ADAR1 regulates dsRNA formation in nuclear and mitochondrial transcripts through editing-dependent and -independent mechanisms

Endogenous (self) double-stranded RNAs (dsRNAs) in human cells can activate innate immune responses. ADAR1, an A-to-I editing enzyme of dsRNAs, suppresses aberrant immune activation by self-dsRNAs. However, how ADAR1 influences the cellular dsRNA landscape remains unclear. We show that human ADAR1 downregulates self-dsRNA abundance through editing-dependent and editing-independent mechanisms. We further conducted quantitative dsRNA sequencing on wild-type and ADAR1-deficient cells. dsRNAs are enriched in protein-coding mRNAs-especially those with repetitive elements and elongated 3' UTRs-and mitochondrial RNAs. ADAR1-regulated dsRNA transcripts consist of nuclear-encoded mRNAs and, unexpectedly, mitochondria-encoded RNAs rarely edited by ADAR1. Accordingly, dsRNAs accumulate to high levels within the mitochondria of ADAR1-deficient cells. Mass spectrometry and biochemical assays can detect ADAR1p150 in mitochondrial fractions. Notably, ADAR1 loss sensitizes cells to inflammation under mitochondrial stress (e.g., herniation and X-ray irradiation). Hence, we show that dsRNAs regulated by ADAR1 go beyond A-to-I edited transcripts and that ADAR1 can control mitochondrial dsRNAs.