Lecture 21 - Cancer Flashcards
What is a tumour?
A group of over-poliferating cells
What is a key characteristic of benign tumours?
They’re not able to invade neighbouring tissues and therefore are not cancerous
What is cancer?
A malignant tumour which can invade nearby tissues
What is a metastatic tumour?
Cancer that spreads to secondary sites - via travel of cancer cells through lymph or blood system
Cancer classifications: Carcinomas
- Arise from epithelial cells
- Most common and most dangerous cancers in humans
Cancer classifications: Sarcomas
Arise from connective tissue or muscle
Cancer classifications: Leukemias and lymphomas
Arise from white blood cells
What can general cancer classifications be further characterized by?
Specific cell type affected
- E.g., melanoma, small-cell lung carcinoma
What are the original theories on the origins of cancer?
- Chemicals and other agents that damage DNA (mutagens) as causes of cancer
- Viruses as causes of cancer
Gene mutations as causes of cancer: 1775
First study linking chemical exposure to cancer
- Observation that chimney sweeps have high incidences of cancer
Gene mutations as causes of cancer: 1918
First experimental evidence that chemical agents can cause cancer in animals
- Tar exposure leads to increased cancer incidence (in rabbits)
Gene mutations as causes of cancer: 1920s
Humans that work with radiation have high incidence of cancer
Gene mutations as causes of cancer: 1930s
Chemicals and radiation result in mutations in DNA
- Carcinogens = mutagens
Gene mutations as causes of cancer: 1960s
In vitro transformation of normal cells to cancer cells using chemical mutagens
What happens to normal cells grown in culture from a monolayer?
- Cells that lose contact with plate surface undergo apoptosis - anchorage dependence
- Cells divide about 30 times before replication-induced senescence or they stop dividing after reaching a uniform monolayer - contact inhibition
- Cells require added serum to survive and grow
What happens when normal cells are turned into cancer cells in vitro by adding chemicals and radiation?
- Loss of anchorage dependence
- Loss of contact inhibition
- Ability to divide indefinitely - immortality
- Reduced requirement for added serum
What was the first characterized tumour virus?
Rous sarcoma virus - RSV
What did characterized RSV lead to?
The model that cancer is caused by infectious agents
What kind of virus is RSV?
Retrovirus
In the 1960s, what was discovered about RSV?
Leads to
- Loss of anchorage dependence
- Loss of contact dependence
- Ability to divide indefinitely - immortality
- Reduced requirement for added serum
Viruses as causes of cancer: 1910
RSV virus causes chicken sarcoma
Viruses as causes of cancer: 1960s
Purified RSV can cause cell transformation in vitro
Viruses as causes of cancer: 1960s
Several unrelated viruses found to also cause cancer
Viruses as causes of cancer: 1968
Tumour virus DNA integrates into host chromosomal DNA
Viruses as causes of cancer: 1974
RSV transformation ability linked to a single viral gene - later named src
At what rate does murine leukemia virus (MLV) cause cancer compared to RSV?
MLV causes slow onset of cancer whereas RSV causes cancer quickly
What does the RSV src gene do?
Confers transformation ability to cells
What was identified in tumour-causing viruses in the 1970s?
Oncogenes
In the late 70s to mid 80s, what was searched for and not found?
“Viral origins of cancer”
- Proponents search for viruses in human tumours
What small percentage of human cancers appear to have viral origins?
Cervical cancer and some sarcomas
After chemical mutagens were identified to cause cancer (60s to 80s), what was not clear?
The understanding of how mutations could cause cancer
Unification of the viral and genetic origin of cancer theories: 1978
Varmus and Bishop identified a gene that is very similar to the viral src gene - in the genome of the infected chicken
- Even uninfected chickens have this gene
- Even animals that cannot be infected by RSV have a src gene
What experiment was carried out once it was discovered that the src related gene was also found in uninfected, non-transformed cells from other vertebrates including humans
A gene was transformed from RSV in a gene that originated in the host (chicken) and got incorporated into the virus at some point to give the virus a selective advantage and allow it to be retained in the viral genome
What did the src-related gene help with determining in terms of the cause of cancer?
Cancer is caused by the overexpression of specific genes that originated from our own genome (proto-oncogenes), that are carried in viruses (viral oncogenes)
How may slower-acting tumour viruses act?
By inserting near and causing high-level expression of host oncogenes which would only occur by chance after many rounds of cell infection
What was suggested for how chemical mutagens cause cancer?
Idea that it’s caused by converting proto-oncogenes into oncogenes
What transfection was done in determining gene mutations as the main cause of cancer?
1) Mix genomic DNA with calcium phosphate
2) Add to cells
3) DNA goes into cell and can be maintained for several generations as a extrachromosomal DNA (transient transfection), or incorporated into chromosomes (stable transfection)
4) If DNA includes a gene, it will be expressed
Gene mutations as main cause of cancer: 1972
Development of transfection technique to introduce DNA into cells
- DNA can integrate into host cell genome and genes can be expressed
Gene mutations as main cause of cancer: 1981
Use transfection method to identify the genes that are altered by mutagen to cause cancer
How were human and mouse cells used in an experiment to determine that gene mutations are the main cause of cancer?
- Transfect normal mouse cells with DNA from a human tumour
- 1 of the transfected cells is transformed into a cancer cell
- These transformed cells can be injected into a healthy mouse –> tumour formation
Gene mutations as main cause of cancer: 1982
Robert Weinberg
- Genomic DNA library from these transfected mouse cells
- Identify a clone that contains human DNA (contains human-specific Alu sequence)
- This clone contains Ras gene (mutation ras-V12 in carcinoma ras gene which makes the protein constitutively active)
What can oncogenes be present in when amplified?
Up to hundreds of copies in cancer cells
How are oncogenes activated in cancer?
1) Viral copy of oncogene expressed at high levels by viral enhancers
2) Virus integration near a proto-oncogene leading to high-level expression of the neighbouring gene by viral enhancers
- Chemical mutagens - translocation - oncogene translocates near the enhancer for a highly expressed gene
- Chemical mutagens - point mutation - leading to constitutively activated form of the protein
- Chemical mutagens - gene amplification
What is the purpose of cell fusion experiments?
Fusing a cancer cell with a normal cell to see if the cancer was caused by oncogenes
- The fusion cell should appear cancerous but in many cases appear normal
What did the results of fusion experiments indicate?
- Argued that something present in normal cells can rescue the fusion cell
- Perhaps normal cells have genes that protect them from getting tumours (and these genes are inactivated in cancer cells)
Retinoblastoma - Knudson’s two-hit hypothesis. Normal, health individual
Occasional cell inactivates one of its two good Rb genes
- No tumour
Retinoblastoma - Knudson’s two-hit hypothesis. Hereditary retinoblastoma
Inherited mutant Rd gene; occasional cell inactivates its only good Rb gene copy; excessive proliferation leading to retinoblastoma
- Most people with inherited mutation develop multiple tumours in both eyes
Retinoblastoma - Knudson’s two-hit hypothesis. Honheredirary retinoblastoma
Occasional cell inactivated one of its two good Rb genes; the second copy of Rb is rarely inactivated in the same line of cells; excessive cell proliferation leading to retinoblastoma
- Only about 1 in 30,000 normal people develop one tumour in one eye
How can loss of heterozygosity occur?
- Genetic changes: nondisjunction, chromosome loss, chromosome duplication, mitotic recombination, gene conversion, deletion, point mutation
- Epigenetic gene silencing
What is epigenetic gene silencing?
Somatically inherited chromatin modifications
What do Rb and INK4 have in common?
Both genes are tumour suppressors necessary for preventing premature entry into S-phase
What mutation do those with Li-Fraumeni syndrome have?
p53
What are most familial cancers caused by?
Inheritance of recessive mutant alleles of tumour suppressor genes
- Retinoblastoma - Rb
- Li-Fraumeni syndrome - p53
- neurofibromatosis I - rasGAP
- Familial melanoma - INK4A
Why do elephants rarely get cancer?
Although elephants live almost as long as humans (up to 70 years) and have 100x more cells than humans, their cancer incidence is 2 to 4x less than humans because their cells are more prone to apoptosis than other cells
- Elephant genome has 20 copies of the p53 gene
Is cancer caused by the activation of oncogenes or by the loss of tumour suppressors?
- RSV and transfection experiments show that a single oncogene can cause cancer, however, the resulting tumour also has many other mutations, including loss of tumour suppressor genes
- Familial cancers result from loss of a single tumour suppressor, however, the resulting tumour also has mutations in other tumour suppressor genes and mutations in oncogenes
What does cancer progressino require?
Multiple genetic events
What do most cancers derive from?
Clones
1) A single cell that acquires a somatic mutation that helps promote (benign) tumour formation
2) This cell produces a clone of daughter cells, one of wich acquires a new mutation that further promotes tumour formation
Is the loss of a single tumour suppressor or gain of a single oncogenic mutation sufficient to cause cancer?
No, multiple somatic mutations must accumulate to produce cancer
What are mutations that contribute to the progression of cancer called?
Driver mutations
What does genomic instability facilitate?
- A rapid acquisition of new mutations - increasing the likeliness of cancer developing
- Failure to arrest the cell cycle in response to double-stranded DNA breaks
No matter what genomic instability occurs, what must happen in all cases?
Checkpoint pathways must be inactivated to allow these cells to survive and keep dividing
What do DNA breaks activate?
A checkpoint that arrests in cell division
What causes genome rearrangements?
Genome instability
What did whole genome sequencing of cancers reveal?
Cases where a single chromosome has incurred massive damage, while other chromosomes are largely unaffected
- Evidence suggests that this can occur almost instantly, within a single cell division
What does chromothripsis typically result from?
One or two chromosomes being isolated in a micronucleus (e.g., failure to segregate at anaphase)
- Cytoplasmic enzymes that damage DNA can get into micronucleus
- Micronucleus lacks DNA repair proteins
- DNA replication is not regulated
- Single chromosome is rapidly pulverized and may instantly lead to alteration of oncogenes and tumour suppressors leading to cancer
What kind of genes being lost can lead to increased genome instability?
Loss of DNA repair genes
- E.g., BRCA1, ATM
What kind of genes being lost allows cells to survive despite genome instability?
Loss of DNA damage checkpoint genes
- E.g., ATM, ATR, Chk1, Chk2, p53, pro-apoptotic genes
What can the loss of SAC lead do?
Microtubule formation and genome instability via chromotripsis
What are Hanahan and Weinberg’s hallmarks of cancer?
Cancer results from the accumulation of mutations that give the cell the following capabilities:
- Self-sufficient in growth signals and/or insensitivity to anti-growth signals (e.g., ras activation, Rb loss)
- Evading apoptosis (e.g., Bcl2 overexpression, -53 loss)
- Limitless replicative potential (e.g., telomerase overexpression, Chk2 loss)
- Angiogenesis
- Metastasis
- Avoiding immune response
- Genome instability - (e.g., Loss of p53 or ATM)
*The order of acquiring these capabilities is not important but once genome instability is acquired, other mutations arise faster
