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Keywords:

  • chronic hepatitis B;
  • cirrhosis;
  • genotype;
  • HBV viral mutation;
  • hepatitis B virus (HBV);
  • hepatocellular carcinoma;
  • interferon-based therapy

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Summary and perspectives
  5. References

Outcomes of chronic hepatitis B virus (HBV) infection are heterogeneous. Estimates of annual incidence of cirrhosis and hepatocellular carcinoma (HCC) are 2–10% and 1–3%, respectively. Several viral factors, including HBV genotype, viral load and specific viral mutations, have been associated with disease progression. Among these, HBV genotype is not only predictive of clinical outcomes but has also been associated with response to interferon treatment. Currently, at least 10 HBV genotypes and several subtypes have been identified; they have distinct geographic distribution. Acute infection with genotypes A and D results in higher rates of chronicity than genotypes B and C. Compared to genotype A and B cases, patients with genotypes C and D have lower rates of spontaneous hepatitis B e antigen (HBeAg) seroconversion; when this occurs, it tends to be delayed. HBV genotype C has a higher frequency of basal core promoter (BCP) A1762T/G1764A mutation, pre-S deletion and is associated with higher viral load than genotype B. Similarly, genotype D has a higher prevalence of BCP A1762T/G1764A mutation than genotype A. These observations suggest important pathogenic differences between HBV genotypes. These may contribute to more severe liver disease, including cirrhosis and HCC with genotypes C and D HBV infection. In addition, genotype A and B patients have better responses to interferon-based therapy than genotypes C and D, but there are few consistent differences for direct HBV antivirals. In conclusion, genotyping of chronic HBV infections can help practicing physicians identify those at risk of disease progression and determine optimal anti-viral therapy.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Summary and perspectives
  5. References

Hepatitis B virus (HBV) infection is endemic in Asia and the Pacific islands, Africa, Southern Europe and Latin America, where the community prevalence of hepatitis B surface antigen (HBsAg) ranges from 2% to 20%. In Asian countries, the majority of those infected with HBV acquire the virus in the perinatal period or early childhood through vertical (mother to child) transmission; however, horizontal transmission is the main route in African and Western countries.1 The long-term outcomes of chronic hepatitis B vary widely in different countries. The annual incidence of cirrhosis is estimated to be 2% to 6% for HBeAg-positive and 8% to 10% for HBeAg-negative chronic hepatitis B patients. In addition, the annual incidence of hepatocellular carcinoma (HCC) is less than 1% for non-cirrhotic HBV-infected “carriers,” and 2% to 3% for patients with cirrhosis.2,3 Recently, several hepatitis B viral factors, including HBV genotype, viral load and specific viral mutations, have been documented to be strongly predictive of clinical outcomes. Although previous population-based studies indicated that HBV genotype influences the natural history of HBV infection, the precise role of genotype in liver disease progression and anti-viral response remains to be firmly established.4,5 Accordingly, HBV genotyping is still not recommended as part of the management of chronic hepatitis B in regional guidelines.6–8 In this article, we describe recent advances in the impact of HBV genotype on the clinical outcomes and responses to antiviral treatments in chronic hepatitis B patients. In addition, the interactions between HBV genotype and other viral factors, such as viral load and viral mutants, will be reviewed.

Geographic distribution of HBV genotypes/subtypes

According to the homogeneity of virus sequences, at least 10 HBV genotypes (A to J) and several subtypes have been defined by divergence in the entire HBV genomic sequences, respectively, >8% for genotypes and 4–8% for subtypes.9–11 Except for the newly identified genotypes I and J, the geographic and ethnic distributions of HBV genotypes and subtypes are well characterized (Table 1). Genotype A is highly prevalent in sub-Saharan Africa (subtype A1), Northern Europe (subtype A2), and Western Africa (subtype A3).

Table 1.  Geographic distribution of hepatitis B virus genotypes and subtypes
Genotypes/subtypesGeographic location
A
A1Sub-Saharan Africa
A2Northern Europe
A3Western Africa
B
B1Japan
B2-5East Asia, Taiwan, China, Indonesia, Vietnam, Philippines
B6Alaska, Northern Canada, Greenland
C
C1-3Taiwan, China, Korea and Southeast Asia.
C4Australia
C5Philippines, Vietnam
D
D1-5Africa, Europe, Mediterranean countries and India
ERestricted to West Africa
F
F1-4Central and South America
GFrance, Germany and the United States
HCentral America
IVietnam and Laos
JRyukyu, Japan

Genotypes B and C are common in Asia. At present, genotype B is divided into B1–B6 subtypes. Among them, B1 is isolated in Japan, B2–5 are found in East Asia, and B6 is found in indigenous populations living in the Arctic, such as Alaska, Northern Canada and Greenland. Genotype C, including subtypes C1–C5, mainly exist in East and Southeast Asia. Genotype D with subtypes D1–D5 is prevalent in Africa, Europe, the Mediterranean region and India. Genotype E is restricted to West Africa. Genotype F with 4 subtypes (F1–F4) is found in Central and South America. Genotype G has been reported in France, Germany and the United States.

The eighth genotype, H, is found in Central America.4,9–13 Recently, genotype I, a novel inter-genotypic recombination among genotypes A, C, and G was isolated in Vietnam and Laos.14–16 The newest HBV genotype, J, was identified in the Ryukyu islands in Japan, and this genotype has a close relationship with gibbon/orangutan genotypes and human genotype C.17

The correlation of HBV genotype distribution with modes of transmission was commented upon in our original landmark review in the Journal of Gastroenterology and Hepatology.4 For example, genotypes B and C are prevalent in highly endemic areas, such as Asian countries, where perinatal or vertical transmission plays an important role in spreading HBV, whereas the remaining genotypes are frequently found in areas where horizontal transmission (close personal conduct between young children, blood or sexual contamination between adults) is the main mode of transmission. Accordingly, genotyping HBV can serve as an epidemiologic tool for the investigation of maternal transmission, familial clustering and geographic distribution of HBV strains.18

The impact of HBV genotype on outcomes of HBV infection

The results of several studies indicate that HBV genotype can influence the short- and long-term outcomes of HBV infection (Table 2).

Table 2.  Comparison of clinical and virological differences among hepatitis B virus genotypes. Due to the unique distribution of HBV genotypes in Asian and Western countries, the available data could only be reliably compared between genotype B and C or genotype A and D
GenotypeBCADE–J
  1. ND, no available data.

Clinical characteristics
Modes of transmissionperinatal /verticalperinatal /verticalhorizontalhorizontalHorizontal
Tendency of chronicityLowerHigherHigherLowerND
Positivity of HBeAgLowerHigherHigherLowerND
HBeAg seroconversionEarlierLaterEarlierLaterND
HBsAg seroclearanceMoreLessMoreLessND
Histologic activityLowerHigherLowerHigherND
Clinical outcome (cirrhosis and hepatocellular carcinoma)BetterWorseBetterWorseWorse in Genotype F
Response to interferon alphaHigherLowerHigherLowerLower in Genotype G
Response to nucleos(t)ide analoguesNo significant difference between genotype A to DND
Virologic characteristics
Serum HBV DNA levelLowerHigherNDNDND
Frequency of precore A1896 mutationHigherLowerLowerHigherND
Frequency of basal core promoter T1762/A1764 mutationLowerHigherHigherLowerND
Frequency of pre-S deletion mutationLowerHigherNDNDND

Tendency to chronicity after acute HBV infection

Recent studies suggested that acute infection with HBV genotype A may increase the risk of progression to chronic infection.19,20 In Japan, the persistence of HBV infection after acute hepatitis B was higher in patients with genotype A (23%) than those with genotype B (11%) or C (7%) infections.20 Of particular note, an increase of acute infections with HBV genotype A would result in a redistribution of HBV genotypes among patients with CHB in any country where universal hepatitis B vaccination has not yet been launched. For example, in a nation-wide survey, Matsuura et al. found that the prevalence of HBV genotype A in chronic hepatitis B patients in Japan increased from 1.7% during 2000 to 3.5% in 2006.21

In an epidemiologic study of acute hepatitis B, Chinese patients with subtype C2 developed chronic infection more often than those infected with subtype B2, and type C2 was an independent factor for the chronicity.22 In a limited number of studies, the rate of chronicity of acute genotype A or D infections has been reported to be higher than genotype B or C infections.23,24 Taken together, the persistence of HBV infection after acute hepatitis B can now be attributed to the inoculum, the variable intensity of host-viral interactions, the mode of transmission, and the varying distribution of genotypes.

HBeAg seroconversion and HBsAg seroclearance

Seroconversion of HBeAg and seroclearance of HBsAg have been recognized as important events in the natural history of chronic HBV infection, with estimated annual incidence of 12% and 2%, respectively.25–28 Earlier HBeAg seroconversion usually confers a favorable outcome, whereas late or absent HBeAg seroconversion after multiple hepatitis flares may accelerate the progression of chronic hepatitis to cirrhosis; it therefore has a poor clinical outcome.29 In our cohort study on 272 Taiwanese patients with chronic HBV infection genotype C patients are more likely to have HBeAg-positive chronic hepatitis B despite multiple hepatitis flares.30 In addition, spontaneous HBeAg seroconversion rate has been observed in 146 HBeAg-positive Taiwanese HBV-infected persons with a mean follow-up of 52 months.27 The results indicated that genotype C infection was associated with lower rates of spontaneous HBeAg seroconversion than genotype B (27% versus 47%, P < 0.025). The estimated annual rates of HBeAg seroconversion in genotype B and C infections were 15.5% and 7.9%, respectively.

Several studies also showed that the mean age at HBeAg seroconversion in genotype C patients is one decade older than that in genotype B patients.27,31 Furthermore, a long-term follow-up study with 460 Taiwanese HBV chronically-infected children indicated that the seropositive rate of HBeAg after 20 years of follow-up was 70% in genotype C and 40% in genotype B carriers.32 Taking these lines of evidence together, genotype C patients may experience delayed HBeAg seroconversion and thus a longer duration of high HBV replication than genotype B patients. With these adverse factors, genotype C patients are correspondingly more prone to develop advanced fibrosis, cirrhosis and HCC than genotype B patients. Similar observations have been reported from Hong Kong and Japan.33–36

Regarding genotypes A and D, one prospective study evaluated the clinical outcomes of 258 Spanish patients with chronic HBV infection; mean follow-up was 94 months.37 Although no differences were observed in the probability of HBeAg seroconversion between patients infected with genotype A and D, the rate of sustained remission after HBeAg seroconversion was higher in genotype A than genotype D (55% versus 32%, P < 0.01). As for spontaneous HBsAg seroclearance, compared to genotypes C and D, genotype A and B patients had a higher rate of HBsAg seroclearance.37,38 Taken together, these facts suggest the phenotype of HBeAg seroconversion differs between genotypes B and C as well as genotypes A and D during the early phase of chronic HBV infection. Further, genotype C and D patients, compared to genotype A and B patients, have late or absent HBeAg seroconversion after multiple hepatitis flares that may accelerate the progression of chronic hepatitis, thereby conferring a poor clinical outcome.

Disease progression to cirrhosis or hepatocellular carcinoma (HCC)

Most retrospective or case-control studies indicated that patients with genotype C infection have more severe liver disease, including cirrhosis and HCC, than those with genotype B.39–42 Recently, a community-based prospective cohort study on 2762 Taiwanese HBV carriers demonstrated that HBV genotype C was associated with an increased risk of HCC than genotype B; the adjusted hazard ratio was 2.35 (95% CI = 1.68 to 3.30; P < 0.001).43 These findings confirm that genotype C correlates with a higher risk of HCC development. Of interest, several reports showed HBV genotype B was associated with the early onset of HCC, whereas genotype C was associated with HCC development at older ages.32,39,44 The predominance of HBV genotype B in HCC patients was more prominent in those younger than 35 years, and most were cases of non-cirrhotic chronic hepatitis B.

HBV genotype also influences the clinicopathological features of patients with resectable HCC. In Taiwan, among 193 resectable HBV-related HCC patients, genotype B patients had a higher rate of solitary tumor (94% versus 86%, P = 0.048) but more satellite nodules (22% versus 12%, P = 0.05) than genotype C patients. These characteristics may contribute to the recurrence patterns and prognosis of HBV-related HCC patients with genotype B or C infection.45,46 As for other genotypes, death related to liver disease is more frequent in patients infected with HBV genotype D and F than those with genotype A infection.37,47,48

Interactions between HBV genotypes, viral load and viral mutants

In addition to HBV genotypes, emerging data reveal that HBV viral load and naturally occurring mutant strains are closely associated with long-term outcomes of HBV-related chronic liver disease.49,50 In an earlier study, we found that genotype C infections conferred a higher frequency of basal core promoter (BCP) A1762T/G1764A mutation than genotype B.51 In another prospective study with 4841 Taiwanese male HBV-infected patients without HCC at enrollment, Yu et al. found that HBV viral load was higher in genotype C than genotype B patients, while genotype C-infected patients who also had very high viral load had a 26-fold higher risk of HCC than those with other genotypes and low or undetectable viral loads.52 Furthermore, Yang et al. reported that among those infected with HBV genotype C, wild-type precore 1896 sequence, and BCP A1762T/G1764A mutation was associated with the highest risk of HCC during 13 year follow-up. The adjusted hazard ratio was 2.99 (95% CI, 1.57 to 5.70, P < 0.001) relative to those with genotype B infection, wild-type precore 1896 and BCP sequences.43

Similarly, patients with genotype D infection, who had more progressive liver disease, also had a higher prevalence of BCP A1762T/G1764A mutation than those with genotype A infection.53 Previous reports also showed that the deletions within the pre-S gene may contribute to more progressive liver cell damage and hepatocarcinogenesis.54,55 In our recent case-control study, the frequency of pre-S deletion was significantly higher in genotype C patients than genotype B patients. In addition, the presence of pre-S deletion was an independent risk factor associated with disease progression (OR, 3.91; 95% CI, 1.57–9.76, P = 0.003) as well as HCC development (OR, 3.72; 95% CI, 1.44–9.65; P = 0.007).56,57 A meta-analysis further confirmed that the odds ratio of HCC for pre-S deletion was 3.77 (95% CI, 2.57 to 5.52). Of particular note, the summary odds ratio for pre-S deletion was higher in genotype C patients than genotype B patients.58 Further investigations have demonstrated that the specific combination of viral load, HBV genotype, BCP A1762T/G1764A mutation and pre-S deletion is strongly associated with disease progression and development of HCC.43,59–61

Recently, several clinical scoring systems, or nomograms, consisting of previously confirmed independent risk predictors such as sex, age, family history of HCC, alcohol consumption, serum alanine aminotransferase (ALT) level, HBeAg status, serum HBV DNA level, and/or HBV genotype have been introduced.62–64 (Fig. 1) These easy-to-use nomograms are based on noninvasive clinical characteristics and have been found to accurately predict HCC risk in either community- or hospital-based HBV-infected persons. Their use could facilitate communication between practicing physicians and patients in daily practice. However, these predictive scoring systems need further validation in different populations across the world.

image

Figure 1. Nomograms for the prediction of the risk of hepatocellular carcinoma. Adapted from Yang HI et al. J Clin Oncol 2010;27:2437–2444.64 Reprinted with permission. © 2008 American Society of Clinical Oncology. All rights reserved. inline image, 5-year risk; inline image, 10-year risk.

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Molecular mechanisms involved in different pathogenesis among HBV genotypes

Sugiyama et al. recently reported the some differences in the immunopathogenesis of chronic hepatitis B between various genotypes. The intracellular expression of HBV DNA and hepatitis B core antigen (HBcAg), as well as extracellular expression of HBV DNA and HBeAg, were higher for genotypes B and C than genotypes A and D. The intracellular accumulation of HBV DNA and viral antigens may play a role in inducing liver cell damage. In addition, the higher replication capacity of genotype C HBV may explain why this genotype is associated with more severe liver disease than others.65,66

HBV genotype and response to anti-viral therapy

Standard and pegylated interferon

The therapeutic endpoints for chronic hepatitis B treatment include sustained suppression of HBV replication to below the detection limit of real-time PCR assays, biochemical remission, histological improvement, HBeAg loss or HBeAg seroconversion for HBeAg-positive patients, and ideally HBsAg loss and HBsAg seroconversion.6–8 Currently, two types of therapy are recommended: standard or pegylated interferon alpha (IFN-α) and five nucleos(t)ide analogues, including lamivudine, telbivudine, entecavir, adefovir dipivoxil and tenofovir disoproxil fumarate.6–8 Although HBV genotyping before anti-viral therapy is not recommended by current guidelines from three regional liver associations, the American Association for the Study of Liver Disease (AASLD),6 the European Association for the Study of Liver (EASL),8 and Asian Pacific Association for the study of liver (APASL),7 the impact of HBV genotype on therapeutic response to both interferon-based and nucleos(t)ide analogues has been increasingly recognized.67,68 In HBeAg-positive patients treated with standard IFN-α, the sustained response rate, defined as normalization of serum ALT level and HBeAg seroconversion post-treatment, is significantly better in genotype A and B patients than for genotype C and D.51,69–71 For HBeAg-positive Asian populations, HBV genotype B patients are more susceptible to IFN-based therapy, regardless of pegylated or standard type IFN products, whereas genotype C patients have a higher likelihood of response to pegylated IFN-α compared to standard IFN-α.72,73

Recently, Zhao et al. assessed the efficacy of low-dose, 24-week standard IFN-α or pegylated IFN-α treatment as well as factors predicting sustained response in Chinese patients with HBeAg-positive chronic hepatitis B.74 They found that HBV genotype B infection and younger age were independent factors associated with sustained response, suggesting low-dose IFN regimen may be cost effective for the treatment of younger patients with genotype B infection. Another multi-center study on pegylated IFN-α for HBeAg-positive patients revealed that the rate of HBeAg clearance also differed according to HBV genotypes: genotype A, 47%; genotype B, 44%; genotype C, 28%; and genotype D, 25%.75 Subsequent analysis consistently demonstrated a higher rate of HBsAg clearance in genotype A compared to other genotypes in both HBeAg-positive and HBeAg-negative chronic hepatitis B.76 In addition, compared to genotype C and D patients, durable loss of HBeAg at 3 years after pegylated IFN-α treatment was higher in genotype A and B patients.77

Among HBeAg-negative patients treated with pegylated IFN-α, a long-term follow-up study also showed that HBsAg clearance was significantly higher in genotype A (20%) than genotype B (6%), genotype C (9%), and genotype D (6%).78 In addition, HBsAg kinetics during pegylated IFN-α treatment also varied between different HBV genotypes. For example, at the end of treatment, mean decrease of HBsAg level was high with genotype A infection, intermediate in genotypes B and D, and low in genotypes C and E. During follow-up, serum HBsAg continued to decrease in genotypes A and D, whereas rebound was observed in genotypes B, C and E.79

Recently, a meta-analysis further confirmed that HBV genotype A has better responses to IFN-α treatment than genotype D patients, regardless HBeAg status. Further, HBV genotype B has a higher response rate to IFN-α treatment than genotype C in HBeAg-positive patients.80 Collectively, patients with genotype A and B infection have better response to IFN-α than those with genotype C and D infections (Fig. 2).81 Recent pooled data from two largest global trials of HBeAg-positive patients with pegylated IFN-α treatment showed that genotype A patients with higher levels of ALT or lower levels of HBV DNA, and genotypes B and C patients having both higher ALT levels and lower HBV DNA levels had a high predicted probability of a sustained response. Of note, genotype D patients had the lowest chance of sustained response, irrespective of ALT or HBV-DNA levels.82 Therefore, in addition to viral factors, the responses to IFN-based therapy are also invariably affected by host factors.83

image

Figure 2. Hepatitis B virus genotypes and response to pegylated interferon therapy. (a) The rate of HBeAg loss/seroconversion in HBeAg-positive patients at 6 months post treatment. Data pooled from Lau and Janssen (73, 75). * The rate of HBsAg clearance/seroconversion in HBeAg-positive patients at 6 months post treatment. Data pooled from Lau and Flink (73, 76). ** The rate of HBsAg clearance in HBeAg-negative patients at 3 years post treatment. Data from Marcellin (78). inline image, HBeAg positive; inline image, HBeAg negative.

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Contrary to genotype A–D, patients infected with genotype E–J are rarer and their responses to IFN-based therapy remain largely unknown. According to a preliminary study, HBV genotypes E, F, and H seem to be more susceptible to IFN-α therapy than genotype G.84 However, further large studies with long-term follow-up are awaited to address this important issue.

Nucleoside and nucleotide analogues

In patients treated with nucleos(t)ide analogues, Chien et al. first reported that the sustained response rate to lamivudine was much higher in genotype B patients than genotype C patients.85 However, two studies from Hong Kong showed contradictory findings.86,87 In addition, the development of lamivudine or telbivudine resistance was similar between genotype B and C.88,89 In Spain, Buti et al. also found that the outcome after lamivudine treatment, as well as the emergence of lamivudine resistance, were comparable between genotype A and D.90 Similarly, no statistical difference was found in response to adefovir dipivoxil,91 entecavir92 and telbivudine89 among patients with different genotypes.

A recent meta-analysis consistently found no significant association between HBV genotype and response to nucleos(t)ide analogues.80 Although HBV genotypes seem not to have impact on the response and resistance to nucleos(t)ide analogue treatment,81 our retrospective study found that HBV genotype B was independently associated with earlier detection of lamivudine-resistant strains. In addition, genotype B was significantly associated with development of lamivudine resistance within the first 12 months of lamivudine therapy compared with genotype C (odds ratio 8.27; P = 0.004).93 Therefore, more frequent monitoring of genotypic resistance might be needed for specific HBV genotypes during nucleos(t)ide analogues therapy.

Marcellin et al. recently reported that 5 of 158 HBeAg-positive patients treated with tenofovir disoproxil fumarate lost HBsAg at 48 weeks of treatment.94 Among these 5 patients with HBsAg loss, 2 and 3 were infected with genotype A and D, respectively. Although the proportion of patients with HBsAg loss is too small to reach any conclusion, the association between HBV genotype and nucleos(t)ide analogues-induced HBsAg loss is anticipated and deserve further studies.

Summary and perspectives

  1. Top of page
  2. Abstract
  3. Introduction
  4. Summary and perspectives
  5. References

In the past decade, we have witnessed advances in research regarding the clinical implications of HBV genotype. In brief, compared to genotype A and B patients, genotype C and D patients have later and infrequent HBeAg seroconversion as well as a higher risk of disease progression (including HCC) and therefore, a poorer clinical outcome. Although genotype A and B patients have a better response to IFN-based therapy than genotype C and D patients, no significant association can be found between HBV genotype and therapeutic response to nucleos(t)ide analogues. On the basis of accumulating lines of evidence, it is recommended that HBV carriers should be routinely genotyped to help identify those who are at higher risk of liver disease progression, and who can benefit most from IFN-based therapy.83,95 At the start of this new decade, clinical trials stratified by different genotypes and treatment regimens are mandatory to determine the optimal treatment strategy for chronic hepatitis B patients.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Summary and perspectives
  5. References