Comment on “Specific and nonhepatotoxic degradation of nuclear hepatitis B virus cccDNA”

Abstract

) report that the hepatitis B virus (HBV) transcriptional template, a long-lived covalently closed circular DNA (cccDNA) molecule, is degraded noncytolytically by agents that up-regulate APOBEC3A and 3B. If these results can be independently confirmed, they would represent a critical first step toward development of a cure for the 400 million patients who are chronically infected by HBV. V iral clearance during acute hepatitis B virus (HBV) infection is mediated by CD8-positive cytotoxic T lymphocytes (CTL) that kill infected cells and secrete antiviral cytokines that noncytolytically inhibit HBV gene expression and replication. In contrast, the CTLinduced mechanisms that mediate clearance of the nuclear pool of viral covalently closed circular DNA (cccDNA), the long-lived transcriptional template of HBV, have been difficult to resolve, largely because of the absence of a cell-based HBV-infection system. Based on in vivo experiments in duck, woodchuck, mouse, and chimpanzee animal models, two competing, but not mutually exclusive, hypotheses of cccDNA clearance have emerged.On one hand, there is ample evidence that cccDNA elimination requires the turnover of infected hepatocytes in ducks infected with duck hepatitis B virus (DHBV) and in woodchucks infected with woodchuck hepatitis virus (WHV), in which a large fraction of the infected hepatocyte population is killed and replaced by hepatocyte cell division (1). On the other hand, CTL-induced, interferon gamma (IFN-g)-mediated clearance of HBV replicative DNA intermediates is well documented in HBV transgenic mice (2), and clearance of HBV in acutely infected chimpanzees is initiated by the influx of IFN-g-producing CD8 T cells and characterized by a disproportionate loss of both cytoplasmic HBV replicative DNA intermediates and nuclear cccDNA relative to multiple simultaneous markers of cell death (3, 4).These latter results suggest that cccDNA may be susceptible to noncytolytic clearance by antiviral cytokines; that is, clearance not requiring the death or proliferation of infected cells. Confirmation of noncytolytic cccDNA clearance in experimentally tractable systems would be a major step forward in finding antiviral therapies for chronic hepatitis B. We were therefore very encouraged by the paper by Lucifora et al. (5) reporting that high-dose interferon alpha (IFN-a) and lymphotoxin b receptor (LTbR)-mediated stimulation of APOBEC3A or 3B expression noncytopathically reduces cccDNA levels in differentiated HepaRG cells and in primary human hepatocytes. We were also encouraged by evidence suggesting that this loss might include, as an intermediate step, a detectable level of cccDNA depurination. However, after a careful reading of this Report, which entails a wealth of technically difficult experiments, we believe several critical points warrant further investigation before these conclusions can be exploited.

HBV cccDNA: viral persistence reservoir and key obstacle for a cure of chronic hepatitis B

“…While various steps of the replication cycle might be affected,132 a recent study137 suggested that very-high-dose interferon-α, or more potently activation of the lymphotoxin-β receptor, could directly target cccDNA integrity via APOBEC3A and 3B-mediated deamination of the (-)-strand and subsequent degradation. Though some aspects are controversial,138 139 the worthiness of activating innate responses is underlined by promising preclinical results with the Toll-like receptor 7 agonist GS-9620 140…”

APOBEC3B lysine residues are dispensable for DNA cytosine deamination, HIV-1 restriction, and nuclear localization

“…Moreover, both of these small DNA viruses specifically upregulate A3B at the transcriptional level (Mori et al, 2017; Starrett et al, 2017; Verhalen et al, 2016; Vieira et al, 2014). Second, A3B knockdown in liver-derived HepG2 cells results in elevated levels of HBV circular DNA replication intermediates suggesting A3B may have a role in suppressing virus replication (Lucifora et al, 2014), however this result has been debated (Chisari et al, 2014; Ding and Robek, 2014; Meier et al, 2017; Shlomai and Rice, 2014; Xia et al, 2014). Disagreement also exists regarding HBV infection rates in individuals lacking the entire A3B gene due to a common deletion allele [higher: (Prasetyo et al, 2015); unchanged: (Ezzikouri et al, 2013)].…”

Cytidine deamination and cccDNA degradation: A new approach for curing HBV?

“…The authors also proposed the use of LTbR agonists as a potential anti-HBV therapy, likely in combination with other antiviral strategies. While the results are intriguing, some important technical concerns have been raised, 14,15 and there are other essential questions that remain to be addressed. First, as multiple APOBEC3 family members have now been shown to target a number of different HBV DNA replication forms, it will be important to understand the specificity and nature of these interactions.…”