Identification of muscle specific autoantibodies for myasthenic syndromes and myositis by proteomic analysis

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We identified two novel antibodies against muscle-specific components in sera of idiopathic inflammatory myopathies (IIMs) patients by proteomic analysis. These target antigens were myosin-binding protein C (MyBP C) and pyruvate kinase isozyme M1/M2 (PK M1/M2). MyBP C belongs to the immunoglobulin superfamily. PK catalyzes the conversion of phosphoenolpyruvate with regeneration of ATPs. Both molecules have been reported as possible myositogenic antigen for experimental autoimmune polymyositis.The number of patients with the anti- MyBP C antibody was significantly higher among the IIM patients (9 of 10) than among the healthy control (1 of 10) and myotonic dystrophy (MyD) patients (5 of 11). The number of patients with the anti-PK M1/M2 antibody was significantly higher among the IIM patients (5 of 10) than among the healthy control (0 of 10) and myotonic dystrophy (MyD) patients (2 of 11). The sensitivity of this combination assay for diagnosis IIM was 50% and specificity was 80%. The sensitivity of this combination assay for diagnosis polymyositis was 71% and specificity was 92%. Therefore we propose that this combination assay is a candidate for further exploration as a diagnostic marker of IIM, particularly polymyositis.To clarify the pathogenic roles of anti- MyBP C and PK M1/M2 antibodies in IIM, we must perform more experiments such as using animal models in which these antibodies are passively administrated.