Cancer II: molecular changes and cancer hallmarks

Question 1

Cancer

Answer 1

Abnormal grwoth – cells continue to proliferate forming a mass (tumour)

Collection of hundreds of disease with the common features of uncontrolled growht

Acquisition of genetic alterations resulting in uncontrolled proliferation

Question 2

Namethe types of tumours and how their masses behave

Answer 2

Benign – growth remains a single mass.

Malignant – cells gain the ability to invade surrounding tissue: Metastasis

Question 3

Describe how cells become malignant and how this allows them to eventually metasatisize.

Answer 3

Cells gradually become malignant through progressive alterations at a genetic level.
Selection for growth advantage and survival, eventually invasion and metastasis.

Question 4

Metastasis:what makes cancer cells undergo metastasis?

Answer 4

Complex, multistep process responsible for >90% of cancer-related deaths. In addition to geentic and external envrionmental factors, the physical interactions of cancer cells wth their microenvironment and their moulation( mechanical stimuli influencing cancer cell behaviour) by mechanical forces are key determinants of the metastatic process

Question 5

Angiogenesis

Answer 5

Angiogenesis (the growth of new blood vessels): important for tumour growth and metastasis.

VEGF (vascular endothelial growth factor): key molecule involved in the production of new blood vessels.

In tumours with low oxygen (hypoxia) HIF stabilised and can transcriptionally activate VEGF

Question 6

Hallmarks of Cancer

Answer 6

Biological capabilities acquired during the development of human tumours.
Cancer cells acquire these capabilities as they develop/evolve.

Question 7

Why are molecular changes important in personalised medicine?

Answer 7

Understanding changes at molecular/genetic levels allow targeted therapies: every tumour could be unique and ultimately this allows for personalised medicine.

Question 8

What causes Unregulated cell growth role in tumour development

Answer 8

Cell proliferation results in increased cell numbers (cell division cycles)
Cancer cells defined by their uncontrolled proliferation and ability to overcome cl death mechanisms
Tumour development is a balance between growth and cell death (increased cell division and decreased apoptosis)
Cell division cycle controls cell number

Question 9

Cyclins and cyclin dependent kinases

Answer 9

• Cyclins bind to and regulate activity of the CDKs.
• Cyclin dependent kinases (CDKs): Family of serine/theronine kinases that can modify various protein substrates involved in cell cycle progression
• CDKs require presence ofcyclinsto become active.
• Cyclins : family of proteins that have no enzymatic activity of their own but activate CDKs by binding to them.
• • CDKs must also be in a particular phosphorylation state — with some sites phosphorylated and others dephosphorylated — in order for activation to occur. Correct phosphorylation depends on: action of other kinases and a second class of phosphatases that are responsible for removing phosphate groups from proteins.
• CDK1 and CDK2 bind to multiple cyclins (cyclin types A, B, D and E),
  CDK4 and CDK6 only partner D-type cyclins.
  D-type cyclins and CDK4 or CDK6 regulate events in early G1 phase, cyclin E-CDK2 triggers S phase, cyclin A-CDK2 and cyclin A-CDK1 regulate the completion of S phase, and CDK1-cyclin B is responsible for mitosis.**

Question 10

Oncogenes vs Tumour suppressor

Answer 10

Oncogene: gene with the potential to cause cancer. are often mutated or expressed at high levels in tumour cells. (Accelerator)
Tumour suppresor is a gene that normally can prevent / repress cancer. (Brake)

Question 11

Two main types of oncogenes

Answer 11

Viral oncogene: gene from the virus itself; E6/E7, E1a/b, large T
(i.e., no cellular homologue).

Cellular oncogene: gene derived from the host cell genes that 
          normally are in an ‘inactive’ form.

Question 12

Proto-oncogene

Answer 12

Normal form of the cellular gene under normal regulation.
Can be converted into oncogenic forms through mutation/alteration that leads to enhanced activity/expression.

Question 13

Functions of oncogenes

Answer 13

Numerous oncogenes identified with a wide variety of specific functions - ultimately they are all growth promoting.

Growth factors: epidermal growth factor (EGF) and platelet derived growth factor (PDGF).
Growth factor receptors: PDGF and EGF receptors (e.g. HER2 in BC).
Signal transduction: Ras, Raf, Src.
Transcription factors: Myc, Jun, Fos, Elk-1.
Anti-apoptotic: Bcl-2.
Cell cycle regulation: Cyclin D1.

Question 14

Function of Ras

Answer 14

Functions as a molecular switch: active when GTP is bound and inactive when GDP is bound.

Question 15

Two classes of signalling protein regulate Ras activity

Answer 15

Ras-GEFs and Ras-GAPs
GEF: Guanine nucleotide exchange factors. Ras-activating protein whereby Ras exchanges GDP for GTP.
GAP: GTPase-accelerating proteins. Ras inactivating protein by increasing the rate of hydrolysis of Ras-GTP

Question 16

Oncogenes: Ras. What does it do?

Answer 16

Mutation results in a constantly active Ras: Consitutive activation.
Ras activation drives signalling cascades from the plasma to the nuceluspromoting cellular proliferation and tumour survival

Question 17

TSGs (Tumour Supressor Genes)

Answer 17

Regulate diverse cellular activities including cell cycle regulation, DNA repair, apoptosis, ubiquitination and protein stability, differentiation, migration and angiogenes.
Some tumour suppressors normally restrict the cell
Tumour suppressor genes (TSG): normally act as brakes for the cell cycle to suppress cell proliferation.
p53, Rb (checkpoints) : p53 controls checkpoints throughout the cell cycle from G1 phase to cytokinesis whereas the classical model associates RB only with cell cycle control during G1 and at the G1/S transition

Question 18

Retinoblastoma (retinal cancer)types and inheritance

Answer 18

Rare childhood tumour of the eye
Most cases (60-70%) are sporadic (not inherited) and present in children 1-4 years of age
remaining 30-40% have hereditary form of retinoblastoma and thus inherited germline cancer predisposing mutation - tend to have tumours earlier and are more likely to have multiple tumours in one (unilateral) or both (bilateral) eyes.

Question 19

Knudson Two-hit hypothesis. How is retinoblasma inherited?

Answer 19

• illustrate how inherited and somatic genetic changes might contribute to cancer.
• Demonstrates recessive nature of some tumour suppressors
• basis for our understanding of how TSGs drive cancer.
• Inherited retinoblastoma: One defective copy of Rb is genetically transmitted - loss of single Rb copy is sufficient for tumour development, but retinoblastoma develops as a result of second somatic mutation at the Rb allele (this explaining dominant pattern of inheritance)
  Noninherited retinoblastoma - rare, development requires two independent somatic mutations to inativate both normal copies of Rb in the same cell.

Question 20

What is Rb and what does it do?
What inhibits Rb?

Answer 20

Rb negatively regulates cell cycle progression through binding to and inactivating E2F - preventing G1/S progression
Rb: 110 kDa nuclear phopshoprotein: a key cyclin/Cdk target. Rb phosphorylation by Cdk/cyclin pervents binding to E2F prallowing E2F to activate cell cycle progression through G1/S.

Question 21

p53 - how was its role as a tumour suppressor confirmed

Answer 21

p53 (TP53) - mutated in over half of all human cancers.
p53 confirmed as a tumour supressor in 1990 by finding that patients with Li-Faumeni syndrome (which predisposes to diverse tumour types) had inherited TP53 mutations. Further confirmed in 1992 by experiments showing thatTrp53(which encodes mouse p53) knockout mice are prone to tumours.
70% of families inherit mutation in p53

Question 22

p53- Role

Answer 22

• Orchestrates cell fate during the stress response.
• Most well characterised role of p53 is as a nuclear transcriptional activator.
• p53 mutated in 50% of cancers - other half thought to have mutationsin pathways impacting on P53 function.

Question 23

p53: the guardian of the genome

Answer 23

Different stressors result in increased p53 levels and this helps to prevent damage/repair damage/kill cells that are too damaged.

Question 24

Two key p53 target genes

Answer 24

p21:Cdk inhibitor which causes cell cycle arrest (G1). Has a P53 promoter region so activated by p53- time to repair damage
Bax: prop-apoptotic - if cell damage is too great then Bax can lead to apoptotic cell death stopping tumour growth

Question 25

Apoptosis- What is it and what is it important for?

Answer 25

One form of cell death - there are others
Involves series of signalling cascades leading to rapid degradation of cellular structure and organelles
Important in removing damaged cells and dveelopment, tissue remodelling and control of tissue heomeostasis
Apoptosis: ubiquitous, energy-consuming from of cell suicide triggered and orchestrated by a defined set of molecular events.
Caenorhabditis elegans - model system created that unravelled some of the fundamental aspects of the biology of apoptosis.

Question 26

Apoptotic cascade - Intrinsic Apoptotic stimuli and pathway

Answer 26

Intrinsic apoptotic stimuli: DNA damage or endoplasmic reticulum (ER) stress: activate BCL-2 (anti) and BH3 (pro) proteins, also activates P53 which activates BAX and BCL2: BCL-2-associated X protein (BAX) (pro) and BCL-2 antagonist or killer (BAK) (pro) activation and mitochondrial outer membrane permeabilisation (MOMP).
- Anti-apoptotic BCL-2 proteins prevent MOMP by binding BH3-only proteins and activated BAX or BAK. Following MOMP, release of various proteins from the mitochondrial intermembrane space (IMS) promotes caspase activation and apoptosis.
- Cytochrome c binds apoptotic protease-activating factor 1 (APAF1), inducing its executioner caspases, caspase 3 and caspase 7, leading to apoptosis.
- Mitochondrial release of second mitochondria-derived activator of caspase (SMAC; also known as DIABLO) and OMI (also known as HTRA2) neutralises the caspase inhibitory function of X-linked inhibitor of apoptosis protein (XIAP).

Question 26

Apoptotic Cascade

Answer 26

Apoptosis triggered by either mitochondrial-dependent mechanism / by ligand binding to cell surface death receptors. This is triigered by Intrinsic and extrinsic stimuli and can be divided into intrinsic and extrinsic pathways.

• BH3- only proteins (Bcl-2 family) activate pro-apoptotic proteins like Bax, Bak.
• Transcription of Bax by p53 can also increase apoptosis.
• MOMP: mitrochondrial outer membrane permeabilisation induced by Bax/Bak oligomerisation.
• Apoptosome formation: cytochrome c induces complex of APAF-1, ATP and initiator caspase 9.
  Cleavage/activation of executioner caspases 3 and 7.

Question 26

How do cancer cells evade apoptosis?

Answer 26

Cancer cells overcome apoptosis in many ways:
-Decrease lelvels/ activity of pro-apoptotic molecules e.g: p53, caspases, Bax/Bak.
- Increase levels/activity of pro-survival/anti-apoptotic molecules e.g GF/receptors, decoy receptors, GF pathways mediators, Bcl2, IAPs.

Question 26

Apoptotic cascade - Extrinsic Apoptotic stimuli and pathway

Answer 26

extrinsic apoptotic pathway simtuli: initiated by the ligation of death receptors with their cognate ligands, leading to the recruitment of adaptor molecules such as FAS-associated death domain protein (FADD) and then caspase 8.
- This results in the dimerisation and activation of caspase 8, which can then directly cleave and activate caspase 3 and caspase 7, leading to apoptosis.
- Crosstalk between the extrinsic and intrinsic pathways occurs through caspase 8 cleavage and activation of the BH3-only protein BH3-interacting domain death agonist (BID), the product of which (truncated BID; tBID) is required in some cell types for death receptor-induced apoptosis. FASL, FAS ligand; TNF, tumour necrosis factor; TRAIL, TNF-related apoptosis-inducing ligand.

Question 26

Senescence

Answer 26

• Is a state of stable exit from the cell cycle. Growth arrested senescent cells are metabolically active.
• Normal cells do not proliferate indefinitely - lose ability to divide, normally after about 50 cell divisions in vitro.
  -Hayflick limit: Number of cell divisions a cell undergoes before it reaches the end of its replicative lifespan.

Question 27

Senescent cells

Answer 27

Eventually stop multiplying but don’t die off when they should.
Remain and continue to release chemicals that can trigger inflammation.
Become enlarged and flattened
Express senescence-associated B-galactosidase.

Question 28

What is Telemore Dependent Senescence

Answer 28

Normal diploid cells: trigger cell cycle arrest when they reach critical length
Germline and cancer cells: Telomerase can add length to the telomere

Question 29

Induction of senescence - Telemore independent Senescence

Answer 29

Simuli include Severe DNA damage/stress/drugs/oncogenes that create stressful conditions for the cell, many f which are present in the tumour environment.
Senescence functions as a self-defence mechanism to prevent proliferation of potentially damaged cells

Question 30

Mediators of Senescence

Answer 30

Depends on p53 and Rb tumour supressor pathways
- Cyclin dependent kinase inhibitors (p21 and p16)
Cancer cells have mutations in p53 and Rb pathways impacting the ability to undergo senescence.