Oncogenic viruses

Basis of viral oncogenesis and the most common viruses causing cancer and their mechanism of causing cancer. Helpful for undergraduate and postgraduate teaching.

1. Oncogenic viruses Dr. Sarvodaya Tripathy

2. Virus family Human cancer Papovaviridae Human papilloma virus Cervical ca Other genital tract ca Esophageal ca Laryngeal ca Oropharyngeal ca Merkel cell virus Merkel cell ca of skin Herpesviridae Epstein Barr virus Burkitt’s lymphoma Hodgkins’s disease Nasopharyngeal ca Human Herpesvirus -8 Kaposi’s sarcoma Primary effusion lymphoma Hepadnaviride Hepatitis B virus Hepatocellular carcinoma Retroviridae HTLV -1 Adult T-cell leukemia /lymphoma HIV AIDS related malignancies Flaviviridae Hepatitis C Hepatocellular carcinoma DNA Viruses RNA Viruses

3. Basic concepts Proto-oncogenes – Oncogenes – Tumour suppressor genes – supress any abnormal proliferation of cells( eg . p53, Retinoblastoma) Apoptosis regulatory genes – eg . bcl-2 (anti-apoptotic) and bax (pro-apoptotic genes) DNA repair genes Normal cellular genes whose products promote cell proliferation. Over-activation can lead to transformation of host cells Mutant or overexpressed versions of proto-oncogenes that function autonomously without a requirement for normal growth-promoting signals When present in viral genome they are designated as V- onc Oncoproteins cause continuous stimulation of nuclear transcription factors that cause increased expression of growth-promoting genes.

4. Cell cycle is controlled by checkpoints at different stages Any stress or damaged DNA is detected at any checkpoint results in increased protein p53 production. p53 is a tumour suppressor gene that stops the progression of the cell cycle (G1 arrest) and starts repair mechanisms for the damaged DNA by inducing the expression of DNA repair genes If this DNA cannot be repaired, then it ensures the cell undergoes apoptosis and can no longer replicate. The p53-encoding tumor suppressor gene is the most commonly mutated gene in human cancer.

5. Cell cycle is also closely regulated by Cyclins which by activating cyclin-dependent kinase (CDK) enzymes control cell progression Rb restricts the ability of a cell to progress from G1 to S phase in the cell cycle. CDK phosphorylates Rb to pRb , making it unable to restrict cell proliferation. This allows cells to divide normally in the cell cycle. p16-INK4A a cell-cycle regulatory protein that interacts with CDK4 and CDK6, inhibiting their ability to interact with cyclins D.

6. Telomere and Telomerase Eukaryotic telomeres are usually identified as the “capping function” They protect chromosome ends from DNA degradation and fusion with other chromosomal ends Their length serves as an intrinsic biological clock that regulates the life span of the cell, i.e. they provide limits on the number of replications a cell can go through . Telomere repeats are lost with each round of cell replication. Enzyme “Telomerase” causes elongation of telomeres, but not completely It is a ribonucleoprotein with reverse transcriptase activity, is composed of two main parts – a telomere RNA component and a telomere reverse transcriptase Most somatic cells express insufficient telomerase, reactivation in malignant conditions

7. Selective tropism for epithelium of skin (squamous epithelium) and mucus membrane Benign warts to malignant condition Non-enveloped, Icosahedral symmetry, ds-circular DNA Papillomavirus Viral genome Early region (E1 –E7) Non-structural proteins E6 protein degrades p53 gene product E7 protein binds to RB gene product Late region (L1, L2) structural proteins L1 – Major capsid proteins L2 – Minor capsid proteins

8. Human papillomaviruses Cervical carcinoma Other genital tract carcinoma -- Anal, Vulval /Vaginal, Penile Esophageal carcinoma Laryngeal carcinoma Oropharyngeal carcinoma Types 1, 2, 4, and 7 cause benign squamous papillomas (warts) in humans Genital warts → low malignant potential and are a/w low-risk HPVs (HPV type-6 & -11) . Squamous cell carcinoma of the cervix and anogenital region are a/w high-risk HPVs (types-16 & -18)

9. E6 E7 p53 Bax p21 Apoptosis RB-E2F CyclinD / CDK4 Growth arrest Increased telomerase activity E6 enhances p53 degradation E6 also stimulates the expression of TERT, the catalytic subunit of telomerase E6 from high-risk HPV types has a higher affinity for p53 p21 tumour suppressor gene stop signal for mitosis in mutated cell E7 inhibits the tumor suppression gene – RB gene (retinoblastoma gene) Displaces the E2F transcription factor that are normally sequestered by RB

10. In benign warts, the HPV genome is maintained in a non-integrated episomal form In cancers, the HPV genome is randomly integrated into the host genome, which interrupts a negative regulatory region in the viral DNA , resulting in overexpression of the E6 and E7 oncoproteins . Cells in which the viral genome has integrated show significantly more genomic instability → pro-oncogenic mutations in host genome.

11. Vaccines Sub-unit vaccine composed of HPV L1 proteins (viral capsid) produced in vectors by DNA recombinant technology Cervarix – L1 proteins of HPV 16, 18 expressed in Baculovirus . Schedule – 0, 2 and 6 months Gardasil – L1 proteins of HPV 6, 11, 16, 18 expressed in Saccharomyces cerevisiae. Schedule – 0, 1 and 6 months 9–26 yrs of females and males recommended in US Gardasil 9 (2014) – against HPV 6, 11, 16, 18, 31, 33, 45, 52, 58.

12. Non-enveloped ds-DNA virus causing subclinical infection in childhood JCV infects 40% to 86% of the general population worldwide. BK virus has more prevalence than JC virus and affects children <10yrs of age Possible route of transmission – respiratory route 5HT2A receptor (glial cells, astrocytes, B lymphocytes, platelets & kidney epithelial cells) Viral genome gets integrated to host genome and persists for life (kidney, BM & brain) . JCV is detected in urine of one third of healthy individuals. Polyomavirus

13. Reactivation occurs during pregnancy – asymptomatic shedding Severe immunosuppression – AIDS, Organ transplant recipients, advanced lymphoid malignancy demonstrable shedding in urine Reactivation leads to lytic productive infection of oligodendrocytes and astrocytes leading to multiple foci of demyelination in brain JC virus – Progressive Multifocal Leukoencephalopathy BK virus causes Haemorrhagic cystitis in BM transplant patients Polyoma associated nephropathy in renal transplant recipients Atypical astrocytes Intra-nuclear inclusion bodies

14. Enveloped ds-DNA virus with icosahedral symmetry In the oropharynx, EBV infects epithelial cells, where it actively replicates and cause lysis, infected epithelial cells infect B cells. Epstein barr virus It binds to CR2 or CD21 (of B cells) through gp360 on its surface Small number of memory B cells retain the virus in form of episome and become latently infected (1 to 50 B cells per million)

15. EBV nuclear antigen (EBNA) 1 protein binds to viral DNA at oriP site and allows the EBV genome to be maintained in the B cell through generations, also it blocks its own degradation by proteasomes EBNA-2 up-regulates the expression of EBV latent membrane protein (LMP) 1 and LMP-2 LMP-2 prevents reactivation of EBV from latently infected cells

16. Survival & Proliferation of B-cells Anti-apoptotic Neo-vascularization B-cell growth

17. Infectious mononucleosis Oral hairy cell leucoplakia Burkitt’s lymphoma Nasopharyngeal carcinoma Progressive lymphoproliferative disease NHL of brain in AIDS patients Hodgkin Lymphoma Autoimmune diseases – Dermatomyositis, SLE, RA, Sjögren's syndrome and multiple sclerosis Oral hairy cell leukoplakia Burkitt’s Lymphoma Nasopharyngeal Carcinoma EBV associated disorders

18. 70% to 85% of hepatocellular carcinomas (HCC) worldwide are caused by HBV or HCV. Do not encode any viral oncoproteins . HBV DNA randomly integrates with the host genome, HCV RNA remains unintegrated HCC has multifactorial causation but dominant effect seems to be immunologically mediated chronic inflammation with hepatocyte death followed by compensatory regenerative process In regenerative process a plethora of growth factors , cytokines, chemokines etc are produced by activated immune cells that promote cell survival, tissue remodeling , and angiogenesis. The activated immune cells also produce reactive oxygen species → mutagenic . Hepatitis B & C viruses

19. Mediators also cause activation of the nuclear factor- κB (NF- κB ) pathway in hepatocytes The HBx gene in HBV genome hepatocellular has been shown to cause HCC in mice. HBx can directly or indirectly activate a variety of transcription factors and several signal transduction pathways May interfere with p53 function In addition, viral integration can cause secondary rearrangements of chromosomes, including multiple deletions → loss of unknown tumor suppressor genes.

20. Human T-cell leukemia virus type 1 (HTLV-1), is firmly implicated in the pathogenesis of adult T-cell leukemia /lymphoma (ATLL) Retrovirideae – enveloped, spherical virus with 2 identical copies of ss -RNA MOT – Sexual intercourse, blood products. Leukemia develops in 3% to 5% of the infected individuals, after a long latent period of 40 to 60 years. Preferentially affects the CD4+ T H cells HTLV- 1

21. Integration in host chromosomes is random (the viral DNA is found at different locations in different cancers), the site of integration is identical within all cells of a given cancer. Transcription activator ( tax) gene essential for viral replication tax gene product modulates host cell function Affected cells express large quantity of IL2-receptors LTR LTR gag pol env tax