Molecular Pathology of Tumours Flashcards
Identify the main properties of malignant cells.
- Disordered proliferation
- Disordered apoptosis
- Disordered differentiation
- Disordered relationship between proliferating cells and surrounding environment (invasion, metastasis, angiogenesis)
Describe the main different steps of cancer.
- Tissue is normal
- Dysplasia (some loss of stratification + immature cells escape from basal cell layer)
- Cancer in situ (total loss of stratification + immature cells throughout + BM intact)
- Invasion (Erosion of BM + tumour gains access to vascular channels)
- Metastasis (Cells escape from tumour via lymphatics or blood vessels)
Describe the concept of cancer clonality.
- A mutation gives one cell an advantage
- This cell survives and proliferates
- A second mutation increases the advantage
- The cell with double advantage survives and proliferates
- A third mutation further increases the advantage
However, most tumors are probably not strictly clonal; all cancer cells in a tumor seldom have the exact same set of mutations. Tumors tend to be subclonal, with all cancer cells having a single common ancestor (clonal) but with later evolution resulting in some mutations shared among all cancer cells but others restricted to sub-populations of cancer cells, which means not all cells in a tumour are genetically identical.
What sorts of genes are altered in tumour cells ?
- Oncogenes (activated)
- Tumour-supressor-gene (inactivated)
Describe the process of activation of oncogenes.
- Normal cells have proto-oncogene
- Single mutation events in proto-oncogenes create the oncogene (because DOMINANT mutation (gain of function) so only need one copy of mutation to affect phenotype)
- Oncogene then drive neoplastic behavior in cells (abnormal proliferation which persists even after stimulus withdrawn)
-Different mechanisms:
1. DIRECT MUTATIONS in coding sequence of gene leading to hyperactive protein made in normal amounts. E.G. Gly12Val ras means that RAS remains active for longer. Hence cell can signal independently of growth factor (sending signals to cell telling it to divide)
2. GENE AMPLIFICATION: multiple copies of same gene, resulting in normal protein being present but overproduced.
E.G. Over expressed growth factors Her2 in breast cancer.
3. CHROMOSOMAL REARRANGEMENT:
Either a) Regulatory part of gene X against coding region for gene Y, resulting in abnormal expression of that rearranged gene product, and thus normal protein overproduction
Or b) Fusion to actively transcribed gene produces hyperactive fusion protein (part of protein X fused with part of protein Y)
E.G. Philadelphia chromosome in MCL.
What are the different functions of oncogenes ? Give an example of oncogen for each function and the cancer they cause.
- Growth Factor: Increased number (sis = PDGF in fibrosarcoma)
- Growth Factor Receptor: Increased number/activated (HER2 in breast cancer)
- Signal Transducer: Interfere with intracellular signaling (ras in colon cancer)
- Transcription Factor: Directly stimulate cell cycle dependent transcription (myc in Burkitt’s lymphoma)
Describe the process of activation of tumor supressor genes.
- 2 copies of tumour supressor gene.
- First mutation event inactivates the first tumour supressor gene copy (Phenotypically, no problem because RECESSIVE MUTATION (loss of function))
- Second mutation event inactivates the second tumour supressor gene copy.
- Two inactivating mutations functionally eliminate tumour supressor gene, stimulating cell survival and proliferation
Describe the main features of retinoblastoma.
- Eye tumour
- Affects children
- Can affect either one or two eyes
- 2/3 cases sporadic/unilateral (older onset 2.5-3 years)
- 1/3 bilateral (early onset 9 months)
- Majority of cases survive
Why does retinoblastoma affect some patients unilaterally and other bilaterally ?
Knudson’s two hit hypothesis:
- Both copies of RB1 gene disrupted in tumor.
- May arise in both copies of the gene in the same cell by chance (rare and sporadic) in which case unilateral retinoblastoma rises
- May arise because patient has inherited one defective copy of the gene already (affecting every cell of body). In this situation only need one further mutation to occur by chance and tumor will arise. Because each eye has already had one predisposing mutation (inherited one) bilateral retinoblastoma more likely.
Why does mutating Rb drive carcinogenesis ?
- RB normally inhibits proliferation, represses action of E2F, a transcription factor required to transcribe genes such as DNA polymerase required for entry into S phase.
- At certain point in cell cycle RB becomes inactivated by phosphorylation, repression is relieved and cells can move into S phase.
- In tumours RB is inactivated so there can be expression of S phase genes and cell driven towards proliferation even when it wouldn’t normally do so.
Describe the main possible functions of tumor repressor genes. Give a couple specific examples of genes for each function, as well as what occurs following their mutation.
1) GATEKEEPER
- Inhibit proliferation or stimulate death of cells, esp. those with DNA damage
- Sends negative signals to cell
2) CARETAKER
- Maintain integrity of genome by promoting DNA repair
- Repair can be through a) nucleotide excision repair b) Mismatch repair c) DNA double strand break repair
Give examples of gatekeeper and caretaker genes, as well as the resulting condition following their mutation.
GATEKEEPERS
- APC (Adenomatous polyposis coli) - Keeps level of transcription of genes required for proliferation very low. Mutated in FAP (Familial adenomatous polyposis) and about 80% of sporadic colon cancers.
- RB1 - mutated in Retinoblastoma
CARETAKERS
- Mutations in genes responsible for nucleotide excision repair lead to Xeroderma Pigmentosa (result in excision mis-formed bases eg following uv thymine dimers)
- Mutations in genes responsible for mismatch repair lead to Hereditary Non-Polyposis Colon Cancer
- BRCA1 or BRCA2(responsible for DNA DS break repair mechanism) mutation linked with breast and ovarian cancer
What is the function of p53 ? What happens when p53 is mutated ?
Both caretaker and gatekeeper
1)
-Senses damage to DNA
2)
-Stops cells from dividing (stops cell cycle) so DNA repair can occur
-Block angiogenesis
-If cell severely damaged, will also activate cell death pathways
Overall, activates genes whose products implement these effects.
Tumour formation will likely occur if p53 is mutated.
What is the Li-Fraumeni Syndrome ?
Inherited familial predisposition to a wide range of certain, often rare, cancers, due to a mutation in p53 tumor suppressor gene.
Describe the nature of P53 mutations.
- P53 is a transcription factor (ie binds DNA at particular sequences and activates transcription of genes with those sequences such as genes involved in controlling cell cycle or repair).
- 95% of mutations occur in central region (part that binds to specific sequences in DNA)
- Most mutations are missense mutations which cause loss of function and prevent DNA binding.
What types of damage does p53 sense ?
- Lack of nucleotides
- UV radiation
- Ionizing radiation
- Oncogene signalling
- Hypoxia
- Blockage of transcription
What else, besides increased cell division, can result in a tumor ?
Decreased apoptosis
Describe the action of BCL2, how BCL2 mutation may arise, and the result of BLC2 mutation.
-BCL2 normally inhibits the cell death pathway.
-Overexpression of BCL2 may occur due to
chromosomal rearrangement, “placeing the coding sequence of Bcl-2 gene from chromosome 18 next to the immunoglobulin heavy chain locus on chromosome 14”
-Overexpression results in increased cell survival, decreased cell death and possibly a tumor (e.g. follicular lymphoma)
Outline the Volgelstein multistep model. Which cancer was this shown with initially ?
- Normal epithelial –> (tumor supressor gene lost) Excessive epithelial proliferation –> (Oncogene activated) Small tumour –> (Other tumor supressor gene lost) Large tumour –> (Third tumor supressor gene lost) Tumor becomes invasive –> (Rapid accumulation of mutatins) Metastasis
Initially, with colon cancer
What are the requirements for a benign tumor to become malignant ?
1) Angiogenesis
2) Limitless replicative potential
3) Escape from immunity
4) Additional mutations
Discuss the potential contribution of telomerase to tumorigenesis.
- Loss of telomeres associated with aging.
- Telomerase highly expressed in embryonic stem cells and cells that are dividing rapidly, but low level expression in most somatic cells.
- Telomerase helps prevent loss of telomeres. Introduction of telomerase to cells can immortalize them (=limitless replicative ability)
- Cancers sometimes express telomerase (Telomerase active tumors associated with poor prognosis)
Identify the key processes and molecules contributing to angiogenesis in tumors.
For a tumor >0.2mm to survive must have blood supply. Specific pathway triggered in response to low oxygen:
- VEGF: Cancer cells start to express vascular endothelial growth factor (VEGF)
- MPP: Those activated endothelial cells start secreting MMPs (Matrix metalloproteinase), which digest surrounding matrix
- MOTILITY AND FILOPODIA: In response to VEGF stimulus, endothelial cell also becomes motile so migrate and divide, extending filopodia that guide the development of capillary sprout
- STALK FORMATION: Leading (tip) cell moves away from capillary as cells behind it migrate in and divide and forming a stalk
- LUMEN FORMATION: Sprout then start to hollow out and form tube: pinocytic vesicle fuse with each other, and large vacuoles formed in this way then fuse with each other, forming lumen that runs through capillary stalk (but cells remain separate)
Are the blood vessels formed in angiogenesis of cancer similar to the normal ones ? Explain why or why not.
No, because much more unstable with abnormal structure and function.
10x more permeable than normal capillaries- partly due to abnormal structure but also increased amounts of VEGF around.
Identify the key processes and molecules contributing to invasion and metastasis.
- Loosening of intracellular junction: Tumor cells detach from each other due to reduced adhesiveness
- Attachment: Cells attach to BM via laminin receptors
- Degradation: Cells secrete proteolytic enzymes including type IV collegenase and plasmogen activator
- Migration: Degradation of BM and tumor cell migration follow
- Cells enter capillary, travelling through bloodstream
- Adhere to blood vessel wall in secondary location
- Escape from blood vessel to form micro-metastasis (once arrive at new site revert to epithelial like)
- Colonize secondary location to form full-blown metastasis
- Motility and invasiveness aided by loss of epithelial characteristics, and gain of mesenchymal characteristics
- Once cells arrive at new site revert to epithelial like again
Describe the steps of 1) Escape from parent tissue 2) Travel through circulation 3) Colonization of remote site in terms of how difficult the process is, briefly explaining why.
1) Escape from parent tissue: Difficult because must acquire many mutations
2) Travel through circulation: Easy
3) Colonization of remote site: Difficult, because must acquire many new mutation, especially to evade immune system (can tell if cell of the body is in the wrong place)
Identify the 7 deadly sins of cancer.
- Self-sufficiency in growth signals
- Insensitivity to growth-inhibitory signals
- Evasion of apoptosis
- Defects in DNA repair
- Limitless replicative potential
- Sustained angiogenesis
- Ability to invade and metastasise
