Transcript Document

TUMOR VIRUSES
AND ONCOGENES
Mohammed El-Khateeb
DSVL-4
April 7th 2015
Types of Cancers
Leukemias (derived from lymphoid cells)
Carcinomas (derived from epithelial or endothelial cells)
Sarcomas (derived from connective tissue cells)
Human Cancer Viruses
• Contributing factor in at least 15% of
human cancers worldwide caused by
viruses
• Major cause of liver & cervical cancer
Characteristics of Cancer Cells
• Undergo unregulated growth
• Become immortal (active growth when they
should be quiescent)
• Have increased nutrient uptake
• In tissue culture become anchorage independent.
• The cell cycle in cancer cells becomes active
Growth signaling pathways activated (oncogenes –
RNA tumor viruses)
Pathways to prevent cell proliferation are disrupted
(tumor suppressors – DNA tumor viruses)
Transformed cells forming foci (clusters) due to
loss of cell-contact inhibition
Cancer
Activated oncogenes transform normal cells into
cancerous cells.
Transformed cells have increased growth, loss of
contact inhibition, tumor specific transplant and T
antigens.
The genetic material of oncogenic viruses becomes
integrated into the host cell's DNA.
Clonality of Tumors
Tumors induced by acutely transforming viruses are
polyclonal in nature, that is, many cells lose growth
control as a result of infection with an oncogenebearing virus
Tumors caused by slow transforming viruses are
monoclonal or possibly oligoclonal in nature, they
occur as a result of an outgrowth of a single rare cell
with virus integrated into a specific site near or within a
proto-oncogene
The clonality of a tumor thus provides clues to the
mechanisms of tumor induction
The hallmarks of cancer comprise
six biological
Capabilities acquired during the multistep development of human
tumors.
•
•
•
•
•
•
Sustaining proliferative signaling
Evading growth suppressors,
Resisting cell death,
Enabling replicative immortality,
Inducing angiogenesis,
Activating invasion and metastasis.
Taxonomy of Tumor Viruses
• DNA viruses:
EBV
KSHV
• RNA viruses:
Papovaviruses
Hepadnaviruses
Herpesviruses
Adenoviruses
Poxviruses
Retroviruses
Flaviviruses
HBV
HPV
SV-40
BK, JC
HTLV-1
Hepatitis C virus
Tumor Viruses
For most viruses:
Replication
Lysis
Progeny virions
Lytic Life Cycle
Genome
all viral proteins
Tumor Viruses
Virus
Latent Life Cycle
Cell
Integration (usually)
Transformation
Some virus-specific proteins expressed - No mature
virus
Viral structural proteins are not expressed
Sometimes latency may terminate – cell must be infected
by complete virus
Changes in the properties of host cell - TRANSFORMATION
Tumor Viruses
Transformation:
♦ Loss of growth control
♦ Reduced adhesion
♦ Motility
♦ Invasion
♦ Ability to form tumors - viral genes interfere
with control of cell replication
♦ Transformed cells frequently exhibit
chromosomal aberrations
Oncogenic Viruses
Oncogenic DNA Viruses
♦ Adenoviridae
♦ Heresviridae
♦ Poxviridae
♦ Papovaviridae
♦ Hepadnaviridae
Oncogenic RNA
viruses
♦ Retroviridae
•
Viral RNA is
transcribed to DNA
which can integrate
into host DNA
• HTLV 1
• HTLV 2
Human Viruses and Associated Malignancies
 HPV 16, 18, 31, 33, 45
Cervical Carcinoma
 Hepatitis B&C viruses
Hepatocellular Carcinoma
 HTLV1
Adult T cell Leukemia
 Epstein-Barr virus (HHV-4) Burkitt’s Lymphoma




Hodgkin’s Disease
PTLD
Nasopharyngeal Carcinoma
Gastric Carcinoma?
 Kaposi sarcoma-associated Kaposi’s Sarcoma herpesvirus (KSHV,
HHV-8)
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
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
Proto-oncogenes  oncogenes:
Proto-oncogenes
•
Proto-oncgenes are genes that possess normal gene products and
stimulate normal cell development.
Oncogenes
•
Oncogenes arise from mutant proto-oncogenes.
•
Oncogenes are more active than normal or active at inappropriate
times and stimulate unregulated cell proliferation.
Some tumor viruses that infect cells possess oncogenes:
•
RNA tumor viruses = possess viral oncogenes (derived form
cellular proto-oncogenes) capable of transforming cells to a
cancerous state.
•
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).
Definitions
Oncogene: “Gain of function”
An altered gene whose product can act in a dominant
fashion to help make a cell cancerous. Usually, an
oncogene is a mutant form of a normal gene (a “protooncogene”) involved in the control of cell growth or
division.
Tumor Suppressor gene: “Loss of function”
A gene whose normal activity prevents formation of a
cancer. Loss of this function by mutation enhance the
likelihood that a cell can become cancerous (a normal
process to maintain control of cell division is lost).
DNA tumor viruses target tumor suppressors
Virus
Gene Product
Cellular target
Adenovirus
E1A
E1B
Rb
p53
SV40
Polyomavirus
Large T antigen
Large T antigen
Middle T antigen
Rb, p53
Rb
Src, PI3K
Papillomavirus
E7
E6
E5
Rb
PDGF receptor
DNA Virus Transforming Activities via Cellular
Homologues
• EBV LMP1 mimics CD40 (tumor necrosis factor receptor)
• E5 gene of bovine papillomavirus is molecular mimic of
growth factor (activates PDGF receptor signaling
cascade)
• Polyomavirus middle T: src signaling pathway
• HHV 8: Encodes viral D cyclin, vIL-6
Human Papillomaviruses
 Virus: 55 nm diameter
 Genome: ds DNA, circular, 8 kbp
 Highly tropic for epithelial cells of the skin and mucous
membrane.
 Viral replication is strictly associated with the
differentiated keratinocytes.
 Papillomaviruses causes warts, including skin warts,
plantar warts, flat warts, genital condylomas and
laryngeal papillomas.
Human Papillomaviruses
 HPVs are accepted as the cause of anogenital cancers
including cervical cancer.
 Cervical cancer is caused most commonly by HPV-16 and
-18 (high risk types) and less commonly by types 31, 33,
35 and 45. Types 6 and 11 are considered low risk types
causing benign tumors.
 Integrated copies of viral DNA are present in cancer cells.
HPV DNA is episomal in non cancerous cells or premalignant lesions.
HPV types
1, 4
2, 4, 26, 27
6, 11
Clinical lesion
Oncogenic
potential
Plantar warts
Benign
Common warts
Benign
Anogenital condylomas
Laryngeal papillomas
Cervical intraepithelial
neoplasia (CIN)
16, 18, 30, 31, Genital carcinoma
33, 35, 45, Laryngeal and
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esophageal
carcinoma
Low
High
Progression of Cervical Carinogenesis
Herpesviruses
 Large viruses (100 – 200 nm diameter), enveloped.
 Linear ds DNA genome (124 – 235 kpb).
 Causes acute infections followed by latency.
 EBV causes acute infectious mononucleosis when it infects B
lymphocytes of susceptible humans. EBV can immortalize such
lymphocytes.
 Pthogenesis of EBV IN:
Immunocompetent Immunocompremised
 Burkitt’s lymphoma
Post-transplant lymphoproliferative diseases (PTLD)
 Nasopharyngeal carcinoma
 Hodgkin’s disease
Hodgkin lymphoma
Non-Hodgkin’s lymphoblastoid malignancies
Herpesviruses
EBV encodes a viral oncoproteins (LMP1) that mimics an
activated growth factor receptor that is essential for
transformation of B lymphocytes. EB nuclear antigens
(EBNAs) are also essential for immortalization of B cells
Most of these tumors show chromosomal translocations
between the c-myc gene and immunoglobulin loci, leading
to the constitutive activation of myc expression
Human herpes virus 8 (HHV-8) is associated with
Kaposi’s sarcoma.
EPSTEIN-BARR VIRUS
EBV has a very limited host range and tissue tropism
defined by the limited cellular expression of its receptor
(CD21).
This receptor is expressed on

B lymphocytes
 Epithelial cells of the oro – and nasopharynx
 Diseases
 Infectious Mononucleosis
 African Burkitt’s Lymphoma
 Nasopharyngeal Carcinoma
 EBV-induced lymphoproliferative disease
African (endemic) Burkitt’s lymphoma
 Poorly differentiated monoclonal B-cell lymphoma
 The tumor cells contain chromosomal translocations that moves the
C-myc oncogene to a very active promoter. (Immunoglobulin gene
promoter)
 African (endemic) Burkitt’s lymphoma
 Poorly differentiated monoclonal B-cell lymphoma
 Affecting the Jaw and face
 Endemic to children of malarial regions of Africa.
 The tumor cells contain chromosomal translocations that moves the C-myc
oncogene to a very active promoter. (Immunoglobulin gene promoter)
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
Acute promyelocytic leukemia
fes
15 to 17
Acute lymphocytic leukemia
myb
6 deletion
Ovarian cancer
myb
6 to 14
* In Burkitt's lymphoma, the c-myc on
chromosome 8 is brought to a site on
chromosome 14 close to the gene for
immunoglobulin heavy chains. It seems
that the proto-oncogene may thus be
brought under the control of the Ig
promotor, which is presumably very active
in B lymphocytes.
THE LATENT CYCLE
EBV in
saliva
Epithelial cells of
oropharynx
B cells
proliferation
Liver
Lymph
node
Spleen
Shedding
in saliva
Atypical
lymphocytes
Heterophile
antibodies
Pharyngitis
T cells
activation
swelling
THE LATENT CYCLE
EB nuclear antigen 1
(EBNA)
EBNA-2
Viral promoter
(ori P)
Monoclonal antibodies
B cell
Heterophile antibodies
immortalization
Antibodies to EBNA persist for life.
Antibodies to viral capsid antigen (VCA)appear during active
disease.
CD8+ T cells are activated against EBNA proteins
Destroy infected B cells
Atypical lymphocytes
T cell immunodeficiencies
B cell lymphoma
•
•
•
EBV Latency Genes
 Non-antigenic:EBNA1 (Epstein Barr Nuclear Antigen 1) - episomal
replication and segregation function
 Antigenic





EBNA2
EBNA3A, 3B, 3C
EBNA-LP
LMP1 (Latent Membrane Protein 1)
LMP2A
 4 different types of latency
 True Latency - no viral gene expression
 EBNA1 only - EBNA1 (non-antigenic)
 Default - EBNA1, LMP1, and LMP2 (moderately antigenic)
 Growth - EBNA1, LMP1, LMP2, EBNA2, EBNA-LP, EBNA3A, 3B, 3C
(highly antigenic)
Kaposi’s Sarcoma
Herpes Virus - HHV-8
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Kaposi’s sarcoma
Hematologic malignancies
• Primary effusion lymphoma
• Multicentric Castleman's disease (MCD) – a rare
lymphoproliferative disorder (AIDS)
• MCD-related immunoblastic/plasmablastic lymphoma
• Various atypical lymphoproliferative disorders
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Hepatitis B virus
Epidemiology:
• Vast public health problem
• 10% of population in underdeveloped countries
are chronic carriers
• Long latency
Strong correlation between HBV andhepatocellular
carcinoma
China: 500,000 - 1 million new cases of hepatocellular
carcinoma per year
Taiwan: Relative risk of getting HCC is 217 x risk of noncarriers
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Hepatitis B Virus
HBV
DNA genome
RNA polymerase II
Host enzyme
RNA Provirus
Reverse transcriptase Viral enzyme
DNA genome
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
RNA TUMOR
VIRUSES
RNA Tumor Viruses
Viral RNA genome
Reverse transcriptase (Virus-encoded)
Viral DNA genome (integrated)
IMPORTANT
DNA-dependent RNA polymerase
(Host RNA pol II)
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
Retrovirus Lifecycle
Simple retrovirus
•LTR-gag-pol-env-LTR
Retroviruses
• RNA tumor viruses “create” oncogenes by acquiring,
modifying, deregulating cellular genes (proto-oncogenes)
• v-onc not essential viral gene & unrelated to strategy of
viral replication
• Replication of RNA viruses is not cytocidal nor is it
required for tumorigenesis
Mechanisms of cell transformation by
retroviruses
1) Retroviral transduction of oncogene (transducing
retrovirus)
2) Oncogene activation by retroviral insertion (cis-acting /
nontransducing retrovirus)
3) Oncogenesis mediated by essential retrovirus proteins
(trans-activating / nontransducing long-latency
retrovirus)
Transducing retroviruses
• Viral acquisition of cellular proto-oncogene with
capacity to transform if deregulated, usually replacing
viral coding sequences (exception is RSV=src
oncogene)
• Overexpression versus structural change in v-onc
mos vs
src
• Becomes replication defective, secondary to the loss of
viral coding information; requires helper virus
v-ONC
Host DNA
c-ONC
cell
Acquired Genes Are Components of
Signaling Networks
• External signal molecules or growth factors (receptor
ligands) (sis)
• Cellular receptors (erbB, fms, kit)
• Second messengers in signaling cascade (kinases: src,
abl, fgr, yes; mos raf)
• Transcription factors (jun, fos, myc, myb, ets, rel)
Outcome of Retroviral Transduction
•“Single hit” carcinogenesis (one event)
•Polyclonal:
tumor growth initiated in every
infected cell
•Tumors form within days
•Characteristic of animal retroviruses
Mechanisms of cell transformation by
retroviruses
1) Retroviral transduction of oncogene (transducing
retrovirus)
2) Oncogene activation by retroviral insertion (cis-acting /
nontransducing retrovirus)
3) Oncogenesis mediated by essential retrovirus proteins
(trans-activating / nontransducing long-latency
retrovirus)
Outcome of Oncogene Activation by
Retrovirus Insertion
• Cell transformation rare event because
insertion near potential oncogenes is
infrequent
• Monoclonal tumors: proviral sequences
integrated at same chromosomal site
• Tumors induced more slowly (months) since
tumor derived from single cell
Human T cell Leukemia Virus type I
(HTLV-I)
• Associated with 2 fatal human diseases
 Adult T cell leukemia (ATL)
 clonal malignancy of infected mature CD4+ T cells
 Tropical spastic paraparesis/HTLV-1 associated myelopathy
 neurodegenerative disease
• Endemic in parts of Japan, South America, Africa, and the Caribbean
 With an estimated 10-20 million people infected worldwide
• Asymptomatic in majority of individuals with approximately 2-5% of
HTLV-I carriers developing disease 20-40yrs post infection.
 The long clinical latency and low percentage of individuals who develop leukemia
suggest that T-cell transformation occurs after a series of cellular alterations and
mutations.
• Infects primarily CD4+ T cells.
HTLV 1 Transmission
• Extended close contact (cell-associated virus)
• Sexual (60% male to female versus 1% female to male
transmission)
• Blood products (screening of blood supply since 1988)
• Mother to child (breast feeding: 20%
seropositive mothers acquire virus)
children with
HTLV-I and ATL
• 1980 Gallo isolated type C retrovirus (HTLV1) from
patient with “cutaneous T cell lymphoma”
• The provirus is present in all cases ATL
• Integration occurs at the same site in all cells derived
from an ATL tumor (monoclonal).
• Integration site varies in different patients
• Integration does not occur at a preferred chromosomal
site (no cis-activation of oncogenes).
HTLV-I genome
• 9 kilobase RNA genome
• HTLV-I does not carry a cellular-derived oncogene
• Unique regulatory proteins Tax and Rev
• Essential for viral replication
• Function in viral gene expression
LTR
LTR
gag
tax
pol
pro
env
rev
Mechanisms of cell transformation by
retroviruses
Virus
category
Tumor
latency
period
Efficiency
of tumor
formation
Oncogenic
effector
Infecting
viral
Genome
Transform
cultured
cells?
Viral-cellular
chimera,
replication
defective
Yes
Transducing
retrovirus
Short (days)
High (can
reach 100%
of animals)
Cell-derived
oncogene carried
in viral genome
Cis-acting/
nontransducing
Intermediate
(wk, mo)
High to
intermediate
Cellular oncogene Intact,
activated in situ by replication
provirus insertion
competent
No
Transactivating/
nontransducing
long latency
Long
(mo, yr)
Very low
(<5%)
Virus-coded
Transcriptional
regulatory protein
No
Intact,
replication
competent
Retroviruses
 Epidemiology
 Typical infectious viruses (exogenous)
• Sexual transmission
• IV drug abusers
• Other, unknown transmission mechanisms
 Germline (endogenous)
• High degree of similarity to retrotransposons
• Some are required for normal functions
• Recombination activation genes 1 & 2 that rearrange antibody and T
cell receptor genes
 Classification
 Leukemia viruses
• Alpharetrovirus
• Gammaretrovirus
 Nontransforming retroviruses
• Deltaretrovirus
• Lentivirus