Disruption of mitochondrial folate metabolism leads to mitochondrial DNA leakage and activation of apoptotic or inflammatory pathways

Folate-mediated one-carbon metabolism (FOCM) is required for the biosynthesis of purines, thymidylate (dTMP), and methionine. Maintenance of adequate cellular dTMP levels is essential to preserve the integrity of the nuclear and mitochondrial genomes. Inadequate dTMP production leads to uracil misincorporation into mitochondrial DNA (mtDNA) and impaired mitochondrial function. However, the mechanisms whereby uracil in mtDNA impairs mitochondrial function are uncharacterized. The release of mtDNA into the cytosol acts as a damage-associated molecular pattern (DAMP), resulting in inflammation and cell death. Previously, we reported that disrupting mitochondrial dTMP synthesis through serine hydroxymethyltransferase 2 (Shmt2) heterozygosity increases uracil in mtDNA and impairs mitochondrial function in mice. This study aimed to investigate whether impairment of mitochondrial FOCM through Shmt2 disruption leads to the release of mtDNA into the cytosol. Shmt2+/- MEF cells showed a > 2-fold increase in cytosolic mtDNA leakage compared to Shmt2+/+ cells (p<0.05). There was no significant difference in total mtDNA content between Shmt2+/+ and Shmt2+/- MEF cells. MEFs with decreased Shmt2 expression activated apoptosis by the ratio of cleaved caspase-3 to caspase-3. In addition, SHMT2 knock-out (SHMT2 KO) haploid chronic myeloid leukemia (HAP1) cells also exhibited increased cytosolic mtDNA content compared to the wild-type (WT) HAP1 cells. HAP1 lacking Shmt2 expression activated the cGAS/STING pathway, but suppressed apoptosis, compared to WT HAP1 cells. This study demonstrates that decreased Shmt2 expression leads to cytosolic mtDNA leakage and that the downstream effects of mtDNA leakage vary by cell type.