Oncogenes and tumour suppressor genes Flashcards
What were the original 6 hallmarks of cancer
Sustaining proliferative signalling Resisting cell death Inducing angiogenesis Enabling replicative immortality Activating invasion and metastasis Evading growth suppressors.
What are the emerging and enabling hallmarks
Emerging:
Deregulating cellular energetics- switch to the more energy dependent anaerobic respiration
Avoiding immune destruction.
Enabling:
Genome instability and mutation- to promote mutagenesis that leads to aberrant growth
Tumour-promoting infiltration.
Summarise the key features of the cell cycle
Cycle checkpoints (growth arrest ensures genetic fidelity).
Specific proteins accumulate/ are destroyed during the cycle.
Cyclins, cycle dependent kinases, cycle dependent kinase inhibitors
Permanent activation of a cyclin can drive a cell through a checkpoint.
Describe the G1-S checkpoint
Check for DNA damage
Restriction point: check for cell size and favourable environmental conditions (growth factor)
Describe the G2-M checkpoint
Check for damage or unduplicated DNA
Describe the metaphase checkpoint
Check for chromosome attachment to mitotic spindle.
What happens at these checkpoints
Cell cycle is arrested
If DNA damaged- tries to repair with DNA repair genes e.g BRCA
If damage too big- apoptosis.
Describe what is meant by proto-oncogenes
Proto-oncogenes code for essential proteins involved in maintenance of cell growth, division and differentiation.
Mutation converts a proto-oncogene to an oncogene, whose protein product no longer responds to control influences.
Oncogenes can be aberrantly expressed, over-expressed or aberrantly active.
E.g. MYC, RAS, ERB, SIS
Proto-oncogenes can be converted to an oncogene by a single mutation.
What is the key difference between photo-oncogenes and oncogenes
Proto-oncogene: code for essential proteins involved in maintenance of cell growth, division and differentiation e.g. Kinases, phosphorylases, transcription factors
Oncogene: mutations in proto-oncogenes that promote uncontrolled cell proliferation - aberrantly expressed, over-expressed (e.g. HER2) or aberrantly active
Describe how normal photo-oncogenes can be activated
The normal proto-oncogene can be activated in 4 ways:
1. Mutation in the coding sequence.
Point mutation or deletion.
2. Gene amplification.
A protein may block the DNA polymerase so the polymerase repeatedly backs up to go over the area a few times creating many identical genes.
3. Chromosomal translocation.
Chimeric genes.
4. Insertional mutagenesis.
Viral infections – some viruses insert their genome into our DNA and usually this isn’t a problem as much of our DNA does not code but if it’s in a coding region, this could be cancer.
What is the end result of gene amplification
You get multiple copies of the protein produced.
Problem with HER2 in Breast Cancers
What are Chimeric genes
Genes that are formed by combinations of portions of one or more coding sequence to produce new genes (e.g. the swapping of tips of chromosomes)
When can the formation of chimeric genes be problematic
If one of the pieces of translocated DNA is a promoter, it could lead to upregulation of the other gene portion (this occurs in Burkitt’s lymphoma)
If the fusion gene codes for an abnormal protein that promotes cancer
What is the Philadelphia chromosome
Chromosome produced by the translocation of the ABL gene on chromosome 9 to the BCR gene on chromosome 22
The BCR-ABL fusion gene encodes a protein that promotes the development of cancer
BCR is anti-apoptotic- so enables tumour to be resistance to stop signals.
Describe the two possible results of chromosomal translocation and insertional mutagenesis
Strong enhancer increases normal protein levels
e.g. Burkitt’s lymphoma- lose normal regulation of gene
Fusion to actively transcribed gene overproduces protein or fusion protein is hyperactive.
e.g. Philadelphia chromosome
Summarise the Philadelphia chromosome
Philadelphia chromosome: end of long arms of Chr9 and Chr22 exchanged to form the BCR-ABL fusion protein - ABL is a very strong promotor region
Summarise the signal transduction pathways
Steroid hormone- Nuclear or cytosolic receptor — transcription/translation — proliferation
Tyrosine Kinase receptor — ligand binds — phosphorylation — proliferation
GPCR- kinase- phosphorylation – proliferaation
Proteins involved in signal transduction are potential critical gene targets (proto-oncogenes)
Activation of proto-oncogenes to oncogenes disrupts normal activity
Give some examples of signal transduction proteins that are photo-oncogenes
Signal transduction proteins are proto-oncogenes.
Examples:
o Tyrosine kinase receptors EC – met, neu.
o Tyrosine kinase receptors IC – src, ret.
o Transcription factors – myc, fos, jun.
o GPCR g-proteins – ras, gip-2.
o Kinases – raf, pim-1.
May habe a point mutation which changes conformation - so inhibitory proteins cannot bind
or changes phosphorylation residues- such that it’s constitutively active.
The higher up the pathway the mutation is- the harder it is to treat- due to a greater dysregulaiton of downstream pathways.
Describe the aberrant activity of mutant Ras
Upon binding GTP, RAS becomes active.
Dephosphorylation of the GTP to GDP switches RAS off.
Mutant RAS fails to dephosphorylate GTP and remains active.
Leading to cell proliferation independent of growth signals
Describe some photo-oncogenes and their associated cancers
SRC
Tyrosine Kinase
Overexpression/ C-terminal deletion
Breast/colon/lung
MYC
TF
Translocation
Burkitt’s lymphoma
JUN
TF
Overexpression/deletion
Lung
Ha-RAS
G-protein
Point mutaation
Bladder
Ki-RAS
G-protein
Point mutation
Colon/lung
Describe Ras signalling
o The binding of GTP allows RAS to bind RAF and pass the signal to RAF deliver the signal further to MEK and ERK.
Phosphorylation of Tfs (myc. jun, fos)
Translocation to the nucleus
increasing RNA synthesis and transcription and translation of effector proteins which lead to cell proliferation and cell survival.
Summarise tumour suppressor genes
Typically proteins whose function is to regulate cellular proliferation, maintain cell integrity.
E.g. RB
Each cell has two copies of each tumour suppressor gene- except in haploinsufficiency- where one mutation is sufficient to produce a tumour
Mutation or deletion of one gene copy is usually insufficient to promote cancer.
Mutation or loss of both copies means loss of control.
Describe Knudson’s two hit hypothesis
Sporadic cancer : 2 acquired mutations in TSG
Hereditary cancer: 1 inherited and 1 acquired mutation- increased risk of cancer- already have 1 hit
Summarise the inherited cancer susceptibility
Family history of related cancers. Unusually early age of onset. Bilateral tumours in paired organs. Synchronous or successive tumours. Tumours in different organ systems in same individual. Mutation inherited through the germline.
Describe the key features of retinoblastoma
Malignant cancer of developing retinal cells.
Sporadic disease usually involves one eye. Hereditary cases can be unilateral or bilateral and multifocal.
Due to mutation of the RB1 tumour suppressor gene on chromosome 13q14.
RB1 encodes a nuclear protein that is involved in the regulation of the cell cycle.
RB1 ensures progression of cell-cycle progression fro G1 – S
Describe the functional classes of tumour suppressor genes
Regulate cell proliferation Maintain cellular integrity Regulate cell growth Regulate the cell cycle Nuclear transcription factors DNA repair proteins Cell adhesion molecules Cell death regulators
All with the aim of:
Suppress the neoplastic phenotype
What is important to remember about tumour suppressor genes
A lot are lethal to the embryo
Describe some examples of tumour suppressor genes
P53
Cell cycle regulation
Many
BRCA1
Cell cycle regulation
Breast/ovarian/prostate
Singl strand break repair, if overwhelmed- double strand break repair (homologous recombination)
PTEN
Tyrosine and lipid phosphotase
Prostate/glioblastoma and Cowdens syndrome
APC
Cell signalling
Colon
p16INK4A
Cell cycle regulation
Colon
MLH1
Mismatch repair
Colon, gastric
Summarise how a cell responds to DNA damage and cellular stress
Detect cellular insult and then switch on genes to repair the cell. Cellular stresses can include: Oxidative stress Nitric Oxide Hypoxia Ribonucleotide depletion mitotic apparatus dysfunction oncogene activation DNA replication stress Double-strand breaks Telomere erosion
These insults are sensed by the tumour suppressor gene p53
P53 is normally inactivated by a partner protein called MDM2
However, these insults cause MDM2 to be lost, activating p53 and allowing it to act as a transcription factor to turn on gene expression for genes involved in DNA repair
Describe the p53 mediated response to mild/physiological stress and severe stress
Mild- regulation of p53 mediated genes involved in: metabolic homeostasis antioxidant defence DNA repair growth arrest
Severe stress- p53 mediated protein-protein interactions which lead to apoptosis.
How many mutations are needed in p53 to get dysregulation of activity
Although p53 is a tumour supressor gene, mutants of p53 act in a dominant manner and mutation of a single copy is sufficient to get dysregulation of activity.
Why is p53 hard to target in therapy
So many ubiquitous roles- would lead to plethora of side effects
Important in healthy cells- side effects
Molecular profiling of tumours to look for mutations - p53 may be driver mutations or passenger mutations-
can’t target p53 to one its functions.
describe the importance of p53 mutations in cancer
Associated with poor prognosis and family histories of sarcomas and rare tumours.
Describe the role of the APC tumour suppressor gene in familial adenomatous polyposis coli
Due to a deletion in 5q21 resulting in loss of APC gene (tumour suppressor gene).
Involved in cell adhesion and signaling- MAPK
Sufferers develop multiple benign adenomatous polyps of the colon.
There is a 90% risk of developing colorectal carcinoma- prophylactic colonectomy and regular colonoscopies from teen years
Adenomatous polyposis coli protein
What signalling pathway is APC involved in
The tumour suppressor gene APC participates in the WNT signalling pathway.
APC protein is a negative regulator of b-catenin, thereby preventing uncontrolled cell division.
Mutation of APC is a frequent event in colon cancer.
Wnt has a stimulatory effect on beta-cantenin
Summarise the APC TSG
APC TSG: mutation of APC gene causing uncontrolled growth creating polyps, each with increased risk of cancer - becomes almost inevitable
Normal function is to degrade beta catenin (WNT pathway - normally bound to cadherin in adherens junctions)
Mutant APC leads to buildup of beta catenin, leading to binding to LEF-1
LEF-1: complex causes gene transcription
What are two features of a normal healthy state
Proto-oncogene
Tumour suppressor gene
Describe the different combinations that can lead to cell growth and proliferation (uncontrolled) and thus cancer
Cancer can be triggered by:
o Oncogene + TSG.
o Proto-oncogene + defective TSG.
o Oncogene + defective TSG.
Describe an analogy that summarises the pathogenesis of cancers
Normal genes (regulated cell growth)- TSGs and port-oncogenes in check- car travels at steady speed
1st mutation (susceptible to cancer)- active oncogenes accelerating car- but TSGs acting as brake to halt progression towards cancer
2nd mutation or loss of TSG- accelerates to cancer
Outline the development of colorectal cancer
Normal epithelium- Apc mutation - leads to hyper proliferative epithelium
This increases the rate of mutations and likelihood go genomic instability:
DNA hypomethylation and k-RAS MUTATION leading to adenoma
p53 mutation then forms carcinoma which can then metastasise
Hyperplasia Metaplasia Dysplasia Carinogenesis
What are the two main functions of b-cantenin
Functions in cell-cell adhesion (1 mark)
Regulates transcription / gene expression (1 mark)
Which protein, defective in familial adenomatous polyposis (FAP), regulates levels of b-catenin by controlling its degradation?
The adenomatous polyposis coli protein (1 mark)
0.5 mark for APC
Summarise Oncogenes
Oncogene
Active in tumours
Translocations/point mutations
Not inherited
Dominant
Leukaemia/lymphoma
Broad tissue specificity
Summarise TSGs
Oncogene
TSG
Inactive in tumours
Deletions/mutations
Inherited
Recessive
Solid tumours
Tissue specificity
Describe the number of mutations needed for each cancer type
Colon- 11 Kidney- 2 Stomach- 4 Lungs- 6 Breast- 4 Brain -6
What is important to remember about cancer
Human cancer involves damage to DNA, or inheritance of aberrant sequences, at critical gene targets.
These targets, proto-oncogenes and tumour suppressor genes, regulate cell cycle decisions (mitosis, arrest, differentiation, apoptosis).
The ‘guardian of the genome’, p53 is a key player in decision making during the cell cycle.
Studies of rare heritable cancers have led to an understanding of tumour suppressor genes.
Colon cancer is a model for many of these factors.
Summarise the genetic mutations that can cause cancer
Genetic mutations causing cancer:
Chromosome translocation
Gene amplification
Point mutations within promotor or enhancer regions of genes
Deletions or insertions
Epigenetic alterations to gene expression
