Oncogenes And Tumour Suppressor Genes

Question 1

What was found in the Rous sarcoma virus?

Answer 1

A extra gene contained in its genome a “oncogene” called v-src

Question 2

What is the fundamental principle of oncogenes?

Answer 2

Oncogenes are alerted forms of normal genes or proto-oncogenes

Question 3

What is the oncogene hypothesis?

Answer 3

It was discovered that come genes of cancer causing viruses were mutated forms of the cellular genes NOT viral genes.

They concluded that the Rous sarcoma gene was in fact a host gene that was kidnapped by the virus.

Question 4

How can the c-src sequence be captured by a retrovirus?

Answer 4

During evolutions, the virus can acquire fragments of genes from the host at integration sites and this process results in the creation of oncogenes.

The oncogenes product was characterised as a 60kDa intracellular tyrosine kinase that can phosphorylase cellular proteins and effect growth.

Question 5

How can DNA viruses become oncoviruses?

Answer 5

Encode various proteins along with environmental factors can initiate and maintain tumours

Question 6

How can RNA viruses become oncoviruses?

Answer 6

Integrate DNA copies of their genomes into the genome of the host cell

as these contain transforming oncogenes they induce cancerous transformation of the host

Question 7

How are oncogenes activated?

Answer 7

mutations,

insertions,

amplification/duplication

translocations.

Question 8

What are the 4 types of proteins are involved in the transduction of growth signals?

Answer 8

• growth factors
• growth factor receptors
• intracellular signal transducers
• nuclear transcription factors

Question 9

What is the RAS oncogene family?

Answer 9

Most commonly mutated oncogene

Ras genes were identified from studies of two cancer causing viruses.

RAS proteins are small GTPase that are normally bound to GDP in a neutral state

Question 10

Explain the intracellular signal transducer pathway.

Answer 10

1. Binding of extracellular growth factor signal
2. Promotes recruitment of RAS proteins to the receptor complex
3. Recruitment promotes Ras to exchange GDP with GTP
4. Activated Ras then initiates the remainder of the signalling cascade (mitogen activated protein kinases)
5. These kinases ultimately phosphorylate targets, such as transcription factor to promote expression of genes important for growth and survival

Question 11

How can oncogenes function?

Answer 11

The oncogene product was characterised as a 60kDa intracellular tyrosine kinase

Can phosphorylate cellular proteins and effect growth.

Question 12

How can oncogenes become activated?

Answer 12

Mutations

Insertions

Amplifications/duplications

Translocations

LOSS OF RESPONSE TO GROWTH REGULATORY FACTORS. ONE ALLELE NEEDS TO BE EFFECTED.

Question 13

What is the MYC oncogene family?

Answer 13

Consists of 3 members:
- C-MYC
- MYCN
- MYCL

Belong to a family of transcription factors that regulate transcription of at least 15% of the entire genome
- cell cycle
- DNA repair
- translation

Question 14

How is MYC activated?

Answer 14

Encodes a helix-loop-helix leucine zipper transcription factor that dimerises with its partner protein (Max) to transactivate gene expression.

Activation is a result of chromosomal translocation

Question 15

Describe how activation of MYC in burkitt’s lymphoma?

Answer 15

BL A high grade lymphoma that effects children from 2-16.

All cases carry one of three characteristic chromosomal translocations:
- chromosome 2
- chromosome 14
- chromosome 22

These translocations place the MYC gene under the regulation of the ig heavy chainb

Question 16

How can chromosomal translocation be responsible for the activation of other oncogenes?

Answer 16

In chronic myelogenous leukaemia - 95% of patients carry the Philadelphia chromosome

The Philadelphia chromosome of translocation is a product of translocation

As a result of this translocation, the tyrosine kinase activity of the oncogene ABL is constitutive leading to abnormal proliferation

Question 17

What is required for a tumour suppressor to become inactivated?

Answer 17

To loose tumour suppressor gene function the inactivation of BOTH ALLELES of the gene are required

Question 18

What are the 3 functions associated with tumour suppressor genes?

Answer 18

Regulators of cell cycle checkpoints

Differentiation

DNA repair

Question 19

What is retinoblastoma Rb?

Answer 19

A rare childhood cancer

developes when immature retinoblasts continue to grow very fast and do not mature

When light is reflected into eye, the light is reflected back - cat eye appearance

Question 20

What are the 2 forms of retinoblastoma?

Answer 20

Familial - 40%

Sporadic - 60%

Question 21

What hereditary mutation causes retinoblastoma?

Answer 21

On Chromosome 13 in the retinoblastoma 1 gene (Rb1)

Question 22

What is the “two-hit” hypothesis?

Answer 22

The development of retinoblastoma requires two mutations, which correspond to the loss of both of the functional copies of the Rb gene

Question 23

What is meant by “loss of heterozygosity”?

Answer 23

The process that leads to the inactivation of the second copy tumour suppressor gene.

A heterozygous cell receives a second hit in its remaining functional copy of the tumour suppressor gene.

Question 24

What types of mutations can activate tumour suppressor genes?

Answer 24

Point mutations

Small deletions

Question 25

Describe the structure of the retinoblastoma protein RB structure.

Answer 25

Includes 3 members, collectively known as pocket proteins
- N terminus
- small pocket
- c terminus

Large pocket made of small pocket and c terminus is where proteins bind.

pRb is a multi functional protein (110kDa) with over 100 binding partners

Main binding partner is the E2F transcription factor

Question 26

What is the function of the retinoblastoma protein?

Answer 26

A transcription co factor that can bind to transcription factors

Involved in apoptosis and cell cycle - regulates these pathways through the stimulation or inhibition of the activity of interacting proteins.

Question 27

How is retinoblastoma protein RB deactivated in the cell cycle?

Answer 27

By inhibiting the G1 to S phase

1. Cyclin D is the first cyclin to be synthesised and drive progression through G1 with CDK4/6
2. The G1 checkpoint leads to the arrest of the cell cycle in response to DNA damage
3. A key substrate for cyclin D is RN protein
4. Cyclin D and E and they’re CDKS phosphorylate RB (deactivated)
5. Upon deactivation of Rb, E2F is released/unbound and migrates to the nucleus where it induces transcription
6. Progression from G1 to S phase

Question 28

How does retinoblastoma protein inhibit the cell cycle after activation?

Answer 28

1. RBC regulates the activity of the E2F transcription factor crucial for the expression of genes required for S phase
2. Rb is regulated by phosphorylation
3. When Rb tumour suppressor is active it can inhibit cell proliferation
4. When Rb is dephosphorylated it is active and remains bound to E2F
5. When Rb is active it blocks the progression of G1 to S phase

Question 29

What are the 3 ways Rb can be inactivated?

Answer 29

Phosphorylation

Mutation

Viral oncoprotein binding

Question 30

How can viral protein binding induce Rb inactivation?

Answer 30

Found in a small number of tumour cells

Mainly by disrupting E2F binding or destabilisation of Rb

Question 31

How can Rb dis regulation lead to tumour formation?

Answer 31

As a direct consequence E2F transcription factors can induce the deregulation of the cell cycle

Cells move through G1 into S without the checkpoints of Rb.

Question 32

What is the function of the p53 tumour suppressor gene?

Answer 32

Involved in sensing DNA damage and regulating cell death/apoptosis

P53 specialises in preventing the appearance of abnormal cells

Transcription factor p53 can bind to 300 promoter regions

Question 33

What is the structure of p53?

Answer 33

Tetrameter complexed with dsDNA

• amino transactivation domain
• a central DNA binding domain
• a tetramization domain
• carboxyl regulatory domain

Question 34

Describe how p53 is regulated in normal cells by MDM2?

Answer 34

Normal p53 protein levels are low inside cells

MDM2 is a ubiquitin ligase (also a ONCOGENE) that binds to the p53 protein

After a complex is formed in the nucleus, MDM modifies the carboxyl terminus of p53 and targets it for degradation by the proteasome

P53 has a 20 minute half life

Question 35

How can stress levels lead to the activation of P53?

Answer 35

Stress signals are sensed by kinases that then phosphorylate p53

Phosphorylated p53 cannot properly interact with MDM2

P53 can thus regulate genes involved in DNA damage repair, apoptosis and cell cycle arrest

Question 36

What are genetic therapeutic strategies using p53 to treat cancer?

Answer 36

Gene therapy

Retroviruses integrate in a stable form into the genome of infected cells

Retrovirus-mediated gene transfer of the wild type TP53 gene into human lung tumour cells COULD lead to the inhibition of tumour cell growth

Question 37

What are the molecular therapeutic strategies using p53 to treat cancer?

Answer 37

PRIMA-1 restores mutant p53 to normal
- by modifying thiol groups in the core domain of the protien

Nutlin is a potent MDM2 antagonist

RITA binds to p53 and can restore mutp53 activity

Inhibitors of CRM1 result in nuclear accumulation of p53

Question 38

How can personalised medicine be used to treat cancer?

Answer 38

Classifies tumours according to their genetic makeup instead of where they grow in the body

Developed drugs that target the molecular faults in a tumour not where the tumour is