Sequence variations in the RYR1 gene encoding the skeletal muscle sarcoplasmic reticulum calcium release channel are the cause of malignant hyperthermia susceptibility and of several myopathies. This article reviews the RYR1-related myopathies and their association with malignant hyperthermia susceptibility, a life-threatening anesthetic complication which is avoidable if anticipated pre-surgically. This is especially important for counseling and improving patient safety during anesthesia, in particular for asymptomatic relatives of patients with recessive RYR1-related myopathies.

Carsana A (2014) RYR1-Related Myopathies and Anesthesiological Implications. Int J Clin Anesthesiol 2(4): 1037.

•    RYR1 gene
•    Sequence variations
•    Malignant hyperthermia
•    Myopathy

CCD: Central Core Disease; CFTD: Congenital Fiber Type Disproportion; CNM: Centronuclear Myopathy; KDS: KingDenborough Syndrome; MH: Malignant Hyperthermia; MmD: Multi-Minicore Disease; RYR1: Ryanodine Receptor Type 1; SV: Sequence Variation

The skeletal muscle sarcoplasmic reticulum calcium release channel, commonly known as ryanodine receptor type 1 (RyR1), is encoded by the RYR1 gene and specifically interacts with the voltage-dependent Ca2+-channel Cav1.1, localized at T-tubular membrane. The depolarization of the plasma membrane results in conformational changes in Cav1.1, which are transmitted directly to the RyR1 channel, causing it to open. RyR1 and Cav1.1 are the two major proteins involved in the excitation-contraction (E-C) coupling in skeletal muscle. RYR1 sequence variations (SVs) have been associated with several distinct skeletal muscle disorders, i.e., malignant hyperthermia susceptibility, central core disease, King-Denborough syndrome, late-onset axial myopathy, congenital “core-rod myopathy”, with mainly autosomal dominant inheritance, and subgroups of multi-minicore disease, congenital fiber type disproportion, centronuclear myopathy, with autosomal recessive inheritance[1]. Autosomal recessively inherited RYR1-related myopathies with wide clinico pathological spectrum have emerged in recent years [2-4].

The present work reviews the RYR1-related myopathies and their association with malignant hyperthermia susceptibility, a life-threatening anesthetic complication which is avoidable if anticipated pre-surgically. This is especially important for counseling and improving patient safety during anesthesia, in particular for asymptomatic relatives of patients with recessive RYR1-related myopathies.

Malignant hyperthermia (MH)

Malignant hyperthermia (MH, OMIM # 145600) is a rare, pharmacogenetic disorder triggered by volatile anesthetics (halothane, isoflurane, desflurane, sevoflurane, enflurane) and/ or depolarizing muscle relaxants (succinylcholine ) in genetically predisposed subjects. In the absence of triggering agents, the affected individuals appear normal without specific pathologic signs. The MH-susceptibility (MHS) condition is transmitted as an autosomal dominant trait in man. On exposure to triggering agents MHS patients may experience an MH crisis as a consequence of an abnormally increasing of intracellular Ca2+ in skeletal muscle. The symptoms are skeletal muscle rigidity, metabolic acidosis, tachycardia and high temperature which correlate with altered biochemical parameters, such as increased pCO2 and lactate production, hyperkalemia, release of muscle proteins, as creatine kinase and myoglobin, into the blood. Frequent late events are damage of kidney function due to massive myoglobin release and/or a diffuse intravascular coagulation, which is often the main cause of death [5]. The incidence of MH reactions is 1:5,000 to 1:50,000–100,000 anesthesias, and the estimated genetic incidence is 1:3,000 to 1:8,500 [5]. Advances in understanding the MH clinical manifestation and pathophysiology, and the treatment with dantrolene, an RyR1 antagonist that blocks Ca2+ release from the intracellular stores, resulted in much lower morbidity and mortality from MH; the mortality dropped from over 80% in the 1960s to less than 5%. Protocols for MH contracture testing of human skeletal muscle have been developed by the European [6] and North American [7] MH Groups, namely, in vitro contracture test (IVCT) and caffeine halothane contracture test (CHCT), respectively. MHS exhibits genetic heterogeneity, with 6 loci (MHS1, OMIM #145600; MHS2, OMIM %154275; MHS3, OMIM %154276; MHS4, OMIM % 600467; MHS5, OMIM# 601887; MHS6, OMIM %601888) so far implicated. Four (MHS2-MHS4, MHS6) of them have been identified only in isolated European families. The MHS1 locus (RYR1 gene) accounts for the majority of MHS cases (about 70%). Many different SVs of uncertain significance have been identified in the gene coding for RyR1, however only 31 RyR1 mutations have been proven to be causative for MH according to the criteria of the European Malignant Hyperthermia Group (www.emhg. org). Moreover, RyR1 SVs, associated or possibly associated to MHS status, have been identified in patients who experienced exertional rhabdomyolysis, and/or myalgia, and stress-induced MH events [8,9]. Less than 1% of MHS cases can be attributed to mutations in the CACNA1S gene (locus MHS5) encoding the alpha1S subunit of the Cav1.1 channel.. Only three MH-causing mutations identified in the CACNA1S gene were hitherto functionally characterized [10-12.]

Central core disease (CCD)

Central core disease (CCD, OMIM # 117000) is an autosomal dominant congenital myopathy characterized by motor developmental delay, hypotrophy and hypotonia in infancy (Table 1), although cases of adult onset have been reported [13]. Recessive transmission has been described for variant forms of CCD [4,14]. Weakness is typically proximal with prominent involvement of the hip girdle and axial muscles. The histological analysis of muscle samples with oxidative enzyme stains reveals the presence of central core lesions extending the length of type 1 muscle fibers. The cores are regions characterized by sarcomeric disorganization, absence of mitochondria, and lack of oxidative activity (Table 1). No association between the number of cores on muscle biopsy and the degree of muscle weakness has been described. RYR1 disease-causing mutations have been identified in the majority of CCD cases. CCD patients are at risk of MHS: some patients experienced clinical episodes of MH and/or were typed MHS by IVCT.

King-Denborough syndrome (KDS)

King-Denborough syndrome (KDS) is a rare condition characterized by susceptibility to MH, hypotonia, delayed motor development, proximal weakness, short stature, cryptorchidism, skeletal abnormalities (scoliosis, kyphosis, lumbar lordosis, pectus carinatum/excavatum), and variable dysmorphic features (ptosis, down-slanting palpebral fissures, malar hypoplasia, micrognathia, high-arched palate, hypertelorism, low set ears). Muscle biopsies show myopathic features: fiber size variation and few, small, or atrophic type 1 muscle fibers and severe paucity of type 2 fibers (Table 1) [15,16]. The triad of MHS, dysmorphic features and myopathy is characteristic. KDS was first described by King and Denborough in 1973 [17], and only recently RYR1 SVs have been identified in KDS patients [18- 20]. All these SVs, but one [18], were non polymorphic, likely pathological and inherited from one parent. Moreover, reduced RyR1 protein expression has been detected in some KDS patients [20]. However, marked phenotypic variability associated with some RYR1 SV has been observed, even in patients/subjects of the same family with the same SV, i.e, parents harboring the same SVs of their KDS children are asymptomatic or are only MHS, or have a different myopathic picture. On the bases of these results, it has been advanced that “KDS is a digenic condition, with a RYR1 mutation accounting for some aspects of the phenotype (susceptibility to MH and mild myopathy) and a second unrelated mutation causing other features like the dysmorphic facies” [20], not detected by routine sequencing methods. Moreover, patients with the KDS phenotype with either dominant or recessive inheritance have been reported [3].

Core-rod myopathy

Core-rod myopathy is a rare, genetically heterogeneous congenital myopathy characterized by the presence of cores and rods in distinct areas in the same or in different muscle fibers (Table 1). SVs in the RYR1 gene [21-24] and in the skeletal muscle α-actin (ACTA1) gene [25] have been identified in families with autosomal dominant inheritance, and in the nebulin (NEB) gene in patients with autosomal recessive inheritance [26,27]. All the RYR1 SVs identified in these patients lie in the transmembrane region of the RyR1 protein, in the Ca2+ channel-forming segment. One patient with Y4796C mutation in the RyR1 channel underwent IVCT and was typed MHS [21]. Patients presented with delayed motor development, lordosis, walking difficulties, weakness, Achilles’ tendon contractures (Table 1) [21-23]. Moreover, monozygotic twins carrying the heterozygous de novo I4898T mutation in the RyR1 channel presented with a severe phenotype: polyhydramnios and loss of fetal motility during pregnancy, hypotonia, arthrogryposis and swallowing impairment at birth, and death before two months of life [24].

Late-onset axial myopathy

Heterozygous missense RYR1 SVs have been reported in 12 patients with late-onset myopathy with predominant axial muscle involvement, variable degrees of lumbar hyperlordosis, scapular winging and / or camptocormia, myalgia (Table 1) [28,29]. The histological analysis of muscle samples revealed variability in fiber size, increased central nuclei, unevenness of oxidative enzyme staining [29] or central and eccentric cores (Table 1) [28,29]. Only one patient underwent the IVCT and was typed MHS.

Fetal akinesia

RYR1 SVs have been identified in 15 patients (fetuses/infants) with decreased fetal movement, arthrogryposis, congenital dislocation of the hips, skeletal and feet deformities, severe hypotonia, respiratory insufficiency at birth (Table 1) [30,31]. Both dominant (6 cases) and recessive (9 cases, compound heterozygous) inheritance of the RYR1 SVs has been reported [30-32]. The histological analysis of muscle samples revealed the presence of central cores and type 1 fiber predominance in patients with dominant RYR1 SVs and histological heterogeneity in patients with recessive RYR1 SVs (Table 1).

Multi-minicore disease (MmD)

Multi-minicore disease (MmD, OMIM #255320) is a recessive clinically heterogeneous condition; general features include neonatal muscle hypotonia, delayed motor development, generalized muscle weakness, and amyotrophy (Table 1). Muscle biopsy shows multiple areas (minicores) of reduced oxidative activity, sarcomere disorganization and mitochondriadepletion in most muscle fibers (Table 1). Clinical presentation in MmD is variable with four subtypes described [33]. MmD is genetically heterogeneous: the classic phenotype characterized by spinal rigidity, early scoliosis and respiratory impairment is due to recessive mutations in the selenoprotein N (SEPN1) gene, whereas recessive RYR1 SVs –homozygous or compound heterozygous- have been identified in patients with a wide range of clinical features including external ophthalmoplegia, distal weakness and wasting or predominant hip girdle involvement (Table 1). MH episodes of MmD patients with recessive RYR1 SVs have been reported [33].

Centronuclear myopathy (CNM)

Centronuclear myopathy (CNM) is a congenital condition with genetic heterogeneity and highly variable clinical phenotypes. X-linked forms have a severe phenotype presenting in males at birth with marked weakness and hypotonia, external ophthalmoplegia and respiratory failure, autosomal dominant forms have a later onset and milder course, and autosomal recessive forms are intermediate. Extraocular muscle involvement is common in all forms of CNM. The X-linked form [OMIM, #310400] is caused by mutations in the myotubularin (MTM1) gene, the autosomal-dominant form [OMIM, #160150] by mutations in the dynamin 2 (DNM2) gene, and the autosomal recessive forms by mutations in the amphiphysin 2 (BIN1) gene [OMIM, #255200] or in the RYR1 gene. The histological picture of CNM muscle biopsies is characterized by prominence of central nuclei and predominance and hypotrophy of type 1 fibers [34] (Table 1). Patients with a severe phenotype and recessive RYR1- CNM have been recently described [2,35]; no MH reactions have been reported in these patients and they were not typed by IVCT.

Congenital fiber type disproportion (CFTD)

Congenital fiber type disproportion (CFTD, OMIM #255310) is characterized by hypotonia and mild-to-severe generalized muscle weakness at birth or within the first year of life (Table 1). CFTD is genetically heterogeneous with six genes involved: alpha actin (ACTA1) gene (~6% of CFTD cases), myosin heavy chain 7 (MYH7 ) gene (rare) , SEPN1 gene (rare), tropomyosin 2 (TPM2) (rare) and 3 (TPM3) (~20%-25% of CFTD cases) genes, and RYR1 gene (10%-20% of CFTD cases). The skeletal muscle histological pattern is characterized by relative hypotrophy of type 1 muscle fibers compared to type 2 fibers (Table 1). No family history of MH has been reported in RYR1- CFTD families [36].

Autosomal-recessive RYR1-related myopathies with wide clinic-pathological spectrum

In recent years an increasing number of myopathy cases with recessive RYR1 SVs and non-specific histological features has been reported [3,4]. Moreover, a homozygous RYR1 SV has been identified in a family with benign Samaritan congenital myopathy [37] and RYR1 SVs at homozygous or at compound heterozygous status have been found in two families with ophthalmoplegia without definite skeletal myopathy [38].

Table 1: Clinical signs, inheritance and histochemistry of some RYR1-congenital myopathies.

* cases with recessive inheritance have been described

In recent years, a wide range of clinical and histopathological variants have been associated with RYR1 SVs; therefore, it has been advanced that RYR1-related myopathies are probably the most frequent form of congenital myopathies in several populations [3,39,40]. The inheritance of RYR1-related myopathies is complex and high phenotypic variability is associated with some RYR1 SVs, even in patients of the same family with the same SV and in the same individual at different ages. Considering the genetic heterogeneity of congenital myopathies and the possible association of RYR1-related myopathies with MHS, the recognition of the genetic bases of myopathies is essential; an approach to the differential diagnosis of congenital myopathies has been recently reported [41]. When indicated, the identification of RYR1 SVs is crucial for counseling and improving patient safety during anesthesia, in particular for asymptomatic relatives of patients with recessive RYR1-related myopathies. In fact, some RYR1 SVs found at the homozygous or at compound heterozygous state in patients with recessive myopathies have been reported at a heterozygous level in several MHS patients, and may determine dominantly inherited susceptibility to MH in heterozygous unaffected relatives [4,19]. Therefore, the affected patients and their carrier relatives should be aware of the potential risk of MHS, and possibly undergo the IVCT. In conclusion, it may be advisable to provide non-MH triggering anesthesia to patients with RYR1- related myopathies and their relatives until definitive diagnostic IVCT can be performed.

This work has been supported by project code PON02_00619_3470457, funded by Italian Ministry of Education, University and Research through PON 2007 -2013 and by the agreement 2010 – 2012 between CEINGE and Campania Regional Authority.

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