2. Oncogenes and tumour suppressors Flashcards
What are the hallmarks of the cancer cell phenotype?
- Disregard of signals to stop proliferating
- Disregard of signals to differentiate
- Capacity for sustained proliferation
- Evasion of apoptosis
- Ability to invade
- Ability to promote angiogenesis
Outline the cell cycle
- G0 - quiescent phase (not replicating)
- G1 - cell makes sure it has enough nutrients, nucleotides etc. to replicate
- G1 checkpoint - growth arrest to ensure the genetic fidelity of the cell
- S - chromosome duplication
- G2 - check for damaged or unduplicated DNA
- M - mitosis (check for chromosome attachment to mitotic spindle)
Specific proteins accumulate/are destroyed during the cycle e.g. cyclins, CDKs, CDK inhibitors
What does the permanent activation of cyclin do?
Can drive a cell through a checkpoint
What are proto-oncogenes and how can a mutation effect it?
- Code for essential proteins involved in maintenance of cell growth, division and differentiation
- Mutation converts a proto-oncogene to an oncogene
- Protein products of oncogenes no longer respond to control influences
- Oncogenes can be aberrantly expressed, over-expressed or aberrantly active
What are the different ways a proto-oncogene may undergo a mutation and how does it affect protein production?
- Mutation in coding sequence (can be single base) - aberrantly active protein
- Gene amplification - lot more protein produced
- Chromosomal translocation (chimeric)/viral mutagenesis: viruses can insert their own DNA into ours - normal DNA transcribed at a higher rate (strong enhancer or fusion of genes - overproduction of gene)
What are chimeric genes?
Genes that are formed by combinations of portions of one or more coding sequences to produce new genes
What is translocated in Burkitt’s lymphoma?
Chimeric genes
• Swap over of genetic material can go wrong
• Promoter is translocated
• Leads to up-regulation of the other gene portion
Describe the Philadelphia Chromosome as an example of chromosomal translocation in cancer
- Translocation of ABL from chromosome 9
- Translocation of BCR from chromosome 22
- BCR-ABL fusion gene => over-expression => development of cancer
List the essential activities that proto-oncogenes code for (functional classes)
- Growth factors
- Growth factor receptors
- Intracellular transducers (signalling proteins)
- Intracellular receptors
- Transcription factors
- Cell cycle regulatory proteins
- Cell death regulators
What is Ras?
- Family of GTPases
* Includes Ki-Ras and Ha-Ras - membrane-bound GTPases important in the stimulation of cell proliferation
How does Ras normally work?
- GTP binds with Ras, activating it
- Active Ras can interact with RAF and signal via phosphorylation
- It activates the kinase cascade
- This leads to the production of gene regulatory proteins
- Ras passes the signal on to other proteins within a signal transduction cascade
- Cell goes into a proliferative phase
- Dephosphorylation of GTP => GDP switches Ras off
How does a mutation affect Ras?
- Ras fails to dephosphorylate GTP
- GTP persists so Ras remains active
- Increased signalling with RAF protein
- Continuous proliferative stimulation
Which complex signalling cascade is the Ras pathway part of?
Mitogen-activated protein kinase cascade (MAPK)
How specific are Ras oncogenes?
• Detected in many human tumours, but there is still tissue specificity:
e.g. rare in breast cancer, very common in pancreatic cancer
• Tumours are commonly form specific e.g. K-RAS in lung, colon, pancreas and N-RAS in AML
• Different isoforms of the enzyme can be problematic in different tumours
• Mutation at codons 12 (Gly) 59 (Ala) and 61 (Gln) inhibit GTP hydrolysis
What do tumour suppressor genes (TSGs) do?
Regulate cell proliferation and maintain cell integrity
How many copies of TSGs in each cell and how many must be damaged to promote cancer?
• Each cell has two copies of each TSG
• Both TSGs must be damaged to promote cancer
• Mutation/deletion of one gene copy is usually insufficient to promote cancer (2-hit hypothesis)
- unless the mutant gene acts dominant e.g. p53
List the functional classes of TSGs
- Regulate cell proliferation/cycle
- Maintain cellular integrity
- Nuclear transcription factors
- DNA repair proteins
- Cell adhesion molecules
- Cell death regulators
Describe the effects, treatment and cause of retinoblastoma
- Malignant cancer of developing retinal cells
- Found in children
- Sporadic disease, usually involves one eye - usually from damage sustained through life
- Can be hereditary - unilateral or bilateral and multifocal
- Results in chalky looking eye
- Treatment is removal of affected eye
- Caused by mutation of the RB1 TSG on chromosome 13q14
- RB1 encodes a nuclear protein involved in the regulation of the cell cycle
What is Knudson’s two-hit hypothesis?
- If you get loss of heterozygosity to the second copy of the gene - you have 2 damaged copies of the TSG
- This promotes cancer
- Sporadic cancer (somatic mutation) - mutation you acquire by the process of living
What is p53?
- Cell cycle regulator (transcription factor)
- Nuclear location
- Expressed in its mutated form in >50% of all human tumours
What is APC?
- Gene involved in cell signalling
* Cytoplasmic and commonly associated with colon cancer
Which cancers do mutations in BRCA1 increase the risk of?
- Breast
- Ovarian
- Prostate
Summarise how p53 works
• p53 is activated in response to DNA damaging agents
• It increases to induce G1 arrest to resolve the problem
• Apoptosis if damage is too bad
• Then transcriptionally activates a series of proteins:
- p21 (Waf1) - binds and inhibits cyclin dependent kinases and cyclins to arrest cell cycle
- MDM2 (hDM2) - binds to and inactivates p53 (autoregulatory)
- BAX - member of the BCl-2 family, promotes apoptosis
• Mutation of p53 affects these activities - damaged cell won’t be repaired and divide with damage or may go into mitotic failure and die
How many copies of a gene do ‘inherited cancers’ usually affect?
One
How many mutated copies does it take for p53 to become dysregulated?
- One, even though it is a TSG
* Acts in a dominant manner
What proportion of p53 transforming mutations occur in DNA binding domains?
98% (40% of which are at 6 ‘hot-spots’ near the DNA-binding surface)
How do the 6 hot-spot sites involved in p53 mutations interact with DNA?
- Arg 248 and 273 directly contact DNA
* The other 4 residues stabilise the DNA binding structure of p53
How many copies of APC need to be damaged for an effect?
2 - more typical TSG
Describe the pathway that APC is involved in?
- WNT pathway
- Signal transduction pathway that leads to transcriptional upregulation in the nucleus
- APC inhibits beta-catenin - a driver of the proliferative process
- Therefore APC prevents uncontrolled growth
What does the loss of the APC gene result in?
• Familial adenomatous polyposis
• Predisposes colonic epithelial cells to a hyperproliferative state
• Causes development of hundreds of thousands of benign adenomatous polyps of the colon
- not cancer but a highly proliferative state
• Highly susceptible to colon cancer later in life (90%)
How can the effect of APC loss be treated?
- Effect: predisposition to hyperproliferative state of colonic epithelial cells
- Treatment: removal of colon in their 20s to prevent chances of getting cancer in the polyps
How does colorectal cancer develop?
- Damage to DNA of epithelium e.g. APC damaged
- Hyperproliferative polyps
- Further damage e.g. mutation of proto-oncogene
- Tumour starts to develop (adenoma)
- Damage to p53 => development of carcinoma, metastatic potential
Compare oncogenes to TSGs (respectively)
Oncogenes/TSGs
• Gene active in tumour / inactive in tumour
• Specific translocations, point mutations / deletions or mutations
• Rarely hereditary / can be inherited
• Dominant / recessive at cell level
• Broad tissue specificity / considerable specificity
• Leukaemia and lymphoma / solid tumours
