Eckhardt J Bachmann C Benucci S Elbaz M Ruiz A Zorzato F Treves S 2020 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. https://sonar.ch/global/documents/92785 eng info:eu-repo/semantics/altIdentifier/doi/10.1093/hmg/ddaa056 info:eu-repo/semantics/altIdentifier/pmid/32242214 Human molecular genetics. - 2020 Genetics(clinical) Genetics Molecular Biology General Medicine Molecular basis of impaired extraocular muscle function in a mouse model of congenital myopathy due to compound heterozygous Ryr1 mutations. http://purl.org/coar/resource_type/c_6501