Tumor Viruses

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Tumor Viruses
For most viruses:
Genome
Replication
viral proteins
Lysis
Progeny virions
Lytic Life Cycle
1
Tumor Viruses
Latent Life Cycle
Virus
Cell
Integration (usually)
Transformation
Virus-specific proteins expressed - No mature virus
Changes in the properties of host cell - TRANSFORMATION2
Tumor Viruses
Transformation:
Loss of growth control
Ability to form tumors - viral genes interfere with
control of cell replication
3
TRANSFORMATION
Both DNA and RNA tumor viruses can transform cells
Integration occurs (usually)
Similar mechanisms
VIRAL TRANSFORMATION
The changes in the biological functions of a cell that result from
REGULATION
of the cell’s metabolism by viral genes and that confer on the
infected cell certain properties characteristic of
NEOPLASIA
These changes often result from the integration of the viral
genome into the host cell DNA
4
TRANSFORMATION
Among the many altered properties of the
TRANSFORMED CELL are:
• Loss of growth control (loss of contact inhibition
in cultured cells)
• Tumor formation
• Mobility
• Reduced adhesion
• Transformed cells frequently exhibit chromosomal
aberrations
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Two Major Classes of Tumor Viruses
DNA Tumor Viruses
DNA viral genome
DNA-dependent
DNA polymerase
(Host or viral)
Host RNA
polymerase
Viral mRNA
Viral protein
6
RNA Tumor Viruses
Viral RNA genome
Reverse transcriptase (Virus-encoded)
Viral DNA genome (integrated)
DNA-dependent RNA polymerase (Host
RNA pol II)
IMPORTANT
Viral genomic RNA
Splicing (Host splicing enzymes)
messenger RNA
viral protein
Virus
Important: Use HOST
RNA polymerase
to make its genome
An enzyme that
normally
makes mRNA
7
DNA Tumor Viruses
DNA genome
Host RNA
polymerase II
mRNA
Host enzymes
protein
virus
OR TRANSFORMATION
In transformation usually only EARLY functions are expressed
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DNA Tumor Viruses In
Human Cancer
Papilloma Viruses
• cause natural cancers in animals
• cause benign warts
• ubiquitous
• epitheliotropic - most human tumors are malignancies of epithelial
cells
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DNA Tumor Viruses In
Human Cancer
Papilloma Viruses
• epidermodysplasia verruciformis
wart
malignant
squamous cell carcinoma
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DNA Tumor Viruses In
Human Cancer
Epidermodysplasia
verruciformis
Papilloma virus
11
DNA Tumor Viruses In
Human Cancer
Papilloma Viruses
urogenital cancer
wart
malignant squamous cell carcinoma
Papilloma viruses are found in 91% of women with cervical cancer
Squamous cell carcinoma:
Larynx
Esophagus
Lung
All histologically similar
10% of human cancers may be HPV-linked
12
DNA Tumor Viruses In
Human Cancer
Papilloma Viruses
• 51 types identified - most common are types 6 and 11
• most cervical, vulvar and penile cancers are ASSOCIATED with
types 16 and 18 (70% of penile cancers)
EPIDEMIOLOGIAL STUDIES BUT:
HPV 16 and HPV 18 do transform human keratinocytes
13
DNA Tumor Viruses In
Human Cancer
Polyoma Viruses
• Simian virus 40 - juvenile hamster sarcomas, transformation
• Polyoma - mouse leukemia, in vitro transformation
• Human polyomas (JC and BK) - monkey sarcoma, transformation
PROGRESSIVE MULTIFOCAL LEUKOENCEPHALOPATHY
Polyoma virus transforms cells when the genome is incomplete
Early functions are necessary
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DNA Tumor Viruses In
Human Cancer
Adenoviruses
Highly oncogenic in animals
Only part of virus integrated
Always the same part
Early functions
E1A region: 2 T antigens
E1B region: 1 T antigen
E1A and E1B = Oncogenes
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DNA Tumor Viruses In
Human Cancer
ONCOGENE
A gene that codes for a protein that potentially can transform
a normal cell into a malignant cell
An oncogene may be transmitted by a virus in which case it is
known as a VIRAL ONCOGENE
v-onc
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DNA Tumor Viruses In
Human Cancer
Herpes Viruses
Considerable evidence for role in human cancer
• Some very tumorigenic in animals
• Viral DNA found in small proportion of tumor cells: “hit and run”
• Epstein-Barr Virus
• Burkitt’s Lymphoma
• Nasopharyngeal cancer
• Infectious mononucleosis
• Transforms human B-lymphocytes in vitro
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DNA Tumor Viruses In
Human Cancer
Hepatitis B Virus
DNA genome
RNA polymerase II
Host enzyme
RNA Provirus
Reverse transcriptase
DNA genome
Viral enzyme
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DNA Tumor Viruses In
Human Cancer
Hepatitis B continued
• Vast public health problem
• 10% of population in underdeveloped countries are chronic carriers
•Long latency
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DNA Tumor Viruses In
Human Cancer
Hepatitis B continued
Epidemiology:
Strong correlation between HBV and
hepatocellular carcinoma
China: 500,000 - 1 million new cases of
hepatocellular carcinoma per year
Taiwan: Relative risk of getting HCC is 217
x risk of non-carriers
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DNA Tumor Viruses In
Human Cancer
Summary
• Can transform cells or have lytic life cycle
• Often integrate into host genome
• In transformation ONLY early genes are transcribed
21
RNA Tumor Viruses
RNA Genome - Retroviruses
RNA-dependent DNA Polymerase encoded by virus
REVERSE TRANSCRIPTASE
RNA genome
Reverse transcriptase
virus
DNA genome
Integrase
virus
Integrates
Host RNA polymerase II
RNA genome
host
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RNA Tumor
Viruses
23
RNA Tumor Viruses
A normal retrovirus has:
3 genes
GAG : internal proteins
ENV: Envelope glycoproteins
POL: Enzymes
Reverse transcriptase
Integrase
Protease
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RNA Tumor Viruses
RNA is:
• Diploid
Capped and polyadenylated
• Positive sense (same as mRNA)
Viral RNA cannot be read as mRNA
New mRNA must be made
Virus must make negative sense DNA before proteins are
made
Therefore virus must carry REVERSE TRANSCRIPTASE
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into the cell
RNA Tumor Viruses
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RNA Tumor Viruses
Groups of Retroviruses
• Oncovirinae
important
Tumor viruses and similar
• Lentiviruses
important
Long latent period
Progressive chronic disease
Visna
HIV
• Spumavirinae
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RNA Tumor Viruses
Retroviruses known to cause human cancer
• Human T cell lymphotropic virus -1 (HTLV-1)
Adult T cell leukemia, Sezary T-cell leukemia
Africa, Caribbean, Some Japanese Islands
• Human T cell lymphotropic virus -2 (HTLV-2)
Hairy cell leukemia
• HIV?
28
RNA Tumor Viruses
Retrovirus Life Cycle
Endocytosis
Fusion of membranes
Release of nucleocapsid to cytoplasm
Nucleus
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RNA Tumor Viruses
Parental RNA
Reverse transcriptase
RNA/DNA Hybrid
Reverse
transcriptase
Linear DNA/DNA duplex
Circular Duplex DNA
Integrase
Host DNA
polymerase
Integration
Replication (DNA genome in cell)
Host RNA pol II
Transcription
Host splicing
enzymes
Viral RNA genome
mRNA
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protein
RNA Tumor Viruses
Drawback to this lifestyle
Genomic RNA
Reverse transcriptase
DNA
Host RNA pol II
Genomic RNA
Pol II is a host enzyme that, in the uninfected cell, makes mRNA
When making mRNA, pol II does not copy entire gene to RNA
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Problem of using RNA pol II to copy a gene
RT
primer
Viral
genomicRNA
Reverse
transcriptase
dsDNA
RNA synthesis
initiation site
promotor
RNA pol II
RNA synthesis termination
site
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Result: New copy of viral RNA is shorter - lacks control sequences
RNA Tumor Viruses
RNA polymerase II will not copy
Upstream sequences from transcription initiation site
• Promotors / Enhancers
Down stream sequences from transcription termination site
• Enhancers / Poly A site / termination site
?
Perhaps virus could integrate downstream of a promotor etc so
that the cell provides sequences
OR
Virus provides its own promotors etc
BUT not copied!
33
RNA Tumor Viruses
Repeat
region
Clue: Difference in the two forms
Repeat
region
RNA
R
U5
GAG
POL
ENV
U3
R
DNA
U3
R
LTR
U5
GAG
POL
ENV U3
R
U5
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R
U5
Viral RNA
U3
R
Reverse
transcriptase
U3
R
U5
U3
R
U5
Long terminal repeats are formed
promotor
POLII
POLII
RNA initiation site
RNA termination site
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Retroviruses can have only one
promotor
Contained in U3
LTR
LTR
POLII
POLII
RNA initiation site
RNA termination site
Therefore only one long RNA can be made
U5
Therefore mRNA requires processing
Explains why RNA has to be positive sense
36
Some retroviruses have an
extra gene
“typical retrovirus”
R
U5
GAG
POL
ENV
U3
R
Rous Sarcoma Virus
R
U5
GAG
POL
ENV
SRC
U3
R
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Some retroviruses have an oncogene
instead of their regular genes
Avian Myeloblastosis
Virus
R
U5
GAG
POL
MYB
U3
R
Feline Sarcoma Virus (FSV)
R
U5
dGAG
FMS
dENV
U3
R
Avian Myelocytoma Virus (MC29)
R
U5
dGAG
MYC
dENV
U3
R
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RNA Tumor Viruses
Viral Oncogene
V-onc
Cellular Proto-oncogene
C-onc
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RNA Tumor Viruses
Proto-oncogene
A cellular (host) gene that is homologous with a
similar gene that is found in a transforming virus
A cellular oncogene can only induce
transformation after
• mutation
• some other change in the cell’s genome
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RNA Tumor Viruses
The discovery of the acutely transforming
retroviruses that contain
v-oncs explains how cancers may arise as a result
of infection
These viruses cause rapid cancer in animals in
the laboratory
41
RNA Tumor Viruses
In contrast:
Chronically transforming retroviruses
cause tumors inefficiently after prolonged period of time
Avian Leukosis Virus (causes lymphomas)
R
U5
GAG
POL
ENV
U3
R
No oncogene! – How does it cause a
tumor?
42
RNA Tumor Viruses
ALV can integrate into the host cell genome at
MANY locations
but in tumor it is always at the SAME site (or
restricted number of sites)
Suggest tumor arose from one cell
• Something must be important about this site for
transformation
• Crucial event must be rare
43
RNA Tumor Viruses
What is special about this site?
Myelocytoma tumors from several birds all have
the oncogene close to this site
It is close to
C-myc!
Oncogenesis by promotor insertion
44
RNA Tumor Viruses
Could C-oncs be involved in NON-VIRAL cancers?
45
RNA Tumor Viruses
myb
mos
myc
Genes can be
assigned to
sites on
specific
chromosomes
mos and myc :
chromosome 8
fes
fes: chromosome 15
46
Cancers often result from gene
translocations
Burkitt’s Lymphoma
8:14 translocation
Break in chromosome
14 at q32
myc
Acute myelocytic leukemia
7:15
9:18
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11:15:17
Oncogenesis by rearrangement
Tumor
c-onc
Burkitt’s lymphoma
myc
(8)
new promotor
Ig heavy (8 to 14)
Ig light (8 to 2)
T cell chronic lymphocytic
myc
T cell receptor (8 to 14)
B-cell chronic lymphocytic
bcl-1
Ig heavy (11 to 14)
leukemia
bcl-2
Ig heavy (18 to 14)
T cell chronic lymphocytic
tcl-1
T cell receptor
leukemia
leukemia
(14 inversion)
48
RNA Tumor Viruses
What do oncogenes encode?
Proteins that are involved in growth control and
differentiation
Growth factors
Growth factor receptors
Signal transduction proteins
Transcription factors
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Oncogenes
Mutations in a proto-oncogene are dominant “gain
of function” mutations
However other oncogenic genes show recessive
mutations
Anti-Oncogenes
• Loss of function mutations
• Retinoblastoma
• p53
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Proto-oncogenes
Heterozygote
Dominant
mutations
Homozygote
Allele 1
Allele 2
Allele 1
Allele 2
Normal
Mutant
Mutant
Mutant
Binds under
special
circumstances
Mutant
always
binds
Always binds
Mutant
always
binds
Mutant
always
binds
Always binds
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Function gained
Function gained
Anti-Oncogenes
Recessive mutations
Mutation
Rb Gene
Mutant Rb
growth
Mutant Rb
Mutant Rb
Rb
Rb protein
Heterozygote
Rb
Binds and controls cell cycle
Homozygote
Function lost
No binding - Growth continues
52
Anti-Oncogenes
Retinoblastoma gene has normal
regulatory function in many cells
Involved in
Retinoblastoma
Lung carcinomas
Breast carcinomas
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Anti-Oncogenes
P53
Inactivated by
• deletion
• point mutation
In a series of colorectal cancers all showed:
• Allele 1: partial or complete deletion
• Allele 2: Point mutation
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DNA Tumor Viruses
Oncogenes
• Adenovirus
E1A region 2
• SV 40
Large T
• Polyoma
Large T
• BK virus
Large T
• Lymphotropic virus
Large T
• Human papilloma Virus-16
E7
All have a sequence in common
Mutations in this region abolish transformation capacity
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Anti-Oncogenes
Retinoblastoma
Adenovirus E1A
Rb Gene
Rb
protein
Rb
105kD
Rb
Rb
Stops replication
Cell cycle continues
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Anti-Oncogenes
p53
P53 gene
P53 gene
Hepatitis C
P53
P53
P53 gene
Papilloma
P53
Papilloma
proteolysis
P53
DNA
Stops replication
replication
replication
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