Hepadnaviruses - Hepadnaviridae
Virion
Genome
Genes and proteins
Viruses and hosts
Diseases
Distinctive characteristics
Hepadnaviruses - Hepadnaviridae
 Virion
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Spherical enveloped particle.
Diameter 42 nm.
Icosahedral capsid, T=4.
Abundant smaller spherical and filamentous
forms lack nucleocapsid and are not infectious.
Hepadnaviruses - Hepadnaviridae
 Genome
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Circular dsDNA with a ss gap on one strand, 3.2
Kb.
Hepadnaviruses - Hepadnaviridae
 Genes and proteins
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mRNAs transcribed by cellular RNA
polymerase II from five promoters.
Four overlapping reading frames, seven viral
proteins.
Three surface proteins: LS, MS, SS.
Core proteins: C, E.
Polymerase (reverse transcriptase) protein: P.
Regulatory protein: X.
Hepadnaviruses - Hepadnaviridae
 Viruses and hosts
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Hepatitis B viruses of human, chimpanzee, duck,
gibbon, gorilla, ground squirrel, heron,
orangutan,snow goose, woodchuck, woolly
monkey.
Hepadnaviruses - Hepadnaviridae
 Diseases
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Hepatitis: incubation period 30–180 days.
Transmission by blood and sexual contact.
Acute disease can be mild or severe.
Chronic or associated diseases: cirrhosis, liver
cancer, serum sickness.
Treatment: interferon, lamivudine.
Prevention: vaccination with recombinant
hepatitis B surface antigens; injection with antihepatitis B immunoglobulin after exposure to
virus.
Hepadnaviruses - Hepadnaviridae
 Distinctive characteristics
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Unusual partly ss, partly ds circular genome
results from incomplete replication in cell.
Pregenome RNA is both an mRNA and a template
for synthesis of genome DNA.
Reverse transcriptase (polymerase protein)
generates genome DNA from pregenome RNA
within capsid during virus assembly.
Makes large amounts of noninfectious spherical
and filamentous particles.
Has a satellite virus: hepatitis delta virus (see
Chapter 29, viroids).
Virion
 At least seven distinct viruses cause human
hepatitis
 The discovery of hepatitis B virus
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1963, unknown protein related to type B hepatitis
1973, Virus-like particles in the serum of type B
hepatitis patients (Dane particle)
1979, DNA genome cloned and sequenced
Virion
 Dane particles are infectious virions;
abundant noninfectious particles lack
nucleocapsids
Fig. 28.1 Structure of virions of hepatitis B
virus.
Virion
 The viral genome is a circular, partly singlestranded DNA with overlapping reading
frames
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Partly double-stranded with a single-stranded
region of variable length
Neither DNA strands forms a covalently closed
circle
 (-) strand joined at 5’ end to P protein
 (+) strrand joined at 5’ end to short capped RNA
 5’ ends of (-) strand and (+) strand genomic DNA are
joined by short complementary regions named DR2
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Four partly overlapped ORFs : C, P, S, X
Genome
(a) The 5 ends of minus-strand (inside
circle) and plus-strand (outside circle)
genomic DNA are joined by short
complementary regions named DR2
(gray boxes). The 5 end of the minus
strand is attached to the viral
polymerase protein (orange sphere)
and the 5 end of the plus strand is
attached to a short, capped RNA (black
line).
(b) DNA polymerase extends the plus
strand, filling in the ss gap.
(c) Nucleases remove the 5 ends of both
strands.
(d) DNA ligase joins the 3 and 5 ends of
each strand, forming a fully covalently
closed double-stranded circular DNA.
Fig. 28.2 Structure of hepatitis B virus genome, and its conversion to a covalently closed, circular DNA.
Genes and proteins
Fig. 28.3 Coding and signaling regions on the
hepatitis B genome.
Genes and proteins
 Nucleocapsids enter the cytoplasm via
fusion and are transported to the nucleus
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No currently available in vitro culture cell lines
Initial steps of virus entry are poorly understood
Cleavage of large hepatitis B surface Ag exposes
fusion peptide, resulting in viral and host cell
membrane fusion
Released nucleocapsid into cytoplasm is
transported to nuclear membrane
Viral genome is converted within nucleus into
covalently closed circular form by host enzymes
Genes and proteins
 Transcription of viral DNA gives rise to
several mRNAs and a pregenome RNA
a. Circular DNA is transcribed
by host RNA polymerase II, which
recognizes five promoters (preC, pg,
preS1, S, and X) to generate five
classes of capped mRNAs that
share a common 3 polyadenylated
end.
b. Open reading frames and
translation products are shown
above each RNA. pg RNA also
serves as a template for virus
replication.
Fig. 28.4 Transcription of
hepatitis B virus DNA.
Genes and proteins
 The roles of hepatitis B virus proteins
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X protein
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Implicated in HCC
Up-regulate c-src, Ras/raf/MAPk, SAPK, PKC etc
Interact with and sequester p53
Inhibit apoptosis
Surface proteins.
 Involved in envelope formation
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Core and E proteins.
 Core : major component of necleocapsid
 E protein : may suppress host immune system
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Polymerase protein.
 RNA/DNA-dependent DNA polymerase (reverse transcriptase)
 Ribonuclease H
Genes and proteins
 The pregenome RNA is packaged by
interaction with polymerase and core protein
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pgRNA serves as mRNA for C and P, and also used
as template to produce DNA genome
5’ e stem-loop is functional in packaging, and only
in the pg RNA
P protein recognizes and interacts with the e stemloop, initiating both encapsidation and reverse
transcription of pg RNA
Genes and proteins
 Genome replication occurs via reverse
transcription of pregenome RNA
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Step 1. Initiation of the reverse transcription
process begins when P binds to the 5 stem-loop.
Steps 2 and 3. Reverse transcription progresses
for only a few base-pairs beyond the 5 stem-loop,
after which the enzyme and the newly synthesized
DNA are translocated onto an identical sequence
in the direct repeat region 1 (DR1) near the 3 end
of the pregenome RNA.
Genes and proteins
 Genome replication occurs via reverse
transcription of pregenome RNA
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Steps 4 and 5. The P protein then makes a
complete (minus strand) copy of the remainder of
the pregenome RNA by extending the DNA chain
to the 5 end of the RNA template.
Steps 6 and 7. The translocated RNA fragment
serves as a primer for subsequent synthesis of
plus-strand DNA.
Genes and proteins
Fig. 28.5 Reverse transcription of
pregenome RNA to form hepatitis
B virus genome DNA.
Genes and proteins
Genes and proteins
Fig. 28.6 Schematic comparison of replication cycles of retroviruses and hepadnaviruses.
Genes and proteins
 Virions are formed by budding in the
endoplasmic reticulum
Fig. 28.7 Assembly of nucleocapsids and virions of hepatitis B virus.
Genes and proteins
 Hepatitis B virus can cause chronic or acute
hepatitis, cirrhosis, and liver cancer
 Hepatitis B virus is transmitted by blood
transfusions, contaminated needles, and
unprotected sex
 A recombinant vaccine is available
 Antiviral drug treatment has real but limited success
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Interferon-a
Lamivudine (3T3)
 Interfere with DNA polymerase activity of P protein
Key Terms
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Apoptosis
Ascites
Cirrhosis
Complement
Enteric
Epitope
Exocytosis
Fulminant hepatitis
Hepatitis
Hepatocellular carcinoma
Immunoprophylaxis
Interferons
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Jak-Stat pathway
Jaundice
Myristate
Myristoylation
NF-B (nuclear factor-B)
Ras
Ribonuclease H
Serum sickness
Src kinases
Tumor necrosis factor (TNF)
Tumor suppressor protein