L7- Oncogenes

Question 1

what is an oncogene

Answer 1

a gene that has the potential to cause cancer when mutated or overexpressed. it is derived from a normal cellular gene called proto-oncogene

Question 2

how do proto-oncogenes become oncogenes

Answer 2

gain of function mutations leading to overexpression of the proteins or altered proteins that have aberrant (usually constitutive) activity.
only ONE allele of the gene needs to mutated.

Question 3

what are the mechanisms of oncogene activation and name an example of each

Answer 3

1. point mutation = to protein with altered activity e.g. RAS, EGFR
2. amplification of proto-oncogene DNA region = overexpression of gene and protein e.g. MYCN, EGFR
3. chromosome translocation that brings proto-oncogene under the control of a different promoter = inappropriate gene and protein expression e.g. c-Myc, BCL-2
4. chromosomal translocation that joins 2 genes together = creates chimeric fusion gene and protein with novel characteristics e.g. PML and RARA

Question 4

what mutations can occur in RAS and where can they occur

Answer 4

missense mutations affecting at least 1 of these 3 residues- G12, G13 or Q61. These residues sit in the GTP binding pocket so mutations here lock RAS into its active GTP bound form.

Question 5

what mutations occur in EGFR

Answer 5

• In frame deletion aa 747-752 = changes protein conformation; prolongs active dimer configuration
• Missense mutation L858R (leu > arg), single aa substitution = increases kinase activity 50-fold

Question 6

in what tumours is MYCN amplification and EGFR amplification present in

Answer 6

MYCN amplification in neuroblastoma, a prognostic biomarker (associated with increased metastatic potential and therapeutic resistance)
EGFR in glioblastoma (associated with reduced survival)

Question 7

what tumour is c-MYC translocation present in and explain how this occurs

Answer 7

Burkitt Lymphoma (BL):
c-MYC translocation on chromosome 8 to on one of the 3 immunoglobulin gene loci – brings c-myc gene next to either IgH (heavy chain) or IgK / IgL (light chain).
If c-Myc gets brought next to the IgH (chromosome 14) regulatory sequence (enhancer region that is highly active in B cells) = overexpression of c-myc protein (assessed using marker for cell poliferation Ki67).

c-myc = increased transcription of genes required for cell growth + poliferation = very high B cell poliferation in BL.

Question 8

what tumour is BCL-2 translocation present in and explain how this occurs

Answer 8

Follicular lymphoma (FL).
BCL2-IgH translocation
> FL is also derived from germinal centre B lymphocytes
> Translocation between the BCL2 gene on chromosome 18 and immunoglobulin gene enhancer on chromosome 14 of IgH.
> Leads to overexpression of BCL2.
> BCL2 is a pro-survival gene that counteracts apoptosis allowing tumour cells to grow and be resistant to killing by chemodrugs.

Question 9

what tumour is PML and RARA translocation present in

Answer 9

Acute promyelocytic leukaemia (APL)
> a failure of myeloid differentiation and accumulation of proliferating progenitor promyelocytes
> T(15;17) translocation in APL: Chr 15- PML (promyelocytic leukaemia protein). Chr 17- RARA (retinoic acid receptor alpha)

Question 10

how does PML and RARA lead to a PML-RARA fusion protein in AML and what is the effect of this in AML

Answer 10

PML - component of PML nuclear bodies
-	Mediates protein-protein interactions
-	Functions in protein PTMs.
-	Enhances p53 protein stability
Important in cell cycle control, apoptosis

RARA- Nuclear receptor and TF
- Binds to retinoic acid receptor elements (RARE)
- Binding of RA ligand -> activates transcription of target genes
- No bound ligand -> repression of transcription of target genes
Important for myeloid differentiation

• PML coding exons 1,2 and 3 translocated to RARA at exon 1 position (which gets removed) = Forms new protein sequence (PML-RARA fusion protein) with novel biological properties:
 Enhances transcriptional repression of target genes
 Expands the repertoire of target genes
 Disrupts PML nuclear bodies
Resulting in:
 Silencing of genes important for myeloid differentiation
 Increased self-renewal of APL cells

Question 11

what are the 2 therapies for APL

Answer 11

Arsenic trioxide (ATO)

• Induces PML/RARA protein degradation
• Binds to PML part of fusion protein causing disruption of PML/RARA aggregates = degredation of fusion protein = loss of oncogenic protein.

Retinoic acid (RA)

• converts PML/RARA from a transcriptional repressor into a transcriptional activator
• RA receptor alpha gene is an activator when bound to RA, allows differentiation to occur.

Question 12

EGFR targeted therapies: what are examples of first, second and third generation tyrosine kinase inhibitors (TKIs)

Answer 12

1. First generation TKI
   e. g. gefitinib, erlotinib. Competitive ATP-mimics. Reversible binding. Frequent drug resistance (acquire other mutations in ATP pocket) . e.g. T790M
2. Second generation TKI
   e. g. Afatinib , Dacomitinib. Irreversible binding in the ATP pocket
3. Third generation TKI
   e. g. Osimertinib. Binds more avidly to EGFR T790M mutants than wild-type EGFR