Oncovirus

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An oncovirus is a virus that can cause cancer. This term originated from studies of acutely transforming retroviruses in the 1950–60s, [1] when the term "oncornaviruses" was used to denote their RNA virus origin. [2] With the letters "RNA" removed, it now refers to any virus with a DNA or RNA genome causing cancer and is synonymous with "tumor virus" or "cancer virus". The vast majority of human and animal viruses do not cause cancer, probably because of longstanding co-evolution between the virus and its host. Oncoviruses have been important not only in epidemiology, but also in investigations of cell cycle control mechanisms such as the retinoblastoma protein.

Contents

The World Health Organization's International Agency for Research on Cancer estimated that in 2002, infection caused 17.8% of human cancers, with 11.9% caused by one of seven viruses. [3] A 2020 study of 2,658 samples from 38 different types of cancer found that 16% were associated with a virus. [4] These cancers might be easily prevented through vaccination (e.g., papillomavirus vaccines), diagnosed with simple blood tests, and treated with less-toxic antiviral compounds.

Background

Generally, tumor viruses cause little or no disease after infection in their hosts, or cause non-neoplastic diseases such as acute hepatitis for hepatitis B virus or mononucleosis for Epstein–Barr virus. A minority of persons (or animals) will go on to develop cancers after infection. This has complicated efforts to determine whether or not a given virus causes cancer. The well-known Koch's postulates, 19th-century constructs developed by Robert Koch to establish the likelihood that Bacillus anthracis will cause anthrax disease, are not applicable to viral diseases. (Firstly, this is because viruses cannot truly be isolated in pure culture—even stringent isolation techniques cannot exclude undetected contaminating viruses with similar density characteristics, and viruses must be grown on cells. Secondly, asymptomatic virus infection and carriage is the norm for most tumor viruses, which violates Koch's third principle. Relman and Fredericks have described the difficulties in applying Koch's postulates to virus-induced cancers. [5] Finally, the host restriction for human viruses makes it unethical to experimentally transmit a suspected cancer virus.) Other measures, such as A. B. Hill's criteria, [6] are more relevant to cancer virology but also have some limitations in determining causality.

Tumor viruses come in a variety of forms: Viruses with a DNA genome, such as adenovirus, and viruses with an RNA genome, like the Hepatitis C virus (HCV), can cause cancers, as can retroviruses having both DNA and RNA genomes (Human T-lymphotropic virus and hepatitis B virus, which normally replicates as a mixed double and single-stranded DNA virus but also has a retroviral replication component). In many cases, tumor viruses do not cause cancer in their native hosts but only in dead-end species. For example, adenoviruses do not cause cancer in humans but are instead responsible for colds, conjunctivitis and other acute illnesses. They only become tumorigenic when infected into certain rodent species, such as Syrian hamsters. Some viruses are tumorigenic when they infect a cell and persist as circular episomes or plasmids, replicating separately from host cell DNA (Epstein–Barr virus and Kaposi's sarcoma-associated herpesvirus). Other viruses are only carcinogenic when they integrate into the host cell genome as part of a biological accident, such as polyomaviruses and papillomaviruses.

A direct oncogenic viral mechanism [7] involves either insertion of additional viral oncogenic genes into the host cell or to enhance already existing oncogenic genes (proto-oncogenes) in the genome. For example, it has been shown that vFLIP and vCyclin interfere with the TGF-β signaling pathway indirectly by inducing oncogenic host mir17-92 cluster. [8] Indirect viral oncogenicity involves chronic nonspecific inflammation occurring over decades of infection, as is the case for HCV-induced liver cancer. These two mechanisms differ in their biology and epidemiology: direct tumor viruses must have at least one virus copy in every tumor cell expressing at least one protein or RNA that is causing the cell to become cancerous. Because foreign virus antigens are expressed in these tumors, persons who are immunosuppressed such as AIDS or transplant patients are at higher risk for these types of cancers. Chronic indirect tumor viruses, on the other hand, can be lost (at least theoretically) from a mature tumor that has accumulated sufficient mutations and growth conditions (hyperplasia) from the chronic inflammation of viral infection. In this latter case, it is controversial but at least theoretically possible that an indirect tumor virus could undergo "hit-and-run" and so the virus would be lost from the clinically diagnosed tumor. In practical terms, this is an uncommon occurrence if it does occur.

Timeline of discovery

Non-human oncoviruses

Human oncoviruses

History

The theory that cancer could be caused by a virus began with the experiments of Oluf Bang and Vilhelm Ellerman in 1908 who first show that avian erythroblastosis (a form of chicken leukemia) could be transmitted by cell-free extracts. [32] This was subsequently confirmed for solid tumors in chickens in 1910-1911 by Peyton Rous, [33] [10] and for liquid cancer in mice by Charlotte Friend. [14]

By the early 1950s, it was known that viruses could remove and incorporate genes and genetic material in cells. It was suggested that such types of viruses could cause cancer by introducing new genes into the genome. Genetic analysis of mice infected with Friend virus confirmed that retroviral integration could disrupt tumor suppressor genes, causing cancer. [34] Subsequently, many viral oncogenes were subsequently discovered and identified to cause cancer.

The main viruses associated with human cancers are human papillomavirus, hepatitis B and hepatitis C virus, Epstein–Barr virus, human T-lymphotropic virus, Kaposi's sarcoma-associated herpesvirus (KSHV) and Merkel cell polyomavirus. Experimental and epidemiological data imply a causative role for viruses and they appear to be the second most important risk factor for cancer development in humans, exceeded only by tobacco usage. [35] The mode of virally induced tumors can be divided into two, acutely transforming or slowly transforming. In acutely transforming viruses, the viral particles carry a gene that encodes for an overactive oncogene called viral-oncogene (v-onc), and the infected cell is transformed as soon as v-onc is expressed. In contrast, in slowly transforming viruses, the virus genome is inserted, especially as viral genome insertion is an obligatory part of retroviruses, near a proto-oncogene in the host genome. The viral promoter or other transcription regulation elements in turn cause overexpression of that proto-oncogene, which in turn induces uncontrolled cellular proliferation. Because viral genome insertion is not specific to proto-oncogenes and the chance of insertion near that proto-oncogene is low, slowly transforming viruses have very long tumor latency compared to acutely transforming viruses, which already carry the viral oncogene.

Hepatitis viruses, including hepatitis B and hepatitis C, can induce a chronic viral infection that leads to liver cancer in 0.47% of hepatitis B patients per year (especially in Asia, less so in North America), and in 1.4% of hepatitis C carriers per year. Liver cirrhosis, whether from chronic viral hepatitis infection or alcoholism, is associated with the development of liver cancer, and the combination of cirrhosis and viral hepatitis presents the highest risk of liver cancer development. Worldwide, liver cancer is one of the most common, and most deadly, cancers due to a huge burden of viral hepatitis transmission and disease.

Through advances in cancer research, vaccines designed to prevent cancer have been created. The hepatitis B vaccine is the first vaccine that has been established to prevent cancer (hepatocellular carcinoma) by preventing infection with the causative virus. In 2006, the U.S. Food and Drug Administration approved a human papilloma virus vaccine, called Gardasil. The vaccine protects against four HPV types, which together cause 70% of cervical cancers and 90% of genital warts. In March 2007, the US Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices (ACIP) officially recommended that females aged 11–12 receive the vaccine, and indicated that females as young as age 9 and as old as age 26 are also candidates for immunization.

DNA Oncoviruses

Introduction

DNA oncoviruses typically impair two families of tumor suppressor proteins: tumor proteins p53 and the retinoblastoma proteins (Rb). It is evolutionarily advantageous for viruses to inactivate p53 because p53 can trigger cell cycle arrest or apoptosis in infected cells when the virus attempts to replicate its DNA. [36] Similarly, Rb proteins regulate many essential cell functions, including but not limited to a crucial cell cycle checkpoint, making them a target for viruses attempting to interrupt regular cell function. [37]

While several DNA oncoviruses have been discovered, three have been studied extensively. Adenoviruses can lead to tumors in rodent models but do not cause cancer in humans; however, they have been exploited as delivery vehicles in gene therapy for diseases such as cystic fibrosis and cancer. [38] Simian virus 40 (SV40), a polyomavirus, can cause tumors in rodent models but is not oncogenic in humans. [39] This phenomenon has been one of the major controversies of oncogenesis in the 20th century because an estimated 100 million people were inadvertently exposed to SV40 through polio vaccines. [39] The Human Papillomavirus-16 (HPV-16) has been shown to lead to cervical cancer and other cancers, including head and neck cancer. [40] These three viruses have parallel mechanisms of action, forming an archetype for DNA oncoviruses. All three of these DNA oncoviruses are able to integrate their DNA into the host cell, and use this to transcribe it and transform cells by bypassing the G1/S checkpoint of the cell cycle.

Integration of viral DNA

DNA oncoviruses transform infected cells by integrating their DNA into the host cell’s genome. [41] The DNA is believed to be inserted during transcription or replication, when the two annealed strands are separated. [41] This event is relatively rare and generally unpredictable; there seems to be no deterministic predictor of the site of integration. [41] After integration, the host’s cell cycle loses regulation from Rb and p53, and the cell begins cloning to form a tumor.

The G1/S Checkpoint

Rb and p53 regulate the transition between G1 and S phase, arresting the cell cycle before DNA replication until the appropriate checkpoint inputs, such as DNA damage repair, are completed. [42] p53 regulates the p21 gene, which produces a protein which binds to the Cyclin D-Cdk4/6 complex. [43] This prevents Rb phosphorylation and prevents the cell from entering S phase. [43] In mammals, when Rb is active (unphosphorylated), it inhibits the E2F family of transcription factors, which regulate the Cyclin E-Cdk2 complex, which inhibits Rb, forming a positive feedback loop, keeping the cell in G1 until the input crosses a threshold. [42] To drive the cell into S phase prematurely, the viruses must inactivate p53, which plays a central role in the G1/S checkpoint, as well as Rb, which, though downstream of it, is typically kept active by a positive feedback loop.

Inactivation of p53

Viruses employ various methods of inactivating p53. The adenovirus E1B protein (55K) prevents p53 from regulating genes by binding to the site on p53 which binds to the genome. [36] In SV40, the large T antigen (LT) is an analogue; LT also binds to several other cellular proteins, such as p107 and p130, on the same residues. [44] LT binds to p53’s binding domain on the DNA (rather than on the protein), again preventing p53 from appropriately regulating genes. [36] HPV instead degrades p53: the HPV protein E6 binds to a cellular protein called the E6-associated protein (E6-AP, also known as UBE3A), forming a complex which causes the rapid and specific ubiquitination of p53. [45]

Inactivation of Rb

Rb is inactivated (thereby allowing the G1/S transition to progress unimpeded) by different but analogous viral oncoproteins. The adenovirus early region 1A (E1A) is an oncoprotein which binds to Rb and can stimulate transcription and transform cells. [36] SV40 uses the same protein for inactivating Rb, LT, to inactivate p53. [43] HPV contains a protein, E7, which can bind to Rb in much the same way. [46] Rb can be inactivated by phosphorylation, or by being bound to a viral oncoprotein, or by mutations—mutations which prevent oncoprotein binding are also associated with cancer. [44]

Variations

DNA oncoviruses typically cause cancer by inactivating p53 and Rb, thereby allowing unregulated cell division and creating tumors. There may be many different mechanisms which have evolved separately; in addition to those described above, for example, the Hepatitis B virus (an RNA virus) inactivates p53 by sequestering it in the cytoplasm. [36]

SV40 has been well studied and does not cause cancer in humans, but a recently discovered analogue called Merkel cell polyomavirus has been associated with Merkel cell carcinoma, a form of skin cancer. [31] The Rb binding feature is believed to be the same between the two viruses. [31]

RNA Oncoviruses

Brief history

In the 1960s, the replication process of RNA virus was believed to be similar to other single-stranded RNA. Single-stranded RNA replication involves RNA-dependent RNA synthesis which meant that virus-coding enzymes would make partial double-stranded RNA. This belief was proven to be incorrect because there were no double-stranded RNA found in the retrovirus cell. In 1964, Howard Temin proposed a provirus hypothesis, but shortly after reverse transcription in the retrovirus genome was discovered.

Description of virus

All retroviruses have three major coding domains; gag, pol and env. In the gag region of the virus, the synthesis of the internal virion proteins are maintained which make up the matrix, capsid and nucleocapsid proteins. In pol, the information for the reverse transcription and integration enzymes are stored. In env, it is derived from the surface and transmembrane for the viral envelope protein. There is a fourth coding domain which is smaller, but exists in all retroviruses. Pol is the domain that encodes the virion protease.

Retrovirus enters host cell

The retrovirus begins the journey into a host cell by attaching a surface glycoprotein to the cell's plasma membrane receptor. Once inside the cell, the retrovirus goes through reverse transcription in the cytoplasm and generates a double-stranded DNA copy of the RNA genome. Reverse transcription also produces identical structures known as long terminal repeats (LTRs). Long terminal repeats are at the ends of the DNA strands and regulates viral gene expression. The viral DNA is then translocated into the nucleus where one strand of the retroviral genome is put into the chromosomal DNA by the help of the virion intergrase. At this point the retrovirus is referred to as provirus. Once in the chromosomal DNA, the provirus is transcribed by the cellular RNA polymerase II. The transcription leads to the splicing and full-length mRNAs and full-length progeny virion RNA. The virion protein and progeny RNA assemble in the cytoplasm and leave the cell, whereas the other copies send translated viral messages in the cytoplasm.

Classification

DNA viruses

RNA viruses

Not all oncoviruses are DNA viruses. Some RNA viruses have also been associated such as the hepatitis C virus as well as certain retroviruses, e.g., human T-lymphotropic virus (HTLV-1) and Rous sarcoma virus (RSV).

Overview table

VirusPercent of cancers [3] Associated cancer types
Hepatitis B (HBV) Hepatocarcinoma [50] [51] [52]
Hepatitis C (HCV)HCV is a known carcinogen, causing hepatocarcinoma [50]
Human T-lymphotropic virus (HTLV)0.03 Adult T-cell leukemia [53]
Human papillomaviruses (HPV)5.2The types 16 and 18 are associated with cancers of cervix, [54] anus, [54] penis, [54] vulva/vagina, [3] and oropharyngeal cancer. [3] According to statistics in the United States, females are more impacted by HPV-associated cancers (83%) than males (74%). [55]
Kaposi's sarcoma-associated herpesvirus (HHV-8)0.9 Kaposi’s sarcoma, multicentric Castleman's disease and primary effusion lymphoma
Merkel cell polyomavirus (MCV)NA Merkel cell carcinoma
Epstein–Barr virus (EBV)NA Burkitt's lymphoma, Hodgkin’s lymphoma, Post-transplant lymphoproliferative disease and Nasopharyngeal carcinoma. [56]

Estimated percent of new cancers attributable to the virus worldwide in 2002. [3] NA indicates not available. The association of other viruses with human cancer is continually under research.

See also

Related Research Articles

Retrovirus family of viruses

A retrovirus is a type of RNA virus that inserts a copy of its genome into the DNA of a host cell that it invades, thus changing the genome of that cell.

Virology study of viruses

Virology is the study of viral – submicroscopic, parasitic particles of genetic material contained in a protein coat – and virus-like agents. It focuses on the following aspects of viruses: their structure, classification and evolution, their ways to infect and exploit host cells for reproduction, their interaction with host organism physiology and immunity, the diseases they cause, the techniques to isolate and culture them, and their use in research and therapy. Virology is considered to be a subfield of microbiology or of medicine.

<i>Papillomaviridae</i> Family of viruses

Papillomaviridae is an ancient taxonomic family of non-enveloped DNA viruses, collectively known as papillomaviruses. Several hundred species of papillomaviruses, traditionally referred to as "types", have been identified infecting all carefully inspected mammals, but also other vertebrates such as birds, snakes, turtles and fish. Infection by most papillomavirus types, depending on the type, is either asymptomatic or causes small benign tumors, known as papillomas or warts. Papillomas caused by some types, however, such as human papillomaviruses 16 and 18, carry a risk of becoming cancerous.

<i>Polyomaviridae</i> family of viruses

Polyomaviridae is a family of viruses whose natural hosts are primarily mammals and birds. As of the most recent (2018) taxonomy release by the International Committee on Taxonomy of Viruses, there were 89 recognized species in this family contained within four genera, as well as 9 species that could not be assigned to a genus. 13 species are known to infect humans, while others, such as Simian Virus 40, have been identified in humans to a lesser extent. Most of these viruses are very common and typically asymptomatic in most human populations studied. BK virus is associated with nephropathy in renal transplant and non-renal solid organ transplant patients, JC virus with progressive multifocal leukoencephalopathy, and Merkel cell virus with Merkel cell cancer.

SV40 is an abbreviation for simian vacuolating virus 40 or simian virus 40, a polyomavirus that is found in both monkeys and humans. Like other polyomaviruses, SV40 is a DNA virus that has the potential to cause tumors in animals, but most often persists as a latent infection. SV40 has been widely studied as a model eukaryotic virus, leading to many early discoveries in eukaryotic DNA replication and transcription.

Mouse mammary tumor virus (MMTV) is a milk-transmitted retrovirus like the HTL viruses, HI viruses, and BLV. It belongs to the genus Betaretrovirus. MMTV was formerly known as Bittner virus, and previously the "milk factor", referring to the extra-chromosomal vertical transmission of murine breast cancer by adoptive nursing, demonstrated in 1936, by John Joseph Bittner while working at the Jackson Laboratory in Bar Harbor, Maine. Bittner established the theory that a cancerous agent, or "milk factor", could be transmitted by cancerous mothers to young mice from a virus in their mother's milk. The majority of mammary tumors in mice are caused by mouse mammary tumor virus.

Kaposis sarcoma-associated herpesvirus species of virus

Kaposi's sarcoma-associated herpesvirus (KSHV) is the ninth known human herpesvirus; its formal name according to the International Committee on Taxonomy of Viruses (ICTV) is Human gammaherpesvirus 8, or HHV-8 in short. Like other herpesviruses, its informal names are used interchangeably with its formal ICTV name. This virus causes Kaposi's sarcoma, a cancer commonly occurring in AIDS patients, as well as primary effusion lymphoma, HHV-8-associated multicentric Castleman's disease and KSHV inflammatory cytokine syndrome. It is one of seven currently known human cancer viruses, or oncoviruses. Even after so many years of discovery of KSHV/HHV8, there is no known cure for KSHV associated tumorigenesis.

Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abnormal cell division. Cell division is a physiological process that occurs in almost all tissues and under a variety of circumstances. Normally the balance between proliferation and programmed cell death, in the form of apoptosis, is maintained to ensure the integrity of tissues and organs. According to the prevailing accepted theory of carcinogenesis, the somatic mutation theory, mutations in DNA and epimutations that lead to cancer disrupt these orderly processes by disrupting the programming regulating the processes, upsetting the normal balance between proliferation and cell death. This results in uncontrolled cell division and the evolution of those cells by natural selection in the body. Only certain mutations lead to cancer whereas the majority of mutations do not.

SV40 large T antigen proto-oncogene derived from polyomavirus SV40

SV40 large T antigen is a hexamer protein that is a dominant-acting oncoprotein derived from the polyomavirus SV40. TAg is capable of inducing malignant transformation of a variety of cell types. The transforming activity of TAg is due in large part to its perturbation of the retinoblastoma (pRb) and p53 tumor suppressor proteins. In addition, TAg binds to several other cellular factors, including the transcriptional co-activators p300 and CBP, which may contribute to its transformation function.

Patrick S. Moore is an Irish and American virologist and epidemiologist who co-discovered together with his wife, Yuan Chang, two different human viruses causing the AIDS-related cancer Kaposi's sarcoma and the skin cancer Merkel cell carcinoma. The couple met while in medical school together and were married in 1989 while they pursued fellowships at different universities.

Yuan Chang American virologist and pathologist

Yuan Chang is an American virologist and pathologist who co-discovered Kaposi's sarcoma-associated herpesvirus (KSHV) and Merkel cell polyomavirus, two of the seven known human oncoviruses.

Rous sarcoma virus (RSV) is a retrovirus and is the first oncovirus to have been described. It causes sarcoma in chickens.

Viral transformation change in cell phenotype resulting from insertion of viral genetic material

Viral transformation is the change in growth, phenotype, or indefinite reproduction of cells caused by the introduction of inheritable material. Through this process, a virus causes harmful transformations of an in vivo cell or cell culture. The term can also be understood as DNA transfection using a viral vector.

TLR9 protein-coding gene in the species Homo sapiens

Toll-like receptor 9 is a protein that in humans is encoded by the TLR9 gene. TLR9 has also been designated as CD289. It is a member of the toll-like receptor (TLR) family. TLR9 is an important receptor expressed in immune system cells including dendritic cells, macrophages, natural killer cells, and other antigen presenting cells. TLR9 preferentially binds DNA present in bacteria and viruses, and triggers signaling cascades that lead to a pro-inflammatory cytokine response. Cancer, infection, and tissue damage can all modulate TLR9 expression and activation. TLR9 is also an important factor in autoimmune diseases, and there is active research into synthetic TLR9 agonists and antagonists that help regulate autoimmune inflammation.

Merkel cell polyomavirus was first described in January 2008 in Pittsburgh, Pennsylvania. It was the first example of a human viral pathogen discovered using unbiased metagenomic next-generation sequencing with a technique called digital transcriptome subtraction. MCV is one of seven currently known human oncoviruses. It is suspected to cause the majority of cases of Merkel cell carcinoma, a rare but aggressive form of skin cancer. Approximately 80% of Merkel cell carcinoma (MCC) tumors have been found to be infected with MCV. Three years later, a team of researchers at the Dana-Farber Cancer Institute developed an antibody that detected MCV expression in 97% of MCC tumors. MCV appears to be a common—if not universal—infection of older children and adults. It is found in respiratory secretions suggesting that it may be transmitted by a respiratory route. But it also can be found shedding from healthy skin, and in gastrointestinal tract tissues and elsewhere, and so its precise mode of transmission remains unknown. In addition, recent studies suggest that this virus may latently infect the human sera and PBMCs.

Virus Small non-cellular infectious agent that only replicates in cells

A virus is a small infectious agent that replicates only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.

Infectious causes of cancer

Estimates place the worldwide risk of cancers from infectious causes at 16.1%. Viral infections are risk factors for cervical cancer, 80% of liver cancers, and 15–20% of the other cancers. This proportion varies in different regions of the world from a high of 32.7% in Sub-Saharan Africa to 3.3% in Australia and New Zealand. Helicobacter pylori is associated with stomach cancer, and Mycobacterium, some other bacteria and parasites also have an effect.

Murine polyomavirus A virus that infects mice and can cause tumors

Murine polyomavirus is an unenveloped double-stranded DNA virus of the polyomavirus family. The first member of the family discovered, it was originally identified by accident in the 1950s. A component of mouse leukemia extract capable of causing tumors, particularly in the parotid gland, in newborn mice was reported by Ludwik Gross in 1953 and identified as a virus by Sarah Stewart and Bernice Eddy at the National Cancer Institute, after whom it was once called "SE polyoma". Stewart and Eddy would go on to study related polyomaviruses such as SV40 that infect primates, including humans. These discoveries were widely reported at the time and formed the early stages of understanding of oncoviruses.

Large tumor antigen

The large tumor antigen is a protein encoded in the genomes of polyomaviruses, which are small double-stranded DNA viruses. LTag is expressed early in the infectious cycle and is essential for viral proliferation. Containing four well-conserved protein domains as well as several intrinsically disordered regions, LTag is a fairly large multifunctional protein; in most polyomaviruses, it ranges from around 600-800 amino acids in length. LTag has two primary functions, both related to replication of the viral genome: it unwinds the virus's DNA to prepare it for replication, and it interacts with proteins in the host cell to dysregulate the cell cycle so that the host's DNA replication machinery can be used to replicate the virus's genome. Some polyomavirus LTag proteins - most notably the well-studied SV40 large tumor antigen from the SV40 virus - are oncoproteins that can induce neoplastic transformation in the host cell.

Small tumor antigen

The small tumor antigen is a protein encoded in the genomes of polyomaviruses, which are small double-stranded DNA viruses. STag is expressed early in the infectious cycle and is usually not essential for viral proliferation, though in most polyomaviruses it does improve replication efficiency. The STag protein is expressed from a gene that overlaps the large tumor antigen (LTag) such that the two proteins share an N-terminal DnaJ-like domain but have distinct C-terminal regions. STag is known to interact with host cell proteins, most notably protein phosphatase 2A (PP2A), and may activate the expression of cellular proteins associated with the cell cycle transition to S phase. In some polyomaviruses - such as the well-studied SV40, which natively infects monkeys - STag is unable to induce neoplastic transformation in the host cell on its own, but its presence may increase the transforming efficiency of LTag. In other polyomaviruses, such as Merkel cell polyomavirus, which causes Merkel cell carcinoma in humans, STag appears to be important for replication and to be an oncoprotein in its own right.

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