What 2 key events are needed for the transformation from normal to neoplastic cells?
1) Oncogene activation
2) Tumour suppressor gene inactivation
What are caretaker genes?
Genes which maintain genetic stability by repairing damaged DNA and replication errors, controlling the accuracy of mitosis - mutant forms of these genes cause genomic instability
What role do caretaker genes play in carcinogenesis?
Mutations in caretaker genes results in genomic instability - genetic instability is important for enabling specific genetic alterations to accumulate in carcinogenesis
Genetic instability is a common feature of most tumour cells
Why is genetic instability important a common feature of tumour cells?
Just clonal expansion is not enough to aquire the necessary mutations for neoplasm as the normal mutation frequency is not high enough so genetic instability is a key factor
What are the 2 types of tumour suppressor genes?
What are gatekeeper genes?
Genes which play an important role in regulating normal growth
Name 3 types of gatekeeper genes?
1) Negative regulators of the cell cycle and proliferation
2) Positive regulators of apoptosis
3) Positive regulators of cell differentiation
Carcinogens induce molecular abnormalities in tumour suppressor genes which lead to what change in function?
loss of function
What is required for TSG’s to become inactivated?
A first and a second hit - loss of heterozygosity
After the first hit, the single remaining normal copy of TSGs is capable of doing the job of 2 genes - the second hit is required for complete loss of function
The first hit in TSG inactivation is normally what kind of mutations?
A point mutation in the coding sequence of the gene
The second hit in TSG inactivation is normally one of what 3 types of mutation?
1) Chromosomal non-dysjunction
2) Epigenetic inactivation through promoter methylation
3) Mitotic recombination
What is different about TSGs in people with familial cancer syndromes?
Every cell in their body will carry the first hit mutation
Individuals with retinoblastoma carry a mutation in what gene, is this a caretaker or gatekeeper gene and what is the principle tumour?
RB1 - gatekeeper gene
Retinoblastoma is principle tumour
Individuals with Li-Fraumeni syndrome carry a mutation in what gene, is a caretaker or gatekeeper and what are the principle tumours?
p53 - gatekeeper/caretaker gene
Sarcomas and breast tumours are the principle tumours
Individuals with familial adenomatous polyposis carry a mutation in what gene, is it a caretaker or gatekeeper and what is the principle tumour?
APC - gatekeeper gene
Individuals with familial breast cancer carry a mutation in what gene, is it a caretaker or gatekeeper gene and what is the other principle tumour?
BRCA-1, BRCA-2 - caretaker gene
(breast) and ovarian tumours
Individuals with hereditary non-polyposis colorectal cancer carry a mutation in what gene, is it a caretaker gene or gatekeeper gene and what is the other principle tumour?
hMLH1, hMSH2 - caretaker gene
(colon) and endometrial cancer
What a proto-oncogenes?
Genes which promote cell proliferation, survival, angiogenesis and negative regulation of apoptosis
What is meant by oncogenes?
Proto-oncogenes which have aquired a mutation. Mutations of proto-oncogenes lead to activated versions or increased expression of proto-oncogenes - oncogenes
Mutations in proto-oncogenes lead to what change in function?
Gain of function
What 4 things do oncogenes cause?
1) Increased levels of cell proliferation
2) Cell survival
4) Inhibition of apoptosis
How many copies of proto-oncogenes need to be activated in order to induce a gain of function?
Only 1 copy of the gene needs to be activated to induce a gain of function, mutated gene is dominant to the other normal parental gene
What are the 3 mechanisms of oncogene activation?
1) Translocation of proto-oncogene from a low transcriptionally active site to an active sight - aberrant expression of the oncogene
2) Point mutation - altering an amino acid and causing it to become hyperactive
3) Amplification - by insertion of multiple copies of an oncogene get increased expression
What is the minimum number of mutations needed to transform a normal cell into a neoplastic cell?
What is the 5 step sequence of genetic alterations in colon carcinoma expression?
1) loss of APC
2) DNA hypomethylation
3) activation of K-ras
4) loss of 18q TSG
5) loss of p53
What are the 6 hallmark acquired functional capabilities of cancer cells?
1) Self-sufficiency in growth signals
2) Insensitivity to antigrowth signals
3) Tissue evasion and metastasis
4) Limitless potential for replication
5) Sustained angiogenesis
6) Evading apoptosis
What is meant by self-sufficiency in growth signals in cancer cells?
Tumour cells have the ability to grow in the absence of the positive growth factors required by normal cells to stimulate them to enter the cell cycle and divide
How do cancer cells aquire the ability to grow in the absence of positive growth factors?
Normal growth factors activate growth factor receptors which cause a cascade of signalling events which culminate in the nucleus with changes in gene expression. In most cancer cells oncogene-encoded proteins are able to trick the cells into believing it has encountered growth factors in its surroundings
What is meant by the fact that cancer cells have an insensitivity to anti-growth factors?
In normal cells once the required level of cell division has taken place they will respond to negative growth factors and leave the cell cycle, tumour cells are unable to respond to these factors
What role does the retinoblastoma (RB) protein play in resistance to negative growth factors in cancer cells?
RB protein is a key regulator of cell cycle progression by preventing progression from G1 to S phase, negative growth factors inhibit progression of cell cycle by activating the RB protein
In activation of RB protein by phosphorylation by kinase enzymes is a common event in tumours and results in resistance to negative growth factors
What is meant by the limitless potential for replication in cancer cells?
Cells have a finite replicative life span, after a number of cells division they senesce and die due to loss of DNA from the telomeres, tumour cells express telomerase that replaces the lost material and cells become immortal
How do health cells limit their life span?
Through the shortening of telomeres
How does the shortening of telomeres limit the life span of healthy cells?
Telomeres act to prevent end to end fusion of chromosomal DNA molecules, the DNA component of each telomere is composed of thousands of repeats of hexanucleotide sequence which don’t get replicated properly and some repeats are lost until eventually the ends of the chromosomes become exposed and are able to fuse with eachother resulting in karyotypic chaos which usually triggers apoptosis
Which gene is a key player in apoptosis?
What does TP53 gene code for?
A transcription factor that induces transcription of over 100 genes
What is the protein product of TP53 and what does it do?
P53 - induces cell cycle arrest to allow repair of DNA damage, but also induces apoptosis if too much damage
What is the most common genetic abnormality in human tumours?
Inherited TP53 inactivation leads to what syndrome?
How do tumour cells aquire the ability of sustained angiogenesis?
Tumours greater than 2mm in diameter require a good blood supply
Hypoxia stabilizes HIF-1 transcription factor which induces vascular endothelial growth factor (VEGF) which is an angiogenic factor
This actively recruits endothelial cells that proceed to construct new capillaries and vessels
Other than tumour growth what else is angiogenesis essential for in malignant tumour cells?
Angiogenesis is key for metastatic spread of malignant tumour cells
Epithelial cells are normally held tightly together by what adhesion molecule?
Through what 2 mechanisms do tumour cells show loss of E cadherin and what transition does this result in?
Mutation or hypermethylation of the gene
Results in the epithelial-mesenchymal transition
In colon cancer progression loss of APC gene leads to what step?
Normal epithelium to hyperplastic epithelium
In colon cancer progression DNA hypomethylation leads to what step?
Hyperplastic epithelium to early adenoma
In colon cancer progression activation of K ras leads to what step?
Early adenoma to intermediate adenoma
In colon cancer progression loss of 18q TSG leads to what step?
Intermediate adenoma to late adenoma
In colon cancer progression loss of p53 leads to what step?
Late adenoma to carcinoma
What are the 7 steps in colon cancer progression?
1) Normal epithelium
2) Hyperplastic epithelium
3) Early adenoma
4) Intermediate adenoma
5) Late adenoma
7) Invasion and metastasis
What are the clinical applications of tumour markers in screening? 2
1) Detection of tumour
2) Detection of predisposition
What is the clinical application of tumour markers in diagnosis?
Identification of type and sub-type
What is the clinical application of tumour markers in prognosis?
Certain mutations confer worse survival
What are the 2 clinical applications of tumour markers in therapy?
1) Predictive markers for therapeutic response
2) Development of targeted drugs or gene therapies
What are the 2 clinical applications of tumour markers in monitoring?
1) Response to treatment
2) Detecting relapse
Name 2 antigens which are tumour markers used for detection of tumours?
1) Prostate specific antigen (PSA) serum protein marker (although a 1/3 with raised PSA do not have prostate cancer)
2) CA-125 serum antigen for ovarian cancer - not good at detecting early stage disease
How is gene profiling used for prognosis in acute myeloid leukaemia (AML)?
Gene expression profiling of AML subtype - AML subtypes with different translocations are clearly distinguished based on differential genomic expressions from 749 probe sets - different subtypes correlate with prognosis outcome
Only patients with what gene overexpression have Herceptin treatment for breast cancer and why?
HER2 overexpression (found in 30% of breast tumours)
Herceptin is an Ab drug targeted to HER2 and dampens the effect of an overactive HER2 receptor
Herceptin will thus have no beneficial effect for patients with overactive HER2 receptors
What effect does overexpression of HER2 in breast tumours have?
Makes cells more responsive to, or independent of positive growth factors