Minimal subenhancer requirements for high-level polyomavirus DNA replication: a cell-specific synergy of PEA3 and PEA1 sites

Polyoma virus: old findings and new challenges

The accidental discovery of the mouse polyoma virus nearly 50 years ago opened up an experimental system unique in opportunities for investigating virus-host interactions leading to the development of tumors. Extensive studies of the virus in tissue culture have provided a detailed understanding of its genetics and molecular biology. Knowledge of the virus as a transforming agent in culture can now be tested in the animal where multiple cell types are targets for tumorigenic conversion and where a variety of host factors, both immunological and nonimmunological, come into play. Studies in the animal using well-characterized wild-type and mutant virus strains have led to some unexpected findings. Some of these run counter to certain widely held beliefs in cancer biology. This minireview focuses on these surprising findings and the challenges they raise.

Novel association of a diverse range of genes with renal cell carcinoma as identified by differential display

We have used differential-display PCR (DD-PCR) to compare renal-cell carcinoma (RCC) and normal kidney gene expression with the aim of identifying genes specifically associated with RCC. Using a modified DD-PCR approach, which was non-radioactive, quicker and simpler than the conventional method, 24 cDNA samples were clearly up- or down-regulated in RCC tissue from 4 patients. Fourteen of these showed high similarity to a number of known genes. Eight of these cDNA clones were chosen for further analysis. These were a regulator of G-protein signalling (RGS-5), Notch-3, Na,K-ATPase alpha subunit, HLA class II antigen, ETS-like protein, transforming growth factor beta-stimulated clone (TSC-22), bladder cancer-related protein (BC10) and adipophilin. Semi-quantitative RT-PCR using specific primers to each of these genes confirmed differential expression in 67% to 83% of a further 12 RCC and normal kidney paired samples from 7 of the 8 cDNA clones. Northern analysis further confirmed the up-regulation in expression of RGS-5 and Notch-3 in RCC. Further characterisation of these differentially expressed genes should lead to a better understanding of the changes that occur at the molecular level during RCC development and progression.

The evolution of small DNA viruses of eukaryotes: past and present considerations

Historically, viral evolution has often been considered from the perspective of the ability of the virus to maintain viral pathogenic fitness by causing disease. A predator-prey model has been successfully applied to explain genetically variable quasi-species of viruses, such as influenza virus and human immunodeficiency virus (HIV), which evolve much faster rates than the host. In contrast, small DNA viruses (polyomaviruses, papillomaviruses, and parvoviruses) are species specific but are stable genetically, and appear to have co-evolved with their host species. Genetic stability is attributable primarily to the ability to establish and maintain a benign persistent state in vivo and not to the host DNA proofreading mechanisms. The persistent state often involves a cell cycle-regulated episomal state and a tight linkage of DNA amplification mechanisms to cellular differentiation. This linkage requires conserved features among viral regulatory proteins, with characteristic host-interactive domains needed to recruit and utilize host machinery, thus imposing mechanistic constrains on possible evolutionary options. Sequence similarities within these domains are seen amongst all small mammalian DNA viruses and most of the parvo-like viruses, including those that span the entire spectrum of evolution of organisms from E. coli to humans that replicate via a rolling circle-like mechanism among the entire spectrum of organisms throughout evolution from E. coli to humans. To achieve benign inapparent viral persistence, small DNA viruses are proposed to circumvent the host acute phase reaction (characterized by minimal inflammation) by mechanisms that are evolutionarily adapted to the immune system and the related cytokine communication networks. A striking example of this is the relationship of hymenoptera to polydnaviruses, in which the crucial to the recognition of self, development, and maintenance of genetic identity of both the host and virus. These observations in aggregate suggest that viral replicons are not recent "escapies" of host replication, but rather provide relentless pressure in driving the evolution of the host through cospeciation.

PEBP2--a modulator of polyoma DNA replication

Previously, we have shown that integrated copies of polyoma DNA can be induced to replicate in rat fibroblasts (H3 cells) exposed to a DNA-damaging agent. In the current study, we demonstrate that UV-irradiation of mouse fibroblasts (WOP cells), transiently transfected with polyoma DNA, results in repression of polyoma replication. Cotransfection of oligomers representing wild-type but not mutated forms of the PEBP2 target sequence restored levels of viral replication indicating a role of PEBP2 binding proteins in mediating this effect. DNA-binding assays revealed that a different subset of complexes was formed with the PEBP2 target sequence when nuclear proteins from sham and UV-irradiated WOP and H3 cells were compared, suggesting that the activities of PEBP2 binding proteins are differentially regulated upon UV-irradiation in these two cell types. The ability of PEBP2 to modulate polyoma replication following UV-irradiation in WOP cells suggests a potential role of PEBP2 proteins in the cellular response to DNA damage.

Fusion of PDGF receptor beta to a novel ets-like gene, tel, in chronic myelomonocytic leukemia with t(5;12) chromosomal translocation

Chronic myelomonocytic leukemia (CMML) is a myelodysplastic syndrome characterized by abnormal clonal myeloid proliferation and by progression to acute myelogenous leukemia (AML). CMML thus offers an opportunity to study early genetic events in the transition to AML. A recently recognized subgroup of CMML has a t(5;12)(q33;p13) balanced translocation. We report that the consequence of the t(5;12) translocation is expression of a fusion transcript in which the tyrosine kinase domain of the platelet-derived growth factor receptor beta (PDGFR beta) on chromosome 5 is coupled to a novel ets-like gene, tel, on chromosome 12. The tel-PDGFR beta fusion demonstrates the oncogenic potential of PDGFR beta and may provide a paradigm for early events in the pathogenesis of AML.

The Ets family of transcription factors

Interest in the Ets proteins has grown enormously over the last decade. The v-ets oncogene was originally discovered as part of a fusion protein expressed by a transforming retrovirus (avian E26), and later shown to be transduced from a cellular gene. About 30 related proteins have now been found in species ranging from flies to humans, that resemble the vEts protein in the so-called 'ets domain'. The ets domain has been shown to be a DNA-binding domain, that specifically interacts with sequences containing the common core trinucleotide GGA. Furthermore, it is involved in protein-protein interactions with co-factors that help determine its biological activity. Many of the Ets-related proteins have been shown to be transcription activators, like other nuclear oncoproteins and anti-oncoproteins (Jun, Fos, Myb, Myc, Rel, p53, etc.). However, Ets-like proteins may have other functions, such as in DNA replication and a general role in transcription activation. Ets proteins have been implicated in regulation of gene expression during a variety of biological processes, including growth control, transformation, T-cell activation, and developmental programs in many organisms. Signals regulating cell growth are transmitted from outside the cell to the nucleus by growth factors and their receptors. G-proteins, kinases and transcription factors. We will discuss how several Ets-related proteins fit into this scheme, and how their activity is regulated both post- and pre-translationally. Loss of normal control is often associated with conversion to an oncoprotein. vEts has been shown to have different properties from its progenitor, which might explain how it has become oncogenic. Oncogene-related products have been implicated in the control of various developmental processes. Evidence is accumulating for a role for Ets family members in Drosophila development, Xenopus oocyte maturation, lymphocyte differentiation, and viral infectious cycles. An ultimate hope in studying transformation by oncoproteins is to understand how cells become cancerous in humans, which would lead to more effective treatments. vEts induces erythroblastosis in chicken. Cellular Ets-family proteins can be activated by proviral insertion in mice and, most interestingly, by chromosome translocation in humans. We are at the beginning of understanding the multiple facets of regulation of Ets activity. Future work on the Ets family promises to provide important insights into both normal control of growth and differentiation, and deregulation in illness.

The yeast GAL4 protein transactivates the polyomavirus origin of DNA replication in mouse cells

We have replaced the polyomavirus (Py) enhancer, which is an essential component of the Py origin of DNA replication (ori), with five repeats of a 17-bp oligonucleotide including the yeast GAL4 upstream activating sequence (5xGAL4 sites). Plasmids containing this modified Py ori, designated test plasmids, and plasmids encoding either the GAL4 transcriptional activator protein or various derivatives of this protein were cotransfected into mouse cells which constitutively synthesize a temperature-sensitive Py large tumor antigen (T-Ag). Replication of the test plasmids was monitored by Southern blot determinations of the amounts of plasmid DNA that became resistant to cleavage by the enzyme DpnI. These studies showed that in the presence of a functional T-Ag, the GAL4 protein, and hybrid proteins including the GAL4 DNA-binding domain and the activating domain of the adenovirus E1a or herpesvirus VP16 protein transactivated the modified Py ori. A truncated protein including just the GAL4 DNA-binding domain was inactive in these assays. The authentic GAL4 protein was found to be a more efficient replication transactivator than the hybrid proteins. In contrast, chloramphenicol acetyltransferase assays showed that the hybrid proteins were more efficient transcriptional activators than the GAL4 protein. The extent of the GAL4-dependent replication of a plasmid in which the Py early promoter was deleted was 55% lower than that of a plasmid including the promoter. However, the extents of replication of plasmids including two tandem repeats of the remaining Py origin core and 5xGAL4 sites or two origin cores flanking a single cluster of 5xGAL4 sites were 4.8- and 1.6-fold higher than that of the plasmid including a single copy of each element. The replication of a plasmid including two clusters of 5xGAL4 sites flanking a single origin core was below the limit of detection of our assays. These results indicate that the GAL4 and hybrid transactivators do not activate the Py ori by virtue of their interactions with transcription factors that bind promoter elements. Rather, it appears that these activator proteins may interact with the replication initiation complexes, thereby facilitating or inhibiting the initiation of replication.