Progress Toward Vector Design for Hematopoietic Stem Cell Gene Therapy

Hematopoietic stem cells (HSCs) are attractive targets for gene therapy because of their capacity for self renewal and the wide systemic distribution of their progeny. Sustained expression of transgenes at clinically relevant levels in the progeny of HSCs would provide novel and potentially curative treatments for a wide range of inherited and acquired blood diseases. Recent improvements in retroviral transduction protocols have resulted in the first successful amelioration of a human hematologic disease-a form of severe combined immunodeficiency-by HSC gene transfer. However, continued advances in gene transfer technology are necessary if the inherent promise of HSC gene therapy is to be fully realized. Ongoing efforts are focused on modifying oncoretroviral vector designs and pseudotyping with alternative envelope proteins. In addition, because of their ability to transduce non-divided cells, safety-modified human immunodeficiency virus-1-based lentiviral vectors have emerged as promising tools for gene modification of HSCs, which reside primarily in the G0_G1 phase of the cell cycle. Irrespective of these advances, accumulated data indicate that stably integrated transgenes are frequently subject to position-effect variegation and extinction of expression. Therefore, the extent to which genetic control elements such as chromatin domain insulators and scaffold_matrix attachment regions in conjunction with posttranscriptional regulatory elements will result in enhanced probability and level of transgene expression is under active investigation. Collectively, these developments increase the likelihood that HSC gene transfer will ultimately become an effective therapeutic strategy.