Generation of competent virus in the REV helper cell line C3

Consistent production of transgenic chickens using replication-deficient retroviral vectors and high-throughput screening procedures

We have developed a novel method of DNA extraction combined with a high-throughput method of gene detection allowing thousands of potentially transgenic chicks to be screened quickly and reliably. By using this method and a replication-deficient retroviral vector based on avian leukosis virus (ALV), we have demonstrated germline transmission of three different transgenes. Several generations of chickens carrying intact transgenes were produced, validating the use of the ALV retroviral vectors for large-scale production of transgenic flocks. Fourth-generation chicks that were nontransgenic, hemizygous, or homozygous for the transgene were identified with the combined genetic screening methods.

To fuse or not to fuse: the effects of electrostatic interactions, hydrophobic forces, and structural amphiphilicity on protein-mediated membrane destabilization

The development of lipid-based delivery vehicles for therapeutic molecules has become a topic of intense research. Recently, much of this effort has been directed towards mimicking the characteristics of viruses that give them an advantage for the delivery of genetic medicines. One of the most desirable properties of viral-based vectors is the ability to promote the destabilization of the host cell membrane to allow the entry of the genetic medicine into the target cell. This has been found to be largely controlled by the coat proteins on the surface of enveloped viruses. Although the exact mechanism by which proteins involved in the fusion process are able to promote the destabilization of membranes has yet to be elucidated, much understanding based upon information gained from a wide variety of studies is advancing the state of knowledge in this area. Parameters such as hydrophobic and electrostatic interactions as well as structural amphiphilicity, control to a large extent, the nature of the interaction of proteins with membranes. Thus, membrane fusion is mediated primarily by these forces acting in concert with one another. Ultimately, the knowledge gained from these studies will help to develop the ideal delivery system for the next generation of therapeutics.

Selectively infective phages (SIP)

We review here advances in the selectively infective phage (SIP) technology, a novel method for the in vivo selection of interacting protein-ligand pairs. A 'selectively infective phage' consists of two components, a filamentous phage particle made non-infective by replacing its N-terminal domains of gene3 protein (g3p) with a ligand-binding protein, and an 'adapter' molecule in which the ligand is linked to those N-terminal domains of g3p which are missing from the phage particle. Infectivity is restored when the displayed protein binds the ligand and thereby attaches the missing N-terminal domains of g3p to the phage particle. Phage propagation becomes strictly dependent on the protein-ligand interaction. This method shows promise both in the area of library screening and in the optimization of peptides or proteins.

Partial reconstitution of a replication-competent retrovirus in helper cells with partial overlaps between vector and helper cell genomes

Contamination of retroviral vector preparations with replication-competent retroviruses is a major safety concern in human gene therapy. These can arise by recombination between retroviral vectors and packaging cell sequences. Recently, we constructed new packaging lines, derived from spleen necrosis virus (SNV) that do not contain overlapping regions of homology between these two components (DSH134G and DSH29B cells). These cell lines were tested for the presence of recombination products and replication-competent viruses in comparison to a similar packaging line (DSN) that contains a partial overlap between vector and viral protein coding regions. No recombination products were detected in DSH cells. However, we found that recombination had occurred in DSN cells, partially reconstituting a provirus-like structure that was capable of being spread, although inefficiently, through an infected cell culture. Our data indicate that even small regions of sequence homology eventually allow homologous recombination between vector and helper cell genomes.

The new frontier: gene and oligonucleotide therapy

Gene and oligonucleotide therapy are emerging as clinically viable therapeutic regimens for genetic, neoplastic, and infectious diseases. Approaches include insertion of human genes in viral vectors including recombinant retrovirus, adenovirus, adeno-associated virus, and herpes simplex virus-1, or recombinant bacterial plasmids. Viral vectors transfect cells directly; plasmid DNA is delivered with the help of cationic liposomes (lipofection), polylysine conjugates, gramicidin S, artificial viral envelopes or other such intracellular carriers. Major areas of interest include replacement of the cystic fibrosis transmembrane regulator gene and the alpha 1-antitrypsin gene; arrest of human immunodeficiency virus infection; and reversal of tumorigenicity and cancer immunization, among others. Oligonucleotide therapy is principally focusing on the same areas, although the approach is to halt DNA transcription or messenger RNA translation with code-blocking triple-helix-forming or "antisense" oligomers. Contributions from the pharmaceutical sciences are expected in pharmaceutical chemistry, drug delivery systems design, analytical chemistry, and biopharmaceutics.

Unusually high frequency of reconstitution of long terminal repeats in U3-minus retrovirus vectors by DNA recombination or gene conversion

Recently, we described a retrovirus vector system with which to study formation of cDNA genes (R. Dornburg and H. M. Temin, Mol. Cell. Biol. 6:2328-2334, 1988; Mol. Cell. Biol. 8:64-72, 1990; J. Virol. 64:886-889, 1990). For these studies, retrovirus vectors were constructed in which the U3 region of the 3' long terminal repeat (LTR) was deleted. After one round of retrovirus replication, such vectors formed a provirus with two U3-minus LTRs. However, the insertion of some additional sequences into such vectors promoted vector rearrangements with an efficiency greater than 95%. Such rearranged vectors behaved like vectors with two wild-type LTRs. Proviruses derived from such vectors were investigated by Southern blot analysis, polymerase chain reaction, and DNA sequencing. We found that the U3 region was reconstituted, resulting in vectors with LTRs like wild-type virus. The sequences that reconstituted the U3 region of the vector LTR were derived from LTR sequences present in the helper cell. Since no retroviral protein coding sequences were detected in infected target cells, recombination of vector sequences with coencapsidated helper cell sequences during reverse transcription seems very unlikely. Thus, it appears that the recombination (or gene conversion) events leading to a vector with reconstituted LTRs occurred at the DNA level. The high frequency of this recombination (or gene conversion) was dependent on internal vector sequences.

Characterization and comparison of avian and murine helper cell lines for production of replication-defective retroviruses for avian transformation

Several approaches were taken to identify improved helper cell lines for the production of replication-defective avian retroviral vectors for avian transformation. Both QT6 and D17 cells were engineered to become helper cell lines for the production of reticuloendotheliosis virus vectors. The results showed that the majority of lines from the D17, QT6, and D17C3 cells produced titers in the 10(2) to 10(3) cfu/mL range, with one QT6 line producing 10(5) cfu/mL. This high producer line was relatively free of helper virus when restricted to low passage. An amphotropic murine cell line produced a 6- to 10-fold higher amount of virus and had a comparable higher titer on chicken cells, suggesting possible application to avian transformation.

Retrovirus packaging cells

Retroviral vectors promote the efficient transfer of genes into a variety of cell types from many animal species. An important contribution to their utility was the development of retrovirus packaging cells, which allow the production of retroviral vectors in the absence of replication-competent virus. Because of their ability to transfer genes efficiently into cells that are difficult to transfect by other methods, retroviral vectors are prime candidates for gene transfer into human somatic cells. Indeed, a retroviral vector recently has been used to mark tumor infiltrating lymphocytes in patients with melanoma to follow the persistence and distribution of these cells following infusion into patients. Hopefully these vectors will soon be used for the treatment of disease by transfer of functional genes, or gene therapy. Here I will review the available packaging cell lines and their properties with a focus on their ultimate application to human gene therapy.

New retrovirus helper cells with almost no nucleotide sequence homology to retrovirus vectors

We prepared retrovirus packaging cell lines containing gag-pol genes from spleen necrosis virus (expressed from a cytomegalovirus promoter and the simian virus 40 (SV40) polyadenylation sequences) and, on a separate vector, either the env gene from spleen necrosis virus (expressed from the Rous sarcoma virus promoter and the SV40 polyadenylation sequences) or the env gene from amphotropic murine leukemia virus (expressed from a cytomegalovirus promoter and the SV40 polyadenylation sequences). The nucleotide sequences in these packaging cell lines have almost no homology to the retrovirus vectors we used. Retrovirus vectors were produced from these new helper cell lines without any genetic interactions between the vectors and sequences in the helper cells and without transfer of the packaging sequences.

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.

Germline transmission of exogenous genes in the chicken

Difficulties associated with in vitro manipulation and culture of the early chicken embryo have restricted generation of transgenic chickens to approaches that use replication-competent retroviruses. The need to produce transgenic chickens in the absence of replicating virus prompted development of a new method of gene transfer into the chicken. Microinjection of the replication-defective reticuloendotheliosis virus (REV) vector ME111 beneath unincubated chicken embryo blastoderms results in infection of germline stem cells. This vector contains genetic information exogenous to the chicken genome, including both the herpes simplex virus type 1 thymidine kinase gene and the Tn5 neomycin phosphotransferase gene. About 8 percent of male birds hatched from injected embryos contained vector DNA in their semen. All four positive males tested passed vector sequences onto their progeny. Analysis of G1 offspring showed that gonads of G0 male birds were mosaic with respect to insertion of vector provirus. Thus, primordial germ cells present in the unincubated chicken embryo blastoderm are susceptible to infection by defective REV vectors.