Oncogenes and tumour suppressor genes Flashcards
describe how the mutations oncogenes and tumour suppressor genes acquire have different effects but the same outcome
mutation in oncogenes make the gene permanently active
mutation in TSG make turn the gene off
these both result in the formation of a tumour
describe how oncogenes gain function
alleles within the protooncogene gain ONE MUTATION = activation of oncogene
An oncogene will produce protein products in large quantities or with increased activity meaning they will act in a dominant manner = uncontrolled cell proliferation
describe the discovery of rous sarcoma virus
• Frances Peyton Rous discovered Rous sarcoma virus (RSV)
• He took the sarcoma from the original chicken and injected it into a healthy chicken, a few weeks later the healthy chicken developed tumours (sarcoma)
• The filter he used excluded bacteria BUT was not small enough to exclude viruses, therefore he concluded that a virus must be responsible for the tumour formation (sarcoma)
• Therefore his discovery was that this sarcoma was transmissible through viruses
Thus = Rous Sarcoma Virus
describe the discovery by Bishop and Varmus
oncogene hypothesis
oncogene hypothesis: an oncogene is any cellular gene that upon activation can transform cells
in their research they found that in the rous sarcoma infection:
• V-src = oncogene = cancer causing
• C-src = proto-oncogene was present in the genome of many species
• C-src gene was normally involved in the positive regulation of cell growth and cell division
HOWEVER following infection that the v-src oncogene was highly expressed, leading to:
- Uncontrolled host cell growth
- Unrestricted host cell division
- Cancer
mechanism of rous sarcoma infection
- Reverse transcription of virus to form dsDNA provirus
- Provirus integrated into sequence adjacent to c-src (protooncogene)
- This is then packaged into v-src (oncogene) = responsible for causing cancer
What did Bishop and Varmus discover about protooncogenes?
found that proto oncogenes are normal genes that can control cell growth
BUT
following exposure to carcinogens (chemicals, viruses, physical) they can become oncogenes = leading to uncontrolled growth and tumour formation
how are viral oncogenes transmitted?
Viral oncogenes can be transmitted by either DNA or RNA viruses.
what is the oncogene mechanism in Epstein-Barr virus?
Epstein-Barr virus can produce its own oncogene called LMP-1 which is ALWAYS switched on and feeds into survival pathways, therefore it promotes tumorigenesis
what effect do DNA viruses have on the host cell
DNA viruses can cause lytic infection leading to the death of the cellular host
OR
can replicate their DNA along with that of the host and promote neoplastic transformation (tumour formation)
difference between mechanism of DNA viruses and RNA viruses in oncogenesis
DNA VIRUSES:
- encode various proteins and (along with environmental factors) can initiate and maintain tumours
RNA VIRUSES:
- integrate DNA copies of their genomes into the genome of the host cell and as these contain transforming oncogenes they induce cancerous transformation of the host.
what changes occur in proto oncogenes to become activated oncogenes
Different oncogenes have different mechanisms by which they are switched on
alteration to their sequence
· Mutations
Point mutation or deletion = alters structure or function of protein encoded by gene
· Insertions
· Amplifications
Gene duplication = increased synthesis of encoded protein
· Translocation
Increased synthesis of encoded protein
only ONE allele needs to be altered to activate the oncogene
oncogene: L-myc
- name the type of cancer
- name the activation mechanism
lung cancer
amplification
oncogene: PI3K
- name the type of cancer
- name the activation mechanism
Breast cancer
amplification
oncogene: PML
- name the type of cancer
- name the activation mechanism
leukaemia
point mutation
oncogene: B-raf
- name the type of cancer
- name the activation mechanism
melanoma
point mutation
what do proto oncogenes encode and what is the function of the proteins they produce
Protooncogenes encode components (proteins) of the growth factor signal transduction pathway.
The majority of oncogene proteins function as elements of the signalling pathways that regulate cell proliferation and survival in response to growth factor stimulation.
Protooncogenes encode components (proteins) of the growth factor signal transduction pathway. What are the four types of proteins involved in the transduction of growth signals?
- Growth factors
- Growth factor receptors
- Intracellular signal transducers (eg- Ras oncogene family)
- Nuclear transcription factors (eg-MYC oncogene family)
what do oncogene proteins act as in the growth factor signal transduction pathway?
- growth factors (e.g.EGF)
- growth factor receptors (e.g. ErbB)
- intracellular signalling molecules (Ras and Raf)
what is the function of Ras and Raf in the growth factor signal transduction pathway
Ras and Raf activate the ERK MAP kinase pathway, leading to the induction of additional genes (e.g. fos) that encode potentially oncogenic transcriptional regulatory proteins.
How were Ras genes identified?
Identified from the studies of 2 cancer-causing viruses:
- Harvey sarcoma virus
- Kirsten sarcoma virus
These viruses were discovered originally in rats hence the name Rat sarcoma
What are Ras proteins?
GTP binding protein that is normally bound to GDP in a neutral state (in its inactive form)
Ras-GDP = inactive Ras-GTP = active
How are Ras proteins activated?
which codons are the mutations in? what does this alter? what type of cancer can each mutation lead to?
Oncogenic activation of Ras is seen in about 30% of human cancer
Most commonly activated by mutated oncogene
Point mutations in codons 12, 13 and 61
- Glycine –> valine = bladder carcinoma (valine instead of glycine)
- Glycine –> cysteine = lung cancer (cysteine instead of glycine)
what is the NORMAL function of Ras in a cell
- Binding of extracellular growth factor signal.
- Promotes recruitment of RAS proteins to the receptor complex.
- Recruitment promotes RAS to exchange GDP (inactive RAS) with GTP (active RAS).
- Activated RAS then initiates the remainder of the signalling cascade (mitogen activated protein kinases).
- These kinases ultimately phosphorylate targets, such as transcription factor to promote expression of genes important for growth and survival.
GTPase activity of RAS hydrolyses GTP to GDP fairly quickly, turning itself “off”.
what happens to Ras function as a result of point mutation
Point mutation results in loss of GTPase activity of the Ras protein normally required to return active Ras —> inactive Ras.
This results in hyperactivity of Ras protein. Some specific mutations in the Ras gene are characteristics for specific cancers.
what is the specific mutation in Ras gene that are characteristic for bladder cancer?
A point mutation in codon 12 that results in the substitution of valine INSTEAD of glycine. This is a characteristic of bladder cancer.
what are the 3 members of the MYC oncogene family
and what are they all?
- C-MYC (encodes c-Myc)
- MYCN (encodes N-Myc)
- MYCL (encodes L-Myc)
these are all transcription factors
MYC oncogene family
where were they originally discovered
Avian myelocytomastosis virus (AMV)
MYC oncogene family
MYC function?
Major downstream effectors of MYC include those involved in:
• ribosome biogenesis
- protein translation
- cell-cycle progression and metabolism
- orchestrating a broad range of biological functions, such as:
- cell proliferation
- Differentiation
- Survival
- immune surveillance
MYC oncogene family
what does MYC encode?
MYC activation and involvement in cancer?
MYC oncogene overexpressed in the majority of human cancers
It encodes a helix-loop-helix leucine zipper transcription factor that dimerizes with its partner protein, Max, to transactivate gene expression.
MYC is activated by chromosomal translocation = overexpression of MYC gene
what oncogenic activity is usually observed in Burkitt’s lymphoma? (which MYC oncogene family member?)
Oncogenic activity of c-myc is commonly observed in Burkitt’s lymphoma
what virus is usually observed in Burkitts lymphoma?
Epstein Barr virus is associated with Burkitt’s lymphoma
who is usually affected by Epstein Barr virus
Effects children age 2-16 yrs (aggressive tumour)
In central Africa children with chronic malarial infections = reduced resistance to Epstein Barr virus
what translocations do people with Burkitts lymphoma usually carry
All Burkitt’s lymphoma cases carry 1 of 3 chromosomal translocations:
- Chromosome 2
- Chromosome 14
- Chromosome 22
any of the above with Chromosomes 8
describe the translocations that occur in Burkitts lymphoma and what effect this has on Myc
In ALL 3 translocations, a region from 1 of the 3 is fused to a section of chromosome 8
In these translocations, the myc gene is moved to be under the control of the Ig heavy chain
Movement from Chr2, 14 or 22 onto fusion with chromosome 8 = switches myc on constantly
what chromosomes are found n nearly all chronic myelogenous leukaemia
Philadelphia chromosomes are found in nearly all CML patients
chronic myelogenous leukaemia
What is the Philadelphia chromosomes?
Product of chromosomal translocation between ABL and BCR
This fusion results in enhances tyrosine kinase activity of the oncogene = abnormal proliferation
chronic myelogenous leukaemia
Therapeutic strategies?
Tyrosine kinase inhibitors
Such as imatinib (Gleevac) = 95% remission in early stage patients
Briefly describe the discovery of tumour suppressor genes
Fusion of normal cell with tumour cell forms a nontumorigenic hybrid cell.
Genes derived from the normal parent acted to inhibit or suppress tumour development
Therefore concluded that their must have been a factor in the normal cell that was transmitted to the tumour cell to suppress its tumorigenic activity.
differences between oncogene and TSG
- consequence of mutation
- type of mutation
There are 2 major classifications of mutated genes that contribute to carcinogenesis:
- Oncogenes
Mutated gene whose protein product is produced in higher quantities or whose altered product has increased activity
Mutation in ONE allele is sufficient for above effect (dominant)
- Tumour suppressor genes
Gene in which the mutation has caused a loss of function = no suppression
Mutation in BOTH alleles required for above effect (recessive)
oncogenes
- dominant or recessive?
dominant
BECAUSE
Mutation in ONE allele is sufficient for effect
TSG
- dominant or recessive?
recessive
BECAUSE
Mutation in BOTH alleles required for effect
What are tumour suppressor gene products?
and their function
- RB
- P53
Tumour suppressor gene products act as stop signs to uncontrolled growth and therefore may:
- Inhibit cell cycle
- Promote differentiation
- Trigger apoptosis
What causes the loss of tumour suppressor gene function?
Inactivation of BOTH alleles of the gene (tumour suppressor genes are recessive genes)
Inactivation can be as a result of mutation OR deletion
Explain the role of tumour suppressor gene products in the cell cycle (RB1)
• Retinoblastoma 1 (RB1) is a regulator of the cell cycle checkpoints
RB-1 is involved in cell cycle regulation
When this protein (RB-1) is inactivated it can be found in different types of cancers. Name 3 of these cancers
retina
lung
breast
What is retinoblastoma?
A rare childhood cancer that affects the retina
Develops when the immature retinoblasts continue to grow very fast and do NOT turn into mature retinal cells
What causes retinoblastoma?
Which Chr is the mutation found on
RB1 (the tumour suppressor gene) causes retinoblastoma when BOTH COPIES are mutated
The hereditary mutation is on chromosome 13, the retinoblastoma 1 (RB1) gene
How can you tell if a child has retinoblastoma? and what is the specific name for this presentation
An eye that contains a tumour cell will reflect light back in a white colour
This appearance is called leukocoria
What are the 2 forms of retinoblastoma?
Familial (inherited) = (40% of cases)
Sporadic = (60% of cases)
mutations in RB1 results in..
retinoblastoma
mutations in BRCA2 results in..
Breast cancer
mutations in CX26 results in..
autosomal recessive neurosensory deafness
mutations in ATP7B results in..
Wilson disease
2 HIT HYPOTHESIS
KNUDSON
describe what he found in hereditary patients
tumour formation requires a mutation in both alleles of the RB gene.
Hereditary (40% of cases) patients: the first mutations (HIT 1) in one of the alleles were inherited and the second mutation (HIT 2) in the other allele was acquired after birth = loss of heterozygosity = TUMOUR early in life
2 HIT HYPOTHESIS
KNUDSON
describe what he found in sporadic patients
tumour formation requires a mutation in both alleles of the RB gene.
Sporadic (60% of cases) patients: no inherited mutation, both mutations were acquired after birth. These patients presented at a much later age.
what mutation is required in order to prevent the expression of RB protein
2 separate mutations, one in each of the 2 retinoblastoma alleles are needed to inactivate the 2 copies of the Rb allele and prevent expression of the RB protein
what are the 3 members of the Rb gene family
- Rb/(p105/110)
- p107
- Rb2/p130
- Rb/(p105/110)
- p107
- Rb2/p130
what are these proteins collectively known as
pocket proteins
pRB (protein retinoblastoma) is the product of which gene?
retinoblastoma tumour suppressor gene
what is the function of pRB
pRB is a multifunctional protein
bind to transcription factors to either inhibit and induce transcription factor activity.
acts to regulate apoptosis and the cell cycle
pRB is considered a transcriptional cofactor. What can this then go on to do?
It is considered as a transcriptional co factor that can bind to transcription factors to either inhibit and induce transcription factor activity.
protein-protein interactions facilitate the function of pRB. what proteins interact with pRB?
Its main binding partner is E2F transcription factor, pRB binds to E2F via its large pocket
Other viral oncoproteins can also bind to pRB
regulation of pRB activity
When RB is DEPHOSPHORYLATED/HYPOPHOSPHORYLATED it is ACTIVE and remains bound to E2F
When RB is active it can inhibit cell proliferation by blocking the progression of the cell to S phase
Cyclin D/CDK4/6 and Cyclin E/CDK2 both phosphorylate the RB = RB unbinds from E2F = inactive RB + E2F can induce transcription of genes required for cell cycle progression
Therefore, RB activity is regulated by phosphorylation via CDK/cyclin complexes
role of RB in terms of E2F activity
Regulates the activity of the E2F transcription factor crucial for the expression of genes required for protein products needed in S phase (eg- thymidine kinase)
What is the role of pRB in the cell cycle?
Main function of pRB is to regulate the cell cycle by inhibiting the G1 —> S phase
Remember: Cyclin E-CDK2 = progression from G1 —> S phase
Passage of a cell through the cell cycle is regulated via cyclins and their associated CDK (cyclin dependent kinase)
what is the first cyclin to be synthesised and what is its role
Cyclin D (along with CDK4/6) is the first cyclin to be synthesised and drives progression through G1
what happens at the G1 checkpoint
The G1 checkpoint leads to cell cycle arrest in response to any DNA damage
what is a key substrate for cyclin D and what is its role?
A key substrate for cyclin D is the pRB
Cyclin D and E families (along with their CDKs) can hyperphosphorylate RB = inactive RB = E2F migrates to the nucleus and can induce transcription of genes required for cell cycle progression from G1 —> S phase (eg- transcription of genes required to produce S phase proteins such as DNA polymerase, thymidine kinase and PCNA)
what are the 3 ways in which pRB can be inactivated
genetic mutation
viral oncoprotein binding
phosphorylation (this is reversible-normal process)
3 ways in which pRB can be inactivated
describe genetic mutation
pRB is functionally inactivated by mutations of partial deletions
3 ways in which pRB can be inactivated
describe viral oncoprotein binding
Viral inactivation found in small DNA tumour viruses mainly by disrupting E2F binding OR destabilisation of RB
- Adenovirus- E1A
- Papilloma- E7
- Polyoma- large T antigen
Describe what happens to RB function in cancer cells
In cancer cells RB phosphorylation is deregulated throughout the cell cycle
As a direct consequence E2F transcription factors can induce the deregulation of the cell cycle
Without RB on watch, cells can move through G1 into S phase and are not checked for things such as DNA damage
Describe how P53 can induce apoptosis
In response to DNA damage (if the cells are unable to repair the DNA) P53 can induce apoptosis
P53 is a transcription factor that can activate the expression of genes
A target gene for P53 is the Bax gene
• DNA damage that cannot be repaired = intracellular stress
• Activation of P53
• P53 acts as a transcription factor and activates the expression of Bax gene = more Bax protein
• This means there will be more Bax molecules present in the mitochondrial membrane
• More pores on the mitochondrial membrane = more cytochrome C can move into the cytosol
• More cytochrome c can act on APAF-1 to activate caspase 9 = inducing apoptosis
why are many cancers treated with DNA damaging drugs
DNA damaging drugs
The aim of this is to damage DNA of cancer and therefore activate P53 to induce apoptosis of the cancerous DNA/cells.
what is the most common mutation in cancer and what does this mean for the therapy method
Mutations in the P53 gene are the most common mutations in cancer
Some mutations destroy the ability of P53 to induce apoptosis
This means that the damaged DNA can go on to replicate and multiply
(this means that these cancers will be resistant to DNA damaging drugs)
describe the role of the P53 tumour suppressor gene
P53 protein is aka the ‘guardian of the genome’
It is involved in sensing DNA damage and regulating cell death/apoptosis as well as other pathways
P53 is specialised in preventing the appearance of abnormal cells
what happens to p53 in 30-50% of cancers and what does this suggest about why cancer cells do this?
P53 is mutated in 30-50% of human cancers
This suggests that tumour cells try to eliminate p53 function so that they can grow and survive
Describe the structure of p53 tumour suppressor
4 domains
- P53 is a transcription factor (because p53 central binding domain has the ability to bind to DNA)
- Protein has an amino transactivation domain, a central binding domain, a tetramerization domain and a carboxyl regulatory domain
Describe the regulation of P53 in normal cells
what is an MDM2 protein?
P53 levels are kept low in cells by MDM2 protein (a ubiquitin ligase and oncogene)
P53 should have a short half life (20 mins)
MDM2 will add a ubiquitin tag onto the lysine residues of p53 protein and this will then target the tagged p53 protein to the lysosome for degradation
Describe the activation of p53 tumour suppressor protein
- DNA damage that cannot be repaired = intracellular stress
- Stress signals are mainly sensed by kinases
- These kinases then phosphorylate P53 = disrupts the interaction between P53 and MDM2
- P53 can therefore regulate genes involved in DNA damage repair, apoptosis and cell cycle arrest
effect of ionising radiation on p53 tumour suppressor protein
Ionising radiation signals through 2 kinases ATM/ATR
activates oncogenes such as Ras to induce activity of p14arf responsible for sequestering MDM2
prevent degradation of p53 by disrupting the interaction between p53&MDM2
Outline what the research on p53 discovered and how this is useful in therapeutic strategies
Mutational inactivation = dysfunction of p53
active p53 = vital role in tumorigenesis suppression
Therapeutic strategies:
Aimed at correcting p53 mutation and restoring wild type p53 function by targeting regulators
explain gene therapy as a therapeutic strategy for cancer (based on p53 function)
retrovirus-mediated gene transfer of the wild-type TP53 gene into both human lung tumour cell lines and xenograft models could lead to the inhibition of tumour cell growth.
what are the alternative therapeutic strategies for cancer
Alternative strategies include the use of inhibitors.
Example of the use of inhibitors:
- PRIMA-1
- Nutlin
- RITA
- Inhibitors of CRM1
Describe personalised medicine’s role in cancer treatment
Uses the understanding of the individual patients tumour (as oppose to where it is located in the body). Even if 2 patients have the same tumour type, their tumour genetics will be very different. Understanding the differences will help to personalise their therapy in a more specific way so it works better for them. This also prevents exposure of treatments to patients that will not work/not work as well as other treatments.
therapeutic strategies for cancer based on p53 function
what is PRIMA-1
restores mutant p53 by modifying the thiol groups in the core domain of the protein
therapeutic strategies for cancer based on p53 function
what is Nutlin
is a potent MDM2 antagonist
therapeutic strategies for cancer based on p53 function
what is RITA
binds to p53 and can restore mutp53 activity
therapeutic strategies for cancer based on p53 function
what are CRM1 inhibitors
Inhibitors of CRM1 result in nuclear accumulation of p53
