The genome of hepatitis B virus.

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Ann. Rev. Microbiol. 1977. 31:357-77 Copyright © 1977 by Annual Reviews Inc. All rights reserved

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THE GENOME

:

··

1 709

OF HEPATITIS B VIRUS William S. Robinson \

Department of Medicine, Stanford University School of Medicine, Stanford, California 94305

CONTENTS INTRODUCTION ............................................................................................................

HBV GENE PRODUCTS ..................................................................................................

HBsAg Forms in the Blood ........................... . .. . .............. . .............................. . ......... HBcAg ...................................................................................................................... HBeAg ...................................................................................................................... Viral-Specified Polypeptides ...................................................................................... Polypeptides of HBsAg Particles .............................................................................. Polypeptides of HBcAg Particles .............................................................................. The Possibility of Additional Viral Polypeptides .... .. . . ... ................ . . ... .................. . . . .

Current Estimate of the Total Viral-Specific Protein

.

......................................... ....

357 358 358 360 360 361 361 364 364 364

DANE PARTICLE DNA STRUCTURE AND THE MECHANISM OF THE

ENDOGENOUS DNA POLYMERASE REACTION ........................................ The Complexity of Dane Particle DNA .................................................................. VIRAL GENE EXPRESSION IN INFECTED LIVER

365

368

. . . .. . .. . . . ............. . .. . ...... . . . ... . ... . . . .. . .

370

SUMMARY AND CONCLUSIONS ..................................................................................

371

. ..

INTRODUCTION

The virus that causes hepatitis B or serum hepatitis appears to infect only man in nature, and experimental infection has been achieved in only a few additional primates (9). Although highly infectious for man, hepatitis B virus (HBV) has not been propagated in tissue culture. The limited host range and failure so far to infect tissue culture cells has prevented development of convenient infectivity assays so that direct identification of the infectious form of the virus has not yet been possible. The most important single discovery concerning this virus was of the antigen fir st known as Australia antigen (Au antigen) in 1964 (11). Only after several years of investigation was the antigen associated with acute hepatitis B (12, 70, 72). It was 357


358

ROBINSON

then named hepatitis-associated antigen (HAA) and currently is designated hepati tis B surface antigen (HBsAg). Much evidence now indicates that HBsAg is an HBV-specified antigen. HBsAg in the blood has proven a convenient marker for HBV infection, and studies of the antigen have provided a wealth of information about the epidemiology and course of infection, and about the structure of viral forms in the blood that bear the antigen. Among the most important discoveries arising from early investigation of HBsAg was the recognition of the relatively common occurrence of persistent or chronic HBV infection. Persistent infection is manifested by circulation for months or years of relatively high concentrations of HBsAg particles; these are mostly defective or incomplete virus forms, as well as infectious virus. The blood of persistently infected patients is a rich source of viral antigen forms for physical and chemical characterization. Although most of the HBsAg in blood is in the form of incomplete viral antigen w ithout nucleic acid, one form, the Dane particle (20), has several properties sug gesting that it may be the complete form of HBV, and it is the only antigen form known to contain nucleic acid. Dane particles contain small circular DNA mole cules w ith an unusual structure and a DNA polymerase that utilizes the DNA as a primer/template. The small size of the Dane particle DNA, however, has raised doubts about whether or not this could be the complete viral DNA (83). Recent studies of Dane particle DNA structure and complexity combined w ith evidence that different infected cells express different viral genes suggest that Dane particle DNA molecules are heterogeneous in base sequence and raise the possibility that the complete viral genome may not be contained in the DNA of a single Dane particle. Here, I review recent information: the number of viral gene products; the size, structure, and complexity of Dane particle DNA; the mechanism of the DNA polymerase reaction; the pattern of viral gene expression in infected cells; and the implications of the findings for the organization of the viral genome. Some other aspects of HBV have been recently reviewed (39, 83, 98). HBV GENE PRODUCTS

The size of the HBV genome is not known. However, several probable viral gene products have been identified; these include HBsAg and two other antigens asso ciated exclusively w ith hepatitis B v irus infection. These antigens occur in the blood in several different structural forms. HBsAg Forms in the Blood

Several particulate structures in serum are known to carry HBsAg determinants on their surfaces; the antigen has not been found in a nonparticulate or low molecular-weight form in serum (4, 55). It is well established that the surface of the H BsAg-bearing p�rticles is antigenically complex. At least five antigenic specificities are present on HBsAg particles. A group-specific antigen (a) and two pairs of subtype determinants (d,y and w,r), which, forthe most part, are mutually exclusive, have been demonstrated (6, 53). The four HBsAg subtypes adw, ayw, adr, and ayr have been identified and are useful epidemiologic markers. Recent evidence suggests


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antigenic heterogeneity of the group-specific determinant a and additional minor determinants may exist (30). Persuasive although i ndirect evidence suggests that HBV is a virus-specified antigen. HBsAg appears in the blood early after infection with HBsAg (90), and an antibody response after infection results in immunity (95). Immunization with highly purified HBsAg particles results in protection against HBV infection and/or disease (75), as does administration of "y-globulin with a high titer of antibody against H BsAg (anti-HBs) (33, 73, 78, 86). The antigenic subtype found in second ary cases of HBV i nfection is regularly the same as the subtype of the index case or of the original source used in experimental infections (54, 63), indicating that the subtype determinants are specified by the viral genome and not by the host. Finally, a large amount of serological and epidemiologic data is consistent with the concept that HBsAg is a viral antigen (90). The HBsAg form with the most complex structure and uniform appearance is the Dane particle (20). It has an overall diameter of 42 nm (20), HBsAg on its surface (2, 20), a lipid-containing outer layer or envelope (83), a 28-nm internal core or nucleocapsid with a unique antigen [hepatitis B core antigen (HBcAg)] on its surface (2), an internal small circular D NA (81), and a D NA polymerase activity (47, 81, 82). In electron micrographs of uranyl acetate-stained preparations, the core of some Dane particles appears to have an electron-dense center and in others an electron translucent center (45). Two populations of Dane particles and Dane particle cores (which can be prepared by detergent treatment of Dane particles) have been demon strated by equilibrium CsCI density gradient centrifugation (42, 45). H igh-buoyant density Dane particles manifest endogenously primed D NA polymerase activity and yield exclusively high-buoyant-density cores w ith electron microscopically "full" centers after detergent treatment. Lower-buoyant-density Dane particles do not manifest D NA polymerase activity and yield m ostly low-densify cores with "empty" centers by electron microscopy. Although there is no direct demonstration that the Dane particle is infectious, several properties suggest that it may be HBV: (a) its surface antigen (HBsAg) induces protective antibody (anti-HBs) (75); (b) it contains a unique internal core antigen (HBcAg), which is probably virus specified (2); (c) its size (20) is consistent with estimates of the size of infectious HBV determined by ultrafiltration (61); (d) its concentration in most sera (1) is consistent with titrations of infectivity in serum (1 0); (e) its concentration in sera appears to correlate with the probability that patients will transmit infection to contacts (5, 69); if) purified and concentrated preparations of Dane particles have been shown to be infectious at higher dilutions than previously shown for unfractionated HBsAg-positive serum (94); (g) it is the only viral antigen form known to contain nucleic acid (83); and (h) DNA reassocia tion experiments have shown that Dane particle D NA base sequences are only present in the DNA and RNA of infected liver and not in uninfected liver, and these sequences appear to be covalently attached to high-molecular-weight DNA (57) as are other viral D NAs when integrated into chromosomal DNA (e.g. 85). Dane particles regularly appear in high concentrations in the blood during the late incubation period of acute hepatitis B and fall rapidly in concentration with the


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onset of disease (46, 50). Their concentration is variable among different chronic carriers, but for individual patients the level appears to remain fairly constant (83). More numerous HBsAg forms in the blood are small spherical particles with diameters between 16 and 25 nm ("20-nm particles") and filamentous forms with diameters around 22 nm and variable lengths up to 1000 nm (reviewed in 83). These forms consist of protein, carbohydrate, and lipid, and no HBcAg or nucleic acid has been found. They are considered to be incomplete viral coat particles. Preparations of 2 0-nm particles consistently appear to contain small amounts of serum protein components (l5, 62, 64) that have not been removed by extensive purification. Whether these are minor intrinsic constituents of the particles, or alternatively only avidly bound to the particle surface, or are just contaminating the preparation and copurifying with HBsAg in some cases (89) is not yet clear. Although the surfaces of these particles clearly share antigenic determinants (and thus some peptides) with the surface of Dane particles (2, 20), it is not known whether the peptide composi tion of the outer layer or envelope of Dane particles is identical in all respects to that in the other antigen forms. HBcAg

HBcAg is found in the blood only as an internal component of Dane particles. as described above. No free HBcAg has been detected in serum. HBcAg-bearing particles with the electron micros cope appearance of Dane particle cores h ave been isolated from homogenates of HBV-infected liver (37). Such particles have been shown to contain DNA polymerase activity like that of Dane particles (84). Particles with the same morphology have been seen in the nuclei of hepato�ytes in electron micrographs of thin sections of HBV -infected liver ( l , 16, 43). HBcAg is considered a virus-specified antigen because it appears to occur only during hepatitus B infec tion, there is an early and brisk antibody response to HBcAg during infection (SO), and it is an internal component of one of the HBsAg forms. Dane particles (2). HBeAg

The e antigen (HBeAg), discovered and named in 1972 (59), is physically and antigenically distinct from HBsAg. It is said to have a sedimentation coefficient of 12S and a buoyant density of 1.29 glm1 in CsCI (60). I t has proven difficult to purify and is not well-characterized chemically. It occurs exclusively in HBsAg-positive sera (60), and an antibody response to HBeAg is frequently observed following HBV infection, suggesting that it may be an HBV-specified l!-ntigen. It occurs commonly in the sera of chronic HBsAg carriers who have high concen trations of Dane particles (5, 67), and thus the presence of HBeAg correlates (as do Dane particles) with a propensity for transmission of infection to patient contacts (5, 34, 69). Since carriers that produce Dane particles in high concentrations also commonly produce HBeAg, it is possible that the two antigen forms are related in some way. It has been reported that anti-HBe reacts with the surface of Dane particles (66). suggesting that HBeAg may be a surface component of Dane particles. However, the antiserum used in that study was not proven monospecific, and other investigators (e.g. 27) have failed to confirm that finding. It seems more likely that if HBeAg is a compo-

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361


nent of Dane particles it is an internal component, although it is known to be distinct from HBcAg (27). The question of its physical relationship to Dane particles re mains unsettled. Recently, Neurath & Strick (65) proposed that HBeAg is an immunoglobulin, in which case it would not be a virus-specified polypeptide. The data supporting this conclusion have not yet been published. Attempts to confirm this have led others (G. L. Gerin, unpublished results; G. N. Vyas, unpublished results) to conclude that HBeAg is not an immunoglobulin. Recently, antigenic subtypes of HBeAg have been described (97). Viral-Specified Polypeptides

As described above, there is reasonable evidence, although not direct proof, that HBV genes specify the two antigens known to be associated with HBV infection: HBsAg and HBcAg. HBeAg may also be a virus-specified antigen, although its origin is less certain than that of HBsAg and HBcAg at this time. A probable additional viral gene product is the protein of the Dane particle core with DNA polymerase activity, since the polymerase enzymes found in other viruses have been shown to be virus specified (19, 44,56). Very. little is known about the polypeptides with HBeAg reactivity and with DNA polymerase activity. Recent studies, how ever, have provided some information about the polypeptides with HBsAg reactivity and in HBcAg particles. Polypeptides of HBsAg Particles

Several investigators have studied the polypeptide composition of HBsAg particles, and widely differing results have been obtained. Early studies of purified 22-nm particles by using sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis to separate peptides identified as few as two (96) or t hree (28) major polypeptides with sizes between 26, 000 and 40,000 daltons (see Table 1). More recently, as many as seven (25 , 87) or nine (18, 22) peptides have been identified by Coomasie blue staining of SDS-polyacrylamide gels with size ranges from 23, 000-97, 000 daltons and from 19, 000-120, 000 daltons, respectively, in two representative studies (Table 1). Two (26, 88) or three (18) polypeptides have been shown to stain with periodic acid-Schiff, suggesting they are glycoproteins. The sum of the sizes of the peptides identified in these two studies was 35 0,000 and 494, 000 daltons, respectively. No consistent differences in the peptides from HBsAg preparations of different subtypes have been found. Although Dreesman et al (18, 22) reported finding small amounts of two peptides (69,000 and 105,000 daltons) in HBsAg/ ayw that were not present in HBsAg/adw, no difference has been found in polypeptides of different subtypes by investigators (Table I). Shih & Gerin (88) found the same sizes, num bers, and relative amounts of polypeptides in purified preparations of HBsAg with SUbtypes adw, ayw, adr (Table 1). To relate the polypeptides found in HBsAg preparations to the size of the HBV genome, it is important to know whether or not each peptide is unique and whether or not it is coded by the viral genome. Antigenic analysis of individual peptides has recently provided some information about these questions. Gerin (26) first reported that individual peptides from HBsAg/ad particles and HBsAg/ ay particles recov-

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ROBINSON

Table 1

Polypeptides of HBsAg particle preparationsa HBsAg

Investigators

subtype

Gerin et al (28)

Vyas et al (96)

Oreesman et al (23)


Chairez et al (IB)

ayw

& Zuckerman (40)

Chairez et al (17)

& Burrell (58)

Shih & Gerin (88)

ay

ayw

Shih & Gerin (87)

Mackay

ad e

adw

Oreesman et at (22)

Gold et at (32)

IB.5b 19b

adwe

('",rin (26) Howard

ad

lOe

24b

ay

& Vyas, (76)

adw I

b

ay

ad

Gerlich & May (29) Rao

26

26 b 26b

ad

Gerin (25)

Reported molecular weight

e

ayw

adw e ayw adT

24 b 6d

32

32

(32) 35

16 ' 22 ' 22 2B 12

23-26 b 30-32'

(4 0)

(40)

(55)

22 ' 27 ' 27

27 ' 32 ' 32

(40)

(55)

(65) 32

40

40

(34)

(50)

(70)

36

42

(54)'

80

(30)b

82

19 b 19b

24

24

23 b

29.5

(l3) b,c

17

(23)

68

23 b,d,f

29.5'

36

41.5

(X

10-3)

(75) 75

39

69

55

68-72

(95)

(95) 39

120

69

105

120

(lOS)

120

97

90 '

'

' 27 ' 27

35

36

41.5

35

' '

40

55

120

53.5

72

97

53.5'

72

97

40

55

(69)

B(). Designated by investigators as minor or variable components; *. stain with PAS indicating glycosylation. bPolypeptide bands detected by Coomasie brilliant blue staining after SOS-gel electrophoresis.

cComponents detected by content of radioactivity after iodination of HBsAg particles with 125( and SDS.gel

electrophoresis.

dComponents detected by absorbance at 280 om after Sephadex G-75 chromatography. All designated subtypes had same polypeptide composition. f Components detected by content of radioactivity after reductive methylation of HBsAg particles with radioactive

e

substrate and electrophoresis in SDS-gels.

ered from SDS-gels after electrophoresis and innoculated into guinea pigs induced group-specific antibody to native HBsAg particles (anti-HBsl a) tested by passive hemagglutination and radioimmunoprecipitation. Most sera also contained type specific antibody (either anti-HBs/y or anti-HBsl d). Since proteins not containing carbohydrate induced anti-HBs, it was clear that the viral-specified antigenic deter minants were contained in polypeptides rather than in carbohydrate of the glyco proteins. The results also suggested that both group- and type-specifi c determinants reside on the same polypeptide. Because all polypeptides shared anti-HBsl a, they were not unique, separate primary gene products. It was not excluded, however, that the preparations used to immunize guinea pigs contained m ore than single peptides. Dreesman et al (21) separated polypeptides of HBsAg particles into five fractions by preparative SDS-gel electrophoresis and used the fractions to immunize guinea pigs. Polypeptides derived from HBsAgladw stimulated the production of both group- and type-specific antibodies based on preferential reaction of the antisera to 125I-labeled HBs Agladw over that to 125I-labeled HBsAglayw. A preference for antigen of homologous subtype was not observed with antisera toayw polypeptides, and some of the antisera to theayw-derived polypeptides actually reacted preferen tially with the adw subtype antigen used in the radioimmunoassays. The polypep-


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363

tide separation achieved by preparative SDS-gel electrophoresis in this study resulted in fractions containing more than a single polypeptide, indicating that the antisera contained more than one antibody specificity and ma king it difficult to interpret the complex reactions based on preferential reactivity. More recently, Gerin and colleagues (32, 87) have achieved more complete sepa ration of individual peptides of highly purified preparations of HBsAg/adw and HBsAg/ayw. Each electrophoretically pure polypeptide induced antibody against the group-specific determinant a (87). Most or all polypeptides from the HBsAg/ adw preparation induced antibody against the d determinant, and those from the HBsAg/ayw preparation induced anti-y (32). In another study (89), monospecific antisera to a and to d prepared from guinea pig anti-HBsl ad by affinity ch romatog raphy both specifically precipitated three purified and radio labeled major polypep tides derived from a purified HBsAg/ad preparation. These results clearly indicate that all individual polypeptides from HBsAg preparations contain identical and m ultiple virus-specified antigenic determinants, and that subtype determinants are found only in polypeptides from HBsAg particles of the same subtype. The only serum component present in large enough amounts in purified HBsAg preparations to be detected by Coomasie blue staining on SDS-gels is apparently h uman serum albumen, which comigrates as a separate polypeptide with the viral specific polypeptide P-6 of Shih & Gerin (68-72 X J03 daltons) (Table 1) (89). These studies suggest that HBsAg particles contain at least seven polypeptides of different apparent molecular weights, all of which contain at least in part the same or very similar virus-specific immunochemical structures and probably amino acid sequences. Furthermore, polypeptides of different HBsAg subtypes may contain a region of constant amino acid sequence specifying the group-specific determinant a and a variable region specifying the type-specific determinant d or y. The findings suggest that the amount of unique amino acid sequence specified by the HBV genome in HBsAg particles may be significantly less than the sum of the sizes of the individual peptides isolated by SDS-gel electrophoresis. The minimal amount of unique amino acid sequence for the protein bearing the known HBsAg determi nants, thus, might be the amount in the largest polypeptide shown to contain these determinants (Le. 97,000-120,000 daltons if these are monomeric polypeptides). Since both group- and type-specific determinants are found in even smaller polypep tides, it is possible that the amount of unique amino acid sequence in HBsAg particle polypeptides is less than the size of the largest component observed by SDS-gel electrophoresis. How the different sized polypeptides arise and their exact amino acid sequence relationship will require further study, including sequence analysis. Mackay & Burrell (58) have suggested that the presence of multiple polypeptides with different sizes but antigenic similarity could result from proteolytic degradation of one or a very few polypeptides within HBsAg particles by proteases found in serum, a process that appears to occur in human erythrocyte membranes (24). The wide variation of polypeptide sizes and n umber in different studies of HBsAg particles (Table 1) would be consistent with such a process. Multiple polypeptides of different sizes, which all con tain the same antigenic determinants, have not been described for other viruses.

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ROBINSON

Polypeptides of HBcAg Particles


Two recent studies have examined the polypeptides of HBcAg particles. Budkow ska, Shih & Gerin (14) isolated HBcAg particles from HBV-infected liver by equilib rium centrifugation in a CsCI density gradient and Bio-Gel A-5m chromatography. The particles were iodinated with 1251 and repurified by sedimentation in a sucrose density gradient. The 12sl-labeled peptides were identified by SDS-polyacrylamide gel electrophoresis of solubilized immune complexes after precipitation of 1251_ labeled HBcAg with anti-HBc. Two major radioactive components with apparent molecular weights of 17,000 and 3 5,000 and multiple minor components were found. Hruska & Robinson (42) purified Dane particles from plasma of chronic HBsAg carriers, prepared Dane particle cores by Nonidet P-40 detergent treatment, and purified the cores by equilibrium centrifugation in CsCI density gradients. Three polypeptide bands with apparent molecular weights of 19;000, 70, 000, and 80,000 were regularly detected by Coomasie blue staining after separation by SDS-poly acrylamide gel electrophoresis. The same polypeptides were detected by Coomasie blue staining after SDS-gel electrophoresis of dissociated HBcAg particles purified by equilibrium centrifugation in CsCI density gradients from HBV-infected chim panzee liver. The sum of the sizes of the major polypeptides reported by Budkowska et al (14) is 5 2,000 daltons and by Hruska & Robinson (42) is 169,000 daltons. The reason for the differences in number and sizes of polypeptides found in HBcAg particles in the two studies is not clear. It is possible that iodination of the HBcAg particle proteins altered their electrophoretic behavior. None of the polypeptides observed has been shown to possess HBcAg reactivity, and the amount of unique virus specified amino acid sequence in Dane particle cores thus is not yet known. Clearly, however, Dane particle cores and HBcAg particles from HBV-infected liver tissue contain polypeptides distinct from those in HBsAg particles and at least some of these must be specified by viral genes. The Possibility of Additional Viral Polypeptides

In addition to the virus-specified protein in HBsAg particles and in Dane particle cores, additional viral-specific proteins are probably necessary for the DNA polyme rase activity of Dane particle cores and possibly for HBeAg. Alternatively, these viral functions could be associated with one or more of the polypeptides described for Dane particle cores and HBcAg particles from infected liver. Current Estimate of the Total Viral-Specific Protein

The total amount of viral-specified protein is not known at this time, but a minimum estimate might be the unique polypeptide sequences found in HBsAg particles (i.e. the largest polypeptide containing type-specific and group-specific determinants) and the polypeptides in HBcAg particles. This would be more than 200,000 daltons of protein.

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365


DANE PARTICLE DNA STRUCTURE AND THE MECHANISM OF THE ENDOGENOUS DNA POLYMERASE REACTION

The Dane particle has been shown to contain a small circular DNA molecule (81). The DNA was first detected as the endogenous template of a DNA polymerase reaction within Dane particles. In 197 1, H irschman, Vernace & Schaffner (36) described small amounts of DNA polymerase activity in crude pellets obtained by high-speed centrifugation of three HBsAg-positive sera. However, they mistakenly postulated that the endogenous template was RNA because the reaction was thought to be interrupted by RNase treatment of the enzyme preparation. They did not identify the enzyme-containing particle. Kaplan et al (47) found DNA polyme rase activity in high-speed pellets from eight of eight HBsAg carriers with high concentrations of Dane particles in their serum. They found that the enzyme activity sedimented with Dane particles and with Dane particle cores after Nonidet P-40 treatment of Dane particles. Robinson & Greenman (82) found that the enzyme conta Ining particle was specifically immunoprecipitated by anti-HBs and not anti HBc before Nonidet P-40 treatment and by anti-HBc and not anti-RBs after Nonidet P-40 treatment, which removes the outer HBsAg layer of Dane particles releasing the inner core. These findings were consistent with the presence of DNA polymerase activity in the core of Dane particles. Kaplan et al (47) showed that the DNA reaction product was reasonably homoge neous and had a sedimentation coefficient of ISS. The reaction was inhibited by actinomycin D and daunomycin, suggesting that the endogenous template was DNA not RNA. The enzyme, the template, and the reaction product appeared to be internal components of the Dane particle core, since DNase (and RNase) would not attack the template or the DNA product before disruption of cores and the enzyme failed to accept a wide variety of added polynucleotides, which are accepted as primer/templates by other DNA polymerases. Robinson, Clayton & Greenman (81) isolated DNA from purified Dane particles and showed that the same small circular molecules were present before and after a DNA polymerase reaction. Radioactive DNA reaction product sedimented with the circular molecules, indicating that these molecules served as the template for the DNA polymerases reaction. Linear DNA molecules of random length were also observed in the total DNA extracted from Dane particles. No radioactivity was incorporated into these molecules, and they were separated from the radioactive template molecules (circles) by sedimentation. The linear DNA was also eliminated by treatment of Dane particle core preparations w ith DNase, suggesting they were not components of the core. The circular molecules appeared to be double stranded and to have a mean length of 0.78 p.m, which corresponds to approximately 1.6 X 106 daltons or 2300 nucleotide pairs (np) before a DNA polymerase reaction. No superhe1ical molecules were observed, suggesting that all were nicked or open circles or, alternately, had a very low superhe1ix density. The thermal transition curve and buoyant density of DNA molecules made radioactive in a Dane particle DNA polymerase reaction were consistent w ith a cytosine plus guanosine content


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ROBINSON

of 48-49%. The presence of circular DNA molecules approximately 0.8 J.Lm in length in purified preparations of Dane particles has been confirmed by other investigators (71, 74). Several observations suggest that the circular DNA molecules from Dane parti cles contain a large single-stranded region heterogeneous in length, and the DNA polymerase reaction closes the single-stranded gaps. Summers, O'Connell & Mill man (93) first proposed the existence of a single-stranded region in Dane particle DNA molecules because avian m yeloblastasis virus (AMY) DNA polymerase, which contains no exonuclease activity and thus cannot introduce radioactive nu cleotides into prexisting double-stranded DNA, successfully used Dane particle D NA as a template for DNA synthesis, presumably by using a single-stranded part of the molecule. Because the endogenous DNA polymerase activity in Dane parti cles introduced radioactive nucleotides into the same restriction endonuclease Rae III-generated DNA fragments as did the AMY DNA polymerase, it was concluded that the endogenous enzyme used the same single-stranded regions as a template. The data published, however, indicated that radioactive nucleotides had been incor porated into almost all Hae III fragments after a reaction with either polymerase, suggesting that DNA synthesis took place in all parts of the molecule and not only in specific regions. This result is consistent with other mechanisms for the endoge nous DNA polymerase reaction, as well as with the possibility that single-stranded gaps in the circular molecules are closed during the reaction. Hruska et al (41) showed that the electron microscope length of the circular DNA molecules from Dane particles increased by an average of 23% when DNA was spread in formamide, which extends single-stranded regions compared with the length under conditions (aqueous spreading) that do not extend such regions. This suggested that the circular DNA molecules contain single-stranded regions averag ing one quarter the length of the circle. After a Dane particle DNA polymerase reaction, the mean circle length (aqueous spreading) increased by 27% and the length became more homogeneous. This suggested that single-stranded regions of variable length were closed during the endogenous DNA polymerase reaction. Most of the circular molecules must have participated in the DNA polymerase reaction because the mean length of the population of molecules increased and the length distribution was greatly narrowed. The mean length of the DNA after an endoge nous DNA polymerase reaction was 1.06 J.Lm, corresponding to a molecular weight of approximately 2.1 X 106 or 3150 np for double-stranded DNA. No superhelical forms were observed in DNA after a DNA polymerase reaction as none were before. Landers, Greenberg & Robinson (51) showed that DNA extracted from Dane particles before a DNA polymerase reaction and detected on polyacrylamide gels by ethidium bromide staining was eiectrophoretically heterogeneous. Treatment with the single-strand-specific endonuclease SI produced double-stranded linear DNA molecules �anging in length from 1700-2800 np, indicating the presence of a nick or single-stranded region susceptible to the enzyme. After an endogenous DNA polymerase reaction, the SI-resistant, double-stranded length was elongated by 15-45% (average 25%) and was made more homogeneous. The mean length of


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the elongated molecules was 3200 np. This length estimate is in good agreement with that determined by electron microscopy, as described above. When the DNA was analyzed by endonuclease Hae III digestion, certain double-str anded DNA frag m ents increased in amount or first appeared only after an endogeneous DNA polymerase reaction. These results are consistant with the existence of a single stranded regions of varying lengths in the circular DNA molecules and closure of the single-stranded gaps during the endogenous DNA polymerase reaction. Although the single-stranded gaps appear to be c losed by the endogenous DNA polymerase reaction, the ends of the open strand do not appear to be joined to make closed circular DNA. The circular molecules after the DNA polymerase reaction as before have a nuclease S l-sensitive site (51), and no closed circular DNA has been detected by alkaline sucrose gradient sedimentation or equilibrium centrifugation in CsCI density gradients containing ethidium bromide (84). The amount of DNA synthesis during the endogeneous Dane particle DNA polymerase reaction has also been determined in two other ways. Lutwick & Robin son (57) showed that the Cot.,., value for the reassociation of the DNA synthesized in an endogenous Dane particle DNA polymerase reaction corresponded to an amount of unique DNA equivalent to about one quarter of the circle length. This figure agrees with the amount of new DNA synthesis estimated by electron micros copy length change and change in electrophoretic mobility, as described above. Hruska & Robinson (42a) carried out a Dane particle DNA polymerase reaction with Br-dUTP in place of TTP, isolated the DNA and denatured it by h eating in dimethyl sulfoxide, and separated DNA strands containing BrdU from light DNA by equilibrium centrifugation in CsCI density gradients. The buoyant density of the h eavy DNA indicated that approximately one third of each stflmd was new DNA (calculated as though 25% was Br-dUMP), in good agreement with the other estimates of the amount of new DNA synthesis. The separated DNA fractions were also tested for ability to reassociate into double-stranded DNA to determine whether new DNA was synthesized on one or both strands of Dane particle DNA (42a). None of the DNA in the heavy DNA component could be shown to reassoci ate into double-stranded DNA until DNA from the light component (or unfrac t ionated Dane particle DNA) was mixed w ith it. This suggests that the n ew DNA synthesized in an endogenous Dane particle DNA polymerase reaction consists of molecules without complementary base sequences, indicating they all represent the same strand of the Dane particle DNA. To identify the primer for DNA synthesis in the endogenous Dane particle reaction, Robinson & Lutwick (84) examined the sedimentation of th e Dane particle DNA in alkaline sucrose gradients after very short reaction times (average chain growth of five to ten nucleotides). They found that the DNA product was attached to a molecule with the sedimentation coefficient ( l IS) corresponding to that of a linear DNA strand of approximately 1900 nucleotides (92). This is near the length of the double-stranded region of the circular molecule before a DNA polymerase reaction (1700-2800 np by gel electrophoresis and an average of 2300 np by electron microscopy. as described above), suggesting that the open strand of the circular


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DNA molecule serves as the primer for the reaction and new DNA is covalently attached to that strand. No evidence for smaller oligodeoxynucleotides or an RNA primer was found. Endogenous Dane particle DNA synthesis appears to be initiated at multiple sites over a large portion of the molecule. After very short reaction times (5 sec or 10 min) with a chain growth rate of 5-10 nucleotides per min under the conditions of low nucleotide concentration used, Landers et al (51) found that radioactive nucleo tides had been incorporated into endonuclease Hae III fragments with sizes adding up to a total of 1600 np, or one half the length of the circular Dane particle DNA molecule. After an extensive reaction (3-4 hr with high nucleotide concentrations), the same Hae III-generated fragments were h eavily labeled and all fragments con tained at least some radioactive nucleotide. The wide distribution of sites for initia tion of DNA synthesis suggests that the single-stranded region must exist at different sites in different molecules, or that there is more than one single-stranded region (and initiation site for DNA synthesis) per molecule, although, as described above, DNA synthesis always appears to involve only one of the two strands of Dane particle DNA. Against the possibility that individual molecules contain multiple gaps is the fact that after very short DNA polymerase reaction times the new DNA sedimented in alkaline sucrose gradients as part of a strand 1900 nucleotides in l ength and none appeared in shorter strands, as might be expected if there existed multiple initiation sites per molecule (84). Another mechanism for Dane particle DNA polymerase reaction has been proposed by Overby et al (71). They observed linear "tails" joining the circular DNA molecules and proposed that such molecules represented DNA forms repli cating by "rolling circle" mechanism. Hruska et al (41), however, carefully quan titated the frequency of occurence and length of such tails in Dane particle DNA before and after an endogenous polymerase reaction. No increase in frequency «8 % of circular molecules had tails) or length of tails was found after a reaction. Robinson & Lutwick (84) found no new DNA sedimenting faster than unit length Dane particle DNA strands after endogenous polymerase reactions of different durations, as observed for DNAs replicating by the rolling circle mechanism (re v iewed in 49). These findings make it unlikely that the endogenous DNA polyme rase reaction involves Dane particle DNA replication by this mechanism. The biological utility of a circular DNA single strand ed over approximately one third of its l ength and a virion enzyme that closes the single-stranded region for a virus such as HBY is not clear. Such features have not been described for other viruses. The Complexity

0/ Dane Particle DNA

Two studies suggest that the total complexity or the amount of unique base sequence in the circular DNA from Dane particles may exceed the amount of DNA in a single circular molecule. Summers et al (93) found that the sum of the sizes of endonu clease Hae III-generated DNA fragm ents, detected by autoradiography after label ing a DNA preparation from Dane particles by n ick translation w ith Escherichia coli DNA polymerase I, corresponded to approximately 3900 base pairs. Landers


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et al (51) found the sum of the sizes of the major Hae III-generated fragments, detected by ethidium bromide staining after an endogenous Dane particle DNA polymerase reaction to fully elongate the double-stranded region, to be 3880 np (Figure I). Two additional fragments not demonstrated with ethidium bromide were detected by autoradiography when 32P-Iabeled deoxyribonucleotides were used in the endogenous DNA polymerase reaction mixture, indicating that these fragments were components of Dane particle core DNA since they serve as template for the endogenous enzyme; however, they were present in less than stoichiometric amounts relative to other fragments (Figure 1). The sum of the sizes of these two fragments and the major fragments detected by ethidium bromide was 4910 np. The fact that these Hae III-generated fragments are each homogeneous suggests that none of them represent random host DNA like that found in some viruses after passage at DANE PARTICLE DNA STRUCTURE

HAE III FRAGMENTS OF ELONGATED DNA

986 957 (800)

a

A_ A' B

••••

C

-

347 D 310 E

-

415

242 F (230) G

-••••

209 H

-

159 I 144 J

-

III

-

strand a - 3,200 nuc1eotides strand

• ••

b

1,700 to 2,800 nuc1eotides

product of the endogenous DNA polymerase reaction

K

-

3,880 np (4,910 np)

Figure 1

Schematic representation of Dane particle DNA structure and the endogenous

DNA polymerase reaction (left), and the result of restriction endonuclease Hae III digestion of Dane particle DNA after elltensive c:longation of the double-stranded region by the endoge nous DNA polymerase rea9tion (right). The Hae III fragments indicated by solid lines are detected by ethidium bromide, and those shown as broken Jines are detected only by autoradi ography after an endogenous DNA polymerase reaction with 32P-labcled deollynucleoside triphosphates. All ethidium bromide straining fragments also contain at least some radioac tivity.


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high multiplicity (52), which appears as a heterogeneous smear after gel electro phoresis. All of the major DNA fragments are almost certainly components of the 3200-np circular DNA molecules rather than some representing components of a larger molecule not yet detected in the DNA preparation. Electron microscope examination of Dane particle DNA (41) indicates that a molecule significantly larger than the 3200-np circles could not occur at a frequency greater than about 1 per 1000 3 2oo-np circles. The relative amount of DNA detected in each major fragment precludes their origin i n such an infrequently occurring molecule. These measurements of the complexity suggest th{lt the amount of unique nucleo tide sequence in Dane particle DNA exceeds the amount of DNA in a single circular molecule by a significant amount. If additional minor fragments observed arose from Dane particle DNA molecules, the complexity would be even greater. This suggests that the DNA molecules in Dane particles are not all identical but are heterogeneous w ith respect to base sequence. The heterogeneity, however, involves a limited total amount of unique DNA (i.e. more than the DNA in a single circular Dane particle DNA molecule but less than that in two such molecules). The DNA used in both of the above studies was from Dane particles from single HBsAg carriers (both with subtype adw) (80), excluding the possibility that the DNA heterogeneity was due to a mixture of D ime particles specifying different HBsAg subtypes. The kind of heterogeneity that appears to occur in Dane particle DNA has not been described for other viruses. VIRAL GENE EXPRESSION IN INFECTED LIVER

The production of 22-nm spherical HBsAg particles appears to exceed the produc tion of other viral antigen forms, such as Dane particles, in all patients persistently infected with HBV. A concentration of 3 X 1013 22-nm HBsAg particles per ml has been accurately measured in the serum of one patient (48). Almeida (1) estimated the concentration of Dane particles to be approximately 105 per ml in the serum of most chronic carriers, and a very few had concentrations as high as 108 or 109 per ml. Thus, 22-nm spheres would appear to outnumber Dane particles by as much as 108 to 1 in some sera and probably by as l ittle as 103 or 104 to 1 in only a few sera. In the serum of one chronic carrier with a very high relative concentration of Dane particles, the ratio of 22-nm HBsAg spheres to Dane particles was found to be 173 0 to 1 (13). S ince the only known form of HBcAg in serum is as a component of Dane particle cores, the relative numbers of 22-nm HBsAg spheres and Dane particles would indicate that HBsAg production probably greatly exceeds produc t ion of HBcAg by the infected liver of most persistently infected patients. Support ing this is the finding of HBsAgl HBcAg complement fixation titer ratios on serum of several chronic HBsAg carriers with high concentrations of Dane particles to be between 32 and 5�2 (27). The relative amount of HBeAg production is not known, although when present in serum it is generally barely detectable by immunodiffu sion. Immunofluorescent staining of liver tissue from persistently infected humans and chimpanzees has revealed different patterns of viral antigen synthesis in different


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ceiIs. Immunofluorescent staining of liver sections for HBsAg and HBcAg has shown that most infected cells synthesize only HBsAg (cytoplasmic), fewer c ells synthesize only HBcAg (nuclear), and even fewer cells appear to synthesize both HBsAg and HBcAg (8, 3 5, 76). The number of infected cells detected by im munofluorescence can vary from less than 1% to virtually 100% of the cells in the liver, and the relative number synthesizing only HBsAg, only HBcAg, or both antigens varies widely in individual patients (8, 3 5, 77). In the liver of some chronic carriers, only cells synthesizing HBsAg exclusively can be found (8, 35, 77). In all chronic carriers producing relatively high concentrations of Dane particles, signifi cant n umbers of cells synthesizing HBcAg can be found in the liver (91). The observation that the number of cells synthesizing HBsAg almost always exceeds the number synthesizing HBcAg in most infected livers correlates with the fact that 22-nm spherical and filamentous HBsAg forms greatly outnumber Dane particles in sera of most infected patients. The dramatic difference in the pattern of viral antigen synthesis in individual cells of chronically infected liver indicates that different viral genes are expressed in different cells, a phenomenon not described for tissues infected with other viruses. SUMMARY AND CONCLUSIONS

1. Several viral antigen forms are found in the blood during HBV infection. The most numerous are 22-nm spherical and filamentous HBsAg particles, which are probably incomplete virus coat particles without HBcAg or nucleic acid. Another form called the Dane particle has several properties, suggesting it may be the complete virus, although its in fectivity has not been directly shown. It has HBsAg on its surface and an internal core containing a unique antigen (HBcAg), a small circular DNA, and a DNA polymerase activity. A third antigen, HBeAg, exclu sively associated with HBV infection is physically and antigenically distinct from the other antigen forms. HBeAg is commonly present in sera containing Dane particles, but whether or not it is physically related to Dane particles (e.g. as an internal component) is not yet known. Claims that HBeAg is an immunoglobulin have yet to be substantiated. 2. The size of the HB genome is not certain at this time, although several probable viral gene products have been identified. The proteins containing HBsAg, HBcAg, and HBeAg reactivity, and that in Dane particles with D NA polymerase activity, are probably all specified by viral genes. The total amount of unique polypeptide involved in these functions is not yet known, but a minimum amount is probably 200,000 daltons or more (the largest polypeptide with HBsAg group and type specific reactivity plus the polypeptides found in Dane particle cores). HBeAg and the DNA polymerase activity may be associated with additional polypeptides. Fur ther study including purification and sequence analysis of the proteins associated with each viral function will be necessary to accurately establish the amount of protein specified by the viral genome. In theory, a DNA the size of the elongated Dane particle DNA (3200 np) could specify only about 125,000 daltons of protein. This suggests that more DNA than that in a single circular molecule from Dane


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particles may be needed to specify all of the unique HBY-specific polypeptide. Whether or not the total amount of unique nucleotide sequence in the DNA from Dane particles (at least 3880 np in restriction endonuclease cleavage fragments) is sufficient to specify the HBY-specific proteins will be known only after the amount of viral-specific polypeptide is more accurately known. 3. Dane particle DNA molecules consist of one closed circular strand of constant length (strand a, 3200 nucleotides) and an open strand of variable length (strand b, 1700-2800 nucleotides) (Figure 1). Thus, 15-45% of the circle length is single stranded in different molecules. The endogenous Dane particle DNA polymerase appears to use the open strand as a primer, and the single-stranded gap is closed during the reaction, resulting in a double-stranded DNA of 3200 np. The free ends of the open strand are not joined within Dane particles to make closed circular DNA. DNA synthesis is initiated at multiple sites over a wide region (50%) of the molecule, suggesting that the single-stranded region exists at different sites in differ ent molecules. Evidence also suggests that the same strand (strand b, Figure 1) is elongated during the endogenous DNA polymerase reaction, i.e. the single-stranded region is on the same strand in all molecules (strand a). 4. The total complexity or amount of unique nucleotide sequence in the DNA isolated from Dane particles appears to be greater than the amoun t of DNA in a single elongated circular molecule (3200 nucleotide pairs) (Figure 1). The sum of the sizes of the major DNA fragments resulting from restriction endonuclease Hae III cleavage of the DNA extracted from Dane particles is 3880 np. Additional minor fragments that also serve as template for the endogenous DNA polymerase of Dane particle cores make the total 4910 np. If the m easure of complexity is correct, the amount of unique nucleotide sequence in the DNA from Dane particles would exceed the amount of DNA in a single circular molecule but would be less than that in two such molecules. In that case, the 3200-np circular molecules would be heterogeneous in nucleotide sequence, although no molecule would be completely different from any other. If each Dane particle contains a single circular DNA molecule, as is likely from the small size of the Dane particle core (the ratio of the elongated Dane particle DNA length to the volume of the core is nearly two times that ratio for simian virus 40), Dane particles must be genetically heterogeneous. 5. Immunofluorescent staining has revealed that in most chronic HBY infections some liver cells appear to synthesize only HBsAg, fewer cells synthesize only HBcAg, and even fewer cells synthesize both antigens. The different pattern of viral antigen synthesis in individual cells indicates that different viral genes are expressed in different cells. The expression of different viral genes in different cells could occur if individual cells contained different complements of viral genes. Alternatively, the regulation of viral gene expression could be different in individual cells containing the same complement of viral genetic information. 6. The findings described above suggest the following working model for the structure of the HBY genome (Figure 2). Four viral genes for the four functions thought to be virus specified can be designated HBsAg (although HBsAg is antigeni cally complex, all antigenic determinants appear to reside on the same polypeptide), HBeAg, HBeAg, and pol (the Dane particle DNA polymerase gene). Additional,


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as yet unrecognized, viral genes may also exist. The base sequence heterogeneity of Dane particle DNA molecules and their small size relative to the probable amount of unique viral-specified polypeptide suggest that Dane particles have heterogeneous and incomplete genomes, and the complete viral genome may be contained in more than a single Dane particle. Liver cells synthesizing different single-viral antigens (HBsAg or HBcAg) would be infected with Dane particles possessing genes for the antigen synthesized and not genes for the other antigen. Infected cells that synthe size HBsAg and not HBcAg would be infected with Dane particles that contain the genes for HBsAg and not for HBcAg. Such cells may produce only the HBsAg forms free of HBcAg, DNA, and DNA polymerase (i.e. 22-nm spheres and fila ments). Because this infected cell type is the most frequent in infected livers of most chronic carriers, greater amounts of 22-nm HBsAg particles than Dane particles appear in the blood of such patients. Cells that synthesize HBcAg and not HBsAg would be infected with Dane particles containing the HBcAg genes and not genes for HBsAg. Cells replicating virus (Dane particles) would be infected with more than one Dane particle to provide the entire complement of viral genes. Such cells would be recognized as containing both HBsAg and HBcAg by immunofluores cence. Most Dane particles in the serum of most chronic carriers probably contain HBsAg genes and not HBcAg genes because the most frequent pattern of viral antigen synthesis in cells of most persistently infected livers is synthesis of HBsAg and not HBcAg. According to this model, infection with low doses of virus (i.e. small numbers of Dane particles) could result in infections in which the liver contains only singly infected cells. In that case, liver cells would be expected to contain only HBsAg or HBcAg genes, and none would contain the entire complement of viral genes neces sary for Dane particle production, which would occur only in multiply infected cells according to the model. Such infections might result in production of incomplete HBsAg forms (22-nm spheres and filaments) exclusively and no Dane particles (i.e. infectious HBY). Infections with HBsAg production but no Dane particles might be expected to occur more frequently after experimental infection with high dilutions of infectious serum or natural transmission by routes other than blood transfusion rather than after high-dose transmission, as must occur where Dane particle-rich blood is transfused. It is not known whether such infections occur although it has been shown that all HBsAg-positive blood units do not result in HBY infection in recipients (31). Whether such blood units actually came from donors producing only HBsAg and no infectious virus (i.e. Dane particles) or rather the recipients of the blood were immune to HBY or were not infected for other reasons is not known. Examination of serum of HBY -infected patients has also shown that Dane particles are not detectable in a significant fraction of patients. Kaplan, Gerin & Alter (46) studied nine patients who developed hepatitis B after blood transfusions for the appearance of Dane-particle-associated DNA polyme rase. High levels of Dane particles appeared in the serum of seven patients during the late incubation period of the disease, but only HBsAg and no measurable Dane particles were found in two patients. Dane particles have not been detected in the serum of some HBsAg carriers by electron microscopy (20, 69, 64) or by DNA

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MODEL FOR REP L I CAT I ON OF DEFECT IVE, GENETI CALLY HETEROGENEOUS HEPAT I T I S B V I R I ONS PATTERN OF V I RAL G E N E EX PRESS I ON IN I N FECTED CELLS

V I RAL FORMS

PRODUCES

GENET I C MAKEUP OF DANE PART I C LE DNA


HBsAg

z

y DANE PARTICLES

HBcAg

x

I

y

Figure 2

Model of Dane particles as defective, genetically heterogeneous hepatitis B virions,

which replicate by complementation in multiply infected cells. X, Y, and Z represent unspeci. fied hepatitis B viral genes and may include the HBeAG gene and the Dane particle DNA polymerase gene (pol).

polymerase assay (81). Whether any of these patients actually produce no Dane particles or only low levels not detectable by the assays used is not yet known. Almost all chronic HBsAg carriers appear to produce anti·HBc, indicating that they synthesize HBcAg (38). It remains to be determined, however, whether all chronic carriers actually produce Dane 'particles (which contain HBcAg) or whether some may produce only HBcAg within liver cell nuclei, as predicted by the model for cells singly infected by Dane particles lacking genes for HBsAg and thus the capacity to produce Dane particles.

375


HEPATITIS B VIRUS

Although the current information about HBV and Dane particles reviewed here suggests the above model, the data are also consistent with other possibilities. Until Dane particles or their 3200-np circular DNA molecules are shown directly to be infectious, it cannot be excluded that a virus particle or a nucleic acid molecule not detected in the studies to date is necessary for HBV infection. The 3200-np circular molecules might then be viral DNAs with small size and some heterogeneity because of large deletions. Under such conditions, almost all Dane particles found in serum (i.e. those containing 3200-np DNA molecules) would be defective, noninfectious virions. If a larger form of DNA does exist, it seems unlikely that it could be contained within the Dane particle core. As described above, the ratio of the 3200-np Dane particle DNA size to the volume of the Dane particle core greatly exceeds that ratio for small spherical viruses such as simian virus 40. An additional virus form not yet identified would probably be needed to contain larger DNA molecules. Further research will be needed to confirm or refute the model. ACKNOWLEDGMENTS

Some of the work described here was supported by U.S. Public Health Service Research grant AI 13526. Literature Cited 1 . Almeida, J. D. 1 972. Am. J. Dis. Child. 1 23:303-9 2. Almeida, J. D., Rubenstein, D., Stott, E. J. 1 97 1 . Lancet 2 : 1225-27 3. Almeida, J. D., Waterson, A.P., Tro well, J. M., Neale, G. 1970. Microbios 2: 145-53 4. Alter, H. J., Blumberg, B. S. 1966. Blood 27:297-309 5. Alter, H. J., Seeff, L. B., Kaplan, P. M., McAuliffe, V. J., Wright, E. C., Gerin, J. L., Purcell, R. H., Holland, P. V., Zimmerman, H. J. 1976. N. Engl. J.

Med. 295:909-1 3 6 . Bancroft, W. H . , Mundon, F . K., Rus sell, P. K. 1972. J. Immunol. 109: 842-48 7. Deleted in proof. 8. Barker, L. F., Chisari, F. V., McGrath, P. P., Dalgard, D. W., Kirschstein, R. L., Almeida, J. D., Edgington, T. S., Sharp, D. G., Peterson, M. R. 1973. J.

In! Dis. 127:648-62

9. Barker, L. F., Maynard, J. E., Purcell, R. H., Hoofnagle, J. H., Berquist, K. R., London, W. T. 1 975. Am. J. Med.

Sci. 270: 1 89-95

10. Barker, L. F., Murray, R. 1972. Am. J. Med. Sci. 263:27-33 1 1 . Blumberg. B. S., Alter, H. 1., Visnich. S. 1 965. J. Am. Med. Assoc. 1 9 1 :541-46 12. Blumberg, B. S., Gerstley, B. J. S., Hun-

gerford, D. A., London, W. T., Sutnick, A. I. 1967. Ann. Int. Med. 66:924 1 3. Bond, H. E., Hall, W. T. 1972. J. Infect.

Dis. 1 2 5:263-68 14. Budkowska. A., Shih, J. W., Gerin. J. L. Submitted for publication

1 5. Burrell, C. J. 1975. J. Gen. Virol. 27: 1 17-26 16. Caramia, F., De Bac, C., Ricci. G. 1 972. Am. J. Dis. Child 123:309-1 1 1 7. Chairez. R., Hollinger. F. B Brun schwig, J. P., Dreesman, G. R. 1 975. J. Virol. 1 5 : 1 82-90 1 8. Chairez, R., Steiner. S., Malnick, G. R., Dreesman, G. R. 1 973. Inte rvirology 1 :224-28 19. Cross. R. K., Fields, B. N. 1972. Virology 50:799-809 20. Dane, D. S., Cameron, C. H., Briggs, M. 1 970. Lance t 1 :695 2 1 . Dreesman, G. R., Chairez, R., Suarez, .•

M., Hollinger, F. B., Courtney. R. J., Melnick, J. L. 1 975. J. Virol. 16:508- 1 5 22. Dreesman, G . R., Hollinger, F . B., Mel nick, J. L. 1 975. Am. J. Med. Sci.

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25. Gerin, J. L. 1972. In Hepatitis and Blood Transfusions, ed. G. N. Vyas, H. A. Perkins, R. Schmidt, pp. 205-20. New York: Grune and Stratton 26. Gerin, J. L. 1974. In Mechanisms of Virus Disease, ed. W. S. Robinson, C. R. Fox, pp. 2 1 5-24. Menlo Park: W. A.

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540--45 3 1 . Gocke, D. J. 1972. J. Am. Med. Assoc. 2 1 9: 1 165-70 32. Gold, J. W. N., Shih, J. W., Purcell, R. H., Gerin, I. L. 1976. J. Immunol. 1 17: 1404-6 33. Grady, G. F. 1975. N. Engl. J. Med. 293: 1 067-70 34. Grady, G. F. 1 976. Lancet 2:492-93 35. Gudat, F., Bianchi, L., Sonnabend, W.,

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64:483-500 44. Hunt, D. M., Wagner, R. R. 1 974. J. Virol. 1 3 :28-35 45. Kaplan, P. M., Ford, E. C., Purcell, R. H., Gerin, I. L. 1976. J. Virol. 17: 885-93 46. Kaplan, P. M., Gerin, J. L., Alter, H. J. 1974. Nature 249:762-64 47. Kaplan, P. M., Greenman, R. L.,

Gerin, I. L., Purcell, R. H., Robinson, W. S. 1973. J. Virol. 12:995-1005 48. Kim, C. Y., Tilles, 1. G. 1973. J. Clin. Invest. 52: 1 1 76-86 49. Kornberg, A. 1974. DNA Synthesis. San Francisco: W. H. Freeman

50. Krugman, S., Hoofnagle, J. H., Gerety, R. I., Kaplan, P. M., Gerin, I. L. 1 974. N. Eng/. J. Med. 290: 1 33 1-35 5 1 . Landers, T., Greenberg, H. B., Robin son, W. S. 1977. J. Virol. In press 52. Lavi, S., Winocour, E. 1972. J. Virol. 9:309-1 6 5 3 . LeBouvier, G . L . 1971. J. Infect. Dis. 123:671-75 54. LeBouvier, G. L. 1972. In Hepatitis and Blood Transfusions, ed. G. N. Vyas, H.

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Res. Commun. Chem. Pathol. Phar macal. 2:667-86 63. Mosley, 1. W., Edwards, V. M., Mei haus, I. E., Redeker, A. G. 1 972. Am J. Epidemiol. 95:529-35 64. Neurath, A. R., Prince, A. M., Lippin, A. 1974. Proc. Natl. A cad. Sci. USA

7 1 :2663-67 65. Neurath, A. R., Strick, N. 1977. Lancet 1 : 146 66. Neurath, A. R., Trepo, C., Chen, M., Prince, A. M . 1976. J. Gen. Virol. 30:277-85 67. Nielsen, J. 0., Dietrichson, 0., Iuhl, E. 1 974. Lancet 2:9 1 3- 1 5 68. Nielsen, J . 0., Nielsen, M . H . , Elling, P . 1 973. N. Engl. J. Med. 288:484-87 69. Okada, K., Kamiyama, I., Inomata, M., Imai, M., Miyakawa, Y. 1 976. N. Engl. J. Med. 294:746-49 70. Okochi, K., Murakami, S. 1 968, Vox Sang. 1 5:374 7 1 . Overby, L. R., Hung, P. P., Mao, J. C. H., Ling, C. M . 1 975. Nature 255:84-85 72. Prince, A. M. 1968. Proc. Natl. Acad. Sci. USA 60:8 1 4 73. Prince, A . M . , Szmuness, W., Mann, M .

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HEPATITIS B VIRUS 74. Purcell, R. H. 1975. In Symp. Viral Hepatitis, Washington D. c., 1975 75. Purcell, R. H., Gerin, G. L. 1975. Am. J. Med. Sci. 270:395-400 76. Rao, K. R., Vyas, G. N. 1973. Nature New Bioi. 241 :240-4 1 77. Ray, M. B., Desmet, V. I., Bradburne, A. F., Desmyter, J., Fevery, J., De Groote, J. 1976. Gastroenterologia 7 1 : 462-67 78. Redeker, A. G., Mosley, I. W., Gocke, D. J., McKee, A. P., Pollack, W. 1975. N. Eng!. J. Med. 293: 1 1 55-59 79. Robinson, W. S. 1975. Am. J. Med. Sci. 270: 1 5 1-59 80. Robinson, W. S. Unpublished results 8 1 . Robinson, W. S., Clayton, D. A., Greenman, R. L. 1974. J. Virol. 14: 384-91 82. Robinson, W. S., Greenman, R. L. 1974. J. Viral 1 3 : 1 23 1-36 83. Robinson, W. S., Lutwick, L. I. 1976. N. Engl. J. Med. 295: 1 1 68-75; 1 232-36 84. Robinson, W. S., Lutwick, L. 1. 1976. In Animal Virology, ed. D. Baltimore, A. Huang, C. F. Fox, pp. 787-8 1 1 . New York: Academic 85. Sambrook, I., Westphal, H., Srinivasan, P. R., Du1becco, R. 1968. Proc. Natl. Acad. Sci. USA 60: 1 288-95 86. Seeff, L. B., Wright, E. C., Finkelstein, I. D., Greenlee, H. B., Hamilton, I., Leevy, C. M., Tamburro, C. H., V1ah-

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Nuclear Import of Hepatitis B Virus Capsids and Genome.

The hepatitis B virus.

Hepatitis B virus genotypes and genome characteristics in China.

A Thermodynamic Model for Genome Packaging in Hepatitis B Virus.

Hepatitis B virus genome asymmetry in hepatocellular carcinoma.

Full genome ultra-deep pyrosequencing associates G-to-A hypermutation of the hepatitis B virus genome with the natural progression of hepatitis B.

Hepatitis B virus and microRNAs: Complex interactions affecting hepatitis B virus replication and hepatitis B virus-associated diseases.

Hepatitis B virus infection.

Hepatitis B e antigen and infectivity of hepatitis B virus.

Hepatitis B virus epidemiology.

Synergistic impact of mutations in Hepatitis B Virus genome contribute to its occult phenotype in chronic Hepatitis C Virus carriers.

Epidemiology of virus B hepatitis.

Reactivation of hepatitis-B virus.

Hepatitis B virus. The importance of "nonepidemics".

Complete Genome Sequence of the WHO International Standard for Hepatitis B Virus DNA.

Hepatitis B virus and hepatitis C virus dual infection.

Full-genome sequences of hepatitis B virus subgenotype D3 isolates from the Brazilian Amazon Region.

Nucleotide sequence of the hepatitis B virus genome (subtype ayw) cloned in E. coli.

The importance of hepatitis B virus genome diversity in Basal core promoter region.

How did hepatitis B virus effect the host genome in the last decade?

Micro-evolution of the hepatitis B virus genome in hepatitis B e-antigen-positive carriers: comparison of genotypes B and C at various immune stages.

Molecular cloning of the human hepatitis C virus genome from Japanese patients with non-A, non-B hepatitis.

Protective effect of hepatitis B virus-active antiretroviral therapy against primary hepatitis B virus infection.

Management of hepatitis B virus infection during treatment for hepatitis B virus-related hepatocellular carcinoma.