Oncogenes and Tumour Suppressor Genes Flashcards
What are the 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)
Proto-oncogenes
normal cellular genes that regulate normal cell growth and division
Tumour suppressor genes
Genes in normal cells that encode products that inhibit the cell cycle or trigger apoptosis
Main genes which contribute to carcinogenesis
Oncogenes
Tumour suppressor genes
Oncogenes
‘Gain of Function’
-mutated proto-oncogenes which promote cell proliferation and are permanently active in cancer
Tumour suppressor genes in cancer
‘Loss of function’
- pick up mutations that switch the gene off
- both genes for the tumour suppressor must be mutated
Rous’s protocol for inducing sarcomas in chickens
· Remove sarcoma and break up into small chunks of tissue
· Grind up sarcoma with sand
· Pass it through a fine-pore filter and collect filtrate
· Inject filtrate into young chicken
· Observe sarcoma in injected chicken
Results of Rous’ experiment on chickens
- Tumours developed weeks later
- Taking the new sarcoma, filtrates produced could also induce tumours in other chickens
- The cycles could be repeated indefinitely. Also, 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
- Discovery that this sarcoma was transmissible through viruses –> Rous Sarcoma Virus
Why does the Rous sarcoma virus cause cancer (sarcoma)?
RSV goes through reverse transcription resulting in a dsDNA provirus. The provirus is integrated next to the host c-rsc sequence. There is co-transcription of viral and c-rsc sequences resulting in the creation of viral oncogene (v-src). This is packaged into a capsid. This causes oncogenic transformation and abnormal growth in host cells.
v-src
An oncogene coding for a 60kDa intracellular tyrosine kinase which can phosphorylate cellular proteins and affect growth
Agents which convert proto-oncogenes to oncogenes
Carcinogens
- chemical
- physical
- hereditary
- viruses
How are viral oncogenes transmitted?
viral oncogenes can be transmitted by either DNA or RNA viruses
How do DNA viruses cause oncogenesis?
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.
Encode various proteins along with environmental factors can initiate and maintain tumours
How do RNA viruses cause oncogenesis?
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 do proto-oncogenes encode?
They are part of normal signal transduction pathways, encoding components of the growth factor signal transduction pathway
How are oncogenes activated?
Alterations to proto-oncogene sequence:
- mutation
- insertion
- amplification
- translocation
These alterations cause a loss of response to growth regulatory factors
-only one allele needs to be altered
Products of proto-oncogenes
Proteins involved in the transduction of growth signals:
- growth factors
- growth factor receptors
- intracellular signal transducers
- nuclear transcription factors
Function of oncogene proteins
The majority of oncogene proteins function as elements of the signalling pathways that regulate cell proliferation and survival in response to growth factor stimulation
-leads to cancer
Oncogenic proteins can act as…
Growth factors
-e.g. EGF
Growth factor receptors
-e.g. ErbB
Intracellular signalling transducers
-e.g. Ras and Raf
Activity of intracellular signalling molecules Ras and Raf
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 Oncogene Family
· Ras genes were identified from studies of two cancer-causing viruses- the Harvey sarcoma virus and the Kirstem sarcoma virus.
What are Ras proteins?
small GTPases that are normally bound to GDP in a neutral/inactive state
How can Ras proteins cause oncogenesis?
Point mutations in one of three codons:
- codon 12
- codon 13
- codon 61
Consequence is a loss of GTPase activity of the RAS protein normally required to return active RAS to the inactive RAS GDP, resulting in hyperactive RAS (constitutive activation) which can cause oncogenesis
Normal function of Ras
1) Extracellular growth factor signal binds to membrane receptor
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
*Ras hydrolyses GTP to GDP fairly quickly, turning itself “off”
Mutations in codon 12 of Ras causes these cancers…
Glycine -> Valine
-bladder carcinoma
Glycine -> Cysteine
-lung cancer
MYC Proto-oncogene Family
The MYC proto-oncogene family consists of 3 members, C-MYC, MYCN, and MYCL, which encode transcription factors (oncoproteins) c-Myc, N-Myc, and L-Myc, respectively which regulate the transcription of at least 15% of the entire genome
Where was MYC oncogene originally identified?
in avian myelocytomatosis virus (AMV)
What are the major effects of MYC?
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 such as cell proliferation, differentiation, survival and immune surveillance
Normal function of MYC proto-oncogene
encodes a helix-loop-helix transcription factor that dimerizes with its partner protein, Max, to transactivate gene expression
MYC in cancer
MYC oncogene is overexpressed/activated in the majority of human cancers by chromosomal translocation in this oncogene:
- MYC comes under the control of foreign transcriptional promoters, causing it to be activated
- this leads to deregulation of the oncogene that drives relentless proliferation
Chromosomal Translocation in Burkitt’s lymphoma
In BL, there are three distinct alternative chromosomal translocations involving chromosomes 2, 14 and 22
Due to the chromosomal translocation, MYC gene is now under foreign regulation of the Ig heavy chain, and therefore c-myc expression is deregulated
In all three translocations, a region from one of these chromosomes is fused to a section of chromosome 8, switching MYC gene on all the time
Chromosomal Translocation in Chronic Myelogenous Leukaemia (CML)
Chromosomal translocation t(9;22)(q34;q11) creating the Philadelphia chromosome
Encodes for BCR-ABL fusion protein. The tyrosine kinase of the oncogene ABL is constitutive leading to abnormal proliferation
Treatment for Chronic Myelogenous Leukaemia
Imatinib (Gleevac)
-tyrosine kinase inhibitor
What counteracts the effect of oncogenes?
Tumour suppressor genes
When activate, tumour suppressor genes will either…
induce cell cycle arrest
OR
induce apoptosis
How is tumour suppressor gene function lost?
Inactivation of BOTH allele of the gene as a result of:
- mutation
- deletion
*causes cancer
Types of tumour suppressors and their MOA
Different functions are associated with each tumour suppressor:
- Regulators of cell cycle checkpoints (e.g. RB1)
- Regulators of differentiation (e.g. APC)
- Regulators of DNA repair (e.g. BRCA1)
Retinoblastoma (Rb)
rare childhood cancer that develops when immature retinoblast cells continue to grow very fast and do not turn into mature retinal cells
Features of retinoblastoma
An eye that contains a tumour will reflect light back in a white colour
-called LEUKOCORIA
Types of retinoblastoma
Familial/Hereditary (40%)
Sporadic (60%)
Where is the hereditary mutation for retinoblastoma?
deletion on chromosome 13 (13q14) in the region containing the retinoblastoma 1 gene
‘Two-hit’ hypothesis
Mutation of both alleles necessary to inactivate tumour suppressor genes
ie the reason cancers are often associated with old age (mutation rates are slow so over a longer time, increased chance of two ‘hits’)
Loss of heterozygosity
describes the process that leads to the inactivation of the second copy of a tumour suppressor gene
a heterozygous cell receives a second hit in its remaining functional copy of the tumour suppressor gene, thereby becoming homozygous for the mutated gene
Mutations which inactivate tumour suppressor genes are called…
Loss-of-function mutations
- point mutations
- small deletions
these disrupt the function of the protein that is encoded by the gene
RB Gene family
Includes three pocket proteins:
- Rb (p105/110)
- p107
- Rb2/p130
Retinoblastoma protein (pRB)
transcriptional co-factor that can bind to transcription factors (over 100 binding partners)
Function of pRB
regulates apoptosis and the cell cycle by stimulating or inhibiting the activity of interacting proteins
What is the main binding partner of pRB?
main binding partner is the E2F transcription factor, interacting with the large pocket of the Rb
How does pRB regulate the cell cycle?
it inhibits the G1 to S phase transition
Important cell cycle regulatory proteins
Cyclins and their associated cyclin dependent kinases (CDKs)
Cyclin D
- First cyclin to be synthesised in response to growth-stimulatory signals
- Drives progression through G1 with CDKs4/6
Dephosphorylated/Hypophosphorylated pRB
pRB is active and remains bound to E2F
-inhibits cell proliferation by inhibiting the G1 to S phase transition
Hyperphosphorylated pRB
When Rb is hyperphosphoprylated in response to extracellular physiological signals it is inactive
- this allows release of E2F
- E2F migrates to nucleus and induces transcription of genes which drive cell cycle progression from G1 to S phase
- deregulated cell cycle leads to cancer
Ways in which Rb can be inactivated
- phosphorylation
- mutation
- viral oncoprotein binding
pRB in retinoblastoma
functionally inactivated by mutations (partial deletions)
pRB inactivation by viral oncoprotein binding
Viral inactivation is found in small DNA tumour viruses mainly by disrupting E2F binding or destabilisation of Rb:
- Adenovirus (E1A)
- Papilloma (E7)
- Polyoma (Large T antigen)
First tumour suppressor gene to be identified is…
p53
p53 protein (transcription factor) function
regulates genes involved in DNA damage repair, apoptosis and cell cycle arrest
MOA of p53
acts as a transcription factor, binding to around 300 different gene promoter regions
Normally, levels of p53 protein in cells are…
low
Regulation of p53
p53 protein levels in cells are kept low by MDM2 protein, a ubiquitin ligase (also an oncogene)
How does Mdm2 regulate p53 (keep them low)?
In normal cells:
MDM2 binds p53 to form a complex in the nucleus where MDM2 protein modifies the carboxyl terminus of p53 and adds a ubiquitin tag onto the lysine residues on that protein, which then gets targeted by the proteosome for degradation
Half-life of wild-type p53
short 20-minute half-life
How is p53 activated?
Stress signals (e.g. ionising radiation)
- sensed by kinases that then phosphorylate p53
- phosphorylation of p53 disrupts interaction with MDM2
p53 inactivation
p53 is inactivated by loss-of-function mutations in more than half of human cancers
-95% in the DNA binding domain
Clinical significance of p53
The role of p53 in suppressing tumorigenesis makes it a promising therapeutic target
Therapeutic strategies against p53 inactivation
Different strategies are aimed at correcting p53 mutation and restoring wild-type p53 function by targeting its regulators:
Gene Therapy:
> 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
Use of inhibitors
Genetic Analysis and oncogenesis
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
