Genetic background affects integration frequency of ecotropic proviral sequences into the mouse germ line

Development of retroviral vectors as safe, targeted gene delivery systems

The transfer of genes of potential therapeutic benefit is presently being attempted in the clinic to treat a number of genetic and virally induced diseases. Many of these protocols use retroviral vectors derived from murine leukemia retroviruses as gene delivery systems. Although these viral delivery systems are well suited for this purpose, a number of their characteristics, some of which are discussed here, are still troublesome. Future retroviral vectors will incorporate nonretroviral features and will be tailored to desired needs for specific uses. These vectors will be safer, more efficient, and targeted in their delivery. Further, expression of the therapeutic genes carried will be limited to the specific target cell type. Some of the recent advances that have been made towards this goal are reviewed here.

Low rates of proviral integration in SWR/J-RF/J hybrid mice

A high frequency of proviral acquisition has previously been reported in the offspring of SWR/J-RF/J hybrid mice. In the present study, it was investigated whether this proviral acquisition would be useful for large-scale insertional mutagenesis studies. A population of SWR/J-RF/J hybrid mice with a predominantly SWR/J background was created. Lines of mice with such a background and partially congenic for two active proviruses from the RF/J strain were generated (the insert lines). Control lines were derived from mice which had no proviral loci but had an otherwise similar genetic background. DNA samples of mice in the insert lines were screened for the appearance of new proviral loci by Southern hybridization. The rate of proviral acquisition, calculated from the observed number of new proviral loci was 0.023 new proviruses per mouse. This rate is lower than found in previous studies and too low for large-scale insertional mutagenesis studies. A sensitivity experiment indicated that there was adequate detection of new proviral loci. The number of segregating proviruses was consistent with the number of newly acquired proviruses actually detected. Two additional crosses between mice in the insert lines and SWR/J mice were performed. The rate of proviral acquisition was greatly increased when SWR/J females were initially mated to insert mice, but remained unchanged when SWR/J males were used. This suggested that mice in the insert lines had acquired a maternally transmitted factor, which was suppressing viral expression and thus reducing the rate of proviral acquisition.

The endogenous ecotropic murine retroviruses Emv-16 and Emv-17 are both capable of producing new proviral insertions in the mouse genome

New germ line proviral insertions are acquired at a high frequency by the progeny of SWR/J-RF/J hybrid female mice that carry the endogenous ecotropic murine leukemia proviruses Emv-16 and Emv-17. The tight linkage of these RF/J strain proviral loci has prevented genetic segregation of the retroviral genomes. Hence, it is not known whether both of these proviruses are capable of giving rise to new proviral insertions. We have molecularly cloned Emv-16 and Emv-17 and have characterized them in vitro and in vivo. Restriction enzyme analysis of the recombinant clones revealed that the proviral genomes are very similar to each other and closely resemble the wild-type AKR virus. A comparison of the flanking cellular DNA suggests that the Emv-16 and Emv-17 loci did not arise by simple duplication of a viral insertion site within the RF/J genome but most likely are independent integration events. Both proviruses produce infectious virus when transfected into NIH 3T3 cells, indicating that they are nondefective retroviruses. Exogenous infection of SWR/J mice with either Emv-16 or Emv-17 leads to viremia in the host animals, and in both cases, progeny of viremic females acquire new proviral insertions. The ability of these retroviruses to generate novel retroviral integration sites in the mouse genome provides a simple method for inducing insertional mutations in mice.

Insertional mutations in mammals and mammalian cells

The retroposon sequences, their mechanisms of transposition and the occurrence of insertional mutation in the mammalian genome are reviewed. Insertional mutations fall into two broad categories: those due to the disruption of a gene following the physical integration of a foreign DNA sequence result in loss of gene product and would be expected to be associated with a recessive mutation. A second class of insertional mutation is well documented in which upon integration the promoter/enhancer activities inherent in the retroposon genome exert their influence on neighboring genes. This promoter/enhancer activity of integrated retroposons may have effects over relatively long distances and thus limit the possibilities of establishing an association between retroposon integration and mutation. It is emphasized that a systematic search for insertional mutations in the mammalian genome involves an extensive two-dimensional array of possible retroposon sequences and mutant alleles. Present results represent only a small portion of the total array. Future studies promise to be fruitful in efforts to isolate genes through insertional tagging, to characterize the mechanisms of retroposon transposition, as well as to study the stability of the mammalian genome.

The polyomavirus early region gene in transgenic mice causes vascular and bone tumors

Transgenic mice carrying the entire polyomavirus (Py) early region consistently develop both vascular and bone tumors. This tumor spectrum represents a subset of the tumors found in mice infected with Py and an expansion of the vascular tumor spectrum seen in Py middle T antigen (MT) transgenic mice (V. L. Bautch, S. Toda, J. A. Hassell, and D. Hanahan, Cell 51:529-538, 1987). Transgenic mice of three independent lineages develop these pathologies, and mice of individual lineages also develop lymphangiomas and fibrosarcomas. All tumors are of mesenchymal origin, and all tumor tissues express the Py transgene. Some unaffected tissues, including the testes of mice of all lineages, also express the Py transgene. The number of transgene expression sites in mice of a given lineage correlates with the severity and latency of the tumor phenotype in these animals. Analysis of transgene transcripts indicates that RNAs for Py large T antigen (LT), MT, and small T antigen (ST) are present in both tumors and testes of transgenic mice. The ratio of LT RNA to MT and ST RNAs, however, is higher in testes than in tumors and other unaffected tissues, indicating that tissue-specific differences in the relative amounts of the alternatively spliced Py RNAs exist. The finding that some sites of Py transgene expression are susceptible to tumor formation while other expressing tissues such as testes are refractory to tumorigenesis suggests that mechanisms of viral pathogenesis are influenced by a tissue specificity in the effects of the Py early region.

Acquisition of endogenous ecotropic MuLV can occur before the late one-cell stage in the genital tract of SWR/J-RF/J hybrid females

Endogenous ecotropic MuLV proviral loci are acquired by the progeny of some [SWR/J x (SWR/J x RJ/J)F1] N2 hybrid females obtained by two successive backcrosses of RF/J mice onto the SWR/J background. This results most likely from an infection of early embryos or oocytes by MuLV particles originating from maternal tissues. However, the time and site of infection are not yet known. Using oviductal transfers of embryos at the one-cell stage, we show here that three of 88 N3 embryos from [SWR/J x (SWR/J x RF/J)F1] N2 hybrid females transferred to virus-free foster mothers harbored new proviral integrations, whereas none of 61 SWR/J embryos transferred to [SWR/J x (SWR/J x RF/J)F1] N2 hybrid females had acquired any proviruses. These data support the infection of oocyte and/or early one-cell embryo as the initial event leading to new proviral insertions.

Spontaneous germ line virus infection and retroviral insertional mutagenesis in eighteen transgenic Srev lines of mice

SWR/J-RF/J hybrid mice spontaneously acquire new germ line ecotropic proviruses at high frequency. In the studies described here, we used these hybrids to produce 18 transgenic mouse lines, each carrying a single newly acquired Srev locus (SWR/J-RF/J ecotropic proviral locus). All of the newly acquired proviruses identified in mosaic founder SWR/J-RF/J mice that could be transmitted through the germ line were also present in somatic tissues, demonstrating that viral integration occurred before the germ line was set aside from the somatic lineages. Quantitative analysis of proviral DNA copy numbers in somatic and germinal tissues of mosaic founder parents combined with structural analysis of Srev loci indicated that these proviruses are acquired after multiple rounds of somatic viral reinfection and that most of these viral integration events occurred after DNA replication in the zygote and before DNA replication in the four-cell embryo. The frequency of provirus acquisition in Srev lines that expressed the infectious ecotropic virus was similar to that in SWR.RF mice carrying Emv-16 and Emv-17, suggesting that the chromosomal integration site of the parental locus is not an important determinant for high-frequency provirus acquisition. The frequency of recessive lethal mutations induced by spontaneous viral integration was 5%, which was similar to that induced by preimplantation embryo infection. This approach represents a simple and viable strategy for inducing and studying mutations that affect mammalian development.

Pattern of expression of ecotropic murine leukemia virus in gonads of inoculated SWR/J mice

An ecotropic murine leukemia virus (MuLV) isolate has recently been shown to be able to infect the germ line or the early embryo or both when inoculated at birth to SWR/J females (J. J. Panthier, H. Condamine, and F. Jacob, Proc. Natl. Acad. Sci. USA 85:1156-1160, 1988). We have used this isolate to further study this phenomenon. By using the techniques of RNA-RNA in situ hybridization, immunocytochemistry, and transmission electron microscopy, the identities of two important cell types that are infected by ecotropic MuLV in the gonads of inoculated mice were determined. These cells are the thecal cells surrounding the follicles in the ovary and the Leydig cells in the testis. Both types actively synthesize viral RNA and express a viral antigen. Furthermore, we documented the production of viral particles by the thecal cells. The expression of ecotropic MuLV by nonlymphoid cells in vivo may play a key role in the vertical transmission of these viruses by females as well as in their horizontal transmission.

Segregation, viral phenotype, and proviral structure of 23 avian leukosis virus inserts in the germ line of chickens

We have artificially introduced 23 avian leukosis virus (ALV) proviral inserts into the chicken germ line by injection of wild-type and recombinant subgroup A ALV near the blastoderm of fertile eggs just before incubation. Eight viremic males were identified as germline mosaics because they transmitted proviral DNA to their generation 1 (G-1) progeny at a low frequency. Eleven female and 9 male G-1 progeny carried 23 distinct proviruses that had typical major clonal proviral-host DNA junction fragments detectable after digestion of their DNA with SacI, Southern blotting and hybridization with a probe representing the complete ALV genome. These proviruses, identified by their typical proviral-host DNA junction fragments, were transmitted to approximately 50% of their G-2 progeny after mating the G-1 parents to a line of chickens lacking endogenous ALV proviral inserts. One G-1 female carried 2 proviruses and another 3. The proviruses appeared to be scattered throughout the genome. One of the 14 proviruses carried by females was on the sex (Z) chromosome. Two of the 3 proviruses carried by a single G-1 female were linked with a recombination frequency of about 0.20. Twenty-one of the proviruses coded for infectious ALV. Two proviruses coded for envelope glycoprotein, and cell cultures carrying them were relatively resistant to subgroup A sarcoma virus, but failed to produce infectious ALV. One of these proviruses coded for internal gag proteins, had a deletion in pol, but produced non-infectious virus particles. The other failed to code for gag proteins and had no detectable internal deletions nor did it produce virus particles. Thus, we have shown that replication-competent ALV can artificially infect germ-line cells and that spontaneous defects in the inherited proviruses occur at a rather low rate.

Spontaneous germ line virus infection and retroviral insertional mutagenesis in eighteen transgenic Srev lines of mice

SWR/J-RF/J hybrid mice spontaneously acquire new germ line ecotropic proviruses at high frequency. In the studies described here, we used these hybrids to produce 18 transgenic mouse lines, each carrying a single newly acquired Srev locus (SWR/J-RF/J ecotropic proviral locus). All of the newly acquired proviruses identified in mosaic founder SWR/J-RF/J mice that could be transmitted through the germ line were also present in somatic tissues, demonstrating that viral integration occurred before the germ line was set aside from the somatic lineages. Quantitative analysis of proviral DNA copy numbers in somatic and germinal tissues of mosaic founder parents combined with structural analysis of Srev loci indicated that these proviruses are acquired after multiple rounds of somatic viral reinfection and that most of these viral integration events occurred after DNA replication in the zygote and before DNA replication in the four-cell embryo. The frequency of provirus acquisition in Srev lines that expressed the infectious ecotropic virus was similar to that in SWR.RF mice carrying Emv-16 and Emv-17, suggesting that the chromosomal integration site of the parental locus is not an important determinant for high-frequency provirus acquisition. The frequency of recessive lethal mutations induced by spontaneous viral integration was 5%, which was similar to that induced by preimplantation embryo infection. This approach represents a simple and viable strategy for inducing and studying mutations that affect mammalian development.

Inoculation of newborn SWR/J females with an ecotropic murine leukemia virus can produce transgenic mice

Endogenous ecotropic murine leukemia proviruses that were not present in the parental stock are acquired by the progeny of some SWR/J X RF/J hybrid females. We have made a stock of an ecotropic murine leukemia virus produced by such a hybrid female and inoculated newborn SWR/J females with it. We show that upon crossing of the inoculated females to SWR/J males, some of their progeny acquire ecotropic proviruses. Although most of these proviruses appear to be distributed in somatic tissues in a mosaic way, some are transmitted through the germ line. Thus an exogenous infection is able to mimic the phenomenon observed in SWR/J X RF/J hybrid mice. Available evidence suggests that this infection occurs during oogenesis in the recipient female. Our results document the conversion of an exogenous infectious ecotropic murine leukemia virus to an endogenous provirus without any manipulation of either eggs or embryos.

Endothelial cell tumors develop in transgenic mice carrying polyoma virus middle T oncogene

Inoculation of newborn mice with the murine polyoma (Py) virus leads to tumor formation in a wide range of tissues. In order to investigate viral oncogenesis, we generated transgenic mice carrying either the Py large T antigen (LT) gene or the Py middle T antigen (MT) gene linked to Py early region regulatory sequences. While Py LT mice exhibit no phenotype, Py MT mice develop multifocal tumors of the vascular endothelium. These hemangiomas are lethal to the animals and can be passaged in vivo. Transgene RNAs and protein are present in both hemangiomas and the testes of these mice, and the Py middle T protein in both tissues is complexed to a cellular tyrosine kinase. The expression of complexed middle T protein in both tumorigenic endothelial cells and unperturbed testes implies that endothelial cells may be particularly susceptible to the action of the middle T oncogene. These observations indicate that Py middle T disrupts the normal strict controls on vascular growth, and suggest that Py MT transgenic mice will provide a model for studying the control of angiogenesis.