Cancer Flashcards
(297 cards)
1
Q
what is our central concept of cancer
A
it is a disease caused by alteration of a cell’s genes
2
Q
why do we say gene changes instead of just mutations
A
these may be any of the following
- Mutations, in the most general sense: any kind of alteration of DNA sequence
- Epigenetic change, such as aberrant DNA methylation or histone modification
- Tumour viruses bringing extra genes into the cell
3
Q
How many critical gene changes are needed for adult cancer
A
> 10
4
Q
what is metastasis
A
formation of new colonies of tumour in other parts of the body, by the of seeding cells into the circulation
5
Q
what is the primary tumour
A
original tumour is called the primary tumour and a metastasis may be called a secondary.
6
Q
what are malignant tumours
A
those capable of
metastasis.
A tumour doesn’t have to have formed any metastases to be
malignant: the actual formation of
metastases is very slow and inefficient
and may not actually have happened yet
7
Q
what are benign tumours
A
Incapable of metastasis (unless subsequent mutation may turn the benign tumour into a malignant one).
8
Q
where do the following cancers often metastasize to:
breast
colorectal
A
breast to lymph nodes and bone
colorectal to liver via HPV
9
Q
why is metastasis important clinically and intellectually
A
most deaths from cancer are caused by metastases;
important intellectually because they are the key mechanisms of pathogenesis
and we don’t understand them.
10
Q
what does a benign tumour look like down a microscope
A
confined to their original site in the body;
they have clearly defined boundaries, and can often be physically separated from surrounding tissue;
often surrounded by a capsule of connective tissue, which can be peeled away
11
Q
what do malignant tumours look like down the microscope
A
ragged edges, infiltrating into
surrounding tissue, and growing there.
Malignant tumours often show other morphological differences, eg abnormalities of nuclear size and shape, and alterations or loss of differentiation;
but ‘invasion’ is the most important.
12
Q
what is the classical idea of development of a colorectal tumour
A
Often a polyp/adenoma precedes the malignant tumour, and the polyp may progress through various degrees of abnormality. Polyps may well be preceded by less visible abnormalities.
13
Q
True or false
Malignant tumours often develop via visible benign precursors
A
true but not all benign can become malignant
eg benign smooth muscle tumour of the uterus rarely if ever turns malignant; the rare invasive tumours in this tissue appear to develop via a different set of mutations.
14
Q
can benign tumours kill
A
yes
for example meningiomas growing in the brain, or hormone-producing benign tumours of the pituitary or adrenal.
15
Q
what are the broad rules for naming a benign tumour
eg
A
tissue name + - oma
e.g.
lipoma = benign fat tumour
Leiomyoma = benign smooth muscle tumour
Papilloma = wart
adenoma [of the colon] = benign glandular lump, including glandular polyp
16
Q
nomenclature of malignant tumours from mesenchyme
A
[name of tissue] sarcomas
E.g.
osteosarcoma (malignant bone tumour),
leiomyosarcoma (malignant smooth muscle tumour)
17
Q
nomenclature for malignant tumours of mesenchyme
A
[name of tissue] carcinoma
e.g. breast carcinoma, colorectal carcinoma
18
Q
give some exceptions to the naming rules of cancer
A
malignant melanoma (‘melanoma’ is not used for benign moles, which are called
nevi (singular nevus))
Neuroblastoma, glioblastoma (malignant neural tumours)
19
Q
how do you name cancers of the haemopoietic system
A
Leukaemias liquid haemopoietic neoplasms
lymphoma solid haemopoietic (usually lymphocytic) neoplasms
20
Q
give examples of how cancer can interfere with normal function (4)
A
Pressure: enlarged prostate obstructing ureter, meningioma on brain
Erosion/destruction e.g. of bone -> fractures and pain
Epithelial ulceration: bleeding from colorectal tumours (picture) -> anaemia
Competition with normal: failure of normal bone marrow in leukaemia
21
Q
what are some of the metabolic effects of cancer
A
general, systemic wasting – ‘cachexia’
specific – tumour specific products, e.g. peptide hormones ACTH, ADH secreted by small cell lung
cancer
22
Q
what causes cachexia
A
Its mechanism, other
than competition for metabolic resources, is not known
23
Q
how do you generally die from liver cancer
A
- Liver overwhelmed by metastatic colon cancer -> liver failure
24
Q
how does leukaemia usually kill you
A
Failure of normal bone marrow -> infection through lack of neutrophils; or haemorrhage through lack
of platelets
25
how can pain from cancer kill you
Pain management -> analgesia -> respiratory depression
26
how common are benign tumours and malignant tumours of mesenchyme
Benign tumours:
very common in all tissues
e.g. Leiomyoma of uterus, lipoma, wart, mole …
Malignant tumours from mesenchyme:
generally rare but often rapidly lethal, e.g. osteosarcoma (malignant bone tumour)
27
how common are malignant cancers of epithelium
Common – the most important cancers
eg breast, colorectal, lung, prostate
28
which cancers have low incidence in the west but high elsewhere
Nasopharyngeal (in Chinese populations), Liver
29
how do cancers present
Tumour may be visible/palpable:
e.g. melanoma, breast carcinoma
but often indirect, through effect of tumour:
prostate: blockage of ureter
colorectal: obstruction of bowel, anaemia due to bleeding
30
how is colorectal cancer usually first noticed
patient is anaemic due to chronic bleeding of the ulcerated tumour, or because s/he is losing weight, or has altered bowel habit due to partial obstruction of the bowel
31
how is breast cancer usually spotted
unusual in that the tumour is often detected directly as a lump in the breast, but occasionally the first indication is bone pain or pathological fracture caused by metastatic tumour growing in bone.
32
what is the object, ideal, and draw backs of screening for cancer
Object: Find cancer earlier
Ideal: Find before malignant
But: have to be able to do something useful
33
what is the ideal screen for cancer
find benign precursors of cancer and remove them even before they turn malignant
eg cervical cancer where benign precursor lesions can be detected by the cervical smear test—cells from the cervix are sampled by brushing—and removed. The fall in cervical cancer in countries that screen strongly suggests that it is very successful
34
How is screening for colorectal cancer usually done
what is a draw back
how could this be improved
by detecting blood in stool collected at home—
reasonably convenient but
only able to detect tumours that have
already ulcerated.
Endoscopy for polyps would be
better, detecting the benign precursor, but is much more expensive and much more demanding on the patient.
35
How can breast cancer be detected by test
by X-ray, ‘mammography’, slightly earlier than it would be detected by palpation.
36
why is breast cancer screening controversial
expensive and demanding for patients, especially as many patients who do not have tumours are recalled for further investigation, and some cancers are discovered and treated unnecessarily
37
why do we not screen for prostate cancer
currently we could screen for malignant prostate cancers but screening is considered counterproductive, because the tumours are often so indolent that they may never cause a problem; and surgery has a very high morbidity—incontinence and/or impotence.
38
what are the stages of the cell cycle
G0+G1 (diploid state),
S (DNA synthesis),
G2 (tetraploid state, tidying up at the end of S phase and preparing for M),
M (mitosis).
39
How does Rb control the cell cycle
e by inhibiting the
| initiation of DNA replication, i.e. holding the cycle at the G1/S checkpoint.
40
How is the cell cycle inhibition by Rb relieved
when CDK4-Cyclin D1 phosphorylates Rb1
41
What inhibits CDK4-cyclin D
p16 (aka CDKN2A) or p21 (aka CDKN1A)
42
what has to happen to Rb to promote cancer
how does this compare to CDK4-cyclin D
has to be inactivated, for example by deletion of all or part of the RB1 gene
overactivity mutations of CDK4 or CyclinD1 can keep Rb-1 phosphorylated.
43
which cyclin complex is often overactive in breast cancer
CCND1/CyclinD1 gene as a result of gene amplification
44
what has to happen p16/INK4A to cause cancer
inactivation (as p16 inhibits CDK4 cyclin D1)
45
define oncogene and tumour suppressor gene
Oncogene mutations are overactivity mutations - requires 1 mutation
Tumour Suppressor Gene mutations are loss of function mutations - requires 2 mutations
46
what is an intermediate case of oncogene vs tumour suppressor
some tumour suppressor gene
mutations, have a significant effect on the cell when only one copy of the gene is mutated, but losing both copies has a stronger effect. These are usually cases where the mutant protein complexes with the normal protein (eg p53)
It functions as a
tetramer, so mutating half the copies of the gene means that most tetramers are faulty
47
what is clonal expansion
A normal cell acquires a mutation/gene change that means that, over time, its progeny compete with neighbouring cells so that they take over more than their normal share of a tissue
48
why is there heterogeneity to a tumour
may contain not just the latest clone but also preceding clones and dead-end branches of the evolutionary tree
49
how did Jones et al demonstrate clonal expansion
They put a mutation and a
fluorescent marker into a few cells in mouse oesophageal epithelium in vivo; over a year the mutant cells’ progeny took over almost the entire epithelium.
50
what is the evidence for tumours being clones
all cells have the same gene changes, except for the most recent ones. Precursor clones, having some of the mutations seen in a tumour, can sometimes be detected in flanking tissue that is superficially normal.
51
How do we know that most cancers have specific defects that make them genetically unstable, and it’s not just that they are cycling so fast they mess up their DNA?
individual cases of cancer show different kinds of genetic instability
eg in colon cancers where there are at least two obviously different types of instability - sequence instability vs chromosome instability
52
How many cancers display sequence instability
15% - stable chromosomes but unstable sequences
53
poor function of what system leads to colorectal cancer 15% of the time
mismatch repair (leading to sequence instability)
54
Is sequence instability the most common cause of colorectal cancer
no
, most
cases have a lot of rearranged chromosomes, chromosomal instability or ‘CIN’,
while generally having a near-normal rate of point mutation
55
what kind of mutation leads to CIN
mutations in managing or repairing chromosomes
56
What are the different types of DNA repair for single strand breaks
NER
BER
MMR
57
What damage does MMR deal with
mismatched bases and also small loops that occur where polymerases slip while replicating repeats and add or delete a copy of the repeat, generating a tiny mismatched loop
58
What happens to mismatched loops when MMR machinery is defective
slippage loops persist, and a striking effect is
| shrinkage or expansion of short repeats known as microsatellites
59
What technique are microsatellites used for
forensic DNA fingerprinting
60
Give 2 'symptoms' of faulty MMR
microsatellite instability
| higher point mutation rate
61
what usually causes the MMR failure in 15% of colon cancers
inactivation of MLH1 or MSH2, key components of mismatch repair
62
how does the mutation rate in colon cancers with dodgy MMR compare with the rate in a normal cell
roughly hundred-fold increased rate of small mutations including both single base changes (mismatches) and frameshifts caused by elongation or shortening of repeats such as AAAAAAA.
Both TGFbetaRII and Bax suffer such mutations
63
How does inactivation of MLH1 often occur
by epigenetic change, methylation of
| the DNA of its promoter
64
which repair system is lost in Lynch Sydrome
MMR
65
What does HR repair of DSB require
Which proteins are important in this repair system
there being 2
copies of the genome: it uses the other copy of the broken sequence—usually the
sister chromatid—as a template to resynthesise the broken bit
BRCA1/2
66
What are the BRCA genes infamous for
hereditary breast cancer
67
Which repair system deals with crosslinked DNA
HR
68
mutations in which protein account for a few percentage of colon cancers
mutations in DNA polymerase epsilon proof-reading domain can cause an extremely high error rate
69
what is a source of chromosome instability other than defects in repair mechanisms
defects in mitosis
| eg multiple centromeres, in some cancer cells, chromosomes sometimes get left behind or broken during anaphase
70
why does a mutation of p53 lead to instability
If DNA is damaged or replication encounters a problem, the cell cycle should be halted at a checkpoint, so defects in checkpoints may leave problems unresolved
71
what is the Vogelstein model of colorectal cancer
model of the development of colorectal cancer (revised 2008), trying to relate genes mutated to stages in cancer development. The model is speculative and oversimplified—e.g. only some colorectal cancers have these particular mutations, and the list of mutations is not complete
72
What is the sequence of mutations in the Vogelstein model of colorectal cancer
1) APC or beta-catenin;
2) CDC4 or CIN;
3) KRAS or BRAF;
4) PIK3CA or PTEN;
5) p53/TP53 or BAX;
6) SMAD4 or TGF-beta.
73
why are the APC and beta-catenin mutations interchangable in the Vogelstein model of colorectal cancer
both may be required for adenoma formation—mutating either gene has much
the same effect so these are alternatives
74
how high is the risk of developing cancer if you have a hereditary predisposition
(not including weak predispositions that increase risk by 1.3x eg)
e.g. hereditary predisposition to breast cancer, the affected individual has a high risk, usually at least 50% and often higher, of developing cancer.
The individual inherits one of the mutations required to get a cancer, and so is one step down the road to cancer.
75
What is an inherited mutation predisposing cancer usually in
why does this increase their chances of cancer
usually (but not always) in one copy of a tumour suppressor gene rather than an oncogene
In a normal individual—known as a ‘sporadic’ case—2 mutations, in the same cell, one on each copy, would be required to inactivate this tumour suppressor gene.
In the predisposed person, all their cells start with one copy inactivated, but one intact so that cells behave normally, and the second mutation occurs in occasional cells during life to alter the cell’s behaviou
76
Why are hereditary predispositions to cancer important
(a) Because they are among the commonest genetic diseases,
| (b) They provided an important route to discovery of tumour suppressor genes including APC and RB1.
77
How common are hereditary predispositions to cancer
among the commonest potentially-lethal hereditary diseases, i.e. are more common than most of the well-known non-cancer genetic diseases that are potentially lethal.
At least a few percent of cancer is due to such predispositions and the individuals affected may have a very high probability of developing the particular cancer, e.g. BRCA2 mutation gives a lifetime risk of around 40-80% of breast cancer
78
true or false
| inhereditary predisposition to cancer mutations are always in genetic instability genes
false
| can either be in growth control genes (e.g. APC, RB1) or in genetic instability genes (MLH1, MSH2, BRCA2).
79
what are the 2 best known examples of hereditary predisposition to colon cancer
Familial Adenomatous Polyposis
Lynch Syndrome
80
What are the other names for familial adenomatous polyposis
how common is it
Adenomatous Polyposis Coli or just polyposis coli
1/10,000
81
What happens in familial adenomatous polyposis
individuals develop ~1000 polyps
(adenomas) in colon in late adolescence
associated with mutation in tumour suppressor APC gene
82
How common is APC mutation in sporadic colorectal cancer
Around 80% of sporadic colorectal cancers also have this mutation, but both copies have to be damaged after birth
83
what proportion of colon cancers are from Lynch Sydrome
What are the mutations in
what do these genes encode
1%
(one copy of) MLH1 and MSH2, both encoding components of DNA mismatch repair
84
How common are MLH1 and MSH2 mutations in sporadic cases of colon cancer
15% have mutation
85
What is Lynch Sydrome also called
old name Hereditary Non-Polyposis Colon Cancer, HNPCC but it afffects other tissues as well
86
How much of breast cancers are in individuals with an inherited predisposition
what genes are often involved in inheritance
5%
Perhaps half of these have a mutant copy of BRCA1 or BRCA2.
87
How are BRCA 1 and 2 related
unrelated proteins but are both components of DNA strand break repair, and loss of both copies of the genes gives genetic instability.
88
what is the major difference between the genes associated with inherited predisposition to breast rather than colon cancer
APC, MSH2 and MLH1 are found in both predisposed and sporadic colon cancers
BRCA1/2 are rarely mutated in non hereditary breast cancer
89
what are BRCA1 and 2 involved in normally
so what happens if they are lost
both components of DNA strand break repair, and loss of both copies of the genes gives genetic instability
90
what is retinoblastoma
a rare tumour arising in the immature retina, in children under a few
years old. About 40% of cases are hereditary, and on average predisposed individuals
develop 3 tumours (range 0 to many)
91
what cancers does a BRCA2 mutation lead to (3)
breast
prostate
ovarian
92
which part of the cell cycle does Rb control
g1/s checkpoint
93
What is the Knudson 2 hit hypothesis
The idea that typical tumour suppressor genes require two mutations to inactivate both copies
developed from studying retinoblastoma incidence
94
true or false
| genetic instability may be separated from growth control
true
When cancers with BRCA2 mutations are treated with DNA-crosslinking agents, treatment selects for cells that can repair the lesions, so selects for revertants of the BRCA2 mutation. These revertants have lost genetic instability but go on to kill the patient
95
What are the hallmarks of cancer that are concerned with loss of growth control (5)
what is another key hall mark
```
independence of growth stimulating signals
resistance to growth inhibitory signals
differentiation block
resistance to apoptosis
immortality
```
genetic instability
96
What are the 4 more controversial hall marks of cancer
metabolic changes
metastasis
angiogenesis
evasion of immune response
97
Why is angiogenesis a controversial hallmark of cancer
arguably any growth in the body will grow extra blood vessels, not just tumours
98
Which receptor type are usually involved in growth factor pathways
RTK
99
name 3 types of signaling pathway in cancers
GFs
Wnts
TGF-beta
100
What does the TGF-beta family usually do in cancer
generally inhibit proliferation in epithelial cells.
101
Which of the 3 main types of cancer signaling pathways are involved in the Vogelstein model of colorectal cancer
pro-proliferation pathways—the Wnt pathway; and two pathways downstream of receptor tyrosine kinase signalling, the KRAS-BRAF MAP kinase pathway and the PI3 kinase-Akt pathway—and a growth-inhibitory pathway—TGF-beta signalling
102
Why is Wnt pathway of particular importance to the colorectal cancers
almost all colorectal cancers have a mutation in it, either in APC, beta-catenin, or, more recently discovered, other components such as a Tcf transcription factor
103
What is the most frequent mutation in the Wnt pathway in colorectal cancer
APC inactivation
104
What does beta catenin do
acting with Tcf transcription factors in the nucleus, drives cell proliferation and/or clonal expansion.
105
How do APC and Wnt relate to beta catenin
APC forms part of a complex that normally degrades beta-catenin, and Wnt signalling prevents the degradation.
106
Which type of mutations are required to make the Wnt pathway cancerous and pro-proliferative
inactivation of APC or activation of beta-catenin
107
How is activation of beta-catenin usually achieved
by preventing degradation, typically by point-mutating the motif on beta-catenin that the degradation machinery recognises.
108
What is the target of Herceptin in breast cancer treatment
HER2
109
Name 2 receptors in the ErbB family
EGFR
| HER2
110
What are the 2 possible downstream pathways from ErbB receptors
1) MAPK (signals via RAS and RAF family members)
| 2) PIP3 (PI3K phosphorylates PIP2 to PIP3, which activates AKT, inhibiting apoptosis)
111
What dephosphylates PIP3
PTEN
112
What would have to happen PTEN to cause cancer
PTEN is a tumour suppressor (dephosphorylates PIP3 so stops inhibition of apoptosis) so must be deleted
113
What are the alternatives to PI3KCA and PTEN in Vogelstein's colorectal cancer model
KRAS/BRAF
114
How do TGF beta family peptide GFs signal
via a transmembrane receptor to the SMAD family (which carry signals to nucleus)
115
How are TGF beta families implicated in cancer
Mutations in TGFbetaRII (TGFbeta receptor two), SMAD4 or SMAD2 are quite common in colon cancer.
All are tumour suppressor genes.
116
How does mutating RAF differ from mutating RAS in cancer
Mutating RAS has much the same effect as mutating RAF, so both mutations can be regarded as activating the RAS-RAF pathway
117
What is an example of differentiation block in the Vogelstein model
inactivation of APC (or activation of beta-catenin).
Mutating APC in the stem cell compartment of the
colon in an animal model blocks differentiation: the stem cell compartment expands and the proliferating cells are no longer able to migrate up the villi.
118
True or false
| Leukaemias must arise as in full differentiated lymphocytes
false
Leukaemias can arise in stem cells or in fully differentiated lymphocytes (as in chronic B-lymphocytic leukaemia and also myeloma which arises in plasma cells).
119
How can chronic and acute leukaemias differ
stem cell leukaemias can either have differentiation blocked giving acute
leukaemias, with stem cells in the blood; or retain differentiation, as in chronic myeloid leukaemia, where cells in the blood are differentiated
120
Is there a difference in severity between stem cell and differentiated leukaemias
The leukaemias with blocked differentiation are more aggressive than the differentiating ones
121
Name 2 anti apoptotic mutations
inactivation of p53 and BAX
122
How do p53 and BAX interact
p53 upregulates BAX to promote cell death via MOMP and casp activation
123
Which of BAX's relatives oppose its effects
BCL2
124
How does the PIP3 pathway affect apoptosis
acts to reduce apoptosis
125
What did Hayflick show in the 1950s
normal human somatic cells in culture would only divide a fixed number of times before entering cycle arrest, a response known as ‘senescence’, and tumour cells escaped this limit to grow indefinitely, they are ‘immortal’. Limited division potential is controlled by telomere length
126
What are telomeres
How are they restored
repeat structures at the ends of chromosomes, and in most somatic cells they are shortened at each division
telomerases
127
How do 10-20% of cancers turn on telomerase
How else might they do it
point mutation in the promoter
a rearrangement of DNA brings in a promoter from another gene
128
Give an estimate for the hayflick limit
50-100 divisions
129
How much do telomeres shorten per cell cycle
100bp
130
What is oncogene induced senescence
senescence due to activation of some oncogenes, e.g. expressing mutant RAS in otherwise normal cells
131
How can cancers remove stress responses
Mutation of p53, with or without mutation of RB1, alleviates stress responses.
132
How do viruses eg HPV associated with cervical cancer overcome senescence
by binding and inactivating Rb-1 and p53.
133
How common is p53 in human neoplasm
It is mutant in around one-third to half of all human neoplasms.
134
Describe levels of p53 levels
can be raised in minutes. Protein is constantly being translated then
degraded, so blocking degradation rapidly
raises the level.
135
What did Folkman postulate
tumour growth might be limited by the need for angiogenesis, and that a tumour had to produce angiogenic factors
136
How do tumour cells migrate through tissue
It used to be assumed that this was a destructive process, involving proteases, but recent videomicroscopy shows cells slipping through pre-existing spaces between cells, along collagen fibres, etc
Cells eventually get into the lymphatics or veins
137
How easily do tumour cells spread once they have reached the circulation
cells can circulate surprisingly freely, to distant sites, e.g. from one breast to the other.
138
Give 2 experiments on metastasis
Cells injected into capillary beds can be isolated from draining lymphatics: they can cross tissue.
Cells injected at different points, e.g.
left ventricle and tail vein, give much the same eventual
distribution of metastatic colonies.
139
How can we show that the site of metastasis is at least a little bit dependent on an intrinsic property of the primary tumour
by selecting variants of a mouse melanoma line that preferentially colonized respectively the brain or the lung independently of where they were injected.
This was done by repeated intravenous injection of cells and recovering colonies from the chosen organ
140
What does the process of metastasis entail
which step(s) are critical
escape of cells into vessels, survival in the circulation, escape out of vessels into tissue, then survival and growth.
only survival and growth in a distant site
141
How do we know that the major barrier to metastasis is survival and growth in the distant site
video microscopy of fluorescent cells injected into the circulation shows
that even normal cells can circulate freely and exit the circulation quite efficiently
142
True or false
| metastasis is very inefficient
true
| Many cells may be released from a tumour with only a few metastatic colonies forming
143
If only a few cells in tumour were capable of metastasis, what would the secondary tumours look like
how do we know this is wrong
highly purified for metastasis-capable cells and would give more metastases if transplanted
this was tested by Weiss who found Cells from metastases were no more metastatic than cells from the primary tumour, so most primary tumour cells are capable of metastasis
144
What is the lecturer's view of the step from malignancy to metastasis
requires little or no additional genetic change; it is first and foremost a rare stochastic process.
145
What do Tcf proteins target
when might you find a mutant version of this
Wnt pathway
in colon cancer instead of APC or beta-catenin (itself a transcription factor)
146
What is MYC
What is ERG
MYC is an oncogene and transcription factor that seems to powerfully upregulate many genes involved in proliferation and is widely activated.
ERG is a transcription factor that may control differentiation; fused in prostate cancer
147
Name a chromatin modifier involved in cancer
MLL, fused to other genes by chromosome translocations in leukaemias, is
a histone methylase
148
Name a complex that modifies chromatin
how is it mutated in cancer
BAF - moves nucleosomes
several components of these are mutated, e.g. ARID1A. overall maybe 20% human cancers have mutations in BAF components.
149
What is E-cadherin
how is it involved in cancer
a cell adhesion molecule
inactivated in some breast cancers, often by epigenetic change: DNA methylation
150
Give an example of mutations to carbohydrate metabolism leading to cancer
mutations in isocitrate dehydrogenase IDH1, or IDH2 an enzyme in the
Krebs cycle. e.g. in some brain tumours
and leukaemias
changes the enzyme specificity to make hydroxyglutarate, which accumulates and interferes with DNA demethylation (which may block differentiation) and histone acylation (which may block DNA damage signalling)
151
What does chromosome translocation involve
pieces of two chromosomes are joined to each other, following breakage of the DNA or an accident during DNA replication.
152
What are the possibly sequence level changes that can occur
SNV
| indel
153
Why do indels usually give truncated proteins
because most of
| them cause frameshifts and frameshifts usually lead to a stop a few amino acids later
154
What are 4 large scale mutations/ changes that can happen to DNA
deletion
duplication
amplification
Chr translocation
155
What is gene fusion
when rearrangement of large chunks of DNA creates a new gene by joining two genes together to create a new gene
156
How do we study sequence level DNA changes
PCR - amplifying a region of interest—say an exon—and sequencing it on a Sanger sequencer
Increasingly this is replaced by Illumina (originally Solexa) sequencing
157
What is Illumina
millions of DNA fragments, are attached to a glass slide and sequenced simultaneously (hence ‘massively-parallel sequencing’)
158
How can structural DNA arrangements be found
d by finding sequence fragments that cross rearrangement junctions—e.g. for the BCR-ABL fusion, finding a DNA fragment with sequence from the BCR gene at one end and sequence from the ABL gene at the other end.
159
What is cytogenetics
study of chromosomes by microscopy
160
How would you perform a cytogenetic study
Cells are arrested in metaphase and their chromosomes spread on a slide and stained. This can show chromosome translocations and some large deletions or inversions.
161
what is the most famous example of a Chr translocation
reciprocal translocation between chromosomes 9 and 22 found in most
chronic myeloid leukaemias. The smaller chromosome formed is called the Philadelphia Chromosome.
162
Why can you not study carcinomas by classical cytogenetics
it’s difficult to get metaphases and there are so many complicated rearrangements they can’t be identified
163
What is FISH
What is the method
fluorescence-in situ hybridization
DNA is labelled with fluorescence and hybridized to metaphase chromosomes. DNA from a chromosome can be labelled, or a small segment of genome can be labelled, e.g. in the example chr12 in red, and a 200kb chunk including the NMYC gene in green. This example shows amplification of N-MYC in a lung tumour.
164
How can FISH be used clinically
e.g. to detect amplification of HER2(ERBB2), the
| target of Herceptin therapy in breast cancer.
165
How have large deletions/ amplifications been detected
by measuring the relative loss or increase in the amount of DNA in different regions of the genome
166
What is RAS
a G-Protein that is activated by GTP binding and inactivated by hydrolysis of the GTP to GDP
167
What do mutations to the RAS pathway affect (2)
mutations block the hydrolysis, at least some of them by preventing access of GTPase-activating proteins that complete the active site.
activation of BRAF
168
Which particular RAS is important to cancer
K-RAS in particular in frequently mutated in a range of human cancers.
169
Give the steps of the RAS pathway (4)
EGF binds to EGFR
EGFR activates Grb2 and SoS
Grb2 activates RAS, which activates BRAF
BRAF activates MAPK ...
170
Which cancers are related to BRAF mutation
notably in melanomas (60% cases) and colorectal carcinomas.
171
How are BRAF and RAS mutations related in cancer
B-RAF and Ras mutations seem to be alternatives with rather similar effects on the cell.
172
What is the commonest change to BRAF in cancer
BRAF V600E
commonest mutation
changes a valine to a glutamic acid at position 600
This is adjacent to an activating phosphorylation site, and the negative charge of the glutamic acid presumably mimics phosphorylation and so activates.
173
What is PI3KCA
the catalytic subunit of PI-3Kinase
174
What is the importance of PI3K to cancer
PIK3CA is mutated in about 1/3 of breast cancers, the most frequent mutation yet found in breast cancer. One of the Common mutations changes one particular negatively charged amino acid to a positively charged one, clearly a dramatic change.
175
Give an example of a mutation that a) stabilises the active conformation of a protein or b) mimics an activating phosphorylation
Are these oncogenes or tumour suppressors
a) RAS
b) BRAF
oncogenes
176
How do the ease of getting an activating mutation in an oncogene compare to deactivating a tumour suppressor
oncogene-activating mutations have to be precise mutations at particular amino acids; in contrast inactivating tumour suppressor genes is much easier—anything that interferes with the protein’s function will do.
177
What is PTEN and how is it associated with cancer
which reverses the action of PIK3CA—is a known tumour suppressor gene and can also suffer inactivating point mutations or deletions.
178
What kind of sequence change usually inactivates tumour suppressors
Indels are thought to be important particularly as inactivators of tumour suppressors,
as they usually truncate.
Many APC mutations are indels.
179
Name 3 proteins inactivated by deletion, leading to cancer
Deletions of tumour suppressors are common.
PTEN is often deleted, as are the cellcycle controllers Rb and p16/INK4A.
180
Give an example of duplication in cancer
a fusion of BRAF to a neighbouring gene KIAA1549, as a result of tandem duplication of a segment of DNA (in paediatric brain tumours)
181
Give 4 examples of amplification gene mutations in cancer
EGFreceptor in brain and ERBB2/HER2 in
around 10-20% breast cancers.
Also Cyclin D1 in breast; and MYC family genes in various cancers
182
True or false
| there is only one way to activate an oncogene
false
BRAF can be activated by point mutation or gene fusion, and fusion can be by tandem duplication or chromosome translocation.
Even RAS can be fused (rarely)
EGFR and ERBB2 can be via SNV or amplification
183
What is BCR-ABL
iconic fusion found in most cases of
chronic myeloid
leukaemia, formed by the reciprocal 9:22 translocation
184
What is ABL
how is it regulated
tyrosine kinase
regulatory domain at the N terminus, and, like most tyrosine kinases, activated by dimerisation
185
Why does the BCR-ABL fusion work
what does the fusion do (3)
N terminus of the BCR protein naturally forms dimers or oligomerises.
The fusion does several things:
the inhibitory N terminus of ABL is removed;
BCR holds the ABL kinase in dimers or oligomers, activating it;
the BCR promoter may be stronger than ABL’s.
186
What is an important example of fusion in a carcinoma
how common is it
TMPRSS2- ERG, found in ~50%
prostate cancers. Since prostate cancer is common, this is the most
prevalent known fusion gene by far.
187
How is the TMPRSS2-ERG fusion gene formed
by deletion between the two genes TMPRSS2 and ERG, rather than by chromosome translocation.
188
Give 2 examples of genes inactivated by DNA methylation
MLH1 (mismatch repair) and E-cadherin (cell adhesion) respectively in colon cancers and lobular breast cancers
189
How common is epigenetic instability in cancer
what does this mean
30-50% colon cancers
analogous to genetic instability, i.e. a high rate of aberrant DNA methylation .
190
Name 2 proteins that can be inhibited by carcinogenic viruses
p53 and Rb1
191
How many driver mutations are required for cancer development
Vogelstein model suggests 6 - but this is incomplete (so >6, more like 10)
192
What is a good way to estimate the proportion of mutations in a tumour that are selected for
compare the relative proportions of mutations that do or do not alter amino acids, since most of the latter will be random
193
What has statistical analysis of tumours revealed about the amount of driver mutations in colon cancers
how does this compare to other adult carcinomas
>10 coding mutations are drivers in colon cancer, and this does not include DNA rearrangements or epigenetic gene changes
other adult carcinomas are similar, but a higher proportion are DNA rearrangements. At least half of these mutations have not been identified yet
194
What is the estimate for the number of driver mutations in breast and ovarian cancers
about 8 mutations per tumour are already known in breast and ovarian
cancer, of which 75% are rearrangements
195
How do we find out what mutating the genes does to the cell and tissue?
Cells in vitro
| Animal models
196
Having found a mutation in cancer, what would we first need to find out
first that it actually does contribute to cancer development and isn’t a random mutation; and second, what sort of effect it might have
197
Give an example of finding out the function of a mutation in vitro
e.g. the classical ‘transforming’ activity of mutant RAS in fibroblasts—mutant-bearing cells overgrow their neighbours
198
Give an example of using an animal model to find out the effect of a cancerous mutation (for an oncogene)
genomic region around the gene CCND1, which encodes CyclinD1 (in Rb1 pathway), is amplified in many breast cancers, but this doesn’t prove CCND1 is an oncogene, because a neighbouring gene
might be the real culprit.
A transgenic mouse was made that expresses a lot of CyclinD1 from the MMTV promoter, which is active specifically in the mammary glands: the mice get mammary hyperplasias that develop into tumours, suggesting CCND1 is indeed the key gene
199
Give an example of using an animal model to find out the effect of a cancerous mutation (for a tumour suppressor)
inactivation of APC in the crypts of the colon in mice, mentioned in
previous lecture, which showed that removing APC profoundly alters the differentiation pattern of the crypt, so that the dividing cell compartment expands and the cells are prevented from migrating up the villus and maturing.
Expression of cre recombinase is activated by administration of a drug and recombines between lox sequences to cut out the APC gene.
200
How can you mimic deletion of APC (tumour suppressor) in a mouse model
Expression of cre recombinase is activated by administration of a drug and recombines between lox sequences to cut out the APC gene.
201
Name some things we are actually sure cause cancer
smoking tobacco, some important tumour viruses, and some occupational exposures such as asbestos
202
How much of the cancer in england does tobacco cause
30% (probs higher in actual fact)
203
true or false
| cancer is more common in some populations
Incidence of particular cancers varies dramatically—sometimes 50-fold—between
populations (after correcting for age distribution), for example prostate, liver, melanoma. But the total amount of cancer does not seem to vary so much
204
Which group of people were studied to see where the difference in cancer type incidence stems from
What was found
give some more evidence to further support this
migrant populations such as Japanese immigrants to the west coast of the United States.
changed from having the Japanese pattern of cancer, a high incidence of stomach cancer, but a low incidence of breast cancer, to the American pattern, which is the other way round, within a generation or two: clearly not primarily genetic
Similar changes have occurred in Japan as it has become westernized.
205
What kinds of environmental factors could affect cancer risk
agents that cause mutations in DNA, but could also include lifestyle factors (e.g. reproductive behaviour, nutrition) that affect the rate of ‘spontaneous’, DNA damage such as errors in replication that go uncorrected
206
What is the principal carcinogenic effect of UV light
dimerisation of pyrimidines (TT or CT)
207
Which group of people demonstrate the effect of UV light on DNA and the importance of DNA repair mechanisms
UV sensitivity of Xeroderma Pigmentosum patients who lack excision repair
208
What are the 5 types of carcinogenic agents
```
UV light
Ionising radiation
Chemical carcinogens
Minerals (eg asbestos)
infectious agents (eg HPV)
```
209
name some chemical carcinogens (5)
smoke
aflatoxin
Dimethyl nitrosamine (in some meat products)
mustard gas
210
True or false
| all carciogens are very reactive
false
some are, eg mustard gas
but the most potent chemical carcinogens are more or less inert in the form we are exposed to, but get activated by metabolism
211
Why are the most potent carcinogens not usually reactive without being metabolised
Highly reactive chemicals if introduced into the body would react with something else before they could reach the DNA
212
What system are enzymes that activate carcinogens part of
e ‘xenobiotic metabolism’ systems that have evolved to render harmless or 'detoxify' lipid-soluble molecules that the body needs to get rid of, by making more soluble derivatives of them, that can be excreted
213
What fact is β–napthylamine an important example of
how
Mutagenic chemicals tend to show tissue specificity
activated by hydroxylation, but then rapidly made soluble and harmless by addition of glucuronic acid.
However, if the glucuronate is hydrolysed off again, the reactive form is regenerated. β-napthylamine acts primarily on the bladder in man, where the glucuronic acid conjugate is removed by glucuronidase. This bladder specificity made it possible to identify it as a carcinogen of importance in humans.
214
True or false
| Mutagenic chemicals often show species-specificity
true
probably down to variations in metabolism, either activation or detoxification of the carcinogen, e.g. β-naphthylamine is a potent bladder carcinogen only in species that have glucuronidase in the bladder, such as dog and man, not rodents
215
What is NMU
What feature of carcinogens has it been used to identify
nitroso-methyl-urea
Mutagenic chemicals (and radiation) often show stage-specificity
216
how was NMU used to show Mutagenic chemicals (and radiation) often show stage-specificity
When the mutagen NMU (nitroso-methyl-urea) was fed to female rats of different ages, the
rats were most susceptible to developing mammary tumours when pubescent, when the mammary epithelium is proliferating.
Similarly breast cancer in human survivors of Japanese atomic bombs occurred mainly in those who were teenagers when exposed
217
Give a reason mutagens may be stage and tissue specific
β-naphthylamine suggests that one factor is tissue-specific metabolism, while the mammary cancers suggest that rapid cell proliferation makes the tissue susceptible
218
Give 2 important examples which debunk the idea that carcinogens are man made
Aflatoxin B1
Aristolochic acid
219
What is aflatoxin B1`
the most powerful carcinogen known to man, at least when assayed in rats, and it is a natural contaminant of peanuts, produced by the fungus Aspergillus flavus, which sometimes grows on peanuts in warm humid conditions.
Like other carcinogens it is activated by metabolism.
220
What is Aristolochic acid
from Aristolochia plant species, causing kidney toxicity and kidney and liver cancer, and is present in some foods and
traditional medicines.
221
What is a carcinogenic signature
eg?
Sequencing shows that some carcinogens give a characteristic pattern of mutation, and this may help in future to identify exposure.
E.g. many mutations found in melanoma are those expected from UV exposure, predominantly C>T.
222
Name 2carcinogen signatures
many mutations found in melanoma are those expected from UV exposure, predominantly C>T.
Aristolochic acid produces a characteristic mutation pattern, found in upper urinary tract carcinomas, predominantly changing A to T at A[C|T]AGG. This ‘signature’ has also been found in a subset of southeast-Asian liver cancers, suggesting it might have been the cause of those cancers.
223
What does asbestos cause
how?
inhalation causes mesothelioma, arising from the mesothelial lining of the pleural cavity. Probably, small fibres that penetrate deep into the lung cause chronic inflammation and cell turnover of these cells
doesn't damage dna but but is a 'tumour promoter'
224
What are tumour promoters
substances that promote the development of tumours without damaging DNA.
225
Why is there a distinction between tumour initiators and promoters
Classical experiments painting substances on mouse skin
distinguished ‘initiators’ (mutagens) from ‘promoters
If an initiator was applied, it would give the occasional tumour but far more tumours would appear if, after the initiator, a promoter was repeatedly applied. Promoters only worked after initiators had been applied.
Promoters behaved like drugs.
226
Name a natural tumour promoter
the most potent promoters we know are natural
The best known is TPA, which comes from the seeds of Croton tiglium
227
How does TPA promote cancer
mimic diacyl glycerol, agonist for protein kinase C.
228
Are the following promoters or initiators of cancer
a) hormones
b) cig smoke
c) asbestos
what links b and c
a) promoter
b) both
c) promoter
Chronic irritation/inflammation behaves as a promoter, perhaps accounting for asbestos’ action and the promoter activity of smoke.
229
Give an example of a non chemical environmental/ life style choice that increases the chance of cancer
breast cancer
Risk is roughly proportional to the interval between menarche(onset of menstrual cycling) and first pregnancy, for childbearing women - probably related to cell proliferation
230
What are the 2 key things we need to find out when assessing a chemical that may cause cancer
(1) Whether a substance is capable of acting as a mutagen or promoter;
(2) How potent it is - how much can we afford to be exposed to.
231
Why do we need to know how potent a carcinogen is
bc we cannot eliminate all of them from the environment (that would include removing O2 and tomatoes for example)
232
What are the 3 general ways to identify a carcinogenic agent
In humans – epidemiology
In vivo – mice and rats
In vitro – the Ames test and others
233
How were asbestos and β napthylamine discovered to be carcinogenic
does this method of discovery always work?
bc a particular set of people developed a particular type of tumours
no: only works well where a specific group of people is exposed to a strong carcinogen, and the cancer is relatively specific to those people—dyestuffs workers were exposed to β-napthylamine and got bladder cancer, which is relatively uncommon; construction workers inhaled asbestos and got mesothelioma, which is almost unique to asbestos-exposed individuals
234
Why did epidemiology work to discover the carcinogenic nature of tobacco smoke
easy to distinguish people heavily exposed (smokers) from those who are only lightly exposed (non-smokers), and the effect is large (5-10X risk even in earliest studies) and distinctive—squamous lung cancers are rare in non-smoking populations.
235
Name 2 things that are strongly associated with liver cancer
what is the increased risk factor
HBV and aflatoxin
combined risk factor of around 50x
236
What is the technique for testing carcinogens on animals
o expose animals to as high a dose as possible for as long as possible and look for tumours. Whether this works is debatable.
237
What did experiments on rats and mice for carcinogens reveal
what does this mean for the amount of cancerous agents we are exposed to
what is the precise difficulty with this method
of 800 compounds that had been tested, 65% gave a significant increase in tumours in at least one organ and species.
Extrapolating to 60,000 compounds we are exposed to, 40,000 would be ‘carcinogenic’
but on average each would only be responsible for 1/40,000th of human cancer and so unimportant.
many compounds can be carcinogens sometimes, but the effective dose and hence the risk they pose is unknown.
238
How do the results of animal models used to find carcinogens compare between mice and rats
2/3 concordance
239
Give 4 advantages of in vitro testing for carcinogens compared to animal models
What is a key disadvantage and how can we overcome it
more humane, cheaper, quicker and more statistically robust
only limited ability to reveal carcinogens that require metabolic activation, though a rat liver extract can be added to try and provide this.
240
which organism is used in the Ames test
salmonella strains which carry mutations in genes involved in histidine synthesis. These strains are auxotrophic mutants, i.e. they require histidine for growth, but cannot produce it.
241
What does the Ames test analyze
the ability of a substance to cause a mutation
tests the capability of the tested substance in creating mutations that result in a return to a "prototrophic" state, so that the cells can grow on a histidine-free medium.
a positive result means the substance is mutagenic and thus potentially carcinogenic
242
Give some sources of ionising radiation (5)
X rays
γ rays,
neutrons,
β particles (free e-),
α particles, (charged helium nuclei)
243
How does radiation damage DNA
by producing free radicals and ions as it passes through tissue; these then react with DNA and alter the structure of bases or cause strand breaks
244
How is exposure to radiation measured
what units are used
as the amount of energy absorbed
per unit of tissue:
Gray (Gy)
(the Gray supersedes the rad: 1 Gray = 100 rads), which is joules absorbed per Kg
tissue.
245
What does damage by alpha particles look like
Alpha particles leave dense tracks of ions and radicals (high LET) so that if they pass by a DNA helix they are likely to cause double-strand breaks or multiple chemical changes, which may be difficult to repair reliably.
246
What does damage by gamma rays look like
leave scattered ions and radicals (low LET) and so will usually only damage one residue on one strand of a DNA molecule, which is usually repairable.
247
What is the radiation in Gray usually multiplied by
Quality Factor for the particular type of radiation, to give an approximate measure of biological damage or dose equivalent measured in Sieverts (Sv).
248
What are the Quality factors for different types of radiation
1 for X-rays and gamma rays and ranges up to 20 for alpha particles
249
What is the average UK exposure to radiation
2.5 mSv/year of which 1/8th medical, 1/100th cosmic rays from long-haul flights, ½ radon escaping from rocks.
250
HOw can we estimate the effects of radiation doses we experiencce (eg 0.01 Gy)
extrapolate back from high doses such as 3 to 5 Gy, where we can measure cancer in people exposed.
eg from the 120,000 survivors of the atomic bombs dropped on Hiroshima and Nagasaki in 1945
251
What data can we use to extrapolate from to estimate the radiation dose-cancer relationship (other than rfom the atom bombs)
people with Ankylosing spondylitis were treated with X-rays in 1935-1954, and increasingly we have data for uranium miners etc.
252
Give an example of a virus directly causing cancer`
inactivation of p53/ Rb by the E6/7 proteins of HPVs
253
Name a parasite associated with cancer
Schistosomes and bladder cancer
254
Name a bacteria associated with cancer
Helicobacter pylori association with gastric adenocarcinoma and MALT lymphoma (lymphoma associated with intestinal mucosa).
255
What are the strong epidemiological links between H pylori and cancer (2)
eradicating H pylori reduces the incidence of gastric adenocarcinoma;
and H pylori infection induces gastric cancer in animal models.
256
Which types of HPV are particularly associated with cancer
`HPV types 16 and 18, where chronic infection creates a high risk of cervical cancer.
257
name a virus common in young Western adults (typically in uni) that can cause cancer
Epstein Barr virus (typically transmitted by kissing)
can cause cancer in the immunosuppressed
258
What cells does Epstein Barr virus infect
who is this particularly dangerous for 92)
B cells and nasopharyngeal
epithelium.
particularly in ethnic Chinese populations, infection can cause
nasopharyngeal carcinoma;
in malaria-endemic areas infected children may develop the aggressive B-cell lymphoma Burkitt’s Lymphoma.
259
What does HBV act synergistically with to result in cancer
Chronic HBV infection gives a high relative risk for primary liver cancer, and in at least some populations acts synergistically with exposure to aflatoxins.
260
What is Kaposi's sarcoma
who does it develop in
seems to develop in immune-suppressed or elderly people infected with HHV8.
It came to prominence as a common problem in AIDS/HIV patients
261
What is HTLV-1
a T-cell tropic retrovirus, which causes leukaemias and lymphomas in a small proportion of infected individuals
262
Where is HTLV-1 endemic
particularly in SW Japan, the Caribbean and parts of Africa and South America, where up to 10% of the population may be infected
263
How can we make a cancer specific therapy (5)
kill normal tissue along with the tumour if it is unnecessary
target mutations or hallmarks
target things that cancers are more vulnerable to eg they proliferate more
treat the viral etc infection
exploit natural defenses
264
What is the current course for cancer treatment typically
primary tumour removed surgically followed/ preceded by cytotoxic drugs or radiotherapy
265
What is our first cancer specific therapy
Breast and prostate patients may receive hormone therapy, which inhibits growth of both normal and cancerous tissue
266
Why is it hard to use targetted drugs to treat cancer on a broad scale
mutation information is not generally available
267
When are cytotoxic drugs usually reserved for
when metastasis becomes evident
268
What are the different ways cytotoxic drugs can damage DNA (4)
what are ways to interfere with DNA synthesis in a less direct way (2)
alkylate bases, intercalate non-covalently
between bases of the DNA helix, crosslink strands, or be toxic analogues of bases (e.g.
5-fluoro-uracil) that interfere with DNA synthesis.
topo inhibitors and drugs that interfere with mitosis (eg Taxol and Vinca alkaloids)
269
Name 2 cancers which can be 'cured'
childhood leukaemia (ALL) and teratomas in young males
treatment can lead to 20 years disease free
270
When was there the breakthrough with testicular cancer
1975 with the introduction of cis-platin (crosslinks purine bases within and between strands)
271
Why is cisplatin more effective in cancer than normal tissue
what is another drug that exploits cancer in a similar way
the crosslinks it creates are usually healed by HR, which requires BRCA2 (mutated in some cancers)
thus it exploits the genetic instability of the cancer
Taxanes target the mitotic spindle, which also seems to be defective in some cancers (lagging chromosomes)
272
What is the Waldmann experiment
made cancer cells that were defective in the p21 that, downstream of p53, arrests the cell cycle in response to DNA damage.
grew them as grafts on mice and X-irradiated them. X-radiation cured
several checkpoint-defective tumours but no wild-type controls.
The checkpoint-defective cells did not arrest when irradiated and presumably died of mitotic catastrophes, while the wild-type cells underwent cycle arrest and then recovered.
273
How are drugs being designed to exploit the genetic instability of cancers
If a DNA repair pathway is defective, cells may be killed by blocking alternative repair routes, leaving the cancer cell unable to deal even with everyday spontaneous DNA damage.
eg PARP inhibitors
274
How do PARP inhibitors work
targets SSB repair pathway, which normally requires PARP
the PARP inhibitors mean SSBs accumulate and halt DNA replication bc at the replication fork the SSB becomes a DSB
DSB usually repaired by HR, so eg BRCA2-defective cells, they cannot repair so die while normal cells survive
275
How does resistance to PARP inhibitors emerge
reverse mutation which restores BRCA2
276
What is Glivec
AKA Imatinib
| inhibits RTK produced by BCR-ABL fusion gene
277
True or false
| Imatinib is only effective against CML stemming from BCR ABL fusion
false
not specific for Bcr-Abl, but inhibits several tyrosine kinases including Abl (and is even used to treat cancers with activation of other kinases)
278
How can cancers become immune to Imatinib
how can this be treated
point mutation
Second-generation drugs that bypass this resistance have been developed.
279
Name some small molecule drugs / antibodies which have been developed to inhibit certain steps of the pathways
Herceptin, the monoclonal antibody to HER2 used in HER2-amplified breast cancer. Anti-BRAF and anti-MEK small-molecule drugs are in use.
There is no anti-RAS drug yet approved.
280
How do anti-EGFR encounter resistance
E.g. treating colorectal cancers with anti-EGFR often results in emergence of variant, resistant tumours that have acquired mutations in genes such as RAS or RAF.
281
How can BRAF V600E be treated
does resistance develop?
The BRAF inhibitor vemurafenib targets the ATP-binding
domain in BRAF melanomas
yes - eg by acquisition of RAS mutations or amplification of the mutant BRAF.
282
What are oncolytic viruses
viruses that t infect and kill cancers, by lysis or exciting an immune
response
eg H101 (an engineered adenovirus)
283
Why are adenoviruses well suited to be designed to be oncolytic
Adenoviruses prepare infected
| cells for virus replication by expressing proteins that inactivate Rb1 (cell cycle control) and p53
284
How are adenoviruses engineered to be oncolytic
how are they administered
what else is in development
what other viruses could be used in the future
have deletions in the E1B_55k gene,
which inactivates p53.
The idea is that without this E1b protein, the virus will only replicate in p53- mutant cancers.
It is injected directly into the tumour.
Derivatives of Herpes viruses, Vaccinia and VSV are in development.
285
What are some of the earliest reasons for thinking the immune system protects against cancer
Early experiments grafting tumours from one animal to another led to the discovery of graft rejection, but this was rejection of allogeneic tissue, not the actual tumour, and led to development of inbred laboratory mice
286
What did Burnet suggest about tumour graft rejection
why did his opinion carry a lot of weight
proposed, in an attempt to explain why graft rejection occurred (long before we understood MHC), that the true role of cellular immunity was to reject tumours. This was the concept of ‘immune surveillance’ against tumours
he proposed the clonal selection of B cells
287
Some experiments have found tumour specific antigens in mouse models. what were these
turned out to be special cases such as env glycoproteins of retroviruses, or mutant MHC alleles, not relevant to most cancers.
288
How does cancer incidence change in the immunosuppressed
careful studies of immunosuppressed humans and mice show no major increase in cancer,
except where the cancer has a viral aetiology .
(e.g. Kaposi’s sarcoma caused by Human
Herpesvirus 8 and lymphomas caused by Epstein-Barr Virus)
289
If studies of the immunosuppressed show no major increase in cancer, what does this suggest about the role of the immune system in cancer
If the acquired immune system played a major role in controlling cancers, immune suppressed people would get a lot more cancer.
It may detect new proteins on cancers, but if so doesn’t do much.
290
Why does contemporary immunology theory would not necessarily predict a response to tumour cells?
Is this backed up experimentally?
Expression of novel (mutant) proteins on a cell would probably induce peripheral tolerance.
DNA sequencing shows that mutant proteins are not selected against: (i) normal cells have many mutations that alter proteins (ii) in cancers overall there is no detectable selection against mutations that create new peptides.
291
Where does most excitement centre in cancer immunotherapy
```
checkpoint inhibitors
(monoclonal antibodies that block signals that hold back cytotoxic T cells)
```
292
which molecules are involved in downregulating cytotoxic T cells and how is this important for cancer treatment
```
by CTLA-4, PD-L1 and PD-1:
CTLA4 binds to
B7, displacing co-stimulator CD28;
PD1 and PD-L1
bind to each other to provide inhibitory signals
```
Monoclonal antibodies to these molecules are licensed.
293
Has immunotherapy made any difference to cancer treatment
some patients, with advanced metastases, who have failed conventional therapy, show dramatic regression of tumour and improvement in health; and secondly, some go on being healthy for at least many months. Resistance seems to develop more slowly.
294
Why must immunotherapy be treated with caution
Only some patients with certain kinds of tumour respond, and treatment is very toxic, giving inflammation and autoimmunity that can be life-threatening
295
What were some of the results from early studies of anti-CTLA4 in melanoma
only about 11%
of patients did better at 3 years, while treatment had to be stopped because of toxicity in 50% of all patients; 1% died of immune disease! It is said that these side-effects can now be managed better.
296
Which cancers tend to respond to immunotherapy
hose with the most mutations and hence the highest numbers of alien peptides.
Lung and melanoma are two of the most mutated cancers, and response in these clearly correlates with number of mutations per tumour
297
Why is it that cancers with the most mutations are affected most by immunotherapy
perhaps there are T cells that recognise mutant peptides but they are mostly tolerised.
If there are enough such antigens and tolerance is downregulated, an effective anti-cancer response may emerge, but
together with some inflammation and autoimmunity.
