Precore and Core Promoter Mutations in HBV Infection
Precore and Core Promoter Mutations in HBV Infection
What is the difference between precore mutations and core promoter mutations in hepatitis B virus infection in terms of disease pattern and response to therapy?
Hepatitis B virus (HBV) e antigen (HBeAg) mutants represent an interesting biologic phenomenon, but their clinical significance is somewhat unclear. The true impact of these variants is difficult to assess because of several confounding factors intrinsic to both the host and the virus. Let's first briefly review the unique interaction between HBV and the infected host.
The HBV genome contains 4 overlapping open reading frames: the S gene, which codes for the surface proteins (HBsAg); the X gene, which codes for the regulatory gene; the P gene, which codes for DNA polymerase; and the C (core) gene. The latter is divided into the precore region (29 amino acid codons) and the core region (181 codons) by 2 in-frame initiating ATG codons. This results in the transcription of the pregenomic RNA that is essential for HBV replication, and of the nucleocapsid protein or the precore RNA that translates into HBeAg protein that is released into the bloodstream of infected patients. HBeAg is considered to be a marker for viral replication; thus, seroconversion from HBeAg to anti-HBe positivity usually indicates a low level of viral production and low serum HBV DNA levels. This generally correlates with improvement of liver disease.
There is a high error rate associated with the reverse transcription step in HBV replication. As a consequence, HBV has a mutation rate 10-fold higher than that of other DNA viruses. The most potent selective force during the natural course of HBV infection is the host immune response. As a consequence of the development of anti-HBe and the reduction in HBV viremia, "escape" mutants of anti-HBe are selected. Because HBeAg expression is not essential for virus replication, the virus can evade anti-HBe immunity by turning off or reducing HBeAg expression.
Precore mutants: These variants were the first major immune escape mutants of HBV to be discovered. These HBV variants appear during HBeAg seroconversion and they carry mutations in the precore region that prevent HBeAg synthesis, despite continuing production of infectious virions. The most common of these mutations is a G to A substitution at nucleotide 1896, which prevents the production of HBeAg by introducing a premature stop codon into the open reading frame of the precore region. This truncates the precore/core protein into a 28-amino acid peptide. Because the precore region is not essential for HBV replication nor hepatitis B core antigen (HBcAg) expression, the G1896A variant is replication-competent and is infectious.
The presence of the G1896A mutation is restricted to specific viral genotypes (B, C, D, and E). These HBV genotypes are not uniformly distributed around the world. This mutation is more prevalent in geographic regions where genotypes B, C, and D are predominant, such as Asia and the Mediterranean area, where it can be detected in more than 50% of individuals with chronic hepatitis B. It is significantly less prevalent in North America and Europe, where genotype A is more common.
The presence of precore escape mutants should be considered in individuals who exhibit HBeAg negativity, HBsAg positivity, anti-HBe positivity, HBV DNA positivity, and elevated serum aminotransferase levels. However, the precore variant is not uniformly pathogenic, and thus, co-mutations or host factors presumably explain the more virulent forms of precore mutant-associated disease.
Core promoter mutants: Another common HBeAg variant is the core promoter mutant, characterized by point mutations in the promoter for both HBeAg mRNA and core protein mRNA The core promoter mutants express less HBeAg through transcriptional downregulation. The most frequent core promoter mutation is the double A1762T and G1764A nucleotide exchange, which results in a substantial decrease in HBeAg expression but enhanced viral genome replication. The reduction of HBeAg expression is apparently mediated by reduced precore RNA transcription, whereas the mechanism for enhanced replication is unclear. These virologic properties lead to enhanced pathogenicity of core promoter mutants in vivo. The enhanced replication capacity and reduced virion secretion may increase viral load in the liver, which triggers liver damage directly or indirectly through the immune response. Massive liver damage during acute infection leads to fulminant hepatitis. Damage during chronic infection increases hepatocyte turnover, induces fibrosis, and increases the chance of hepatocellular transformation and malignancy.
As opposed to precore variants, core promoter variants can be detected in patients who are either HBeAg-positive or -negative. The prevalence of the core promoter variant is about 40%; it is evenly distributed among the major HBV genotypes. Although fulminant forms of HBV-related hepatitis have been linked to infection with core promoter mutants, case-controlled studies are needed to prove the association. These studies should also take into account other viral mutations and the genetic makeup of the host. The association of core promoter mutants and liver cancer is also controversial; carefully done epidemiologic studies are needed to demonstrate a link between core promoter mutations and hepatocellular carcinogenesis.
Impact on treatment: Despite the high prevalence of precore and core promoter mutants as the predominant viral species in HBeAg-negative chronic hepatitis B, treatment of these variants is challenging. At present, options include the use of interferon-alfa-based therapy and nucleos(t)ide analogues, such as lamivudine, entecavir, or adefovir. The response rate for precore mutants to interferon-alfa is low, even with a longer duration of therapy, than that for wild-type strains or re-treatment strategies. Lamivudine, entecavir, and adefovir are regarded as safe and efficacious, but they usually require treatment over several years.
Bottom line: According to published guidelines from the American Association for the Study of Liver Diseases, patients with HBeAg-negative chronic hepatitis B (serum HBV DNA > 10 copies/mL, elevated aminotransferase levels, or moderate/severe hepatitis on biopsy) should be considered for treatment. According to these guidelines (published in 2004), treatment may be initiated with interferon-alfa, lamivudine, or adefovir. In view of the need for long-term treatment, IFN-alfa or adefovir is preferred.
What is the difference between precore mutations and core promoter mutations in hepatitis B virus infection in terms of disease pattern and response to therapy?
Hepatitis B virus (HBV) e antigen (HBeAg) mutants represent an interesting biologic phenomenon, but their clinical significance is somewhat unclear. The true impact of these variants is difficult to assess because of several confounding factors intrinsic to both the host and the virus. Let's first briefly review the unique interaction between HBV and the infected host.
The HBV genome contains 4 overlapping open reading frames: the S gene, which codes for the surface proteins (HBsAg); the X gene, which codes for the regulatory gene; the P gene, which codes for DNA polymerase; and the C (core) gene. The latter is divided into the precore region (29 amino acid codons) and the core region (181 codons) by 2 in-frame initiating ATG codons. This results in the transcription of the pregenomic RNA that is essential for HBV replication, and of the nucleocapsid protein or the precore RNA that translates into HBeAg protein that is released into the bloodstream of infected patients. HBeAg is considered to be a marker for viral replication; thus, seroconversion from HBeAg to anti-HBe positivity usually indicates a low level of viral production and low serum HBV DNA levels. This generally correlates with improvement of liver disease.
There is a high error rate associated with the reverse transcription step in HBV replication. As a consequence, HBV has a mutation rate 10-fold higher than that of other DNA viruses. The most potent selective force during the natural course of HBV infection is the host immune response. As a consequence of the development of anti-HBe and the reduction in HBV viremia, "escape" mutants of anti-HBe are selected. Because HBeAg expression is not essential for virus replication, the virus can evade anti-HBe immunity by turning off or reducing HBeAg expression.
Precore mutants: These variants were the first major immune escape mutants of HBV to be discovered. These HBV variants appear during HBeAg seroconversion and they carry mutations in the precore region that prevent HBeAg synthesis, despite continuing production of infectious virions. The most common of these mutations is a G to A substitution at nucleotide 1896, which prevents the production of HBeAg by introducing a premature stop codon into the open reading frame of the precore region. This truncates the precore/core protein into a 28-amino acid peptide. Because the precore region is not essential for HBV replication nor hepatitis B core antigen (HBcAg) expression, the G1896A variant is replication-competent and is infectious.
The presence of the G1896A mutation is restricted to specific viral genotypes (B, C, D, and E). These HBV genotypes are not uniformly distributed around the world. This mutation is more prevalent in geographic regions where genotypes B, C, and D are predominant, such as Asia and the Mediterranean area, where it can be detected in more than 50% of individuals with chronic hepatitis B. It is significantly less prevalent in North America and Europe, where genotype A is more common.
The presence of precore escape mutants should be considered in individuals who exhibit HBeAg negativity, HBsAg positivity, anti-HBe positivity, HBV DNA positivity, and elevated serum aminotransferase levels. However, the precore variant is not uniformly pathogenic, and thus, co-mutations or host factors presumably explain the more virulent forms of precore mutant-associated disease.
Core promoter mutants: Another common HBeAg variant is the core promoter mutant, characterized by point mutations in the promoter for both HBeAg mRNA and core protein mRNA The core promoter mutants express less HBeAg through transcriptional downregulation. The most frequent core promoter mutation is the double A1762T and G1764A nucleotide exchange, which results in a substantial decrease in HBeAg expression but enhanced viral genome replication. The reduction of HBeAg expression is apparently mediated by reduced precore RNA transcription, whereas the mechanism for enhanced replication is unclear. These virologic properties lead to enhanced pathogenicity of core promoter mutants in vivo. The enhanced replication capacity and reduced virion secretion may increase viral load in the liver, which triggers liver damage directly or indirectly through the immune response. Massive liver damage during acute infection leads to fulminant hepatitis. Damage during chronic infection increases hepatocyte turnover, induces fibrosis, and increases the chance of hepatocellular transformation and malignancy.
As opposed to precore variants, core promoter variants can be detected in patients who are either HBeAg-positive or -negative. The prevalence of the core promoter variant is about 40%; it is evenly distributed among the major HBV genotypes. Although fulminant forms of HBV-related hepatitis have been linked to infection with core promoter mutants, case-controlled studies are needed to prove the association. These studies should also take into account other viral mutations and the genetic makeup of the host. The association of core promoter mutants and liver cancer is also controversial; carefully done epidemiologic studies are needed to demonstrate a link between core promoter mutations and hepatocellular carcinogenesis.
Impact on treatment: Despite the high prevalence of precore and core promoter mutants as the predominant viral species in HBeAg-negative chronic hepatitis B, treatment of these variants is challenging. At present, options include the use of interferon-alfa-based therapy and nucleos(t)ide analogues, such as lamivudine, entecavir, or adefovir. The response rate for precore mutants to interferon-alfa is low, even with a longer duration of therapy, than that for wild-type strains or re-treatment strategies. Lamivudine, entecavir, and adefovir are regarded as safe and efficacious, but they usually require treatment over several years.
Bottom line: According to published guidelines from the American Association for the Study of Liver Diseases, patients with HBeAg-negative chronic hepatitis B (serum HBV DNA > 10 copies/mL, elevated aminotransferase levels, or moderate/severe hepatitis on biopsy) should be considered for treatment. According to these guidelines (published in 2004), treatment may be initiated with interferon-alfa, lamivudine, or adefovir. In view of the need for long-term treatment, IFN-alfa or adefovir is preferred.