L3: Cell Signalling and the Cell Cycle in Cancer

Question 1

Which cell cycle phase is responsive to mitogens?

Answer 1

G1 phase

Question 2

What are the phases of the cell cycle?

Answer 2

• G0
• G1
• S
• G2
• M

Question 3

Which phase of the cell cycle is responsive to mitogens?

Answer 3

G1 phase

Question 4

Which is the quiescence state?

Answer 4

Non-dividing (resting) state is G0

Question 5

What are the features of G1 phase?

Answer 5

Growth phase 1, cell growth and preparation for DNA synthesis
- responsive to environmental signals
- contains G1 resitriction checkpoint (R)
- G1/S DNA damage checkpoint

Question 6

What are the features of S phase?

Answer 6

Synthesis phase, DNA replication

Question 7

What are the features of G2 phase?

Answer 7

GRowth phase 2, preparation for mitosis, checkpoint for DNA damage before entering M phase

Question 8

What happens during M phase?

Answer 8

mitosis, nuclear division

Question 9

How is the cell cycle regulated? Explain using relevant cyclins and CDKs

Answer 9

• In G1 phase, we’ve got favourable conditions: increased expression of cyclin D (inhibits the cell cycle exit); cyclin D then binds to CDK 4/6 (can be oncogenic, often amplified in cancers); this then drives expression of cyclin E; makes a complex with CDK2 and inhibits the retinoblastoma gene product
• In S phase, E2F then drives the conversion of Cyclin E to A, which drives cells into the S phase
• In G2 phase: eventually then leaves CDK2 conversion to CDK1
• In M phase: cyclin A is converted to Cyclin B

Question 10

What are the checkpoints and where in the cell cycle are they located?

Answer 10

• In the G1 phase CDK4/6 and cyclin D complex, DNA damage inhibits this complex formation, pauses cell cycle, if can’t repair cells become quiescent or senescent (irreversibly go out of cell cycle); similar with CDK2 and Cyclin E/A complex
• In S phase: replication stress checkpoint, damage here leads to apoptosis because it’s a very complicated process
• In M phase: spindle checkpoint, defects lead to apoptosis – in cancer many of these are dysregulated

Question 11

What happens when receptor tyrosine kinases mutate? give examples

Answer 11

• In a normal cell GF binds to its ligand, dimerises it and sends off its signal in this way
• There can be mutations affecting the structure of the receptor: point mutations can sit in the juxtamembrane, will affect the structure of the receptor, will allow the activation of the receptor in an easier way; or you can have truncations, ligand binding domain is lost but the receptor can still dimerise (examples gastrointestinal tumours – KIT mutation; or glioblastoma or lung cancer where we have mutations of EGF receptor
• Another effect can be overexpression or overamplification of the genes – lots more receptors (examples: breast cancer – HER2 amplification, glioblastoma – EGF amplification

Question 12

Give a GTPase drive cancer example and describe what happens during it

Answer 12

GTPase driven cancer example – Uveal melanoma
- Harbouring a hotspot mutation (at Q209) in the genes GNAQ or GNA11, which activates the Galphaq pathway
- Sporadically also occur in CYSLTR2 or PLCB4
- these mutations activate the co-transcriptional regulators YAP/TAZ, stimulating cell proliferation, which is considered to be one of the main oncogenic pathways in UM.
- These mutations also activate the protein kinase C (PKC), MAPK and AP1 pathways similarly activating proliferation

Question 13

Give examples of ErbB alterations in cancer

Answer 13

Examples of ErbB alterations in cancer
- there can be overexpression of ErbB1 (head and neck cancer) and ErbB2 (breast cancer)
- deletions within ErbB1 (brain tumours)
- kinase domain mutations in ErbB1 (lung cancer)
- kinase domain mutations in ErbB2 (lung cancer)

Question 14

What is EGFR? What happens during alteration?

Answer 14

• ErbB1 receptor
• evidence that patients with EGFR mutation have lower survival

Question 15

What are the ways of targeting EGFRs therapeutically?

Answer 15

Two options possible:
- can block the action by targeting extracellular portion by using antibodies
o useful if cancer is driven by overexpression of extracellular domain
o Cetuximab used for colorectal cancer treatment
o not useful in K-ras mutant (40%) cancers but can be effective in K-Ras wild type colon cancers
- or target intracellular usually kinase activity by using small molecule inhibitors
o gefitinib – an EGFR targeted kinase inhibitor used for lung cancer treatment

Question 16

Describe ErbB2 overexpression and gene amplification. What are the possible treatments?

Answer 16

• Taking biopsy and using immunohistochemistry and fluorescence in situ hybridization to define the type of overexpression
• Outcome of ErbB2 positive cancers used to be very poor
• This led to development of targeted therapy
• First approved therapy – trastuzumab (monoclonal antibody) HER2+ - given to metastatic or as adjuvant as well
• Also therapies developed targeting tyrosine kinases

Question 17

Explain the mechanism of trastuzumab

Answer 17

• Trastuzumab binds to domain IV – blocks receptor, can’t dimerise anymore, physically gets in the way, induces receptor to be internalized, can also activate ADCC (antibody dependent cellular cytotoxicity) – immune system now recognizes that there is an antibody now bound to the cancer cell, signal to kill the cell
• Kinase inhibitors bind to intracellular tyrosine kinase domain, cross the membrane and block its function
• Often these therapies are combined to get better critical outcome

Question 18

Describe gastrointestinal stromal tumours (mutations, treatment)

Answer 18

• Soft tissue sarcoma
• Around 80% of GIST have mutually exclusive activating KIT or PDGFRA mutations
• These occur most often in the juxtamembrane (bit between the plasma membrane and the kinase domain) domain making them partially active
• KIT/PDGFR receptor kinase inhibitors – imatinib, sunitinib
• Normally when receptor dimerises the juxtamembrane domain gets phosphorylated and stabilized releasing the kinase domain, so it can phosphorylate other things

Question 19

What is the KRAS pathway?

Answer 19

• KRAS –> RAF –> MERK –> ERK
• Small GTPase involved in transmitting signals within cells
• Activated by exchange of GDP to GTP
• Inactivated by hydrolysis of GTP to GDP
• Common hotspot cancer activating mutations reduce the interaction with gamma-phosphate of GTP and reducing its hydrolysis

Question 20

What is the most common KRAS mutation? What is used for its therapy?

Answer 20

• RAS was considered undruggable as many strategies have failed
• KRAS-G12C mutations occur in 13-16% of lung adenocarcinomas
• KRAS-G12C is nearly mutually exclusive with other known oncogenic drivers
• Sotorasib irreversibly forms an irreversible bond with the mutant cysteine 12 and blocks GTP binding trapping KRAS-G12C in the inactive state – if you don’t have this mutation you wouldn’t benefit from it

Question 21

What is BRAF? what is the most common mutation?

Answer 21

• BRAF V600E most common mutation
• V600 sits in the hydrophobic pocket of the activation loop which normally stabilize the inactive form
• BRAF V600E mutant melanoma – selective inhibitors include vemurafenib and dabrafenib are effective in mutant disease but not wild-type
• Targeted inhibitors can produce impressive responses
• However, these may be short lived if person develops resistance