Background/Purpose: Approximately 70% of patients with Systemic Lupus Erythematosus (SLE) show a striking Type I Interferon (IFN-I) signature in peripheral blood. Although we have some understanding of how this signature can be generated in vitro (by immune complex activation of Toll-Like Receptors, TLRs), we have little understanding of mechanisms operative in SLE patients. Cyclic GMP-AMP (cGAMP) synthase (cGAS) is a cytosolic DNA sensor that is triggered by both microbial as well as self DNA and has been shown to be the most important cytosolic DNA sensor responsible for IFN-I production. Using PBMC samples from patients, we have shown that the cGAS-STING (stimulator of interferon genes) pathway is activated in ~15% of patients with SLE. In a separate study, we also reported that oxidized mitochondrial DNA (Ox-mtDNA comprising 8-hydroxydeoxyguanine or 8-OHdG) released from neutrophils undergoing NETosis stimulates IFN-I via the cGAS-STING pathway. The aim of our study was to determine the relationship between Ox-mtDNA and activation of the cGAS-STING pathway in SLE patients.

Methods: Cell-free DNA was isolated from the plasma of SLE patients (n=16) and Healthy Controls (HC, n=12) by standard procedures.  Genes unique to mitochondria (COXII and MT-TL1, abbreviated MtDNA) in plasma were quantified by Real time qPCR. 8-OHdG was quantified by DNA Damage ELISA. Multiple Reaction Monitoring (MRM) by Ultra-Performance Liquid Chromatogram coupled with tandem Mass Spectrometer (UPLC-MS/MS) was used to quantify cGAMP in peripheral blood mononuclear cells (PBMC). Disease activity was assessed using SLEDAI and defined as either low (SLEDAI< =2) or high (SLEDAI >4). Statistical analyses were performed with GraphPad Prism 7.

Results: Levels of mtDNA were increased in plasma from SLE patients compared to HC (p< 0.01), and related to disease activity, e.g. patients with active disease (n=8) had higher levels of mtDNA in plasma as compared to patients with low disease activity (n=8, p< 0.05) as well as HC (p< 0.0001). Further, SLE patients with active disease had higher concentrations of 8-OHdG DNA as compared to patients with low disease activity (p< 0.05) as well as HC (p< 0.05). Patients that were cGAMP+ (n=5) had higher concentrations of 8-OHdG compared to cGAMP- (n=11) SLE patients although the difference was not statistically significant. Of interest, the levels of mtDNA was significantly higher in cGAMP+ patients compared to HC (p< 0.01) whereas the levels of mtDNA in cGAMP- patients was not statistically different from HC (p=0.1).

Conclusion: SLE patients, in particular those with active disease, have elevated levels of mtDNA and 8-OHdG DNA in peripheral blood. The increased levels of mtDNA in cGAMP positive SLE plasma is consistent with a role of mtDNA in activation of the cGAS pathway in SLE patients although other ligands cannot be excluded.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/mitochondrial-dna-a-potential-trigger-of-cyclic-gmp-amp-synthase-activation-in-systemic-lupus-erythematosus/