Hepatitis B virus cccDNA: Formation, regulation and therapeutic potential

Abstract

Hepatitis B virus (HBV) infection remains a major public health concern worldwide with about 257 million individuals chronically infected. Current therapies can effectively control HBV replication and slow down disease progress, but cannot cure HBV infection. Upon infection, HBV establishes a pool of covalently closed circular DNA (cccDNA) in the nucleus of infected hepatocytes. The cccDNA exists as a minichromosome and resists to antivirals, thus a therapeutic eradication of cccDNA from the infected cells remains unattainable. In this review, we summarize the state of knowledge on the mechanisms underlying cccDNA formation and regulation, and discuss the possible strategies that may contribute to the eradication of HBV through targeting cccDNA.

Keywords: Drug target; HBV cccDNA minichromosome; HBV cure; Host-virus interaction; Transcriptional regulation; cccDNA eradication.

Figure 1.. The formation of HBV cccDNA

The formation of HBV cccDNA involves: 1. the release of viral polymerase, which may mediated by tyrosyl-DNA-phosphodiesterase 2 (TDP2) or its related proteins (Koniger et al., 2014) (Cui et al., 2015b).; 2. removal of RNA primer from the positive strand by some yet unknown enzymes; 3. cleavage of terminally redundant sequences (r) from the negative strand, which may require flap structure-specific endonuclease 1 (FEN1) activity (Kitamura et al., 2018); 4. repair of the positive strand, with the help of DNA polymerase κ(Qi et al., 2016) or polymerase α, δ and ɛ (Tang et al., 2019), and DNA topoisomerase I and II (Sheraz et al., 2019); 5. ligation of minus strand (Luo et al., 2017b) and 6. Plus strand DNA separately or simultaneously by DNA ligase 1 and 3 (Long et al., 2017); 7. chromatinization, which involves histone chaperones, chromatin remodelers, transcription factors and viral proteins. (Note: the numbers (1–7) are indicative of each specific step involved in cccDNA formation, but not the time sequence of these reactions, which remains obscure, and some reactions/steps may occur simultaneously.)

Figure 2.. Mechanisms of cccDNA loss or inactivation

Cell division, or direct killing of infected cells by T cells can cause HBV cccDNA loss. Additionally, cccDNA destabilizer, cytokine-induced deamination or gene editing tools can affect the integrity of cccDNA. Furthermore, epigenetic drugs may be able to silence cccDNA transcription, resulting in cccDNA inactivation.