Stem Cell Transplantation for Sickle Cell Disease: Can We Reduce the Toxicity?

Long-Term Engraftment and Fetal Globin Induction upon BCL11A Gene Editing in Bone-Marrow-Derived CD34+ Hematopoietic Stem and Progenitor Cells

To develop an effective and sustainable cell therapy for sickle cell disease (SCD), we investigated the feasibility of targeted disruption of the BCL11A gene, either within exon 2 or at the GATAA motif in the intronic erythroid-specific enhancer, using zinc finger nucleases in human bone marrow (BM) CD34⁺ hematopoietic stem and progenitor cells (HSPCs). Both targeting strategies upregulated fetal globin expression in erythroid cells to levels predicted to inhibit hemoglobin S polymerization. However, complete inactivation of BCL11A resulting from bi-allelic frameshift mutations in BCL11A exon 2 adversely affected erythroid enucleation. In contrast, bi-allelic disruption of the GATAA motif in the erythroid enhancer of BCL11A did not negatively impact enucleation. Furthermore, BCL11A exon 2-edited BM-CD34⁺ cells demonstrated a significantly reduced engraftment potential in immunodeficient mice. Such an adverse effect on HSPC function was not observed upon BCL11A erythroid-enhancer GATAA motif editing, because enhancer-edited CD34⁺ cells achieved robust long-term engraftment and gave rise to erythroid cells with elevated levels of fetal globin expression when chimeric BM was cultured ex vivo. Altogether, our results support further clinical development of the BCL11A erythroid-specific enhancer editing in BM-CD34⁺ HSPCs as an autologous stem cell therapy in SCD patients.

Hydroxyurea treatment for sickle cell disease: impact on haematopoietic stem cell transplantation's outcome

Since 1988, 24 children have undergone haematopoietic stem cell transplantation (HSCT) for severe sickle cell disease (SCD) in our unit, 13 being grafted after having been exposed to hydroxyurea (HU) to control SCD-related complications. Different pre-transplant conditioning regimens were given over time: Bu14/Cy200 in six patients (group 1), Bu16/Cy200/antithymocyte globulin (ATG) in five (group 2) and Bu16/Cy200/ATG with HU prior to HSCT in 13 (group 3). The aim of this study is to compare the outcome after HSCT of these groups of patients, which differ according to pre-transplant drug exposure. Overall, 20 of the 24 transplanted children had stable engraftment and have remained free of SCD-related symptoms after HSCT; 19 of them are currently alive and cured of SCD. In group 1 (HU-, ATG-), we observed one unexplainable late death, one absent engraftment, one late rejection and one mixed stable chimerism. In group 2 (HU-, ATG+), we observed the absence of engraftment in two patients and one early rejection. In group 3 (HU+, ATG+), we observed no cases of either absent engraftment, mixed stable chimerism or late rejection. In our experience, pre-transplant treatment with HU seems to be associated with a lower incidence of rejection/absent engraftment in severe SCD patients. These results need to be confirmed with a larger number of patients.

Cell-based therapies for birth defects: a role for adult stem cell plasticity?

Cell therapy can offer a reasonable approach to the treatment of specific birth defects, particularly those for which hematopoietic stem cells (HSCs) can be used to restore (even partially) the number of cells, protein levels, or enzyme activity. Relatively few clinical experiences have been published on this subject, but when a natural selective advantage exists for the cell graft, a degree of "rescue" is possible. Strategies have been developed to confer a selective advantage through genetic engineering of donor cells, and this approach may prove valuable in the treatment of birth defects, as it is in hematological malignancy. Stem cell (SC) plasticity, or transdifferentiation, may offer another route for delivery of cells to established or developing organs. A wide variety of studies support the concept that adult tissue-specific SCs can, if displaced from their normal niche to another, be reprogrammed to produce cell types appropriate to their new environment. Clinical observations reveal that persistent tissue microchimerism develops not only in blood lineages after transfusion, but also in thyroid follicular epithelium via transplacental exchange. In addition, hepatic and renal parenchyma also become chimeric following allografts or bone marrow transplantation (BMT). Experimental models indicate that a renal glomerulosclerosis phenotype can be transferred by grafting whole BM, and that a severe liver disorder in fah-/- mice can be overcome by grafting HSCs and then exerting a selection pressure. It may be possible in the future to exploit the ability of adult SCs to contribute to diverse tissues; however, our understanding of the processes involved is at a very early stage.

Milestones in the development of pediatric hematopoietic stem cell transplantation--50 years of progress

In the 1950s, the first infusions of hematopoietic stem cells were given as a form of treatment for childhood leukemia. This heralded the beginning of a field that has expanded to include the treatment of immune deficiencies, a variety of leukemias and solid tumors, and then genetic diseases. A number of milestones are highlighted, particularly in regard to the use of alternative sources of hematopoietic stem cells such as unrelated donors, peripheral blood stem cells and umbilical cord stem cells. In addition, newer techniques of using non-myeloablative preparative regimens helped to reduce the toxicity and long-term consequences of hematopoietic stem cell transplant. Many diseases now benefit from the replacement of the marrow stem cells and the provision of a new immune system and improved immune surveillance.