Epidemiology of Precore and Core Promoter Mutations
Prior to 1980, most experts considered patients with positive hepatitis B surface antigen (HBsAg) and negative hepatitis B e antigen (HBeAg) as "healthy carriers" with nonreplicative hepatitis B virus (HBV) infection; these individuals were considered to have a favorable prognosis compared with HBeAg-positive patients and treatment was not recommended. In the early 1980's, however, investigators described a group of Mediterranean patients with chronic hepatitis B who had detectable serum hepatitis B DNA (by non-PCR based assays) despite having persistently negative HBeAg. These patients also had evidence of active liver disease on liver biopsy. In 1989, researchers identified two types of mutations in the HBV genome in these patients: precore and core promoter mutations. These mutations prevent or diminish HBeAg formation by an otherwise normally replicating hepatitis B virus. The prevalence of HBeAg-negative chronic hepatitis B varies geographically, with a higher prevalence found in regions where patients predominantly have non-A genotype HBV. In China and the Mediterranean region, patients with chronic hepatitis B typically have genotype B, C, or D infection and these areas accordingly have a relatively high prevalence of patients with HBeAg-negative chronic hepatitis[2,3]. Nevertheless, these mutations exist in a significant proportion of persons with chronic HBV infection in the United States, as shown by a study that estimated a prevalence of precore and core promoter variants in 27% and 44% of patients in the United States with chronic HBV infection[4,5].
Normal Synthesis of HBeAg
The normal synthesis of HBeAg originates in the precore-core region of the HBV DNA--a region of the HBV DNA that also codes for the hepatitis B core antigen proteins that assemble to form the viral core (Figure 1). The process HBeAg synthesis begins with initiation of transcription at the ATG codon at location 1814. After translation of the mRNA, the 212 amino acid long product, also known as the p25 protein, is shuttled to the endoplasmic reticulum whereby a signal peptidase cleaves 19 amino acids off the C terminus and a basic endopeptidase cleaves 34 amino acids off the N terminus region. This final truncated protein, a 159 amino acid long product, is HBeAg. The formed HBeAg does not become incorporated into HBV, but instead is secreted outside the cell. Although the function of the HBeAg remains poorly understood, most experts believe this antigen provides an immunologic advantage for HBV, particularly in perinatally-acquired hepatitis B infection where the expression of e antigen is thought to induced immune tolerance. Loss of HBeAg and development of anti-HBe, when it occurs spontaneously without antiviral therapy, is usually associated with a decrease in serum HBV viral load and a favorable prognosis. One of the main endpoints of antiviral therapy is loss of HBeAg and development of antibody to HBeAg (anti-HBe)[8,18].
Mechanism of Precore and Core Promoter Mutations
The precore and core promoter mutations are mutations in the HBV DNA that alter the normal HBeAg synthesis (Figure 1). The most common mutation, known as the precore mutation, involves a G-to-A base pair substitution at nucleotide 1896 in the precore region of the HBV genome. This mutation transforms codon 23 in the mRNA from TGG to a TAG stop codon. The creation of the stop codon results in the formation of a truncated 28 amino-acid peptide and thus the formation of normal HBeAg is completely halted. In contrast, the core-promoter mutation involves paired nucleotide substitutions in the core promoter region, most often A-to-T at nucleotide 1762 and G-to-A at nucleotide 1764, leading to reduced HBeAg production. Investigators have identified other less common mutations in the precore and core promoter regions.
Identifying Precore and Core Promoter Mutations
Although several selected commercial laboratories now offer the assays that identify the precore and core promoter mutations, in clinical practice, the diagnosis of HBeAg-negative chronic hepatitis B is generally made by identifying a high HBV DNA load in an HBeAg-negative patient. Unfortunately, there is no clear viral load 'cutoff' indicating infection with preocore or core promoter mutants.
Clinical Implications of Precore and Core-Promoter Mutations
Infection with an HBeAg-negative HBV strain usually does not occur de novo, but rather emerges during immune clearance of a wild-type strain, when increased immune pressure on the wild-type strain leads to selection of the HBeAg-negative mutant. Several reports have also described person-to-person transmission of mutant strains. Because precore and core-promoter mutations generally occur during spontaneous or treatment-induced clearance of HBeAg, patients with HBeAg-negative chronic hepatitis B infection tend to be older than patients with wild-type infection. Additional characteristics of these patients include having a lower HBV DNA load and a higher prevalence of advanced fibrosis[14,15]. Although the increased prevalence of advanced fibrosis in patients with HBeAg-negative chronic hepatitis B may be partially explained by the older age of these patients (and thus the longer duration of infection), some studies have suggested that increased virulence of these mutant strains may lead to more aggressive clinical disease[16,19,20]. Studies have also suggested that core promoter mutation is a risk factor for hepatocellular carcinoma[21,22]. Two main patterns of disease activity have been observed among these variants: (1) persistent three- to four-fold elevations of serum ALT (approximately one-third of patients), and (2) intermittent normal ALT levels with frequent serum ALT flares (approximately two-thirds of patients). Unlike wild-type infection, spontaneous remission in HBeAg-negative mutant infection occurs in less than 15% of patients during their lifetime. When taking into account the higher prevalence of advanced fibrosis, the tendency to have elevated serum ALT, and the low likelihood of spontaneous remission, many experts consider infection with HBeAg-negative viral strains to correlate with a poor prognosis when compared with wild-type infection.
Management of HBeAg-Negative Chronic Hepatitis B
Two different organizations have recently published sets of recommendations regarding the treatment approach to patients with positive HBsAg and negative HBeAg[8,17]. The 2009 American Association for the Study of Liver Diseases (AASLD) Practice Guidelines recommends considering treatment for those HBeAg-negative patients with ALT levels greater than 2 times the upper limit of normal (defined as 30 IU/L in men and 19 IU/L in women) or with moderate-to-severe inflammation or significant fibrosis on biopsy and HBV DNA greater than 2,000 IU/ml. In a separate publication in 2008, a panel of United States hepatologists also recommended treatment for patients with HBV DNA greater than 2,000 IU/ml and elevated ALT levels (greater than the upper limit of normal). This panel also recommended that for patients who have HBV DNA levels greater than 2,000 IU/mland normal ALT levels, liver biopsy should be considered and treatment initiated if significant disease (characterized by moderate fiborsis (stage 2) or greater, significant necroinflammation or both) is present, citing data from several studies that suggest that up to a third of patients with persistently normal ALT can have histologic evidence of moderately severe liver disease, particularly if they are older than 35 to 40 years. The duration of therapy for HBeAg-negative infection is generally prolonged in the case of oral antiviral agents since relapse frequently occurs with treatment of shorter duration (less than a year); the optimal duration and treatment endpoints however remain unclear. Because the conversion of positive HBeAg to negative HBeAg is not a parameter that can be followed in these patients, the treatment endpoint in patients with HBeAg-negative chronic hepatitis B is based largely on suppression of HBV DNA and normalization of ALT.
Case Summary and RecommendationsIn the patient discussed above in the case history, the 40-year-old Asian man had a HBV DNA level of 130,000 IU/ml. A liver biopsy yielded Grade 3, Stage 2 histological disease. In this clinical setting, with an HBV DNA greater than 2,000 IU/ml and active disease on liver biopsy, his provider started antiviral treatment for HBeAg-negative chronic hepatitis B infection. An important distinction must be made between the treatment approaches to HBeAg-positive (wild-type) chronic infection versus HBeAg-negative chronic infection. For patients with HBeAg-positive infection, the treatment endpoint consists primarily of HBeAg seroconversion. In contrast, for HBeAg-negative infection, the treatment endpoint consists of durable HBV DNA suppression and serum biochemical response, with or without demonstrated histological improvement.
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