Compound heterozygous RYR1-RM 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 mice). Ryr1 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 mice display reduced respiratory function, locomotor activity, and in vivo muscle strength. Extensor digitorum longus muscles from Ryr1 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 Ca 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 mice at 4 weeks and 12 weeks of age, RYR1 Ca 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 mice, while proteins involved in the extracellular matrix, dystrophin-associated glycoprotein complex, and fatty acid metabolism were upregulated in Ryr1 mice that survive to 12 weeks of age. These findings suggest that adaptations that optimize RYR1 expression/Ca leak balance, sarcolemmal stability, and fatty acid biosynthesis provide Ryr1 mice with an increased survival advantage during postnatal development.