MBB 446 Lecture 4 Flashcards
Describe the 4 steps in outlined by Weinburg on metastasis
- Arrest by size restriction
- Active extravasation
- Strict perivascular position
- Cooperative growth OR angiogenic growth
Define transdifferentiation
one mature cell type converts to another mature cell type
Define dedifferentiation
a mature cell type regresses to a simpler state
What are the steps in Rous’s protocol for inducing sarcomas in chickens
- Chicken with sarcoma in breast muscle
- Remove sarcoma and break up into small chunks of tissue
- Grind up sarcoma with sand
- Collect filtrate that has passed thru fine-pore filter
- infectious agents trapped in pores were considered to be bacteria
- Infectious agents that passed through filters were classified as viruses - inject filtrate into young chicken
- Observe sarcoma in injected chicken
What is a virion
= individual virus particle
5 structures of a virion?
- nucleic acid (RNA in the case of RSV)
- capsid (protein coat encoded by gag)
- a lipid membrane surrounding the capsid (in some cases)
- Protruding from the lipid bilayer are GLYCOPROTEINS (encoded by env), enable adsorption to the surface of a cell
- viral pol gene encodes REVERSE TARNSCRIPTASE molecules to replicate the viral RNA genome (makes DNA copy)
Virion is said to be _____ if the virus destroys the host cell
virulent
Virion is ____ if the host cell survives for extended periods while harboring the viral genome and releasing progeny virus particles
temperate
Define foci? Morphology?
- infected cells form foci
- clusters of tranformed cells growing amid normal cells
- rounded morphology and altered metabolism
Define transformation
conversion of a normal cell into a cell having some or many of the attributes of a cancer cell
Describe how normal cells grow in a petri dish and how RSV-infected cells that are transformed grow different.
- normal cells in a Petri dish form islands that scatter across the bottom of the dish;; they proliferate and eventually fill up the space in the bottom of the dish, creating confluent cultures;; once confluence is reached, normal cells stop proliferating, resulting in a monolayer.
- Cessation of growth in this monolayer of normal cells is called contact inhibition
(or density inhibition).
- normal cells in a Petri dish form islands that scatter across the bottom of the dish;; they proliferate and eventually fill up the space in the bottom of the dish, creating confluent cultures;; once confluence is reached, normal cells stop proliferating, resulting in a monolayer.
- RSV-infected cells are transformed;; they lose contact inhibition and continue to proliferate, creating multilayered clumps of cells (ie. foci)
Infection of a chick embryo fibroblast with RSV results in cell _____ and formation of a ____
Infection of a chick embryo fibroblast with RSV results in cell TRANSFORMATION and formation of a FOCUS
Question: Did the transformed state of descendant cells depend on the continuing presence of RSV?
Yes RSV is required to both initiate and maintain the transformed phenotype
Examples of viruses that contain DNA genomes that can also induce cancer
- Shope papillomavirus
2. Human papillomavirus
What does the SM40 (simian virus 40) cause in permissive host cells
Launches lytic cycle (causes lysis of cells)
9 properties of TRANSFORMED cells
- Altered morphology (rounded shape)
- Loss of contact inhibition
- Ability to grow without attachment to solid substrate
- Ability to proliferate indefinitely
- Reduced requirement for mitogenic growth factors
What is anchorage independence?
ability to multiply without attachment to a solid surface; cells are suspended in agar
What is a good predictor of ability of cells to form tumors in vivo (= tumorigenicity)
anchorage independence
What is syngeneic?
= host and injected cells come from the same genetic strain; immune system does not recognize the transformed cells as foreign and a tumor can grow
Alternate strategy besides syngeneic to grow tumor in mice
- Alternate strategy is to use an immunocompromised host; (eg. Nude, RAG, NOD/SCID mice)
- tolerate engrafted tissues or cells (= xenografts)
- candidate tumor cells often injected subcutaneously
- monitor progress of tumor formation
How is a xenograft given
- cells often embedded in a matrix (e.g. Matrigel) and the mixture is innoculated into mice
- Matrigel is composed primarily of structural proteins like collagen, lamin, and entactin, also contains Growth Factors
Describe the sucrose gradient sedimentation experiment
Viral genomes (portions) become integrated into host chromosomal DNA
How does info flow in retroviruses
Info flows “backwards” from RNA to DNA
What is a provirus
dsDNA copy of a retroviral genome that is the product of reverse transcription
What are the steps in the life cycle of a RNA tumor virus
- Introduction of nucleocapsid into cell
- Association of reverse transcriptase and integrase with viral RNA
- Synthesis of (-) strand viral DNA by reverse transcriptase
- Removal of viral RNA and synthesis of (+) strand viral DNA by reverse transcriptase
- Integration of viral dsDNA into cell chromosome to form provirus
- Transcription of provirus by host cell RNA polymerase II
- Translation of viral RNA to make new viral proteins
- Assembly of viral proteins and viral RNA genomes into progeny virions
- Progeny virions leave cell and initiate new infectious cycles
How did Bishop and Varmus labs search for the gene(s) causing transformation in RSV?
- Bishop and Varmus labs made a DNA probe that recognized RSV src
- Unexpectedly found that src sequences were already present in uninfected chicken cells
- Present as 2 copies per genome (ie. normal cellular gene)
- Found to be conserved in all vertebrate species
- Revolution in thinking about the origins of cancer: normal vertebrate cells contain genes that have the intrinsic potential to induce cancer
What is a proto-oncogene?
a normal cellular gene that, upon alteration by DNA- damaging agents or viral genomes, can acquire the ability to function as an oncogene (c-src was the first to be discovered)
What is an oncogene?
a cancer-inducing gene; a gene that can transform cells
a single oncogene can act ____ to evoke a series of distinct changes in cellular traits
pleiotropically (one gene influences two or more seemingly unrelated phenotypic traits)
How do non-transforming retroviruses induce cancer? Contrast it to oncogene-bearing retroviruses that can transform cells
- only after an extended time period (months) compared to the oncogene-bearing retroviruses (days-weeks)
- insertional mutagenesis = inserting a site next to gene to lead to tumour genesis
What was the first virus to be directed implicated in human cancer?
Epstein-Barr Virus (EBV)
EBV is a member of what family
herpesvirus family (DNA virus)
EBV infects >___% of the world’s adult population;;= delayed infection can result in ________
EBV infects >90% of the world’s adult population;= delayed infection can result in MONONUCLEOSIS
EBV is implicated in pathogenesis of what types of cancer
Burkitt’s lymphoma, nasopheryngeal carinoma, other lymphomas
Why has it been so difficult to identify Infectious agents as causative factors for human tumors?
- Certain infectious agents may be necessary but not sufficient?
- Cancer is a multi-step process
- No human cancer arises as the acute consequence of infection.
- Latency periods between primary infection and cancer development are frequently 15-40 yrs
- Besides some rare exceptions, no synthesis of the infectious agents occurs in cancer cells
- Most of the infections are common in human populations; only a small proportion of infected individuals develop cancer
- Mutations in host cell genes or within the viral genome are mandatory for malignant conversion.
What are the 3 direct and 4 indirect mechanisms of cancer induction by infections? Provide an example for each
I. Direct mechanisms:
1. Introduction of viral oncogenes into host cells
(high risk HPV, EBV, HHV-8, HTLV-1)
- Modified viral oncogenes after integration into host cell DNA
(Merkel cell polyomavirus) - Modified host cell genes integrated into viral genomes act as oncogenes
(human endogenous retroviruses;; HERV?)
II. Indirect mechanisms:
1. Virus-induced immunosuppression activates other tumor viruses
(HIV-1 and HIV-2)
- Chronic inflammation, induction of oxygen radicals
(Hepatitis B and C, Helicobacter pylori, Parasites) - Prevention of apoptosis (some cutaneous HPV types)
- Induction of chromosomal instability and translocations
(Adenoviruses, Herpesviruses, TT viruses? Endogenous retroviruses?)
Tumor viruses: why are they not recognized as foreign by the host and eliminated?
Evasion of host immune responses by multiple mechanisms:
- Antigenic variability (selection of variants that can evade recognition by neutralizing antibodies
- Interfere with interferons (which normally protect cells from viral infection)
- Block cytokines
- Express viral anti-apoptosis proteins (i.e. evade or subvert the host immune response)
Why do viruses insert into other genomes? Ie. What is the advantage to the virus?
They can replicate and produce many copies.
How to determine if an infectious agent is really causative?
N/A
