Background/Purpose: Sporadic inclusion body myositis (sIBM) is an autoimmune disease manifesting with muscle degeneration, inflammatory infiltrates and inclusion vacuoles. Diagnosis of sIBM is hampered by its imprecise characteristics, at times indistinguishable from other Idiopathic Inflammatory Myopathies, but may now be assisted by detection of sIBM-specific autoantibodies targeting muscle antigen Mup44, identified as cytosolic 5’-nucleotidase 1A (cN-1A; Mup44; NT5c1A). This study evaluated sensitivity and specificity of an anti-cN-1A IgG serological assay in sera from patients with and without sIBM.

Methods: Serum from patients with clinically and pathologically diagnosed sIBM (n=68) and from suspected sIBM (encompassing biopsy readings of possible, probable and doubtful sIBM patients, n=15), as well as from patients with other forms of myositis [including dermatomyositis (DM; n=4), polymyositis (PM; n=7); unspecified myositis without sIBM (n=94), muscle atrophy (n=1), myonecrosis (n=4)], systemic lupus erythematosus (n=33), scleroderma (n=20), Sjögren’s syndrome (n=20), rheumatoid arthritis (n=20) and from healthy controls (n=254) were tested for anti-cN-1A IgG using an anti-cN-1A ELISA (full-length antigen, EUROIMMUN).

Results: Anti-cN-1A was most frequent among definite sIBM (41.2%). The overall specificity was 96.3% with individual specificities from 90% (scleroderma) to 100% (PM or DM).

Conclusion: The presence of anti-cN-1A in serum appears to be disease-specific for sIBM. These antibodies are found at a moderate prevalence but are only rarely detected in other autoimmune conditions. Thus, anti-cN-1A ELISA may support the diagnostics of sIBM and accelerate the diagnosis, where muscle biopsy is delayed or unfeasible.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/presence-of-anti-cn-1a-mup44-nt5c1a-igg-is-specific-for-sporadic-inclusion-body-myositis/