N6-Methyladenosine Modification of Hepatitis B and C Viral RNAs Attenuates Host Innate Immunity via RIG-I Signaling.

Kim, Geon-Woo; Imam, Hasan; Khan, Mohsin; Siddiqui, Aleem

N6 methyladenosine (m6A), the methylation of the adenosine base at the nitrogen-6 position, is the most common epitranscriptomic modification of mRNA that affects a wide variety of biological functions. We have previously reported that hepatitis B viral RNAs are m6A modified, displaying a dual functional role in the viral life cycle. Here, we show that cellular m6A machinery regulates host innate immunity against hepatitis B and C viral (HBV/HCV) infections by inducing m6A modification of viral transcripts. The depletion of the m6A writer enzymes (METTL3 and METTL14) leads to an increase in viral RNA recognition by retinoic acid-inducible gene I (RIG-I), thereby stimulating type-I interferon production. This is reversed in cells, in which m6A METTL3 and METTL14 are overexpressed. The m6A modification of viral RNAs renders RIG-I signaling less effective, and while single nucleotide mutation of m6A consensus motif of viral RNAs, enhances RIG-I sensing activity. Importantly, m6A reader proteins (YTHDF2 and YTHDF3) inhibit RIG-I transduced signaling activated by viral RNAs by occupying m6A modified RNAs and inhibiting RIG-I recognition. Collectively, our results provide new insights into the mechanism of immune evasion via m6A modification of viral RNAs.