Conformational specificity of the C4F6 SOD1 antibody; low frequency of reactivity in sporadic ALS cases

Greater than 160 missense mutations in copper-zinc superoxide dismutase-1 (SOD1) can cause amyotrophic lateral sclerosis (ALS). These mutations produce conformational changes that reveal novel antibody binding epitopes. A monoclonal antibody, clone C4F6 -raised against the ALS variant G93A of SOD1, has been identified as specifically recognizing a conformation shared by many ALS mutants of SOD1. Attempts to determine whether non-mutant SOD1 adopts a C4F6-reactive conformation in spinal tissues of sporadic ALS (sALS) patients has produced inconsistent results. To define the epitope recognized by C4F6, we tested its binding to a panel of recombinant ALS-SOD1 proteins expressed in cultured cells, producing data to suggest that the C4F6 epitope minimally contains amino acids 90-93, which are normally folded into a tight hairpin loop. Multiple van der Waals interactions between the 90-93 loop and a loop formed by amino acids 37-42, particularly a leucine at position 38, form a stable structure termed the beta-plug. Based on published modeling predictions, we suggest that the binding of C4F6 to multiple ALS mutants of SOD1 occurs when the local structure within the beta-plug, including the loop at 90-93, is destabilized. In using the antibody to stain tissues from transgenic mice or humans, the specificity of the antibody for ALS mutant SOD1 was influenced by antigen retrieval protocols. Using conditions that showed the best discrimination between normal and misfolded mutant SOD1 in cell and mouse models, we could find no obvious difference in C4F6 reactivity to spinal motor neurons between sALS and controls tissues.