Molecular changes in cancer Flashcards
What are carcinomas?
Arise from epithelial cells and account for around 90% cancers.
What is an adenocarcinoma?
A cancer which has arisen from glandular tissue e.g. breast.
What are sarcomas?
Arise from connective tissue and muscle.
What are leukaemias?
Blood cell derived sarcomas
What is a benign tumour?
Cells resemble normal cells. Tend to be localised. Often surrounded by a fibrous capsule. Usually require little treatment. Surgical removal may be needed.
What are malignant tumours?
Less well differentiated than normal cells.
Grow and divide more rapidly.
High nucleus to cytoplasm ratio, fewer specialised structures.
More difficult to treat, less definition.
Invade surrounding tissues.
Enter circulation, seed at different sites - metastasis.
What are protooncogenes?
These are genes which are normally involved with cell growth control, which can be converted to oncogenes.
What are oncogenes?
Genes which when undergo GAIN OF FUNCTION mutation result in uncontrolled cell growth.
What are tumour suppressor genes?
Genes which restrain cell growth, promote cell death and promote DNA repair.
Loss of function leads to excessive growth or damaged cells.
How does the nucleus to cytoplasm ratio differ between tumour cells and normal cells?
Higher in tumour cells, there are fewer specialised structures found.
Why must both copies of a tumour suppressor gene be lost before malignant effects occur?
Two hit hypothesis.
If one copy remains, the protein can still be created.
How do TSGenes and oncogenes differ regarding the two hit hypothesis?
THH implies that both alleles that code for a particular protein must be affected before an effect is manifested. This is because if only one allele is damaged, the second can still produce the correct protein.
What are some exceptions to the ‘two-hit’ rule regarding tumour suppressor proteins?
p53 mutations can function as a “dominant negative”, meaning that a mutated p53 protein can prevent the function of normal protein from the un-mutated allele.
The transcription of tumour suppressor genes can be blocked by ________ of ______ residues in the ______ region.
Transcription of Tumour suppressor genes can be blocked by methylation of cytosine residues in promoter region.
What is epigenetics?
The study of changes in organisms caused by modification of gene expression rather than alteration of the genetic code itself.
Epigenetic changes usually involve modifications to________. Also related are: _____
Epigenetic changes usually involve modifications to histone proteins. Also related are: miRNAs.
Cancer cells usually contain _____ mutations.
3-7.
What are the lifestyle factors for cancer development? [4]
- Environmental exposure
- Occupation
- Smoking = 40% cancer deaths. Tobacco smoke contains ~81 carcinogens.
- UV/Radiation exposure
What type of diet is best for cancer prevention?
Mediterranean, eastern is worst.
What is a proto-oncogene?
A proto-oncogene is an unmutated gene functioning normally to control cell growth, that has the potential to be mutated into an oncogene.
Ras is an example of a proto-oncogene which can be mutated via____________
Point mutation.
HER2 is an example of a proto-oncogene which can be mutated via___________
Gene amplification caused by template slipping leading to multiple copies of a gene coding for more protein.
Bcl-abl Philadephia is an example of an oncogene caused by________
Chromosomal translocations where two separate chromosomes switch locations.
What does the c- prefix to c-jun and c-fos indicate?
Indicate these are the wildtype/normal version of genes identified as proto-oncogenes.
What is the only human oncogenic retrovirus?
HTLV – Human T cell Leukaemia Virus
What is the Src gene?
It is a protooncogene, codes for the Src protein, a non-receptor tyrosine-protein kinase. The role of this protein is to phosphorylate specific tyrosine residues in other proteins, ultimately enhancing cell growth.
What is the structure of the Src protein?
Src protein contains a C-terminus tyrosine residue, which when phosphorylated, binds to its own SH2 domain inactivating the protein.
How does the Src protein become active in normal function?
The Src protein becomes active and unfolds following C-terminus tyrosine dephosphorylation by phosphatases. This goes against the normal convention of protein phosphorylation resulting in activation.
How does V-src (the protein resulting from the mutated v-src oncogene) differ from normal Src protein structure and activity?
V-src differs from normal srs in that the C-terminus is truncated, leading to an inability to enter the inactive state and thus having constitutive/basal activity, and abnormal cell growth.
What is the normal role of Bcl-abl?
Anti-apoptotic, pro-survival genes. Translocates next to genes coding for B cell immunoglobulin production, hence highly expressed and highly pro-survival.
What is HPV? (4)
- Human papilloma virus
- Oncogenic DNA virus - integrates viral DNA into host genome - permanently transforming host cells.
- Causes warts and other benign epithelial growths
- Causes cervical cancer
What are the main 3 proteins produced as a result of HPV infection?
E5
E6
E7
How does HPV have oncogenic effects?
The proteins it encodes for interefere with proteins that control cell growth.
The HPV produced E5 subunit binds to:
E5 binds to platelet derived growth factor receptor (PDGFR) causing constitutive activation.
The HPV produced E6 subunit binds to:
E6 and E7 inhibit the TSGs p53 and pRb respectively.
The HPV produced E7 subunit binds to:
E6 binds to the TSG p53.
E7 binds to pRb.
How does HPV produced E6 lead to p53 degrdation?
E6, in associated with host E6-associated protein (which has ubiquitin ligase activity), acts to ubiquinate p53, leading to proteosomal degradation.
How does HPV produced E7 result in the cell cycle progressing?
E7 competes for retinoblastoma protein (pRb) binding, freeing the transcription factor E2F to transactivate its targets, pushing the cell cycle forward.
EGFs drive cell proliferation by binding and activating _______ leading to activation of the _________ pathway and the __________ pathway.
EGFs drive cell proliferation by binding and activating EGFRs leading to activation of the Ras/MAPK pathway (cell growth) and the PI3K-PKB pathway (cell survival).
The Ras/MAPK pathway is involved with____
Cell growth regulation
The PI3K-PKB pathway is involved with____
Cell survival
What happens to the EGFR when EGF binds?
Two EGFR dimerise.
Kinase domains trans- and auto-activate.
These active kinase domains can then phosphorylate signalling molecules.
What causes the ErbB oncoprotein?
The loss/deletion of the extracellular ligand binding domain of EGF. Leading to constitutive activation of kinase domains.
What causes the Neu oncoprotein?
Point mutation leading to replacement of the hydrophobic Valine in the transmembrane region of HER2 with a charged Glutamine.
This Gln is energetically unfavourable in a lipid environment, leading to the two chains dimerising, independent of ligand presence, to minimise disruption to the membrane.
What is HER2 positive breast cancer?
This is where amplification of WT HER2 has occured. Person becomes overly sensitive to low levels of the ligand. However there is no mutation in the receptor itself.
This is caused by the template slipping leading to multiple copies of the gene or the gene being near to areas of high rates of transcription.
What is iressa?
A small molecule inhibitor that blocks ATP binding site in the kinase domain of EGFR. The first selective inhibitor developed.
What is tarceva?
Small molecule inhibitor that blocks the ATP binding site in the kinase domain of EGFR.
What is herceptin?
This is a drug that targets HER2 positive cancer only.
What percentage of women with breast cancer are HER2 positive?
25%.
HER2 + is associated with aggressive tumours and reduced survival rate.
How can we target HER2 positive breast cancer?
Herceptin/trastuzumab (recombinant humanised monoclonal antibody.
How does herceptin/trastuzumab work?
It is a recombinant humanised monoclonal antibody (MAb) against the extracellular domain of the HER2 protein.
Multiple MOAs: blocks MAPK + pI3K activation, downregulates receptor, decreases srs activation, increases PTEN activation etc.
What are the negatives to herceptin use? (4)
- Expensive MAb
- Resistance can occur if mutations alter epitope.
- Only targets HER2 positive
- Potentially cardiotoxic (expressed in heart)
What is Ras?
Ras protein family members belong to a class of protein called small GTPase, and are involved in transmitting signals within cells, subsequently switching on other proteins and ultimately genes involved with cell growth.
Mutations often occur at what positions on Ras?
Positions 12 or 61, present in many human tumours, e.g. 90% of pancreatic tumours.
Why is EGFR targeting less effective in tumours caused by Ras mutations?
Ras is downstream of the EGFR and therefore targeting the receptor itself may not be efficacious if Ras is constitutively active.
What are the less defined mechanisms of action of herceptin? [5]
- Downregs receptor
- Decreases srs activation
- Increases PTEN activation
- Induces cell cycle arrest.
- Potentially increase apoptosis through Fc region being detected by cytotoxic T cells and NK cells.
How can Ras mutations be targeted if the EGFR is not a valid target?
Ras has a fatty acid modification which tethers it to membranes.
Inhibitors of this modification (farnesyl transferase inhibitors, FTI) have been developed - these are peptidomimetics.
Don’t work well in humans due to compensatory mechanisms that exist.
How can mutations to Raf/ how can Raf be targeted during cancer treatment?
Nexavar is a kinase inhibitor approved for the treatment of kidney and liver cancers. It inhibits Raf, but also has MOA by inhibiting VEGFR and PDGFR.
What are tumour suppressor genes?
Genes that code for proteins that normally play a role in inhibiting both growth and tumour formation.
What does loss of function of tumours suppressor genes lead to?
Tumours! Loss of growth inhibition occurs when mutations cause a loss of function of these genes.
Tumour suppressor gene mutations are normally recessive, why is this?
One intact allele is usually sufficient to inhibit unchecked growth, so mutations in both alleles would be needed for full loss of function.
What is haploinsufficiency?
Alternative mechanism suggested for particular tumour suppressor genes.
One normal allele produces half (haplo-) of the quantity of protein produced by normal cells and this is not enough to suppress tumour formation. Particularly occurs in genes that regulate DNA repair and the DNA-damage response.
What is the APC hereditary mutation?
Precancerous intestinal polyps, increased risk of colon cancer.
What is the BRCA1 hereditary mutation?
60% probability of inheriting breast cancer compared to 2% with two WT alleles.
What is the retinoblastoma protein?
pRb is a tumour suppressor protein that is dysfunctional in several major cancers. Gene name: RB or RB1.
What is the function of pRb, the retinoblastoma protein?
- Prevent excessive cell growth by inhibiting cell cycle progression until a cell is ready to divide. When the cell is ready to divide, pRb is phosphorylated, becomes inactive and allows the cell cycle to progress.
- It is a recruiter for several chromatin remodeling enzymes such as methylases and acetylases.
How many forms of retinoblastoma are there?
2.
One is a familial form which is inherited.
The other is a sporadic form.
60% inherited, 40% sporadic.
Why do those sufferers of inherited retinoblastoma have an increased risk of developing other cancers later in life whereas those with sporadic retinoblastoma do not?
In the familial form, a mutated allele is inherited along with a normal allele. In this case, should a cell sustain only one mutation in the other RB gene, all Rb in that cell would be ineffective at inhibiting cell cycle progression, allowing cells to divide uncontrollably and eventually become cancerous. Furthermore, as one allele is already mutated in all other somatic cells, the future incidence of cancers in these individuals is observed with linear kinetics. However, in the sporadic form, both alleles would need to sustain a mutation before the cell can become cancerous. This explains why sufferers of sporadic retinoblastoma are not at increased risk of cancers later in life, as both alleles are functional in all their other cells.
How does Rb restrict the cells ability to replicate DNA?
It inhibits the progression from G1 (first gap phase) to S (synthesis phase) of the cell division cycle.
pRb is normally bound to E2F - preventing the transcription of genes required for S phase.
a LOF in pRb means that E2F is free all the time, independent of growth factors, and cells are constantly pushed through S phase leading to uncontrolled proliferation.
How does pRb become phosphorylated during the cell cycle?
Growth factors are activated eg through ras/MAPK pathway from EGFR, this leads to transcription of cyclin D1 which binds to Cyclin dependent kinase 4, which then phosphorylates pRb.
A loss of function to p16 has what impact on the cell?
LOF p16 removes the ability of the cell to halt the cell cycle in order to repair damaged DNA. Mutations are passed on to daughter cells and accumulate.
What is p16?
p16 is an inhibitor of cyclin dependent kinases such as CDK4 and CDK6. These latter kinases phosphorylate pRB which results in progression from G1 phase to S phase.
What is p53?
An evolution to prevent tumour development. It is usually present at low levels in cells, complexed to inhibitor protein MDM2.
What impact do stress signals have on MDM2 inhibitor protein?
They inhibit MDM2 which allows the activation of p53: transcription regulation occurs.
What type of stress responses can inhibit MDM2 and thus allow p53 to function?
DNA damage
Oncogenic activity
Cytokine activation
Hypoxia
