Lecture 4 - Tumor Viruses Flashcards
What are tumor viruses?
Tumor viruses cause the host cell to proliferate instead of causing death.
Give a brief history of tumor virus research and discoveries. Include 4-6 scientists by name and the outcome of their research.
- Pasteur/Koch: micro-organisms cause disease in humans
- Peyton Rous: Hen-to-Hen sarcoma transplantation experiments. Breast muscle, filtered = must be a virus. More fluid/virus recovered than originally injected, worked on a predictable schedule.
- Fibiger: stomach lesions (thought cancer) in rats caused by nutrient deficiency (not worms)
- Temin/Rumin: RSV induces chicken embryo fibroblasts that can survive indefinitely.
Describe the Peyton Rous Experiment in more detail. What are the 6 steps involved?
1 - Get a chicken with a sarcoma in breast muscle
2 - remove sarcoma in small chunks of tissue
3 - Grind up sarcoma tissue with sand
4 - Collect filtrate that passes through a fine-pore filter
5 - Inject filtrate into healthy young chicken
6 - Observe formation of new sarcoma in injected host
What key characteristic to RSV transformed cells show in vitro? Why is this: what do they lack?
RSV transformed cells do not form a confluent monolayer as expected (normal conditions), but will rather form a cluster as transformed cells lack “contact inhibition”
Describe the maintenance of RSV transformation. What role does temperature play and what effect does it have when adjusted?
Ex: chicken fibroblasts are injected with a temperature sensitive (ts) mutant of RSV. Under normal conditions, infection and transformation occurs naturally. However, if the temperature is increased, the viral proteins denature and are non-functional leading to rejection and reformation of normal cells. If the temperature is decreased again to regular levels, transformation occurs as it did before, producing cancer-like cells.
Describe 3 types of cancer inducing viruses that are characteristically non-human. Give their typical host and defining attributes.
1 - Papillomavirus (Rabbits), skin warts
2 - Polyomavirus (Mice), various tumors
3 - SV40 virus (Monkeys), polio vaccine, lytic/transformative
What are the “Big 3” types of cancer-inducing viruses that affect ~90% if the human population?
1 - Human Papillomavirus (HPV)
2 - SD40 Virus
3 - Cytoplasmic Vacuoles
What are some properties of transformed cells? Give 6 our of 9 possible altered morphological characteristics.
1 - Abnormal/micro-nuclei 2 - Unusual growth 3 - Loss of contact inhibition 4 - Anchorage-independent growth 5 - High saturation density 6 - Immortalization 7 - Reduced requirement for mitogenic growth factors 8 - Increased glucose transport 9 - Tumorigenicity
What are some important factors when infecting lab animals with tumor inducing viruses?
Usually requires a syngenic host that is immunocompromised. This way the immune system of the “nude mouse” can’t fight back. Mouse usually lacks hair/thymus.
How do RSV and related viruses transform a cell? What are some of the key steps taken in the transformation of a normal cell into a host cell?
RSV and related viruses have limited genetic material and have pleiotropic effects (more than one) depending on what kind of cell they are attacking.
1 - Nucleocapsid introduced into cell that contains a reverse transcriptase and an integrase with viral DNA
2 - Reverse transcriptase synthesizes a viral strand of DNA from viral RNA
3 - Foreign DNA incorporated into host cell genome
4 - Integrase used in final step, reverse transcription NOT required.
How does SV40 and related viruses transform a cell? What are some of the key steps taken in the transformation of a normal cell into a host cell?
1 - Healthy cell is unable to distinguish between viral DNA and host DNA
2 - (now) host cell integrates portions of viral DNA into its own genome
3 - Since viral genome is replicated into episomal copies (some monomers, some concatemers etc…) regions of viral DNA are stretched out randomly next to that of the host. (low concentration, similar to plasmids)
4 - Non-homologous recombination occurs between sections of viral DNA and the host strand
5 - May not immediately effect host depending on what’s been integrated from the viral genetic material. Effects can be passed down for generations before becoming active.
What is the Src gene? Where does it come from? What are proto-oncogenes and how is Src classified as one? Refer to a study in your answer.
Proto-oncogenes are genes that have the potential to become oncogenes if altered or constitutively turned “on”. Proto-oncogenes have pleiotropic effects (causing transformation & cancer) and can be generated by over-expression via tumor viruses. Src is the gene most commonly responsible for transformative changes. To determine the genetic origin of tumor virus gene expression, scientists used a labelled probe to test the viral DNA. They discovered Src in controls as well which means its a naturally occurring proto-oncogene.
How do certain viruses lacking oncogenes cause cancer?
Viruses lacking oncogenes can cause cancer by containing genes such as myc. These myc RNAs are then translated into myc oncoproteins which, in high number, lead to an uncontrollable proliferative signal to the cell.
What is the third mechanism that only select retroviruses use to cause tumorigenesis. Give an example.
Some select retroviruses can contain a viral tax gene that activates a proviral DNA sequence.
Ex: T-cell leukemia virus (HTLV-1)
