Caspr2 autoantibodies target multiple epitopes (1)
Autoantibodies to the voltage-gated potassium channel (VGKC) complex are found in patients with acquired neuromyotonia (Isaac syndrome),1,2 limbic encephalitis, or Morvan syndrome. Caspr2 has a large extracellular domain with 8 distinct subdomains and 12 potential N-linked glycosylation sites. Autoantibodies to synaptic proteins, including the NMDA receptor (NMDAR) and the nicotinic acetyl- choline receptor (nAChR), may target either a single dominant epitope or a major immunogenic region.
All 10 patients’ sera recognized a surface epitope on transfected cells but not on untransfected cells.
Tunicamycin treatment prevented cell surface expression of Caspr2 and resulted in a shift of the Caspr2 band on Western blot, but it did not prevent antibody recognition. Caspr2 autoantibodies therefore recognize the protein under denaturing conditions and glycosylation is not required for antibody recognition.
Starting from the N-terminus of the protein and moving to the transmembrane region, these domains are Discoidin (Disc), LamininG (Lam1), LamininG (Lam2), Egf (Egf1), FibrinogenC (FibC), LamininG (Lam3), Egf (Egf2), and LamininG (Lam4).
This pattern of findings showed that no single extracellular subdomain was absolutely necessary for antibody recognition and that multiple epitopes must exist in the extracellular domain. The Disc domain may contain a target epitope.
This construct was recognized by 9 of 10 serum samples. An epitope in the Disc domain is still recognized under denaturing conditions.
Testing with a panel of 7 sera showed that target epit- opes for most patients (6 of 7) were confined entirely to the 4 subdomains on the N-terminal of the protein. Only 1 of 7 samples reacted (weakly) when these 4 subdomains were deleted.
Antibodies to the nAChR that target the major immunogenic region may be more pathogenic than antibodies to other parts of the receptor.