Oncogenes And Tumour Suppressor Genes Flashcards
What are the major functional changes in cancer?
- Increased growth
- Failure to undergo apoptosis or senescence
- Loss of differentiation
- Failure to repair DNA damage
How does increased growth happen in cancer?
Loss of regulation and stimulation of environment promoting growth
What do oncogenes cause?
Oncogenes will normally make cells divide and drive cell division forward, mutations mean that they may be permanently active
- mutation gains function
What are oncogenes?
Altered gene whos product can act in a dominant fashion to help make a cell cancerous
What is the normal version of oncogenes?
Protoconcogenes
What do tumour supressor genes cause?
Tumour suppressors will do the opposite (and may also be involved in cell cycle arrest?) if you have a mutated oncogene, tumour suppressor genes may still be strong enough to stop proliferation
- mutation loses function
Explain why tumour suppressor genes need to undergo two mutations as opposed to oncogenes
Tumour suppressors become oncogenic when they lose function unlike oncogenes that become oncogenic when they gain function.
- TS genes need BOTH alleles (one on each chromosome) to be inactivated in order to cause oncogenesis
What is Rous’s protocol for inducing sarcoma in chickens?
- Remove sarcoma and break up into small tissue chunks
- Grind up with sand
- Filter and collect filtrate through a fine-pore filter
- Inject filtrate into young chickens
- Observe sarcoma in injected chickens
Name the extra gene found in Rous sarcoma virus
V-SRC
What is this gene (V-SRC)?
- V-SRC (viral SRC) is a mutated cellular gene, so in uninfected chickens there was found to be a SRC homologue that was not pathogenic.
- So some genes from cancer causing viruses are mutated forms of cellular genes that have been ‘kidnapped’ by the virus
- C-SRC is usually involved in the positive regulation of cell growth and division
How can this ‘kidnapping’ happen?
- Retroviruses convert their RNA into DNA by reverse transcriptase and then integrate this into the host genome - in one of the viruses, this integration was accidentally next to C-SRC (cellular SRC) and then there is co-transcription of the viral DNA AND C-SRC - this is then packaged into the capsid and the new RSV produced has C-SRC in it
So explain how RSV infection can lead to oncogenesis
V-SRC is expressed at high levels in the host cell leading to uncontrolled host cell growth → cancer
- I am not sure if there is an actual mutation in V-SRC or if it is the same as C-SRC but just expressed more
Discuss a different mechanism for viral oncogenesis
Viral oncogenes also exist (so not genes that have come from a host originally), these can be in DNA or RNA viruses
- 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 is the capture of c-src by retrovirus?
During evolution, the virus can acquire fragments of genes from the host at integration sites and this process results in the creation of oncogenes
What is the c-src oncogene product?
60 kDa intracellular tyrosine kinase
What can the C-Src oncogene do?
Phosphorylate cellular proteins and affect growth
Name 3 ways that mutations in a proto-oncogene can cause it to become an oncogene
- translocation or transportation to a new promoter
- gene amplification so multiple copies of the gene
- point mutation within the gene causing it to become resistant to degradation or hyperactive
What do gene duplications/amplifications do for oncogenes?
Increase synthesis of encoded proteins
Name the 4 types of proteins that are normally involved in the transduction of growth signals to a cell so different protein types encoded by proto-oncogenes
- Growth factors
- Growth factor receptors
- Intracellular signal transducers
- Nuclear transcription factors
What is the name of the pathway that RAS and RAF activate?
- ERK MAP kinase pathway
- Ictivated by growth factor receptors and leads to the induction of additional genes such as fos that encode potentially oncogenic proteins (so for proliferation)
What do the majority of oncogene proteins function as?
Elements of the signalling pathways that regulate cell proliferation and surivival in response to growth factor stimulation
Describe the RAS oncogene family - what are RAS proteins, what do they do?
- RAS proteins are small GTPases that are normally bound to GDP in a neutral state
- Is a signal transducer in proliferation, making it an oncogene
RAS is activated in one third of all cancers - this makes it the most commonly mutated oncogene
What do the majority of oncogene proteins function as?
Growth factors, growth backer receptors and intracellular signalling molecules
What were RAS oncogenes identified from?
Studies of two cancer causing cell lines
What proportion of oncogenic activation of RAS is seen in human cancer?
RAS is activated in one third of all cancers - this makes it the most commonly mutated oncogene
What codons are the mutations in on RAS oncogenes?
12, 13 and 61
Glycine → Valine = bladder carcinoma
Glycine → Cysteine = lung cancer
What do the point mutations lead to in RAS oncogenes?
Loss of GTPase acting on the RAS protein that is normally used to inactivate the RAS GDP
How do the RAS oncogene intracellular signal transducers work?
Binding of extracellular growth factor signal
→ Promotes recruitment of RAS proteins to the receptor complex
→ recruitment promotes RAS to exchange GDP for GTP (activates RAS)
→ activated RAS then initiates the remainder of the signalling cascade (mitogen activated protein kinases)
→ kinases ultimately phosphorylate targets such as transcription factor to promote expression of genes important for growth and survival
Name the 3 members of the MYC oncogene family and the protein they encode
- C-MYC → c-Myc
- MYCN → N-Myc
- MYCL → L-Myc
Describe the function of these MYC proteins
They are transcription factors that regulate the transcription of at least 15% of the entire genome
- some of the major effectors include ribosome synthesis, protein translation, metabolism, proliferation … SO they are really important!
- overexpressed in the majority of tumours
Describe how MYC is activated into an oncogene
This is NOT via a mutation, but by chromosomal translocation
- so it is placed downstream of a foreign transcriptional promoter
Where was the MYC oncogene family originally identified?
In avian myelocytomatosis virus
What are MYC oncoproteins?
Transcription factors
How much of the genome do the MYC oncogenes transcribe?
15%
What are the major downstream effectors of MYC?
Those involved in:
- Ribosome biogenesis
- Protein translocation
- Cell cycle progression and metabolism
- Cell proliferation, differentiation, survival and immune surveillance
What proportion of human cancers are affected by overexpression of MYC?
40%
What does MYC encode?
Helix-loop-helix leucine zipper transcription factor
How does MYC transactivate gene expression?
Dimerises with MAX (its partner protein)
When is MYC activated (generally)?
Comes under the control of foreign transcriptional promotors
What does activated MYC lead to?
Deregulation of the oncogene that drives proliferation as a result of chromosomal translocation
What is burkitts lymphoma?
High grade lymphoma
Explain how Burkitt’s lymphoma can progress from Epstein Barr virus
Activation of MYC by chromosomal translocation is what causes Burkitt’s lymphoma which is the tumour type you get when infected from Epstein Barr virus
Who is affected by burkitts lymphoma?
Children aged 2-16
What are the 3 chromosomal translocations associated with BL?
Parts of chromosomes 2, 14 and 22 attach themselves to chromosome 8 → lose control of MYC so uncontrolled proliferation
Which type of leukaemia accounts for 15-20% of leukaemia?
Chronic myelogenous leukaemia
What proportion of chronic myelogenous leukaemia patients carry the philadelphia chromosome?
95%
what is the Philadelphia chromosome?
An abnormality of chromosome 22 in which part of chromosome 9 is transferred to it
Which two proteins fuse in this translocation (Philadelphia) and which one is the oncogene?
- BCR (22) fuses with ABL (9)
- So tyrosine kinase activity of the oncogene ABL is constitutive, leading to abnormal proliferation
Name the treatment for CML
- Tyrosine kinase inhibitor
- This has a 96% remission in early-stage patients!
Name some different ways that tumour suppressor genes can control the cell
- regulators of cell cycle checkpoints, like RB1
- differentiation regulators like APC
- DNA repair proteins like BRCA1
What does loss of tumour supressor gene function require?
Inactivation of both alleles of the gene
Are tumour supressor genes dominant or recessive?
Recessive
When do retinoblastomas occur?
When immature retinoblasts continue to grow very fast and do not turn into mature retinal cells
How can you see retinoblastomas?
Tumour will reflect light back in a white colour
What is leukocoria?
When the tumour reflects light back in a white colour
What are the two forms of retinoblastoma?
- Familial
- Sporadic
On what chromosome and what gene does retinoblastoma have a mutation?
Chromosome 13 on Retinoblastoma 1 gene
Explain the 2 hit hypothesis, using Rb as an example
- There is familial (hereditary) and sporadic retinoblastoma.
- You need one mutation on each allele for tumour suppressor genes to become non-functional (and therefore cancerous) so we may call them recessive.
- So if you have the inherited mutation, you are much more likely to develop the disease!
- this is why sporadic tumour occurs much later on as both alleles need to be mutated
Describe the structure of Rb
Is made of 3 proteins (p105/110, p107 and p130) which are known as pocket proteins
How many binding partners does prb have?
Over 100
Describe the function of Rb - how it works and what it does
Is a transcriptional co-factor that can bind to transcription factors such as E2F by interacting with the large pocket in Rb
Regulates the cell cycle by regulating the transition of G1 to S (major checkpoint)
Which is the first cyclin to be synthesised?
Cyclin D
What does Cyclin D do?
Drives progression through G1 with CDK 4/6
So what is the function of Rb in this step through G1 - how does it do it?
- Rb protein is a substrate for cyclin D in this step
- Cyclin D and cyclin E and their CDKs phosphorylate Rb
- so phosphorylated Rb can NOT bind (is detached) to E2F, meaning that transcription can occur (as Rb is inhibiting E2F) so the cell can now progress to S phase
- DNA damage would cause de-phosphorylation of Rb so that cell can not progress through G1 checkpoint
What does the g1 checkpoint lead to if the DNA is damaged?
Arrest of the cell cycle
What is a key substrate for cyclin D?
Rb protein
What is the main binding factor in G1?
E2F transcription factor
What is Rb activity regulated by in G1?
Phosphorylation
What happens when the Rb tumour suppressor is active?
Inhibits cell proliferation
What happens when RB is dephosphorylated in G1?
Active and remains bound to E2F
What happens when RB is phosphorylated in G1?
E2F is released and migrates to the nucleus to induce transcription
What can RB be inactivated by in G1?
Phosphorylation, mutation or viral oncoprotein binding
How can viral oncogenes affect this Rb pathway?
- As well as mutations in Rb, viral oncogenes can also cause inactivation.
- This would normally be by disrupting E2F binding or destabilisation of Rb
Describe how the tumour suppressor p53 is different in function to retinoblastoma
- Rb controls the progression through the G1→S phase checkpoint of the cell cycle
- p53 on the other hand works to regulate apoptosis in the event of cell damage as well as other pathways
Describe the structure of p53 and what it is exactly
- p53 has a central domain that allows it to bind to DNA
- p53 is a transcription factor that can bind to around 300 different gene promoters
what is the prevelance of p53 mutation in cancers?
30-50%
In a normal cell, what molecule keep p53 levels low?
MDM2 protein, is a ubiquitin ligase (also an oncogene)
Describe how, with MDM2, p53 levels are regulated in a normal cell
- MDM2 binds to p53 to form a complex in the nucleus where MDM2 modifies the carboxyl terminus of p53 and targets it for proteasomal degradation
- so p53 has a short half life of 20 minutes
What is the MDM2 protein?
Ubiquitin ligase (also an oncogene)
What activates p53?
Stress signals
HOW does DNA damage/stress signals activate p53?
- ATM/ATR activated by ROS (reactive oxygen species) which activates CHK1/2 which phosphorylates p53
- phosphorylated p53 disrupts its interaction with MDM2 so it is not degraded by the proteasome
Describe some therapeutic strategies for p53 mutations
Targeting p53 to correct any mutations and restoring the wild type function by targeting its regulators
- 95% of p53 mutations are in the central DNA binding domain
We can use inhibitors to re-fold p53 into wild-type (by PRIMA-1) or regulate p53 to increase its half life in cells, we can do this with MDM2 inhibitors or CRM1 to stop nuclear export of p53
Explain why tumour suppressor genes need to undergo two mutations as opposed to oncogenes
Tumour suppressors become oncogenic when they lose function unlike oncogenes that become oncogenic when they gain function.
- TS genes need BOTH alleles (one on each chromosome) to be inactivated in order to cause oncogenesis
Describe the function of these MYC proteins
They are transcription factors that regulate the transcription of at least 15% of the entire genome
- some of the major effectors include ribosome synthesis, protein translation, metabolism, proliferation … SO they are really important!
- overexpressed in the majority of tumours
