Megakaryocyte- and megakaryocyte precursor-related gene therapies

Abstract

Hematopoietic stem cells (HSCs) can be safely collected from the body, genetically modified, and re-infused into a patient with the goal to express the transgene product for an individual's lifetime. Hematologic defects that can be corrected with an allogeneic bone marrow transplant can theoretically also be treated with gene replacement therapy. Because some genetic disorders affect distinct cell lineages, researchers are utilizing HSC gene transfer techniques using lineage-specific endogenous gene promoters to confine transgene expression to individual cell types (eg, ITGA2B for inherited platelet defects). HSCs appear to be an ideal target for platelet gene therapy because they can differentiate into megakaryocytes which are capable of forming several thousand anucleate platelets that circulate within blood vessels to establish hemostasis by repairing vascular injury. Platelets play an essential role in other biological processes (immune response, angiogenesis) as well as diseased states (atherosclerosis, cancer, thrombosis). Thus, recent advances in genetic manipulation of megakaryocytes could lead to new and improved therapies for treating a variety of disorders. In summary, genetic manipulation of megakaryocytes has progressed to the point where clinically relevant strategies are being developed for human trials for genetic disorders affecting platelets. Nevertheless, challenges still need to be overcome to perfect this field; therefore, strategies to increase the safety and benefit of megakaryocyte gene therapy will be discussed.

Figure 1

Promising strategies for megakaryocyte gene transfer. Displayed is a schematic diagram that summarizes 3 strategies currently being examined for megakaryocyte modification including: transplantation of cytokine-mobilized CD34⁺ PBSCs transduced with a lentiviral vector; direct injection of lentiviral vector into the bone marrow space to transduce HSCs; and lentiviral vector transduction of iPSCs dedifferentiated from peripheral blood mononuclear cells followed by transfusion of genetically altered platelets into the patient. Although each method focuses on modification of the HSCs with a lentiviral vector under the transcriptional control of a megakaryocyte-specific gene promoter, there is significant contrast in procuring the HSC target cell as well as different strategies for accomplishing megakaryocyte manipulation. HSC differentiation along the megakaryocyte lineage is depicted by stage from HSCs within the bone marrow to formation of proplatelets, preplatelets, and finally mature platelets within the vascular space. The cartoon person illustrates the routes of collection and transfusion of genetically modified cells and a potential point of injection for LV within the bone marrow space. PBSCs are all HSCs. BM, bone marrow; LV, lentivector.