Rapid generation of a tetracycline-inducible BCR-ABL defective retrovirus using a single autoregulatory retroviral cassette

Abstract

The development of chronic myelogenous leukemia (CML) models in mice using an inducible BCR-ABL gene has been hampered by the requirement of sequential expression of tTA (Tet repressor-VP16 fusion protein) and Tet-OP sequences in the same cells after separate transfection. This double transfection strategy is time consuming as it requires screening of many hundreds of individual clones and cannot be applied to primary hematopoietic cells. To generate a tetracycline-inducible BCR-ABL retrovirus, we have subcloned BCR-ABL p210 cDNA in the SIN-Retro-TET vector, which allows regulated expression of a gene of interest in a single autoregulatory cassette, containing both tTA and Tet OP sequences. Retroviral particles were obtained by transfecting the SIN-BCR-ABL p210 construct into the 293 cells and by VSVG pseudotyping. To determine the functionality of the retrovirus, the IL-3-dependent murine Ba/F3 cell line was retrovirally transduced and clones were grown in the absence of both IL-3 (to select for transformed cells) and a tetracycline analog, doxycycline (to induce BCR-ABL expression). Using this technique, polyclonal Ba/F3 cells and several growth factor-independent Ba/F3 clones expressing BCR-ABL were obtained within 2–3 weeks. A single dose of doxycycline added to the medium (1 μg/ml), induced in different clones, a reduction of BCR-ABL protein levels by 60–90% at 24 h, leading to cell death in the absence of IL-3. In several individual clones, BCR-ABL expression was further reduced to become almost undetectable at 48 h. The doxycycline-regulated BCR-ABL expression was stable, as many clones maintained in culture for >8 months showed a persistent inhibitory response to doxycycline addition in the medium. In in vivo experiments, subcutaneous injection of 2 × 10⁶ Ba/F3-SIN p210 cells in nude mice induced visible tumors in 2 weeks and all established tumors completely regressed upon addition of doxycycline in the drinking water (200 μg/ml). To determine the functionality of the inducible BCR-ABL retrovirus in vivo, primary Lin⁻ bone marrow cells were transduced with SIN-p210 and transplanted in lethally irradiated mice. All transplanted mice had successful hematopoietic reconstitution and BCR-ABL integration was found in the peripheral blood of seven out of 14 mice available for long-term analysis (>6 months). However, despite evidence of retrovirus-mediated gene transfer, there was no evidence of leukemia, due either to low viral titers or to the relative inefficiency of the minimal CMV promoter in primary hematopoietic cells. Thus, these results demonstrate for the first time, to our knowledge, the feasibility to generate an inducible BCR-ABL retrovirus in a single step, in the context of an immortalized cell line. Our data suggest that with further improvements of the retrovirus-mediated gene transfer technology, it might be possible to generate inducible leukemia models in mice by the use of single retroviral constructs.