Background/Purpose:   Antibodies to small ubiquitin-like modifier activating enzyme (SAE) are associated with diagnosis of dermatomyositis (DM). The aims of this study were to establish the frequency, demographic and clinical characteristics of SAE antibody-positive patients in a referral setting, and to evaluate the performance of a line immunoblot assay (LIA) for detection of SAE antibodies.

Methods:   Consecutive samples (n=3,921) received at ARUP Laboratories for myositis antibodies testing were screened by protein immunoprecipitation (IP) of S³²-labeled K562 cells. Antinuclear antibodies (ANA) were detected by indirect immunofluorescent antibody (IFA) test. Of these, 13 samples with distinct bands corresponding to approximately 40 and 90 kDa were identified as suspicious of SAE and tested for SAE antibodies by LIA. As controls, 170 disease controls and 44 self-proclaimed healthy controls were also evaluated by LIA. Clinical and laboratory data for all SAE antibody-positive patients were sought to confirm diagnosis.

Results:   Twelve out of 13 patient samples with 40 and 90 kDa bands observed in the IP assay were positive in the LIA. SAE antibodies were identified in 2 of the disease control samples, one signal recognition particle (SRP) antibody-positive sample with additional bands at 40 and 90 kDa and one transcriptional intermediary factor 1-γ (TIF1γ) antibody-positive sample with no apparent bands corresponding to 40 and 90 kDa bands (false positive rate = 0.5%). SAE antibodies were not identified in any of the self-proclaimed healthy controls. Overall very good qualitative agreement was observed between the two methods for SAE antibody marker (Cohen’s kappa = 0.92, 95% CI 0.81-1.00). The positive, negative, and total percent agreements were 92.3% (95% CI 64.0-99.8%), 99.5% (95% CI 97.4-100%), and 99.5% (95% CI 97.5-100%), respectively. SAE antibodies were detected by IP and LIA in 12/3,921 sera (0.3%). Patients positive for SAE in both methods all had a confirmed diagnosis of DM, with characteristic cutaneous manifestations and varying degrees of muscle involvement. Presence of SAE antibodies was mainly associated with homogeneous and/or speckled antinuclear antibody patterns by IFA.

Conclusion:   Identification of SAE antibodies is important not only for diagnosis, but also for prognosis and disease management in patients with DM since some of the other DM-associated autoantibodies have been associated with malignancy or rapidly progressive interstitial lung disease. Testing for SAE antibodies is currently only performed by a small number of labs, primarily on a research basis. LIA confirmed the presence of SAE antibodies in patients with 40 and 90 kDa bands observed in the IP assay. Combined use of both methods improves reliability for the diagnosis of DM in patients undergoing evaluation for inflammatory myopathies.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/antibodies-to-small-ubiquitin-like-modifier-activating-enzyme-frequency-and-characteristics-of-antibody-positive-patients-in-an-unselected-cohort/