Molecular basis of impaired muscle function in a mouse model of congenital myopathy due to compound heterozygous RYR1 mutations

Congenital myopathies (CM) are a group of early-onset, genetically diverse muscle disorders of variable severity with characteristic muscle biopsy findings. Mutations in RYR1, the gene encoding the RYR1, are the most common genetic cause, responsible for ∼30% of all human CM. They are linked to the pharmacogenetic disorder malignant hyperthermia susceptibility and to various disease phenotypes, including central core disease (which is primarily dominantly inherited), multiminicore disease (which is predominantly recessively inherited), some forms of centronuclear myopathy and congenital fiber-type disproportion (which can be either dominantly or recessively inherited), and King–Denborough syndrome (a CM characterized by skeletal abnormalities, dysmorphic features, and malignant hyperthermia susceptibility). The recessive forms of RYR1-linked CM are more severe, affecting children at birth and, in addition to profound muscle weakness, may also affect facial and extraocular muscles and cause skeletal deformities and feeding difficulties. To study the mechanism leading to the profound muscle weakness characterized by recessive RYR1-CM, we created transgenic mice knocked in for the compound heterozygous RYR1 p.Q1970fsX16+p.A4329D mutations (double knock-in mouse, or DKI) identified in a severely affected child. The in vivo and ex vivo physiological functions of fast twitch, slow twitch, and extraocular muscles were severely impaired in DKI mice; in addition, the mutations were accompanied by a >50% decrease in RYR1 protein in all muscles examined, as well as changes in the expression of many proteins important for muscle function and chromatin structure. Muscle ultrastructure was disorganized, with fewer CRU and mitochondria and presence of cores. MyHC-EO, the superfast and ocular-muscle−specific myosin heavy isoform, was almost undetectable in EOMs from DKI mutant mice. Thus, the DKI mouse model faithfully recapitulates the human disease and could be exploited for preclinical studies aimed at developing therapeutic strategies to treat neuromuscular disorders linked to recessive RYR1 mutations.