Muscle-specific receptor tyrosine kinase autoantibodies--a new immunoprecipitation assay.

Background: It has been reported that measurement of autoantibodies to a muscle-specific receptor tyrosine kinase (MuSK) may be useful in the assessment and management of patients with acetylcholine receptor antibody (AChRAb)-negative myasthenia gravis (MG).

Methods: A new and convenient assay for MuSKAb measurement based on 125I-labeled purified recombinant MuSK is described. In the assay, serum samples (5 microl) were incubated with 50 microl of 125I-MuSK and any immune complexes formed precipitated with antihuman IgG (50 microl).

Results: With this assay, MuSKAbs were detected in 8/33 (24%) sera from AChRAb-negative MG patients studied. In contrast, no MuSKAb was detected in 53 AChRAb-positive MG patient sera; furthermore, 0/18 Lambert-Eaton myasthenic syndrome sera, 0/5 non-MG neuromuscular disease sera, 0/95 control autoimmune disease sera, and 0/50 healthy blood donor sera contained detectable MuSKAb. In this assay, inter-assay coefficients of variation (n = 5) were 6.8%, 5.9%, and 3.6% for samples with MuSKAbs at 0.73, 0.32, and 0.11 nmol/l, respectively. Similar results were obtained with 125I-labeled rat and human MuSK.

Conclusion: The 125I-MuSK immunoprecipitation assay we have described provides a simple, specific, and precise procedure for detecting MuSKAbs.