The Role of Plasma Cell-Free Mitochondrial DNA and Nuclear DNA in Systemic Lupus Erythematosus

Background: The roles of plasma cell-free (pcf) mitochondrial DNA (mtDNApcf) and nuclear DNA (nDNApcf) in the pathogenesis of systemic lupus erythematosus (SLE) remain unclear. We analyzed the relative copies of mtDNApcf and nDNApcf and investigated their association with the levels of plasma 8-hydroxy-2’-deoxyguanosine (8-OHdG), plasma malondialdehyde (MDA) and mRNA of leukocyte C-type lectin domain family 5 member A (CLEC5A) in SLE patients. Methods: A total of 80 SLE patients and 43 healthy controls (HCs) were enrolled. Their plasma samples were subjected to the measurements of mtDNApcf copies, nDNApcf copies, 8-OHdG and MDA, respectively. Their leukocytes were analyzed for CLEC5A mRNA expression. Results: SLE patients had higher nDNApcf copies (2.84 ± 1.99 vs. 2.00 ± 0.88, p = 0.002), lower mtDNApcf copies (4.81 ± 6.33 vs. 9.83 ± 14.20, p = 0.032), higher plasma 8-OHdG (0.227 ± 0.085 vs. 0.199 ± 0.041 ng/mL, p = 0.016), lower plasma MDA (3.02 ± 2.20 vs. 4.37 ± 2.16 μM, p = 0.001) and similar leukocyte CLEC5A mRNA expression levels (1.21 ± 1.17 vs. 1.26 ± 1.05, p = 0.870), as compared with those of HCs. Among the HCs, SLE patients with SLE Disease Activity Index (SLEDAI) ≤8, and SLE patients with SLEDAI >8, their respective mtDNApcf copies decreased stepwisely (9.83 ± 14.20 vs. 6.28 ± 7.91 vs. 3.19 ± 3.35, p = 0.054). The nDNApcf copies of HCs, SLE patients without nephritis, and SLE patients with nephritis were increased stepwisely (2.00 ± 0.88 vs. 2.63 ± 1.74 vs. 3.16 ± 2.34, p = 0.043). Among SLE patients, higher nDNApcf copies were associated with higher levels of plasma 8-OHdG (p < 0.001) but lower plasma MDA (p = 0.019). Among HCs but not SLE patients, higher nDNApcf copies (p = 0.013) or lower mtDNApcf copies (p < 0.001) were related to higher levels of leukocyte CLEC5A mRNA expression. Conclusions: Higher nDNApcf, lower mtDNApcf, increased ROS-elicited oxidative DNA damage and dysregulated leukocyte CLEC5A expression might be implicated in the pathogenesis of SLE.