mtDNA breaks compromise mitochondrial membrane ultrastructure and trigger an integrated stress response

Mitochondrial DNA (mtDNA) breaks are deleterious lesions that lead to degradation of mitochondrial genomes and subsequent reduction in mtDNA copy number. The signaling pathways activated in response to mtDNA damage remain ill-defined. Using mitochondrial targeted restriction enzymes, we show that cells with mtDNA breaks exhibit reduced respiratory complexes, loss of membrane potential, and mitochondrial protein import defect. Furthermore, mtDNA damage activates the integrated stress response (ISR) through phosphorylation of eIF2a by the OMA1-DELE1-HRI pathway. Electron microscopy reveals concomitant defects in mitochondrial membranes and cristae ultrastructure. Notably, inhibition of the ISR exacerbates mitochondrial defects and delays the recovery of mtDNA copy number, thereby implicating this stress response in mitigating mitochondrial dysfunction following mtDNA damage. Last, we provide evidence suggesting that ATAD3A, a membrane-anchored protein that interacts with nucleoids, relays the signal from mtDNA breaks to the inner mitochondrial membrane. Altogether, our study fully delineates the sequence of events linking damaged mitochondrial genomes with the cytoplasm and uncovers an unanticipated role for the ISR in response to mitochondrial genome instability. Overall design: We performed RNA-seq analysis on GFP11 tagged RRBP1 and MTA2 cells subjected to the following treatments: ApaLI* + Dox, ApaLI + Dox, ApaLI + Dox GFP- population, ApaLI + Dox GFP+ population. RNA-seq was done in 2 replicates.