More than 300 million people worldwide are chronically infected with hepatitis B virus (HBV). Considering the very short generation time for a virus, and the high error rate associated with the reverse transcription step of HBV replication, decades of HBV infection are probably equivalent to million years of human evolution. The most important selective force during the natural course of HBV infection appears to be the immune response. The development of anti-HBe antibody in hepatitis B patients usually correlates with reduction of HBV viremia. As a consequence, escape mutants of anti-HBe are selected.
Since HBeAg expression is not essential for virus replication, the simplest way for the virus to evade the anti-HBe immunity is to switch off HBeAg expression altogether. The so-called "precore mutants" are the first discovered major immune escape mutants of HBV. These mutants are characterized by a G1896A nonsense mutation in the precore region that truncates the precore/core protein into a 28-aa peptide. Other nonsense and frameshift mutations inside the precore region have also been found, although less frequently. Point mutations of the precore ATG codon have also been observed, which prevent initiation of translation. We have found that triple mutation at the -5, -3, and -2 positions of the precore ATG codon, as occasionally found in some South African strains of HBV, greatly reduced translation efficiency. The selective disruption of HBeAg expression through mutations affecting the precore region rather than the core gene can be easily understood in terms of the indispensable role of core protein for viral replication.
The other common, but more important, HBeAg variants are the core promoter mutants. They are characterized by point mutations in the promoter for both HBeAg mRNA and core protein mRNA (also called pregenomic RNA). These mutations were found by transfection experiments to down regulate HBeAg mRNA production, resulting in reduced protein levels. Core promoter mutants are the dominant viral species at not only the anti-HBe stage, but also the late HBeAg stage of infection. The common core promoter mutations, A1762T/G1764A, reduced HBeAg expression by a mere 20% in a genotype A clone that we examined. The core promoter mutants express less HBe antigen (HBeAg) through transcriptional down regulation, while precore mutants express truncated products. We have identified additional mutations that modulate HBeAg translation initiation, proteolytic cleavage, and secondary structure maintenance through a disulfide bond.
The core promoter mutants have been associated with the development of fulminant hepatitis during acute infection and liver cancer during chronic infection. Consistent with their enhanced pathogenicity, core promoter mutants were found to replicate at up to 10-fold higher levels in transfected human hepatoma cells than the wild-type virus. Moreover, some core promoter mutants are impaired in virion secretion due to missense mutations in the envelope gene. These virological properties help explain enhanced pathogenicity of core promoter mutants in vivo. It is suggesting that the enhanced replication capacity and reduced virion secretion may increase viral load in the liver, thus triggering liver damage either directly or indirectly through the immune response. When massive liver damage occurs during acute infection, fulminant hepatitis may ensue. When such damage occurs during chronic infection, it increases hepatocyte turnover, induces fibrosis, and enhances the chance of hepatocellular transformation and malignancy.
The paper is published by International Journal of Medical Sciences. Full text of the article is available in http://www.medsci.org/v02p0002
Author contact: Dr. Shuping Tong or Jisu Li, The Liver Research Center, Rhode Island Hospital and Brown Medical School, 55 Claverick Street, Providence, RI 02906. Telephone: 401-444-7365 (ST); 401-444-7387 (JL). Fax: 401-444-2939. E-mail: [email protected]
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