Novel TPM3 mutation in a family with cap myopathy and review of the literature

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.

A novel variant in the tropomyosin 3 gene presenting as an adult-onset distal myopathy - a case report

BACKGROUND

We report a patient with a novel c.737 C > T variant (p.Ser246Leu) of the TPM3 gene presenting with adult-onset distal myopathy.

CASE PRESENTATION

A 35-year-old Chinese male patient presented with a history of progressive finger weakness. Physical examination revealed differential finger extension weakness, together with predominant finger abduction, elbow flexion, ankle dorsiflexion and toe extension weakness. Muscle MRI showed disproportionate fatty infiltration of the glutei, sartorius and extensor digitorum longus muscles without significant wasting. Muscle biopsy and ultrastructural examination showed a non-specific myopathic pattern without nemaline or cap inclusions. Genetic sequencing revealed a novel heterozygous p.Ser246Leu variant (c.737C>T) of the TPM3 gene which is predicted to be pathogenic. This variant is located in the area of the TPM3 gene where the protein product interacts with actin at position Asp25 of actin. Mutations of TPM3 in these loci have been shown to alter the sensitivity of thin filaments to the influx of calcium ions.

CONCLUSION

This report further expands the phenotypic spectrum of myopathies associated with TPM3 mutations, as mutations in TPM3 had not previously been reported with adult-onset distal myopathy. We also discuss the interpretation of variants of unknown significance in patients with TPM3 mutations and summarise the typical muscle MRI findings of patients with TPM3 mutations.

A review of major causative genes in congenital myopathies

In this review, we focus on congenital myopathies, which are a genetically heterogeneous group of hereditary muscle diseases with slow or minimal progression. They are mainly defined and classified according to pathological features, with the major subtypes being core myopathy (central core disease), nemaline myopathy, myotubular/centronuclear myopathy, and congenital fiber-type disproportion myopathy. Recent advances in molecular genetics, especially next-generation sequencing technology, have rapidly increased the number of known causative genes for congenital myopathies; however, most of the diseases related to the novel causative genes are extremely rare. There remains no cure for congenital myopathies. However, there have been recent promising findings that could inform the development of therapy for several types of congenital myopathies, including myotubular myopathy, which indicates the importance of prompt and correct diagnosis. This review discusses the major causative genes (NEB, ACTA1, ADSSL1, RYR1, SELENON, MTM1, DNM2, and TPM3) for each subtype of congenital myopathies and the relevant latest findings.

Nemaline Myopathy in Brazilian Patients: Molecular and Clinical Characterization

Nemaline myopathy (NM), a structural congenital myopathy, presents a significant clinical and genetic heterogeneity. Here, we compiled molecular and clinical data of 30 Brazilian patients from 25 unrelated families. Next-generation sequencing was able to genetically classify all patients: sixteen families (64%) with mutation in NEB, five (20%) in ACTA1, two (8%) in KLHL40, and one in TPM2 (4%) and TPM3 (4%). In the NEB-related families, 25 different variants, 11 of them novel, were identified; splice site (10/25) and frame shift (9/25) mutations were the most common. Mutation c.24579 G>C was recurrent in three unrelated patients from the same region, suggesting a common ancestor. Clinically, the “typical” form was the more frequent and caused by mutations in the different NM genes. Phenotypic heterogeneity was observed among patients with mutations in the same gene. Respiratory involvement was very common and often out of proportion with limb weakness. Muscle MRI patterns showed variability within the forms and genes, which was related to the severity of the weakness. Considering the high frequency of NEB mutations and the complexity of this gene, NGS tools should be combined with CNV identification, especially in patients with a likely non-identified second mutation.

Whole-body muscle magnetic resonance imaging in patients with muscle symptoms: incidental findings and outcomes

BACKGROUND AND PURPOSE

Whole-body muscle magnetic resonance imaging (MRI) has become widely used for diagnostic workup in patients with muscle diseases. The prevalence of incidental findings in diagnostic whole-body muscle MRI is unknown. Here, the prevalence and outcomes of incidentalomas in whole-body muscle MRI in a large cohort of patients with muscle symptoms were studied.

METHODS

Two hundred and six patients who consulted at our neuromuscular clinic with muscle weakness and/or myalgia and/or increased serum creatine kinase and in whom a whole-body muscle MRI was performed between January 2016 and March 2020 were included.

RESULTS

Whole-body muscle MRI revealed at least one incidentaloma in 132 patients (64.1%), with mean age at MRI examination 50.4 years (19-74 years). Most of the incidental findings were benign. However, diagnostic examinations were indicated in 16.3% of the incidentalomas, treatment was needed in 4.7% and, in retrospect, symptoms related to the incidental findings were identified in 14.3%. Three malignant (glioblastoma multiforme, renal cell carcinoma and hepato-splenomegaly related to a lymphoma) and one precancerous (low-grade appendiceal mucinous neoplasm) incidental findings were identified. In one patient an abdominal aortic aneurysm was detected and in another a large cerebral arteriovenous malformation. In 1.2% of the incidentalomas, i.e. periventricular white matter lesions (LAMA2 mutations) and fibrous dysplasia (ANO5 mutations), an indirect link with the muscle disease could be established.

CONCLUSIONS

It is concluded that incidental findings in diagnostic whole-body muscle MRI in patients with muscle symptoms occur frequently. Most of them are benign, but in some timely detecting the incidentaloma leads to early treatment and can thus impact prognosis.

Update on Congenital Myopathies in Adulthood

Congenital myopathies (CMs) constitute a group of heterogenous rare inherited muscle diseases with different incidences. They are traditionally grouped based on characteristic histopathological findings revealed on muscle biopsy. In recent decades, the ever-increasing application of modern genetic technologies has not just improved our understanding of their pathophysiology, but also expanded their phenotypic spectrum and contributed to a more genetically based approach for their classification. Later onset forms of CMs are increasingly recognised. They are often considered milder with slower progression, variable clinical presentations and different modes of inheritance. We reviewed the key features and genetic basis of late onset CMs with a special emphasis on those forms that may first manifest in adulthood.

Congenital myopathies: an update

Congenital myopathies comprise a clinical, histopathological, and genetic heterogeneous group of rare hereditary muscle diseases that are defined by architectural abnormalities in the muscle fibres. They are subdivided by the predominant structural pathological change on muscle biopsy, resulting in five subgroups: (1) core myopathies; (2) nemaline myopathies; (3) centronuclear myopathies; (4) congenital fibre type disproportion myopathy; and (5) myosin storage myopathy. Besides the clinical features, muscle biopsy, muscle imaging, and genetic analyses are essential in the diagnosis of congenital myopathies. Using next-generation sequencing techniques, a large number of new genes are being identified as the cause of congenital myopathies as well as new mutations in known genes, broadening the phenotype-genotype spectrum of congenital myopathies. Management is performed by a multidisciplinary team specialized in neuromuscular disorders, where the (paediatric) neurologist has an essential role. To date, only supportive treatment is available, but novel pathomechanisms are being discovered and gene therapies are being explored. WHAT THIS PAPER ADDS: Many new genes are being identified in congenital myopathies, broadening the phenotype-genotype spectrum. Management is performed by a multidisciplinary team specialized in neuromuscular disorders.

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.

Myopathic mutations in the β-chain of tropomyosin differently affect the structural and functional properties of ββ- and αβ-dimers

Tropomyosin (Tpm) is an actin-binding protein that plays a vital role in the regulation of muscle contraction. Fast skeletal muscles express 2 Tpm isoforms, α (Tpm 1.1) and β (Tpm 2.2), resulting in the existence of 2 forms of dimeric Tpm molecule: αα-homodimer and αβ-heterodimer. ββ-Homodimer is unstable and absent in the native state, despite which most of the studies of myopathy-relating Tpm mutations have been performed on the ββ-homodimer. Here, we applied different methods to investigate the effects of myopathic mutations R133W and N202K in the β-chain of Tpm on properties of αβ-heterodimers and to compare them with the features of ββ-homodimers with the same mutations. The results show that properties of αβ-Tpm and ββ-Tpm with substitutions in the β-chain differ significantly, and this indicates that the effects of myopathic mutations in the Tpm β-chain should be studied on the Tpm αβ-heterodimer.-Bershitsky, S. Y., Logvinova, D. S., Shchepkin, D. V., Kopylova, G. V., Matyushenko, A. M. Myopathic mutations in the β-chain of tropomyosin differently affect the structural and functional properties of ββ- and αβ-dimers.

Recessive MYPN mutations cause cap myopathy with occasional nemaline rods

Congenital myopathies are phenotypically and genetically heterogeneous. We describe homozygous truncating mutations in MYPN in 2 unrelated families with a slowly progressive congenital cap myopathy. MYPN encodes the Z-line protein myopalladin implicated in sarcomere integrity. Functional experiments demonstrate that the mutations lead to mRNA defects and to a strong reduction in full-length protein expression. Myopalladin signals accumulate in the caps together with alpha-actinin. Dominant MYPN mutations were previously reported in cardiomyopathies. Our data uncover that mutations in MYPN cause either a cardiac or a congenital skeletal muscle disorder through different modes of inheritance. Ann Neurol 2017;81:467-473.

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.

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.