Double-strand breaks in mitochondrial DNA upregulated IFN signaling via cytosolic RNA sensing.
Major Finding: Double-strand breaks in mitochondrial DNA upregulated IFN signaling via cytosolic RNA sensing.
Concept: Mitochondrial DNA damage led to mitochondrial membrane herniation and release of mitochondrial RNA.
Impact: This work provides insight into the effects of mitochondrial DNA–damaging agents such as chemotherapy.
Exposure to ionizing radiation and treatment with some chemotherapeutic agents can cause double-strand breaks (DSB) in mitochondrial DNA (mtDNA). To investigate the impact of mtDNA DSBs, Tigano and colleagues employed mtDNA-targeted transcription activator–like effector nucleases (TALEN), which are restriction enzymes engineered to cut DNA at specific sequences. In vitro, RNA-sequencing and RT-qPCR analyses showed that expression of the mtDNA-targeting TALENs caused changes in expression of nuclear genes, including several important IFN-stimulated genes. This was associated with a time-dependent increase in IFN-mediated phosphorylation of protein STAT1 at Y701, a modification required for STAT1 activation and consequent transcriptional induction of IFN-stimulated genes. This increase in phosphorylated STAT1 was not seen in cells lacking mtDNA, supporting the notion that the TALEN-induced mtDNA DSBs were the cause of the observed STAT1 activation in cells with normal mtDNA. Notably, the mtDNA DSBs did not appear to cause significant mitochondrial dysfunction; however, immunofluorescence microscopy analyses revealed herniation of the inner mitochondrial membrane triggered by the formation of BAX–BAK macropores, a process previously shown to expose mtDNA and mitochondrial RNA (mtRNA) to sensors in the cytoplasm. Further investigation showed that it was specifically the cytoplasmic mtRNA (and not mtDNA) that caused the immune response to mtDNA damage. The immunogenic effects of cytoplasmic mtRNA in cells with mtDNA damage was mediated by the RIG-I–MAVS pathway, in which the pattern recognition receptor RIG-I detects cytoplasmic RNA and then binds the signaling protein MAVS to activate a type I IFN response. Importantly, mtDNA damage was shown to underlie the innate immune response caused by exposure to ionizing radiation, which also exerted its effects via RIG-I–mediated mtRNA sensing. In summary, this work provides a detailed characterization of the immune effects of mtDNA DSBs in cells, offering insight into this effect of chemotherapy and irradiation.
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