Hematopoietic stem cells reside within the bone marrow post-natally, providing all hematopoietic lineages for the life of the host, and their extensive characterization over the last several decades has led to clinical application including bone marrow transplantation for benign and malignant disorders affecting the hematopoietic compartment. Isolation of bone marrow cells based upon expression of the CD34 antigen enriches for the primitive compartment, and techniques to isolate the CD34-positive population are commonly used in clinical practice. Assays for true hematopoietic stem cells require transplantation and repopulation, and thus studies querying such potential among human cells are difficult. We have thus developed a xenograft model with robust human hematopoietic stem cell engraftment, and have demonstrated engraftment of all cell lineages. This assay has allowed us to test globin vectors for the first time among engrafted human cells in a relevant model. Furthermore, ex vivo differentiation of hematopoietic stem cells along the erythroid lineage can be utilized to assay viral vector directed beta-globin expression after transfer of the human beta globin gene. These systems have proven useful in developing viral vector systems for clinical application including a forward oriented globin vector which has been developed by our group and demonstrates 10 fold higher vector titer as well as 10 fold higher transduction efficiency for long term repopulating cells. Furthermore, we are now developing the tools to allow transfer of gene editing elements to HSCs using lentiviral vector constructs that have been modified to package these elements without integration. Finally, we are using gene editing strategies to query the manipulation of human HSCs and the consequences.
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