A new de novo missense mutation in MYH2 expands clinical and genetic findings in hereditary myosin myopathies

A genetic variant in SMAD7 acts as a modifier of LMNA-associated muscular dystrophy, implicating SMAD signaling as a therapeutic target

Mutations in LMNA cause multiple types of muscular dystrophy (LMNA-MD). The symptoms of LMNA-MD are highly variable and sensitive to genetic background. To identify genetic contributions to this phenotypic variability, we performed whole-genome sequencing on four siblings possessing the same LMNA mutation with differing degrees of skeletal muscle disease severity. We identified a variant in SMAD7 that segregated with severe muscle disease. To functionally test the SMAD7 variant, we generated a Drosophila model possessing the LMNA mutation and the SMAD7 variant in the orthologous fly genes. The SMAD7 variant increased SMAD signaling and enhanced muscle defects caused by the mutant lamin. Conversely, overexpression of wild-type SMAD7 rescued muscle function. These findings were extended to humans by showing that SMAD signaling is increased in muscle biopsy tissue from individuals with LMNA-MD compared to age-matched controls. Collectively, our findings support SMAD7 as the first functionally tested genetic modifier for LMNA-MD and suggest components of the SMAD pathway as therapeutic targets.

Dominantly inherited muscle disorders: understanding their complexity and exploring therapeutic approaches

Treatments for disabling and life-threatening hereditary muscle disorders are finally close to becoming a reality. Research has thus far focused primarily on recessive forms of muscle disease. The gene replacement strategies that are commonly employed for recessive, loss-of-function disorders are not readily translatable to most dominant myopathies owing to the presence of a normal chromosome in each nucleus, hindering the development of novel treatments for these dominant disorders. This is largely due to their complex, heterogeneous disease mechanisms that require unique therapeutic approaches. However, as viral and RNA interference-based therapies enter clinical use, key tools are now in place to develop treatments for dominantly inherited disorders of muscle. This article will review what is known about dominantly inherited disorders of muscle, specifically their genetic basis, how mutations lead to disease, and the pathomechanistic implications for therapeutic approaches.

Dominantly inherited myosin IIa myopathy caused by aberrant splicing of MYH2

Background

Myosin heavy chain (MyHC) isoforms define the three major muscle fiber types in human extremity muscles. Slow beta/cardiac MyHC (MYH7) is expressed in type 1 muscle fibers. MyHC IIa (MYH2) and MyHC IIx (MYH1) are expressed in type 2A and 2B fibers, respectively. Whereas recessive MyHC IIa myopathy has been described in many cases, myopathy caused by dominant MYH2 variants is rare and has been described with clinical manifestations and muscle pathology in only one family and two sporadic cases.

Methods

We investigated three patients from one family with a dominantly inherited myopathy by clinical investigation, whole-genome sequencing, muscle biopsy, and magnetic resonance imaging (MRI).

Results

Three siblings, one woman and two men now 54, 56 and 66 years old, had experienced muscle weakness initially affecting the lower limbs from young adulthood. They have now generalized proximal muscle weakness affecting ambulation, but no ophthalmoplegia. Whole-genome sequencing identified a heterozygous MYH2 variant, segregating with the disease in the three affected individuals: c.5673 + 1G > C. Analysis of cDNA confirmed the predicted splicing defect with skipping of exon 39 and loss of residues 1860–1891 in the distal tail of the MyHC IIa, largely overlapping with the filament assembly region (aa1877–1905). Muscle biopsy in two of the affected individuals showed prominent type 1 muscle fiber predominance with only a few very small, scattered type 2A fibers and no type 2B fibers. The small type 2A fibers were frequently hybrid fibers with either slow MyHC or embryonic MyHC expression. The type 1 fibers showed variation in fiber size, internal nuclei and some structural alterations. There was fatty infiltration, which was also demonstrated by MRI.

Conclusion

Dominantly inherited MyHC IIa myopathy due to a splice defect causing loss of amino acids 1860–1891 in the distal tail of the MyHC IIa protein including part of the assembly competence domain. The myopathy is manifesting with slowly progressive muscle weakness without overt ophthalmoplegia and markedly reduced number and size of type 2 fibers.

Novel mutation in the MYH2 gene in a symptomatic neonate with a hereditary myosin myopathy

INTRODUCTION

Hereditary myosin myopathies are muscle disorders caused by mutations in myosin heavy chain genes. The MYH2 gene encodes the fast 2A skeletal muscle isoform, and mutations manifest as joint contractures, muscle weakness, and external ophthalmoplegia. Muscle biopsy shows decreased type 2A fibers, and vacuoles are sometimes present in adults with progressive disease.

PRESENTATION OF CASE

This case describes a full term baby boy with hypotonia, dysmorphic features, dysphagia, and aspiration. Whole genome sequencing detected a novel heterozygous variant in the MYH2 gene. Muscle biopsy showed decreased type 2A fibers and vacuoles in myofibers.

DISCUSSION

Hypotonia and dysphagia are common in infants with a MYH2 myopathy. However, dysmorphic features and vacuoles on biopsy have not previous been described in infants with MYH2 myopathies.

CONCLUSION

This case reports an unusual phenotype of a rare neonatal-onset congenital myopathy associated with a novel heterozygous variant in MYH2.

A case of GNE myopathy mimicking hereditary motor neuropathy

A 36-year-old woman who presented with upper limb distal weakness since the age of 15 years, with gradual progression to the lower limbs, is reported. Hereditary motor neuropathy was initially suspected based on distal weakness and hyporeflexia; however, whole exome sequencing accidentally revealed a compound heterozygous variant in the GNE gene, and ultrasound revealed increased homogeneous echogenicity in the involved muscles, which is characteristic of myopathic changes. Muscle magnetic resonance imaging revealed fatty infiltration in all limb muscles, sparing the triceps brachii, vastus lateralis and vastus medialis. Muscle biopsy revealed intracytoplasmic rimmed vacuole, supporting the diagnosis of GNE myopathy.

Filamentous tangles with nemaline rods in MYH2 myopathy: a novel phenotype

The MYH2 gene encodes the skeletal muscle myosin heavy chain IIA (MyHC-IIA) isoform, which is expressed in the fast twitch type 2A fibers. Autosomal dominant or recessive pathogenic variants in MYH2 lead to congenital myopathy clinically featured by ophthalmoparesis and predominantly proximal weakness. MYH2-myopathy is pathologically characterized by loss and atrophy of type 2A fibers. Additional myopathological abnormalities have included rimmed vacuoles containing small p62 positive inclusions, 15–20 nm tubulofilaments, minicores and dystrophic changes. We report an adult patient with late-pediatric onset MYH2-myopathy caused by two heterozygous pathogenic variants: c.3331C>T, p.Gln1111* predicted to result in truncation of the proximal tail region of MyHC-IIA, and c.1546T>G, p.Phe516Val, affecting a highly conserved amino acid within the highly conserved catalytic motor head relay loop. This missense variant is predicted to result in a less compact loop domain and in turn could affect the protein affinity state. The patient’s genotype is accompanied by a novel myopathological phenotype characterized by centralized large myofilamentous tangles associated with clusters of nemaline rods, and ring fibers, in addition to the previously reported rimmed vacuoles, paucity and atrophy of type 2A fibers. Electron microscopy demonstrated wide areas of disorganized myofibrils which were oriented in various planes of direction and entrapped multiple nemaline rods, as corresponding to the large tangles with rods seen on light microscopy. Nemaline rods were rarely observed also in nuclei. We speculate that the mutated MyHC-IIA may influence myofibril disorganization. While nemaline rods have been described in myopathies caused by pathogenic variants in genes encoding several sarcomeric proteins, to our knowledge, nemaline rods have not been previously described in MYH2-myopathy.

MYH2 myopathy, a new case expands the clinical and pathological spectrum of the recessive form

Background

Hereditary myosin myopathies are a group of rare muscle disorders, caused by mutations in genes encoding for skeletal myosin heavy chains (MyHCs). MyHCIIa is encoded by MYH2 and is expressed in fast type 2A and 2B muscle fibers. MYH2 mutations are responsible for an autosomal dominant (AD) progressive myopathy, characterized by the presence of rimmed vacuoles and by a reduction in the number and size of type 2A fibers, and a recessive early onset myopathy characterized by complete loss of type 2A fibers. Recently, a patient with a homozygous mutation but presenting a dominant phenotype has been reported.

Methods

The patient was examined thoroughly and two muscle biopsies were performed through the years. NGS followed by confirmation in Sanger sequencing was used to identify the genetic cause.

Results

We describe the second case presenting with late‐onset ophthalmoparesis, ptosis, diffuse muscle weakness, and histopathological features typical for AD forms but with a recessive MYH2 genotype.

Conclusion

This report contributes to expand the clinical and genetic spectrum of MYH2 myopathies and to increase the awareness of these very rare diseases.

Diagnosis of 'possible' mitochondrial disease: an existential crisis

Primary genetic mitochondrial diseases are often difficult to diagnose, and the term 'possible' mitochondrial disease is used frequently by clinicians when such a diagnosis is suspected. There are now many known phenocopies of mitochondrial disease. Advances in genomic testing have shown that some patients with a clinical phenotype and biochemical abnormalities suggesting mitochondrial disease may have other genetic disorders. In instances when a genetic diagnosis cannot be confirmed, a diagnosis of 'possible' mitochondrial disease may result in harm to patients and their families, creating anxiety, delaying appropriate diagnosis and leading to inappropriate management or care. A categorisation of 'diagnosis uncertain', together with a specific description of the metabolic or genetic abnormalities identified, is preferred when a mitochondrial disease cannot be genetically confirmed.

Clinical remission of myopathy with MYH2 deficiency after precision medicine-developed rehabilitation: a case report

Here, we describe the case of a motor developmental disorder associated with intellectual disability accompanied by MYH2 mutations (c.2266G>A and c.4258C>T) in a female child in China. Her initial detailed functional rehabilitation evaluation gauged motor skills, balance, verbal language, and daily living skills. A general therapy plan was then established to enhance balance, muscle strength in the lower extremities, walking, gross and fine motor function, and family education. Clinicians and therapists later modified her rehabilitation regimen after her MYH2 mutations were identified by adding specific mobility and endurance exercise to the original plan. The clinical remission of myopathy with MYH2 missense mutations was observed in the patient after this targeted rehabilitation, indicating that precision therapy is very effective for developing a suitable rehabilitation program for patients with unexplained myopathies.

An E321G MYH1 mutation is strongly associated with nonexertional rhabdomyolysis in Quarter Horses

BACKGROUND

An E321G mutation in MYH1 was recently identified in Quarter Horses (QH) with immune-mediated myositis (IMM) defined by a phenotype of gross muscle atrophy and myofiber lymphocytic infiltrates.

HYPOTHESIS/OBJECTIVES

We hypothesized that the MYH1 mutation also was associated with a phenotype of nonexertional rhabdomyolysis. The objective of this study was to determine the prevalence of the MYH1 mutation in QH with exertional (ER) and nonexertional (nonER) rhabdomyolysis.

ANIMALS

Quarter Horses: 72 healthy controls, 85 ER-no atrophy, 56 ER-atrophy, 167 nonER horses selected regardless of muscle atrophy.

METHODS

Clinical and histopathologic information and DNA was obtained from a database for (1) ER > 2 years of age, with or without atrophy and (2) nonER creatine kinase (CK) ≥ 5000 U/L, <5 years of age. Horses were genotyped for E321G MYH1 by pyrosequencing.

RESULTS

The MYH1 mutation was present in a similar proportion of ER-no atrophy (1/56; 2%) and in a higher proportion of ER-atrophy (25/85; 29%) versus controls (4/72; 5%). The MYH1 mutation was present in a significantly higher proportion of nonER (113/165; 68%) than controls either in the presence (39/42; 93%) or in absence (72/123; 59%) of gross atrophy. Lymphocytes were present in <18% of muscle samples with the MYH1 mutation.

CONCLUSIONS AND CLINICAL IMPORTANCE

Although not associated with ER, the MYH1 mutation is associated with atrophy after ER. The MYH1 mutation is highly associated with nonER regardless of whether muscle atrophy or lymphocytic infiltrates are present. Genetic testing will enhance the ability to diagnose MYH1 myopathies (MYHM) in QH.

Homozygous recessive MYH2 mutation mimicking dominant MYH2 associated myopathy

Mutations in MYH2 that encodes myosin heavy chain IIa cause both dominant and recessively inherited myopathies. Patients with dominantly inherited MYH2 missense mutations present with ophthalmoplegia and progressive proximal limb weakness. Muscle biopsy reveals rimmed vacuoles and inclusions, prompting this entity to initially be described as hereditary inclusion body myopathy 3. In contrast, patients with recessive MYH2 mutations have early onset, non-progressive, diffuse weakness and ophthalmoplegia. Muscle biopsy reveals near or complete absence of type 2A fibers with no vacuole or inclusion pathology. We describe a patient with childhood onset ophthalmoplegia, progressive proximal muscle weakness beginning in adolescence, and muscle biopsy with myopathic changes and rimmed vacuoles. Although this patient's disease course and histopathology is consistent with dominant MYH2 mutations, whole exome sequencing revealed a c.737 G>A p.Arg246His homozygous MYH2 variant. These findings expand the clinical and pathologic phenotype of recessive MYH2 myopathies.

Congenital myopathy due to myosin heavy chain 2 mutation presenting as chronic aspiration pneumonia in infancy

A 7-week-old infant presented with persistent noisy breathing and aspirations during swallowing. Neurological examination and brain MRI were normal. His 12-year-old brother underwent pneumonectomy at the age of 10 years due to recurrent aspirations leading to severe lung damage. The older brother developed subsequently ophthalmoplegia and nystagmus along with mild weakness of the neck flexors and proximal muscles. Exome analysis revealed homozygosity for a novel truncating mutation p.G800fs27* in the Myosin Heavy Chain 2 (MYH2) gene in both brothers, while parents and an unaffected sibling were heterozygous. A muscle biopsy from the older brother showed absence of type-2 muscle fibers and predominance of type-1 fibers. The aspirations causing pneumonia likely result from weakness of the laryngeal muscles, normally rich in type-2 fibers. The findings expand the phenotypic spectrum of MYH2 deficiency. MYH2 mutations should be included in the differential diagnosis of infants presenting with recurrent aspirations.

Identification, modeling, and characterization studies of Tetrahymena thermophila myosin FERM domains suggests a conserved core fold but functional differences

Myosins (MYO) define a superfamily of motor proteins which facilitate movement along cytoskeletal actin filaments in an ATP-dependent manner. To date, over 30 classes of myosin have been defined that vary in their roles and distribution across different taxa. The multidomain tail of myosin is responsible for the observed functional differences in different myosin classes facilitating differential binding to different cargos. One domain found in this region, the FERM domain, is found in several diverse proteins and is involved in many biological functions ranging from cell adhesion and actin-driven cytoskeleton assembly to cell signaling. Recently, new classes of unconventional myosin have been identified in Tetrahymena thermophila. In this study, we have identified, modeled, and characterized eight FERM domains from the unconventional T. thermophila myosins as their complete functional MyTH4-FERM cassettes. Our results reveal notable sequence, structural, and electrostatic differences between T. thermophila and other characterized FERM domains. Specifically, T. thermophila FERM domains contain helical inserts or extensions, which contribute to significant differences in surface electrostatic profiles of T. thermophila myosin FERMs when compared to the conventional FERM domains. Analyses of the modeled domains reveal differences in key functional residues as well as phosphoinositide-binding signatures and affinities. The work presented here broadens the scope of our understanding of myosin classes and their inherent functions, and provides a platform for experimentalists to design rational experimental studies to test the functional roles for T. thermophila myosins.

Adult onset distal and proximal myopathy with complete ophthalmoplegia associated with a novel de novo p.(Leu1877Pro) mutation in MYH2

An MYH2 mutation p.(Glu706Lys) was originally described in a family with autosomal dominant inheritance, where the affected family members presented with multiple congenital contractures and ophthalmoplegia, progressing to a proximal myopathy in adulthood. Another patient with a dominant mutation p.(Leu1870Pro) was described, presenting as a congenital myopathy with ophthalmoplegia. Here, we present a patient with symptoms beginning at age 16 years, of prominent distal but also proximal weakness, bulbar involvement and ophthalmoplegia. Initially, clinically classified as oculopharyngodistal myopathy, the patient was found to carry a novel, de novo MYH2 mutation c.5630T>C p.(Leu1877Pro). This expands the phenotype of dominant MYH2 myopathies with the clinical phenotype overlapping the oculopharyngodistal myopathy spectrum.

Pathophysiological concepts in the congenital myopathies: blurring the boundaries, sharpening the focus

The congenital myopathies are a diverse group of genetic skeletal muscle diseases, which typically present at birth or in early infancy. There are multiple modes of inheritance and degrees of severity (ranging from foetal akinesia, through lethality in the newborn period to milder early and later onset cases). Classically, the congenital myopathies are defined by skeletal muscle dysfunction and a non-dystrophic muscle biopsy with the presence of one or more characteristic histological features. However, mutations in multiple different genes can cause the same pathology and mutations in the same gene can cause multiple different pathologies. This is becoming ever more apparent now that, with the increasing use of next generation sequencing, a genetic diagnosis is achieved for a greater number of patients. Thus, considerable genetic and pathological overlap is emerging, blurring the classically established boundaries. At the same time, some of the pathophysiological concepts underlying the congenital myopathies are moving into sharper focus. Here we explore whether our emerging understanding of disease pathogenesis and underlying pathophysiological mechanisms, rather than a strictly gene-centric approach, will provide grounds for a different and perhaps complementary grouping of the congenital myopathies, that at the same time could help instil the development of shared potential therapeutic approaches. Stemming from recent advances in the congenital myopathy field, five key pathophysiology themes have emerged: defects in (i) sarcolemmal and intracellular membrane remodelling and excitation-contraction coupling; (ii) mitochondrial distribution and function; (iii) myofibrillar force generation; (iv) atrophy; and (v) autophagy. Based on numerous emerging lines of evidence from recent studies in cell lines and patient tissues, mouse models and zebrafish highlighting these unifying pathophysiological themes, here we review the congenital myopathies in relation to these emerging pathophysiological concepts, highlighting both areas of overlap between established entities, as well as areas of distinction within single gene disorders.