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

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.