Candidate therapeutic strategies to target HBV covalently closed circular (ccc)DNA. The circles represent P protein-linked and P protein-free relaxed circular (RC)-DNA and cccDNA in the host cell nucleus. (A) Prevention of cccDNA accumulation by blocking host factors involved in the multiple steps of RC-DNA to cccDNA conversion. (B) Prevention of nuclear import of RC-DNA by capsid-targeting drugs; cytoplasmic release of RC-DNA may, in addition, trigger DNA sensors like cyclic GMP-AMP synthase (cGAS) and activate STING to induce antiviral cytokines. (C) Silencing of cccDNA transcriptional activity by inducing the host cell’s epigenetic machinery, or by blocking the de-silencing activity of HBV X protein (HBx). (D) Degradation of existing cccDNA by immune-mediated mechanisms, perhaps via APOBEC enzymes, or (E) by direct targeting with designer nucleases as used in genome editing. See text for details.
HBV cccDNA: viral persistence reservoir and key obstacle for a cure of chronic hepatitis B
At least 250 million people worldwide are chronically infected with HBV1 and at a greatly increased risk to develop liver fibrosis, cirrhosis and hepatocellular carcinoma, causing an estimated 650 000 deaths per year.2 While an efficient prophylactic vaccine is available,3 current treatments for chronic hepatitis B are limited to type 1 interferons and five approved nucleos(t)ide analogues (NAs), which target the viral polymerase, P protein, a multifunctional reverse transcriptase (see below). Due to severe side effects, only a fraction of patients are eligible for interferon therapy, and <10% of them show a sustained virological response, measured as loss of hepatitis B surface antigen (HBsAg; see below).4 NAs are much better tolerated, and the most potent drugs, entecavir and tenofovir, can reduce viraemia by 5–6 logs, often below detection limit, and with low rates of viral resistance development.5 However, HBsAg clearance is very rare (0–5%) even after prolonged treatment,4 and the frequent viral rebound upon therapy withdrawal indicates a need for lifelong treatment.6 Reactivation can even occur, upon immunosuppression, in patients who resolved an acute HBV infection decades ago,7 indicating that the virus can be immunologically controlled but is not eliminated.
The virological key to this persistence is an intracellular HBV replication intermediate, called covalently closed circular (ccc) DNA, which resides in the nucleus of infected cells as an episomal (ie, non-integrated) plasmid-like molecule that gives rise to progeny virus. A cure of chronic hepatitis B will therefore require elimination of cccDNA. However, despite >30 years of research, little is known about the molecular mechanisms of cccDNA formation and degradation, foremostly due to the lack of suitable experimental systems. Recent discoveries are about to change this situation, particularly the identification of a liver-resident bile acid transporter, sodium taurocholate cotransporting polypeptide (NTCP; also known as SLC10A1), as an entry receptor for HBV and hepatitis delta virus (HDV), which usurps HBV’s envelope to enter cells8 ,9 (box 1). Various aspects of this finding have recently been reviewed.10 ,11
A second key for HBV persistence is a flawed immune response, typically including the functional exhaustion and depletion of cytotoxic T cells, a lack of adequate CD4+ T cell help, and failure to mount neutralising antibodies. While immune restoration will likely be indispensable even if other ways are found to reduce cccDNA,12 for more information readers are referred to pertinent reviews.13–16 The focus here will be on a brief history on cccDNA research and its experimental difficulties, and on recent developments and how they may translate into new, curative treatments for chronic hepatitis B.
keywords: CHRONIC VIRAL HEPATITIS; DNA DAMAGE; HEPATITIS B; MOLECULAR MECHANISMS