1. Hepadnaviruses - Hepadnaviridae
Virion
Genome
Genes and proteins
Viruses and hosts
Diseases
Distinctive characteristics

2. Hepadnaviruses - Hepadnaviridae
Virion
Spherical enveloped particle.
Diameter 42 nm.
Icosahedral capsid, T=4.
Abundant smaller spherical and filamentous forms lack nucleocapsid and are not infectious.

3. Hepadnaviruses - Hepadnaviridae
Genome
Circular dsDNA with a ss gap on one strand, 3.2 Kb.

4. Hepadnaviruses - Hepadnaviridae
Genes and proteins
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.

5. Hepadnaviruses - Hepadnaviridae
Viruses and hosts
Hepatitis B viruses of human, chimpanzee, duck, gibbon, gorilla, ground squirrel, heron, orangutan,snow goose, woodchuck, woolly monkey.

6. Hepadnaviruses - Hepadnaviridae
Diseases
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 anti-hepatitis B immunoglobulin after exposure to virus.

7. Hepadnaviruses - Hepadnaviridae
Distinctive characteristics
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).

8. Virion At least seven distinct viruses cause human hepatitis
The discovery of hepatitis B virus
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

9. Virion
Dane particles are infectious virions; abundant noninfectious particles lack nucleocapsids
Fig Structure of virions of hepatitis B virus.

10. Virion
The viral genome is a circular, partly single-stranded DNA with overlapping reading frames
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
Four partly overlapped ORFs : C, P, S, X

11. Genome
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).
DNA polymerase extends the plus strand, filling in the ss gap.
Nucleases remove the 5 ends of both strands.
DNA ligase joins the 3 and 5 ends of each strand, forming a fully covalently closed double-stranded circular DNA.
Fig Structure of hepatitis B virus genome, and its conversion to a covalently closed, circular DNA.

12. Genes and proteins
Fig Coding and signaling regions on the hepatitis B genome.

13. Genes and proteins
Nucleocapsids enter the cytoplasm via fusion and are transported to the nucleus
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

14. 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 Transcription of
hepatitis B virus DNA.

15. Genes and proteins The roles of hepatitis B virus proteins X protein
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
Core and E proteins.
Core : major component of necleocapsid
E protein : may suppress host immune system
Polymerase protein.
RNA/DNA-dependent DNA polymerase (reverse transcriptase)
Ribonuclease H

16. Genes and proteins
The pregenome RNA is packaged by interaction with polymerase and core protein
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 stem-loop, initiating both encapsidation and reverse transcription of pg RNA

17. Genes and proteins
Genome replication occurs via reverse transcription of pregenome RNA
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.

18. Genes and proteins
Genome replication occurs via reverse transcription of pregenome RNA
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.

19. Genes and proteins
Fig Reverse transcription of pregenome RNA to form hepatitis B virus genome DNA.

20. Genes and proteins

21. Genes and proteins
Fig Schematic comparison of replication cycles of retroviruses and hepadnaviruses.

22. Genes and proteins
Virions are formed by budding in the endoplasmic reticulum
Fig Assembly of nucleocapsids and virions of hepatitis B virus.

23. 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
Interferon-a
Lamivudine (3T3)
Interfere with DNA polymerase activity of P protein

25. Key Terms Jak-Stat pathway Apoptosis Jaundice Ascites Myristate
Myristoylation
NF-B (nuclear factor-B)
Ras
Ribonuclease H
Serum sickness
Src kinases
Tumor necrosis factor (TNF)
Tumor suppressor protein
Apoptosis
Ascites
Cirrhosis
Complement
Enteric
Epitope
Exocytosis
Fulminant hepatitis
Hepatitis
Hepatocellular carcinoma
Immunoprophylaxis
Interferons