Serum from patients with MuSK antibody-positive myasthenia gravis triggers transcriptomic changes leading to muscle atrophy and weakness in human myotube cells

Myasthenia gravis (MG) is an autoimmune disease characterized by autoantibodies targeting the acetylcholine receptor (AChR) or muscle-specific tyrosine kinase (MuSK). These autoantibodies inhibit ACh signal transmission at the neuromuscular junction, leading to muscle weakness and fatigue. Anti-MuSK antibody-positive MG (MuSK+MG) appears more rarely than anti-AChR antibody-positive MG but more frequently results in muscle atrophy. However, the underlying mechanism is unknown. In this study, we analyzed whether serum from MuSK+MG patients has any pathogenic effect on cultured myotube cells. Primary human skeletal muscle myoblasts were differentiated into myotubes, which were then treated with serum from healthy control individuals or MuSK+MG patients. After one day, RNA-seq analysis and Western blotting were performed and myotube diameters were measured. Calcium dynamics following caffeine stimulation was also assessed. Comparing myotube cells treated with healthy control serum with those treated with serum from MuSK+MG patients, RNA-seq analysis showed suppression of pathways associated with muscle function and Western blotting analysis revealed reduced expression of Type II myosin heavy chain. These changes are consistent with muscle atrophy and weakness, although myotube diameter remained unchanged. Caffeine stimulation induced higher cytoplasmic calcium levels. Expression of sarcomere components was significantly reduced. Serum from MuSK+MG patients directly affected gene and protein expression in cultured human myotube cells, leading to changes associated with muscle atrophy and weakness. These findings suggest that there are mechanisms in addition to impaired ACh signal transmission at the neuromuscular junction that can cause muscle weakness and fatigue in MG patients.