Background/Purpose: The Ro60 protein is a prominent autoantigen in systemic lupus erythematosus (SLE) and Sjogren’s syndrome (SS). Anti-Ro antibodies are strongly associated with UV-mediated skin rashes in lupus, especially in subacute cutaneous lupus erythematosus (SCLE), the most photosensitive form of lupus. Surprisingly, Ro60 knockout (KO) mice spontaneously develop a lupus-like syndrome associated with autoantibodies and glomerulonephritis. An association with type I interferon (IFN) has not previously been investigated. While it is known that Ro binds to small non-coding Y RNAs, it was recently reported that Ro also binds to Alu RNAs derived from short interspersed retroelements (SINEs). Loss of Ro60 in human cell lines resulted in the accumulation of Alu RNAs and the dysregulation of IFN-stimulated genes (ISGs). Since SINE transcripts have been shown to be increased following DNA damage, we examined the relationship between Ro60, SINEs, and the inflammatory response after UV irradiation in mice.

Methods: C57BL/6 and Ro60 KO mice (n = 3) were irradiated with UVB 500 mJ/cm² once. Skin biopsies were performed at baseline and 1 day following UVB exposure. RNA was extracted from the skin, and RNA expression of retroelements [B1 and B2 SINEs (rodent equivalents of human Alu)], inflammatory markers including type I IFNs, ISGs and Y RNAs were quantified by RT-qPCR.

Results: At baseline, Ro60 KO mice exhibited increased expression of ISGs compared to C57BL/6 mice with a 7-fold increase found in Isg15, 4-fold increase in Isg20, and 18-fold increase in Mx1. Y1 and Y3 RNAs were virtually undetectable in Ro60 KO mice even after UVB irradiation. This supports the current theory that Ro60 is involved in RNA processing and quality control including the stabilization of RNA polymerase III transcripts as seen with La protein. While B1 and B2 SINEs were not elevated in Ro60 KO mice compared to C57BL/6 mice at baseline, SINEs were increased in both Ro60 KO and C57BL/6 mice after UVB. These results are consistent with the possibility that retroelements play a role in the inflammatory response following UVB irradiation, although a causal relationship remains to be determined.

Conclusion: Increases in ISGs observed at baseline in Ro60 KO mice compared to control C57BL/6 mice may contribute to autoimmunity and lupus-like syndrome reported previously. Furthermore, the accumulation of SINE transcripts after UV irradiation may overwhelm the binding capacity of Ro60, leading to stimulation of innate immune response. A clearer understanding of the relationship between autoimmunity in Ro60 KO mice and changes in the relative amounts of SINEs versus Ro60 protein could provide a new paradigm of how environmental stimuli trigger lupus.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-role-of-interferon-in-autoimmune-susceptible-ro60-knockout-mice/