Cre-mediated recombination at the murine whey acidic protein (mWAP) locus

Construction of targeting vector for expressing human GDNF in cattle mammary gland

Glial cell line-derived neurotrophic factor (GDNF) is a type of neurotrophic factor with significant potential in treatment of Parkinson's disease. Combining gene targeting of animal somatic cells with nuclear transfer technique has provided a powerful method to produce transgenic animal mammary gland bioreactor. The aim of this study was to construct a gene-targeting vector for the human gdnf gene knockin at the bovine beta-casein gene locus so that human GDNF protein can be produced in the mammary gland of the gene-targeted bovine. The constructed vector contains the 2.2 kb 5' homologous arm and the 5.7 kb 3' homologous arm. The human gdnf cDNA was located at the downstream of the 5' homologous arm. The neo gene placed between the 5' and 3' homologous arms as positive selection marker gene. The HSV-tk gene and DsRed2 gene were located outside the homologous recombinant area as negative selection marker genes, respectively. The recombinant plasmids were identified by restriction fragment analysis and partial DNA sequencing. The results show that the structure of the final constructed vector accords with the designed plasmid map. In order to analyze the bioactivity of the vector, the plasmid DNA was transfected into human mammary tumor cell line Bcap-37 by lipofectamine. Reverse transcription polymerase chain reaction and Western-blotting analysis showed that the transfected cells produced human GDNF mRNA and protein. The results show that the constructed targeting vector pNRTCNbG has bioactivity to efficiently express GDNF in mammary gland cells. At the same time, it is first time to confirm that human mammary tumor cell line Bcap-37 is valid for bioactivity analysis of mammary gland specific expression vector.

Folate Transport Gene Inactivation in Mice Increases Sensitivity to Colon Carcinogenesis

Low dietary folate intake is associated with an increased risk for colon cancer; however, relevant genetic animal models are lacking. We therefore investigated the effect of targeted ablation of two folate transport genes, folate binding protein 1 (Folbp1) and reduced folate carrier 1 (RFC1), on folate homeostasis to elucidate the molecular mechanisms of folate action on colonocyte cell proliferation, gene expression, and colon carcinogenesis. Targeted deletion of Folbp1 (Folbp1+/− and Folbp1−/−) significantly reduced (P < 0.05) colonic Folbp1 mRNA, colonic mucosa, and plasma folate concentration. In contrast, subtle changes in folate homeostasis resulted from targeted deletion of RFC1 (RFC1+/−). These animals had reduced (P < 0.05) colonic RFC1 mRNA and exhibited a 2-fold reduction in the plasma S-adenosylmethionine/S-adenosylhomocysteine. Folbp1+/− and Folbp1−/− mice had larger crypts expressed as greater (P < 0.05) numbers of cells per crypt column relative to Folbp1+/+ mice. Colonic cell proliferation was increased in RFC1+/− mice relative to RFC1+/+ mice. Microarray analysis of colonic mucosa showed distinct changes in gene expression specific to Folbp1 or RFC1 ablation. The effect of folate transporter gene ablation on colon carcinogenesis was evaluated 8 and 38 weeks post-azoxymethane injection in wild-type and heterozygous mice. Relative to RFC1+/+ mice, RFC1+/− mice developed increased (P < 0.05) numbers of aberrant crypt foci at 8 weeks. At 38 weeks, RFC1+/− mice developed local inflammatory lesions with or without epithelial dysplasia as well as adenocarcinomas, which were larger relative to RFC1+/+ mice. In contrast, Folbp1+/− mice developed 4-fold (P < 0.05) more lesions relative to Folbp1+/+ mice. In conclusion, Folbp1 and RFC1 genetically modified mice exhibit distinct changes in colonocyte phenotype and therefore have utility as models to examine the role of folate homeostasis in colon cancer development.

Mutational analysis of loxP sites for efficient Cre-mediated insertion into genomic DNA

The Cre/loxP system has been used in transgenic models primarily to excise DNA flanked by loxP sites for gene deletion. However, the insertion reaction is more difficult to control since the excision event is kinetically favored. Mutant loxP sites favoring integration were identified using a novel, bacterial screening system. Utilizing lambda integrase, mutant loxP sites were placed at the E. coli attB site and the excision-insertion ratios of incoming DNA plasmids carrying a second, complementary mutant loxP site were determined. Comparison of 50 mutant loxP sites combinations to the native loxP site revealed that mutations to the inner 6 bp of the Cre binding domain severely inhibited recombination, while those in the outer 8 bps were more tolerated. The most efficient loxP combinations resulted in 1421-fold and 1529-fold increases in relative integration rates over wild-type loxP sites. These loxP mutants could be exploited for site-directed "tag and insert" recombination experiments.

Cre/lox: one more step in the taming of the genome

Cre recombinase has become an important tool in the precise manipulation of the genome, and its adoption has led to the development of increasingly accurate mouse models for the understanding of gene function. Although much of current work exploits the alacrity and precision with which Cre catalyzes excisive DNA recombination, Cre also is adept at the insertion of heterologous DNA into the genome. The precision and efficiency with which Cre can target DNA to a predesignated locus in the genome promises to facilitate understanding of mutant genes and allelic variants in their natural chromosomal context.

A single vector containing modified cre recombinase and LOX recombination sequences for inducible tissue-specific amplification of gene expression

The selective alteration of the genome using Cre recombinase to target the rearrangement of genes flanked by LOX recognition sequences has required the use of two separate genetic constructs in trans, one containing cre and the other containing the gene of interest flanked by LOX sites. We have developed a strategy in which both the cre recombinase gene and LOX recombination sites may be cloned within a single vector in cis. This method uses a modified form of Cre (CREM) that contains alterations to the 5' region including the introduction of a Kozak consensus sequence and insertion of a functional intron. This system allows for the inducible, tissue-specific activation or inactivation of gene expression in a single vector and can be utilized for the 300-fold amplification of gene expression from a weak promoter. This approach can be applied to targeting strategies for generating genetically altered mice and gene therapy.

Bcl-x and Bax regulate mouse primordial germ cell survival and apoptosis during embryogenesis

Restricted germ cell loss through apoptosis is initiated in the fetal gonad around embryonic day 13.5 (E13.5) as part of normal germ cell development. The mechanism of this germ cell attrition is unknown. We show that Bcl-x plays a crucial role in maintaining the survival of mouse germ cells during gonadogenesis. A bcl-x hypomorphic mouse was generated through the introduction of a neomycin (neo) gene into the promoter of the bcl-x gene by homologous recombination. Mice that contained two copies of the hypomorphic allele had severe reproductive defects attributed to compromised germ cell development. Males with two mutant alleles lacked spermatogonia and were sterile; females showed a severely reduced population of primordial and primary follicles and exhibited greatly impaired fertility. Primordial germ cells (PGCs) in bcl-x hypomorph mice migrated to the genital ridge by E12.5 but were depleted by E15.5, a time when Bcl-x and Bax were present. Two additional bcl-x transcripts were identified in fetal germ cells more than 300 bp upstream of previously reported start sites. Insertion of a neo cassette led to a down-regulation of the bcl-x gene at E12.5 in the hypomorph. Bax was detected by immunohistochemistry in germ cells from bcl-x hypomorph and control testes at E12.5 and E13.5. Bcl-x function was restored, and animals of both genders were fertile after removal of the neo selection cassette using Cre-mediated recombination. Alternatively, the loss of Bcl-x function in the hypomorph was corrected by the deletion of both copies of the bax gene, resulting in a restoration of germ cell survival. These findings demonstrate that the balance of Bcl-x and Bax control PGC survival and apoptosis.

Pre-selection of integration sites imparts repeatable transgene expression

Variable gene expression amongst transgenic lines occurs due to copy number and to random associations of incoming DNA with chromosomal elements at the site of integration. Here we describe a method of identifying sites permissive for transgene expression and their use for efficient introduction of single copy transgenes by homologous recombination. ES clones were selected in HAT medium for expression of a randomly integrated HPRT marker lying 5' to an Oct4/ lacZ transgene. 794 clones were assessed in vitro for appropriate down-regulation of lacZ following differentiation. Two clones were chosen for further analysis which displayed appropriate and inappropriate gene regulation (clones 710 and 91, respectively). Three developmental promoters (thyroglobulin, Hox2.6 and Myf5) were then sequentially introduced into the original insertion sites in each clone (710 and 91) by homologous recombination, to drive expression of lacZ. Transgenic embryos were assessed for their ability to direct lacZ expression to tissues in which the respective promoter sequences are normally active. The site which appropriately down-regulated lacZ in vitro (710) also showed appropriate in vivo regulation of lacZ from the three developmental promoters. Site 91, however, directed an additional pattern of ectopic expression, which was common to all four promoters. Pre-selection of genomic sites for the introduction of transgenes by gene targeting improves the repeatability of transgene expression and provides an efficient means of single copy transgene introduction by homologous recombination.

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.

Insertion of a foreign gene into the beta-casein locus by Cre-mediated site-specific recombination

The expression of foreign genes in transgenic animals is generally unpredictable as transgenes are integrated at random after pro-nuclear injection into fertilized oocytes. In many cases, transgene expression is inhibited by neighbouring chromatin structures or by the repeated nature of the multiple transgene copies present at the integration site. A strategy involving homologous and site-specific recombination has been devised by which single copies of a foreign gene can be inserted specifically into the locus of a highly expressed gene. As a first step, a loxP recombination target site is introduced by homologous recombination into a predetermined gene locus such that the loxP sequence is placed next to the promoter region and replaces the translational initiation signal. In a subsequent site-specific recombination reaction, a gene of interest can be integrated into the pre-existing loxP site. This biphasic recombination strategy was used to integrate a luciferase reporter gene into the locus of the murine beta-casein gene in embryonic stem cells.

Utilization of gene mapping information in livestock animals

A number of recent advances in genomic research will change and improve livestock production in the near future. Genetic linkage maps have been developed for a number of livestock species including cattle, sheep, and pigs. These maps allow scientists to identify chromosomal regions that influence traits of economic importance. This information will lead to improved genetic selection practices by identifying animals with superior copies of the chromosomal regions that affect the selected trait. This mapping information will also be used to identify the genes controlling the trait. A number of genomic regions or loci have already been reported that affect production, carcass or disease traits, and in a few cases, a specific gene has been identified. Production of transgenic animals with sequence changes in these genes may be beneficial for evaluating the effect of the gene upon the selected trait and more specifically the effect of certain polymorphisms (mutations) within the gene.

Factors affecting the efficiency of introducing precise genetic changes in ES cells by homologous recombination: tag-and-exchange versus the Cre-loxp system

The introduction of genetic modifications in specific genes by homologous recombination provides a powerful tool for elucidation of structure-function relationships of proteins of biological interest. Presently, there are several alternative methods of homologous recombination that permit the introduction of small genetic modifications in specific loci. Two of the most widely used methods are the tag-and-exchange, based on the use of positive-negative selection markers, and the Cre-loxP system, based on the use of a site-specific recombinase. The efficiency of detection of targeting events at different loci using the two systems was compared. Additionally, we analysed how the distance between two gene markers placed within the region of homology of a targeting vector affects the rate at which both markers are introduced into the locus during the homologous recombination event. Our results indicate that the method based on the use of positive-negative selection markers was less efficient than the Cre-loxP based system, irrespective of locus or type of positive-negative selection. It was also determined that as the distance between the selectable marker and the genetic modification being introduced increases, there is a progressive reduction in the efficiency of detecting events with the desired genetic modification.