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BACKGROUND: Approximately 10% to 20% of patients with autoimmune MG do not have antibodies to the acetylcholine receptor (AChR), so-called seronegative MG (SNMG). IgG antibodies from up to 70% of SNMG patients bind to the muscle-specific receptor tyrosine kinase, MuSK. The plasmas and non-IgG fractions from SNMG patients (and some with AChR antibodies) also contain a factor, perhaps an IgM antibody, that inhibits AChR function, but it is not clear how this factor acts and whether it is related to the MuSK IgG antibodies. METHODS: The authors studied 12 unselected SNMG plasmas and their non-IgG fractions; seven were positive for MuSK IgG antibodies. Ion flux assays, electrophysiology, phosphorylation, and kinase assays were used to look at mechanisms of action. RESULTS: Eight of the 12 plasmas and their non-IgG fractions inhibited AChR function, but the inhibitory activity was transient and did not correlate with the presence of MuSK IgG antibodies. Two of three plasmas added outside of a cell-attached patch pipette inhibited AChR function within the patch, and these two plasmas also increased AChR phosphorylation. CONCLUSIONS: The authors propose that a plasma factor(s) in SNMG patients, distinct from MuSK IgG antibodies, binds to a muscle membrane receptor and activates a second messenger pathway leading to AChR phosphorylation and reduced AChR function. Identifying the target for this factor should lead to improved diagnosis of MG in MuSK antibody-negative patients and may provide new insights into the function of the neuromuscular junction and pathophysiological mechanisms in MG.

Type

Journal article

Journal

Neurology

Publication Date

10/12/2002

Volume

59

Pages

1682 - 1688

Keywords

Adenosine Triphosphate, Adolescent, Adult, Aged, Autoantibodies, Cell Line, Child, Child, Preschool, Cholinergic Antagonists, Cyclic AMP-Dependent Protein Kinases, Electrophysiology, Female, Humans, Immunoglobulin G, Infant, Male, Middle Aged, Myasthenia Gravis, Patch-Clamp Techniques, Phosphorylation, Protein Kinases, Receptor Protein-Tyrosine Kinases, Receptors, Cholinergic, Sodium