Identification of ISG20 as the nuclease involved in interferon-induced decline of hepatitis B virus cccDNA

Hepatitis B virus (HBV) persists by depositing a covalently closed circular DNA (cccDNA) in the nucleus that cannot be targeted by available antivirals. Cytokine treatments can purge cccDNA from the nucleus of infected cells by APOBEC3-mediated deamination. cccDNA levels in HBV-infected cells were not reduced by overexpression of APOBEC3A alone, but only after simultaneous addition of interferon. This indicated the need of an additional, interferon-induced factor (ISG). Microarray analysis identified ISG20 as the only nuclease located to the nucleus induced by type I and II interferons. ISG20 was confirmed to be expressed in acute, self-limiting but not in chronic hepatitis B in human livers. ISG20 knockdown abolished the interferon-induced loss of cccDNA, and co-expression of ISG20 and APOBEC3A was sufficient to diminish cccDNA without further treatment. To conclude, nucleolar ISG20 was identified as the nuclease contributing to interferon-induced purging of HBV cccDNA, opening new avenues for antiviral targeting. Transcriptional profiles of HBV infected HepaRG cells expressing A3A in presence of Interferon Alpha were compared with HepaRG cells expressing A3A without Interferon Alpha.