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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., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2019. Published by The Company of Biologists Ltd.)

Recessive ryanodine receptor 1 (RYR1) mutations cause congenital myopathies including multiminicore disease (MmD), congenital fiber-type disproportion and centronuclear myopathy. We created a mouse model knocked-in for the Q1970fsX16+A4329D RYR1 mutations, which are isogenic with those identified in a severely affected child with MmD. During the first 20 weeks after birth the body weight and the spontaneous running distance of the mutant mice were 20% and 50% lower compared to wild-type littermates. Skeletal muscles from mutant mice contained 'cores' characterized by severe myofibrillar disorganization associated with misplacement of mitochondria. Furthermore, their muscles developed less force and had smaller electrically evoked calcium transients. Mutant RyR1 channels incorporated into lipid bilayers were less sensitive to calcium and caffeine, but no change in single-channel conductance was observed. Our results demonstrate that the phenotype of the RyR1Q1970fsX16+A4329D compound heterozygous mice recapitulates the clinical picture of multiminicore patients and provide evidence of the molecular mechanisms responsible for skeletal muscle defects., (© The Author(s) 2019. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Introduction: The objective of this study was to obtain a 6-month natural history of motor function performance in individuals with RYR1- related myopathy (RYR1-RM) by using the Motor Function Measure-32 (MFM-32) and graded functional tests (GFT) while facilitating preparation for interventional trials., Methods: In total, 34 participants completed the MFM-32 and GFTs at baseline and 6-month visits., Results: Motor deficits according to MFM-32 were primarily observed in the standing and transfers domain (D1; mean 71%). Among the GFTs, participants required the most time to ascend/descend stairs (>7.5 s). Functional movement, determined by GFT grades, was strongly correlated with MFM-32 (D1; r ≥ 0.770, P < 0.001). Motor Function Measure-32 and GFT scores did not reflect any change in performance between baseline and 6-month visits., Discussion: The MFM-32 and GFTs detected motor impairment in RYR1-RM, which remained stable over 6 months. Thus, these measures may be suitable for assessing change in motor function in response to therapeutic intervention. Muscle Nerve 60: 80-87, 2019., (© 2019 The Authors. Muscle & Nerve published by Wiley Periodicals, Inc.)

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.

Malignant hyperthermia is a genetic defect of uncontrolled hypermetabolic skeletal muscle response to anesthetic triggering drugs. Some congenital myopathies are regarded as risk increasing factors. The use of volatile anaesthetics or suxamethonium (succinylcholine) in patients who are predisposed to malignant hyperthermia leads to an increase in Ca2+ release from sarcoplasmic reticulum, which in turn causes a set of biochemical and clinical symptoms, which can be a cause of death, if dantrolene is not administered adequately. The aim of the study was to draw attention to the problem of malignant hyperthermia, which is hardly ever described in Polish literature, and requires the necessity of intensifying the cooperation between the dentist and specialists from other medical fields. The origin of the article was a case of congenital myopathy with recognized malignant hyperthermia in an 18-year-old patient, in whom surgical extraction of teeth was indicated. The course of diagnostics and treatment showed once more that contemporary medicine is in need of holistic approach, and in consequence, promising and effective cooperation of many specialists.

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., (© 2013 John Wiley & Sons Ltd.)

Objective: To report a series of 11 patients on the severe end of the spectrum of ryanodine receptor 1 (RYR1) gene-related myopathy, in order to expand the clinical, histologic, and genetic heterogeneity associated with this group of patients., Methods: Eleven patients evaluated in the neonatal period with severe neonatal-onset RYR1-associated myopathy confirmed by genetic testing were ascertained. Clinical features, molecular testing results, muscle imaging, and muscle histology are reviewed., Results: Clinical features associated with the severe neonatal presentation of RYR1-associated myopathy included decreased fetal movement, hypotonia, poor feeding, respiratory involvement, arthrogryposis, and ophthalmoplegia in 3 patients, and femur fractures or hip dislocation at birth. Four patients had dominant RYR1 mutations, and 7 had recessive RYR1 mutations. One patient had a cleft palate, and another a congenital rigid spine phenotype-findings not previously described in the literature in patients with early-onset RYR1 mutations. Six patients who underwent muscle ultrasound showed relative sparing of the rectus femoris muscle. Histologically, all patients with dominant mutations had classic central cores on muscle biopsy. Patients with recessive mutations showed great histologic heterogeneity, including fibrosis, variation in fiber size, skewed fiber typing, very small fibers, and nuclear internalization with or without ill-defined cores., Conclusions: This series confirms and expands the clinical and histologic variability associated with severe congenital RYR1-associated myopathy. Both dominant and recessive mutations of the RYR1 gene can result in a severe neonatal-onset phenotype, but more clinical and histologic heterogeneity has been seen in those with recessive RYR1 gene mutations. Central cores are not obligatory histologic features in recessive RYR1 mutations. Sparing of the rectus femoris muscle on imaging should prompt evaluation for RYR1-associated myopathy in the appropriate clinical context.

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., (Crown Copyright © 2012. Published by Elsevier B.V. All rights reserved.)

The large and rapidly increasing number of potentially pathological mutants in the type 1 ryanodine receptor (RyR1) prompts the need to characterize their effects on voltage-activated sarcoplasmic reticulum (SR) Ca(2+) release in skeletal muscle. Here we evaluated the function of the R4892W and G4896V RyR1 mutants, both associated with central core disease (CCD) in humans, in myotubes and in adult muscle fibers. For both mutants expressed in RyR1-null (dyspedic) myotubes, voltage-gated Ca(2+) release was absent following homotypic expression and only partially restored following heterotypic expression with wild-type (WT) RyR1. In muscle fibers from adult WT mice, both mutants were expressed in restricted regions of the fibers with a pattern consistent with triadic localization. Voltage-clamp-activated confocal Ca(2+) signals showed that fiber regions endowed with G4896V-RyR1s exhibited an ∼30% reduction in the peak rate of SR Ca(2+) release, with no significant change in SR Ca(2+) content. Immunostaining revealed no associated change in the expression of either α1S subunit (Cav1.1) of the dihydropyridine receptor (DHPR) or type 1 sarco(endo)plasmic reticulum Ca(2+) ATPase (SERCA1), indicating that the reduced Ca(2+) release resulted from defective RyR1 function. Interestingly, in spite of robust localized junctional expression, the R4892W mutant did not affect SR Ca(2+) release in adult muscle fibers, consistent with a low functional penetrance of this particular CCD-associated mutant.

Albuterol, a selective beta-adrenergic agonist, has been used experimentally in combination with exercise therapy in a few inherited neuromuscular disorders to increase muscle strength and muscle volume . We report on a 9-year-old boy with central core disease and mitochondrial dysfunction due to compound heterozygous RYR1 mutations receiving albuterol treatment for 1 year. Throughout the period of albuterol administration, the patient underwent an aerobic exercise regime of training sessions three times a week that lasted 20 min each. No side effects of albuterol use were seen. Significant clinical progress, including self care, sitting up, raising arms above the shoulders, independent feeding, and better speech and writing were observed compared with minimal development of these abilities in the previous years on physiotherapy. Improved forced expiratory volume in 1 s (FEV1) score was detected and increased muscle strength was noted: progress was measured using various functional tests and assessment scales. The only complication observed was a mild progression of the joint contractures, possibly due to an unbalance between the flexor and extensor musculature. In general, in this pilot study in a complex case of metabolic myopathy our patient has shown promising results following albuterol treatment and aerobic exercise therapy.

Ryanodine receptors (RyR) are intracellular Ca2+-permeable channels that provide the sarcoplasmic reticulum Ca2+ release required for skeletal and cardiac muscle contractions. RyR1 underlies skeletal muscle contraction, and RyR2 fulfills this role in cardiac muscle. Over the past 20 years, numerous mutations in both RyR isoforms have been identified and linked to skeletal and cardiac diseases. Malignant hyperthermia, central core disease, and catecholaminergic polymorphic ventricular tachycardia have been genetically linked to mutations in either RyR1 or RyR2. Thus, RyR channelopathies are both of interest because they cause significant human diseases and provide model systems that can be studied to elucidate important structure-function relationships of these ion channels.

Purpose of Review: To summarize advances in our understanding of the clinical phenotypes, genetics, and molecular pathophysiology of the periodic paralyses, the nondystrophic myotonias, and other muscle channelopathies., Recent Findings: The number of pathogenic mutations causing periodic paralysis, nondystrophic myotonias, and ryanodinopathies continues to grow with the advent of exon hierarchy analysis strategies for genetic screening and better understanding and recognition of disease phenotypes. Recent studies have expanded and clarified the role of gating pore current in channelopathy pathogenesis. It has been shown that the gating pore current can account for the molecular and phenotypic diseases observed in the muscle sodium channelopathies, and, given that homologous residues are affected in mutations of calcium channels, it is possible that pore leak represents a pathomechanism applicable to many channel diseases. Improvements in treatment of the muscle channelopathies are on the horizon. A randomized controlled trial has been initiated for the study of mexiletine in nondystrophic myotonias. The class IC antiarrhythmia drug flecainide has been shown to depress ventricular ectopy and improve exercise capacity in patients with Andersen-Tawil syndrome., Summary: Recent studies have expanded our understanding of gating pore current as a disease-causing mechanism in the muscle channelopathies and have allowed new correlations to be drawn between disease genotype and phenotype.

This article is dedicated to our teacher, Prof. Erich Kuhn, Heidelberg, on the occasion of his 88th birthday on 23rd November 2008. In contrast to muscular dystrophies, the muscle channelopathies, a group of diseases characterised by impaired muscle excitation or excitation-contraction coupling, can fairly well be treated with a whole series of pharmacological drugs. However, for a proper treatment proper diagnostics are essential. This article lists state-of-the-art diagnostics and therapies for the two types of myotonic dystrophies, for recessive and dominant myotonia congenita, for the sodium channel myotonias, for the primary dyskalemic periodic paralyses, for central core disease and for malignant hyperthermia susceptibility in detail. In addition, for each disorder a short summary of aetiology, symptomatology, and pathogenesis is provided.

The term congenital myopathy is applied to muscle disorders presenting with generalized muscle weakness and hypotonia from early infancy with delayed developmental milestones. The congenital myopathies have been classified into various categories based on morphological findings on muscle biopsy. Although the clinical symptoms may seem homogenous, the genetic basis is remarkably variable. This review will focus on myotubular myopathy, centronuclear myopathy, central core disease, and congenital neuromuscular disease with uniform Type 1 fiber, myopathies that are subjects of our ongoing examinations.

A 10-month-old boy presented with gross motor delay and muscle weakness, especially in both lower limbs. At age 5, he developed lordosis, talipes, and planovalgus. His grandmother died of malignant hyperthermia during surgery. On neurological examination, he had mild proximal muscle weakness and atrophy, decreased deep tendon reflexes and Gowers' sign, but his intelligence was normal. The electromyogram showed myopathic pattern. Muscle biopsy revealed type 1 fiber atrophy and central core abnormalities. We report this case of central core disease with a family history of malignant hyperthermia during surgery.

Diploid/triploid mosaicism is an uncommon malformation syndrome thought to result from incorporation of the second polar body into a blastomere nucleus of the developing embryo. Clinical manifestations include mental and growth retardation, truncal obesity, body asymmetry, hypotonia, syndactyly, clino-/camptodactyly, malformed low-set ears, and small phallus. Although muscular atrophy has been documented in 35% of cases of diploid/triploid mosaicism, to our knowledge histologic evidence of myopathy has not been reported. We present a novel case of diploid/triploid mosaicism with evidence of central core disease and nemaline bodies (rods). The histologic and ultrastructural features are described. A review of the literature is provided, including discussion of the various theories regarding the co-expression of central cores and nemaline rods.

Background: Multi-minicore disease is a rare form of myopathy characterized by slowly progressive or nonprogressive muscle weakness and characteristic multiple cores within the muscle fibers. To the best of our knowledge, this is first documentation of the clinicopathological features of this rare entity from India., Materials and Methods: A ll cases of multi-minicore disease diagnosed in our laboratory were retrieved. Clinical and pathological features were reviewed., Result: During a period of two years (January 2004 to December 2005), we received 985 muscle biopsies for various reasons. Of which, 15 were diagnosed as myopathies and four of which were of multi-minicore disease. Thus, multi-minicore disease comprises 0.40% of all muscle diseases and 26.6% of all myopathies. All were male and presented in early childhood (first decade of life) with generalized hypotonia and muscle weakness. All of them had dysmorphic facies and three had high arched palate. CPK levels were normal and EMG was myopathic except in one patient. Microscopic examination revealed minimal changes with Type I fibers' predominance but characteristic multiple cores in the myofibers. Ultrastructural examination showed both structured and unstructured cores., Conclusions: Multi-minicore disease, although a rare form of myopathies, should be suspected in children who present with generalized hypotonia and slowly progressive muscle weakness along with dysmorphic facies.

Mutations in two intracellular Ca2+ release channels or ryanodine receptors (RyR1 and RyR2) are associated with a number of human skeletal and cardiac diseases. This chapter discusses these diseases in terms of known mechanisms, controversies, and unanswered questions. We also compare the cardiac and skeletal muscle diseases to explore common mechanisms.

Objective: To describe the characteristic clinical features and the diagnostic findings in muscle histochemistry of multi-minicore disease in 5 children from two unrelated families in Kuwait., Clinical Presentation: The 5 children who presented with muscle weakness, 2 siblings from family 1 and the remaining 3 from the other (family 2), represent the classical type of multi-minicore disease; however, the two families differ in the course of the disease. Family 1 had the non-progressive form while family 2 had progressive weakness with respiratory complications and scoliosis. The diagnosis was confirmed in the index patients by muscle histochemistry, which demonstrated the typical minicores, which are devoid of oxidative enzyme activity., Conclusion: Both families represent the classical form, however, clinical variability in the course of the illness was demonstrated in these two families.

Several studies have documented positive effects of beta-adrenergic agonists on human skeletal muscle with regard to muscle mass and strength. The aim of this pilot study was to evaluate the effect of the beta2-agonist salbutamol (albuterol) in a group of children with central core disease and multi-minicore disease. Thirteen patients, 8 with central core disease (mean age 17.5 years) and 5 with minicore disease (mean age 13.6 years) received oral salbutamol at a dose of 2 mg four times a day. Measures of efficacy were the change from baseline at 3 and 6 months in muscle strength, assessed by MRC score, myometry, functional measures and forced vital capacity. Statistical analysis was performed using repeated measures ANOVA (significance level < 0.05). Two patients with central core disease stopped the medication after one month because they did not notice any improvement and another one with minicore disease after 4 months because of increased tremors and palpitations. The remaining ten (6 with central core and 4 with minicore disease) completed the course of salbutamol without any significant adverse effects. There were significant increases in myometry, MRC scores and forced vital capacity between baseline and the six-month assessments. For both myometry and MRC the difference was already significant at 3 months and this was associated with a significant increase in functional abilities assessed with a structured functional scale. Our results suggest that salbutamol was overall well tolerated and might be beneficial in both central core and minicore patients. Larger prospective randomised, double-blind, placebo-controlled trials with salbutamol will be needed to confirm these preliminary findings.

Myoplasmic calcium homeostasis is an essential feature of skeletal muscle contraction. The calcium mobilisation complex (CMC) located at the level of the triadic junction plays a major role for the regulation of calcium fluxes between extra-cellular, cytoplasmic and intra-cellular compartments. The ryanodine receptor type I (RYR1), which is located at the level of the terminal cisternae of the sarcoplasmic reticulum is a key component of the CMC. RYR1 allow the release into the myoplasm of the intralumenal stores of calcium. RYR1 interacts with other proteins: DiHydroPyridine Receptor, triadin, calsequestrin, FKBP12, calmodulin. Malignant hyperthermia (MHS) and congenital core myopathies have been associated with a dysfunction of the CMC. MHS is an autosomic dominant pharmacogenetic disease. The MH crisis is induced by exposure of the predisposed patients to halogenated volatile anaesthetics. MHS is characterised by a genetic heterogeneity and two genes, RYR1 and CACNA1S, have been associated so far with the disease. Mutations in the RYR1 gene have been recently associated with heat stroke, a related syndrome. Central Core Disease (CCD) and Multi minicore Disease (MmD) are congenital myopathies presenting with clinical variability and characterized by the presence of specific although heterogeneous muscle histological features: the cores. Clinical boundaries between the two diseases may overlap and the specific diagnosis is often based on the nature of the cores. These diseases show genetic heterogeneity with both autosomic dominant and recessive mode of inheritance and mutations in the SEPN1, RYR1, ACTA1, TPM3 genes have been reported. Mutations associated with MHS were mainly identified into 2 regions of the N-terminal part of RYR1. Functional role of these two domains is still unclear. Mutations responsible for congenital myopathies mainly mapped to the C terminal region of RYR1 that form the transmembrane calcium channel. Functional studies of the RYR1 mutations have shown that MHS mutations were mainly associated with an alteration of the calcium fluxes in response to caffeine or halothane while CCD mutations would result in a leaky RYR1 channel or would alter the Excitation-Contraction coupling at the level of the CMC.

The congenital myopathies encompass a group of neuromuscular disorders with characteristic morphologic abnormalities in skeletal muscle, including nemaline myopathy, central core disease, multi-minicore disease, and myotubular myopathy. Giant steps have been made in our understanding of the molecular bases of these disorders, all of which show remarkable genetic heterogeneity. This review of congenital myopathies examines progress in defining clinical diagnostic criteria and novel genetic advances that have provided important clues regarding their pathogeneses.

Causes of hypotonia in the newborn can be broadly categorized into two classifications. Hypotonia with a supraspinal origin may be seen with systemic disease, hypoxic ischemic encephalopathy, cerebral malformations, syndromes (for example: Down, Prader-Willi, Lowe, Zellweger, Smith-Lemli-Opitz), and c-spine injury. Disorders of the motor unit that present with hypotonia in the newborn period include SMA, congenital myotonic dystrophy, congenital myasthenia gravis, and congenital myopathies. Central core disease is one of the classic congenital myopathies that can be differentiated based on characteristic histologic findings. Muscle fiber samples from patients with central core disease possess distinct morphology that can be diagnostic. Many infants may not exhibit muscle weakness in the newborn period, although there have been rare cases of profound hypotonia and respiratory failure. Clearly, muscle biopsy is the gold standard and is indicated for any infant with marked hypotonia that is not thought to be supraspinal in origin.

Central core disease is a congenital myopathy with muscle weakness defined pathologically by the presence of extensive areas in muscle fibres that are devoid of oxidative enzyme activity. The gene responsible has been shown to be the ryanodine receptor 1 on chromosome 19q13 and mutations have now been identified in several patients. Some cases with the morphological defect remain molecularly undefined, particularly those studied before molecular studies were available. We have studied three families with congenital onset, each with a dominantly inherited mutation in a C-terminal exon of the ryanodine receptor 1. They illustrate the spectrum of pathology that can be observed in patients with the myopathic features of central core disease. We show that extensive fibrosis and fat may be present, type 1 fibre uniformity may occur in the absence of cores; cores may be central or peripheral, single or multiple; and that an appearance of multiple focal minicores might cause a diagnostic pathological dilemma. In addition, we show the value of immunocytochemistry in identifying cores, in particular the use of antibodies to desmin and gamma-filamin.

Purpose of Review: Considerable progress has been made in molecular genetic research and in identifying the underlying pathogenesis of congenital myopathies, with implications for genetic counseling. Therefore an overview of such advances in the last two years is most timely and relevant for a more precise delineation of these disorders., Recent Findings: New mutations have been described on the ryanodine receptor gene, including the carboxyl-terminus region, and experimental models developed to explain their role in central core disease. Phenotype-genotype correlations for nemaline myopathy have improved our understanding of those related to gene mutations. In multi-minicore disease, collaborative studies support genetic heterogeneity and autosomal-recessive inheritance. Research on X-linked myotubular myopathies has revealed a high percentage of mothers of sporadic cases as carriers. Although not initially included within the congenital myopathies, desmin-related or myofibrillar myopathies are described here because they are closely related to other congenital myopathies with intracytoplasmic inclusions. Western blot for myotubularin and desmin has been proposed as a useful diagnostic test for both X-linked myotubular myopathy and desmin-related myopathy, and in-vitro and mouse models for the latter have provided insights into its pathogenesis. Several entities still await genetic characterization. Here we focus on clinical features, inheritance, and molecular genetics., Summary: Advances in immunohistochemistry and molecular genetics in congenital muscular dystrophies have enriched our knowledge of this heterogeneous group of disorders, leading to more accurate classification and differentiation between the various congenital myopathies.

Multi-minicore disease is an autosomal recessive congenital myopathy characterized by the presence of multiple, short-length core lesions (minicores) in both muscle fiber types. These lesions being nonspecific and the clinical phenotype being heterogeneous, multi-minicore disease boundaries remain unclear. To identify its genetic basis, we performed a genome-wide screening in a consanguineous Algerian family in which three children presented in infancy with moderate weakness predominant in axial muscles, pelvic girdle and hands, joint hyperlaxity (hand involvement phenotype), and multiple minicores. We mapped the disease to chromosome 19q13 in this family and, subsequently, in three additional families showing a similar phenotype, with a maximum LOD score of 5.19 for D19S570. This locus was excluded in 16 other multi-minicore disease families with predominantly axial weakness, scoliosis, and respiratory insufficiency ("classical" phenotype). In the Algerian family, we identified a novel homozygous missense mutation (P3527S) in the ryanodine receptor type 1 gene, a positional candidate gene responsible for the autosomal dominant congenital myopathy central core disease. New muscle biopsies from the three patients at adulthood demonstrated typical central core disease with rods; no cores were found in the healthy parents. This subgroup of families linked to 19q13 represents the first variant of central core disease with genetically proven recessive inheritance and transient presentation as multi-minicore disease.

The excitation-contraction-relaxation cycle of skeletal muscle fibres depends on the finely tuned interplay between the voltage-sensing dihydropyridine receptor, the junctional ryanodine receptor Ca2+-release channel and the sarcoplasmic reticulum Ca2+-ATPase. Inherited diseases of excitation-contraction coupling and muscle relaxation such as malignant hyperthermia, central core disease, hypokalemic periodic paralysis or Brody disease are caused by mutations in these Ca2+-regulatory elements. Over twenty different mutations in the Ca2+-release channel are associated with susceptibility to the pharmacogenetic disorder malignant hyperthermia. Other mutations in the ryanodine receptor trigger central core disease. Primary abnormalities in the alpha-1 subunit of the dihydropyridine receptor underlie the molecular pathogenesis of both hypokalemic periodic paralysis and certain forms of malignant hyperthermia. Some cases of the muscle relaxation disorder named Brody disease were demonstrated to be based on primary abnormalities in the Ca2+-ATPase. Since a variety of other sarcoplasmic reticulum proteins modulate the activity of the voltage sensor, Ca2+-release channel and ion-binding proteins, mutations in these Ca2+-regulatory muscle components might be the underlying cause for novel, not yet fully characterized, genetic muscle disorders. The cell biological analysis of knock-out mice has been helpful in evaluating the biomedical consequences of defects in ion-regulatory muscle proteins.

Malignant hyperthermia (MH) is a pharmacogenetic disorder of skeletal muscle that manifests in response to anesthetic triggering agents. Central core disease (CCD) is a myopathy closely associated with MH. Both MH and CCD are primarily disorders of calcium regulation in skeletal muscle. The ryanodine receptor (RYR1) gene encodes the key channel which mediates calcium release in skeletal muscle during excitation-contraction coupling, and mutations in this gene are considered to account for susceptibility to MH (MHS) in more than 50% of cases and in the majority of CCD cases. To date, 22 missense mutations in the 15,117 bp coding region of the RYR1 cDNA have been found to segregate with the MHS trait, while a much smaller number of these mutations is associated with CCD. The majority of RYR1 mutations appear to be clustered in the N-terminal amino acid residues 35-614 (MH/CCD region 1) and the centrally located residues 2163-2458 (MH/CCD region 2). The only mutation identified outside of these regions to date is a single mutation associated with a severe form of CCD in the highly conserved C-terminus of the gene. All of the RYR1 mutations result in amino acid substitutions in the myoplasmic portion of the protein, with the exception of the mutation in the C-terminus, which resides in the lumenal/transmembrane region. Functional analysis shows that MHS and CCD mutations produce RYR1 abnormalities that alter the channel kinetics for calcium inactivation and make the channel hyper- and hyposensitive to activating and inactivating ligands, respectively. The likely deciding factors in determining whether a particular RYR1 mutation results in MHS alone or MHS and CCD are: sensitivity of the RYR1 mutant proteins to agonists; the level of abnormal channel-gating caused by the mutation; the consequential decrease in the size of the releasable calcium store and increase in resting concentration of calcium; and the level of compensation achieved by the muscle with respect to maintaining calcium homeostasis. From a diagnostic point of view, the ultimate goal of development of a simple non-invasive test for routine diagnosis of MHS remains elusive. Attainment of this goal will require further detailed molecular genetic investigations aimed at solving heterogeneity and discordance issues in MHS; new initiatives aimed at identifying modulating factors that influence the penetrance of clinical MH in MHS individuals; and detailed studies aimed at describing the full epidemiological picture of in vitro responses of muscle to agents used in diagnosis of MH susceptibility., (Copyright 2000 Wiley-Liss, Inc.)