Oncogenesis and tumour suppressor genes Flashcards
1
Q
What are the major functional changes in cancer?
A
- Increased growth
• Loss of growth regulation, stimulation of environment promoting growth e.g. angiogenesis) - Failure to undergo apoptosis/senescence
- Loss of differentiation
○ Including alterations in cell migration and adhesion - Failure to repair DNA damage
-Including chromosomal instability
2
Q
What are oncogenes normally components of?
A
Many oncogenes are normally components of growth factor signalling pathways
3
Q
What happens when oncogenes are mutated?
A
When mutated produce products in higher quantities or whose altered products have increased activity and therefore act in a dominant manner
4
Q
What do tumour suppressor gene products act as?
A
Many tumour suppressor gene products act as a stop signal to uncontrolled growth, may inhibit the cell cycle or trigger apoptosis
5
Q
What is the normal job of oncogenes?
A
• Normal job: make cells divide, driving cell division forward
6
Q
What happens in cancer to oncogenes?
A
In cancer:Pick up mutations that mean they are permanently active
7
Q
What does a mutation cause to oncogenes?
A
Mutation causes gain of function
8
Q
What has to occur in order to completely knock out tumour suppressor function?
A
In order to completely knock out tumour suppressor function, must have mutations in both alleles – two hit hypothesis
9
Q
What does a mutation cause to tumour suppressor gene?
A
Mutation causes loss of function
10
Q
How can viral oncogene be transmitted?
A
Viral oncogenes can be transmitted by either DNA or RNA viruses
11
Q
What can DNA viruses cause?
A
• 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
12
Q
What do DNA viruses encode?
A
○ Encode various proteins along with environmental factors can initiate and maintain tumours
13
Q
What do RNA viruses integrate and induce?
A
○ Integrate DNA copies of their genomes into the genome of the host cell and as these contain transforming oncogenesthey induce cancerous transformation of the host
14
Q
What causes activation of oncogenes?
A
○ Mutations
○ Insertion
○ Amplifications
○ Translocations
15
Q
What does activation of oncogenes do?
A
○ Loss of response to growth regulatory factors
○ One allele needs to be altered
16
Q
What are the 4 types of proteins involved in the transduction of growth signals?
A
- Growth factors
- Growth factor receptors
- Intracellular signal transducers
- Nuclear transcription factors
17
Q
What do the majority of oncogene proteins function as?
A
○ The majority of oncogene proteins function as elements of the signalling pathways that regulate cell proliferation and survival in response to growth factor stimulation
18
Q
What do oncogene proteins act as?
A
Oncogene proteins act as growth factors (e.g.EGF), growth factor receptors (e.g. ErbB) and intracellular signalling molecules (Ras and Raf)
19
Q
What pathway do Ras and Raf activate and what does this lead to?
A
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
20
Q
What are RAS proteins?
A
○ RAS proteins are small GTPases that are normally bound to GDP in a neutral state
21
Q
What percentage of cancers is oncogenic activation of Ras seen in?
A
○ Oncogenic activation of ras is seen in about 30% of human cancer
22
Q
What is the most commonly mutated oncogene?
A
Ras gene
23
Q
Where do the point mutations for Ras occur?
A
In codons12, 13, 61
24
Q
What does a mutation from glycine to valine in Ras cause?
A
Glycine to valine - bladder carcinoma
25
What does a mutation from glycine to cysteine in Ras cause?
Glycine to cysteine - lung cancer
26
What are the steps involved in activating Ras and what do these activated Ras do?
1. Binding of extracellular growth factor signal
2. Promotes recruitment of RAS proteins to the receptor complex
3. Recruitment promotes Ras to exchange GDP (inactive Ras) with GTP (active Ras)
4. Activated Ras then initiates the remainder of the signalling cascade (mitogen activated protein kinases)
5. These kinases ultimately phosphorylate targets, such as transcription factor to promote expression of genes important for growth and survival
27
How is Ras inactivated?
1. Ras hydrolyzes GTP to GDP fairly quickly, turning itself “off”
28
What does mutations in the codons of Ras result in?
• Mutation in codons results in hyperactive Ras protein
○ Always switched on
Cells are continually dividing
– leads to tumorigenesis
29
What does the MYC oncogene family consist of and what do they encode?
○ The MYC oncogene family consists of 3 members, C-MYC, MYCN, and MYCL, which encode c-Myc, N-Myc, and L-Myc, respectively
30
What family does the MYC oncoproteins belong to?
○ The MYC oncoproteins belong to a family of transcription factors that regulate the transcription of at least 15% of the entire genome
31
What are the main downstream effectors of MYC include?
○ 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
32
In what is MYC oncogene overexpressed?
○ The MYC oncogene is overexpressed in the majority of human cancers and contributes to the cause of at least 40% of tumours
33
What does MYC oncogene encode and what does it dimerize?
○ It encodes a helix-loop-helix leucine zipper transcription factor that dimerizes with its partner protein, Max, to transactivate gene expression
34
When is MYC generally activated and what does this lead to?
○ Generally MYC is activated when it comes under the control of foreign transcriptional promoters. This leads to a deregulation of the oncogene that drives relentless proliferation.
35
What is activation of MYC a result of?
○ Such activation is a result of chromosomal translocation
36
What virus is associated with Burkitt's lymphoma?
Epstein Barr virus is associated with Burkitt's lymphoma
37
What is burkitt's lymphoma?
• BL is a high grade lymphoma that can effect children from the age of 2 to 16 years
38
When does EBV infection usually happen and what happens if this infection is delayed to teenage years?
• Usually this infection happens in childhood but if it is delayed to teenage years, you get infection mononucleosis or glandular fever
39
What is the main transmission of EBV?
Main transmission is through saliva
40
What does EBC affect?
• Affects jawbone
| -Highly aggressive, doubles every 24 hours
41
What do all burkitt's lymphoma cases carry and what does this therefore mean?
• All BL cases carry one of three characteristic chromosomal translocations that place the MYC gene under the regulation of the Ig heavy chain
-Therefore c-myc expression is deregulated
42
What are the chromosomes involved in the translocations in burkitt's lymphoma and what does each translocation fuse to?
• In BL three distinct, alternative chromosomal translocations involving chromosomes 2, 14 and 22
-In all three translocations a region form one of these three chromosomes is fused to a section of chromosome 8
43
What percent of all leukemias does chronic myelogenous leukaemia account for?
• Chronic myelogenous leukaemia (CML) accounts for 15-20% of all leukaemias
44
What do 95% of chronic myelogenous leukemia patients carry and what is this the product of?
• 95% of CML patients carry the Philadelphia chromosome, that is the product of the chromosomal translocation t(9;22)(q34;q11) generating the BCR-ABL fusion protein
45
As a result of the translocation of the philadelphia chromosome, what happens?
• As a result of this translocation the tyrosine kinase activity of the oncogene ABL is constitutive leading to abnormal proliferation
46
What are the therapeutic strategies for chronic myelogenous leukaemia?
• Therapeutic strategies for CML include Imatinib (Gleevac) a tyrosine kinase inhibitor-96% remission in early-stage patients
47
What do tumour suppressor gene products act as and what are they usually the regulators of?
• Tumour suppressor gene products act as stop signs to uncontrolled growth, promote differentiation or trigger apoptosis
-Therefore they are usually regulators of cell cycle checkpoints (e.g. RB1), differentiation (e.g. APC) or DNA repair (e.g. BRCA1)
48
What does loss of tumour suppressor gene function require?
• Loss of tumour suppressor gene function requires inactivation of both alleles of the gene
49
What are tumour suppressor genes defined as?
• Tumour suppressor genes are defined as recessive genes
| ○ Sometimes referred to as ‘anti-oncogenes’
50
What is retinoblastoma?
Retinoblastoma is a rare childhood cancer
51
Why do retinoblastoma develop?
• Develops when immature retinoblasts continue to grow very fast and do not turn into mature retinal cells
52
What is leukocoria?
An abnormal white reflection from the retina of the eye when the eye contains a tumour
53
What are the 2 forms of the retinoblastoma disease?
Two forms of the disease: familial (40%) and sporadic (60%)
54
What is the hereditary mutation for retinoblastoma on?
○ The hereditary mutation is on chromosome 13 (13q14), the retinoblastoma 1 (Rb1) gene
55
What is loss of heterozygosity often used to describe in retinoblastomas?
• "Loss of heterozygosity” often used to describe the process that leads to the inactivation of the second copy of a tumour suppressor gene
56
What are mutations that inactivate tumour suppressor genes?
• Mutations that inactivate tumour suppressor genes, called loss-of-function mutations, are often point mutations or small deletions
○ Disrupt the function of the protein that is encoded by the gene
57
What does the Rb gene family include?
• The Rb gene family includes three members: Rb/(p105/110), p107 and Rb2/p130
58
What are the retinoblastoma genes collectively known as?
• Collectively known as pocket proteins
| -Have large and small pockets
59
What can large pocket areas interact with?
§ Through large pocket area they can interact with E2F transcription factor
60
What is the retinoblastoma protein?
• A transcriptional co factor that can bind to transcription factors
61
What does the retinoblastoma function in and how does it regulate these pathways?
• RB functions in diverse cellular pathways, such as apoptosis and the cell cycle
-RB regulates these pathway through the stimulation or inhibition of the activity of interacting proteins.
62
What is the main binding partner of retinoblastoma?
• Main binding partner is the E2F transcription factor, interacting with the large pocket
63
What is the main function of Rb?
Regulate the cell cycle by inhibiting the G1 to s phase transition
64
What are the important proteins involved in the cell cycle?
1. Cyclins
| 2. CDKs
65
What is cyclin D
Cyclin D is the first cyclin to be synthesised and drive progression through G1 together with cdks4/6
66
What does the G1 checkpoint lead to and in response to what?
• The G1 checkpoint leads to the arrest of the cell cycle in response to DNA damage
67
What is a key substrate for cyclin D?
• A key substrate for cyclin D is RB protein
68
What do cyclins D and E families and their CDKs do?
• Cyclin D and E families and their cdks phosphorylate RB
69
What do Rb protein regulate the activity of?
• Rb protein regulates the activity of the E2F transcription factor which crucial for the expression of genes required for S phase
70
What is Rb activity regulated by?
Rb activity is regulated by phosphorylation
71
What form does Rb exist in?
Rb exists in hypophosphorylated form
72
When Rb is bound to E2F, what does it prevent?
When bound to E2F, it prevents progression into S phase so triggers cell cycle death
73
What can the Rb tumour suppressor do when active?
• When the Rb tumour suppressor is active it can inhibit cell proliferation
74
What state is Rb active in and remains bound to E2F?
• When Rb is dephosphorylated/hypophosphorylated it is active and remains bound to E2F
75
What state is Rb inactive in?
• When Rb is hyperphosphorylated in response to extracellular physiological signals, it is inactive
76
What happens upon the phosphorylation of Rb?
• Upon phosphorylation of RB, E2F is released and migrates to the nucleus to induce transcription
77
What progression occurs when Rb is inactive?
• When RB is inactive cell cycle progression from G1 to S occurs
78
In retinoblastoma, what is pRb functionally inactivated by?
• In retinoblastoma, pRb is functionally inactivated by mutations or partial deletions
79
What happens to Rb phosphorylation in cancer cells and what happens as a direct consequence?
• In cancer cells RB phosphorylation is deregulated throughout cell cycle
○ As a direct consequence E2F transcription factors can induce the deregulation of the cell cycle
80
What is the p53 gene involved in?
• It is involved in sensing DNA damage and regulating cell death/apoptosis as well as other pathways
81
What does frequent mutation of p53 in tumour cell genomes suggest?
• Frequent mutation of p53 in tumour cell genomes suggests that tumour cells try to eliminate p53 function before they can thrive
82
What is the structure of p53?
• Protein has an amino transactivation domain, a central DNA binding domain, a tetramerization domain and a carboxyl regulatory domain
83
How many gene promoter regions can p53 bind to?
• Can bind to around 300 different gene promoter regions-main role as a transcription factor
84
What is MDM2 and what does it regulate?
Is a ubiquitin ligase and regulates p53
85
What does MDM2 do?
○ Adds ubiquitin to lycine residues of molecule so is targeted to proteasome for proteolytic degradation
86
What keeps p53 levels low?
○ These levels are kept low by MDM2 protein, a ubiquitin ligase (also an oncogene)
87
How does MDM2 regulate p53 levels?
○ MDM2 binds p535 to form a complex in the nucleus where MDM modifies the carboxyl terminus of p53 and targets it for degradation by the proteasome
88
Half life of WT p53?
○ WT p53 has a short 20 min half life
89
What signals activate p53?
Stress signals are able to activate p53
90
What are stress signals sensed by?
• Signals are sensed by mainly kinases that then phosphorylate p53
91
What does phosphorylation of p53 disrupt?
• Phosphorylation of p53 disrupts the interaction between it and MDM2
92
What can p53 regulate?
• P53 can thus regulate genes involved in DNA damage repair, apoptosis and cell cycle arrest
93
What is the most common molecular mechanism behind the dysfunction of p53?
• Mutational inactivation is considered to be one of the most common molecular mechanisms behind the dysfunction of p53
94
What are the therapeutic strategies for p53 mutations?
• Correcting p53 mutation and restoring wild-type p53 function by targeting its regulators
95
How does retrovirus as a therapeutic strategy for p53 mutation work?
• Retroviruses integrate in a stable form into the genome of infected cells. It has been demonstrated that 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
96
How are inhibitors used in p53 mutations?
○ Inhibitors can be used in two ways:
1. Refold mutant p53 by modifying the thiol groups in the core domain of the protein e.g. PRIMA 1
2. Regulate the regulators of p53
• i.e. MDM2; the half life would therefore be greater so more ability to function as a tumour suppressor
-Nutlin (MDM2 inhibitor) prevents the ubiquitination of the molecule
