Oncogenes And Tumour Suppressor Genes Flashcards
What was found in the Rous sarcoma virus?
A extra gene contained in its genome a “oncogene” called v-src
What is the fundamental principle of oncogenes?
Oncogenes are alerted forms of normal genes or proto-oncogenes
What is the oncogene hypothesis?
It was discovered that come genes of cancer causing viruses were mutated forms of the cellular genes NOT viral genes.
They concluded that the Rous sarcoma gene was in fact a host gene that was kidnapped by the virus.
How can the c-src sequence be captured by a retrovirus?
During evolutions, the virus can acquire fragments of genes from the host at integration sites and this process results in the creation of oncogenes.
The oncogenes product was characterised as a 60kDa intracellular tyrosine kinase that can phosphorylase cellular proteins and effect growth.
How can DNA viruses become oncoviruses?
Encode various proteins along with environmental factors can initiate and maintain tumours
How can RNA viruses become oncoviruses?
Integrate DNA copies of their genomes into the genome of the host cell
as these contain transforming oncogenes they induce cancerous transformation of the host
How are oncogenes activated?
mutations,
insertions,
amplification/duplication
translocations.
What are the 4 types of proteins are involved in the transduction of growth signals?
- growth factors
- growth factor receptors
- intracellular signal transducers
- nuclear transcription factors
What is the RAS oncogene family?
Most commonly mutated oncogene
Ras genes were identified from studies of two cancer causing viruses.
RAS proteins are small GTPase that are normally bound to GDP in a neutral state
Explain the intracellular signal transducer pathway.
- Binding of extracellular growth factor signal
- Promotes recruitment of RAS proteins to the receptor complex
- Recruitment promotes Ras to exchange GDP with GTP
- 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
How can oncogenes function?
The oncogene product was characterised as a 60kDa intracellular tyrosine kinase
Can phosphorylate cellular proteins and effect growth.
How can oncogenes become activated?
Mutations
Insertions
Amplifications/duplications
Translocations
LOSS OF RESPONSE TO GROWTH REGULATORY FACTORS. ONE ALLELE NEEDS TO BE EFFECTED.
What is the MYC oncogene family?
Consists of 3 members:
- C-MYC
- MYCN
- MYCL
Belong to a family of transcription factors that regulate transcription of at least 15% of the entire genome
- cell cycle
- DNA repair
- translation
How is MYC activated?
Encodes a helix-loop-helix leucine zipper transcription factor that dimerises with its partner protein (Max) to transactivate gene expression.
Activation is a result of chromosomal translocation
Describe how activation of MYC in burkitt’s lymphoma?
BL A high grade lymphoma that effects children from 2-16.
All cases carry one of three characteristic chromosomal translocations:
- chromosome 2
- chromosome 14
- chromosome 22
These translocations place the MYC gene under the regulation of the ig heavy chainb
How can chromosomal translocation be responsible for the activation of other oncogenes?
In chronic myelogenous leukaemia - 95% of patients carry the Philadelphia chromosome
The Philadelphia chromosome of translocation is a product of translocation
As a result of this translocation, the tyrosine kinase activity of the oncogene ABL is constitutive leading to abnormal proliferation
What is required for a tumour suppressor to become inactivated?
To loose tumour suppressor gene function the inactivation of BOTH ALLELES of the gene are required
What are the 3 functions associated with tumour suppressor genes?
Regulators of cell cycle checkpoints
Differentiation
DNA repair
What is retinoblastoma Rb?
A rare childhood cancer
developes when immature retinoblasts continue to grow very fast and do not mature
When light is reflected into eye, the light is reflected back - cat eye appearance
What are the 2 forms of retinoblastoma?
Familial - 40%
Sporadic - 60%
What hereditary mutation causes retinoblastoma?
On Chromosome 13 in the retinoblastoma 1 gene (Rb1)
What is the “two-hit” hypothesis?
The development of retinoblastoma requires two mutations, which correspond to the loss of both of the functional copies of the Rb gene
What is meant by “loss of heterozygosity”?
The process that leads to the inactivation of the second copy tumour suppressor gene.
A heterozygous cell receives a second hit in its remaining functional copy of the tumour suppressor gene.
What types of mutations can activate tumour suppressor genes?
Point mutations
Small deletions
Describe the structure of the retinoblastoma protein RB structure.
Includes 3 members, collectively known as pocket proteins
- N terminus
- small pocket
- c terminus
Large pocket made of small pocket and c terminus is where proteins bind.
pRb is a multi functional protein (110kDa) with over 100 binding partners
Main binding partner is the E2F transcription factor
What is the function of the retinoblastoma protein?
A transcription co factor that can bind to transcription factors
Involved in apoptosis and cell cycle - regulates these pathways through the stimulation or inhibition of the activity of interacting proteins.
How is retinoblastoma protein RB deactivated in the cell cycle?
By inhibiting the G1 to S phase
- Cyclin D is the first cyclin to be synthesised and drive progression through G1 with CDK4/6
- The G1 checkpoint leads to the arrest of the cell cycle in response to DNA damage
- A key substrate for cyclin D is RN protein
- Cyclin D and E and they’re CDKS phosphorylate RB (deactivated)
- Upon deactivation of Rb, E2F is released/unbound and migrates to the nucleus where it induces transcription
- Progression from G1 to S phase
How does retinoblastoma protein inhibit the cell cycle after activation?
- RBC regulates the activity of the E2F transcription factor crucial for the expression of genes required for S phase
- Rb is regulated by phosphorylation
- When Rb tumour suppressor is active it can inhibit cell proliferation
- When Rb is dephosphorylated it is active and remains bound to E2F
- When Rb is active it blocks the progression of G1 to S phase
What are the 3 ways Rb can be inactivated?
Phosphorylation
Mutation
Viral oncoprotein binding
How can viral protein binding induce Rb inactivation?
Found in a small number of tumour cells
Mainly by disrupting E2F binding or destabilisation of Rb
How can Rb dis regulation lead to tumour formation?
As a direct consequence E2F transcription factors can induce the deregulation of the cell cycle
Cells move through G1 into S without the checkpoints of Rb.
What is the function of the p53 tumour suppressor gene?
Involved in sensing DNA damage and regulating cell death/apoptosis
P53 specialises in preventing the appearance of abnormal cells
Transcription factor p53 can bind to 300 promoter regions
What is the structure of p53?
Tetrameter complexed with dsDNA
- amino transactivation domain
- a central DNA binding domain
- a tetramization domain
- carboxyl regulatory domain
Describe how p53 is regulated in normal cells by MDM2?
Normal p53 protein levels are low inside cells
MDM2 is a ubiquitin ligase (also a ONCOGENE) that binds to the p53 protein
After a complex is formed in the nucleus, MDM modifies the carboxyl terminus of p53 and targets it for degradation by the proteasome
P53 has a 20 minute half life
How can stress levels lead to the activation of P53?
Stress signals are sensed by kinases that then phosphorylate p53
Phosphorylated p53 cannot properly interact with MDM2
P53 can thus regulate genes involved in DNA damage repair, apoptosis and cell cycle arrest
What are genetic therapeutic strategies using p53 to treat cancer?
Gene therapy
Retroviruses integrate in a stable form into the genome of infected cells
Retrovirus-mediated gene transfer of the wild type TP53 gene into human lung tumour cells COULD lead to the inhibition of tumour cell growth
What are the molecular therapeutic strategies using p53 to treat cancer?
PRIMA-1 restores mutant p53 to normal
- by modifying thiol groups in the core domain of the protien
Nutlin is a potent MDM2 antagonist
RITA binds to p53 and can restore mutp53 activity
Inhibitors of CRM1 result in nuclear accumulation of p53
How can personalised medicine be used to treat cancer?
Classifies tumours according to their genetic makeup instead of where they grow in the body
Developed drugs that target the molecular faults in a tumour not where the tumour is
