B-076 Comparison of Three Methods for the Detection of Antibodies Against Muscle-Specific Kinase

Abstract

Abstract Background Myasthenia gravis (MG) is a neuromuscular disease characterized by skeletal muscle weakness and fatigue. These symptoms are due to a reduction in synaptic signal transduction caused by autoantibodies against post-synaptic proteins in the neuromuscular junction. Approximately 85% of patients with MG have autoantibodies against acetylcholine receptors (AChR); however, autoantibodies against muscle specific kinase (MuSK) and low-density lipoprotein receptor-related protein 4 (LRP4) may also be found in patients with MG. Detection and identification of these antibodies is important in the diagnosis and treatment of patients under evaluation for MG. MuSK, a transmembrane protein expressed on skeletal muscles, is essential to the formation and maintenance of the neuromuscular junction. Approximately 6% of MG patients have MuSK autoantibodies. Traditionally, radioimmunoprecipitation assays (RIA) have been used to detect MuSK antibodies in patient sera. However, RIA has several limitations, including safety and regulatory concerns associated with using radioactive materials and the short half-life of reagents. Alternatives to RIA, such as enzyme-linked immunosorbent assays (ELISA) and cell-based assays (CBA), do not have the same challenges. Recent studies outside of the USA suggest that cell-based assays may have equal or superior sensitivity than RIA; particularly in the detection of MuSK antibodies in patients previously diagnosed with seronegative MG (MG patients without detectable AChR or MuSK antibodies by RIA). The objective of this study was to compare the performance of two commercially available ELISA and fixed cell-based assay (f-CBA) kits to RIA for the detection of MuSK antibodies. Methods A total of 237 sera were evaluated by RIA, ELISA and/or f-CBA. These sera include 47 MuSK RIA positive sera, 55 MuSK RIA negative sera, 70 self-reported healthy control sera, and 65 sera positive for other neural antibodies associated with MG, Lambert-Eaton Myasthenic Syndrome, paraneoplastic neurologic syndromes, or autoimmune encephalitis. Results Both f-CBA and ELISA demonstrated 100% specificity for MuSK antibodies in sera from self-reported healthy controls and sera from patients positive for other antibodies. Forty-seven sera tested positive for anti-MuSK antibodies by RIA; of those 47 sera, 7 sera tested negative by ELISA and 5 tested negative by f-CBA kits while the remaining 40 were concordant between ELISA, f-CBA and RIA. The discrepancy between RIA and the commercial kits for ELISA and f-CBA was only observed in low positive sera (0.04–0.21 nmol/L ARUP RIA). Clinical information was not available for discrepant specimens. However, a review of MuSK RIA results for University of Utah patients shows that of 7 patients with an ARUP RIA result of 0.04–0.21 nmol/L, only 1 had symptoms that may possibly be consistent with MG. The remaining 6 low positive patients did not have symptoms consistent with a diagnosis of MG. This may suggest an issue with RIA specificity rather than ELISA and f-CBA sensitivity. However, further studies are needed to confirm this. Conclusion Commercially available ELISA and f-CBA kits offer viable alternatives to traditional RIA for detection of MuSK antibodies.