W8 Carcinogenesis (GN) Flashcards
Carcinogenesis requires several cellular changes
Can Somatic and Germline DNA changes be inherited?
- 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
Aetiology of cancer
How can oncogenic mutations occur?
Examples of Carcinogens? and what they do
- 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
Direct vs Indirect initiator carcinogens & promoters:
How do initiators and promoters work together to inc the likelihood of cancer formation?
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
- Initiators (direct + indirect)
-Irreversible DNA damage in structure/sequence - Promoters stimulate excess cell division
=Increased likelihood of
cancer transformation
Radiation carcinogens
Carcinogenicity depends on what? (3)
2 types of radiation?
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
Biological Carcinogens
What are the 4 types?
- Oncoviruses- Responsible for 20% of tumours
- Bacteria- H. Pylori ↑ risk for Gastric cancer
- Fungus- Aspergillus flavus (producing Aflatoxins B1) ↑ risk for Liver cancer
- Parasites- Schistosoma haematobium ↑ risk for Bladder cancer
What is a carcinogen?
A carcinogen is a substance, organism, or agent that can cause cancer.
Physical (radiation)
Chemical (asbestos, tobacco smoke, nickel…)
Biological (viruses, bacteria..)
Cancer rates differ by geographic location (for info)
- Lifestyle choices significantly contribute to some types of cancer.
➢Cancer incidence differ by geographic location
➢Cancer incidence usually matches lifestyles and environmental risk factors
Cancer prevention
What are the lifestyle change can reduce the risk for some types of cancer? (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)
What is Carcinogenesis?
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
What do tumoral cells/tissue undergo:?
➢Hyperplasia → Increased/irregular proliferation
➢Dysplasia → Loss/Abnormal cell/tissue typical architecture → loss of cell specialisation
4 stages of carcinogenesis?
Initiation
Promotion
Progression
Metastasis
Carcinogenesis - preinitiation
what occurs?
- 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
Carcinogenesis - initiation (1)
what are the steps?
- 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
Carcinogenesis - promotion (2)
what are the steps?
- 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
Carcinogenesis - progression (3)
what are the steps?
- 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
Carcinogenesis – invasion/metastasis (4)
what are the steps?
- Malignant tumours → cancer
- Angiogenesis
- Colonisation of the closer tissues → Invasion
- Metastasis
Clonality of cancer
- 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
Cancer ecosystem and treatment
Possible outcomes of cancer treatments? (3)
a) The tumour has responded
completely to treatment
b) treatment-resistant clones are
selected → treatment failure
c) Complete lack of response
Key genes in cancer development?
- 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**
Protooncogenes and oncogenes
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
Oncoproteins and functions
Functional categories of oncoproteins? (6)
- Growth factors
- Growth factors receptors
- Signal transduction protein
- Cell cycle control system (cyclins or CDKs)
- Anti-apoptotic factor
- Transcription factor → mutated ras
→ oncogenes
Oncoproteins of relevance - RAS
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
Tumour suppressor genes:
2 types?
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
p53 -Tumour suppressor genes
- 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
