Clonal selection of hematopoietic stem cells after gene therapy for sickle cell disease.

Gene therapy (GT) provides a potentially curative treatment option for patients with sickle cell disease (SCD); however, the occurrence of myeloid malignancies in GT clinical trials has prompted concern, with several postulated mechanisms. Here, we used whole-genome sequencing to track hematopoietic stem cells (HSCs) from six patients with SCD at pre- and post-GT time points to map the somatic mutation and clonal landscape of gene-modified and unmodified HSCs. Pre-GT, phylogenetic trees were highly polyclonal and mutation burdens per cell were elevated in some, but not all, patients. Post-GT, no clonal expansions were identified among gene-modified or unmodified cells; however, an increased frequency of potential driver mutations associated with myeloid neoplasms or clonal hematopoiesis (DNMT3A- and EZH2-mutated clones in particular) was observed in both genetically modified and unmodified cells, suggesting positive selection of mutant clones during GT. This work sheds light on HSC clonal dynamics and the mutational landscape after GT in SCD, highlighting the enhanced fitness of some HSCs harboring pre-existing driver mutations. Future studies should define the long-term fate of mutant clones, including any contribution to expansions associated with myeloid neoplasms.

Perivascular deletion of murine Rac reverses the ratio of marrow arterioles and sinusoid vessels and alters hematopoiesis in vivo.

Hematopoietic stem cells (HSCs) are localized within specialized microenvironments throughout the BM. Nestin-expressing (Nestin(+)) mesenchymal stromal cells (MSCs) are important in the perivascular space. Rac is critical for MSC cell shape in vitro, whereas its function in MSCs in vivo remains poorly characterized. We hypothesized that deletion of Rac in the Nestin(+) cells would perturb the perivascular space, altering HSC localization and hematopoiesis. Nestin-Cre-directed excision of Rac1 in Rac3(-/-) mice reduces Nestin(+) cells in the marrow. We observed a 2.7-fold decrease in homing of labeled wild-type hematopoietic cells into Rac1(Δ/Δ)Rac3(-/-) mice compared with control mice. Rac1(Δ/Δ)Rac3(-/-) mice demonstrated a marked decrease in arterioles and an increase in the number and volume of venous sinusoids in the marrow that was associated with a reduction in the numbers of immunophenotypically and functionally-defined long-term HSCs in the marrow, a decrease in colony-forming cells and a reduction in circulating progenitors. Rac-deleted animals demonstrated a significant increase in trabecular bone. These data demonstrate that Rac GTPases play an important role in the integrity of perivascular space. Increased trabecular bone and sinusoidal space and decreased arteriolar volume in this model were associated with decreased HSC, underscoring the complexity of regulation of hematopoiesis in the perivascular space.

Retroviral transduction of murine and human hematopoietic progenitors and stem cells.

Genetic modification of cells using retroviral vectors is the method of choice when the cell population is difficult to transfect and/or requires persistent transgene expression in progeny cells. There are innumerable potential applications for these procedures in laboratory research and clinical therapeutic interventions. One paradigmatic example is the genetic modification of hematopoietic stem and progenitor cells (HSPCs). These are rare nucleated cells which reside in a specialized microenvironment within the bone marrow, and have the potential to self-renew and/or differentiate into all hematopoietic lineages. Due to their enormous regenerative capacity in steady state or under stress conditions these cells are routinely used in allogeneic bone marrow transplantation to reconstitute the hematopoietic system in patients with metabolic, inflammatory, malignant, and other hematologic disorders. For patients lacking a matched bone marrow donor, gene therapy of autologous hematopoietic stem cells has proven to be an alternative as highlighted recently by several successful gene therapy trials. Genetic modification of HSPCs using retrovirus vectors requires ex vivo manipulation to efficiently introduce the new genetic material into cells (transduction). Optimal culture conditions are essential to facilitate this process while preserving the stemness of the cells. The most frequently used retroviral vector systems for the genetic modifications of HSPCs are derived either from Moloney murine leukemia-virus (Mo-MLV) or the human immunodeficiency virus-1 (HIV-1) and are generally termed according to their genus gamma-retroviral (γ-RV) or lentiviral vectors (LV), respectively. This chapter describes in a step-by-step fashion some techniques used to produce research grade vector supernatants and to obtain purified murine or human hematopoietic stem cells for transduction, as well as follow-up methods for analysis of transduced cell populations.