Mutation in the polyomavirus genome that activates the properties of large T associated with neoplastic transformation

High-affinity Rb binding, p53 inhibition, subcellular localization, and transformation by wild-type or tumor-derived shortened Merkel cell polyomavirus large T antigens

Interference with tumor suppressor pathways by polyomavirus-encoded tumor antigens (T-Ags) can result in transformation. Consequently, it is thought that T-Ags encoded by Merkel cell polyomavirus (MCPyV), a virus integrated in ∼90% of all Merkel cell carcinoma (MCC) cases, are major contributors to tumorigenesis. The MCPyV large T-Ag (LT-Ag) has preserved the key functional domains present in all family members but has also acquired unique regions that flank the LxCxE motif. As these regions may mediate unique functions, or may modulate those shared with T-Ags of other polyomaviruses, functional studies of MCPyV T-Ags are required. Here, we have performed a comparative study of full-length or MCC-derived truncated LT-Ags with regard to their biochemical characteristics, their ability to bind to retinoblastoma (Rb) and p53 proteins, and their transforming potential. We provide evidence that full-length MCPyV LT-Ag may not directly bind to p53 but nevertheless can significantly reduce p53-dependent transcription in reporter assays. Although early region expression constructs harboring either full-length or MCC-derived truncated LT-Ag genes can transform primary baby rat kidney cells, truncated LT-Ags do not bind to p53 or reduce p53-dependent transcription. Interestingly, shortened LT-Ags exhibit a very high binding affinity for Rb, as shown by coimmunoprecipitation and in vitro binding studies. Additionally, we show that truncated MCPyV LT-Ag proteins are expressed at higher levels than those for the wild-type protein and are able to partially relocalize Rb to the cytoplasm, indicating that truncated LT proteins may have gained additional features that distinguish them from the full-length protein.

IMPORTANCE

MCPyV is one of the 12 known polyomaviruses that naturally infect humans. Among these, it is of particular interest since it is the only human polyomavirus known to be involved in tumorigenesis. MCPyV is thought to be causally linked to MCC, a rare skin tumor. In these tumors, viral DNA is monoclonally integrated into the genome of the tumor cells in up to 90% of all MCC cases, and the integrated MCV genomes, furthermore, harbor signature mutations in the so-called early region that selectively abrogate viral replication while preserving cell cycle deregulating functions of the virus. This study describes comparative studies of early region T-Ag protein characteristics, their ability to bind to Rb and p53, and their transforming potential.

Comparisons between murine polyomavirus and Simian virus 40 show significant differences in small T antigen function

Although members of a virus family produce similar gene products, those products may have quite different functions. Simian virus 40 (SV40) large T antigen (LT), for example, targets p53 directly, but murine polyomavirus LT does not. SV40 small T antigen (SVST) has received considerable attention because of its ability to contribute to transformation of human cells. Here, we show that there are major differences between SVST and polyomavirus small T antigen (POLST) in their effects on differentiation, transformation, and cell survival. Both SVST and POLST induce cell cycle progression. However, POLST also inhibits differentiation of 3T3-L1 preadipocytes and C2C12 myoblasts. Additionally, POLST induces apoptosis of mouse embryo fibroblasts. SVST reduces the proapoptotic transcriptional activity of FOXO1 through phosphorylation. On the other hand, SVST complements large T antigen and Ras for the transformation of human mammary epithelial cells (HMECs), but POLST does not. Mechanistically, the differences between SVST and POLST may lie in utilization of protein phosphatase 2A (PP2A). POLST binds both Aα and Aβ scaffolding subunits of PP2A while SVST binds only Aα. Knockdown of Aβ could mimic POLST-induced apoptosis. The two small T antigens can target different proteins for dephosphorylation. POLST binds and dephosphorylates substrates, such as lipins, that SVST does not.

Polyomavirus small t antigen prevents retinoic acid-induced retinoblastoma protein hypophosphorylation and redirects retinoic acid-induced G0 arrest and differentiation to apoptosis

Polyomavirus small t antigen (ST) impedes late features of retinoic acid (RA)-induced HL-60 myeloid differentiation as well as growth arrest, causing apoptosis instead. HL-60 cells were stably transfected with ST. ST slowed the cell cycle, retarding G2/M in particular. Treated with RA, the ST transfectants continued to proliferate and underwent apoptosis. ST also impeded the normally RA-induced hypophosphorylation of the retinoblastoma tumor suppressor protein consistent with failure of the cells to arrest growth. The RA-treated transfectants expressed CD11b, an early cell surface differentiation marker, but inducible oxidative metabolism, a later and more mature functional differentiation marker, was largely inhibited. Instead, the cells underwent apoptosis. ST affected significant known components of RA signaling that result in G0 growth arrest and differentiation in wild-type HL-60. ST increased the basal amount of activated ERK2, which normally increases when wild-type cells are treated with RA. ST caused increased RARalpha expression, which is normally down regulated in RA-treated wild-type cells. The effects of ST on RA-induced myeloid differentiation did not extend to monocytic differentiation and G0 arrest induced by 1,25-dihydroxy vitamin D3, whose receptor is also a member of the steroid-thyroid hormone superfamily. In this case, ST abolished the usually induced G0 arrest and retarded, but did not block, differentiation without inducing apoptosis, thus uncoupling growth arrest and differentiation. In sum, the data show that ST disrupted the normal RA-induced program of G0 arrest and differentiation, causing the cells to abort differentiation and undergo apoptosis.

Polyomavirus large T antigen mutants affected in viral DNA replication

We have characterized two polyomavirus large T antigen mutants with different properties in viral DNA replication. dl-97, a mutant active in immortalization, exerts a dominant negative effect in viral DNA replication. 13val, which is defective in both immortalization and viral DNA replication, has a lesion in the putative DnaJ domain affecting the block of Rb function.

Signaling from polyomavirus middle T and small T defines different roles for protein phosphatase 2A

Polyomavirus causes a broad spectrum of tumors as the result of the action of its early proteins. This work compares signaling from middle T antigen (MT), the major transforming protein, to that from small T antigen (ST). The abilities of MT mutants to promote cell cycle progression in serum-starved NIH 3T3 cells were compared. Transformation-defective mutants lacking association with SHC or with phosphatidylinositol 3-kinase (PI3-K) retained the ability to induce DNA synthesis as measured by bromodeoxyuridine incorporation. Only when both interactions were lost in the Y250F/Y315F double mutant was MT inactive. ST promoted cell cycle progression in a manner dependent on its binding of protein phosphatase 2A (PP2A). Since the Y250F/Y315F MT mutant was wild type for PP2A binding yet unable to promote cell cycle progression, while ST was capable of promoting cell cycle progression, these experiments revealed a functional difference in MT and ST signaling via PP2A. Assays testing the abilities of MT and ST to induce the c-fos promoter and to activate c-jun kinase led to the same conclusion. ST, but not Y250F/Y315F MT, was able to activate the c-fos promoter through its interaction with PP2A. In contrast, MT, but not ST, was able to activate c-jun kinase by virtue of its interaction with PP2A.

Genetic analysis of polyomavirus large T nuclear localization: nuclear localization is required for productive association with pRb family members

Polyomavirus large T antigen (LT) is a multifunctional nuclear protein. LT has two nuclear localization signals (NLS2), one spanning residues 189 to 195 (NLS1) and another spanning residues 280 to 286 (NLS2). Site-directed mutagenesis showed that each signal contains at least two critical residues. The possibility of connections between NLSs and adjacent phosphorylations has attracted much attention. Cytoplasmic LT (CyT) mutants were underphosphorylated, particularly at sites adjacent to NLS2. However, since a nuclear LT bearing an inactivated NLS2 was phosphorylated normally at adjacent sites, the signal was not directly required for phosphorylation. Conversely, LT could be translocated to the nucleus via NLS2 even when the adjacent phosphorylation sites were deleted. CyT was examined to probe the importance of LT localization. CyT was unable to perform LT functions related to interactions with retinoblastoma susceptibility gene (pRb) family members. Hence, CyT was unable to immortalize primary cells or to transactivate an E2F-responsive promoter. Consistent with these findings, CyT, though capable of binding pRb in vitro, did not cause relocalization of pRb in cells. Assays of transactivation of the simian virus 40 late promoter and of the human c-fos promoter showed that defects of CyT were not limited to functions dependent on pRb interactions.

Intrachromosomal recombination mediated by the polyomavirus large T antigen

We used a spleen necrosis virus-based retroviral vector to introduce the polyomavirus replication origin into rat cells and developed a system to analyze homologous recombination events that do not reconstitute a selectable marker. Introduction of the gene coding for the polyomavirus large T antigen into the cell lines by DNA transfection promoted high-frequency recombination between the two retroviral LTRs, leading to amplification and excision of DNA sequences. To analyze homology requirements, we constructed cell lines carrying only the replication origin without exogenous repeats. Most of the cell lines sustained high-frequency recombination, presumably by undergoing homologous recombination between repetitive DNA lying in the vicinity of the integrated origin. Our results indicate that homologous recombination promoted by large T antigen does not require recombination hot spots in the viral genome other than the replication origin and they explain the cytotoxicity observed in some cell types when large T antigen is expressed in the presence of a functional origin.

The cytotoxicity of the autonomous parvovirus minute virus of mice nonstructural proteins in FR3T3 rat cells depends on oncogene expression

The nonstructural (NS) proteins of the autonomous parvovirus minute virus of mice are involved in viral DNA replication and in the regulation of homologous and heterologous promoters. Moreover, NS products have proved to be cytotoxic, especially for transformed cells. We show here that intracellular accumulation of NS products is not sufficient to kill rat fibroblasts from the established cell line FR3T3, which is phenotypically normal in several respects. FRNS cell lines were obtained by stable transfection of FR3T3 cells by a vector carrying the NS genes under the control of the hormone-inducible long terminal repeat promoter of the mouse mammary tumor virus. In the presence of dexamethasone, the NS proteins were synthesized without associated cell death. Transformation of FRNS cells with the c-Ha-ras oncogene or polyomavirus oncogenes had little effect on their capacity for NS induction, as measured at both concentration and transactivating activity levels, yet the transformants were now dying within a few days in the presence of the inducer. The same results were obtained with cells stably transfected by a vector expressing the NS1 product alone, suggesting that in this system there is no cooperation between NS1 and NS2 for maximal cytopathic effect. Cell mortality after NS protein induction was quantitatively related to the yield of oncogene expression, while NS-1 was not limiting in this respect. Our results show that the NS1 protein is not lethal unless cellular factors that may depend on oncogene expression trigger its cytotoxicity.

Characterization of an immortalizing N-terminal domain of polyomavirus large T antigen

Polyomavirus large T antigen has an N-terminal domain of approximately 260 amino acids which can immortalize primary cells but lacks sequences known to be required for DNA binding and replication. Treatment of full-length large T with either V8 protease or chymotrypsin yields an N-terminal fragment of 36 to 40 kDa and a C-terminal fragment of approximately 60 kDa. This finding suggests a division of the protein into two domains. Proteolysis experiments show that the N-terminal domain does not have strong physical association with the rest of the protein. It also does not self-associate. A construct expressing only the N-terminal 259 amino acids is sufficient for immortalization. The independently expressed N-terminal domain is multiply phosphorylated, although at a lower level than the same region in full-length large T. The 259-residue protein binds to both pRb and p107 with somewhat lower efficiency than the full-length protein.

High-frequency recombination mediated by polyomavirus large T antigen defective in replication

We investigated the mechanism by which the large T antigen (T-Ag) of both polyomavirus and simian virus 40 (SV40) promotes homologous recombination in mammalian cells. To this end, we constructed a rat cell line, designated Hy5, that carries two mutated copies of the polyomavirus middle-T-Ag (pmt) oncogene lying as direct repeats on the same chromosome. The structure of the viral insert was devised so that intrachromosomal recombination between the pmt repeats reconstitutes wild-type pmt and yields cell populations amenable to selection for the transformed phenotype. Correction of pmt by gene conversion occurred spontaneously at a rate of ca. 1.7 x 10(-7) per cell generation and was masked by another recombination event that also led to the transformation of the Hy5 cell line. This event was identified as chromosomal inversion and overexpression of the upstream pmt copy as a result of homologous recombination between adjacent pBR322 sequences. Both events were promoted by the polyomavirus large T-Ag by several orders of magnitude, as well as by mutants defective in the initiation of viral DNA synthesis. Large T-Ag also promoted reconstitution of wild-type pmt by unequal exchange between sister chromatids, yielding structures compatible with some of the chromosomal aberrations commonly observed in transformed cells. Our data indicate that large T-Ag has a recombination-promoting activity that can be dissociated from its replicative function.

A mathematical model of homologous recombination in cultured cells

This work presents a model describing the rate of recombination between homologous segments of DNA stably integrated into the genome of cultured cells. The model has been applied to rat cell lines carrying the polyomavirus middle T oncogene and a functional origin of viral DNA replication. Introduction of the gene coding for the polyoma large T antigen or the SV40 large T antigen into cells by DNA transfection promotes homologous recombination in the resident viral inserts with rates varying between 0.1 x 10(-3) and 3.7 x 10(-1) per cell generation.

Amplification of polyomavirus DNA sequences stably integrated in rat cells

To investigate the mechanism by which the polyomavirus large T antigen (T-Ag) promotes amplification of integrated viral sequences, we constructed a rat cell line, Hy2-ts5, carrying two different inserts of polyomavirus DNA. The first insert, designated the middle T (pmt) locus, was devised to analyze homologous recombination between two defective copies of pmt lying 3.3 kb apart on the same chromosome. Reconstitution of a functional pmt by spontaneous recombination occurred at a rate of about 2 x 10(-7) per cell generation. The second locus contained the polyomavirus large T (plt) gene carrying a temperature-sensitive mutation and producing a nonfunctional large T-Ag at 39 degrees C. A shift to the permissive temperature for as little as 24 h induced the production of a functional large T-Ag which, in turn, promoted homologous recombination in the pmt locus at a rate close to 1.0 per cell generation. The particularity of this system is that it allowed recombination products to be analyzed as early as a single cell doubling following the initial recombinational event. Amplification occurred by successive duplications of a discrete sequence in the viral insert. Unequal sister chromatid exchange was ruled out as the recombination mechanism promoted by large T-Ag. Instead, we proposed a model of nonconservative recombination involving mispairing between homologous sequences.

Polyomavirus large T mutants affected in retinoblastoma protein binding are defective in immortalization

To clarify the relationship between the various activities of the polyomavirus large T antigen and the contribution of this oncogene to neoplastic transformation, we constructed a series of mutants with small deletions or single-amino-acid substitutions in two separate regions of the protein. These sequences were targeted because they showed considerable similarity to conserved regions 1 and 2 of adenovirus E1A which are thought to be binding sites for the retinoblastoma gene product (pRB). The pRB-binding properties of the large T mutants were assessed with an in vitro coimmunoprecipitation assay. pRB binding was readily detected with wild-type large T, but coprecipitation was completely abolished by as little as a single amino acid substitution (Asp-141----Glu or Glu-146----Asp) in region 2 of the polyomavirus large T antigen. Mutants defective in pRB binding were unable to immortalize primary rat embryo fibroblasts, suggesting that association with pRB is an important component of immortalization mediated by polyomavirus large T. The mutations in region 1 affected pRB binding only marginally, yet some of them severely impaired immortalization, indicating that pRB binding may be essential but not sufficient for immortalization.

Intrachromosomal recombination mediated by papovavirus large T antigens

To investigate the mechanism by which the large T antigen (T-Ag) of polyomavirus and simian virus 40 can promote recombination in mammalian cells, we analyzed homologous recombination events occurring between two defective copies of the polyomavirus middle T (pmt) oncogene lying in close proximity on the same chromosome in a rat cell line. Reconstitution of a functional pmt gene by spontaneous recombination occurred at a rate of about 2 x 10(-7) per cell generation. Introduction of the polyomavirus large T (plt) oncogene into the cell line by DNA transfection promoted recombination very efficiently, with rates in the range of 10(-1) to 10(-2) per cell generation. Recombination was independent of any amplification of viral sequences and could even be promoted by the large T-Ag from simian virus 40, which cannot activate polyomavirus DNA replication. To explain the role of large T-Ag, we propose a novel mechanism of nonconservative recombination involving slipped-strand mispairing between the two viral repeats followed by gap repair synthesis.

Mutations in polyomavirus large T affecting immortalization of primary rat embryo fibroblasts

To clarify the relationship between various functions of the polyomavirus large T antigen and the contribution of this oncogene toward neoplastic transformation, we have analyzed the properties of mutants with in-frame deletions in the second large T exon. dl45, dl96, and dl97 have retained the ability to immortalize primary rat embryo fibroblasts and to trans-activate viral promoters. dl8, dl23, and dl300, which are deficient immortalization, are also deficient in transactivation. However, a newly constructed mutant, designated dl141, which is deficient in immortalization, is still able to trans-activate both the polyoma and SV40 late promoters. This indicates that the ability to trans-activate promoters is not sufficient to confer on the large T antigen the ability to immortalize primary cells.

Mutations in polyomavirus middle T antigen affecting tumorigenesis

P155 is a polyomavirus mlt mutant with normal transforming ability but impaired tumorigenic potential. The mutation, a 12-bp deletion (nucleotides 1348-1359), removes amino acids 372 to 375 from middle T and affects its ability to function in tumorigenesis (C. Gelinas, S. Masse, and M. Bastin, 1984, J. Virol. 51, 242-246). We used deletion loop mutagenesis to introduce point mutations within the wild-type sequence spanned by the P155 deletion. A mutant phenotype resembling that of P155 could be produced by as little as one alanine to valine substitution at residue 373. The mutants were impaired in their ability to induce tumors in rats but they could still transform established cell lines or primary fibroblasts in culture. To define the biochemical defect, we examined the mutant middle T antigen both for association with pp60c-src, the cellular src gene product, as well as its pattern of phosphorylation. No obvious differences explaining the phenotype were observed. The mutant middle T associated with, and activated pp60c-src, but exhibited a slightly altered pattern of phosphorylation, presumably because of additional sites on the middle T protein.

Transfer of mlt mutations into polyomavirus intronless genomes by intramolecular recombination in bacteria

We describe a modification of the procedure of Weber and Weissmann [Nucl. Acids Res. 11 (1983) 5661-5669] for the formation of hybrid genes by in vivo recombination to introduce two separate mutations into the same gene. The mutants of interest are inserted as head-to-tail tandems in a bacterial plasmid in such a way that the 5'-proximal mutation is located upstream from the mutant with the more distal mutation. Propagation of the plasmid in a rec+ strain of Escherichia coli allows recombination between homologous sequences in the insert. DNA with the size expected for the recombinant plasmid is isolated by agarose gel electrophoresis, cloned in a recA strain, and characterized by restriction endonuclease mapping. Using this procedure, we have transferred the deletion from polyomavirus mutant dl-8 into other mutant genomes lacking the intervening sequences for either middle T or large T.

Site-directed mutagenesis of the polyomavirus genome: replication-defective large T mutants with increased immortalization potential

We used the deletion loop mutagenesis procedure to direct point mutations into a small region of the polyomavirus genome, at 0.97 map units, affecting the structure of both the middle and large T antigens. We describe the construction of six middle T mutants which have retained the ability to transform rat cells in culture and four large T mutants, three of which are deficient in viral DNA replication and capable of immortalizing primary rat embryo fibroblasts very efficiently.

Expression of the malignant phenotype in rat fibroblasts transfected with the polyomavirus transforming genes

As a step toward understanding the molecular mechanism of cooperation between viral and cellular genes in oncogenic transformation, we examined various properties of rat cells transfected with the polyomavirus transforming genes without selecting for a neoplastic phenotype. The cell lines displayed a phenotype ranging from nontumorigenic (flat) to fully transformed (tumorigenic). In the established FR3T3 cell line, acquisition of the fully transformed phenotype correlated with effective expression of the polyomavirus middle T (pmt) antigen. Flat cells carrying silent copies of pmt mutated spontaneously to the fully transformed state with a frequency of 2 to 6 X 10(-5) per cell per generation. In unestablished rat fibroblasts, simultaneous transfer of either pmt and small T or pmt and large T in the presence of the neo marker conferred only a partially transformed phenotype to most of the cell lines. The same results were obtained when wild-type genomic DNA was cotransfected with pSV2-neo. The flat transformants progressively acquired properties characteristic of fully transformed cells with passage in culture. However, in contrast to FR3T3 cells, the generation of fully transformed variants from the flat, unestablished fibroblasts was not caused by activation of pmt expression. This indicates that the functions conferred by the large and small T antigens, alone or in combination with each other, cannot substitute for all the functions expressed by the FR3T3 cell line as a result of in vitro establishment. Thus, polyomavirus-mediated transformation may require additional cellular alterations beyond the acquisition of the three viral oncogenes.

Ability of a T-antigen transport-defective mutant of simian virus 40 to immortalize primary cells and to complement polyomavirus middle T in tumorigenesis

The oncogenic potential of polyomavirus in newborn rats could not be expressed by a genome encoding only the middle T antigen but required the presence of one of the other two viral early genes, small T or large T. The tumorigenicity defect could also be complemented by other viral or cellular genes that are known to be implicated in immortalization and establishment functions. The simian virus 40(cT)-3 mutant (R. E. Lanford and J. S. Butel, Cell 37:801-813, 1984), which fails to localize to the nucleus, has the capacity to complement polyomavirus middle T in tumorigenesis and to immortalize primary rat embryo fibroblasts when it was cotransfected in the presence of pSV2-neo. Our data suggested that under the conditions of DNA-mediated tumor induction and cotransfection with a dominant selection marker, the cellular alterations achieved by nonnuclear oncogenes such as polyomavirus small T and simian virus 40(cT)-3 were sufficient to complement polyomavirus middle T in transformation and tumorigenesis.