The virological and clinical significance of mutations in the overlapping envelope and polymerase genes of hepatitis B virus

Abstract

The potential for hepatitis B virus (HBV) to alter its genome is considerable. This occurs because the virus utilizes a reverse transcription step in replicating the viral genome. Like human immunodeficiency virus, the reverse transcriptase of HBV is error prone and as a consequence of specific selection pressures within a host a population of viral quasispecies emerges. HBV mutants with survival advantages over the wild type virus appear within the selective in vivo environment. Some of these viruses include HBV vaccine escape and anti-viral resistant mutants that have changes in the envelope (S) and polymerase genes, respectively. In addition, the genome of HBV is organised in to overlapping reading frames. The S gene is completely overlapped by the polymerase gene. As a consequence, mutations in the S gene may produce changes in the overlapping polymerase gene. Similarly, mutations in the polymerase gene may produce changes in the S gene. The virological and clinical significance of such overlapping mutations is unclear. However, we have shown that certain mutations in either the S or polymerase gene produce functionally significant changes in the respective overlapping gene. Treatment of chronic hepatitis B carriers with long-term lamivudine (LMV) results in the selection of HBV mutants that are resistant to this nucleoside analogue. The polymerase mutations associated with LMV resistance produce changes in the overlapping S gene and in its envelope protein (hepatitis B small antigen, HBsAg) that results in a reduced antigenicity of the HBsAg protein. The selection of vaccine escape mutants by HBV vaccination or hepatitis B immune globulin is associated with changes in the S gene that are accompanied by mutations in the fingers sub-domain of the polymerase protein. When combined with polymerase mutations that are associated with resistance to LMV the changes within the fingers sub-domain of the viral enzyme behave as compensatory mutations that are able to restore the replication of LMV resistant HBV. The ability to change a viral protein by mutations in an overlapping but unrelated viral gene may produce HBV mutants with altered antigenicity and/or replication and a natural history that may be distinctly different to wild type HBV.

Introduction

Hepatitis B infection remains a major health problem in many regions including Asia, Africa, the Middle East and the Pacific Islands. In some developing countries the carrier rates for chronic hepatitis B infection range from 10 to 20% in contrast to areas of eastern and southern Europe where chronic carrier rates are 2–5%. In South East Asia where chronic carrier rates vary between 8 and 20%, up to 50% of carrier mothers are HBeAg positive resulting in high rates of perinatal infection (Vryheid et al., 2001).

A high percentage of individuals who acquire hepatitis B virus (HBV) in infancy or childhood develop chronic sequelae of hepatitis B infection later in life including cirrhosis and hepatocellular carcinoma (HCC). Chronic hepatitis B infection is the leading cause of HCC in countries like China and Africa where serological evidence of HBV can be found in 70–90% of cases of HCC. Chronic hepatitis B infections increases the risk of HCC approximately 100-fold (Beasley, 1988, Ganem, 1982, Obata et al., 1980).

HBV is an enveloped, partly double-stranded DNA virus containing a genome of approximately 3200 base pairs. The major viral translational products include the precore and core, polymerase, large, middle and small envelope proteins and the transcriptional regulator the X protein that are encoded by overlapping genes. The compact organization of the viral genome in to overlapping reading frames is characteristic of hepadnaviridae (Fig. 1). In addition, hepadnaviridae are the only DNA viruses that, like RNA viruses, utilise a reverse transcriptase step in the replication of the viral genome. Reverse transcription is an error prone process (Domingo and Holland, 1997) and as with retroviruses, like human immunodeficiency virus (HIV), this process results in the selection of HBV quasispecies that contain several mutations within the viral genome. Some of these mutations are detrimental to the virus while others may provide the virus with a survival advantage in a specific environment. Although it is likely that most of these changes do not result in functionally significant changes, it is possible that some of these mutations will produce changes in the overlapping gene and its translational product. The complete overlap of the envelope gene by the polymerase gene within the HBV viral genome creates a unique situation in which a change within the S gene selected as a consequence of immunotherapies such as HBV vaccination or hepatitis B immune globulin (HBIg) may produce a functionally significant alteration of the viral polymerase. These changes may influence viral replication fitness. Similarly, changes selected within the polymerase gene as a consequence of nucleoside analogue therapy may result in structural changes in the hepatitis B small antigen (HBsAg) protein with a consequent reduction in the antigenicity of this protein.

To date, there have been few reports, on the effects of complimentary mutations in overlapping reading fames and there influence on HBV replication or antigenicity. This review will focus on the significance of such mutations and discuss the potential impact on the replication fitness of HBV and the antigenicity of the HBsAg protein.