C. HESDORFFER 4/93 
genes, as well as the human MDR gene (31,32), have all been 
successfully and stably transferred into the bone marrow cells of 
live mice, and, in most cases, expressed. 
B. The Use of the MDR Gene as a Selectable Marker ; 
The human MDR gene, when expressed in cells, provides 
resistance to a variety of commonly used anti-cancer compounds 
including the anthracyclines, vinca alkaloids, etoposides, and 
taxol (33,34). (These and other drugs susceptible to MDR 
activity are referred to as "MDR-responsive" agents or drugs.) 
The cDNA for the human MDR gene has been cloned and is expressed 
in both mouse and human cells in culture (35,36). Retroviruses 
containing the MDR gene are also available (37,38). The 
expression of this gene leads to resistance of cells in culture 
to mitomycin and colchicine (34,38). The MDR gene has been 
transferred into mouse and human bone marrow cells in culture and 
leads to drug resistance of the transduced cells (39) . A 
transgenic mouse line expressing the MDR gene only in the bone 
marrow is resistant to the effect of daunomycin, a drug that 
decreases the white blood cell count of normal mice (40,41). 
Inhibitors of MDR gene expression are available that lower the 
level of MDR and allow increased cell killing with MDR-responsive 
drugs (34,41,42). Specific oligonucleotide primers can be used 
to detect the MDR gene and its RNA product in cells by polymerase 
chain reaction (PCR) analysis (43) . Expression of the human MDR 
gene at the protein level can be detected by antibodies (44-46) . 
The MDR gene has recently been shown to be stably transferred and 
expressed in both MELC (47) and the bone marrow cells of live 
mice (31,32) . 
C. Methods to Increase Infection and Expression of 
Transferred Genes : 
Repeated administration of retrovirus containing an 
ADA gene in long-term bone marrow culture increases the 
efficiency of gene transfer (48) . Stromal cell lines support the 
growth of bone marrow cells optimally in short- and long-term 
cultures (49,50). Certain growth factors alone and in 
combination, primarily IL-3, IL-6, GM-CSF, and stem cell factor 
(SCF) , also known as mast cell factor and c-Kit ligand, enhance 
retroviral gene transfer into murine bone marrow stem cells (51- 
61) . These factors increase the cycling and proliferation of 
bone marrow stem cells (56-62) . 
Monoclonal antibodies have been used to enrich human bone marrow 
cells for early hematopoietic cells containing the majority of 
bone marrow stem cells (BM-SC) (63-67) . These BM-SC are defined 
here as cells capable of long-term reconstitution of all 
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