Viral carcinogenesis copy

Viral carcinogenesis

1. VIRAL CARCINOGENESIS By Dr Ajish M Saji Deparment of Oral Pathology Malabar Dental College Edappal

2. Contents Introduction RNA viruses History RNA Genome Reverse Transcription Human T- Lymphotropic virus type I and II HIV Hepatitis C virus DNA Viruses Hepadna virus HPV Epstein Barr virus Polyoma virus Conclusion References

3. INTRODUCTION A large number of DNA and RNA viruses have proved to be oncogenic in a wide variety of animals, ranging from amphibia to primates, and the evidence grows stronger that certain forms of human cancer are of viral origin.

4. RNA Viruses Members of two RNA virus families - Retroviridae and Flaviviridae . Retroviruses - human T- lymphotropic virus (HTLV) and human immunodeficiency virus (HIV). Flavivirus hepatitis C virus (HCV).

5. History Classified on the basis of pathogenesis - Oncoretrovirus , Lentivirus , and Spumavirus groups. Basis of virus particle morphology-virus types A-D.

6. Now, organized into seven genera based on molecular genetic analysis: Alpha-, Beta-, Gamma-, Delta-, and Epsilon-retroviruses, Lentiviruses , and Spumaviruses . Deltaretroviruses HTLV-1 and HTLV-2. Lentiviruses HIV-1 and HIV-2.

7. Human T- lymphotropic virus type 1 (HTLV-1) contribute directly to the development of adult T-cell leukemia (ATL). HIV and HCV are associated with human malignancy, but likely contribute to its development in an indirect manner.

8. RNA genome 5 untranslated region, the three genes common to all retroviruses gag, pol , and env , and a 3 untranslated region and polyadenylated tail.

9. gag gene encodes viral structural proteins. pol encodes viral enzymatic proteins. env encodes viral envelope glycoproteins .

10. Following entry into a cell - single-stranded viral genome is converted to a double-stranded DNA copy by reverse transcriptase, an RNA-dependent DNA polymerase.

11. Then, the retroviral integrase protein inserts the double-stranded DNA viral genome into a host cell chromosome where it permanently resides as a provirus.

12. It is duplicated along with the cell's genome, passed on to daughter cells during mitosis, and subsequently transcribed and processed into mRNA.

13. Mechanisms of Retroviral Oncogenesis Reverse transcription and integration of the viral genome into the cell favors the three major mechanisms by which oncogenic retroviruses may participate in the malignant transformation process.

14. 1. Slowly transforming viruses -alter cellular gene expression by random integration of a provirus within or adjacent to cellular protooncogenes ( insertional mutagenesis). Direct physical disruption of a gene or effects of viral promoters and enhancers on cellular gene expression can lead to a malignant phenotype in infected cells.

15. 2. Acutely transforming retroviruses have incorporated into their genomes viral oncogenes derived from cellular protooncogenes ( protooncogene capture). Subsequently transfer these altered or deregulated oncogenes into newly infected cells, thus leading to development of a malignant phenotype.

16. 3. Trans-acting retroviruses alter cellular gene expression and function and, consequently, the control of cell growth via viral proteins that act in trans.

17. Human T- Lymphotropic Virus Type 1 (HTLV-1) Adult T cell Leukemia, an aggressive malignancy of CD4+ T cells. Transmission of HTLV-1 - blood transfusion, needle sharing, breast feeding, and from male to female (rarely the reverse) by sexual intercourse.

18. The Viral Replication Cycle and its Implications for Virus Spread In addition to gag, pol , and env , the virus genome contains additional open reading frames (ORF I-IV) located in the pX region of the genome. The two best characterized are the trans-regulating proteins Tax and Rex.

19. Rex promotes the cytoplasmic accumulation of singly-spliced ( env ) and unspliced (genomic) mRNAs. Tax activates transcription from the HTLV-1 LTR by associating with a number of cellular transcription factors. Additional HTLV-1 proteins - p12 I , p13 II , p30 II.

20. Virus spreads to uninfected cells by cell-to-cell transmission of the virus. This cell-to-cell spread of HTLV-1 appears to involve polarization of the cytoskeleton of infected cells to a cell-cell junction, promoting spread of virus to new cells.

21. In addition to cell-to-cell virus transmission, the number of HTLV-1 infected cells within an individual increases by simple mitosis of provirus-containing T-cells - amplifying, the number of infected T cells. In individuals infected with HTLV-1, significant viremia is not detected.

22. Models for Human T- Lymphotropic Virus Type 1 Leukemogenesis Outcome of HTLV-1 infection is an asymptomatic carrier state. The genome of HTLV-1 contains, in addition to the usual retroviral genes, a unique region called pX . This region encodes protein called TAX. The secrets of its transforming activity are locked in the TAX gene.

23. The TAX protein can activate the transcription of several host cell genes, including genes encoding the cytokine IL-2 and its receptor and the gene for GM-CSF. TAX can repress the function of several tumor suppressor genes that control the cell cycle including the CDKIs CDKN2A/p16 and TP53 .

24. Clinical Features In acute ATL, tumor cells aggressively infiltrate multiple organs, commonly involving lymph nodes, liver, spleen, skin, and lung. The age of onset averages 58 years (range, 24 to 85 years), with a male-to-female ratio of 1.4:1

25. ATL has been classified into four stages: acute, chronic, smoldering, and lymphomatous . Characteristics of the malignancy may include hypercalcemia , elevated lactate dehydrogenase levels, cutaneous leukemic infiltrates, lytic bone lesions, lymphadenopathy , and liver or spleen lesions. Death is often the result of opportunistic infection.

26. Human T- Lymphotropic Virus Type 2 The virus was isolated from a patient with atypical T-cell variant hairy cell leukemia. HTLV-2 is transmitted by: contaminated blood, breast feeding, and sexual intercourse.

27. Human Immunodeficiency Virus HIV-1 and HIV-2 are members of the Lentivirus genus of retroviruses. HIV-2 can also cause AIDS in humans and monkeys, the majority of AIDS cases worldwide are the result of HIV-1 infection

28. HIV replicates actively following initial infection, which results in high levels of viremia . The high rate of viral replication, combined with a high mutation rate due to lack of proof-reading ability of reverse transcriptase during reverse transcription, results in the extreme genetic variability-HIV-1.

29. Highly cytopathic for CD4-positive T cells. HIV encodes two trans-acting proteins, Tat and Rev. In HIV-infected persons, non-Hodgkin's lymphoma - Burkitt's , immunoblastic , and primary CNS, Kaposi's sarcoma, anal squamous cell carcinoma and cervical cancer are all AIDS-defining illnesses.

30. Many of the neoplasms common to AIDS patients are associated with infection by DNA viruses. These viruses include Kaposi's sarcoma-associated herpesvirus /human herpes virus-8, Epstein-Barr virus, and human papilloma virus.

31. Hepatitis C Virus HCV infection is a well-established risk factor for the development of hepatocellular carcinoma . HCV belongs to the Hepacivirus genus of the Flaviviridae family of viruses

32. HCV infection is strongly associated with the development of hepatic cirrhosis and HCC. Following initial infection by HCV, about 25% of people develop acute clinical hepatitis while others are asymptomatic. HCV infection is chronic in 50% to 80% of cases. Of these cases, 60% to 70% will develop chronic hepatitis, with about 20% of this group progressing to cirrhosis.

33. The estimated proportion of individuals chronically infected with HCV who develop HCC is estimated to be 1% to 5%. Surgical resection of tumors or liver transplantation are the only two curative therapies for HCC; therefore, early detection of HCC is important

34. DNA Viruses General characteristics that have emerged for many human cancer viruses are: (1) the viruses that have been implicated in human carcinogenesis are frequently ubiquitous (e.g., Epstein-Barr virus [EBV], human papillomavirus [HPV], hepatitis viruses); (2) cancer is a rare outcome of virus infection and only a small percentage of infected individuals develops cancer;

35. (3) the time intervals between the initial infection and cancer development is long (usually decades); (4) the cancers are usually clonal ; (5) chemical or physical agents are often implicated as playing cofactor roles.

36. Hepadnaviruses and Hepatocellular Carcinoma Hepatocellular carcinoma (HCC) is one of the world's commonest malignancies. HBV is a small DNA virus classified as a member of the hepadnavirus family .

37. Primary HBV infection produces either a subclinical infection or acute liver injury-95% of such infections resolve. 5% of patients develop persistent hepatic infection and viremia , and most of the demonstrated HCC risk falls within this subgroup of infections. Another factor that adds to risk is the severity of chronic liver injury: asymptomatic carriers have lower HCC risk than those with chronic active hepatitis or cirrhosis.

38. The induction of hepatocellular injury is thought to be important in HCC pathogenesis because it triggers in the liver a stereotyped proliferative response. Proliferation increases opportunities for replicative errors (mutations) that over time can contribute to the loss of normal cellular growth control.

39. Papillomaviruses and Human Cancer The papillomaviruses are nonenveloped DNA viruses that induce squamous epithelial and fibroepithelial tumors in their natural hosts.

40. History In 1951, a Canadian cytologist Ernest Ayre demonstrated squamous epithelial cells with a ‘perinuclear halo’ in smears from the uterine cervix - precancer cells and some long standing infection or inflammation. Koss and Durfee , in 1956, named these squamous cells as “koilocytes”, from the Greek word ‘ koilos ’ meaning ‘hollow cell’.

41. It was in 1968 - the ultrastructural finding of koilocytes and viral particles within them in genital condylomas. The year 1977 - Human Papilloma Viruses (HPV) and they play a role in the etiology of Squamous Cell Carcinoma and its precursors.

42. Koilocytes are actually virus-infected squamous epithelial cells and the virus found in the nuclei of koilocytes is consistent with HPV.

43. It can be classified to α – HPV predominate in mucosal sites such as genital and oral mucosa; β – HPV and γ – HPV predominate at skin sites. Further categorized as high – risk( oncogenic ) strains – cervical cancer. Low risk(non oncogenic ) – benign diseases.

44.  HPV-6 and -11 are the two most common low-risk types; they account for the majority of genital warts, which rarely progress to malignancy. HPV-16 and -18 are the major high-risk types and predominate in invasive anogenital cancers.

45. The buccal mucosa, being the site that is most exposed to chemical carcinogens, infections, and trauma, is most vulnerable to carcinogenesis. Abrasions caused due to this continuous exposure might make this mucosal surface more susceptible to HPV by making it easier for the virus to gain entry into the basal cells.

46. Of the HPV family, more than 12 types have been found in oral lesions, including HPVs 1, 2, 4, 6, 7, 11, 13, 16, 18, 30, 32, and 57.   HPV-13 and -32 appears to be restricted to oral lesions.  

47.  A higher risk of oral cancer has been found to be associated with, Number of sexual partners. Younger age at first sexual intercourse. Practice of oral sex. History of genital warts.

48. Carcinogenic Potential of HPV Following the HPV infection of the host tissue, The HPV genome is integrated into the host genome and two products are formed – E6 and E7 protein.

49. ‘E6 protein’ that forms a complex leading to the degradation of p53 gene thereby inhibiting apoptosis. ‘E7 protein’ that disturbs the retinoblastoma tumor suppressor gene thereby causing increased DNA synthesis and proliferation

50. Pathogenesis A breakpoint in the E1/ E2 sequence allows integration of HPV into the host genome and significantly increases its tumorigenicity through upregulation of E6 and E7 encoded in the early open reading frame of the virus. Expression of E6 and E7 is negatively regulated by E2 protein, which is also encoded in the early open reading frame of the virus.

51. By altering host genome functions, HPV E6 and E7 disrupts the p53 and pRb tumor suppressor genes, as well as numerous cellular proteins involved in carcinogenesis. Subsequently, infected cells develop defects in gene expression controlling apoptosis, DNA repair, and cell cycle, thus paving the way for cellular transformation.

52. HPV and OSCC Nearly 100 distinct types of HPV have been identified, only a few are seen to be associated with OSCC. Balram et al have reported high prevalence of HPV-16 and HPV-18 (42% & 47% respectively) in their study on oral cancer from Indian betel quid chewers.

53. Chang et al, Shroyer & Greer, Mork et al and Charfi et al have, in separate studies demonstrated the high association rate of HPV-16 in OSCC cases as between 62% and 95%. Cabibi et al shows that the sensitivity of detection of HPV infection by identification of koilocytes was 74% and the specificity was 72%.

54. Allen et al diagnosed severe epithelial dysplasia of oral mucosa, concluded that ‘Oral Koilocytic Dysplasia’ (OKD) represented a unique pathological entity and the presence of HPV could be predicted under light microscopy with 80% accuracy.

55. Studies by Fornatora et al have also reported OKD in association with OSCC, which exhibits features of both HPV infection (koilocytes) and oral epithelial dysplasia.

56. Balram et al, suggested that the high prevalence of HPV in OSCC point to the continuous viral infection being an etiologic factor with the betel quid (tobacco) causing additional mutagenic steps in the carcinogenic process.

57. Recent studies - increasing incidence of OSCC of the tongue in patients below 40 years of age, in the absence of any known habit of using tobacco. It has been reported to have increased over the past decade and is thought to be the second most common site for malignancy oral cavity. Microscopic evidence of koilocytes and HPV DNA have been demonstrated in around 75% of these tongue cancer patients, suggesting a direct etiologic role for HPV in these cases, in the absence of any other known factor.

58. HPV infection of the mouth and of the oropharynx , like HPV infection of the uterine cervix, is associated with high-risk sexual behaviour , in particular with orogenital sex.

59. Epstein-Barr Virus EBV is a remarkably unusual virus that potentially induces malignant-infected B-lymphocyte proliferation in primary human infection. EBV-infected B cells are the source of reactivated lytic virus infection.

60. EBV infects epithelial cells of the oropharynx and B lymphocytes. It gains entry into B cells via the CD21 molecule, which is expressed on all B cells. Within B lymphocytes, the linear genome of EBV circularizes to form an episome in the cell nucleus.

61. The infection of B cells is latent; that is, there is no replication of the virus and the cells are not killed, but the latently infected B cells are immortalized and acquire the ability to propagate indefinitely in vitro.

62. The latent membrane protein-1 (LMP-1) binds to and activates a signaling molecule that is normally activated by the CD40 receptor in B cells. This receptor is the key recipient of helper T-cell signals, which are normally required for full B-cell responses .

63. Mimicking CD40, LMP-1 activates the NFκB and JAK/STAT signaling pathways and promotes B-cell survival and proliferation, all of which are helper T cell-induced responses that occur in the absence of T cells (or any other signals) in EBV-infected B cells.

64. Thus, the virus has efficiently co-opted a normal pathway of B-cell activation in order to increase the number of cells it can infect and inhabit.

65. The EBV-encoded EBNA-2 gene transactivates several host genes, including CYCLIN D and members of the SRC family, promoting the transition of resting B cells from G to G 1. EBNA-2 also activates the transcription of LMP-1 and is a key regulator of viral gene expression. Thus, several viral genes collaborate to render B cells immortal.

66. EBV has been implicated in the pathogenesis of several human tumors: Burkitt lymphoma, post-transplant lymphoproliferative disease, primary central nervous system lymphoma in AIDS patients.

67. Polyomaviruses SV40 DNA or antigens in different human cancers - osteosarcomas , mesotheliomas , brain tumors, and non-Hodgkin's lymphomas. SV40 is a nonhuman primate virus that naturally infects Asian macaques.

68. Source - contaminated poliovirus vaccines given between 1955 and 1963. SV40 is a highly oncogenic virus in rodent cells. Valuable model for determining the various mechanisms by which DNA tumor viruses contribute to tumor formation.

69. Conclusion Understanding the role of virus in the etiology of oral cancer may be essential in determining prognosis and treatment and for disease prevention. Over the past few years, there have been significant advances in the understanding of the tumor pattern, and various options are now available as far as treatment modalities and techniques of surgical reconstruction are concerned.

70. References DeVita , Hellman and Rosenberg’s; Cancer Principles And Practices In Oncology; 8 th edition. Kumar , Abbas , Fasusto ,Robbins And Cotran ,Pathologic Basics Of Diseases ,Elsevier, Seventh Edition. Kumar, Robbins And Cotran , Basic Pathology, Seventh Edition.

71. Rakesh S et al; ASSOCIATION OF HUMAN PAPILLOMA VIRUS WITH ORAL SQUAMOUS CELL CARCINOMA – A BRIEF REVIEW; Oral & Maxillofacial Pathology Journal; Vol 1, No 2 Jul- Dec 2010. Liviu Feller et al, Human papillomavirus -mediated carcinogenesis and HPV-associated oral and oropharyngeal squamous cell carcinoma. Part 2: Human papillomavirus associated oral and oropharyngeal squamous cell carcinoma, Head & Face Medicine 2010, 6:15

72. Chocolatewala NM, Chaturvedi P, Role of human papilloma virus in the oral carcinogenesis: an Indian perspective, J Cancer Res Ther . 2009 Apr-Jun;5(2):71-7. Kari Syrjänen et al, Morphological and immunohistochemical evidence suggesting human papillomavirus (HPV) involvement in oral squamous cell carcinogenesis, International Journal of Oral Surgery,Volume 12, Issue 6, December 1983, Pages 418–424.

73. Thank you