CBIO 2: Oncogenes and Tumour Suppressor Genes Flashcards
1
Q
Observe the learning outcomes of this session
A
2
Q
What are the two distinct types of genetic changes involved in tumour development?
A
- Activation of oncogenes
- Inactivation of tumour suppressor genes
3
Q
Define proto-oncogene
A
- a normal gene which, when changed by a mutation becomes an oncogene that can contribute to cancer
4
Q
Define oncogene
A
- a gene that encodes for a protein whose activation, overexpression or mutation have the potential to promote oncogenesis
5
Q
Define oncogenesis
A
- a process through which healthy cells become transformed into cancer cells (carcinogenesis)
6
Q
Define oncovirus
A
- a virus that can cause cancer
7
Q
Define retrovirus
A
- retroviruses are a class of RNA viruses that can produce double-stranded DNA copies of their genome that then integrate into the chromosome of the host cell
- They are grouped together based on how they are structured and how they replicate within a host
- An example would be human immunodeficiency virus (HIV), the virus that causes AIDS.
8
Q
Define tumour suppressor
A
- a gene that encodes for a protein which can slow down cell growth/division and promote apoptosis when needed
- The absence, repression, inactivation or mutation of tumour suppressors can promote oncogenesis.
9
Q
What do tumour suppressor genes do?
What happens when they don’t work properly?
A
- they are normal genes that slow down cell division, repair DNA mistakes or tell the cell when to undergo apoptosis
- when they don’t work properly, cells can grow out of control, leading to cancer
10
Q
How many copies of a tumour suppressor gene do we need for us to prevent the formation of tumours?
A
- as long as we have one functional copy that is able to produce enough of the tumour suppressor protein
- both copies need to be mutated for the genes to become inactivated
11
Q
What is an important difference between oncogenes and tumour suppressor genes?
A
- oncogenes results from the activation of proto-oncogenes
- tumour suppressor genes cause cancer when they are inactivated
12
Q
Give an example of some inherited abnormalities of tumour suppressor genes
A
- they cause certain types of cancer to run in families
- e.g. the APC (Adenomatous Polyposis Coli) gene
- Mutations in APC are associated with an increased risk of colon cancer in people with familial adenomatous polyposis.
13
Q
Are most tumour suppressor gene mutations inherited or acquired?
Give an example of this
A
- most are acquired, not inherited
- e.g. acquired mutations of the TP53 gene (coding for the p53 protein) have been found in more than half of human cancers
14
Q
What kind of issues could tumour suppressor gene mutations cause?
A
- loss-of-function changes in proteins
- complete loss of gene expression
- could arise as a result of gene deletion or epigenetic changes
- e.g. promoter methylation that results in loss of gene transcription
15
Q
What are the two classifications of tumour suppressor genes?
A
- Gatekeeper tumour suppressor genes:
- these genes with ‘gatekeeper’ function act to negatively regulate cell growth
- they may encode proteins that inhibit proliferation, induce apoptosis, inhibit angiogenesis or induce cell adhesion
- e.g. retinoblastoma protein (encoded by RB1) - Caretaker tumour suppressor genes:
- these genes maintain chromosomal integrity
- the cell cycle and DNA damage are closely linked so that if there is damaged DNA in the cell it should not divide
- if the DNA can be repaired, the cell can then divide, but if the DNA is irreparable, then the cell should undergo apoptosis to remove the damage it poses
- DNA damage repair genes can act as this type of tumour suppressor genes
- e.g. p53 encoded by TP53, but it has gatekeeper properties as well
16
Q
Observe this diagram of the main roles of caretaker and gatekeeper tumour suppressor genes
A
17
Q
Describe retinoblastoma
- causes
- what it is
A
- a rare type of cancer most commonly affecting children in one or both eyes
- during a baby’s early development, the retinal cells grow rapidly, then stop growing, but in some rare cases cells continue to grow uncontrollably, forming retinoblastoma cancer
- 40% of the cases arise because of the mutation in the retinoblastoma gene, RB1
- this usually affects both eyes
- the gene can either be inherited from a parent or the mutation may occur during the baby’s development in the womb
- it is not understood how the remaining 60% of cases arise, as in some cases there is no mutated gene
18
Q
Explain in detail how the tumour suppressor gene causes retinoblastoma
A
- when a cell reaches the G1 checkpoint to pass into S-phase, where DNA replication occurs, it needs to build the proteins or enzymes needed to carry out DNA replication
- this involves a protein called a transcription factor
- they bind to sequences of DNA and cause the transcription of specific genes that are turned into proteins
- at the G1 checkpoint, the most important transcription factor to start the building of proteins for DNA replication is E2F, normally bound to another protein such as retinoblastoma
- retinoblastoma’s role is to inactivate E2F protein and stop the cell’s DNA from replicating
- stopping the cells from going past the G1 checkpoint
- for a cell to divide, retinoblastoma needs to be inactivated
- so it is phosphorylated
- this allows E2F to allow cells to enter S-phase and later control the phosphorylation of retinoblastoma, the production of retinoblastoma protein
- if the retinoblastoma gene is mutated, and starts producing a protein that cannot bind to E2F, then the E2F will move the cell into S-phase
- however, if you have one functional copy of the retinoblastoma gene, you can produce sufficient retinoblastoma protein to regulate the function of E2F
19
Q
What is the two-hit hypothesis?
A
- the two-hit hypothesis that most tumor suppressor genes require both alleles to be inactivated
- applies to the majority of tumour suppressor genes
- those that carry a mutated, non-functional allele will not start developing tumours until the normal copy has been mutated in the body
- this means that people with inherited mutations get tumours at a younger age and they tend to develop multiple tumours
- those with spontaneous mutations, the person has two normal copies to begin with
- for tumours to arise, they need acquire two spontaneous mutations
- the chances of this occurring are far lower and requires more time to occur
- those with spontaneous retinoblastoma tend to develop tumours later in life and rarely have more than one tumour
20
Q
Why are mutations in tumour suppressor genes both dominant and recessive?
A
- at the cellular level, tumour suppressor gene (TSG) mutations behave in a recessive manner
- meaning that for cells to display the mutant phenotype, both copies of the TSG allele must be altered by mutation
- however, TSG mutations can behave in a dominant manner
- predisposition to retinoblastoma is passed on as a dominant trait
- inherited dominant mutated RB allele = increased susceptibility to retinoblastoma
- as they only need to gain an additional mutation for the mutant phenotype is expressed, their augmented cancer risk is dominant
21
Q
What does this diagram show?
A
- the retinoblastoma gene is mutated in the vast majority of cancers
22
Q
Why are DNA repair proteins also considered tumour suppressor genes?
A
- because mutations in their genes augment the chances of getting cancer
- and not always in a recessive fashion
- they can have indirect effects
- as higher mutation rates caused by the decreased DNA repair can cause increased inactivation of other tumour suppressor genes/activation of oncogenes
23
Q
What is p53?
- endoded by
- location of gene
- functions
A
- p53 is a transcription factor that acts in numerous pathways which prevent mutations and stabilise the genome
- it is encoded by TP53 (tumour protein p53)
- found in humans on chromosome 17
- p53 is expressed in nearly all cells
- its function is altered in the majority of cancers by:
- loss of nucleotides
- different types of radiation
- oncogene signalling
- hypoxia
- inhibition of transcription
- see diagram of function and factors that alter its function
24
Q
What percent of all cancers have a mutation in the p53 gene?
A
- about 50%
25
Q
What are the two mechanisms P53 has for stopping tumour development?
What would happen if p53 is mutated?
A
- Activating a gene that stops the cell cycle in response to DNA damage or stress
- Trigger apoptosis in damaged cells
- if p53 is mutated, mutations can be passed on and cells can divide indefinitely
26
Q
How are p53 mutations acquired normally?
A
- the majority of cancers acquire p53 mutations spontaneously
- in some rare cases, people can inherit one copy of the damaged p53 gene
- people with this develop Li Fraumeni syndrome, where approximately 50% of people develop cancer by the age of 30
27
Q
Observe this diagram of TP53 mutation prevalence in cancers
A
33
Q
How is p53 regulated?
A
- regulated by Mdm2 E3-ubiquitin:
- in normal conditions, Mdm2 keeps cellular levels of p53 low
- does this by binding to the transactivation domain of p53, which prevents it acting as a transcription factor.
- It also promotes the movement of p53 out of the nucleus into the cytosol, where p53 is bound by ubiquitin and degraded by the proteasome via the ubiquitin pathway
- in stressed cells, DNA damage triggers a group of kinases to phosphorylate p53 at various residues
- This phosphorylation of p53 prevents Mdm2 from binding to it, which has the effect of stabilising p53 protein
- also stabilised by the ARF protein, acting as a ‘decoy’:
- ARF binds to Mdm2, which means it is unable to bind to p53 and p53 is therefore undegraded and builds up in the nucleus
- (for information, ARF is an Alternate Reading Frame product of the INK4a/p16 locus)
34
Q
Study the figure of how the cell cycle is regulated
A
35
Q
Summarise the key points about the roles of RB and p53
A
- The Retinoblastoma gene, RB1, codes for the protein pRB which is a tumour suppressor.
- Tumour suppressors control cell growth, cell death and cell division.
- pRB is a negative regulator of cycle cell control.
- Low level of the tumour suppressor p53 allows the cell cycle to progress.
- Stability of p53 is controlled by MDM2.
- When stressed, p53 is stabilised by phosphorylation or inhibition of MDM2.
- p53 mutations are found in many cancers.
- Loss of p53 means there is no p53 to cause cycle cell arrest.
- Mutations of p53 can result in it being unable to bind to MDM2, causing elevated levels of p53 which inhibit target genes.
- The 2-hit hypothesis: summarised in this figure:
37
Q
Briefly describe the history of viral oncogenes
A
- In 1911, the pathologist Peyton Rous of Rockefeller University showed that a filtered cell-free extract from a spontaneous spindle cell sarcoma obtained from a Plymouth Rock chicken could cause tumours in healthy chickens (see the figure below)
- Thus, he was able to show that the cancer could be transmitted by a virus (now known as the Rous sarcoma virus [RSV
- In 1958 experiments were carried out to further investigate Rous Sarcoma Virus (RSV)
- When cells were infected in vitro with RSV, it was found that this led to changes in the cellular phenotypes (such as loss of anchorage dependence and contact inhibition, see the figure below) commonly found in cancer cell cultures but not in healthy cultures.
- This research led to the finding that RSV had the ability to transform primary fibroblasts, suggesting that it was carrying a tumorigenic factor.
- In 1976, this factor was identified as the gene v-src.
- This pivotal study was the first to demonstrate that a cellular gene could cause cancer and was the first step towards our understanding of oncogenes today.
- Since this discovery, retroviral oncogenes have played a vital role in the understanding of cancer as a genetic disease.
38
Q
Use the diagram of transformed cells to spot these attributes:
- Altered plasma membrane: e.g. blebbing of membrane
- Altered adherence: e.g. rounded shape, loss of monolayer
- Altered cell proliferation, e.g. high cell densities, immortality
A
43
Q
What are the three main types of proto-oncogene activation that can occur?
A
- Increased protein concentration
- Increased enzyme or protein activity
- Regulation loss
- Diagram summarises the first two genetic mechanisms
45
Q
Describe how the RAS protein is activated and deactivated in a normal cell
A
- in its inactive state, the RAS protein encoded by the RAS proto-oncogene is bound to a molecule of guanosine di-phosphate - or GDP for short
- For RAS to become active, this GDP must be substituted for GTP, or guanosine tri-phosphate
- The exchange is made by another protein, called GTP exchange factor
- The resulting active RAS protein then activates a number of transcription factors that in turn activate genes to promote cell cycle and cell division.
- In the normal cell, RAS protein then deactivates again.
- It does this with its enzymatic GTPase activity, which breaks down the GTP back to GDP.
- This is assisted by another protein called GTPase activity accelerating protein – GA for short.
- This inactivation is critical because it is the way in which RAS activity, and therefore cell cycle and cell division, is controlled.
49
Q
Study this diagram of what RAS mutations do
A
51
Q
Observe the list of point mutated Ras oncogenes identified in different human tumour cells
The letters H, K and N refer to the human HRAS, KRAS and NRAS respectively.
A
53
Q
Describe Burkitt’s lymphoma and how it relates to chromosomal rearrangements
A
- Burkitt’s lymphoma was the first human tumour in which the key molecular abnormality underlying tumorigenesis was found
- Cytogenetic analysis of these tumours in the 1970s identified a set of unusual chromosomal translocations
- In these tumours, a region of chromosome 8 is exchanged, by breakage and rejoining, most commonly with a region of chromosome 14 (and less commonly with regions of chromosomes 2 or 22).
- A cellular oncogene called MYC was mapped at the chromosome 8 breakpoint
- MYC is the cellular equivalent of the avian myelocytomatosis virus (an RNA virus or retrovirus) which causes carcinomas and sarcomas in chickens.
- The chromosome 14, 2, and 22 breakpoints contain the immunoglobulin heavy chain (IGH), κ light chain (IGHK), and λ light chain (IGHL) genes respectively
- Investigation of the gene structure changes caused by these translocations showed that in each case, the oncogene’s promoter was replaced by that of the immunoglobulin gene without disturbing its protein-coding region.
- The MYC gene encodes a transcription factor, which is usually itself very precisely regulated
- The Burkitt’s lymphoma translocations remove this specific regulation and bring the gene under the transcriptional regulation of the immunoglobulin gene promoters, which are designed to promote continuous expression
- This deregulation leads to incorrect (and increased) expression of the oncogene’s normal protein product, leading in turn to activation of other genes that should not be expressed, and eventually leading to cell transformation.
54
Q
Observe this list of translocations found in human tumours that cause the formation of oncogenic fusion of novel structure and function
A
- It should be noted that translocations are most frequently found in leukaemias and lymphomas.
62
Q
Describe the oncogene: MYC
- normal function/role
- mutation type and change in function/role
- cancer type or syndrome
A
- normal function/role:
- The Myc protein is a transcription factor and controls expression of several genes.
- Myc is thought to be involved in avoiding the cell death mechanism
- mutation type and change in function/role:
- The MYC family of oncogenes may become activated by gene rearrangement or amplification.
- cancer type or syndrome:
- Mutations in the MYC gene have been found in many different cancers, including Burkitt’s lymphoma, B-cell leukaemia, and lung cancer.
64
Q
Describe the function of p53
A
a) Growth arrest
b) Apoptosis
65
Q
What is oncogenic addiction?
A
- In this model, a tumour is considered to be a heterogeneous cell population containing largely cancer cells addicted to oncogene X (blue), but containing a small subpopulation of non-addicted cancer cells, “persisters” (orange) that are characterised by very slow growth, a lack of dependency on the same survival signals as the majority of the tumour population, and resistant to an initial dose of drug.
- Upon treatment with a drug specifically targeted to the addicting oncoprotein, tumour regression results from destruction of the addicted cells, but persisters, because they are not oncogene addicted, are maintained. These persisters have the ability to regenerate the tumour in three different ways:
(1) The tumour will remain composed largely of cancer cells that remain addicted to the same oncogene X (blue: centre scenario), in which case resistance appears to be reversible, possibly involving an epigenetic mechanism;
(2) the tumour is largely composed of cells that have switched their addiction to an alternative oncogene Y (green: scenario shown on the right); or
(3) the tumour will be comprised largely of cells that exhibit co- addiction to an oncogene Y in addition to the original addicting oncogene X (purple: scenario shown on the left).
68
Q
Give some example of double-agent genes (POTSF)
A
- transcription factors
- kinases
- others
see image
