ABSTRACT
Hepatitis B virus (HBV) transcription and replication increase progressively throughout postnatal liver development with maximal viral biosynthesis occurring at around 4 weeks of age in the HBV transgenic mouse model of chronic infection. Increasing viral biosynthesis is associated with a corresponding progressive loss of DNA methylation. The loss of DNA methylation is associated with increasing levels of 5-hydroxymethylcytosine (5hmC) residues which correlate with increased liver-enriched pioneer transcription factor Forkhead box protein A (FoxA) RNA levels, a rapid decline in postnatal liver DNA methyltransferase (Dnmt) transcripts, and a very modest reduction in ten-eleven translocation (Tet) methylcytosine dioxygenase expression. These observations are consistent with the suggestion that the balance between active HBV DNA methylation and demethylation is regulated by FoxA recruitment of Tet in the presence of declining Dnmt activity. These changes lead to demethylation of the viral genome during hepatocyte maturation with associated increases in viral biosynthesis. Consequently, manipulation of the relative activities of these two counterbalancing processes might permit the specific silencing of HBV gene expression with the loss of viral biosynthesis and the resolution of chronic HBV infections.IMPORTANCE HBV biosynthesis begins at birth and increases during early postnatal liver development in the HBV transgenic mouse model of chronic infection. The levels of viral RNA and DNA synthesis correlate with pioneer transcription factor FoxA transcript plus Tet methylcytosine dioxygenase-generated 5hmC abundance but inversely with Dnmt transcript levels and HBV DNA methylation. Together, these findings suggest that HBV DNA methylation during neonatal liver development is actively modulated by the relative contributions of FoxA-recruited Tet-mediated DNA demethylation and Dnmt-mediated DNA methylation activities. This mode of gene regulation, mediated by the loss of DNA methylation at hepatocyte-specific viral and cellular promoters, likely contributes to hepatocyte maturation during liver development in addition to the postnatal activation of HBV transcription and replication.
