Selectable Markers for Gene Therapy

One of the major problems with current approaches to gene 9 therapy is the instability of expression of genes transferred into 10 recipient cells. Although in theory, homologous recombination 11 or use of artificial chromosomes can stabilize sequences with 12 wild-type regulatory regions, such approaches to gene therapy 13 are not yet feasible and may not be efficient for some time to 14 come. In most high efficiency DNA transfer in current use in 15 intact organisms, selectable markers must be used to maintain 16 transferred sequences; in the absence of selection the trans17 ferred DNAs or their expression is rapidly lost. 18 There are several different selectable markers that might 19 be used for in vivo selection, including genes whose expres20 sion has been associated with resistance of cancers to antican21 cer drugs. Examples include: (a) methotrexate resistance due 22 to mutant dihydrofolate reductase [DHFR] (1); (b) alkylating 23 agent resistance due to expression of methylguanine methyl24 transferase [MGMT] (2); and (c) the expression of the multid25 rug transporting proteins P-glycoprotein (P-gp, the product of 26 theMDR1 gene) (3) andMRP (multidrug resistance associated 27 protein) (4). In this chapter, we will detail our experience with 28 the MDR1 gene. 29