each of the major structural components of the sarcomere, the thick and thin filaments, related to skeletal muscle disease. The genetics and clinical manifestations of these disorders will be discussed.

Congenital myopathies

This review focuses on congenital myopathies, a distinct but markedly heterogeneous group of muscle disorders that present with muscle weakness and typically appear at birth or in infancy. These myopathies have characteristic histopathologic abnormalities on muscle biopsy, allowing a preliminary morphologic classification. Advances in molecular genetics have allowed a more rational classification of these disorders and have reshuffled taxonomy for some of these conditions. Here, we focus on recent research advances in specific congenital myopathies, including nemaline myopathy, myotubular myopathy, centronuclear myopathy, central core myopathy, multi-minicore myopathy, congenital fiber-type disproportion myopathy, and hyaline body myopathy. Scientific progress has not only elucidated the pathologic mechanisms of these disorders, but it has also provided the basis for therapeutic strategies.

Mutations in TPM3 are a common cause of congenital fiber type disproportion

OBJECTIVE

Congenital fiber type disproportion (CFTD) is a rare form of congenital myopathy in which the principal histological abnormality is hypotrophy of type 1 (slow-twitch) fibers compared with type 2 (fast-twitch) fibers. To date, mutation of ACTA1 and SEPN1 has been associated with CFTD, but the genetic basis in most patients is unclear. The gene encoding alpha-tropomyosin(slow) (TPM3) is a rare cause of nemaline myopathy, previously reported in only five families. We investigated whether mutation of TPM3 is a cause of CFTD.

METHODS AND RESULTS

We sequenced TPM3 in 23 unrelated probands with CFTD or CFTD-like presentations of unknown cause and identified novel heterozygous missense mutations in five CFTD families (p. Leu100Met, p.Arg168Cys, p.Arg168Gly, p.Lys169Glu, p.Arg245Gly). All affected family members that underwent biopsy had typical histological features of CFTD, with type 1 fibers, on average, at least 50% smaller than type 2 fibers. We also report a sixth family in which a recurrent TPM3 mutation (p.Arg168His) was associated with histological features of CFTD and nemaline myopathy in different family members. We describe the clinical features of 11 affected patients. Typically, there was proximal limb girdle weakness, prominent weakness of neck flexion and ankle dorsiflexion, mild facial weakness, and mild ptosis. The age of onset and severity varied, even within the same family. Many patients required nocturnal noninvasive ventilation despite remaining ambulant.

INTERPRETATION

Mutation of TPM3 is the most common cause of CFTD reported to date.

Beta-tropomyosin mutations alter tropomyosin isoform composition

BACKGROUND AND PURPOSE

Tropomyosin (TM) is an actin-binding protein, which is localized head to tail along the length of the actin filament. There are three major TM isoforms in human striated muscle. Mutations in beta-tropomyosin (TPM2) have recently been identified as an important cause of neuromuscular disorders.

MATERIALS AND METHODS

The expression of TM isoforms in patients carrying mutations in TPM2 was detected using a combination of SDS-PAGE, Western blotting, and a new method to measure the relative abundance of the various TM transcripts.

RESULTS

The level of gamma-TM is reduced in patients with mutations in TPM2. Beta-tropomyosin was expressed at high levels in muscle specimens of the patients.

DISCUSSION

Our study indicates that beta-TM gene mutations can alter the expression of other sarcomeric TM isoforms and that the perturbation of TM isoform levels may affect the dimer preference within the thin filaments, which may contribute to muscle weakness as a result of both functional and structural changes in muscle.

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.

Tropomyosin-based regulation of the actin cytoskeleton in time and space

Tropomyosins are rodlike coiled coil dimers that form continuous polymers along the major groove of most actin filaments. In striated muscle, tropomyosin regulates the actin-myosin interaction and, hence, contraction of muscle. Tropomyosin also contributes to most, if not all, functions of the actin cytoskeleton, and its role is essential for the viability of a wide range of organisms. The ability of tropomyosin to contribute to the many functions of the actin cytoskeleton is related to the temporal and spatial regulation of expression of tropomyosin isoforms. Qualitative and quantitative changes in tropomyosin isoform expression accompany morphogenesis in a range of cell types. The isoforms are segregated to different intracellular pools of actin filaments and confer different properties to these filaments. Mutations in tropomyosins are directly involved in cardiac and skeletal muscle diseases. Alterations in tropomyosin expression directly contribute to the growth and spread of cancer. The functional specificity of tropomyosins is related to the collaborative interactions of the isoforms with different actin binding proteins such as cofilin, gelsolin, Arp 2/3, myosin, caldesmon, and tropomodulin. It is proposed that local changes in signaling activity may be sufficient to drive the assembly of isoform-specific complexes at different intracellular sites.

Tropomyosins in skeletal muscle diseases

A number of congenital muscle diseases and disorders are caused by mutations in genes that encode the proteins present in or associated with the thin filaments of the muscle sarcomere. These genes include alpha-skeletal actin (ACTA1), beta-tropomyosin (TPM2), alpha-tropomyosin slow (TPM3), nebulin (NEB), troponin I fast (TNNI2), troponin T slow (TNNT1), troponin T fast (TNNT3) and cofilin (CFL2). Mutations in two of the four tropomyosin (Tm) genes, TPM2 and TPM3, result in at least three different skeletal muscle diseases and one disorder as distinguished by the presence of specific clinical features and/or structural abnormalities--nemaline myopathy (TPM2 and TPM3), distal arthrogryposis (TPM2), cap disease (TPM2) and congenital fiber type disproportion (TPM3). These diseases have overlapping clinical features and pathologies and there are cases of family members who have the same mutation, but different diseases (Table 1). The relatively recent discovery of nonmuscle or cytoskeletal Tms in skeletal muscle adds to this complexity since it is now possible that a disease-causing mutation could be in a striated isoform and a cytoskeletal isoform both present in muscle.