21. Carcinogenesis - molecular hallmarks of cancer cells Flashcards
What happens due to a carcinogen when normal cells are turned into a neoplasia?
- oncogene activation
- tumour supressor gene inactivation
How do mutations occur?
- induced by carcinogens
- arise as random spontaneous replication error in a cell population
What is the basic process of mutations and clonal expansion?
- initiating mutation
- clonal expansion (more cells with the mutation)
- second mutation + clonal expansion
- third mutation + clonal expansion (more cells with multiple mutations)
etc
What do caretaker genes do?
Maintain genetic stability by repairing damaged DNA and replication errors
- DNA repair genes
- controlling accuracy of mitosis
What do mutated forms of caretaker genes cause?
Genomic instability
What is genetic instability important for?
Enabling specific genetic alterations to accumulate in carcinogenesis
Cells require a greatly enhanced level of mutational frequency in order to become malignant. In some tumours, what is this enhanced level of mutation the result of?
High level of exposure to mutagenic agents, such as lung cancer and tobacco smoke or skin cancer and UV irradiation
In most tumours, level of exposure to mutagenic agents is not high enough for transformation. What is found in those tumours?
Mutational inactivation of genes that are involved in protecting the integrity of the genome (caretaker genes) - this results in higher levels of mutation than normal
How are caretaker genes different from TSGs?
Caretaker genes maintain genetic stability and suppress carcinogenesis.
TSGs act as drivers of carcinogenesis when inactivated
What are 2 types of TSG?
- gatekeepers
- caretakers
What do gatekeepers do?
Play important roles in regulating normal growth
- negative regulators of the cell cycle and proliferation
- positive regulators of apoptosis
- positive regulators of cell differentiation
Carcinogens induce molecular abnormalities in TSGs. What do these cause?
Reduced/lack of protein expression or inactivation of protein - LOSS OF FUNCTION
What types of mutations occur in TSGs?
- point mutations
- deletions/insertions
- chromosomal rearrangements
- epigenetic silencing (promoter methylation)
Inactivating mutations in caretaker TSGs do not contribute directly to the tumorigenic phenotype. What do they instead do?
They create the conditions whereby mutations have a chance to arise in gatekeeper TSGs
What is normally the ‘1st hit’ in TSG inactivation?
A point mutation in the coding sequence of the gene
Every cell in the body will carry the ‘1st hit’ in what circumstances?
In the case of familial cancer syndromes
Why is a ‘2nd hit’ required for complete loss of function of a TSG ?
Because the single remaining normal copy of the TSG is capable of doing the job of two genes (the mutant version is recessive to the normal version)
How common are the types of event that causes the ‘2nd hit’ in TSG inactivation?
Three orders of magnitude more common than point mutations (1st hit)
What types of event cause the ‘2nd hit’ in TSG inactivation?
- chromosomal non-disjunction
- gene conversion
- mitotic recombination
What is the most common feature of tumour cells?
Aneuploidy caused by chromosomal non-disjunction
What is the main consequence of aneuploidy in tumour cells?
Providing the ‘2nd hit’ that completely inactivates an important tumour suppressor gene (TSG)
Chromosomal recombination through crossovers can happen in somatic cells during mitosis, as well as in meiosis. What can this create?
In the presence of a ‘1st hit’, it can create a daughter cell that is now homozygous for this mutation and has therefore lost the function of a TSG
Familial cancer syndromes can involve inheritance of a mutant copy of what?
A gatekeeper or caretaker gene
What is the risk of cancer for a carrier of a mutant copy of a gatekeeper/caretaker gene?
70-90% lifetime risk of developing cancer, depending on the syndrome
What TSG gene is involved in retinoblastoma cancer syndrome?
RB1 (gate)
What TSG gene is involved in Li-Fraumeni cancer syndrome?
p53 (gate/care)
What TSG gene is involved in Familial adenomatous polyposis cancer syndrome?
APC (gate)
What TSG gene is involved in Familial breast cancer syndrome?
BRCA1, BRCA2 (care)
What TSG gene is involved in Hereditary non-polyposis colorectal cancer syndrome?
hMLH1, hMSH2 (care)
What are the principal tumours in Li-Fraumeni syndrome?
Sarcomas
Breast
What are the principal tumours in FAP?
Colorectal
What are the principal tumours in HNPCC?
Colon
Endometrial
What are the stages from proto-oncogene to cancerous cell?
- proto-oncogene
- cancer-promoting agent (UV light, chemicals etc)
- oncogene
- cancerous cell
What do proto-oncogenes do?
Promote cell proliferation, survival, angiogenesis, and negative regulation of apoptosis
What do mutations in proto-oncogenes lead to?
Activated versions of or increased expression of proto-oncogenes (gain of function)
Oncogenes
What do oncogenes do?
Cause increased levels of cell proliferation, survival, angiogenesis and inhibition of apoptosis
How many copies of the oncogene needs to be activated to induce a gain in function?
Only 1 copy
Mutated gene is dominant to the other normal parental gene
What are the different types of mechanism of oncogene activation?
- translocation
- point mutation
- amplification
Translocation is one of the possible mechanisms of oncogene activation. Explain this works
Translocation of a proto-oncogene from a low transcriptionally active site to an active site = aberrant expression of the oncogene
Point mutation is one of the possible mechanisms of oncogene activation. Explain how this works
Substitution of a single base pair can alter an amino acid in the protein causing it to become hyperactive
Amplification is one of the possible mechanisms of oncogene activation. Explain how this works
Amplification by insertion of multiply copies of an oncogene = increased expression
What is involved in multi-step tumorigenesis?
- multi-step process involving activation of oncogenes/inactivation of TSGs
- minimum of 3 genetic alterations needed to transform a normal cell into a neoplastic cell
In multi-step tumorigenesis, how many genetic alterations are needed to transform a normal cells into a neoplastic cell?
At least 3
What are the acquired functional capabilities of cancer cells?
- self-sufficiency in growth signals
- insensitivity to antigrowth signals
- tissue invasion and metastasis
- limitless potential for replication
- sustained angiogenesis
- evading apoptosis
Cancer cells have self-sufficiency in growth signals. How is this different to normal cells?
Normal cells require the stimulus of positive growth factors before they enter the cell cycle and divide. Tumour cells acquire the ability to grow in the absence of these factors
What occurs in growth-stimulatory signal transduction?
Signals are processed and integrated by complex circuits within the cell, which decide where cell growth and division is appropriate or not. Signal transduction is the passage of these signals from outside the cell, through the cell membrane, across the cytoplasm and into the nucleus, where changes in gene expression can take place
What are signals passed between cells carried by and what do they activate?
Small proteins called growth factors (GFs)
These bind to and activate complex proteins straddling the cell membrane called growth factor receptors (GFRs)
What do activated GFRs do?
They stimulate a cascade of signalling events that culminate in the nucleus with changes in gene expression
What happens in terms of growth factor signalling in most cancer cells?
There is deregulation of signal transduction
Oncogene-encoded proteins are able to trick the cell into believing that it has encountered growth factors in its surroundings and as a result, the cells will begin to proliferate
Name one of the most common oncoproteins
RAS
A membrane bound signal transduction protein
Which family is the oncoprotein RAS part of?
It is a member of a family of guanine nucleotide binding proteins called G-proteins
RAS will be in what form in a normal cells that is not proliferating?
It will be in its inactive form, bound to a GDP molecule
What happens to RAS when a stimulatory signal is received?
The RAS protein discards the GDP and acquires a GTP molecule instead. The shifts the RAS into its active signal transducing configuration
How does RAS become inactive again after the stimulatory signal subsides?
It cleaves a phosphate group from the GTP, which reverts the protein into its inactive GDP-bound state
What occurs in cancer cells that carry an activated RAS oncogene?
Mutations alter one of the amino acids involved in the cleaving of the phosphate group from GTP. This means that an activated RAS cannot revert back into its inactive state. This fools the cell into believing that a continual positive growth signal is being received from its surroundings.
Cancer cells have an insensitivity to antigrowth signals. How is this different from normal cells?
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 do RB protein do?
It is a key regulator of the cell cycle by preventing progression from G1 to S phase
What is RB protein activated by?
Negative growth factors - they inhibit progression of cell cycle by activating Rb protein
Inactivation of RB gene is a common event in tumours. What does it result in?
Resistance to negative growth regulation
What does RB protein stand for?
Retinoblastoma protein
What does RB do in a non-proliferating cell?
It binds to and suppresses the activity of transcription factors whose function is to switch on genes that are required for proliferation ie. progression through the cell cycle
What happens to the RB protein in a proliferating cell?
It is phosphorylated and therefore inactivated by kinase enzymes that have been switched on via a proliferation signal transduction pathway
Give an example of a negative growth factor
Transforming growth factor beta (TGFbeta)
confusing name!
How do negative growth factors activate RB protein?
The stimulate the expression of proteins that inhibit the kinase enzymes
How can cancer cells escape inhibition of proliferation by negative growth factors?
They can acquire mutational inactivation, or epigenetic silencing, of the RB tumour suppressor gene.
Cancer cells have a limited potential for replication. How is this different from normal cells?
Normal cells have a finite life span. After a number of cell 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
What do cancer cells express that allows them to become immortal?
Telomerase
In normal cells, what shortens after each cell division?
Telomeres (the chromosome ends)
How many times are normal cells able to divide before the enter a state called senescence?
No more than 50-60 times - after this, they can survive for an extended period of time but are unable to re-enter the proliferative cell cycle
What can telomeres be compared to?
The aglets at the end of shoelaces
Telomeres act to prevent end to end fusion of chromosomal DNA molecules
What is the DNA component of telomeres composed of?
Thousands of repeats of hexanucleotide sequence
Why is a section of telomere lost from each chromosome after every cell division?
The tips of chromosomes cannot be replicated completely and a section of the hexanucleotide sequences are lost
How do loss of sections of telomere eventually causes apoptosis of the cell?
As eventually the ends of chromosomes become exposed and are able to fuse with each other resulting in karyotypic chaos, which usually triggers apoptosis
Telomere regeneration can be accomplished by which enzyme?
Telomerase
Can replace lost telomere sequences using an RNA template
Is there telomerase expression in normal cells?
Normally expressed early in embryogenesis but expression is lost in cells once they differentiate
How many human tumour cells express telomerase?
85-90%
Cancer cells can evade apoptosis. How is this different to normal cells?
Normal cells have an inbuilt protection against abnormal stress or growth called programmed cell death/apoptosis
Tumour cells tend to lose the ability to respond in this way
What gene is a key player in apoptosis?
TP53
What does P53 do?
Induces cell cycle arrest to allow repair of DNA damage
But also induces apoptosis if there is too much damage
What does TP53 inactivation lead to?
Loss of apoptotic response
This is the most common genetic abnormality in human tumours (>50% of tumours)
Which syndrome does inherited mutation of TP53 cause?
Li-Fraumeni syndrome
Initially it was found that DNA damage was most effective at triggering P53-induced apoptosis but we now know that a range of insults are just as effective including what?
- hypoxia
- demethylation of DNA
- viral infection
- depletion of ribonucleotides
- blockage of RNA or DNA synthesis
Why is sustained angiogenesis important in cancer cells?
Tumours greater than 2mm need to stimulate a new blood supply (angiogenesis) or the cells in the middle of the growth will die from lack of oxygen/nutrients
How do cancer cells carry out angiogenesis?
Growth factors/angiogenic factors such as vascular endothelial growth factor are often produced by tumours and these stimulate growth of new vessels
How is vascular endothelial growth factor (VEGF) induced?
Hypoxia stabilises HIF-1 transcription factor, this induces VEGF
What do VEGF do?
Stimulates growth of new vessels
Actively recruits endothelial cells that process to construct new capillaries and vessels
How significant is angiogenesis?
Angiogenesis is small in dysplasia but extensive in invasive tumours
Also key for metastatic spread of malignant tumour cells
Cancer cells are often able to invade tissue and metastasise. How is this different from normal cells?
Normal cells are unable to detach from their neighbouring cells or grow into new compartments outside their own tissue
However, malignant tumour cells acquire the ability to invade new tissues and detach and migrate to other sites in the body
What are epithelial cells held tightly together by?
Adhesion molecule E-cadherin
Tumour cells which are able to invade new tissues show which feature?
Loss of E-cadherin through mutation or hypermethylation of the gene
What does loss of E-caherin result in?
Epithelial-mesenchymal transition (EMT)
Mesenchymal cells are motile and secrete proteases. What does this allow them to do?
Break through the basement membrane and invade the underlying stroma
What does metastasis involve?
The spread of malignant cells via the blood/lymphatic system to secondary sites and the formation of secondary tumours
PSA is used to diagnose prostate cancer. How accurate is it?
1/3 with raised PSA do not have prostate cancer
Not very sensitive or specific
What is CA-125 serum antigen used for?
Used in the detection and monitoring of ovarian cancer - but not very sensitive or specific
What is a better technique to diagnose and predict prognosis than serum protein markers?
Gene expression profiling
Give an example of what gene expression profiling has been used in?
Acute myeloid leukaemia (AML)
- different subtypes with different translocations - the different types correlate with prognosis outcome
Overexpression of which gene is found in around 30% of breast tumours? And what does it code for?
HER2
Codes for a positive growth factor receptor
So over expression causes cells to become more responsive to, or independent of, positive growth factors.
Which drug targets HER2 and what type of drug is it?
Herceptin
It is an antibody drug targeted to HER2
What does herceptin do?
It dampens the effects of an overactive HER2 receptor
