oncogenes and tumour suppressor genes Flashcards
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)
growth factor signalling, oncogenes and tumour suppressor genes
Many oncogenes are normally components of growth factor signalling pathways that when mutated produce products in higher quantities or whose altered products have increased activity and therefore act in a dominant manner
Many tumour suppressor gene products act as a stop signal to uncontrolled growth, may inhibit the cell cycle or trigger apoptosis
describe oncogenes
make cells divide, driving cell division forward
in cancer, pick up mutation mean they are permanently active
= gain of function
describe tumour suppressor genes
counteract the oncogene
loss of function
rou’s protocol for inducing sacroma in chickens
tumour developed weeks later
taking new sarcoma, filtrates produced could induce tumour in other chickens
cycles repeated indefinitely. carcinogenic agent small enough to pass through filter
filter used excluded bacteria it was not small enough to exclude viruses
rous concluded that virus must be responsible for induction of tumour formation
capture of c-src by retrovirus
during evolution, virus can acquire fragments of genes from host at integration and process results In creation of oncogenes
oncogene product was characterised as a 60kDa intracellular tyrosine kinase
can phosphorylate cellular proteins and effect growth
exception to central dogma DNA-RNA protein
the oncogene hypothesis
Bishop and Varmus used different strains of Rous sarcoma virus in their research, they:
Identified the v-src oncogene as responsible for causing cancer.
Used hybridization experiments, and they found that the c-src gene was present in the genome of many species.
They then showed that the host cell c-src gene was normally involved in the positive regulation of cell growth and cell division.
Following infection, however, the 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.
viral oncogenesis
Approximately 15%-20% of all human cancers are caused by oncoviruses
Viral oncogenes can be transmitted by either DNA or RNA viruses.
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
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
activation of oncogenes
over 100 identified oncogenes
examples of oncogenes for every type of protein involved in a growth factor signal transduction pathway
these genes captured by animal retroviruses are altered in human cancer, activation can involve mutations, insertions amplifications and translocations
describe activation of oncogenes
mutation
amplification/duplication
translocation
porto-oncogenes encode components of the growth factor signal transduction pathway
4 types of proteins are involved in the transduction of growth signals Normally Growth factors Growth factor receptors Intracellular signal transducers Nuclear transcription factors
Growth factors, signal transduction and cancer
The majority of oncogene proteins function as elements of the signalling
pathways that regulate cell proliferation and survival in response to growth
factor stimulation
Oncogene proteins act as growth factors (e.g.EGF),
growth factor receptors (e.g. ErbB) and 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
To date-over 100 identified oncogenes
intracellular signal transducers
RAS oncogene family
ras genes identified from 2 cancer causing viruses
RAS proteins are small GTPases that are normally bound to GDP in neutral state
oncogenic activation of ras is seen in about 30% human cancers
most commonly mutated oncogene
intracellular signal transducers 2
RAS oncogene family
- Binding of extracellular growth factor signal
- 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)
- These kinases ultimately phosphorylate targets, such as
transcription factor to promote expression of genes
important for growth and survival
Ras hydrolyzes GTP to GDP fairly quickly, turning itself “off”
intracellular signal transducers 3
Consequence of each of these mutations is a
loss of GTPase activity of the RAS protein
normally required to return active RAS to
the inactive RAS GDP
transcription factors
MYC oncogene family
The MYC oncogene family consists of 3 members,
C-MYC, MYCN, and MYCL, which encode c-Myc, N-Myc,
and L-Myc, respectively
Originally identified in avian myelocytomatosis virus (AMV)
The MYC oncoproteins belong to a family of transcription factors that regulate the transcription of at least 15% of the entire genome
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
MYC oncogene family 2
The MYC oncogene is overexpressed in the majority of human cancers and contributes to the cause of at least 40% of tumours
It encodes a helix-loop-helix leucine zipper transcription factor that dimerizes with its partner protein, Max, to transactivate gene expression
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.
Such activation is a result of chromosomal translocation
activation of MYC in Burkitts lymphoma
Epstein Barr virus is associated with Burkitt’s lymphoma (BL)
BL is a high grade lymphoma that can effect children from the age of 2 to 16 years
In central Africa, children with chronic malaria infections have a reduced resistance to the virus. This is known as classical African or endemic BL
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
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
chromosomal translocation is responsible for the activation of other oncogenes
Chronic myelogenous leukaemia (CML) accounts for 15-20%
of all leukaemias
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
As a result of this translocation the tyrosine kinase activity
of the oncogene ABL is constitutive leading to abnormal
proliferation
Therapeutic strategies for CML include Imatinib (Gleevac) a tyrosine kinase inhibitor-96% remission in early-stage patients
discovery and identification of tumour suppressor genes
In 1969 Henry Harris and his colleagues performed somatic cell hybridization experiments
Fusion of normal cells with tumour cells yielded hybrid cells containing chromosomes form both parents. These cells were not capable of forming tumours
Genes derived from the normal parent acted to inhibit or suppress tumour development
The first tumour suppressor gene was identified by studies of retinoblastoma, a rare childhood eye tumour
logic of tumour suppressor genes
Discovery of oncogenes, an explanation for proliferation
Like other well designed control systems biological systems follow a similar logic-component promoting a process
must be counterbalanced by others that oppose the process-tumour suppressor genes
tumour suppressor genes
Body has mechanisms to ‘police’ processes that regulate cell numbers
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)
Loss of tumour suppressor gene function requires inactivation of both alleles of the gene
Inactivation can be a result of mutation or deletion
Tumour suppressor genes are defined as recessive genes
Sometimes referred to as ‘anti-oncogenes’
the retinoblastoma gene, Rb
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.
Two forms of the disease, familial (40%) and sporadic (60%)
The hereditary mutation is on chromosome 13 (13q14),
the retinoblastoma 1 (Rb1) gene
discovery of retinoblastoma
The existence of the RB1 gene was predicted in 1971 by Alfred Knudson
Whilst studying the development of retinoblastoma he proposed that the development of retinoblastoma requires two mutations, which are now known to correspond to the loss of both of the functional copies of the Rb gene - “two-hit” hypothesis
“Loss of heterozygosity“ often used to describe
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 that inactivate tumour suppressor
genes, called loss-of-function mutations, are
often point mutations or small deletions that
disrupt the function of the protein that is
encoded by the gene
the retinoblastoma protein RB structure
The Rb gene family includes three members: Rb/(p105/110), p107 and Rb2/p130
-collectively known as pocket proteins
pRb is a multi functional protein (110kDa) with over 100 binding partners
A transcriptional co factor that can bind to transcription factors
RB functions in diverse cellular pathways, such as apoptosis and the
cell cycle, it has also become clear that RB regulates these pathways
through the stimulation or inhibition of the activity of interacting proteins.
Therefore, an important starting point for understanding RB function is its
structure, which acts as a scaffold for these multiple protein interactions
It’s main binding partner is the E2F transcription factor,
interacting with the large pocket
Other viral oncoproteins can bind to Rb
retinoblastoma protein RB and cell cycle
- main function of Rb is to regulate the cell cycle by inhibiting the G1 to S phase transition
- 2 important proteins involved in cell cycle are: cyclins and their associated cyclin dependent kinases
- passage of a cell through the cell cycle is regulated cyclins and cyclin dependent kinases
retinoblastoma protein RB and cell cycle 2
Cyclin D is the first cyclin to be synthesized and drive progression through G1 together with cdks4/6
The G1 checkpoint leads to the arrest of the cell cycle in response to DNA damage
A key substrate for cyclin D is RB protein
Cyclin D and E families and their cdks phosphorylate RB
retinoblastoma protein RB, function, phosphorylation and activity
Rb protein regulates the activity of the E2F transcription factor crucial for the expression of genes required for S phase
Rb activity is regulated by phosphorylation
When the Rb tumour suppressor is active it can inhibit cell proliferation
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
When Rb is hyperphosphorylates , in response to extracellular physiological signals it is inactive
Upon phosphorylation of RB, E2F is released and migrates to the nucleus to induce transcription
When RB is inactive cell cycle progression from G1 to S occurs
inactivation of Rb - loss of function
Rb can be inactivated by 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
P53 tumour suppressor
The p53 gene was the first tumour suppressor gene to be identified
The p53 protein is at the heart of the cell’s tumour suppressive mechanism and has been nicknamed the ‘guardian of the genome’
It is involved in sensing DNA damage and regulating cell death/apoptosis
as well as other pathways
p53 is mutated in 30-50% of commonly occurring human cancers
Frequent mutation of p53 in tumour cell genomes suggests that tumour
cells try to eliminate p53 function before they can thrive
p53 specializes in preventing the appearance of abnormal cells
p53 tumour suppressor 2
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
Normally levels of p53 protein are low in cells
These levels are kept low by MDM2 protein, a ubiquitin ligase (also an oncogene)
In unstressed normal cells both p53 and MDM2 move between the nucleus and cytosol
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
WT p53 has a short 20 min half life
activation of p53 tumour suppressor
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
p53 mutation/therapeutic strategies
Mutational inactivation is considered to be one of the most common molecular mechanisms behind the dysfunction of p53.
Extensive mutation search revealed that more than half of human cancers carry loss of function mutations of p53
Among them, 95% of mutations were detectable within the DNA-binding domain
Role of p53 a s star player in suppressing tumorigenesis makes it a promising therapeutic target
Different strategies aimed at:
- Correcting p53 mutation and restoring wild-type p53 function by targeting its regulators
therapeutic strategies
Gene therapy obvious approach
Many vectors and retroviruses have been examined
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
Alternative strategies- use of inhibitors 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
Inhibitors of CRM1 result in nuclear
accumulation of p53
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
