RNA Helicase Encoded by Melanoma Differentiation-Associated Gene 5 Is a Major Autoantigen in Patients With Clinically Amyopathic Dermatomyositis Association With Rapidly Progressive Interstitial Lung Disease

Objective. To identify the autoantigen recognized by the autoantibody that is associated with clinically amyopathic dermatomyositis (C-ADM) and rapidly progressive interstitial lung disease (ILD). Methods. An anti-CADM-140 antibody-positive prototype serum sample was used to screen a HeLa cell-derived complementary DNA (cDNA) library. Selected cDNA clones were further evaluated by immunoprecipitation of their in vitro-transcribed and in vitro-translated products using anti-CADM-140 antibody-positive and anti-CADM-140 antibody-negative sera. The lysates of COS-7 cells transfected with the putative antigen were similarly tested. An enzyme-linked immunosorbent assay (ELISA) to detect the anti-CADM-140 antibody was established using a recombinant CADM-140 antigen, and its specificity and sensitivity for C-ADM and rapidly progressive ILD were assessed in 294 patients with various connective tissue diseases. Results. By cDNA library screening and immunoprecipitation of in vitro-transcribed and in vitro-translated products, we obtained a cDNA clone encoding melanoma differentiation-associated gene 5 (MDA-5). The anti-CADM-140 antibodies in patients' sera specifically reacted with MDA-5 protein expressed in cells transfected with full-length MDA-5 cDNA, confirming the identity of MDA-5 as the CADM-140 antoantigen. The ELISA, using recombinant MDA-5 protein as the antigen, showed an analytical sensitivity of 85% and analytical specificity of 100%, in comparison with the gold standard immunoprecipitation assay, and was useful for identifying patients with C-ADM and/or rapidly progressive ILD. Conclusion. Given that RNA helicase encoded by MDA-5 is a critical molecule involved in the innate immune defense against viruses, viral infection may play an important role in the pathogenesis of C-ADM and rapidly progressive ILD. Moreover, our ELISA using recombinant MDA-5 protein makes detection of the anti-CADM-140 antibody routinely available.