Human OTULIN mutations can cause distinct inflammatory disease phenotypes depending on the protein domain that is mutated

Abstract

Abstract Linear ubiquitination is a post-translational modification that controls many immune signaling pathways. Linear ubiquitin chains are added to targets by LUBAC and removed by OTULIN, thus the opposing actions of LUBAC and OTULIN are critical for immune homeostasis. To understand mechanisms of how ubiquitination controls inflammation, we leveraged the use of biospecimens from patients with a novel bi-allelic OTULIN mutation. These patients have pyoderma gangrenosum (PG), an extremely rare inflammatory skin disease. OTULIN mutations have been previously described to cause a systemic autoinflammatory disease called ORAS, yet these patients define a new monogenic disease. We sought to understand this discrepancy through functional analyses of OTULIN mutations that culminate in PG vs ORAS. ORAS-causing OTULIN mutations impair the catalytic domain. However, the mutation in PG patients affects the domain responsible for binding LUBAC. Further, OTULIN’s ability to downregulate linear ubiquitin or NF-kB activity was unaffected. Patients’ transcriptional signatures showed elevated expression of genes involved in neutrophil activation and antigen presentation, and reduced expression of genes involved in Th17 differentiation, TLR signaling, and NOD-like receptor signaling. Deep immunophenotyping by CyTOF revealed that PG patients have higher frequencies of T cells and subtle defects of B cells and myeloid cells. These patients’ unique phenotype suggests an unrecognized function for OTULIN in skin inflammation. This discovery adds to the emerging spectrum of human immune-mediated diseases caused by defects in the ubiquitin pathway.