Cancer Biology

Question 1

Give an overview of tumour development?

Answer 1

Cancer cells arise from normal tissues
Mutated cell -> benign tumour -> malignant tumour progressively invading neighbouring tissue, then sending cancer cells into the bloodstream

Tissue architecture is deranged in tumours
It goes from ordered to a disordered phenotype

Question 2

What specialised cell types do types of cancer arise from?

Answer 2

Carcinomas - from epithelial cells
Two types - squamous cell carcinomas or adenocarcinomas
Adenocarcinomas - arise from specialised cells in the epithelia that secrete substances into ducts or cavities e.g. Stomach or colon (goblet cell in small intestine)

Haematopoietic cancer - leukaemia’s and lymphomas
E.g. Multiple myelomas (plasma cells of B-cell lineage)

Sarcomas - derive from connective tissue (muscle, bone, cartilage, adipocytes)
E.g. Leiomyosarcoma (smooth muscle)

Tumours of the nervous system - central and peripheral
E.g. Medulloblastoma (from granular layer precursor’s)

Question 3

Describe the stages of tumour development?

Answer 3

Example - Lung epithelium tumour
1. Benign tumour
2. Cells break through the basal lamina
This is due to genetic properties changing
3. Cells invade capillary (intravasation) - travel through bloodstream
4. Cells adhere to capillary wall in liver
5. Cells escape from capillary (extravasation)
Cells proliferate to form liver metastasis

Question 4

What is the origin of tumours?

Answer 4

Tumours are of monoclonal origin
So they originate from a single mutation in a cell - called a driver mutation
A succession of additional mutations cause a change in phenotype - leading to cancerous tumour growth

Question 5

Describe some properties of cancer cells?

Answer 5

Cancer cells have a reduced dependence on serum (growth factors)
They show loss of contact inhibition - when culturing, the proliferation continues and the cells don’t become quiescent when coming into contact with other cells
They show anchorage-independent growth
Appear refractile
Actin cables are disarrayed
Often over proliferating and failing to differentiate
Tumour cells often show abnormalities in their ability to undergo apoptosis or to senesce

Question 6

Describe the designated hallmarks of cancer?

Answer 6

Deregulated proliferation
Block in differentiation
Limitless replicative potential
Evasion of apoptosis
Invasion and metastasis
Sustained angiogenesis
Evasion of immune attack

(Hanahan and Weinberg, 2000, 2011)

Question 7

How is cancer and age related?

Answer 7

Cancer incidence as a function of age
As we age the incidence of cancer increases exponentially - meaning your more susceptible to get cancer
Starting around age 50

Question 8

How was cancer identified as a genetic disease?

Answer 8

Both physical and chemical carcinogens act as mutagens
X-ray exposure linked to the development of cancer
1927 - Hermann Muller - induced mutations in Drosophila melanogaster by exposing flies to X-rays

Alkylating agents are carcinogenic in laboratory animals
Late 1940s - these chemicals were shown to be mutagenic in Drosophila melanogaster

Chromosome abnormalities in cancer
1914 Theodor Boveri

Question 9

What test detects mutations?

Answer 9

The Ames test
Combine - histidine-dependent Salmonella bacteria, test chemical and mammalian liver extract (added for metabolic activation of the test chemical)
This is incubated 37°C 48 hours onto minimal agar (i.e. No histidine)
Count the number of histidine-independent colonies

Colonies grow when the bacteria have acquired a mutation in the his-degrading enzyme (revertant to wild-type activity) due to the mutagenic action of the test chemical

Question 10

What is the correlation between carcinogenicity and mutagenicity?

Answer 10

As mutagenicity increases along the bottom the tumour induction (carcinogenicity) increases proportionally with this

Question 11

What was involved in the early discovery of target proteins for mutagenesis?

Answer 11

Rous was investigating sarcomas
He found a filter was too small to pass bacterial - so sarcoma must be transmitted by a virus - Rous Sarcoma Virus (RSV)

Rous Sarcoma virus - is an acutely (rapidly) transforming retrovirus

Question 12

Describe the structure and genome of RSV?

Answer 12

The structure is a sphere with a lipid envelope and protein spikes
The core contains - diploid viral RNA genome and reverse transcriptase

Genome contains:
Gag - core proteins
Pol - reverse transcriptase
Env - envelope protein 
Src - viral oncogene

Question 13

How was the c-Src proto-oncogene discovered?

Answer 13

Radiolabelled v-Src probe
Hybridise to uninfected chicken DNA
Detected hybridising DNA = c-Src

The cellular c-Src gene is also found in other species that cannot be infected by RSV ie it is a “genuine” cellular gene

Question 14

Describe proto-oncogenes?

Answer 14

Proto-oncogenes regulate cell proliferation and apoptosis:
Normal proto-oncogene function is to promote cell proliferation or to inhibit apoptosis (tightly regulated)

Altered proto-oncogenes i.e. increased/de-regulated turns into a oncogene
This oncogene activity in tumours leads to uncontrolled proliferation or loss of apoptotic potential = an uncontrolled increase in cell numbers

Question 15

Describe oncogenes?

Answer 15

Mutations leading to oncogene activation in cancer are gain-of-function leading to:
Increased levels of wild-type protein (eg myc in Burkitt’s lymphoma) or
Normal levels of a mutant protein that has increased activity (eg Ras in many types of malignancy)

Mutant oncogene allele is dominant over wild-type
Development of cancer requires mutation (“activation”) of only one allele

Question 16

Describe the lifecycle of retroviruses?

Answer 16

Entry into cell and shedding of envelope
Reverse transcriptase makes DNA/RNA and then DNA/DNA double helix
Integration of DNA copy into host chromosome
Transcription and translation
Assembly of many new virus particles, each containing reverse transcriptase, into protein coats

Question 17

How does the ALV capture the Src gene in the retroviral lifecycle?

Answer 17

The Avian Leukosis virus
ALV virion (with no viral oncogene) - undergoes infection, reverse transcription and integration into the host cell
During transcription, c-Src which is downstream of the proviral DNA, has become packaged into the capsid RSV virion - called transduction of oncogenes

Question 18

Describe transduction of oncogenes?

Answer 18

The transduced v-src oncogene is expressed at higher levels (than the host c-src gene) under control of strong retroviral enhancers/promoters (LTRs = long terminal repeats)
The transduced oncogene sometimes acquires additional (activating) mutations eg point mutations and deletions
The transduced oncogene has pleiotropic effects on the host cell

Question 19

Describe insertional mutagenesis by retroviruses?

Answer 19

Identification of previously unknown oncogenes (i.e. genes located at sites of integration of slow retroviruses)
Showed that proto-oncogenes can participate in tumorigenesis while still residing in their natural location in the chromosomes of cells
We can study genes present at regions of known chromosomal abnormality in human cancers - chromosome translocations and region of amplified DNA

Question 20

What are the biochemical mechanisms of oncogene action?

Answer 20

Growth factor receptors acting via tyrosine-specific protein kinase activity
GTP-binding proteins
Membrane/cytoskeleton-associated tyrosine-specific protein kinases
Nuclear transcription factors
Steroid-type growth factor receptors
Serine/threonine-specific protein kinases

Question 21

Describe the Src protein?

Answer 21

This is responsible for the transforming activity of the ALV virion
It functions as a tyrosine kinase
When active it can bind ATP - and transfers a Pi to a substrate which binds in the active site
We can detect this y an anti-Src antibody

Question 22

What is the EGF receptor?

Answer 22

This functions as a tyrosine kinase
It is an integral membrane protein - with an ectodomain outside the cells
This can bind an EGF - this signal is transduced to the intracellular domain, which has a region of homology to Src
The intracellular domain is switched on and goes onto phosphorylated signalling molecules in many pathways

Question 23

Describe the relationship of EGFR and ErbB oncogene?

Answer 23

The EGF receptor is related to the ErbB Oncogene
It is homologous with a truncated ectodomain
v-ErbB = oncogene carried by the avian erythroblastosis virus (AEV)

This lead to thinking:
Is the EGF receptor involved in human cancer?
If so, is tyrosine kinase activity important?
If so, how is the cellular gene “activated” in human cancer?

Question 24

What is involved in the EGFR signalling network?

Answer 24

There are a large number of ligands that can bind to the extracellular domain
This can activate different cellular pathways e.g. MAPK involving Ras (p21-GTP)

Question 25

Describe the coupling receptor signalling to Ras?

Answer 25

Grb2 = growth factor receptor bound-protein 2
Contains an SH2 domain that recognises phosphotyrosine of the EGF receptor and an SH3 domain that recognises the proline-rich sequence of Sos
Functions as an adaptor that links EGFR to Sos

SOS = son of sevenless (sevenless = Drosophila EGFR)
Role is to activate Ras - induces Ras to release GDP and bind GTP
Functions as a guanine nucleotide exchange factor (GEF)

Question 26

Describe signalling downstream from Ras?

Answer 26

Ras is a small GTPase that functions as a molecular switch
GTP-bound = ON – binds to downstream effectors -> signal
GDP-bound = OFF – does not bind -> no signal
GTP effect the conformational change in the switch 2 region

Question 27

What pathways are Ras involved in?

Answer 27

AKT pathway:
Stimulation of cell proliferation and growth
Inhibition of apoptosis

RHO pathway
Regulation of cell shape, motility and invasiveness

Mitogen activated protein kinase pathway
Activation of protein synthesis
Activation of growth-promoting genes
Anchorage-independence
Loss of contact inhibition
Change in cell shape

Question 28

What is the link between oncogenes and transcription factors?

Answer 28

Many oncogenes also function as transcription factors
TFs are “activated” by diverse mechanisms
eg. c-Myc - alterations in the c-Myc genes themselves
c-Myc overexpressed as a result of gene amplification or chromosome translocation
“indirect” activation - overexpressed as a result of aberrantly activated signalling pathways