Activation of the cGAS-STING innate immune response in cells with deficient mitochondrial topoisomerase TOP1MT

Abstract

Systemic lupus erythematosus (SLE) is an autoimmune condition where the underlying dysfunction is often unknown. Here, we identify a novel link between the mitochondrial topoisomerase TOP1MT and SLE, as a novel variant predicted to affect TOP1MT function, P193L, was identified in a family with SLE and other autoimmune diseases. Although there was no previous genetic association between TOP1MT and SLE, the role of TOP1MT as a regulator of mtDNA led us to investigate whether TOP1MT could mediate the release of mtDNA to the cytosol, where it could then activate the cGAS-STING innate immune pathway. This pathway increases the expression of type-I interferons and is known to be activated in some patients with SLE. Through analysis of cells lacking TOP1MT, or re-expressing the P193L variant, we established a new association by which TOP1MT dysfunction leads to increased release of mtDNA to the cytosol, causing activation of the cGAS-STING pathway. We also characterized the P193L variant for its ability to rescue several TOP1MT functions in TOP1MT knockout cells. While re-expression of the P193L variant was able to rescue steady state mtDNA copy number, most functions investigated only showed partial rescue, including repletion of mtDNA replication following depletion, nucleoid size, steady state mtDNA transcripts levels, and mitochondrial morphology. Meanwhile, the P193L variant failed to rescue decreased mitochondrial respiration. Overall, these findings support the notion that P193L variant is not fully functional and likely influences susceptibility to SLE via cGAS-STING mediated activation of the innate immune system.

Mitochondrial DNA Release in Innate Immune Signaling

“…The TFAM paradigm suggested that other forms of mtDNA stress might also lead to mtDNA-release-mediated cGAS-STING signaling, which has turned out to be the case. Cells either lacking TOP1MT (EC 5.6.2.1) or expressing a pathogenic variant associated with lupus exhibit mtDNA release and activation of the cGAS-STING pathway (73). TOP1MT knockout cells have reduced mtDNA copy number and mitochondrial transcripts, defects in nucleoid organization, elongated mitochondria, and lower OXPHOS.…”

Circadian rhythm in systemic autoimmune conditions: Potential of chrono-immunology in clinical practice: A narrative review

“…This, in turn, activates the cGAS-STING innate immune signaling pathway, which is known to be active in SLE and other autoimmune diseases. [49,60]…”

“…This, in turn, activates the cGAS-STING innate immune signaling pathway, which is known to be active in SLE and other autoimmune diseases. [49,60] SLE-associated dysregulated pathways have been linked to circadian abnormalities. Patients with SLE or those at risk of developing it often experience sleep disruptions, which significantly impact their quality of life.…”

The MFN2 Q367H variant from a patient with late-onset distal myopathy reveals a novel pathomechanism connected to mtDNA-mediated inflammation

“…All imaging was performed as previously described (21). The Olympus Spinning Disk Confocal System (Olympus SD OSR) was used with the UAPON 100XOTIRF/1.49 oil objective lens.…”

“…Beyond its role of encoding OXPHOS proteins, mtDNA is now also recognized to play an important role as an inflammatory signalling molecule when released from mitochondria. Examples of inflammatory signalling pathways that detect mtDNA-mediated include TLR9 (4,18), cGAS-STING (21,22) and the inflammasome (23,24). Notably, a recent study showed that inducible muscle-specific loss of MFN1 causes mtDNA release via early endosomes, leading to activation of TLR9/NF-kB inflammation and myopathy (4).…”