HBeAg negative variants and their role in the natural history of chronic hepatitis B virus infection

Abstract

Molecular virology methods including polymerase chain reaction, cloning and sequencing have revolutionised our understanding of viral genome variation. In the case of hepatitis B virus (HBV), sequencing studies have identified a number of virus variants normally found during the natural course of chronic infection. The appearance of the precore stop codon (with G-for-A substitution at position 1896) and basal core promoter (BCP) (with A-for-T and G-for-A, at positions 1762 and 1764, respectively) variants which reduce or abrogate hepatitis B e antigen (HBeAg) production, heralds the initiation of the seroconversion phase from HBeAg to anti-HBe positivity. The gradual removal of the tolerogenic effect of HBeAg leads to the awakening of the immune response (immune clearance phase). Most patients after HBeAg seroconversion become "inactive HBsAg carriers". However during the course of infection precore and/or BCP variants may emerge and be selected leading to HBeAg negative chronic hepatitis B (CHB) with high viremia levels (reactivation phase). The prevalence of HBeAg negative CHB has been increasing over the last few decades and has become the commonest type of HBV infection in many countries of the world. This probably reflects the aging of existing HBV carriers and the effective prevention measures restricting new HBV infections. Frequent acute exacerbations accompanied by high viral replication, elevated alanine aminotransferase levels and histological activity are a common feature of HBeAg negative CHB leading to cirrhosis much faster than in HBeAg positive CHB patients.

Keywords: Hepatitis B virus-DNA replication; Precore stop codon variants, basal core promoter variants; Re-activation; hepatitis B e antigen negative chronic hepatitis B.

Figure 1

Course of the natural history of hepatitis B virus infection showing the four possible phases of disease. These are referred to as the immune tolerant, the immune clearance, the non-replicative (inactive) and the reactivation phase characterized by increased viral replication and liver damage (shaded area). During the first two phases the patient is hepatitis B e antigen (HBeAg) positive and negative during the last two. Moreover, the wild type hepatitis B virus (HBV) is dominant in the Immune tolerant and immune clearance phases, whilst the precore and basic core promoter variants are the main isolates seen in patients during the reactivation phase, and such patients constitute the HBeAg negative chronic hepatitis B group. Patients in the immune clearance and the reactivation phases are at greater risk of developing cirrhosis and eventually hepatocellular carcinoma. ALT: Alanine aminotransferase.

Figure 2

Protein products encoded by the precore/core open reading frames in wild type and precore stop codon variant isolates. A: The 3.2 kb in length HBV DNA genome in linear form (normally circular as cccDNA) and the two longer than genome length precore mRNA and pgRNA, the transcription of which is under the control of the BCP. The transcripts encoding for HBsAg and HBxAg are not shown. The precore mRNA encodes for the precore/core protein (grey open reading frame) and the pgRNA for the core and polymerase (red and orange open reading frames); B: Effect of the precore stop codon mutation on HBeAg production in comparison to that of the wild type virus. The precore/core protein is the precursor of HBeAg and consists of the precore region (in brown) and the core encoding region (in pink). HBeAg is formed through proteolytic cleavage of both its amino- and carboxyl-ends (see text). The presence of the stop codon mutation leads to the abrogation of HBeAg production as a result of pre-mature termination of the synthesis of the precore/core precursor as shown. HBcAg synthesis on the other hand which utilises the pgRNA transcript remains unaffected, thus permitting continued virion production. Please note that the proteins share complete amino acid homology over the overlapped regions. Yet, HBeAg and HBcAg display different antigenic epitopes. ALT: Alanine aminotransferase; HBeAg: Hepatitis B e antigen; HBV: Hepatitis B virus; HBsAg: Hepatitis B surface antigen; BCP: Basic core promoter; HBcAg: Hepatitis B c Antigen; HBxAg: Hepatitis B x Antigen.