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

Arthritis Rheumatol. 2023 Jun;75(6):1058-1071. doi: 10.1002/art.42416. Epub 2023 Mar 30.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adaptor Proteins, Signal Transducing / genetics
  • Adaptor Proteins, Signal Transducing / metabolism
  • Animals
  • Child
  • Humans
  • Interferon Type I* / metabolism
  • Interferon-alpha
  • Lupus Erythematosus, Systemic* / metabolism
  • Mice
  • Neurons / metabolism
  • Protein Kinase C / metabolism
  • TNF Receptor-Associated Factor 4 / metabolism
  • TNF Receptor-Associated Factor 6 / genetics
  • TNF Receptor-Associated Factor 6 / metabolism
  • Toll-Like Receptor 7 / genetics
  • Toll-Like Receptor 7 / metabolism
  • Toll-Like Receptor 9

Substances

  • Toll-Like Receptor 7
  • Interferon-alpha
  • Protein Kinase C
  • TNF Receptor-Associated Factor 4
  • TNF Receptor-Associated Factor 6
  • Interferon Type I
  • Toll-Like Receptor 9
  • PACSIN1 protein, human
  • Adaptor Proteins, Signal Transducing
  • TRAF4 protein, human