Development and evaluation of a standardized ELISA for the determination of autoantibodies against cN-1A (Mup44, NT5C1A) in sporadic inclusion body myositis

Abstract

Purpose: Sporadic inclusion body myositis (sIBM) is an autoimmune degenerative disease of the muscle, with inflammatory infiltrates and inclusion vacuoles. Its pathogenesis is not fully understood and the diagnosis is hampered by its imprecise characteristics, at times indistinguishable from other idiopathic inflammatory myopathies such as polymyositis and dermatomyositis. The diagnosis may be assisted by the detection of autoantibodies targeting Mup44, a skeletal muscle antigen identified as cytosolic 5'-nucleotidase 1A (cN-1A, NT5C1A). A novel standardized anti-cN-1A IgG ELISA was developed and its diagnostic performance was evaluated by two reference laboratories.

Methods: Recombinant human full-length cN-1A was expressed and purified, and subsequently utilized to set up a standardized ELISA. To evaluate the novel assay, laboratory A examined sera from North American patients with clinically and pathologically diagnosed definite sIBM (n = 17), suspected sIBM (n = 14), myositis controls (n = 110), non-myositis autoimmune controls (n = 93) and healthy subjects (n = 52). Laboratory B analyzed a Dutch cohort of definite sIBM patients (n = 51) and healthy controls (n = 202).

Results: Anti-cN-1A reactivity was most frequent in definite sIBM (39.2-47.1%), but absent in biopsy-proven classic polymyositis or dermatomyositis. Overall diagnostic sensitivity and specificity amounted to 35.5 and 96.1% (laboratory A) and 39.2 and 96.5% (laboratory B).

Conclusions: Anti-cN-1A autoantibodies were detected by ELISA with moderate sensitivity, but high specificity for sIBM and may therefore help diagnose this infrequent and difficult-to-diagnose myopathy. The novel anti-cN-1A IgG ELISA can improve and accelerate the diagnosis of sIBM using sera where muscle biopsy is delayed or unfeasible.

Keywords: Anti-Mup44; Anti-cN-1A; Autoantibodies; Autoimmunity; ELISA; Myopathy; Sporadic inclusion body myositis.

Fig. 1

Analysis of purified recombinant cN-1A. Molecular weight markers (lane 1) and 1 µg of purified cN-1A (lane 2) were separated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis followed by Coomassie staining. The molecular masses (kDa) of the size markers are indicated on the left

Fig. 2

Correlation analysis. Reactivity of serum samples from Dutch patients with sporadic inclusion body myositis was measured by anti-cN-1A IgG ELISA (Euroimmun) based on recombinant full-length cN-1A, by ELISAs based on three synthetic peptides representing major cN-1A epitopes (n = 51), and by immunoprecipitation (IP) of in vitro translated full-length cN-1A (n = 30). Results by anti-cN-1A IgG ELISA were plotted against a the maximum OD450 values measured by either of the three peptide ELISAs and b against the efficiency of immunoprecipitation. Dotted lines represent cutoff values (anti-cN-1A IgG ELISA, ratio ≥1.0; IP high efficiency, >5% of input; IP low efficiency, >1% of input). In panel a, triangles/circles depict serum specimens with a maximum OD450 value below/above the cutoff value of the respective peptide ELISA (peptide 1, OD450 ≥ 0.384; peptide 2, OD450 ≥ 0.270; peptide 3, OD450 ≥ 0.280). Correlation was evaluated by Pearson’s correlation coefficients (r) as indicated. P values <0.05 were considered significant