The role of N6-methyladenosine modification in the life cycle and disease pathogenesis of hepatitis B and C viruses

Abstract

N6-methyladenosine (m⁶A) is the most prevalent modification of mammalian cellular RNAs. m⁶A methylation is linked to epigenetic regulation of several aspects of gene expression, including RNA stability, splicing, nuclear export, RNA folding, and translational activity. m⁶A modification is reversibly catalyzed by methyltransferases (m⁶A writers) and demethylases (m⁶A erasers), and the dynamics of m⁶A-modified RNA are regulated by m⁶A-binding proteins (m⁶A readers). Recently, several studies have shown that m⁶A methylation sites have been identified in hepatitis B virus (HBV) transcripts and the hepatitis C virus (HCV) RNA genome. Here, we review the role of m⁶A modification in HBV/HCV replication and its contribution to liver disease pathogenesis. A better understanding of the functions of m⁶A methylation in the life cycles of HBV and HCV is required to establish the role of these modifications in liver diseases associated with these viral infections.

Fig. 1. The roles of the cellular m⁶A machinery and m⁶A reader proteins in regulating cellular RNA and viral RNA.

m⁶A modification occurs in consensus DRACH motifs of cellular and viral RNAs. This modification is reversibly catalyzed by an m⁶A ‘writer’ or ‘eraser’. The m⁶A ‘writer’ (methyltransferase) complex is composed of METTL3, METTL14, and WTAP, and FTO or ALKBH5 is m⁶A ‘eraser’ (demethylase). The dynamics of m⁶A-modified RNAs are regulated by the m⁶A ‘reader’ proteins, including YTHDF1/2/3, YTHDC1/2, and IGF2BP1/2/3.

Fig. 2. The role of m⁶A modification in differentially regulating the HBV life cycle.

HBV transcripts are cotranscriptionally m⁶A-methylated at a consensus DRACH motif in the epsilon stem-loop region. HBV pgRNA contains two such motifs at the 5′ and 3′ termini owing to terminal redundancy, but other viral transcripts contain only one such motif, in the 3′ terminal sequence. m⁶A methylation of the 5′ terminus occurs in the area surrounding the priming site for reverse transcription initiation and induces reverse transcription of HBV DNA from pgRNA, whereas m⁶A at the 3′ terminus in all viral transcripts reduces RNA stability by interacting with YTHDF2.

Fig. 3. The role of m⁶A modification in regulating the HCV virion packaging.

The HCV genome is m⁶A-methylated in several regions (~19 regions), including the HCV E1 region. m⁶A methylations in the HCV E1 region decrease extracellular viral RNA and virion production via recognition by YTHDF proteins. YTHDF proteins sequester the m⁶A-methylated HCV genome to inhibit interaction with the HCV core protein in the lipid droplets.