Gene Therapy (2000) 7, 451–457 2000 Macmillan Publishers Ltd All rights reserved 0969-7128/00 $15.00 www.nature.com/gt MILLENNIUM REVIEW Stem cell therapy and gene transfer for regeneration T Asahara, C Kalka and JM Isner Cardiovascular Research and Medicine, St Elizabeth’s Medical Center, Tufts University School of Medicine, Boston, MA, USA The committed stem and progenitor cells have been recently In this review, we discuss the promising gene therapy appli- isolated from various adult tissues, including hematopoietic cation of adult stem and progenitor cells in terms of mod- stem cell, neural stem cell, mesenchymal stem cell and ifying stem cell potency, altering organ property, accelerating endothelial progenitor cell. These adult stem cells have sev- regeneration and forming expressional organization. Gene eral advantages as compared with embryonic stem cells as Therapy (2000) 7, 451–457. their practical therapeutic application for tissue regeneration. Keywords: stem cell; gene therapy; regeneration; progenitor cell; differentiation Introduction poietic stem cells to blood cells. The determined stem cells differentiate into ‘committed progenitor cells’, which The availability of embryonic stem (ES) cell lines in mam- retain a limited capacity to replicate and phenotypic fate. malian species has greatly advanced the field of biologi- In the past decade, researchers have defined such com- cal research by enhancing our ability to manipulate the mitted stem or progenitor cells from various tissues, genome and by providing model systems to examine including bone marrow, peripheral blood, brain, liver cellular differentiation. ES cells, which are derived from and reproductive organs, in both adult animals and the inner mass of blastocysts or primordial germ cells, humans (Figure 1). While most cells in adult organs are can be maintained continuously in an undifferentiated composed of differentiated cells, which express a variety pluripotent state. They have the potential to contribute of specific phenotypic genes adapted to each organ’s to a variety of differentiated cell lineages in the fetus, environment, quiescent stem or progenitor cells are main- including germ cells.1,2 Furthermore, targeted mutations in ES cells have been introduced into the mouse germ tained locally or in the systemic circulation and are acti- line to elucidate target gene function in vivo and create vated by environmental stimuli for physiological and mouse models of human genetic diseases and abnormali- pathological tissue regeneration. ties.3,4 ES cells have also been used to study the events Tissue replacement in the body takes place by two regulating the differentiation of various cell types and mechanisms. One is the replacement of differentiated tissues in vitro, for example, into multiple hematopoietic cells by newly generated populations derived from lineages, endothelial cells, neural cells and mesenchymal residual cycling stem cells. Blood cells are a typical cell lineages. The transfer of ES-derived cells into fetal example of this kind of regeneration. Whole hematopo- and adult mice has demonstrated their functional ietic lineage cells are derived from a few self-renewal integration into organs of recipient animals.5–10 stem cells by regulated differentiation under the influ- In 1998, Thomson’s group at the University of Wiscon- ence of appropriate cytokines and/or growth factors. The sin and Gearhart’s group at the Johns Hopkins Hospital second mechanism is the self-repair of differentiated isolated human cells with the pluripotent property of ES functioning cells preserving their proliferative activity. cells.11,12 This exciting achievement opened the way to the Hepatocytes, endothelial cells, smooth muscle cells, kera- clinical application of stem cell therapy: the replacement tinocytes and fibroblasts are considered to possess this of diseased or degenerating cell populations, tissues ability. Following physiological stimulation or injury, fac- and organs. tors secreted from surrounding tissues stimulate cell rep- lication and replacement. In contrast, those cells which Stem cell in adult species are more fully differentiated are limited in terms of their proliferative potential by senescence, and by their ES cells are not the only candidates for stem cell gener- inability to incorporate into remote target sites. ation. During embryonic development, the pluripotency Thus, quiescence stem or progenitor cells in most adult of ES cell is narrowed to ‘determined stem cells’ which tissues are mobilized in response to environmental stim- 13 give rise to cells of a particular tissue. Thus neural stem uli when an emergent regenerative process is required, cells give rise to cells of the nervous system and hemato- while during a minor event, neighboring differentiated cells are relied upon. Correspondence: JM Isner, St Elizabeth’s Medical Center, 736 Cambridge Street, Boston, MA 02135, USA Millennium review T Asahara et al 452 Figure 1 Cell stages of embryonic and adult stem/progenitor cell during lineage differentiation. Stem cell therapy Thus, while ES cells represent a promising cell source for gene therapy in the future, this review will focus Recently, the regenerative potential of stem cells has been primarily on gene-modified cell therapy using adult under intense investigation. In vitro, stem and progenitor stem cells. cells possess the capability of self-renewal and differen- tiation into organ-specific cell types. In vivo, transplan- tation of these cells may reconstitute organ systems, as Gene-modified stem cell therapy shown in animal models of diseases.14–18 In contrast, dif- ferentiated cells do not exhibit such characteristics. Considering their regenerative ability, gene modification Human endothelial progenitor cells (EPCs), for example, of stem cells has several advantages over conventional have been isolated from the peripheral blood of adult gene therapy. Ex vivo gene transfection of stem cells may individuals, expanded in vitro and committed into an avoid administration of vectors and vehicles into the endothelial lineage in culture.15 The transplantation of recipient organism. Possible targets of gene-modified human EPCs has been shown to facilitate successful stem cells include the following (Figure 2). salvage of limb vasculature and perfusion in athymic nude mice with severe hindlimb ischemia, while Modification of stem cell potency/stem cell target differentiated endothelial cells (human microvascular Certain properties of stem cells can be modified by gene endothelial cells, HUMEC) failed to accomplish limb-saving transfer. Stem cells isolated from adults may exhibit age- neovascularization.19 related, genetic, or acquired disease-related impairment The committed stem and progenitor cells isolated from of their regenerative ability. Transcriptional or enzymatic the adult species have several advantages as compared gene modification may constitute effective means to with ES cell in terms of their possible therapeutic appli- maintain, enhance or inhibit their capacity to proliferate cation for tissue regeneration. First, committed stem and or differentiate. progenitor cells can be transplanted autologously with- out immunologic consequences. As adult stem and pro- genitor cells are already determined or committed to cell Modification of organ property/progeny target types for targeted organs, they spontaneously differen- Genes introduced into stem cells are inherited by their tiate under controlled conditions in vitro into specific lin- progeny through differentiation cascades. Specifically, eage cells, and have the ability to reconstitute target expression of the inserted gene may persist through the organs in vivo. In contrast, ES cells are still under investi- life of the organ system which is reconstructed by the gation for direct induction of specific cell differentiation gene-modified stem cells. Genetically disordered organs, for therapeutic application. Furthermore, ethical issues especially in the case of hematopoietic pathology, could about the isolation and the use of human ES are currently be replaced by this strategy following bone marrow quite controversial. transplantation.20–24 Gene Therapy Millennium review T Asahara et al 453 Figure 2 Targets of stem cell gene modification for therapeutic regeneration. Acceleration of regeneration/process target Stem cell gene transfer and associated Regeneration of injured tissues is in part or primarily target disorders achieved via stem cell expansion and differentiation. Examples include endothelial progenitor cells for neovas- Hematopoietic stem cells cularization, neural stem cells for neurogenesis, and hem- atopoietic stem cells (HSCs) and mesenchymal stem cells A hematopoietic stem cell is a determined multipotent (MSCs) for bone marrow reconstruction. Natural repara- stem cell, which is able to differentiate along a number tory processes are often too impaired from unexpectedly of pathways and thereby generate erythrocytes, granulo- severe injury, basic diseases, or aging to accomplish cytes, monocytes, mast cells, lymphocytes and megakary- regeneration. Gene modification of stem cells to sup- ocytes. These stem cells are limited in number, occurring with a frequency of one stem cell per 104 bone marrow plement mitogens25 or to deliver inhibitory factors for negative control might accelerate retarded regenerative cells. By virtue of their capacity for self-renewal, stem processes following stem cell proliferation, incorporation, cells are maintained at homeostatic levels