Is “seronegative” MG explained by autoantibodies to MuSK?

Jon Lindstrom1 poses the question “Is ‘seronegative' MG explained by autoantibodies to MuSK?” We agree that muscle-specific receptor tyrosine kinase (MuSK) antibodies are not found in all myasthenia gravis (MG) patients seronegative for acetylcholine receptor (AchR) antibodies, their pathogenicity is still unclear, and there are other immune factors in seronegative MG,2–4 but we believe that MuSK antibodies are important. The editorial states, “autoantibodies to MuSK were not found where they were expected.” This referred to a Taiwanese report in which only 1/26 individuals with seronegative MG had MuSK antibodies. But 7/17 were positive in an accompanying Japanese study,5 closer to the 70% reported initially,2 and similar to the 41% in a more inclusive UK study.4 The observations support our growing evidence that the prevalence of MuSK antibodies …