Comment on Puthenveetil et al, page 3445
Investigators use gene therapy to express therapeutic levels of β-globin in primary erythroblasts from patients with β-thalassemia.
Gene therapy offers a potential curative approach for sickle cell anemia and β-thalassemia but has proven to be a difficult and elusive goal. It has recently been discovered that stable transmission of globin gene expression cassettes can be achieved using lentiviral vectors derived from the human immunodeficiency virus. These newer vectors have been used to correct mouse models of β-thalassemia1 and sickle cell anemia,2 important milestone achievements for the field. However, globin gene expression was still less than optimal and was influenced by the site of vector integration.3 Furthermore, it has not been clear how these vectors would perform in primary human erythroid cells, the ultimate target for gene expression.
In this issue of Blood, Puthenveetil and colleagues describe the use of a novel lentiviral vector for expressing a normal human β-globin gene in primary erythroblasts obtained from patients with β-thalassemia major. This vector contained a chromatin insulator fragment that flanked the globin expression cassette in order to shield the vector from locally repressive chromatin effects, a particularly troublesome problem in the developing erythroblast. Therapeutic levels of β-globin gene expression were obtained in transduced erythroblasts both in culture (Figure 4E) and in erythroid cells that had engrafted in immunodeficient mice. Expression of the transferred β-globin gene was remarkably consistent in clonally derived erythroid colonies, indicating that the insulator likely reduced position-site variability in gene expression. These results demonstrate continued progress in the design of globin vectors and establish the feasibility of obtaining therapeutic levels of β-globin in human thalassemic erythroblasts. Other issues remain to be addressed before a clinical trial can be expected, most notably the clinical safety of this approach.
Insertional mutagenesis remains a significant concern following the development of T-cell leukemia in a trial for X-linked severe common immunodeficiency (XSCID). In these cases, vector integration events contributed to transformation by causing aberrant expression of the LMO2 oncogene.4 This unintended gene activation was due, at least in part, to the strong viral enhancers located in the viral long terminal repeats. No viral enhancers are present in the lentiviral vectors used for globin gene therapy, but there are strong erythroid enhancers included in the globin transcription cassette. The presence of an insulator element provides another advantage; it should decrease the likelihood of unintended gene activation since insulators act by blocking enhancer effects on nearby promoters. It is also possible that globin gene therapy will be inherently safer than that for XSCID if specific risk factors exist for XSCID gene therapy and are absent in myeloid disorders. Another hurdle for gene therapy is the difficulty in transducing an adequate number of hematopoietic stem cells (HSCs). In this regard, it is notable that this study largely focused on transduced erythroblasts rather than HSCs. It will be important to validate new globin vectors in nonhuman primate models of HSC transplantation and to develop strategies for the selective amplification of transduced, genetically corrected HSCs.5