Главная Cancer-Causing Viruses and Their Inhibitors
Cancer-Causing Viruses and Their Inhibitors
Satya Prakash Gupta
''Cancer-causing viruses, also called oncoviruses, play a key role in the development of certain cancers. They contribute to genetic changes that disrupt the cell cycle machinery, interfering with functions such as cell growth. Cancer-Causing Viruses and Their Inhibitors presents a plethora of research from internationally reputed contributors who discuss different types of oncoviruses, their mechanisms of invasion and growth, and their life cycles. The book begins with an overview of the oncoviruses discovered to date and includes a brief description of their structures, genotypes, replication, and mechanisms of infection leading to cancers. It then explores several of these viruses in detail, including: Human T-cell leukemia virus type 1 (HTLV-1), Hepatitis C virus (HCV), Epstein-Barr virus (EBV), Human papilloma virus (HPV), Human herpes virus 8 (HHV-8)/Kaposi's sarcoma-associated herpes virus (KSHV), Human immunodeficiency virus (HIV/AIDS), Oncolytic viruses. This book is an essential reference for those working in virology, oncology, and biotechnology. The discoveries presented will enable researchers and clinicians to optimize both historical and current approaches to anti-viral therapies''--Provided by publisher. Read more...
Categories: Medicine\\Oncology
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Carbohydrate Chemistry: Proven Synthetic Methods, Volume 2
Pavol Kováč
Building a Programmable Logic Controller with a PIC16F648A Microcontroller
Murat Uzam
Cancer-Causing
Viruses and Their
National Institute of Technical Teachers' Training and Research (NITTTR), Bhopal, India
Version Date: 20140327
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Preface......................................................................................................................vii
About the Editor.........................................................................................................xi
Contributors............................................................................................................ xiii
Cancer-Causing Viruses: An Introduction............................................ 1
Satya P. Gupta and Vertika Gautam
Targeting Human T-Cell Leukemia Virus Type 1: Pathogenesis
and Treatment Strategies..................................................................... 21
Jun-ichirou Yasunaga
Hepatitis C Virus and Its Inhibitors: The Polymerase as a Target
for Nucleoside and Nucleotide Analogs.............................................. 41
Jerome Deval and Martin Huber
Mechanisms of Hepatitis C Virus Clearance by Interferon
and Ribavirin Combination: Lessons Learned from
In Vitro Cell Culture........................................................................... 85
Srikanta Dash, Partha K. Chandra, Kurt Ramazan,
Robert F. Garry, and Luis A. Balart
Hepatitis B Virus Infection and Hepatocellular Carcinoma............. 121
Rakhi Maiwall and Manoj Kumar Sharma
Epstein–Barr Virus and Treatment of Its Infection.......................... 157
Tarun Jha, Amit Kumar Halder, and Nilanjan Adhikari
Landscape of Papillomavirus in Human Cancers: Prevention
and Therapeutic Avenues..................................................................207
Susri Ray Chaudhuri (Guha), Anirban Roy, Indranil Chatterjee,
Rahul Roy Chowdhury, and Snehasikta Swarnakar
Role of High-Risk Human Papillomaviruses in
Breast Carcinogenesis....................................................................... 243
Ala-Eddin Al Moustafa
Prospective Therapies of Genital Human
Papillomavirus Infections................................................................. 261
Ricardo Ney Oliveira Cobucci, Janaina Cristiana de Oliveira
Crispim, and Ana Katherine da Silveira Gonçalves
Chapter 10 Human Herpesvirus Type 8/Kaposi Sarcoma Herpesvirus:
An Overview of Viral Pathogenesis and Treatment
of Associated Diseases...................................................................... 279
Hiba El Hajj, Raghida Abou Merhi, and Ali Bazarbachi
Chapter 11 Tea Polyphenolic Compounds against Herpes Simplex Viruses........ 321
Tin-Chun Chu, Sandra D. Adams, and Lee H. Lee
Chapter 12 HIV Integrase Inhibitors: Qualitative and Quantitative
Structure–Activity Relationship Studies........................................... 345
Rajni Garg and Gene M. Ko
Chapter 13 Virus Wars: Oncolytic versus Oncogenic Viruses............................407
Markus Vähä-Koskela, Fabrice Le Boeuf, and Vincenzo Cerullo
Chapter 14 Oncolytic Viruses and Histone Deacetylase Inhibitors.................... 437
Vaishali M. Patil and Satya P. Gupta
Cancer-causing viruses are also called oncoviruses or tumor viruses. Their infection
in the human body leads to cancer. According to the World Health Organization
Cancer Report 2008, cancer-causing viruses are responsible for up to 20% of cancer
deaths all over the world; approximately 20%–30% of all cancers worldwide can
be linked to viral infections. Cancer develops when cells start to divide uncontrollably because the "cell cycle machinery" that regulates this process stops working
properly. These cancer cells can then invade other tissues. Cancer development is
a complex process involving a series of genetic changes that disrupt the cell cycle
machinery, interfering with cellular functions such as cell growth. Cancer-causing
viruses play a key role in the development of certain cancers by contributing to
these genetic changes. Although cancer itself is not an infectious disease, a significant number of human viruses have been linked to specific cancers, such as adult
T-cell leukemia (ATL), hepatocellular carcinoma (HCC), Burkitt's lymphoma (BL),
nasopharyngeal carcinoma (NPC), posttransplant lymphomas, Hodgkin's disease,
cervical cancer, skin cancer in patients with epidermodysplasia verruciformis (EV),
head and neck cancers, primary effusion lymphoma, and Castleman's disease. The
involvement of these viruses in human cancer development means that the frequency
of these cancers can be reduced either prophylactically by vaccinating against the
viruses or therapeutically by treating the infections. Extensive studies of these
viruses are currently taking place with respect to their life cycle, their mechanism of
infection, the inhibition of their growth, and the development of vaccines to treat the
cancer caused by them. This book will cover in detail all of these aspects of oncoviruses, and it may be of great interest to medicinal and pharmaceutical chemists as
well as to those working in the area of biotechnology. Cancer researchers may also
find it of immense value.
There are a total of fourteen chapters in this volume. In the first chapter, Gupta
and Gautam discuss the cancer-causing viruses discovered to date and include a
brief description of their structure, genotypes, replication, and mechanisms of infection leading to cancers. The second chapter, by Yasunaga, presents the pathogenesis
of human T-cell leukemia virus type 1 (HTLV-1) as well as current and upcoming
therapeutic strategies against HTLV-1-mediated diseases. Chapter 3, by Deval and
Huber, discusses the hepatitis C virus (HCV)—which leads to hepatocellular carcinoma—and its inhibitors. The number of HCV infection cases has now reached
more than 170 million worldwide; one of the main protein targets for the generation
of HCV therapeutics is the viral RNA-dependent RNA polymerase (RdRp) NS5B.
This chapter summarizes what is known to date about the structure and function(s)
of NS5B, with a particular focus on its role in HCV replication, and it reviews the
recent advances in the development of nucleoside and nucleotide analogs as inhibitors of NS5B. Combination therapy utilizing interferon-alpha (IFN-α), ribavirin
(RBV), and a protease inhibitor is the current standard of care for HCV genotype 1
infection. In Chapter 4, Dash et al. discuss the hypothesis that a better understanding
of interferon's and ribavirin's HCV clearance mechanisms will lead to i­ mprovement
in treatment response and will reduce the burden of liver cirrhosis and primary
liver cancer. They present an overview of HCV infection clearance by treatment with type I and type III interferon (IFN), ribavirin, and other new e­ merging
antivirals. Dash et al. also review novel antiviral strategies that aim to eradicate
HCV ­infection and reduce the burden of liver cirrhosis and primary liver cancer.
Like HCV, ­hepatitis B virus (HBV) is also a leading risk factor for ­hepatocellular
­carcinoma (HCC), with more than 80% of HCC cases occurring in regions that are
also endemic for hepatitis B. In Chapter 5, Maiwall and Sharma ­discuss the factors
associated with increased risk of developing HCC in chronic HBV-infected subjects
and the preventive strategies for HBV-related HCC including the role of vaccination,
nucleoside and nucleotide analogs, and interferon.
Epstein–Barr virus (EBV) is a ubiquitous pathogen that has adopted a unique and
effective strategy for infection, persistence, and spread in its only biological host—
human beings. It infects more than 90% of the human population worldwide and
is associated with a diverse array of proliferative diseases in different parts of the
world. It leads to BL, nasopharyngeal carcinoma (NPC), posttransplant lymphomas,
and Hodgkin's disease. In Chapter 6, Jha et al. present the biology and pathophysiology of EBV and discuss currently available techniques for preventing EBV infection
as well as for treating EBV-mediated malignancies.
More than four decades have elapsed since the initial hypothesis regarding the
possible role of human papillomavirus (HPV) in the genesis of human cancers was
postulated. It is now a matter of common knowledge that HPV infection accounts for
almost 100% of cervical cancers and that HPV is responsible for 5.2% of all cancers
including 25% of head and neck cancers. In Chapter 7, Chaudhury et al. present the
structure and properties of HPV, its association with cervical and neck and head
cancers, and the prevention and therapeutics of these cancers. Several recent studies
have reported that high-risk HPVs are also present in human breast cancer tissues.
Although several investigations did not confirm this fact, in Chapter 8, Al Moustafa
presents an overview of the presence and role of high-risk HPVs in human breast
carcinogenesis and focuses on the function of E5, E6, and E7 oncoproteins of highrisk HPVs in the progression of breast cancer.
Genital infection by HPV is the most common sexually transmitted disease in
the world. Its global prevalence among women without cervical abnormalities is
12%. Chapter 9, by Cobucci et al., presents research data on promising prospective genital HPV infection therapies along with a brief description of current and
future therapies.
Another of the viruses that infect humans, human herpesvirus 8 (HHV-8)/
Kaposi's sarcoma-associated herpesvirus (KSHV), is described as a novel gamma-2
herpesvirus closely related to the human gamma-1 herpesvirus, Epstein–Barr virus
(EBV). In Chapter 10, El Hajj et al. discuss the pathogenesis of HHV-8 and the
treatment of the diseases associated with it, such as primary effusion lymphoma
and Castleman's disease. Herpes simplex virus 1 and 2 (HSV-1 and HSV-2), also
known as human herpesvirus 1 and 2 (HHV-1 and HHV-2), are two members of the
Herpesviridae virus family that infects humans. Although HSV-1 and HSV-2 are
closely related, they differ in several aspects of their pathology. HSV-1 is primarily
associated with throat and mouth diseases as well as ocular and genital infections,
while HSV-2 is the leading cause of recurring genital herpes cases worldwide. HSVs
currently have no cure, and some have developed drug resistance. However, in
Chapter 11, Chu et al. discuss the importance of tea polyphenolic compounds, such
as epigallocatechin gallate (a green tea polyphenol) and theaflavin (a black tea polyphenol), against these HSVs.
People with AIDS have a weak immune system and thus are at an increased risk
of developing infections, lymphoma, and other types of cancer. Therefore, HIV is
also considered a cancer-causing virus. The most common types of AIDS-related
cancers are Kaposi sarcoma and non-Hodgkin lymphoma. Other AIDS-related cancers include Hodgkin's disease and cancers of the lung, mouth, cervix, and digestive system. Anti-HIV drugs target multiple pathways in the life cycle of HIV and
provide a large source of potential anticancer drugs. HIV integrase (IN) is one of the
major viral targets for the development of new anti-HIV drugs. In Chapter 12, Garg
and Ko have presented a review of HIV-related cancers and repositioning efforts to
target some HIV IN inhibitors as anticancer agents as well as quantitative structure–­
activity relationship (QSAR) studies on HIV IN inhibitors that might be of great help
in the design of more effective HIV IN inhibitors relevant to cancer treatment.
Oncolytic (tumor-killing) viruses vis-à-vis oncoviruses represent a heterogeneous group of viruses that have been observed/engineered to preferentially replicate in and destroy tumor cells but not normal cells. In Chapter 13, Vähä-Koskela
et al. present a brief history of oncolytic viruses and their mechanism of action and
­clinical development. The authors assess whether there is any rationale—either
molecular or immunological—for deliberate targeting of oncovirus-associated
disease by oncolytic viruses. In this regard, they discuss a limited but expanding
number of studies that indicate that it may be feasible to deliberately infect oncovirus tumors with oncolytic viruses and that such "virus wars" may result in substantial therapeutic benefit and long-term tumor control. Whereas oncolytic viruses
have shown significant promise in cancer therapy, histone deacetylase inhibitors
have also shown great promise for inhibiting cell proliferation in various cancer
cell lines. Therefore, current research is directed toward combined use of these
approaches. A detailed description of this aspect is presented by Patil and Gupta
in Chapter 14.
In this volume, an attempt has been made to present a detailed account of
­cancer-causing viruses—their structure, genotypes, and replication; the mechanisms of infection leading to human cancers; modes of infection prevention; and the
­treatment of the cancers produced by these viruses. This book will be of great value
to those working on virology and cancers and in the area of biotechnology as well
as to general readers interested in viral infections. As the editor of this book, I have
greatly enjoyed reading all the chapters and hope the readers do as well. I gratefully
acknowledge the interest and zeal of all the authors for contributing such important,
timely, and useful material for this book.
Satya P. Gupta
National Institute of Technical Teachers' Training and Research (NITTTR)
Dr. Satya P. Gupta, formerly a renowned professor of chemistry at Birla Institute
of Technology and Science (BITS), Pilani, India, is presently a professor in the
Department of Applied Science at the National Institute of Technical Teachers'
Training and Research (NITTTR), Bhopal, India. Prior to this, he served as a distinguished professor in the Department of Pharmaceutical Technology at the Meerut
Institute of Engineering and Technology (MIET), Meerut, India. Initially, he spent
a few years at the Tata Institute of Fundamental Research (TIFR), Bombay, under
Professor Girjesh Govil, where he worked on the structure and functions of biomembranes. He later moved to BITS, where he initiated work on the quantitative
­structure–activity relationship (QSAR), a very prominent area of medicinal chemistry. Dr. Gupta developed a deep understanding of QSAR, the modes of drug–receptor
interactions, and the roles of physicochemical properties in drug–receptor interactions. Dr. Gupta earned an MSc in physical chemistry and PhD in quantum chemistry.
In 1985, Dr. Gupta was made a fellow of the National Academy of Sciences,
India, and in 1989, he was awarded the coveted Ranbaxy Research Foundation
Award. Dr. Gupta is one of the pioneers of QSAR. He has published approximately
200 papers in various journals of repute on the topic and has contributed dozens of
authoritative reviews in prestigious journals such as Chemical Reviews (American
Chemical Society), Progress in Drug Research (Birkhauser Verlag Basel), and
Current Medicinal Chemistry (Bentham Science, the Netherlands and the United
States). Dr. Gupta has also contributed several chapters in books produced by internationally reputed publishers such as J. R. Prous Science Spain, Elsevier Science
BV (the Netherlands), Springer Heidelberg, and Springer Basel. He has edited several books published by Springer Heidelberg/Basel, namely, Topics in Heterocyclic
Chemistry, Vol. 3 and 4 (2006); Ion Channels and Their Inhibitors (2011); Matrix
Metalloproteinase Inhibitors—Specificity of Binding and Structure–Activity
Relationships (2012); and Hydroxamic Acids: A Unique Family of Chemicals with
Multiple Biological Activities (2013). In addition, Dr. Gupta has been the editor-inchief of four international journals published by Bentham Science: Cardiovascular
and Hematological Agents in Medicinal Chemistry, Current Computer-Aided Drug
Design, Current Enzyme Inhibition, and Current Bioinformatics. Dr. Gupta also
serves on the editorial boards of several journals.
In 1996, Dr. Gupta's book, Quantum Biology (New Age International Publishers,
New Delhi), was highly acclaimed by theoretical chemists and biologists. His recent
book, QSAR and Molecular Modeling (published in 2011 by Anamaya, New Delhi, in
collaboration with Springer, the Netherlands), has become the most popular v­ olume
among those interested in QSAR and molecular modeling—the most ­fascinating
area of drug design. This book is a result of his vast work experience in QSAR.
Sandra D. Adams
Department of Biology and
Partha K. Chandra
Tulane University Health
Sciences Center
Nilanjan Adhikari
Natural Science Laboratory
Division of Medicinal and
Department of Pharmaceutical
Indranil Chatterjee
Department of Surgical Oncology
Saroj Gupta Cancer Centre and
Department of Oncology
Syrian Research Cancer Centre
Syrian Society against Cancer
Aleppo, Syria
Luis A. Balart
Tulane University Health Sciences Center
Ali Bazarbachi
Vincenzo Cerullo
Immunovirotherapy Group
Division of Biopharmaceutics
and Pharmacokinetics
Susri Ray Chaudhuri (Guha)
Drug Development, Diagnostics
and Biotechnology Division
CSIR–Indian Institute of Chemical
Rahul Roy Chowdhury
Department of Gynaecological Oncology
Tin-Chun Chu
Ricardo Ney Oliveira Cobucci
Potiguar University
Natal, Brazil
Ana Katherine da Silveira Gonçalves
Department of Obstetrics and
Federal University of
Srikanta Dash
Departments of Pathology and
Janaina Cristiana de Oliveira Crispim
Federal University of Rio Grande
Jerome Deval
Alios BioPharma, Inc.
Hiba El Hajj
Rajni Garg
Computational Science Research
Robert F. Garry
Vertika Gautam
National Institute of Technical
Teachers' Training and Research
(NITTTR)
Amit Kumar Halder
Humapharm
Liestal, Switzerland
Tarun Jha
Gene M. Ko
Fabrice Le Boeuf
Centre for Innovative Cancer
Ottawa Hospital Research Institute
Lee H. Lee
Department of Biology and Molecular
Rakhi Maiwall
Assistant Professor of Hepatology
Institute of Liver and Biliary Sciences
Raghida Abou Merhi
Vaishali M. Patil
Bharat Institute of Technology
Meerut, India
Kurt Ramazan
Anirban Roy
Drug Development, Diagnostics and
Biotechnology Division
CSIR–Indian Institute of
Associate Professor of Hepatology
Snehasikta Swarnakar
Markus Vähä-Koskela
Translational Virology Group
Haartman Institute
Laboratory of Virus Control
Institute for Virus Research
1 An Introduction
Cancer-Causing Viruses
Till the dawn of 19th century, viruses were classified as small infectious agents
that, unlike cells and bacteria, pass through fine-pore filters. After a decade, Peyton
Rous realized a relation between cancer and viruses. In 1966, he was awarded the
Nobel Prize in recognition for his seminal discovery of tumor-inducing viruses.
Later, a series of studies found viruses as physical and chemical initiators of cancer.
Rapid progress in tumor virology has taken place over the last 40 years. According
to the World Health Organization (WHO) cancer report 2008, some cancer-causing
viruses are responsible for up to 20% of cancer deaths worldwide. Approximately
20%–30% of all cancers can be linked to viral infections (Goedert 2000).
Cancers are the result of a disruption of the normal restraints on cellular
­proliferation. There are several cellular genes in which mutation or some other
disruption of their expression can lead to unrestrained cell growth. There are
two classes of such genes in which altered expression can lead to loss of growth
1. Oncogenes: These genes are stimulatory for growth and cause cancer when
hyperactive. Mutations in these genes will be dominant.
2. Tumor suppressor genes or anti-oncogenes: These genes inhibit cell growth
and cause cancer when they are turned off. Mutations in these genes will be
recessive.
Viruses are involved in cancers because they can either carry a copy of one of
these genes or alter expression of the cell's copy of one of them. Such viruses are
called cancer-causing viruses, oncoviruses, or tumor viruses. The majority of tumor
virus-infected individuals do not develop cancer; many years may pass between
initial infection and tumor appearance. Additional cofactors, such as host immunity and chronic inflammation, as well as additional host cellular mutations, play
an important role in the transformation process. Actually, the long-term interactions
between virus and host are the key features of the oncogenic viruses because they
set the stage for a variety of molecular events that may contribute to eventual virusmediated carcinogenesis (Cohen et al. 1999).
FAMILIES OF CANCER-CAUSING VIRUSES
Broadly, there are two different families of oncogenic viruses that depend on the
type of nucleic acid present in the viral genome and the type of strategy to induce
malignant transformation. One is the RNA virus family with two subfamilies known
as Retroviridae and Flaviviridae, and the other is the DNA virus family with the
subfamilies Hepadnaviridae, Herpesviridae, and Papillomaviridae. These two
major classes of viruses utilize diverse mechanisms in cancer development but often
have one aspect of their life cycle in common—the ability to integrate their own
genome into that of the host cell. However, such integration is not a prerequisite for
tumor formation. Moreover, tumor viruses have the ability to infect but not kill their
host cell. So, unlike other disease causing viruses, they establish long-term persistent
Tumor viruses induce oncogenesis through manipulating an array of different
­cellular pathways. These viruses initiate a series of cellular events that lead to immortalization and proliferation of the infected cells by disrupting the mitotic checkpoint
upon infection of the host cell. This is often accomplished by functional inhibition or
proteasomal degradation of many tumor suppressor proteins by virally encoded gene
products. The virally infected cells can either be eliminated via cell-mediated apoptosis
or persist in a state of chronic infection. Importantly, the chronic persistence of infection
by tumor viruses can lead to oncogenesis (Saha et al. 2010). The two ­different families
of tumor viruses or cancer-causing viruses are described as follows.
1.2.1 RNA Tumor Viruses/Retroviruses
RNA tumor viruses are retroviruses. A retrovirus is an RNA virus that replicates in a host cell through the process of reverse transcription. First, it uses its
own reverse transcriptase enzyme to produce DNA from its RNA genome—the
reverse of the usual pattern, thus, retro (backward). This new DNA is then incorporated into the host's genome by an enzyme called integrase. The cell then treats
the viral DNA as part of its own instructions, making the proteins required to
­assemble new copies of the virus (Figure 1.1). Retroviruses are enveloped viruses
that belong to the viral family of Retroviridae. The incorporation of the viral
DNA into the host's genome converts proto-oncogenes—the cellular genes that
code for components of the cellular machinery that regulates cell proliferation,
differentiation, and death—into oncogenes in two ways: the gene sequence may
be altered or truncated so that it codes for proteins with abnormal activity, or the
gene may be brought under the control of powerful viral regulators that cause its
product to be made in excess or in inappropriate circumstances. Retroviruses may
also exert similar oncogenic effects by insertional mutation when DNA copies of
the viral RNA are integrated into the host-cell genome at a site close to or even
within proto-­oncogenes. Cellular oncogenes are expressed by the cell at some
period in the life of the cell, often when the cell is growing, replicating, and differentiating normally. They are usually proteins that are involved in growth control.
Cellular oncogenes are highly conserved.
Parental RNA
RNA/DNA hybrid
Linear DNA/DNA duplex
Circular duplex DNA
Integrase
Host DNA
Host RNA pol II
Replication (DNA genome in cell)
Host splicing
Viral RNA genome
FIGURE 1.1 (See color insert.) The pathway of RNA virus infection.
So far only two RNA viruses have been recognized: human T-cell leukemia
virus of type 1 (HTLV-1) and hepatitis C virus (HCV), described in the following
sections.
1.2.1.1 Human T-Cell Leukemia Virus of Type 1
Human T-cell leukemia virus of type 1 (HTLV-1) is the first human retrovirus to
be discovered that is clearly associated with human malignancy (Poiesz et al. 1980;
Yoshida et al. 1982). It is a delta-type complex retrovirus and is the etiologic agent of
adult T-cell leukemia (ATL). It is endemic to Japan, South America, Africa, and the
Caribbean (Wong-Staal and Gallo 1985; Gessian et al. 1991). Although exact cellular events involved in the leukemogenesis of ATL by HTLV-1 are not well known, a
variety of steps, including virus, host cell, and immune factors, have been implicated
(Matsuoka 2003). Several studies, such as Pozzatti et al. (1990), have suggested that
Tax, a multifunctional viral accessory protein, is a major protein, transforming the
activities of HTLV-1 to immune modulation, apoptosis regulation, proliferation, and
genetic instability.
To date, four types of HTLVs (HTLV-1, HTLV-2, HTLV-3, and HTLV-4) have
been identified, of which HTLV-1 is the most widely studied for its association
with human malignancy. Its genome is diploid, composed of two copies of a singlestranded RNA virus whose genome is copied into a double-stranded DNA form that
integrates into the host-cell genome, at which point the virus is referred to as a provirus. The association of HTLV-1 with ATL was demonstrated by the presence of
extracellular type C viral particles in cells from a patient with ATL (Hinuma et al.
1981; Gallo 2005) that has been confirmed by further studies (Matsuoka and Jeang
2007; Boxus and Willems 2009). HTLV-1 proviral DNA can be detected in virtually
every ATL patient tested. Attempts have been made to understand the biological and
pathogenic properties of HTLV-1; this has ultimately led to the d­ evelopment of v­ arious
­experimental v­ accination and therapeutic strategies to combat HTLV-1 ­infection.
The therapeutic strategies include the development of envelope glycoprotein-derived
B-cell epitopes for the induction of neutralizing antibodies and the ­generation of a
multivalent cytotoxic T-lymphocyte (CTL) response against the HTLV-1 Tax antigen.
1.2.1.2 Hepatitis C Virus
HCV is a single-stranded RNA virus of the Hepacivirus genus in the Flaviviridae
family. It is the only positive-stranded RNA virus among the human oncogenic
viruses. The length of the HCV genome is approximately 9.6 kb with a polyprotein
of about 3,000 amino acids encoded in it. This polyprotein precursor is cleaved by
cellular and viral proteases into three structural proteins (core, E1, E2) and seven
nonstructural proteins (p7, NS2, NS3, NS4A, NS4B, NS5A, and NS5B) (Lindenbach
and Rice 2005). Both envelope proteins (E1 and E2) are highly glycosylated and
important in cell entry. E1 serves as the fusogenic subunit, and E2 acts as the
­receptor-binding protein. Of the nonstructural proteins, the NS5B protein (65 kDa)
is the viral RNA-dependent RNA polymerase, which has the key function of replicating the HCV's viral RNA by using the viral positive RNA strand as its template.
NS5B catalyzes the polymerization of ribonucleoside triphosphates (rNTP) during
RNA replication (Moradpour et al. 2007; Rigat et al. 2010).
Persistent HCV infection can lead to hepatitis, hepatic steatosis, cirrhosis, and
hepatocellular carcinoma (HCC) (Saito et al. 1990; Alter 1995; Lauer and Walker
2001; Poynard et al. 2003; Pawlotsky 2004). HCV infection also causes chronic
inflammation and fibrosis, which can progress to cirrhosis. Though the chronic
inflammation and cirrhosis are thought to play key roles in HCV-induced carcinogenesis, the exact mechanism of carcinogenesis is not well understood (Fattovich
et al. 2004). However, the immune modulation by quasispecies variation, ­apoptosis
regulation through core protein, proliferation through NS2, and genetic stability
through NS5B are thought to be the major biological activities involved in the induction of carcinogenesis by HCV (McLaughlin-Drubin and Munger 2008).
1.2.2 The DNA Tumor Viruses
The family of DNA tumor viruses consists of papilloma, polyoma, adeno, hepadna,
and herpesviruses and produces tumors of different types in various species. DNA
tumor viruses are thought to play a role in the pathogenesis of about 15%–20% of
human cancers. These include Burkitt's lymphoma (BL), nasopharyngeal carcinoma (NPC), immunoblastic lymphomas in immunosuppressed individuals, and a
proportion of Hodgkin's lymphomas that are all associated with the Epstein–Barr
virus (EBV) of the herpes family and with liver carcinoma associated with chronic
­hepatitis B virus (HBV) infection. Unlike retroviral oncogenes, DNA virus oncogenes
are essential components of the viral genome and have no counterpart in the normal
host cells. Also, they are of viral, not cellular, origin and are necessary for the replication of the virus (Damania 2007). The analysis of DNA tumor virus oncoproteins
has not only elucidated the etiology of several human diseases but also has revealed
the mechanisms controlling mammalian cell growth, ultimately leading to discovery
of a cellular tumor suppressor gene. Most of the DNA tumor viruses, such as human
­papillomavirus (HPV), HBV, EBV, and Kaposi's sarcoma-associated herpesvirus
(KSHV), cause malignancy in their natural hosts, but a few, such as human adenoviruses, transform cultured cells and cause tumors only in heterologous animal models.
The transforming ability of DNA tumor viruses has been traced to several
viral proteins that cooperate to stimulate cell proliferation, overriding some of the
­normal growth control mechanisms in the infected cell and its progeny. Some of these
viral proteins bind to the protein products of two key tumor suppressor genes of the
host cells, the retinoblastoma and the p53, deactivating them and thereby ­permitting
the cell to replicate its DNA and divide. Studies on small DNA viruses, such as
adenoviruses, polyomaviruses, and papillomaviruses, have helped in elucidating the
underlying molecular mechanisms of virus-induced cell transformation. Although
these small viruses differ in their evolution, they have striking similarity in their
­transforming functions, which emphasize their mutual need to utilize the host cell's
replication machinery for efficient viral replication (McLaughlin-Drubin and Munger
2008). The following different types of DNA tumor viruses have been recognized.
1.2.2.1 Epstein–Barr Virus
EBV or human herpesvirus 4 is a B-lymphotropic gammaherpesvirus that belongs
to a subfamily of Lymphocryptovirus (LCV) genus that infects more than 95% of the
world's population (Evans and Niederman 1989). It was the first human virus to be
classified as carcinogenic. The majority of EBV infection occurs during ­childhood
without causing overt symptoms. The most common manifestation of ­
infection with this organism is acute infectious mononucleosis, a ­
self-limiting
­lymphoproliferative disease that most frequently affects adolescents and young
adults. Classic symptoms include sore throat, fever, and lymphadenopathy that may
or may not be accompanied by a faint, generalized maculopapular rash. Humans
are the only known reservoir of EBV. The relatively low incidence of EBV-related
tumors compared with this prevalence of infection shows that there are definitely
many other factors that contribute to tumor development.
After exposure, EBV establishes a persistent infection in the host and is intermittently shed in oropharyngeal secretions. In uncomplicated EBV infections, the main
target cells are B lymphocytes that express CD21, which along with HLA class II molecules serve as the viral entry receptor and coreceptor, respectively. Infected B lymphocytes resemble antigen-activated B cells. The virus can also infect epithelial cells and
may initially replicate in the oropharynx before infecting B cells. Naive B cells become
infected in local lymphoid tissue, most often Waldeyer's tonsillar ring, and establish
latent infected memory B-cell pools that migrate to other lymphoid tissues. Periodic
reactivations of latently infected memory B cells are thought to facilitate infection of
epithelial cells in the oropharynx that permits shedding of virus into saliva for transmission to new hosts. Latently infected memory B cells are long-lived and, due to low levels
of expression of viral proteins, they escape detection by CD8+ cytotoxic T cells (Kutok
and Wang 2006); thus, persistence of EBV in the body is lifelong. EBV gene expression
in these cells is limited to a B-cell growth program, termed Latency III, that includes
nine latency-associated viral proteins, including six nuclear antigens (Epstein–Barr
nuclear antigen [EBNA]-1, -2, -3A, -3B, -3C, and -LP) and three membrane proteins
(latent membrane protein [LMP]-1, -2A, and -2B) that include LMP-1 and LMP-2a/b;
EBNA-1, -2, -3a, -3b, -3c, and -LP; miRNAs; BamHI A Rightward Transcripts, BamHI:
a type II restriction endonuclease from Bacillus amyloliquefaciens, having the capacity
for recognizing short sequences (6 b.p.) of DNA and specifically cleaving them at a target site (BARTs); and EBV-encoded RNAs (EBERs) (Gruhne et al. 2009). These cells
are eliminated by a robust immune response to EBNA-3 proteins, resulting in Latency
I, a reservoir of latently infected resting memory B cells expressing only EBNA-1 and
LMP-2. The differentiation of memory cells to plasma cells results in reactivation of the
replication phase of the viral life cycle that includes expression of Latency III gene products. In addition, there is likely another amplification step by reinfection of the epithelial
cells followed by shedding virus in the saliva to the next host (McLaughlin-Drubin and
Munger 2008). Apart from B cells, which are known to be primarily infected by EBV,
other non-B-cell NHLs associated with EBV infection include T-cell lymphoproliferative disorders that include a subset of peripheral T-cell lymphomas such as extranodal
nasal-type NK/T-cell lymphoma.
All phases of the EBV life cycle are associated with human disease (Ocheni et al.
2010). In immunocompetent individuals, infected cells increase in number, and eventually B-cell growth control pathways are activated, inducing transformation and
leading to malignancies such as BL, NPC, and Hodgkin's lymphoma. Other types
of malignancies include immunoblastic lymphomas, such as X-linked and acquired
immunodeficiency syndrome (AIDS)-associated lymphoproliferative disorders, and
posttransplant ­lymphoproliferative disorders (PTLDS).
1.2.2.2 Hepatitis B Virus
HBV is a small DNA virus that is maintained in circular conformation within
the virions. HBV replicates via a reverse transcription of an RNA pregenome and
­predominantly infects human hepatocytes in the liver. This marked hepatotropism is
a prominent feature of hepadnaviruses. The majority of individuals that are chronically infected with HBV ultimately experience severe liver disease and are at a
high risk of developing HCC (Beasley et al. 1981). HBV is supposed to be a major
etiological factor in the development of HCC. HBV-infected individuals develop
chronic active hepatitis (CAH) that can in turn lead to cirrhosis, liver failure, or
HCC (Feitelson 1999; Lok 2004). Patients with cirrhosis are more prone to develop
HCC than those without cirrhosis (Brechot 2004; Tanaka et al. 2005). It is ­suggested
that there is a direct oncogenic contribution of HBV to the carcinogenic process. The
cause of HBV-associated HCC is supposed to be a combination of HBV encoded
oncogenic activities along with the synergistic effects of chronic inflammation. It is
estimated that more than 400 million people worldwide are infected with HBV.
In the life cycle of HBV, the replication of HBV is dependent on reverse transcription, which is a trait of retroviruses. However, unlike retroviruses, integration of the
viral genome into the host chromosome is not necessary for viral replication but does
allow for persistence of the viral genome (Feitelson and Lee 2007). The 3.2 kb HBV
DNA genome codes for four genes named C, S, P, and X. The C gene codes for the
core protein and the serum e antigen; the S gene codes for three related viral envelope
proteins known as surface antigen; the P gene codes for the viral DNA polymerase;
and the X gene codes for a 16.5 kDa protein. The viral transcription is governed by
four promoters and two enhancer elements that are located in the HBV genome. The
compact nature of this small genome necessitates an ­extensive overlapping arrangement of the genetic information. Therefore, it is likely that a number of complex
mechanisms regulate the temporal and differential expression of the viral RNAs.
Among the proteins encoded by the HBV genome, the X gene product, termed HBx,
which is essential for productive infection of the mammalians HBV (Chen et al.
1993; Zoulim et al. 1994), has drawn considerable attention due to its pleiotropic
functions. HBx does not directly bind with DNA but functions via protein–protein
interaction (Maguire et al. 1991). It can activate cellular signaling pathways including mitogen-activated protein kinase (MAPK), c-Jun N-terminal kinase (JNK), and
Src tyrosine kinases (Benn and Schneider 1994; Klein and Schneider 1997). It also
regulates proteasomal function (Huang et al. 1996; Hu et al. 1999; Zhang et al. 2000),
affects mitochondrial function (Rahmani et al. 2000; Huh and Siddiqui 2002), and
modulates calcium homeostasis (Bouchard et al. 2001; Chami et al. 2003), and its
expression causes genomic instability (Forgues et al. 2003).
HBV-associated liver carcinogenesis is thought to be a multifactorial process in
which the mutations in core protein preS of HBV lead to immune modulation; HBx
leads to apoptosis regulation and proliferation; and preS mutation followed by HBx
action leads to genetic instability. Further studies are, however, necessary to find the
molecular mechanisms for the development of HBV-associated HCC.
1.2.2.3 Human Papillomavirus
HPVs belong to a family of small DNA viruses. Their circular double-stranded DNA
genomes of less than 8000 base pairs are packaged into nonenveloped particles of
approximately 55 nm. They show a marked preference for infection of epithelial cells
and have been isolated from a wide range of hosts, including humans. More than
200 different HPV types have been described to date (de Villiers et al. 2004), and
they can be classified into two groups: mucosal and cutaneous. Of these, mucosal
HPVs are further designated into low-risk HPVs (HPV-6 and HPV-11) that can cause
benign hyperplastic lesions (warts) that have little or no probability for malignant
progression and high-risk HPVs (HPV-16 and HPV-18) that are associated with the
vast majority of cervical carcinoma and a pathologically distinct group of oropharyngeal tumors.
HPV infects squamous epithelial cells by interacting with putative host-cell
­surface receptors such as heparan sulfate proteoglycans (Patterson et al. 2005) and
alpha 6 integrins (Doorbar 2006). Depending on the HPV type, the virus enters
the cell via either clathrin-mediated (Day et al. 2003) or caveolin-mediated endocytosis (Bousarghin et al. 2003). After uncoating, the virus delivers its genome of
eight genes to the host-cell nucleus, which is expressed as autonomous replicating
episomal or extrachromosomal elements. Viral DNA is replicated using host-cell
machinery and then segregated into progeny cells. Afterward, one of the daughter
cells remains in the basal layer to serve as a reservoir for the replication of more viral
DNA, while the other daughter cell migrates suprabasally; HPV inhibits the cell
from its normal exit from the cell cycle. Differentiation of infected cells is necessary
for productive infection of the host cell and is coupled with expression of late viral
capsid genes for completion of the viral life cycle (Doorbar 2005).
The HPV genome is comprised of eight genes with multiple promoters and a
number of splice variants that are expressed either early (E) or late (L) in the HPV
life cycle. The early genes encode nonstructural proteins that participate in DNA
replication, transcriptional regulation, cell transformation, and viral assembly and
release. The late (L) genes, L1 and L2, encode viral capsid proteins. An additional
1000 base pairs noncoding region of the 8000 base pairs HPV genome contains transcriptional regulatory sequences and the viral origin of replication (Bernard 2002).
All these E and L genes and their functions are shown in Table 1.1.
Papillomaviruses are supposed to have two modes of replication: stable and vegetative. The former mode exists as a circular episomal genome in the cells of the basal
and parabasal layers of low-risk HPV-infected squamous epithelia, while the latter
form occurs in highly differentiated cells at the epithelial surface. E6 and E7, the
first viral genes to be expressed, are involved in cell transformation in HPV high-risk
types. E6 complexes with the cellular ubiquitin ligase. The E6AP and the E6AP/
E6 complex act as a p53-specific ubiquitin protein ligase to increase the rate of p53
degradation, thereby negating the antiproliferative and pro-apoptotic functions of
p53 in cases of DNA damage and cellular stress (Scheffner et al. 1993). E7, similar to E6, is also an oncoprotein. E7 complexes with hypophosphorylated pocket
proteins pRb, p107, or p130, facilitating the release of the transcription factor E2F,
which then c­ onstitutively activates DNA synthesis and cell proliferation (Munger and
HPV Genes and Their Functions
DNA-dependent ATPase, ATP-dependent helicase: Allow unwinding of the viral genome and
act as an elongation factor for DNA replication.
Responsible for recognition and binding of origin of replication. Exists in two forms: full
length (transcriptional transactivator) and truncated (transcriptional repressor). The ratio of
these found in the heterotrimeric complex formed before complexing with E1 regulates
transcription of viral genome.
Late expression: C terminal binds intermediate filament, allowing release of virus-like
particles. Also involved in transformation of host cell by deregulation of host-cell mitogenic
signaling pathway.
Obstruction of growth suppression mechanisms, for example, EGF receptor; activation of
mitogenic signaling pathways via transcription factors: c-Jun and c-Fos (important in
ubiquitin pathway degradation of p53 complex by E6). Inactivation of p21 (p53-induced
expression halts cell cycle until DNA is proofread for mutations).
E6 transformation of host cell by binding p53 tumor suppressor protein.
Transforming protein: Binds to pRB/p107.
Major capsid protein: Can form virus-like particles.
Minor capsid protein: Possible DNA-packaging protein.
Source: www.microbiologybytes.com/virology/Papillomaviruses.html, accessed on April 8, 2009.
Howley 2002). The Rb tumor suppressor protein normally binds to and inactivates
E2F. E7 has also been shown to inactivate the cyclin-dependent kinase (Cdk) inhibitors, p21/Cip1 and p27/Kip1, and to complex with cyclins A and E (Duensing and
Munger 2004). However, in low-risk HPV types, such as HPV-6 and HPV-11, E6 may
or may not bind to p53. In either case, it does not stimulate p53 degradation, and E7
does not bind pRb with high affinity. E1 and E2 are both DNA-binding proteins. E1 is
a DNA helicase/ATPase involved in viral genome replication, while E2 is a transcription factor that binds to four sites within the viral noncoding region and recruits E1
to the viral origin of replication (Hughes and Romanos 1993). E1 and E2 have been
shown to be sufficient for transient replication of the HPV genome.
In addition, increased levels of active E1 and E2 have been correlated with
increased viral copy number (Del Vecchio et al. 1992). E2 also binds to the early
promoter and decreases the expression of E6 and E7. Therefore, loss of E2 during
integration of viral DNA into the host genome is the first stage in transformation
(Talora et al. 2002). E2 exists in a full-length form that acts as a transcriptional
activator of the early promoter at low levels of E2 expression and a truncated form
at high levels of E2 expression that acts as a transcriptional repressor of the early
promoter production of oncoproteins E6 and E7 (Bernard et al. 1989; Ushikai et al.
1994). At higher concentrations of E2, displacement of basal transcription factors is
thought to be responsible for the inactivation of the promoter and for the repression
of E6 and E7 expression (Doorbar 2006). Additionally, a switch in promoter usage to
the differentiation-dependent late promoter (which is not repressible by E2) during
differentiation is due to changes in cellular signaling and not to genome amplification. This switch results in increased levels of E1 and E2, and subsequent E4 gene
sequences are not highly conserved among the various HPV types. Although E4 has
been shown to associate with cytokeratins to destabilize the cytokeratin network
and to aid in viral release at the epithelial cell layer surface, its primary function is
still unknown (Sterling et al. 1993). However, E4 has been shown to cause G2 cell
cycle arrest and to antagonize E7-mediated cell proliferation. Researchers also have
hypothesized that E4 associates with E2 (Doorbar 2006). E5 proteins are hydrophobic transmembrane proteins that seem to be weakly involved in cellular transformation and that modulate the activity of the epidermal growth factor (EGF) receptor
tyrosine kinase signaling pathway (Dimaio and Mattoon 2001). E5 is present on
plasma and on Golgi and endoplasmic reticulum (ER) membranes.
Researchers also believe that E5 associates with the vacuolar ATPase and delays
endosomal acidification in addition to inhibiting gap junction mediated cell–cell
­communication in keratinocytes. This results in an increase in EGF receptor recycling to the cell membrane and an inability to respond to paracrine growth inhibitory
signals (Thomsen et al. 2000; Doorbar 2006). E5 expression in high-risk HPV types
is thought to occur early during carcinogenesis because expression is inhibited by
incorporation of the viral genome during progression to cervical cancer (Oelze et al.
1995). The functions of E3 and E8 are still unknown. As infected host cells become
terminally differentiated, the late genes, L1 and L2, which encode the major and
minor structural capsid proteins, are expressed until the viral copy number increases
enough that viral particles are sloughed off at the skin surface to infect other cells and
hosts. L2 expression precedes that of L1, and the presence of E2 is thought to increase
the efficiency of this process (Zhao et al. 2000). The lytic release of viral progeny
requires terminal differentiation of stratified epithelium, a prerequisite for the activation of differentiation-dependent late viral promoters. Most HPV infections are latent
or asymptomatic in immunocompetent hosts but can become reactivated in cases of
cell-mediated immunosuppression replication of the viral genome (Doorbar 2006).
Progression to cancer with many HPV types results after years of active infection.
Although it has been shown that E6 and E7 expression is not sufficient to transform
human keratinocytes in culture, E6 inactivation of the p53 response to DNA damage
and cellular stress and repeated E7-mediated S phase entry and cell proliferation do
result in the accumulation of multiple mutations in the host genome (Hawley-Nelson
et al. 1989; Doorbar 2006). However, cases of rapid onset of carcinogenesis after
initial infection have also been noted (Woodman et al. 2001). The highest incidences
of HPV-related cancers have been found to be associated with cancer of the cervix in
females and of the anus in homosexual men (Bosch and de Sanjosé 2003). Research
has suggested that HPV can more easily access the basal epithelial cells at both of
these sites due to the presence of reserve cells in the transformation zone that will
eventually take on the basal cell phenotype (Doorbar 2006). Integration of HPV DNA
at specific sites in the host-cell genome has been suggested to be an early and critical
step in cancer progression (Bosch and de Sanjosé 2003). The E6 and E7 oncoproteins
of HPV are supposed to mechanistically contribute to initiation and progression of
cervical cancer. The participation of certain flanking sequences in the integration of
viral DNA has also been suggested to be a step in carcinogenesis (Yu et al. 2005).
1.2.2.4 Kaposi's Sarcoma-Associated Herpesvirus/Human Herpesvirus 8
KSHV is the eighth human herpesvirus, the formal name of which according to
the International Committee on Taxonomy of Viruses (ITCV) is human herpesvirus 8 (HHV-8). This virus causes Kaposi's sarcoma, a cancer commonly occurring
in patients with AIDS (Boshoff and Weiss 2002), as well as primary effusion lymphoma (PEL) (Cesarman et al. 1995) and some types of multicentric Castleman's
disease. In the 1980s, frequent occurrence of aggressive Kaposi's sarcoma (KS) was
noted in AIDS patients and, subsequently, an increased incidence rate of KS was
noticed specifically among homosexual human immunodeficiency virus (HIV)infected ­individuals (Sarid and Gao 2011). This led to the suggestion that KS might
be caused by a sexually transmitted agent distinct from HIV (Beral et al. 1990,
1992; Cohen et al. 2005). The use of advanced molecular techniques led Chang
et al. (1994) to identify the infectious agent responsible for KS, which was named
as KSHV. Shortly after its discovery, KSHV was shown to be associated with PEL
(a rare subgroup of B-cell non-Hodgkin lymphoma) and a subset of multicentric
Castleman's disease (Cesarman et al. 1995; Soulier et al. 1995). Among patients with
AIDS, KSHV infection was shown to precede KS onset, corroborating a causative
role of KSHV in KS (Whitby et al. 1995; Gao et al. 1996; Moore et al. 1996).
KSHV is a large double-stranded DNA virus with a protein that packages its
nucleic acids, called capsid, which is then surrounded by an amorphous protein
layer called tegument, and is finally enclosed in a lipid envelop derived in part from
the cell membrane. KSHV has a genome of about 165,000 nucleic acid bases in
length. KSHV contains several proteins that promote cell proliferation and survival
and thus contribute to cellular transformation. These proteins include open reading
frames 73-71 (Orf 73-71), locus-encoded latency-associated antigen (LANA, Orf 73),
viral cyclin (v-cyclin, Orf 72), viral FLICE inhibitory protein (vFLIP, Orf 71), viral
interferon regulatory factor 1 (vIRF-1), and the kaposin/K12 gene. Among the other
proteins that have been mentioned as contributing to the transforming activities of
KSHV/HHV-8 are modulator of immune response (MIR), complement control protein homolog (CCPH), viral G-protein-coupled receptor (vGPCR), latency-associated
membrane protein (LAMP), and viral B-cell lymphoma 2 (vBlc-2) (McLaughlinDrubin and Munger 2008).
1.2.3 Miscellaneous
In addition to the aforementioned RNA and DNA viruses, there are some oncoviruses that have been identified, but their association with human cancer is yet to be
firmly established. Some of them are described here.
1.2.3.1 Polyomaviruses
Polyomaviruses belong to Polyomaviridae family, which is a group of nonenveloped,
small, double-stranded DNA viruses. They are potentially oncogenic, and the first
polyomavirus, known as mouse polyomavirus (Gross 1953; Stewart et al. 1958), is
one of the most aggressive oncogenic viruses. It causes tumors in almost every tissue
of susceptible mouse strains (Eddy 1969; Gross 1983). The name polyoma refers to
the ability of viruses to produce multiple (poly-) tumors (-oma). Polyomaviruses have
approximately 5000 base pairs, a circular genome and icosahedral shape, and do not
have a lipoprotein envelope.
Several polyomaviruses have been found in humans. Four of these viruses, John
Cunningham (JC), BK, Karolinska Institute (KI), and Washington University (WU),
are closely related to simian virus 40 (SV40), the second polyomavirus discovered
after the mouse polyoma and isolated as a contaminant of the early batches of the
polio vaccine, which was produced in African green monkey kidney cells (Sweet
and Hilleman 1960). Therefore, infection of these four viruses can be confused
with SV40 infection (zur Hausen 2003; Poulin and DeCaprio 2006). Infection with
BK and JC viruses is widespread in humans and is usually subclinical. However,
in immune-suppressed patients, the former is associated with renal nephropathy
(Randhawa and Demetris 2000; Hirsch et al. 2002) and the latter can cause progressive multifocal leukoencephalopathy (Safak and Khalili 2003; Berger and Koralnik
2005). However, no conclusive proof exists whether either of them causes or acts as
a cofactor in human cancer.
The Merkel cell carcinoma (MCC) was the first human cancer that was found to
harbor an integrated polyomavirus genome. Merkel cell polyomavirus (MCPyV) is
highly divergent from the other human polyomaviruses and is most closely related
to murine polyomavirus. After it was cloned, several studies suggested a casual role
of this virus in MCC.
Polyomaviruses replicate in the nucleus of the host. Because their genomic structure is homologous to that of the mammalian host, they utilize the host machinery
for replication. Their replication occurs in two distinct phases: early and late gene
expression, separated by genome replication. The early gene expression is ­responsible
for the synthesis of nonstructural proteins, whereas the late gene expression synthesizes the structural proteins, which are required for the viral particle composition.
Prior to genome replication, there occur the processes of viral attachment, entry, and
uncoating. Cellular receptors for polyomaviruses are sialic acid residues of gangliosides. The attachment of polyomaviruses to host cells is mediated by viral protein 1
(VP1) via the sialic acid attachment region. Then polyomavirus virions are endocytosed and transported first to the endoplasmatic reticulum where a conformational
change occurs revealing VP2. After this, the virus is exported to the nucleus for
which the mechanism is not known.
1.2.3.2 Adenoviruses
Adenoviruses belong to the family of Adenoviridae. They are so named because
they were initially isolated from human adenoids (in 1953). They are mediumsized (90–100 nm), nonenveloped (without an outer lipid bilayer) viruses with an
icosahedral nucleocapsid containing a double-stranded DNA genome. In humans,
more than 50 different adenovirus serotypes have been described. Adenoviruses
are able to replicate in the nucleus of mammalian cells using the host's replication machinery. Once the virus has successfully gained entry into the host cell,
the endosome acidifies, altering virus topology by causing capsid components to
disassociate. These changes as well as the toxic nature of the pentons result in the
release of the virions into the cytoplasm. They are then transported to the nuclear
pore complex with the help of cellular microtubules, whereby the adenovirus particles disassemble. Viral DNA is subsequently released, which can enter the nucleus
via the nuclear pore (Meier and Greber 2004). After this, the DNA associates with
histone molecules. Thus, viral gene expression can occur, and new virus particles
can be generated.
The transforming and oncogenic potential of adenoviruses has been ascribed to
E1A and E1B oncoproteins, where the former is similar to SV40 TAg and the latter to
high-risk HPV E7/E6 target pRb and p53. However, the possible role of adenoviruses
in producing any kind of tumor is to be carefully ascertained. Adenoviruses typically cause respiratory illnesses such as the common cold, conjunctivitis (an infection in the eye), croup, bronchitis, or pneumonia. In children, adenoviruses usually
cause infections in the respiratory tract and the intestinal tract.
1.2.3.3 Torque Teno Virus
Torque teno virus (TTV) is a nonenveloped small virus containing a circular, singlestranded, negative-sense DNA genome. It belongs to the Circoviridae family. It was
first found in a patient with non-A-E-hepatitis (Nishizawa et al. 1997). Within a few
years after its discovery, TTV was found to have several genotypes (Niel et al. 2000)
and to be highly prevalent globally among healthy individuals; up to 94% of subjects are viremic. Persistent infections and coinfections with several genotypes occur
frequently. However, the pathogenicity of TTV is at present unclear. A study had
suggested that a high TTV load may have prognostic significance in HCV-associated
liver disease, but whether the high TTV load mediates HCV-associated disease
progression remains to be ascertained. However, a relatively high prevalence of
TTV-related DNA sequences in human cancers, specifically gastrointestinal cancer,
lung cancer, breast cancer, and myeloma, has been observed (de Villiers et al. 2002).
1.2.3.4 Human Mammary Tumor Virus/Pogo Virus
The inspiration for searching for human mammary tumor virus (HMTV) came
from the existence of mouse mammary tumor virus (MMTV). MMTV is a
­m ilk-transmitted retrovirus such as HTLV, HIV, and BLV (bovine leukemia virus).
It belongs to the genus Betaretrovirus and was formerly known as Bittner virus
because it was demonstrated by Bittner that MMTV can cause breast cancer in
mice (Bittner 1936). Previously, MMTV was also known as "milk factor" because
Bittner established the theory that it could be transmitted by cancerous mothers
to young mice from a virus in their mother's milk (Bittner 1936). The majority of
mammary tumors in mice are caused by MMTV. From several studies, it has been
suggested that an MMTV-related virus, HMTV, sometimes also referred to as Pogo
virus, could be associated with human breast cancer (Holland and Pogo 2004).
Primary breast cancer cells have been found to produce HMTV particles in vitro
(Melana et al. 2007). In fresh breast cancers, the entire HMTV genome, which
contains hormone response elements, has also been observed (Liu et al. 2001).
HMTV particles in primary breast cancer cells were reported to have morphogenic
and molecular characteristics similar to MMTV (Melana et al. 2007). However,
further studies failed to corroborate this (Bindra et al. 2007). Also, it could not be
well established that HMTV makes any major contribution to the development of
breast cancer.
1.2.3.5 Human Endogenous Retrovirus
Human endogenous retrovirus (HERV) is an HIV-like retrovirus, but where HIV
is exogenous (meaning that it is originated outside the organism's body), HERV is
endogenous (meaning that it is the part of human genome). There are many families
of endogenous retroviruses that exist throughout the human genome. Unlike HIV,
these retroviruses purportedly integrated themselves into the genome long ago and
have since accumulated mutations that have rendered them unable to produce infectious, exogenous viruses. HERVs are supposed to be "junk DNA." The regulatory
role of HERVs has been demonstrated in the liver, placenta, colon, and other locations
(Bannert and Kurth 2004). One of the families of HERVs, HERV-K, is proposed to
be fairly young (less than five million years old) because it still contains a complete
set of genes (albeit with mutations) necessary for a retrovirus to produce infectious
viruses (Bannert and Kurth 2004; Dewanneiux et al. 2006). Some HERVs have been
implicated in human malignancy because of increased expression of HERV messenger RNA (mRNA) (Andersson et al. 1998), functional protein (Sauter et al. 1995),
and retrovirus-like particles (Lower et al. 1993) in certain cancers. Studies are being
made on the association of HERV-K with a number of cancers such as germ cell
tumors like seminomas (Sauter et al. 1995, 1996), breast cancer, myeloproliferative
disease, ovarian cancer (Wang-Johanning et al. 2007a, b), melanoma (Muster et al.
2003; Buscher et al. 2005), and prostate cancer (Tomlins et al. 2007). However, it
could not be firmly established so far that HERV-K expression contributes to the
development of any kind of cancer (McLaughlin-Drubin and Munger 2008).
1.2.3.6 Xenotropic Murine Leukemia Virus-Related Virus
Xenotropic murine leukemia virus-related virus (XMRV) is a chimeric virus that
arose from the recombination of two endogenous mouse retroviruses during the mid1990s. First described in 2006 as an apparently novel retrovirus and potential human
pathogen, it was linked to prostate cancer and later to chronic fatigue syndrome (CFS).
However, no evidence gathered so far proves that XMRV can infect humans, nor has it
been demonstrated that XMRV is associated with or causes any human disease.
XMRV is a murine leukemia virus (MLV) that formed through the recombination
of the genomes of two parent MLVs known as preXMRV-1 and preXMRV-2 (Cingöz
et al. 2012). MLV is a gammaretrovirus, a genus of the Retroviridae family, and has
a single-stranded RNA genome that replicates through a DNA intermediate. The
name XMRV was given because the discoverers of the virus initially thought that it
was a novel potential human pathogen that was related to but distinct from MLVs.
XMRV has now been established as a laboratory contaminant. This contamination
hypothesis has been supported by several studies (Smith 2010).
In conclusion, we find that currently acknowledged viruses that are associated with
human malignancies include two RNA viruses—namely HTLV-1 and HCV—and
four DNA viruses—namely EBV, HBV, HPV, and KSHV/HHV-8. Of the two RNA
viruses, HTLV-1 causes ATL (Poiesz et al. 1980; Giam and Jeang 2007). HCV and
HBV, which belongs to the class of DNA viruses, lead to HCC (Dane et al. 1970;
Choo et al. 1989; Levrero 2006; Jin 2007). EBV (of the DNA class) produces BL,
NPC, posttransplant lymphomas, and Hodgkin's disease (Epstein et al. 1964; Pagano
1999; Tao et al. 2006; Klein et al. 2007). Of the remaining two DNA viruses, HPV
leads to cervical cancer, skin cancer in patients with epidermodysplasia verruciformis, head and neck cancers, and other anogenital cancers (Durst et al. 1983; Boshart
et al. 1984; Hausen 1996; Munger et al. 2004), and KSHV/HHV-8 causes PEL and
Castleman's disease (Chang et al. 1994; Cathomas 2003). The oncogenic roles of
other viruses discovered are not well established.
All tumor viruses induce oncogenesis through manipulating an array of different cellular pathways. These viruses initiate a series of cellular events, which lead
to immortalization and proliferation of the infected cells by disrupting the mitotic
checkpoint upon infection of the host cell. This is often accomplished by functional
inhibition or proteasomal degradation of many tumor suppressor proteins by virally
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Targeting Human T-Cell
Leukemia Virus Type 1
Pathogenesis and
Treatment Strategies
Human T-cell leukemia virus type 1 (HTLV-1) was the first retrovirus that was
­identified as a causative agent of human diseases (Gallo 2005; Takatsuki 2005;
Matsuoka and Jeang 2007). HTLV-1 induces malignancy of CD4+ CD25+
T-lymphocytes, adult T-cell leukemia (ATL), and several inflammatory diseases,
such as HTLV-1-associated myelopathy (HAM)/tropical spastic paraparesis (TSP)
and HTLV-1 u­ veitis (HU), in a small fraction of the HTLV-1-infected individuals
(Matsuoka and Jeang 2007; Yasunaga and Matsuoka 2011).
At present, about 10–20 million people worldwide are infected with HTLV-1
(Yasunaga and Matsuoka 2007). HTLV-1 is endemic in Japan, the Caribbean
islands, the areas surrounding the Caribbean basin, Central Africa, South
America, Papua New Guinea, and the Solomon islands. HTLV-1 belongs to the
delta-type retroviruses, which also include bovine leukemia virus (BLV), human
T-cell leukemia virus type 2 (HTLV-2), and simian T-cell leukemia virus. Like
other retroviruses, the HTLV-1 proviral genome has the structural genes, gag,
pol, and env, which are flanked by long terminal repeats (LTRs) at both ends.
A unique structure, termed the pX region, is found between env and the 3′-LTR
(Figure 2.1). The plus strand of the pX region encodes the regulatory proteins
p40 tax (Tax), p27rex (Rex), p12, p13, p30, and p21 in four different open reading
frames (ORFs) (Franchini et al. 2003; Matsuoka 2003). The HTLV-1 bZIP factor
(HBZ) is encoded in the minus strand of the provirus, and alternative splicing
generates the isoforms of HBZ, the spliced and unspliced HBZ forms (Gaudray
et al. 2002; Cavanagh et al. 2006; Murata et al. 2006; Satou et al. 2006). The
major host of HTLV-1 is CD4+ T lymphocytes in HTLV-1 carriers, although it is
known that HTLV-1 can infect many types of cells in vitro (Koyanagi et al. 1993;
Yasunaga et al. 2001; Satou et al. 2012).
Plus strand transcripts
Viral genome
Genomic RNA
core and enzymes
Structural genes
Regulatory/accessory genes
ORF I
p12/p8
ORF II
ORF III
ORF IV
HTLV-1 provirus
Host genome
5'LTR gag
3'LTR
Spliced HBZ
Unspliced HBZ
Minus strand transcripts
FIGURE 2.1 (See color insert.) HTLV-1-encoded genes. The HTLV-1 proviral genome
consists of gag, pol, and env genes flanked by long terminal repeat (LTR) sequences at both
ends. The pX region encodes several regulatory/accessory proteins in four open reading
frames (ORFs) and the minus strand.
Targeting Human T-Cell Leukemia Virus Type 1
LIFE CYCLE OF HTLV-1
Viral Transfer via Cell-to-Cell Contact
It is well known that infection by free HTLV-1 virions is very inefficient and
that HTLV-1 viral particles are hardly detectable in serum of the HTLV-1 carriers (Matsuoka and Jeang 2007). Indeed, HTLV-1 replication is not active in vivo;
this fact indicates that the mechanism of HTLV-1 transmission is quite different
from that of another human retrovirus, human immunodeficiency virus 1 (HIV-1),
which ­vigorously replicates in vivo to promote efficient transmission through cellfree virions. Instead, HTLV-1 is transmitted from infected cells to uninfected cells
via cell-to-cell contact (Igakura et al. 2003; Pais-Correia et al. 2010; Van Prooyen
et al. 2010). Therefore, living HTLV-1-infected cells are required for efficient
transmission. There are three major routes of HTLV-1 infection: mother-to-infant
­transmission (mainly through breast milk), parenteral transmission, and sexual transmission (Matsuoka 2003). Infected lymphocytes in the fluids are the main source of
HTLV-1. Because the replication level of HTLV-1 in infected cells is generally low,
efficient HTLV-1 viral transmission is established directly between cells.
Viral Entry into Target Cells
HTLV-1 is an enveloped virus. After transfer of the virions from infected to uninfected cells, viral particles attach to the receptor complex on the cell surface and
enter target cells by fusion of the viral membrane with the cellular membrane (Jones
et al. 2011). The receptor complex consists of at least three molecules: a glucose
transporter (GLUT1) (Manel et al. 2003), heparan sulfate proteoglycan (HSPG)
(Jones et al. 2005), and neuropilin 1 (Ghez et al. 2006), which are important for
the interaction between the HTLV-1 envelope and the cell membrane. It has been
suggested that the virus may first contact HSPG and then form complexes with
­neuropilin 1, followed by association with the GLUT1 on the cell surface prior to
membrane fusion and entry into the cell.
After entry into target cells and uncoating of the virus, HTLV-1 genomic RNA is
reverse transcribed into double-stranded DNA by reverse transcriptase (RT). RT and
integrase (IN) are incorporated in virions and released along with the viral genome
after viral entry.
2.2.4 Integration
Viral IN mediates integration of viral double-stranded DNA into the cellular
genome; the integrated viral DNA is called the provirus. Integration of HTLV-1 has
been shown to occur at random sites in the host genome, whereas proviruses in leukemic cells tend to be integrated near the transcriptional start sites of cellular genes,
indicating positive selection of cells with this feature during leukemogenesis (Doi
et al. 2005; Gillet et al. 2011).
Viral Transcription
To produce HTLV-1 progeny, viral transcripts are expressed from the provirus.
The LTRs at each end of the provirus are used as promoters: the 5′-LTR and
3′-LTR control the transcription of the viral genes encoded in the plus and minus
strands of the provirus, respectively (Figure 2.1). Because the plus strand of the
provirus encodes all the structural proteins and the viral genomic RNA, 5′-LTRmediated transcription is required for viral replication and transmission. In contrast, the 3′-LTR drives the transcription of the spliced form of HBZ, which is
constitutively expressed in HTLV-1-infected cells. The regulatory genes (tax and
rex) and accessory genes (p12, p13, p30, and HBZ) encoded in the pX region regulate viral replication and the proliferation of infected cells. Among the products
of these genes, Tax is a critical activator of viral transcription through the 5′-LTR.
Tax does not bind to DNA, but rather activates the transcription of target genes
by recruiting various transcription factors and modifying the epigenetic status
of promoter regions (Kashanchi and Brady 2005). The association between Tax
and cAMP response element-binding protein (CREB) is required for viral gene
transcription. There are three 21-bp repeat elements, called Tax-responsive elements (TREs), located in the 5′-LTR, and the Tax-CREB complex recruits several
histone acetyl transferases, including CREB-binding protein (CBP), p300, and
p300/CBP-associated factor, to the LTR, resulting in induction of viral expression. Another regulatory protein, Rex, is an RNA-binding protein and a positive
post-transcriptional regulator of virus production (Inoue et al. 1986; Hidaka et al.
1988). Rex increases nuclear export of viral genomic RNA and singly spliced
messenger RNA (mRNA) encoding viral structural genes, such as gag, pol, and
env, which are required for the production of an infectious virus. In contrast, p30
and HBZ negatively regulate 5′-LTR-mediated transcription (Gaudray et al. 2002;
Nicot et al. 2004; Yoshida et al. 2008). Because viral antigens, especially Tax, are
the targets of cytotoxic T-lymphocytes (CTLs) (Kannagi et al. 2004, 2011), silencing of viral expression by p30 and HBZ supports the escape of HTLV-1-infected
cells from host immune surveillance. Transcription from the 5′-LTR is transiently
induced and complicatedly regulated by many factors, whereas 3′-LTR-driven
transcription is constitutively activated by a ubiquitously expressed transcription factor, Sp1 (Yoshida et al. 2008; Gazon et al. 2012). These data suggest that
HBZ has important roles not only in viral latency but also in the proliferation of
infected cells.
Assembly and Budding
Translation of unspliced viral RNA produces Gag, Gag-Protease (Pro), and
Gag-Pro–Polymerase (Pol) polyproteins, and mature structural proteins and
enzymes are generated by cleavage with Pro (Nam et al. 1988; Trentin et al. 1998;
Kadas et al. 2004). The singly spliced mRNA encodes the envelope protein that is
cleaved into the surface glycoprotein gp46 and the transmembrane glycoprotein
gp21 (Pique et al. 1990). HTLV-1 particles are assembled at the host cellular membrane and released from cells by budding (Poiesz et al. 1980).
CLONAL EXPANSION OF HTLV-1-INFECTED CELLS
Replication of HTLV-1 is known to be generally suppressed in vivo (Hanon et al.
2000). Because Tax is a highly immunogenic protein, cells expressing Tax protein are
eliminated by the host immune system. Therefore, vigorous replication of HTLV-1
viruses is not observed in HTLV-1-infected individuals. Instead, HTLV-1 increases
its copy number by promoting proliferation of the infected cells; HTLV-1-infected
T-cell clones can proliferate and survive for a long time (Wattel et al. 1995; Etoh et al.
1997). The number of HTLV-1-infected cells has been shown to be almost constant
(Okayama et al. 2004; Tanaka et al. 2005; Umeki et al. 2009), and some HTLV-1infected clones persist for many years in the same HTLV-I carriers (Etoh et al. 1997).
Among the viral proteins, HBZ is thought to be important for clonal proliferation of
HTLV-1-infected cells. HBZ is the only viral protein that is conserved and constitutively expressed in HTLV-1-infected cells (Satou et al. 2006; Miyazaki et al. 2007;
Fan et al. 2010) because nonsense mutations were reported in all other viral genes
except the HBZ gene, even in asymptomatic HTLV-1 carriers. In addition, knockdown of HBZ inhibits growth of ATL and HTLV-1-infected cell lines (Satou et al.
2006), indicating that HBZ is critical for the expansion of HTLV-1-infected cells.
HTLV-1-INDUCED DISEASES
Most HTLV-1-infected individuals are asymptomatic and do not develop any
HTLV-1-mediated diseases during their lifetime. In a small fraction of carriers,
HTLV-1 induces a malignant disease, ATL, and several inflammatory diseases,
such as HAM/TSP and HU. It is also known that HTLV-1 carriers have mild immunodeficiency and develop some opportunistic infections. These diseases are pictorially represented and briefly described in Figure 2.2.
Adult T-Cell Leukemia
ATL is a fatal T-cell malignancy induced by HTLV-1 and an outcome of the
malignant transformation of one HTLV-1-infected T-cell clone. Among individuals infected with HTLV-1, a small proportion of carriers (6% of males and 2% of
females in Japan) develop ATL (Matsuoka and Jeang 2007). Most HTLV-1 carriers
do not develop any HTLV-1-associated diseases. There is a long latency period from
the initial infection until the onset of ATL—about 60 years in Japan (Iwanaga et al.
2012) and 40 years in Jamaica (Hanchard 1996). These findings suggest that multistep leukemogenic mechanisms are involved in the genesis of ATL. The diagnostic
criteria for ATL have been defined as (Takatsuki 2005):
1. Presence of a morphologically proven lymphoid malignancy expressing
T-cell surface antigens (typically, CD4+ and CD25+). These abnormal
T ­lymphocytes have hyper-lobulated nuclei in acute ATL and are called
"flower cells."
2. Presence of antibodies to HTLV-1 in serum.
3. Demonstration of monoclonal integration of HTLV-1 provirus in tumor
cells by Southern blotting.
Malignant disease
Asymptomatic
>90% of carriers
HTLV-1
5% of carriers
HAM/TSP
0.25%–4% of carriers
FIGURE 2.2 (See color insert.) A pictorial representation of HTLV-1-associated diseases.
Patients are subdivided into four clinical subtypes according to the clinical features (Shimoyama 1991): acute, lymphoma, chronic, and smoldering types. The acute
type is the prototypic ATL in which patients exhibit an increased number of ATL
cells, frequent skin lesions, systemic lymphadenopathy, and hepatosplenomegaly.
Lymphoma-type ATL is characterized by prominent systemic lymphadenopathy,
with few abnormal cells in the peripheral blood. In chronic ATL, the white blood cell
count is somewhat higher, and skin lesions, lymphadenopathy, and hepatosplenomegaly are sometimes exhibited. Smoldering ATL is characterized by the presence
of a few ATL cells in which monoclonal integration of HTLV-1 provirus can be
confirmed in peripheral blood. Acute- and lymphoma-type ATL are ver