Chapter 3:
Tumor Viruses
Copyright © Garland Science 2007

• Viruses discovered by research of Pasteur and Koch
• Infectious agents are responsible for Dysentary, Cholera, Rabies…
• Viruses classified as agents that survive filtration (too small to be removed by filtration)
1909 Chicken Sarcoma Virus discovered by Peyton Rous
Figure 3.1 The Biology of Cancer (© Garland Science 2007)

Experiment showing viral origin of a chicken breast sarcoma
Viral agent named RSV (Rous Sarcoma Virus)
Figure 3.2 The Biology of Cancer (© Garland Science 2007)

DNA Virus Life Cycle
Figure 3.3 The Biology of Cancer (© Garland Science 2007)

RSV Viron
Figure 3.4a The Biology of Cancer (© Garland Science 2007)

INFLUENZA A VIRUS HAS 11 PROTEINS ONLY

• 1913 Danish scientist Fibiger reported that stomach cancer in rats can be traced to Spirochet worms.
• Received Nobel Prize in 1926. Died after a few months
• Tumors found to be metaplasic epithelium from vitamin deficiency due to sugar cane diet.
• Viral cause of cancer discredited – focus shifted to carcinogens.

• Temin , Rubin, Dulbecco
• Chicken embryo fibroblasts infected with
RSV became virtually immortal and continued to make virus
• Infected cells in vitro:
– formed foci
– lost contact inhibition
Figure 3.5 The Biology of Cancer (© Garland Science 2007)

Formation of focus in transformed cells
Figure 3.7a The Biology of Cancer (© Garland Science 2007)

• Descendents of transformed cells continued to harbor copies of RSV virus
• Did the transformed state DEPEND on the continued presence of the virus?

• Mutant RSV could only transform at 37 o C – not at 41 o C
• Morphology depended on temperature. At 41 o C, cells had normal morphology.
Figure 3.8 The Biology of Cancer (© Garland Science 2007)

Other viruses: Shope Papilloma Virus – 1950
DNA Virus, had protein coat
Figure 3.9a The Biology of Cancer (© Garland Science 2007)

Table 3.1 The Biology of Cancer (© Garland Science 2007)

1950-1960:
• SV40 Virus found in Polio vaccine stocks prepared from monkey kidney cells
• Some cells would lyse, releasing virons
• 10-30 million people exposed to SV40 by Polio vaccine
• No additional cancer risk from exposure
Figure 3.10a The Biology of Cancer (© Garland Science 2007)

Table 3.2 The Biology of Cancer (© Garland Science 2007)

Anchorage Independent Growth of Transformed Cells
Figure 3.12 The Biology of Cancer (© Garland Science 2007)

Immunocompromised mouse
No thymus – so no self/non-self recognition
Receptive to grafted cells
Hairless: subcutaneous tumor formation can be seen
Figure 3.13 The Biology of Cancer (© Garland Science 2007)

The viron in SV40 infected cells was tightly associated with the chromosomal DNA:
• SV40 DNA did not sediment with small (~5 kb) viral genome
• Instead, it came associated with a protein (T-antigen) that elicited a strong immune response
• Implied that the SV40 DNA in transformed cells was covalently linked to chromosomal DNA
Figure 3.15 The Biology of Cancer (© Garland Science 2007)

Howard Temin:
• Retroviral genomes become integrated with chromosome of infected cells.
• Against Central Dogma
• Nobel Prize for co-discovery (with David
Baltimore) of reverse transcriptase
Figure 3.6 The Biology of Cancer (© Garland Science 2007)

Life Cycle of an RNA Tumor Virus like RSV
Figure 3.17 The Biology of Cancer (© Garland Science 2007)

• Retroviruses like RSV integrate easily into cellular DNA.
• DNA viruses like SV40 do not carry reverse transcriptase.They only occasionally integrate into cellular DNA.

• Comparison of genomes of viruses which could and could not transform cells showed that
– Transformation seemed to depend on a single gene (the “src” or “sarcoma” gene)

Preparing src specific probes:
These probes were labeled with radioisotope, and hybridized with virus infected cells.
Cells that carried viral src would hybridize with these probes, showing the presence of viral src.
Figure 3.20 The Biology of Cancer (© Garland Science 2007)

Experiment went terribly wrong !!
• There was src in uninfected chicken cells !
• And in other species as well
Figure 3.21 The Biology of Cancer (© Garland Science 2007)

Table 3.3 The Biology of Cancer (© Garland Science 2007)

• The cellular src gene seemed normal
– Present in single copy per haploid genome
– Evolving slowly
– Functioning normally (no associated transformation property in uninfected cells).
• RSV kidnapped a cellular gene to transform cells

Possible Mechanism by which src gene was captured by avian leukosis virus
ALV proviral DNA gets integrated (by chance) near c-src
And gets copied into viral RNA as v-src
Figure 3.22 The Biology of Cancer (© Garland Science 2007)

• RSV already had an oncogene and could readily induce tumors
• ALV, MLV (murine leukemia virus) acquired a proto-oncogene from genome of some host
• But precursor ALV (without its oncogene), was also able to slowly transform cells ! !
• How could this be ? How did they induce a malignancy without an oncogene?

Insertional Mutagenesis found by mapping viral sites in host DNA:
ALV inserts near c-myc oncogene which now is under control of the viral transcriptional promoter.
Figure 3.23a The Biology of Cancer (© Garland Science 2007)

How viruses cause tumors !!
Figure 3.23b The Biology of Cancer (© Garland Science 2007)

Table 3.4 The Biology of Cancer (© Garland Science 2007)

• Perhaps x-rays, mutagenic agents can alter the normal function of oncogenes at their normal sites – and make them able to cause cancers !