1. VIRAL& Bacterial ONCOGENESIS
HA MWAYOMA, MD

2. PROTO-ONCOGENES:
These are cellular oncogenes (c-oncs) which are detected in normal animal and human cells and promote normal growth and differentiation of cells.
These are genes that are active in normal cells and have normal action.
These same genes, behaving in an abnormal fashion are known as oncogenes.

3. Carcinogenesis 1 – Oncogenes and Tumor Suppresor Genes
Two categories of cell regulatory genes
Proto-oncogenes (cellular oncogene, c-onc)
Tumor suppressor gene
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

4. Carcinogenesis 1 – Oncogenes and Tumor Suppresor Genes
Gene Activation and Inactivation
Proto-oncogene is activated and
tumor suppressor gene is inactivated
normal growth regulation is diverted into oncogenesis
Activated proto-oncogene = activated oncogene, mutant oncogene, cellular oncogene

5. II: ONCOGENES:
These are cancer-causing genes and are derived from proto-oncogenes
They are regularly overexpressed or mutated and contribute to converting a normal cell into a cancer cell.
Proto-oncogenes may be activated and transformed into oncogenes by one of the following mechanism:-

6. Mechanisms of transformation of proto-oncogenes to oncongenes-1
Point mutation:
There is alteration in the structure of proto-ocogene protein (e.g. Ras oncogenes in Urinary bladder
Amplification:
More copies of the gene are made either within the chromosome or floating free as double minute structures
Examples :
.n- myc gene of neuroblastoma,
erb-B in breast cancer and ovarian cancer)

7. Mechanisms of transformation of proto-oncogenes to oncongenes--2
Translocation:
Many genes in their normal sites are only transcribed in special restricted circumstances.
Examples:- proto-oncogene is translocated to another site just below the enhancer region in;
chronic mylogenous leukaemia (9:22), c-abl gene
Burkitt’s lymphoma (8:14), c- myc gene

8. Mechanisms of transformation of proto-oncogenes to oncongenes--3
Depression:
Proliferation occurs in absence of a repressor.
The Rb1 gene located on 13q 142 is important in suppressing mitosis in cells that have differentiated. If it is absent or mutated, then the cells will proliferate and the tumour known as Retinoblastoma develops.

9. Mechanisms of transformation of proto-oncogenes to oncongenes---
Viral insertion:
An oncogenic virus may provide the oncogene or enhancer sequence that turns on a proto-oncogene in the affected cell which will contribute to tumour development.

10. TRANSMISSION:
Oncogenic viruses can be transmitted by one of the 3 routes;
Vertical transmission:- when the infection is genetically transmitted from infected parents to offsprings.
Horizontal transmission:- when the infection passes from one to another by direct contact as occurs in most contagious diseases.
By inoculation:- as is done in experimental animals.

11. CLASSIFICATION:
Classification of oncogenic viruses is based on their nucleic acid content. They fall into 2 broad groups.
Those containing deoxyribonucleic acid are called DNA oncogenic viruses and
Those containing ribonucleic acid are termed RNA oncogenic viruses.

12. DNA ONCOGENIC VIRUSES:
DNA oncogenic viruses are classified into 5 subgroups each of which is capable of producing neoplasms in different hosts. These are:-
Papovaviruses
Herpesviruses
Adenoviruses
Poxviruses and
Hepadnaviruses

13. 1. PAPOVAVIRUSES: Papillomavirus including Human papillomavirus
This group consists of;
Papillomavirus including Human papillomavirus
Polyomavirus
SV-40 (Simian Vacuolating virus)
These viruses have aetiologic role in a variety of benign and malignant neoplasms in animals and humans

14. PAPOVAVIRUSES: Papilloma viruses.
These viruses were the first to be implicated in the aetiology of human neoplasia.
These viruses appear to replicate in the layers of stratified squamous epithelium
About 50 different types of HPV (Human papillomaviruses) have been identified

15. Papilloma viruses cont--
HPV types 1, 2, 4, and 7 - benign squamous papilloma
HPV 6, and 11 – genital warts (Condyloma accuminata)
HPV 16 and 18 – Cervical cancer

16. Polyoma virus. Occurs as a natural infection in mice
Causes carcinomas and sarcomas in immunodeficient (Nude) mice and other rodents but NOT in man.
Is not known to produce any tumours in humans.

17. SV-40 virus.
Occurs in monkeys without causing any harm but can induce sarcoma in hamsters.
There is no evidence of involvement of SV-40 infection in causing any human tumour.

18. 2. HERPESVIRUSES Important members of herpesvirus family include;
Epstein-Barr virus (EBV)
Herpes simples virus type 2 (HSV-2).
Human herpes virus 8 (HHV8).
Cytomegalovirus (CMV)
Only EBV and HHV8 are implicated in human tumours.
Primary infection of all herpes viruses in man persists probably for life in a latent stage which can get reactivated later

19. 2. HERPESVIRUSES:cont--
EPSTEIN-BARR VIRUS (EBV):
EBV infects lymphocytes and stimulates them to proliferate.
EBV is implicated in 2 human tumours.
Burkitt’s lymphoma and
Anaplastic nasopharyngeal carcinoma
It is also shown to be the cause of infectious mononucleosis in man.

20. 2. HERPESVIRUSES:cont--
HUMAN HERPES VIRUS 8 (HHV8):
HHV8 or Kaposi’s sarcoma associated herpes virus (KSHV) is associated with Kaposi’s sarcoma, a vascular neoplasm common in patients with AIDS.
HHV8 has lymphotropism and is also implicated in causation of B cell lymphoma and MUlticentric variant of Castleman’s disease.

21. HERPESVIRUSES:cont--
ADENOVIRUSES:
In man, they are not known to be involved in any tumour.
In animals (hamsters), they may induce sarcoma.
POXVIRUSES:
humans, poxviruses cause;
Molluscum contagiosum and
Squamous cell papilloma.

22. HERPESVIRUSES:cont--
HEPADNAVIRUSES:
Hepatitis B virus is a member of Hepadnavirus family
It has been implicated in the causation of cancer of the liver known as Hepatocellular carcinoma.

23. II: RNA ONCOGENIC VIRUSES:
RNA oncogenic viruses are retroviruses i.e. they contain the enzyme reverse transcriptase, though not all retroviruses are oncogenic.
The enzyme reverse transcriptase is required for reverse transcription of viral RNA to synthesize viral DNA strands i.e. reverse of normal in which DNA is transcribed into messenger RNA (mRNA).

24. RETROVIRUSES
Based on their activity to transform target cells, they are divided into 3 subgroups.
Acute transforming viruses
Slow transforming viruses
Human T-cell Lymphotropic viruses (HTLV).
The former two are implicated in inducing a variety of tumours in animals
while HTLV is a causative for Human T-cell leukaemia and lymphoma.

25. RETROVIRUSES CONT-- ACUTE TRANSFORMING VIRUSES:
This group includes retroviruses which transform all cells infected by them into malignant cells rapidly (acute).
All viruses in this group posses one or more viral oncogenes (V-oncos)
All members in this group are defective viruses in which the particular V-onc has substituted other essential material such as gag, pol and env.

26. ACUTE TRANSFORMING VIRUSES:CONT--
These defective viruses cannot replicate by themselves unless the host cell is infected by another “ helper virus”
Acute oncogenic viruses have not been detected in any human tumour so far, though they have been identified in tumours in different animals.

27. RETROVIRUSES CONT-- SLOW TRANSFORMING VIRUSES:
These oncogenic retroviruses cause development of leukaemia and lymphomas in different species of animals (e.g. mice, cats and bovine.
These viruses have long incubation period between infection and development of neoplastic transformation (‘slow’)

28. SLOW TRANSFORMING VIRUSES:CONT--
They cause neoplastic transformation by insertional mutagenesis
i.e. viral DNA synthesized by viral RNA via reverse transcriptase is inserted or intergrated near the proto-oncogene of the host cell resulting in enhanced expression of proto-oncogene as well
causes genetic damage (Mutagenesis) to the host genome leading to neoplastic transformation

29. RETROVIRUSES HUMAN T-CELL LYMPHOTROPIC VIRUSES:
HTLV is a form of slow transforming virus but is described separately because of 2 reasons:
This is the only retrovirus implicated in human cancer
The mechanism of neoplastic transformation is different from slow transforming as well as from acute transforming viruses.

30. HUMAN T-CELL LYMPHOTROPIC VIRUSES--
4 types of HTLVs are recognized
HTLT-I
HTLV-II
HTLV-III and
HTLV-IV
Note that the aetiologic agent of AIDS, HIV,is an HTLV (HTLV-III).

31. HTLV CONT---
There is a strong link between HTLV-I infection and Adult T-cell leukaemia- lymphoma syndrome (ATLL).
Similarly, HTLV-II is implicated in causation of T-cell variant of hairy cell leukaemia.
HTLV-I is transmitted through sexual contact, by blood, or to infants during breast feeding.

32. HTLV CONT---
HTLV-I has tropism for T-lymphocytes (especially CD4 subset of T cells, similar to HIV infection of AIDS)
Immunosuppression plays a supportive role in the neoplastic transformation by HTLV-I infection.

33. HTLV CONT---
HTLV-I infection differs from acute transforming viruses with regard to mechanism of neoplastic transformation in that it does not contain V-onc as from other slow transforming viruses because it does not have fixed site of insertion for insersional mutagenesis.

34. MECHANISM OF VIRAL ONCOGENESIS
Mode of oncogenesis by DNA and RNA oncogenic viruses are different as follows:-
I: MODE OF DNA VIRAL ONCOGENESIS:-
Host cells infected by DNA oncogenic viruses may have one of the following 2 results:-
Replication: – the virus may replicate in the host cell with consequent lysis of the infected cell and release of the virions.
Integration:- the viral DNA may integrate in the host DNA. This results in neoplastic transformation,

35. MODE OF DNA VIRAL ONCOGENESIS:- CONT--
A feature essential for host cell transformation is the expression of virus-specific T-antigen (transforming protein) immediately after infection of the host cell by DNA oncogenic virus.

36. Replication of viruses:
VIRAL REPLICATION STAGE:
The Stages of Replication
attachment (adsorption) stage.
The first stage in viral replication is called the attachment (adsorption) stage.
Like bacteriophages, animal viruses attach to host cells by means of a complementary association between attachment sites on the surface of the virus and receptor sites on the host cell surface

37. 1. attachment (adsorption) stage cont--
This accounts for specificity of viruses for their host cells.
Attachment sites on the viruses (usually called virus receptors) are distributed over the surface of the virus coat (capsid) or envelope, and are usually in the form of glycoproteins or proteins
Cells lacking receptors for a certain virus are resistant to it and cannot be infected.

38. 1. attachment (adsorption) stage cont--
Attachment can be blocked by antibody molecules that bind to viral attachment sites or to host cell receptors.
Since antibodies block the initial attachment of viruses to their host cells, the presence of these antibodies in the host organism are the most important basis for immunization against viral infections

39. 2. The penetration stage follows attachment.
Penetration of the virus occurs either by engulfment of the whole virus, or by fusion of the viral envelope with the cell membrane allowing only the nucleocapsid of the virus to enter the cell.
Animal viruses generally do not "inject" their nucleic acid into host cells as do bacteriophages, although occasionally non enveloped viruses leave their capsid outside the cell while the genome passes into the cell

40. 3. uncoating:
Once the nucleocapsid gains entry into the host cell cytoplasm, the process of uncoating occurs.
The viral nucleic acid is released from its coat.
Uncoating processes are apparently quite variable and only poorly understood

41. 3. uncoating: cont--
Most viruses enter the host cell in an engulfment process called receptor mediated endocytosis and actually penetrate the cell contained in a membranous structure called an endosome
Acidification of the endosome is known to cause rearrangements in the virus coat proteins which probably allows extrusion of the viral core into the cytoplasm
Some antiviral drugs such as amantadine exert their antiviral effect my preventing uncoating of the viral nucleic acid

42. DNA VIRUES Early viral proteins:
In DNA viruses, such as Herpes, the viral DNA is released into the nucleus of the host cell where it is transcribed into early mRNA for transport into the cytoplasm where it is translated into early viral proteins.
The early viral proteins are concerned with replication of the viral DNA,

43. Early viral proteins:
so they are transported back into the nucleus where they become involved in the synthesis of  multiple copies of viral DNA
These copies of the viral genome are then templates for transcription into late mRNAs which are also transported back into the cytoplasm for translation into late viral proteins

44. late viral proteins
The late proteins are structural proteins (e.g. coat, envelope proteins) or core proteins (certain enzymes) which are then transported back into the nucleus for the next stage of the replication cycle
In the case of some RNA viruses (e.g. picornaviruses), the viral genome (RNA) stays in the cytoplasm where it mediates its own replication and translation into viral proteins.

45. late viral proteins cont--
In other cases (e.g. orthomyxoviruses), the infectious viral RNA enters into the nucleus where it is replicated before transport back to the cytoplasm for translation into viral proteins.

46. 5. The assembly stage:
Once the synthesis of the various viral components is complete, the assembly stage begins
The capsomere proteins enclose the nucleic acid to form the viral nucleocapsid. The process is called encapsidation
If the virus contains an envelope it will acquire that envelope and asssociated viral proteins in the next step

47. 6. The release stage: is the final event in viral replication, and
it results in the exit of the mature virions from their host cell
Virus maturation and release occurs over a considerable period of time.
Some viruses are released from the cell without cell death, by egestion, whereas others are released when the cell dies and disintegrates.

48. 6. The release stage: cont--
In the case of enveloped viruses, the nucleocapsid acquires its final envelope from the nuclear or cell membrane by a budding off process (envelopment) before egress (exit) out of the host cell.
Whenever a virus acquires a membrane envelope, it always inserts specific viral proteins into the that envelope which become unique viral antigens and which will be used by the virus to gain entry into a new host cell

49. DIAGRAMS:
Below are illustrated the modes of replication of two viruses that conform to this model. Herpes simplex virus (HSV) is an enveloped, double stranded DNA virus; Influenza virus is an enveloped, single stranded (-)RNA virus that contains a segmented genome

50. Replication cycle of Herpes Simplex Virus

52. Replication of DNA virus: (see diagram).
Step 1: The DNA virus invades the host cell
Step 2: Viral DNA is incorporated into the host cell nucleus and T-antigen is expressed immediately after infection.
Step 3: Replication of viral DNA occurs and other components of virion are formed.
Step 4: The new virions are assembled accompanied by host cell lysis.

53. Integration: of DNA virus (see diagram).
Steps 1 and 2 are similar as in replication
Step 3: integration of viral genome into the host cell genome occurs which requires essential presence of functional T-antigens
Step 4: A” transformed (Neoplastic) cell” is formed

54. MODE OF RNA VIRAL ONCOGENESIS.
RNA retroviruses contain two identical strands of RNA and the enzyme reverse transcriptase
Reverse transcriptase is RNA-dependent DNA synthetase that acts as a template to synthesise a single strand of matching viral DNA i.e. reverse of the normal in which DNA is transcribed into messenger RNA.

55. MODE OF RNA VIRAL ONCOGENESIS cont--
The single strand of viral DNA is then copied by DNA dependent DNA synthetase to form another strand of complementary DNA resulting in double stranded DNA of provirus.

56. MODE OF RNA VIRAL ONCOGENESIS.cont--
The provirus is then integrated into the DNA of the host cell genome and may transform the cell into neoplasti cell
Retroviruses are replication-competent. The host cells which allow replication of integrated retrovirus are called permissive cells. Non-permissive cells do not permit replication of the integrated retrovirus.

57. The RNA virus invades the host cell.
INTEGRATION AND REPLICATION OF RNA VIRUS (RETROVIRUS) IN THE HOST CELL:1
Step 1:
The RNA virus invades the host cell.
The virus envelope fuses with the plasma membrane of the host cell.
Viral RNA genome as well as reverse transcriptase are release into the cytosol.

58. Reverse transcriptase acts as template to synthesize
INTEGRATION AND REPLICATION OF RNA VIRUS (RETROVIRUS) IN THE HOST CELL--2
Step 2:
Reverse transcriptase acts as template to synthesize
a single strand of matching viral DNA which is then copied to form
Complementary DNA resulting in double stranded DNA(provius)

59. INTEGRATION AND REPLICATION OF RNA VIRUS (RETROVIRUS) IN THE HOST CELL---3
Step 3:
The provirus is integrated into the host cell genome, Producing “transformed host cell”
Step 4: Integration of the provirus brings about replication of viral Components which are then assembled and released by budding.

60. INTEGRATION AND REPLICATION OF RNA VIRUS (RETROVIRUS) IN THE HOST CELL----4
Viral replication begins after integration of the provirus into the host cell genome
Integration results in transcription of the proviral genes or progenes into messenger RNA which then forms components of virus particle:-
virion core protein from gag gene
, reverse transcriptase from pol gene and
envelope glycoprotein from env gene.

61. The three components of virus particle are then;
INTEGRATION AND REPLICATION OF RNA VIRUS (RETROVIRUS) IN THE HOST CELL-----5
The three components of virus particle are then;
assembled at the plasma membrane of the host cell and
the virus particles released by budding off from the plasma membrane, thus completing the process of replication.

62. VIRUS AND HUMAN CANCER: A SUMMARY.
There are 8 human cancers which have enough epidemiological and serological evidence that viruses, in addition to other concomitant factors, are implicated in their aetiology:-
Epstein Barr Virus (EBV).
Burkitt’s lymphoma
Anaplastic nasopharyngeal carcinoma
Hepatitis B virus & Hepatitis C virus
Primary Hepatocellular carcinoma

63. VIRUS AND HUMAN CANCER: A SUMMARY.cont--
high risk human papillomavirus (HPV16&18).
Cervical cancer
Human herpes virus type 8 (HHV8).
Kaposi’s sarcoma
B cell lymphoma
HTLV-I:
Adult-cell leukaemia and lymphoma
HTLV-II:
T-cell variant of hairy cell leukaemia

64. Bacterial carcinogenesis
Helicobacter pylori-induced epithelial cell signalling in gastric carcinogenesis

65. Helicobacter pylori
Produces adhesins which bind to membrane- associated lipids and carbohydrates and help its adhesion to epithelial cells
Breaks down urea (NH2CONH2) to NH4+ and CO2
Stomach acidity 
Possible for H. pylori to survive

66. Helicobacter pylori Highly successful human microbial pathogen
classified as a class I carcinogen
cellular and molecular signalling pathways are used during H. pylori infection to promote epithelial hyperproliferation and transformation
Gastric inflammation
Chronic gastritis
peptic ulcers
gastric cancer
gastric adenocarcinoma
mucosa associated lymphoid tissue (MALT) lymphoma
gastric epithelial hyperproliferation.

69. Hpylori affects apoptosis and cell cycle control
Apoptosis and cell cycle control are processes required for the regulation of cellular homeostasis
chronic imbalance between apoptosis and cell proliferation is the first step of gastric carcinogenesis, as in all tumours.
H. pylori infection could lead to an overall increase in cellular turnover and persistence of mutated cells, which will favour the development of neoplasia

70. Hpylori affects apoptosis and cell cycle control
The cell cycle, the programme for cell growth and division (proliferation), consists of four phases that are known as G1 and G0, S, G2 and M. The important protein families used during this cycle include the cyclins, the cyclin dependent kinases (Cdks), the Cdk inhibitors and the tumour-supressor genes (in particular, Rb and p53)

71. H. pylori affects apoptosis and cell cycle control(2)
The H. pylori toxin VacA induces gastric epithelial cell apoptosis, suggesting that differences in levels of gastric mucosal apoptosis among infected persons might result from strain-dependent variations in VacA structure.

72. H. pylori affects apoptosis and cell cycle control(3)
Exposure of epithelial cells to H. pylori alters cell cycle control both in vitro and in vivo. Mucosal expression of cyclin D1, the tumour-suppressor p53 and the cell cycle inhibitor p21 was significantly higher in H. pylori- infected patients with intestinal metaplasia
A clear effect of H. pylori on cell cycle progression has been described in infected patients with intestinal metaplasia that overexpress cyclin D2 and show reduced expression of the cell cycle inhibitor p27

73. H. pylori
Chronic colonization of the human stomach by Helicobacter pylori, a Gram-negative bacterium, is the major cause of chronic gastritis, peptic ulcers and gastric cancer.
Recent progress has elucidated important bacterial and host factors that are responsible for H. pylori-induced gastric inflammation and gastric malignancy. H. pylori cytotoxin-associated antigen A is the major oncogenic factor injected into host cells from bacteria and it disrupts epithelial cell functions

74. Helicobacter Pylori

75. ANY PARASITIC INFECTION?
Schistosomiasis (haematobium)
---- urinary bludder >>>>> Squamous cell carcinoma, through squamous metaplasia.
ANY FUNGAL INFECTION ?
Aspergillus flavus LIVER >>>>> Hepatocellular carcinoma, through Aflatoxin B1

76. Thanks for your kind attention