De Novo PACSIN1 Gene Variant Found in Childhood Lupus and a Role for PACSIN1/TRAF4 Complex in Toll‐like Receptor 7 Activation

Objective Increased Toll‐like receptor 7 (TLR‐7) signaling leading to the production of type I interferon (IFN) is an important contributor to human systemic lupus erythematosus (SLE). Protein kinase C and casein kinase substrate in neurons 1 (PACSIN1), a molecule that regulates synaptic vesicle recycling, has been linked to TLR‐7/TLR‐9–mediated type I IFN production in humans and mice, but the underlying mechanism is unknown. We undertook this study to explore the pathogenicity and underlying mechanism of a de novo PACSIN1 missense variant identified in a child with SLE. Methods PACSIN1 Q59K de novo and null variants were introduced into a human plasmacytoid dendritic cell line and into mice using CRISPR/Cas9 editing. The effects of the variants on TLR‐7/TLR‐9 signaling in human and mouse cells, as well as PACSIN1 messenger RNA and IFN signature in SLE patients, were assessed using real‐time polymerase chain reaction and flow cytometry. Mechanisms were investigated using luciferase reporter assays, RNA interference, coimmunoprecipitation, and immunofluorescence. Results We established that PACSIN1 forms a trimolecular complex with tumor necrosis factor receptor–associated factor 4 (TRAF4) and TRAF6 that is important for the regulation of type I IFN. The Q59K mutation in PACSIN1 augments binding to neural Wiskott‐Aldrich syndrome protein while it decreases binding to TRAF4, leading to unrestrained TRAF6‐mediated activation of type I IFN. Intriguingly, PACSIN1 Q59K increased TLR‐7 but not TLR‐9 signaling in human cells, leading to elevated expression of IFNβ and IFN‐inducible genes. Untreated SLE patients had high PACSIN1 expression in peripheral blood cells that correlated positively with IFN‐related genes. Introduction of the Pacsin1 Q59K mutation into mice caused increased surface TLR‐7 and TRAIL expression in B cells. Conclusion PACSIN1 Q59K increases IFNβ activity through the impairment of TRAF4‐mediated inhibition of TLR‐7 signaling, possibly contributing to SLE risk. image