1. Viral transformation and oncogenesis

2. Taxonomy of Tumor Viruses
HPV SV-40 BK, JC
DNA viruses: papovaviruses hepadnaviruses herpesviruses adenoviruses poxviruses
RNA viruses: retroviruses flaviviruses
HBV
EBV KSHV
HTLV-1
Hepatitis C virus

3. How do viruses transform cells?
Virus infection provides a “hit” towards the genesis of cancer.
Act as a “mutagen”
Other cofactors (genetic, immunological, or enviromental) may be needed for development of cancer
Cell transformation is accompanied by the persistence of all or part of the viral genome and continual expression of a limited number of viral genes.
Viral oncogenes are expressed that alter normal cellular gene expression and signal transduction pathways.

4. Generalization about Viral Transfomration
RNA viruses activate oncogenes
DNA viruses negate tumor suppressors

5. RNA TUMOR VIRUSES

6. Retroviridae
Retroviruses (family Retroviridae) are enveloped, single stranded (+) RNA viruses that replicate through a DNA intermediate using reverse transcriptase.
This large and diverse family includes members that are oncogenic, are associated with a variety of immune system disorders, and cause degenerative and neurological syndromes.

7. General Introduction to Retroviruses
- Ubiquitous; found in all vertebrates
- Large, diverse family
- Includes HIV, FIV and FeLV
Definition and classification of retroviruses
- Common features- structure, composition and replication
- Distinctive life cycle: RNA-DNA-RNA
- Nucleic acid is RNA in virus, and DNA in infected cell
Transmission may be either:
- Horizontal- by infectious virus (exogenous virus) or vertical- by proviruses integrated in germ cells (endogenous virus)
- Can transmit either as free viral particle or (for some retroviruses) through cell-cell contact

8. Retroviruses Transmission EM matrix Env RNA capsid
Scanning EM
Schematic cross section of retroviral particle.
Retroviruses:
Core- surrounded by an envelope. “Enveloped virus”. Core- thought to be icosohedral- not true.
Inside core:
Dimer RNA genome
Enzymes and other proteins required for replication
3. OUTSIDE the core-Envelope
Derived from the lipid bilayer of the cell
2 proteins encoded by the virus- Env- SU/TM
3D representation of HIV virion:

9. Retrovirus replication cycle
Attachment of the virion to a specific cell surface receptor
Penetration of the virion core into the cell
Reverse transcription within the core structure to copy the genome RNA into DNA
Transit of the DNA to the nucleus
Integration of the viral DNA into random sites in cellular DNA to form the provirus
Synthesis of viral RNA by cellular RNA polymerase II using the integrated provirus as a template
Processing of the transcripts to genome and mRNAs
Synthesis of virion proteins
Assembly and budding of virions
Proteolytic processing of capsid proteins

10. Retrovirus
life cycle:

11. Retroviral Life Cycle Early events: Late events:
from viral binding and entry until the time the DNA copy of the viral genome is integrated into the host cell’s chromosome
Late events:
From time when integrated provirus is expressed until virus has been released

12. Latent vs. active infection
In latent infection- retroviral genome is present but is not transcribing viral genome or mRNA for structural proteins.

13. Retroviruses
RNA tumor viruses “create” oncogenes by acquiring, modifying, deregulating cellular genes (proto-oncogenes)
v-onc not essential viral gene & unrelated to strategy of viral replication
Replication of RNA viruses is not cytocidal nor is it required for tumorigenesis

14. Mechanisms of cell transformation by retroviruses
1) Retroviral transduction of oncogene (transducing retrovirus)
2) Oncogene activation by retroviral insertion (cis-acting / nontransducing retrovirus)
3) Oncogenesis mediated by essential retrovirus proteins (trans- activating / nontransducing long-latency retrovirus)

15. Mechanism of Acquisition of cellular sequences
Model for retroviral transduction of a cellular proto-oncogene (cONC) to form an acute transforming virus. A provirus is integrated upstream of a cONC,; the insertion may increase the level of transcription of the oncogene. Either a viral oncogene readthrough transcript is made (A) or the provirus and the cONC gene are fused by a deletion (B) Either event gives rise to a hybrid RNA transcript initiating in the 5’ LTR of the provirus and extending into the oncogene. Additional proviruses integrated elsewhere in the cellular genome can provide helper function, forming virion particles (C) that contain both helper and hybrid RNAs. Recombination between these two RNAs during the process of reverse transcription (D) joins the ends of the viral genome to the hybrid RNA. Either one or two crossovers are required depending on the structure of the starting RNA. Reverse transcription gives rise to a fully transmissible retroviral genome carrying the oncogene. Subsequent transmission of the new genome (E) from doubly infected cells can occur at high efficiency without further rearrangements.

16. Acquired Genes Are Components of Signaling Networks
External signal molecules or growth factors (receptor ligands) (sis)
Cellular receptors (erbB, fms, kit)
Second messengers in signaling cascade (kinases: src, abl, fgr, yes; mos raf)
Transcription factors (jun, fos, myc, myb, ets, rel)

17. DNA TUMOR VIRUSES

18. DNA tumor viruses
Diverse group of viruses with different structures, genome organization, and strategies of replication
Some induce tumors in natural host
Papilloma
EBV, KSHV
Hepatitis B
Others induce tumors in experimental systems:
Adenovirus
Polyomaviruses , SV40

19. DNA tumor viruses
Oncogenic potential linked to virus replication strategy
Oncogenes are essential viral genes without cellular homologues (for small DNA tumor viruses)
Transformation occurs ONLY in “aborted” viral life cycle (early genes expressed but replication, which is cytocidal, does not occur)
Adenovirus, SV40, and polyomavirus frequency of transformation is less than 1 in 105 infected cells.
For small DNA tumor viruses, integration of viral genome may enable abortive viral lifecycle.

20. DNA tumor viruses target tumor suppressors
Virus Gene Product Cellular target
Adenovirus
SV40
Polyomavirus
Papillomavirus
E1A E1B
Rb p53
Large T antigen Large T antigen Middle T antigen
Rb, p53 Rb Src, PI3K
E7 E6 E5
Rb p53
PDGF receptor