Physiological Promoters Reduce the Genotoxic Risk of Integrating Gene Vectors

Chemoprotection by transfer of resistance genes

Dose-limiting toxicity of chemotherapeutic agents, i.e., myelosuppression, can limit their effectiveness. The transfer and expression of drug-resistance genes might decrease the risks associated with acute hematopoietic toxicity. Protection of hematopoietic stem/progenitor cells by transfer of drug-resistance genes provides the possibility of intensification or escalation of antitumor drug doses and consequently an improved therapeutic index. This chapter reviews drug-resistance gene transfer strategies for either myeloprotection or therapeutic gene selection. Selecting candidate drug-resistance gene(s), gene transfer methodology, evaluating the safety and the efficiency of the treatment strategy, relevant in vivo models, and oncoretroviral transduction of human hematopoietic stem/progenitor cells under clinically applicable conditions are described.

Hematopoietic stem cell gene transfer for the treatment of hemoglobin disorders

Hematopoietic stem cell (HSC)-targeted gene transfer is an attractive approach for the treatment of a number of hematopoietic disorders caused by single gene defects. Indeed, in a series of gene transfer trials for two different primary immunodeficiencies beginning early in this decade, outstanding success has been achieved. Despite generally low levels of engrafted, genetically modified HSCs, these trials were successful because of the marked selective advantage of gene-corrected lymphoid precursors that allowed reconstitution of the immune system. Unlike the immunodeficiencies, this robust level of in vivo selection is not available to hematopoietic repopulating cells or early progenitor cells following gene transfer of a therapeutic globin gene in the setting of beta-thalassemia and sickle cell disease. Both preclinical and clinical transplant studies involving bone marrow chimeras suggest that 20% or higher levels of engraftment of genetically modified HSCs will be needed for clinical success in the most severe of these disorders. Encouragingly, gene transfer levels in this range have recently been reported in a lentiviral vector gene transfer clinical trial for children with adrenoleukodystrophy. A clinical gene transfer trial for beta-thalassemia has begun in France, and one patient with transfusion-dependent HbE/beta-thalassemia has demonstrated a therapeutic effect after transplantation with autologous CD34(+) cells genetically modified with a beta-globin lentiviral vector. Here, the development and recent progress of gene therapy for the hemoglobin disorders is reviewed.