A replication-competent promoter-trap retrovirus

Effects of viral strain, transgene position, and target cell type on replication kinetics, genomic stability, and transgene expression of replication-competent murine leukemia virus-based vectors

The limited efficiency of in vivo gene transfer by replication-deficient retroviral vectors remains an obstacle to achieving effective gene therapy for solid tumors. One approach to circumvent this problem is the use of replication-competent retroviral vectors. However, the application of such vectors is at a comparatively early stage and the effects which virus strain, transgene cassette position, and target cell can exert on vector spread kinetics, genomic stability, and transgene expression levels remain to be fully elucidated. Thus, in this study a panel of vectors allowing the investigation of different design features on an otherwise genetically identical background were analyzed with respect to these readout parameters in cultures of both murine and human cells and in preformed tumors in nude mice. The obtained data revealed that (i) Moloney murine leukemia virus (Mo-MLV)-based vectors spread with faster kinetics, drive higher levels of transgene expression, and are more stable than equivalent Akv-MLV-based vectors; (ii) vectors containing the transgene cassette directly downstream of the envelope gene are genomically more stable than those containing it within the 3'-long terminal repeat U3 region; and (iii) the genomic stability of both strains seems to be cell line dependent.

Highly efficient and tumor-restricted gene transfer to malignant gliomas by replication-competent retroviral vectors

The first large randomized phase III trial in gene therapy demonstrated no improvement in the survival of patients injected with packaging cells that produced conventional replication-defective retroviral vectors carrying the herpes simplex virus thymidine kinase gene, a disappointing result that was attributed to extremely poor levels of transduction efficiency. To circumvent this problem, we have developed a modified replication-competent retrovirus (RCR) that is capable of transducing human glioma cell lines A-172, U-87, T-98G, U-373, and U-138 and rat glioma cell lines C6 and 9L, over multiple infection cycles in vitro, resulting in a tremendous enhancement in transduction efficiency over conventional replication-defective retroviral vectors at the same dose. Whereas the transduction efficiency of conventional retroviral vectors injected into preestablished subcutaneous U-87 tumors at a dose of 1.0 x 10(5) transducing units (TU) was only 0.2% at 6 weeks postinjection, the same dose of RCR vector resulted in up to 97.2% transduction. When RCR vectors at a dose of 1.0 x 10(4) TU were injected into preestablished intracranial U-87 tumors, transduction efficiency at 2 and 3 weeks was 74 and 98.1%, respectively. Notably, however, intracranial injection of RCR vectors did not result in detectable infection of normal brain cells. Furthermore, using a sensitive polymerase chain reaction assay, no detectable RCR signal could be observed in any extracerebral tissues, including lung, liver, kidney, upper gastrointestinal tract (esophagus and stomach), lower gastrointestinal tract (colon and small intestine), skin, spleen, and bone marrow. Treatment of U-87 intracranial gliomas with RCR vectors carrying the yeast cytosine deaminase suicide gene followed by 5-fluorocytosine prodrug administration resulted in 100% survival over a 60-day follow-up period, compared with 0% survival of control groups receiving vector alone or prodrug alone. Our results demonstrate that RCR vectors can achieve therapeutically significant levels of transduction in malignant human gliomas, and that RCR vector spread after intratumoral injection is restricted to the tumor itself.

A replication-competent feline leukemia virus, subgroup A (FeLV-A), tagged with green fluorescent protein reporter exhibits in vitro biological properties similar to those of the parental FeLV-A

We previously established that lymphoid tumors could be induced in cats by intradermal injection of ecotropic feline leukemia virus (FeLV), subgroup A, plasmid DNA. In preparation for in vivo experiments to study the cell-to-cell pathway for the spread of the virus from the site of inoculation, the green fluorescent protein (GFP) transgene fused to an internal ribosome entry site (IRES) was inserted after the last nucleotide of the env gene in the ecotropic FeLV-A Rickard (FRA) provirus. The engineered plasmid was transfected into feline fibroblast cells for production of viruses and determination of GFP expression. The virions produced were highly infectious, and the infected cells could continue to mediate strong expression of GFP after long-term propagation in culture. Similar to parental virus, the transgene-containing ecotropic virus demonstrated recombinogenic activity with endogenous FeLV sequences in feline cells to produce polytropic recombinant FeLV subgroup B-like viruses which also contained the IRES-GFP transgene in the majority of recombinants. To date, the engineered virus has been propagated in cell culture for up to 8 months without diminished GFP expression. This is the first report of a replication-competent FeLV vector with high-level and stable expression of a transgene.

A uniquely stable replication-competent retrovirus vector achieves efficient gene delivery in vitro and in solid tumors

A major obstacle in cancer gene therapy is the limited efficiency of in vivo gene transfer by replication-defective retrovirus vectors in current use. One strategy for circumventing this difficulty would be to use vectors capable of replication within tumor tissues. We have developed a replication-competent retrovirus (RCR) vector derived from murine leukemia virus (MuLV). This vector utilizes a unique design strategy in which an internal ribosome entry site-transgene cassette is positioned between the env gene and the 3' long terminal repeat (LTR). The ability of this vector to replicate and transmit a transgene was examined in culture and in a solid tumor model in vivo. The RCR vector exhibited replication kinetics similar to those of wildtype MuLV and mediated efficient delivery of the transgene throughout an entire population of cells in culture after an initial inoculation with 1 plaque-forming unit (PFU) of vector per 2000 cells. After injection of 6 x 10(3) PFU of vector into established subcutaneous tumors, highly efficient spread of the transgene was observed over a period of 7 weeks, in some cases resulting in spread of the transgene throughout the entire tumor. MuLV-based RCR vectors show significant advantages over standard replication-defective vectors in efficiency of gene delivery both in culture and in vivo. This represents the first example of the use of an RCR vector in an adult mammalian host, and their first application to transduction of solid tumors.

Expression of heterologous genes from an IRES translational cassette in replication competent murine leukemia virus vectors

We describe replication competent retroviruses capable of expressing heterologous genes during multiple rounds of infection. An internal ribosome entry site (IRES) from encephalomyocarditis virus was inserted in the U3 region of Akv- and SL3-3-murine leukemia viruses (MLV) to direct translation of neo or the enhanced green fluorescence protein gene (EGFP). Akv-MLV's with IRES-neo and IRES-EGFP cassettes replicated with titers of about 10(6) infectious units/ml while SL3-3-MLV with IRES-neo gave about 10(3)-fold lower titers. Interestingly, RNA analysis showed a drastic reduction in the amount of spliced env mRNA for the SL3-3 derived vector relative to the Akv derived vectors, seemingly contributing to its low replication capacity. The EGFP expressing Akv-MLV was genetically stable for multiple rounds of infection; marker-cassette deletion revertants appeared after several replication rounds and these revertants only slowly became dominant in the virus population.