Oncogenic viruses DNA viruses Herpesviridae Papovaviridae Human Herpes Virus 8 (HHV8) a.k.a Kaposi’s sarcoma associated virus Epstein-Barr virus (EBV) Papovaviridae human papilloma virus (HPV) Hepadnaviridae hepatitis B virus-(HBV) RNA viruses Flaviviridae (hepatitis C virus HCV) Retroviridae Human T-cell lymphotropic virus (HTLV type I)

Why are they oncogenic? Viral genomes show the presence of several human gene homologues that are responsible for cellular transformation e.g. v-myc and c-myc (myc oncogene) or vIL6 and IL6 (interleukin 6)

Overview of viral replication

Human Herpes Virus 8 (HHV8) or Kaposi’s sarcoma associated virus KSHV Herpes virus family Type 1 - causes ‘cold sores’ on lips (~90% of population) Type 2 - sexually transmitted disease that causes "cold sores" on the genitals (~ 25% of US adults).

Human Herpes Virus 8 (HHV8) a. k Human Herpes Virus 8 (HHV8) a.k.a Kaposi’s sarcoma associated virus HHV8 endemic regions

Kaposi’s sarcoma

HHV8 and transformation

EBV- Epstein Barr Virus most potent transforming agent, widespread in all human populations usually carried as an asymptomatic persistent infection. virus sometimes associated with the pathogenesis of certain types of lymphoid and epithelial cancers, including Burkitt lymphoma (BL), Hodgkin disease and nasopharyngeal carcinoma (NPC). Virus infection involves two cellular compartments: (1) B cells, where infection is predominantly latent and has the potential to induce growth-transformation of infected cells; and (2) epithelial cells, where infection is predominantly replicative. Although the exact mode of primary and persistent EBV infection and the relative contributions of B cells and epithelial cells are uncertain, recent data point to the B-cell compartment as the main mediator of primary as well as persistent infection. Following primary infection of B cells, a chronic virus carrier state is established in which the outgrowth of EBV-transformed B cells is controlled by an EBV-specific cytotoxic T lymphocyte response re-activated from a pool of virus-specific memory T cells. At certain sites, latently infected B cells can become permissive for lytic EBV infection. Infectious virus released from these cells can be shed directly into the saliva or might infect epithelial cells and other B cells. In this way a virus-carrier state is established that is characterised by persistent, latent infection in circulating B cells and occasional EBV replication in B cells and epithelial cells

EBV associated cancers

Nasopharyngeal carcinoma Burkitt’s lymphoma Nasopharyngeal carcinoma Hodgkin’s lymphoma Burkitt's lymphoma is endemic in regions with mean minimum temperatures that exceed 15·5°C and yearly rainfall of higher than 50 mL, which stretch from about 10° north to 10° south of the equator. 40-50% of patients are EBV seropositive NPC tissue stained for the presence of EBV late antigens.

in vivo interactions between EBV and host cells Putative in vivo interactions between Epstein–Barr virus and host cells.   a | Primary infection. Incoming virus establishes a primary focus of lytic replication in the oropharynx (possibly in the mucosal epithelium), after which the virus spreads throughout the lymphoid tissues as a latent (latency III) growth-transforming infection of B cells. Many of these proliferating cells are removed by the emerging latent-antigen-specific primary-T-cell response, but some escape by downregulating antigen expression and establishing a stable reservoir of resting viral-genome-positive memory B cells, in which viral antigen expression is mostly suppressed (latency 0). Different views of these events are shown. One view is that naive B cells are the main targets of new EBV infections in vivo. In this scenario, viral transformation drives naive cells into memory by mimicking the physiological process of antigen-driven memory-cell development in lymphoid tissues, a process involving somatic immunoglobulin-gene hypermutation during transit through a germinal centre. However, this is difficult to reconcile with the finding that EBV-infected B cells in tonsils from patients with infectious mononucleosis (IM) localize to extrafollicular areas — not to germinal centres — and show no evidence of ongoing hypermutation within expanding clones. An alternative view therefore envisages infection of pre-existing memory cells as a direct route into memory; this is consistent with the above observations on IM tonsils, but still leaves unexplained the apparent disappearance of the infected naive cell population. b | Persistent infection. The reservoir of EBV-infected memory B cells becomes subject to the physiological controls governing memory-B-cell migration and differentiation as a whole. Occasionally, these EBV-infected cells might be recruited into germinal-centre reactions, entailing the activation of different latency programmes, after which they might either re-enter the reservoir as memory cells or commit to plasma-cell differentiation — possibly moving to mucosal sites in the oropharynx and, in the process, activating the viral lytic cycle. Virions produced at these sites might initiate foci of lytic replication in permissive epithelial cells, allowing low-level shedding of infectious virus in the oropharynx, and might also initiate new growth-transforming latency III infections of naive and/or memory B cells; these new infections might possibly replenish the B-cell reservoir, but are more likely to be efficiently removed by the now well-established memory-T-cell response.

EBV infection and Burkitt’s lymphoma A model for the evolution of Burkitt's lymphoma. Epstein–Barr virus (EBV) infects a resting B cell, expresses multiple viral genes, including EBV-encoded RNAs (EBERs), Epstein–Barr nuclear antigen (EBNA)-1, EBNA2, latent membrane protein (LMP)-1 and LMP2a, and induces the cell to proliferate. Some infected cells evolve to be resting memory cells, in which LMP2a might be expressed [46], although recent findings indicate that LMP2a might be absent from the memory cells [47]. Most infected cells are killed by an immune-mediated cytotoxic response. Malaria is endemic where Burkitt's lymphoma is endemic and reduces the immune response to EBV, enabling more infected cells to accumulate [24]. Rarely, an immunoglobulin (Ig)–c-myc translocation occurs, which enables the cells to proliferate even when EBNA2, LMP1, and LMP2a are not expressed [23 and 25]. The absence of expression of these viral immune targets enables the EBNA1+, Ig–c-myc+ cells to proliferate, survive and further evolve to become a Burkitt's lymphoma [27, 28 and 29]. Trends in Molecular Medicine Volume 10, Issue 7 , 1 July 2004, Pages 331-336

Nasopharyngeal carcinoma (NPC) Burkitt's lymphoma is endemic in regions with mean minimum temperatures that exceed 15·5°C and yearly rainfall of higher than 50 mL, which stretch from about 10° north to 10° south of the equator.

EBV and pathogenesis of NPC

Summary of EBV aetiology of several different lymphoid and epithelial malignancies. EBV-encoded latent genes induce B-cell transformation in vitro by altering cellular gene transcription and constitutively activating key cell-signalling pathways. EBV exploits the physiology of normal B-cell differentiation to persist within the memory-B-cell pool of the immunocompetent host.

Putative life cycle of HBV Putative life cycle of HBV and HCV.   a | After entry to the cell, hepatitis B virus (HBV) nucleocapsids transport their cargo, the genomic HBV DNA, to the nucleus, where the relaxed circular DNA is converted into covalently closed circular (ccc) DNA. The cccDNA functions as the template for the transcription of four viral RNAs (of 0.7 kilobases (kb), 2.1 kb, 2.4 kb and 3.5 kb), which are exported to the cytoplasm and used as mRNAs for the translation of the HBV proteins. The longest (pre-genomic) RNA also functions as the template for replication, which occurs within nucleocapsids in the cytoplasm16. Nucleocapsids are enveloped during their passage through the endoplasmic reticulum (ER) and/or Golgi complex and are then secreted from the cell.

Human papilloma virus (HPV) 90% of cervical cancers contain HPV DNA. 4 types (HPV-16, HPV-18, HPV-31, and HPV-45) accounted for about 80% of the HPV-positive cancers. HPV-16 most common type of HPV found in cervical cancers. HPV-16 is the most common type in squamous cell cancers. HPV-18 is the predominant type in adenocarcinomas,   Copyright © 1998 - 2000 David Reznik, D.D.S. All Rights Reserved

Regions of stratified epithelium where HPV is maintained and amplified

Cancer transformation Transforming activity of HPV16 is associated with mainly E6 and E7proteins E6 and E7 are multifunctional proteins that can increase cell proliferation and survival by interfering with tumour suppressor activity.

Cancer transformation E6 binds to the E6-associated protein (E6-AP) and p53 (tumour suppressor) in a heterotrimeric complex. The result of this binding is the premature degradation of p53 through the ubiquitin pathway. E6-AP is in fact a ubiquitin protein ligase. Since one of the functions of p53 is to control the passage of cells through the G1 phase of the cell cycle, any abrogation of this activity could lead to uncontrolled cell cycle progression. E7 binds to the retinoblastoma family of tumour suppressor proteins, RB, p107 and p130. Normally, late in G1 RB is phosphorylated, and this releases transcription factors such as E2F, important for DNA synthesis. E7 can cause this same release of factors in the absence of RB phosphorylation and drive cells into unregulated S phase. E7 has also been shown to bind the AP-1 family of transcription factors and bind/compete with histone deacetylase for binding to RB

RNA viruses Unstable RNA genome prone to mutations Generates genetic diversity and escape antiviral therapy Can be oncogenic (e.g.hepatitis C virus HCV)

hepatitis C virus HCV Affects 3% of global population Infects primarily hepatocytes 50-80% of infected individuals go on to develop hepatocellular carcinoma (HCC) At least 6 genotypes known Putative life cycle of HBV and HCV.   b | After entry to the cell, hepatitis C virus (HCV) nucleocapsids are delivered to the cytoplasm, where the viral RNA functions directly as an mRNA for translation of a long polyprotein. Replication occurs within cytoplasmic, membrane-associated replication complexes in a perinuclear membranous web24. Genomic RNA-containing plasmids bud through intracellular membranes into cytoplasmic vesicles, which fuse with the plasma membrane. E, envelope protein; HBeAg, HBV e antigen; HBsAg, HBV surface antigen; HBx, HBV X protein; NS, non-structural protein; POL, polymerase

HCV life cycle Putative life cycle of HBV and HCV.   b | After entry to the cell, hepatitis C virus (HCV) nucleocapsids are delivered to the cytoplasm, where the viral RNA functions directly as an mRNA for translation of a long polyprotein. Replication occurs within cytoplasmic, membrane-associated replication complexes in a perinuclear membranous web24. Genomic RNA-containing plasmids bud through intracellular membranes into cytoplasmic vesicles, which fuse with the plasma membrane. E, envelope protein; HBeAg, HBV e antigen; HBsAg, HBV surface antigen; HBx, HBV X protein; NS, non-structural protein; POL, polymerase

What causes hepatocellular carcinoma? Current hypothesis is HBV and HCV infection HBV integrate into genome and a protein Hbx is known to cause HCC HCV does not integrate into the genome but can interact with host proteins and cause an inflammatory response, which can transform cells e.g. HCV proteins NS3 and NS5A can disrupt transcription factors leading to proliferation and inhibition of apoptosis Putative life cycle of HBV and HCV.   b | After entry to the cell, hepatitis C virus (HCV) nucleocapsids are delivered to the cytoplasm, where the viral RNA functions directly as an mRNA for translation of a long polyprotein. Replication occurs within cytoplasmic, membrane-associated replication complexes in a perinuclear membranous web24. Genomic RNA-containing plasmids bud through intracellular membranes into cytoplasmic vesicles, which fuse with the plasma membrane. E, envelope protein; HBeAg, HBV e antigen; HBsAg, HBV surface antigen; HBx, HBV X protein; NS, non-structural protein; POL, polymerase

Human Immunodeficiency Virus HIV

HIV life cycle See animation at http://www.roche-hiv.com/home/home.cfm

HIV genome 3 structural genes gag (group specific antigen) encodes matrix, capsid, nucleocapsid proteins pol (polymerase) encodes reverse transcriptase, integrase, protease env (envelope) encodes surface & transmembrane proteins 6 regulatory genes rev (regulatory virus protein) tat (transactivator) nef (negative regulatory factor) vif, vpr, vpu, env (envelope) encodes surface & transmembrane protein The HIV genome, transcripts and proteins.   a | HIV transcripts. Integrated into the host chromosome, the 10-kb viral genome contains open reading frames for 16 proteins that are synthesized from at least ten transcripts. Black lines denote unspliced and spliced transcripts, above which coding sequences are given, with the start codons indicated. Of these transcripts, all singly spliced and unspliced transcripts shown above those encoding the transcriptional transactivator (Tat) require regulator of virion gene expression (Rev) for their export from the nucleus to the cytoplasm. The RNA target for Rev, the Rev response element (RRE), is contained in the gene encoding envelope protein (Env).

Course of HIV infection HIV invades certain cells of the immune system--including CD4, or helper T lymphocytes or certain macrophages --replicates inside them and spreads to other cells. HIV infections start with a dramatic drop in CD4 cells (acute phase) within 3-6 weeks, followed by a steady state of viral replication (set point) at about 6 months. CD4 T cell concentrations gradually fall over a period of 8 – 10 years (chronic phase) to below 200 cells / mm3 of blood ( onset of AIDS)

Antiretroviral or anti HIV therapy All approved anti-HIV drugs attempt to block viral replication within cells by inhibiting either RT or HIV protease. Nucleoside analogues mimic HIV nucleosides preventing DNA strand completion e.g. Zidovudine (AZT), ddI, ddC, Stavudine Non nucleoside RT inhibitors (NNRTI) e.g Delavirdine and Nevirapine Protease inhibitors block active, catalytic site of HIV protease Multidrug therapy HAART (highly active antiretroviral therapy) usually consists of triple therapy including 2 nucleoside analogues + 1 protease inhibitor 1 non nucleoside RT inhibitor + 1(2) prot. inhibitor

EBV genome Diagram showing the location and transcription of the EBV latent genes on the double-stranded viral DNA episome. The origin of plasmid replication (OriP) is shown in orange. The large green solid arrows represent exons encoding each of the latent proteins, and the arrows indicate the direction in which the genes encoding these proteins are transcribed. The latent proteins include the six nuclear antigens (EBNAs 1, 2, 3A, 3B and 3C, and EBNA-LP) and the three latent membrane proteins (LMPs 1, 2A and 2B). EBNA-LP is transcribed from a variable number of repetitive exons. LMP2A and LMP2B are composed of multiple exons, which are located on either side of the terminal repeat (TR) region, which is formed during the circularization of the linear DNA to produce the viral episome. The blue arrows at the top represent the highly transcribed non-polyadenylated RNAs EBER1 and EBER2; their transcription is a consistent feature of latent EBV infection. The long outer green arrow represents EBV transcription during a form of latency known as latency III (Lat III), in which all the EBNAs are transcribed from either the Cp or Wp promoter; the different EBNAs are encoded by individual mRNAs that are generated by differential splicing of the same long primary transcript. The inner, shorter red arrow represents the EBNA1 transcript, which originates from the Qp promoter during Lat I and Lat II. Transcripts from the BamHIA region can be detected during latent infection, but no protein arising from this region has been definitively identified. The locations of the BARF0 and BARF1 coding regions are shown here. c | Location of open reading frames for the EBV latent proteins on the BamHI restriction-endonuclease map of the prototype B95.8 genome. The BamHI fragments are named according to size, with A being the largest. Lowercase letters indicate the smallest fragments. Note that the LMP2 proteins are produced from mRNAs that splice across the terminal repeats (TRs) in the circularized EBV genome. This region is referred to as Nhet, to denote the heterogeneity in this region due to the variable number of TRs in different virus isolates and in different clones of EBV-infected cells

EBV-encoded nuclear antigen 2 (EBNA2) The EBV-encoded nuclear antigens.   a | Epstein–Barr virus (EBV)-encoded nuclear antigen 2 (EBNA2) functions as a transcriptional activator by interacting with the DNA-binding J -recombination-binding protein (RBP-J ) and relieving the transcriptional repression that is mediated by a large multiprotein complex consisting of SMAT, SIN3A, histone deacetylase 1 (HDAC1) and HDAC2 (Refs 4, 17, 18). SKIP (Ski interacting protein) is another RBP-J -interacting protein that also interacts with the SMRT–HDAC corepressor complex. EBNA2 abolishes RBP-J mediated repression by competing for the SMRT–HDAC corepressor complex through binding to both RBP-J and SKIP130. b | The acidic domain of EBNA2 then recruits the basal transcription machinery (TFIIB, TFIIH and p300; not shown) to activate transcription. EBNA-LP cooperates with EBNA2 in RBP-J -mediated transcriptional activation by interacting with the acidic activation domain of EBNA2 (Ref. 4). The EBNA3 family of proteins modulate EBNA2-mediated RBP-J activation by interacting with RBP-J and competing for binding and activation by EBNA2. The RBP-J homologue in Drosophila is involved in signal transduction from the Notch receptor, a pathway that is important in cell-fate determination in Drosophila and has also been implicated in the development of T-cell tumours in humans131. EBNA2 can functionally replace the intracellular region of Notch132. BTM, basal transcription machinery; CIR, CBFI (RBP-J )-interacting corepressor; SAP30, SIN3-associated protein 30.

latent membrane protein 1 Structure and function of LMP1.   The Epstein–Barr virus latent membrane protein 1 (LMP1) is an integral membrane protein of 63 kDa and can be subdivided into three domains: first, an amino-terminal cytoplasmic tail (amino acids 1–23), which tethers LMP1 to the plasma membrane and orientates the protein; second, six hydrophobic transmembrane loops, which are involved in self aggregation and oligomerization (amino acids 24–186); third, a long carboxy-terminal cytoplasmic region (amino acids 187–386), which possesses most of the signalling activity of the molecule. Two distinct functional domains referred to as C-terminal activation regions 1 and 2 (CTAR1 and CTAR2) have been identified on the basis of their ability to activate the nuclear factor- B (NF- B) transcription-factor pathway133. The signalling effects of LMP1 result from the ability of tumour necrosis factor receptor (TNFR)-associated factors (TRAFs) to interact either directly with CTAR1 or indirectly by interacting with the death-domain-containing protein TRADD, which binds to CTAR2 (Ref. 37). These adaptor proteins subsequently recruit a multiprotein catalytic complex containing the NF- B-inducing kinase (NIK) and the I B kinases (IKKs). This results in the activation of both the classic I B -dependent NF- B pathway (involving p50–p65 heterodimers) and the processing of p100 NF- B2 to generate p52–p65 heterodimers134. Other kinases are recruited to LMP1 through interactions with TRAF molecules including the mitogen-activated protein kinase kinase kinases (MAPKKKs) TPL2 and TAK1, and these contribute to the activation of the NF- B, MAPK and phosphatidylinositol 3-kinase (PI3K) pathways. ERK, extracellular signal-regulated kinase; JNK, c-JUN amino-terminal kinase.

LMP2 Structure and function of LMP2.   The structures of the Epstein–Barr virus (EBV) latent membrane proteins LMP2A and LMP2B are similar; both have 12 transmembrane domains and a 27-amino-acid cytoplasmic carboxyl terminus. In addition, LMP2A has a 119-amino-acid cytoplasmic amino-terminal domain that contains eight tyrosine residues, two of which (Tyr74 and Tyr85) form an immunoreceptor tyrosine-based activation motif (ITAM)38. The phosphorylated ITAM recruits members of the SRC family of protein tyrosine kinases and the SYK tyrosine kinase and negatively regulates their activities. A membrane-proximal tyrosine residue (Tyr112) binds the LYN tyrosine kinase and mediates the constitutive phosphorylation of the other tyrosine residues in LMP2A38. The LMP2A ITAM blocks signalling from the B-cell receptor (BCR) by sequestering these tyrosine kinases and by blocking the translocation of the BCR into lipid rafts135. LMP2A also recruits NEDD4-like ubiquitin protein ligases through phosphotyrosine (PY) motifs, and these promote the degradation of LYN and LMP2A by a ubiquitin-dependent mechanism136. LMP2A interacts with the extracellular signal-regulated kinase 1 (ERK1) mitogen-activated protein kinase (MAPK), and this results in the phosphorylation of two serine residues (Ser15 and Ser102) in LMP2A, and might contribute to LMP2A-induced activation of JUN137. MAPKKK, MAPK kinase kinase; PI3K, phosphatidylinositol 3-kinase.