Oncogenes and Tumour Suppressor Genes Flashcards
Define what a hallmark of cancer is
Characteristic that normal cells have to require to become tumour cells
Major functional changes in cancer
Increased growth (regulation, environment )
Failure to undergo apoptosis
Loss of differentiation
Failure to repair DNA damage
Oncogenes function
Components of GF signalling pathways = reg. cell proliferation + survival in response to GF stim
↳ mutated = ↑ product or altered products have ↑ activity = act in a dominant manner
Tumour suppressor gene function
Stop signal to uncontrolled growth, may inhibit cell cycle or trigger apoptosis
Describe the effect of gain of function in oncogenes
an altered gene whose product can act
in a dominant fashion to help make a cell cancerous.
= leads to stimulated cell proliferation
Describe how there is a loss of function in TSG and their its effect
2 inactivating mutations for TS functionally eliminate TSG = stim cell proliferation
=
a normal process to maintain control of cell division is lost
= enhances the likelihood that a cell can become cancerous
Explain how tumour development in chickens led to the discovery of Rous Sarcoma Virus
The carcinogenic agent was small enough to pass through a filter
Although the filter used excluded bacteria it was not small enough to exclude viruses
Rous concluded that a virus must be responsible for the induction of tumour formation
How to induce sarcomas in other chickens
Chicken w/sarcoma in breast muscle
Remove sarcoma and break up into small chunks of tissue
Grind up sarcoma with sand
Collect filtrate that has passed through fine pore filter
Inject filtrate into young chicken
Observe sarcoma in injected chicken
List the reaasons why retroviruses were important experimentally
Technological advances
Funding
Improved tissue culture techniques
The discovery of reverse transcriptase, RNA genome, replicates via DNA intermediate and that they are enveloped
Describe + explain discovery of the fundamental principle of oncogenes
Oncogenic transformation by RSV was found to be caused by an extra gene contained in its genome = ‘oncogene’ v-src
Homologous sequences were found in uninfected chickens and other OG
Fundamental principle: Oncogenes are alerted forms of normal genes or proto-oncogenes
Describe oncogene hypothesis
V-src homologous seq in uninfected
So some genes of cancer causing viruses were mutated forms of the cellular gene not viral genes
Conclusion = rous sarcoma viral gene was in fact a host gene that had been ‘kidnapped’ by the virus (and ‘transformed’ into an oncogene)
Describe capture of c-src by retrovirus
c-src (in host cell chromosomal DNA) included into viral sequences with the dsDNA provirus (from infection then reverse transcription)
Which is accidentally integrated next to c-src
Then packaged into capsid leading to RSV virion carrying src sequences
Describe and explain the effect of integration of gene fragments in viruses
During evolution, virus can acquire gene fragments from host at integration sites = creation of oncogenes
Can phosphorylate cellular proteins and effect growth
Explained how Bishop/varmus showed that an oncogene was responsible for causing cancer
Identified v-src oncogene for causing cancer
Hybridization experiments = c-src gene was present in genome of many species
Showed that host cell c-src gene was normally involved in +ve regulation of cell growth and cell division
Following infection - v-src oncogene was expressed at high levels in host cell = uncontrolled cell division/growth and cancer
Explain how cells can be transformed
Various agents, including radiation, chemical carcinogens,
and
Exogenously added viruses, may transform cells by “switching on” the endogenous oncogenic information
What do DNA viruses:
Encode
What can they cause
Encode various proteins along with environmental factors can initiate and maintain tumours
Can cause lytic infection = death of cellular host or can replicate own/hosts’ DNA and promote neoplastic transformation
What do RNA viruses do
Integrate DNA copies of their genomes into HC genome and as these contain transforming oncogenes they induce cancerous transformation of the host
Viral oncogenes transmission - how
Viral oncogenes can be transmitted by either DNA or RNA viruses
Describe what can lead to the activation of oncogenes
OG captured by animal retroviruses
+
altered in human cancer, activated by mutations, insertions, amplifications and translocations
Describe oncogene activation
Translocation = DNA regulatory seq translocated from distant site alters expression of downstream gene
Mutation/deletion = protein w/altered structure/function
Duplication = ↑ synth. of encoded protein - activates by insertion nearby P-OG
- ↑ synth. of encoded protein
OR - Synthesis of a protein containing portions encoded by different genes. The fusion protein is no longer under normal control
Thus a protein-coding gene translocated from distant site fuses w/portion of gene causing formation of a fusion gene
4 types of proteins are involved in the transduction of growth signals
Growth factors
Growth factor receptors
Intracellular signal transducers
Nuclear transcription factors
Explain the involvement of oncogenes with ras
Oncogenes act as GF, GFR and intracellular signalling molecules
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
Ras proteins - definition
RAS proteins are small GTPases that are normally bound to GDP in a neutral state
Ras mutations - effect
Loss of GTPase activity of the RAS protein
Hyperactive ras due to mutation so DNA damage repair is not allowed to happen
Ras activation - description
- Binding of EC GF signal
2. Promotes recruitment of RAS proteins to the receptor complex
- 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 TF to promote expression of genes = growth + survival
Ras hydrolyzes GTP to GDP fairly quickly
MYC onogenes - family description
C-MYC, MYCN, and MYCL, which encode c-Myc, N-Myc,
and L-Myc, respectively
MYC oncogenes - regulate what
Belong to a family of TFs that regulate transcription of at least 15% of the entire genome
MYC oncogenes - major downstream effectors
Include those involved in
RS biogenesis, translation, cell-cycle progression and metabolism,
orchestrating a broad range of biological functions,
such as cell proliferation, differentiation, survival, and immune surveillance
MYC oncogenes - structure
Encodes a helix-loop-helix leucine zipper TF
that dimerizes with its partner protein, Max, to transactivate gene expression
MYC oncogenes - activation
Activated when under control of foreign transcriptional promoters (chromosomal translocation)
=
deregulation of the oncogene
Burkitt’s lymphoma - activation
BL = carry ⅓ CS translocation = MYC under reg of Ig heavy chain = CS 2/14/22 = a region from one of these is fused w/CS 8
Chronic myelogenous leukaemia - mechanism
Carry the Philadelphia CS,
that is the product of CST
t(9;22)(q34;q11) = BCR-ABL fusion protein
= tyrosine kinase activity
of the oncogene ABL is constitutive leading to abnormal
proliferation
Chronic myelogenous leukaemia - therapeutic strategies
Therapeutic strategies for CML include Imatinib (Gleevac) a tyrosine kinase inhibitor
Identification of tumour suppressor genes - description
Somatic hybridization experiments
= fuse normal cells w/tumour cells
= cells produced can’t form tumours
= genes from normal inhibit/suppress tumour development
Anti-oncogenes - description of how they arise
TSG products act as stop signs to uncontrolled growth, promote differentiation or trigger apoptosis
Usually regulators of cell cycle checkpoints, differentiation or DNA repair
Loss of TSG function requires inactivation (mutation/deletion) of both alleles of the gene
TSGs = recessive genes
Sometimes referred to as ‘anti-oncogenes’
Retinoblastoma - define
Retinoblastoma is a rare childhood cancer (1 in 20,000) that develops when immature retinoblasts continue to grow very fast and do not turn into mature retinal cells.
An eye that contains a tumour will reflect light back in a white colour.
Often called a “cat’s eye appearance,” the technical term for this is leukocoria.
Retinoblastoma - forms + difference between them
Two forms of the disease, familial (40%) and sporadic (60%)
Heriditary tumour = 1 inherited and 1 acquired mutation thus 1 hit on somatic level
Sporadic tumour = 2 acquired mutations thus 2 hits on somatic level
Retinoblastoma - cause
The hereditary mutation is on chromosome 13 (13q14),
the retinoblastoma 1 (Rb1) gene
Retinoblastoma - loss of heterozygosity description
“Loss of heterozygosity“ = inactivation of second TSG copy
heterozygous cell receives second hit in remaining copy of TSG = homozygous for mutated gene
Retinoblastoma - family description
The Rb gene family includes three members: Rb/(p105/110), p107 and Rb2/p130
-collectively known as pocket proteins
pRb - define
pRb is a multi functional protein (110kDa) with over 100 binding partners
A transcriptional co factor that can bind to transcription factors
Retinoblastoma - structure
structure = scaffold for multiple protein interactions
Large pocket = small pocket and C-terminus
N terminus at the end
Space w/in large pocket
It’s main binding partner is the E2F transcription factor,
interacting with the large pocket
Retinoblastoma - interaction w/cell cycle
Main function of Rb is to regulate the cell cycle by inhibiting the G1 to S phase transition
Cyclin D function in cell cycle
Cyclin D is first cyclin to be synthesized and drive progression through G1 together with cdks4/6
A key substrate for cyclin D is RB protein
Cyclin D and E families and their cdks phosphorylate RB
Rb interaction w/E2F - description and effects
Rb protein regulates activity of E2F = crucial for expression of genes required for S phase
When Rb is dephosphorylated/hypophosphorylated it is active and remains bound to E2F
When Rb is active it blocks the progression of to S phase
E2F release and effect - description
Upon phosphorylation of RB, E2F is released and migrates to the nucleus to induce transcription
Rb inactivation - list 3 ways
Phosphorylation
Mutation
Viral oncoprotein binding
Retinoblastoma - state of pRb
In retinoblastoma, pRb is functionally inactivated by mutations or partial deletions
Viral inactivation in DNA viruses - cause
Viral inactivation found in small DNA tumour viruses mainly by disrupting E2F binding or destabilisation of Rb
RB phosphorylation in cancer cells and effect - description
Rb phosphorylation deregulated throughout cell cycle
= E2F TF can deregulate CC
= cells move from G1-S phase w/out checks
p53 role and specialization
Sensing DNA damage and regulating cell death/apoptosis as well as other pathways
p53 specializes in preventing the appearance of abnormal cells
Frequent mutation of p53 - effect
Frequent mutation of p53 in tumour cell genomes
suggests that tumour cells try to eliminate p53 function before they can thrive
p53 - structure and binding capabilities
Protein has an amino transactivation domain, a central DNA binding domain, a tetramerization domain and a carboxyl regulatory domain
Can bind to around 300 different gene promoter regions-main role as a transcription factor
Regulation of P53 by MDM2 - description
p53 levels kept low by MDM2 (OG, ubiquitin ligase),
unstressed cells = p53/MDM2 move between nucleus/cytosol → MDM2 binds p53 = complex in nucleus,
MDM adds ubiquitin tag onto lysine residues at carboxyl terminus = targeted for degradation by proteasome
Activation of p53 tumour suppressor and effect - description w/example
Stress signals are able to activate p53
Signals are sensed by mainly kinases that then phosphorylate p53
Phosphorylation of p53 disrupts the interaction between it and MDM2
e.g. ionizing radiation signals through two kinases ATM/ATR activate oncogenes such as ras, induce activity of p14arf responsible for sequestering MDM2.
P53 can thus regulate genes involved in DNA damage repair, apoptosis and cell cycle arrest
Why is p53 a promising therapeutic target
Role of p53 as star player in suppressing tumorigenesis makes it a promising therapeutic target
Therapeutic strategies for p53
Correcting p53 mutation and restoring wild-type p53 function by targeting its regulators
Describe use of PRIMA-1 in therapy
PRIMA-1, Restores mutant p53 by modifying the thiol groups in the core domain of the protein
Nutlin- is a potent MDM2 antagonist
RITA binds to p53 and can restore mutp53 activity
Inhibition of CRM1 - effect
Inhibitors of CRM1 result in nuclear accumulation of p53
4 uses of genetic analysis and personalised medicine
A detailed readout of the molecular faults in a patient’s tumour, and new generation of drugs that precisely target them
Classifies tumours according to their genetic make-up instead of where they grow in the body
People with the ‘same’ cancer can have different forms of the disease so responses to treatment vary
Cancers growing in different parts of the body may also share the same genetic faults so respond to similar treatments
