W8 Carcinogenesis (GN)

Question 1

Carcinogenesis requires several cellular changes

Can Somatic and Germline DNA changes be inherited?

Answer 1

• Cancers derived from somatic cells (~95%) are not passed to offspring
• Cancer is a complex genetic disease at the level of somatic cells
• Tumours can be inherited only if cancer susceptibility mutations related to germ cells
  → producing gametes (sex cells)

Somatic- No, acquire over a persons lifetime in single cells, can lead to cancer
Germline- Yes, present in every body cell inc egg and sperm, can inc cancer susceptibility

Question 2

Aetiology of cancer
How can oncogenic mutations occur?
Examples of Carcinogens? and what they do

Answer 2

• Oncogenic mutations can result from DNA-copying errors or induced by carcinogens
  (initiators)
• Chemical, Physical-radiation, Microbes (oncoviruses) — cause DNA damage in structure/sequence
  -If not repaired, they increase the likelihood of
  carcinogenic transformation

Question 3

Direct vs Indirect initiator carcinogens & promoters:
How do initiators and promoters work together to inc the likelihood of cancer formation?

Answer 3

Carcinogens - Initiators:
* Direct → directly binding DNA by highly reactive electrophile groups and inducing
damage
* Indirect → inert chemicals that are metabolised into reactive intermediates with carcinogenicity potential

Promoters:
* Does not have cancerogenic properties
* Stimulate continuous/excessive cell divisions
promoting potential further genetic instability

Promoters
stimulating excess cell division
Initiators (direct or indirect)
Irreversible DNA damage in structure/sequence

1. Initiators (direct + indirect)
   -Irreversible DNA damage in structure/sequence
2. Promoters stimulate excess cell division
   =Increased likelihood of
   cancer transformation

Question 4

Radiation carcinogens
Carcinogenicity depends on what? (3)
2 types of radiation?

Answer 4

the type of radiation, time of exposure, and penetration

Ionising
➢ Chromosome breakage/rearrangements
➢ Inducing secondary effectors – e.g. ROS

✓ X-rays → ↑ risk for Leukaemia
✓ Radio isotopes → ↑ risk for thyroid cancer
✓ Atomic explosion → ↑ risk for leukaemia, thyroid cancer

Non- ionising
➢ Direct DNA damage/mutations

✓ UV-A and UV-B→ ↑ risk for melanoma and
other skin cancers
* Melatonin in darker phototypes
→ risk carcinogenicity

Question 5

Biological Carcinogens
What are the 4 types?

Answer 5

1. Oncoviruses- Responsible for 20% of tumours
2. Bacteria- H. Pylori ↑ risk for Gastric cancer
3. Fungus- Aspergillus flavus (producing Aflatoxins B1) ↑ risk for Liver cancer
4. Parasites- Schistosoma haematobium ↑ risk for Bladder cancer

Question 6

What is a carcinogen?

Answer 6

A carcinogen is a substance, organism, or agent that can cause cancer.
Physical (radiation)
Chemical (asbestos, tobacco smoke, nickel…)
Biological (viruses, bacteria..)

Question 7

Cancer rates differ by geographic location (for info)

Answer 7

• Lifestyle choices significantly contribute to some types of cancer.
  ➢Cancer incidence differ by geographic location
  ➢Cancer incidence usually matches lifestyles and environmental risk factors

Question 8

Cancer prevention
What are the lifestyle change can reduce the risk for some types of cancer? (8)

Answer 8

-Avoid Smoking, Avoid Excessive Sun Exposure
-Limit Alcohol intake
-Vaccinations (HBV and HPV) & infection prevention
-Preventive occupational health
-Healthy diet and avoid obesity
-Exercise regularly
-Cancer screening programmes and follow-up
-Limit processed food and sugars

• 75% melanoma (UV exposure)
• 75% stomach cancer (H. pylori)
• 90% lung cancers (smoke)
• 90% esophageal cancers
  (smoke, alcohol, diet)
• > 90% uterine cervix cancers
  (HPV vaccine)

Question 9

What is Carcinogenesis?

Answer 9

Multi-stage process acquiring or inheriting a set of DNA mutations in key genes (1% of the human DNA) regulating vital cell repair/control systems

Question 10

What do tumoral cells/tissue undergo:?

Answer 10

➢Hyperplasia → Increased/irregular proliferation
➢Dysplasia → Loss/Abnormal cell/tissue typical architecture → loss of cell specialisation

Question 11

4 stages of carcinogenesis?

Answer 11

Initiation
Promotion
Progression
Metastasis

Question 12

Carcinogenesis - preinitiation
what occurs?

Answer 12

• Random exposure to various types of carcinogens
• **DNA repair systems efficiency **maintains genetic stability
• Cell cycle is strictly regulated at checkpoints
• In healthy and unaltered cells, this stage lasts for the entire life

Question 13

Carcinogenesis - initiation (1)
what are the steps?

Answer 13

• First DNA alteration occurs (induced by initiator carcinogens or spontaneous), which is not repaired by the cells
• It can alter an oncogene or a tumour suppressor gene → initiating the dysregulation of proliferative signalling
• Irreversible stage with no detectable phenotypic cell change
• It might last decades without progression

Question 14

Carcinogenesis - promotion (2)
what are the steps?

Answer 14

• Promoters (e.g. growth factors) stimulate cell divisions of the initiated cell → giving chances to acquire further mutations → precancerous cell
• Oncogenes activations AND tumour suppressor inhibition allow tumoral clonal expansion
• Last few years
• Reversible at initial stages by chemo preventive agents

Question 15

Carcinogenesis - progression (3)
what are the steps?

Answer 15

• Clinically-evident but localised tumour (premalignant)
• Changes in the cell’s morphology/specialisation → dysplasia
• Heterogenicity of tumoral cells (different clones/variants)
• Establishing of the internal tumoral microenvironment

Question 16

Carcinogenesis – invasion/metastasis (4)
what are the steps?

Answer 16

• Malignant tumours → cancer
• Angiogenesis
• Colonisation of the closer tissues → Invasion
• Metastasis

Question 17

Clonality of cancer

Answer 17

• Cancer arises from a single cell
  (Monoclonal initially)
• Cancer cells evolve independently generating heterogenous clonal subpopulations (subclones), with distinct features
• Posing challenges for treatment
• During progression, tumoral clones are subjected to immune and non-immune (e.g. treatment) selective pressure
  ➢ E.g. highly antigenic clones are
  destroyed by host defences
  ➢ Low antigenicity clones are selected
• A growing tumour tends to be enriched for subclones capable to survival, growth, invasion,
  metastasis and resistance to treatment.

Treatments can be effective or select/induce treatment-resistant clones

Question 18

Cancer ecosystem and treatment
Possible outcomes of cancer treatments? (3)

Answer 18

a) The tumour has responded
completely to treatment
b) treatment-resistant clones are
selected → treatment failure
c) Complete lack of response

Question 19

Key genes in cancer development?

Answer 19

• Proto-oncogene transformation→ Oncogenes
• promoting mitogen-independent cell growth
• Tumour suppressor genes inactivation
  ▪ Cell cycle control (gatekeeper) genes
  ▪ DNA repair (caretaker) genes

Affecting ultimately cellular behaviour → hallmarks acquisition → malignant transformation

Tumour : Oncogenes activation and Tumour suppressor genes *deactivation**

Question 20

Protooncogenes and oncogenes

Answer 20

in normal cells
* Proto-oncogenes →. Normal genes encoding proteins that promote cell progression only in response to signals

In tumoral cells
* Oncogenes → coding abnormal oncoproteins, resemble the normal protooncogene products, but their activity does NOT depend on proliferative signals

• Oncoproteins are constitutive ON in cancer cells → bypass the cell checkpoint control →
  unregulated proliferative signalling

Question 21

Oncoproteins and functions
Functional categories of oncoproteins? (6)

Answer 21

1. Growth factors
2. Growth factors receptors
3. Signal transduction protein
4. Cell cycle control system (cyclins or CDKs)
5. Anti-apoptotic factor
6. Transcription factor → mutated ras
   → oncogenes

Question 22

Oncoproteins of relevance - RAS

Answer 22

In normal cells:
▪ Proto-oncogene RAS encodes for a single unit GTP-binding proteins involved in signal
transduction of the receptor tyrosine kinase
▪ RAS switches with “on” (GTP-bound) and “off” (GDP-bound) states
▪ RAS is activated the signal/receptor binding to exchange GDP for GTP
▪ Lack of signal → RAS has GTPase activity → hydrolysing GTP into GDP
▪ GTPase activating proteins (GAPs) augment this process (1000 fold)

In tumoral cells:
* 15%-20% of all cancers have RAS mutated

▪ Point mutations → oncogene transformation
▪ Mutated RAS proteins cannot be regulated
-Its GTPase activity is defective → always ON
(GTP-bound)

• Mutated RAS are cancer-specific
  ✓ KRAS → colon and pancreas
  ✓ HRAS → bladder tumours
  ✓ NRAS → haematological tumours

Question 23

Tumour suppressor genes:
2 types?

Answer 23

Tumour suppressor genes are genes that produce proteins that normally prevent
uncontrolled cell cycle progression and genetic instability.

➢ Gatekeeper genes → cell cycle “Brakes”
▪ Inhibit cell progression at checkpoints in
response to anti-growth factors, DNA damage or internal stress stimuli
✓ p53 → transcriptional factor → mutated in 50% cancer
✓ Rb → transcriptional factor → mutated in retinoblastoma
✓ APC → regulatory protein → mutated in colorectal canc.

➢ Caretaker genes
▪ Maintain integrity of the genome by
detecting and repairing DNA damage
✓ BRCA1 and BRCA2 → DNA repair→ mutated in breast and ovarian cancer

Question 24

p53 -Tumour suppressor genes

Answer 24

• p53 gene codes for a p53 transcription factor regulating the transcription levels of genes
  involved in many cell responses, including cell proliferation
• In normal cells, p53 is inactive (unphosphorylated and bound to MDM2) and rapidly degraded
• “Guardian of the genome” → DNA damage activates p53 that transcribes genes to induce
  cell cycle arrests at 2 checkpoints (G1/S & G2/M), DNA repair and apoptosis

Expression target genes
* Cell division suppression
* DNA repair
* Apoptosis
* Angiogenesis

Question 25

Mechanisms of p53 inactivation in tumours:
In tumoral cells:

Answer 25

➢ Mutations in p53 gene – (Alteration in both alleles ) → LOSS OF FUNCTION tumour progression
➢ MDM2 protein overexpression → involved in p53 degradation
➢ Viral oncogenes (e.g. HPV E6) → binds and degrades p53 → HPV-induced cervix cancer
In tumoral cells

Question 26

The p53 gene is often referred to as the ‘guardian of the genome’ because, when activated, it is involved in all the following cell activities except for:
A) Inhibition of cell cycle progression
B) Induction of DNA repair mechanisms
C) Angiogenesis inhibition
D) Induction of apoptosis
E) Activation of telomerases to block the cell senescence

Answer 26

=E

Question 27

Summary

Answer 27

• Genetic changes that lead to cancer can be triggered by numerous natural and human-
  made carcinogens
• Most cancer-causing mutations occur in somatic cells; 5-10% of cancers have a hereditary component (mutations in germ-line cells).
• Cancer cells in primary and secondary tumours are clonal, arising from one cell that
  accumulated several cancer-causing mutations and developed multiple subclones (heterogenicity)
• The development of cancer is a multistep process requiring multiple mutations and clonal
  expansions (initiation, promotion, progression and metastasis).
• Mutations in proto-oncogenes (activating oncogenes) and tumour-suppressor genes
  contribute to uncontrolled cell proliferation (also apoptosis inhibition and DNA repair block)
  and the development of cancers.