COMMENTARY Hepatitis B

COMMENTARY

Hepatitis B: The Next Generation

Commentary on the article by Ni et al. on page 396

EVE A. ROBERTS Division of Gastroenterology and Nutrition, The Hospital for Sick Children, Toronto, Ontario, Canada M5G 1X8 and Departments of Paediatrics, Medicine and Pharmacology, University of Toronto, Toronto, Ontario, Canada M5S 1A8

nfection with hepatitis B virus (HBV) remains an important protein which acts as a transactivator). The S open reading Iglobal health problem. Approximately 350 million people frame is divided into the S, preS1, and preS2 regions. “S” worldwide have chronic HBV infection, of whom three- encodes the small protein; SϩpreS2 encodes the middle quarters are in Asia, Southeast Asia and Polynesia. Chronic protein; SϩpreS1ϩpreS2 encodes the large protein. Both HBV infection often leads to cirrhosis and hepatocellular the core protein HBcAg and the secreted HBeAg protein are carcinoma. An important success story of 20th century hepa- encoded by the preC/C gene. These two proteins share ~160 tology is determining the biology of HBV and devising effec- amino acids. Interference with production of HBcAg will tive strategies for prevention and medical management of HBV result in failure to produce and secrete HBeAg. Detectable infection. Although distinctive clinical patterns of acute hepa- HBeAg in serum indicates ongoing viral replication and titis were recognized in the 1940s, the real breakthrough came high infectivity. The DNA polymerase and the X protein are with the identification of the “Australia antigen” (later called the principal nonstructural proteins. the hepatitis B surface antigen, HBsAg) in 1965. By the end of the The HBV replication cycle is complicated (4). Viral entry 1970s, the entire HBV genome had been cloned. In 1982 the first into the hepatocyte appears to involve structural components of anti-HBV vaccine was introduced, and by the mid-1990s antiviral the large and middle HBsAg proteins and annexins, and pos- treatment with interferon-␣ was shown to be effective, and even sibly other hepatocellular membrane proteins. Inside the hepa- curative, in some patients. Antivirals designed to interfere with tocyte the virus uncoats. Its DNA is converted within the HBV replication were developed, and one of them, lamivudine, nucleus by host apparatus into closed circular DNA (cccDNA), is now in wide clinical use. Recent reports show that universal which is then organized into viral minichromosomes. This infant vaccination programs have reduced both the incidence of becomes the template for production of viral RNAs including hepatocellular carcinoma in young adults by eliminating ver- the pregenomic RNA (pgRNA), which is essential for HBV tically transmitted chronic hepatitis B (1,2) and occurrence of replication. The pgRNA is translated to produce the core acute hepatitis B in adolescents (3). Despite this impressive protein and DNA polymerase, and it plays a role in nucleo- progress, important questions remain unanswered. New chal- capsid formation. The HBV DNA is then generated by the viral lenges are emerging. DNA polymerase from the pgRNA. Thus HBV functionally HBV is a 42-nm coated DNA virus with a partially double- resembles a retrovirus. Recent work has shown that these stranded 3.2 kb genome. The nucleocapsid, or core particle, a processes depend on the formation of a unique structure known 27-nm icosahedron, is composed of the core protein (HBcAg), as epsilon (⑀), which is a stem-loop structure found at both the viral DNA and polymerase. The envelope is composed of 5' and 3'-ends of the pgRNA. The interaction of viral DNA lipids derived from the host combined with the viral surface polymerase with ⑀ is stabilized by host chaperone proteins such antigen (HBsAg), which is made up of three related proteins as heat shock protein 90. After the partial double-stranded viral (small, middle, large), variably glycosylated. Excess HBsAg DNA is generated, it is either recycled back to augment the spontaneously aggregates to form noninfectious spherical and amount of cccDNA available for generating pgRNA or sent on tubular forms. HBV has an extremely compact genome utiliz- to the endoplasmic reticulum and Golgi apparatus for further ing multiple transcriptional start sites and four overlapping modification, followed by assembly of the complete virion and open reading frames to produce an astonishingly large number secretion from the cell. Some cccDNA persists indefinitely in of proteins. The open reading frames are S (surface), C infected hepatocytes. (core), P (polymerase) and X (encoding a nonstructural HBV is a successful virus because it establishes a stable symbiosis within hepatocytes. It is for the most part not Received December 8, 2003; accepted May 21, 2004. cytopathic, and its utilizing hepatocellular proteins and or- Correspondence: Eve A. Roberts, M.D., FRCPC, The Hospital for Sick Children, Division of Gastroenterology and Nutrition, 555 University Avenue, Toronto, Ontario ganelles does not seem to interfere very much with normal M5G 1X8, Canada; e-mail: [email protected] hepatocellular function. Liver damage is due to the host’s

318 COMMENTARY 319 immune reaction (5). In the typical case of acute infection in thal. Thus, although mutations occur frequently in all HBV adolescents and adults, this is a brisk polyclonal cytotoxic T genes, only a few appear to have clinical impact (11,12). lymphocyte response to multiple viral epitopes, which elimi- Mutations in response to external selection pressures include S nates the virus at a cost of substantial, but transient, hepato- gene mutations that permit escape from anti-HBs protection cellular damage, with elevated serum aminotransferases and through vaccination and DNA polymerase mutations (usually sometimes jaundice. The host elaborates the protective anti- the “YMDD” mutation affecting mainly the methionine), body to HBsAg (anti-HBs) and becomes immune to further which render HBV resistant to certain antivirals like lamivu- HBV infection. When HBV infection is acquired around birth dine. Precore and core promoter mutants may be selected by or in infancy, the most frequent scenario worldwide, the natural the strength of the host immune response. These mutations history is much more complicated. Most infants develop tol- interfere with production of HBeAg; they are highly prevalent, erance for HBV, which proliferates freely in the liver, without especially in Asian populations (13,14). The most common liver damage. At some point this tolerance breaks down. HBV mutation affecting the precore region isaGtoAtransversion clearance appears to depend on the concerted action of Th1 at nucleotide 1896, which produces a stop codon terminating cells and cytotoxic T lymphocytes. An immune response dom- protein translation. This mutation is not found in genotype A inated by Th2 cells appears to be associated with HBV persis- virus because in genotype A the 1896 guanosine is paired to tence. Hepatocyte damage ensues, although some of these cytosine at nucleotide 1858, and the G3A transversion results antiviral processes are not necessarily cytolytic. Low-grade in a A-C pair which destabilizes ⑀. In contrast, in genotypes hepatitis may be intermittent, with accruing liver injury. Clin- B-D the resulting base pair with an 1858 thymidine actually ically, the turning point in chronic hepatitis B is characterized stabilizes the ⑀ structure and enhances viral replication. The by a loss of detectable serum HBeAg, elaboration of antibody core promoter region is important because it regulates tran- to this viral protein (anti-HBe), and marked decrease in the scription of pgRNA and the precore mRNA. A double muta- serum concentration of HBV DNA. Serum aminotransferases tion at nucleotides 1762 and 1764 occurs and is associated with normalize. Most of the residual hepatocellular HBV subse- decreased expression of HBeAg (15). Core promoter region quently integrates into host hepatocellular DNA. HBsAg re- mutations have been described in genotypes A-D. Individuals mains positive because a small actively replicating viral pop- chronically infected with HBV may harbor wild type viruses of ulation persists, but liver damage becomes negligible. Two multiple genotypes and/or various HBV mutants. A specific clinical disease patterns may later develop: bursts of self- mutant HBV may become the dominant viral form over the limited hepatitis presumably with further hepatic damage, or course of chronic infection. hepatocellular carcinoma. Over time, in relatively rare individ- Despite its importance, how HBeAg seroconversion occurs uals HBV may be essentially eliminated: they become negative is not known. Multiple mechanisms may exist. In both adults for HBsAg and make anti-HBs. Whether such elimination of and children development of core promoter or precore muta- HBV occurs spontaneously or as a result of treatment, highly tions may play a role, possibly by affecting viral susceptibility sensitive analytical methods can reveal the presence of HBV to the host immune reaction. Studies in adults from Hong Kong DNA in hepatocytes or other tissues, whose clinical signifi- (16,17) and from France (18) showed high prevalence of cance is uncertain. Accomplishing HBeAg seroconversion is precore and core promoter mutants in anti-HBe positive pa- sufficiently important biologically that it is endpoint for anti- tients. Data from 343 Hong Kong patients with chronic HBV viral treatment efficacy. At very low levels of viral load hepatic infection showed that HBeAg seroconversion occurred earlier fibrosis may resolve extensively. with genotype B than with genotype C, but occurrence of HBV displays important diversity. This viral diversity was severe hepatic disease did not differ between the two geno- initially described in terms of serological subtypes based on types; genotype B was associated with a higher percentage of features of the HBsAg, useful as epidemiologic markers but precore mutations and C with a higher percentage of core devoid of major clinical implication. HBV has recently been promoter mutations (19). Studies of small numbers of Euro- classified genetically. Genotypes are distinguished on the basis pean children with HBV infection have revealed precore and of at least 8% genetic nonidentity: to date, eight genotypes core promoter mutations in children with fulminant hepatitis, (A-H) have been identified (6–8). Notably, the genotypes and genotype A3D changes after HBeAg seroconversion, also serological subtypes do not have a one-to-one correspondence. mutations in the preS region (20,21). The paper by Ni and HBV genotypes display geographical variation: A and D are colleagues in this issue of Pediatric Research extends our pandemic, B and C mainly in China, Japan, Viet Nam and knowledge of precore/core mutations in children with HBV Polynesia, E in Africa, F in North American indigenous peo- infection. Their previous observations in pediatric patients ples and Polynesia, G identified in North America and Europe, from Taiwan showed that the 1896G3A mutation was found H in Central America. Differences between genotypes may in one-quarter of patients before and after HBeAg seroconver- explain some of the variations in disease course observed sion, only after HBeAg seroconversion in another quarter, and clinically (9). Different genotypes may also respond differently not at all in one-half of the patients (22). Core mutations were to interferon-␣ treatment (10). found in all children with chronic HBV infection but a char- HBV diversity extends to mutant viruses. The imprecision of acteristic clustering of such mutations in children who devel- its DNA polymerase and lack of editing/correction capacity oped hepatocellular carcinoma (23). In the current prospective predisposes HBV to mutation, but its compact genome with study of children with chronic hepatitis B undergoing HBeAg overlapping open reading frames renders many mutations le- seroconversion, ~90% of children had core promoter or pre- 320 COMMENTARY core mutations during the seroconversion process; however, 8. Arauz-Ruiz P, Norder H, Robertson BH, Magnius LO 2002 Genotype H: a new ϩ Amerindian genotype of hepatitis B virus revealed in Central America. J Gen Virol the 1762 1764 paired core promoter mutation played a 83:2059–2073 minimal role in HBeAg seroconversion. 9. Clarke B, Bloor S 2002 Molecular genotyping of hepatitis B virus. J Clin Virol 25:S41–S45 10. Wai CT, Chu CJ, Hussain M, Lok AS 2002 HBV genotype B is associated with better CONCLUSION response to interferon therapy in HBeAg(ϩ) chronic hepatitis than genotype C. Hepatology 36:1425–1430 In summary, the complexity of disease course with chronic 11. Feitelson MA 1994 Biology of hepatitis B virus variants. Lab Invest 71:324–349 12. Francois G, Kew M, Van Damme P, Mphahlele MJ, Meheus A 2001 Mutant hepatitis HBV infection reflects viral diversity—different genotypes and B viruses: a matter of academic interest only or a problem with far-reaching multiple mutational events—and host diversity in terms of implications? Vaccine 19:3799–3815 13. Papatheodoridis GV, Hadziyannis SJ 2001 Diagnosis and management of pre-core immune response. Developmental considerations also apply, mutant chronic hepatitis B. J Viral Hepat 8:311–321 and children differ from adults in how they handle HBV 14. 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