Increased RNA Editing and Inhibition of Hepatitis Delta Virus Replication by High-Level Expression of ADAR1 and ADAR2

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Abstract
Hepatitis delta virus (HDV) is a subviral human pathogen that uses specific RNA editing activity of the host to produce two essential forms of the sole viral protein, hepatitis delta antigen (HDAg). Editing at the amber/W site of HDV antigenomic RNA leads to the production of the longer form (HDAg-L), which is required for RNA packaging but which is a potent trans-dominant inhibitor of HDV RNA replication. Editing in infected cells is thought to be catalyzed by one or more of the cellular enzymes known as adenosine deaminases that act on RNA (ADARs). We examined the effects of increased ADAR1 and ADAR2 expression on HDV RNA editing and replication in transfected Huh7 cells. We found that both ADARs dramatically increased RNA editing, which was correlated with strong inhibition of HDV RNA replication. While increased HDAg-L production was the primary mechanism of inhibition, we observed at least two additional means by which ADARs can suppress HDV replication. High-level expression of both ADAR1 and ADAR2 led to extensive hyperediting at non-amber/W sites and subsequent production of HDAg variants that acted as trans-dominant inhibitors of HDV RNA replication. Moreover, we also observed weak inhibition of HDV RNA replication by mutated forms of ADARs defective for deaminase activity. Our results indicate that HDV requires highly regulated and selective editing and that the level of ADAR expression can play an important role: overexpression of ADARs inhibits HDV RNA replication and compromises virus viability.