Viral oncogenesis can be caused by retroviruses, DNA viruses, or RNA viruses. Retroviruses may contain cellular oncogenes called v-onc genes. Some retroviruses induce cancer by inserting near cellular proto-oncogenes (c-onc genes), deregulating their expression. c-onc genes encode proteins involved in growth control and differentiation. Their mutation or overexpression can lead to cancer. Both DNA and RNA tumor viruses can integrate into host genomes and induce transformation by expressing early genes. Human viruses linked to cancer include EBV, HBV, HCV, HPV, and HIV. HIV causes AIDS by depleting CD4+ T cells, leaving the immune system vulnerable to opportunistic infections.

1. Viral Oncogenesis
Retroviruses
AIDS

2. Oncogenic viruses
DNA viruses RNA viruses
 Herpesvirus  Retrovirus
 Adenovirus  Flavivirus
 Hepadnavirus
 Papillomavirus
 Poliomavirus
 Poxvirus

3. Feature of malignant
transformation (1)
Feature Description
Altered Loss of differentiated shape
morphology Rounded as a result of disaggregation of actin
filaments and decreased adhesion to surface
Altered Loss of contact inhibition of growth
growth Loss of contact inhibition of movement
control Reduced requirement for serum growth factors
Increased ability to grow in suspension
Increased ability to be cloned from a single cell
Increased rapidity of multiplication
Increased ability to continue growing
(“immortalization”)

4. Feature of malignant
transformation (2)
Feature Description
Altered Induction of DNA synthesis
cellular Chromosomal changes
properties
Appearance of new antigens
Altered Reduced level of cyclic AMP
biochemical Increased anaerobic glycolysis
properties
Loss of fibronectin
Changes in glycoproteins and glycolipids

5. The 2 major concepts of the way viral
tumorigenesis (1)
 The provirus modes. The genes enter
the cell at the time of infection by the
tumor virus
 The oncogene model.

6. Only 2 viruses are considered to be
human tumor viruses:
 Human T cell leukemia virus. There are 2 types:
HTLV-I and HTLV-II
 Human papillomavirus. Especially serotypes 16,
18, 33, 35, 11
Most cervical, vulvar and penile cancers are ASSOCIATED
with types 16 and 18 (70% of penile cancers)
Several other candidate viruses are implicated by
epidemiologic correlation, by serologic relationship,
or by recovery of virus from tumor cells

7. 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

8. Retroviral oncogenes
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

9. Not all tumor viruses of the retrovirus
family contain onc genes.
How do these viruses cause malignant
transformation?
The DNA copy of the viral RNA integrates near a
cellular oncogene, causing a marked increase in its
expression.
Overexpression of the cellular oncogene may play
a key role in malignant transformation by these
viruses.

10. The 2 major concepts of the way viral
tumorigenesis (2)
 The provirus modes.
 The oncogene model. The genes for malignancy
are already present in all cells of the body. These
oncogenes encode proteins that encourage cell growth, eg,
fibroblast growth factor. In this model, carcinogenes such
as chemicals, radiation, and tumor viruses activate cellular
oncogenes to overproduce these growth factors. This
initiates inappropriate cell growth and malingant
transformation.

11. What do oncogenes encode?
Proteins that are involved
in growth control and
differentiation:
Growth factors
Growth factor receptors
Signal transduction proteins
Transcription factors

12. Oncogenes
Viral Oncogene is named V-onc
Cellular Proto-oncogene is named
C-onc

13. 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

14. Cellular oncogenes
Genes can be
assigned to sites
on specific
chromosomes
myb mos
myc
mos and myc :
chromosome 8
fes: chromosome 15
fe
s

15. Evidence that cellular oncogenes
(c-onc) can cause tumors (1)
Evidence Description
Mutation of DNA isolated from tumor cells can transform
c-onc gene nirmal cells. This DNA has a c-onc gene
with a mutation consisting of a single base
change
Translocati Movement of c-onc gene to a new site on a
on of c-onc different chromosome results in malignancy
gene accompanied by increased expression on
the gene

16. Cancers often result from gene
translocations
Burkitt’s Lymphoma
8:14 translocation
myc

17. Evidence that cellular oncogenes
(c-onc) can cause tumors
Evidence Description
Amplification of The numbers of copies of c-onc genes is
c-onc gene increased, resulting in enhanced expression of
their mRNA and proteins
Insertion of Proviral DNA inserts near c-onc gene, which
retrovirus near alters its expression and causes tumors
c-onc gene
Overexpression Addition of an active promoter site enhances
of c-onc gene expression of the c-onc gene, and malignant
by modification transformation occurs
in the laboratory

18. Both DNA and RNA tumor viruses can
transform cells
Integration of viral genome into the host
chromosomes occurs (usually)
Similar mechanisms of transformation by
each type of tumor virus

20. DNA Tumor Viruses
DNA genome
Host RNA
polymerase II
mRNA
Host enzymes
protein
virus
OR TRANSFORMATION
In transformation usually only EARLY genes are
expressed

21. DNA Tumor Viruses In Human
Cancer
• Can transform cells or have lytic life cycle
• Often integrate into host genome
• In transformation ONLY early genes are
transcribed
• These are genes that are also necessary for
a PRODUCTIVE infection

22. Human viruses that can cause
tumor growth in human
Epstein-Barr virus (Herpesviridae). Burkitt’s
lymphoma, nasopharyngeal carcinoma
Herpes simplex virus type 2 (Herpesviridae).
Carcinoma of the cervix
Hepatitis B virus (Hepadnaviridae). Hepatoma –
hepatocellular carcinoma
Hepatitis C virus (Flaviviridae). Hepatocellular
carcinoma

23. retroviruses
HIV and AIDS

24. Family Retroviridae
(have approximately 150 species)
Subfamily:
Oncovirinae
Lentivirinae
Spumavirinae

25. Retroviruses
Groups of Retroviruses
• Oncovirinae important
Tumor viruses
• Lentiviruses important
Long latent period
Progressive chronic disease
Visna HIV

26. Retroviruses
Enveloped viruses round shape
Size – 80-130 nm
Nucleocalsid with cubical type
of symmetry
Structural enzyme - REVERSE
TRANSCRIPTASE
Structural genes – pol, env,
gag
Possibility to integrate into the
host chromosome

27. Retroviral genome
RNA
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 structural
REVERSE TRANSCRIPTASE into the cell

28. Retroviral structural genes
Gag: encodes internal proteins
Env: encodes envelope glycoproteins
Pol: encodes enzymes
Reverse transcriptase
Integrase
Protease

29. Structure of retroviruses

30. Retrovirus life cycle
Bind to
surface receptor
Fusion of membranes
Release of nucleocapsid to cytoplasm
Nucleus

31. Parental RNA
Reverse transcriptase
RNA/DNA Hybrid
Reverse transcriptase
Linear DNA/DNA duplex
Circular Duplex DNA
Integrase
Integration Host DNA polymerase
Replication (DNA genome in cell)
Host RNA pol II
Transcription Viral RNA genome mRNA protein

33. Some retroviruses have an oncogene
instead of their regular genes

34. HIV and AIDS
Acquired Immunodeficiency Syndrome
Disease caused by an infectious agent:
a retrovirus – Human Immunodeficiency Virus

35. History of HIV discovering
 1980 – isolation of Human T cell lymphotropic
virus (HTLV-І) – causative agent of T-cell
leukemia virus
 1982 – isolation of HTLV-ІІ – agent of hairy cell
leukemia
 1983 – isolation of HIV-1
 1985 – isolation of HIV-2
Robert Gallo
Luc Montagnier

36. Classification HIV
Family Retroviridae
Subfamily Lentivirinae
Genus Lentivirus
Type HIV-1 HIV-2
Gropes M, N, O A, B
Subtypes At least 10 (A, B, No
(genotypes) C, D, E, F, and
others)

37. AIDS Statistics
• Approximately 44,000,000 people in the world are HIV-infected
• Approximately 14,000 new HIV infections occur daily around
the world
• Over 90% of these are in developing countries
• 1000 are in children less than 15 years of age.
• Of adult infections, 48% are in women and 15% in individuals
15-25 years
• As of December 2003, 929,985 Americans reported with AIDS.
• At least 501,669 of them have died (2002 figures)
• 5,315 children under 15

38. Characteristic HIV-infection and
AIDS
IMMUNOSUPPRESSION
OPPORTUNISTIC INFECTIONS
Also
Lymphadenopathy
Hodgkin’s Lymphoma

39. HIV-associated opportunistic
infection, that usually are signs of AIDS
Agent Disease
Pneumocystis carinii Pneumonia
Mycobacterium Tuberculosis
tuberculosis
Mycobacterium avium Lung infection
Herpesviruses 6. 7 Kaposi’s sarcoma, Hodgkin’s
lymphoma
Cytomegalovirus CMV-infection
Cabndida albicans Affection of skin and oral mucosa
Cryptococcus Skin affection
neoformans
Cryptosporidia species Acute diarrhea
Toxoplasma gondii Neurological pathology

40. HIV and AIDS
The Cellular Picture
Loss of one cell type throughout the course of the disease
CD4+ T4 helper cells
A fall in the CD4+ cells always precedes disease
The virus only grows on T4 cells that are proliferating in
response to an immune stimulus
In advanced disease: the loss of another cell type
CD8+ cytotoxic killer cells

41. The Genome of HIV

42. HIV
Membrane: host derived
Three genes
GAG – POL – ENV
Three polyproteins

43. HIV
ENV gene
Two glycoproteins: gp160 gp120 and gp41
• gp120 – adherence to cellular CD4 receptors
• gp41 – fusion of viral envelope with cell cytoplasmic
membrane

44. HIV
GAG gene
Polyprotein
Group-Specific Antigens
p17: inner surface - myristoylated
p24: nucleocapsid
p9: nucleocapsid associated with RNA

45. HIV
• POL gene
Enzymes
• Polymerase (reverse transcriptase –
RNA dependent DNA polymerase)
• Integrase
• Protease (cuts polyproteins)

46. Structure of HIV
gp 120
gp 41
Matrix proteins
Envelope
Nucleocapsid with
RNA and enzymes

47. Reproduction of HIV
Budding virion Adherence
to CD4
Penetration and
Cell
synthesis DNA
membrane
modified by
on the RNA
viral proteins template
New viral Cell nucleus
RNA
New viral Viral progeny RNA Viral DNA enter into the
proteins leave the nucleus nucleus and integrate
with chromosome

48. Budding virions

49. T-cells infected by HIV (releasing
viruses)

50. HIV - Life History
Latency
Specific destruction of
CD4+ cells

51. HIV - Life History
•Syncytia formation
Profound significance for
AIDS progression and
therapy:
spread from cell to cell and
as result escaping of
antibodies.
Humoral antibody will not
stop spread – need cell-
mediated response

52. HIV - Life History
Latency – Cellular – The problem of memory T4 cells
Only activated T4 cells can replicate virus
Most infected T4 cells are rapidly lyzed but are replaced
Some T4 cells revert to resting state as memory cells which are long-lived
Memory T4 cells cannot replicate the virus unless they become activated
Clinical Latency
HIV infection is not manifested as disease for years
During apparent clinical latency, virus is being replicated and cleared

53. Transmission of HIV
 By sexual contact (57%)
 By transfer of infected blood (13%)
 By injection (13%)
 Vertical transmission - from infected
mother to neonate, either at birth or via
breast milk (17%)

54. HIV and AIDS
The cellular and immunological picture - The course of the disease

55. HIV and AIDS
The cellular and immunological picture - The course of the disease

56. HIV and AIDS
The cellular and immunological picture
The course of the disease
1. Acute Infection
High virus titer
Mild symptoms
Fall in CD4+ cells but recovers
Rise in CD8+ cells but recovers
A high virus titer (up to 10 million viruses per ml
blood)
Macrophages infected

57. HIV and AIDS
2. A strong immune response
Virus almost disappears from circulation
Good cytoxic T cell response
Soluble antibodies appear later against both
surface and internal proteins
Most virus at this stage comes from recently
activated (dividing) and infected CD4+ cells
CD4+ cell production compensates for loss due
to lysis of cells by virus production and
destruction of infected cells by CTLs

58. HIV and AIDS
3. A latent state
Latency of virus and of symptoms
Virus persists in extra-vascular
tissues
Lymph node dendritic cells
Resting CD4+ memory cells (last a
very long time - a very stable
population of cells) carry provirus

59. HIV and AIDS
• 10 billion HIV particles per day
• Virus half life 5.7 hours
• 100-10 million virions per ml blood (set point)
• Small minority of T4 cells are infected
• Virus found in lymph nodes

60. HIV and AIDS
4. The beginning of disease
Massive loss of CD4+ cells
CD4+ cells are the targets of the virus
Cells that proliferate to respond to the
virus are killed by it
Dendritic cells present antigen and virus
to CD4 cells
Epitope variation allows more and more HIV to
escape from immune response just as response wanes
Apoptosis of CD4+ cells
HIV patients with high T4 cell counts
do not develop AIDS

61. HIV and AIDS
5. Advanced disease - AIDS
CD8+ cells destroy more CD4+ cells
CD4 cell loss means virus and infected
cells no longer controlled
As CD4+ cells fall below 200 per cu mm
virus titer rises rapidly and remaining
immune response collapses
CD8+ cell number collapses
Opportunistic infections
Death in ~2 years without intervention

62. Virus destroys the cell as a
result of budding
Why do all T4
cells
1. PUNCTURED
MEMBRANE disappear?

63. Why do all T4 cells
disappear? - 2
Most T4 cells are
not HIV+
Infected CD4 Could “sweep
cell Cells Fuse up” uninfected
Gp120 positive cells
Uninfected
Killing of CD4 cells CD4 cell
2. Syncytium Gp120
Formation negative

64. Why do all T4
cells
disappear?
Cytotoxi
c T cell
Killing of CD4 cells
3. Cytotoxic T cell-mediated
lysis
BUT: Most cells
are not infected

65. Killing of CD4+ cells
4. Complement-
mediated lysis
Binding of free Gp120 to
CD4 antigen makes
uninfected T4 cell look
like an infected cell

66. 5. Apoptosis of T cells and
macrophages

67. Laboratory diagnosis
 Detection of viral antigens in the patient
blood by immuno-enzyme assay
 Detection of antiviral antibodies by
immuno-enzyme assay ELISA
 Detection of viral RNA by PCR
 Detection of proviral DNA in infected cells
by PCR

68. Strategies for drugs to treat AIDS (1)
A prominent group of drugs (AZT, ddC) are molecular
mimics called nucleoside analogs or reverse
transcriptase inhibitors

69. Strategies for drugs to treat AIDS (2)
Protease inhibitors plug into the active sites in HIV
protease

70. Strategies for drugs to treat AIDS (3)
Ribozyme. The enzyme that effectively cleave the viral
RNA in half

71. vaccine problem
Population Polymorphism
Retroviruses use host cell RNA polymerase II to replicate
their genome
Pol II has a high error rate 1:2,000-10,000
HIV genome 9749 nucleotides
Therefore EVERY new virus has at least one mutation!
Every possible single mutation arises daily
1% of all possible double mutations arise daily
The HIV that infects a patient is very different
from that seen by the time AIDS appears

72. Population Polymorphism
• Variation in reverse transcriptase leads to resistance to
nucleoside analogs
drug problem
• Variation in protease leads to resistance to protease
inhibitors
drug problem
Polymorphism due to high mutation rate as a result of
lack of proof-reading in reverse transcriptase and RNA
pol II
Sub-populations arise with altered cell tropism