King Saud University
College of Science
Disclaimer
Department of
Biochemistry
• The texts, tables and images contained in this course
presentation are not my own, they can be found on:
– References supplied
– Atlases or
– The web sites
BCH 475
Biochemistry of Carcinogenesis
Professor A. S. Alhomida
Summer, 2008
Part 2
1
2
Oncogenesis
3
Oncogenesis Arises from
• Spontaneous gene or chromosome
mutations
• Exposure to mutagens or radiation
• Activity of genes introduced by
tumor viruses
• Some cancers are inherited
(individuals may be predisposed)
4
Classes of Cancer Genes
5
Classes of Cancer Genes
• Three classes of genes are mutated
in cancer:
(1) Proto-oncogenes whose products
stimulate cell proliferation
• Cab be mutated into oncogene that induce
cell transformation (cancer cells)
6
Proto-oncogenes
• Proto-oncgenes
– Are genes that possess normal gene
products and stimulate normal cell
development
• Oncogenes
– Arise from mutant proto-oncogenes
– Are more active than normal or active at
inappropriate times and stimulate
unregulated cell proliferation
7
Classes of Cancer Genes
(2) Tumor suppressor genes whose
products normally inhibit proliferation,
negative regulatory protein
8
Classes of Cancer Genes
(3) Mutator genes whose products ensure
accurate replication and maintenance of
the genome
• Types of genes which may mutate to
cause cancer (mutators)
• DNA repair genes
• Telomerase
9
10
Oncogenic Viruses
(Viral Oncogenes)
11
Oncogenic Viruses
• Oncogenic viruses or viral
Carcinogenesis
– Viruses which produce cancer
• There is no single mechanism by
which viruses cause tumors
12
Viral Oncogenes
• Tumor viruses induce infected cells
to proliferate and produce a tumor
• There are two types, based on the
viral genome:
(1) RNA tumor viruses transform cells by
introducing viral oncogenes
•
An oncogene is any gene that stimulates
unregulated proliferation
13
Viral Oncogenes
– RNA tumor viruses
• Possess viral oncogenes
• Derived from cellular proto-oncogenes
capable of transforming cells to a
cancerous state
14
Viral Oncogenes
(2) DNA tumor viruses
• Another class of tumor viruses; do not carry
oncogenes, but induce cancer by activity of
viral gene products on the cell (no
transformation per se)
15
Cell Cycle and Cancer
16
Cell Cycle and Cancer
• Cell differentiation occurs as cells
proliferate to form tissues
• Cell differentiation correlates with
loss of ability to proliferate; highly
specialized cells are terminally
differentiated
17
Cell Cycle and Cancer
• Terminally differentiated cells have a
finite life span, and are replaced with
new cells produced from stem cells
• Stem cells are capable of selfrenewal; cells divide without
undergoing terminal differentiation
18
Incidence of Cancer
19
Incidence of Cancer
(1) Sporadic cancers
– The more frequent type, do not appear
to have an hereditary cause
20
Incidence of Cancer
(2) Familial (hereditary) cancers
– Run in families
• Retinoblastoma
– Is the most common eye tumor in children birth
to 4 years
– Early treatment (usually gamma radiation) is over
90% effective
21
Two-hit Mutation Model
for Cancer
22
Two-hit Mutation Model
• Cancers can be caused by viruses,
but most result from mutations in
cellular genes
• Usually these mutations have
accumulated over time, and research
has identified the genes involved
23
Retroviruses and
Oncogenes
24
Retroviruses and
Oncogenes
• Retrovirus
– Single-stranded RNA virus that
replicates via double-stranded DNA
intermediate
– RNA is converted to cDNA by reverse
transcriptase
– DNA integrates into host chromosome
and is transcribed
25
Retroviruses Genes
• Three types of genes occur in most
retroviruses:
– Gag (group antigen)
• Codes the protein core
– Pol (polymerase)
• Codes reverse transcriptase and an enzyme
for proviral integration
– Env (envelope)
• Codes envelope glycoproteins
26
Retroviruses and
Oncogenes
• Oncogenic retroviruses (v-onc)
transform the cell and cause cancer
(also called transducing viruses)
• Different retroviruses carry different
oncogenes responsible for different
types of cancer
– e.g. v-src in RSV
27
Retroviruses and
Oncogenes
• Most oncogenic retroviruses (but not
RSV) are defective and do not
possess a full set of virus life-cycle
genes
– Transform cells but do not produce
progeny viruses
28
Retroviruses and
Oncogenes
• Defective retroviruses produce
progeny with the help of a normal
virus that co-infects cell and supplies
missing gene products
– Helper virus supplies missing gene
products → viral expression
29
Transducing Retroviruses
• Retroviruses that carry an oncogene
(v-onc) are transducing retroviruses
• Different types of cancer are caused
by different v-onc genes (e.g., the
sarcoma gene, v-src, of RSV)
– RSV-infected cells rapidly transform,
– Produce progeny RSV particles
– Because RSV is unusual in having
intact gag, pol, and env genes
30
Transducing Retroviruses
– All other transducing retroviruses are
defective, lacking one or more genes
needed to replicate
– If a helper virus supplies the missing
gene product(s) progeny can be made
31
Structures of Four
Defective Transducing
Viruses
32
Structures of Four
Defective Transducing
Viruses
33
RNA Tumor Viruses
• They are all retroviruses, and their
oncogenes are altered forms of
normal host genes
– Examples include:
•
•
•
•
Rous sarcoma virus
Feline leukemia virus
Mouse mammary tumor virus
Human immunodeficiency virus (HIV-1,
cause of AIDS)
34
35
RNA Tumor Viruses
• Structurally, retroviruses have:
– Two copies of the 7-10 kb ssRNA
genome
– A protein core (often icosahedral)
– An envelope derived from host
membrane and bearing viral
glycoproteins used to enter a host cell
36
RNA Tumor Viruses
• The retroviral life cycle was first
characterized (1910) for a “filterable
agent” from a chicken tumor, later
named the Rous sarcoma virus (RSV)
• RSV’s genome organization is known
37
RNA Tumor Viruses
38
Mechanism of RSV Action
39
Mechanism of RSV Action
40
Mechanism of RSV Action
• Upon retroviral infection, the ssRNA
genome is released from the virus
particle, and reverse transcribed to
dsDNA (proviral DNA) by reverse
transcriptase carried in the virus
particle
41
Mechanism of RSV Action
– Proviral DNA integrates into host
chromosome:
• The 5’ (left) end of the viral genome has
sequences R and U5, while the 3’ (right) end
has sequences U3 and R
• During proviral synthesis, genome ends are
duplicated to produce long repeats (LTR) of
U3-R-U5
• The LTRs contain transcription regulatory
signals for viral genes
42
Mechanism of RSV Action
• Proviral DNA is ligated to produce a circular
dsDNA with two adjacent LTRs
• Staggered nicks in proviral and host DNA
are used for integration of the viral genome
into the host chromosome
• Single-stranded gaps are filled, producing
short, direct repeats in host DNA flanking
the provirus
43
Mechanism of RSV Action
– Host RNA polymerase II transcribes the
proviral DNA, and viral mRNAs are
produced by alternative splicing
– Three genes are characteristic of
retroviruses:
• The gag (group antigen) gene encodes a
precursor protein that is cleaved to form the
protein core (capsid)
44
Mechanism of RSV Action
• The pol (polymerase) gene produces a
precursor protein cleaved to make reverse
transcriptase and an enzyme for proviral
integration
• The env (envelope) gene encodes the
envelope glycoprotein, used to infect a host
cell
45
Features of LTRs
46
Oncogenic Retroviruses
Not Involved in the Cell
Cycle
47
Oncogenic Retroviruses
Not Involved in the Cell
Cycle
• Oncogenic retroviruses carry an
oncogene that is not involved in the
cell cycle
– Different retroviruses carry different
oncogenes
– In RSV the oncogene is src
48
Oncogenic Retroviruses
Not Involved in the Cell
Cycle
– Most retroviruses cannot replicate due
to missing life-cycle genes (RSV is an
exception)
– Retroviruses without oncogenes
(nononcogenic retroviruses) direct their
own life cycle, and do not change the
growth properties of infected cells
49
Nononcogenic
Retroviruses
50
Life Cycle of
Nononcogenic Retrovirus
51
Nononcogenic
Retroviruses
• They include HIV-1, a virus that
causes AIDS, rather than cancer
– The bullet-shaped capsid is surrounded
by an envelope containing viral gp120
glycoproteins
– The genome contains gag, pol and env
genes, and several other genes used for
gene regulation (e.g., tat regulates gag
and pol expression)
52
Nononcogenic
Retroviruses
– Infection begins when the gp120
glycoprotein in the HIV-1 envelope
binds:
• Most commonly, the CD4 receptor of a
helper T cell
• A different receptor on a different type of
cell (e.g., macrophage, glial cell in brain,
regulatory cell of intestinal lining)
53
Mechanism of
Nononcogenic
Retroviruses
54
Mechanism of
Nononcogenic Retroviruses
– The virus particle enters the cell, the
protein capsid is lost and the viral life
cycle begins
– Normal viral replication causes death of
cells infected with HIV-1, depleting the
helper T cells needed to mount an
immune response
– Unable to combat infection, AIDS
patients frequently die of infections and
cancers
55
Cellular Proto-oncogenes
56
Cellular Proto-oncogenes
• Mid-1970s: J. Michael Bishop &
Harold Varmus (Nobel Prize 1989)
• Demonstrated normal animal cells
contain non-cancer causing genes
closely related to viral oncogenes
57
Cellular Proto-oncogenes
• Early-1980s: R. A. Weinberg & M.
Wigler demonstrated a variety of
human tumor cells contain
oncogenes, which transform normal
cells growing in culture to cancer
cells
58
Cellular Proto-oncogenes
• Most human and animal oncogenes
are mutated forms of normal cellular
genes (proto-oncogene = normal
state)
59
Cellular Proto-oncogenes
• v-onc
– Viral oncogene, carried by a virus
• c-onc
– Cellular oncogene, resides in host
chromosome
60
Chicken c-src Proto-oncogene
and v-src Oncogene
61
Genomic Changes that Cause
Proto-oncogene Activation
• Genomic changes (amplification,
insertion & translocation) that cause
proto-oncogene activation:
– Amplification
• c-myc, c-abl, c-myb, c-erbB, c-K-ras, mdm-2
• Presence of known oncogenes in amplified
region
– Amplification of same oncogenes in
many cancers
62
Genomic Changes that Cause
Proto-oncogene Activation
• Insertion
– Insertion of retrovirus LTR overexpresses c-myc
• Insertion of ALV activates c-myc gene
• Translocation
– Reciprocal translocation by illegitimate
recombination
• Immunoglobulin or TCR gene and c-myc
oncogene
63
Genomic Changes that Cause
Proto-oncogene Activation
• Increased c-myc expression after
translocation
• c-myc coding sequences are unaltered in all
cases
64
Genomic Changes that Cause
Proto-oncogene Activation
65
Genomic Changes that Cause
Proto-oncogene Activation
66
Genomic Changes that Cause
Proto-oncogene Activation
67
Genomic Changes that Cause
Proto-oncogene Activation
• Evidence of oncogenic potential of cmyc gene:
– Transgenic mice carrying c-myc that:
• Linked to B lymphocyte enhancer →
lymphoma
• Under mouse mammary tumor virus LTR →
various cancers
68
Genomic Changes that Cause
Proto-oncogene Activation
• Translocation can generate hybrid
oncogenes and human cancers
– CML & Philadelphia chromosome
• c-abl gene on chromosome 9 and bcr gene
on chromosome 22
• Why is the hybrid bcr-abl protein
oncogenic?
– Activation of ras pathway for transformation
69
70
Mechanism of
Retroviruses
Oncogenesis
71
Mechanism of Retroviruses
Oncogenesis
• Retrovirus integrates into host
chromosome near a cellular protooncogene by random recombination
• Deletion fuses retrovirus
transcription signal sequences with
proto-oncogene sequences
72
Mechanism of Retroviruses
Oncogenesis
• In the process, parts of the viral DNA
sequences typically are deleted (this
is how the defective oncogene is
created)
• Viral “progeny” carry the cellular
gene, but now under the influence of
viral promoters
73
Mechanism of Retroviruses
Oncogenesis
• Most transducing viral oncogenes
are defective and cannot replicate
independently
74
Mechanism of Retroviruses
Oncogenesis
• If mRNA is packaged into a virus
particle along with a normal virus
genome (co-infection), reverse
transcriptase produces a new
defective oncogene by switching
templates during cDNA synthesis
75
Mechanism of Retroviruses
Oncogenesis
• Template switching + lack of
proofreading during DNA replication
result in rapid evolution of oncogenic
retroviruses
76
Formation of Transducing
Retrovirus Oncogene
77
DNA Tumor Viruses
78
DNA Tumor Viruses
• Do not carry oncogenes
• Transform cells to the cancerous
state through actions of genes in the
viral genome
79
DNA Tumor Viruses
• Examples include viruses in the
following groups:
– Papovaviruses (warts and human
cervical cancer)
– Hepatitis B
– Herpes
– Adenoviruses
– Pox viruses
80
DNA Tumor Viruses
• DNA viruses induce production of
cellular DNA replication enzymes,
which are used in viral replication
• Rarely, viral DNA integrates into the
host genome instead, and may
produce protein(s) that stimulate the
cell to proliferate
81
DNA Tumor Viruses
• An example:
– The papovavirus group includes many
different papillomaviruses, some of
which cause:
• Human warts
• Human cervical cancer (HPV-16, HPV-18),
due to action of the E6 and E7 genes, which
influence cell growth and division
82
Proto-oncogenes
83
Oncogenes
• Oncogene is mutated form of normal
genes called proto-oncogene
– Control of cell proliferation and
differentiation
84
Oncogenes
• Oncogenic virus
– Oncogenic DNA virus
– Oncogenic RNA virus
• Viruses are named because of reverse
transcriptase
• Retrovial oncogene, src
85
Proto-oncogen Activation
• Normal cell genes from which
oncogene originated, encoding
proteins that function in:
– Signal transduction pathway
– Controlling normal cell proliferation
86
Proto-oncogen Activation
87
Functions of Oncogene
Products
• Uncotrolled proliferation of cancer
cell
• Defective differentiation
• Failure to programmed cell death
88
ras Proto-oncogenes
• Involved in signal transduction
pathway as are many protooncogene products
• ras family genes mutated in 40% of
all cancers
89
ras Proto-oncogenes
• Involved in signal transduction
pathway from growth factor receptor
to nucleus
– G protein
• Mutant form lacks GTPase activity and
remains active
90
Proto-oncogene and Oncogene
Protein Products
• ~100 different oncogenes have been
identified
• To understand the cancer
– Understand the function of protein
products coded by the proto-oncogenes
91
Protein Products of
Proto-oncogenes
• Proto-oncogenes fall into classes
with characteristic protein products,
all of which stimulate cell growth
• Examples:
92
Examples of Protein Products
of Proto-oncogenes
• An example of growth factors is the
viral oncogene v-sis, which encodes
platelet-derived growth factor (PDGF)
– Deriving from mammalian blood
platelets, PDGF causes fibroblasts to
grow as part of wound-healing
– Introduction of a cloned PDGF gene into
cells that normally do not express it
(e.g., fibroblasts) transformed the cells
93
Examples of Protein Products
of Proto-oncogenes
– Inappropriately expressed growth
factors, therefore, can cause cancer
• An example of protein kinases is the
src gene product, which encodes
pp60src, a nonreceptor protein
kinase
– Both cellular and viral versions of the
pp60src protein phosphorylate tyrosine
(rather than serine or threonine
94
Examples of Protein Products
of Proto-oncogenes
– Protein kinases are known to be
involved in many aspects of cell
signaling and growth regulation
95
Proto-oncogene and Oncogene
Protein Products
• All known proto-oncogenes are
involved in positive control of cell
growth and division
96
Proto-oncogene and Oncogene
Protein Products
• Two classes:
– Growth factors
• Regulatory genes involved in the control of
cell multiplication
– Protein kinases
• Add phosphate groups to target proteins,
important in signal transduction pathways
97
Mechanism of
Conversion of Protooncogenes to Oncogenes
98
Mechanism of Conversion of
Proto-oncogenes to Oncogenes
• Conversion of proto-oncogenes to
oncogenes relaxes cell control,
allowing unregulated proliferation
Examples:
– Point mutations in the coding or
controlling sequences can either
change the gene product or alter its
expression
99
Mechanism of Conversion of
Proto-oncogenes to Oncogenes
• The ras genes are an example:
– A point mutation produces a mutant protein that
can cause cancer in many different types of cells
– G proteins lose regulation, and constitutive
growth signals are transmitted to the cell
100
Mechanism of Conversion of
Proto-oncogenes to Oncogenes
– Deletions of coding or controlling
sequences can change the amount of
activity of growth stimulatory proteins,
allowing proliferation. The myc gene is
an example:
• The myc gene product is a transcription
factor that activates genes involved in cell
division
101
Mechanism of Conversion of
Proto-oncogenes to Oncogenes
• Deletions can remove upstream sequences,
allowing expression from an alternative
promoter and changing the amount or
activity of the protein product
– Gene amplification, caused by random
overreplication of regions of genomic
DNA, has been found in tumor cells
– Multiple copies of ras in mouse
adrenocortical tumors are an example
102
Mechanism of Conversion of
Proto-oncogenes to Oncogenes
• Point mutations that result in
constitutively active protein products
• Localized gene amplification of a
proto-oncogene leading to overexpression
103
Mechanism of Conversion of
Proto-oncogenes to Oncogenes
• Chromosomal translocation that
brings a growth-regulatory gene
under control of a different promoter
that causes inappropriate expression
• Note: These are generally gain-offunction mutations
104
Gain-of-function
Mutations Convert Protooncogenes into
Oncogenes
105
Gain-of-function
Mutations
• Oncogenes were first identified in
cancer-causing retroviruses
– The Rous sarcoma virus (RSV) contains
a gene (src) that is required for cancerinduction but is not required for viral
function
• Normal cells contain a related gene
that codes for a protein-tyrosine
kinase
106
Gain-of-function
Mutations
• The normal gene (c-src) is the protooncogene, while the viral gene (v-src)
is an oncogene that codes for a
constitutively active mutant proteintyrosine kinase
• Many DNA viruses also contain
oncogenes but these have integral
functions in viral replication
107
Slow-acting Carcinogenic
Retroviruses can Activate
Cellular Proto-oncogenes
108
Retroviruses Activate
Cellular proto-oncogenes
109
Tumor Suppressor Genes
110
Tumor Suppressor Genes
• Harris (1960s) showed that fusion of
cancer cells and normal cells did not
always result in a tumor, indicating
the existence of tumor suppressor
genes
111
Tumor Suppressor Genes
• In certain cancers, both homologous
chromosomes show deletion of
specific regions, the sites of tumor
suppressor genes that inhibit cell
growth and division
112
Tumor Suppressor Genes
• Human examples include:
– Breast cancer
– Colon cancer
– Lung cancer
• Action of tumor suppressors is the
opposite of proto-oncogenes
113
Tumor Suppressor Genes
• Both tumor suppressor genes must
be lost for unregulated growth to
occur (they are recessive), while only
one mutation is needed to change a
proto-oncogene to an oncogene (it is
dominant)
114
Tumor Suppressor Genes
• The gene’s normal function is to
regulate cell division
• Both alleles need to be mutated or
removed in order to lose the gene
activity
• The first mutation may be inherited
or somatic
115
Tumor Suppressor Genes
• The second mutation will often be a
gross event leading to loss of
heterozygosity in the surrounding
area
• Block abnormal growth and
malignant transformation
116
Tumor Suppressor Genes
• Proto-oncogene; dominant in action
• Tumor-suppressor gene; recessive
in action
– Examples:
• Rb, p53, INK4, APC, DCC
117
Tumor Suppressor Genes
• Functions of tumor suppressor gene
products
– Tumor development by eliminating
negative regulatory proteins
• Examples:
– WT1, Rb and INK4, p53 gene product, APC and
DCC
118
Five Proteins are Encoded by
Tumor-Suppressor Genes
1. Intracellular proteins that regulate
or inhibit progression through a
specific stage of the cell cycle
2. Receptors for secreted hormones
that inhibit cell proliferation
3. Checkpoint-control proteins that
arrest the cell cycle
119
Five Proteins are Encoded by
Tumor-Suppressor Genes
4. Proteins that promote apoptosis
5. Enzymes that participate in DNA
repair
Note: These are usually loss-offunction mutations
120
Loss-of-function Mutations
in Tumor-suppressor Genes
• The first tumor-suppressor gene was
identified in patients with inherited
retinoblastoma
121
Loss-of-function Mutations
in Tumor-suppressor Genes
122
Loss-of-function Mutations
in Tumor-suppressor Genes
123
Loss of Heterozygosity of TumorSuppressor Genes Occurs by Chromosome
Mis-segregation or Mitotic Recombination
124
Loss of Heterozygosity of TumorSuppressor Genes Occurs by Chromosome
Mis-segregation or Mitotic Recombination
125
Comparison of the
Effects of Tumor
Suppressor gene and
Proto-oncogene
Mutations
126
127
Protooncogenes and
tumorsuppressor
genes: the
seven types of
proteins that
participate in
controlling cell
growth
128
p53 Tumor Suppressor
Gene
• Mutated (inactivated) in more than
50% of all cancers
• p53 regulates (activates or
represses) transcription of more than
50 different genes
129
Mutations in p53 Abolish
G1 checkpoint Control
• Some human
carcinogens cause
inactivating
mutations in the
p53 gene and
• p53 activity is also
inhibited by certain
proteins encoded
by DNA tumor
viruses
130
Mutations in p53 Abolish
G1 checkpoint Control
131
p53 Tumor Suppressor
Gene
• Mutations in the p53 gene occur in
more than 50% of human cancers
• Because p53 is a tetramer, a point
mutation in a single allele can inhibit
all p53 activity
132
p53 Tumor Suppressor
Gene
• MDM2, a protein that normally
inhibits the ability of p53 to halt
the cell cycle, is overexpressed in
certain cancers
• One human papillomavirus protein,
E6, binds to and inhibits p53
133
p53 Tumor Suppressor
Gene
• The large T protein from the DNA
monkey papovavirus binds to both
p53 and Rb, inhibiting their function.
• Carcinogens such as benzo(a)pyrene
and aflotoxin induce inactivating
mutations in p53
134
p53 Tumor Suppressor
Gene
• p53 regulated by Mdm2 (prevents the
phosphorylations and acetylations
that activate inactive p53)
• Activated p53 levels rise rapidly if
DNA is damaged or repair
intermediates accumulate
135
p53 Tumor Suppressor
Genes
• Mutations in p53 are implicated in
~50% of human cancers, including
cancers of the:
– breast, brain, liver, lung, colorectal,
bladder, and blood
• Development of tumors requires
mutations on two p53 alleles
136
p53 Tumor Suppressor
Genes
• Codes a 393 amino acid protein
involved in transcription, cell cycle
control, DNA repair, and apoptosis
(programmed cell death)
• p53 binds to several genes, including
WAF1, and interacts with at least 17
cellular and viral proteins
137
p53 Tumor Suppressor
Genes
• Transgenic mice with deletions of
both p53 alleles are viable, but 100%
develop cancer by ten months of age
• Effects of DNA damage and normal
(non-mutant) p53 lead to cell growth
arrest
138
p53 Function
• Suppresses progression through the
cell cycle in response to DNA
damage
• Initiates apoptosis if the damage to
the cell is severe acts as a tumor
suppressor
• It is a transcription factor and once
activate
139
p53 Function
• It represses transcription of one set
of genes (several of which are
involved in stimulating cell growth)
while stimulating expression of other
genes involved in cell cycle control
140
p53 Function
• Activated p53 acts as transcription
factor to turn on genes that
– Arrest the cell cycle so DNA can be
repaired
• Initiates synthesis of p21, which inhibits
CDK4/cyuclinD1 complex, blocking entry
into S phase
• Genes expressed which retard rate of DNA
replication
• Other products block G2/M progression
141
p53 Function
– Initiate apoptosis if DNA cannot be
readily repaired
• Turns on Bax gene, represses Bcl2 gene
• Bax homodimers activate process of cell
destruction
• Cancer cells lacking p53 do not initiate
pathway even if DNA/cellular damage is
great
142
pRB Function
• Tumor suppressor protein that
controls the G1/S checkpoint
• Found in nucleus and activity
regulated by level of phosphorylation
(by CDK4/cyclinD1 complex)
143
pRB Function
• Nonphosphorylated version binds to
TFs such as E2F, inactivating them
• Free E2F and the other regulators
turn on >30 genes required for
transition to S phase
144
Role of pRB in Regulating
Cell Division
145
Mechanism of Tumor
Suppressor Genes
146
Mechanism of Tumor
Suppressor Genes
• Oncogenic virus enters the cell
• DNA of virus enters cells
chromosome
– If retrovirus, first, the RNA is converted
to DNA (provirus)
– The provirus DNA enters the cell
147
Mechanism of Tumor
Suppressor Genes
• Virus has cancer causing gene =
oncogene transformation:
– Cell becomes cancer cell
• In 1976 (Varmus and Bishop from
UCSF received Nobel Prize 1989)
• Oncogenes found in normal cells
– (Same nucleotide sequence or genetic
recipe) called protooncogene
148
Mechanism of Tumor
Suppressor Genes
• Most oncogenes are dominant genes
• Whole lists of oncogenes now
discovered for animal and a few for
humans
• We all have protooncogenes:
– Human’s total genome is 100,000 genes
probably < 100 are protooncogenes
149
Mechanism of Tumor
Suppressor Genes
– 40 discovered so far (50 Talaro)
– They have normal functions that are
important
• Now we know that all cancers are the
result of an oncogene
150
RB1 Tumor Suppressor
Gene
• Retinoblastoma 1 gene
• Involved in breast, bone, lung,
bladder and retinal cancers (among
others)
151
RB1 Tumor Suppressor
Gene
• Inheriting one mutated (inactivated)
copy of gene increases chances of
retinoblastoma formation from
1/14,000-20,000 to 85% (plus
increases other cancer rates)
– Loss of second copy in a cell eliminates
function
– Normal cells unlikely to lose both good
copies
152
DNA Repair Genes
(Mutator Genes)
153
DNA Repair Genes
(Mutator Genes)
• Third category of cancer-causing
genes
– Excision, mismatch repair
• Cancer effects are indirect
• Defective DNA repair = increase rate
of failure to repair mutations
– Mutations accumulate in the genome
154
DNA Repair Genes
(Mutator Genes)
• Significance
– Have an increased chance of mutation
in a proto-oncogene and/or tumor
suppressing gene
155
Mutator Genes
156
Mutator Genes
• Mutator gene increases spontaneous
mutation rate of other genes
• Mutator gene products are involved
in DNA replication and repair;
mutations make the cell error prone
• HNPCC-OMIM 120435, human nonpolyposis colon cancer
157
Mutator Genes
• Mutation at any one of two genes
(hMSH2, hMLH1, hPMS1, hPMS2)
leads to predisposition
• Tumor formation requires mutation
at the second allele
• All four genes have homologs in
yeast
• DNA blood tests are available for all
158
four genes
Defects in DNA-repair Systems Perpetuate
Mutations and are Associated with Certain
Cancers
159
Virus-encoded Activators of
Growth-factor Receptors Act as
Oncoproteins
160
Activating Mutations or
Overexpression of Growth-factor
Receptors can Transform Cells
161
Constitutively Active SignalTransduction Proteins are Encoded by
Many Oncogenes
162
Constitutively Active SignalTransduction Proteins are Encoded by
Many Oncogenes
163
Chromosomal Abnormalities are
Common in Human Tumors
• In Burkitt’s lymphoma c-myc is
translocated to chromosome 14
near an antibody-gene enhancer
164
Chromosomal Abnormalities are
Common in Human Tumors
• A translocation between
chromosomes 9 and 22 causes the
formation of the chimeric bcr-abl
oncogene found in virtually all
patients with chronic myelogenous
leukemia
165
Chromosomal Abnormalities are
Common in Human Tumors
166
Passage from G1 to S Phase is
Controlled by Proto-oncogenes and
Tumor-suppressor Genes
167
Passage from G1 to S Phase is
Controlled by Proto-oncogenes and
Tumor-suppressor Genes
168
Loss of TGF Signaling
Contributes to Abnormal Cell
Proliferation and malignancy
169
Finding Tumor
Suppressor Genes
• Recessive genes, like those for
tumor suppression, are more difficult
to detect than dominant genes
• Positional cloning, the search for
DNA variations between normal and
tumor cells, was finally successful in
isolating several tumor suppressor
genes
170
Retinoblastoma
171
Retinoblastoma
• Retinoblastoma has two forms:
– Sporadic retinoblastoma (60%)
develops in children with no family
history of retinoblastoma, and occurs in
one eye (unilateral tumor)
172
Retinoblastoma
– Hereditary retinoblastoma (40%)
patients typically develop multiple
tumors involving both eyes (bilateral
tumors)
• Onset is usually earlier in the hereditary
form
• Siblings and offspring often develop the
same type of tumor
• Pedigrees of affected families are
consistent with a single gene responsible
for retinoblastoma
173
Knudson’s Two-hit
Mutation Model for
Retinoblastoma
174
Knudson’s Model for
Retinoblastoma
• Two mutations are required for the
development of retinoblastoma
• Sporadic retinoblastoma
– Child starts with two wild type alleles
(RB+/RB+)
– Both alleles must mutate to produce the
disease (RB/RB)
175
Knudson’s Model for
Retinoblastoma
– Probability of both mutations occurring
in the same cell is low; only one tumor
forms (e.g., one eye)
176
Knudson’s Model for
Retinoblastoma
• Hereditary retinoblastoma
– Child starts with heterozygous alleles
(RB/RB+)
– Only one mutation is required to
produce disease (RB/RB)
– Mutations resulting in loss of
heterozygosity (LOH) are more probable
in rapidly dividing cells, and multiple
tumors occur (e.g., both eyes)
177
178
Knudson’s Model for
Retinoblastoma
• Retinoblastoma alleles are recessive;
only homozygotes (RB/RB) develop
tumors
• Retinoblastoma appears as dominant
in pedigree analysis:
– RB/RB+ individuals are predisposed
and have a significant incidence of the
disease
179
Knudson’s Model for
Retinoblastoma
– Homozygous dominant individuals
(RB+/RB+) require two mutations in the
same cell to develop the cancer
• Retinoblastoma was mapped to the
long arm of chromosome 13
(13q14.1-q14.2)
180
Knudson’s Model for
Retinoblastoma
• Mutations occur in a gene that
encodes a growth inhibitory factors
(tumor suppressor gene)
• Retinoblastoma is rare among
cancers; most cancers result from a
series of mutations in many different
genes
181
Retinoblastoma
182
Retinoblastoma
183
The Retinoblastoma
Tumor Suppressor Gene
• The human RB tumor suppressor
gene has been mapped (13q14.1q14.2) and sequenced
– Its 180 kb of DNA encodes a 4.7 kb
mRNA that produces a 928-amino-acid
nuclear phosphoprotein, pRB
– pRB is expressed in every tissue type
examined, regulating cell cycle and all
major cellular processes
184
The Retinoblastoma
Tumor Suppressor Gene
– Tumor cells have point mutations or
deletions in the gene, leading to loss of
pRB function
185
The Retinoblastoma
Tumor Suppressor Gene
– Karyotype analysis detects about 5% of
RB mutants, and the remainder are
difficult to detect even with molecular
techniques
• Mitotic recombination
• Chromosomal nondisjunction
• Gene conversion
186
The Retinoblastoma
Tumor Suppressor Gene
• The cell cycle transition from G1 to S
is regulated by pRB, committing the
cell to the rest of the cycle
– In a normal G1 cell, pRB binds two
transcription factors, E2F and DP1
– As long as pRB stays bound to the
factors, the cell remains in G1 or enters
G0
187
The Retinoblastoma
Tumor Suppressor Gene
– At the signal to progress through the
cell cycle, cyclin/cyclin-dependent
kinase (Cdk) phosphorylates pRB so
that it is unable to bind E2F
– Free E2F now binds and activates
transcription of genes required for entry
into S phase
– After the cell completes mitosis, pRB is
dephosphorylated
188
The Retinoblastoma
Tumor Suppressor Gene
• In a cell with two mutant RB alleles:
– If pRB is present, it is unable to bind
E2F/DP1
– Target genes are activated, and the cell
enters S phase
189
The Retinoblastoma
Tumor Suppressor Gene
• Several viruses (e.g., adenovirus,
SV40) make proteins that complex
with pRB, blocking its ability to bind
E2F, and so allowing the S phase
genes to be activated
190
The Retinoblastoma
Tumor Suppressor Gene
• pRB bind other cellular proteins,
including those involved with all
three RNA polymerases:
– A component of the RNA polymerase II
basal transcription machinery
– Factors for rRNA synthesis by RNA
polymerase I
– Factors for tRNA synthesis by RNA
polymerase III
191
The Retinoblastoma
Tumor Suppressor Gene
• Retinoblastoma indicates that pRB
may also play a role in regulating
development, perhaps by causing
cells to become terminally
differentiated
192
Retinoblastoma Tumor
Suppressor Genes
• Mapped to gene chromosome 13 and
sequenced.
• 180 kb; codes a 4.7 kb mRNA that
produces a 928 amino acid nuclear
phosphoprotein, pRB
• pRB is expressed in every tissue that
has been examined and regulates the
cell cycle
193
Retinoblastoma Tumor
Suppressor Genes
• Retioblastoma tumor cells possess
point mutations or deletions, which
render pRB defective
• In hereditary retinoblastoma, second
RB mutation often is identical to the
inherited one (a possible example of
gene conversion)
194
Effects of DNA Damage
and Normal p53
195
Breast Cancer Tumor
Suppressor Genes
• Breast cancer affects 1 in 10 women
and represents 31% of cancers in
women (~185,000 women diagnosed
each year)
196
Breast Cancer Tumor
Suppressor Genes
• ~ 5% of breast cancers are
hereditary; age of onset for
hereditary breast cancer is earlier
than other forms (mutations at two
alleles)
• Many genes involved; BRCA1 and
BRCA2 are thought to be tumor
suppressor genes
197
Breast Cancer Tumor
Suppressor Genes
• BRCA1 is important for homologous
recombination, cellular repair of DNA
damage, and transcription of mRNA
• Mutations in BRCA1 also are
involved in ovarian cancer
• BRCA2 plays a role in timing of
mitosis in the cell cycle
198
Multi-step Nature of
Cancer
• Cancer is a stepwise process,
typically requiring accumulation of
mutations in a number of genes
• ~6-7 independent mutations typically
occur over several decades:
199
Multi-step Nature of
Cancer
• Cancer is a stepwise process,
typically requiring accumulation of
mutations in a number of genes
• ~ 6-7 independent mutations typically
occur over several decades:
– Conversion of proto-oncogenes to
oncogenes
– Inactivation of tumor suppressor genes
200
Bert
Vogelstein’s
model of
colorectal
cancer
OMIM175100
201
How does HPV cause
cancer?
• Gene products of certain sub-type
(eg 16 and 18) interfere with normal
cellular proteins
• Early viral proteins E6 and E7 bind
p53 and RB proteins respectively
202
How does HPV cause
cancer?
203