Lecture 2 - Protooncogenes and Oncogenes Flashcards
What is a proto-oncogene?
- a gene whose normal function is to control cell growth, proliferation and pro survival genes
- have a high propensity to develop into oncogenes
How are protooncogenes converted to oncogenes?
- a point mutation
- gene amplification
- chromosomal translocation
How does gene amplification cause a protooncogene to become an oncogene?
- Cell copying DNA in S phase it accidently copies it twice
- then have twice as many copies of the gene making twice as many prosurvival and pro proliferation proteins
- still the wild type protein just more of it
What is chromosomal translocation?
- where bits of a chromosome snap off and swap with other chromosome parts
What is an oncogene?
A gene which encodes a protein able to transform cells
- 100+ identified
How was the oncogene Src discovered?
- Discovered in chickens
- RSV-transducing retrovirus, RT then incorporated into host, c Src then converted to V Src form by an excision mutation. RSV potent as SRC is potent, others insert near protooncogene and act as enhancers to increase transcription.
How does Src cause cancer?
- Src contains SH2 domains which bind to phosphorylated tyrosine residues.
- in the phosphorylated state src folds up and masks the active site keeping in the inactive form
- the virus cut the c terminal off when it excised from the gene, therefore the src couldn’t be deactivated and fold up and was signalling all the time
What effect does overexpression of Src gene have on cells?
- causes cells to pile up into cellular masses
What do mutations in src correlate with?
- metastatic potential in cancers
- caused by deletion of the c terminal tyrosine
If a drug ends in ‘nib’ what is it?
Small molecule inhibitor
Name 3 Src small molecule inhibitors
Dasatinib, saracatinib, bosutinib
Overview of how small molecule inhibitors work against Src
Src is a tyrosine kinase so requires binding of ATP, small molecule inhibitors block the ATP binding site preventing downstream signalling
How does HPV cause cervical cancer?
Produces protein subunits E5, E6 and E7 instead of causing direct mutations to the normal cells
What are epidermal growth factor receptors in the context of cancer?
Important proto oncogenes which are involved in many cancers
What does binding of EGF to cells cause?
Stimulate growth through RAS/MAPK and PI3K-PKB pathways
Example of cancer type where EGF are overexpressed
HER2+ breast cancer
What are the two mutations of the same EGF receptor which can cause cancer?
- Ligand independence where there is constitutive activation of the receptor in lung cancer
- overexpression where there are more receptors in breast cancer
Overview of the EGFR signalling pathway
- EGF binds to the receptor which leads to activation of EGF receptor
- causes recruitment of intracellular signalling molecules
- Activation of Ras then occurs, which is a GTPase. SOS exchanges the GDP for GTP making Ras active.
- Ras activates kinases Raf, MEK and ERK.
- ERK feeds into the nucleus to phosphorylate transcription factors which drive expression of cyclin.
What is EGF receptor also referred to as in humans?
HER receptor (1,2,3,4)
Why is it important to know what is driving the breast cancer?
So that we can effectively treat it
- different methods of detecting receptors present
How can we detect the breast cancer status?
- via biopsy detecting
- via florescent in situ hybridisation
- to detect HER2 via HER2-ECD ELISA
Breast cancer types
HER2+
Oestrogen receptor+
Progesterone receptor positive+
Or absence of all 3 = triple negative
What do we usually target lung cancer with?
Small molecule inhibitors as it is constitutively active and ligand dependent
What do we usually target breast cancer with?
Monoclonal antibodies
What are HER2+ cells associated with in breast cancer?
More aggressive tumour phenotype, increased recurrence, increased chance of metastasis and reduced survival rate
What kind of drug ends in ‘mab’?
Monoclonal antibodies
Examples of drugs to treat HER2+ breast cancer
Herceptin (Trastuzumab)
Cetuximab
Pertuzumab
What does the E5 subunit of HPV do?
- Causes prolonged activation of platelet derived growth factor receptor (PDGFR) which can send out growth signals
What does the E6 and E7 subunits of HPV do?
E6 and E7 subunits inhibit two tumour suppressor genes pRb and p53 and decrease their function
What are the other names for EGF receptor?
HER receptor or ERBB receptors
What is the normal role of EGF/EGF receptors?
- proto-oncogenes which are usually well controlled
- stimulate cell growth when signalled to do so via their release and binding to EGF receptor
Pathways which EGF signal by
- Ras/MAPK which drives proliferation via the cell cycle
- PI3K-PKB which is pro survival
What are the 2 types of mutations which can occur in EGF receptors which can lead to cancer?
- Ligand independent mutations where a point mutation occurs and changes an amino acids within the transmembrane or intracellular kinase region, causing the receptor to be constitutively active signalling growth.
- Over expression of the wildtype gene due to gene amplification producing more receptors on the surface which can lead to increased growth of cells.
Which cancers do the 2 mutations in the EGF receptor correlate with?
- Ligand independent mutation –> lung cancer
- Overexpression –> HER2+ in breast cancer
What are the combination of transcription factors in the nucleus which drives cyclin D in the EGRF signalling pathway?
- c-fos and c-jun combine to form AP-1 which drives cyclin D
What does a c- in front of jun and fos mean?
means they have been identified as a proto-oncogenes and are frequently mutated in a lot of cancers
Overview of the point mutation of EGF receptor in lung cancer
(what is it what does it cause?)
- Point mutation of valine to glutamine can destabilise the receptor therefore it constitutively dimerises which activates the intracellular kinase domains.
- receptor is always active and always signalling for growth.
What is the name of the EGF receptor which has undergone a point mutation from valine to glutamine in lung cancer?
Neu oncoprotein
Overview of the deletion mutation of HER2/EGF receptor in lung cancer
- deletion of the extracellular domain via a mutation can lead to an unstable molecule which dimerises to stabilise
- leading to cell growth being signalled
Overview of the amplification mutation of HER2/EGF receptor in breast cancer
- amplification of HER2 receptor gene ending up with multiple copies of WT receptor making multiple copies of HER2 receptor protein on the surface of breast cancer cells
- therefore get more activation of growth receptors and therefore signalling of growth
How do lung cancer small molecule inhibitors work?
- via blocking the intrinsic kinase domain to block the ATP binding site
Overtime what happened to the responses seen in lung cancers to 1st and 2nd generation small molecule inhibitor therapies targeting EGFR?
- inhibitors no longer worked because the ATP binding site had undergone further mutations
- T790M is a mutation; threonine –> methionine
- changed structure of the ATP binding site
- blocked small molecule inhibitors from binding but ATP could still bind
- still got cellular growth
What is the mode of action of the monoclonal antibody herceptin used to treat breast cancer?
- block the natural ligand binding
- recruit immune cells
- Binding of Herceptin to HER2 induces down regulation of the receptor
- binding induces uncoupling of Src activation
- increase P10 tumour suppressor activation
- Switch off PI3K pathway which leads to prosurvival
What are the 3 examples of oncogenes we looked at?
Src, EGF family, Ras
What mutation typically occurs in Ras which gives rise to cancer?
Point mutation of an amino acid
What does the mutation of Ras cause?
- causes Ras to become constitutively active
- and lose its GTPase activity
Ras point mutations and their cancer types
Bladder - valine (12) to a glycine
Lung - valine to serine and glycine to cystine
Outline the normal function of Ras
- When stimulated, Ras exchanged GDP for GTP
- Ras activated in its GTP bound form
- Hydrolyses GTP back to GDP and during this process activates downstream signalling to drive gene transcription
Which would you target if you had mutation in EGFR and Ras?
Ras as it is further down the pathway
Always target the last one on the pathway
What normal modification does Ras have and what does this do?
- Fatty acid modification
- Tethers it to the membrane making it close to the receptor and substrate
What does peptidomimetics do?
- Inhibits the fatty acid modification of Ras so it isn’t in the vicinity of the receptor or substrate in the membrane
- However, wasn’t effective in humans
What is the name of the drug which was more efficient in targeting Ras?
Sotorasib
What is the mode of action of sotarasib?
- First selective K-Ras inhibitor, blocking the glycine to cysteine
- targets this mutation which is common in lung cancer
- however it is selective for this mutation so wouldn’t work on others
What is Herceptin sometimes given with?
- in combination with chemotherapy- doxorubicin and cyclophosphamide or paclitaxel
What is the issue with Herceptin treatment? (side effects)
- HER2 receptors in the heart which can be problematic
- cardiotoxicity
- build up of resistance
EGFR Lung cancer inhibitors
- Gefitinib/- erlotinib
- 2nd Lapatinib
- 3rd Rociletinib
What combination of therapies might you target lung cancer with caused by mutations in the EGFR pathway?
Sotorasib (if Ras is mutated) and Pembrozilubmab (PD-1 CPI)
