L05 - Oncogenes

Question 1

What is a proto-oncogene?

Answer 1

A normal regulatory gene, that promotes normal cell growth & proliferation in a regulated manner, encoding proteins involved in signal transduction that can be mutated to become an oncogene

Question 2

What is an oncogene?

Answer 2

A mutated form of the corresponding proto-oncogene that induces abnormal cell proliferation and tumour development

Question 3

In what 4 ways can a proto-oncogene gain a function mutation to become an oncogene?

Answer 3

1 - Point mutation

2 - Amplification of a DNA segment that includes a proto-oncogene

3 - Chromosome translocation that brings a growth regulatory gene under the control of a different promoter

4 - Chromosome translocation that joins two genes together, creating a fusion gene

Question 4

Give two examples of proto-oncogenes that gain their function mutations by point mutation.

Answer 4

1 - Ras

2 - EGFR

Question 5

Give two examples of proto-oncogenes that gain their function mutations by amplification.

Answer 5

1 - MYCN

2 - EGFR

Question 6

Give two examples of proto-oncogenes that gain their function mutations by chromosome translocations that bring a growth regulatory gene under the control of a different promoter.

Answer 6

1 - c-MYC

2 - BCL-2

Question 7

Give an example of a proto-oncogene that gains its function mutation by chromosome translocations that join two genes together, creating a fusion gene.

Answer 7

• BCR & ABL

Question 8

What are the 3 members of the RAS oncogene family?

Answer 8

1 - KRas

2 - NRas

3 - HRas

Question 9

Which 3 codons on Ras genes do oncogenic point/missense mutations invariably involve?

Answer 9

1 - G12

2 - G13

3 - Q61

*G = glycine and Q = glutamine

Question 10

How do G12, G13 and Q61 point mutations produce oncogenic activity in Ras?

Answer 10

They make Ras unresponsive to GAP activity, which is necessary to deactivate Ras by converting the GTP on Ras to GDP

Question 11

In which area of the Ras protein are the products of the G12, G13 and Q61 genes?

Answer 11

The nucleotide-binding pocket (where GTP binds)

Question 12

How many alleles of a proto-oncogene must be mutated in order to create oncogenic activity of that particular gene?

Answer 12

Only one

Question 13

Which mutations in EGFR also cause increased activity of Ras?

Answer 13

• In frame deletions aa 747-752: these change the protein conformation, which prolongs the active dimer configuration
• Missense mutation L858R (leu > arg): this increases kinase activity 50-fold

Question 14

Which cancers are caused by amplification of which genes?

Answer 14

• MYCN - Neuroblastoma
• c-MYC - Small cell lung cancer, Breast cancer, Ovarian cancer, Oesophageal cancer
• Cyclin D1 - Breast cancer, Oesophageal cancer
• EGFR - Glioblastoma

Question 15

What are double minutes?

Of which gene are they a common feature?

Answer 15

• Small fragments of extrachromosomal DNA as a result of gene amplification
• They are a common feature of the MYCN gene

Question 16

What are homogeneously staining regions?

Of which gene are they a common feature?

Answer 16

• Multiple copies of a gene on the same chromosome as a result of gene amplification
• They are a common feature of the MYCN gene

Question 17

What is MYCN amplification associated with?

Answer 17

Aggressive disease, metastatic potential, therapeutic resistance and poor patient outcomes

Question 18

What is MYC?

How does it work?

Answer 18

• MYC is a transcription factor, and MYC proteins compete with MAD for binding MAX

1 - MYC/MAX – activation of gene expression

2 - MAD/MAX – repression

• ∴ if MYC > MAD, MYC outcompetes MAD for MAX, leading to increased expression
• MYC binds promoters of a large proportion of genes already active in the cell; wide activation leads to a generally more aggressive cancer
• It significantly increases the expression of these actively transcribed genes

Question 19

What is a hallmark of Burkitt lymphoma?

Answer 19

Translocations involving c-MYC

Question 20

What chromosomal translocation do all Burkitt lymphoma tumour cells carry?

Answer 20

• All BL tumour cells carry a chromosomal translocation involving c-MYC gene on chromosome 8 and one of the immunoglobulin gene loci:

1 - t(8;14) MYC – IgH (85%)

2 - t(2;8) MYC – IgK

3 - t(8;22) MYC – IgL

Question 21

What is the outcome of the MYC / IgH translocation?

Answer 21

• The translocation places the IgH enhancer adjacent to c-MYC
• Since IgH is highly expressed by the IgH enhancer to produce antibodies, the translocation causes the IgH enhancer to powerfully up regulate c-MYC
• c-MYC binds with MAX to E-box sequences
• Genes required for cell growth, proliferation, ribosomal & protein synthesis, metabolism, & energy generation are unregulated (i.e. the hallmarks of cancer)

Question 22

What chromosomal translocation occurs in follicular lymphoma?

Answer 22

• BCL12-IgH translocation

- BCL12 is a pro-survival gene

Question 23

Which chromosomal translocation is a hallmark of chronic myeloid leukaemia?

Answer 23

• BCR-ABL
• Philadelphia chromosome
  t(9;22)-(q34;q11)

Question 24

What is oncogene addiction?

Answer 24

• A process in which cancers with genetic, epigenetic, or chromosomal irregularities become dependent on one or several genes for maintenance and survival
• As a result, cancer cells rely on continuous signalling from these oncogenes for their survival
• Growth and survival can be significantly impaired by the inactivation of a single driver mutation

Question 25

What is the treatment for cancers caused by oncogene addiction?

Answer 25

Targeted therapy - tyrosine kinase inhibitors (TKIs)

Question 26

Describe 1st generation TKIs.

Answer 26

• e.g. gefitinib, erlotinib
• Competitive ATP-mimics with reversible binding
• Frequent drug resistance e.g. T790M (as the ATP out-competes the inhibitor)

Question 27

Describe 2nd generation TKIs.

Answer 27

• e.g. Afatinib, Dacomitinib

- Irreversible binding in the ATP pocket

Question 28

Describe 3rd generation TKIs.

Answer 28

• e.g. osimertinib

- Bind more avidly to EGFR T790M mutants than wild-type EGFR