Ribonucleotide incorporation into mtDNA

Metabolic dysregulation can lead to inflammatory responses. Imbalanced nucleotide synthesis triggers the release of mitochondrial DNA (mtDNA) to the cytosol and an innate immune response through cGAS-STING signaling. However, how nucleotide deficiency drives mtDNA-dependent inflammation has not been elucidated. Here, we show that nucleotide imbalance leads to an increased misincorporation of ribonucleotides into mtDNA during age-dependent renal inflammation in a mouse model lacking the mitochondrial exonuclease MGME1 and in cells lacking the mitochondrial i-AAA protease YME1L. Similarly, reduced deoxyribonucleotide synthesis increases the ribonucleotide content of mtDNA in cell cycle-arrested senescent cells. This leads to mtDNA release into the cytosol, cGAS-STING activation and the mtDNA-dependent senescence-associated secretory phenotype (SASP), which can be suppressed by exogenously added deoxyribonucleotides. Our results explain age- and mtDNA-dependent inflammatory responses and SASP in senescent cells by imbalanced nucleotide metabolism and highlight the sensitivity of mtDNA to aberrant ribonucleotide incorporation.