Stability of retrovirally transduced markers in a rat cell line

Expression of exogenous DNA in rat liver cells after liposome-mediated transfection in vivo

Liposomes containing the expression vector pRSVneo coding for neomycin phosphotransferase-II were used for a gene transfer into rat liver cells in vivo. After intravenous application of liposomes to male Wistar-rats, non-integrated vector DNA was detected by blot-hybridisation in isolated nuclei of hepatocytes. Presence of messenger RNA specific for the transferred construct was demonstrated by Northern analyses of poly(A)+RNA. The biological activity of the neomycin phosphotransferase-II gene and its enzyme could be demonstrated in hepatocytes for at least 7d post injection. Liposome-mediated gene transfer into adult animals is discussed as a model system to study the expression and fate of transferred genes in adult rat hepatocytes under physiological conditions.

Inactivation of the Moloney murine leukemia virus long terminal repeat in murine fibroblast cell lines is associated with methylation and dependent on its chromosomal position

The expression of a retroviral vector with the Moloney murine leukemia virus (Mo-MuLV) long terminal repeat (LTR) promoter after integration into the genome of murine fibroblast cell lines was monitored with the Escherichia coli-derived beta-galactosidase (beta-gal) gene as the reporter. Monoclonal cell lines derived after retroviral infection exhibited a marked heterogeneity in their expression of the reporter gene. We studied two monoclonal cell lines with a single unrearranged copy of the vector provirus integrated into their genome. The first, BB10, expressed the marker enzyme in only 8% of its cell population, whereas in the second, BB16, beta-gal expression could be detected in over 98% of the cells. Treatment of BB10 with the DNA-demethylating agent 5-azacytidine raised the number of beta-gal-positive cells to over 60%. Transfection experiments showed that the Mo-MuLV LTR promoter-enhancer is potentially fully functional in both the BB10 and BB16 cell lines. The inactivated provirus from BB10 cells was cloned and subsequently used to generate retrovirus stocks. The promoter-enhancer activity of its LTR after infection with these BB10-derived viruses showed a variation similar to that of the original virus stocks. Our data showed that (1) inactivation of the Mo-MuLV LTR is a frequent event in murine fibroblast cell lines, (2) inactivation is associated with de novo methylation of cytidine residues, (3) the frequency of inactivation of the provirus must be determined by its chromosomal position, (4) the process of methylation of sequences within the LTR is not necessarily the same as the transcription-repression mechanism that is operating in undifferentiated embryonal carcinoma cells.

Construction and hormone regulation of a novel retroviral vector

We report the analysis of a self-inactivating retroviral vector, constructed to allow inducible gene expression of inserted sequences from the mouse mammary tumour virus hormonal response element. The cloning strategy has been designed to allow for ease of insertion of the genes of interest. The vector contains the aph gene, allowing geneticin-resistance selection in mammalian cells. We have characterised dexamethasone (Dex)-induced increase in gene expression using the reporter gene encoding chloramphenicol acetyltransferase (CAT) inserted into the retroviral vector. We observe low basal levels of CAT activity in infected cells which is increased up to 50-fold by induction with Dex. The induction of pooled clones is 13.3-fold. Variation in Dex-induced CAT activity is observed in independently infected clones, which is not explained by proviral copy number.

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.

Progress toward human gene therapy

Current therapies for most human genetic diseases are inadequate. In response to the need for effective treatments, modern molecular genetics is providing tools for an unprecedented new approach to disease treatment through an attack directly on mutant genes. Recent results with several target organs and gene transfer techniques have led to broad medical and scientific acceptance of the feasibility of this "gene therapy" concept for disorders of the bone marrow, liver, and central nervous system; some kinds of cancer; and deficiencies of circulating enzymes, hormones, and coagulation factors. The most well-developed models involve alteration of mutant target genes by gene transfer with recombinant pathogenic viruses in order to express new genetic information and to correct disease phenotypes--the conversion of the swords of pathology into the plowshares of therapy.