Increased noncanonical splicing of autoantigen transcripts provides the structural basis for expression of untolerized epitopes

Background: Alternative splicing is important for increasing the complexity of the human proteome from a limited genome. Previous studies have shown that for some autoantigens, there is differential immunogenicity among alternatively spliced isoforms. Objectives: Herein, we tested the hypothesis that alternative splicing is a common feature for transcripts of autologous proteins that are autoantigens. The corollary hypothesis tested was that nonautoantigen transcripts have a lower frequency of alternative splicing. Methods: The extent of alternative splicing within 45 randomly selected self-proteins associated with autoimmune diseases was compared with 9554 randomly selected proteins in the human genome by using bioinformatics analyses. Isoform-specific regions that resulted from alternative splicing were studied for their potential to be epitopes for antibodies or T-cell receptors. Results: Alternative splicing occurred in 100% of the autoantigen transcripts. This was significantly higher than the approximately 42% rate of alternative splicing observed in the 9554 randomly selected human gene transcripts (P <.001). Within the isoform-specific regions of the autoantigens, 92% and 88% encoded MHC class I and class II-restrieted T-cell antigen epitopes, respectively, and 70% encoded antibody binding domains. Furthermore, 80% of the autoantigen transcripts underwent noncanonical alternative splicing, which is also significantly higher than the less than 1% rate in randomly selected gene transcripts (P <.001). Conclusion: These studies suggest that noncanonical alternative splicing may be an important mechanism for the generation of untolerized epitopes that may lead to autoimmunity. Furthermore, the product of a transcript that does not undergo alternative splicing is unlikely to be a target antigen in autoimmunity.