RNA 5-methylcytosine marks mitochondrial double-stranded RNAs for degradation and cytosolic release

Mitochondria are essential regulators of innate immunity. They generate long double-stranded RNAs (mt-dsRNAs) and release them to the cytosol to trigger immune response under pathological stress conditions. Yet, the regulation of these self-immunogenic RNAs remains largely unknown. Here, we employ CRISPR screening on RNA-binding proteins residing in mitochondria and identify NOP2/Sun RNA methyltransferase 4 (NSUN4) as a key regulator of mt-dsRNA expression. We find that NSUN4 induces 5-methylcytosine (m5C) modification on mitochondrial RNAs, especially on the termini of light-strand long noncoding RNAs. These m5C-modified RNAs are recognized by complement C1q binding protein (C1QBP), which recruits polyribonucleotide nucleotidyltransferase to facilitate RNA turnover. Suppression of NSUN4 or C1QBP results in increased mt-dsRNA expression while C1QBP deficiency also leads to increased cytosolic mt-dsRNAs and subsequent immune activation. Collectively, our study unveils the mechanism underlying the selective degradation of light-strand mitochondrial RNAs and establishes a molecular mark for mitochondrial RNA decay and cytosolic release. RNA Bisulfite-seq following NSUN4, C1QBP or PNPT1 downregulation in HEK-293T cells.