Antisense and ribozyme constructs in transgenic animals

Genetically engineered livestock for agriculture: a generation after the first transgenic animal research conference

At the time of the first Transgenic Animal Research Conference, the lack of knowledge about promoter, enhancer and coding regions of genes of interest greatly hampered our efforts to create transgenes that would express appropriately in livestock. Additionally, we were limited to gene insertion by pronuclear microinjection. As predicted then, widespread genome sequencing efforts and technological advancements have profoundly altered what we can do. There have been many developments in technology to create transgenic animals since we first met at Granlibakken in 1997, including the advent of somatic cell nuclear transfer-based cloning and gene editing. We can now create new transgenes that will express when and where we want and can target precisely in the genome where we want to make a change or insert a transgene. With the large number of sequenced genomes, we have unprecedented access to sequence information including, control regions, coding regions, and known allelic variants. These technological developments have ushered in new and renewed enthusiasm for the production of transgenic animals among scientists and animal agriculturalists around the world, both for the production of more relevant biomedical research models as well as for agricultural applications. However, even though great advancements have been made in our ability to control gene expression and target genetic changes in our animals, there still are no genetically engineered animal products on the market for food. World-wide there has been a failure of the regulatory processes to effectively move forward. Estimates suggest the world will need to increase our current food production 70 % by 2050; that is we will have to produce the total amount of food each year that has been consumed by mankind over the past 500 years. The combination of transgenic animal technology and gene editing will become increasingly more important tools to help feed the world. However, to date the practical benefits of these technologies have not yet reached consumers in any country and in the absence of predictable, science-based regulatory programs it is unlikely that the benefits will be realized in the short to medium term.

Application of RNAi technology to the inhibition of zebrafish GtHalpha, FSHbeta, and LHbeta expression and to functional analyses

Zebrafish (Danio rerio) were used as a model fish, and the technique of RNA interference (RNAi) was employed to knockdown three subunits of the gonadotropin alpha (GtHalpha, common alpha), follicle-stimulating hormone beta (FSHbeta), and luteinizing hormone beta (LHbeta) genes. Three short-hairpin RNA (shRNA) expression vectors and three mismatched shRNA expression vectors as controls for each subunit gene were constructed, and the depression efficiency was tested in vivo by microinjection; the RNA or protein expression levels of the GtH genes were monitored by RT-PCR, Southern blotting, and green fluorescent protein (GFP) analyses. Expression of GtH mRNA was obviously and more efficiently depressed by GtHalpha RNAi expression compared with the other two subunits. A GtHalpha morpholino analysis showed that the GtHalpha morpholino led to suppression of embryonic development and the production of embryonic mutants as a result of an injection of GtHalpha -shRNA. Taken together, these results show that GtHalpha-shRNA, which more efficiently targets RNAi, may have an essential role in the further development of sterility technology of transgenic fish for biosafety purposes.

Efficient and specific down-regulation of prion protein expression by RNAi

Prion diseases are fatal neurodegenerative disorders associated with an abnormal isoform of the PrPc host-encoded protein. Invalidation of the Prnp gene, that encodes PrPc, led to transgenic mice that are viable, apparently healthy, and resistant to challenge by the infectious agent. These results indicated that a down-regulation of the Prnp gene expression is a potential therapeutic approach. In the present report, we demonstrate that RNAi targeted towards the Prnp mRNA can efficiently and highly specifically reduce the level of PrPc in transfected cells. It, thus, indicates that RNAi is an attractive therapeutic approach to fight against prion diseases.

Gene transfer in higher animals: theoretical considerations and key concepts

Gene transfer technology provides the ability to genetically manipulate the cells of higher animals. Gene transfer permits both germline and somatic alterations. Such genetic manipulation is the basis for animal transgenesis goals and gene therapy attempts. Improvements in gene transfer are required in terms of transgene design to permit gene targeting, and in terms of transfection approaches to allow improved transgene uptake efficiencies.

Transgenic tilapia and the tilapia genome

The tilapia fish (Oreochromis niloticus) has an important place in the aquaculture of the developing world. It is also a very useful laboratory animal, and readily lends itself to the transgenic technology. Through the use of reporter genes, a range of potential gene promoters have been tested in tilapia, both through transient and stable expression of the reporter construct. Using the transgenic technology, growth enhanced lines of tilapia have been produced. These fish have no abnormalities and offer a considerable growth advantage for future exploitation. It is however crucial that transgenic fish, to be exploited in aquaculture, be sterile, and various methods of achieving sterility are considered. These include triploidy, gene knock out of crucial hormone encoding genes via homologous recombination, and knock down of the function of the same genes via ribozyme or antisense technologies. Transgenic tilapia also offer the potential for exploitation as biofactories in the production of valuable pharmaceutical products, and this is also discussed.

Loss-of-function genetics in mammalian cells: the p53 tumor suppressor model

Using an improved system for the functional identification of active antisense fragments, we have isolated antisense fragments which inactivate the p53 tumour suppressor gene. These antisense fragments map in two small regions between nt 350 and 700 and nt 800 and 950 of the coding sequence. These antisense fragments appear to act by inhibition of p53 mRNA translation both in vivo and in vitro. Expression of these antisense fragments overcame the p53-induced growth arrest in a cell line which expresses a thermolabile mutant of p53 and extended the in vitro lifespan of primary mouse embryonic fibroblasts. Continued expression of the p53 antisense fragment contributed to immortalisation of primary mouse fibroblasts. Subsequent elimination of the antisense fragment in these immortalised cells led to restoration of p53 expression and growth arrest, indicating that immortal cells continuously require inactivation of p53. Expression of MDM2 or SV40 large T antigen, but not E7 nor oncogenic ras, overcomes the arrest induced by restoration of p53 expression. Functional inactivation of both p21 and bax (by overexpression of Bcl2), but not either alone, allowed some bypass of p53-induced growth arrest, indicating that multiple transcriptional targets of p53 may mediate its antiproliferative action. The ability to conditionally inactivate and subsequently restore normal gene function may be extremely valuable for genetic analysis of genes for which loss-of-function is involved in specific phenotypes.

Generation and application of asymmetric hammerhead ribozymes

Most researchers who intend to suppress a particular gene are interested primarily in the application of ribozyme technology rather than its mechanistic details. This article provides some background information and describes a straightforward strategy to generate and test a special design of a ribozyme: the asymmetric hammerhead ribozyme. This version of a hammerhead ribozyme carries at its 5' end the catalytic domain and at its 3' end a relatively long antisense flank that is complementary to the target RNA. Asymmetric hammerhead ribozymes can be constructed via polymerase chain reaction amplification, and rules are provided on how to select the DNA oligonucleotides required for this reaction. In addition to details on construction, we describe how to test asymmetric hammerhead ribozymes for association with the target RNA in vitro, so that RNA constructs can be selected and optimized for fast hybridization with their target RNA. This test can allow one to minimize association problems caused by the secondary structure of the target RNA. Additionally, we describe the in vitro cleavage assay and the determination of the cleavage rate constant. Testing for efficient cleavage is also a prerequisite for reliable and successful application of the technology. A carefully selected RNA will be more promising when eventually used for target suppression in living cells.

Genetic modification of animals in the next century

Since the initial demonstration in 1982 of profound phenotypic effects stemming from the expression of a single transgene, genetic engineering has revolutionized fundamental biological and biomedical research. The application of transgenic technology to farm animals has held the promise of being able to improve animal agriculture significantly and has resulted in a new industry, i.e., the successful expression of foreign proteins in the mammary gland for the pharmaceutical industry. Work over the last few years in model species (e.g., the mouse) and new technical developments such as cloning have now set the stage for the initial application of transgenic technology for the improvement of farm animals. Major limitations that remain are the lack understanding of which genes we should transfer in order to alter quantitative production traits usefully and the low efficiency of producting transgenic founders. Furthermore, more research is needed concerning the consequences and potential problems arising from the integration of transgenes into populations with varying genetic backgrounds. Recent advances suggest that within the first decade of the 21 st century the first transgenic animals will become available to the livestock industry, with acceptance depending upon their cost versus their potential economic benefit to the producers.

Genetic engineering in the pig. Gene knockout and alternative techniques

Since endothelial cells (EC) are the major target cells during hyperacute rejection and are likely in delayed graft rejection, most of the genetic engineering of the xenotransplant donor is aimed at modifying their properties. Among the various strategies that are reviewed are the genotypic or phenotypic knockout of the alpha 1,3Gal antigen, which is a major target of xenoantibodies and is also probably involved in innate cellular response. In addition, the success of the transgeny of complement regulatory proteins is well established. In vitro data from analyses of the mechanisms of endothelial cell activation also suggest that other molecules could be used to regulate apoptosis or thrombotic microenvironment or to minimize recipient T-cell activation by inhibiting costimulatory proteins such as CD40 or B7. Alternative to usual knockout techniques (thus far not available in pigs, where no ES cells have been derived) will be presented.

Examination of antisense RNA and oligodeoxynucleotides as potential inhibitors of avian leukosis virus replication in RP30 cells

Avian leukosis virus (ALV) is an economically important pathogen of chickens. Both antisense RNA and antisense oligodeoxynucleotides (ODN) have been used to diminish the replication and spread of other retroviruses. The use of antisense RNA and ODN to inhibit ALV replication has been examined in cultured RP30 cells. Using an expression system that constitutively transcribes antisense ALV RNA, one transfected cell clone showed a significant reduction in virus growth. However, this effect was not reproducibly observed in other transfected cell lines or in cells in which the antisense transcript was expressed from a regulatable promoter, even though a substantial amount of antisense transcript was generated. Antisense ODN complementary to several different target sites near the 5' end of the ALV genome were also tested for antiviral activity, by comparison of antisense ODN effects to those of randomized sequence controls. An antisense ODN complementary to the ALV primer binding site demonstrated a reproducible reduction in viral replication. However, when the corresponding region was specifically employed as a target for intracellular antisense RNA expression, there again was no significant inhibition of ALV. These results suggest that in vivo expression of antisense RNA is unlikely to be an effective way to generate transgenic poultry that are resistant to field strains of ALV.

Redefining body composition: nutrients hormones, and genes in meat production

Growth rate and body composition of livestock can be optimized to meet consumer needs for a leaner product and to improve the efficiency of meat-animal production. Optimization strategies have traditionally focused on genetic selection and cost-effective ration formulation to achieve the genetic potential. Advances in understanding the mechanisms of growth and its control have led to additional opportunities for its manipulation. These include nutritional manipulation,the use of growth promotants, and, more recently, the ability to change the genetic potential through genetic engineering. Selection of appropriate candidate genes for manipulation depends on understanding the mechanisms underlying differentiation and growth of embryonic muscle cells. Recent advances in genetic engineering techniques, including gene therapy and germline transgenesis, will likely hasten the genetic progress toward a leaner carcass in domestic livestock. Such strategies may prove to be more beneficial then the controlled enhancement of somatotropin expression.

Modification and repression of genes expressed in the mammary gland using gene targeting and other technologies

Transgenic experiments using oocyte micro-injection methodology are often performed in order to target expression of a foreign gene in a specific tissue or, to a lesser extent, to study the regulation of gene expression. However, the isolation of embryonic stem cells in mice and the development of antisense and ribozyme technologies have allowed more subtle alterations of endogenous gene expression to be achieved. The mammary gland is one of the few organs able to undergo several cycles of development, differentiation and apoptosis through complex multihormonal regulation during adult life. It is thus an attractive model to assess the in vivo function of some genes potentially involved in these mechanisms, either by silencing them or by partially repressing their expression. Furthermore, such alterations of gene expression have also been performed for more applied objectives such as the modification of milk composition for nutritional and technological purposes. This review will describe the experimental procedures used toward these aims and the results already obtained in this field. Some potential new targets will be suggested.

Repressible antisense inhibition in B lymphocytes

The tetracycline-responsive promoter (TRP) system has been adopted in an attempt to obtain repressible antisense inhibition in a B lymphocyte model in vitro. Levels of secreted IgM protein and mRNA were assessed following the stable transfection of B cell line, HO-2.2, with a series of plasmid constructs containing antisense or sense target sequence DNA (the 3'-untranslated region adjacent to the secreted exon of IgM gene) under the control of the TRP. Significant reduction (approximately 90%) in IgM secretion was observed for clones transfected with antisense plasmids driven by the TRP and containing the IgH enhancer element and the polyadenylation signal sequence from membrane IgM, when compared with untransfected and sense controls. Tetracycline (1 microgram/ml) addition to the culture medium restored the level of IgM secretion in these clones to control values, demonstrating repressibility of antisense inhibition. Transfection of HO-2.2 cells with antisense (or sense) constructs had no detectable effect on membrane IgM protein levels. Hybridisation studies demonstrated that decreased protein production observed in the antisense-transfected clones was most likely attributable to reduced RNA levels. These data show that the TRP can be used for repressible and specific antisense inhibition of gene product expression in B lymphocytes.

Controlling your losses: conditional gene silencing in mammals

Methods are now widely used in mice, and to a lesser extent in mammalian-cell culture, for the constitutive silencing of target genes in order to assess their function. For a variety of reasons, not least because many genes are essential for viability, it is important that these methods can be adapted to allow the controlled silencing of target genes. Reviewed here are the ways in which gene-silencing methods can be combined with a growing number of genetic control systems to generate cell lines or mice that are, in effect, conditional mutants. These approaches are still being developed and promise to open up key areas of cell and animal biology to genetic analysis.

The Therapeutic Potential of Ribozymes

Ribozymes are catalytic RNA molecules that recognize their target RNA in a highly sequence-specific manner. They can therefore be used to inhibit deleterious gene expression (by cleavage of the target mRNA) or even repair mutant cellular RNAs. Targets such as the mRNAs of oncogenes (resulting from base mutations or chromosome translocations, eg, ras or bcr-abl) and viral genomes and transcripts (human immunodeficiency virus–type 1 [HIV-1]) are ideal targets for such sequence-specific agents. The aim of this review is therefore to introduce the different classes of ribozymes, highlighting some of the chemistry of the reactions they catalyze, to address the specific inhibition of genes by ribozymes, the problems yet to be resolved, and how new developments in the field give hope to the future for ribozymes in the therapeutic field.

The Therapeutic Potential of Ribozymes

Ribozymes are catalytic RNA molecules that recognize their target RNA in a highly sequence-specific manner. They can therefore be used to inhibit deleterious gene expression (by cleavage of the target mRNA) or even repair mutant cellular RNAs. Targets such as the mRNAs of oncogenes (resulting from base mutations or chromosome translocations, eg, ras or bcr-abl) and viral genomes and transcripts (human immunodeficiency virus–type 1 [HIV-1]) are ideal targets for such sequence-specific agents. The aim of this review is therefore to introduce the different classes of ribozymes, highlighting some of the chemistry of the reactions they catalyze, to address the specific inhibition of genes by ribozymes, the problems yet to be resolved, and how new developments in the field give hope to the future for ribozymes in the therapeutic field.

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.

G protein-coupled receptors: functional and mechanistic insights through altered gene expression

G protein-coupled receptors (GPCRs) comprise a large and diverse family of molecules that play essential roles in signal transduction. In addition to a constantly expanding pharmacological repertoire, recent advances in the ability to manipulate GPCR expression in vivo have provided another valuable approach in the study of GPCR function and mechanism of action. Current technologies now allow investigators to manipulate GPCR expression in a variety of ways. Graded reductions in GPCR expression can be achieved through antisense strategies or total gene ablation or replacement can be achieved through gene targeting strategies, and exogenous expression of wild-type or mutant GPCR isoforms can be accomplished with transgenic technologies. Both the techniques used to achieve these specific alterations and the consequences of altered expression patterns are reviewed here and discussed in the context of GPCR function and mechanism of action.

A ribozyme-mediated, gene "knockdown" strategy for the identification of gene function in zebrafish

The zebrafish system offers many unique opportunities for the study of molecular biology. To date, only random mutagenesis, and not directed gene knockouts, have been demonstrated in this system. To more fully develop the potential of the zebrafish system, an approach to effectively inhibit the expression of any targeted gene in the developing zebrafish embryo has been developed. This approach uses a transient, cytoplasmic, T7 expression system, injected into the fertilized zebrafish egg to rapidly produce high levels of a ribozyme directed against the mRNA encoded by the targeted gene to inhibit its expression. In a demonstration of this strategy, expression of the recessive dominant zebrafish no tail gene was effectively inhibited by using this strategy to yield a phenotype identical to that resulting from a known defective mutation in this same gene. This, ribozyme-mediated, message deletion strategy may have use in determining the function of genetic coding sequences of unknown function.