CELL TRANSFORMATION
ONCOGENIC VIRUSES
ONCOGENIC VIRUSES OR
TUMOR VIRUSES
– ability to induce cell
transformation and to form tumors
Tumor Viruses
For most viruses:
Genome viral proteins
Replication Lysis Progeny virions
Lytic Life Cycle
Tumor Viruses
Latent Life Cycle
Virus
Cell
Integration (usually)
Transformation
Virus-specific proteins expressed - No mature virus
Changes in the properties of host cell - TRANSFORMATION
Both DNA and RNA tumor viruses can transform cells
Integration the viral genome into the host cell DNA 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
Major human Oncogenic Viruses
DNA Viruses
Small DNA tumor viruses
- Adenovirus
- SV40
- Human Papilloma virus (HPV)
Herpesviruses (large)
- Epstein Barr virus (EBV)
- Kaposi’s Sarcoma Herpesvirus (KSHV)
Other
- Hepatitis virus B
RNA viruses
Retroviruses - Human T-cell Leukemia Virus 1 and 2 (HTLV1, HTLV2)
Hepatitis virus C
Tumor Viruses
Transformation:
Loss of growth control
Ability to form tumors - viral genes interfere with
control of cell replication
Control of cell replication
Two categories of cell regulatory genes
Proto-oncogenes (cellular oncogene, c-onc)
Tumor suppressor genes
Proto-oncogenes code for
Growth factors
Receptors
Signal-relay or transduction factors
Tumor suppressor genes code for factors that down-regulate
the cell cycle
P53
Rb
TRANSFORMATION
Properties of the TRANSFORMED CELL are:
• Loss of growth control (loss of contact inhibition in
cultured cells)
• Cell morphology changes
• Tumor formation
• Transformed cells frequently exhibit chromosomal
aberrations
• Mobility
• Reduced adhesion
Changes When a Cell Becomes Cancer Cell
Multiple Genetic Changes
Changes lead the conversion of normal cells transformed cells:
Multiple genetic changes: 6-7 events over 20-40 years
Factors (carcinogens) that increase the conversion:
Initiate/Promote suggest stages in cancer development
Genes that cause transformation:
Oncogenes (100+)
Viral oncogenes and cellular counterparts (proto-oncogenes)
Gain-of-function or Activated
Tumor suppressor genes (~10)
Loss-of-function or Inactivated
Tumor Viruses
Mechanisms of oncogenesis:
- Presence of viral oncogene (v-onc)
- Activation of cellular proto-oncogenes or
inactivation of tumor suppressor genes
V-onc
- early viral genes
- altered form of c-onc
Activation of proto-oncogenes
- mutation
- amplification
- translocation
- insertion of viral
genome
Inactivation of tumor suppressor genes
- mutation
- viral oncoproteins
- viral degradation
Anti-Oncogenes
Retinoblastoma
Rb Gene Adenovirus E1A
Rb Rb
protein 105kD
Rb
Rb
Stops replication Cell cycle continues
Anti-Oncogenes
p53
P53 gene P53 gene P53 gene
Hepatitis C Papilloma
P53 P53 P53
Papilloma
proteolysis
P53 DNA
Stops replication replication replication
DNA Tumor Viruses
DNA genome
Integration
TRANSFORMATION
-Typically nonproductive infections
-Transformation of nonpermissive cells
- usually only EARLY functions are expressed
DNA Tumor Viruses In Human Cancer
HUMAN PAPILLOMAVIRUSES
urogenital cancer
benign warts malignant squamous cell carcinoma
Papilloma viruses are found in 99.7% of women with cervical cancer
Squamous cell carcinoma:
Larynx
Esophagus All histologically similar
Lung
10% of human cancers may be HPV-linked
HUMAN PAPILLOMAVIRUSES
• 118 types identified - most common are benign types 6 and 11
• most cervical, vulvar and penile cancers are ASSOCIATED with
types 16 and 18
EPIDEMIOLOGIAL STUDIES BUT:
HPV 16 and HPV 18 do transform human keratinocytes
The important transforming genes in papilloma
viruses are the non-structural regulatory genes,
E6 and E7 – inactivation of tumor supressor
genes (Rb, p53)
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 = Viral Oncogenes
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”
(Virus cause mutation in cell genes and then virus is no
longer needed)
HERPES VIRUSES
Epstein-Barr Virus
• Burkitt’s Lymphoma
• Nasopharyngeal cancer
• Lymphomas
• Transforms human B-lymphocytes in vitro
Human herpes virus 8 (HHV8)
• Kaposi’s Sarcoma Herpesvirus (KSHV)
Gene translocation
Burkitt’s Lymphoma
8:14 translocation
Break in chromosome
14 at q32
myc
DNA Tumor Viruses In Human Cancer
HEPATITIS B VIRUS
Strong correlation between HBV and
hepatocellular carcinoma
• Mechanism of carcinogenesis still unknown
• X gene – oncogene ?
• activation of proto-oncogenes ?
RNA Tumor
Viruses
• Retroviruses
• Hepatitis C virus
Transducing retroviruses
“typical retrovirus”
R U5 GAG POL ENV U3 R
“transducing retrovirus” Have v-onc instead of env
R U5 GAG POL V-onc U3 R
Rous Sarcoma Virus
R U5 GAG POL ENV SRC U3 R
V-onc
Have an extra gene (v-onc)
Cis-acting retroviruses
Nondefective viruses
Near c-onc and LTR
activation
Insertional inactivation
of tumor suppressor
genes
Chronic-transforming
Trans-acting retroviruses
R U5 GAG POL ENV TAX U3 R
C-onc
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
RNA Tumor Viruses In Human Cancer
HEPATITIS C VIRUS
Strong correlation between HCV and
hepatocellular carcinoma
• Mechanism of carcinogenesis still unknown
• inactivation of p53 ?
