A novel, patient-derived RyR1 mutation impairs muscle function and calcium homeostasis in mice.

RYR1 is the most commonly mutated gene associated with congenital myopathies, a group of early-onset neuromuscular conditions of variable severity. The functional effects of a number of dominant RYR1 mutations have been established; however, for recessive mutations, these effects may depend on multiple factors, such as the formation of a hypomorphic allele, or on whether they are homozygous or compound heterozygous. Here, we functionally characterize a new transgenic mouse model knocked-in for mutations identified in a severely affected child born preterm and presenting limited limb movement. The child carried the homozygous c.14928C>G RYR1 mutation, resulting in the p.F4976L substitution. In vivo and ex vivo assays revealed that homozygous mice fatigued sooner and their muscles generated significantly less force compared with their WT or heterozygous littermates. Electron microscopy, biochemical, and physiological analyses showed that muscles from RyR1 p.F4976L homozygous mice have the following properties: (1) contain fewer calcium release units and show areas of myofibrillar degeneration, (2) contain less RyR1 protein, (3) fibers show smaller electrically evoked calcium transients, and (4) their SR has smaller calcium stores. In addition, single-channel recordings indicate that RyR1 p.F4976L exhibits higher Po in the presence of 100 µM [Ca2+]. Our mouse model partly recapitulates the clinical picture of the homozygous human patient and provides significant insight into the functional impact of this mutation. These results will help understand the pathology of patients with similar RYR1 mutations.

Functional characterization of RYR1 variants identified in malignant hyperthermia susceptible individuals.

Malignant hyperthermia is a pharmacogenetic disorder triggered by halogenated anesthetic agents in genetically predisposed individuals. Approximately 70 % of these individuals carry mutations in RYR1, the gene encoding the ryanodine receptor calcium channel of skeletal muscle. In this study, we performed functional analysis of 5 RYR1 variants identified in members from 8 families who had been diagnosed by the IVCT. Of the 68 individuals enrolled in the study, 43 were diagnosed as MHS, 23 as MHN, and 2 individuals were not tested. Here we demonstrate that the 5 RyR1 variants cause hypersensitivity to RyR1 agonist-mediated calcium release. According to the EMHG scoring matrix these five genetic variants can be classified as follows: c.8638G>A (p.E2880K) and c.11314C>T (p.R3772W) likely pathogenic, c.11416G>A (p.G3806R), c.14627A>G (p.K4876R) and c.14813T>C (p.I4938T), pathogenic (RefSeq NM_000540.3). We propose that the newly functionally characterized RYR1 variants, be included in the panel of variants to be used for the molecular diagnosis of MHS.

Improvement of muscle strength in a mouse model for congenital myopathy treated with HDAC and DNA methyltransferase inhibitors.

To date there are no therapies for patients with congenital myopathies, muscle disorders causing poor quality of life of affected individuals. In approximately 30% of the cases, patients with congenital myopathies carry either dominant or recessive mutations in the ryanodine receptor 1 (RYR1) gene; recessive RYR1 mutations are accompanied by reduction of RyR1 expression and content in skeletal muscles and are associated with fiber hypotrophy and muscle weakness. Importantly, muscles of patients with recessive RYR1 mutations exhibit increased content of class II histone deacetylases and of DNA genomic methylation. We recently created a mouse model knocked-in for the p.Q1970fsX16+ p.A4329D RyR1 mutations, which are isogenic to those carried by a severely affected child suffering from a recessive form of RyR1-related multi-mini core disease. The phenotype of the RyR1 mutant mice recapitulates many aspects of the clinical picture of patients carrying recessive RYR1 mutations. We treated the compound heterozygous mice with a combination of two drugs targeting DNA methylases and class II histone deacetylases. Here, we show that treatment of the mutant mice with drugs targeting epigenetic enzymes improves muscle strength, RyR1 protein content, and muscle ultrastructure. This study provides proof of concept for the pharmacological treatment of patients with congenital myopathies linked to recessive RYR1 mutations.

Bi-allelic expression of the RyR1 p.A4329D mutation decreases muscle strength in slow-twitch muscles in mice.

Mutations in the ryanodine receptor 1 (RYR1) gene are associated with several human congenital myopathies, including the dominantly inherited central core disease and exercise-induced rhabdomyolysis, and the more severe recessive phenotypes, including multiminicore disease, centronuclear myopathy, and congenital fiber type disproportion. Within the latter group, those carrying a hypomorphic mutation in one allele and a missense mutation in the other are the most severely affected. Because of nonsense-mediated decay, most hypomorphic alleles are not expressed, resulting in homozygous expression of the missense mutation allele. This should result in 50% reduced expression of the ryanodine receptor in skeletal muscle, but its observed content is even lower. To study in more detail the biochemistry and pathophysiology of recessive RYR1 myopathies, here we investigated a mouse model we recently generated by analyzing the effect of bi-allelic versus mono-allelic expression of the RyR1 p.A4329D mutation. Our results revealed that the expression of two alleles carrying the same mutation or of one allele with the mutation in combination with a hypomorphic allele does not result in functionally equal outcomes and impacts skeletal muscles differently. In particular, the bi-allelic RyR1 p.A4329D mutation caused a milder phenotype than its mono-allelic expression, leading to changes in the biochemical properties and physiological function only of slow-twitch muscles and largely sparing fast-twitch muscles. In summary, bi-allelic expression of the RyR1 p.A4329D mutation phenotypically differs from mono-allelic expression of this mutation in a compound heterozygous carrier.

Molecular basis of impaired extraocular muscle function in a mouse model of congenital myopathy due to compound heterozygous Ryr1 mutations.

Mutations in the RYR1 gene are the most common cause of human congenital myopathies, and patients with recessive mutations are severely affected and often display ptosis and/or ophthalmoplegia. In order to gain insight into the mechanism leading to extraocular muscle (EOM) involvement, we investigated the biochemical, structural and physiological properties of eye muscles from mouse models we created knocked-in for Ryr1 mutations. Ex vivo force production in EOMs from compound heterozygous RyR1p.Q1970fsX16+p.A4329D mutant mice was significantly reduced compared with that observed in wild-type, single heterozygous mutant carriers or homozygous RyR1p.A4329D mice. The decrease in muscle force was also accompanied by approximately a 40% reduction in RyR1 protein content, a decrease in electrically evoked calcium transients, disorganization of the muscle ultrastructure and a decrease in the number of calcium release units. Unexpectedly, the superfast and ocular-muscle-specific myosin heavy chain-EO isoform was almost undetectable in RyR1p.Q1970fsX16+p.A4329D mutant mice. The results of this study show for the first time that the EOM phenotype caused by the RyR1p.Q1970fsX16+p.A4329D compound heterozygous Ryr1 mutations is complex and due to a combination of modifications including a direct effect on the macromolecular complex involved in calcium release and indirect effects on the expression of myosin heavy chain isoforms.