Recent Advances in Understanding, Diagnosing, and Treating Hepatitis B Virus Infection

Abstract

Chronic hepatitis B virus (HBV) infection affects 292 million people worldwide and is associated with a broad range of clinical manifestations including cirrhosis, liver failure, and hepatocellular carcinoma (HCC). Despite the availability of an effective vaccine HBV still causes nearly 900,000 deaths every year. Current treatment options keep HBV under control, but they do not offer a cure as they cannot completely clear HBV from infected hepatocytes. The recent development of reliable cell culture systems allowed for a better understanding of the host and viral mechanisms affecting HBV replication and persistence. Recent advances into the understanding of HBV biology, new potential diagnostic markers of hepatitis B infection, as well as novel antivirals targeting different steps in the HBV replication cycle are summarized in this review article.

Keywords: HBV; HBV RNA; HBcrAg; direct-acting antivirals; novel viral markers.

Figure 1

HBV replication cycle and new direct-acting antivirals for HBV. Entry: the replication cycle begins with the reversible binding of S-HBsAg to the glypican 5 (GPC5) protein [17,18], followed by specific binding between preS1 region of L-HBsAg and HBV entrance receptor—Sodium Taurocholate Cotransporting Peptide (NTCP) [19]. Upon the entrance, capsids are transported within the cytosol to the nucleus during which uncoating and release of the viral genome are initiated [14]. cccDNA formation: the molecular basis of rcDNA to cccDNA conversion remains unclear but it requires (1) the release of the viral polymerase covalently attached to the minus DNA strand by tyrosyl-DNA-phosphodiesterase 2 (TDP2) or its related proteins [20]; (2) the removal of the RNA primer from plus DNA strand by unrecognized enzymes; (3) cleavage of terminally redundant sequences (r) from the negative strand by structure-specific endonuclease 1 (FEN1) activity [21]; (4) the competition of positive DNA strand by the cellular replicative machinery: DNA polymerase κ [22] or polymeraseα, δ, and ε [23], and DNA topoisomerase I and II [24]; (5) the ligation of both viral DNA strands by DNA ligase LIG I and LIG III [25]; (6) chromatinization involving histone chaperones, chromatin remodelers, transcription factors, and viral proteins [26]. Transcription and translation: cccDNA transcribes into all viral RNAs necessary for protein production and viral replication utilizing the cellular transcriptional machinery: 3.5-kb pregenomic RNA (pgRNA)/preC RNA and 2.4-kb, 2.1-kb, and 0.7-kb subgenomic RNA [15]. The pregenomic RNA encodes both the polymerase and core protein and works as a template for viral DNA replication. The three subgenomic RNAs encode envelope preS1 protein, preS2, and HBsAg proteins and the X protein [27]. The precore mRNA is translated into precore protein, which is processed at the N-terminal and C-terminal ends to HBeAg, a secretory protein. Pregenomic RNA is reverse transcribed into HBV DNA and also translated into core protein (HBcAg), which overlaps with HBeAg [28]. Encapsidation: the first step of HBV genome replication is the encapsidation of the pgRNA by core protein, forming an immature nucleocapsid. This process requires the cis-acting packaging signal (a stem-loop structure termed as epsilon, ‘ε’) at the 5′ end of pgRNA, and phosphorylation of C-terminal of the core protein [15]. Reverse transcription: tyrosine residue hydroxyl group of polymerase protein covalently binds at the ε region of 5′ pgRNA initiating the reverse transcription [14]. Next, the first three nucleotides from the bulge region of ε stem-loop are synthesized, and the polymerase with the covalently attached trinucleotide sequence translocates from ε to direct repeat, DR1 located at the 3′-end of the 3.5-kb pregenomic RNA. Following the minus-strand elongation, pgRNA template is degraded by an RNase H encoded within the pol protein [29]. Pus-strand DNA synthesis: the terminal 16–18 nts from the 5′ end of pgRNA remains uncleaved and serves as the primer for plus-strand DNA synthesis after it translocates to a complementary sequence on the minus-strand template [30]. Circularization then occurs, facilitated by the short terminal redundancy on the minus strand. Elongation and completion of plus-strand DNA synthesis yield a relaxed-circular DNA genome [14]. Virions secretion: HBV genome is packaged into an icosahedral capsid composed of the HBV core protein (HBc), termed nucleocapsid. RC DNA-containing mature nucleocapsids are enveloped by HBV envelope glycoproteins proteins through the ESCRT machinery in the Golgi and secreted extracellularly as complete virions. A portion of capsids are transported back to the nucleus to maintain the pool of cccDNA [31]. cccDNA, covalently closed circular DNA; pgRNA, pregenomic RNA; RC DNA, relaxed circular DNA; HBeAg, hepatitis B e antigen; HBsAg, hepatitis B surface antigen; mRNA, messenger RNA; Pol, HBV polymerase ; Core, HBV core protein.