Identification of tumor precursor cells in the brains of primates with radiation-induced de novo glioblastoma multiforme

The pathogenesis of de novo glioblastoma multiforme (GBM) is poorly understood and precursor cells are not known. To gain insight into the pathogenesis of GBM we analyzed brains from primates that developed de novo tumors ten years after whole brain radiation. Four animals had clinical and radiological evidence of GBM, and two animals had no evidence of GBM at the time of euthanization. Tumor precursor cells were identified diffusely scattered in the grossly normal white matter of all animals including two monkeys without evidence of GBM by MR-imaging or on autopsy examination. Tumor precursors demonstrated cellular atypia and mitoses, and were negative for tumor-associated markers GFAP, EGFR and p53. The cells were positive for Ki67 and N-CoR, the nuclear corepressor of astroglial differentiation. These results suggest that radiation-induced nuclear damage to neural stem cells or early astrocytic precursor cells can prevent normal differentiation and lead to tumor development. The findings provide insight into the tumorigenesis of de novo GBMs and suggest a new strategy for treatment of these lethal tumors by targeting both inactivation of N-CoR and inhibition of EGFR.

Purification and Characterization of the Simian Virus 40 Transcription Elongation Complex

The transcriptional regulatory region of the simian virus 40 minichromosome that is being transcribed in the cell is nucleosome-free, while that of the non-transcribed minichromosome is nucleosome covered. Although additional studies have shown that the two structures are otherwise similar, the precision of these indirect studies has not been sufficient to determine if the transition between the two involves nucleosome displacement or nucleosome sliding. In order to address this question directly, we have developed a new function-based affinity isolation method that is capable of purifying the native transcription elongation complex of a single gene from mammalian cells. The simian virus 40 transcription elongation complex was purified by this method and the topological linking number of its DNA was compared directly to that of the bulk, non-transcribed minichromosome. The results show that the two types of minichromosome contain the same number of nucleosomes as well as nucleosomal structure. These findings indicate that interconversion between the non-transcribing and transcribing states is accomplished by a remodeling event involving nucleosome sliding rather than nucleosome displacement.

Investigation of simian virus 40 large T antigen in 18 autopsied malignant mesothelioma patients in Japan

It has been reported that Simian virus 40 (SV40) is linked to human beings by inoculation of contaminated poliovaccines and may have a role in the etiology of malignant mesothelioma. However, there have been no reports describing the relationship between SV40 and malignant mesothelioma in Japan. A study was undertaken to investigate whether SV40 was related to patients of malignant mesothelioma in Japan by the polymerase chain reaction (PCR) assay, DNA sequence analysis, and immunohistochemical methods. Paraffin-embedded samples of the 18 autopsied patients with pleural malignant mesothelioma were collected from five hospitals in Japan. After isolation of DNA from paraffin blocks, PCR analyses followed by sequencing were performed using three different sets of primers for detection of SV40 large T antigen (TAg) gene. All 18 malignant mesothelioma samples were also immunohistochemically evaluated for expression of SV40 TAg protein with two different anti-SV40 TAg antibodies. SV40 TAg genome was detected in eight malignant mesothelioma cases. Only one of three primer pairs successfully amplified SV40 genome in the samples, whereas all pairs yielded a PCR product in the controls, suggesting a low content of virus DNA. No immunopositive staining for SV40 TAg was found in any of the samples. This study shows that SV40 genome was present in a subset of Japanese malignant mesothelioma patients who were unlikely to have received a contaminated polio vaccine based on their age.

T antigen origin-binding domain of simian virus 40: determinants of specific DNA binding

To better understand origin recognition and initiation of DNA replication, we have examined by NMR complexes formed between the origin-binding domain of SV40 T antigen (T-ag-obd), the initiator protein of the SV40 virus, and cognate and noncognate DNA oligomers. The results reveal two structural effects associated with "origin-specific" binding that are absent in nonspecific DNA binding. The first is the formation of a hydrogen bond (H-bond) involving His 203, a residue that genetic studies have previously identified as crucial to both specific and nonspecific DNA binding in full-length T antigen. In free T-ag-obd, the side chain of His 203 has a pK(a) value of approximately 5, titrating to the N(epsilon)(1)H tautomer at neutral pH (Sudmeier, J. L., et al. (1996) J. Magn. Reson., Ser. B 113, 236-247). In complexes with origin DNA, His 203 N(delta)(1) becomes protonated and remains nontitrating as the imidazolium cation at all pH values from 4 to 8. The H-bonded N(delta1)H resonates at 15.9 ppm, an unusually large N-H proton chemical shift, of a magnitude previously observed only in the catalytic triad of serine proteases at low pH. The formation of this H-bond requires the middle G/C base pair of the recognition pentanucleotide, GAGGC. The second structural effect is a selective distortion of the A/T base pair characterized by a large (0.6 ppm) upfield chemical-shift change of its Watson-Crick proton, while nearby H-bonded protons remain relatively unaffected. The results indicate that T antigen, like many other DNA-binding proteins, may employ "catalytic" or "transition-state-like" interactions in binding its cognate DNA (Jen-Jacobson, L. (1997) Biopolymers 44, 153-180), which may be the solution to the well-known paradox between the relatively modest DNA-binding specificity exhibited by initiator proteins and the high specificity of initiation.

Expression, refolding, crystallization and preliminary crystallographic study of MHC H-2Kkcomplexed with octapeptides and nonapeptides

Major histocompatibility complex (MHC) molecules are heterodimeric cell-surface receptors that play a crucial role in the cellular immune response by presenting epitope peptides to T-cell antigen receptors (TCR). Although the structural basis of the peptide-MHC binding mechanism is becoming better understood, it is still difficult to predict a binding mode for an MHC of unknown structure. Therefore, as the first stage of a TCR-MHC interaction study, the crystal structures of the mouse H-2K(k) molecule in complex with both an octapeptide from Influenza A virus and a nonapeptide from simian virus SV40 were solved. Here, the expression, refolding, purification and crystallization of the two complexes are reported. For the H-2K(k)-HA(259-266) complex, crystals were obtained via an extensive screen using a nanodrop-dispensing robot and diffracted to 2.5 A resolution. For the H-2K(k)-SV40(560-568) complex, microscopic needles were initially obtained and their size was improved by macroseeding and a stepwise increase in precipitant concentration. Diffraction data to a resolution of 3.0 A were collected at a synchrotron facility.

Structure of the replicative helicase of the oncoprotein SV40 large tumour antigen

The oncoprotein large tumour antigen (LTag) is encoded by the DNA tumour virus simian virus 40. LTag transforms cells and induces tumours in animals by altering the functions of tumour suppressors (including pRB and p53) and other key cellular proteins. LTag is also a molecular machine that distorts/melts the replication origin of the viral genome and unwinds duplex DNA. LTag therefore seems to be a functional homologue of the eukaryotic minichromosome maintenance (MCM) complex. Here we present the X-ray structure of a hexameric LTag with DNA helicase activity. The structure identifies the p53-binding surface and reveals the structural basis of hexamerization. The hexamer contains a long, positively charged channel with an unusually large central chamber that binds both single-stranded and double-stranded DNA. The hexamer organizes into two tiers that can potentially rotate relative to each other through connecting alpha-helices to expand/constrict the channel, producing an 'iris' effect that could be used for distorting or melting the origin and unwinding DNA at the replication fork.

Cys(9), Cys(104) and Cys(207) of simian virus 40 Vp1 are essential for infectious virion formation in CV-1 cells

Structural studies have implicated Cys(9), Cys(104) and Cys(207) of simian virus 40 (SV40) Vp1 in disulfide bond formation. Recently, we have shown the three cysteines to be essential for disulfide linkage of Vp1 complexes in vitro. Here, the role of the three cysteines was explored during the course of SV40 infection. Single-, double- and triple-mutant Vp1 at Cys(9), Cys(104) and Cys(207) continued to localize to the nuclei of transfected CV-1 cells and to bind DNA, but showed a range of abilities to form plaques. Only mutants containing the Cys(9)-->Ser change showed defects in plaque formation. Single mutants at Cys(9) formed small plaques; mutants at Cys(9). Cys(104), Cys(9). Cys(207) and Cys(9). Cys(104). Cys(207) formed no plaques. All three isolated revertants contained back-mutations at the Vp1 Cys(9) codon. These results further confirm the involvement of the three Vp1 cysteines in protein-protein interactions during virus assembly. Cys(9) is critical for production of wild-type infectious virions, whereas Cys(104) and Cys(207) play secondary roles.

Insertion of capsid proteins from nonenveloped viruses into the retroviral budding pathway

Retroviral Gag proteins direct the assembly and release of virus particles from the plasma membrane. The budding machinery consists of three small domains, the M (membrane-binding), I (interaction), and L (late or "pinching-off") domains. In addition, Gag proteins contain sequences that control particle size. For Rous sarcoma virus (RSV), the size determinant maps to the capsid (CA)-spacer peptide (SP) sequence, but it functions only when I domains are present to enable particles of normal density to be produced. Small deletions throughout the CA-SP sequence result in the release of particles that are very large and heterogeneous, even when I domains are present. In this report, we show that particles of relatively uniform size and normal density are released by budding when the size determinant and I domains in RSV Gag are replaced with capsid proteins from two unrelated, nonenveloped viruses: simian virus 40 and satellite tobacco mosaic virus. These results indicate that capsid proteins of nonenveloped viruses can interact among themselves within the context of Gag and be inserted into the retroviral budding pathway merely by attaching the M and L domains to their amino termini. Thus, the differences in the assembly pathways of enveloped and nonenveloped viruses may be far simpler than previously thought.

Additional cytotoxic polyacetylenes from the marine sponge Petrosia species

Ten new polyacetylenic alcohols (1-6, 8-11), along with a known compound, petrocortyne C (7), were isolated from the marine sponge Petrosia sp. The gross structures were established based on NMR and MS data, and the absolute configuration was determined by the modified Mosher's method. These compounds displayed considerable cytotoxicity against a small panel of human solid tumor cell lines. Compounds 1-11 were further evaluated for in vitro inhibitory activity on DNA replication.

Role of simian virus 40 Vp1 cysteines in virion infectivity

We have developed a new nonoverlapping infectious viral genome (NO-SV40) in order to facilitate structure-based analysis of the simian virus 40 (SV40) life cycle. We first tested the role of cysteine residues in the formation of infectious virions by individually mutating the seven cysteines in the major capsid protein, Vp1. All seven cysteine mutants-C9A, C49A, C87A, C104A, C207S, C254A, and C267L-retained viability. In the crystal structure of SV40, disulfide bridges are formed between certain Cys104 residues on neighboring pentamers. However, our results show that none of these disulfide bonds are required for virion infectivity in culture. We also introduced five different mutations into Cys254, the most strictly conserved cysteine across the polyomavirus family. We found that C254L, C254S, C254G, C254Q, and C254R mutants all showed greatly reduced (around 100,000-fold) plaque-forming ability. These mutants had no apparent defect in viral DNA replication. Mutant Vp1's, as well as wild-type Vp2/3, were mostly localized in the nucleus. Further analysis of the C254L mutant revealed that the mutant Vp1 was able to form pentamers in vitro. DNase I-resistant virion-like particles were present in NO-SV40-C254L-transfected cell lysate, but at about 1/18 the amount in wild-type-transfected lysate. An examination of the three-dimensional structure reveals that Cys254 is buried near the surface of Vp1, so that it cannot form disulfide bonds, and is not involved in intrapentamer interactions, consistent with the normal pentamer formation by the C254L mutant. It is, however, located at a critical junction between three pentamers, on a conserved loop (G2H) that packs against the dual interpentamer Ca(2+)-binding sites and the invading C-terminal helix of an adjacent pentamer. The substitution by the larger side chains is predicted to cause a localized shift in the G2H loop, which may disrupt Ca(2+) ion coordination and the packing of the invading helix, consistent with the defect in virion assembly. Our experimental system thus allows dissection of structure-function relationships during the distinct steps of the SV40 life cycle.

Structure of simian virus 40 DNA replicated by herpes simplex virus type 1

Replicating herpes simplex virus type 1 (HSV-1) DNA is known to form large branched structures. The aim of this study was to define whether HSV-1-specific DNA elements in cis play a critical role in formation of this structure. We did this by investigating the structure of heterologous simian virus 40 (SV40) DNA, which is replicated in HSV-infected cells by SV40 large T-antigen and defined HSV-encoded replication factors (e.g., DNA polymerase, single-stranded DNA-binding protein, and helicase-primase). During this process, extrachromosomal concatemeric DNA replication products are formed, indicating a herpesvirus-specific replication mode. In this study, we found that the replicating SV40 DNA consisted of a complex branched structure indistinguishable from that of replicating HSV DNA. Thus, no HSV-specific DNA element is necessary in cis for the formation of the large branched structure during HSV DNA replication. The trans-acting HSV DNA replication proteins seem to be sufficient to generate these complex structures. Moreover, replicating SV40 DNA showed a high frequency of homologous recombination events, which is typical for HSV DNA replication. However, in contrast to HSV origin-bearing amplicon plasmids, SV40 plasmids bearing the HSV cleavage-packaging signal were not efficiently processed to linear 150-kb DNA packaged into HSV capsids. This indicates that initiation of DNA synthesis on HSV-ori determines some, yet undefined, property of replicating HSV DNA, which is crucial for regular processing of the replication intermediates to daughter genomes.

Molecular quantification of cell cycle-related gene expression at the protein level

BACKGROUND

Immunofluorescence cytometry of antigen and DNA content provides relative measurements of the cell cycle phase distribution of a specific epitope. Measurement of correlated expression of epitopes on signaling and regulatory proteins will be useful in the study of the complex pathways involved in cell cycle regulation and carcinogenesis. However, to formulate regulatory pathway models, measurements of molecules per cell would be more useful than relative measurements of intensity. Here, we report on a system in which the relationship between molecules and fluorescence is determined for a reference set of cell lines that are then used to directly calculate the number of molecules for unknowns. To demonstrate the process, we calculated the cell cycle phase distribution of SV40 large T antigen (Tag) in the reference cells.

METHODS

A set of cell line clones expressing different levels of Tag were isolated. Quantitative Western blots of these cells and purified, recombinant Tag were performed. Cells from the same sample were stained and analyzed by flow cytometry for Tag and DNA. The relationship between molecules and fluorescence was established and calculations were performed for the phase distributions of Tag.

RESULTS

The five cell lines had 0.11, 0.27, 1.06, 2.44, and 2.63 x 10(6) molecules of Tag per cell, determined by Western blot. The average coefficient of variation was 10.6%. The relationship of molecules to fluorescence fit a linear equation (r(2) = 0.96) over the range, 0.11 - 2.63 x 10(6) molecules, however, the same equation did not fit the relationship between 0 molecules, defined by isotype staining controls, and the lowest expressing cell line. To calculate the phase distributions of molecules in the lowest cell line, a second linear equation from 0 to 110,000 molecules was used.

CONCLUSIONS

This work describes a system where fixed cells expressing various levels of a target antigen quantified by Western blots can be used to standardize flow cytometric measurements of gene expression in absolute terms.

Cys9, Cys104 and Cys207 of simian virus 40 Vp1 are essential for inter-pentamer disulfide-linkage and stabilization in cell-free lysates

Previous studies have implicated disulfide bonds between Vp1 molecules in the stabilization of the simian virus 40 (SV40) capsid. To identify the cysteine residues involved in intermolecular disulfide interactions, systematic oligo-directed mutagenesis of cysteine codons to serine codons was initiated. Wild-type and mutant Vp1 proteins were produced in rabbit reticulocyte lysates and were allowed to interact post-translationally. Disulfide-linked Vp1 complexes were assessed via non-reducing SDS-PAGE and via sucrose-gradient sedimentation. Wild-type Vp1 forms 7S pentamers followed by 12S disulfide-linked multi-pentameric complexes in cell-free lysates. Mutagenesis of all seven cysteine codons abolished Vp1 12S complexes, but did not affect pentamer formation. A quadruple Vp1 mutant at Cys49, Cys87, Cys254 and Cys267 continued to form 12S complexes, whereas the major products of the Cys9, Cys104 and Cys207 triple mutant Vp1 were 7S pentamers. Single and double mutant Vp1 proteins at the three cysteines affected continued to form 12S complexes, but to a lesser extent. Thus, inter-pentamer disulfide bonds at Cys9, Cys104 and Cys207 are essential and sufficient for stabilization of Vp1 complexes in cell-free lysates. These results are in agreement with previous structural studies of SV40 that implicated the same three residues in disulfide linkage in the capsid. Possible parameters for the involvement of the three cysteines are discussed.

Bizelesin, a bifunctional cyclopropylpyrroloindole alkylating agent, inhibits simian virus 40 replication in trans by induction of an inhibitor

Bizelesin, a bifunctional DNA minor groove alkylating agent, inhibits both cellular and viral (SV40) DNA replication in whole cells. Bizelesin inhibition of SV40 DNA replication was analyzed in SV40-infected cells, using two-dimensional (2D) neutral agarose gel electrophoresis, and in a cell-free SV40 DNA replication assay. Within 1 h of bizelesin addition to infected cells, a similar rapid decrease in both the level of SV40 replication intermediates and replication activity was observed, indicating inhibition of initiation of SV40 DNA replication. However, prolonged bizelesin treatment (>/=2 h) was associated with a reduced extent of elongation of SV40 replicons, as well as the appearance on 2D gels of intense spots, suggestive of replication pause sites. Inhibition of elongation and induction of replication pause sites may result from the formation of bizelesin covalent bonds on replicating SV40 molecules. The level of in vitro replication of SV40 DNA also was reduced when extracts from bizelesin-treated HeLa cells were used. This effect was not dependent upon the formation of bizelesin covalent bonds with the template DNA. Mixing experiments, using extracts from control and bizelesin-treated cells, indicated that reduced DNA replication competence was due to the presence of a trans-acting DNA replication inhibitor, rather than to decreased levels or inactivation of essential replication factor(s).

Reappraisal of progressive multifocal leukoencephalopathy due to simian virus 40

Several cases of progressive multifocal leukoencephalopathy (PML) have been associated with simian virus 40 (SV40), rather than with JC virus (JCV), the polyomavirus originally isolated from PML tissue. PML has, therefore, been defined as a demyelinating syndrome with possible multiple viral etiologies. Tissues from three of the cases thought to be associated with SV40 were available for reexamination. Monoclonal antibodies specific for SV40 capsid antigen VPI, virus-specific biotinylated DNA probes for in situ hybridization, and virus-specific primers in the polymerase chain reaction (PCR) were used. Macaque PML brain served as a positive control tissue for SV40 brain infection. Monoclonal antibodies to SV40 VPI failed to recognize viral antigen in lesions from all three human PML cases. The biotinylated DNA probe, which reacted with SV40 in macaque PML, failed to detect SV40 in human PML. However, JCV could be detected by in situ hybridization with a JCV-specific DNA probe. Moreover, JCV DNA sequences were amplified by PCR from the human PML tissues, whereas SV40 DNA sequences were amplified only from the macaque brain. Thus, we could not confirm the original reports that the demyelinating agent in these three cases of PML was SV40, rather than JCV. We conclude that SV40 infection of the central nervous system need not be ruled out in the differential diagnosis of PML.

Crystallographic analysis of the recognition of a nuclear localization signal by the nuclear import factor karyopherin alpha

Selective nuclear import is mediated by nuclear localization signals (NLSs) and cognate transport factors known as karyopherins or importins. Karyopherin alpha recognizes "classical" monopartite and bipartite NLSs. We report the crystal structure of a 50 kDa fragment of the 60 kDa yeast karyopherin alpha, in the absence and presence of a monopartite NLS peptide at 2.2 A and 2.8 A resolution, respectively. The structure shows a tandem array of ten armadillo repeats, organized in a right-handed superhelix of helices. Binding of the NLS peptide occurs at two sites within a helical surface groove that is lined by conserved residues. The structure reveals the determinants of NLS specificity and suggests a model for the recognition of bipartite NLSs.

Salt effects on the structure and internal dynamics of superhelical DNAs studied by light scattering and Brownian dynamics

Using laser light scattering, we have measured the static and dynamic structure factor of two different superhelical DNAs, p1868 (1868 bp) and simian virus 40 (SV40) (5243 bp), in dilute aqueous solution at salt concentrations between 1 mM and 3 M NaCl. For both DNA molecules, Brownian dynamics (BD) simulations were also performed, using a previously described model. A Fourier mode decomposition procedure was used to compute theoretical light scattering autocorrelation functions (ACFs) from the BD trajectories. Both measured and computed autocorrelation functions were then subjected to the same multiexponential decomposition procedure. Simulated and measured relaxation times as a function of scattering angle were in very good agreement. Similarly, computed and measured static structure factors and radii of gyration agreed within experimental error. One main result of this study is that the amplitudes of the fast-relaxing component in the ACF show a peak at 1 M salt concentration. This nonmonotonic behavior might be caused by an initial increase in the amplitudes of internal motions due to diminishing long-range electrostatic repulsions, followed by a decrease at higher salt concentration due to a compaction of the structure.

SV40 and adenovirus may act as cocarcinogens by downregulating glutathione S-transferase expression

We have discovered a novel function of the SV40 T antigen and the adenovirus E1A proteins: the ability to downregulate the endogenous expression of an important detoxification enzyme, glutathione S-transferase alpha (GST alpha). GST alpha mRNA is much less abundant in rat and human cells that express SV40 T antigen than in the parental cell lines. This GST alpha downregulation does not require expression of SV40 small t antigen or complex formation between large T antigen and p53, p300, or the pRb family of proteins. As might be predicted, cells that express SV40 T antigen are more sensitive than normal cells to alkylating drugs, which GST alpha is known to detoxify. Finally, GST alpha expression is also downregulated in cells that express the adenovirus E1A proteins. We propose that by downregulating GST alpha expression and inactivating p53 function, SV40 and adenovirus may contribute to the initiation of, or the progression toward, malignancy. Thus, in their quest to establish persistent infections, these viruses may inadvertently make the cellular environment more permissive for tumorigenesis.

Identification of a novel antiapoptotic functional domain in simian virus 40 large T antigen

The ability of DNA tumor virus proteins to trigger apoptosis in mammalian cells is well established. For example, transgenic expression of a simian virus 40 (SV40) T-antigen N-terminal fragment (N-termTag) is known to induce apoptosis in choroid plexus epithelial cells. SV40 T-antigen-induced apoptosis has generally been considered to be a p53-dependent event because cell death in the brain is greatly diminished in a p53-/- background strain and is abrogated by expression of wild-type (p53-binding) SV40 T antigen. We now show that while N-termTags triggered apoptosis in rat embryo fibroblasts cultured in low serum, expression of full-length T antigens unable to bind p53 [mut(p53-)Tags] protected against apoptosis without causing transformation. One domain essential for blocking apoptosis by T antigen was mapped to amino acids 525 to 541. This domain has >60% homology with a domain of adenovirus type 5 E1B 19K required to prevent E1A-induced apoptosis. In the context of both wild-type T antigen and mut(p53-)Tags, mutation of two conserved amino acids in this region eliminated T antigen's antiapoptotic activity in REF-52 cells. These data suggest that SV40 T antigen contains a novel functional domain involved in preventing apoptosis independently of inactivation of p53.

Measurement of the linking number change in transcribing chromatin

The in vivo-initiated, transcribing simian virus 40 (SV40) minichromosome was analyzed to determine its DNA linking number change, i.e. the difference between the linking number of the minichromosomal DNA and that of relaxed bare DNA. As part of this measurement, the linking number change due to the in vivo-initiated RNA polymerase II was determined, the first time a value for this quantity has been reported. The topological contribution of the polymerase was combined with values determined for constrained and non-constrained linking number contributions from the native transcription complex chromatin to yield the linking number change for the complex. The linking number change of the native non-transcribed SV40 minichromosome was independently determined and was found to be virtually the same as that for the chromatin of the transcription complex. This indicates that there is little difference between the two structures. The plausibility of several current models for the contribution of chromatin structure to transcription regulation is discussed in light of this finding.

A DNA polymerase alpha accessory protein exhibits structural and functional similarities to SV40 large tumor antigen

Untransformed cells have been proposed to require a protein homologous to SV40 large tumor antigen (TAg) which functions as a component of the replicase complex during the initiation of DNA synthesis. By definition, this should be a phosphoprotein which interacts with the retinoblastoma protein (pRb) in G0 or early G1, and is capable of binding to and potentiating the activity of DNA polymerase alpha (pol alpha). This protein should also be an ATP-dependent helicase which interacts with the single-stranded DNA (ssDNA) binding protein, RP-A. Because of these requirements, a TAg homologous protein could be expected to contain epitopes with amino acid sequences similar to those of TAg at critical functional sites, such as ATP, pRb and pol alpha binding sites. TAg and a putative cellular homolog of TAg, DNA pol alpha accessory protein (alpha AP), were compared for pRb and pol alpha interaction, and for immunological identity. The analyses utilized immunoaffinity-purified TAg and pRb from a baculovirus expression system, and DNA pol alpha/primase and alpha AP chromatographically isolated from a mouse lymphocytic leukemia cell line. Monoclonal antibodies specific for the pol alpha or pRb binding sites on TAg interacted with alpha AP strongly enough to be employed for immunoaffinity purification of alpha AP. Anti-pRb and anti-TAg reciprocally coimmunoprecipitated pRb bound to TAg and pRb bound to alpha AP. The functional consequences of pol alpha interaction with TAg or alpha AP in the presence or absence of pRb was determined using pol alpha nucleotide incorporation assays. alpha AP exhibited the capacity to stimulate pol alpha activity, a capacity which was diminished in the presence of pRb. Lastly, TAg and alpha AP independently co-purified with pol alpha through a multi-step chromatographic protocol. These data indicate that a pol alpha accessory protein, alpha AP, exhibits functional and immunological similarities to SV40 TAg, suggest that alpha AP is involved in regulation of the initiation of DNA synthesis, and support the proposal that alpha AP may be a normal cell protein homologous to SV40 large T antigen.

Quantitation of ethidium-stained closed circular DNA in agarose gels

The fluorescence of ethidium bromide (EB) bound to equimolar amounts of supercoiled form I and unstrained linear form III pBR322, SV40 and PM2 DNA in agarose gels has been measured by scanning a photographic negative of the gel with a microdensitometer. For SV40 and PM2 DNA, commonly used staining conditions cause both forms, i.e. linear and supercoiled, to fluoresce to the same extent. This obviates the need to use a correction factor for the fluorescence of form I DNA when measuring the amount of this form relative to the amounts of unstrained forms in agarose gels. In the case of PBR322 DNA, form I was found to fluoresce approximately 20% more than form III DNA.

SV40 early region and large T antigen in human brain tumors, peripheral blood cells, and sperm fluids from healthy individuals

SV40 T antigen (Tag) coding sequences were detected by PCR amplification followed by Southern blot hybridization in human brain tumors and tumor cell lines, as well as in peripheral blood cells and sperm fluids of healthy donors. SV40 early region sequences were found in 83% of choroid plexus papillomas, 73% of ependymomas, 47% of astrocytomas, 33% of glioblastoma multiforme cases, 14% of meningiomas, 50% of glioblastoma cell lines, and 33% of astrocytoma cell lines and in 23% of peripheral blood cell samples and 45% of sperm fluids from normal individuals. None of the 13 normal brain tissues were positive for SV40 DNA, nor were seven oligodendrogliomas, two spongioblastomas, one neuroblastoma, one meningioma, or four neuroblastoma cell lines. Expression of SV40 early region was found by reverse transcription PCR, and SV40-specific Tag was detected by indirect immunofluorescence in glioblastoma cell lines. DNA sequence analysis, performed in four positive samples, confirmed that the amplified PCR products belong to the SV40 early region. Sixty-one % of the neoplastic patients positive for SV40 sequences had an age excluding exposure to SV40-contaminated polio vaccines, suggesting a contagious transmission of SV40. The possible role of SV40 Tag in the etiopathogenesis of human brain tumors and the spread of SV40 by horizontal infection in the human population are discussed.