Construction of a defective retrovirus containing the human hypoxanthine phosphoribosyltransferase cDNA and its expression in cultured cells and mouse bone marrow

Mutatect: a mouse tumour model for detecting radiation-induced mutations in vivo

A new mouse model (Mutatect) that permits detection of mutations at the hprt (hypoxanthine phosphoribosyltransferase) locus is described. It is highly sensitive to detection of mutants induced by clastogenic agents such as ionizing radiation. MN-11 cells are grown as a subcutaneous tumour in C57BL/6 mice for a period of 2 weeks, during which time they can be exposed to mutagenic treatments. Cells taken from the animal are cultured ex vivo and 6-thioguanine (6-TG)-resistant mutant clones can be readily identified and scored. This model system may have special utility for detecting multi-locus deletion events (chromosomal mutations) induced by high LET forms of radiation that might be encountered in space.

Effects of infection rate and selection pressure on gene expression from an internal promoter of a double gene retroviral vector

Many commonly used retroviral vectors express one gene from the viral long terminal repeat (LTR) promoter and another gene from an internal promoter. We have investigated factors affecting the expression of the luciferase reporter gene from the internal cytomegalovirus-derived promoter of the retroviral vector, LNCX, which contains a LTR-driven neo gene as a selectable marker. A subline of human HT1080 cells, expressing the murine ecotropic receptor, was infected with retrovirus generated by transient transfection of BOSC 23 packaging cells. Mass populations of cells infected under conditions resulting in different initial infection rates (IIR) and selected with G418, showed highly variable luciferase activity. Luciferase expression in cell populations with IIR < or = 5% was generally low; many populations with IIR < 1% had marginal or no luciferase activity. The loss of luciferase expression in low-IIR populations was associated with G418 selection. In contrast, cell populations with IIR > or = 6% showed higher luciferase expression, which was strongly correlated with the IIR. Southern hybridization analysis showed that most cells of the low-IIR populations carried one integrated provirus, with a high incidence of structural rearrangements that abolished luciferase activity. In contrast, populations with IIR > or = 6% contained two or more copies of integrated provirus per cell, and their luciferase activity correlated with the provirus copy number. Luciferase expression was relatively stable in the populations with IIR > 1% maintained in the absence of G418. Increasing the selective concentration of G418 or prolonged maintenance of cell populations in the presence of G418 resulted in higher incidence of provirus rearrangements and decreased luciferase expression. These results indicate that the negative effect of selection for the LTR-driven gene on gene expression from an internal promoter depends on the selection stringency and can be obviated by increasing the infection rate.

Flow cytometric titration of retroviral expression vectors: comparison of methods for analysis of immunofluorescence histograms derived from cells expressing low antigen levels

Few quantitative studies addressing immunofluorescence histogram analysis have been published. One study by Overton (Cytometry 9:619-626, 1988) has shown threshold and histogram subtraction methods to be accurate for analysis of well-separated immunofluorescence distributions of positive and negative cells. An evaluation of methods to analyze immunofluorescence histograms when positive and negative immunofluorescence distributions overlap has not, to our knowledge, been reported. In this paper, data obtained from flow cytometry of immunofluorescently stained cells infected with recombinant retroviruses that produce a range of simian virus 40 large T antigen levels were analyzed by threshold, histogram subtraction, and distribution modeling methods. This analysis showed that as the separation between the immunofluorescence distributions of positive and negative cell populations decrease the best methods for histogram analysis are modeling followed, in order, by histogram subtraction, and threshold analysis.

Expression of introduced genetic sequences in hematopoietic cells following retroviral-mediated gene transfer

The use of retroviral vectors allows efficient transfer of genes into a variety of mammalian cells. A focus of research over the past 6 years has been the use of retroviral vectors to effect gene transfer into hematopoietic cells. These transduced cells might then be used for gene therapy of severe genetic diseases affecting blood cells. In spite of early optimism concerning the transfer and expression of a variety of gene sequences in hematopoietic cells, progress in obtaining the goal of stable and long-term expression of introduced genes in progeny of hematopoietic stem cells has been slow, frustrating, and only partially successful. This slow progress has been due, in part, to lack of understanding of the control of gene regulation in primary cells but also to the complexity of hematopoietic stem cell biology in both murine and large animal species. This review attempts to summarize the progress that has been made in the expression of genes introduced into hematopoietic cells and the difficulties still remaining before meaningful application of gene transfer methods can be expected to cure human diseases of bone marrow-derived cells.

Resolution of the pathway taken by implanted Schwann cells to a spinal cord lesion by prior infection with a retrovirus encoding beta-galactosidase

This series of experiments is designed to follow the fate of implanted Schwann cells by first labeling them with a recombinant retrovirus encoding the bacterial beta-galactosidase gene, then injecting them into the spinal cord after a demyelinating lesion has been produced. The label provides a means of distinguishing the exogenous Schwann cells from endogenous ones and of determining their travel pattern and myelinating or ensheathing behavior in the central nervous system (CNS). Neonatal rat primary Schwann cells were stimulated to divide by administering glial growth factor and forskolin. Fresh virus-containing supernatant from Psi2 cells producing retrovirus LZ1 was placed in cell culture to label the cells. The capacity of infected Schwann cells to form myelin was verified by coculturing in vitro with neurons from embryonic dorsal root ganglia. Infected cells were injected into the right side of adult syngenic rat spinal cords after a lysolecithin-induced demyelinating lesion had been produced 1 cm caudal on the left side. After 3 weeks the animals were killed, perfused for electron microscopy, and spinal cord sections histochemically stained for beta-galactosidase activity using the chromogenic substrate 5-bromo-4-chloro-3-indoyl-beta-D-galactosidase (X-Gal) which forms a blue precipitate in infected cells. The labeled cells, easily recognized macro- and microscopically, were clustered at the cell injection site, in the dorsal meninges and, at the area of demyelination, bilaterally in the superficial aspect of the dorsal funiculi. Labeled cells were not evident in the neuropil midway between the injection and demyelination sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of retroviral packaging lines for generation of replication-competent virus

Amplification of retroviral vector sequences occurs in cocultures of ecotropic and amphotropic packaging cell lines. Mixed packaging line cocultures were used to determine both the host range and time of appearance of replication-competent virus after introduction of a retroviral vector. Replication-competent virus was generated at a characteristic time for a given ecotropic and amphotropic packaging line combination. The time required to generate replication-competent virus in a packaging line varied with the number of recombination events necessary to generate replication-competent virus from the retroviral sequences present in the line. The psi 2 packaging line generated replication-competent virus within 10 days after transfection of the N2 vector into a mixture of psi 2 (ecotropic) and PA317 (amphotropic) packaging cells. Under the same conditions, it took only 3 days to develop replication-competent virus in psi 2/PA12 cocultures. The host range of replication-competent virus was used to identify the packaging line that initially generates virus. Each packaging line combination generated replication-competent virus at a characteristic time and this time period can be used as a measure of the "safety" of the packaging line and vector combination.

Expression of human HPRT mRNA in brains of mice infected with a recombinant herpes simplex virus-1 vector

Complete deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) results in a devastating neurological disease, the Lesch-Nyhan syndrome. This disorder has been identified as a candidate for initial attempts at somatic cell gene therapy. We have previously reported the construction of a recombinant herpes simplex virus type 1 (HSV-1) vector containing human hprt cDNA sequences under the regulatory control of the viral thymidine kinase gene (tk) [Palella et al., Mol. Cell. Biol. 8 (1988) 457-460]. Infection of HPRT- cultured rat neuronal cells with these vectors resulted in transient expression of human hprt. In this paper, we report the expression of human hprt mRNA transcripts in the brains of mice infected in vivo with this vector by direct intracranial inoculation. Human hprt transcripts were distinguished from endogenous mouse transcripts by RNase A mapping using riboprobes transcribed from human hprt cDNA. These initial studies demonstrate the transfer and transcription of a human gene in brain cells by direct in vivo infection with recombinant HSV-1 vectors.

Factors affecting long-term stability of Moloney murine leukemia virus-based vectors

We have examined the long-term functional and structural stability of retroviral vectors in infected murine cells. We have used Moloney murine leukemia virus-based vectors expressing human HPRT, firefly luciferase (luc), and Escherichia coli beta-galactosidase (lacZ) as reporter genes, and the human HPRT and the transposon Tn5 neomycin resistance (neo) gene as selectable markers. All vectors, whether single or double gene, yielded both stable and unstable clones. Stability of the proviruses was dependent on a number of factors, including the nature of the infected cell, the reporter gene, the integration site of the provirus, the relative positions of the component genes in multigene vectors, and the presence or absence of selection pressure. Selection pressure was helpful, but not universally effective, in maintaining provirus structural and functional integrity. Reporter gene expression from an internal promoter was likely to be unstable with or without selection for an upstream, LTR-driven neo gene. In some clones, loss of proviral gene expression was accompanied by deletions, while other inactive clones retained an apparently intact provirus. In the latter clones, treatment with 5-azacytidine failed to reactivate the reporter genes, but superinfection with helper virus resulted in the reappearance of transmissible vector, indicating a reversible epigenetic mechanism for proviral shutdown. The design of effective retroviral vectors and their possible use in vivo will require further characterization of these determinants of provirus stability.

Infection of bovine cells of embryonic origin by amphotropic retroviral vectors

Two amphotropic-based mouse retroviral vectors carrying the neomycin-resistance gene were used to infect four bovine cell lines. Two cell lines, bovine kidney and spleen cells, were refractory to the infection while two independent bovine cells of apparent embryonic origin were infected by the amphotropic retroviral vectors at a measurable titer. Southern blot analysis reveals the presence of neomycin-resistance gene in the G418-resistant bovine cells. The results demonstrate the successful transfer of a gene to bovine cells of embryonic origin using a murine retroviral vector system.

Prospects for gene therapy now and in the future

Advances in gene technology and cell biology have supplied the means to undertake human gene therapy in the near future. Techniques have been developed for the efficient introduction of gene sequences into the pluripotential stem cells of the haematopoietic system and our increased understanding of gene-regulatory mechanisms should allow therapeutic gene expression levels to be obtained. Gene therapy should, at present, be termed gene supplementation since it will involve the addition of corrective genes to the host cell genome. It may only be used to treat recessively inherited disorders. Prospects for the future include the use of homologous recombination to correct or replace defective genes, allowing the treatment of dominantly inherited diseases.

Lineage-specific expression of a human beta-globin gene in murine bone marrow transplant recipients reconstituted with retrovirus-transduced stem cells

Recombinant retroviral genomes encoding a chromosomal human beta-globin gene have been used to transduce murine haematopoietic stem cells in vitro. After permanent engraftment of lethally irradiated recipients with the transduced cells, the human beta-globin gene is expressed at significant levels only within the erythroid lineage. These results indicate that it is possible to obtain stable expression of exogenous chromosomal DNA sequences introduced into mature haematopoietic cells in vivo via stem cell infection, and that human disorders of haemoglobin production may be more feasible candidates for somatic cell gene therapy than previously suspected.