Final Exam: Pathology: Moleclar Basis Of Cancer I

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What is the SOMATIC MUTATION HYPOTHESIS OF CANCER? (3)

1) Cell growth..

2) Malignant transformation..

3) Tumor mass results..

Back 1

1) Cell growth, differentiation, and survival are under genetic control

2) Malignant transformation is a result of accumulated mutations in specific classes of genes

3) Tumor mass results from the clonal expansion of a single progenitor cell that has incurred the genetic damage

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What are the PHENOTYPIC HALLMARKS of CANCER? (6)

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1) Dysregulation of cell proliferation by constitutive activation of growth stimulatory pathways (GF) or independence of proliferation signals (ECM)

2) Insensitivity to growth inhibitory signals or loss of growth inhibition pathways

3) Evasion of apoptosis

4) Limitless replicative potential

5) Angiogenesis

6) Invasion and Metastasis

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Which GENES are involved in carcinogenesis and what is their function? (5)

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1) ONCOGENES AND PROTO-ONCOGENES: which are required for and promote cell proliferation

2) TUMOR SUPPRESSOR GENES: which are genes that inhibit cell proliferation or block a step in the cell cycle

3) PRO- or ANTI- APOPTOTIC PROTEINS/ IMMORTALIZATION GENES (like telomerase):
are genes involved in apoptosis or cell senscence

4)"CARETAKER GENES": genes that produce DNA repair enzymes

5) LANDSCAPER GENES: required for malignant phenotype
- involved in regulating angiogenesis
- involved in cell- cell and cell- matrix adhesion
- proteolytic enzymes required for invasion

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* ONCOGENES *

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*

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PROTO-ONCOGENE

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Gene that encodes a protein that mediates or stimulates cell proliferation.

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ONCOGENE

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Inappropriately activated proto- oncogene, either by mutation or aberrant expression (over- expression or ectopic expression)

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What are the different types of proteins encoded by ONCOGENES?

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1) GF
2) GF RECEPTORS
3) SIGNAL TRANSDUCTION MOLECULES
4) STEROID HORMONE RECEPTORS
5) TRANSCRIPTION FACTORS
6) CELL CYCLE PROTEINS

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1) GROWTH FACTORS ex)

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A) v-sis is the oncogenic form of the proto-oncogenic c-sis, which encodes for the platelet- derived growth factor (PDGF)

B) AUTOCRINE STIMULATION LOOP

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2) GROWTH FACTOR RECEPTORS ex)

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A) Transmembrane receptor tyrosine kinase

B) Oncogenic versions of these receptors are over- active

C) EGF-R family most often involved
- EGFR overexpressed in 80% of squamous cell carcinomas of the lung
- HER2- neu amplified in some breast, ovary, lung, stomach CAs

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3) SIGNAL TRANSDUCTION MOLECULES ex)

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A) Non- receptor protein- tyrosine kinases (src)

B) Cytoplasmic serine/ threonin kinases (raf)

C) GTP- binding proteins
- critical in signal transduction from RTK's
- mutation of the ras gene is the SINGLE MOST COMMON ABNORMALITY of dominant oncogenes in human tumors

- Ki- ras: involved in lung, ovarian, colon, and pancreatic cancers
- N- ras: involved in leukemias

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4/ 5) NUCLEAR REGULATORY FACTORS

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A) TF such as MYC, MYB, FOS, and JUN: persistent expression or overexpression of myc transcription factor family members occurs in

- NEUROBLASTOMA and GLIOBLASTOMA (N- myc)
- SMALL CELL LUNG CANCER (L- myc)

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6) CELL CYLCE PROTEINS

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A) CYCLINS: synthesized during specific phases of the cell cycle to activate cyclin- dependent kinases

B) Cyclin- Dependent Kinases (CDK's): drive the cell cycle by P of critical target proteins required for progression to the next phase of the cell cycle

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How are PROTO- ONCOGENES activated?

What are two ways in chich proto- oncogenes may be activated?

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*Gain- of- function mutations "DOMINANT" mutations:

1) QUALITATIVE: changes in the structure of the gene (loss of regulatory elements), resulting in an abnormal gene product (ONCOPROTEIN) with uncontrolled, aberrant function (ex. ras)

2) QUANTITATIVE: up- regulation or ectopic expression of a structurally normal growth- promoting protein

- breast cancer cells often produce excess cyclin D and cyclin E

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EXAMPLE of QUALITATIVE activation of proto- oncogene:

POINT MUTATIONS OF RAS family proteins

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An example of changes in coding sequence of a proto- oncogene that may result in an abnormal gene product is POINT MUTATIONS OF RAS FAMILY PROTEINS.

Normally, ligand- activated growth factor receptors stimulate the MAP kinase pathway, which activate target nuclear TF.

RAS signaling usually negatively regulated by the instrinsic GTPase activity of ras, BUT

ONCOGENIC point muations in ras diminish this GTPase activity and generate a constitutive and unregulated signaling molecule.

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EXAMPLE of QUAnITATIVE activation of proto- oncogene:

Deals with changes in structure of gene regulatory element that causes an increase in expression of protein...

What karyotypic abnormalities does this give rise to?

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1) Double- minute chromosomes (DM)
2) Homoegenous staining regions (HSR)

Ex)
1) MDM2 (p53 antagonist)
2) N-myc amplification in 25% of neuroblastomas
3) HER2-neu/c-cerbB2 in breast cancer

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1) MDM2 (p53 antagonist)

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*

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2) HER2-neu/c-cerbB2 in breast

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*

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3) CML and the Philadelphia Chromosome

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This is the first chromosomal abnormality ever linked to a specific cancer

t( 9;22) fuses with the proto- oncogene c-abl with bcr
bcr-abl fusion protein has increased tyrosine kinase activity and abnormal cellular localization

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4) ST1571: Gleevec

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*

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* INSENSITIVITY TO GROWTH INHIBITORY SIGNALS OR "GROWTH INHIBITION" PATHWAYS: LOSS OF TUMOR SUPRESSOR GENE *

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*

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A) What are TUMOR SUPRESSOR GENES?

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1) Genes that inhibit cell proliferation

2) The inactivation or loss of TSG lead to tumor development by eliminating negative regulatory signals
- by removing an inhibitor of proliferation

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B) What are pieces of evidence for TSGs? (2)

What did they conclude and what did the cell fusion experiment show?

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1) Deletion of genetic material is the most frequently observed genetic abnormality in solid tumors

2) In a CELL FUSION experiment, they fused a normal cell and a tumor cell together, and they found that the resulting cell was a NONTUMORIGENIC HYBRID CELL.

*From this experiment they concluded that genes derived from the normal cell can inhibit (or supress) the tumorigenic phenotype

*This showed that cancer is a RECESSIVE TRAIT

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C) What is the TWO-HIT HYPOTHESIS of Alfred Knudson (1971)

What are a few facts about RB?

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With regards to RETINOBLASTOMA, most people with an INHERITED mutation developed a tumor, whereas only 1 in 30,000 normal people develop such tumors.

HEREDITARY RB: occurs in both eyes of very young children- - usually inheritance of one single mutant TSG increases risk of developing a tumor in an autosomal dominant fashion.

Inherited cases occur early in life and are multi focal because one defective copy of RB is transmitted, and the "second" hit occurs in many of the retinal cells present.

NONHEREDITARY: occus in only one eye in late childhood

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D) What does the LOSS OF HETEROZYGOSITY have to do with developing cancer?

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With regards to cancer, patients with familial cancer syndromes usually show reduction to hemizygosity at specific genetic sites that correspond to TSG.

So this means that when the cell becomes HOMOZYGOUS or loses HETEROZYGOSITY at the TSG locus, transformation occurs.

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What happens if you consistently get LOH's at the same locus?

IMPORTANCE OF LOH: What does LOH show? What is it's significance?

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If you consistently get LOH's at the same genetic loci in sporadic tumors, it's suggestive of non- familal cancers as well

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What is the MECHANISM of LOH?

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1) Mitotic recombination
2) Chromosome missegregation during mitosis (nondisjunction) because of failure of G2/M mitotic checkpoint, resulting ni deletion of teh wild type allele
3) Epigenetic mechanisms of gene inactivation (like methylation
4) Point mutations are unlikely because they're usually repaired

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What kinds of genes are TSGs?

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1) Many are involved in cell cycle checkpoints

2) Cell cycle checkpoints ensure that progression does not occur until the preceding phase has been completed with high fidelity

3) Failed checkpoints result in either apoptosis or genomic instability

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What is RETINOBLASTOMA (RB, pRB)?

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1) Primarily regulatory protein of the G1/ S phase transition

2) At the G1/ S restriction point:
- RB inhibits passage through the restriction point by repressing trascription of a number of genes involved in cell cycle progression and DNA synthesis
- It's regulated by CYCLIN D/CDK complees, which phosphorylate and inactivate pRB, releasing critical transcription factors

3) Mutational inactivation of RB tumors remove a key negative regulator of cell cycle progression

4) LOH in retinoblastomas is seen in
- osteosarcomas
- SCLC
- and many adenocarcinomas

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What is the function of CDKI's?

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CDKI p16 induces G1 cell cycle arrest by inhibiting CDK4 and CDK6

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What is the function of p53?

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*p53 is the "GUARDIAN OF THE GENOME"

1) It is the TF responsible for CELL CYCLE ARREST and induction of APOPTOSIS upon DNA DAMAGE - - so that damaged DNA can be repaired before replication or division

2) Loss of p53 results in increased mutation frequencies and general genome instability

3) It's involved in up to 50% of cancers
- the MOST COMMON TARGET of genetic alteratons in sporadic human malignancies
- Li- Fraumeni syndrome

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What is Li- Fraumeni syndrome an example of?

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*

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What happens when DNA is damaged?

What is the function of p53?

What is the function of p21?

What does cell cycle arrest allow for?

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When DNA is damaged, it leads to rapid induction of p53 levels and transactivation ability (post- translational modifications, mainly phosphorylation)

p53 activates transcription of CDK-Inhibitor p21 (WAF1/ CIP1)

p21 blocks cell cycle progression in G1 and G2 phases

Cell cycle arrest is important as it allows for DNA to be repaired before replication/ division

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What happens if DNA damage cannot be repaired?

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If DNA damage cannot be repaired, then p53 triggers APOPTOSIS by activation of pro-apoptotic genes (BAX and others)

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What is a type of p53 mutation?

What is a DOMINANT NEGATIVE MUTATION?

Why might p53 levels be upregulated in cancer?

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1) Unlike most other TSGs, mutations in p53 can be "dominant"

DOMINANT MUTATION: it is a mutation in one allele which prevents function of the other (usually by physical associateion, such as dimerization)

2) p53 levels are often up-regulated in cancer because many mutations not only reduce/ alter p53 function, but it also increases its half- life

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What are VIRAL ONCOGENES?

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1) Some DNA viruses have transforming sequences not derived from cellular genes

2) These viruses may have proteins that complex with and inactivate TSGs

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What is an example of a virus that may cause cancer?

Back 36

HPV

1) high risk genotypes are found in 82%- 85% of invasive squamous cell carcinomas of the cervix

2) HPV 6: binds to and promotes the degradation of p53

3) HPV E7 binds pRB and displaces E2F TF

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What is the difference between a GATEKEEPER and CARETAKER?

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GATEKEEPERS are genes that DIRECTLY inhibit cell proliferation or promote cell death.

They are responsible for maintaining tissue homeostasis with respect to cell number.

Mutations in this disrupts equilibrium, and is an early step towards CARCINOGENESIS.

ex) APC (colon), NF1 (schwann cells), RB (epithelial cells)

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What are the CARETAKER GENES?

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The CARETAKER genes are involved in the maintainance of genetic stability and fidelity of DNA replication and prevention of mutation (DNA repair genes)

They do not promote tumorgenesis directly,

It's inactivation leads to increased mutation of all genes, leading to accelerated tumorigenesis, thus they are also considered TSGs (recessive oncogenes)

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* See last slide for summary of development from normal cell to malignant neoplasm. *

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*

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What are 3 examples of DNA repair systems TO KNOW FOR EXAM?

(See notes for details!)

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1) Mismatch repair genes (HNPCC)
2) Nucleotide excision repair (XERODERMA PIGMENTOSUM)
3) Recombination repair genes (BRCA 1 and 2)