Restoration of transgene expression in hematopoietic cells with drug-selectable marker genes.

Somatic gene therapy is supposed to cure life-threatening hematopoietic disorders but is limited by unstable transgene expression. Efficient gene transfer to hematopoietic progenitor cells does not ensure long-term gene expression. It would therefore be advantageous if the expression of transgenes could be restored in bone marrow. Transfer of drug resistance genes such as the multidrug resistance (MDR1) or mutated dihydrofolate reductase (DHFR) genes to hematopoietic cells protects them from the toxicity of anticancer drugs. In addition, transduced cells obtain a selective growth advantage in the presence of anticancer drugs. This can be used to introduce and enrich otherwise non-selectable genes by cotransfer to target cells. Bicistronic vectors have been constructed for coexpression of drug resistance genes and non-selectable, therapeutic genes with the use of an internal ribosomal entry-site (IRES). With the use of bicistronic vectors, expression and function of therapeutic genes have been increased in tissue culture and in animal models. Further preclinical investigations are needed to identify optimal conditions for selection.