Gene therapy for the hemoglobin disorders: past, present, and future.

W ith the development in the early 1980s of technology to transfer genes to murine hematopoietic stem cells by using recombinant murine oncoretroviral vectors (1), the possibility of genetic therapy for a number of human disorders of the lympho-hematopoietic system seemed an attainable goal. Hemoglobin disorders were among the first diseases to be considered for gene therapy. Both sickle cell disease and the thalassemias are common monogenic disorders worldwide that cause serious morbidity and mortality. The pathophysiology of these disorders is amenable to correction by the addition of a functional globin gene to stem cells with subsequent high level expression in maturing erythroid cells (Fig. 1). Encouraging reports of successful transfer of a b-globin gene into the hematopoietic stem cells of mice using an oncoretroviral vector appeared in 1988 (2, 3). Erythroid lineage-specific expression of the globin gene was observed, but the levels of expression were much too low (,1% of mouse b-globin) for a potential therapeutic effect. Recent studies using murine models of both sickle cell disease and b-thalassemia and g-globin expressing transgenic mice substantiate the clinical impression that sustained expression of a transferred globin gene in the range of 10–20% of the level of endogenous globin in a majority of developing erythroblasts will be required for a therapeutic benefit (4).† Over the past 12 years, progress toward obtaining a globin vector capable of achieving these therapeutic levels of expression in the erythroid progeny of hematopoietic stem cells has been painstakingly slow. Only four additional reports have presented data consistent with the expression of a transferred globin gene in the erythroid progeny of genetically modified murine stem cells (5–8). Unfortunately, despite inclusion of elements from the locus control region (see below), the levels of expression obtained were subtherapeutic and subject to decay over time, suggesting silencing of the globin gene. In this issue of PNAS, Kalberer et al. use an updated b-globin gene vector coupled with preselection and transplantation of vector-expressing bone marrow cells to achieve, for the first time, sustained vector-encoded globin gene expression over time in a cohort of long-term murine transplant recipients (9). Despite preselection of vector-expressing stem cells for transplantation, significant vector silencing andyor position effect variegation (PEV) of expression still occurred, and the overall levels of human b-globin gene expression averaged only 3% of endogenous globin in long-term recipients.