Expression of reporter genes introduced by microinjection and electroporation in fish embryos and fry.

The technique for foreign gene transfer in fish is becoming a novel method for genetic engineers to produce useful transgenic fish as well as for experimental purposes. Our studies of transgenic fish have been focused on methods of gene transfer and regulatory elements for transgene expression. I describe the characteristics of 3 gene transfer methods we have established (i.e., microinjection into fertilized eggs, microinjection into oocytes, and electroporation). Also described is a series of experiments to estimate activities of several promoters and enhancers in a fish cell line. Finally, experiments to examine activities of the elements shown to be active in the cell line in fish embryos and fry are described. SV2, miw, and metallothioneine promoters of trout and mouse were shown to be active in these experiments.     

Silencing of fat-1 transgene expression in sheep may result from hypermethylation of its driven cytomegalovirus (CMV) promoter

The fat-1 gene was isolated from roundworm Caenorhabditis elegans, and built into pIRES2-EGFP expression vectors driven by cytomegalovirus (CMV) promoter or cytomegalovirus enhancer and chickenβ-actin (CAG) promoter. Both CMV- and CAG-driven expression vectors were transfected to sheep fetal fibroblast cells. Positive transfected cells were used as donors for somatic cell nuclear transfer (SCNT) and the cloned embryos were transferred into the oviducts of synchronized recipient sheep. Two lambs derived from CMV vector and three lambs derived from CAG vector developed to term. Although Southern analyses using tissues from the two lambs derived from CMV vectors indicated integration of fat-1 gene into the genome, fat-1 mRNAs were not detected by RT-PCR. However, there was fat-1 expression (detected by RT-PCR) in tissues from transgenic lambs driven by CAG vectors. To investigate potential mechanisms involved in the two transgene models, methylation state of the vector promoters were examined. In CMV-driven transgenics, CMV promoters had almost no methylation in transfected cells and the resultant cloned embryos, whereas high methylations were detected in tissues and organs in transgenic lambs. In the CAG-driven transgenics, there were almost no methylations in transgenic cells and transgenic cloned embryos, and cloned lambs expressed fat-1 mRNA (detected by RT-PCR). Moreover, although SV40 promoters which drove neo/kan marker gene in CMV vectors were highly methylated in tissues from transgenic lambs, they were without methylation in cells and embryos. Therefore, we concluded that highly methylated CMV promoters induced the silence of fat-1 transgene expression in sheep. Furthermore, CAG promoter, but not CMV promoter was suitable for generation of fat-1 transgenic sheep.     

Targeted chromosomal insertion of large DNA into the human genome by a fiber-modified high-capacity adenovirus-based vector system

A prominent goal in gene therapy research concerns the development of gene transfer vehicles that can integrate exogenous DNA at specific chromosomal loci to prevent insertional oncogenesis and provide for long-term transgene expression. Adenovirus (Ad) vectors arguably represent the most efficient delivery systems of episomal DNA into eukaryotic cell nuclei. The most advanced recombinant Ads lack all adenoviral genes. This renders these so-called high-capacity (hc) Ad vectors less cytotoxic/immunogenic than those only deleted in early regions and creates space for the insertion of large/multiple transgenes. The versatility of hcAd vectors is been increased by capsid modifications to alter their tropism and by the incorporation into their genomes of sequences promoting chromosomal insertion of exogenous DNA. Adeno-associated virus (AAV) can insert its genome into a specific human locus designated AAVS1. Trans- and cis-acting elements needed for this reaction are the AAV Rep78/68 proteins and Rep78/68-binding sequences, respectively. Here, we describe the generation, characterization and testing of fiber-modified dual hcAd/AAV hybrid vectors (dHVs) containing both these elements. Due to the inhibitory effects of Rep78/68 on Ad-dependent DNA replication, we deployed a recombinase-inducible gene switch to repress Rep68 synthesis during vector rescue and propagation. Flow cytometric analyses revealed that rep68-positive dHVs can be produced similarly well as rep68-negative control vectors. Western blot experiments and immunofluorescence microscopy analyses demonstrated transfer of recombinase-dependent rep68 genes into target cells. Studies in HeLa cells and in the dystrophin-deficient myoblasts from a Duchenne muscular dystrophy (DMD) patient showed that induction of Rep68 synthesis in cells transduced with fiber-modified and rep68-positive dHVs leads to increased stable transduction levels and AAVS1-targeted integration of vector DNA. These results warrant further investigation especially considering the paucity of vector systems allowing permanent phenotypic correction of patient-own cell types with large DNA (e.g. recombinant full-length DMD genes).     

Characterization of in vivo recombination activities in the mouse embryo

Homologous recombination makes use of sequence homology to repair DNA and to rearrange genetic material. In mammals, these processes have mainly been characterized using cultured cell systems. We have developed an assay that allows us to quantitatively analyze homologous recombination in vivo in the mouse embryo. Transgenic mouse lines were generated by microinjection into a fertilized mouse ovum of a vector containing two homologous LINE-1 (L1) sequences arranged as a direct repeat: these sequences can recombine with each other and with endogenous L1 sequences before, during or after integration of the vector into the genome. Using a plasmid rescue procedure, we determined the composition of the integrated vector array in several transgenic mice and their descendants. Homologous recombination frequencies were found to be strikingly high, involving 70% of integrated vectors in some arrays, with homologous deletions being five times more frequent than gene conversion without crossing-over. Interestingly, non-homologous recombination was found to be much less frequent. We also found that endogenous L1 sequences could be involved in homologous recombination events in the mouse embryo, and that the integrated arrays could be modified from generation to generation by homologous recombination between the integrated L1 sequences.     

Generation and characterization of a transgenic mouse with a functional human TSPY

To generate an animal model that is suitable for the analysis of regulation and expression of human testis-specific protein, Y-encoded TSPY, a transgenic mouse line, TgTSPY9, harboring a complete structural human TSPY gene was generated. Fluorescence in situ hybridization and Southern analyses show that approximately 50 copies of the human TSPY transgene are integrated at a single chromosomal site that maps to the distal long arm of the Y chromosome. The transgene is correctly transcribed and spliced according to the human pattern and is mainly expressed in testicular tissue, with spermatogonia and early primary spermatocytes (leptotene and zygotene) as expressing germ cells. TSPY transgenic mice are phenotypically normal, and spermatogenesis is neither impaired nor enhanced by the human transgene. The present study shows that a human TSPY gene integrated into the mouse genome follows the human expression pattern although murine tspy had lost its function in rodent evolution millions of years ago.     

From the first to the third generation adenoviral vector: What parameters are governing the production yield?

Human adenoviral viral vector serotype 5 (AdV) is presently the primary viral vector used in gene therapy trials. Advancements in AdV process development directly contribute to the clinical application and commercialization of the AdV gene delivery technology. Notably, the development of AdV production in suspension culture has driven the increase in AdV volumetric and specific productivity, therefore providing large quantities of AdV required for clinical studies. This review focuses on detailing the viral, cell and cell culture parameters governing the productivity of the three generations of AdV vectors.     

Stable and strictly controlled expression of LTR-flanked autoregulated expression cassettes upon adenoviral transfer

An autoregulatory bidirectional expression cassette encoding all components necessary for regulated gene expression in a one-step gene transfer was evaluated for use in adenoviral vectors. Adenoviral vectors transducing this cassette provide about 1000-fold regulation. Regulation could be further improved by integrating the cassette as a retroviral vector into the adenoviral backbone. Moreover, with these adeno/retroviral hybrid vectors, the frequency of chromosomal integration is enhanced and about 1% of infected cells show stable chromosomal integration of the autoregulated cassette. In these stably transduced cells high regulation capacity is maintained. To elucidate the molecular mechanism underlying this unexpected observation we investigated the regulation capacity of these cassettes in a viral and non-viral vector background after stable integration into the host's DNA. While naked cassettes show regulated expression that is strongly influenced by the chromosomal surrounding sequences the regulatory capacity of LTR flanked cassettes is highly comparable amongst different cell clones. This strict regulation with little influence from the flanking sequences is obtained when LTR-flanked cassettes are transduced as DNA, by retroviral or by adenoviral infection.     

Muscle-specific overexpression of the adenovirus primary receptor CAR overcomes low efficiency of gene transfer to mature skeletal muscle

Significant levels of adenovirus (Ad)-mediated gene transfer occur only in immature muscle or in regenerating muscle, indicating that a developmentally regulated event plays a major role in limiting transgene expression in mature skeletal muscle. We have previously shown that in developing mouse muscle, expression of the primary Ad receptor CAR is severely downregulated during muscle maturation. To evaluate how global expression of CAR throughout muscle affects Ad vector (AdV)-mediated gene transfer into mature skeletal muscle, we produced transgenic mice that express the CAR cDNA under the control of the muscle-specific creatine kinase promoter. Five-month-old transgenic mice were compared to their nontransgenic littermates for their susceptibility to AdV transduction. In CAR transgenics that had been injected in the tibialis anterior muscle with AdVCMVlacZ, increased gene transfer was demonstrated by the increase in the number of transduced muscle fibers (433 +/- 121 in transgenic mice versus 8 +/- 4 in nontransgenic littermates) as well as the 25-fold increase in overall beta-galactosidase activity. Even when the reporter gene was driven by a more efficient promoter (the cytomegalovirus enhancer-chicken beta-actin gene promoter), differential transducibility was still evident (893 +/- 149 versus 153 +/- 30 fibers; P < 0.001). Furthermore, a fivefold decrease in the titer of injected AdV still resulted in significant transduction of muscle (253 +/- 130 versus 14 +/- 4 fibers). The dramatic enhancement in AdV-mediated gene transfer to mature skeletal muscle that is observed in the CAR transgenics indicates that prior modulation of the level of CAR expression can overcome the poor AdV transducibility of mature skeletal muscle and significant transduction can be obtained at low titers of AdV.     

A limited number of transducible hepatocytes restricts a wide-range linear vector dose response in recombinant adeno-associated virus-mediated liver transduction

Recombinant adeno-associated virus (rAAV) vectors are promising vehicles for achieving stable liver transduction in vivo. However, the mechanisms of liver transduction are not fully understood, and furthermore, the relationships between rAAV dose and levels of transgene expression, total number of hepatocytes transduced, and proportion of integrated vector genomes have not been well established. To begin to elucidate the liver transduction dose response with rAAV vectors, we injected mice with two different human factor IX or Escherichia coli lacZ-expressing AAV serotype 2-based vectors at doses ranging between 4.0 x 10(8) and 1.1 x 10(13) vector genomes (vg)/mouse, in three- to sixfold increments. A 2-log-range linear dose-response curve of transgene expression was obtained from 3.7 x 10(9) to 3.0 x 10(11) vg/mouse. Vector doses above 3.0 x 10(11) vg/mouse resulted in disproportionately smaller increases in both the number of transduced hepatocytes and levels of transgene expression, followed by saturation at doses above 1.8 x 10(12) vg/mouse. In contrast, a linear increase in the number of vector genomes per hepatocyte was observed up to 1.8 x 10(12) vg/mouse concomitantly with enhanced vector genome concatemerization, while the proportion of integrated vector genomes was independent of the vector dose. Thus, the mechanisms that restrict a wide-range linear dose response at high doses likely involve decreased functionality of vector genomes and restriction of transduction to fewer than 10% of total hepatocytes. Such information may be useful to determine appropriate vector doses for in vivo administration and provides further insights into the mechanisms of rAAV transduction in the liver.     

Integrating Adenovirus–Adeno-Associated Virus Hybrid Vectors Devoid of All Viral Genes

Recently, we demonstrated that inverted repeat sequences inserted into first-generation adenovirus (Ad) vector genomes mediate precise genomic rearrangements resulting in vector genomes devoid of all viral genes that are efficiently packaged into functional Ad capsids. As a specific application of this finding, we generated adenovirus-adeno-associated virus (AAV) hybrid vectors, first-generation Ad vectors containing AAV inverted terminal repeat sequences (ITRs) flanking a reporter gene cassette inserted into the E1 region. We hypothesized that the AAV ITRs present within the hybrid vector genome could mediate the formation of rearranged vector genomes (DeltaAd.AAV) and stimulate transgene integration. We demonstrate here that DeltaAd.AAV vectors are efficiently generated as by-products of first-generation adenovirus-AAV vector amplification. DeltaAd.AAV genomes contain only the transgene flanked by AAV ITRs, Ad packaging signals, and Ad ITRs. DeltaAd.AAV vectors can be produced at a high titer and purity. In vitro transduction properties of these deleted hybrid vectors were evaluated in direct comparison with first-generation Ad and recombinant AAV vectors (rAAVs). The DeltaAd.AAV hybrid vector stably transduced cultured cells with efficiencies comparable to rAAV. Since cells transduced with DeltaAd.AAV did not express cytotoxic viral proteins, hybrid viruses could be applied at very high multiplicities of infection to increase transduction rates. Southern analysis and pulsed-field gel electrophoresis suggested that DeltaAd.AAV integrated randomly as head-to-tail tandems into the host cell genome. The presence of two intact AAV ITRs was crucial for the production of hybrid vectors and for transgene integration. DeltaAd.AAV vectors, which are straightforward in their production, represent a promising tool for stable gene transfer in vitro and in vivo.     

Replication-defective vectors of reticuloendotheliosis virus transduce exogenous genes into somatic stem cells of the unincubated chicken embryo.

Replication-defective vectors derived from reticuloendotheliosis virus were used to transduce exogenous genes into early somatic stem cells of the chicken embryo. One of these vectors transduced and expressed the chicken growth hormone coding sequence. The helper cell line, C3, was used to generate stocks of vector containing about 10(4) transducing units per ml. Injection of 5- to 20-microliters volumes of vector directly beneath the blastoderm of unincubated chicken embryos led to infection of somatic stem cells. Infected embryos and adults contained unrearranged integrated proviral DNAs. Embryos expressed the transduced chicken growth hormone gene and contained high levels of serum growth hormone. Blood, brain, muscle, testis, and semen contained from individuals injected as embryos contained vector DNA. Replication-defective vectors of the reticuloendotheliosis virus transduced exogenous genes into chicken embryonic stem cells in vivo.     

慢病毒载体介导的转人ahsp基因的β654地中海贫血小鼠的制备

目的：经慢病毒载体介导,制备转人α血红蛋白稳定蛋白基因（ahsp）的β654地中海贫血小鼠。方法：用巢式PCR从人血DNA中获得人ahsp基因,构建含有人ahsp基因的慢病毒载体,制备假病毒,通过卵周隙显微注射手段将其导入β654地贫小鼠的受精卵,经移植至假孕母鼠输卵管,最终孕育出转人ahsp基因的β654地贫小鼠;分析小鼠体内外源ahsp基因的表达情况及其遗传稳定性。结果：共获得了8只人ahsp阳性小鼠,转基因阳性率为32%（8/25）,其中3只同时具有β654突变基因;人ahsp基因在小鼠体内的表达水平维持在小鼠自身ahsp表达量的1%左右,且可稳定遗传至子代。结论：制备了转人ahsp基因小鼠,并可遗传至子代,为在个体水平上研究α血红蛋白稳定蛋白与β地贫之间的关系提供了工具。     

Rapid localization of transgenes in mouse chromosomes with a combined Spectral Karyotyping/FISH technique

We explored the feasibility of combined Spectral Karyotyping (SKY) and Fluorescence In Situ Hybridization (FISH) as means to rapidly map a chromosomally integrated renin/green fluorescent protein (GFP) fusion gene construct (Ren-GFP) in the transgenic mouse, Tg(Ren-GFP)1Kwg. A sequential hybridization with SKY probes followed by FISH gave consistently satisfactory results, demonstrating that multiple copies of the Ren-GFP transgene in this transgenic mouse line are integrated into a single chromosomal site of Chromosome (Chr) 4, most probably in the juxta-centromeric euchromatic region consisting of the A2-A3 domain. Chr 4 as a sole carrier of the transgene also was confirmed by co-hybridization to p1 BAC clone DNA containing telomeric sequences specific for mouse Chr 4 and the Ren-GFP construct in pGEM4Z vector. The hemizygosity of the Ren-GFP transgene is maintained not only in bone marrow cells, but also in lung cells proliferating in vitro, indicating that stable integration of the Ren-GFP transgene into chromosomal DNA was established at a very early embryonic stage. We conclude that the SKY/FISH technique is a reliable and facile method for establishing the integration site of a transgene. As such, this protocol has obvious advantages over traditional backcross methods in terms of time, cost and labor for determining the chromosomal location of transgenes.     

Activation of transgene expression by early region 4 is responsible for a high level of persistent transgene expression from adenovirus vectors in vivo.

The persistence of transgene expression has become a hallmark for adenovirus vector evaluation in vivo. Although not all therapeutic benefit in gene therapy is reliant on long-term transgene expression, it is assumed that the treatment of chronic diseases will require significant persistence of expression. To understand the mechanisms involved in transgene persistence, a number of adenovirus vectors were evaluated in vivo in different strains of mice. Interestingly, the rate of vector genome clearance was not altered by the complete deletion of early region 4 (E4) in our vectors. The GV11 (E1- E4-) vector genome cleared with a similar kinetic profile as the GV10 (E1-) vector genome in immunocompetent and immunocompromised mice. These results suggest that the majority of adenovirus vector genomes are eliminated from transduced tissue via a mechanism(s) independent of T-cell, B-cell, and NK cell immune mechanisms. While the levels of persistence of transgene expression in liver or lung transduced with GV10 and GV11 vectors expressing beta-galactosidase, cystic fibrosis transmembrane conductance regulator, or secretory alkaline phosphatase were similar in immunocompetent mice, a marked difference was observed in immunocompromised animals. Levels of transgene expression initially from both GV10 and GV11 vectors were the same. However, GV11 transgene expression correlated with loss of vector genome, while GV10 transgene expression persisted at a high level. Coadministration and readministration of GV10 vectors showed that E4 provided in trans could activate transgene expression from the GV11 vector genome. While transgene expression activity per genome from the GV10 vector is clearly activated, expression from a cytomegalovirus promoter expression cassette in a GV11 vector appeared to be further inactivated as a function of time. Understanding the molecular mechanisms underlying these expression effects will be important for developing persistent adenovirus vectors for chronic applications.     

New insights into stability of recombinant adenovirus vector genomes in mammalian cells

Recombinant adenoviruses are widely used in basic virology research, therapeutic applications, vaccination studies or simply as a tool for genetic manipulation of eukaryotic cells. Dependent on the application, transient or stable maintenance of the adenoviral genome and transgene expression are required. The newest generation of recombinant adenoviral vectors is represented by high-capacity adenoviral vectors (HC-AdVs) which lack all viral coding sequences. HC-AdVs were shown to result in long-term persistence of transgene expression and phenotypic correction in small and large animal models with negligible toxicity.Although there is evidence that adenoviral vectors predominantly persist as episomal DNA molecules with a low integration frequency into the host genome, detailed information about the nuclear fate and the molecular status of the HC-AdV genome once inside the nucleus is lacking. In recent years we have focused on analyzing and modifying the nuclear fate of HC-AdVs after infection of mammalian cells. We have focused on investigating the molecular DNA forms of HC-AdV genomes and we have designed strategies to excise and stably integrate a transgene from an episomal adenovirus vector genome into the host chromosomes by recombinases. This review article provides a state-of-the art overview of the current knowledge of episomal HC-AdV persistence and it discusses strategies for changing the nuclear fate of a transgene inserted into the HC-AdV genome by somatic integration into host chromosomes.     

Flp recombinase transgenic mice of C57BL/6 strain for conditional gene targeting

We constructed an expression vector of Flp recombinase modified by adding a nuclear localization signal. Injection of the expression vector into fertilized eggs of the C57BL/6 strain yielded transgenic mouse lines expressing the Flp recombinase transgene in the testis. We crossed the transgenic mice to reporter mice carrying the neomycin phosphotransferase gene flanked by target sites of Flp recombinase. Examination of the deletion of the neomycin phosphotransferase gene in the progeny showed that Flp-mediated recombination took place efficiently in vivo in FLP66 transgenic mouse line. These results suggest that the Flp recombinase system is effective in mice and in combination with the Cre recombinase system extends the potentials of gene manipulation in mice. One of the useful applications of FLP66 transgenic mouse line is the removal of marker genes from mice manipulated for the conditional gene targeting with the Cre/loxP system in the pure C57BL/6 genetic background.     

Isolation of Recombinant Adeno-Associated Virus Vector-Cellular DNA Junctions from Mouse Liver

Recombinant adeno-associated virus (rAAV) vectors allow for sustained expression of transgene products from mouse liver following a single portal vein administration. Here a rAAV vector expressing human coagulation factor F.IX (hF.IX), AAV-EF1alpha-F.IX (hF.IX expression was controlled by the human elongation factor 1alpha [EF1alpha] enhancer-promoter) was injected into mice via the portal vein or tail vein, or directly into the liver parenchyma, and the forms of rAAV vector DNA extracted from the liver were analyzed. Southern blot analyses suggested that rAAV vector integrated into the host genome, forming mainly head-to-tail concatemers with occasional deletions of the inverted terminal repeats (ITRs) and their flanking sequences. To further confirm vector integration, we developed a shuttle vector system and isolated and sequenced rAAV vector-cellular DNA junctions from transduced mouse livers. Analysis of 18 junctions revealed various rearrangements, including ITR deletions and amplifications of the vector and cellular DNA sequences. The breakpoints of the vector were mostly located within the ITRs, and cellular DNA sequences were recombined with the vector genome in a nonhomologous manner. Two rAAV-targeted DNA sequences were identified as the mouse rRNA gene and the alpha1 collagen gene. These observations serve as direct evidence of rAAV integration into the host genome of mouse liver and allow us to begin to elucidate the mechanisms involved in rAAV integration into tissues in vivo.     

Kinetics of pronuclear development and the effects of vector type and timing of injection on the efficiency of gene transfer into rhesus macaque embryos

A series of experiments was performed to determine the dynamics of pronuclear development as well as the efficiency of either adenovirus-associated (AAV) or lentivirus-derived vectors to introduce a green fluorescent protein (GFP) reporter gene into rhesus macaque (Macaca mulatta) embryos. Assessment of pronuclear development at various times after fertilization revealed that the appearance of pronuclei was determined by the presence of the first and the timing of the second polar body. The dynamics of pronuclear formation was a significant determinant of whether an oocyte reached the blastocyst stage, however, when the percentage of blastocysts were based on the number of zygotes, the timing of the appearance of polar bodies did not appear to have any effect on subsequent development. Injection of different AAV-derived vectors showed that the serotype of the vector did not affect development or the proportion of transgenic embryos. Moreover, all putative transgenic embryos proved to be expression mosaics. Injection of embryos with lentiviral vectors showed that timing of injection (before or after fertilization) had no effect on subsequent transgene expression, but that the type of reporter gene determined post-injection development and rate of transgenesis. The transfer of embryos following injection of a lentiviral vector into three recipients resulted in one pregnancy which was lost during the second trimester. Analysis of fetal tissues showed ubiquitous presence of the transgene and GFP expression in all tissues examined. These results show that lentivirus-derived vectors can efficiently transform rhesus embryos and are suitable for the generation of transgenic rhesus monkeys.