Cellular DNA rearrangements and early developmental arrest caused by DNA insertion in transgenic mouse embryos

Reverse genetic morpholino approach using cardiac ventricular injection to transfect multiple difficult-to-target tissues in the zebrafish larva

The zebrafish is an important model to understand the cell and molecular biology of organ and appendage regeneration. However, molecular strategies to employ reverse genetics have not yet been adequately developed to assess gene function in regeneration or tissue homeostasis during larval stages after zebrafish embryogenesis, and several tissues within the zebrafish larva are difficult to target. Intraventricular injections of gene-specific morpholinos offer an alternative method for the current inability to genomically target zebrafish genes in a temporally controlled manner at these stages. This method allows for complete dispersion and subsequent incorporation of the morpholino into various tissues throughout the body, including structures that were formerly impossible to reach such as those in the larval caudal fin, a structure often used to noninvasively research tissue regeneration. Several genes activated during larval finfold regeneration are also present in regenerating adult vertebrate tissues, so the larva is a useful model to understand regeneration in adults. This morpholino dispersion method allows for the quick and easy identification of genes required for the regeneration of larval tissues as well as other physiological phenomena regulating tissue homeostasis after embryogenesis. Therefore, this delivery method provides a currently needed strategy for temporal control to the evaluation of gene function after embryogenesis. 

Integration, stability and expression of the E. coli phytase transgene in the Cassie line of Yorkshire Enviropig™

The genomic structure and generational stability of the transgene carried by the Cassie (CA) line of the transgenic Enviropig™, a prospective food animal, are reported here. This transgene is composed of the Escherichia coli phytase coding sequence regulated by the mouse parotid secretory protein promoter to direct secretion of phytase in the saliva. In the CA line the transgene integrated in chromosome 4 is present as a concatemer of three copies, two in a head to tail orientation and the third in a reverse orientation 3' to the other copies with a 6 kbp deletion in the 5' promoter region. The overall size of the integrated transgene complex is 46 kbp. During integration a 66 kbp segment of the chromosome was deleted, but a BLAST search of the segment from a GenBank clone did not reveal any essential genes. The transgene integration site was stable through 9 generations analyzed. Phytase activity in the saliva was similar among 11 day old hemizygous boars and gilts and remained relatively constant through nine generations of hemizygous pigs. However, as the pigs grew there generally was a gradual decrease in activity that stabilized when pigs reached the finisher phase of growth (4-6 months old). Homozygous pigs exhibited 1.5 fold higher phytase activity (P < 0.0001) than that of hemizygous littermates. Moreover, no differential salivary phytase activity was seen in hemizygotes arising from CA-Yorkshire and CA-Duroc breed outcrosses, suggesting that expression of the transgene is unaffected by genetic background. This data demonstrates that an exogenous phytase gene can be stably transmitted and expressed in the salivary glands of a domestic food animal.

Fertility comparison between wild type and transgenic mice by in vitro fertilization

Transgenic mice are increasingly used as animal models for studies of gene function and regulation of mammalian genes. Although there has been continuous and remarkable progress in the development of transgenic technology over several decades, many aspects of the resulting transgenic model's phenotype cannot be completely predicted. For example, it is well known that as a consequence of the random insertion of the injected DNA construct, several founder mice of the new line need to be analyzed for possible differences in phenotype secondary to different insertion sites. The Knock out technique for transgenic production disrupts a specific gene by insertion or homologous recombination creating a null expression or replacement of the gene with a marker to localize it expression. This modification could result in pleiotropic phenotype if the gene is also expressed in tissues other than the target organs. Although the future breeding performance of the newly created model is critical to many studies, it is rarely anticipated that the new integrations could modify the reproductive profile of the new transgenic line. To date, few studies have demonstrated the difference between the parent strain's reproductive performance and the newly developed transgenic model. This study was designed to determine whether a genetic modification, knock out (KO) or transgenics, not anticipated to affect reproductive performance could affect the resulting reproductive profile of the newly developed transgenic mouse. More specifically, this study is designed to study the impact of the genetic modification on the ability of gametes to be fertilized in vitro. We analyzed the reproductive performance of mice with different background strains: FVB/N, C57BL/6 (129Sv/J x C57Bl/6)F1 and outbred CD1((R)) and compared them to mice of the same strain carrying a transgene or KO which was not anticipated to affect fertility. In vitro Fertilization was used to analyze the fertility of the mice. Oocytes from superovulated females were inseminated with sperm of same background. Fertility rate was considered as the percentage of two cell embryos scored 24 h after insemination. The data collected from this study shows that the fertilization rate is affected (reduced to half fold) in some of the transgenic mice compared to the respective Wild Type (WT) mice. For the WT the average fertility rate ranged from 80% (C57BL/6), 90% (FVB/N), 45% (129Sv/J x C57Bl/6)F1 and 43% (CD1). For transgenic mice it was 52% (C57BL/6), 65% (FVB/N), 22% (129Sv/J x C57Bl/6)F1 and 25% (CD1).

A model for the mechanism of precise integration of a microinjected transgene

A unique transgenic mouse line has undergone transgene integration in a very precise fashion. The phenotype displayed by mice of the line followed the predicted inheritance patterns for X-linked transgene insertion which has been confirmed. In order to investigate the mechanism of integration the DNA sequence of the transgene and cellular junctions have been determined. A comparison between wild type and transgenic mutant sequences at the site of insertion revealed that there was no loss or rearrangement of cellular DNA upon integration of the transgene. The cellular sequences at the transgene 5' and 3' joins are contiguous in the wild type. The integrant exists as a head to tail tandem dimer with minimal loss of sequence compared with the injected monomer. Analysis of the site of insertion has revealed a 5 bp homology between the 5' end of the transgene and the cellular sequences. In addition, adjacent to the site of insertion within the cellular sequences, there are several sequence motifs implicated in recombination events including a clustering of strong consensus sites of DNA topoisomerase type I and a region of homology to the human minisatellite consensus core sequence, the Escherichia coli Chi site and the meiotic recombination hotspot within the E beta gene of the murine major histocompatibility complex. This clustering of features is likely to have been factorial in the integrity of the insertion event. A model depicting the mechanism of this precise integration is proposed.

Transgene detection during early murine embryonic development after pronuclear microinjection

The polymerase chain reaction (PCR) technique was used to detect a whey acidic protein (WAP) gene and transgene presence in mouse ova cultured to various stages of development after pronuclear microinjection at the one-cell stage. The PCR technique detected an endogenous 442 bp WAP DNA sequence in 78% of one-cell, 88% of two-cell and 94% of four-cell ova, and in 95% of morulae and 97% of blastocysts. The heterologous WAP-human protein C transgene was detected in 88% of one-cell, 88% of two-cell and 44% of four-cell ova, and in 40% of morulae and 29% of blastocysts. For comparison, the integration frequency for transgenic mouse production using the same DNA construct was 22%. After five days of in vitro culture, embryos that were either developmentally arrested or fragmented were tested for the presence of the transgene. The injected construct was detected in 83% of arrested one-cell, 85% of arrested two-cell, and 85% of fragmented ova. In culture, only 28% of zygotes microinjected with DNA developed to the blastocysts stage compared to 74% of noninjected zygotes, while 63% of zygotes developed to the blastocyst stage after injection of buffer alone. Pronuclear injection of the transgene at concentrations of 1.5, 15 and 50 micrograms ml-1 resulted in 28, 11 and 9% development to blastocysts and 29, 86 and 88% transgene detection, respectively. Transgene detection was 85, 96 and 97% in degenerate embryos at the respective doses of DNA. These data show that pronuclear microinjection of the transgene is detrimental to subsequent embryonic development.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescence-activated sorting of totipotent embryonic stem cells expressing developmentally regulated lacZ fusion genes

Murine embryonic stem (ES) cells were infected with a retrovirus promoter trap vector, and clones expressing lacZ fusion genes (LacZ+) were isolated by fluorescence-activated cell sorting (FACS). Of 12 fusion genes tested, 1 was repressed when ES cells were allowed to differentiate in vitro. Two of three lacZ fusion genes tested were passed into the germ line, indicating that FACS does not significantly affect stem cell totipotency. The pattern of lacZ expression observed in vivo was consistent with that seen in vitro. Both fusion genes were expressed in preimplantation blastulas. However, a fusion gene whose expression was unaffected by in vitro differentiation was ubiquitously expressed in day-10 embryos, while the other, which showed regulated expression in vitro, was restricted to cells located along the posterior neural fold, the optic chiasm, and within the fourth ventricle. These results demonstrate the utility of using promoter trap vectors in conjunction with fluorescence sorting to disrupt developmentally regulated genes in mice.

Structure of clonal and polyclonal cell arrays in chimeric mouse retina

One of the most striking results of recent cell-lineage studies of vertebrate retina is the marked variability in the size and types of clones marked by retroviral transfection and dye injection of embryonic progenitor cells. Is this variability due to microenvironmental modulation of cell determination, to lineage restriction, or to experimental perturbation of the progenitor cells? We have taken advantage of species-specific DNA probes to mark groups of lineage-related cells in experimental mouse chimeras. This method of marking cells has two distinct advantages over previous methods: direct manipulation of progenitor cells is avoided, and clones are established at an earlier stage of retinal development. The most notable feature of retinal cohorts in chimeras is their structural uniformity--each is a solid radial array that contains the same ratio of major cell types as the retina itself. This is true even of the smallest monoclonal cohorts, which contain fewer than 200 cells. Our results provides compelling empirical support for the hypothesis that the murine retina is made up of hundreds of relatively homogeneous radial units, each derived from single retinal precursor cells. This finding is inconsistent with micro-environmental modulation of clone structure early in development. We raise the possibility that the heterogeneity among clones marked by dye injection and transfection is due to progressive lineage restriction or to experimental perturbation of the retinal progenitor cells.

Direct sequencing of PCR-amplified junction fragments from tandemly repeated transgenes

When microinjected foreign genes integrate into the genomes of mice, multiple copies are frequently found clustered together at one location. How they concatamerize--by the integration of large linearized concatamers that are formed by simple end-to-end linkage, by circularization of individual DNA fragments and recombination, or by some other means--is not understood. In the transgenic animals studied thus far by ourselves and others, integration frequency and transgene copy number do not seem to be significantly influenced by the complementarity of the ends of the DNA fragments that have been microinjected. We have utilized PCR amplification and DNA sequence analysis to study selected transgene junctions at the nucleotide level. In two transgenic mice carrying the synthetic RSVcat gene (injected with noncomplementary overhangs on the fragment ends), ends were 'nibbled' from 1 to 62 bases before being joined to an adjacent gene copy. Repeated dinucleotides, providing the most minimal of homologies, are present in half of the characterized junctions. Determination of the relative copy number of the junctions in each mouse supports the idea that transgene complexes can undergo additional rearrangements after the initial formation event.

Symplastic spermatids (sys): a recessive insertional mutation in mice causing a defect in spermatogenesis

A line of transgenic mice that carries an insertional mutation in a gene essential for spermatogenesis is described. Males homozygous for the transgenic insert are sterile, while female homozygotes and both male and female heterozygotes exhibit normal fertility. Developing spermatids in homozygous males form prominent abnormal multinucleated syncytia (symplasts) and do not complete maturation. In addition, abnormal cytoplasmic vacuolation is commonly seen in Sertoli cells. One flank of the transgenic integration site within the genome has been cloned and used to show linkage between homozygosity for the transgene and the mutant phenotype. The flank maps to mouse chromosome 14 approximately 4 centimorgans proximal to the gene encoding esterase-10 (Es-10). As no other gene that is known to be essential for spermatogenesis has been mapped to this region of the genome and as the mutant phenotype is unique, the transgenic insert appears to affect a previously unidentified gene. We have named the mutation "symplastic spermatids" (sys).

Expression of a foreign gene in a line of transgenic mice is modulated by a chromosomal position effect

Unusual aberrant expression of a foreign gene in a particular transgenic mouse line is often attributed to chromosomal position effect, although proof of this is lacking. An alternative explanation is that expression has been modified by the arrangement of multiple copies of the foreign gene at the insertion site or by mutation or gene rearrangement. We have distinguished between these explanations in the case of one particular transgenic line by recovering the aberrantly expressed foreign DNA and reintroducing it into the mouse genome to produce secondary transgenic mice. The expression pattern of the gene in the secondary transgenic mice was normal, showing that this case of aberrant expression is due to a chromosomal position effect.

Expression of a foreign gene in a line of transgenic mice is modulated by a chromosomal position effect

Unusual aberrant expression of a foreign gene in a particular transgenic mouse line is often attributed to chromosomal position effect, although proof of this is lacking. An alternative explanation is that expression has been modified by the arrangement of multiple copies of the foreign gene at the insertion site or by mutation or gene rearrangement. We have distinguished between these explanations in the case of one particular transgenic line by recovering the aberrantly expressed foreign DNA and reintroducing it into the mouse genome to produce secondary transgenic mice. The expression pattern of the gene in the secondary transgenic mice was normal, showing that this case of aberrant expression is due to a chromosomal position effect.

Prenatal lethality in a transgenic mouse line is the result of a chromosomal translocation

We have produced a line of transgenic mice that is characterized by prenatal lethality. These mice bear a chimeric plasmid containing the long terminal repeat of the Rous sarcoma virus linked to the coding region of the chloramphenicol acetyltransferase gene (pRSV-CAT). Mice heterozygous for the pRSV-CAT integration site are semisterile, producing litters approximately equal to 40% of the average size when crossed to normal mice. Approximately 50% of the progeny from such a cross bear the pRSV-CAT sequences and also produce litters of smaller size. An analysis of embryogenesis revealed that normal numbers of embryos implanted, but 60% failed to develop past day 7. Eight other independent transgenic lines containing RSV-CAT show no evidence of embryonic lethality; thus, it is unlikely that the defect observed is due to the direct effects of RSV-CAT expression. We have found that carrier mice bear a reciprocal translocation between chromosomes 6 and 17, T(6A2-6A3;17D-17E1), that can explain the apparent dominant embryonic lethality seen in this line. The site of integration has been localized by in situ hybridization at or near the translocation breakpoint in one of the translocated chromosomes (6(17)). Because the foreign DNA is present in one of the translocated chromosomes, we propose that this rearrangement was elicited by the introduction of foreign DNA.