Small ubiquitin-related modifiers: a novel and independent class of autoantigens in primary biliary cirrhosis

Serum autoantibodies against components of nuclear dots (anti-NDs), namely PML and Sp100, are specifically detected in 20% to 30% of patients with primary biliary cirrhosis (PBC). Although anti-ND antibodies are nonpathogenic, the mechanisms that lead to this unique reactivity are critical to understanding the loss of immune tolerance in PBC. Importantly, Sp100 and PML are both covalently linked to small ubiquitin-related modifiers (SUMOs). Therefore, we investigated whether SUMO proteins are independent autoantigens in PBC and studied 99 PBC sera samples for reactivity against NDs, PML, and Sp100, as well as against SUMO-2 and SUMO-1 recombinant proteins. Autoantibodies against SUMO-2 and SUMO-1 were found in 42% and 15% of anti-ND-positive PBC sera, respectively. Anti-SUMO reactivity was not observed in anti-ND-negative sera. Anti-SUMO-2 autoantibodies were found in 58% of sera containing autoantibodies against both PML and Sp100 and were detected exclusively in sera containing anti-Sp100 autoantibodies. In conclusion, SUMO proteins constitute a novel and independent class of autoantigens in PBC. Furthermore, we believe our data emphasize the post-translational modification of lysine by either lipoylation in the case of AMA or SUMOylation in the case of specific anti-ND autoantibodies as the pivotal site for autoantibody generation in PBC.