Selection with a regulated cell growth switch increases the likelihood of expression for a linked gamma-globin gene.

Several lines of evidence indicate that in vivo drug selection can be used to overcome the low rates of gene transfer and engraftment encountered in many hematopoietic stem cell gene therapy settings. However, whether selection imposed on one transcription cassette effects the likelihood of expression from a second, independent transcription cassette within the same vector has been less well studied. In order to address this issue, we engineered an oncoretrovirus vector to express two separate transcription units: (i) a bicistronic cassette encoding both GFP and a pharmacologically regulated cell growth switch based on the thrombopoietin receptor Mpl; and (ii) a highly position-dependent second cassette encoding human gamma-globin.