Cell-free mtDNA drives pyroptosis and inflammation in m.3243A>G mitochondrial disease models

Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke-like episodes (MELAS) syndrome is primarily caused by the heteroplasmic m.3243A>G/MT-TL1 pathogenic variant. Patients exhibit elevated circulating cell-free mtDNA (cf-mtDNA) in plasma, which acts as a damage-associated molecular pattern. Using patient-derived fibroblasts and neuronal progenitors, as well as transmitochondrial cytoplasmic hybrids (cybrids), we show that mutant cells release higher levels of cf-mtDNA than wild-type controls, demonstrating that the m.3243A>G pathogenic variant drives mtDNA release. Mechanistically, increased mitochondrial oxidative stress promotes mtDNA oxidation and fragmentation, leading to Ca2+ overload and subsequent mtDNA extrusion. This, in turn, triggers inflammasome activation and pyroptosis, resulting in the secretion of pro-inflammatory cytokines and the activation of innate immune pathways. Pharmacological inhibition of the Mitochondrial Calcium Uniporter (MCU) or Voltage-Dependent Anion Channel (VDAC) reduced mtDNA release, confirming their involvement. Overall, our findings reveal a previously unrecognized mechanism in MELAS linking mitochondrial dysfunction to innate immune activation, with potential implications for therapeutic intervention.