Diagnostic utility of cortactin antibodies in myasthenia gravis

Patients with myasthenia gravis (MG) without antibodies to the acetylcholine receptor (AChR) or muscle-specific tyrosine kinase (MuSK) have been classified as having double-seronegative myasthenia gravis (dSNMG). We used the sera from six dSNMG patients with positive immunohistochemistry assays in a protein array to screen reactivity with 9000 human proteins. We identified cortactin, an intracellular protein that interacts with agrin/MuSK favoring AChR aggregation, as a new antigen in dSNMG. We then designed an in-house enzyme-linked immunosorbent assay as a screening assay and confirmed these results by western blot. We found that 19.7% of dSNMG patients had anti-cortactin antibodies. In contrast, patients with AChR⁺ MG or other autoimmune disorders and healthy controls were positive at significantly lower rates. Five percent of healthy controls were positive. In a recent study, we screened sera from 250 patients (AChR⁺ MG, MuSK⁺ MG, dSNMG) and 29 healthy controls. Cortactin antibodies were identified in 23.7% of dSNMG and 9.5% AChR⁺ MG patients (P = 0.02). None of the MuSK⁺ MG patients, patients with other autoimmune disorders, or healthy controls had antibodies against cortactin. Patients with dSNMG cortactin⁺ MG were negative for anti-striated muscle and anti-LRP4 antibodies. Patients with dSNMG cortactin⁺ MG presented ocular or mild generalized MG without bulbar symptoms. We conclude that cortactin autoantibodies are biomarkers of MG that, when present, suggest that the disease will be mild.