Compound heterozygous <scp>RYR1</scp> ‐ <scp>RM</scp> mouse model reveals disease pathomechanisms and muscle adaptations to promote postnatal survival

Pathogenic variants in the type I ryanodine receptor (RYR1) result in a wide range of muscle disorders referred to as RYR1-related myopathies (RYR1-RM). We developed the first RYR1-RM mouse model resulting from co-inheritance of two different RYR1 missense alleles (Ryr1^TM/SC-ΔL mice). Ryr1^TM/SC-ΔL mice exhibit a severe, early onset myopathy characterized by decreased body/muscle mass, muscle weakness, hypotrophy, reduced RYR1 expression, and unexpectedly, incomplete postnatal lethality with a plateau survival of ~50% at 12 weeks of age. Ryr1^TM/SC-ΔL mice display reduced respiratory function, locomotor activity, and in vivo muscle strength. Extensor digitorum longus muscles from Ryr1^TM/SC-ΔL mice exhibit decreased cross-sectional area of type IIb and type IIx fibers, as well as a reduction in number of type IIb fibers. Ex vivo functional analyses revealed reduced Ca²⁺ release and specific force production during electrically-evoked twitch stimulation. In spite of a ~threefold reduction in RYR1 expression in single muscle fibers from Ryr1^TM/SC-ΔL mice at 4 weeks and 12 weeks of age, RYR1 Ca²⁺ leak was not different from that of fibers from control mice at either age. Proteomic analyses revealed alterations in protein synthesis, folding, and degradation pathways in the muscle of 4- and 12-week-old Ryr1^TM/SC-ΔL mice, while proteins involved in the extracellular matrix, dystrophin-associated glycoprotein complex, and fatty acid metabolism were upregulated in Ryr1^TM/SC-ΔL mice that survive to 12 weeks of age. These findings suggest that adaptations that optimize RYR1 expression/Ca²⁺ leak balance, sarcolemmal stability, and fatty acid biosynthesis provide Ryr1^TM/SC-ΔL mice with an increased survival advantage during postnatal development.