Use of PCR-based methods for selection of integrated transgenes in preimplantation embryos

Contemporary Transposon Tools: A Review and Guide through Mechanisms and Applications of Sleeping Beauty, piggyBac and Tol2 for Genome Engineering

Transposons are mobile genetic elements evolved to execute highly efficient integration of their genes into the genomes of their host cells. These natural DNA transfer vehicles have been harnessed as experimental tools for stably introducing a wide variety of foreign DNA sequences, including selectable marker genes, reporters, shRNA expression cassettes, mutagenic gene trap cassettes, and therapeutic gene constructs into the genomes of target cells in a regulated and highly efficient manner. Given that transposon components are typically supplied as naked nucleic acids (DNA and RNA) or recombinant protein, their use is simple, safe, and economically competitive. Thus, transposons enable several avenues for genome manipulations in vertebrates, including transgenesis for the generation of transgenic cells in tissue culture comprising the generation of pluripotent stem cells, the production of germline-transgenic animals for basic and applied research, forward genetic screens for functional gene annotation in model species and therapy of genetic disorders in humans. This review describes the molecular mechanisms involved in transposition reactions of the three most widely used transposon systems currently available (Sleeping Beauty, piggyBac, and Tol2), and discusses the various parameters and considerations pertinent to their experimental use, highlighting the state-of-the-art in transposon technology in diverse genetic applications.

Germline transgenesis in rodents by pronuclear microinjection of Sleeping Beauty transposons

We describe a protocol for high-efficiency germline transgenesis and sustained transgene expression in two important biomedical models, the mouse and the rat, by using the Sleeping Beauty transposon system. The procedure is based on co-injection of synthetic mRNA encoding the SB100X hyperactive transposase, together with circular plasmid DNA carrying a transgene construct flanked by binding sites for the transposase, into the pronuclei of fertilized oocytes. Upon translation of the transposase mRNA, enzyme-mediated excision of the transgene cassettes from the injected plasmids followed by permanent genomic insertion produces stable transgenic animals. Generation of a germline-transgenic founder animal by using this protocol takes ∼3 months. Transposon-mediated transgenesis compares favorably in terms of both efficiency and reliable transgene expression with classic pronuclear microinjection, and it offers comparable efficacies to lentiviral approaches without limitations on vector design, issues of transgene silencing, and the toxicity and biosafety concerns of working with viral vectors.

Transgenic animals: current and alternative strategies

Transgenic animal technology is one of the most fascinating technologies developed in the last two decades. It allows us to address questions in life sciences that no other methods have achieved. The impact on biomedical and biological research, as well as commercial interests are overwhelming. The questions accompanying this fast growing technology and its diversified applications attract the attention from a variety of entities. Still, one of the most fundamental problems remaining is the search for an efficient and reliable gene delivery system for creating transgenic animals. The traditional method of pronuclear microinjection has displayed great variability in success among species. While an acceptable efficiency in the production of transgenic mice has been attained, the relative low efficiency (<1%) in creating transgenic livestock has become one of the barriers for its application. In the past decades, improvements in producing transgenic livestock have made a slow progression, however, the recent advancement in cloning technology and the ability to create transgenic livestock in a highly efficient manner, have opened the gate to a new era in transgenic technology. Discoveries of new gene delivery systems have created an enthusiastic atmosphere that has made this technology so unique. This review focuses on gene delivery strategies as well as various approaches that may assist the advancement of transgenic efficiency in large animals.

Preimplantation screening for transgenesis using an embryonic specific promoter and green fluorescent protein

We report enrichment in the efficiency of generating mice transgenic for expression of a human protein in their milk using GFP-mediated preimplantation screening. The transgene array consisted of a functional gene (human alpha-1 antitrypsin under the control of the ovine BLG promoter) linked 5' to a reporter gene (GFP under the control of the murine Oct-4 promoter). GFP expression was detected in blastocysts by fluorescence microscopy and green and nongreen embryos were transferred to recipients in separate groups. In the first experiment, of seven pups that resulted from the transfer of blastocysts expressing GFP, five (71%) were transgenic. The experiment was repeated and of 12 pups that resulted from transfer of GFP-expressing blastocysts, 11 were transgenic (92%). The presence of the reporter cassette used for preimplantation screening did not affect the expression level of alpha-1-antitrypsin in the milk of the transgenic mice. In addition, in a related experiment wherein the GFP reporter gene was co-injected with a second mammary-specific transgene, pINC, no effect on transgene expression was observed. For mice transgenic for the mammary-specific gene alone, expression levels for four different lines were 192, 197, 382, and 415 microg/mL. For mice transgenic for both the mammary-specific transgene and the Oct4-GFP reporter cassette, expression levels for seven different lines were 282, 321, 468, 497, 499, 516, and 806 microg/mL.

Transgenic production from in vivo-derived embryos: effect on calf birth weight and sex ratio

We examined transgenic-cattle production by DNA microinjection into 1-, 2-, and 4-cell embryos, analyzing the impact on calf size and subsequent viability. Embryos were either collected at an abattoir by flushing oviducts from superovulated and artificially inseminated cows (in vivo-derived) or obtained by in vitro maturation and in vitro fertilization of oocytes aspirated from excised ovaries (in vitro-derived). A human serum albumin (hSA) milk-expression DNA construct was microinjected, either in one of the visible pronuclei of in vitro- and in vivo-derived 1-cell embryos or in the nuclei of two blastomeres of 2- and 4-cell in vivo-derived embryos. Microinjection-induced mortality (lysis and developmental block) was equivalent ( approximately 40%) for all microinjected embryos. Embryos were co-cultured with BRL cells in B-2 medium containing 10% fetal calf serum (FSC). Overall, embryo development to morulae/blastocysts was significantly greater for in vivo-derived ova (15.5%) than for in vitro-derived oocytes (9.3%). All morulae and blastocysts were transferred to synchronized recipient females on Days 6-8 post-fertilization. A total of 189 calves were delivered. Birth weights were significantly greater for calves generated from in vitro-derived oocytes compared with those generated from in vivo-derived oocytes. One transgenic bull calf was obtained from the microinjection of a 2-cell embryo. Fluorescence in situ hybridization (FISH) analysis of lymphocytes detected one transgenic integration site in all cells. Transmission frequency of the hSA transgene in embryos obtained through IVM/IVF/IVC utilizing the semen of the transgenic calf confirmed that it was not mosaic.

Selection of in vitro produced, transgenic embryos by nested PCR for efficient production of transgenic goats

The production of valuable pharmaceutical proteins using transgenic animals as bioreactors has become one of the goals of biotechnology. However, the efficiency of producing transgenic animals by means of pronuclear microinjection is low. This may be attributed in part to the low integration rate of foreign DNA. Therefore, a large number of recipients are required to produce transgenic animals. We recently developed a transgenic procedure that combined the techniques of goat oocyte in vitro maturation (IVM), in vitro fertilization (IVF), microinjection, preimplantation selection of the transgenic embryos with nested PCR and transferring the transgenic embryos into the recipient goat uterus to produce transgenic goats. Thirty-seven transgenic embryos determined by nested PCR were transferred to thirty-two recipient goats. In the end, four live-born kids were produced. As predicted, all the live kids were transgenic as identified by PCR as well as Southern blot hybridization, The integration rate was 100% (4/4) which was completely in accordance with the results of embryo preimplantation detection. The results showed a significant decrease in the number of recipients required as only 8 recipients (32/4) were needed to obtain one live transgenic goat. We suggest that the transgenic system described herein may provide an improved way to efficiently produce transgenic goats on a large scale.

TransgenICSI reviewed: foreign DNA transmission by intracytoplasmic sperm injection in rhesus monkey

This brief review considers the status of transgenesis by intracytoplasmic sperm injection (ICSI) with nonhuman primates. GFP expressing rhesus macaques embryos (mean = 34.6%; N = 81) were produced by ICSI using rhodamine-tagged DNA encoding the green fluorescence protein (GFP) gene bound on sperm. Rhodamine signal was lost at the egg surface during in vitro fertilization (IVF) but could be traced by dynamic imaging during ICSI within the egg cytoplasm. GFP gene was expressed as early as the 4-cell stage in ICSI embryos but not in embryos produced by in vitro fertilization (IVF). The percentage of GFP expressing blastomeres increased during embryogenesis to the blastocyst stage. Three offspring resulted from seven embryo transfers-a set of anatomically normal twins (a male and a female) stillborn 35 days premature, and a healthy male born at term. Although transgene was not detected in the offspring, the successful production of live primates using DNA bound sperm by ICSI suggests an alternative route to creating transgenic animals. It also raises concern regarding transmission of infectious material during ICSI.

Timing of DNA integration, transgenic mosaicism, and pronuclear microinjection

Selection of transgenic embryos prior to embryo transfer is a means to increase the efficiency of transgenic livestock production. Among transgenic reporters, cytoplasmic expression of green fluorescent protein (GFP) has features that make it ideal for transgenic embryo selection. The primary objective of this study was to assess cytoplasmic expression of a specially designed GFP reporter as a tool for transgenic bovine embryo selection. A second objective was to evaluate this reporter for studying transgenic mosaicism related to timing of integration of pronuclear microinjected DNA. Transgenic embryos produced by pronuclear injection showed a discrete pattern of GFP expression with clusters at 25, 50, and 100% of blastomeres expressing GFP. This pattern of mosaicism is interpreted to indicate that the integration of microinjected DNA occurred, not only at the pronuclear stage, but also in the subsequent cell divisions. Among the GFP-positive transgenic embryos, only in 21% did all the blastomeres show the green fluorescence. Using the fraction of positive blastomeres within an embryo, the timing of integration of microinjected DNA was estimated. The frequency of nonmosaic embryos expressing GFP is consistent with published germline transmission success rates of transgenic cattle derived from pronuclear microinjected embryos. These results indicate the possible application of GFP as a marker of transgenic embryos and graphically illustrate underlying complexities in DNA integration in embryos subjected to pronuclear microinjection.

Prediction of transgene integration by noninvasive bioluminescent screening of microinjected bovine embryos

Transgenesis in domestic species, as a research tool and in biotechnological applications, has been limited by the expense of producing transgenic offspring by standard microinjection techniques. A major factor is the inefficiency of maintaining large numbers of recipient females, when a high percentage of these carry nontransgenic fetuses. There are two approaches to reduce this cost, the fusion of transfected fetal fibroblasts with enucleated oocytes, and the screening of microinjected embryos for transgene integration in blastocysts, prior to transfer. Here, we develop a luminescent screening system to select transgenic bovine embryos. A transgene with scaffold attachment regions flanking the murine HSP70.1 promoter linked to firefly luciferase cDNA, was microinjected into pronuclei of in vitro produced zygotes. At the blastocyst stage, the transgene was induced by heat shock (45 degrees C, 15 min) and 4-6 h later, luciferase expression was analyzed by photon counting imaging. Screened blastocysts were transferred to recipients and day 50 fetuses or calves were analyzed by PCR and Southern blot for transgene integration. When nonluminescent blastocysts were transferred, transgene integration was never observed. Of 13 fetuses derived from luminescent blastocysts, 3 contained integrated transgenes that were functional in all tissues examined. Image analysis of the signal emitted by positive blastocysts revealed that 9 nontransgenic fetuses were obtained from blastocysts that exhibited a localized luminescent signal. On the other hand, 3 of 4 fetuses derived from blastocysts that emitted light over more than 70% of their surface were transgenic. Thus, by selecting luminescent blastocysts on the basis of both signal intensity and distribution, the number of recipient females required to produce transgenic offspring can be greatly reduced. Using this technique it should also be possible to improve the efficiency of transgenesis by microinjection through studies in which vector design and integration conditions are examined at the blastocyst stage.

Detection of microinjected genes in bovine preimplantation embryos with combined DNA digestion and polymerase chain reaction

We have developed a simple digestion-polymerase chain reaction (PCR) assay for a simultaneous transgene detection and sexing of pronucleus-injected bovine preimplantation embryos. Bovine embryos were microinjected with dam-methylated gene construct and cultured in vitro for 6-7 days after the injections. The developed blastocysts and compact morulae were bisected and the embryonic biopsies representing mainly trophoblasts were subjected to the digestion-PCR, while the biopsied embryos remained in culture. Embryonic DNA was released with proteinase K and the samples were digested with a Dpnl-Bal31 mixture before the PCR amplification of the transgene, bovine alpha S1-casein, and bovine Y-chromosome fragments in the same reaction. The whole assay from biopsy to electrophoresis took less than 6 hr. The digestion removed up to 50 fg of dam-methylated transgene copies (unintegrated or contaminants) and also a few hundred copies of contaminating PCR products from the embryonic samples. The digestion-PCR assay eliminated all transgene contaminations from noninjected blastocysts, which were exposed to the microinjection DNA during the stay in injection chambers, and reduced the amount of transgene-positive embryos among pronucleus-injected blastocysts as compared with unmodified PCR. Analysis of 486 microinjected bovine embryo biopsies in 13 separate experiments revealed that we were able to sex 398 (82%) of the biopsies and 77 (19%) of the biopsies were scored as transgene positive and 57 (14%) as transgene questionable. Upon reanalysis of 41 of the biopsied embryos, 38 (93%) of the embryos were observed to be transgene negative and 2 questionable in both assays and uneven distribution of transgene copies was observed in one embryo. The results from sexing were in accordance with biopsies and remaining embryos in 38 (93%) of the embryos.