A Selective Amplifier Gene for Tamoxifen-Inducible Expansion of Hematopoietic Cells

Abstract

Background: We have developed a novel system for expansion of gene-modified hematopoietic stem/progenitor cells to overcome the low efficiency of current gene transfer methodology. This system involves 'selective amplifier genes', that encode fusion proteins between the granulocyte colony-stimulating factor receptor (GCR) and the hormone-binding domain of estrogen receptor (ER). Hematopoietic progenitors expressing the chimeras showed estrogen-responsive growth in a controllable manner. However, endogenous estrogen may activate the fusion proteins in vivo, depending on the hormonal status of the subjects. Methods: We replaced ER with a mutant receptor (TmR) which specifically binds to 4-hydroxytamoxifen (Tm), to overcome limitations with wild-type ER. Interleukin-3 (IL-3)-dependent Ba/F3 cells and hematopoietic progenitor cells transduced with the resultant fusion proteins (GCRTmR and ΔGCRTmR) were examined for ligand-inducible growth. Results: GCRTmR- and ΔGCRTmR-expressing Ba/F3 showed IL-3-independent growth in response to Tm, while the cells were unresponsive to estrogen at concentrations up to 10-7-10-6 M. Furthermore, murine bone marrow cells transduced with GCRTmR and ΔGCRTmR formed colonies in methyl-cellulose medium in response to Tm, while virtually no colonies appeared with 10-7 M estrogen or without cytokines. Conclusions: These results suggest that influences of the endogenous estrogen can be almost eliminated by using the GCRTmR/Tm or ΔGCRTmR/ Tm system to expand gene-modified hematopoietic stem/progenitor cells. Copyright © 1999 John Wiley & Sons, Ltd.