Tumor Viruses

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Tumor Viruses
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Life Cycles
Lytic Life Cycle Virus Infection Cell
Replication Lysis Progeny
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Life Cycles
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Tumor Viruses
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Tumor Viruses
Transformation Loss of growth control Ability
to form tumors - viral genes interfere with
control of cell replication DNA may
be Integrated - replicates with DNA -
provirus Independent - plasmid
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Tumor Viruses

• Lecture Outline We shall see that
• Two basic classes of tumor viruses
• DNA tumor viruses
• RNA tumor viruses (RETROVIRUSES)
• These form tumors
• Virus genome may integrate
• Virus genes are expressed
• Viruses often have oncogenes that transform the
  cell

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Tumor Viruses

• Lecture Outline 2
• Oncogenes are not unique to tumor viruses
• Cells have homologous genes
• Proto-oncogenes
• Transforming virus may carry its own oncogene or
  activate a cellular proto-oncogene
• Factors other than viruses can activate a
  cellular oncogene so that cell is transformed
• Cellular oncogenes have growth control and
  differentiation functions
• Cells also have anti-oncogenes that suppress
  transformation

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(No Transcript)
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Important Use HOSTRNA polymerase to make its
genome
An enzyme that normallymakes mRNA
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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 cells 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
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• 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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DNA Tumor Viruses
DNA genome mRNA protein virus
Host RNA polymerase II
Host enzymes
OR TRANSFORMATIONIn 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 are malignancies of
  epithelial cells

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DNA Tumor Viruses In Human Cancer
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DNA Tumor Viruses In Human Cancer
Epidermodysplasia verruciformis Papilloma virus
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DNA Tumor Viruses In Human Cancer
Squamous cell carcinoma Larynx Esophagus
All histologically similar Lung
10 of human cancers may be HPV-linked
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DNA Tumor Viruses In Human Cancer
EPIDEMIOLOGIAL STUDIES BUT HPV 16 and HPV 18 do
transform human keratinocytes
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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
• No polyoma virus has been shown to cause
  transformation in its natural host species
• PROGRESSIVE MULTIFOCAL LEUKOENCEPHALOPATHY
• Polyoma virus transforms cells when the genome is
  incomplete
• Viral DNA is linearized and integrated at many
  sites

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DNA Tumor Viruses In Human Cancer
ASSOCIATION OF JC WITH PROGRESSIVE MULTIFOCAL
LEUKOENCEPHALOPATHY

• 70-80 of the population JC seropositive
• Causes progressive multifocal leukoencephalopathy
  
• Disease associated with immunosuppression.

• 1979 the rate of occurrence - 1.5 per 10
  million population.
• Much more common because of AIDS
• Seen in 5 of AIDS patients

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DNA Tumor Viruses In Human Cancer

• Deletions in early functions result in
• No viral replication
• No transformation
• Early functions needed for both lytic infection
  and transformation

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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
SV 40 Oncogenes Large T antigen Small T
antigenLarge T is found in nucleus and at the
cell surface
Polyoma Oncogenes Large T Middle T Small T
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DNA Tumor Viruses In Human Cancer
In Polyoma Viruses
T antigens Oncogenes T antigens are true viral
genes They are necessary for Lytic
Infection Transformation
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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

• 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
• Burkitts Lymphoma
• Nasopharyngeal cancer
• Infectious mononucleosis
• Transforms human B-lymphocytes in vitro

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DNA Tumor Viruses In Human Cancer
Herpes Viruses continued

• Human cytomegalovirus

• Human herpes virus 8 (Kaposi sarcoma-associated
  virus)

• Herpes simplex 2 (cervical carcinoma?)

Herpes virus DNA is found in only a small number
of herpes-transformed cells Complex mode of
transformation
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DNA Tumor Viruses In Human Cancer
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DNA Tumor Viruses In Human CancerHepatitis 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 CancerHepatitis 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 51 of deaths of HB S
antigen carriers are from HCC or liver disease 2
of non- HB S positives
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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

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DNA Tumor Viruses In Human Cancer
Summary
T ANTIGENS
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RNA Tumor Viruses
RNA Genome - Retroviruses RNA-dependent DNA
Polymerase encoded by virus REVERSE
TRANSCRIPTASE RNA genome Reverse
transcriptase DNA genome
Integrase Integrates Host RNA
polymerase II RNA genome
host
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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?

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RNA Tumor Viruses
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RNA Tumor Viruses
A normal retrovirus has 3 genes
GAG internal proteins
ENV Envelope glycoproteins
POL Enzymes Reverse transcriptase Integrase Prot
ease
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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 into the cell
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RNA Tumor Viruses
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RNA Tumor Viruses

• Groups of Retroviruses
• Oncovirinae
• Tumor viruses and similar
• Lentiviruses
• Long latent period
• Progressive chronic disease
• Visna HIV
• Spumavirinae

important
important
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RNA Tumor Viruses
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RNA Tumor Viruses
Parental RNA RNA/DNA Hybrid Linear
DNA/DNA duplex Circular Duplex
DNA Integration Replication (DNA
genome in cell) Transcription Viral RNA genome
mRNA protein
Reverse transcriptase
Reverse transcriptase
Integrase
Host DNA polymerase
Host splicing enzymes
Host RNA pol II

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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
Viral genomicRNA
RT
primer
Reverse transcriptase
dsDNA
Result New copy of viral RNA is shorter - lacks
control sequences
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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!
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RNA Tumor Viruses
Clue Difference in the two forms RNA
R U5 GAG POL ENV
U3 R
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R U5
Viral RNA
U3 R
Reverse transcriptase
U3 R U5
U3 R U5
Long terminal repeats are formed
POLII
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Long Terminal Repeat Formation
movie
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Retroviruses can have only one promotor
LTR
LTR
POLII
Therefore only one long RNA can be made Therefore
mRNA requires processing Explains why RNA has to
be positive sense
U5
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Some retroviruses have an extra gene
typical retrovirus
R U5 GAG POL ENV
U3 R
SRC
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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 cells genome

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RNA Tumor Viruses

• Characteristics of Cellular Proto-Oncogenes
• 1) Normal cellular genes
• Typical Mendelian inheritance
• Contain introns (c.f. v-onc)
• No LTR-like features
• Always at same place in genome
• 2) Expressed at some point in many tissues
• Suggests basic function
• Growth control - Differentiation - Morphogenesis
• Highly conserved in evolution

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RNA Tumor Viruses

• 3) v-onc is most like the c-onc of animal species
  that is host to the virus

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RNA Tumor Viruses
Acute transforming retroviruses (e.g. RSV) have
acquired cellular genes that have an important
function in the host cell The v-onc can cause
malignancy in the host cell when part of the
virus Why?

• V-oncogenes have
• Amino acid substitutions
• Deletions
• Fusion protein characteristics
• Enhancer/Promotor LTRs

There may be many v-oncs in a infected cell but
only one c-onc v-oncs are integrated randomly
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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 but there is no evidence that in nature
such viruses cause cancer in humans These viruses
cause rapid cancer in animals in the laboratory
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RNA Tumor Viruses
In contrast Chronically transforming
retroviruses cause tumors inefficiently after
prolonged period of time
No oncogene! How does it cause a tumor?
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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)

• Suggest tumor arose from one cell
• Something must be important about this site for
  transformation
• Crucial event must be rare

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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!
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RNA Tumor Viruses
POL II
POL II
Host DNA U3 R U5 Gag Pol
Env U3 R U5 Host DNA
MYC
Initiation of c-myc transcription from a viral
promotor inserted UPSTREAM c-myc mRNA often
contains a piece of viral RNA suggesting second
LTR is lost ONCOGENESIS BY PROMOTOR INSERTION
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The virus does not have to be inserted upstream
POL II
Host DNA U3 R U5
Gag Pol Env U3 R U5 Host
DNA
MYC
Promotor Host cell
Enhancers may be upstream OR downstream The
viral enhancer may stimulate transcription of the
oncogene from the promotor of the
oncogene ONCOGENESIS BY ENHANCER INSERTION
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RNA Tumor Viruses

• Could C-oncs be involved in NON-VIRAL cancers?
• C-oncs are a class of genes that could be
  involved if
• over expressed
• mutated
• There is an abundance of evidence that c-oncs can
  cause cancers in absence of virus

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RNA Tumor Viruses
Genes can be assigned to sites on specific
chromosomes
mosmycabl
mos myc and abl chromosome 8
fes
fes chromosome 15
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Cancers often result from gene translocations
Translocations near a c-onc may give rise to
neoplasia Disease c-onc translocation Burkitt
s lymphoma myc 8 to 14 Acute myeloblastic
leukemia mos 8 to 21 Chronic myelogenous
leukemia abl 9 to 22
(Philadelphia Chromosome)) Acute promyelocytic
leukemia fes 15 to 17 Acute lymphocytic
leukemia myb 6 deletion Ovarian
cancer myb 6 to 14 break at same site
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RNA Tumor Viruses
Burkitts Lymphoma 814 translocation Break in
chromosome 14 at q32
myc
Acute myelocytic leukemia715918111517
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RNA Tumor Viruses
Oncogenesis by rearrangement Tumor c-onc
new promotor Burkitts lymphoma myc Ig heavy
(8 to 14) Ig light (8 to 2) T cell chronic
lymphocytic myc T cell receptor (8 to
14) leukemia 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 (14 inversion)
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RNA Tumor Viruses

• SO Some cancers may result from a c-onc being
  under the wrong promotor
• The promotor may be
• viral
• cellular
• One basis of viral oncogenesis is mutation of the
  viral oncogene
• Can cellular oncogenes be mutated to cause cancer?

Yes! Bladder carcinoma single point mutation in
a cellular oncogene (HaRas)
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RNA Tumor Viruses
What do oncogenes encode? Proteins that are
involved in growth control and differentiation
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Altered growth factor e.g. sis (PDGF)
Altered receptor e.g. erbB (EGF receptor), fms
Cytoplasm
sos
grb2
Altered GTP-binding protein e.g. h-ras/ k-ras
mek
raf-1
P
myc
Map kinase
Altered transcription factors
P
fos/jun
nucleus
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Oncogenes
Mutations in a proto-oncogene are dominantgain
of function mutations However other oncogenic
genes show recessive mutations

• Anti-Oncogenes
• Loss of function mutations
• Retinoblastoma
• p53

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Proto-oncogenes
Dominant mutations
Heterozygote
Homozygote
Allele 1 Allele 2 Allele 1
Allele 2 Normal Mutant
Mutant Mutant
Binds under special circumstances
Always binds
Always binds
Always binds
Function gained
Function gained
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Anti-Oncogenes
Recessive mutations
Rb Gene
Mutant Rb
Mutant Rb
Mutant Rb
Rb
Rb protein
Heterozygote
Homozygote
Function lost
Rb
Binds and controls cell cycle
No binding - Growth continues
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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 VirusesOncogenes

• 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
Rb
Rb
Cell cycle continues
Stops replication
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Anti-Oncogenes
p53
P53 gene
P53 gene
P53 gene
Hepatitis C
P53
Papilloma protease
P53
P53
P53
DNA
Stops replication
replication
replication