Converting the JCV T antigen Rb binding domain to that of SV40 does not alter JCV's limited transforming activity but does eliminate viral viability

Viruses in colorectal cancer

Increasing evidence suggests that microorganisms might represent at least highly interesting cofactors in colorectal cancer (CRC) oncogenesis and progression. Still, associated mechanisms, specifically in colonocytes and their microenvironmental interactions, are still poorly understood. Although, currently, at least seven viruses are being recognized as human carcinogens, only three of these – Epstein–Barr virus (EBV), human papillomavirus (HPV) and John Cunningham virus (JCV) – have been described, with varying levels of evidence, in CRC. In addition, cytomegalovirus (CMV) has been associated with CRC in some publications, albeit not being a fully acknowledged oncovirus. Moreover, recent microbiome studies set increasing grounds for new hypotheses on bacteriophages as interesting additional modulators in CRC carcinogenesis and progression. The present Review summarizes how particular groups of viruses, including bacteriophages, affect cells and the cellular and microbial microenvironment, thereby putatively contributing to foster CRC. This could be achieved, for example, by promoting several processes – such as DNA damage, chromosomal instability, or molecular aspects of cell proliferation, CRC progression and metastasis – not necessarily by direct infection of epithelial cells only, but also by interaction with the microenvironment of infected cells. In this context, there are striking common features of EBV, CMV, HPV and JCV that are able to promote oncogenesis, in terms of establishing latent infections and affecting p53‐/pRb‐driven, epithelial–mesenchymal transition (EMT)‐/EGFR‐associated and especially Wnt/β‐catenin‐driven pathways. We speculate that, at least in part, such viral impacts on particular pathways might be reflected in lasting (e.g. mutational or further genomic) fingerprints of viruses in cells. Also, the complex interplay between several species within the intestinal microbiome, involving a direct or indirect impact on colorectal and microenvironmental cells but also between, for example, phages and bacterial and viral pathogens, and further novel species certainly might, in part, explain ongoing difficulties to establish unequivocal monocausal links between specific viral infections and CRC.

Host-Immune Interactions in JC Virus Reactivation and Development of Progressive Multifocal Leukoencephalopathy (PML)

With the advent of immunomodulatory therapies and the HIV epidemic, the impact of JC Virus (JCV) on the public health system has grown significantly due to the increased incidence of Progressive Multifocal Leukoencephalopathy (PML). Currently, there are no pharmaceutical agents targeting JCV infection for the treatment and the prevention of viral reactivation leading to the development of PML. As JCV primarily reactivates in immunocompromised patients, it is proposed that the immune system (mainly the cellular-immunity component) plays a key role in the regulation of JCV to prevent productive infection and PML development. However, the exact mechanism of JCV immune regulation and reactivation is not well understood. Likewise, the impact of host factors on JCV regulation and reactivation is another understudied area. Here we discuss the current literature on host factor-mediated and immune factor-mediated regulation of JCV gene expression with the purpose of developing a model of the factors that are bypassed during JCV reactivation, and thus are potential targets for the development of therapeutic interventions to suppress PML initiation. Graphical Abstract.

Neuroimmune Regulation of JC Virus by Intracellular and Extracellular Agnoprotein

JC virus (JCV) is a human polyomavirus and the etiologic agent of the demyelinating disease progressive multifocal leukoencephalopathy (PML). PML is observed in patients with underlying immunocompromising conditions, suggesting that neuro-immune interactions between peripheral immune cells and neuro-glia play an important role in controlling viral reactivation in the brain. There is little known about the immunobiology of JCV reactivation in glial cells and the role of immune, glial, and viral players in this regulation. We have previously showed that agnoprotein, a small JCV regulatory protein, is released from infected cells and internalized by neighboring bystander cells. Here we have investigated the possible role of extracellular and intracellular agnoprotein in the neuroimmune response to JC virus. Our findings suggest that glial cells exposed to agnoprotein secrete significantly less GM-CSF, which is mediated by agnoprotein induced suppression of GM-CSF transcription. Likewise, monocytes treated with agnoprotein showed altered differentiation and maturation. In addition, monocytes and microglial cells exposed to agnoprotein showed a significant reduction in their phagocytic activities. Moreover, when an in vitro blood-brain barrier model was used, agnoprotein treatment resulted in decreased monocyte migration through the endothelial cell layer in response to activated astrocytes. All together, these results have revealed a novel immunomodulatory function of agnoprotein during JCV infection within theCNS and open a new avenue of research to better understand the mechanisms associated with JCV reactivation in patients who are at risk of developing PML.

Removal of a small C-terminal region of JCV and SV40 large T antigens has differential effects on transformation

The large T antigen (LT) protein of JCV and SV40 polyomaviruses is required to induce tumors in rodents and transform cells in culture. When both LTs are compared side-by-side in cell culture assays, SV40 shows a more robust transformation phenotype even though the LT sequences are highly conserved. A complete understanding of SV40׳s enhanced transforming capabilities relative to JCV is lacking. When the least conserved region of the LT proteins, the variable linker and host range region (VHR), was removed, changes in T antigen expression and cellular p53 post-translational modifications occurred, but interaction with the pRB pathway was unaffected. Transformation assessed by growth in low serum was reduced after VHR truncation of the SV40, but not the JCV, T antigen. Conversely, anchorage independent transformation was enhanced only by truncation of the JCV VHR. This is the first report to link the SV40 or JCV VHR region to transformation potential.

Molecular biology, epidemiology, and pathogenesis of progressive multifocal leukoencephalopathy, the JC virus-induced demyelinating disease of the human brain

Progressive multifocal leukoencephalopathy (PML) is a debilitating and frequently fatal central nervous system (CNS) demyelinating disease caused by JC virus (JCV), for which there is currently no effective treatment. Lytic infection of oligodendrocytes in the brain leads to their eventual destruction and progressive demyelination, resulting in multiple foci of lesions in the white matter of the brain. Before the mid-1980s, PML was a relatively rare disease, reported to occur primarily in those with underlying neoplastic conditions affecting immune function and, more rarely, in allograft recipients receiving immunosuppressive drugs. However, with the onset of the AIDS pandemic, the incidence of PML has increased dramatically. Approximately 3 to 5% of HIV-infected individuals will develop PML, which is classified as an AIDS-defining illness. In addition, the recent advent of humanized monoclonal antibody therapy for the treatment of autoimmune inflammatory diseases such as multiple sclerosis (MS) and Crohn's disease has also led to an increased risk of PML as a side effect of immunotherapy. Thus, the study of JCV and the elucidation of the underlying causes of PML are important and active areas of research that may lead to new insights into immune function and host antiviral defense, as well as to potential new therapies.

Human papillomavirus type 18 chimeras containing the L2/L1 capsid genes from evolutionarily diverse papillomavirus types generate infectious virus

Papillomaviruses (PVs) comprise a large family of viruses infecting nearly all vertebrate species, with more than 100 human PVs identified. Our previous studies showed that a mutant chimera HPV18/16 genome, consisting of the upper regulatory region and early ORFs of HPV18 and the late ORFs of HPV16, was capable of producing infectious virus in organotypic raft cultures. We were interested in determining whether the ability of this chimeric genome to produce infectious virus was the result of HPV18 and HPV16 being similarly oncogenic, anogenital types and whether more disparate PV types could also interact functionally. To test this we created a series of HPV18 chimeric genomes where the ORFs for the HPV18 capsid genes were replaced with the capsid genes of HPV45, HPV39, HPV33, HPV31, HPV11, HPV6b, HPV1a, CRPV, and BPV1. All chimeras were able to produce infectious chimeric viral particles, although with lower infectivity than wild-type HPV18. Steps in the viral life cycle and characteristics of the viral particles were examined to identify potential causes for the decrease in infectivity.

JC virus small T antigen binds phosphatase PP2A and Rb family proteins and is required for efficient viral DNA replication activity

BACKGROUND

The human polyomavirus, JC virus (JCV) produces five tumor proteins encoded by transcripts alternatively spliced from one precursor messenger RNA. Significant attention has been given to replication and transforming activities of JCV's large tumor antigen (TAg) and three T' proteins, but little is known about small tumor antigen (tAg) functions. Amino-terminal sequences of tAg overlap with those of the other tumor proteins, but the carboxy half of tAg is unique. These latter sequences are the least conserved among the early coding regions of primate polyomaviruses.

METHODOLOGY AND FINDINGS

We investigated the ability of wild type and mutant forms of JCV tAg to interact with cellular proteins involved in regulating cell proliferation and survival. The JCV P99A tAg is mutated at a conserved proline, which in the SV40 tAg is required for efficient interaction with protein phosphatase 2A (PP2A), and the C157A mutant tAg is altered at one of two newly recognized LxCxE motifs. Relative to wild type and C157A tAgs, P99A tAg interacts inefficiently with PP2A in vivo. Unlike SV40 tAg, JCV tAg binds to the Rb family of tumor suppressor proteins. Viral DNAs expressing mutant t proteins replicated less efficiently than did the intact JCV genome. A JCV construct incapable of expressing tAg was replication-incompetent, a defect not complemented in trans using a tAg-expressing vector.

CONCLUSIONS

JCV tAg possesses unique properties among the polyomavirus small t proteins. It contributes significantly to viral DNA replication in vivo; a tAg null mutant failed to display detectable DNA replication activity, and a tAg substitution mutant, reduced in PP2A binding, was replication-defective. Our observation that JCV tAg binds Rb proteins, indicates all five JCV tumor proteins have the potential to influence cell cycle progression in infected and transformed cells. It remains unclear how these proteins coordinate their unique and overlapping functions.

Transforming Activities of JC Virus Early Proteins

Polyomaviruses, as their name indicates, are viruses capable of inducing a variety of tumors in vivo. Members of this family, including the human JC and BK viruses (JCV, BKV), and the better characterized mouse polyomavirus and simian virus 40 (SV40), are small DNA viruses that commandeer a cell's molecular machinery to reproduce themselves. Studies of these virus-host interactions have greatly enhanced our understanding of a wide range of phenomena from cellular processes (e.g., DNA replication and transcription) to viral oncogenesis. The current chapter will focus upon the five known JCV early proteins and the contributions each makes to the oncogenic process (transformation) when expressed in cultured cells. Where appropriate, gaps in our understanding of JCV protein function will be supplanted with information obtained from the study of SV40 and BKV.

JC virus T'135, T'136 and T'165 proteins interact with cellular p107 and p130 in vivo and influence viral transformation potential

The JC virus (JCV) regulatory proteins, large T antigen, small t antigen, T'135, T'136, and T'165, are encoded by five transcripts alternatively spliced from the viral early precursor mRNA. T antigen and the T' proteins share N-terminal amino acid sequences that include the L x CxE and J domains, motifs in SV40 T antigen known to mediate binding to the retinoblastoma (Rb) proteins and Hsc70, respectively. In this study, G418-resistant cell lines were created that express wild-type or mutant JCV T antigen and T' proteins individually or in combination. These cell lines were used to evaluate the ability of each viral protein to bind p107 and p130 in vivo, and to influence cellular growth characteristics. Differences were observed in the abilities of individual T' proteins to bind p107 and p130 and to alter their phosphorylation status. The T' proteins were also found to localize to the cell's nucleus and to be phosphorylated in a cell cycle-dependent manner. JCV T antigen and T' proteins expressed from a cytomegalovirus promoter failed to induce dense focus formation in Rat2 cells, but they did cooperate with a mutant Ras protein to overcome cellular senescence and immortalize rat embryo fibroblasts. These data indicate that, despite their sequence similarities, JCV early proteins exhibit unique activities that, in combination, effect the inactivation of cell cycle regulators, a requirement for polyomavirus-induced transformation.

Interaction of retinoblastoma protein family members with large T-antigen of primate polyomaviruses

The retinoblastoma gene product pRb and other members of the Rb family of pocket proteins have a central role in the regulation of cell cycle progression. Soon after its discovery, pRb was found to interact with the transforming oncoproteins of DNA tumor viruses and this led to rapid advances in our understanding of the mechanisms of viral transformation and cell cycle progression. DNA viruses of the polyomavirus family have small, circular, double-stranded DNA genomes contained within non-enveloped icosahedral capsids and are highly tumorigenic in experimental animals. At least three types of polyomavirus infect humans: JC virus (JCV), BK virus (BKV) and Simian Vacuolating virus-40. The early region of these viruses encodes the transforming proteins large T-antigen and small t-antigen, which are involved in viral replication and also promote transformation of cells in culture and oncogenesis in vivo. Binding of T-antigen to pRb promotes the activation of the E2F family of transcription factors, which induce the expression of cellular genes required for S phase. In the context of lytic infection, this cell cycle progression is necessary for viral replication because polyomaviruses rely on S phase-specific host factors for their DNA synthesis. In the context of cellular transformation and tumorigenesis, T-antigen/pRB interaction is an indispensable event.

The polyomavirus, JCV and its involvement in human disease

The human neurotropic polyomavirus, JC virus (JCV), is the etiologic agent of progressive multifocal leukoencephalopathy (PML), a fatal demyelinating disease of the central nervous system that occurs mainly in immunosuppressed patients. JCV has also been found to be associated with human tumors of the brain and other organs. In this chapter, we describe JC virus and its role in human diseases.

Stability and function of JC virus large T antigen and T' proteins are altered by mutation of their phosphorylated threonine 125 residues

JC virus (JCV), a human polyomavirus, exhibits oncogenic activity in rodents and primates. The large tumor antigens (TAgs) of the polyomaviruses play key roles in viral replication and oncogenic transformation. Analyses of JCV TAg phosphorylation mutants indicated that the amino-terminal phosphorylation site at threonine 125 (T125) is critical to TAg replication function. This site is also conserved in the TAg splice variants T'(135), T'(136), and T'(165). By constructing stable cell lines expressing JCV T125A and T125D mutants, we show that mutation of this phosphorylation site to alanine generates an unstable TAg; however, the stability of the three T' proteins is unaffected. JCV T125A mutant proteins bind the retinoblastoma protein (RB) family members p107 and p130 with slightly reduced efficiencies and fail to induce the release of transcriptionally active E2F from RB-E2F complexes. On the other hand, cell lines expressing JCV T125D mutant proteins produce stable TAg and T' proteins which bind p107 and p130 more efficiently than do the wild-type proteins. In addition, T125D mutant proteins efficiently induce the release of E2F from RB-E2F complexes. T125D mutant cell lines, unlike the T125A mutant lines, continue to grow under conditions of low serum concentration and anchorage independence. Finally, both T125A and T125D mutant viruses are replication defective. Phosphorylation of the T125 site is likely mediated by a cyclin-cyclin-dependent kinase, suggesting that JCV TAg and T' protein functions that mediate viral replication and oncogenic transformation events are regulated in a cell cycle-dependent manner.

Expression of JC virus regulatory proteins in human cancer: potential mechanisms for tumourigenesis

JC virus (JCV) is a human polyomavirus that is the etiologic agent of the fatal demyelinating disease of the central nervous system known as progressive multifocal leukoencephalopathy (PML). JCV is also linked to some tumours of the brain and other organs as evidenced by the presence of JCV DNA sequences and the expression of viral proteins in clinical samples. Since JCV is highly oncogenic in experimental animals and transforms cells in culture, it is possible that JCV contributes to the malignant phenotype of human tumours with which it is associated. JCV encodes three non-capsid regulatory proteins: large T-antigen, small t-antigen and agnoprotein that interact with a number of cellular target proteins and interfere with certain normal cellular functions. In this review, we discuss how JCV proteins deregulate signalling pathways especially ones pertaining to transcriptional regulation and cell cycle control. These effects may be involved in the progression of JCV-associated tumours and may represent potential therapeutic targets.

Human polyomaviruses and brain tumors

Polyomaviruses are DNA tumor viruses with small circular genomes. Three polyomaviruses have captured attention with regard to their potential role in the development of human brain tumors: JC virus (JCV), BK virus (BKV), and simian vacuolating virus 40 (SV40). JCV is a neurotropic polyomavirus that is the etiologic agent of progressive multifocal leukoencephalopathy (PML), a fatal demyelinating disease of the central nervous system occurring mainly in AIDS patients. BKV is the causative agent of polyomavirus-associated nephropathy (PVN) which occurs after renal transplantation when BKV reactivates from a latent state during immunosuppressive therapy to cause allograft failure. SV40, originating in rhesus monkeys, gained notoriety when it entered the human population via contaminated polio vaccines. All three viruses are highly oncogenic when injected into the brain of experimental animals. Reports indicate that these viruses, especially JCV, are associated with brain tumors and other cancers in humans as evidenced from the analysis of clinical samples for the presence of viral DNA sequences and expression of viral proteins. Human polyomaviruses encode three non-capsid regulatory proteins: large T-antigen, small t-antigen, and agnoprotein. These proteins interact with a number of cellular target proteins to exert effects that dysregulate pathways involved in the control of various host cell functions including the cell cycle, DNA repair, and others. In this review, we describe the three polyomaviruses, their abilities to cause brain and other tumors in experimental animals, the evidence for an association with human brain tumors, and the latest findings on the molecular mechanisms of their actions.

Polyomaviruses and human cancer: molecular mechanisms underlying patterns of tumorigenesis

Polyomaviruses are DNA tumor viruses with small circular genomes encoding only six proteins including three structural capsid proteins. Despite this simplicity, our understanding of the mechanisms of polyomavirus-mediated tumorigenesis is far from complete. The archetypal primate polyomavirus, SV40, was isolated more than 40 years ago and has been used extensively as a model system for the study of basic eukaryotic cellular processes such as DNA replication and transcription. Two human polyomaviruses have been isolated from clinical samples: JC virus (JCV) and BK virus (BKV). In this review, SV40, JCV, and BKV will be compared based on what is known about their molecular biology from experiments performed in vitro, in cell culture and in laboratory animals. The association of these viruses with clinical tumors is discussed along with the possible roles of these polyomaviruses in the etiology of human malignant disease.

SV40-positive brain tumor in scientist with risk of laboratory exposure to the virus

Simian virus 40 (SV40) is a DNA tumor virus known to induce cancers in laboratory animals. There are numerous reports of the detection of SV40 DNA and/or proteins in human malignancies of the same types as those induced by SV40 in animals, including brain cancers. However, known exposure to the virus has not yet been linked directly to cancer development in a specific individual. Here we describe the detection of SV40 sequences in the meningioma of a laboratory researcher who had a probable direct exposure to SV40 and subsequently developed a tumor positive for viral DNA sequences indistinguishable from those of the laboratory source. This case suggests a link between viral exposure and tumor development.

Infectious virions produced from a human papillomavirus type 18/16 genomic DNA chimera

The organotypic raft culture system has allowed the study of the differentiation-dependent aspects of the human papillomavirus (HPV) life cycle. However, genetic strategies to more completely understand the HPV life cycle are limited. The generation of chimeric viruses has been a useful tool in other virus systems to analyze infection and replication. To investigate the specificity of the interaction of nonstructural genes of one HPV type with the structural genes of another HPV type, we have replaced the L2 and L1 open reading frames (ORFs) of HPV type 18 (HPV18) with the L2 and L1 ORFs of HPV type 16 (HPV16). The resulting HPV18/16 chimeric construct was introduced into primary keratinocytes, where it was stably maintained episomally at a range of 50 to 100 copies of HPV genomic DNA, similar to that typically found in HPV-infected cells in vivo. The integrity of the HPV18/16 genomic DNA chimera was demonstrated. Upon differentiation in raft cultures, late viral functions, including viral DNA amplification, capsid gene expression, and virion morphogenesis, occurred. Chimeric HPV18/16 virions were purified from the raft cultures and were capable of infecting keratinocytes in vitro. Additionally, infection was specifically neutralized with human HPV16 virus-like particle (VLP)-specific antiserum and not with human HPV18 VLP-specific antiserum. Our data demonstrate that the nonstructural genes of HPV18 functionally interact with the structural genes of HPV16, allowing the complete HPV life cycle to occur. We believe that this is the first report of the propagation of chimeric HPV by normal life cycle pathways.

JC virus large T protein transforms rodent cells but is not involved in human medulloblastoma

JC virus (JCV) together with Simian virus 40 (SV40) and BK virus (BKV), belong to the polyomavirus group and these viruses are neuro-oncogenic to rodents by expression of large T antigen (LT), which binds to cellular p53 and pRB thus reducing the anticancer potential of the cell. The function of LT has not been clarified because small t antigen (st) is transcribed from the same start codon as the overlapping reading frame of LT, and is translated as a different protein with the same N-terminal residues (1-81 amino acids) by a splice-site variant of mRNA. To elucidate the function of LT without st, we constructed plasmids that express LT only by deleting the splicing region including the C-terminus of st, and consequently stable cell lines were established that express only JCLT, SV40LT and BKLT. The growth rates of these cells were examined in colonies on soft agar and it was found that LT alone has a transforming capacity; the order of efficiency being SV40LT, BKLT and JCLT. In addition, to verify the involvement of JCV in human medulloblastoma, eight cases of medulloblastoma, six cases of frozen material and five cases of paraffin-embedded tissues which included three cases of frozen tissues, were examined. PCR assay, genomic Southern blotting, and in situ hybridization were applied to detect the JCV genome, and LT and st were examined by immunohistochemistry; the results were compared with JCV-infected tissues as a positive control. All methods failed to detect not only JCV genome but also LT protein in medulloblastoma and it was concluded that JCV LT has transforming activities in rodent cells, but is not related to human medulloblastoma.

Human cytomegalovirus immediate-early gene 2 expression leads to JCV replication in nonpermissive cells via transcriptional activation of JCV T antigen

Human papovavirus JCV is the causative agent of the demyelinating brain disease progressive multifocal leukoencephalopathy (PML) that typically develops as a complication of impaired immunocompetence. JCV displays a strong tropism for glial cells which is correlated by glial-specific transcriptional regulation of viral gene expression. In a previous report HCMV was shown to overcome the restricted cell specificity of JCV by inducing DNA replication of a PML-derived JCV strain in human fibroblasts which are nonpermissive for the replication of JCV alone. Here we show that productive JCV replication is induced by HCMV in human glioblastoma cells. Both in fibroblasts and in glioblastoma cells, the HCMV immediate-early transactivator 2 (IE2) is sufficient to mediate JCV replication. Furthermore, IE2 induces DNA replication of several structurally different brain- or kidney-derived JCV variants. IE2-induced JCV DNA replication is accompanied by the induction of JCV T antigen expression due to stimulation of the JCV early promoter. Our results indicate that stimulation of JCV early gene expression by HCMV-IE2 is sufficient to overcome the restricted cell specificity of JCV.

Purified JC virus T and T' proteins differentially interact with the retinoblastoma family of tumor suppressor proteins

The amino termini of polyomavirus T antigens contain LXCXE and J domains, which are necessary for binding and inactivating the retinoblastoma family of tumor suppressors. Both of these motifs are found in the JC virus (JCV) early proteins T'(135), T'(136), and T'(165), leading to the suggestion that these recently discovered proteins complement the cell-cycle-deregulating function of the JCV large T antigen (TAg). To investigate this hypothesis, the three JCV T' proteins were produced in a baculovirus expression system and purified by immunoaffinity chromatography. To facilitate purification, hybridomas that secrete antibodies recognizing amino-terminal epitopes of JCV early proteins were produced. Potential interactions between the early viral proteins and the cellular proteins pRB, p107, and p130 were investigated by incubating purified JCV TAg and T' proteins with extracts of MOLT-4 cells, a human T cell line. The four viral proteins preferentially bound hypophosphorylated species of the cellular proteins and exhibited the highest binding affinity to p107 and the lowest affinity to pRB. TAg and T'(165) bound more pRB and less p107 than did T'(135) and T'(136); T'(165) also bound less p130 than the other three early proteins. Results of these in vitro interactions were compared to those obtained in vivo using POJ cells, a transformed human glial cell line that expresses JCV early proteins, relatively high levels of pRB and p107, and low levels of p130. Most of the pRB in POJ cells is hyperphosphorylated, and only a fraction of the hypophosphorylated form(s) of pRB is bound by the viral proteins. In contrast, only hypophosphorylated p130 is detected in the transformed cells, and most of this protein was found in complex with the viral proteins. Finally, nearly all of the p107 in POJ cells is bound by the JCV proteins.

Detection of human neurotropic JC virus DNA sequence and expression of the viral oncogenic protein in pediatric medulloblastomas

Medulloblastoma represents greater than 25% of childhood intracranial neoplasms and is considered a highly malignant tumor. This tumor, which arises predominantly in the cerebellar vermis, preferentially affects children between the ages of 5 and 15. Although the etiology of medulloblastomas in humans remains unknown, results from several experiments have indicated that the human neurotropic JC virus (JCV) is able to induce cerebellar neoplasms in rodents that exhibit a phenotype similar to that of human medulloblastomas. JCV is a polyomavirus that is widespread in the human population, with infection occurring most frequently in early childhood. In this study, we have examined the possible association of JCV with human medulloblastomas. By using PCR techniques we demonstrate that 11 of 23 samples of tumor tissue contain DNA sequences corresponding to three different regions of the JCV genome. More importantly, we demonstrate the presence of DNA sequences encoding the N- and C-terminal regions of the JCV oncogenic protein, T antigen, in 11 of 23 samples and the production of T antigen in the nuclei of 4 samples of tumor tissue. These observations provide evidence for a possible association of JCV with human medulloblastomas.

Adding an Rb-binding site to an N-terminally truncated simian virus 40 T antigen restores growth to high cell density, and the T common region in trans provides anchorage-independent growth and rapid growth in low serum concentrations

The simian virus 40 large T antigen is sufficient to confer on cells multiple transformed cell growth characteristics, including growth to a high cell density, rapid growth in medium containing low serum concentrations, and anchorage-independent growth. We showed previously that distinct regions of the protein were involved in conferring these properties and that removal of the first 127 amino acids of T antigen abrogated all three activities. At least three large-T-antigen transformation-related activities have been localized to that region: binding of the tumor suppressor gene product Rb and two independent activities contained within the common region shared by large T and small t antigens. The experiments described here were directed toward determining whether these were the only activities from the N terminus that were needed. To do so we reintroduced an Rb-binding region into the N-terminally truncated T antigen (T128-708) and examined the growth properties of cells immortalized by it in the presence and absence of small t antigen, which can provide the T-common-region transformation-related activities in trans. We show that an Rb-binding region consisting of amino acids 101 to 118, when introduced into a heterologous site in T128-708, is capable of physically binding Rb and that binding is sufficient for cells expressing the protein to acquire the ability to grow to a high saturation density. However, in low-serum medium, the growth rate of the cells and maximal cell density are reduced relative to those of wild-type-T-antigen-expressing cells, and the cells cannot divide without anchorage. This result suggests that although Rb binding is sufficient in the context of T128-708 to confer growth to a high density, one or more other N-terminally located T-antigen activities are needed for cells to acquire the additional growth properties. Small t antigen in trans supplied those activities. These results indicate that the T-common-region activities and Rb binding are the only activities from the T-antigen N terminus needed to restore full transforming activity to the N-terminally truncated T antigen.

Functional implications of mutations within polyomavirus large T antigen Rb-binding domain: effects on pRb and p107 binding in vitro and immortalization activity in vivo

In this study, we have extensively modified the Rb-binding domain of polyomavirus large T antigen. Mutant polyomavirus large T antigens were tested for their ability to bind pRb and p107 in vitro and assayed for their capacity to immortalize primary rat embryo fibroblasts in vivo. Polyomavirus large T antigen bound pRb and p107 through a common region located between amino acids 141 to 158, containing the consensus Rb-binding sequence D/N-L-X-C-X-E. Substitution of any amino acid within the core Rb-binding sequence abolished pRb and p107 binding in vitro and immortalization activity in vivo. Substitution of amino acids outside the core Rb-binding sequence reduced pRb and p107 binding in vitro and decreased or abolished immortalization of rat embryo fibroblasts in vivo. Although duplication of the Rb-binding domain within the polyomavirus large T antigen results in a molecule that can bind at least twice as much pRb and p107 in vitro, this mutant displayed an essentially wild-type level of immortalization activity. More importantly, we found that the addition of acidic residues within the casein kinase II consensus phosphorylation region flanking the Rb-binding domain, or the deletion of amino acids 256 to 272, increased the immortalizing activity of the mutant polyomavirus large T antigen. These two mutants displayed a greater than wild-type level of pRb binding in vitro, while in contrast, a decreased affinity for p107 binding in vitro was observed. Together, these results indicate that while pRb binding appears to be an essential event for immortalization, there is no tight correlation between the frequency of immortalization and the absolute level of pRb binding in vitro, indicating that other large T antigen functions are important for cellular immortalization.