Department of Infectious Diseases, University of Lund, SE-221 85, Lund, Sweden1
Miyakawa Memorial Research Foundation, Tokyo 107-0062, Japan2
Department of Virology, University of Ume, SE-901 85 Ume
, Sweden3
Author for correspondence: Karin Kidd-Ljunggren. Fax +46 46 137414. e-mail Karin.Kidd{at}infek.lu.se
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Abstract |
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Introduction |
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Before the advent of PCR, the molecular characterization of HBV was a cumbersome process, as it was shown very early on that HBV would not grow in cell culture and was highly species specific, only infecting humans and some other primates. HBV was the first member to be discovered of a family of viruses, later designated Hepadnaviridae. This family has since been divided into two groups, the orthohepadnaviruses and the avian hepadnaviruses. These are hepatotropic, partially double-stranded DNA viruses. Their replication strategy is unique for animal DNA viruses and only shared by cauliflower mosaic virus (also a DNA virus), in that they use an RNA intermediate and a reverse transcription step (Seeger, 1991 ). The circular genome is very compact, with four partially overlapping open reading frames (ORFs) (Fig. 1
). There are no non-coding regions in the genome, so that all regulatory signals are also part of protein-encoding sequences; HBV can encode approximately 50% more protein than would be expected from its genome length (Ganem & Varmus, 1987
). In terms of HBV evolution, this leads to two opposing tendencies: the use of reverse transcriptase with its lack of proofreading tends to maintain a relatively high mutational rate, whereas the extreme compactness of the genome will prevent a large degree of genetic variability from occurring.
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Subtypes |
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During the 1980s, it became increasingly clear that the subtype determinants are specified by one single amino acid, at positions 122 (d or y) and 160 (r or w) in the S protein, respectively (Okamoto et al., 1987b , c
; Ashton-Rickardt & Murray, 1989a
, b
; Norder et al., 1991
). Subtype determinants d and w have a lysine at both positions, whereas an arginine at both positions indicates subtype determinants y and r (Table 1
). Additional subtype determinant reactivities have been mapped to amino acid positions 127, 144, 145, 158, 159, 177 and 178 (Okamoto et al., 1989
; Norder et al., 1992a
).
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Genotypes, history and classification |
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An extended study, where the complete nucleotide sequences of several strains were compared, showed that the previously described group E found in West Africa, although closely related to group D, constituted a genotype of its own (Norder et al., 1994 ). The year before, a highly divergent (15%) strain from Brazil had been reported (Naumann et al., 1993
). It expressed the adw4 phenotype and constitutes genotype F. It has often been used as an outgroup in phylogenetic studies of HBV, as it is the most divergent human-derived genotype reported. Recently, Stuyver et al. (2000
) described an additional genotype, G (Table 2
).
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Non-human hepadnaviruses |
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A third mammalian hepadnavirus (GSHV) was found in California Beechey ground squirrels (Spermophilus beecheyi) (Marion et al., 1980 ). Its genome is more related to WHV (82% nucleotide identity) than to HBV (55% homology) (Tiollais et al., 1988
). More recently, another ground squirrel hepatitis virus, this time found in wild arctic ground squirrels (Spermophylus parryi kennicotti) in Alaska, showed approximately 84% identity to GSHV as well as WHV and was designated arctic squirrel hepatitis virus (Testut et al., 1996
). The death of a woolly monkey (Lagothrix lagotricha) from the Louisville Zoological Gardens from fulminant hepatitis led to the discovery of woolly monkey HBV (WMHBV). Among the orthohepadnaviruses, this virus is most closely related to HBV, the largest variability being seen between preS1 regions (Lanford et al., 1998
).
The first avian hepadnavirus to be identified was the duck HBV (DHBV) infecting Pekin ducks (Anas platyrhynchus) in China (Zhou, 1980 ) and the USA (Mason et al., 1980
). It is highly divergent from the other hepadnaviruses, with only about 40% nucleotide identity to HBV (Tiollais et al., 1988
). Related to DHBV, but diverging 22% in nucleotide sequence, the heron HBV (HHBV) was found in German grey herons (Ardea cinerea) by Sprengel et al. (1988
). Stork HBV was recently isolated and appears most closely related to HHBV (Pult et al., 2001b
).
The host range of all hepadnaviruses is narrow. DHBV has been shown to infect geese and GSHV has been transmitted to woodchucks, but HHBV could not be shown to induce infection in ducklings (Sprengel et al., 1988 ). HBV can infect chimpanzees and some other primates but there also appear to be separate non-human primate genotypes within HBV.
The HBV strain from a persistently infected chimpanzee in the London Zoo was characterized and shown to diverge by about 10% from other human HBV strains (Vaudin et al., 1988 ). Recently, there have been a large number of reports about variant HBV strains isolated from different primates. Serum from a chimpanzee that had been inoculated with serum from a white-handed gibbon (Hybolates lar), infected in the wild, showed the presence of an HBV genome most closely related to the chimpanzee strain described by Vaudin et al. (1988
) (Norder et al., 1996
). Phylogenetic analysis suggested that the reported non-human HBV strains were indigenous to their respective hosts and not acquired recently. An extended study by Lanford et al. (2000
) confirmed the existence of the gibbon HBV as a separate group within the human hepadnaviruses but suggested that the gibbon strains had diverged recently from the human HBV strains. The strains isolated from orangutans (Pongo pygmeaeus) in captivity and also in the wild (Warren et al., 1999
; Verschoor et al., 2001
) are more distantly related to human HBV. Two independent reports on wild chimpanzee strains from West Africa (Hu et al., 2000
; MacDonald et al., 2000
) found a close relationship with the strain above described from the London Zoo. A strain isolated from a captive gorilla (Gorilla gorilla) originating from Cameroon clustered with the chimpanzee strains (Grethe et al., 2000
). Relationships between representative primate HBV strains are illustrated in Fig. 2
.
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Structural differences between hepadnavirus genomes |
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It is noteworthy that amongst HBV strains belonging to HBV genotype D, there is a 33 nt deletion in the preS1 region (Heermann & Gerlich, 1991 ). No such deletion has been seen in other genotypes, even in members of the more closely related genotype E (Norder et al., 1994
; Bowyer et al., 1997
).
One salient feature of all hepadnaviruses is the secondary structure formed by the pregenomic RNA in the precore core region. Due to the ca. 130 nt terminal redundancy in pregenomic RNA, there are two copies of this structure, one at each end of the genome. The 5' version of this stemloop structure forms the encapsidation signal , which directs the packaging of pregenomic RNA into immature core particles during replication. Although there is a considerable sequence variation between different hepadnaviruses, they all form a stable stemloop structure in this region and there is a large degree of sequence conservation within the different hepadnavirus groups. The stability of the stemloop structure depends on strict conservation of base pairing in the stem region; mutations disrupting base pairing may lead to less efficient replication or non-viable virus particles. It is interesting to note that a G to U nucleotide change in the distal part of the lower stem, changing the sequence 1893UUUGGGG1899 to UUUUGGG is seen in non-human HBV strains isolated from chimpanzees, orangutans, gorilla and gibbon, but also shared by WMHBV, a non-HBV strain (Fig. 3
). This would affect the stability of the stemloop structure in a similar way as does the G to A mutation at position 1896 in HBV genotype A strains (Li et al., 1993
).
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The phenomenon of RNA splicing in HBV has been known for some time (Günther et al., 1997 ). In a recent study of spliced HBV genomes by Sommer et al. (2000
), the authors found that minor splice variants differed between genotypes, implying that some minor splice sites are active only in certain genotypes. The same authors support the suggestion put forward by others (Rosmorduc et al., 1995
; Soussan et al., 2000
) in that splicing events may contribute to the persistence of HBV.
In a study where the secondary structure of the whole pregenomic RNA was predicted by computer modelling, some differences in RNA folding between genotypes could be seen (Kidd-Ljunggren et al., 2000 ).
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Distribution of HBV genotypes |
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Genotype B and C strains belong in the indigenous population of Southeast Asia (Okamoto et al., 1988 ; Kidd-Ljunggren et al., 1995
; Theamboonlers et al., 1999
). Their distribution is fairly intermixed, with a tendency toward more genotype C strains being found in the Northern mainland regions and in mainland Japan (Orito et al., 2001a
). However, genotype C especially is also found in the populations of the South Pacific islands, where the prevalence of HBV carriers is sometimes very high (Gust, 1984
). Interestingly, it is possible to differentiate genotype C strains geographically by subtype. The genotype C strains isolated from the Pacific islands are more often of the adrq subtype, as compared to those strains from Southeast Asia (Norder et al., 1993b
).
Genotype D is the most widely distributed genotype and has been found universally, with its highest prevalence in a belt stretching from Southern Europe and North Africa (Norder et al., 1993b ; Borchani-Chabchoub et al., 2000
) to India, in West and South Africa (Bowyer et al., 1997
), and among intravenous drug users on all continents (Kidd-Ljunggren et al., 1999
; Bläckberg et al., 2000
; Flodgren et al., 2000
). Genotype E is the most similar to genotype D genetically (Norder et al., 1993a
, b
, 1994
) and has been interpreted as a subset of genotype D when using the X gene for phylogenetic analysis (Kidd-Ljunggren et al., 1995
). It is found in West and South Africa and one of the main differences from genotype D is that it does not have the 33 nt deletion at the beginning of the preS1 region which is common to all genotype D strains (Bowyer et al., 1997
; Norder et al., 1994
; Odemuyiwa et al., 2001
).
The most divergent genotype, F, is found in South and Central America (Norder et al., 1993a ; Arauz-Ruiz et al., 1997a
, b
; Blitz et al., 1998
; Mbayed et al., 1998
; Nakano et al., 2001
). Although it shares some structural features with genotype A strains (see above), it is believed to be the original genotype of the New World. It shows less homology than the other genotypes to the different primate strains that have been described. Genotype G has been found in France and the USA (Stuyver et al., 2000
) but not in Japan (Kato et al., 2001
).
In many countries where well-known waves of migration have occurred over time, the prevalence of different HBV genotypes reflects the origin of the immigrants and other patterns of migration. This is exemplified by South Africa, where the most prevalent genotypes, A and D (Bowyer et al., 1997 ), correlate with migration from Northwestern Europe (UK and the Netherlands), Southern Europe and India. The same genotypes in Argentina, A and D (Mbayed et al., 1998
), reflect migratory waves from Northwestern Europe, Italy and Spain. In New Zealand and Australia, the same genotypes feature strongly, together with a number of genotype C strains contributed by immigrants from Southeast Asia and the Pacific Islands (Kidd-Ljunggren et al., 1995
; Sugauchi et al., 2001
). In a study of Belgian children who had received interferon treatment for chronic HBV infection, a child originating from Haiti harboured a genotype E strain (Liu et al., 2001
). As HBV genotype E strains are found exclusively in West and South Africa, this provides a parallel to the chain of events reported for the transmission of human immunodeficiency virus (HIV) to the New World. Another unexpected genotype E infection was found in a Swedish sailor with acute HBV who had received a vitamin injection in West Africa 3 months earlier (unpublished data).
Not only migration but also behavioural patterns may change the prevailing genotype in a given region. In a recent study by Koibuchi et al. (2001 ), Japanese homosexual men coinfected with HIV were unexpectedly found to harbour HBV genotype A instead of C or B, which are the prevailing genotypes in Japan.
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Mutation rate and evolution pattern of HBV |
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The rate of substitutions in vivo depends on a number of factors, which are both separate and interdependent of each other. Thus, the conflicting virus strategies of a compact genome and replication through reverse transcription combine with host factors, such as immune response, and also with the risk of mutations arising from antiviral treatments. The host response appears to be an important factor, as there is evidence that the mutation rate over many decades is negligible in silent or occult HBV infection, where there is minimal host response (Bläckberg & Kidd-Ljunggren, 2000b ). In a survey of Australian HBV-carrying schoolchildren, some of whom had been followed for 2 years, no sequence variations were found over time (McIntosh et al., 1998
), which implies that there may not be much change in healthy carriers either. Preliminary results from a family where a carrier mother transmitted the infection to all of her five children, showed that there was no sequence variation between the HBV strain of the mother and those of the children, two of whom were identical twins aged 8 years (unpublished data). The study of the evolutionary rate of a viral genome in patients with genetically identical immune responses may partly eliminate one of the confounding factors present when assessing in vivo mutation rates.
The evolutionary rates of other hepadnaviruses, namely WHV and DHBV, have been studied experimentally (Girones & Miller, 1989 ; Argentini et al., 1999
; Pult et al., 2001a
). By measuring the number of revertants of a cytopathic DHBV strain injected into ducklings, a mathematical model was used to estimate the number of substitutions to between 0·8 and 4·5x10-5 per site per generation (Pult et al., 2001a
), which approximates the mutation rate for HBV suggested by others (Okamoto et al., 1987a
; Orito et al., 1989
).
There have been several recent attempts to analyse the evolutionary history of HBV, both in terms of the origin of HBV and the time point when it diverged from the other hepadnaviruses (Norder et al., 1996 ; Warren et al., 1999
; Lanford et al., 2000
; Takahashi et al., 2000
) and also from the point of divergence between HBV genotypes (Mizokami et al., 1997
; Bollyky & Holmes, 1999
). MacDonald et al. (2000
) have summarized the attempts by several authors to determine the historical relationship of human and non-human HBV strains and suggested that a much larger number of both primate strains and human HBV strains, from poorly investigated areas with high endemicity, need to be analysed before any firm conclusions can be made.
Highlighting the significance of the overlapping ORFs for the mutation rate of HBV, Mizokami et al. (1997 ) proposed the term constrained evolution for the evolution of HBV. Due to the variability of substitution rates observed in an in-depth phylogenetic analysis of a large number of complete genomes and S gene sequences, no reliable molecular clock for the development of the HBV genome could be obtained and the origin of HBV remains obscure (Bollyky & Holmes, 1999
).
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Recombination |
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Clinical differences between HBV genotypes |
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Early studies demonstrating subtype-related clinical differences include the association of Gianottis disease with subtype ayw in Japan (Ishimaru et al., 1976 ) and a higher frequency of liver dysfunction in adr-infected patients compared to those infected with adw (Shiina et al., 1991a
, b
; Noguchi et al., 1994
). Taking into account that genotype C strains are most often of subtype adr, the latter results have been confirmed by several studies of Southeast Asian chronic carriers (Lindh et al., 1999
; Ding et al., 2001
; Orito et al., 2001b
).
The correlation between chronic HBV infection and HCC has been proven both epidemiologically (Beasley et al., 1981 ) and experimentally (Brechot et al., 1980
). Most large-scale reports of HCC and HBV have come from Southeast Asia, reflecting the high prevalence of chronic HBV infection in this region of the world. In a recent study by Kao et al. (2000a
), a large number of Taiwanese HCC patients was studied and compared to patients with cirrhosis and asymptomatic carriers. These authors found that genotype C was more common in cirrhotic patients. There was also a striking difference in genotypes found in HCC patients, depending on the age of the patient. In HCC patients older than 50 years, genotype C was the most prevalent, whereas in patients younger than 50 years, genotype B was the most common. This difference was even more pronounced in the HCC patients under 35 years in whom no genotype C was found. A possible explanation would be that genotype C infection leads to HCC through cirrhosis. These results are partly in conflict with those described by Orito et al. (2001a
) from Japanese patients, where genotype B infected patients with HCC were older than HCC patients with genotype C. In a prospective study where a large number of cirrhotic patients was followed, genotype C-infected patients developed HCC more frequently than genotype B-infected patients. The genotype C-associated HCC was also more resistant to treatment (Tsubota et al., 2001
).
There have been few reports of genotype correlations in fulminant hepatitis. It is notable that von Weizsäcker et al. (1995 ) found heterogeneous virus populations in sera from three carrier mothers who transmitted neonatal fulminant hepatitis to their babies. Subtypes adw2 and ayw were present simultaneously in the mothers sera. In one baby who survived, both subtypes were transmitted. In the two babies who died, only subtype ayw could be detected. Analysing strains from fulminant hepatitis patients in Vietnam, Yuasa et al. (2000
) found that they mostly belonged to genotype B and that fulminant genotype B strains differed from non-fulminant strains by a specific mutation in the X gene. Outbreaks of fulminant virus hepatitis in some parts of the world have been associated with concomitant infection with hepatitis D virus (HDV). HBV genotype F together with HDV genotype 3, seen in outbreaks in Northern South America, is believed to be more highly correlated with the development of fulminant hepatitis. However, in a recent study of fulminant hepatitis in Samara, Russia, the prevailing strains were HBV genotype D and HDV genotype 1 (Flodgren et al., 2000
).
In a cross-sectional study by Mayerat et al. (1999 ), genotype A was suggested to lead more often to chronicity as it was found more often in chronic hepatitis patients than genotype D, whereas the opposite situation was found in patients with acute hepatitis. An overrepresentation of drug addicts in the acute case group could well explain the higher prevalence of genotype D, the genotype predominantly infecting intravenous drug users in the Western world. In another study, genotype D was found to be associated with more severe disease in post-transplant patients with recurrence of HBV infection (McMillan et al., 1996
). However, most of the genotype D strains described had single or double mutations at the end of the precore gene (1896G to A and 1899G to A). These mutations have been associated with more severe disease and have been found in many studies where strains leading to fulminant hepatitis were analysed. In order to minimize the influence of confounding factors when interpreting results such as these, large-scale studies are necessary. Ideally, a prospective study where neonatally infected babies infected by genotype A, B, C, D, E, F or G were followed until adulthood would give an answer to the question of genotype differences in the long-term outcome of HBV infection.
Interferon, which has been used to treat HBV infections, was shown to give better response in patients from Northwestern Europe than in vertically infected patients from Southeast Asia (Thomas et al., 1987 ). It is quite possible that the origin of the patient plays a low role in these observations and that the differences rather reflect the HBV genotypes prevailing in these separate geographical regions. Two recent studies support this theory. A retrospective analysis of the results of interferon treatment in chronic carriers (genotypes B and C) demonstrated that genotype C had a lower response to interferon (Kao et al., 2000b
). Treatment with nucleoside analogues has largely replaced interferon. Patients infected with adw strains had a 20-fold increased risk of lamivudine-resistance than patients infected with ayw strains (Zollner et al., 2001
).
Serological and genotypic shift
Genotype differences and genotype shifts correlated to seroconversion have been reported from different groups. Seroconversion from HBeAg to anti-HBe has been believed to be associated with either the emergence of a translational stop codon in the precore gene (1896G to A mutation), precluding the expression of HBeAg (Carman et al., 1989 ) or the appearance of a double mutation (1762A to G and 1764G to A) in the upstream core promoter, regulating the transcription of the precore gene (Okamoto et al., 1994
). In many cases, both changes have been seen in anti-HBe-positive patients. Although there have been numerous cross-sectional studies analysing these mutations, little has been known about the temporal sequence of seroconversion correlated to the appearance of mutations. In a study of Chinese patients in Hong Kong, 92% of the samples showed precore and/or core promoter changes after seroconversion to anti-HBe, thereby implying a definite role for these mutations in seroconversion (Chan et al., 1999
). Different results were obtained from a study of patients in Sweden, where only 50% of the strains showed any mutations after seroconversion (Bläckberg & Kidd-Ljunggren, 2000a
). The difference may be explained by the distribution of genotypes in both studies. Although all samples were not genotyped in the Chinese study, the 11 strains in which this was performed belonged to genotype B or C. In the Swedish study, where genotypes A to E were represented, genotypes A and D were by far the most common and HBeAg seroconversion was confirmed to occur earlier than core promoter or precore mutations in genotype D.
That seroconversion from HBeAg to anti-HBe and from HBsAg to anti-HBs can lead to change of genotype in the infected patient has been reported in several studies by the one group. In a small group of chronic carrier children who remained HBV DNA-positive while they became serologically HBsAg-negative, three showed a change from subtype determinant d to y (Bahn et al., 1997 ). In two other studies, children who seroconverted to anti-HBe changed their HBV genotype from A to D in seven cases and from D to A in three cases (Gerner et al., 1998
; Friedt et al., 1999
). This latter change was also seen in a neonatally infected baby with fulminant hepatitis who survived. It is not clear how a complete genotypic change would occur in the one patient, unless the patient had originally been infected with more than one genotype and an immune selection occurred during seroconversion.
Nosocomial infections
Characterization of HBV strains by subtype or genotype has been used to investigate chains of infection in different settings. Using the X and S genes, Hawkins et al. (1996 ) linked an outbreak of acute HBV with subtype adw in a haematology unit to contamination of a cryopreservation tank storing bone marrow. By subtype comparison, a large number of silent HBV infections among the members of an American football team were traced to one member of the team who was a chronic carrier of HBV subtype adr (Tobe et al., 2000
). By finding the same genotype, and by further sequence analysis, identical strains in two elderly women and a number of intravenous drug users with acute HBV infection, a likely transmission through multiple-dose vials could be implicated (Kidd-Ljunggren et al., 1999
). In an unusual chain of events, a neonatally infected infant transmitted the infection to two paediatricians who both died from fulminant hepatitis (Kosaka et al., 1991
). The strains were found to be identical and were subsequently used to transmit the infection to a chimpanzee (Ogata et al., 1993
).
Subtyping and sequencing of a region of the core gene were used to show the transmission of HBV from a thoracic surgeon to several patients on whom he had operated (Harpaz et al., 1996 ). The risk of nosocomial infection with HBV during thoracic surgery is further highlighted by two studies on heart-transplant patients. In Hannover, ayw2 was found in previously HBV-susceptible patients who had undergone a heart transplant (Petzold et al., 1999
). When the whole genomes were sequenced, they were found to differ by 18 nucleotides from the most similar published strain. Osterhaus et al. (1998
) found one strain, ayw2, in 20 of 21 heart transplant recipients in Rotterdam. Additional support for a nosocomial infection in such cases is the low prevalence, less than 10%, of subtype ayw2 among the HBV-infected patients in the Netherlands. Knowledge of the prevalence of HBV genotypes in our local setting made it more likely that an acute HBV infection in a kidney transplant patient was caused by reactivation rather than by a new infection (Kidd-Ljunggren & Simonsen, 1999
). Haemodialysis centres were found very early on to be at risk for nosocomial HBV transmission (Löfgren et al., 1982
), both between patients and from patients to staff. A recent study from a city in Brazil by Teles et al. (1999
) shows that this problem still persists. In their survey of patients attending haemodialysis centres, two centres were shown to harbour exclusively infection with genotype D, subtype ayw2, whereas three centres showed predominance of genotype A, subtype adw2.
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Test methods |
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The search for methods that would not have to involve sequencing led to the technique of restriction endonuclease analysis of HBV PCR products, also called restriction fragment length polymorphism (RFLP) (Shih et al., 1991 ; Niel et al., 1994
; Lindh et al., 1997
, 1998
; Mizokami et al., 1999
). Often, the S gene or a combination of the preS and S genes have been used. In the study by Mizokami et al. (1999
), a comparison of RFLP results from full-length genomes with those from S genes demonstrated that using the S gene alone could be accurate enough to differentiate between the six genotypes A to F. Another method which is also based on further analysis of PCR products is known as post-PCR hybridization or line probe assay (Grandjacques et al., 2000
). Naito et al. (2001
) recently described a PCR discriminating between different genotypes by using genotype-specific primers.
Taking advantage of the genotypic variability of the preS2 gene, Usuda et al. (1999 ) raised monoclonal antibodies to genotype-specific epitopes in this region and developed an ELISA discriminating between genotypes A to F (Usuda et al., 2000
).
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Conclusions and future perspectives |
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We have seen that the strict geographical pattern of HBV genotype prevalence has shifted, especially in areas of the world where more migration has occurred. It is probable that this geographical pattern will become even more loose with increases in migration. It is also possible that the events of recombination between genotypes that have been reported lately (A with D, A with C and B with C) will increase as different genotypes circulate in the same region. There are structural and clinical differences between genotypes and it remains to be seen whether recombination will create strains with other, or even larger, differences between them.
The most thoroughly analysed region of the HBV genome in the context of structural differences between genotypes is the precore region, where the stability of the secondary structure formed by pregenomic RNA depends on strict base-pairing, which differs between some genotypes. The importance of this structure for the replication of HBV is undisputed; however, few studies have concentrated on the importance of structures elsewhere in the genome and the impact of genotypic differences on these. Both at the nucleic acid and at the protein levels, genotypic variability may lead to structural changes which could have far-reaching effects. New techniques, such as nuclear magnetic resonance, will undoubtedly expand our knowledge about different HBV structures and genotypic changes within these. This will be important from a clinical perspective also, as many new antiviral compounds are targeted against specific structures in the replication process and the response rate may depend on variability of these structures.
Parts of the preS region constitute the most variable part of the HBV genome and this region is important for virus attachment and cell entry. Advances are currently being made in the development of cell culture systems to sustain HBV replication. It will be interesting to study differences between genotypes in infectivity and virus viability in these and future systems.
There is increasing evidence of clinical differences between subtypes and genotypes at various levels, including seroconversion age from HBeAg to anti-HBe, the risk for development of severe liver injury, including HCC, and the response to antiviral treatment. Unfortunately, and due to the geographical pattern of genotype distribution, most studies have compared genotype A with D or genotype B with C. Thus, no general consensus has appeared about the degree of virulence of different genotypes. It appears, though, as if genotype C, with its highest prevalence in Southeast Asia, may lead to more severe disease than some of the other genotypes. In this context, it is necessary to remember that the genetic make-up of the host may have a strong significance and may affect the long-term interactions between virus and host. This is an issue that has been studied in terms of vaccination success or failure but more studies about its effect on long-term HBV infection are needed.
There are many hundreds of hepadnavirus sequences in the databases. Despite many excellent studies using various phylogenetic methods to elucidate the history of hepadnaviruses, the origin of HBV is still obscure. It will be a strong challenge to try to answer some of the complex questions outlined above, and analysis of similarities and differences between strains will continue to form an integral part of this work.
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Acknowledgments |
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Received 19 May 2000;
accepted 30 August 2000.