Congenital neuromuscular disease with uniform type 1 fiber and RYR1 mutation

Congenital myopathy presenting as recurrent pneumonia with lung collapse and pulmonary artery hypertension

A boy presented with cough, breathlessness for 1 month, fever for 1 week with similar previous episodes without hospitalisation. He had generalised muscle wasting, acute chronic malnutrition and required immediate ventilation. Provisional diagnosis of recurrent pneumonia with failure to thrive was made. As serial chest X-rays showed recurrent lung collapse, congenital lung anomalies were ruled out. 2D-echocardiography showed pulmonary arterial hypertension. Workup for congenital immunodeficiency and cystic fibrosis was negative. There was no improvement in muscle mass despite total parenteral nutrition. He was noticed to have myopathic facies. History was reviewed when the mother reported reduced fetal movements in this pregnancy. The patient had low voice amplitude. Creatine kinase levels were normal. Muscle biopsy followed by whole exome sequencing identified frameshift duplication NM_020451.3(SELENON):c.249_250dupGG (p.Asp84Glyfs*17), thus, confirming diagnosis of SEPN1-related congenital myopathy (CM) with fibre-type disproportion. Respiratory system involvement was distracter, emphasising consideration of CM while evaluating persistent lung collapse with muscle wasting.

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.

Congenital myopathies

The congenital myopathies are inherited muscle disorders characterized clinically by hypotonia and weakness, usually from birth, with a static or slowly progressive clinical course. Historically, the congenital myopathies have been classified according to major morphological features seen on muscle biopsy as nemaline myopathy, central core disease, centronuclear or myotubular myopathy, and congenital fiber type disproportion. However, in the past two decades, the genetic basis of these different forms of congenital myopathy has been further elucidated with the result being improved correlation with histological and genetic characteristics. However, these notions have been challenged for three reasons. First, many of the congenital myopathies can be caused by mutations in more than one gene that suggests an impact of genetic heterogeneity. Second, mutations in the same gene can cause different muscle pathologies. Third, the same genetic mutation may lead to different pathological features in members of the same family or in the same individual at different ages. This chapter provides a clinical overview of the congenital myopathies and a clinically useful guide to its genetic basis recognizing the increasing reliance of exome, subexome, and genome sequencing studies as first-line analysis in many patients.

Expanding the clinical-pathological and genetic spectrum of RYR1-related congenital myopathies with cores and minicores: an Italian population study

Mutations in the RYR1 gene, encoding ryanodine receptor 1 (RyR1), are a well-known cause of Central Core Disease (CCD) and Multi-minicore Disease (MmD). We screened a cohort of 153 patients carrying an histopathological diagnosis of core myopathy (cores and minicores) for RYR1 mutation. At least one RYR1 mutation was identified in 69 of them and these patients were further studied. Clinical and histopathological features were collected. Clinical phenotype was highly heterogeneous ranging from asymptomatic or paucisymptomatic hyperCKemia to severe muscle weakness and skeletal deformity with loss of ambulation. Sixty-eight RYR1 mutations, generally missense, were identified, of which 16 were novel. The combined analysis of the clinical presentation, disease progression and the structural bioinformatic analyses of RYR1 allowed to associate some phenotypes to mutations in specific domains. In addition, this study highlighted the structural bioinformatics potential in the prediction of the pathogenicity of RYR1 mutations. Further improvement in the comprehension of genotype-phenotype relationship of core myopathies can be expected in the next future: the actual lack of the human RyR1 crystal structure paired with the presence of large intrinsically disordered regions in RyR1, and the frequent presence of more than one RYR1 mutation in core myopathy patients, require designing novel investigation strategies to completely address RyR1 mutation effect.

A review of core myopathy: central core disease, multiminicore disease, dusty core disease, and core-rod myopathy

Core myopathies are clinically, pathologically, and genetically heterogeneous muscle diseases. Their onset and clinical severity are variable. Core myopathies are diagnosed by muscle biopsy showing focally reduced oxidative enzyme activity and can be pathologically divided into central core disease, multiminicore disease, dusty core disease, and core-rod myopathy. Although RYR1-related myopathy is the most common core myopathy, an increasing number of other causative genes have been reported, including SELENON, MYH2, MYH7, TTN, CCDC78, UNC45B, ACTN2, MEGF10, CFL2, KBTBD13, and TRIP4. Furthermore, the genes originally reported to cause nemaline myopathy, namely ACTA1, NEB, and TNNT1, have been recently associated with core-rod myopathy. Genetic analysis allows us to diagnose each core myopathy more accurately. In this review, we aim to provide up-to-date information about core myopathies.

Mutational and clinical spectrum of centronuclear myopathy in 9 cases and a literature review of Chinese patients

Centronuclear myopathy (CNM) is a group of congenital myopathies with the histopathological findings of centralized nuclei in muscle fibres. In this study, we summarized the mutational spectrum and phenotypic features of nine Chinese patients with CNM and reanalysed the existing data on 32 CNM patients reported in China. In a cohort comprising nine patients, 14 variants were found in three CNM-related genes, including DNM2, RYR1, and TTN, in 4, 3, and 2 patients, respectively. Of the total 14 variants identified, nine were reported, and 5 were novel including one pathogenic, one likely pathogenic, and 3 of undetermined significance (VUS). Pathologically, we identified the percentage of muscle fibres with central nuclei was much higher in the DNM2-related CNM patients than that in other genetic type of CNM. Of the 32 genetic-diagnosed CNM patients previously reported from China, DNM2, MTM1, SPEG, RYR1, and MYH7 mutations accounted for 59.4%, 25.0%, 9.4%, 3.1%, and 3.1%, respectively. Notably, all of the 20 variants of DNM2 were missense mutations, and the missense mutations in exon 8 were found in 60.0% of DNM2 variants. The c.1106G > A/ p.R369Q (NM_001005360) occurred in 26.3% patients of this Chinese cohort with DNM2-CNM. In conclusion, CNM showed a highly variable genetic spectrum, with DNM2 as the most common causative gene in Chinese CNM patients.

A case of dermatomyositis in a patient with central core disease: unusual association with autoimmunity and genetic muscle disease

BACKGROUND

Dermatomyositis is an inflammatory muscle disease caused by immune-mediated muscle injury, and central core disease (CCD) is a congenital myopathy associated with disturbed intracellular calcium homeostasis and excitation-contraction coupling. To date, CCD has not been reported to have autoantibodies or coexist with inflammatory myopathy.

CASE PRESENTATION

Here, we described the case of a 25-year-old woman who had progressive proximal muscle weakness, myalgia, pruritic macular rash, skin ulcers, and calcinosis. Dermatomyositis was initially suspected based on the clinical symptoms accompanied by elevated muscle enzyme levels, electromyography abnormalities, and a positive antinuclear antibody test. However, the patient's muscle biopsy revealed the characteristic findings of both dermatomyositis and CCD, suggesting that dermatomyositis occurred in this patient with previously asymptomatic CCD. The patient did not have any pathogenic gene mutations associated with congenital myopathy, including RYR1 and SEPN1 in targeted next-generation sequencing. She received high-dose glucocorticoid therapy and azathioprine with a significant improvement in muscle strength.

CONCLUSIONS

We present a case of rare coexistence of dermatomyositis and CCD. Clinicians should be aware that patients with CCD may have inflammatory myopathy that responds well to immunosuppressive therapy.

Evaluation of the Core Formation Process in Congenital Neuromuscular Disease With Uniform Type 1 Fiber and Central Core Disease

Typical central core disease (CCD) is characterized pathologically by the presence of a core and is accompanied by type 1 fiber uniformity. Congenital neuromuscular disease with uniform type 1 fiber (CNMDU1) is characterized pathologically by the presence of type 1 fiber uniformity but without the abnormal structural changes in muscle fibers. Interestingly, typical CCD and 40% of CNMDU1 cases are caused by the same mutations in RYR1, and thus CNMDU1 has been considered an early precursor to CCD. To better understand the nature of CNMDU1, we re-evaluated muscle biopsies from 16 patients with CNMDU1 using immunohistochemistry to RYR1, triadin and TOM20, and compared this to muscle biopsies from 36 typical CCD patients. In CCD, RYR1, and triadin were present in the core regions, while TOM20 was absent in the core regions. Interestingly, in 5 CNMDU1 cases with the RYR1 mutation, RYR1, and triadin were similarly present in core-like areas, while TOM20 was absent in the subsarcolemmal region. Furthermore, there was a correlation between the core position and the disease duration or progression-the older patients in more advanced stages had more centralized cores. Our results indicate that CNMDU1 due to RYR1 mutation is a de facto core myopathy.

Core myopathies - a short review

Congenital myopathies represent a clinically and genetically heterogeneous group of early-onset neuromuscular diseases with characteristic, but not always specific, histopathological features, often presenting with stable and/or slowly progressive truncal and proximal weakness. It is often not possible to have a diagnosis on clinical ground alone. Additional extraocular, respiratory, distal involvement, scoliosis, and distal laxity may provide clues. The "core myopathies" collectively represent the most common form of congenital myopathies, and the name pathologically corresponds to histochemical appearance of focally reduced oxidative enzyme activity and myofibrillar changes on ultrastructural studies. Because of the clinical, pathological, and molecular overlaps, central core disease and multiminicore disease will be discussed together.

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.

Ryanodine receptor 1-related disorders: an historical perspective and proposal for a unified nomenclature

The RYR1 gene, which encodes the sarcoplasmic reticulum calcium release channel or type 1 ryanodine receptor (RyR1) of skeletal muscle, was sequenced in 1988 and RYR1 variations that impair calcium homeostasis and increase susceptibility to malignant hyperthermia were first identified in 1991. Since then, RYR1-related myopathies (RYR1-RM) have been described as rare, histopathologically and clinically heterogeneous, and slowly progressive neuromuscular disorders. RYR1 variants can lead to dysfunctional RyR1-mediated calcium release, malignant hyperthermia susceptibility, elevated oxidative stress, deleterious post-translational modifications, and decreased RyR1 expression. RYR1-RM-affected individuals can present with delayed motor milestones, contractures, scoliosis, ophthalmoplegia, and respiratory insufficiency.

Historically, RYR1-RM-affected individuals were diagnosed based on morphologic features observed in muscle biopsies including central cores, cores and rods, central nuclei, fiber type disproportion, and multi-minicores. However, these histopathologic features are not always specific to RYR1-RM and often change over time. As additional phenotypes were associated with RYR1 variations (including King-Denborough syndrome, exercise-induced rhabdomyolysis, lethal multiple pterygium syndrome, adult-onset distal myopathy, atypical periodic paralysis with or without myalgia, mild calf-predominant myopathy, and dusty core disease) the overlap among diagnostic categories is ever increasing. With the continuing emergence of new clinical subtypes along the RYR1 disease spectrum and reports of adult-onset phenotypes, nuanced nomenclatures have been reported (RYR1- [related, related congenital, congenital] myopathies). In this narrative review, we provide historical highlights of RYR1 research, accounts of the main diagnostic disease subtypes and propose RYR1-related disorders (RYR1-RD) as a unified nomenclature to describe this complex and evolving disease spectrum.

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.

Cored in the act: the use of models to understand core myopathies

The core myopathies are a group of congenital myopathies with variable clinical expression - ranging from early-onset skeletal-muscle weakness to later-onset disease of variable severity - that are identified by characteristic 'core-like' lesions in myofibers and the presence of hypothonia and slowly or rather non-progressive muscle weakness. The genetic causes are diverse; central core disease is most often caused by mutations in ryanodine receptor 1 (RYR1), whereas multi-minicore disease is linked to pathogenic variants of several genes, including selenoprotein N (SELENON), RYR1 and titin (TTN). Understanding the mechanisms that drive core development and muscle weakness remains challenging due to the diversity of the excitation-contraction coupling (ECC) proteins involved and the differential effects of mutations across proteins. Because of this, the use of representative models expressing a mature ECC apparatus is crucial. Animal models have facilitated the identification of disease progression mechanisms for some mutations and have provided evidence to help explain genotype-phenotype correlations. However, many unanswered questions remain about the common and divergent pathological mechanisms that drive disease progression, and these mechanisms need to be understood in order to identify therapeutic targets. Several new transgenic animals have been described recently, expanding the spectrum of core myopathy models, including mice with patient-specific mutations. Furthermore, recent developments in 3D tissue engineering are expected to enable the study of core myopathy disease progression and the effects of potential therapeutic interventions in the context of human cells. In this Review, we summarize the current landscape of core myopathy models, and assess the hurdles and opportunities of future modeling strategies.

Adult Diagnosis of Type 1 Fiber Predominance Myopathy Caused by Novel Mutations in the RYR1 Gene

We describe a 57-year-old patient with mild diffuse weakness that was incidentally detected when he was evaluated for restless leg syndrome. An electromyography confirmed the presence of a myopathy without suggestion of inflammatory myopathy. A muscle biopsy demonstrated type 1 fiber predominance with minimal inflammatory features suggesting a genetic myopathy. Exome sequencing revealed c.10648C > T variant (p.R3550W), and a novel variant, c.10749_10753delGGAGG (E3584Rfs*3), in the ryanodine receptor 1 (RYR1) gene transmitted through his asymptomatic father indicating these mutations are in trans. Prompted by these results, a 47-year-old sister presented for evaluation. Her examination showed mild proximal muscle weakness, and an electromyography confirmed a noninflammatory myopathy. Her genotype was identical to her affected brother confirming that in these siblings, the RYR1 mutations, transmitted in an autosomal recessive pattern, are the cause of their myopathy. The adult age at diagnosis of these affected siblings likely reflects the mild and minimally progressive nature of the myopathy.

Interactions among ryanodine receptor isotypes contribute to muscle fiber type development and function

Mutations affecting ryanodine receptor (RyR) calcium release channels commonly underlie congenital myopathies. Although these channels are known principally for their essential roles in muscle contractility, mutations in the human RYR1 gene result in a broad spectrum of phenotypes, including muscle weakness, altered proportions of fiber types, anomalous muscle fibers with cores or centrally placed nuclei, and dysmorphic craniofacial features. Currently, it is unknown which phenotypes directly reflect requirements for RyRs and which result secondarily to aberrant muscle function. To identify biological processes requiring RyR function, skeletal muscle development was analyzed in zebrafish embryos harboring protein-null mutations. RyR channels contribute to both muscle fiber development and function. Loss of some RyRs had modest effects, altering muscle fiber-type specification in the embryo without compromising viability. In addition, each RyR-encoding gene contributed to normal swimming behavior and muscle function. The RyR channels do not function in a simple additive manner. For example, although isoform RyR1a is sufficient for muscle contraction in the absence of RyR1b, RyR1a normally attenuates the activity of the co-expressed RyR1b channel in slow muscle. RyR3 also acts to modify the functions of other RyR channels. Furthermore, diminished RyR-dependent contractility affects both muscle fiber maturation and craniofacial development. These findings help to explain some of the heterogeneity of phenotypes that accompany RyR1 mutations in humans.

Myopathology of Congenital Myopathies: Bridging the Old and the New

Congenital myopathies (CM) are a genetically heterogeneous group of neuromuscular disorders most commonly presenting with neonatal/childhood-onset hypotonia and muscle weakness, a relatively static or slowly progressive disease course, and originally classified into subcategories based on characteristic histopathologic findings in muscle biopsies. This enduring concept of disease definition and classification based on the clinicopathologic phenotype was pioneered in the premolecular era. Advances in molecular genetics have brought into focus the increased blurring of the original seemingly "watertight" categories through broadening of the clinical phenotypes in existing genes, and continuous identification of novel genetic backgrounds. This review summarizes the histopathologic landscape of the 4 "classical" subtypes of CM-nemaline myopathies, core myopathies, centronuclear myopathies, and congenital fiber type disproportion and some of the emerging and novel genetic diseases with a CM presentation.

Inositol trisphosphate receptor-mediated Ca2+ signalling stimulates mitochondrial function and gene expression in core myopathy patients

Core myopathies are a group of childhood muscle disorders caused by mutations of the ryanodine receptor (RyR1), the Ca²⁺ release channel of the sarcoplasmic reticulum. These mutations have previously been associated with elevated inositol trisphosphate receptor (IP3R) levels in skeletal muscle myotubes derived from patients. However, the functional relevance and the relationship of IP3R mediated Ca²⁺ signalling with the pathophysiology of the disease is unclear. It has also been suggested that mitochondrial dysfunction underlies the development of central and diffuse multi-mini-cores, devoid of mitochondrial activity, which is a key pathological consequence of RyR1 mutations. Here we used muscle biopsies of central core and multi-minicore disease patients with RyR1 mutations, as well as cellular and in vivo mouse models of the disease to characterize global cellular and mitochondrial Ca²⁺ signalling, mitochondrial function and gene expression associated with the disease. We show that RyR1 mutations that lead to the depletion of the channel are associated with increased IP₃-mediated nuclear and mitochondrial Ca²⁺ signals and increased mitochondrial activity. Moreover, western blot and microarray analysis indicated enhanced mitochondrial biogenesis at the transcriptional and protein levels and was reflected in increased mitochondrial DNA content. The phenotype was recapitulated by RYR1 silencing in mouse cellular myotube models. Altogether, these data indicate that remodelling of skeletal muscle Ca²⁺ signalling following loss of functional RyR1 mediates bioenergetic adaptation.

The histopathological spectrum of malignant hyperthermia and rhabdomyolysis due to RYR1 mutations

Objective

The histopathological features of malignant hyperthermia (MH) and non-anaesthetic (mostly exertional) rhabdomyolysis (RM) due to RYR1 mutations have only been reported in a few cases.

Methods

We performed a retrospective multi-centre cohort study focussing on the histopathological features of patients with MH or RM due to RYR1 mutations (1987–2017). All muscle biopsies were reviewed by a neuromuscular pathologist. Additional morphometric and electron microscopic analysis were performed where possible.

Results

Through the six participating centres we identified 50 patients from 46 families, including patients with MH (n = 31) and RM (n = 19). Overall, the biopsy of 90% of patients showed one or more myopathic features including: increased fibre size variability (n = 44), increase in the number of fibres with internal nuclei (n = 30), and type I fibre predominance (n = 13). Abnormalities on oxidative staining, generally considered to be more specifically associated with RYR1-related congenital myopathies, were observed in 52%, and included unevenness (n = 24), central cores (n = 7) and multi-minicores (n = 3). Apart from oxidative staining abnormalities more frequently observed in MH patients, the histopathological spectrum was similar between the two groups. There was no correlation between the presence of cores and the occurrence of clinically detectable weakness or presence of (likely) pathogenic variants.

Conclusions

Patients with RYR1-related MH and RM exhibit a similar histopathological spectrum, ranging from mild myopathic changes to cores and other features typical of RYR1-related congenital myopathies. Suggestive histopathological features may support RYR1 involvement, also in cases where the in vitro contracture test is not informative.

Electronic supplementary material

The online version of this article (10.1007/s00415-019-09209-z) contains supplementary material, which is available to authorized users.

Ryanodine Receptor 1-Related Myopathies: Diagnostic and Therapeutic Approaches

Ryanodine receptor type 1-related myopathies (RYR1-RM) are the most common class of congenital myopathies. Historically, RYR1-RM classification and diagnosis have been guided by histopathologic findings on muscle biopsy. Main histological subtypes of RYR1-RM include central core disease, multiminicore disease, core-rod myopathy, centronuclear myopathy, and congenital fiber-type disproportion. A range of RYR1-RM clinical phenotypes has also emerged more recently and includes King Denborough syndrome, RYR1 rhabdomyolysis-myalgia syndrome, atypical periodic paralysis, congenital neuromuscular disease with uniform type 1 fibers, and late-onset axial myopathy. This expansion of the RYR1-RM disease spectrum is due, in part, to implementation of next-generation sequencing methods, which include the entire RYR1 coding sequence rather than being restricted to hotspot regions. These methods enhance diagnostic capabilities, especially given historic limitations of histopathologic and clinical overlap across RYR1-RM. Both dominant and recessive modes of inheritance have been documented, with the latter typically associated with a more severe clinical phenotype. As with all congenital myopathies, no FDA-approved treatments exist to date. Here, we review histopathologic, clinical, imaging, and genetic diagnostic features of the main RYR1-RM subtypes. We also discuss the current state of treatments and focus on disease-modulating (nongenetic) therapeutic strategies under development for RYR1-RM. Finally, perspectives for future approaches to treatment development are broached.

Characterization of congenital myopathies at a Korean neuromuscular center

INTRODUCTION

Congenital myopathies are muscle diseases characterized by specific histopathologic features, generalized hypotonia from birth, and perinatal complications, although some cases develop during childhood or, rarely, in adulthood. We undertook this study to characterize congenital myopathies among patients registered at our institution.

METHODS

Clinical, histopathologic, and genetic features were evaluated in 34 patients recruited for this study.

RESULTS

The majority of patients experienced a childhood onset, and no disease-related mortality was recorded during follow-up. Functional outcomes were no better for those with late-onset disease, indicating later disease progression can be significant. Nemaline myopathy was the most frequent pathology, followed by central core disease and centronuclear myopathy. Among the 18 (54.5%) genetically confirmed patients, NEB and RYR1 mutations were the most common, followed by DNM2 mutations.

DISCUSSION

This study shows features not previously reported and suggests that congenital myopathy should be considered an important issue among adult patients. Muscle Nerve 58: 235-244, 2018.

RYR1-Related Myopathies: Clinical, Histopathologic and Genetic Heterogeneity Among 17 Patients from a Portuguese Tertiary Centre

BACKGROUND

Pathogenic variants in ryanodine receptor type 1 (RYR1) gene are an important cause of congenital myopathy. The clinical, histopathologic and genetic spectrum is wide.

OBJECTIVE

Review a group of the patients diagnosed with ryanodinopathy in a tertiary centre from North Portugal, as an attempt to define some phenotypical patterns that may help guiding future diagnosis.

METHODS

Patients were identified from the database of the reference centre for Neuromuscular Disorders in North Portugal. Their data (clinical, histological and genetic) was retrospectively accessed.

RESULTS

Seventeen RYR1-related patients (including 4 familial cases) were identified. They were divided in groups according to three distinctive clinical characteristics: extraocular muscle (EOM) weakness (N = 6), disproportionate axial muscle weakness (N = 2) and joint laxity (N = 5). The fourth phenotype includes patients with mild tetraparesis and no distinctive clinical features (N = 4). Four different histopathological patterns were found: centronuclear (N = 5), central core (N = 4), type 1 fibres predominance (N = 4) and congenital fibre type disproportion (N = 1) myopathies. Each index case, except two patients, had a different RYR1 variant. Four new genetic variants were identified. All centronuclear myopathies were associated with autosomal recessive inheritance and EOM weakness. All central core myopathies were caused by pathogenic variants in hotspot 3 with autosomal dominant inheritance. Three genetic variants were reported to be associated to malignant hyperthermia susceptibility.

CONCLUSIONS

Distinctive clinical features were recognized as diagnostically relevant: extraocular muscle weakness (and centronuclear pattern on muscle biopsy), severe axial weakness disproportionate to the ambulatory state and mild tetraparesis associated with (proximal) joint laxity. There was a striking genetic heterogeneity, including four new RYR1 variants.

Novel Variants in Individuals with RYR1-Related Congenital Myopathies: Genetic, Laboratory, and Clinical Findings

The ryanodine receptor 1-related congenital myopathies (RYR1-RM) comprise a spectrum of slow, rare neuromuscular diseases. Affected individuals present with a mild-to-severe symptomatology ranging from proximal muscle weakness, hypotonia and joint contractures to scoliosis, ophthalmoplegia, and respiratory involvement. Although there is currently no FDA-approved treatment for RYR1-RM, our group recently conducted the first clinical trial in this patient population (NCT02362425). This study aimed to characterize novel RYR1 variants with regard to genetic, laboratory, muscle magnetic resonance imaging (MRI), and clinical findings. Genetic and histopathology reports were obtained from participant's medical records. Alamut Visual Software was used to determine if participant's variants had been previously reported and to assess predicted pathogenicity. Physical exams, pulmonary function tests, T1-weighted muscle MRI scans, and blood measures were completed during the abovementioned clinical trial. Six novel variants (two de novo, three dominant, and one recessive) were identified in individuals with RYR1-RM. Consistent with established RYR1-RM histopathology, cores were observed in all biopsies, except Case 6 who exhibited fiber-type disproportion. Muscle atrophy and impaired mobility with Trendelenburg gait were the most common clinical symptoms and were identified in all cases. Muscle MRI revealed substantial inter-individual variation in fatty infiltration corroborating the heterogeneity of the disease. Two individuals with dominant RYR1 variants exhibited respiratory insufficiency: a clinical symptom more commonly associated with recessive RYR1-RM cases. This study demonstrates that a genetics-led approach is suitable for the diagnosis of suspected RYR1-RM which can be corroborated through histopathology, muscle MRI and clinical examination.

Congenital myopathies: disorders of excitation-contraction coupling and muscle contraction

The congenital myopathies are a group of early-onset, non-dystrophic neuromuscular conditions with characteristic muscle biopsy findings, variable severity and a stable or slowly progressive course. Pronounced weakness in axial and proximal muscle groups is a common feature, and involvement of extraocular, cardiorespiratory and/or distal muscles can implicate specific genetic defects. Central core disease (CCD), multi-minicore disease (MmD), centronuclear myopathy (CNM) and nemaline myopathy were among the first congenital myopathies to be reported, and they still represent the main diagnostic categories. However, these entities seem to belong to a much wider phenotypic spectrum. To date, congenital myopathies have been attributed to mutations in over 20 genes, which encode proteins implicated in skeletal muscle Ca²⁺ homeostasis, excitation-contraction coupling, thin-thick filament assembly and interactions, and other mechanisms. RYR1 mutations are the most frequent genetic cause, and CCD and MmD are the most common subgroups. Next-generation sequencing has vastly improved mutation detection and has enabled the identification of novel genetic backgrounds. At present, management of congenital myopathies is largely supportive, although new therapeutic approaches are reaching the clinical trial stage.

Review of RyR1 pathway and associated pathomechanisms

Ryanodine receptor isoform-1 (RyR1) is a major calcium channel in skeletal muscle important for excitation-contraction coupling. Mutations in the RYR1 gene yield RyR1 protein dysfunction that manifests clinically as RYR1-related congenital myopathies (RYR1-RM) and/or malignant hyperthermia susceptibility (MHS). Individuals with RYR1-RM and/or MHS exhibit varying symptoms and severity. The symptoms impair quality of life and put patients at risk for early mortality, yet the cause of varying severity is not well understood. Currently, there is no Food and Drug Administration (FDA) approved treatment for RYR1-RM. Discovery of effective treatments is therefore critical, requiring knowledge of the RyR1 pathway. The purpose of this review is to compile work published to date on the RyR1 pathway and to implicate potential regions as targets for treatment. The RyR1 pathway is comprised of protein-protein interactions, protein-ligand interactions, and post-translational modifications, creating an activation/regulatory macromolecular complex. Given the complexity of this pathway, we divided these interactions and modifications into six regulatory groups. Three of several RyR1 interacting proteins, FK506-binding protein 12 (FKBP12), triadin, and calmodulin, were identified as playing important roles across all groups and may serve as promising target sites for treatment. Also, variability in disease severity may be influenced by prolongation or hyperactivity of post-translational modifications resulting from RyR1 dysfunction.

The sensitivity of exome sequencing in identifying pathogenic mutations for LGMD in the United States

The current study characterizes a cohort of limb-girdle muscular dystrophy (LGMD) in the United States using whole exome sequencing. Fifty-five families affected by LGMD were recruited using an institutionally-approved protocol. Exome sequencing was performed on probands and selected parental samples. Pathogenic mutations and co-segregation patterns were confirmed by Sanger sequencing. Twenty-two families (40%) had novel and previously reported pathogenic mutations, primarily in LGMD genes, but also in genes for Duchenne muscular dystrophy, facioscapulohumeral muscular dystrophy, congenital myopathy, myofibrillar myopathy, inclusion body myopathy, and Pompe disease. One family was diagnosed via clinical testing. Dominant mutations were identified in COL6A1, COL6A3, FLNC, LMNA, RYR1, SMCHD1, and VCP, recessive mutations in ANO5, CAPN3, GAA, LAMA2, SGCA, and SGCG, and X-linked mutations in DMD. A previously reported variant in DMD was confirmed to be benign. Exome sequencing is a powerful diagnostic tool for LGMD. Despite careful phenotypic screening, pathogenic mutations were found in other muscle disease genes, largely accounting for the increased sensitivity of exome sequencing. Our experience suggests that broad sequencing panels are useful for these analyses due to the phenotypic overlap of many neuromuscular conditions. The confirmation of a benign DMD variant illustrates the potential of exome sequencing to help determine pathogenicity.

Next generation sequencing reveals ryanodine receptor 1 mutations in a Chinese central core disease cohort

INTRODUCTION

Ryanodine receptor 1 (RYR1), myosin heavy chain 7 (MYH7), and selenoprotein N1 (SEPN1) mutations are associated with core myopathies. RYR1 mutations cause most cases of central core disease (CCD).

METHODS

We screened 8 Chinese patients with clinicopathological diagnosis of CCD. Genetic analysis was carried out by targeted next generation sequencing (NGS) to identify causative genes. Variants were assessed for pathogenicity using bioinformatic approaches, and NGS results were confirmed by Sanger sequencing.

RESULTS

One novel (p.L4578V) and heterozygous missense mutations in RYR1 were identified in 7 patients. Two patients carried a novel mutation, 1 had p.M4640R, 3 had p.R4861H, and 1 had p.R4861C. All patients had mild to moderate severity phenotypes. Histopathological findings demonstrated central cores and type I fiber predominance.

CONCLUSIONS

NGS is an efficient strategy to identify variants in RYR1 in CCD. However, genetic results revealed by NGS must be combined with clinicopathologic features to validate the diagnosis. Muscle Nerve, 2016 Muscle Nerve 54: 432-438, 2016.

Ophthalmoplegia in congenital neuromuscular disease with uniform type 1 fiber

Congenital neuromuscular disease with uniform type 1 fiber (CNMDU1) is a rare type of congenital myopathy. It is characterized by early onset of symptoms, mild proximal muscle weakness, hyporeflexia or areflexia, normal serum creatine kinase (CK) levels and myopathic electromyography finding, uniform type 1 fibers, and nonprogression. We report a 2-year-old boy who presented with congenital hypotonia, breathing and feeding difficulty, myopathic facies, proximal muscle weakness, ptosis, total external ophthalmoplegia and delayed motor developmental milestones. Normal serum muscle enzyme and short duration of motor unit potentials on electromyography were noted. Muscle biopsy showed uniformity of type 1 fibers (greater than 99%) and moderate variation in fiber size without specific structural abnormality. Total external ophthalmoplegia may be one of the important clinical manifestations of CNMDU1. It is important to recognize this disorder because it is nonprogressive in nature.

Ryanodine myopathies without central cores--clinical, histopathologic, and genetic description of three cases

BACKGROUND

Mutations in ryanodine receptor 1 gene (RYR1) are frequent causes of myopathies. They classically present with central core disease; however, clinical variability and histopathologic overlap are being increasingly recognized.

PATIENTS

Patient 1 is a 15-year-old girl with mild proximal, four-limb weakness from age 5, presenting with a progressive scoliosis starting at age 10. Patient 2 is an 18-year-old girl with progressively worsening muscle hypotrophy and mild proximal, four-limb weakness. She developed a rapidly progressive scoliosis from age 11 and needed surgical treatment at age 14 years. Patient 3 is an 11-year-old boy with moderate proximal limb weakness and progressive neck flexor weakness, first noticed at age 2. Muscle biopsies revealed type 1 fiber predominance (Patients 1 and 2) or abnormal type 1 fiber uniformity (Patient 3). Different RYR1 variants were identified in all patients. In Patients 1 and 3, these changes were validated as being pathogenic.

CONCLUSIONS

These patients illustrate early-onset, progressive myopathies with predominant axial involvement. Histopathologic findings were abnormal but not specific for a diagnosis, particularly central core myopathy. Genetic testing helped broaden the range of phenotypes included in the RYR1-related myopathies. Our patients reinforce the need to recognize the broad histopathologic variability of RYR1-related myopathies and sometimes lack of pathognomonic findings that may reduce the diagnostic threshold of this disease. We suggest that the predominance of type 1 fibers and involvement of axial muscles may be an important element to consider the RYR1 gene as candidate.

Novel RYR1 missense mutations in six Chinese patients with central core disease

Central core disease (CCD) is a genetically heterogeneous congenital myopathy, and ryanodine receptor 1 (RYR1, gene ID6261) is the only pathogenicity gene until now. Data on mutation characteristics of RYR1 in the Chinese CCD population are scarce. This study searched for mutations in the C-terminal-encoding domain of RYR1 in six Chinese patients with CCD, and identified five missense mutations (N4807F, R4861H, R4893P, G4897D, and I4898T). Among them, N4807F, G4897D were novel while R4861H, R4893P, and I4898T were previously reported. All missense mutations were highly conserved across the species of human, mouse, rabbit, fish, and pig. This study found that mutations could be identified in about 85% CCD patients, even if only the C-terminal-encoding region of RYR1 was screened. Many mutations clustered in exons 100-102.

Core myopathies and malignant hyperthermia susceptibility: a review

The core myopathies are a subset of myopathies that present in infancy with hypotonia and muscle weakness. They were formerly considered a rare type of congenital myopathy but are now recognized as being more prevalent. Due to their genetic linkage to mutations in the ryanodine receptor gene (RYR1), core myopathies (in particular, central core disease) carry a high risk of malignant hyperthermia susceptibility. In this review article, we describe the phenotypical, genetic, and histopathological characteristics of core myopathies and further describe the currently understood nature of their risk of malignant hyperthermia. We also review the level of suspicion a clinician should exhibit with a child who has a possible core myopathy or other congenital myopathy presenting for an anesthetic prior to a definitive genetic analysis. For this review article, we performed literature searches using the key words anesthesiology, core myopathies, pediatric neurology, malignant hyperthermia, genetics, ryanodine receptor, and molecular biology. We also relied on literature accumulated by the two authors, who served as hotline consultants for the Malignant Hyperthermia Hotline of the Malignant Hyperthermia Association of the United States (MHAUS) for the past 12 years.

Variable myopathic presentation in a single family with novel skeletal RYR1 mutation

We describe an autosomal recessive heterogeneous congenital myopathy in a large consanguineous family. The disease is characterized by variable severity, progressive course in 3 of 4 patients, myopathic face without ophthalmoplegia and proximal muscle weakness. Absence of cores was noted in all patients. Genome wide linkage analysis revealed a single locus on chromosome 19q13 with Zmax = 3.86 at θ = 0.0 and homozygosity of the polymorphic markers at this locus in patients. Direct sequencing of the main candidate gene within the candidate region, RYR1, was performed. A novel homozygous A to G nucleotide substitution (p.Y3016C) within exon 60 of the RYR1 gene was found in patients. ARMS PCR was used to screen for the mutation in all available family members and in an additional 150 healthy individuals. This procedure confirmed sequence analysis and did not reveal the A to G mutation (p.Y3016C) in 300 chromosomes from healthy individuals. Functional analysis on EBV immortalized cell lines showed no effect of the mutation on RyR1 pharmacological activation or the content of intracellular Ca²⁺ stores. Western blot analysis demonstrated a significant reduction of the RyR1 protein in the patient’s muscle concomitant with a reduction of the DHPRα1.1 protein. This novel mutation resulting in RyR1 protein decrease causes heterogeneous clinical presentation, including slow progression course and absence of centrally localized cores on muscle biopsy. We suggest that RYR1 related myopathy should be considered in a wide variety of clinical and pathological presentation in childhood myopathies.

Congenital myopathies--clinical features and frequency of individual subtypes diagnosed over a 5-year period in the United Kingdom

The congenital myopathies are a group of inherited neuromuscular disorders mainly defined on the basis of characteristic histopathological features. We analysed 66 patients assessed at a single centre over a 5 year period. Of the 54 patients where muscle biopsy was available, 29 (54%) had a core myopathy (central core disease, multi-minicore disease), 9 (17%) had nemaline myopathy, 7 (13%) had myotubular/centronuclear myopathy, 2 (4%) had congenital fibre type disproportion, 6 (11%) had isolated type 1 predominance and 1 (2%) had a mixed core-rod myopathy. Of the 44 patients with a genetic diagnosis, RYR1 was mutated in 26 (59%), ACTA1 in 7 (16%), SEPN1 in 7 (16%), MTM1 in 2 (5%), NEB in 1 (2%) and TPM3 in 1 (2%). Clinically, 77% of patients older than 18 months could walk independently. 35% of all patients required ventilatory support and/or enteral feeding. Clinical course was stable or improved in 57/66 (86%) patients, whilst 4 (6%) got worse and 5 (8%) died. These findings indicate that core myopathies are the most common form of congenital myopathies and that more than half can be attributed to RYR1 mutations. The underlying genetic defect remains to be identified in 1/3 of congenital myopathies cases.

Severe stroke: patient profile and predictors of favorable outcome

BACKGROUND

Severe stroke carries high rates of mortality and morbidity. The aims of this study were to determine the characteristics of patients who initially presented with severe ischemic stroke, and to identify acute and subacute predictors of favorable clinical outcome in these patients.

METHODS

An observational cohort study, Acute Stroke Registry and Analysis of Lausanne (ASTRAL), was analyzed, and all patients presenting with severe stroke - defined as a National Institute of Health Stroke Scale score of ≥ 20 on admission - were compared with all other patients. In a multivariate analysis, associations with demographic, clinical, pathophysiologic, metabolic and neuroimaging factors were determined. Furthermore, we analyzed predictors of favorable outcome (modified Rankin scale score of ≤ 3 at 3 months) in the subgroup of severe stroke patients.

RESULTS

  Of 1915 consecutive patients, 243 (12.7%) presented with severe stroke. This was significantly associated with cardio-embolic stroke mechanism (odds ratio [OR] 1.74, 95% confidence interval [CI] 1.19-2.54), unknown stroke onset (OR 2.35, 95% CI 1.14-4.83), more neuroimaging signs of early ischemia (mostly computed tomography; OR 2.65, 95% CI 1.79-3.92), arterial occlusions on acute imaging (OR 27.01, 95% CI 11.5-62.9), fewer chronic radiologic infarcts (OR 0.43, 95% CI 0.26-0.72), lower hemoglobin concentration (OR 0.97, 95% CI 0.96-0.99), and higher white cell count (OR 1.05, 95% CI 1.00-1.11). In the 68 (28%) patients with favorable outcomes despite presenting with severe stroke, this was predicted by lower age (OR 0.94, 95% CI 0.92-0.97), preceding cerebrovascular events (OR 3.00, 95% CI 1.01-8.97), hypolipemic pretreatment (OR 3.82, 95% CI 1.34-10.90), lower acute temperature (OR 0.43, 95% CI 0.23-0.78), lower subacute glucose concentration (OR 0.74, 95% CI 0.56-0.97), and spontaneous or treatment-induced recanalization (OR 4.51, 95% CI 1.96-10.41).

CONCLUSIONS

Severe stroke presentation is predicted by multiple clinical, radiologic and metabolic variables, several of which are modifiable. Predictors in the 28% of patients with favorable outcome despite presenting with severe stroke include hypolipemic pretreatment, lower acute temperature, lower glucose levels at 24 h, and arterial recanalization.

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.

Novel excitation-contraction uncoupled RYR1 mutations in patients with central core disease

Central core disease, one of the most common congenital myopathies in humans, has been linked to mutations in the RYR1 gene encoding the Ca(2+) release channel of the sarcoplasmic reticulum (RyR1). Functional analyses showed that disease-associated RYR1 mutations led to impairment of skeletal muscle Ca(2+) homeostasis; however, thorough understanding of the molecular mechanisms underlying central core disease and other RyR1-related conditions is still lacking. We screened by sequencing the complete RYR1 transcripts in ten unrelated patients with central core disease and identified five novel, p.M4640R, p.L4647P, p.F4808L, p.D4918N and p.F4941C, and four recurrent mutations. Four of the novel mutations involved amino acid residues that were positioned within putative transmembrane segments of the RyR1. The pathogenic character of the identified mutations was demonstrated by bioinformatic analyses and by the in vitro functional studies in HEK293 cells and RYR1-null (dyspedic) myotubes. Characterization of Ca(2+) channel properties of RyR1s carrying one recurrent and two novel mutations upholds the view that diminished intracellular Ca(2+) release caused by impaired Ca(2+) channel gating and/or Ca(2+) permeability is an important component of central core disease etiology. This study expands the list of functionally characterized disease-associated RyR1 mutations, increasing the value of genetic diagnosis for RyR1-related disorders.

Genetics of neuromuscular disorders

Neuromuscular disorders affect the peripheral nervous system and muscle. The principle effect of neuromuscular disorders is therefore on the ability to perform voluntary movements. Neuromuscular disorders cause significant incapacity, including, at the most extreme, almost complete paralysis. Neuromuscular diseases include some of the most devastating disorders that afflict mankind, for example motor neuron disease. Neuromuscular diseases have onset any time from in utero until old age. They are most often genetic. The last 25 years has been the golden age of genetics, with the disease genes responsible for many genetic neuromuscular disorders now identified. Neuromuscular disorders may be inherited as autosomal dominant, autosomal recessive, or X-linked traits. They may also result from mutations in mitochondrial DNA or from de novo mutations not present in the peripheral blood DNA of either parent. The high incidence of de novo mutation has been one of the surprises of the recent increase in information about the genetics of neuromuscular disorders. The disease burden imposed on families is enormous including decision making in relation to presymptomatic diagnosis for late onset neurodegenerative disorders and reproductive choices. Diagnostic molecular neurogenetics laboratories have been faced with an ever-increasing range of disease genes that could be tested for and usually a finite budget with which to perform the possible testing. Neurogenetics has moved from one known disease gene, the Duchenne muscular dystrophy gene in July 1987, to hundreds of disease genes in 2011. It can be anticipated that with the advent of next generation sequencing (NGS), most, if not all, causative genes will be identified in the next few years. Any type of mutation possible in human DNA has been shown to cause genetic neuromuscular disorders, including point mutations, small insertions and deletions, large deletions and duplications, repeat expansions or contraction and somatic mosaicism. The diagnostic laboratory therefore has to be capable of a large number of techniques in order to identify the different mutation types and requires highly skilled staff. Mutations causing neuromuscular disorders affect the largest human proteins for example titin and nebulin. Successful molecular diagnosis can make invasive and expensive diagnostic procedures such as muscle biopsy unnecessary. Molecular diagnosis is currently largely based on Sanger sequencing, which at most can sequence a small number of exons in one gene at a time. NGS techniques will facilitate molecular diagnostics, but not for all types of mutations. For example, NGS is not good at identifying repeat expansions or copy number variations. Currently, diagnostic molecular neurogenetics is focused on identifying the causative mutation(s) in a patient. In the future, the focus might move to prevention, by identifying carriers of recessive diseases before they have affected children. The pathobiology of many of the diseases remains obscure, as do factors affecting disease severity. The aim of this review is to describe molecular diagnosis of genetic neuromuscular disorders in the past, the present and speculate on the future.

Clinical and genetic findings in a large cohort of patients with ryanodine receptor 1 gene-associated myopathies

Ryanodine receptor 1 (RYR1) mutations are a common cause of congenital myopathies associated with both dominant and recessive inheritance. Histopathological findings frequently feature central cores or multi-minicores, more rarely, type 1 predominance/uniformity, fiber-type disproportion, increased internal nucleation, and fatty and connective tissue. We describe 71 families, 35 associated with dominant RYR1 mutations and 36 with recessive inheritance. Five of the dominant mutations and 35 of the 55 recessive mutations have not been previously reported. Dominant mutations, typically missense, were frequently located in recognized mutational hotspot regions, while recessive mutations were distributed throughout the entire coding sequence. Recessive mutations included nonsense and splice mutations expected to result in reduced RyR1 protein. There was wide clinical variability. As a group, dominant mutations were associated with milder phenotypes; patients with recessive inheritance had earlier onset, more weakness, and functional limitations. Extraocular and bulbar muscle involvement was almost exclusively observed in the recessive group. In conclusion, our study reports a large number of novel RYR1 mutations and indicates that recessive variants are at least as frequent as the dominant ones. Assigning pathogenicity to novel mutations is often difficult, and interpretation of genetic results in the context of clinical, histological, and muscle magnetic resonance imaging findings is essential.

Kyphoscoliosis associated with congenital neuromuscular disease with uniform type 1 fibers

OBJECTIVE

To report the first case of surgical treatment for severe kyphoscoliosis associated with respiratory disorder in a patient with congenital neuromuscular disease with uniform type 1 fibers (CNMDU1), including management of the possible onset of malignant hyperthermia (MH) in general anesthesia. CNMDU1 is rare among congenital neuromuscular diseases, and surgery for spinal deformity in CNMDU1 has not been described. Onset of MH in general anesthesia is a concern in this disease.

METHODS

A 13-year-old female with motor retardation, suspected myopathy, and severe spinal deformity was followed at another pediatric hospital before referral to Meijo Hospital. Symptoms at the initial consultation were mild general muscular weakness and muscular atrophy. The rib hump was 60° and trunk balance was poor. The tendon reflex showed hyporeflexia, and blood tests were normal. Vital capacity was 0.69 L and forced expiratory volume percentage in 1 s was 75.5%, showing a restrictive and obstructive ventilatory defect. A plain radiograph showed severe kyphoscoliosis with thoracic scoliosis of 130° (T5-L1) and thoracic kyphosis of 110° (T2-T12) with almost no flexibility in bending or traction film.

RESULTS

After preoperative halo traction for 2 months, one-stage anterior and posterior correction and fusion from T2 to L3 was conducted. MH did not occur, but recovery of respiratory function required 8 days by intubation after surgery. Postoperatively, thoracic kyphosis improved to 25° and thoracic scoliosis was 66° (correction rate: 49%). Pathological results of an intraoperative muscle biopsy from the paraspinal muscles confirmed the diagnosis of CNMDU1. At 6 years after surgery, the patient has no problems in daily life and no respiratory difficulty.

CONCLUSION

Spinal deformity in CNMDU1 has a risk of severe progression, which makes early diagnosis by biopsy important. The surgery may be recommended before severe progression of spinal deformity and respiratory disorder. Perioperative MH is a concern, but can be managed by appropriate procedures.

Core myopathies

The core myopathies, Central Core Disease and Multiminicore Disease, are heterogeneous congenital myopathies with the common defining histopathological feature of focally reduced oxidative enzyme activity (central cores, multiminicores). Mutations in the gene encoding for the skeletal muscle ryanodine (RyR1) receptor are the most common cause. Mutations in the selenoprotein N (SEPN1) gene cause a less common variant. Pathogenic mechanisms underlying dominant RYR1 mutations have been extensively characterized, whereas those associated with recessive RYR1 and SEPN1 mutations are emerging. Identifying a specific genetic defect from the histopathological diagnosis of a core myopathy is complex and ought to be informed by a combined appraisal of histopathological, clinical, and, increasingly, muscle magnetic resonance imaging data. The present review aims at giving an overview of the main genetic and clinicopathological findings, with a major emphasis on features likely to inform the diagnostic process, as well as current treatments and perspectives for future research.

Mutated p.4894 RyR1 function related to malignant hyperthermia and congenital neuromuscular disease with uniform type 1 fiber (CNMDU1)

BACKGROUND

Ryanodine receptor 1 (RyR1) is a Ca(2+) release channel located in the sarcoplasmic reticulum membrane of skeletal muscle. More than 200 variants in RyR1 have been identified in DNA from patients with malignant hyperthermia (MH) and congenital myopathies; only 30 have been sufficiently studied so as to be identified as MH-causative mutations. The Ala4894Thr RyR1 variant was found in a Japanese patient with susceptibility to MH, and the Ala4894Pro variant in a rare case of myopathy: congenital neuromuscular disease with uniform type 1 fiber (CNMDU1). We hypothesized that different Ala4894 variants of RyR1 cause different pathophysiological changes that are identifiable by having differing pharmacological sensitivities to RYR1 agonists.

METHODS

Expression vector with a mutation in RYR1 corresponding to the Ala4894Thr, Ala4894Pro, Ala4894Ser, or Ala 4894Gly variant of human RyR1 was transfected into human embryonic kidney 293 cells. At 72 hours after transfection, we determined the intracellular Ca(2+) changes induced by caffeine and 4-chloro-m-cresol (4CmC), in the presence or absence of dantrolene.

RESULTS

Ala4894Thr-transfected cells and Ala4894Ser-transfected cells were more sensitive to caffeine than the wild type, and Ala4894Thr-transfected cells were also more sensitive to 4CmC than the wild type, whereas Ala4894Pro-transfected cells had no response to caffeine or 4CmC. Ala4894Gly-transfected cells were significantly less sensitive to caffeine than the wild type. In addition, the responses of Ala4894Thr-transfected cells and Ala4894Ser-transfected cells to caffeine were suppressed by dantrolene.

CONCLUSION

We concluded that different Ala4894 variants of RyR1 lead to different agonist/antagonist sensitivities, which may predict differing RYR1 functionality during excitation-contraction coupling and sensitivity to MH. The hypersensitive Ala4894Thr-RyR1 is associated with MH and the poorly functional Ala4894Pro-RyR1 with CNMDU1.

T-tubule biogenesis and triad formation in skeletal muscle and implication in human diseases

In skeletal muscle, the excitation-contraction (EC) coupling machinery mediates the translation of the action potential transmitted by the nerve into intracellular calcium release and muscle contraction. EC coupling requires a highly specialized membranous structure, the triad, composed of a central T-tubule surrounded by two terminal cisternae from the sarcoplasmic reticulum. While several proteins located on these structures have been identified, mechanisms governing T-tubule biogenesis and triad formation remain largely unknown. Here, we provide a description of triad structure and plasticity and review the role of proteins that have been linked to T-tubule biogenesis and triad formation and/or maintenance specifically in skeletal muscle: caveolin 3, amphiphysin 2, dysferlin, mitsugumins, junctophilins, myotubularin, ryanodine receptor, and dihydhropyridine Receptor. The importance of these proteins in triad biogenesis and subsequently in muscle contraction is sustained by studies on animal models and by the direct implication of most of these proteins in human myopathies.

Recessive RYR1 mutations cause unusual congenital myopathy with prominent nuclear internalization and large areas of myofibrillar disorganization

AIMS

To report the clinical, pathological and genetic findings in a group of patients with a previously not described phenotype of congenital myopathy due to recessive mutations in the gene encoding the type 1 muscle ryanodine receptor channel (RYR1).

METHODS

Seven unrelated patients shared a predominant axial and proximal weakness of varying severity, with onset during the neonatal period, associated with bilateral ptosis and ophthalmoparesis, and unusual muscle biopsy features at light and electron microscopic levels.

RESULTS

Muscle biopsy histochemistry revealed a peculiar morphological pattern characterized by numerous internalized myonuclei in up to 51% of fibres and large areas of myofibrillar disorganization with undefined borders. Ultrastructurally, such areas frequently occupied the whole myofibre cross section and extended to a moderate number of sarcomeres in length. Molecular genetic investigations identified recessive mutations in the ryanodine receptor (RYR1) gene in six compound heterozygous patients and one homozygous patient. Nine mutations are novel and four have already been reported either as pathogenic recessive mutations or as changes affecting a residue associated with dominant malignant hyperthermia susceptibility. Only two mutations were located in the C-terminal transmembrane domain whereas the others were distributed throughout the cytoplasmic region of RyR1.

CONCLUSION

Our data enlarge the spectrum of RYR1 mutations and highlight their clinical and morphological heterogeneity. A congenital myopathy featuring ptosis and external ophthalmoplegia, concomitant with the novel histopathological phenotype showing fibres with large, poorly delimited areas of myofibrillar disorganization and internal nuclei, is highly suggestive of an RYR1-related congenital myopathy.

Anesthetic case in a child with congenital neuromuscular disease with uniform type 1 fibers (CNMDU1)

Congenital neuromuscular disease with uniform type 1 fibers (CNMDU1) is an extremely rare, non-progressive, congenital neuromuscular disorder. Although the etiology is unknown, ryanodine receptor gene mutation is reportedly involved. No descriptions of anesthetic practice in patients with this disease have been reported around the world. We report a case of safe perioperative management with general anesthesia, using total intravenous anesthesia, propofol, fentanyl and a non-depolarizing muscle relaxant but avoiding the use of any inhaled anesthetics or depolarizing muscle relaxants to prevent malignant hyperthermia and postoperative respiratory failure, during anesthetic management for cranioplasty for premature synostosis of the cranial sutures in a pediatric patient of CNMDU1.

Ca2+ dysregulation in Ryr1(I4895T/wt) mice causes congenital myopathy with progressive formation of minicores, cores, and nemaline rods

Ryr1(I4895T/wt) (IT/+) mice express a knockin mutation corresponding to the human I4898T EC-uncoupling mutation in the type 1 ryanodine receptor/Ca(2+) release channel (RyR1), which causes a severe form of central core disease (CCD). IT/+ mice exhibit a slowly progressive congenital myopathy, with neonatal respiratory stress, skeletal muscle weakness, impaired mobility, dorsal kyphosis, and hind limb paralysis. Lesions observed in myofibers from diseased mice undergo age-dependent transformation from minicores to cores and nemaline rods. Early ultrastructural abnormalities include sarcomeric misalignment, Z-line streaming, focal loss of cross-striations, and myofibrillar splitting and intermingling that may arise from defective myofibrillogenesis. However, manifestation of the disease phenotype is highly variable on a Sv129 genomic background. Quantitative RT-PCR shows an equimolar ratio of WT and mutant Ryr1 transcripts within IT/+ myofibers and total RyR1 protein expression levels are normal. We propose a unifying theory in which the cause of core formation lies in functional heterogeneity among RyR1 tetramers. Random combinations of normal and either leaky or EC-uncoupled RyR subunits would lead to spatial differences in Ca(2+) transients; the resulting heterogeneity of contraction among myofibrils would lead to focal, irreversible tearing and shearing, which would, over time, enlarge to form minicores, cores, and nemaline rods. The IT/+ mouse line is proposed to be a valid model of RyR1-related congenital myopathy, offering high potential for elucidation of the pathogenesis of skeletal muscle disorders arising from impaired EC coupling.

Congenital myopathies

PURPOSE OF REVIEW

The present review aims to discuss the pathological and clinical heterogeneity of congenital myopathies, and the overlap between the different variants highlighted by recent studies.

RECENT FINDINGS

The spectrum of pathological changes associated with known gene defects has widened, and new genes responsible for rare structural defects have been identified. The complexity of the classification of these conditions is highlighted by the realization that defects in the same gene can result in diverse phenotypes, including disorders traditionally classified as congenital myopathies with structural abnormalities, adult-onset disorders, conditions characterized by distal weakness and wasting, or distal arthrogryposis. There is a wider appreciation of the complexities of inheritance and of the value of noninvasive assessment, such as muscle MRI. New animal models provide a better understanding of pathogenesis and are highlighting therapeutic possibilities.

SUMMARY

The overlap of clinical and pathological features in the congenital myopathies has led to the recognition that diverse disorders are often associated with the same causative gene, and is challenging traditional classifications. Identification of further causative genes and development of new models will further the understanding of pathogenesis and development of therapies.