High level production of human growth hormone in the milk of transgenic mice: the upstream region of the rabbit whey acidic protein (WAP) gene targets transgene expression to the mammary gland

Mice with gene alterations in the GH and IGF family

Much of our understanding of GH's action stems from animal models and the generation and characterization of genetically altered or modified mice. Manipulation of genes in the GH/IGF1 family in animals started in 1982 when the first GH transgenic mice were produced. Since then, multiple laboratories have altered mouse DNA to globally disrupt Gh, Ghr, and other genes upstream or downstream of GH or its receptor. The ability to stay current with the various genetically manipulated mouse lines within the realm of GH/IGF1 research has been daunting. As such, this review attempts to consolidate and summarize the literature related to the initial characterization of many of the known gene-manipulated mice relating to the actions of GH, PRL and IGF1. We have organized the mouse lines by modifications made to constituents of the GH/IGF1 family either upstream or downstream of GHR or to the GHR itself. Available data on the effect of altered gene expression on growth, GH/IGF1 levels, body composition, reproduction, diabetes, metabolism, cancer, and aging are summarized. For the ease of finding this information, key words are highlighted in bold throughout the main text for each mouse line and this information is summarized in Tables 1, 2, 3 and 4. Most importantly, the collective data derived from and reported for these mice have enhanced our understanding of GH action.

The main WAP isoform usually found in camel milk arises from the usage of an improbable intron cryptic splice site in the precursor to mRNA in which a GC-AG intron occurs

Background

Whey acidic protein (WAP) is a major protein identified in the milk of several mammalian species with cysteine-rich domains known as four-disulfide cores (4-DSC). The organization of the eutherian WAP genes is highly conserved through evolution. It has been proposed that WAP could play an important role in regulating the proliferation of mammary epithelial cells. A bacteriostatic activity was also reported. Conversely to the other mammalian species expressing WAP in their milk, camel WAP contains 4 additional amino acid residues at the beginning of the second 4-DSC domain, introducing a phosphorylation site. The aim of this study was to elucidate the origin of this specificity, which possibly impacts its physiological functions.

Results

Using LC-ESI-MS, we identified in Camelus bactrianus from Kazakhstan a phosphorylated whey protein, exhibiting a molecular mass (12,596 Da), 32 Da higher than the original WAP (12,564 Da) and co-eluting with WAP. cDNA sequencing revealed a transition G/A, which modifies an amino acid residue of the mature protein (V12 M), accounting for the mass difference observed between WAP genetic variants. We also report the existence of two splicing variants of camel WAP precursors to mRNA, arising from an alternative usage of the canonical splice site recognized as such in the other mammalian species. However, the major camel WAP isoform results from the usage of an unlikely intron cryptic splice site, extending camel exon 3 upstream by 12-nucleotides encoding the 4 additional amino acid residues (VSSP) in which a potentially phosphorylable Serine residue occurs. Combining protein and cDNA sequences with genome data available (NCBI database), we report another feature of the camel WAP gene which displays a very rare GC-AG type intron. This result was confirmed by sequencing a genomic DNA fragment encompassing exon 3 to exon 4, suggesting for the GC donor site a compensatory effect in terms of consensus at the acceptor exon position.

Conclusions

Combining proteomic and molecular biology approaches we report: the characterization of a new genetic variant of camel WAP, the usage of an unlikely intron cryptic splice site, and the occurrence of an extremely rare GC-AG type of intron.

Ingestion of milk containing the Dp2 peptide, a dust mite allergen, protects mice from allergic airway inflammation and hyper-responsiveness

Background

Allergen-specific immunotherapy has been demonstrated to have potential for the treatment of allergic diseases. Transgenic animals are currently the best available bioreactors to produce recombinant proteins, which can be secreted in milk. It has not been clearly demonstrated whether milk from transgenic animals expressing recombinant allergens has immunomodulatory effects on allergic asthma.

Methods

We aimed to determine whether the oral administration of milk containing a mite allergen can down-regulate allergen-specific airway inflammation. Transgenic CD-1 mice that express a recombinant group 2 allergen from Dermatophagoides pteronyssinus (Dp2) in their milk were generated using an embryonic gene-microinjection technique. Mouse pups were fed transgenic Dp2-containing milk or wild-type milk. Subsequently, these mice were sensitized and challenged with Dp2 to induce allergic airway inflammation.

Results

Upon sensitization and challenge, mice fed transgenic Dp2 milk had decreased T-helper 2 (Th2) and increased T-helper 1 (Th1) responses in the airway compared with mice fed wild-type milk. Moreover, pre-treatment with transgenic Dp2 milk attenuated airway inflammation and decreased airway hyper-responsiveness.

Conclusions

This study provides new evidence that oral administration of transgenic milk containing the Dp2 allergen down-regulated and moderately protected against allergic airway inflammation. Milk from transgenic animals expressing allergens may have potential use in the prevention of allergic asthma.

Sequential activation of genetic programs in mouse mammary epithelium during pregnancy depends on STAT5A/B concentration

The transcription factors Signal Transducer and Activator of Transcription (STAT) 5A/B mediate prolactin-induced mammary development during pregnancy. However, it is not clear how the different processes, expansion and maturation of alveolar precursor cells and the differential induction of milk protein genes are regulated on a molecular level. We have used mouse genetics and genome-wide analyses to determine how altering concentrations of STAT5A and STAT5B impacts mammary epithelial development during pregnancy and the regulation of target genes. The presence of only a single Stat5a or Stat5b allele was sufficient for the establishment of histologically undifferentiated alveolar units and two alleles permitted the execution of a differentiation program similar to that found with all four alleles. While one copy of Stat5 induced limited expression of target genes, two copies activated a lactation-like gene signature. Using ChIP-seq analyses on intact tissue under physiological conditions, we found that highly expressed and regulated genes were bound by STAT5 in their promoter proximal regions, whereas upstream binding had minor biological consequences. Remarkably, 80% of the genes bound by STAT5 in vivo were not under STAT5 control. RNA polymerase II intensity was directly proportional to STAT5 concentration only on STAT5 regulated genes providing mechanistic insight by which STAT5 activates mammary specific genes.

Establishment of transgenic fibroblasts for producing recombinant human interferon-α and erythropoietin in bovine milk

Human interferon α (IFN-α) and erythropoietin (EPO) have been used for a variety of purposes in clinical medicine. Human IFN-α has been used to treat several types of viral infection and cancer, as well as renal anemia, via stimulation of erythrocyte formation in the bone marrow. Transgenic cattle are excellent candidates for pharmaceutical production for humans due to their ability to produce recombinant proteins in milk. The purpose of the present study was to generate bovine transgenic fibroblasts capable of producing recombinant human IFN-α and EPO proteins in transgenic cattle milk. First, we analyzed the promoter activities of various bovine milk protein genes in HC11 mouse mammary epithelial cells. The bovine milk protein gene promoters were cloned into the Luc gene in a promoter-less pGL3-Basic vector. Presence of the αS1-casein promoter (-175 to +796 nt) resulted in an up to 16-fold increase in luciferase activity compared with that of the promoter-less construct. In addition, the human IFN-α and EPO genes were identified as significantly overexpressed in HC11 cells compared with the promoter-less construct. Together, the present results demonstrate that the construct with the αS1-casein promoter may induce secretion of recombinant human IFN-α and EPO into bovine milk. Furthermore, we generated transgenic fibroblasts expressing human IFN-α and EPO cDNA controlled by the αS1-casein promoter and two screening markers, enhanced green fluorescent protein and neomycin resistance. These transgenic fibroblasts may be a source of somatic cells for generating transgenic cattle that produce recombinant human IFN-α and EPO proteins during lactation.

Expression of human growth hormone in the milk of transgenic rabbits with transgene mapped to the telomere region of chromosome 7q

The advent of transgenic technology has provided methods for the production of pharmaceuticals by the isolation of these proteins from transgenic animals. The mammary gland has been focused on as a bioreactor, since milk is easily collected from lactating animals and protein production can be expressed at very high levels, including hormones and enzymes. We demonstrate here the expression pattern of recombinant human growth hormone (rhGH) in transgenic rabbits carrying hGH genomic sequences driven by the rat whey acidic protein (WAP) promoter. The transgene was mapped to the q26-27 telomere region of chromosome 7q by fluorescence in situ hybridization (FISH). Nearly 30 % of the F1 generation demonstrated the presence of transgene. The recombinant growth hormone was detected in the milk of the transgenic rabbit females, but not in serum, up to the level of 10 μg/ml. Ectopic expression of the transgene in the brain, heart, kidney, liver, and salivary gland was not observed, indicating that a short sequence of rat WAP promoter (969 bp) contained essential sequences directing expression exclusively to the mammary gland. The biological activity of recombinant growth hormone was measured by immunoreactivity and the capability to stimulate growth of the hormone-dependent Nb211 cell line.

Production of transgenic goats expressing human coagulation factor IX in the mammary glands after nuclear transfer using transfected fetal fibroblast cells

There are growing numbers of recombinant proteins that have been expressed in milk. Thus one can consider the placement of any gene of interest under the control of the regulatory elements of a milk protein gene in a dairy farm animal. Among the transgene introducing techniques, only nuclear transfer (NT) allows 100 % efficiency and bypasses the mosaicism associated with counterpart techniques. In this study, in an attempt to produce a transgenic goat carrying the human coagulation factor IX (hFIX) transgene, goat fetal fibroblasts were electroporated with a linearized marker-free construct in which the transgene was juxtaposed to β-casein promoter designed to secret the recombinant protein in goat milk. Two different lines of transfected cells were used as donors for NT to enucleated oocytes. Two transgenic goats were liveborn. DNA sequencing of the corresponding transgene locus confirmed authenticity of the cloning procedure and the complementary experiments on the whey demonstrated expression of human factor IX in the milk of transgenic goats. In conclusion, our study has provided the groundwork for a prosperous and promising approach for large-scale production and therapeutic application of hFIX expressed in transgenic goats.

A 3,387 bp 5'-flanking sequence of the goat alpha-S1-casein gene provides correct tissue-specific expression of human granulocyte colony-stimulating factor (hG-CSF) in the mammary gland of transgenic mice

A new expression vector containing the 1,944 bp 5'-flanking regulatory region together with exon 1 and intron 1 of the goat alpha-S1-casein gene (CSN1S1), the full-sized human granulocyte colony-stimulating factor gene (hGCSF) and the 3'-flanking sequence of the bovine CSN1S1, was created. The vector DNA was used for generation of four mouse transgenic lines. The transgene was integrated into chromosomes 8 and 12 of two founders as 2 and 5 copies, respectively. Tissue-specific secretion of hG-CSF into the milk of transgenic mice was in the range of 19-40 μg/ml. RT-PCR analysis of various tissues of the transgenic mice demonstrated that expression of hGCSF was detected in only the mammary gland in the progeny of all founders. Moreover, cells were shown to be positive for hG-CSF by immunofluorescent analysis in the mammary glands but not in any other tissues. There were no signs of mosaic expression in the mammary gland. Trace amounts of hG-CSF were detected in the serum of females of two transgenic lines during lactation only. However, no transgenic mice showed any changes in hematopoiesis based on the number of granulocytes in blood. Immunoblotting of hG-CSF in the milk of transgenic mice revealed two forms, presumably the glycosylated and non-glycosylated forms. The hematopoietic activity of hG-CSF in the milk of transgenic females is comparable to that of recombinant G-CSF. In general, the data obtained in this study show that the new expression vector is able to provide correct tissue-specific expression of hG-CSF with high biological activity in transgenic mice.

Transgenic mammalian species, generated by somatic cell cloning, in biomedicine, biopharmaceutical industry and human nutrition/dietetics--recent achievements

Somatic cell cloning technology in mammals promotes the multiplication of productively-valuable genetically engineered individuals, and consequently allows also for standardization of transgenic farm animal-derived products, which, in the context of market requirements, will have growing significance. Gene farming is one of the most promising areas in modern biotechnology. The use of live bioreactors for the expression of human genes in the lactating mammary gland of transgenic animals seems to be the most cost-effective method for the production/processing of valuable recombinant therapeutic proteins. Among the transgenic farm livestock species used so far, cattle, goats, sheep, pigs and rabbits are useful candidates for the expression of tens to hundreds of grams of genetically-engineered proteins or xenogeneic biopreparations in the milk. At the beginning of the new millennium, a revolution in the treatment of disease is taking shape due to the emergence of new therapies based on recombinant human proteins. The ever-growing demand for such pharmaceutical or nutriceutical proteins is an important driving force for the development of safe and large-scale production platforms. The aim of this paper is to present an overall survey of the state of the art in investigations which provide the current knowledge for deciphering the possibilities of practical application of the transgenic mammalian species generated by somatic cell cloning in biomedicine, the biopharmaceutical industry, human nutrition/dietetics and agriculture.

Genetically modified animals for use in research and biotechnology

Transgenic animals are used extensively in the study of in vivo gene function, as models for human diseases and in the production of biopharmaceuticals. The technology behind obtaining these animals involves molecular biology techniques, cell culture and embryo manipulation; the mouse is the species most widely used as an experimental model. In scientific research, diverse models are available as tools for the elucidation of gene function, such as transgenic animals, knockout and conditional knockout animals, knock-in animals, humanized animals, and knockdown animals. We examined the evolution of the science for the development of these animals, as well as the techniques currently used in obtaining these animal models. We review the phenotypic techniques used for elucidation of alterations caused by genetic modification. We also investigated the role of genetically modified animals in the biotechnology industry, where they promise a revolution in obtaining heterologous proteins through natural secretions, such as milk, increasing the scale of production and facilitating purification, thereby lowering the cost of production of hormones, growth factors and enzymes.

Rabbit milk protein genes: from mRNA identification to chromatin structure

Milk protein genes are among the most intensively expressed and they are active only in epithelial mammary cells of lactating animals. They code for proteins which represent 30% of the proteins consumed by humans in developed countries. Mammary gland development occurs essentially during each pregnancy. This offers experimenters attractive models to study the expression mechanisms of genes controlled by known hormones and factors (prolactin, glucocorticoids, progesterone, insulin-like growth factor-1 and others) as well as extracellular matrix. In the mid-1970s, it became possible to identify and quantify mRNAs from higher living organisms using translation in reticulocyte lysate. A few years later, the use of radioactive cDNAs as probes made it possible for the quantification of mRNA in various physiological situations using hybridisation in the liquid phase. Gene cloning offered additional tools to measure milk protein mRNAs and also to identify transcription factors. Gene transfer in cultured mammary cells and in animals contributed greatly to these studies. It is now well established that most if not all genes of higher eukaryotes are under the control of multiple distal regulatory elements and that local modifications of the chromatin structure play an essential role in the mechanisms of differentiation from embryos to adults. The technique, known as ChIP (chromatin immunoprecipitation), is being implemented to identify the factors that modify chromatin structure at the milk protein gene level during embryo development, mammogenesis and lactogenesis, including the action of hormones and extracellular matrix. Transgenesis is not just a tool to study gene regulation and function, it is also currently used for various biotechnological applications including the preparation of pharmaceutical proteins in milk. This implies the design of efficient vectors capable of directing the secretion of recombinant proteins in milk at a high concentration. Milk protein gene promoters and long genomic-DNA fragments containing essentially all the regulatory elements of milk protein genes are used to optimise recombinant protein production in milk.

Producing recombinant human milk proteins in the milk of livestock species

Recombinant human proteins produced by the mammary glands of genetically modified transgenic livestock mammals represent a special aspect of milk bioactive components. For therapeutic applications, the often complex posttranslational modifications of human proteins should be recapitulated in the recombinant products. Compared to alternative production methods, mammary gland production is a viable option, underlined by a number of transgenic livestock animal models producing abundant biologically active foreign proteins in their milk. Recombinant proteins isolated from milk have reached different phases of clinical trials, with the first marketing approval for human therapeutic applications from the EMEA achieved in 2006.

In vitro and in vivo effects of a multimerized alphas 1-casein enhancer on whey acidic protein gene promoter activity

Experimental data obtained in previous works have led to postulate that enhancers increase the frequency of action of a linked promoter in a given cell and may have some insulating effects. The multimerized rabbit alpha s1-casein gene enhancer, the 6i multimer, was added upstream of the rabbit whey acidic protein gene (WAP) promoter (-6,300; +28 bp) fused to the firefly luciferase (luc) gene (6i WAP-luc construct). The 6i multimer increased reporter gene expression in mouse mammary HC11 cells. In transgenic mice, a very weak but significant increase was also observed. More noticeable, no silent lines were found when the 6i multimer was associated to the WAP-luc construct. This reflects the fact that the 6i multimer tends to prevent the silencing of the WAP-luc construct. After addition of the 5'HS4 insulator region from the chicken beta-globin locus upstream of the 6i multimer, similar luciferase levels were measured in 6i WAP-luc and 5'HS4 WAP-luc transgenic mice. Our present data and previous ones, which show that the 6i multimer has no insulating activity on a TK gene promoter construct indicate that the insulating activity of the 6i multimer is construct-dependent and not amplified by the 5'HS4 insulator.

Mammary gland-specific secretion of biologically active immunosuppressive agent cytotoxic-T-lymphocyte antigen 4 human immunoglobulin fusion protein (CTLA4Ig) in milk by transgenesis

A major challenge in the field of transplantation is to prevent graft rejection and prolong graft survival. Tolerance induction is a promising way to achieve long-term graft survival without the need for potent immunosuppression and its associated side effects. The recent success of co-stimulatory blockade by the chimeric protein CTLA4Ig in the modulation of the recipient's immune system and the prolongation of graft survival in animal models suggests a possible application of CTLA4Ig in clinical transplantation. To produce sufficient amounts of CTLA4Ig for future clinical application, we sought to use the mammary gland as a bioreactor and produce CTLA4Ig in the milk of transgenic farm animals. Prior to the generation of transgenic farm animals, we tested our strategy in mice. Using the promoter of the sheep beta-lactoglobulin gene, we expressed our CTLA4Ig chimeric gene in the mammary gland of transgenic mice. The yield of CTLA4Ig was fivefold higher in transgenic milk than that from transfected cells. Purified milk-derived CTLA4Ig is biologically active and suppresses T cell activation. We showed that the production of CTLA4Ig in the milk has no adverse immunosuppression effect on the transgenic animals and the offsprings that were fed with the transgenic milk. The findings suggest that the approach to produce CTLA4Ig in milk by transgenesis is feasible; further studies involving farm animals are warranted.

Gene transfer strategies in animal transgenesis

Position effects in animal transgenesis have prevented the reproducible success and limited the initial expectations of this technique in many biotechnological projects. Historically, several strategies have been devised to overcome such position effects, including the progressive addition of regulatory elements belonging to the same or to a heterologous expression domain. An expression domain is thought to contain all regulatory elements that are needed to specifically control the expression of a given gene in time and space. The lack of profound knowledge on the chromatin structure of expression domains of biotechnological interest, such as mammary gland-specific genes, explains why most standard expression vectors have failed to drive high-level, position-independent, and copy-number-dependent expression of transgenes in a reproducible manner. In contrast, the application of artificial chromosome-type constructs to animal transgenesis usually ensures optimal expression levels. YACs, BACs, and PACs have become crucial tools in animal transgenesis, allowing the inclusion of distant key regulatory sequences, previously unknown, that are characteristic for each expression domain. These elements contribute to insulating the artificial chromosome-type constructs from chromosomal position effects and are fundamental in order to guarantee the correct expression of transgenes.

The murine whey acidic protein promoter directs expression to human mammary tumors after retroviral transduction

The whey acidic protein (WAP) promoter is known to be active in pregnant and lactating mammary epithelial cells as well as mammary tumors of mice. Here we show that a proximal fragment of the murine WAP promoter, including most elements postulated as being responsible for mammary-specific regulation, confers mammary-specific expression upon a marker gene in transgenic mice even though the distal promoter region, known to be important for rat WAP promoter activity, is lacking. The relatively small size of this fragment allows its insertion into a murine leukemia virus-based retroviral vector in place of the viral promoter. Infection of a number of established human mammary and nonmammary cell lines with such a retroviral vector revealed that the WAP promoter was limited in its activity to mammary tumor cell lines. Expression in tumorigenic mammary cells was even more pronounced when these cells were introduced into the mammary fat pads of mice. This is the first demonstration that the WAP promoter is active in human mammary cells and mammary tumor cells in general, and suggests that the extended proximal WAP promoter may be useful for directing therapeutic gene expression to human mammary tumors.

Expression of exogenous protein in the egg white of transgenic chickens

Using a replication-deficient retroviral vector based on the avian leukosis virus (ALV), we inserted into the chicken genome a transgene encoding a secreted protein, beta-lactamase, under the control of the ubiquitous cytomegalovirus (CMV) promoter. Biologically active beta-lactamase was secreted into the serum and egg white of four generations of transgenic chickens. The expression levels were similar in successive generations, and expression levels in the magnum of the oviduct were constant over at least 16 months in transgenic hens, indicating that the transgene was stable and not subject to silencing. These results support the potential of the hen as a bioreactor for the production of commercially valuable, biologically active proteins in egg white.

A new beta-lactoglobulin-based vector targets luciferase cDNA expression to the mammary gland of transgenic mice

A beta-lactoglobulin (BLG)/luciferase gene vector (p907), composed of a luciferase intronless gene inserted between the second and sixth BLG exons was constructed. Stable transfections of CID-9 cells with this vector, as well as with a series of additional vectors, were performed to define regulatory regions within the BLG sequence, and the contribution of the SV40 polyadenylation (PA) site to luciferase expression. A relatively low level of luciferase activity was supported by vector p907. It was partially rescued by vector p906, in which the BLG 3' region, downstream of the luciferase cDNA, was replaced with the SV40 PA site. Flanking the SV40 region of vector p906, at its 3' end, with BLG sequences of exon 6/intron 6/exon 7 and the 3' region of the gene resulted in vector p904. This vector supported the highest luciferase activity, 10 times or 2.5 times higher than that measured in cells transfected with vectors p907 and p906, respectively. The induced activity supported by vector p904 is attributed to interaction between the SV40 PA site and elements of the distal part of the BLG 3' flanking sequences. The BLG 5' regulatory region of vector p904 encompasses a 3-kb promoter sequences. Deletion of 935 bp of its proximal end resulted in a 60% decrease in luciferase activity. Reduced activity was also seen with vector p915 lacking sequences of exon 1/intron 1/exon 2. This decrease could not be rescued with heterologous sequences of insulin intron 1, inserted upstream of the luciferase cDNA. Two sets of transgenic mice carrying vectors p907 and p904 were generated. Vector p907 supported only marginal luciferase activity in the mammary gland of all transgenic mice tested and luciferase RNA could not be detected by northern analysis. In contrast, 50% of the transgenic mice carrying vector p904 expressed luciferase RNA in the mammary gland and tissue-specific, hormonal-dependent activity was determined. However, the new p904 vector was not able to insulate the transgene from surrounding host DNA sequences, as reflected by its copy number-independent manner of expression. Nevertheless, vector p904 may represent a valuable tool for the expression of cDNAs in the mammary gland of transgenic animals.

A transgenic mouse model for investigating the response of the upstream region of whey acidic protein (WAP) gene to various steroid hormones

The limitations of studies of clarification of response elements of whey acidic protein (WAP) gene to hormones using mammary cell lines has been shown. We studied the response of the upstream region (2.6 kb) of WAP to various steroid hormones using gonadectomized mWAP/hGH transgenic mice. Ovariectomy or castration for transgenic mice was performed at 10 days or 30 days post partum. Various steroid hormones were administered daily for 10 days to the gonadectomized transgenic mice after they reached 2 months of age. Prior to the hormonal administration and 24 hr after the final administration, blood was collected and the hGH levels in the plasma was measured by RIA. Daily doses of estradiol-17 beta were significantly more effective at increasing hGH levels in transgenic females ovariectomized at 10 days post partum than progesterone of an equal dose. A combined dose of progesterone and of estradiol-17 beta significantly amplified the increase of hGH levels accompanied by the great development of mammary glands, compared to a dose of progesterone alone. Corticosterone induced only a slight increase of hGH, while testosterone had no effect. The doses of gonadal steroid hormones did not induce an increase in hGH levels and development of mammary glands in the castrated transgenic males. The results showed that the response of 5' region of WAP requires at least some extended development of the mammary gland and that the 2.6 kb upstream region of the exogenous WAP gene contained the element responsive to ovarian hormones.

Transgenic animal bioreactors

The production of recombinant proteins is one of the major successes of biotechnology. Animal cells are required to synthesize proteins with the appropriate post-translational modifications. Transgenic animals are being used for this purpose. Milk, egg white, blood, urine, seminal plasma and silk worm cocoon from transgenic animals are candidates to be the source of recombinant proteins at an industrial scale. Although the first recombinant protein produced by transgenic animals is expected to be in the market in 2000, a certain number of technical problems remain to be solved before the various systems are optimized. Although the generation of transgenic farm animals has become recently easier mainly with the technique of animal cloning using transfected somatic cells as nuclear donor, this point remains a limitation as far as cost is concerned. Numerous experiments carried out for the last 15 years have shown that the expression of the transgene is predictable only to a limited extent. This is clearly due to the fact that the expression vectors are not constructed in an appropriate manner. This undoubtedly comes from the fact that all the signals contained in genes have not yet been identified. Gene constructions thus result sometime in poorly functional expression vectors. One possibility consists in using long genomic DNA fragments contained in YAC or BAC vectors. The other relies on the identification of the major important elements required to obtain a satisfactory transgene expression. These elements include essentially gene insulators, chromatin openers, matrix attached regions, enhancers and introns. A certain number of proteins having complex structures (formed by several subunits, being glycosylated, cleaved, carboxylated...) have been obtained at levels sufficient for an industrial exploitation. In other cases, the mammary cellular machinery seems insufficient to promote all the post-translational modifications. The addition of genes coding for enzymes involved in protein maturation has been envisaged and successfully performed in one case. Furin gene expressed specifically in the mammary gland proved to able to cleave native human protein C with good efficiency. In a certain number of cases, the recombinant proteins produced in milk have deleterious effects on the mammary gland function or in the animals themselves. This comes independently from ectopic expression of the transgenes and from the transfer of the recombinant proteins from milk to blood. One possibility to eliminate or reduce these side-effects may be to use systems inducible by an exogenous molecule such as tetracycline allowing the transgene to be expressed only during lactation and strictly in the mammary gland. The purification of recombinant proteins from milk is generally not particularly difficult. This may not be the case, however, when the endogenous proteins such as serum albumin or antibodies are abundantly present in milk. This problem may be still more crucial if proteins are produced in blood. Among the biological contaminants potentially present in the recombinant proteins prepared from transgenic animals, prions are certainly those raising the major concern. The selection of animals chosen to generate transgenics on one hand and the elimination of the potentially contaminated animals, thanks to recently defined quite sensitive tests may reduce the risk to an extremely low level. The available techniques to produce pharmaceutical proteins in milk can be used as well to optimize milk composition of farm animals, to add nutriceuticals in milk and potentially to reduce or even eliminate some mammary infectious diseases.

Transgenic milk as a method for the production of recombinant antibodies

Recombinant antibodies and their derivatives are increasingly being used as therapeutic agents. Clinical applications of antibodies often require large amounts of highly purified molecules, sometimes for multiple treatments. The development of very efficient expression systems is essential to the full exploitation of the antibody technology. Production of recombinant protein in the milk of transgenic dairy animals is currently being tested as an alternative to plasma fractionation for the manufacture of a number of blood factors (human antithrombin, human alpha-1-antitrypsin, human serum albumin, factor IX). The ability to routinely yield mg/ml levels of antibodies and the scale-up flexibility make transgenic production an attractive alternative to mammalian cell culture as a source of large quantities of biotherapeutics. The following review examines the potential of transgenic expression for the production of recombinant therapeutic antibodies.

A hybrid bovine beta-casein/bGH gene directs transgene expression to the lung and mammary gland of transgenic mice

We investigated spatial and temporal expression of bGH controlled by two different sizes (1.8 kb and 15 kb) of 5'-flanking sequences of the bovine beta-casein in transgenic mice. In the 1.8-kb promoter-containing mice, bGH expression was specifically confined to lung and mammary gland at lactation. While mammary gland expression was highly variable depending on the lines, lung expression was relatively constant with a high level in most lines. Moreover, this dual-tissue specificity of bGH expression was consistently retained in all of the 15 kb-promoter-containing mice, although a low ectopic expression was sometimes detected in salivary gland or brain. During mammary gland development in the 1.8-kb promoter-containing mice was mammary gland expression first detected at lactation, following the bovine rather than murine pattern of beta-casein expression. In contrast, lung expression was almost constant regardless of mammary gland developmental state or sex. Therefore, it can be concluded that a combination of the bovine beta-casein promoter and bGH gene directs a distinct dual-tissue specific bGH expression with different regulatory mechanisms between mammary gland and lung and as little as 1.8-kb promoter is sufficient for the proper regulation of the bovine beta-casein gene in mammary gland.

Species-specific alternative splicing of transgenic RNA in the mammary glands of pigs, rabbits, and mice

Gene-farming techniques provide an effective tool for the production of recombinant proteins in livestock. Transgenes consisting of genomic DNA sequences are as a rule more efficiently expressed than those in which the product of interest is encoded by a cDNA. However, the processing of pre-mRNA from genomic constructs may yield unexpected messenger RNAs and subsequently protein variants. We describe the appearance of different alternative mRNA splice patterns of a gene construct in which a mutant human growth hormone (hGH-N) gene is transcriptionally controlled by 2.5 kb of mouse whey acidic protein (WAP2) regulatory sequences in the mammary gland of different livestock species. Compared to the transcription products in transgenic mice harboring the same gene construct and to cell transfection experiments, expression analysis in transgenic pigs and rabbits revealed different mRNA splice patterns with regard to the proportion of the processed transcripts. Apart from already-known physiological mRNA splice products, previously undescribed processed hGH transcripts were observed in these species. Sequence analysis of the transgenes suggests that the species-specific hGH mRNA patterns may be caused by species- and tissue-specific differences in trans-acting splice factors.

High-level expressing YAC vector for transgenic animal bioreactors

The position effect is one major problem in the production of transgenic animals as mammary gland bioreactors. In the present study, we introduced the human growth hormone (hGH) gene into 210-kb human alpha-lactalbumin position-independent YAC vectors using homologous recombination and produced transgenic rats via microinjection of YAC DNA into rat embryos. The efficiency of producing transgenic rats with the YAC vector DNA was the same as that using plasmid constructs. All analyzed transgenic rats had one copy of the transgene and produced milk containing a high level of hGH (0.25-8.9 mg/ml). In transgenic rats with the YAC vector in which the human alpha-lactalbumin gene was replaced with the hGH gene, tissue specificity of hGH mRNA was the same as that of the endogenous rat alpha-lactalbumin gene. Thus, the 210-kb human alpha-lactalbumin YAC is a useful vector for high-level expression of foreign genes in the milk of transgenic animals.

Transgenic bioreactors

Since the generation of the first transgenic mice in 1980, transgene technology has also been successfully applied to large farm animals. Although this technology can be employed to improve certain production traits of livestock, this approach has not been very successful so far owing to unwanted effects encountered in the production animals. However, by using tissue-specific targeting of the transgene expression, it is possible to produce heterologous proteins in the extracellular space of large transgenic farm animals. Even though some recombinant proteins, such as human hemoglobin, have been produced in the blood of transgenic pigs, in the majority of the cases mammary gland targeted expression of the transgene has been employed. Using production genes driven by regulatory sequences of milk protein genes a number of valuable therapeutic proteins have been produced in the milk of transgenic bioreactors, ranging from rabbits to dairy cattle. Unlike bacterial fermentors, the mammary gland of transgenic bioreactors appear to carry out proper postsynthetic modifications of human proteins required for full biological activity. In comparison with mammalian cell bioreactors, transgenic livestock with mammary gland targeted expression seems to be able to produce valuable human therapeutic proteins at very low cost. Although not one transgenically produced therapeutic protein is yet on the market, the first such proteins have recently entered or even completed clinical trials required for their approval.

Transgenic animal bioreactors in biotechnology and production of blood proteins

The regulatory elements of genes used to target the tissue-specific expression of heterologous human proteins have been studied in vitro and in transgenic mice. Hybrid genes exhibiting the desired performance have been introduced into large animals. Complex proteins like protein C, factor IX, factor VIII, fibrinogen and hemoglobin, in addition to simpler proteins like alpha 1-antitrypsin, antithrombin III, albumin and tissue plasminogen activator have been produced in transgenic livestock. The amount of functional protein secreted when the transgene is expressed at high levels may be limited by the required posttranslational modifications in host tissues. This can be overcome by engineering the transgenic bioreactor to express the appropriate modifying enzymes. Genetically engineered livestock are thus rapidly becoming a choice for the production of recombinant human blood proteins.

Production of biologically active salmon calcitonin in the milk of transgenic rabbits

Salmon calcitonin (sCT) is an example of one of the many bioactive peptides that require amidation of the carboxy terminus for full potency. We describe a method for the production of amidated sCT in the mammary gland of transgenic rabbits. Expression of a fusion protein comprising human alpha lactalbumin joined by an enterokinase cleavable linker to sCT was directed to the mammary gland under the control of the ovine beta lactoglobulin promoter. C-terminal amidation in vivo was achieved by extending the sCT by a single glycine residue that provides a substrate for endogenous amidating activity in the mammary gland. Full characterization of the released sCT demonstrated it to be equivalent to synthetic standard in terms of structure, purity, and potency.

Expression of human erythropoietin transgenes and of the endogenous WAP gene in the mammary gland of transgenic rabbits during gestation and lactation

An understanding of the expression of transgenes in the mammary gland during gestation and lactation is crucial for the use of transgenic mammals as bioreactors. Here we describe the temporal pattern of expression of the endogenous rabbit WAP gene and human erythropoietin (hEPO) transgenes under the control of rabbit WAP promoter and 3' flanking sequences. The endogenous rabbit WAP gene was expressed throughout gestation including the day of mating, as well as during lactation in transgenic rabbits bearing a minigene construct. In non-pregnant cycling females, WAP expression was found independent of transgenic status; however, WAP expression was not detected in non-cycling females. The significance of this new finding is not clear at present. hEPO mRNA was detected in mammary gland biopsies from pregnant transgenic rabbits only on day 28 of gestation. During lactation, transcripts were present in mammary gland biopsy samples taken on days 0, 7, 14 and 21. A sharp decline in the levels of transcripts was found for an hEPOcDNA gene construct at the end of lactation (day 28). Although the levels of hEPO were too low to allow a conclusion to be drawn regarding temporal or position-dependent expression, this finding may reflect an integration position effect.

Regulation of CAT protein by ribozyme and antisense mRNA in transgenic mice

Transgenic mouse lines were engineered to express stably antisense mRNA or antisense mRNA containing catalytic ribozyme (rbz) structures complementary to bacterial chloramphenicol acetyltransferase (CAT) gene transcripts. One transgenic line expressed antisense mRNA that specifically targeted full-length CAT coding sequences (ACAT). Another transgenic line expressed full-length antisense CAT mRNA which was modified by mutagensis to include four rbz cassettes (rbz-ACAT) in order to compare antisense versus antisense-rbz function in vivo. Preliminary data were also collected from a transgenic mouse line expressing antisense mRNA targeting 72% of the 5' region of CAT coding sequences (5' ACAT). All constructs contained similar control elements in their design. Promoter elements were derived from the bovine alpha s1-casein gene, while the small t intron and 3' control sequences were derived from SV40. The ability of these various constructs to down-regulate CAT protein levels was compared by analysis of CAT protein production in lactating double-hemizygous transgenic female mice. Every double-hemizygous mouse analysed expressed mRNA from the alpha s1-casein-CAT construct (Clarke et al., 1994) and equivalent levels of mRNA from one of the three antisense constructs. Transgenic mouse lines expressing both ACAT and CAT mRNA down-regulated CAT protein levels by 90% of that found in the CAT only transgenic population. Similarly, double-hemizygous transgenic lines expressing both rbz-ACAT and CAT mRNA regulated CAT protein levels by 87%. Preliminary data suggests that expression of mRNA from 5' ACAT/CAT double-hemizygote mice allowed approximately 67% down-regulation of normal CAT protein levels. We conclude that incorporation of multiple ribozymes within the full-length antisense CAT construct does not enhance the effectiveness of antisense mRNA in the down-regulation of CAT protein production in our system.

Adenovirus vector-infected cells can escape adenovirus antigen-specific cytotoxic T-lymphocyte killing in vivo

The recent findings that prolonged expression of certain adenovirus (Ad) vector-encoded proteins, including human alpha1-antitrypsin (huAAT), mouse erythropoietin (EPO), and human factor IX, can be achieved in animals that do not mount an immune response to the reporter protein were obtained with mouse strains which have been shown to be capable of mounting a cellular immune response against Ad vector antigens. This suggests either that Ad vectors expressing nonimmunogenic transgenes fail to elicit a cellular immune response or that an Ad-specific cellular immune response does develop but is ineffective against cells expressing nonimmunogenic transgenes. Here we demonstrate that an Ad vector expressing huAAT administered by intravenous injection does stimulate an Ad-specific cellular immune response but that this response fails to abolish vector-directed gene expression in vivo. Moreover, expression of huAAT remained stable in animals stimulated by concurrent and multiple administrations of different Ad vectors or viruses. We also demonstrate prolonged expression of huAAT in CD1 mice transgenic for the huAAT gene, indicating that long-term expression is not restricted to C57BL/6 mice. These results demonstrate that under some circumstances, an Ad vector can direct prolonged expression of a nonimmunogenic transgene despite the presence of a robust Ad-specific cellular immune response.

Recombinant expression and secretion of a natural splicing variant containing the ectodomain of porcine LH receptor in HC11 mammary epithelial cells

Large-scale synthesis of active recombinant porcine luteinizing hormone/chorionic gonadotropin receptor (pLHR) is required for biophysical and structural studies. This study was undertaken to improve expression of the corresponding cDNA already obtained with a number of other systems, (i) by turning to cells from mammalian origin able to perform adequate glycosylation, (ii) by using an expression vector containing the acknowledged high-performance rabbit WAP gene upstream region together with transcription and translation stimulating sequences, and (iii) by expressing natural splicing variants. Selection of the transfected HC11 cells was performed in terms of pLHR expression using specific radioligand binding and immunoradiometric assays. Secretion of pLHR ectodomain into the culture medium of the HC11 clones was quantified, and reached 70 ng/ml, which represents the highest active amount ever produced. However, this level of expression was relatively low in comparison to that currently observed with bGH cDNA used as reporter gene. Additional investigations were performed in order to gain further insight into the limitation of the production of pLHR relative to bovine or human growth hormone using the same expression system. A high number of copies of cDNA in the genome of HC11 cells was found, provided that an antibiotic selection pressure was maintained to avoid drifting. The low mRNA levels detected for pLHR relative to hGH mRNAs correlate well with the relative protein production levels. They could arise from poor stability of mRNAs, a fact already observed for the natural receptor in gonadal cells. These results thus constitute a promising indicator for possible expression of pLHR in the milk of transgenic animals.

Bovine alpha s1-casein gene sequences direct high level expression of human granulocyte-macrophage colony-stimulating factor in the milk of transgenic mice

The generation is reported of transgenic mice expressing human granulocyte-macrophage colony-stimulating factor (GM-CSF) or human erythropoietin (EPO) under the control of bovine alpha s1-casein regulatory sequences. GM-CSF expression was specific to the mammary gland, and levels of human GM-CSF in transgenic mouse milk were in the range of mg ml-1. The specific activity of the milk GM-CSF was similar to that of the recombinant protein produced in Escherichia coli, and the glycosylation-derived size heterogeneity corresponded to that of the native human protein. In spite of the identical bovine regulatory sequences of the fusion genes, the levels of human EPO in transgenic mouse milk were 10(3)-10(6) times lower than those of GM-CSF, ranging from 0.003 to 3 micrograms ml-1. There appeared to be a positive correlation between the amount of EPO in the milk of lactating females and blood haematocrit values. In view of this, other type of constructs should be used to achieve more efficient EPO expression and to circumvent concomitantly-occurring adverse effects. In contrast, the high-level production of recombinant GM-CSF, its resemblance to the native mammalian protein, and mild adverse consequences of transgene expression imply that the current construct could be used for generation of larger GM-CSF transgenic animals to produce this protein in quantities sufficient for therapeutic purposes.

Recombinant protein production in transgenic animals

The engineering of animals for recombinant protein production has gone beyond the stage of identifying proper regulatory sequences. Efforts are now spent on the generation of transgenic animals that process heterologous proteins more efficiently. Another line of research is the development of strategies aimed at bypassing pronuclear microinjection.

An efficient expression of human growth hormone (hGH) in the milk of transgenic mice using rat beta-casein/hGH fusion genes

In order to produce human growth hormone (hGH) in the milk of transgenic mice, two expression vectors for hGH differing in their 3' flanking sequences were constructed by placing the genomic sequences of hGH gene under the control of the rat beta-casein gene promotor. The 3' flanking sequences of the expression constructs were derived from either the hGH gene (pBCN1GH) or the rat beta-casein gene (pBCN2GH). Transgenic lines bearing pBCN1GH expressed hGH more efficiently than those bearing pBCN2GH in the milk (19-5500 micrograms/mL vs 0.7-2 micrograms/mL). In particular, one of the BCN1GH lines expressed hGH as much as 5500 +/- 620 micrograms/mL. Northern blot analysis showed that the transgene expression was specifically confined to the mammary gland and developmentally regulated like the endogenous mouse beta-casein gene in the mammary gland. However, a low level of nonmammary expression was also detected with more sensitive assay methods. In conclusion, the rat beta-casein/hGH fusion gene could direct an efficient production of hGH in a highly tissue-and stage-specific manner in the transgenic mice and the 3' flanking sequences of hGH gene had an important role for the efficient expression.

Elements within the beta-lactoglobulin gene inhibit expression of human serum albumin cDNA and minigenes in transfected cells but rescue their expression in the mammary gland of transgenic mice

Two new beta-lactoglobulin (BLG)/human serum albumin (HSA) hybrid gene vectors were constructed and tested for expression in COS-7 cells and in transgenic mice. The HSA sequences were inserted between the second and sixth BLG exons. Transient transfection experiments with these vectors as well as a series of additional vectors with either the BLG 5'- or 3'- intragenic sequences revealed that sequences within BLG exon 1/intron 1/exon 2 abrogated BLG- directed HSA expression in vitro, regardless of the presence of HSA introns or the origin of the 3' polyadenylation signal. In contrast, the same BLG expression cassette enabled the efficient expression of HSA cDNA or minigene in the mammary gland of transgenic mice with subsequent secretion of the corresponding protein into the milk of 56 and 82%, respectively of the mouse strains at levels up to 0.3 mg/ml. Previous attempts to express HSA cDNA inserted into exon 1 of the BLG gene had failed [Shani,M., Barash,I., Nathan,M., Ricca,G., Searfoss,G.H., Dekel,I., Faerman,A., Givol,D. and Hurwitz,D.R. (1992) Transgenic Res. 1, 195- 208]. The new BLG expression cassette conferred more stringent tissue specific expression than previously described BLG/HSA constructs [Barash,I, Faerman,A., Ratovitsky,T, Puzis,R., Nathan,M., Hurwitz,D.R. and Shani, M. (1994) Transgenic Res. 3, 141-151]. However, it was not able to insulate the transgenes from the surrounding host DNA sequences and did not result in copy number dependent expression in transgenics. Together, the in vitro and in vivo results suggest both positive and negative regulatory elements within the BLG intragenic sequences evaluated. The new BLG construct represents an extremely valuable vector for the efficient expression of cDNAs in the mammary gland of transgenic animals.

The effect of matrix attached regions (MAR) and specialized chromatin structure (SCS) on the expression of gene constructs in cultured cells and in transgenic mice

The flanking sequences of several genes have been shown to direct a position independent expression of transgenes. Attempts to completely identify the insulating sequences have failed so far. Some of these sequences contain a matrix attached region (MAR) located in the flanking part of the genes. This article will show that the MARs in cultured cells located in the 3' OH region of the human apolipoprotein B100 (Apo B100) and within the SV40 genome were unable to stimulate and insultate transgene expression directed by the promoters from a rabbit whey acidic protein (WAP) gene or from human cytomegalovirus (hCMV) early genes. In transgenic mice, the MAR from the Apo B100 and SV40 genes did not enhance the expression of a transgene containing the rabbit whey acid protein (WAP) promotor, the late gene SV40 intron (VP1 intron), the bovine growth hormone (bGH) cDNA and the SV40 late gene terminator. This construct was even toxic for embryos. Similarly, the specialized chromatin structure (SCS) from the Drosophila 87A7 HSP70 gene reduced chloramphenicol acetyl transferase (CAT) activity when added between a cytomegalovirus (CMV) enhancer and a Herpes simplex thymidine kinase (TK) gene promoter. This inhibitory action was almost complete when a second SCS sequence was added before the CMV enhancer. Sequences from the firefly luciferase and from the human gene cathepsin D cDNA used as control unexpectedly showed a similar inhibitory effect when added to the CMVTKCAT construct instead of SCS. When added before the CMV enhancer and after the transcription terminator in the CMVTKCAT construct, the SCS sequence was unable to insulate the integrated gene as seen by the fact that the level of CAT in cell extracts were by no means correlated with the number of copies in individual clones. From these data, it is concluded that i) a MAR containing the canonical AT rich sequences does not amplify the expression of all gene constructs ii) At rich MAR sequences do not have per se an insulating effect iii) Drosophila SCS from the 87A7 HSP70 gene has no insulating effect in all gene constructs (at least in mammalian cells) iv) and the addition of a DNA fragment between an enhancer and a promoter in a gene construct cannot be used as a reliable test to evaluate its insulating property.

Mammary gland expression of transgenes and the potential for altering the properties of milk

Transgenic animals are a useful in vivo experimental model for assessing the ability and impact of foreign gene expression in a biological system. Transgenic mice are most commonly used, while transgenic sheep, goats, pigs and cows have also been developed for specific, "applied" purposes. Most of the work directed at targeting expression of transgenes to the mammary gland of an animal, by using a milk gene promoter, has been with the intent of either studying promoter function or recovering the desired protein from the milk. Transgenic technology can also be used to alter the functional and physical properties of milk resulting in novel manufacturing properties. The properties of milk have been altered by adding a new protein with the aim of improving the milk, not of recovering the protein for other uses.

The effect of mammary gland expression of human lysozyme on the properties of milk from transgenic mice

Transgenic mice were used as model systems to evaluate the impact of human lysozyme expression in the mammary gland. We previously generated two lines of transgenic mice that express human lysozyme mRNA in the mammary gland under the tissue-specific and developmentally correct control of the bovine gene promoter for alpha s1-casein. Concentrations of human lysozyme protein in milk of transgenic mice varied from .25 to .71 micrograms/microliters of milk. Human lysozyme secreted into mouse milk retained its antimicrobial activity, as determined by a denaturing polyacrylamide gel activity assay. The physical and functional properties of the milk were also altered, because mouse milk containing human lysozyme had a 35% decrease in rennet clotting time, a smaller median micelle size (157 nm vs. 172 nm), and a 2.5- to 3-fold greater gel strength than control milk. From these results, we conclude that the use of transgenic animals producing lysozyme in the milk is feasible and potentially useful to the dairy industry.

Rabbit whey acidic protein gene upstream region controls high-level expression of bovine growth hormone in the mammary gland of transgenic mice

Transgenic mice were produced which secreted high levels of bGH into milk. The 6.3-kb upstream region of the rabbit whey acidic protein (rWAP) gene was linked to the structural part of the bovine growth hormone (bGH) gene, and the chimeric gene was radioimmunoassay into mouse oocytes. bGH was detected by radioimmunoassay in the milk of all resulting transgenic mice. bGH concentrations in milk varied from line to line, from 1.0-16 mg/ml. This expression was not correlated to the number of transgene copies. In all lines studied, the mammary gland was the major organ expressing bGH mRNA during lactation. bGH mRNA concentrations were barely detectable in the mammary gland of cyclic females; they increased during pregnancy. These results show that the upstream region of the rWAP gene harbors powerful regulatory elements which target high levels of bGH transgene expression to the mammary gland of lactating transgenic mice.

Prolactin-mediated gene activation in mammary epithelial cells

Mammary epithelial cells grow and develop with the onset of sexual maturity. In addition, lobular alveolar structures are formed during pregnancy, and quiescent differentiated cells secrete high levels of milk proteins after parturition. These events are governed by multiple hormones and growth factors and involve the sequential and synergistic action of functionally distinct signal transduction pathways. Milk protein genes have been analyzed and composite response elements have been identified in the promoter sequences. Transcription factors, which relay the hormonal signals, bind to these sequences. The factor that confers prolactin simulation to milk protein gene transcription has recently been identified. MGF/Stat5 is a latent transcription factor that becomes activated by a tyrosine-specific protein kinase, Jak2, associated with the prolactin receptor. Tyrosine phosphorylation converts the latent factor into one with DNA-binding and transcriptional activation potential. The regulation of MGF/Stat5 in vitro and in vivo indicates that it is a central component of the lactogenic hormone signaling pathway. Involvement of MGF/Stat5 in the signaling by other cytokines indicates that the same factor might be involved in regulation of growth-promoting genes, primarily in hematopoietic cells.

Human parathyroid hormone as a secretory peptide in milk of transgenic mice

In a transgenic mouse model we have targeted the expression of recombinant human parathyroid hormone (hPTH) to the mammary gland yielding hPTH as a secretory, soluble peptide in milk. A 2.5 kb upstream regulatory sequence of the murine whey acidic protein (WAP) directed the expression of the hPTH cDNA in a fusion gene construct (WAPPTHSV2) containing the SV40 small t-antigen intron and polyadenylation site in the 3' end. Established lines of transgenic mice secreted hPTH to milk in concentrations up to 415 ng/ml. Recombinant hPTH recovered from the milk was purified by HPLC and shown to be identical to hPTH standard as analyzed by SDS-PAGE followed by immunoblotting. Expression of the WAPPTHSV2 was limited to the mammary gland as analyzed by polymerase chain reaction (PCR) and Southern blot of reversed transcribed mRNA from different tissues. hPTH is an important bone anabolic hormone and may be a potentially important pharmaceutical for treatment of demineralization disorders such as osteoporosis. We present the transgenic animal as a possible production system for hPTH.

The effect of various introns and transcription terminators on the efficiency of expression vectors in various cultured cell lines and in the mammary gland of transgenic mice

Various combinations of promoters, introns and transcription terminators were used to drive the expression of bovine growth hormone (bGH) cDNA in different cell types. In constructs containing the human cytomegalovirus (hCMV) promoter and the SV40 late genes terminator, the intron from SV40 genes (VP1) was much more efficient, than the intron from the early genes (t). The synthetic intron SIS generated by the association of an adenovirus splice donor and an immunoglobulin G splice acceptor showed the highest activity. The respective potency of these introns was similar in several mammalian (CHO, HC11 and COS) and fish (TO2 and EPC) cells. The rabbit whey acidic protein (WAP) gene promoter was highly efficient to drive the expression of bGH gene in the HC11 mammary cell lines. In contrast, the bGH cDNA under the control of the same promoter was much less efficiently expressed when the SV40 VP1 intron and transcription terminator were used. The rabbit WAP gene and the human GH gene terminators did not or only moderately enhanced the expression of the construct WAP bGH cDNA. Introduction of a promoter sequence from the mouse mammary tumor virus (MMTV) LTR in the VP1 intron increased very significantly the expression of the WAP bGH cDNA. Although several of these vectors showed high potency when expressed stably in HC11 cells, all of them were only moderately efficient in transgenic mice. These data indicate that the VP1 and the SIS introns may be used to express foreign cDNAs with good efficiency in different cell types. The addition of an enhancer within an intron may still reinforce its efficiency. However, transfection experiments, even when stable expression is carried out, are poorly predictive of the potential efficiency of a vector in transgenic animals.