Prevalence and course of muscle-specific receptor tyrosine kinase (MuSK) antibodies in myasthenia gravis - A retrospective study.

Myasthenia gravis is a neuromuscular disease typically associated with acetylcholine receptor (AChR) autoantibodies. However, in AChR-negative cases, autoantibodies against muscle-specific receptor tyrosine kinase (MuSK) may be present. Given the rarity of MuSK autoantibodies, there is limited information on their development and significance as biomarkers of disease activity. In a retrospective 10-year study, we tested 749 serum samples from 641 patients for MuSK autoantibodies. For MuSK-positive Myasthenia gravis cases, we extended the observation period to almost 17 years and analyzed the correlation between MuSK titers and clinical severity (Besinger score). We also examined the association between elevated but formally negative MuSK titers and other autoimmune diseases. Of the 749 samples, 21 (3 %, n = 7 patients) were MuSK-positive. In these patients, MuSK titers did not correlate with disease severity or treatment changes. 78 samples (10 %, n = 73 patients) showed elevated but formally negative MuSK titers, with no link to other autoimmune diseases. Our results contradict previous studies suggesting that MuSK autoantibodies could serve as biomarkers of Myasthenia gravis severity, and they shed light on the clinical relevance of elevated but formally negative MuSK titers.