Molecular Basis of Carcinogenesis (Exam 3) Flashcards
Name some of the properties of a malignant, or transformed, cell (11 total)
- Loss of contact inhibition/high saturation density
- Immortalization (avoid apoptosis)
- Ability to grow w/o attachment to solid substrate
- Reduced requirement for mitogenic growth factors (constitutive activation of pathways) and inability to halt cell cycle in the absence of growth factors
- Altered morphology (little/no cytoplasm; rounded)
- Invasive, that is capable of outgrowth into neighboring normal tissues to extend the boundaries of the tumor.
- Metastatic, that is capable of shedding cells that can drift through the circulatory system and proliferate at other sites in the body.
- Increased transport of glucose (lower O2 requirement–ferment sugar in place of oxidative phosphorylation)
- De-differentiated, that is, lack many of the specialized structures and functions of the tissue in which they grow.
- Sustained angiogenesis
- Clonal in origin (derived from a single cell)
What differentiates a benign tumor from a malignant one?
Benign tumors have lost cell controls (are immortal), but are not invasive or metastatic.
What are two critical types of genes in which mutations are likely to cause cancer?
- Oncogenes, which normally stimulate cellular proliferation (analogous to the “gas pedal” of your car), are activated.
- Anti-oncogenes or tumor suppressors, which normally inhibit cellular proliferation (analogous to the “brake pedal” of your car), are inactivated.
What is the Knudson theory, and how does this apply to Loss of Heterozygosity? What is LOH?
The Knudson “two hit” theory is that two mutations must occur to knock out both copies of a gene. Refers to a tumor suppressor like Rb. This is why sporadic retinoblastoma is so much less less common than hereditary; two mutations in the same spot is much more rare, and takes longer. People born with a mutation are at a much higher risk because only one recombination event is required to turn off the brakes. LOH occurs as an “inappropriate” crossing over during mitosis between non-homologous chromosomes. Odds are 50% that heterozygosity is maintained, 50% that one cell gets both mutations and one gets none, in which case heterozygosity is lost.
Examples of cancers that are inherited as autosomal dominant disorders are (4):
- Familial Adenomatous Polyposis (FAP-APC gene)
- Familial Retinoblastoma (RB gene)
- Familial Breast and Ovarian Cancer (BRCA1 and BRCA2 genes)
- Wilms tumor syndromes
Examples of cancers that are inherited as autosomal recessive disorders are (4):
- Xeroderma pigmentosa (XP genes)
- Ataxia-telangiectasia (AT gene)
- Bloom’s syndrome
- Fanconi’s congenital aplastic anemia (FA genes)
This is the hallmark of a antioncogene or tumor suppressor gene.
In cases of “inherited” retinoblastoma (i.e. when there was a parent and other family members who also had the disease), the DNA from normal tissue of the patient or from other unaffected family members often shows a defect in the retinoblastoma gene, but has one normal copy of the gene per cell.
+++++++ In these patients it appears that normal, nonmalignant retinal cells, are heterozygous for the retinoblastoma gene, but the tumor cells have descended as a clone from a single cell that has acquired homozygosity for the retinoblastoma susceptibility gene. ++++
The Rb protein is ______ in rapidly proliferating cells at ____ of the cell cycle, but is ____ in non-proliferating cells in ____ of the cell cycle. ______ are responsible for phosphorylating Rb, allowing it to proceed from G1 to S phase.
hyperphosphorylated; S or G2; hypophosphorylated; G0 or G1; CDK (Cyclin Dependent Kinases);
HPV encodes two proteins, ____ which targets Rb, and ___ which targets p53.
E7, E6
____ encodes a T antigen (Large T) that targets ____ and ____.
SV40; Rb and p53
HeLa cells were isolated from a cervical carcinoma and have been growing in culture for over 60 years. These cells express HPV _____
E7 and E6 protein (E6 inhibits p53, another important tumor suppressor). If E7 and E6 expression is blocked, the cells return to normal phenotype. This bodes well for therapy as affecting just two proteins can have a drastic effect.
Kaposi Sarcoma encodes a ____, which binds ___ to push cells into S phase. Often seen in patients who have ____.
cyclin; CDK; AIDS
Rb appears to have a dominant inheritance pattern. We know, however, that a cell must be homozygous for the mutation to become tumorigenic. Explain.
What is inherited in a dominant fashion is the susceptibility to retinoblastoma. People who are heterozygotes for retinoblastoma have only one normal RB gene in each cell of their body including the cells of the retina. These cells will regulate their proliferation normally and will be non-malignant. However, loss of the single, normal RB gene by any number of events will produce a tumor. Thus if one cell among the millions of retinal cells has no RB protein, it will lose the ability to regulate its proliferation, grow out of control, thereby generating a clone of cells, which will become a malignant tumor. Thus people who are heterozygotes for the RB gene are likely to develop the disease and will pass on the defective gene to 1/2 of their children, so it appears to be autosomal dominant in its inheritance.
Persons who survive inherited retinoblastoma have an increased risk for developing a second neoplasm, which is typically mesenchymal in origin, for example,______. Cells of these tumors are also defective in RB function.
osteosarcoma
Lets go over CML one more time. Called the ______ chromosome, this involves a translocation between chromosomes ___ and ____. This creates a novel gene product called the _____, which is a ______. This protein is constitutively activated, and inhibits p27, an inhibitor of the cell cycle. The drug for this is _____ aka _____.
Philadelphia; 9/22; Bcr-Abl; protein kinase; Imantinib; Gleevec. (We conclude that Bcr-Abl promotes cell cycle progression and activation of cyclin-dependent kinases by interfering with the regulation of the cell cycle inhibitory protein p27.)
What is FAP? What gene encodes it? This disease appears to have autosomal dominant inheritance, but is actaully another example of _____.
FAP (Familial Adenomatous Polyposis); Encoded by the APC gene; Loss of heterozygosity.
[Like RB, FAP is inherited in an autosomal dominant fashion, in which patients that inherit one defective APC gene will be at higher risk (90% will develop colon cancer by age 50) to develop colon cancer. Cancer develops when the wild-type gene is lost by LOH in cells in adenomatous polyps of the colon during the first 20 years. Thus, these benign adenomatous polyps may become malignant by LOH.]
Describe the Wnt signaling pathway and how a mutation in the gene encoding APC disrupts normal function to produce cancer.
WNT binds Frizzled, APC complex is bound, which frees Beta-catenin to relocate to nucleus and transcribe c-myc. W/o APC, Beta-catenin is perpetually in the nucleus.
From lecture notes: The APC gene encodes a cytoplasmic protein that regulates the localization of the Beta-catenin protein. Beta-catenin is kept at the plasma membrane by being bound to E-cadherin in normal cells. The APC protein causes the degradation of any unbound and free Beta-catenin in the cytoplasm. When the APC is lost in FAP patients, Beta-catenin goes to the nucleus to produce transcription of oncogenes like c-myc. Thus, loss of APC tumor suppressor causes an overexpression of the c-myc oncogene, resulting in cancer!
In what two ways does p54 suppress tumorgenesis?
p53 restrains tumor formation by two different mechanisms. In the first, p53activates the p21 Cdk inhibitor gene in response to DNA damage and stress. Loss of p53in cells prevents the p21 gene from being transcribed, leading to the increased activity of the multiple Cdks normally turned off by p21 and resulting in increased cell proliferation.
A second way in which p53 restrains tumor formation is by inducing apoptosis.
Why are mutations in BRCA1/BRCA2 a big deal?
BRCA1 and BRCA2 function as “caretaker” genes, like p53, which serve to maintain genomic integrity. The gene products encoded by BRCA1 and BRCA2 are nuclear proteins that co-localize with RAD-51 at sites of DNA damage, and play a role in:
- homologous recombination repair of double-stranded breaks
- There is also evidence that BRCA1 and BRCA2 interact with the p53-mediated DNA damage checkpoint
- Loss of BRCA1 or BRCA2 leads to the accumulation of other genetic defects, which can then lead to cancer formation.
In addition to their roles in DNA repair, BRCA1 and BRCA2 have been implicated in a variety of cellular processes, including DNA synthesis, regulation of gene transcription (similar to p53, one target of BRCA1 transcriptional activation is the Cdk inhibitor p21), cell cycle checkpoint control, centrosome duplication and ubiquitination.
Inherited mutations in BRCA1/BRCA2 display what inheritance pattern?
LOH
The p53 gene is a very, important cancer predisposing gene because mutant p53 is found in about _____% of all cancers!
50
The p53 gene has several important properties, which explain why it is one of the most important genes in human cancer (3):
- p53 protein acts as a transcription factor important for the expression of genes, which prevent cells from replicating damaged or foreign DNA. p53 is also required for apoptosis, in which cells commit suicide if their DNA is damaged beyond repair. In p53 defective cells, damaged DNA is replicated, thereby producing additional mutations including chromosomal rearrangements, which can lead to cancer. In this manner, p53 acts as a “guardian of the genome”.
- Certain p53 point mutations are found more frequently than others in human cancers. These mutations are called “hotspots”. For example, mutational “hotspots” produce alterations in amino acids 248 or 273 of p53 in all human cancers. Some “hotspots” are unique to specific cancers. For example, an alteration in amino-acid 157 of p53 is found mainly in lung cancer and is the result of the mutagenic chemicals found in cigarette smoke
- p53 interferes with some viral infections. Adenovirus and HPV have oncogenes that act by inactivating p53, (for example, Adenovirus E1B and HPV E6 proteins). Remember that these viruses also inactivate RB protein. In fact, destruction of both RB and p53 either by cellular mutations or viruses is a major route to cancer.
Why was p53 originally thought to be an oncogene?
One mutant subunit (of tetramer) will inactivate 3wt subunits. Tumors then, have both wt and mutant p53, and it was thought that the presence of wt indicated that p53 was an oncogene (remember that Rb was the model, and 100% of Rb in a tumor will be mutant b/c of clonal nature of cells.) The mutant p53 protein is MORE stable than wt, so is virtually impossible to get wt/wt/wt/wt.
Definition of a proto-oncogene (aka c-onc):
A host gene that is homologous to an oncogene that is found in a virus but which can induce transformation only after being altered (such as mutation or a change of context such as coming under the control of a highly active promotor). It usually encodes a protein that functions in DNA replication or growth control at some stage of the normal development of the organism.
[Once retroviral oncogenes had been discovered, a surprising observation was made: Unlike the situation with DNA virus oncogenes which are true viral genes, there are homologs of all retrovirus oncogenes in cells that are not infected by a retrovirus. These cellular homologs are often genes involved in growth control and development/differentiation (as might be expected)]
[Note that c-oncs are not identical to their corresponding v-oncs. It appears that the virus has picked up a cellular growth controlling or differentiation gene and, after the gene was acquired by the virus, it has been subject to mutation.]
