O.5 Mutations in MuRF1 and MuRF3 cause a novel protein aggregate myopathy and cardiomyopathy

Abstract

MuRF1 and MuRF3 are microtubule-associated proteins that localize to the sarcomeric M-band and Z-line of striated muscle. They are E3 ubiquitin ligases that mediate degradation of sarcomeric proteins and participate in diverse biological functions including stabilisation of microtubules and myogenesis. Double knock-out mice for MuRF1 and MuRF3 developed cardiomyopathy and skeletal myopathy. Morphological features included myosin accumulation and aggregates of fragmented sarcomeres with preserved A-bands and M-lines. We studied a family with three affected members suffering from cardiomyopathy or cardioskeletal myopathy. We performed sequencing analysis of TRIM63 and TRIM54 encoding MuRF1 and MuRF3, respectively. Transfection of wild-type MuRF1 in cultured patient myoblast was performed to assess the rescue assay. To characterize the behaviour of TRIM54 mutation, control myoblasts were transfected with wild-type or mutant MuRF3. We identified a homozygous MuRF1 null mutation and heterozygous MuRF3 mutation in the patient with cardioskeletal myopathy. Myopathological features were highly reminiscent of that of MuRF1−/−/MuRF3−/− mice. Cultured myotubes from patient showed perturbed myofibrillogenesis and abnormal organization of the microtubule network. Transfection of wild-type MuRF1 in cultured patient myocytes rescued the phenotype. Transfection of control myoblasts with mutant MuRF3 induced the formation of irregular filamentous structures demonstrating the pathogenic effect of the MuRF3 mutation. We describe a novel protein aggregate myopathy and cardiomyopathy due to MuRF1 deficiency and a deleterious MuRF3 missense mutation. Unique morphological features including myosin accumulation, disrupted microtubule structures and fragmented sarcomeres with preserved A-bands and M-lines in the absence of thin filaments and Z-lines characterize the disease associated with combined MuRF1 and MuRF3 deficiency.