Progesterone and glucocorticoid response elements occur in the long control regions of several human papillomaviruses involved in anogenital neoplasia

We have previously identified in the long control region of the genome of human papillomavirus type 16 (HPV-16) a DNA segment which functions as a cell-type-specific enhancer as well as mediating glucocorticoid response. It contains multiple transcription-factor-binding sites, including several for nuclear factor I and one for the glucocorticoid receptor, which binds to the partially palindromic sequence TGTACANNNTGTCAT. We report here that this sequence element, when separated from the surrounding transcription-factor-binding sites and placed as an oligonucleotide into a test vector, retains its function as a glucocorticoid response element (GRE) in HeLa cells. In T47D cells, which express the progesterone receptor, the HPV-16 enhancer fragment mediates progesterone responsiveness. A point mutant in this fragment and the response of the oligonucleotide clone to both steroids prove the identity of the progesterone response element (PRE) with the GRE. The antiprogesterone and antiglucocorticoid RU486 interferes with both hormonal responses. In SiHa cells, the HPV-16 GRE mediates an increase in transcripts encoding E6 and E7 proteins, which are involved in transformation by HPV-16. Hormonal regulation is not restricted to HPV-16: DNA segments containing the cell-type-specific enhancers of HPV-11 and HPV-18 also mediate glucocorticoid and progesterone response. We identified sequence elements in the long control regions of HPV-11 and HPV-18 which function as GRE/PREs when tested as oligonucleotides. These findings suggest that GRE/PREs are an integral part of gene expression regulation in genital HPVs.

Nucleic acid hybridization and the polymerase chain reaction in biology and medicine, with special reference to the detection of human papillomaviruses

Nucleic acid hybridization, and more recently, gene amplification by the polymerase chain reaction, have contributed immensely to the development of molecular genetics. These powerful tools have been greeted and employed with great enthusiasm, in view of their wide-ranging applications in biological research and clinical diagnosis. Here we review the basic principles and some applications of these techniques, particularly in infectious diseases and oncogenesis. In order to illustrate their use, data on specific methods as applied to the detection of human papillomaviruses is presented. Gene probe and DNA amplification technology will undoubtedly continue to provide critical molecular pathways in the biomedical research endeavour.

Clusters of nuclear factor I binding sites identify enhancers of several papillomaviruses but alone are not sufficient for enhancer function

The long control region (LCR) of human papillomaviruses (HPV) encompasses 5-12% of the viral genome and contains an intricate network of cis responsive elements. In the LCR of seven unrelated HPV-types, namely HPV-1, 6, 8, 11, 16, 18 and 33, we have identified clusters of 4 to 7 5-TTGGC-3 motifs suggesting nuclear factor I (NFI) binding sites. We randomly selected 20 (out of a total of 38) of these motifs and showed that pure NFI from porcine liver protects virtually the same nucleotides as a factor present in crude HeLa nuclear extracts. The footprints obtained with HeLa extracts in the LCR of HPV-16 are eliminated in competition experiments by an oligonucleotide representing the palindromic adenovirus NFI binding site. Restriction fragments from the genome of HPV-11, 16 and 18, which contain this cluster of NFI binding sites associated with binding sites of unrelated transcription factors, function as transcriptional enhancers. In contrast, a fragment from HPV-8 exhibiting exclusively NFI binding sites, or polymerized NFI sites from HPV-16, are functionally inactive. NFI seems to be necessary but not sufficient for HPV enhancer activation.

Numerous nuclear proteins bind the long control region of human papillomavirus type 16: a subset of 6 of 23 DNase I-protected segments coincides with the location of the cell-type-specific enhancer

The long control region of the human papillomavirus type 16 genome is 856 base pairs (bp) long. It contains a cell-type-specific enhancer, a glucocorticoid response element, and sequences mediating the response to the viral gene products of open reading frame E2; all three regulate the promoter P97. We mapped binding sites of trans-acting proteins relevant for the cell-type-specific enhancer and other cis-acting elements by DNase I footprint experiments with nuclear extracts from HeLa cells. Throughout the human papillomavirus type 16 long control region 23 footprints protect 557 of 900 bp. Nine footprints fall into a 400-bp segment that was previously identified to contain the cell-type-specific enhancer. Variations of the protein concentration in the footprint reaction do not affect six of these nine footprints. At high protein concentrations, three footprints fuse to a 106-bp protected region, suggesting that this segment specifically binds several proteins of lower affinity or abundance. Unexpectedly, extracts from human MCF7 and mouse 3T3 cells, in which the enhancer is inactive, give footprints identical to those obtained with HeLa extracts. Seven footprints contain the sequence 5'-TTGGC-3'. Footprint competition experiments suggest that factor NFI binds to these seven motifs. Competition with cloned oligonucleotides in transfections suggests that these elements contribute to the enhancer function. Subcloning identifies a 232-bp fragment between positions 7524 and 7755 as sufficient for full enhancer activity. Several of the six footprinted elements on this segment may cooperate functionally, since subclones of this region show decreased or no cell-type-specific enhancer function.

Human papillomavirus-16 and genital cancer: are tests for the viral gene expression in vitro indicators for risk factors in vivo?

Human papillomaviruses are found in up to 90% of all cervical carcinomas and are considered to play a causal role in the etiology of this malignancy. The genome of human papillomaviruses consists of a single circular DNA molecule with a size of approximately 8000 basepairs. 90% of this genome encodes proteins involved in functions such as neoplastic transformation of the host cell or formation of the viral capsid. The remaining 10% of the genome, which is termed upstream regulatory region (URR), harbours elements to control expression of the viral genes. We have identified in the URR DNA elements that regulate viral gene expression in the presence of glucocorticoid hormones or tumour promoting substances. This was done by DNase I protection experiments and functional analysis of fusion genes. Our data predict that the transforming potential of the virus might be stimulated by certain steroid hormones, polypeptide hormones and tumour promoting chemicals.

The upstream regulatory region of the human papilloma virus-16 contains an E2 protein-independent enhancer which is specific for cervical carcinoma cells and regulated by glucocorticoid hormones

The upstream regulatory region of the human papilloma virus-16 (HPV-16) genomic DNA contains a sequence element with a large degree of homology to the partially palindromic sequence GGTACANNNTGTTCT, which is the consensus sequence of the glucocorticoid responsive elements of known genes regulated by this steroid hormone. DNase I and dimethylsulfate protection experiments reveal the binding of this sequence by rat glucocorticoid receptor protein. A 400-bp DNA segment centrally containing this sequence confers strong inducibility by dexamethasone to the promoter p97 of HPV-16 and to the Herpes simplex virus thymidine kinase promoter, as judged by chloramphenicol acetyltransferase activity and RNase protection assays. The same DNA segment, that does not contain the consensus sequences of all papilloma viruses relevant for E2 protein-mediated transcription enhancement, functions in an enhancer-like fashion in addition to its glucocorticoid responsive action. This hormone-independent transcription enhancement is absent in human MCF7 cells, but is strong in human HeLa cells where the combined activity of the constitutive and the steroid hormone-dependent enhancer elements stimulate transcription by a factor of 500. This cell type specificity of the HPV-16 enhancer may be responsible for the tissue tropism of the virus. These observations and the presence of numerous homologies to known enhancers of cellular and viral genes suggest a complex pattern of activation of the human papilloma virus-16 promoters.

Expression of the human papillomavirus type 18 E7 gene by a cassette-vector system for the transcription and translation of open reading frames in eukaryotic cells

We have constructed and functionally tested a cassette-vector-system for the transcription and translation of open reading frames (ORFs) in cells of higher eukaryotes. The vectors are derived from the plasmid pBR322 and can be selected and amplified in Escherichia coli. Alternative eukaryotic promoters can be inserted between the restriction sites SphI and KpnI, translation initiation motifs between KpnI and BglII, linkers for the adjustment of the translation reading frame and the insertion of genes or gene segments between BglII and HindIII, followed by a HindIII-EcoRI segment with splicing and polyadenylation signals derived from SV40. A prototype vector system, pORFEX11, 12 and 13, contains the strong cytomegalovirus immediately early promoter and a 10-bp motif of the SV40 T-antigen translation start. Polylinkers derived from pUC18 permit the insertion of ATG-less ORFs downstream from the ATG of the vector. Either of the three alternative polylinkers adjusts the appropriate translation frame. A similar construct contains the regulatable promoter of the Drosophila heat shock gene 70. We inserted genes or gene segments, that code for the bacterial chloramphenicol acetyltransferase, the bacterial gene conferring resistance against hygromycin, and the ORF E7 of the human papillomavirus type 18 into these vectors. After transfection of mouse L fibroblasts, all proteins and functions were expressed in accordance with the prediction. In transiently transfected L cells, the E7 protein expressed from pORFEX12 constitutes approximately 2.0% of total cell protein. This E7 protein could be localized by immunocytochemistry as a cytoplasmic component.

Physical state, expression and regulation of two glucocorticoid-controlled genes on bovine papilloma virus vectors

We have studied the extrachromosomal maintenance and the transcription regulation of two glucocorticoid-inducible genes on bovine papilloma virus (BPV) vectors in c127 mouse fibroblasts. These genetic elements were the rat tryptophan oxygenase (TOase) gene promoter, which is active in vivo only in hepatocytes, and the long terminal repeat of the mouse mammary tumor virus (MMTV-LTR). From both genes, fusions of the 5'-flanking region of the transcription unit to the bacterial gene for chloramphenicol acetyltransferase (CATase) were constructed. These fusion genes were inserted either into pCGBPV9, a BPV vector encoding G418 resistance, into pBPV-BV1, a vector containing "stabilizing" segments of the human beta-globin gene, or into a BPV construct, whose bacterial plasmid sequences could be removed before transfection. Five constructs of the two latter groups, selectable in c127 cells only as foci, were normally maintained in the extrachromosomal state. In contrast, three out of five constructs based on pCGBPV9 and selectable for resistance against G418 were maintained in a high molecular weight form, most probably of intrachromosomal concatemeric nature, while the remaining two G418-resistant constructs appeared alternatively in this or the extrachromosomal monomeric form. In contrast to its absence of expression in fibroblasts in vivo, the TOase gene element present on BPV vectors was found to be active in fibroblasts in these transfection experiments. As judged by CATase activities and for TOase also by mapping of the transcription start sites, transcription of both genes was under hormonal regulation. All BPV vectors proved to be useful tools in the study of these regulated genes, and in only one out of ten constructs was regulation atypical, possibly due to effects from flanking vector sequences.

Construction of a fusion gene that confers resistance against hygromycin B to mammalian cells in culture

Mouse L fibroblasts and other mammalian cells are killed by the translation inhibitor hygromycin B. We have modified the gene conferring resistance against hygromycin B in E. coli in such a way that it can be transcribed in mammalian cells from the promoter of the HSVtk gene. The resulting plasmid, pHMR272, was transfected into mouse L fibroblasts and HeLa cells by the calcium phosphate method and upon selection produced clones resistant against hygromycin B. The transfection rate was similar to that obtained with other selective markers. This plasmid is a useful addition to the relatively small number of dominant selectable markers available for mammalian cells.

New cosmid vectors developed for eukaryotic DNA cloning

A series of ColE1 and pSC101 cosmid vectors have been constructed suitable for cloning large stretches of DNA. All contain a single BamHI site allowing cloning of Sau3A, MboI, BglII, BclI , and BamHI-generated fragments. These vectors have the following characteristics: (i) they are relatively small (1.7-3.4 kb); (ii) the BamHI cloning site is flanked by restriction enzyme sites enabling direct cloning of unfractionated insert DNA without generating multiple insert or vector ligation products [ Ish - Horowitz and Burke, Nucl . Acids Res. 9 (1981) 2989-2998]; (iii) two vectors ( pHSG272 and pHSG274 ) contain a hybrid Tn5 KmR/ G418R gene which is selectable in both prokaryotic and eukaryotic cells, making them suitable for transferring DNA into eukaryotic cells, and (iv) the different prokaryotic selectable markers available in the other vectors described facilitate cosmid rescue of the transferred DNA sequences from the eukaryotic cell: CmR, ApR, KmR, ( pHSG429 ), CmR, ( pHSG439 ), colicin E1 immunity ( pHSG250 ), (v) the cosmid pHSG272 was used successfully to construct a shuttle vector based on the BPVI replicon [ Matthias et al., EMBO J. 2 (1983) 1487-1492].

A bovine papilloma virus vector with a dominant resistance marker replicates extrachromosomally in mouse and E. coli cells

We describe the construction of a bovine papilloma virus-based vector (pCGBPV9) which contains a dominant selectable marker and replicates autonomously in both mouse and Escherichia coli cells. This vector contains the complete bovine papilloma virus genome, a ColE1 replication origin and a dominant selectable marker conferring resistance to kanamycin in bacteria and G418 in eukaryotic cells. A high number of G418R colonies are obtained after transfer of pCGBPV9 into mouse C127 cells. These G418R colonies contain vector DNA which replicates autonomously at approximately 10-30 copies per cell. The molecules are in most cases unrearranged and can be rescued into E. coli cells by bacterial transformation.

Isolation of a genomic clone partially encoding human hypoxanthine phosphoribosyltransferase

Mouse cells deficient in the enzyme hypoxanthine phosphoribosyltransferase (HPRT; EC 2.4.2.8) have been transfected with total human DNA, and cells producing human enzyme were isolated by growth in selective medium. DNA from several such cell lines has been used to generate secondary transfectants that make human HPRT. Blots of the DNA of these secondary cells have been hybridized with total human DNA probes or with cloned human Alu sequences, and one of several common bands has been cloned in pBR322. Colonies of transformed Escherichia coli containing human sequences were detected by their homology with human DNA, and subclones of resulting recombinant plasmids were prepared. Two subclones free of Alu sequences were found to contain human sequences that hybridized to human X chromosome DNA. One of these, pBR1.5, also hybridized to a single RNA band on gel blots of human and secondary transfectant cytoplasmic poly(A)+RNA but not to RNA from the parent mouse cell line. These results indicate that these clones represent human HPRT gene fragments. This has been confirmed by using pBR1.5 as a probe to isolate an authentic and expressible human HPRT cDNA clone from a library prepared by H. Okayama and P. Berg.

Construction of plasmid cloning vehicles that promote gene expression from the bacteriophage lambda pL promoter

Two multiple-copy, ColE1-type, plasmid cloning vehicles, pHUB2 and pHUB4, have been constructed that carry four different single restriction sites down-stream from the phage lambda promoter pL. The promoting activity of pL is switched off at low temperature in the presence of a cIts gene that specifies a temperature-sensitive repressor but could be activated by heat induction. cIts was located either on the host chromosome, or on a second plasmid pRK248 that is compatible with the cloning vehicle, or on the vehicle itself. Three different restriction fragments, each carrying the gene trpA of Salmonella typhimurium or Shigella dysenteriae, have been inserted into the EcoRI, BamHI and SalI sites, respectively, of these plasmids and pL dependent expression of the inserted gene in Escherichia coli was determined by measuring the enzymatic activity of the trpA gene product. Heat induction resulted in a level of expression of trpA corresponding to 1 to 6.6% of the total soluble cell protein as trpA protein. The level of trpA protein production depended on the particular insert and the plasmid used.