W1-Molecular Basis of Carcinogenesis Flashcards
1.Describe the properties of malignant cancer cells. You should be able to give at least five different properties.
MC UDI
-Unresponsive, to normal signals for proliferation control.
- De-differentiated, (lack specialized structures/functions of the tissue where they grow).
- Invasive, (capable of outgrowth into other tissues).
- Metastatic, (capable of shedding cells that can drift through circulatory system and proliferate at other sites in the body).
-Clonal in origin, (derived from a single aberrant cell).
Transformed cells have altered morphology, loss of contact inhibition, the ability to grow without attachment to a solid substrate (‘anchorage independence’), high saturation density, inability to halt proliferation in response to deprivation of GFs, reduced requirement for mitogenic GFs, increased transport of glucose, and tumorigenicity.
For malignant cancer cells MC UDI gon’ be “AAL RIIIHT” (#kingkendrick)
AAL RIIIHT
- Altered morphology
- Anchorage independence
- Loss of contact inhibition
- Reduced requirement for mitogenic growth factors
-immortalization
-inability to halt proliferation in response to deprivation
-increased transport of glucose
-high saturation density
of growth factors
-tumorigenicity
2.What is the multi-step process for carcinogenesis? You should be able to discuss the relative importance of heredity and the environment and why early events may include mutations in DNA repair genes.
Two ways to think about this: The first: 1. Insensitivity to growth signals 2. Tissue invasion and metastasis 3. Limitless replicative potential 4. Sustained angiogenesis 5. Evading Apoptosis 6.Self-sufficiency in growth signals
Second way to think of it: 1.Normal cell 2.Increased proliferation 3. Early neoplassia 4.Progressive neoplasia 5.Carcinoma 6. Metastasis From 1-3, it may include mutations in DNA repair genes From 4 and on, increasing chromosomal aneuploidy
The last part of the learning goal:
Cancer is not inherited, generally due to somatic mutations produced by environmental factors (especially aging). To go from normal to neoplastic state –> changes in cellular heredity –> due to mutagenic events in somatic cells early in life that can produce tumors many years later. (Ex: Australians exposed to UV).
Can inherit a susceptibility to cancer that later requires less mutations in the long run –> you have one defective and one normal allele. Can make it easier to get loss of heterozygosity –> cells lose both their active alleles –> deficient –> tumorigenic event. (Ex: Familial retinoblastoma –>auto dominant, inherit one bad Rb copy.)
3.What types of genes are usually mutated in tumor initiation? Describe the effect on cellular proliferation that the product of these genes has.
Tumor initiation occurs by mutations in either: -Oncogenes:
(normally stimulate cell proliferation –> are over-expressed in cancer and stimulate too much cellular proliferation (the gas pedal is all the way down)
Usually only 1 copy needed
-Tumor suppressors:
(normally inhibit cellular proliferation –> get inactivated in cancer so they can’t play this regulatory role). ( who cut the fuckin brakes)
Usually 2 copies needed
- What type of cytogenetic abnormalities are associated with malignancy? You should be able to give at least two different examples.
Ex1)
-chromosomal translocation. CML caused by reciprocal translocation between C9 and C22 –> creates fusion Philadelphia chromosome that makes an active BCR-ABL tyrosine kinase –> leukemia.
Ex2)
-Inactivation of tumor suppressors via LOH; some examples are retinoblastoma and APC gene in familial adenomatous polyposis (FAP)
- What events can produce LOH? Give at least two examples and state how they support Knudson’s theory.
Background Info:
Normal people are heterozygous. LOH = you become homozygous for 2 defective alleles (generally TSGs) that allow for a tumorigenic event.
Knudson’s theory refers to a 2-hit hypothesis:
-1st hit = being heterozygous (inherited a bad copy)
-2nd hit = some mutation that breaks the other copy (like UV exposure).
Generally happens through mutation, mitotic recombination, chromosome loss, and/or environmental factors.
EXAMPLE 1):
Familial retinoblastoma:
-inherit one normal and one mutant Rb allele (via a germline mutation). Some event causes the other allele to break = 2 defective genes = tumor of retina (high penetrance). Happens in autosomal dominant fashion with some somatic mutation to get LOH. Familial retinoblastoma tends to result in multiple tumors, bilateral, with early onset.
Sporadic retinoblastoma, on the other hand, is the result of 2 somatic mutations, which is really rare. Generally get single tumors, unilateral, with later onset.
EXAMPLE 2)
Familial APC gene mutation:
-(chromosome 5q) also inherited in auto dominant fashion –>requires 2 bad copies. Polyps are precursors to colon cancer (LOH = cancer). APC sequesters beta-catenin in the cell unless Wnt binds the Frizzled transmembrane receptor –> APC releases B-cat so it can go to the nucleus and cause expression of c-myc oncogene. LOH = APC mutated such that even without Wnt, B-cat is active all the time and you have an overactive oncogene. Stem cells in colon crypts over-proliferate –> polyps.
EXAMPLE 3)
BRCA:
-involved in familial breast and ovarian cancer. Involved in tumor suppression and DNA repair (during damage, BRCA causes ATM to bind ChK2 to block cell cycle while DNA gets fixed). 2 damaged BRCA copies = mutation accumulation.
- Are cancers associated with both dominant and recessive syndromes? You should be able to give a different example of each type.
Cancer susceptibility inherited in auto dominant or auto recessive fashion.
Auto Dominant:
- Familial adenomatous polyposis (FAP-APC gene)
- Familial retinoblastoma (RB gene)
- Familial breast and ovarian cancer (BRCA 1 and 2 genes)
- Wilms tumor syndromes.
Auto Recessive: -xeroderma pigmentosa (XP genes)
- Ataxia-telangiectasia (AT genes)
- Bloom’s syndrome
- Fanconi’s congenital aplastic anemia (FA genes).
- Describe how the RB (retinoblastoma) gene was first identified. You should be able to describe the important cytogenetic and molecular evidence.
Inherited Retinoblastoma is a relatively rare, pediatric disorder (1/20,000 infants). Yet the study of this rare cancer has shed considerable light on etiology of all cancers. The RB gene is the best understood of the cancer susceptibility genes. RB gene also represents the first clearcut example of an antioncogene or a tumor suppressor gene. Therefore, we will focus on the molecular biology of the RB gene.
Cytogenetic analysis of cells from retinoblastomas showed region around chromosome 13q14 generally had abnormal structure (like a deletion).
Some of the retinoblastoma cells lack RB completely (both deletion; as detected by PCR or southern blots). Can also have partial deletions or rearrangements of Rb.
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. This is the hallmark of a antioncogene or tumor
suppressor gene.
- What are the properties of the protein product of the RB gene? List at least three biochemical properties.
- Describe how the RB protein functions during the cell cycle and why it is important in cancer.
Rb is a tumor suppressor protein that inhibits G1–> S phase progression.
GF and EGFR activate –>CDK4,6 and cycD1-3 —>activate CDK2 and cycE–> phosphorylate Rb. Phosphorylating Rb inhibits it. Thus allowing cell cycle to progress to S phase.
Rb protein is hyperphosphorylated in rapidly proliferating cells at S or G2 phase.
Rb protein is hypophosphorylated in non-proliferating cells in G0 or G1.
Hypophosphorylated Rb represses entry of cells into S phase.
Hyperphosphorylation causes Rb inactivation which means cell cycle can go like crazy.. No Rb = cells can’t down-regulate cell division = out of control growth.
Phosphorylation by CDKs (cyclin-dependent protein kinases) inactivates
the RB protein, thereby allowing the cell to proceed from G1 to the S phase of the cell cycle
- What is the hallmark of a tumor suppressor gene? You should be able to use the RB gene as an example.
Tumor suppressors become defective via LOH. Heterozygotes have one normal RB gene in every cell of their body –> they regulate proliferation properly and are non-malignant. Loss of the other, normal RB = tumor because nothing there to regulate proliferation – malignancy. (more info on LO 7)
- Explain why APC, BRCA1 and BRCA2 genes are tumor suppressors.
APC( tumor suppressor in FAP):
- Random fact FAP incidence is 1/10,000.
- APC normally sequesters/targets for degradation Beta-catenin in the cytoplasm in the absence of Wnt signal.
- When Wnt signal binds to Frizzled transmembrane receptor –> APC releases B-cat so it can go to the nucleus and cause expression of the c-myc oncogene.
- In cancer, LOH renders APC defective = B-cat active all the time –> stem cells in colon crypt proliferate and give you polyps.
BRCA1 and BRCA 2
(5% of breast cancers:
BRCA1 and BRCA 2 are tumor suppressors involved in breast and ovarian cancer. Heterozygous = at risk; homozygosity via LOH allows for DNA damage and instability = cancer. BRCA involved in DNA repair; regulates DNA damage checkpoints (causes ATM to bind Chk2 to block cell cycle before S phase). Mutation allows for buildup of a lot of other mutations that can cause cancer.
However in the acquired cases, the situation is different from that seen for RB in that somatic
mutations in these genes have not been found in tumors. Therefore, it is believed that mutations
in other genes may affect BRCA1 and BRCA2 function indirectly. Both BRCA1 and BRCA2 function in DNA repair and their loss may give rise to the many mutations needed for full-blown
malignancy.
Recently, BRCA2 has been shown to be allelic with the Fanconi’s anemia D1 gene,
FANCD1 (Howlett et al., 2002). This means that individuals with homozygous mutations in
BRCA2 get Fanconi’s anemia, while heterozygotes get breast cancer from rare recombinant
cells in the mammary gland that lose the wild-type allele.
- Know why p53 was originally incorrectly thought to be an oncogene.
Certain p53 mutant genes were dominant to the WT gene in producing cellular transformation. This is because oncogenic p53 mutations produce mutant p53 protein that actually quite stable –> binds WT p53 protein and inactivates it in a dominant negative fashion (‘monkey wrench’). *Just one mutant subunit (p53 is a tetramer) messes up the whole tetramer; like an oncogene would (only needs one defective copy) . These findings were found in Li-Fraumeni syndrome. Most mutations are in the DNA binding domain.
- Explain why p53 is the “guardian of the genome.”
When there’s DNA damage, p53 becomes stabilized and available in increased amounts. P21 blocks CDKs so the whole cell cycle stops and p53 can come in and fix things, or target cell for apoptosis if conditions are bad.
Very important genes because cells missing p53 accumulate mutations at much higher rate = greater chance of becoming malignant. It’s a guardian because it prevents these potentially deleterious mutations.
-p53 found in about 50% of all cancers
- Know the cellular function of the p53 protein.
p53 acts as a TF involved in expression of genes that prevent replication of damaged DNA. Also required for apoptosis (if DNA damage is beyond repair). If p53 is defective, damaged DNA gets replicated = more mutations (like chromosomal rearrangements).
- Know that oncogenic viruses make proteins to inactivate both Rb and p53.
Adenovirus targets Rb and p53 proteins with its E1 proteins. HPV targets Rb with E7 and p53 with E6. Destruction of these proteins by mutation or viruses = major route to cancer.
E7 with Rb.
E6 with p53.
Know that HPV (human papilloma virus) is an example of an oncogenic virus in humans
The RB protein is a target for many animal tumor viruses, for example,
SV40 and HPV (human papilloma virus). These viruses drive a quiescent cell into the S phase
of the cell cycle and to proliferate by producing a viral protein(s), SV40 T antigen (T stands for
transforming) or HPV E7 protein, that binds to and inactivate the RB protein.
