Oncogenic Viruses

C H A P T E R

18 Oncogenic Viruses Manoj Kumar, Kumari Seema, Ashok Kumar Sharma, Amber Prasad, Nikesh Sinha, Zulfiquar Ali Bhuttoo and Poonam Kumari Department of Microbiology, Rajendra Institute of Medical Sciences, Ranchi, Jharkhand, India

INTRODUCTION, HISTORICAL ASPECT, AND CLASSIFICATION The first observations about a possible infectious etiology of cancer arose at the beginning of the past century. Ellermann and Bang in 1908 and Rous in 1911 transmitted avian leukemias and sarcomas, respectively, through cell-free tumor extracts, suggesting a viral etiology (Rous, 1910, 1911; Ellerman and Bang, 1908). About 50 years later the first human tumor virus was discovered. Sir Anthony Epstein, Bert Achong, and Yvonne Barr observed viral particles in cell cultures from equatorial African pediatric patients with Burkitt’s lymphoma; this virus was named Epstein
Barr virus (EBV) in honor of their discoverers (Epstein et al., 1964). In the following years a set of experimental evidence demonstrated that EBV was the causative agent of endemic Burkitt’s lymphoma and other neoplasias. Currently, there is clear evidence that several viruses are oncogenic to humans and the first century of tumor virology research has culminated with the Medicine Nobel Price granted to Harald zur Hausen for the discovery of HPV as the causative agent of cervical cancer (Durst et al., 1983; Boshart et al., 1984). To date, EBV, Kaposi’s sarcoma-associated herpesvirus (KSHV), human high-risk papillomaviruses (HPV), Merkel cell polyomavirus (MCPV), hepatitis B virus (HBV), hepatitis C virus (HCV), and human T-cell lymphotropic virus type 1 (HTLV-1) have been classified as type

Emerging and Reemerging Viral Pathogens DOI: https://doi.org/10.1016/B978-0-12-819400-3.00018-1

375

© 2020 Elsevier Inc. All rights reserved.

376

18. ONCOGENIC VIRUSES

1 carcinogenic agents by the International Agency for Research on Cancer (Moore and Chang, 2010). It is estimated that infections are responsible for up to 15% of cancer cases worldwide and about 20% in the developing countries (Parkin, 2006). With the advent of new technologies allowing genetic identification, it is very likely that these numbers will continue to increase. Virus-mediated oncogenesis results from the cooperation of multiple events, including different mechanisms bound to the viral life cycle. The knowledge derived from the study of tumor viruses has allowed the construction of a conceptual biological framework to understand not only cancers of infectious origin but also of almost any type of cancer. However, to change the traditional scientific thinking to accept the participation of infectious agents in cancer was difficult, mostly because the biological processes involved do not adjust to the causation dogmatic principles postulated by Koch (1876). Koch original observations about the transmission of acute infectious agents are difficult to apply to cancer because of the multifactorial nature of cancer and because tumorigenic viruses are generally present in a large part of the population without causing disease. Cancer develops from a succession of genetic changes that cumulatively provide growth advantage for the cancerous cells. Despite the heterogeneity of human cancers in cellular origin, etiology, and pathogenesis, they more or less share the same characteristics self-sufficiency in growth stimulation, insensitivity to antigrowth signals, evasion of apoptosis, infinite lifespan, sustained angiogenesis, tissue invasion and metastasis ability, and genetic instability. These cellular changes may be inherited genetically or acquired as a combinational aftermath of environmental mutagens and infectious agents, such as viruses (Hanahan and Weinberg, 2000). The initial observation that viruses may cause cancer was made in 1908 when it was found that cell-free filtrates could transfer leukemia from one chicken to another (Ellerman and Bang, 1908). This observation was reinforced in 1911 when Peyton Rous demonstrated that healthy chickens injected with cell-free sarcoma filtrates, in which only viruses could be present, developed sarcoma (Peyton, 1911). It is now clear that the causative infectious agent in chicken leukemia and Rous sarcoma are both retroviruses, with the former named avian leukosis virus and the latter named Rous sarcoma virus. The viruses that can induce tumors were reaffirmed in the 1950s and 1960s, when three additional DNA viruses, simian vacuolating virus 40 (SV40), mouse polyomavirus, and adenovirus, were found to cause tumors in newborn rodents (Girardi et al., 1962; Stewart et al., 1958; Trentin et al., 1962). These three different viruses, however, did not cause tumors in their natural hosts. The early research on retroviruses and DNA tumor viruses, which include the aforementioned SV40, mouse polyomavirus, and adenovirus, has generated much valuable information for understanding viral tumorigenesis and cancer biology. The discovery of EMERGING AND REEMERGING VIRAL PATHOGENS

INTRODUCTION, HISTORICAL ASPECT, AND CLASSIFICATION

377

transducing retroviruses, which carry an oncogene (v-onc) in their genome, led to the discovery of cellular homologs (c-onc) of these viral oncogenes (Stehelin et al., 1976). Retroviruses are now known to induce tumors through three different mechanisms. The first mechanism involves transducing retroviruses. These viruses induce tumors at a high frequency and require a short latency period. The second mechanism involves cis-acting retroviruses. These cis-acting retroviruses do not carry a cellular oncogene. However, they can activate cellular oncogenes through the insertion of the provirus into the cellular chromosomes (Hayward et al., 1981). This mechanism induces tumors at an intermediate frequency and requires a longer latency period. The third mechanism involves trans-acting retroviruses. These viruses encode regulatory proteins to affect cell growth and death. These viruses induce tumors at a low frequency and often require a long-latency period. The research on DNA tumor viruses also led to a better understanding of viral oncogenesis and facilitated the later research on human oncogenic viruses. SV40 and mouse polyomavirus, which both belong to the Polyomaviridae family, encode early gene products that are capable of causing tumors. These gene products were thus termed tumor (T) antigens. The large T antigen of SV40 and mouse polyomavirus is a multifunctional protein that can immortalize primary cells, bind to the tumor suppressor p53 to different degrees and to the retinoblastoma protein RB, and activate gene expressions to promote cell-cycle progression (Conzen and Cole, 1995; Imperiale and Major, 2006). The mouse polyomavirus middle T antigen, which is absent in SV40, possesses additional activities (Ichaso and Dilworth, 2001). This middle T antigen can bind to and constitutively activate c-src, an important tyrosine kinase that activates cellular signaling pathways, and can also serve as a substrate of c-src to activate phosphotyrosine-binding proteins, such as phosphoinositol-3-kinase and phospholipase C-γ (Dilworth, 2002). SV40 and mouse polyomavirus also produce a small T antigen that can enhance the transformation activity of the virus (Khalili et al., 2003). Similarly, the research on adenovirus led to the finding that it’s E1A and E1B proteins have oncogenic potential. These two proteins can bind to RB and p53, respectively. Of particular interest is the highly oncogenic Ad12 adenovirus strain. The E1A protein of this particular adenovirus strain can suppress the expression of the major histocompatibility complex class I antigen and inhibit the cytotoxic T lymphocyte response (Schrier et al., 1983; Vasavada et al., 1986). This indicates that the modulation of the immune system is also important for viral oncogenesis. Despite the research progresses on animal tumor viruses, conclusive evidence on a similar viral etiology in human malignancies was not available until 1964, when EBV was identified in B-cell lines derived from Burkitt’s lymphoma samples (Epstein et al., 1964). The causal relationship between a viral infection and human cancer has always been EMERGING AND REEMERGING VIRAL PATHOGENS

378

18. ONCOGENIC VIRUSES

difficult to determine due to many reasons: the long incubation period between the infection and the onset of tumors, relative rarity of cancer cases among the virus-infected population, requirement of other potent and nonviral-related cofactors during oncogenesis, and inconsistent oncogenic potentials among different serological strains of the same virus (Henderson, 1989). To date nearly 20% of human cancer cases worldwide can be attributed to viral infections (Farrell, 2002). Human oncogenic viruses consist of both RNA and DNA viruses and encompass several different taxonomic groups. Human oncogenic RNA viruses include HCV and HTLV-1 and human oncogenic DNA viruses that include HBV, HPV, EBV, and KSHV. The possibility that polyomaviruses may also cause human cancers has also been raised (Khalili et al., 2003), although this topic remains highly controversial (Feng et al., 2008; Lowe et al., 2007; Poulin and DeCaprio, 2006). Recently, Merkel cell carcinoma, a rare skin cancer, was found to contain DNA from a previously unknown polyomavirus (Liu et al., 2005). The DNA of this polyomavirus, which has since been named MCPV, is found integrated in the chromosomes of 80% of Merkel cell carcinoma tissues (Cohen et al., 2007), indicating that this virus may play a role in Merkel cell carcinogenesis. Further research, however, will be required to confirm this possibility. For all the human oncogenic viruses, oncogenic transformation is not the prerequisite for the production of viral progeny. Neither is the productive viral replication required for cellular transformation. In addition, the fact that only a small fraction of the virus-infected population develops malignancy after a long-latency period indicates that viral infection may only contribute to some of the steps of carcinogenesis.

HISTORICAL TIMELINE OF DISCOVERY • 1908: Vilhelm Ellerman and Olaf Bang, University of Copenhagen, first demonstrated that avian sarcoma leukosis virus could be transmitted after cell-free filtration to new chickens, causing leukemia. • 1910: Peyton Rous at Rockefeller extended Bang and Ellerman’s experiments to show filtrable cell-free transmission of a solid tumor sarcoma to chickens (now known as Rous sarcoma). The reasons why chickens are so receptive to such transmission may involve unusual characteristics of stability or instability as they relate to endogenous retroviruses. • 1933: Richard Edwin Shope discovered cottontail rabbit papillomavirus or Shope papillomavirus, the first mammalian tumor virus. • 1936: Mouse mammary tumor virus shown by John J. Bittner to be an “extrachromosomal factor” (i.e., virus) transmitted among laboratory

EMERGING AND REEMERGING VIRAL PATHOGENS

HISTORICAL TIMELINE OF DISCOVERY

•

•

•

•

• •

•

•

379

strains of mice by breastfeeding. This was an extension of work on murine breast cancer caused by a transmissible agent as early as 1915, by A. F. Lathrop and L. Loeb. 1954
59: Ludwik Gross, working at the Bronx VA medical center isolated murine polyomavirus, which caused a variety of salivary gland and other tumors in specific strains of newborn mice, subsequently confirmed by Sarah Stewart and Bernice Eddy. 1961: SV40 discovered by Eddy at NIH, and Hillman and Sweet at Merck laboratory as a rhesus macaque virus contaminating cells used to make of Salk and Sabin polio vaccines. Several years later, it was shown to cause cancer in Syrian hamsters, raising concern about possible human health implications. Scientific consensus now strongly agrees that this is not likely to cause human cancer. 1964: Anthony Epstein, Bert Achong, and Yvonne Barr identify the first human oncovirus from Burkitt lymphoma cells. A herpesvirus, this virus is formally known as human herpesvirus type 4 (HHV4) but more commonly called Epstein
Barr virus or EBV. Mid-1960s: Baruch Blumberg first physically isolated and characterized hepatitis B while at NIH and later Fox Chase Laboratory, receiving the 1976 Nobel Prize in medicine or physiology. Although this agent was the clear cause of hepatitis and might contribute to liver cancer hepatocellular carcinoma (HCC), this link was not firmly established until epidemiologic studies were performed in the 1980s by R. Palmer Beasley and others. 1980: Human T-lymphotropic virus 1 (HTLV-1), the first human retrovirus, was discovered by Bernard Poiesz and Robert Gallo at NIH and Mitsuaki Yoshida and coworkers in Japan. 1984
86: Harald zur Hausen, together with Lutz Gissman, discovered first HPV16 and then HPV18 responsible for approximately 70% of cervical cancers. For discovery that human papillomaviruses (HPV) cause human cancer, zur Hausen won a 2008 Nobel Prize. 1987: HCV was discovered by panning a cDNA library made from diseased tissues for foreign antigens recognized with patient sera. This work was performed by Michael Houghton at Chiron, a biotechnology company, and D.W. Bradley at CDC. Controversy erupted when Chiron claimed all rights to the discovery although the work had been performed under contract with the CDC using Bradley’s materials and ideas. Eventually, this was amicably settled. HCV was subsequently shown to be a major contributor to liver cancer (HCC) worldwide. 1994: Patrick S. Moore and Yuan Chang (a husband and wife team then at Columbia University) working together with Frank Lee and Ethel Cesarman isolated Kaposi sarcoma-associated herpesvirus

EMERGING AND REEMERGING VIRAL PATHOGENS

380

18. ONCOGENIC VIRUSES

(KSHV or HHV-8) using representational difference analysis. This search was prompted from the work by V. Beral, T. Peterman, and H. Jaffe who showed from accumulating evidence from the epidemic of Kaposi sarcoma associated with AIDS, that this cancer must have another infectious cause besides HIV itself. This agent was predicted to be a new virus. Subsequent studies revealed that KSHV is indeed the “KS agent” and is responsible for the epidemiologic patterns of KS and related cancers. • 2008: Chang and Moore, now at the University of Pittsburgh Cancer Institute, developed a new method to identify cancer viruses based on computer subtraction of human sequences from a tumor transcriptome, called digital transcriptome subtraction (DTS). DTS was used to isolate DNA fragments of MCPV from a Merkel cell carcinoma, and it is now believed that this virus causes 70%
80% of these cancers. This is the first polyomavirus to be well established as the cause for a human cancer.

Classification of Oncogenic Viruses DNA virus

Associated cancer types

HBV

Hepatocarcinoma

Herpes simplex 1 and 2

Cervical neoplasia

Cytomegalovirus

Cervical neoplasia

Kaposi’s sarcoma-associated herpesvirus (HHV-8)

Kaposi’s sarcoma, multicentric Castleman’s disease, and primary effusion lymphoma

EBV

Burkitt’s lymphoma, Hodgkin’s lymphoma, hyperlink “https://en.wikipedia.org/wiki/Posttransplant_lymphoproliferative_disease” Posttransplant lymphoproliferative disease and nasopharyngeal carcinoma

HPV

The types 16 and 18 are associated with cancers of genital tract and oropharyngeal cancer

RNA virus

Associated cancer types

Merkel cell polyomavirus (MCV)

Merkel cell carcinoma

HCV

HCV is a known carcinogen, causing hepatocarcinoma

Human virus (HTLV)

T-lymphotropic adult T-cell leukemia

EBV, Epstein
Barr virus; HBV, hepatitis B virus; HCV, hepatitis C virus; HHV-8, human herpesvirus type 8; HPV, human papillomaviruses; HTLV, human T-cell lymphotropic virus.

EMERGING AND REEMERGING VIRAL PATHOGENS

MECHANISMS OF VIRAL ONCOGENESIS

381

GENERAL PRINCIPLES OF VIRAL ONCOGENIC MECHANISMS Oncogenic viruses generally maintain chronic infections in which there is not or little production of viral particles, and that last for the whole life of the infected individual. These mechanisms of viral persistency and/or latency are biologically compatible with the carcinogenic process, because they avoid cell death most common in acute lytic infections, while maintaining the infectious agent hidden from the immune system. Viral persistence in the host is achieved by integrating the viral genome into the cell genome or by expressing viral proteins that equally segregate the viral genome into daughter cells during cell partitioning. Both mechanisms ensure that the virus is not lost during cellular replication. Viral persistence is usually characterized by the expression of proteins that control cell death and proliferation; in this manner, oncogenic viruses nurture infection of a controlled number of cells establishing a balance between virus and host, preserving the integrity of both. Cell transformation is probably not an evolutionary viral strategy, but rather a biological accident that rarely occurs in the virus
host interaction. Cancer leads to the death of the host, and thus, it also represents the end of the virus. The existence of viral oncogenes is explained as part of the viral persistence mechanisms, which only under altered conditions may lead to cancer. All virus-associated tumors result from the cooperation of various events, involving more than persistent infection and viral transformation mechanisms. Additional oncogenic hits are necessary for full-blown transformation. The occurrence of mutations impairing expression and function of viral and/or cellular oncogenes is necessary in the carcinogenic process, in line with that, an increased mutation rate of infected over normal cells is frequently observed. In this scenario, latently infected cells by oncogenic viruses might be more susceptible targets of additional oncogenic hits, for example, due to smoking, a diet scarce in fruits and vegetables or/and increased exposure to environmental oncogenic agents. All these insults, plus the host genetic component driving inflammatory responses triggered by the infection itself, result in the cell transformation and cancer development (Table 18.1).

MECHANISMS OF VIRAL ONCOGENESIS Direct and Indirect Viral Carcinogenesis Infectious agents can contribute to carcinogenesis by direct and/or indirect mechanisms (Fig. 18.1). The direct-acting carcinogenic agents

EMERGING AND REEMERGING VIRAL PATHOGENS

382 TABLE 18.1

18. ONCOGENIC VIRUSES

Concepts in Cancer Biology Advanced by Viral Carcinogenesis.

1 Cellular origin of viral oncogenes—oncogenes carried by transforming retroviruses are derived from cellular genes 2 Genetic basis of cancer—oncogenes involved in cancer development are aberrant versions of normal cell genes that function in growth control 3 Multistep carcinogenesis—multiple genetic changes are required for tumor development 4 Positive and negative regulatory genes as cancer genes—both proto-oncogenes and tumor suppressor genes are altered in cancer cells 5 Unification of molecular basis of cancer—different classes of carcinogens affect the same regulatory network of cellular growth control pathways

FIGURE 18.1 Direct mechanisms of viral carcinogenesis. After infecting target cells, tumor viruses are persistently maintained as genetic elements; viral genomes can form episomes (upper panel example, herpesviruses) or integrate into the host genomic DNA (lower panel example, retroviruses and HBV).

are generally found in a monoclonal form within the tumor cells. These agents help to keep the tumor phenotype through the expression of either viral or cellular oncogenes. Retroviruses, whose replication cycle requires the integration of the viral genome into the host genome, commonly transform because integration deregulates expression of cellular oncogenes or tumor suppressor genes. On the other hand, EBV is an example of a virus that does not need to integrate and transforms through the expression of its own oncogenes.

EMERGING AND REEMERGING VIRAL PATHOGENS

MECHANISMS OF VIRAL ONCOGENESIS

383

The indirect transforming viruses are not conditioned to exist within the cell that forms the tumor. These agents act through two main mechanisms: (1) triggering chronic inflammation and oxidative stress that persistently damage local tissues and (2) by producing immunosuppression that reduces or eliminates antitumor immune surveillance mechanisms (Fig. 18.2). Among the most documented viral agents belonging to the first group are HBV and HCV; chronic inflammation produced by persistent infection associated with any of these viruses is a major risk to develop HCC (Coleman, 2003; Zucman-Rossi and Laurent-Puig, 2007). On the other hand, HIV belongs to the second group; patients with noncontrolled infection and low T-cell counts frequently develop lymphomas associated with EBV or Kaposi Sarcoma Virus (KSV) infection (Chadburn et al., 2013).

FIGURE 18.2 Indirect mechanisms of viral carcinogenesis. (A) Chronic inflammation. Infected cells produce chemokines attracting immune cells, which establish a chronic inflammatory microenvironment that persistently damage the local tissue. Cancer evolves within this cycle of infection, induced inflammation and tissue damage. (B) Immunosuppression. The prototype agent for immunosuppression is HIV. In immunocompetent individuals EBV infection is efficiently controlled by cytotoxic CD8 T cells; as HIV infection progresses and immune responses collapse, individuals become at increased risk of developing EBV associated lymphomas.

EMERGING AND REEMERGING VIRAL PATHOGENS

384

18. ONCOGENIC VIRUSES

TABLE 18.2 Oncogenesis Is Achieved by the Following Manner. Mechanisms of virus-induced cellular transformation 1 Perturbation of signaling pathways a Mimicking the signaling ligands b Mimicking the cellular signaling receptors c Mimicking the intracellular signaling adaptors d Activation of cell-surface receptors 2 Deregulation of the cell cycle a Abrogation of the RB function b Enhancement of CDK activities c Targeting of cyclin 3 Escape of apoptosis a Inactivation of the “gatekeeper” p53 b Expression of viral version of Bcl-2 4 Immortalization of cells 5 Induction of genetic instability 6 Insertional mutagenesis 7 Induction of chronic inflammation

Tumorigenic viruses were previously considered either exclusively direct or indirect transforming agents. However, some agents may require both mechanisms to induce carcinogenesis, for instance, HBV and HCV (Coleman, 2003; Zucman-Rossi and Laurent-Puig, 2007). The oncogenic mechanisms used by different viruses differ significantly. We will summarize some of the common mechanisms in this section (Table 18.2). Perturbation of Signaling Pathways The behaviors of cells are subjected to regulations by external stimuli. These stimuli are transmitted inside the cell often through specific receptors either on the cell surface or inside the cell. These stimuli are then further passed, integrated, furcated, and interpreted by intracellular signaling networks for cell growth, death, differentiation, or selfrenewal. Human oncogenic viruses have evolved ways to perturb these signal pathways to result in cellular transformation. Some of the common mechanisms are listed as follows: 1 2 3 4

Mimicking the signaling ligands, Mimicking the cellular signaling receptors, Mimicking the intracellular signaling adaptors, and Activation of cell-surface receptors.

EMERGING AND REEMERGING VIRAL PATHOGENS

HUMAN ONCOGENIC VIRUSES AND ASSOCIATED CANCERS

385

Mimicking the Signaling Ligands Cytokine interleukin (IL)-6 plays an important role in many human malignancies, especially those with a B-cell origin, in a paracrine or autocrine manner. KSHV encodes an IL-6 analog (vIL-6) that has about 25% sequence homology with cellular IL-6 (cIL-6) and can functionally substitute for cIL-6 to support the proliferation of IL6-dependent B9 cell proliferation (Moore et al., 1996).

HUMAN ONCOGENIC VIRUSES AND ASSOCIATED CANCERS DNA Oncogenic Viruses Hepatitis B Virus In 1965, Baruch S. Blumberg discovered the Australia antigen, later known as the HBV surface antigen, when working with the serum from an Australian aborigine. He soon recognized its relationship with hepatitis and developed a serological test for it (Blumberg et al., 1965, 1967). However, HBV was not identified until 1970 (Dane et al., 1970), and the viral genome was sequenced in 1979 (Galibert et al., 1979). The Hepadnaviridae family groups a series of viruses that cause liver disease in animals, with HBV infecting humans. HBV is an enveloped virus with an approximate 3.2 kb genome of a partially double-stranded DNA chain and a single-stranded fragment. HBV replicates through an intermediary RNA via a viral reverse transcriptase (RT). The main target of infection by HBV is the hepatocyte, and infection can occur through vertical or horizontal transmission starting in the first years of life or during adulthood (Kew, 2010; Moore et al., 1996). As stated HBV genome is around 3.2 kb in length and presents a highly compact genetic organization with four overlapping openreading frames (ORFs) that cover the entire genome. The pre-S/S ORF encodes the three viral surface proteins, the pre-C/C ORF encodes the Hepatitis B envelope antigen (HBeAg) and the core antigen (HBcAg), the P ORF encodes the terminal protein, and the viral polymerase that possesses DNA polymerase, RT, and ribonuclease H (RNase H) activities. The X gene encodes a small protein that is essential for virus replication but whose function remains partially understood. Finally, a viral protein termed Hepatitis B Spliced Protein (HBSP) has been shown to be encoded by a spliced viral transcript (Soussan et al., 2000). The initial phases of hepatocyte infection, namely, virion attachment, uncoating, and entry, remain poorly defined, and a cell-surface receptor has not been identified so far. These limitations may be explained by the low

EMERGING AND REEMERGING VIRAL PATHOGENS

386

18. ONCOGENIC VIRUSES

efficiency of HBV infection in few available in vitro systems. In the absence of candidate receptor, a less conventional hypothesis has been proposed, in which scavenging liver sinusoidal endothelial cells, rather than hepatocytes themselves, mediate the initial uptake of the viral pathogen into the liver (Breiner et al., 2001). After viral entry the nucleocapsid is released into the cytoplasm and transported along the microtubules to the nuclear membrane. Nuclear import of the nucleocapsid through the nuclear pore complex appears to be mediated by importins α and β, and it is followed by disintegration of the nucleocapsids and liberation of the HBV genome in the nuclear pore basket (Rabe et al., 2003). In the nucleus the relaxed circular, partially duplexed DNA genome, is converted into a covalently closed circular DNA (cccDNA) (Beck and Nassal, 2007; Mason et al., 1980; Weiser et al., 1983). The cccDNA serves as template for transcription of all viral RNAs, including the pregenomic RNA (pgRNA) as well as subgenomic RNAs. This step is critical for amplification of the viral progeny and for maintaining productive infection in the host. Four promoters and two enhancers that are preferentially active in hepatocytes regulate the transcription of viral RNAs, and the HBx protein has been reported to play an essential role in activating HBV transcription (Tang et al., 2005). It has been shown that the cccDNA is organized into a minichromosome harboring a chromatin-like structure in the nuclei of infected cells (Bock et al., 1994). Few studies led to demonstrate that HBV replication is regulated by the acetylation status of the cccDNA-bound H3/H4 histones, and that H3/ H4 acetylation status is tightly correlated with the level of viremia in the infected patient. The regulatory protein, HBx, has been found to play a key role in HBV transcription and replication. Early studies have shown that HBx stimulates the activity of viral promoters and enhancers, and that the closely related virus Woodchuck Hepatitis Virus (WHV) deficient for the expression of WHx cannot replicate in the animal host (Chen et al., 1993; Zoulim et al., 1994). More recently, it was found that the replication of X-deficient HBV genomes was strongly compromised in established cell lines and in the mouse liver, and that HBx provided in “trans” was able to restore HBV replication to wildtype levels. In most studies, this effect has been attributed to the transactivator activity of nuclear HBx protein. In the cytoplasm the pgRNA is selectively packaged into progeny capsids and reverse transcribed by the viral polymerase into minus strand DNA, followed by the synthesis of relaxed circular DNA (RC-DNA). Capsids containing RC-DNA can be either recycled for intracellular cccDNA amplification (Ikeda et al., 2009) or assemble with the viral surface proteins in the endoplasmic reticulum (ER) to form the viral particles that will be released from the cell (Collado et al., 2007; Fattovich et al., 2008). So far, inspection of the

EMERGING AND REEMERGING VIRAL PATHOGENS

HUMAN ONCOGENIC VIRUSES AND ASSOCIATED CANCERS

387

viral genome has not led to identify a dominant viral oncogene, and accordingly, introduction of the HBV genome into primary cells in culture or into transgenic murine models has not provided convincing evidence of cell transformation or liver tumorigenesis. Typically, the initial phase of chronic HBV infection is characterized by HBeAg positivity, high levels of HBV DNA in serum, and variable elevation of serum alanine aminotransferase (ALT). However, in a proportion of HBeAg-positive patients, ALT levels are normal, and liver damage is minimal despite high levels of HBV replication. This state referred to as the “immune tolerant phase” is frequent after perinatal infections, and it can persist for years. The “immune-active phase” also referred as “chronic hepatitis B phase” is typified by elevated ALT levels, low levels of HBV DNA in serum, and moderate-to-severe liver necroinflammation that may progress to fibrosis. While this phase can occur several years after experiencing the “immune tolerant phase” in perinatally infected patients, it can be detected shortly after infection during adulthood. Large cell dysplasia has been frequently found in advanced chronic hepatitis B with increased activity. It involves hepatocytes with senescent features and no sign of proliferative or apoptotic activity and develops during periods of frequent neoplastic transformation, thus raising the possibility that it might occur as a safeguard against the development of HCC (Honda et al., 2001). Senescence in tumorigenesis is regarded as a defense mechanism, and evasion of senescence is thought to lead to malignancy (Sumi et al., 2003). Among patients in the “immune-active phase” a majority eventually progress into the “inactive HBV carrier phase,” characterized by the loss of HBeAg, seroconversion to anti-HBe antibodies, and decline of serum HBV DNA to barely detectable levels, as well as improvement of fibrosis and inflammation over time. However, up to one-third of patients will undergo seroconversion to anti-HBe antibodies and transition to the “HBeAg-negative chronic hepatitis B state,” characterized by periodic reactivation associated with fluctuating levels of ALT and HBV DNA, and active hepatitis with variable degree of fibrosis. This transition has been attributed to the occurrence of nucleotide substitutions in the precore and/or the basal core promoter (BCP) region that preclude the expression of the e antigen. Emergence of these HBV variants is associated with active liver disease, and with a high risk of developing cirrhosis and HCC. While a majority of liver cancers develop in cirrhotic livers, a significant fraction of HBV-related HCCs occurs in a background of chronic hepatitis B in the absence of liver cirrhosis. In addition, distinctive gene expression profiles have been detected in the nontumoral livers of chronic HBV carriers, such as activated expression of genes implicated

EMERGING AND REEMERGING VIRAL PATHOGENS

388

18. ONCOGENIC VIRUSES

in proapoptotic, inflammatory, and DNA repair responses, suggesting specific pathways triggered by chronic hepatitis B (Zoulim et al., 1994; Yotsuyanagi et al., 2002). Thus, besides the effects of host immune responses, HBV replication might trigger various signaling cascades in the infected hepatocyte. Integration of viral, host, and environmental parameters with molecular changes might lead to identify novel targeted strategies to improve the management of chronic hepatitis B. Risk scores have been established to estimate the risk of developing HCC in less than 10 years after presentation. Such scores based on age, gender, HBV DNA levels, core promoter mutations and cirrhosis, can be used to identify high-risk patients for treatment and screening of HCC.

Chronic HBV infection and hepatocarcinogenesis. HBV DNA integration into the host genome and persistent expression of viral proteins such as HBx and Hepatis B Virus large surface protein (LHBs) can activate cellular cancer-related genes and induce oxidative stress and genetic instability. In the inflammatory context triggered by host immune responses, the viral functions contribute to ceaseless hepatocyte destruction
regeneration and provide a favorable ground for emergence of genetic and epigenetic alterations leading to hepatocyte transformation. HBV, Hepatitis B virus.

Hepatitis B Virus Mutants and Genotypes During the progression of chronic hepatitis B from the asymptomatic carrier state to HCC, mutations accumulate in the viral genome in the preS region and in the carboxy-terminal region of the X gene, which overlaps with the enhancer II and the BCP (Sumi et al., 2003). Whether the risk of developing HCC may be influenced by the viral status of the patients, such as HBV genotypes, HBeAg serostatus, and mutations arising during chronic infections are important issues. It has been shown

EMERGING AND REEMERGING VIRAL PATHOGENS

HUMAN ONCOGENIC VIRUSES AND ASSOCIATED CANCERS

389

that these parameters play important role, not only in the progression of chronic liver disease but also in the response to antiviral therapies (Zoulim et al., 1994; Yotsuyanagi et al., 2002; Rickinson and Kieff, 1996). The HBx protein encoded by the X gene has been designated “viral oncoprotein,” this protein has been involved in liver cell transformation because of its pleiotropic activities on cell cycle regulation, signaling pathways, and DNA repair. The viral protein has also been shown to cooperate with Ras in the transformation of primary human fibroblasts, either by activating the phosphatidylinositol-3-kinase and Akt pathway or by overcoming Ras-induced senescence. A weak tumorigenicity has been attributed to HBx in transforming growth factor-alpha (TGF-α) immortalized murine hepatocytes, but no cooperation could be evidenced mutated p53.

Multiple biological activities of HBx. HBx binds various transcription factors, coactivators, and components of the basal transcription machinery and participates directly in the control of viral and cellular transcription. HBx can also regulate transcription indirectly by acting on cellular signaling pathways such as NF-κB, MAPKs, and JAK/STAT. Moreover, HBx may play a role in protein degradation and in apoptosis via its interaction with proteasome subunits, mitochondrial proteins, DDB1 and p53.

Studies of HBx oncogenicity in transgenic mice have yielded divergent results that may be related to different murine genetic backgrounds and various promoters yielding different expression levels of the viral

EMERGING AND REEMERGING VIRAL PATHOGENS

390

18. ONCOGENIC VIRUSES

protein. HCC development has been associated with high-level hepatic expression of HBx in a transgenic mouse line generated in the outbred CD-1 background. In most other transgenic lines in different backgrounds, expression of HBx by itself did not lead to HCC development, although slight histopathologic alterations could be observed in the liver.

Herpes Simplex Virus Herpesviruses are enveloped viruses with double-stranded linear DNA that after infecting the host cell remain in the nucleus as episomes (Henle et al., 1969). Both EBV and KSHV show a biphasic life cycle consisting of a latent and a lytic phase. The latent phase seems to be the primary choice in which most of viral gene expression is shut down. This phase allows these viruses to coexist with the host generally asymptomatically and only in unusual situations may cause disease, for example, during pharmacological or HIV-induced immunosuppression. The lytic phase occurs in healthy individuals only in poorly understood sporadic events of reactivation. EBV, also known as HHV4, is found in approximately 95% of the adult population worldwide (Gerber et al., 1969); its principal routes of transmission are oral and blood (Hoagland, 1955; Klein et al., 2010). Early acquisition of this agent does not cause disease but when primary infection occurs during adolescence or early adulthood, it causes infectious mononucleosis. B cells are the main target of EBV infection (Cinatl et al., 2004) more rarely and less understood; EBV can also infect epithelial cells, mainly in the upper digestive tract, which is thought to occur in viral reactivation events. EBV has mainly been associated with the malignancies of B and epithelial cells of the upper digestive tract, which provides biological plausibility and coherence to the role of EBV in these neoplasias.

CYTOMEGALOVIRUS Human cytomegalovirus (HCMV) is a ubiquitous herpes virus that leads to a life-long persistence. The frequency of infection ranges from 50% to 100% in the general adult population. HCMV causes severe and often fatal disease in immunocompromised individuals, including recipients of organ transplants and AIDS patients. It routinely reactivates in healthy virus carriers, but this is usually controlled by the host immune response (Sinclair, 2008; So¨derberg-Naucle´r, 2008; Geder et al., 1976).

EMERGING AND REEMERGING VIRAL PATHOGENS

CYTOMEGALOVIRUS

391

Monocytes may be an important reservoir for latent HCMV; however, the primary reservoir may be a more primitive cell from the myeloid lineage. Reactivation may result from cellular differentiation or inflammation. The relationship between HCMV infection and cancer has been investigated for decades. Detection of viral DNA, mRNA, and/or antigens in tumor tissues as well as seroepidemiologic evidence suggested a role of HCMV infection in the etiology of several human malignancies. In the 1970s the group of Fred Rapp reported HCMV to transform normal human embryonal cells in vitro (Geder et al., 1977; Jiang et al., 2008). It is generally accepted that infection of normal permissive cells with HCMV does not result in malignant transformation (cells actively expressing virus no longer divide and eventually die), so that the virus is not considered to be oncogenic.

Influence of Human Cytomegalovirus on Cancer Cell Apoptosis Resistance to apoptosis is a common feature of cancer cells and represents a relevant chemoresistance mechanism (Park et al., 2008; Plati et al., 2008; Zhu et al., 1995). The first study on HCMV infection and apoptosis revealed that HCMV protects fibroblasts from apoptosis induced by adenovirus E1A protein (Zhu et al., 1995). Moreover, the HCMV IE1-72 and IE2-86 proteins inhibited apoptosis induced by the adenovirus E1A and TNF-α but not by irradiation with UV light in cervix carcinoma HeLa cells. The authors speculated that HCMV exerts its antiapoptotic effects through IE proteins by both p53-independent and p53-dependent mechanisms. In fact, other investigators revealed that, in some cells, HCMV IE2-86 binds to p53, inhibits its transactivating function, and protects from p53-mediated apoptosis under conditions that would otherwise induce the pathway. IE2-86 inhibited doxorubicininduced apoptosis in smooth muscle cells, indicating that IE2-86 can suppress p53-mediated apoptosis after DNA damage (Tanaka et al., 1999). Furthermore, ts13 cells with a temperature-sensitive mutation in TAFII250 do not undergo p53-dependent apoptosis when IE2-86 is expressed, and the cells are grown at the nonpermissive temperature. Direct antiapoptotic activity of HCMV proteins was ascribed mainly to some distinct transcripts encoded by the HCMV UL36
UL38 genes. The product of UL36, a viral inhibitor of caspase activation, binds to the prodomain of caspase-8, thus inhibiting Fas-mediated apoptosis (Lukac and Alwine, 1999; McCormick, 2008; Michaelis et al., 2004; Skaletskaya et al., 2001; McCormick et al., 2003). The UL37 gene product, UL37 exon 1, a viral mitochondrial inhibitor of apoptosis, inhibits the recruitment

EMERGING AND REEMERGING VIRAL PATHOGENS

392

18. ONCOGENIC VIRUSES

of the proapoptotic endogenous Bcl-2 family member Bax and Bak to mitochondria, resulting in their functional neutralization. Notably, cervix carcinoma HeLa cells expressing the viral mitochondrial inhibitor of apoptosis were resistant to apoptosis triggered by doxorubicin. The HCMV UL38 gene encodes a protein that protects infected cells from apoptosis induced by a mutant adenovirus lacking the antiapoptotic E1B-19K protein or by thapsigargin, which disrupts calcium homeostasis in the ER. In cells infected with wild-type virus, pUL38 was shown to interact with tuberous sclerosis tumor suppressor protein complex, resulting in a failure to regulate the mammalian target of rapamycin complex 1. Viral 2.7-kb noncoding RNA (β2.7) inhibited apoptosis in infected U373 glioma cells subjected to mitochondrial stress by stabilizing the mitochondrial respiratory chain complex. HCMV infection was shown to protect tumor cells from apoptosis by the induction of cellular proteins, including AKT, Bcl-2, and Np73α. HCMV binding to its cellular receptors, such as integrins, initiates activation of AKT through the phosphatidylinositol-3-kinase (PI3K) pathway. Moreover, stable expression of the IE1-72 protein was sufficient to sustain increased AKT activity in glioblastoma cells independently of virus receptor signaling. Recently, HCMV glycoprotein B was shown to bind to the platelet-derived growth factor receptor (PDGFR) and to initiate activation of AKT through the PI3K pathway in different cell types, including U87 glioma cells. Furthermore, HCMV activated AKT by selective phosphorylation of the upstream nonreceptor cellular kinase focal adhesion kinase at Tyr397, in glioblastoma and prostate carcinoma cell lines. Activation of other signaling pathways including those mediated by mitogen-activated protein kinase (MAPK) kinase (MEK) 1/2 or the c-Jun NH2-terminal kinase may be induced through viral regulatory proteins and/or binding of HCMV glycoproteins to PDGFR or virus coreceptors, including integrins and Toll-like receptor 2. These complex events may result in gene transcription, which alters apoptotic responses and other malignant properties of tumor cells. Increased expression of the antiapoptotic protein Bcl-2 and decreased sensitivity to chemotherapeutic agents were reported in HCMV-infected neuroblastoma and colon carcinoma cell lines. Other observations showed that HCMV infection induced accumulation of the Np73α isoform in glioblastoma and neuroblastoma cell lines (Allart et al., 2002; Blaheta et al., 2004). HCMVinduced Np73α exerted a dominant negative effect on p73α- and p53dependent apoptosis in both p53-negative and p53 wild-type tumor cells. In persistently infected neuroblastoma cells, HCMV decreased the expression of p73 with a concomitant increase in N-myc expression, suggesting that HCMV could also enable neuroblastoma cells to escape the apoptotic properties of p73 through N-myc
dependent pathway.

EMERGING AND REEMERGING VIRAL PATHOGENS

PAPILLOMAVIRUSES

393

Signaling pathways activated by HCMV binding to the PDGFR and/or by HCMV IE proteins that may contribute to oncomodulation by HCMV. Akt, Murine thymoma viral (vakt) oncogene homolog-1 (protein kinase B); ERK, extracellular signal
regulated kinase; HCMV, human cytomegalovirus; IE, immediate early; IKK, inhibitor of NF-κBkinase; JNK, c-jun N-terminal kinase; MEK, MAPK/ERK kinase; MEKK, MEK kinase; mTOR, mammalian target of rapamycin; NF-κB, nuclear factor κB; p70S6K, 70-kDa ribosomal S6 kinase; PI3K, phosphatidylinositol-3-kinase; PDGFR, platelet-derived growth factor receptor.

PAPILLOMAVIRUSES Human papilloma viruses belong to the Papillomaviridae family; they contain a double-strand DNA genome of approximately 8000 bp and are not enveloped viruses. More than 100 members of this family have been described, and from them, more than a dozen (types 16, 18, 31, 33, 35, 45, 51, 52, 56, 58, 59, 62, 66, and 68) have been classified as high risk due to their epidemiological association with cervical and other cancers (Zur Hausen, 2008). HPV subtypes 16 and 18 are the most frequently found in tumors; the first is mainly associated with invasive cervical cancer, and the second is the most frequent in squamous cell carcinoma (Ghittoni et al., 2010). Low-risk HPVs generally cause benign lesions, such as warts. HPV is transmitted by skin contact, including genital contact during sexual intercourse; thus HPV infection in the

EMERGING AND REEMERGING VIRAL PATHOGENS

394

18. ONCOGENIC VIRUSES

genital area tents to be common in sexually active persons. Infection is generally controlled by the immune system, and only in a low number of people, HPV persists, increasing the risk to develop epithelial lesions. Viral persistence seems to be greatly helped by the inability of infected cells to present antigenic epitopes to adaptive immune cells, which is common in individuals with alterations in the human leukocyte antigen presentation pathway (Bollag et al., 2000). The neoplastic progression involves a series of histological changes that have been stratified in clinical stages, which correlate with differential expression of viral oncogenes and accumulation of mutations in the host genome. The main oncogenic proteins are E6 and E7, which are required since the first lesions and are necessary for the maintenance of the malignant phenotype. HPV is usually not integrated into the host genomic DNA, and E2 negatively regulates the expression of E6 and E7. An important event in the oncogenic process is the integration of the viral genome, a step usually resulting in the loss of E2 and overexpression of E6 and E7. Increased expression of E6 and E7 correlates with progression to high-grade lesions and eventually to carcinoma in situ.

RNA ONCOGENIC VIRUSES Polyomaviruses The human polyomaviruses, polyomavirus hominis 1 virus (BKV) and John Cunningham Virus (JCV), were first isolated 30 years ago, BKV from the urine of a renal transplant patient and from the brain of a patient with progressive multifocal leukoencephalopathy, a neurodegenerative disease in which glial cells, the cells that produce myelin, are destroyed. These viruses share much in common with their well-studied simian cousin, SV40, but each also has its own unique features that distinguish it from the monkey virus. In this review, I will provide a brief overview of the biology of the two human viruses, an analysis of their transforming potential and their oncoproteins, the T antigens, and a discussion of the possible role of these viruses in human cancer JCV and BKV are small, nonenveloped particles with an icosahedral capsid consisting of three viral proteins, VP1, VP2, and VP3, and containing a double-stranded, circular chromosome that is approximately 5.4 kb in length. They enter the cell

EMERGING AND REEMERGING VIRAL PATHOGENS

RNA ONCOGENIC VIRUSES

395

through an as-yet undetermined receptor and are transported to the nucleus where the DNA is uncoated, and the viral transcription program begins. The early transcription unit is the first to be expressed and produces a series of alternatively spliced mRNAs that encode the major tumor antigens of the virus, the most well studied of which are large T antigen and small t antigen. These two proteins are identical in their amino termini but have distinct carboxy termini as a result of the use of alternative RNA splice acceptors. A number of other minor species, also derived by differential premRNA splicing, have been reported in JCV and BKV (Trowbridge and Frisque, 1993; Weber et al., 2001). One of these, a 17 kDa protein, called T that shares its first 131 amino acids with T antigen and then terminates after 4 or 5 unique amino acids, has also been identified in SV40. Polyomavirus genomes do not encode replication proteins and so they must drive cells into S phase where host DNA replication proteins are produced. A major mechanism by which this is achieved is through the action of the early proteins large T-Ag and small t-Ag (discussed below). T-Ag interferes with two tumor suppressor proteins that regulate cell cycle progression, pRb and p53. This aberrant stimulation of the cell cycle is a driving force for oncogenic transformation. pRb sequesters the E2F family of transcription factors that promote the G1/ S phase transition, thereby preventing expression of E2F-dependent genes, such as c-fos and cmyc whose products are needed for entry into S phase. During the cell cycle, CDK4 and CDK6 cause phosphorylation of pRb in late G1 allowing release of the E2F transcription factors allowing S phase progression (Cress and Nevins, 1996). The T-Ags of all three polyomaviruses bind to pRb and displace E2F, thereby promoting cell cycle progression. This is a major mechanism whereby T-Ag promotes the inappropriate cell proliferation characteristic of oncogenically transformed cells (Harris et al., 1996; DeCaprio et al., 1988; Krynska et al., 1997; Tavis et al., 1994). The Rb-binding domain of T-Ag is highly conserved between JCV and BKV but not between JCV/ BKV and SV40 perhaps explaining the lower affinity for pRb exhibited by JCV and BKV T-Ags compared to SV40 T-Ag (Tellinghuisen and Rice, 2002). p53 protein is a tumor suppressor encoded by a gene whose disruption is associated with 50%
55% of all human cancers. p53 protein acts as a checkpoint in the cell cycle and regulates apoptosis. The release of E2F from pRb by T-Ag activates p14ARF expression that leads to the stabilization of p53. However, polyomavirus T-Ags bind to and inactivate p53 and thus prevent inhibition of the cell cycle or apoptosis.

EMERGING AND REEMERGING VIRAL PATHOGENS

396

18. ONCOGENIC VIRUSES

Functions of large T-Ag. Large T-Ag binds to the retinoblastoma virus susceptibility protein, pRb, causing the release of the transcription factor E2F that is responsible for inducing the expression of many genes involved in the advancement of the cell cycle. However, E2F also induces p14ARF gene expression. p14ARF protein interferes with the activity of mouse double minute 2 homolog (MDM2) that is in turn a negative regulator of p53. T-Ag, T antigen.

Hepatitis C Virus HCV is a member of the Flaviviridae family of enveloped, positivestrand RNA viruses and is the only member of the genus Hepacivirus (Waleswski et al., 2001). The HCV genome consists of an RNA molecule, of approximately 9.6 kb, that contains a large ORF flanked by structured 50 and 30 nontranslated regions (NTRs). Viral proteins are translated as a polyprotein precursor from an internal ribosome entry site located in the 50 NTR. The polyprotein undergoes a complex series of co- and posttranslational cleavage events catalyzed by both host and viral proteinases to yield the individual HCV proteins. The structural proteins include the core protein and the envelope glycoproteins, E1 and E2. The nonstructural proteins include the P7 polypeptide, the NS2-3 autoprotease, and the NS3 serine protease, an RNA helicase located in the Cterminal region of NS3, the NS4A polypeptide, the NS4B and NS5A proteins, and the NS5B RNA-dependent RNA polymerase (Waleswski et al., 2001). An additional HCV protein, F (for frameshift protein) or

EMERGING AND REEMERGING VIRAL PATHOGENS

RNA ONCOGENIC VIRUSES

397

ARFP (for alternate reading frame protein), generated by an overlapping reading frame in the core (C) protein coding sequence, has been proposed (Xu et al., 2001; Varaklioti et al., 2002; Lohmann et al., 1999; Guo et al., 2001; Fukutomi et al., 2005; Pavio et al., 2005).

HCV life cycle. The HCV genome consists of an RNA molecule, of approximately 9.6 kb. Viral proteins are translated as a polyprotein precursor from an IRES located in the 50 NTR. HCV, Hepatitis C virus; IRES, internal ribosome entry site; NTR, nontranslated region. The polyprotein undergoes a complex series of co- and posttranslational cleavage events catalyzed by both host and viral proteinases to yield the individual HCV proteins. The nonstructural proteins form the viral replicase complex. Once released from NS2, the amino-terminal domains of the NS3 proteins serve as serine proteinases for the release of the remaining nonstructural proteins from the polyprotein. Viral RNA replication is thought to occur in the perinuclear membrane. The replication complex contains the viral polymerase, helicase as well as a number of host-cell factors.

The study of HCV life cycle and replication has been limited by the lack of efficient cell culture systems that support high-level HCV replication, and HCV molecular biology has mainly been addressed by Guo et al. (2001), Fukutomi et al. (2005), Pavio et al. (2005), and Thimme et al. (2002) expressing individual viral proteins. The development of efficient cell culture HCV RNA replication systems based on the replicon technology has allowed to one perform molecular genetic studies to determine the function of individual HCV proteins during replication. Replicons are self-replicating HCV genomic RNA molecules. Replicons have been instrumental in defining the cellular localization and topology of the HCV nonstructural proteins.

EMERGING AND REEMERGING VIRAL PATHOGENS

398

18. ONCOGENIC VIRUSES

The structural proteins, namely C, E1, and E2, are cleaved from the polyprotein by the ER signal peptidases and, after maturation in the ER, are assembled into the progeny virions in internal membrane compartments. E2 is also responsible for binding the putative hostcell receptor, including the CD81 tetraspanin. The function of the small hydrophobic p7 protein, located in the polyprotein at the junction of the structural and nonstructural proteins, is unknown. The nonstructural proteins form the viral replicase complex. The NS2 protein, together with the amino-terminal region of the NS3 protein, constitutes the NS2
NS3 proteinase, which catalyzes a single autocatalytic cleavage between NS2 and NS3. Once released from NS2, the amino-terminal domains of the NS3 proteins serve as serine proteinases for the release of the remaining nonstructural proteins from the polyprotein. The carboxy-terminal region of the NS3 protein has RNA helicase activity. The NS4A protein acts as a cofactor/enhancer for the activities of NS3. The function of the hydrophobic integral membrane protein NS4B is unknown, but it interacts with the viral replicase. NS5A is a hydrophilic membrane-associated protein and exists in multiple phosphorylation states. The NS5B protein is the RNAdependent RNA polymerase. Viral RNA replication is thought to occur in the perinuclear membrane. The replication complex contains the viral polymerase, helicase as well as a number of host-cell factors. The mechanism by which HCV core regulates transcription has been proposed to be indirect, with the core protein interacting with cytoplasmic signal-transduction molecules and leading to modulation of transcription for genes dependent on these cascades. The Raf1/ MAPK pathway has consistently been reported to be activated by HCV core, resulting in relieve from serum starvation growth arrest and cell proliferation. Conflicting reports have shown both activation and repression of the NF-κB pathways by HCV core. Expression of HCV core protein has been shown to induce cell proliferation DNA synthesis, and cell-cycle progression either alone or in the context of HCV replication, which is mediated by transcriptional upregulation of growth-related genes, in particular wnt-1 and its downstream target gene, wisp-2 (Nelson et al., 1997). Abrogation of wnt-1 expression by specific small interfering RNA blunts core-mediated cell growth and, consistent with secretion of the wnt-1 protein, conditioned medium from wnt-1-transfected cells accelerated cell growth. Microarray analysis revealed significant transcriptional changes in 372 of 12,500 human genes in core protein-expressing cells, with upregulation, besides wnt-1 and wisp-2 of many genes involved in cell growth, oncogenic signaling, and cell lipid metabolism and downregulation of genes associated with immunity, cellular defense systems, and inflammatory responses.

EMERGING AND REEMERGING VIRAL PATHOGENS

RNA ONCOGENIC VIRUSES

399

HCV proteins. Genetic organization and polyprotein processing of HCV. The 9.6 kb positivestrand RNA genome is composed of a 50 NCR, a long ORF encoding a polyprotein precursor of about 3000 amino acids and a 30 NCR. The polyprotein precursor is processed into structural and nonstructural proteins by cellular and viral proteases. Solid rods denote cleavage sites of the endoplasmic reticulum signal peptidase. The open rod indicates the additional C-terminal processing of the core protein by signal peptide peptidase. Arrow heads indicate cleavages by HCV NS2-3 and NS3 proteases. Besides their roles in the viral life cycle, HCV proteins interact with many host-cell factors and impact on a wide range of cellular activities, including cell signaling, transcriptional modulation, transformation, apoptosis, membrane rearrangements, vesicular trafficking, and translational regulation. HCV, Hepatitis C virus; NCR, noncoding region, ORF, open-reading frames.

HCV core variants isolated from liver tumor but not from adjacent nontumor tissue interact with Smad3 and inhibit the TGF-β pathway (Marcellin et al., 2002), suggesting that during chronic infection, viral variants that promote cell transformation by providing clonally expanding cells with resistance to TGF-β antiproliferative effects are actively selected. Indeed, HCV exists as quasispecies in patient sera or tissues, and the switch from acute to chronic infection has been associated with a wider variety of viral quasispecies (Neuman et al., 2002). TGF-β signaling not only controls cell proliferation, differentiation, and apoptosis but also plays an important role in liver repair processes and fibrogenesis through its action on the extracellular matrix. HCV-infected patients have high levels of TGF-β, which correlate with the degree of fibrosis (Koch, 1876; Galibert et al., 1979; Kew, 2010). On the other hand, wildtype core has been shown to upregulate TGF-β expression at the transcriptional level and to induce TGF-β synthesis in hepatic stellate cells, thus promoting fibrogenesis. Induction of TGF-β would at the same

EMERGING AND REEMERGING VIRAL PATHOGENS

400

18. ONCOGENIC VIRUSES

time favor viral persistence by limiting the antiviral immune response. To reconcile these apparently conflicting findings, it has been proposed that HCV core might have a dual action on the TGF-β system depending on the phase of the disease. Early in HCV infection, HCV core would contribute to fibrogenesis by increasing TGF-β synthesis, and then, after long-term of fibrosis and inflammation and in the context of cirrhotic livers, HCV core variants that contribute to clonal cell expansion and cellular transformation by inhibiting TGF-β-dependent antiproliferative pathways may arise and be selected. The impact of other structural proteins on malignant transformation is more indirect. The E2 glycoprotein has been shown to interfere with interferon actions in vitro by inhibition of protein kinase R, an important intermediate of interferon effects. The NS3 serine protease domain alone can transform mammalian cells, although the link between this interaction and HCC is not clear (Poiesz et al., 1980). The oncogenic properties of NS3 may involve an interaction with the tumor suppressor p53. Sequences in NS3 have been shown to serve as substrates for protein kinase C (PKC) phosphorylation and can inhibit PKC signaling via competition with normal substrates and also been shown to inhibit interferon response factor-3-mediated induction of type I interferon in response to viral infection, which may be important in the ability of HCV to escape immune surveillance. Expression profile analysis of HCC samples by comparative gene expression clustering has identified a number of transcripts “upregulated” (mainly cell growth genes) or “downregulated” (mainly growth inhibition genes) in HCC tissues A number of overexpressed genes encoding for secreted (e.g., GPC3, LCN2, and DKK1) or membranebound proteins (e.g., GPC3, IGSF1, and PSK-1), which may be attractive candidates for the diagnosis of HCC, have also been identified. However, as for the study of viral genes, no specific gene signature has been found that might clearly explain as to how HBV or HCV mediates oncogenesis.

HUMAN T-LYMPHOTROPIC VIRUS Human T-lymphotropic virus type 1 (HTLV-1) is a complex leukemogenic retrovirus with a single-stranded positive sense RNA genome that expresses unique proteins with oncogenic potential. There are four known strains of HTLV, of which HTLV-1 and HTLV-2 are the most prevalent worldwide. HTLV-1 was originally identified in 1980 in a Tcell line derived from a patient with cutaneous T-cell lymphoma (Hinuma et al., 1981) and was also detected in adult T-cell leukemia (ATL) patients (Yoshida et al., 1982; Kalyanaraman et al., 1982). Subsequently, HTLV-2 was identified in a cell line derived from a

EMERGING AND REEMERGING VIRAL PATHOGENS

HUMAN T-LYMPHOTROPIC VIRUS

401

patient with a variant form of hairy T-cell leukemia (Saxon et al., 1978a, b; Boxus et al., 2008). Since then, HTLV-2 has not been associated with leukemia/lymphoma; nevertheless, it has been associated with a few sporadic cases of neurological disorders. HTLV-1 can infect T cells, B cells, monocytes, dendritic cells, and endothelial cells with equal efficiency; yet, it can transform only primary T cells. HTLV-1 is an enveloped virus that is approximately 100 nm in diameter. The inner membrane of the virion envelope is lined by the viral matrix protein (MA). This structure encloses the viral capsid (CA), which carries two identical strands of the genomic RNA as well as functional protease (Pro), integrase (IN), and RT enzymes. A newly synthesized viral particle attaches to the target cell receptor through the viral envelope (Env) and enters via fusion, which is followed by the uncoating of the capsid and the release of its contents into the cell cytoplasm. The viral RNA is reverse transcribed into double-stranded DNA by the RT. This doublestranded DNA is then transported to the nucleus and becomes integrated into the host chromosome forming the provirus. The provirus contains the promoter and enhancer elements for transcription initiation in the long terminal repeats (LTRs); the polyadenylation signal for plus strand transcription is located in the 30 LTR107. HTLVs are intracellular proviruses that pass through the formation of a “virological synapse,” allowing the viral genome to be passed from one cell to another. Once infection has occurred, little replication takes place. Infection affects the expression of T-lymphocyte gene expression, leading to increased proliferation of affected T lymphocytes. HTLV primarily affects T lymphocytes: specifically, HTLV-1 predominantly affects CD4 lymphocytes, while HTLV-2 predominantly affects CD8 lymphocytes. In vitro, HTLV-1 is also capable of infecting other cell types, possibly accounting for the diverse pathogenesis of HTLV-1. Recently, GLUT-1, a ubiquitous glucose transporter, has been identified as a receptor for HTLV-1; this may explain its ability to infect various cell types. Acute HTLV infection is rarely seen or diagnosed, as most infections are latent and asymptomatic. Infection might be diagnosed after an attempted blood donation or through workup of a disease caused by the virus. For example, HTLV-1 is associated primarily with two diseases, ATL and HTLV-1
associated myelopathy/tropical spastic paraparesis. HTLV-1 and HTLV-2 have similar transmission patterns, although the transmission efficiency of HTLV-2 is uncertain because of a lack of unbiased data gathering. Both can be transmitted via breast milk, sexual contact, and intravenous drug use, and both can be introduced directly into the vascular system. HTLV-3 and HTLV-4 seem to be transmitted through direct human contact with primates (e.g., through hunting, butchering, keeping them as pets), but data are lacking.

EMERGING AND REEMERGING VIRAL PATHOGENS

402

18. ONCOGENIC VIRUSES

HTLV-l transmission, infection, virus
host interactions, and onset of ATL. Cell-to-cell spreading is the main route of HTLV-1 transmission. Tax mainly promotes T-cell proliferation of HTLV-1-infected cells. However, other HTLV-l proteins can enhance this process. CTL cells can prevent the proliferation by killing of Tax-expressing targets cells. The expression of Tax is inactivated by several mechanisms, suggesting that Tax is not necessary in ATL stage and HB2 may be more effective. ATL, Adult T-cell leukemia; CTL, cytolytic T lymphocyte. Source: Idea from Cinatl, J., Scholz, M., Kotchetkov, R., Vogel, J.U., Doerr, H.W., 2004. Molecular mechanisms of the modulatory effects of HCMV infection in tumor cell biology. Trends Mol. Med. 10, 19
23 with modification.

On the molecular level, as with all retroviruses, HTLV has a gag
pol
env motif with flanking LTR sequences. Unique to the Deltraviruses, however, it includes a fourth sequence named Px, which participates in ORF transcription, in turn encoding for regulatory proteins Tax, Rex, p12, p13, and p30. All these proteins are important for the infectivity of cells, as well as in stimulating replication. In ATL the main pathogenic protein, Tax, leads to leukogenesis and immortalization of T lymphocytes in vitro (Feuer and Green, 2005). This is achieved by stimulation of IL-15 and IL-2, in turn leading to T-cell growth and transformation. Research on this subject is ongoing, and the expression of this gene is not always found in ATL cells. Furthermore, Tax is inherent to both HTLV-1 and HTLV-2, although HTLV-1 is more pathogenic. Recently, the HTLV-1 basic zipper (HBZ) factor gene has been found to be consistently expressed in ATL cells, suggesting a role in cellular transformation and leukemogenesis. This might correlate with the increased pathogenesis of HTLV-1. The expression of the HBZ gene also correlates with the provirus load of HTLV-1. Many studies have demonstrated that HTLV-I genes (e.g., Tax and HBZ) can induce various cellular dysfunctions, genetic and epigenetic alterations, and the host immune system may be involved in the leukemogenesis of ATL (Klein et al., 2010). Immortalization of lymphocytes infected by HTLV-I requires Tax, however, Tax-immortalized cells, but nontransformed cells are still IL-2 dependent. Therefore infected cells could not pass the G1 check point without exogenous growth factors such as IL-2. In addition the inactivation of tumor suppressor gene p53 contributes to the

EMERGING AND REEMERGING VIRAL PATHOGENS

OPPORTUNITIES AND FUTURE DIRECTIONS

403

accumulation of genetic mutations (Cinatl et al., 2004). Alternatively, alterations and errors are accumulated progressively by several viral proteins in the host genome during the latent period, finally leading to the onset of ATL (Sinclair, 2008). CD41
CD81 cells and immature CD42
CD82 cells from bone marrow can be transformed by HTLV-I. Transformed cells not only display an activated phenotype but also in some cases retain T-cell functional properties. Among the functional activities of HTLV-I, transformed cells have reported to be able to induce suppressor cell activity (So¨derberg-Naucle´r, 2008) and retain antigen-specific responses and cytotoxic function.

OPPORTUNITIES AND FUTURE DIRECTIONS It is difficult to predict what the future holds for viral oncology, as no one could have foreseen the revolutionary advances of the last 20 years that emanated from virus studies. However, several areas of opportunity and avenues of investigation seem likely to keep viral carcinogenesis at the forefront of cancer studies in the new millennium.

Molecular Mechanisms of Viral Oncogenesis The general patterns are now established of how transforming genes function and collaborate, including cellular oncogenes, tumor suppressor genes, and viral oncogenes. This outline will continue to be finetuned and more molecular details and variations added in the future. Viruses are the ultimate model systems for unraveling molecular and cellular mechanisms, and just as tumor viruses shaped modern cancer biology, it is expected that viral systems will prove equally useful in the future. We can anticipate the development of targeted interventions based on the biochemical pathways disrupted by oncoproteins and on new knowledge of the structural basis of molecular interactions. Practically any gene or protein that plays a significant role in carcinogenesis is a legitimate therapeutic target. Many new drugs are under development, such as antibodies that block the activity of cell-surface receptors and small-molecule drugs that block oncogene growth signal transmission inside the cell. The hope is that such new therapies will be more specific to cancer cells than current chemotherapies and less harmful to normal cells. The choice of therapy for a cancer patient may 1 day be guided by the molecular fingerprints of a given tumor. Studies will continue to characterize the molecular mechanisms of viral involvement in human cancers. The precise roles played by viruses in those human cancers with viral cofactors remain puzzling, and

EMERGING AND REEMERGING VIRAL PATHOGENS

404

18. ONCOGENIC VIRUSES

unraveling the molecular complexities of viral contributions is a serious challenge for the future. More attention needs to be focused on early steps in tumorigenesis. As studies of late-stage tumors cannot reveal the initiating early events, analyses of early-stage lesions are necessary in order to learn how a rare rogue cell is able to escape host control and progress into a recognizable tumor. It is possible that, on occasion, viral gene functions involved in the early stages of tumorigenesis may become redundant as growing tumor cells become more autonomous, and the viral genes may be lost from advanced tumors. Additional indirect mechanisms of action by human tumor viruses will probably be realized. There are the current indications that viral infections may affect the cellular DNA repair system, supposedly allowing the accumulation of mutations in growth regulatory genes, or that processes and reactants associated with a virus-induced inflammatory response may predispose to cancer. It is possible that other normal mechanisms of host homeostasis and response to infection, under the pressure of chronic viral replication, can go awry and promote tumor outgrowth. Finally, the hit-and-run mechanism of viral involvement in carcinogenesis should be considered a possibility. The concept that a virus can initiate the transformation process through a mutagenic mechanism and then disappear without leaving viral traces has fallen out of favor. As more examples of indirect mechanisms of viral involvement in human tumors are found, this discarded theory of viral action should be remembered and evaluated.

References Allart, S., Martin, H., Detraves, C., Terrasson, J., Caput, D., Davrinche, C., 2002. Human cytomegalovirus induces drug resistance and alteration of programmed cell death by accumulation of deltaN-p73alpha. J. Biol. Chem. 277, 29063
29068. Beck, J., Nassal, M., 2007. Hepatitis B virus replication. World J. Gastroenterol. 13, 48
64. Blaheta, R.A., Beecken, W.D., Engl, T., Jonas, D., Oppermann, E., Hundemer, M., et al., 2004. Human cytomegalovirus infection of tumor cells downregulates NCAM (CD56): a novel mechanism for virus-induced tumor invasiveness. Neoplasia 6, 323
331. Blumberg, B.S., Alter, H.J., Visnich, S., 1965. A “new” antigen in Leukemia Sera. JAMA 191, 541
546. Blumberg, B.S., Gerstley, B.J., Hungerford, D.A., London, W.T., Sutnick, A.I., 1967. A serum antigen (Australia antigen) in Down’s syndrome, leukemia, and hepatitis. Ann. Intern. Med. 66 (5), 924
931. Bock, C.T., Schranz, P., Schroder, C.H., Zentgraf, H., 1994. Hepatitis B virus genome is organized into nucleosomes in the nucleus of the infected cell. Virus Genes 8, 215
229. Bollag, B., Prins, C., Snyder, E.L., Frisque, R.J., 2000. Virology 274, 165
178. Boshart, M., Gissmann, L., Ikenberg, H., Kleinheinz, A., Scheurlen, W., zur Hausen, H., 1984. A new type of papillomavirus DNA, its presence in genital cancer biopsies and in cell lines derived from cervical cancer. EMBO J. 3, 1151
1157. Boxus, M., Twizere, J.-C., Legros, S., Dewulf, J.-F., Kettmann, R., Willems, L., 2008. The HTLV1 Tax interactome. Retrovirology 5, 76. ,https://doi.org/10.1186/1742-4690-5-76..

EMERGING AND REEMERGING VIRAL PATHOGENS

REFERENCES

405

Breiner, K.M., Schaller, H., Knolle, P.A., 2001. Endothelial cell-mediated uptake of a hepatitis B virus: a new concept of liver targeting of hepatotropic microorganisms. Hepatology 34, 803
808. Chadburn, A., Abdul-Nabi, A.M., Teruya, B.S., Lo, A.A., 2013. Lymphoid proliferations associated with human immunodeficiency virus infection. Arch. Pathol. Lab. Med. 137, 360
370. Chen, H.S., Kanako, S., Girones, R., Anderson, R.W., Hornbuckle, W.E., Tennant, B.C., et al., 1993. The woodchuck hepatitis virus X gene is important for establishment of virus infection in woodchucks. J. Virol. 67, 1218
1226. Cinatl, J., Scholz, M., Kotchetkov, R., Vogel, J.U., Doerr, H.W., 2004. Molecular mechanisms of the modulatory effects of HCMV infection in tumor cell biology. Trends Mol. Med. 10, 19
23. Cohen, S.B., Graham, M.E., Lovrecz, G.O., Bache, N., Robinson, P.J., Reddel, R.R., 2007. Protein composition of catalytically active human telomerase from immortal cells. Science 315, 1850
1853. Coleman, W.B., 2003. Mechanisms of human hepatocarcinogenesis. Curr. Mol. Med. 3, 573
588. Collado, M., Blasco, M.A., Serrano, M., 2007. Cellular senescence in cancer and aging. Cell 130, 223
233. Conzen, S.D., Cole, C.N., 1995. The three transforming regions of SV40 T antigen are required for immortalization of primary mouse embryo fibroblasts. Oncogene 11 (11), 2295
2302. Cress, W.D., Nevins, J.R., 1996. Use of the E2F transcription factor by DNA tumor virus regulatory proteins. Curr. Top. Microbiol. Immunol. 208, 63
78. Dane, D.S., Cameron, C.H., Briggs, M., 1970. Virus-like particles in serum of patients with Australia-antigen-associated hepatitis. Lancet 1 (7649), 695
698. DeCaprio, J.A., Ludlow, J.W., Figge, J., Shew, J.Y., Huang, C.M., Lee, W.H., et al., 1988. SV40 large tumor antigen forms a specific complex with the product of the retinoblastoma susceptibility gene. Cell 54, 275
283. Dilworth, S.M., 2002. Polyoma virus middle T antigen and its role in identifying cancerrelated molecules. Nat. Rev. Cancer 2 (12), 951
956. Durst, M., Gissmann, L., Ikenberg, H., zur Hausen, H., 1983. A papillomavirus DNA from a cervical carcinoma and its prevalence in cancer biopsy samples from different geographic regions. Proc. Natl. Acad. Sci. U.S.A. 80, 3812
3815. Epstein, M.A., Achong, B.G., Barr, Y.M., 1964. Virus particles in cultured lymphoblasts from Burkitt’s lymphoma. Lancet 1, 702
703. Farrell, P.J., 2002. Tumour viruses—could they be an Achilles’ heel of cancer? Eur. J. Cancer 38 (14), 1815
1816. Fattovich, G., Bortolotti, F., Donato, F., 2008. Natural history of chronic hepatitis B: special emphasis on disease progression and prognostic factors. J. Hepatol. 48, 335
352. Feng, H., Shuda, M., Chang, Y., Moore, P.S., 2008. Clonal integration of a polyomavirus in human Merkel cell carcinoma. Science 319 (5866), 1096
1100. Feuer, G., Green, P.L., 2005. Comparative biology of human T-cell lymphotropic virus type 1 (HTLV-1) and HTLV-2. Oncogene 24 (39), 5996
6004. ,https://doi.org/10.1038/sj. onc.1208971.. Fukutomi, T., Zhou, Y., Kawai, S., Eguchi, H., Wands, J.R., Li, J., 2005. Hepatology 41, 1096
1105. Galibert, F., Mandart, E., Fitoussi, F., Tiollais, P., Charnay, P., 1979. Nucleotide sequence of the hepatitis B virus genome (subtype ayw) cloned in E. coli. Nature 281 (5733), 646
650. Geder, K.M., Lausch, R., O’Neill, F., Rapp, F., 1976. Oncogenic transformation of human embryo lung cells by human cytomegalovirus. Science 192, 1134
1137.

EMERGING AND REEMERGING VIRAL PATHOGENS

406

18. ONCOGENIC VIRUSES

Geder, L., Kreider, J., Rapp, F., 1977. Human cells transformed in vitro by human cytomegalovirus: tumorigenicity in athymic nude mice. J. Natl. Cancer Inst. 58, 1003
1009. Gerber, P., Walsh, J.H., Rosenblum, E.N., Purcell, R.H., 1969. Association of EB-virus infection with the post-perfusion syndrome. Lancet 1, 593
595. Ghittoni, R., Accardi, R., Hasan, U., Gheit, T., Sylla, B., Tommasino, M., 2010. The biological properties of e6 and e7 oncoproteins from human papillomaviruses. Virus Genes 40, 1
13. Girardi, A.J., Sweet, B.H., Slotnick, V.B., Hilleman, M.R., 1962. Development of tumors in hamsters inoculated in the neonatal period with vacuolating virus, SV-40. Proc. Soc. Exp. Biol. Med. 109, 649
660. Guo, J.T., Bichko, V.V., Seeger, C., 2001. J. Virol. 75, 8516
8523. Hanahan, D., Weinberg, R.A., 2000. The hallmarks of cancer. Cell 100 (1), 57
70. Harris, K.F., Christensen, J.B., Imperiale, M.J., 1996. BK virus large T antigen: interactions with the retinoblastoma family of tumor suppressor proteins and effects on cellular growth control. J. Virol. 70, 2378
2386. Hayward, W.S., Neel, B.G., Astrin, S.M., 1981. Activation of a cellular onc gene by promoter insertion in ALV-induced lymphoid leukosis. Nature 290 (5806), 475
480. Henderson, B.E., 1989. Establishment of an association between a virus and a human cancer. J. Natl. Cancer Inst. 81 (5), 320
321. Henle, G., Henle, W., Clifford, P., Diehl, V., Kafuko, G.W., Kirya, B.G., et al., 1969. Antibodies to Epstein-Barr virus in Burkitt’s lymphoma and control groups. J. Natl. Cancer Inst. 43, 1147
1157. Hinuma, Y., Nagata, K., Hanaoka, M., Nakai, M., Matsumoto, T., Kinoshita, K.-I., et al., 1981. Adult T-cell leukemia: antigen in an ATL cell line and detection of antibodies to the antigen in human sera. Proc. Natl. Acad. Sci. U.S.A. 78, 6476
6480. Hoagland, R.J., 1955. The transmission of infectious mononucleosis. Am. J. Med. Sci. 229, 262
272. Honda, M., Kaneko, S., Kawai, H., Shirota, Y., Kobayashi, K., 2001. Differential gene expression between chronic hepatitis B and C hepatic lesion. Gastroenterology 120, 955
966. Ichaso, N., Dilworth, S.M., 2001. Cell transformation by the middle T-antigen of polyoma virus. Oncogene 20 (54), 7908
7916. Ikeda, H., Sasakia, M., Satoa, Y., Haradaa, K., Zenc, Y., Mitsuid, T., et al., 2009. Large cell change of hepatocytes in chronic viral hepatitis represents a senescent-related lesion. Hum. Pathol. 40 (12), 1774
1782. Imperiale, M.J., Major, E.O., 2006. Polyomaviruses. In: fifth ed. Knipe, D.M., Howley, P.M. (Eds.), Fields Virology, vol. 61. Lippincott Williams & Wilkins, Philadelphia, PA, pp. 2263
2298. Jiang, W., Cazacu, S., Xiang, C., Zenklusen, J.C., Fine, H.A., Berens, M., et al., 2008. FK506 binding protein mediates glioma cell growth and sensitivity to rapamycin treatment by regulating NF-kappaB signaling pathway. Neoplasia 10, 235
243. Kalyanaraman, V.S., Sarngadharan, M.G., Robert-Guroff, M., Miyoshi, I., Golde, D., Gallo, R.C., 1982. A new subtype of human T-cell leukemia virus (HTLV-II) associated with a T-cell variant of hairy cell leukemia. Science 218, 571
573. Kew, M.C., 2010. Epidemiology of chronic hepatitis B virus infection, hepatocellular carcinoma, and hepatitis B virus-induced hepatocellular carcinoma. Pathol. Biol. 58, 273
277. Khalili, K., Del Valle, L., Wang, J.Y., Darbinian, N., Lassak, A., Safak, M., et al., 2003. Tantigen of human polyomavirus JC cooperates with IGF-IR signaling system in cerebellar tumors of the childhood-medulloblastomas. Anticancer Res. 23 (3A), 2035
2041. Klein, G., Klein, E., Kashuba, E., 2010. Interaction of Epstein-Barr virus (EBV) with human B-lymphocytes. Biochem. Biophys. Res. Commun. 396, 67
73. Koch, R., 1876. Untersuchungen u¨ber bakterien: V. Die a¨tiologie der milzbrand-krankheit, begru¨ndet auf die entwicklungsgeschichte des bacillus anthracis [Investigations into bacteria: V. The etiology of anthrax, based on the ontogenesis of bacillus anthracis]. Cohns Beitr. Biol. Pflanz. 2, 277
310.

EMERGING AND REEMERGING VIRAL PATHOGENS

REFERENCES

407

Krynska, B., Gordon, J., Otte, J., Franks, R., Knobler, R., DeLuca, A., et al., 1997. Role of cell cycle regulators in tumor formation in transgenic mice expressing the human neurotropic virus, JCV, early protein. J. Cell. Biochem. 67, 223
230. Liu, B., Hong, S., Tang, Z., Yu, H., Giam, C.Z., 2005. HTLV-I Tax directly binds the Cdc20associated anaphase-promoting complex and activates it ahead of schedule. Proc. Natl. Acad. Sci. U.S.A. 102, 63
68. Lohmann, V., Korner, F., Koch, J., Herian, U., Theilmann, L., Bartenschlager, R., 1999. Science 285, 110
113. Lowe, D.B., Shearer, M.H., Jumper, C.A., Kennedy, R.C., 2007. SV40 association with human malignancies and mechanisms of tumor immunity by large tumor antigen. Cell. Mol. Life Sci. 64 (7
8), 803
814. Lukac, D.M., Alwine, J.C., 1999. Effects of human cytomegalovirus major immediate-early proteins in controlling the cell cycle and inhibiting apoptosis: studies with ts13 cells. J. Virol. 73, 2825
2831. Marcellin, P., Asselah, T., Boyer, N., 2002. Hepatology 36, S47
S56. Mason, W.S., Seal, G., Summers, J., 1980. Virus of Pekin ducks with structural and biological relatedness to human hepatitis B virus. J. Virol. 36, 829
836. McCormick, A.L., 2008. Control of apoptosis by human cytomegalovirus. Curr. Top. Microbiol. Immunol. 325, 281
295. McCormick, A.L., Skaletskaya, A., Barry, P.A., Mocarski, E.S., Goldmacher, V.S., 2003. Differential function and expression of the viral inhibitor of caspase 8
induced apoptosis (vICA) and the viral mitochondria
localized inhibitor of apoptosis (vMIA) cell death suppressors conserved in primate and rodent cytomegaloviruses. Virology 316, 221
233. Michaelis, M., Kotchetkov, R., Vogel, J.U., Doerr, H.W., Cinatl Jr, J., 2004. Cytomegalovirus infection blocks apoptosis in cancer cells. Cell. Mol. Life Sci. 61, 1307
1316. Moore, P.S., Chang, Y., 2010. Why do viruses cause cancer? Highlights of the first century of human tumour virology. Nat. Rev. Cancer 10, 878
889. Moore, P.S., Boshoff, C., Weiss, R.A., Chang, Y., 1996. Molecular mimicry of human cytokine and cytokine response pathway genes by KSHV. Science 274 (5293), 1739
1744. Nelson, D.R., Gonzalez-Peralta, R.P., Qian, K., Xu, Y., Marousis, C.G., Davis, G.L., et al., 1997. J. Viral. Hepat. 4, 29
35. Neuman, M.G., Benhamou, J.P., Bourliere, M., Ibrahim, A., Malkiewicz, I., Asselah, T., et al., 2002. Cytokine 17, 108
117. Park, D.C., Yeo, S.G., Wilson, M.R., Yerbury, J.J., Kwong, J., Welch, W.R., et al., 2008. Clusterin interacts with paclitaxel and confer paclitaxel resistance in ovarian cancer. Neoplasia 10, 964
972. Parkin, D.M., 2006. The global health burden of infection-associated cancers in the year 2002. Int. J. Cancer 118, 3030
3044. Pavio, N., Battaglia, S., Boucreux, D., Arnulf, B., Sobesky, R., Hermine, O., et al., 2005. Oncogene 24, 6119
6132. Peyton, R., 1911. Transmission of a malignant new growth by means of a cell-free filtrate. J. Am. Med. Assoc. 56, 198. Plati, J., Bucur, O., Khosravi-Far, R., 2008. Dysregulation of apoptotic signaling in cancer: molecular mechanisms and therapeutic opportunities. J. Cell. Biochem. 104, 1124
1149. Poiesz, B.J., Ruscetti, F.W., Gazdar, A.F., Bunn, P.A., Minna, J.D., Gallo, R.C., 1980. Detection and isolation of type C retrovirus particles from fresh and cultured lymphocytes of a patient with cutaneous T-cell lymphoma. Proc. Natl. Acad. Sci. U.S.A. 77, 7415
7419. Poulin, D.L., DeCaprio, J.A., 2006. Is there a role for SV40 in human cancer? J. Clin. Oncol. 24 (26), 4356
4365.

EMERGING AND REEMERGING VIRAL PATHOGENS

408

18. ONCOGENIC VIRUSES

Rabe, B., Vlachou, A., Pante´, N., Helenius, A., Kann, M., 2003. Nuclear import of hepatitis B virus capsids and release of the viral genome. Proc. Natl. Acad. Sci. U.S.A. 100, 9849
9854. Rickinson, A.B., Kieff, E., 1996. Epstein-Barr virus, Fields Virology, vol. 2. LippincottRaven Publishers, Philadelphia, PA, pp. 2397
2446. Rous, P., 1910. A transmissible avian neoplasm. (Sarcoma of the common fowl.). J. Exp. Med. 12, 696
705. Rous, P., 1911. A sarcoma of the fowl transmissible by an agent separable from the tumor cells. J. Exp. Med. 3, 397
411. Saxon, A., Stevens, R.H., Golde, D.W., 1978a. T-lymphocyte variant of hairy-cell leukemia. Ann. Intern. Med. 88, 323
326. Saxon, A., Stevens, R.H., Quan, S.G., Golde, D.W., 1978b. Immunologic characterization of hairy cell leukemias in continuous culture. J. Immunol. 120, 777
782. Schrier, P.I., Bernards, R., Vaessen, R.T., Houweling, A., van der Eb, A.J., 1983. Expression of class I major histocompatibility antigens switched off by highly oncogenic adenovirus 12 in transformed rat cells. Nature 305 (5937), 771
775. Sinclair, J., 2008. Human cytomegalovirus: latency and reactivation in the myeloid lineage. J. Clin. Virol. 41, 180
185. Skaletskaya, A., Bartle, L.M., Chittenden, T., McCormick, A.L., Mocarski, E.S., Goldmacher, V.S., 2001. A cytomegalovirus-encoded inhibitor of apoptosis that suppresses caspase-8 activation. Proc. Natl. Acad. Sci. U.S.A. 98, 7829
7834. So¨derberg-Naucle´r, C., 2008. HCMV microinfections in inflammatory diseases and cancer. J. Clin. Virol. 41, 218
223. Soussan, P., Garreau, F., Zylberberg, H., Ferray, C., Brechot, C., Kremsdorf, D., 2000. In vivo expression of a new hepatitis B virus protein encoded by a spliced RNA. J. Clin. Invest. 105, 55
60. Stehelin, D., Varmus, H.E., Bishop, J.M., Vogt, P.K., 1976. DNA related to the transforming gene(s) of avian sarcoma viruses is present in normal avian DNA. Nature 260 (5547), 170
173. Stewart, S.E., Eddy, B.E., Borgese, N., 1958. Neoplasms in mice inoculated with a tumor agent carried in tissue culture. J. Natl. Cancer Inst. 20 (6), 1223
1243. Sumi, H., Yokosuka, O., Seki, N., Arai, M., Imazeki, F., Kurihara, T., et al., 2003. Influence of hepatitis B virus genotypes on the progression of chronic type B liver disease. Hepatology 37, 19
26. Tanaka, K., Zou, J.P., Takeda, K., Ferrans, V.J., Sandford, G.R., Johnson, T.M., et al., 1999. Effects of human cytomegalovirus immediate-early proteins on p53-mediated apoptosis in coronary artery smooth muscle cells. Circulation 99, 1656
1659. Tang, H., Delgermaa, L., Huang, F., Oishi, N., Liu, L., He, F., et al., 2005. The transcriptional transactivation function of HBx protein is important for its augmentation role in hepatitis B virus replication. J. Virol. 79, 5548
5556. Tavis, J.E., Trowbridge, P.W., Frisque, R.J., 1994. Converting the JCV T antigen Rb binding domain to that of SV40 does not alter JCV’s limited transforming activity but does eliminate viral viability. Virology 199, 384
392. Tellinghuisen, T.L., Rice, C.M., 2002. Curr. Opin. Microbiol. 5, 419
427. Thimme, R., Bukh, J., Spangenberg, H.C., Wieland, S., Pemberton, J., Steiger, C., et al., 2002. Proc. Natl. Acad. Sci. U.S.A. 99, 15661
15668. Trentin, J.J., Yabe, Y., Taylor, G., 1962. The quest for human cancer viruses. Science 137, 835
841. Trowbridge, P.W., Frisque, R.J., 1993. Virology 196, 458
474. Varaklioti, A., Vassilaki, N., Georgopoulou, U., Mavromara, P., 2002. J. Biol. Chem. 277, 17713
17721.

EMERGING AND REEMERGING VIRAL PATHOGENS

FURTHER READING

409

Vasavada, R., Eager, K.B., Barbanti-Brodano, G., Caputo, A., Ricciardi, R.P., 1986. Adenovirus type 12 early region 1A proteins repress class I HLA expression in transformed human cells. Proc. Natl. Acad. Sci. U.S.A. 83 (14), 5257
5261. Waleswski, J.L., Keller, T.R., Stump, D.D., Branch, A.D., 2001. RNA 7, 710
721. Weber, F., Goldmann, C., Kramer, M., Kaup, F.J., Pickhardt, M., Young, P., et al., 2001. Cellular and humoral immune response in progressive multifocal leukoencephalopathy. Ann. Neurol. 49, 636
642. Weiser, B., Ganem, D., Seeger, C., Varmus, H.E., 1983. Closed circular viral DNA and asymmetrical heterogeneous forms in livers from animals infected with ground squirrel hepatitis virus. J. Virol. 48, 1
9. Xu, Z., Choi, J., Yen, T.S., Lu, W., Strohecker, A., Govindarajan, S., et al., 2001. EMBO J. 20, 3840
3848. Yoshida, M., Miyoshi, I., Hinuma, Y., 1982. Isolation and characterization of retrovirus from cell lines of human adult T-cell leukemia and its implication in the disease. Proc. Natl. Acad. Sci. U.S.A. 79, 2031
2035. Yotsuyanagi, H., Hino, K., Tomita, E., Toyoda, J., Yasuda, K., Iino, S., 2002. Precore and core promoter mutations, hepatitis B virus DNA levels and progressive liver injury in chronic hepatitis B. J. Hepatol. 37, 355
363. Zhu, H., Shen, Y., Shenk, T., 1995. Human cytomegalovirus IE1 and IE2 proteins block apoptosis. J. Virol. 69, 7960
7970. Zoulim, F., Saputelli, J., Seeger, C., 1994. Woodchuck hepatitis virus X protein is required for viral infection in vivo. J. Virol. 68, 2026
2030. Zucman-Rossi, J., Laurent-Puig, P., 2007. Genetic diversity of hepatocellular carcinomas and its potential impact on targeted therapies. Pharmacogenomics 8, 997
1003. Zur Hausen, H., 2008. Papillomaviruses—to vaccination and beyond. Biochem. Biokhim. 73, 498
503.

Further Reading Lairmore, M.D., Anupam, R., Bowden, N., Haines, R., Haynes, R.A.H., Ratner, L., et al., 2011. Molecular determinants of human T-lymphotropic virus type 1 transmission and spread. Viruses 3 (7), 1131
1165. ,https://doi.org/10.3390/v3071131.. Westland, C., Delaney, W., Yang, H., Chen, S.S., Marcellin, P., Hadziyannis, S., et al., 2003. Hepatitis virus genotypes and virologic response in 694 patients in phase III studies of adefovir dipivoxil1. Gastroenterology 125, 107
116.

EMERGING AND REEMERGING VIRAL PATHOGENS