Cancer Flashcards
Why are cancer trends changing?
Increase in lung cancer due to smoking
Increase in breast cancer due to delayed childbirth
Increasing obesity and diet changes affects risk of cancer
How are politics involved in cancer today?
Can’t afford to treat someone if it increase lifespans by 6months
Can afford to treat if it increases lifespan by 5 years
Circumstances- mother with 2 kids likely to get correct treatment compared to single middle aged male
If smoking was stopped how would cancer incidence change?
Incidence would reduce by 25% worldwide
Why are 90% of cancers of epithelial origin?
Proliferating tissues have increased risk of mutation
Exposure to uv light, diet and smoking which can enter blood and cause cancer anywhere
What factors increase the likelihood of a cell turning cancerous?
6-10 mutations in same cell
Live longer - more likely to accumulate mutations
Weak immune system
Changes in gene expression
Shape of nucleus changes with age - can have impact on gene expression
Genetic predisposition in young people
What gene is involved in bowel cancer?
Adenomatous polyposis coli APC gene
Tumour suppressor gene
Inherit one mutant copy - 100% risk bowel cancer
What are the key events in colorectal tumorigenesis?
- Mutated APC causes early adenoma
- Mutated K-ras proto-oncogene and overexpression of epidermal growth factor ….. Late adenoma
- Mutated p53 causes carcinoma
What is cancer?
Breakdown in the control of proliferation, differentiation and cell death Movement of cancerous cells to invade other tissues
What is the role of Bcl-2 in the colonic crypt?
Protects from apoptosis
Found in the base of the crypt
What is the role of TGFB in the colonic crypt?
Inhibits epithelial cell growth
Found at the top of the crypt
What is the role of Bax in the colonic crypt?
Gene involved in apoptosis
Found at the top of the crypt
As well as increased cell growth, what must there also be for tumours to develop?
Decreased cell death
What pathway is affected in colon cancer?
Apoptotic pathway
Bcl-2 overexpression
Baxmutated
Number of cells increases
How might some genetic changes that uncouple growth also confer resistance to current therapies for cancer?
Bcl-2 can protect against chemotherapy as it prevents programmed cell death
How does aspirin reduce the risk of bowel cancer?
Chronic inflammation increases risk
Aspirin is an NSAID so decreases inflammation
What do cancer cells behave like?
Embryonic cells
Display retrodifferentiation and revert back to embryonic phenotype
What behaviour do tumours acquire?
Evading apoptosis Self sufficiency in growth signal Insensitivity to anti growth signals - resistance to TGFB Tissue invasion and metastasis Limitless replication potential Sustained angiogenesis
How do carcinogens cause cancer?
Mutate growth control genes (TSG and Proto-oncogenes)
Alter gene expression
Does where you live affect your risk of developing cancer?
Yes
People who migrate from low to high incidence places, within one generation their cancer risk increases
Proof lifestyle is a factor in cancer development
What is a proto-oncogene?
Normal gene involved in normal growth control and differentiation.
Often involved in controlling the cell cycle- if they mutate or overexpress they become an oncogene
Single base change of c-ras turns it into oncogene
Overexpression of c-myc means it is an oncogene but still has normal function
What is an oncogene?
Gene whose product can act in a dominant fashion to make a normal cell cancerous
Mutant form of a proto-oncogene
2 alleles - mutate one - becomes oncogene
What are compete carcinogens?
Produce tumours on their own
Don’t need extra chemicals such as tumour promotors
E.g. Radiation at high dose
What is an incomplete carcinogen?
Sometimes called initiating agents
Require subsequent exposure of the treated cells to tumour promoting agents
Damage cells in such a way that if it comes into contact with another chemical it will turn cancerous
What is a papilloma?
Benign skin tumour caused by initiation followed by tumour promotion
Some undergo tumour progression
Most regress
(Similar to adenoma in colorectal cancer)
What do tumour promoting agents do?
Not mutagenic
Cause irritation and inflammation
Alter gene expression and inhibit metabolic cooperation
Prevents communication between cell gap junctions allowing clinal expansion
Prevents normal surrounding cells restraining mutant cell
Why is clonal expansion a key part in cancer?
Increase pool of benign cells
Increase risk of another mutation
Increase development of carcinoma
Example of an initiating agent
B(a)P found in tobacco
DMBA
Example of a tumour promoting agent
TPA
Phorbol ester that causes inflammation
Activates many signalling pathways
How is B(a)P a carcinogen?
Indirect
P450 inactivates it
Metabolic activation in the liver in order to damage DNA
Chemical itself does not damage DNA
Genetic variation in individuals - enzyme to activate may be more active in one person compared to another and vice versa
How does 7,8-diol-9,10-epoxide work as a carcinogen?
Direct
Bunds directly to guanine causing mutations
Can a tumour develop if exposed to promoter before cell is initiated?
No
Must be exposed to incomplete carcinogen first
How can mutations in proto-oncogenes be detected?
Direct DNA sequencing of whole gene to see if mutated
Direct DNA sequencing of the known hotspots to be mutated
DNA transfection assays
In mouse skin carcinogenesis how is the cellular proto-oncogene c-H-ras mutated?
Depends on carcinogen
B(a)P causes codon 12 mutations
DMBA causes codon 61 mutations
Single base change causes constituitive activation as growth factor can’t bind to receptor on surface
Different carcinogens leave different DNA fingerprints
At what stage is ras mutated in morse skin carcinogenesis?
Smallest papilloma had a ras mutation - is it the initiating event?
Specificity of the mutation depended on the initiating agent and not the tumour promoter - promoter could be swapped but ras had same mutation
Initiating agents target specific DNA bases consistent with codon 12 and 61 mutations in H-ras suggesting the carcinogens target c-H-ras in skin stem cells
Take active oncogene from virus and scrape into skin - tumour
Some papillomas merely require more mutations for them to become cancerous - p53 mutant or amplified ras mutation
What is the significance of viruses causing cancer?
10-20% cancers have a viral involvement
Viruses are relatively simple hence can understand mechanisms of cancer induction
Can reduce cancer incidence by screening, vaccines and avoiding risks of infection (prostitutes)
Changing trends of cancer incidence..increased sexual activity - cervical cancer
How does a proto-oncogene become an oncogene?
Single base pair point mutation - ras
Over expression of normal gene causing high levels of normal protein - c-myc
Knowing if a gene is switched on is just as important as knowing if it’s mutated or not.
What is the product of the ras oncogene?
p21 protein which has GTPase activity
The mutant form has reduced activity
How does the ras oncogene function?
As a G protein involved in signal transduction at the cell membrane
What do ras mutations do to the gene?
Constitutively switched on to continuously signal for growth
Usually dominant acting, gain of function mutations
Does not require growth factor binding to be activated
In what cancers is ras mutated?
60-80% pancreatic cancer
50% colorectal cancer
In some ras is not mutated but pathway is activated by mutations in receptors
What is the definition of a tumour virus?
Viruses which are capable either alone or in cooperation with other agents of converting normal cells to tumour cells.
Most viruses are not oncogenic
How is acute transforming virus oncogenic?
p60src (rous sarcoma virus) oncoprotein Dominant acting Has protein kinase activity Quick at inducing tumours Virus can transform regardless of where it integrates in host DNA
How is it proven that acute virus and cellular transfections cause foci?
Isolate gene from virus and sheer it
Add calcium phosphate
Precipitate it onto normal cells in culture
DNA taken up into cells
How does avian leukosis virus cause cancer?
No oncogene
Integrates next to c-myc proto-oncogene
- insertional mutagenesis
Viral promotor drives over expression of c-myc
Why is integration site of non oncogenic viruses crucial?
Integration is random, can take a while for it to integrate into the right place in a cells DNA
Not as potent as viral oncogenes
Which two methods are a form of insertional mutagenesis?
Promoter insertion next to proto-oncogene
Viral insertion directly onto a growth control gene, mutating it. E.g. p53 or Rb TSGs and causing inactivation
What are the properties of slow transforming viruses?
No oncogene- don’t transform cells in culture
3-14months for tumour to appear
Specific integration site - KEY
Slow due to randomness of integration and time it takes
Direct effect of viral genome and not viral encoded proteins
What type of secretion do tumour cells respond to?
Autocrine secretion
Decreased growth factor requirement as produce their own
What are the requirements for growth factor action?
Specific receptors
Some secreted latent and require activation
Response to a GF may be determined by what other GF are present
More than one GF to a receptor
Can have a positive (EGF) or negative (TGFB) effect
Do all cells respond in the same way to GF?
No
TGFB stimulates normal fibroblasts but inhibits normal epithelial cells (G1arrest)
What are the domains of the receptor/growth factor?
Receptor has outer domain
Transmembrane domain
Inner cytoplasmic domain which associates with intrinsic tyrosine kinase cascade and activates it - autophosphorylation of the receptor and other cellular proteins for stimulation of DNA synthesis
What is platelet derived growth factor?
Blood serum supported growth of connective tissue cells
Released from platelets at wounded sites
Major mitogen in serum for cells of mesenchymal origin
Fibroblasts, smooth muscle, glial cells have receptors
What is the relationship between PDGF and role in transformation?
Viral transformed cells have reduced GF need. Cancer cells conditions medium by secreting GF to promote growth
Are viral oncogenes encoding growth factors?
PDGF sequence made and compared with p28sis of simian sarcoma virus oncogene - VERY SIMILAR 104 contiguous aa showed virtual identity
V-sis may have picked up mutant version of proto-oncogene
Properties of Epidermal Growth Factor?
Single polypeptide chain
All 3 germ layers
Not restricted to epidermis
Receptor has intrinsic tyrosine kinase activity - DNA synthesis
Structural similarity between receptor and v-erb B oncogene of avian erythroblastosis - viral encodes truncated receptor
How does the viral oncogene affect epidermal growth factor?
EGF receptor is truncated so that there is no outer domain
Receptor is constitutively activated with no need for GF binding