Cancer 5

Question 1

How are receptor tyrosine kinases activated?

Answer 1

• Exist as monomers on surface
• Ligand induces dimerisation = active dimers (aka reeptor mediated dimerisation)
• Active dimers go through trans-autophosphorylation and act as docking sites (RTK) for SH2/ PTB domain

Question 2

What are the mechanisms of activation of GF induced pathways?

(5 marks)

Answer 2

• Mutation made hyperactive growth factor (uncommon)
• Increased levels of RTK on growth surface
• Mutation in RTK
  
  • constitutively active
• Activation of RTK by viral proteins
• Loss of RTK regulatory elements
  
  • dephosphorylation by phosphotases
  • internalisation and degradation

Question 3

What does the epidermal growth factor receptor (EGFR) do?

(4 marks)

Answer 3

• Binds lots of ligands including EGF
• Metastasis and advanced disease
• Resistance to treatment, poor coutcome
• Tumours filled with EGFR on surface

Question 4

What are the active EGFR dimers?

(4 marks)

Answer 4

• Ras-Raf kinase pathway = cell proliferation (recruitment of Raf activates Ras-GTP)
• PI3K - Akt kinase signalling pathway = cell proliferationa and survival

Question 5

What is the main role of the molecule c-Cbl?

Answer 5

• Central role in EGFR down-regulation and helps it ‘escape’

Question 6

How is EGFR regulated?

Answer 6

Degradation of its receptor

Question 7

How do you ‘switch’ off EGFR?

(4 marks)

Answer 7

• Autophosphorylated EGFR recruits ubiquitin ligase Cbl
• Receptor is ubiquitinylated and internalised
• Have docking sites for the switch on and off signal
• c-Cbl ubiquitn ligase, when bound to TK releases ubiquitin ligase molecules which indicate internalisation and degradation of receptor by hydrolytic enzymes

Question 8

What are some abnormalities that occur in EGFR signalling?

(2 marks)

Answer 8

• Increase in amount of EGFR tumour releases it to cell surface aka autocrine stimulation
• Amplification of EGFR gene on cell surface

Question 9

In abnormal EGFR signalling what does p53 do?

Answer 9

• Mutant p53 protein can bind to EGFR to promote transcription

Question 10

What haopens with Cbl in abnormal EGFR signalling?

(2 marks)

Answer 10

• Mutation in receptor can’t bind to Cbl
• Or mutation in Cbl stops it binding to the receptor

Question 11

What is increased ligand production in EGFR signalling caused by?

Answer 11

EGFR ligand (particularly EGF) are frequently over-expressed in cancer by autocrine stimulation

Question 12

Why do increased EGFR receptor signals occur?

(3 marks)

Answer 12

• Gene amplification
• Defective gene promoter activity (binding of p53 proteins)
• Defective receptor downregulation (inability to bind c-Cbl)

Question 13

What is the most common variant EGFR mutation and what does it do?

(2 marks)

Answer 13

• EGFR variant III
• Puts receptor in permanently active state as has loss in ligand binding region - (common in agressive brain tumour)

Question 14

What is the function of the EGFR variant V?

(3 marks)

Answer 14

• Strongly activates PI3 kinase/ Akt signalling pathway
• Involved in increased cell survival, proliferation and motility
• Hard to treat as ensures cell undegoes apoptosis

Question 15

Which domains are affected by mutations in EGFR?

(3 marks)

Answer 15

• Extracellular domain
• Intracellular domain
• Intracellular tyrosine kinase domain

Question 16

How does evasion of apoptosis occur via the intrinsic route?

(4 marks)

Answer 16

• ‘Mitochondiral route’
• Can be activated by UV exposure
• Over-expression of anti-apoptotic Bcl-2 gene causing insufficient apoptotic turnover and accumulation of cells
• p53 mutations contribute to mutations in pathway

(most commonly disrupted in cancer)

Question 17

How does evasion of apoptosis occur in the extrinsic route?

(3 marks)

Answer 17

• Can be activated by UV exposure
• Involves binding to death receptors
• Suppresion of caspase gene in cancers i.e. neuroblastoma - affects this pathwat and so does loss of caspase 8 expression

Question 18

How does DNA damage affect the intrinsic pathway?

(3 marks)

Answer 18

• Need cytochrome C to be released from mitochondria
• Regulated by balance between molecules that either promote or inhibit apoptosis
• Regulation needs to tip in favour of anti-apoptotic molecules i.e. BCL-2, BCL-X

Question 19

What are the series of events involved in the intrinsic/ mitochondrial induction of apoptosis?

(4 marks)

Answer 19

• Mitochondria forms channels for cytochrome c to leak out
• Once in cytoplasm cytochrome c bonds to adaptor protein which recruits procaspase molecules
• This binds to adaptors to form apoptosome
• the active caspase 9 can go and cleave its target

Question 20

What is another name for the intrinsic pathway?

Answer 20

Mitochondrial induction

Question 21

How does deregulation of the intrinsic pathway occur by over-expression of anti-apoptotic molecules?

(3 marks)

Answer 21

• anti-apoptotic molecules = BCL-2, BCL-XL
• By activating mutations in these genes e.g. chromosomal translocation
• excessive survival signalling promoted by RTKs

Question 22

How does dergulation of the intrinsic pathway occur by loss of activity/ levels of pro-apoptotic molecules?

(3 marks)

Answer 22

• e.g. p53, BAX
• Due to loss of function of mutations
• Excessive survival signalling promoted by RTKs

Question 23

Where gene expression iss regulated, how do you get growth factor thats outside the cell to inside the nucleus?

(6 marks)

Answer 23

• Binding growth factor to receptor
• Receptor dimerization
• Autophosphorylysation
• Activation of intracellular transducers
• Activation of cascade of serine/threonine kinases
• Regulation of transcription factors for gene expression

Question 24

How is c-Cbl incorporated into the cell?

(3 marks)

Answer 24

• c-Cbl recruited from plasma membrane where it is associated with 3-ubiquitinates and activated by EGFR prior to receptor internalisation
• c-Cbl binds to activated EGFR directly - mediated by phosphorylated tyrosine residue 1045 on EGFR
• c-Cbl binds to activated EGFR indirectly - mediated through adaptor protein Gbr2