Oncogenes and tumour suppressor genes Flashcards
What are the four major functional changes in cancer?
- Increased growth (loss of growth regulation, stimulation of environment promoting growth e.g angiogenesis
- Failure to undergo programmed cell death (apoptosis) or senescence
- Loss of differentiation (including alterations in cell migration and adhesion)
- Failure to repair DNA damage (including chromosomal instability) , those damaged cells are not repaired and not removed by apoptosis
What is an oncogene?
Gain of function, ‘accelerator’ of a car, if activated it speeds up cell division
An altered gene whose product can act in a dominant fashion to help make a cell cancerous
Oncogene is a mutant form of a normal gene (a proto oncogene) involved in the control of cell growth or division
What is a tumour suppressor gene?
Loss of function
the ‘brakes’, counteracts oncogene.
A gene whose normal activity prevents formation of a cancer
Both genes for the tumour suppressor must be mutated to knock out tumour suppressor function.
Loss of this function by mutation enhances the likelihood that a cell can become cancerous (a normal process to maintain control of cell division is lost)
How does the retrovirus capture c-src (cellular oncogene)?
During evolution virus acquires fragments of genes from host at integration sites process results in creation of oncogenes
oncogene is 60kDa intracellular tyrosine kinase, phosphorylate cellular proteins and effect growth
go from RNA to DNA (normally DNA to RNA)
pro-virus is accidentally integrated next to host c-src sequence, fusion and packaged into capsid
end up with Rous Sarcoma Virus carrying src sequence
What did the oncogene hypothesis show?
Identified v-src (protooncogene altered form transduced by retrovrius) oncogene as responsible for causing cancer
Used hybridisation experiments, found that the c-src gene was present in the genome of many species and that host cell c-src gene is involved in the positive regulation of cell growth and division
Following infection v-src oncogene was expressed at high levels in the host cell, leading to uncontrolled host cell growth, unrestricted host cell division and cancer
Proto-oncogenes are normal genes that can control growth
Various agents, including radiation, chemical carcinogens and perhaps exogenously added viruses may transform cells by switching on the endogenous oncogenic information
15-20% of human cancers caused by onocviruses, what can viral oncogenes be transmitted by?
DNA viruses
- Encode various proteins 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 activates oncogenes?
Mutations, amplification/duplication, translocations
These alter structure and function of protein or increase synthesis of the protein.
Proto-oncogenes encode components of growth factor signal transduction pathway
What 4 types of proteins are involved in transduction of growth signal?
Growth factors
Growth factor receptors
Intracellular signal transducers
Nuclear transcription factors
Ras Oncogene family
What are Ras genes?
Identified from 2 cancer causing viruses, Harvey sarcoma virus and Kristen sarcoma virus
Ras proteins are small GTPases, normally bound to GDP in a neutral state
oncogenic acitvation of ras seen in 30% of human cancer
glycine to valine - bladder carcinoma - mutation at codon 12
glycine to cysteine - lung cancer - codon 13
What normally happens with Ras?
- Binding of the extracellular growth factor signal
- Promotes recruitment of RAS proteins to the receptor complex
- Recruitment promotes Ras to exchange GDP (inactive ras) with GTP (activate ras)
- Activated Ras then initiated the remainder of the signalling cascade (mitogen activated protein kinase)
- These kinases ultimately phosphorylate targets such as transcription factor to promote expression of genes important for growth and survival
What happens to ras pathway when you have mutations in codons?
hyperactive ras
point mutations in codons 12, 13, 61
Consequence of these mutations is loss of GTPase activity of the RAS protein normally required to return active RAS to the inactive RAS GDP
Causes constitutive activation, always switched on
Cells are continually dividing, can’t stop, leading to tumour.
What does the myc oncogene family consist of?
C-MYC, MYCN, MYCL which encode c-Myc, N-myc and L-myc
family of transcription factors that regulate transcription of 15% of the entire genome
encodes helix-loop-helix leucine zipper transcription factor which dimerises with max to transactivate gene expression
What is Myc activated by?
What does over expression cause?
Chromosomal translocation, Myc activated when it comes under the control of foreign transcriptional promoters
leads to deregulation of oncogene which drives relentless proliferation
contributes to cause of 40% of tumours
What is Burkitt’s lymphoma?
How is c-myc expression deregulated?
what are the 3 chromosomal translocations that can occur in BL?
- A high grade lymphoma, effects children from 2-16.
in central africa kids with chronic malaria infections have reduced resistance to virus - endemic BL
- All BL cases carry 1 of 3 chracteresitc chromosomal translocations, places MYC gene under regulation of Ig heavy chain
Therefore c-myc deregulated
- In chromosomes 2, 14, 22, places Myc in control of Ig chain 8, lost control of MYC - lots of proliferation - aggressive tumour
in all 3 translocations, a region from one of the chromosomes is fused to section of chr8
What is the phildaelphia chromosome and which patients carry it?
What therapeutic strategy is used for CML?
- Chronic myelogenous leukaemia accounts for 15-25% of leukaemias, 95% of CML patients carry philadelphia chromosome.
The product of chromosomal translocation t(9:22) (q34:q11), generates BCR-ABL fusion protein
as a result, tyrosine kinase activity of oncogene ABL is constitutive - abnormal proliferation
- Imatinib (Gleevac), tyrosine kinase inhibitor - 96% remission in early stage patients
How do tumour supressors and oncogenes work together?
Oncogenes:
- Target cell proliferation
- And cell survival
Tumour suppressors
- Repair mutation
- Cell cycle checkpoints
- Remove damaged cells by apoptosis
How many tumour suppressor genes are there?
15, different functions associated with each
Regulators of cell cycle checkpoints (e.g RB1, retinoblastoma. )
Differentiation (e.g APC, adenomatous polyposis coli, colon carcinoma)
DNA repair (e.g BRCA1, breast ovary)
What is retinoblastoma?
Is it hereditary or sporadic?
rare childhood cancer, 1 in 20,000
develops when immature retinoblasts continue to grow very fast and don’t turn into mature retinal cells
eye with tumour reflects light back in white colour - leukcoria
two forms of disease, familial (40%) and sporadic (60%)
hereditary mutation on chr13 (13q14), retinoblastoma 1 gene (Rb1)
What is the two hit hypothesis for Rb gene?
Retinboblastoma requires two mutations
Loss of heterozygosity used to describe process that leads to inacitvation of second copy of tumour - heterozygous cell recieves second hit in remaining functional copy of tumour suppressor gene
becomes homozygous for mutated gene
mutation which inactivates tumour supressor genes = loss of function mutation
often point mutations or deletions
in sporadic tumour need 2 mutations ine ach of alleles, tumour developed later
What are the three proteins in RB gene family
Rb/(p105/110)
P107
Rb2/p130
pocket proteins, has small and large pocket, through large pocket interacts with E2F
What is the main function of Rb?
Regulate cell cycle by inhibiting G1 to S phase transition (growth to DNA synthesis)
2 important proteins involved - cyclins and their associated Cdks
Passage of a cell through the cell cycle is regulated by cyclins and CDKS
Cyclin D is the first cyclin to be synthesised and drives progression through G1 together with cdks1/6
G1 checkpoint leads to arrest of the cell cycle in response to DNA damage
Key substrate for cyclin D is RB protein
Cyclin D and E families and their cdks phosphorylate RB
What function does RB have with E2F transcriptionf actor?
Regulates E2F, crucial for expression of genes needed for S phase
RB activity regulated by phosphorylation
When RB is dephosphorylated it is active, remains bound to E2F -
When RB tumour suppressor is active it inhibits cell proliferation
Blocks progression to S phase, encourages cell cycle arrest.
Extracellular signals hyperphosphorylate Rb as a result of cyclin - inactive
Upon phosphorylation, E2F released, migrates to nucleus, induces transcription, cell cycle from G1 to S occurs
Inactivating tumour suppressor cells = progression in cell cycle
What can inactivate RB?
phosphorylation, mutation or viral oncoprotein binding
In retinoblastoma pRb is functionally inactivated by mutations or partial deletions
Viral inactivation found in small DNA tumour viruses mainly by disrupting E2F binding or destabilisation of Rb
- Adenovirus – E1A
- Papilloma – E7
- Polyoma – large T antigen
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
Without RB on watch, cells move through G1 into S and are not subjected to usual checks
What is p53?
Tumour supressor gene
involved in sensing DNA damage and regulating cell death/apoptosis and other pathways
- oxidative stress, nutrient deprivation, oncogene supression, hypoxia, ribosomal dysfunction
P53 mutated in 30-50% of commonly occuring cancers, suggests tumour cells try to eliminate p53 function before they can thrive
How does MDM2 regulate P53?
Normally levels of p53 proteins are low in cells
kept low by MDM2 protein, a ubiquitin ligase (also an oncogene).
Adds ubiquitin onto lysine residues of molecule and it gets targeted to proteosome for proteasomal degradation
In unstressed normal cells both p53 and MDM2 move between the nucleus and cytosol
MDM2 binds p53 to form a complex in the nucleus where MDM modifies the carboxyl terminus of P53 and targets it for degradation by the proteasome
Wt P53 has a short 20 min half life
What activates P53 tumour suppressor?
stress signals
Signals sensed by kinases that then phosphorylate p53
Phosphorylation of p53 disrupts interactions between it and MDM2
P53 can thus regulate genes involved in DNA damage repair, apoptosis and cell cycle arrest
What causes dysfunction of P53?
Mutational inactivation.
More than half of cancers carry mutations of P53.
Therapeutic strategies aimed at correcting P53 mutations and restoring wild-type p53 function by targeting its regulators
What are some therapeutic strategries for treating p53 mutations?
Gene therapy
Inhibitors, can refold mutant P53 back into wild type or can regulate regulators of P53
Primar-1 restores mutant P53 by modifying the thiol groups in the core domain of protein
Nutlin – potent MDM2 antagonist
RITA binds to p53 and can restore mutp53 activity
Inhibitors of CRM1 result in nuclear accumulation of p53
