TUMOR%20VIRUSES%20AND%20ONCOGENES

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TUMOR VIRUSESAND ONCOGENES

• Mohammed El-Khateeb
• DSVL-4
• April 7th 2015

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Types of Cancers

• Leukemias (derived from lymphoid cells)
• Carcinomas (derived from epithelial or
  endothelial cells)
• Sarcomas (derived from connective tissue cells)

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Human Cancer Viruses

• Contributing factor in at least 15 of human
  cancers worldwide caused by viruses
• Major cause of liver cervical cancer

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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)

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Transformed cells forming foci (clusters) due to
loss of cell-contact inhibition
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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.

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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 oncogene-bearing 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

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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.

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Taxonomy of Tumor Viruses

• DNA viruses Papovaviruses Hepadnaviruses
  Herpesviruses
  Adenoviruses Poxviruses
• RNA viruses Retroviruses Flaviviruses

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Tumor Viruses
For most viruses
Replication Lysis
Progeny virions
Lytic Life Cycle
Genome all viral proteins
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Tumor Viruses
Virus Cell Integration (usually) Transformatio
n
Latent Life Cycle
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
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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

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Oncogenic Viruses

• Oncogenic RNA viruses
• Retroviridae
• Viral RNA is transcribed to DNA which can
  integrate into host DNA
• HTLV 1
• HTLV 2

• Oncogenic DNA Viruses
• Adenoviridae
• Heresviridae
• Poxviridae
• Papovaviridae
• Hepadnaviridae

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Human Viruses and Associated Malignancies

• HPV 16, 18, 31, 33, 45 Cervical Carcinoma
• Hepatitis BC viruses Hepatocellular Carcinoma
• HTLV1 Adult T cell Leukemia
• Epstein-Barr virus (HHV-4) Burkitts Lymphoma
• Hodgkins Disease
• PTLD
• Nasopharyngeal Carcinoma
• Gastric Carcinoma?
• Kaposi sarcoma-associated Kaposis Sarcoma
  herpesvirus (KSHV, HHV-8)

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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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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).

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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 proto-oncogene) 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).

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DNA tumor viruses target tumor suppressors
Virus Gene Product Cellular target
Adenovirus SV40 Polyomavirus Papillomavirus
E1A E1B
Rb p53
Large T antigen Large T antigen Middle T antigen
Rb, p53 Rb Src, PI3K
E7 E6 E5
Rb
PDGF receptor
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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

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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.

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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 pre-malignant lesions.

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Oncogenic potential Clinical lesion HPV types
Benign Plantar warts 1, 4
Benign Common warts 2, 4, 26, 27
Low Anogenital condylomas Laryngeal papillomas Cervical intraepithelial neoplasia (CIN) 6, 11
High Genital carcinoma Laryngeal and esophageal carcinoma 16, 18, 30, 31, 33, 35, 45, 51
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Progression of Cervical Carinogenesis
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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
• Burkitts lymphoma Post-transplant
  lymphoproliferative diseases (PTLD)
• Nasopharyngeal carcinoma
• Hodgkins disease Hodgkin lymphoma
• Non-Hodgkins lymphoblastoid malignancies

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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
  Kaposis sarcoma.

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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 Burkitts Lymphoma
• Nasopharyngeal Carcinoma
• EBV-induced lymphoproliferative disease

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African (endemic) Burkitts 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) Burkitts 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)

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translocation C-onc  Disease
8 to 14 myc Burkitt's lymphoma
8 to 21 mos Acute myeloblastic leukemia
9 to 22 abl Chronic myelogenous leukemia
15 to 17 fes Acute promyelocytic leukemia
6 deletion myb Acute lymphocytic leukemia
6 to 14 myb Ovarian cancer
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.
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THE LATENT CYCLE
EBV in saliva
B cells proliferation
T cells activation
Liver Lymph node Spleen
Epithelial cells of oropharynx
Shedding in saliva
Atypical lymphocytes
Heterophile antibodies
swelling
Pharyngitis
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THE LATENT CYCLE
EB nuclear antigen 1 (EBNA)
EBNA-2
Viral promoter (ori P)
B cell immortalization
Monoclonal antibodies Heterophile antibodies

• 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
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EBV Latency Genes

• Non-antigenicEBNA1 (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)

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Kaposis Sarcoma Herpes Virus - HHV-8

• 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 non-carriers

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Hepatitis B Virus
Host enzyme
Viral enzyme
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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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RNA TUMOR VIRUSES
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IMPORTANT
Important Use HOSTRNA polymerase to make its
genome
An enzyme that normallymakes mRNA
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Retrovirus Lifecycle
Simple retrovirus

• LTR-gag-pol-env-LTR

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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

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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)

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Transducing retroviruses

• Viral acquisition of cellular proto-oncogene with
  capacity to transform if deregulated, usually
  replacing viral coding sequences (exception is
  RSVsrc 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

Host DNA
cell
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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)

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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

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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)

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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

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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.

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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 children
  with seropositive mothers acquire virus)

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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).

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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
pol
tax
env
pro
rev
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Mechanisms of cell transformation by retroviruses
Virus category Tumor latency period Efficiency of tumor formation Oncogenic effector Infecting viral Genome Transform cultured cells?
Transducing retrovirus Short (days) High (can reach 100 of animals) Cell-derived oncogene carried in viral genome Viral-cellular chimera, replication defective Yes
Cis-acting/ nontransducing Intermediate (wk, mo) High to intermediate Cellular oncogene activated in situ by provirus insertion Intact, replication competent No
Trans-activating/ nontransducing long latency Long (mo, yr) Very low (lt5) Virus-coded Transcriptional regulatory protein Intact, replication competent No
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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