T7: Oncogenes

Question 1

Proto-oncogenes def + some examples

Answer 1

genes coding for proteins that favour cell proliferation. Usually these proteins are either TFs, membrane receptors or molecules involved in signal transduction

eg. TRK receptors, Ras (protein kinase), MAP kinases, growth factors

Question 2

Oncogene def

Answer 2

a mutated gene that has the potential to cause cancer. Before an oncogene becomes mutated, it is called a proto-oncogene, and it plays a role in regulating normal cell division.

Question 3

types of mutations that alter protooncogenes

Answer 3

1. TRANSLOCATION/ TRANSPORTATION: when 1+ portion of a chromosome is in an altered position
2. GENE AMPLIFICATION: duplication of a specific gene
3. POINT MUTATION: change in 1 nucleotide which can potentially change the amino acid the codon is coding for

Question 4

wild type gene def

Answer 4

the gene, characteristic, or phenotype that occurs most frequently in the natural population

Question 5

consequence of transportation mutation on proto-oncogene

Answer 5

changed position causes the wild type gene to be controlled by a different promoter, increased production of protein arising from that gene

Question 6

consequence of gene amplification mutation on proto-oncogene

Answer 6

duplication of gene causes the gene to be present in multiple copies leading to an excess in transcribed and translated normal proteins

Question 7

consequence of point mutation on proto-oncogene

Answer 7

1. mutation in control element of gene: amount of normal protein produced in increased
2. mutation in gene: results either in hyperactive protein or resistance to degradation

Question 8

Categories of proto-oncogenes and examples (5)

Answer 8

1. Membrane receptors: binding for growth factors (Erb b, Erb b2)
2. growth factors: synthesized by the ribosomes on the ER and transported to Golgi to be modified and secreted out of cell (PDGF, FGF, IGF1)
3. Signaling molecules: synthesized by ribosomes or polysomes in cytoplasm (Ras, MAPk)
4. TFs: molecules modulating transcription and synthesized by free ribosomes (Myc)
5. Cyclins and CDKs: synthesised by free ribosomes and regulate cell cycle complexes

Question 9

constitutive activation meaning

Answer 9

The ability of a receptor to spontaneously achieve a signaling conformation resulting in the production of a second messenger without binding natural or pharmacological agonist

Question 10

TRK dimerization mutation

Answer 10

-change in the physiological presence of the TRK so that instead of being naturally present as a monomer and dimerizing upon ligand binding, the receptor is found as a dimer from the beginning
-RESULT: ligand independent firing will occur which causes signal of proliferation to be sent constantly instead of just upon ligand stimulation

!! ERB B2 AND ERB B RECEPTORS CAN HAVE THIS MUTATION

Question 11

pathology caused by Philadelphia chromosome mutation

Answer 11

-9-22 chromosome translocation causes chronic myeloid leukemia
-occurs due to the constitutive activation of tyrosine kinase enzyme bcr-abl1
-this enzyme then phosphorylates a huge number of intracellular substrates
-causes a range of responses within the cell
eg: cytoskeletal elements , STAT1,5, Myc

Question 12

Constitutively-activated Ras

Answer 12

ONCOGENE:
-in normal conditions GDP bound Ras is inactive and GTP bound Ras is active
-mutated Ras is unable to hydrolyze its GTP group hence is in its active form constantly
-causes constant activation of MAPk cascades
-this affects mitochondrial (intrinsic) cell apoptosis mechanism - cell death is BLOCKED

Question 13

oncosuppressor gene def

Answer 13

genes that are involved in negative modulation of cell cycle due to the fact that they code for peptides inhibiting TFs, cyclin/CDK complex activity or by promoting apoptosis

!! also code for proteins responsible in DNA repair

Question 14

In what situation do oncosuppressor genes completely lose function?

Answer 14

DOUBLE ALLELE MUTATION (recessive) which allows excessive cell proliferation with similar effects to oncogenes

Question 15

How is the double allele mutation for oncosuppressor gene loss of function achieved?

Answer 15

1. first allele mutation: creates a pre-tumour cell which is able to code for a sufficient number of proteins
2. second allele mutation:: creation of tumour cell from pre-tumour cell, where the wild type gene can no longer code for proteins.

Question 16

role of TGF beta receptor

Answer 16

ONCOSUPPRESSOR
induces an increase in p16 protein which inhibits cyclin D/cdk complex –> negative modulation of cell cycle

Question 17

role of cadherins

Answer 17

ONCOSUPPRESSOR: e-cadherin
-molecules involved in formation of cell junctions
-inhibit cell proliferation because of cell contact (physically joining cells together) so that they are prevented by growing on top of eachother (property of tumours)

CELL INVASIVENESS (migration of cells within a tissue) is a property of tumorigenic cells
HIGH CADHERIN: invasiness inhibited
LOW CADHERIN: invasiveness stimulated

Question 18

role of APC (adenomatous polyposis coli)

Answer 18

ONCOSUPPRESSOR:
-isolated in adenomatous colon polyposis in infected patients
-inhibits signals triggered by beta catenin which is involved in cell adhesion

Question 19

role of neurofibromin

Answer 19

ONCOSUPPRESSOR
-play a role in NF1 disorder
-GTPase activating proteins which transform active Ras into inactive Ras

Question 20

Role of Retinoblastoma

Answer 20

ONCOSUPPRESSOR:
-binds to TF E2F which allow entrance into the S phase and allow DNA replication
active RB inhibits these TFs hence reduced proliferation

Question 21

Process of retinoblastoma inactivation (and effect)

Answer 21

RB is an oncosuppressor that can be inhibited via phosphorylation by cyclin-cdk:

-TRK activated by mitogen
-mitogen activates Ras which activates MAPk cascade
-Myc activation which promotes synthesis of cyclins
-allows the formation of cyclin-cdk complex
-cyclin-cdk complex phosphorylates RB allowing synthesis of proteins involved in S phase for DNA replication

Question 22

role of BRCA1 and BRCA2

Answer 22

ONCOGENES:
-DNA repair and transcriptional regulation in response to DNA damage
-maintenance of chromosomal stability
-found in breast cancer

Question 23

what are levels of p53 expression dependent on

Answer 23

detection of extracellular stress:
-unfavourable environment
-oncogene activation
-telomerase erosion
-malfunction in mitotic apparatus
-oxidative stress

INCREASED STRESS: higher concentration of p53 (to promote apoptosis)

Question 24

general role of p53

Answer 24

ONCOSUPPRESSOR
-induction of apoptosis
-acts as a TF to induce cell cycle arrest
-sensor of stress and DNA damage

Question 25

Intracellular control of p53 gene using binding partner

Answer 25

CONTROL DEPENDS ON MDM2 (murine double minute 2)

Absence of MDM2: p53 moves into nucleus to act as a TF. NEGATIVE FEEDBACK: promotes the formation of MDM2 so that it induces its own inhibition

Presence of MDM2: increases p53 ubiquitination and degradation (in cytoplasm)

Question 26

result of p53 phosphorylation

Answer 26

-occurs following DNA damage
-blocks interaction of p53 with MDM2 and stabilises p53 in nucleus
-increases its amount
-activated target gene expression (acting as a TF)
-increases apoptosis

Question 27

cancer inducing mechanisms that cause mutation to p53 (5)

Answer 27

1. aa substitution: changes p53 DNA binding domain so it no longer acts as TF and control genes that induce apoptosis
2. deletion of c terminal domain: prevention of tetramer formation which plays a role in TF function
3. duplication of MDM2 encoding gene: excess ubiquitination of p53 which reduces apoptosis
4. viral infection: negatively modulates p53 activity
5. deletion of p14ARF gene which inhibits MDM2: creates excessive MDM2 and increases p53 ubiquitination which reduces apoptosis