Multi-minicore disease and atypical periodic paralysis associated with novel mutations in the skeletal muscle ryanodine receptor (RYR1) gene

The skeletal muscle ryanodine receptor plays a crucial role in excitation–contraction (EC) coupling and is implicated in various congenital myopathies. The periodic paralyses are a heterogeneous, dominantly inherited group of conditions mainly associated with mutations in the SCN4A and the CACNA1S genes. The interaction between RyR1 and DHPR proteins underlies depolarization-induced Ca²⁺ release during EC coupling in skeletal muscle. We report a 35-year-old woman presenting with signs and symptoms of a congenital myopathy at birth and repeated episodes of generalized, atypical normokalaemic paralysis in her late teens. Genetic studies of this patient revealed three heterozygous RYR1 substitutions (p.Arg2241X, p.Asp708Asn and p.Arg2939Lys) associated with marked reduction of the RyR1 protein and abnormal DHPR distribution. We conclude that RYR1 mutations may give rise to both myopathies and atypical periodic paralysis, and RYR1 mutations may underlie other unresolved cases of periodic paralysis with unusual features.     

King-Denborough syndrome with and without mutations in the skeletal muscle ryanodine receptor (RYR1) gene

King-Denborough syndrome (KDS), first described in 1973, is a rare condition characterised by the triad of dysmorphic features, myopathy, and malignant hyperthermia susceptibility (MHS). Autosomal dominant inheritance with variable expressivity has been reported in several cases. Mutations in the skeletal muscle ryanodine receptor (RYR1) gene have been implicated in a wide range of myopathies such as central core disease (CCD), the malignant hyperthermia (MH) susceptibility trait and one isolated patient with KDS.Here we report clinical, pathologic and genetic features of four unrelated patients with KDS. Patients had a relatively uniform clinical presentation but muscle biopsy findings were highly variable. Heterozygous missense mutations in RYR1 were uncovered in three out of four families, of which one mutation was novel and two have previously been reported in MH. Further RyR1 protein expression studies performed in two families showed marked reduction of the RyR1 protein, indicating the presence of allelic RYR1 mutations not detectable on routine sequencing and potentially explaining marked intrafamilial variability.Our findings support the hypothesis that RYR1 mutations are associated with King-Denborough syndrome but that further genetic heterogeneity is likely.     

Abnormal distribution of calcium-handling proteins: a novel distinctive marker in core myopathies

Central core disease (CCD) and multi-minicore disease (MmD) are muscle disorders characterized by foci of mitochondria depletion and sarcomere disorganization ("cores") in muscle fibers. Although core myopathies are the most frequent congenital myopathies, their pathogenesis remains elusive and specific diagnostic markers are lacking. Core myopathies are mostly caused by mutations in 2 sarcoplasmic reticulum proteins: the massive Ca-release channel RyR1 or the selenoprotein N (SelN) of unknown function. To search for distinctive markers and to obtain further pathophysiological insight, we identified the molecular defects in 12 core myopathy patients and analyzed the immunolocalization of 6 proteins of the Ca-release complex in their muscle biopsies. In 7 cases with RYR1 mutations (6 CCD, one MmD), RyR1 was depleted from the cores; in contrast, the other proteins of the sarcoplasmic reticulum (calsequestrin, SERCA1/2, and triadin) and the T-tubule (dihydropyridine receptor-alpha1subunit) accumulated within or around the lesions, suggesting an original modification of the Ca-release complex protein arrangement. Conversely, all Ca-related proteins were distributed normally in 5 MmD cases with SelN mutations. Our results provide an appropriate tool to orientate the differential and molecular diagnosis of core myopathies and suggest that different pathophysiological mechanisms lead to core formation in SelN- and in RyR1-related core myopathies.     

Principal mutation hotspot for central core disease and related myopathies in the C-terminal transmembrane region of the RYR1 gene

The congenital myopathies are a group of disorders characterised by the predominance of specific histological features observed in biopsied muscle. Central core disease and nemaline myopathy are examples of congenital myopathies that have specific histological characteristics but significantly overlapping clinical pictures. Central core disease is an autosomal dominant disorder with variable penetrance which has been linked principally to the gene for the skeletal muscle calcium release channel (RYR1). Two recent reports have identified the 3' transmembrane domain of this gene as a common site for mutations. Two other studies have reported single families that have features of both central core disease and nemaline myopathy (core/rod disease) caused by mutations in RYR1. Screening of the 3' region (exons 93-105) of the RYR1 gene for mutations in 27 apparently unrelated patients with either central core disease or core/rod disease by single strand conformation polymorphism analysis and DNA sequencing identified three described and nine novel mutations in 15 patients.     

Aging impairs regulation of ryanodine receptors from extensor digitorum longus but not soleus muscles

ABSTRACT: Introduction: Because impaired excitation‐contraction coupling and reduced sarcoplasmic reticulum (SR) Ca²⁺ release may contribute to the age‐associated decline in skeletal muscle strength, we investigated the effect of aging on regulation of the skeletal muscle isoform of the ryanodine receptor (RyR1) by physiological channel ligands. Methods: [³H]Ryanodine binding to membranes from 8‐ and 26‐month‐old Fischer 344 extensor digitorum longus (EDL) and soleus muscles was used to investigate the effects of age on RyR1 modulation by Ca²⁺ and calmodulin (CaM). Results: Aging reduced maximal Ca²⁺‐stimulated binding to EDL membranes. In 0.3 μM Ca²⁺, age reduced binding and CaM increased binding to EDL membranes. In 300 μM Ca²⁺, CaM reduced binding, but the age effect was not significant. Aging did not affect Ca²⁺ or CaM regulation of soleus RyR1. Discussion: In aged fast‐twitch muscle, impaired RyR1 Ca²⁺ regulation may contribute to lower SR Ca²⁺ release and reduced muscle function. Muscle Nerve 57 : 1022–1025, 2018     

Modulation of big K+ channel activity by ryanodine receptors and L‐type Ca2+ channels in neurons

As metabotropic glutamate receptor type 1 (mGluR1) is known to couple L-type Ca²⁺ channels and ryanodine receptors (RyR, 1 ) in cerebellar granule cells, we examined if such a coupling could activate a Ca²⁺-sensitive K⁺ channel, the big K⁺ (BK) channel, in cultured cerebellar granule cells. We observed that (±)-1-amino-cyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) and quisqualate (QA) stimulated the activity of BK channels. On the other hand, (2S, 3S, 4S)-α-carboxycyclopropyl-glycine (L-CCG-I) and l -(+)-2-amino-4-phosphonobutyrate (L-AP4) had no effect on BK channels, indicating a specific activation by group I mGluRs. Group I mGluRs stimulation of the basal BK channel activity was mimicked by caffeine and both effects were blocked by ryanodine and nifedipine. Interestingly, carbachol stimulated BK channel activity but through a pertussis toxin (PTX)-sensitive pathway that was independent of L-type Ca²⁺ channel activity. Our report indicates that unlike the muscarinic receptors, group I mGluRs activate BK channels by mobilizing an additional pathway involving RyR and L-type Ca²⁺ channels.     

Late-onset axial myopathy with cores due to a novel heterozygous dominant mutation in the skeletal muscle ryanodine receptor (RYR1) gene

Mutations in the skeletal muscle ryanodine receptor (RYR1) gene have been associated with a wide range of phenotypes including the malignant hyperthermia (MH) susceptibility trait, Central Core Disease (CCD) and other congenital myopathies characterized by early onset and predominant proximal weakness.We report a patient presenting at 77 years with a predominant axial myopathy associated with prominent involvement of spine extensors, confirmed on MRI and muscle biopsy, compatible with a core myopathy. RYR1 mutational analysis revealed a novel heterozygous missense mutation (c.119G>T; p.Gly40Val) affecting the RYR1 N-terminus, previously predominantly associated with MH susceptibility.This case expands the spectrum of RYR1-related phenotypes and suggests that MH-related RYR1 mutations may give rise to overt neuromuscular symptoms later in life, with clinical features not typically found in CCD due to C-terminal hotspot mutations. Late-onset congenital myopathies may be under-recognised and diagnosis requires a high degree of clinical suspicion.     

New Tmc1 Deafness Mutations Impact Mechanotransduction in Auditory Hair Cells

Transmembrane channel-like protein isoform 1 (TMC1) is a major component of the mechano-electrical transducer (MET) channel in cochlear hair cells and is subject to numerous mutations causing deafness. We report a new dominant human deafness mutation, TMC1 p.T422K, and have characterized the homologous mouse mutant, Tmc1 p.T416K, which caused deafness and outer hair cell (OHC) loss by the fourth postnatal week. MET channels showed decreased Ca²⁺ permeability and resting open probability, but no change in single-channel conductance or expression. Three adjacent deafness mutations are TMC1 p.L416R, p.G417R, and p.M418K, the last homologous to the mouse Beethoven that exhibits similar channel effects. All substitute a positive for a neutral residue, which could produce charge screening in the channel pore or influence binding of an accessory subunit. Channel properties were compared in mice of both sexes between dominant (Tmc1 p.T416K, Tmc1 p.D569N) and recessive (Tmc1 p.W554L, Tmc1 p.D528N) mutations of residues near the putative pore of the channel. Tmc1 p.W554L and p.D569N exhibit reduced maximum current with no effect on single-channel conductance, implying a smaller number of channels transported to the stereociliary tips; this may stem from impaired TMC1 binding to LHFPL5. Tmc1 p.D528N, located in the pore''s narrowest region, uniquely caused large reductions in MET channel conductance and block by dihydrostreptomycin (DHS). For Tmc1 p.T416K and Tmc1 p.D528N, transduction loss occurred between P15 and P20. We propose two mechanisms linking channel mutations and deafness: decreased Ca²⁺ permeability, common to all mutants, and decreased resting open probability in low Ca²⁺, confined to dominant mutations.SIGNIFICANCE STATEMENT Transmembrane channel-like protein isoform 1 (TMC1) is thought to be a major component of the mechanotransducer channel in auditory hair cells, but the protein organization and channel structure are still uncertain. We made four mouse lines harboring Tmc1 point mutations that alter channel properties, causing hair cell degeneration and deafness. These include a mouse homolog of a new human deafness mutation pT416K that decreased channel Ca²⁺ permeability by introducing a positively-charged amino acid in the putative pore. All mutations are consistent with the channel structure predicted from modeling, but only one, p.D528N near the external face of the pore, substantially reduced channel conductance and Ca²⁺ permeability and virtually abolished block by dihydrostreptomycin (DHS), strongly endorsing its siting within the pore.     

First genomic rearrangement of the RYR1 gene associated with an atypical presentation of lethal neonatal hypotonia

Neonatal hypotonia is frequently observed with a highly variable clinical presentation. Congenital myopathies that are classically characterized by the presence of structural changes of the muscle fibres such as cores, rods and aggregates have been reported to be occasionally associated with this presentation. However, the identification of the causing defect can be a challenging task in severe neonatal forms of the disease since specific structural changes might not always be present in affected newborn’s muscles. The RYR1 gene encodes the skeletal muscle isoform of a calcium channel, the ryanodine receptor, and has been involved in both dominant and recessive congenital myopathies associated with structural changes and presenting with various degree of severity. Here we report the case of a child presenting at birth with a lethal form of neonatal hypotonia associated with an atypical congenital myopathy. Molecular investigations showed that the disease was caused by two novel RYR1 mutations. One of the mutations was a large-sized genomic deletion. This is the first genomic rearrangement identified into the RYR1 gene to our knowledge. This new class of mutation of the RYR1 gene will clearly have consequences for the molecular investigation of RYR1-related diseases.     

A novel homozygous variant of RYR1 p.Ala3072Asp in a neonate with dusty core disease: A new entity with clinicopathological implications

Dusty core disease (DuCD) is a recently described form of congenital myopathy with clinicopathological implications. The presence of “dusty core fibers” is the defining myopathological feature of DuCD. Most cases have a recessive inheritance and harbor RYR1 mutations. I hereby describe a novel homozygous variant of RYR1 p.Ala3072Asp clinicopathologically compatible with DuCD. To the best of my knowledge, this is the first documented case of DuCD from India.     

Histamine resets the circadian clock in the suprachiasmatic nucleus through the H1R‐CaV1.3‐RyR pathway in the mouse

Histamine, a neurotransmitter/neuromodulator implicated in the control of arousal state, exerts a potent phase‐shifting effect on the circadian clock in the rodent suprachiasmatic nucleus (SCN). In this study, the mechanisms by which histamine resets the circadian clock in the mouse SCN were investigated. As a first step, Ca²⁺‐imaging techniques were used to demonstrate that histamine increases intracellular Ca²⁺ concentration ([Ca²⁺]_i) in acutely dissociated SCN neurons and that this increase is blocked by the H1 histamine receptor (H1R) antagonist pyrilamine, the removal of extracellular Ca²⁺ and the L‐type Ca²⁺ channel blocker nimodipine. The histamine‐induced Ca²⁺ transient is reduced, but not blocked, by application of the ryanodine receptor (RyR) blocker dantrolene. Immunohistochemical techniques indicated that Ca_V1.3 L‐type Ca²⁺ channels are expressed mainly in the somata of SCN cells along with the H1R, whereas Ca_V1.2 channels are located primarily in the processes. Finally, extracellular single‐unit recordings demonstrated that the histamine‐elicited phase delay of the circadian neural activity rhythm recorded from SCN slices is blocked by pyrilamine, nimodipine and the knockout of Ca_V1.3 channel. Again, application of dantrolene reduced but did not block the histamine‐induced phase delays. Collectively, these results indicate that, to reset the circadian clock, histamine increases [Ca²⁺]_i in SCN neurons by activating Ca_V1.3 channels through H1R, and secondarily by causing Ca²⁺‐induced Ca²⁺ release from RyR‐mediated internal stores.     

Autoantibodies against TRPC3 and ryanodine receptor in myasthenia gravis

The transient receptor potential canonical type-3 (TRPC3, receptor- and store-operated Ca²⁺ influx channel) participates in skeletal muscle contraction; its functional interactions with ryanodine receptor-1 (RyR1) are independent of sarcoplasmic Ca²⁺ content and dihydropyridine receptor. In 25 generalized myasthenia gravis (MG), we detected antibodies against human TRPC3 peptide in 9 patients (8 with thymoma and one with hyperplastic thymus) and those against human RyR1 peptides in 16 patients (15 with thymoma and one with hyperplastic thymus). Both antibodies were found in patients with more severe myasthenia and could contribute to the contractile abnormalities in MG.     

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

J. A. Bevilacqua, N. Monnier, M. Bitoun, B. Eymard, A. Ferreiro, S. Monges, F. Lubieniecki, A. L. Taratuto, A. Laquerrière, K. G. Claeys, I. Marty, M. Fardeau, P. Guicheney, J. Lunardi and N. B. Romero (2011) Neuropathology and Applied Neurobiology 37, 271–284
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.     

RYR1-related congenital myopathy with fatigable weakness,responding to pyridostigimine

The spectrum of RYR1 mutation associated disease encompasses congenital myopathies, exercise induced rhabdomyolysis, malignant hyperthermia susceptibility and King-Denborough syndrome. We report the clinical phenotype of two siblings who presented in infancy with hypotonia and striking fatigable ptosis. Their response to pyridostigimine was striking, but genetic screening for congenital myasthenic syndromes was negative, prompting further evaluation. Muscle MRI was abnormal with a selective pattern of involvement evocative of RYR1-related myopathy. This directed sequencing of the RYR1 gene, which revealed two heterozygous c.6721C>T (p.Arg2241X) nonsense mutations and novel c.8888T>C (p.Leu2963Pro) mutations in both siblings. These cases broaden the RYR1-related disease spectrum to include a myasthenic-like phenotype, including partial response to pyridostigimine. RYR1-related myopathy should be considered in the presence of fatigable weakness especially if muscle imaging demonstrates structural abnormalities. Single fibre electromyography can also be helpful in cases like this.     

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.     

A Double Mutation of the Ryanodine Receptor Type 1 Gene in a Malignant Hyperthermia Family with Multiminicore Myopathy

Background and purpose

At least 100 Ryanodine receptor type 1 (RYR1) mutations associated with malignant hyperthermia (MH) and central core disease (CCD) have been identified, but 2 RYR1 mutations accompanying multiminicore myopathy in an MH and/or CCD family have been reported only rarely.

Methods

Fifty-three members of a large MH family were investigated with clinical, histopathologic, RYR1 mutation, and haplotyping studies. Blood creatine kinase (CK) and myoglobin levels were also measured where possible.

Results

Sequencing of the entire RYR1 coding region identified a double RYR1 mutation (R2435H and A4295V) in MH/CCD regions 2 and 3. Haplotyping analysis revealed that the two missense heterozygous mutations (c.7304G>A and c.12891C>T) were always present on a common haplotype allele, and were closely cosegregated with histological multiminicores and elevated serum CK. All the subjects with the double mutation showed elevated serum CK and myoglobin, and the obtained muscle biopsy samples showed multiminicore lesions, but only two family members presented a late-onset, slowly progressive myopathy.

Conclusions

We found multiminicore myopathy with clinical and histological variability in a large MH family with an unusual double RYR1 mutation, including a typical CCD-causing known mutant. These results suggest that multiminicore lesions are associated with the presence of more than two mutations in the RYR1 gene.     

Tubular aggregate myopathy with features of Stormorken disease due to a new STIM1 mutation

STIM1 is a reticular Ca²⁺ sensor composed of a luminal and a cytosolic domain. Missense mutations in the luminal domain have been associated with tubular aggregate myopathy (TAM), while cytosolic mutations can cause Stormorken syndrome, a multisystemic disease associating TAM with asplenia, thrombocytopenia, miosis, ichthyosis, short stature and dyslexia. Here we present the case of a 41-year-old female complaining of exercise intolerance. Clinical examination showed short stature, scoliosis, proximal muscle weakness with lower limb predominance, and ophthalmoplegia. Laboratory tests revealed hypocalcemia, mild anemia and elevated creatine kinase (CK) levels. Whole-body muscle magnetic resonance imaging (MRI) revealed asplenia. Muscle biopsy was consistent with TAM. STIM1 gene analysis disclosed the novel c.252T>A, p.D84E missense mutation which was shown to induce constitutive STIM1 clustering in a functional study. This study reports a novel STIM1 mutation located in the Ca²⁺-binding EF domain causing TAM with features of Stormorken syndrome.     

A novel late-onset axial myopathy associated with mutations in the skeletal muscle ryanodine receptor (RYR1) gene

Mutations in the skeletal muscle ryanodine receptor (RYR1) gene are a common cause of inherited neuromuscular disorders and have been associated with a wide clinical spectrum, ranging from various congenital myopathies to the malignant hyperthermia susceptibility (MHS) trait without any associated weakness. RYR1-related myopathies are usually of early-childhood onset. Here we present 11 patients from 8 families with a late-onset axial myopathy associated with RYR1 variants. Patients presented between the third and seventh decade of life to neuromuscular centres in Norway, the Netherlands and the United Kingdom with predominant axial muscle involvement, comprising variable degrees of lumbar hyperlordosis, scapular winging and/or camptocormia. Marked myalgia was commonly associated. Serum creatine kinase levels were normal or moderately elevated. Muscle imaging showed consistent involvement of the lower paravertebral muscles and the posterior thigh. Muscle biopsy findings were often discrete, featuring variability in fibre size, increased internal nuclei and unevenness of oxidative enzyme staining, but only rarely overt cores. RYR1 sequencing revealed heterozygous missense variants, either previously associated with the MHS trait or localizing to known MHS mutational hotspots. These findings indicate that MHS-related RYR1 mutations may present later in life with prominent axial weakness but not always typical histopathological features. We propose a combined effect of RyR1 dysfunction, aging and particular vulnerability of axial muscle groups as a possible pathogenic mechanism. RYR1 is a candidate for cases with “idiopathic” camptocormia or bent spine syndrome (BSS).     

A novel gain-of-function mutation in the <Emphasis Type="Italic">ITPR1</Emphasis> suppressor domain causes spinocerebellar ataxia with altered Ca<Superscript>2+</Superscript> signal patterns

We report three affected members, a mother and her two children, of a non-consanguineous Irish family who presented with a suspected autosomal dominant spinocerebellar ataxia characterized by early motor delay, poor coordination, gait ataxia, and dysarthria. Whole exome sequencing identified a novel missense variant (c.106C>T; p.[Arg36Cys]) in the suppressor domain of type 1 inositol 1,4,5-trisphosphate receptor gene (ITPR1) as the cause of the disorder, resulting in a molecular diagnosis of spinocerebellar ataxia type 29. In the absence of grandparental DNA, microsatellite genotyping of healthy family members was used to confirm the de novo status of the ITPR1 variant in the affected mother, which supported pathogenicity. The Arg36Cys variant exhibited a significantly higher IP₃-binding affinity than wild-type (WT) ITPR1 and drastically changed the property of the intracellular Ca²⁺ signal from a transient to a sigmoidal pattern, supporting a gain-of-function disease mechanism. To date, ITPR1 mutation has been associated with a loss-of-function effect, likely due to reduced Ca²⁺ release. This is the first gain-of-function mechanism to be associated with ITPR1-related SCA29, providing novel insights into how enhanced Ca²⁺ release can also contribute to the pathogenesis of this neurological disorder.     

Mild Clinical Features and Histopathologically Atypical Cores in Two Korean Families with Central Core Disease Harboring RYR1 Mutations at the C-Terminal Region

Background

Central core disease (CCD) is a congenital myopathy characterized by distinctive cores in muscle fibers. Mutations in the gene encoding ryanodine receptor 1 (RYR1) have been identified in most CCD patients.

Case Report

Two unrelated patients presented with slowly progressive or nonprogressive proximal muscle weakness since childhood. Their family history revealed some members with the same clinical problem. Histological analysis of muscle biopsy samples revealed numerous peripheral cores in the muscle fibers. RYR1 sequence analysis disclosed a novel mutation in exon 101 (c.14590T>C) and confirmed a previously reported mutation in exon 102 (c.14678G>A).

Conclusions

We report herein two families with CCD in whom missense mutations at the C-terminal of RYR1 were identified. Although it has been accepted that such mutations are usually associated with a severe clinical phenotype and clearly demarcated central cores, our patients exhibited a mild clinical phenotype without facial muscle involvement and skeletal deformities, and atypical cores in their muscle biopsy specimens.