Lecture 27: Cancer Cells: Excessive Birth Rate Flashcards
What are cancer cells (in terms of chromosomes?)
Aneuploid
What is euploid?
Abnormal number of chromosomes (humans= 46)
What is aneuploid?
An abnormal number of chromaomes
Cancer cells may also have what kind of altered chromosomes?
Structurally altered
Describe the nature of abnormalities in cancer cells
Many abnormalities are random but some are characteristic of a particular type of tumour
Cancer cells containing excess copies of certain genes can increase what?
Gene copy number (amplification)
What can amplification arise from the accumulation of?
Extra chromosomes in which the gene is found,
Multiple small “double minute” chromosomes each composed of a segment of chromosomal DNA (an amplicon) containing the relevant gene.
Multiple copies of the gene arranged in head to tail tandem configuration in a chromosomes which appears as a homogenously staining region
Which gene is often amplified in cancer?
The gene containing the epidermal growth factor receptor EGFR
How does EGFR bind to growth factors like EGF and TGFα?
Epidermal growth factor receptor has an extracellular ligand binding domain which binds growth factors,
And a transmembrane domain and a cytoplasmic tyrosine kinase domain which phosphorylates protein substrates in tyrosine
What may happen to the EGFR gene in advanced cancers?
It may undergo a deletion where the ligand binding domain is removed from the EGFR protein .
What happens in a normal EGFR receptor?
The ligand binding domain in the absence of the ligands (EGF and TGFα) suppress tyrosine kinase activity
What happens in the truncated protein?
It has deregulated tyrosine kinase activity
What kind of chromosome translocations do many cancers have?
Characteristic chromosome translocations
Due to exchange of genetic material between non homologous chromosomes
What is the effect of the exchange of genetic material between non homologous chromosomes?
They place genes into novel genetic environments and deregulate their expression.
What are mantle cell tumours?
Tumours of b lymphocytes
What break points do mantle cell lymphomas have?
Chromosome (c) 11 near the B cell lymphoma-1 gene and
C-14 within the immunoglobulin heavy chain gene (IgH)
What does the translocated BCL-1 gene become controlled by and why?
IgH gene transcriptional enhances as the translocated BCL-1 gene is placed near it
How can the mutant cancer causing genes in cancer cell DNA be assayed?
Transferring (transfecting) the DNA from cancer cells into partially transformed mouse cells and observing phenotypic change in the latter
What are the effects of transfecting DNA from partially transformed cells (which are immortalised, as they have no limits to proliferation)
They become fully transformed cells and have
Reduced dependence on growth factors Reduced density dependent inhibition of proliferation producing multiple layers of cells Reduced adherence to substrate Anchorage - independent proliferation Tumor formation in mice
What type of genes was discovered by transfecting?
One of the RAS genes which causes 25% of human cancers
What do the oncogenic versions of the RAS genes contain?
Mutations at 1 of 3 amino acid positions (codons 12, 13 or 61)
What is retinoblastoma?
Rare childhood cancer of the eye
How can the tumour be predisposed?
It can either be inherited or it can occur sporadically
What is the difference between hereditary and sporadic retinoblasomas?
Hereditary retinoblastomas develop with single hit kinetics
Sporadic retinoblastomas show two-hit kinetics
What did Knudson propose?
That two genetic mutations are required for cancer development
Hit1: inheritance or mutation of one allele of the RB1 gene
Hit2: loss of the remaining (normal wild type) allele due to nondisjunctionvloss of chromosome 13 at mitosis, mitotic crossing over, deletion
What is required for mutations to become oncogenic.
The RB1 gene function must be totally lost.
This is lost in 25% of human cancers
What is gene activation by promoter methylation?
When cancers possess a normal RB1 genes but do not express its product.
This gene is transcriptionally silenced when cytosines in 5’-C-G-3’ dinucleotides aka CpG are methylated in the promoter
The methylated base binds depressor proteins that suppress gene transcription
What are the different ways that genes can be activate?
Amplification, deletion, Point mutation Mutation Promoter methylation
How are genes activated by amplification?
Increased gene copy number in cancerous cells, over expression of certain genes
How does gene activation occur with deletion?
Deletion of gene, results in truncated protein which has deregulated tyrosine kinase activity
TK normally is suppressed
How does gene activation occur by point mutation
-
How does gene activation occur by mutation?
Loss of the gene function due to mutation can cause it to be oncogenic
How does gene inactivation occur with promoter methylation?
The gene is transcriptionally silenced when the promoter is methylated
What are proto-oncogenes and tumour suppressor genes?
Genes that regulate cell proliferation and differentiation processes
What can happen during carcinogenesis?
Proto-oncogenes may be mutated into oncogenes that drive cancer growth.
Tumour suppressor genes are inactivated
What happens in normal POG and TSG function?
Proto oncogenes activate proliferation and suppress differentiation
And tumour suppressor genes inhibit proliferation and promote differentiation
What are the roles of POG and TSG in different mechanisms of oncogenic change?
Proto oncogenes encode the amplification (of EGFR), deletion of domain (EFGR) translocation (cyclin D) point mutation (Ras)
while tumour suppressor genes encode the
gene inactivation (RB1)
Gene silencing by methylation (RB1)
What are the roles of POG and TSG resulting from changes to protein function?
Proto oncogenes encode mutations, which gives increased or altered activity and gain of function
Tumour suppressor genes encode mutation to give less activity
What are the roles of POG and TSG as a result of changes to cell behaviour?
Proto - oncogenes drives abnormal growth
Tumour suppressor genes releases cells to an abnormal phenotype
What is the overall effect of proto oncogenes?
Too much throttle
What is the overall effect of tumour suppressor genes?
Not enough break
How many stages to proliferating cells traverse in each cycle?
Four stages:
Mitosis, growth1, synthetic phase, growth 2
What is the mitosis phase?
Culminates in cytokinesis
Produces two daughter. Cells
What is the gap 1 or growth 1 phase?
This is when cells sense their environment for growth factors, space and substrate attachment to decide whether or not they should complete a round of cell division.
Cells may exit the cycle and enter a non proliferating phase of G0
Or they can choose to cycle and prepare to synthesise their DNA
What is phase G0
The phase where cells are not proliferating. It is a state that cells can choose to enter to
What is the restriction point?
The point at which the cell decides to continue cycling
What is lost in cancer cells? (In terms of the cell cycle)
The control of progression through G1 phase
What is the S (DNA synthetic) phase?
DNA is replicated in preparation for cell division
Cells in this phase can be identified by supplying cells with precursors used to make DNA
What are the precursors used to make DNA used to identify cells in the s phase ?
Tritiated thymidine followed by radioautography or
5- bromo- 2’-deoxyuridine followed by anti-BUdr antibody conjugated with an appropriate label
What is the G2 phase of the cell cycle?
It is when quality control pathways check for replication completion.
This requires a tetrapods DNA content, 4n
Cells prepare for mitosis
The cell checks it has completed one phase of the cycle before entering the next phase,
How is the control of cell checking the completion of one cycle before entering the next, mediated?
Cyclin-dependent kinases which consist of a catalytic unit and a regulatory unit (aka the cyclin) are necessary for activity
What do the cyclins do?
They oscillate in concentration through the cell cycle
When do the cyclins reach their maximum?
At the G1/S transition (cyclin E-Cdk2)
During the S phase (A-Cdk2)
At the G2/M transition (cyclin B-Cdk1)
What do growth factors like TGFα and PDGF promote?
Cell division .
They induce receptor dimerisation , tyrosine kinase activation, receptor phosphorylation and phosphorylation of signalling molecules
What do growth factor receptors show in cancer?
Excessive tyrosine kinase activity
The epidermal growth factor receptor may also be excessively active as a result of
High concentrations of growth factors
High concentration of receptor which may follow gene amplification
Mutant receptor which lacks the ligand binding domain,
Mutant receptor with single base changes in the tyrosine kinase domain
What do ras proteins do?
They integrate signals from growth factor receptors.
Receptor activation causes ras proteins to bind to GTP, they then change conformation and can bind to other signalling proteins
When does signalling stop?
When ras GTPase activity hydrolysis GTP to GDP and phosphate.
What do mutant ras proteins have that causes them to remain active.,
Reduced GTPase activity
How is the BCL-1 gene activated?
By ras signalling
What does the BCL-1 gene encode?
It encodes a cyclin (cyclin D1) that activates the cyclin dependent kinase Cdk4
This also stimulates entry into the cell cycle
In cancer cells its degranulation will cause uncontrolled entry into the cycle
What are the two forms that pRB protein encoded by the RB1 gene exists (these vary through the cell cycle)
1) in early G1 it is weakly phosphorylated and prevents entry into the cycle
2) in late G1, it becomes heavily phosphorylated, (ppRb) and allows entry into the cycle
What is the initial kinase that phosphorylates pRb and prevents its cell cycle-suppressive effect
D-Cdk4
What does the conversion of pRb to ppRB allow?
The transcription of genes needed for S phase, including the gene for cyclin E
