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Review
. 2021 Apr 14;27(14):1369-1391.
doi: 10.3748/wjg.v27.i14.1369.

Intracellular interferon signalling pathways as potential regulators of covalently closed circular DNA in the treatment of chronic hepatitis B

Affiliations
Review

Intracellular interferon signalling pathways as potential regulators of covalently closed circular DNA in the treatment of chronic hepatitis B

Zhi Yi Goh et al. World J Gastroenterol. .

Abstract

Infection with the hepatitis B virus (HBV) is still a major global health threat as 250 million people worldwide continue to be chronically infected with the virus. While patients may be treated with nucleoside/nucleotide analogues, this only suppresses HBV titre to sub-detection levels without eliminating the persistent HBV covalently closed circular DNA (cccDNA) genome. As a result, HBV infection cannot be cured, and the virus reactivates when conditions are favorable. Interferons (IFNs) are cytokines known to induce powerful antiviral mechanisms that clear viruses from infected cells. They have been shown to induce cccDNA clearance, but their use in the treatment of HBV infection is limited as HBV-targeting immune cells are exhausted and HBV has evolved multiple mechanisms to evade and suppress IFN signalling. Thus, to fully utilize IFN-mediated intracellular mechanisms to effectively eliminate HBV, instead of direct IFN administration, novel strategies to sustain IFN-mediated anti-cccDNA and antiviral mechanisms need to be developed. This review will consolidate what is known about how IFNs act to achieve its intracellular antiviral effects and highlight the critical interferon-stimulated gene targets and effector mechanisms with potent anti-cccDNA functions. These include cccDNA degradation by APOBECs and cccDNA silencing and transcription repression by epigenetic modifications. In addition, the mechanisms that HBV employs to disrupt IFN signalling will be discussed. Drugs that have been developed or are in the pipeline for components of the IFN signalling pathway and HBV targets that detract IFN signalling mechanisms will also be identified and discussed for utility in the treatment of HBV infections. Together, these will provide useful insights into design strategies that specifically target cccDNA for the eradication of HBV.

Keywords: APOBECs; Covalently closed circular DNA; Epigenetic modification; Hepatitis B virus therapeutics; Interferons.

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Conflict of interest statement

Conflict-of-interest statement: Authors declare no conflicts-of-interest for this article.

Figures

Figure 1
Figure 1
Activation of antiviral interferon-stimulated genes by different interferon subtypes. Different subtypes of interferons (IFNs) bind to their cognate receptors to trigger IFN signalling pathways. The activate janus kinase/signal transducers and activators of transcription-dependent signalling results in the formation of transcription complexes that induce the expression several IFN-stimulated response elements-dependent and gamma activated site-dependent antiviral IFN-stimulated genes (ISGs), which target covalently closed circular DNA (cccDNA) stability and function through a variety of mechanisms. The potency of IFN sub-type for cccDNA degradation or silencing is dependent on the types and number of ISGs induced. IFN: Interferon; TYK2: Tyrosine kinase 2; SOCS: Suppressor of cytokine signalling; PKC-δ: Protein kinase C-delta; IRF: Interferon regulatory factor; JAK: Activate janus kinase; STAT: Signal transducers and activators of transcription; ISGF: Interferon-stimulated gene factor; GAF: Gamma-activated factor; ISGs: Interferon-stimulated genes; ISRE: Interferon-stimulated response elements; GAS: Gamma activated site.
Figure 2
Figure 2
Interferon treatment silences covalently closed circular DNA transcription and recruits APOBECs to actively degrade covalently closed circular DNA. Covalently closed circular DNA (cccDNA) exists as an episomal mini chromosome that is epigenetically modified to support active transcription. Interferon (IFN)-α treatment results in the recruitment of histone modifying complexes that remove activating transcription post-translational modifications (PTMs), and add repressive PTMs that silence cccDNA function. These complexes can be recruited by IFN-stimulated genes (ISGs) such as IFI16. ISGs may also bind directly to cccDNA to repress transcription. Other ISGs such as APOBECs induced by IFN signalling are also recruited by HBc, generating AP sites which lead to cccDNA degradation. HBV: Hepatitis B virus; IFN: Interferon; IRF: Interferon regulatory factor; JAK: Activate janus kinase; STAT: Signal transducers and activators of transcription; ISGs: Interferon-stimulated genes; ISRE: Interferon-stimulated response elements; cccDNA: Covalently closed circular DNA.
Figure 3
Figure 3
Anti-covalently closed circular DNA strategies and antagonistic hepatitis B virus proteins that modulate the interferon signalling pathway. Hepatitis B virus (HBV) has developed multiple mechanisms that target different parts of the intracellular interferon (IFN) signalling pathway to avert elimination by IFN treatment. Therapeutic agents that counter HBV anti-IFN signalling activities or enhance the strength of host cell IFN signalling may be beneficial to increase the efficiency of IFN treatment for the elimination or silencing of covalently closed circular DNA. HBV: Hepatitis B virus; IFN: Interferon; PEG: Pegylated form; SOCS: Suppressor of cytokine signalling; PKC-δ: Protein kinase C-delta; IRF: Interferon regulatory factor; JAK: Activate janus kinase; STAT: Signal transducers and activators of transcription; Pol/RT: Polymerase/reverse transcriptase; TNF: Tumour necrosis factor; ISGs: Interferon-stimulated genes; ISRE: Interferon-stimulated response elements; cccDNA: Covalently closed circular DNA.

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