Interferon-induced degradation of the hepatitis B virus persistence form, cccDNA, depends on ISG20

Hepatitis B virus (HBV) persists by depositing a covalently closed circular DNA (cccDNA) in the nucleus of infected cells that cannot be targeted by available antivirals. Cytokine treatments can diminish HBV cccDNA via APOBEC3-mediated deamination. Here we show that overexpression of APOBEC3A alone, however, was not sufficient to reduce cccDNA in HBV-infected cells. This required addition of interferon indicating that cccDNA degradation requires an additional, interferon-stimulated gene (ISG). Transcriptome analyses identified ISG20 as the only type I and II interferon-induced, nucleus-resident protein with annotated nuclease activity. ISG20 expression was detected in human livers in acute, self-limiting but not in chronic hepatitis B. ISG20 depletion abolished the interferon-induced loss of cccDNA, and co-expression of ISG20 and APOBEC3A was sufficient to diminish cccDNA. In conclusion, non-cytolytic HBV cccDNA decline requires induction of a deaminase and nuclease. Our findings highlight that ISGs cooperate for their antiviral function and this cooperativity may be explored for therapeutic targeting.