Lecture 2: Genetics of Malignancy

Question 1

In a normal cell, what controls entry into the cell cycle?

Answer 1

External mitogens signal to allow cell to overcome the G1/S checkpoint

Question 2

What phase of the cell cycle are most adult (differentiated) cells in?

Answer 2

G0

Question 3

How can cancer cells circumvent normal regulation of the cell cycle?

Answer 3

• produce their own growth factors that signal in an autocrine manner
• overexpress growth factor receptors
• acquire mutations in growth factor signalling pathways that results in constitutive activation

Question 4

What are the two main types of genes that acquire changes that can lead to cancer?

Answer 4

Mutation leading to gain of oncogene

Mutation leading to loss of tumour suppressor gene

Question 5

Are oncogenes genetically dominant or recessive?

Answer 5

Dominant (only need one allele to be mutated to get the effect)

Question 6

How are oncogenes derived?

Answer 6

Proto-oncogenes normally involved in proliferation and survival are either mutated or dysregulated leading to gain of oncogene and uncontrolled proliferation.

Question 7

True or false: oncogenes are formed by genetic alteration of a proto-oncogene?

Answer 7

True

Question 8

How can cellular oncogenes be identified?

Answer 8

• Add carcinogen to chemically transform cells
• Isolate DNA from cells
• transfect DNA into normal cells
• some cells will form a cancerous focus
• inject some of the cancerous cells into a mouse host - forms a tumour

Question 9

Describe the 5 mechanisms for oncogene activation

Answer 9

1. Deletion or point mutation of coding sequence (normal dose of protein but increased activity)
2. Mutation in regulatory region (results in more mRNA so protein overexpressed)
3. Gene amplification (more copies of the gene results in protein overexpression)
4. Chromosome rearrangement of regulatory sequence (gene now under control of more active regulatory sequence resulting in protein overproduction)
5. Chromosome rearrangement resulting in fusion protein (fusion to actively transcribed gene produces hyperactive fusion protein)

Question 10

What are the 7 types of growth regulatory genes that can give rise to oncogenes?

Answer 10

1. growth factors
2. growth factor receptors
3. G proteins
4. Intracellular ser/thr kinases
5. Intracellular tyrosine kinases
6. Transcription factors
7. negative regulators of apoptosis (E.g. Bcl)

Question 11

Give an example for each of the 7 types of growth regulatory genes that can give rise to oncogenes

Answer 11

1. growth factors = TGF-alpha
2. growth factor receptors = receptor tyrosine kinases (E.g. HER2)
3. G proteins = Ras GTPases (E.g. KRas)
4. Intracellular ser/thr kinases = Raf
5. Intracellular tyrosine kinases = ABL
6. Transcription factors = Myc
7. negative regulators of apoptosis = Bcl

Question 12

How can mutation in genes encoding receptor tyrosine kinase (growth factor receptor) result in cancer formation?

Answer 12

normally cells require external growth factor to cause dimerisation of receptor and resultant firing of the signalling pathway.

• in cancer cells mutations can occur within the cytosolic region that result in dimerisation and firing in the absence of ligand binding
• mutations may cause overexpression of the receptor that results in molecular crowding that induces dimerisation and ligand independent firing

Question 13

How can mutation in genes encoding Ras (and/or Raf Ser/thr kinase) result in cancer formation?

Answer 13

Ras G-proteins act downstream of growth factor signalling pathways from RTKs
- mutations in Ras can result in constitutively active (bound to GTP) Ras
- this leads to constitutively active growth factor signalling pathway that activates proteins involved in the G1/S transition and abnormal proliferation (unchecked)

Question 14

Mutations in which Ras proteins are most common in cancer?

Answer 14

KRAS

Question 15

What are the Ras hotspots for mutations? and how do they promote Ras activation?

Answer 15

amino acids: 12, 13 and 61
Mutations in these amino acids correlate to decreased GTPase activity so Ras can bind GTP but not hydrolyse and release GDP resulting in constitutively active Ras

Question 16

How do the amino acid changes/mutations differ between oncogenes and tumour suppressor genes?

Answer 16

Oncogenes typically have mutational hotspots and show a specific change in ammino acids signatures of oncogenes

Tumour suppressor genes don’t have mutational hotspots that correlate to loss of function

Question 17

How does the bcr-abl oncogene form and how can this mutation (intracellular tyrosine kinase) result in cancer formation?

Answer 17

Breakage events in chromosomes 9 and 22 followed by translocation result in the formation of the Philadelphia chromosome and the bcr-abl oncogene
- this oncogene encodes the Bcr-Abl fusion protein that acts as a constitutively active tyrosine kinase in GF signalling pathways

Question 18

the bcr-abl mutation is found in more than 95% of people with which cancer?

Answer 18

Chronic myeloid leukaemia

Question 19

How can CML be diagnosed based on the bcr-abl mutation?

Answer 19

karyotype for the Philadelphia chromosome

Question 20

How does the Bcr-Abl fusion protein become constitutively active tyrosine kinase?

Answer 20

The Bcr protein contains a dimerisation domain and the Abl protein contains kinase domain
- two Bcr-Abl fusion proteins can dimerise via the Bcr dimerisation domain
- the Abl domains are brought into close proximity for transphosphorylation to constitutively activate the kinase domain

Question 21

What are the two ways Myc transcription factor can become oncogenic and which cancers this can occur in?

Answer 21

Myc regulates expression of genes involved in promoting cell proliferation and survival and can become oncogenic by:
1. amplification at the myc locus (can occur in leukaemias and carcinomas)
2. chromosomal translocation resulting in deregulation of myc (can occur in Burkitt’s lymphoma) - the myc gene is translocated downstream of a heavily transcribed IgH gene

Question 22

What are the Myc homologues that can also be overexpressed in cancers?

Answer 22

N-Myc and L-Myc

Question 23

How does number of N-Myc copies relate to event free survival in neuroblastoma patients?

Answer 23

more than 10 copies of N-myc gene correlates with greatly reduced survival

Question 24

Give 4 examples of proto-oncogenes and their mechanism of activation to become oncogenic

Answer 24

ras (G-protein) and B-raf (Ser/Thr kinase) are proto-oncogenes that become activated oncogenes by deletion or point mutation within the coding sequence

EGFR (growth factor receptor), HER2 (growth factor receptor) and myc (transcription factor) are all proto-oncogenes that become activated oncogenes by gene amplification

In Burkitt’s lymphoma, myc (transcription factor) becomes oncogenic by chromosome rearrangement where the gene becomes under regulation of a transcriptionally active promoter

The bcr-abl fusion protein becomes activated by chromosome rearrangement where two coding sequences come together.

Question 25

Give an example of a cancer and how BCL-2 gene becomes oncogenic in this cancer?

Answer 25

Cancer: follicular lymphoma

BCL-2 gene becomes oncogenic after chromosomal translocation between chromosome 14 and 18 results in the BCR-2 gene being under the control of a highly active IgH promoter resulting in overexpression of Bcl-2 that promotes anti-apoptosis and promotes cell survival