Lecture 3: Genetics of malignancy part 2 - tumour suppressor genes

Question 1

Where do tumour suppressors normally act in the cell cycle?

Answer 1

at the G1/S phase checkpoint

Question 2

What is the normal function of tumour suppressor genes?

Answer 2

to suppress growth or promote apoptosis

Question 3

Are tumour suppressor genes dominant or recessive at the cellular genetic level?

Answer 3

Recessive (both alleles need to be mutated)

Question 4

When may tumour suppressor genes show dominant inheritance?

Answer 4

often show dominant inheritance in cancer predisposition syndromes (I.e. when one allele is already mutated, then only the other allele needs to acquire a mutation to get the phenotype)

Question 5

How do oncogenes signal to overcome the G1/S checkpoint?

Answer 5

Oncogenes ultimately result in upregulation of transcription factors (such as Myc) involved in expression of proliferation-associated genes.
- this leads to increased cyclin D-Cdk4 activity (the G1-Cdk) to promote cell cycle progression through G1/S checkpoint

Question 6

How does Rb function as a tumour suppressor in normal cells?

Answer 6

In normal cells, growth factor signalling results in activation of the cyclin D-Cdk4 complex that positively feedback and phosphorylates Rb causing the release of active E2F required to promote expression of S-phase genes (E.g. cyclin E and A) that promotes progression into S phase.

Rb binds to E2F and inactivates it. This prevents inappropriate progression into S phase

Question 7

How does loss of Rb tumour suppressor promote cancer?

Answer 7

Loss of Rb (by inactivating mutations) results in E2F being constantly available (no regulatory inhibition of E2F) so cells are able to proceed into S-phase without mitogen signalling (unregulated entry into S-phase)

Question 8

How was Rb first identified?

Answer 8

Was first identified because loss of Rb was found to cause the childhood cancer retinoblastoma.

Question 9

What are the two forms in which retinoblastoma can present and how do they differ?

Answer 9

1. Unilateral - sporadic mutations (no family history), patients can be cured by removing the tumour and usually go on to live healthy lives
2. Bilateral - often have a family history so have inherited a mutant allele. Even if tumours are removed, patient have an increased long-life risk of developing other cancers since all cells contain one mutated allele.

Question 10

What is the name of the tumour suppressor gene often mutated in cancer and the protein for which it encodes?

Answer 10

TP53 gene, which encodes the p53 tumour protein

Question 11

How does p53 function in a normal cell as a tumour suppressor?

Answer 11

p53 is a transcription factor that is activated in response to cellular stresses, such as UV radiation, ionising radiation, hypoxia etc…
In response to these stresses, p53 upregulates transcription of genes involved in:
- cell cycle arrest (E.g. p21)
- DNA repair (E.g. XPA)
- Inhibition of angiogenesis (E.g. TSP-1)
- Apoptosis (E.g. Killer/DR5)

Question 12

Give an example of a gene target of p53 transcription factor to result in cell cycle arrest

Answer 12

p21 (which binds to CDKs and inhibits their kinase activity resulting in inhibition of cell cycle progression)

Question 13

Give an example of a gene target of p53 transcription factor to result in DNA repair

Answer 13

XPA (responsible for regulating Nucleotide Excision Repair mechanisms by ATR in response to UV damage)

Question 14

Give an example of a gene target of p53 transcription factor to result in inhibition of angiogenesis

Answer 14

TSP-1 (encode thrombospondin-1, which antagonises the effects of VEGF resulting in direct effects on endothelial cell migration, proliferation, survival, and apoptosis)

Question 15

Give an example of a gene target of p53 transcription factor to result in apoptosis

Answer 15

Killer/DR5
(upregulation of Killer/DR5 means there is more receptor available for the ligand TRAIL to bind to, which signals into the cell and promotes apoptotic pathways)

Question 16

How does p53 become stable and active in response to cell stress?

Answer 16

• DNA damage causes activation of ATM/ATR kinase
• ATM/ATR kinase phosphorylates and activates Chk1/Chk2 kinase
• Chk1/Chk2 phosphorylate p53
• this results in disassociation of p53 from Mdm2 and autoubiquitylation of Mdm2
• p53 is now active and stable and can promote transcription of genes involved in apoptosis, cell cycle arrest, DNA damage etc.

Question 17

How is p53 normally maintained at low levels in the cell?

Answer 17

p53 binds to Mdm2 E3 ubiquitin ligase.
Mdm2 ubiquitylates p53, which targets it for proteasomal degradation

Question 18

Which hallmark of cancer is governed by loss of tumour suppressor genes only?

Answer 18

Evasion of growth suppressors
- by loss of tumour suppressors that normally inhibit cell growth

Question 19

Which hallmark of cancer is governed by gain of oncogenes only?

Answer 19

Sustaining proliferation signalling
- by gain of growth-promoting oncogenes

Question 20

Which hallmark of cancer is governed by both gain of oncogenes and loss of tumour suppressor genes?

Answer 20

resisting cell death
- by gain of oncogenes that negatively regulate apoptosis
- by loss of tumour suppressor genes that are positive regulators of apoptosis

Question 21

Which hallmark of cancer is governed by telomerase?

Answer 21

evading replicative immortality

Question 22

Under normal conditions, what would be the result of oncogenic signalling using overexpressed Myc as an example of the oncogenic signal?

Answer 22

Excessive Myc production leads to activation of Arf, which binds to Mdm2 and inhibits interaction between Mdm2 and p53.
- p53 is no longer ubiquitylated and degraded
- p53 become active and stable and promotes transcription of genes involved in cell cycle arrest and apoptosis

Question 23

How can loss of p53 lead to evasion of apoptosis and unregulated cell growth (use Myc as example of oncogenic signal)?

Answer 23

p53 not activated in response to oncogenic signalling so cannot upregulate genes involved in cell cycle arrest or apoptosis.
- this oncogenic signalling therefore promotes evasion of apoptosis and unregulated cell growth

Question 24

How does telomerase enable replicative immortality in cancer

Answer 24

Telomerase gene (TERT) is often overexpressed in cancer and is responsible for re-synthesising the telomeres. This prevents the progressive shortening of the telomeres with successive replications (which would normally lead to crisis eventually and result in cell death)
- this allows cancer cells to escape crisis and exhibit replicative immortality

Question 25

In an individual without cancer, where is telomerase normally expressed?

Answer 25

only in germ cells and stem cells

Question 26

What are two emerging hallmarks of cancer?

Answer 26

1. deregulating cellular energetics
2. Avoiding immune detection

Question 27

What are two enabling characteristics of cancer?

Answer 27

1. genome instability and mutation
2. tumour-promoting inflammation (microenvironment)

Question 28

What is the Warburg effect?

Answer 28

Normally, pyruvate from aerobic glycolysis is diverted to the Krebs cycle for production of large amounts of ATP. However this requires oxygen and leaves little pyruvate for biosynthetic pathways.

The Warburg effect refers to the tendency of cancer cells to preferentially use solely aerobic glycolysis for ATP production even in the presence of oxygen
- this is believed to be due to the increased number of biosynthetic building blocks that can be generated that can be used to promote growth.
(this means that cancer cells take up more glucose than normal cells)

Question 29

How can PET/CT scanning be used to image increased glucose uptake by cancer cells?

Answer 29

• Patients are given small amount of PET radiotracer (18F fluorodeoxyglucose (FDG), which is a glucose analogue)
• cancer cells will take up large amounts of the FDG
• combined PET/CT scans can detect radiation and reveal precise location of tumours in the body