Dynamics of Hepatitis B Virus Covalently Closed Circular DNA: A Mini-Review

Abstract

Eradication of cccDNA is an ideal goal of chronic hepatitis B (CHB) therapy. Understanding the changes in the cccDNA pool during therapy provides a basis for developing CHB treatment strategies. On the other hand, the shift in the balance of the cccDNA pool following therapies allowed researchers to investigate the dynamics of cccDNA. Central to the description of cccDNA dynamics is a parameter called cccDNA half-life. CccDNA half-life is not an intrinsic property of cccDNA molecules, but a description of an observed phenomenon characterized by cccDNA pool decline. Since cccDNA has to be in the nuclei of host cells to function, the half-life of cccDNA is determined by the state and destiny of the host cells. The major factors that drive cccDNA decay include noncytopathic effects and hepatocyte turnover (death and division). In some cases, the determining factor is not the half-life of cccDNA itself, but rather the half-life of the hepatocyte. The main purpose of this review is to analyze the major factors affecting cccDNA half-life and determine the areas requiring further study. In addition, the discrepancy in cccDNA half-life between short-term and long-term nucleot(s)ide analog (NUC) therapy was reported. Hypotheses were proposed to explain the multi-phasic decline of cccDNA during NUC therapy, and a framework based on cccDNA dynamics was suggested for the consideration of various anti-HBV strategies.

Keywords: cccDNA; half-life; hepatitis B virus; hepatocyte; nucleot(s)ide analogues; turnover.

Figure 2

Monitoring the lifespan and proliferation of hepatocytes. (A) H³-thymidine was injected into rat and mouse models to label hepatocytes and monitor the changes in the labeled cells [42,43]. (B) Ki67 expression induced a constitutive expression of GFP which was used to monitor the fate of liver cells. (C) From week 2 to week 12, the number of hepatocytes with Ki67-expression-induced GFP expression increased to more than 50%, which represent newly produced hepatocytes. The figure is a modification of a figure from reference [45]. (D) Structure of the liver lobule. Hepatocytes in the liver lobule are organized into three zones and those in zones 2 and 1 have higher proliferation rates than those in zone 3 during homeostasis.

Figure 1

Replication cycle of HBV. The viruses infect hepatocytes by interacting with the receptor sodium taurocholate cotransporting polypeptide (NTCP). Relaxed circular DNA (rcDNA) is released into the nuclei after nucleocapsid uncoating. With the help of the host factors, rcDNA is converted into cccDNA, which serves as the template of viral RNA transcription. The productive nucleocapsid containing progeny rcDNA either secretes through the multivesicular body (MVB) pathway or recycles to the nuclei.

Figure 3

cccDNA decline is responsible for the second phase of serum viral load decrease during NUC therapy. (A) Serum HBV has a half-life of one day [17], and new viruses derived from the transcription of cccDNA replenish serum viral load during homeostasis prior to treatment. NUC treatment alters the balances in serum viral load and cccDNA pool size by inhibiting the production of new viruses [58]. (B) The serum HBV decay rate (amount of viruses that decay each day) is determined by the current viral load, and the production rate of new viruses is determined by the cccDNA pool size. Both rates decrease over the course of NUC treatment because the total viral load and the cccDNA pool size both decline. However, the decay rate declines faster than the production rate since serum HBV has a much shorter half-life than cccDNA (or infected hepatocyte). At the transition point, the viral decay rate equals the production rate. Subsequently, the production rate (or cccDNA) shows a faster decline than the serum viral load.

Figure 4

Explanations for the multi-phasic decline of cccDNA during long-term NUC therapy. (A) cccDNA declined in a multi-phasic model during long-term NUC therapy ([70], Table 1 and Table 2). (B) NUC shifts the balance of the cccDNA pool by reducing replenishment. (C) There are several hypotheses for the multi-phasic decline of cccDNA during long-term NUC therapy.

Figure 5

A framework for considering various anti-HBV strategies. Effective therapies shift the balance of the cccDNA pool by either blocking replenishment, accelerating cccDNA decay or both. NUCs, CpAMs, entry inhibitors, siRNA/ASO/LNA and PEG-IFNα all can inhibit cccDNA replenishment indirectly by suppressing steps upstream of cccDNA synthesis. SiRNA/ASO/LNA, NAPs, PEG-IFNα and immune modulators affect cccDNA decay directly or indirectly by facilitating immune restoration.