Expansion of genetically modified primary human hemopoietic cells using chemical inducers of dimerization

The inability to deliver a therapeutic gene to a sufficient percentage of hematopoietic stem cells is the major obstacle to using gene therapy to treat blood disorders. Providing genetically corrected stem cells with a reversible growth advantage could solve this problem. To this end we have employed small synthetic molecules that can reversibly dimerize and activate fusion proteins which contain a growth factor receptor signaling domain. We have shown that the thrombopoietin receptor (mpl) signaling domain can be used in this system to expand transduced multipotential progenitor cells from mouse bone marrow. In the present study we tested a similar retroviral vector in human CD34-selected cord blood cells. Following transduction, cells cultured in the presence of the dimerizing molecule AP1903 expanded 13.8- to 186-fold relative to cells cultured in the absence of AP1903. The cell type that emerged in suspension culture was erythroid. Contrary to our results in the murine system, cell expansion was transient. Activation of mpl caused the disappearance of BFU-E followed by a transient increase in CFU-E. In contrast, mpl activation had no discernable effect on transduced myeloid progenitor cells. AP1903-mediated expansion was restricted to transduced cells, as demonstrated by immunohistochemical staining. These findings indicate that synthetic dimerizing molecules can be used to expand primary human hematopoietic cells. (Blood. 2000;95:430-436)