Lentiviral vector-mediated gene delivery into human embryonic stem cells

Advances in human pluripotent stem cell reporter systems

The capability of human pluripotent stem cells (hPSCs) to self-renew and differentiate into any cell type has greatly contributed to the advancement of biomedicine. Reporter lines derived from hPSCs have played a crucial role in elucidating the mechanisms underlying human development and diseases by acting as an alternative reporter system that cannot be used in living humans. To bring hPSCs closer to clinical application in transplantation, scientists have generated reporter lines for isolating the desired cell populations, as well as improving graft quality and treatment outcomes. This review presents an overview of the applications of hPSC reporter lines and the important variables in designing a reporter system, including options for gene delivery and editing tools, design of reporter constructs, and selection of reporter genes. It also provides insights into the prospects of hPSC reporter lines and the challenges that must be overcome to maximize the potential of hPSC reporter lines.

Manipulating and studying gene function in human pluripotent stem cell models

Human pluripotent stem cells (hPSCs) are uniquely suited to study human development and disease and promise to revolutionize regenerative medicine. These applications rely on robust methods to manipulate gene function in hPSC models. This comprehensive review aims to both empower scientists approaching the field and update experienced stem cell biologists. We begin by highlighting challenges with manipulating gene expression in hPSCs and their differentiated derivatives, and relevant solutions (transfection, transduction, transposition, and genomic safe harbor editing). We then outline how to perform robust constitutive or inducible loss-, gain-, and change-of-function experiments in hPSCs models, both using historical methods (RNA interference, transgenesis, and homologous recombination) and modern programmable nucleases (particularly CRISPR/Cas9 and its derivatives, i.e., CRISPR interference, activation, base editing, and prime editing). We further describe extension of these approaches for arrayed or pooled functional studies, including emerging single-cell genomic methods, and the related design and analytical bioinformatic tools. Finally, we suggest some directions for future advancements in all of these areas. Mastering the combination of these transformative technologies will empower unprecedented advances in human biology and medicine.

CHD1L augments autophagy-mediated migration of hepatocellular carcinoma through targeting ZKSCAN3

Autophagy is an important biological process in normal cells. However, how it affects tumor progression still remains poorly understood. Herein, we demonstrated that the oncogenic protein Chromodomain-helicase-DNA-binding-protein 1-like gene (CHD1L) might promote HCC cells migration and metastasis through autophagy. CHD1L could bind to the promotor region of Zinc finger with KRAB and SCAN domain 3 (ZKSCAN3), a pivotal autophagy suppressor, and inhibit its transcription. We established inducible CHD1L conditional knockout cell line (CHD1L-iKO cell) and found that the deletion of CHD1L significantly increased ZKSCAN3 expression both at mRNA and protein level. Deletion of CHD1L impaired the autophagic flux and migration of HCC cells, while specifically inhibiting ZKSCAN3 blocked these effects. Further exploration demonstrated that the enhanced tumor cell migration and metastasis induced by CHD1L was mediated through ZKSCAN3-induced autophagic degradation of Paxillin. In summary, we have characterized a previously unknown function of CHD1L in regulating tumor migration via ZKSCAN3-mediated autophagy in HCC. Further inhibition of CHD1L and its downstream autophagy signaling might shed new light on cancer therapeutics.

Systemically transplanted mesenchymal stem cells induce vascular-like structure formation in a rat model of vaginal injury

The beneficial effect of mesenchymal stem cells (MSCs) on wound healing is mostly attributed to a trophic effect that promotes angiogenesis. Whether MSCs can contribute to the formation of new blood vessels by direct differentiation is still controversial. Pelvic floor dysfunction (PFD) is a group of disorders that negatively affect the quality of women's lives. Traditional vaginal surgical repair provides disappointing anatomical outcome. Stem cell transplantation may be used to supplement surgery and improve its outcome. Here we aimed to examine the engraftment, survival, differentiation and angiogenic effect of transplanted MSCs in a vaginal injury rat model. MSCs were obtained from the bone marrow of Sprague Drawley (SD) rats, expanded and characterized in vitro. The MSCs expressed CD90 and CD29, did not express CD45, CD34, CD11b and CD31 and could differentiate into osteogenic, chondrogenic and adipogenic lineages. Cells were labeled with either PKH-26 or GFP and transplanted systemically or locally to female SD rats, just after a standardized vaginal incision was made. Engraftment after local transplantation was less efficient at all-time points compared to systemic administration. In the systemically transplanted animal group, MSCs migrated to the injury site and were present in the healed vagina for at least 30 days. Both systemic and local MSCs transplantation promoted host angiogenesis. Systemically transplanted MSCs created new vascular-like structures by direct differentiation into endothelium. These findings pave the way to further studies of the potential role of MSCs transplantation in improving surgical outcome in women with PFD.

CXCR4 Promotes Neuroblastoma Growth and Therapeutic Resistance through miR-15a/16-1-Mediated ERK and BCL2/Cyclin D1 Pathways

CXCR4 expression in neuroblastoma tumors correlates with disease severity. In this study, we describe mechanisms by which CXCR4 signaling controls neuroblastoma tumor growth and response to therapy. We found that overexpression of CXCR4 or stimulation with CXCL12 supports neuroblastoma tumorigenesis. Moreover, CXCR4 inhibition with the high-affinity CXCR4 antagonist BL-8040 prevented tumor growth and reduced survival of tumor cells. These effects were mediated by the upregulation of miR-15a/16-1, which resulted in downregulation of their target genes BCL-2 and cyclin D1, as well as inhibition of ERK. Overexpression of miR-15a/16-1 in cells increased cell death, whereas antagomirs to miR-15a/16-1 abolished the proapoptotic effects of BL-8040. CXCR4 overexpression also increased miR-15a/16-1, shifting their oncogenic dependency from the BCL-2 to the ERK signaling pathway. Overall, our results demonstrate the therapeutic potential of CXCR4 inhibition in neuroblastoma treatment and provide a rationale to test combination therapies employing CXCR4 and BCL-2 inhibitors to increase the efficacy of these agents.Significance: These results provide a mechanistic rationale for combination therapy of CXCR4 and BCL-2 inhibitors to treat a common and commonly aggressive pediatric cancer.Cancer Res; 78(6); 1471-83. ©2017 AACR.

MicroRNA-202 maintains spermatogonial stem cells by inhibiting cell cycle regulators and RNA binding proteins

miRNAs play important roles during mammalian spermatogenesis. However, the function of most miRNAs in spermatogenesis and the underlying mechanisms remain unknown. Here, we report that miR-202 is highly expressed in mouse spermatogonial stem cells (SSCs), and is oppositely regulated by Glial cell-Derived Neurotrophic Factor (GDNF) and retinoic acid (RA), two key factors for SSC self-renewal and differentiation. We used inducible CRISPR-Cas9 to knockout miR-202 in cultured SSCs, and found that the knockout SSCs initiated premature differentiation accompanied by reduced stem cell activity and increased mitosis and apoptosis. Target genes were identified with iTRAQ-based proteomic analysis and RNA sequencing, and are enriched with cell cycle regulators and RNA-binding proteins. Rbfox2 and Cpeb1 were found to be direct targets of miR-202 and Rbfox2 but not Cpeb1, is essential for the differentiation of SSCs into meiotic cells. Accordingly, an SSC fate-regulatory network composed of signaling molecules of GDNF and RA, miR-202 and diverse downstream effectors has been identified.

Notch signaling regulates motor neuron differentiation of human embryonic stem cells

In the pMN domain of the spinal cord, Notch signaling regulates the balance between motor neuron differentiation and maintenance of the progenitor state for later oligodendrocyte differentiation. Here, we sought to study the role of Notch signaling in regulation of the switch from the pMN progenitor state to differentiated motor neurons in a human model system. Human embryonic stem cells (hESCs) were directed to differentiate to pMN-like progenitor cells by the inductive action of retinoic acid and a Shh agonist, purmorphamine. We found that the expression of the Notch signaling effector Hes5 was induced in hESC-derived pMN-like progenitors and remained highly expressed when they were cultured under conditions favoring motor neuron differentiation. Inhibition of Notch signaling by a γ-secretase inhibitor in the differentiating pMN-like progenitor cells decreased Hes5 expression and enhanced the differentiation toward motor neurons. Conversely, over-expression of Hes5 in pMN-like progenitor cells during the differentiation interfered with retinoic acid- and purmorphamine-induced motor neuron differentiation and inhibited the emergence of motor neurons. Inhibition of Notch signaling had a permissive rather than an inductive effect on motor neuron differentiation. Our results indicate that Notch signaling has a regulatory role in the switch from the pMN progenitor to the differentiated motor neuron state. Inhibition of Notch signaling can be harnessed to enhance the differentiation of hESCs toward motor neurons.

Stem cells as drug delivery methods: application of stem cell secretome for regeneration

Mesenchymal stem cells (MSCs) are a unique cell population defined by their ability to indefinitely self-renew, differentiate into multiple cell lineages, and form clonal cell populations. It was originally thought that this ability for broad plasticity defined the therapeutic potential of MSCs. However, an expanding body of recent literature has brought growing awareness to the remarkable array of bioactive molecules produced by stem cells. This protein milieu or "secretome" comprises a diverse host of cytokines, chemokines, angiogenic factors, and growth factors. The autocrine/paracrine role of these molecules is being increasingly recognized as key to the regulation of many physiological processes including directing endogenous and progenitor cells to sites of injury as well as mediating apoptosis, scarring, and tissue revascularization. In fact, the immunomodulatory and paracrine role of these molecules may predominantly account for the therapeutic effects of MSCs given that many in vitro and in vivo studies have demonstrated limited stem cell engraftment at the site of injury. While the study of such a vast protein array remains challenging, technological advances in the field of proteomics have greatly facilitated our ability to analyze and characterize the stem cell secretome. Thus, stem cells can be considered as tunable pharmacological storehouses useful for combinatorial drug manufacture and delivery. As a cell-free option for regenerative medicine therapies, stem cell secretome has shown great potential in a variety of clinical applications including the restoration of function in cardiovascular, neurodegenerative, oncologic, and genitourinary pathologies.

Advances in homology directed genetic engineering of human pluripotent and adult stem cells

The ability to introduce precise genomic modifications in human cells has profound implications for both basic and applied research in stem cells, ranging from identification of genes regulating stem cell self-renewal and multilineage differentiation to therapeutic gene correction and creation of in vitro models of human diseases. However, the overall efficiency of this process is challenged by several factors including inefficient gene delivery into stem cells and low rates of homology directed site-specific targeting. Recent studies report the development of novel techniques to improve gene targeting efficiencies in human stem cells; these methods include molecular engineering of viral vectors to efficiently deliver episomal genetic sequences that can participate in homology directed targeting, as well as the design of synthetic proteins that can introduce double-stranded breaks in DNA to initiate such recombination events. This review focuses on the potential of these new technologies to precisely alter the human stem cell genome and also highlights the possibilities offered by the combination of these complementary strategies.

Nanodentistry: combining nanostructured materials and stem cells for dental tissue regeneration

Regenerative dentistry represents an attractive multidisciplinary therapeutic approach that complements traditional restorative/surgery techniques and benefits from recent advances in stem cell biology, molecular biology, genomics and proteomics. Materials science is important in such advances to move regenerative dentistry from the laboratory to the clinic. The design of novel nanostructured materials, such as biomimetic matrices and scaffolds for controlling cell fate and differentiation, and nanoparticles for diagnostics, imaging and targeted treatment, is needed. The combination of nanotechnology, which allows the creation of sophisticated materials with exquisite fine structural detail, and stem cell biology turns out to be increasingly useful in regenerative medicine. The administration to patients of dynamic biological agents comprising stem cells, bioactive scaffolds and/or nanoparticles will certainly increase the regenerative impact of dental pathological tissues. This overview briefly describes some of the actual benefits and future possibilities of nanomaterials in the emerging field of stem cell-based regenerative dentistry.

Embryonic template-based generation and purification of pluripotent stem cell-derived cardiomyocytes for heart repair

Cardiovascular disease remains a leading cause of death in Western countries. Many types of cardiovascular diseases are due to a loss of functional cardiomyocytes, which can result in irreversible cardiac failure. Since the adult human heart has limited regenerative potential, cardiac transplantation is still the only effective therapy to address this cardiomyocyte loss. However, drawbacks, such as immune rejection and insufficient donor availability, are limiting this last-resort solution. Recent developments in the stem cell biology field have improved the potential of cardiac regeneration. Improvements in reprogramming strategies of differentiated adult cells into induced pluripotent stem cells, together with increased efficiency of directed differentiation of pluripotent stem cells toward cardiac myocytes, have brought cell-based heart muscle regeneration a few steps closer to the clinic. In this review, we outline the status of research on cardiac regeneration with a focus on directed differentiation of pluripotent stem cells toward the cardiac lineage.

Directed evolution of adeno-associated virus for enhanced gene delivery and gene targeting in human pluripotent stem cells

Efficient approaches for the precise genetic engineering of human pluripotent stem cells (hPSCs) can enhance both basic and applied stem cell research. Adeno- associated virus (AAV) vectors are of particular interest for their capacity to mediate efficient gene delivery to and gene targeting in various cells. However, natural AAV serotypes offer only modest transduction of human embryonic and induced pluripotent stem cells (hESCs and hiPSCs), which limits their utility for efficiently manipulating the hPSC genome. Directed evolution is a powerful means to generate viral vectors with novel capabilities, and we have applied this approach to create a novel AAV variant with high gene delivery efficiencies (~50%) to hPSCs, which are importantly accompanied by a considerable increase in gene-targeting frequencies, up to 0.12%. While this level is likely sufficient for numerous applications, we also show that the gene-targeting efficiency mediated by an evolved AAV variant can be further enhanced (>1%) in the presence of targeted double- stranded breaks (DSBs) generated by the co-delivery of artificial zinc finger nucleases (ZFNs). Thus, this study demonstrates that under appropriate selective pressures, AAV vectors can be created to mediate efficient gene targeting in hPSCs, alone or in the presence of ZFN- mediated double-stranded DNA breaks.

Derivation, propagation and controlled differentiation of human embryonic stem cells in suspension

Undifferentiated human embryonic stem cells (hESCs) are currently propagated on a relatively small scale as monolayer colonies. Culture of hESCs as floating aggregates is widely used for induction of differentiation into embryoid bodies. Here we show that hESC lines can be derived from floating inner cell masses in suspension culture conditions that do not involve feeder cells or microcarriers. This culture system supports prolonged propagation of the pluripotent stem cells as floating clusters without their differentiation into embryoid bodies. HESCs cultivated as aggregates in suspension maintain the expression of pluripotency markers and can differentiate into progeny of the three germ layers both in vitro and in vivo. We further show the controlled differentiation of hESC clusters in suspension into neural spheres. These results pave the way for large-scale expansion and controlled differentiation of hESCs in suspension, which would be valuable in basic and applied research.

In vitro osteogenic differentiation of adipose stem cells after lentiviral transduction with green fluorescent protein

BACKGROUND

Adipose-derived stem cells (ASCs) have the potential to differentiate into osteogenic cells that can be seeded into scaffolds for tissue engineering for use in craniofacial bone defects. Green fluorescent protein (GFP) has been widely used as a lineage marker for mammalian cells. The use of fluorescent proteins enables cells to be tracked during manipulation such as osteogenic differentiation within three-dimensional scaffolds. The purpose of this study was to examine whether ASCs introduced with GFP-encoding lentivirus vector exhibit adequate GFP fluorescence and whether the expression of GFP interfered with osteogenic differentiation of ASCs in both monolayer and three-dimensional scaffolds in vitro.

METHODS

Primary ASCs were harvested from the inguinal fat pad of Sprague Dawley rats. Isolated ASCs were cultured and infected with a lentiviral vector encoding GFP and plated into both monolayers and three-dimensional scaffolds in vitro. The cells were then placed in osteogenic medium. Osteogenic differentiation of the GFP-ASCs was assessed using alizarin red S, alkaline phosphate staining, and immunohistochemistry staining of osteocalcin with quantification of alizarin red S and osteocalcin staining.

RESULTS

The efficacy of infection of ASCs with a lentiviral vector encoding GFP was high. Cell-cultured GFP-ASCs remained fluorescent over the 8 weeks of the study period. The GFP-ASCs were successfully induced into osteogenic cells both in monolayers and three-dimensional scaffolds. Whereas the quanitification of alizarin red S revealed no difference between osteoinduced ASCs with or without GFP, the quantification of osteocalcin revealed increased staining in the GFP group.

CONCLUSIONS

Transduction of isolated ASCs using a lentiviral vector encoding GFP is an effective method for tracing osteoinduced ASCs in vitro. Quantification data showed no decrease in staining of the osteoinduced ASCs.

Nongenetic method for purifying stem cell-derived cardiomyocytes

Several applications of pluripotent stem cell (PSC)-derived cardiomyocytes require elimination of undifferentiated cells. A major limitation for cardiomyocyte purification is the lack of easy and specific cell marking techniques. We found that a fluorescent dye that labels mitochondria, tetramethylrhodamine methyl ester perchlorate, could be used to selectively mark embryonic and neonatal rat cardiomyocytes, as well as mouse, marmoset and human PSC-derived cardiomyocytes, and that the cells could subsequently be enriched (>99% purity) by fluorescence-activated cell sorting. Purified cardiomyocytes transplanted into testes did not induce teratoma formation. Moreover, aggregate formation of PSC-derived cardiomyocytes through homophilic cell-cell adhesion improved their survival in the immunodeficient mouse heart. Our approaches will aid in the future success of using PSC-derived cardiomyocytes for basic and clinical applications.

New opportunities: the use of nanotechnologies to manipulate and track stem cells

Nanotechnologies are emerging platforms that could be useful in measuring, understanding, and manipulating stem cells. Examples include magnetic nanoparticles and quantum dots for stem cell labeling and in vivo tracking; nanoparticles, carbon nanotubes, and polyplexes for the intracellular delivery of genes/oligonucleotides and protein/peptides; and engineered nanometer-scale scaffolds for stem cell differentiation and transplantation. This review examines the use of nanotechnologies for stem cell tracking, differentiation, and transplantation. We further discuss their utility and the potential concerns regarding their cytotoxicity.

MicroRNA-145 regulates OCT4, SOX2, and KLF4 and represses pluripotency in human embryonic stem cells

MicroRNAs (miRNAs) are posttranscriptional modulators of gene expression and play an important role in many developmental processes. We report here that expression of microRNA-145 (miR-145) is low in self-renewing human embryonic stem cells (hESCs) but highly upregulated during differentiation. We identify the pluripotency factors OCT4, SOX2, and KLF4 as direct targets of miR-145 and show that endogenous miR-145 represses the 3' untranslated regions of OCT4, SOX2, and KLF4. Increased miR-145 expression inhibits hESC self-renewal, represses expression of pluripotency genes, and induces lineage-restricted differentiation. Loss of miR-145 impairs differentiation and elevates OCT4, SOX2, and KLF4. Furthermore, we find that the miR-145 promoter is bound and repressed by OCT4 in hESCs. This work reveals a direct link between the core reprogramming factors and miR-145 and uncovers a double-negative feedback loop involving OCT4, SOX2, KLF4, and miR-145.

Embryonic stem cell-derived neural precursor grafts for treatment of temporal lobe epilepsy

Complex partial seizures arising from mesial temporal lobe structures are a defining feature of mesial temporal lobe epilepsy (TLE). For many TLE patients, there is an initial traumatic head injury that is the precipitating cause of epilepsy. Severe TLE can be associated with neuropathological changes, including hippocampal sclerosis, neurodegeneration in the dentate gyrus, and extensive reorganization of hippocampal circuits. Learning disabilities and psychiatric conditions may also occur in patients with severe TLE for whom conventional anti-epileptic drugs are ineffective. Novel treatments are needed to limit or repair neuronal damage, particularly to hippocampus and related limbic regions in severe TLE and to suppress temporal lobe seizures. A promising therapeutic strategy may be to restore inhibition of dentate gyrus granule neurons by means of cell grafts of embryonic stem cell-derived GABAergic neuron precursors. "Proof-of-concept" studies show that human and mouse embryonic stem cell-derived neural precursors can survive, migrate, and integrate into the brains of rodents in different experimental models of TLE. In addition, studies have shown that hippocampal grafts of cell lines engineered to release GABA or other anticonvulsant molecules can suppress seizures. Furthermore, transplants of fetal GABAergic progenitors from the mouse or human brain have also been shown to suppress the development of seizures. Here, we review these relevant studies and highlight areas of future research directed toward producing embryonic stem cell-derived GABAergic interneurons for cell-based therapies for treating TLE.

The biological restoration of central nervous system architecture and function: part 1-foundations and historical landmarks in contemporary stem cell biology

Since their discovery, stem cells have fascinated scientists with their ultimate potential: the ability to cure disease, repair altered physiology, and reverse neurological deficit. Stem cell science unquestionably promises to eliminate many of the tragic limitations contemporary medicine must acknowledge, and cloning may provide young cells for an aging population. Although it is widely believed that stem cells will transform the way medicine is practiced, therapeutic interventions using stem cell technology are still in their infancy. The 3 most common stem cell sources studied today are umbilical cord blood, bone marrow, and human embryos. Although cord blood is currently used to treat dozens of disorders and bone marrow stem cells have been used clinically since the 1960s, human embryonic stem cells have yet to be successfully applied to any disease. Undeniably, stem cell therapy has the potential to be one of the most powerful therapeutic options available. In this introductory article of a 5-part series on stem cells, we narrate the evolution of modern stem cell science, delineating major landmarks that will prove responsible for taking stem cell technology from the laboratory into revolutionary clinical applications: from the first milestone of identifying the mouse hematopoietic stem cell to the latest feats of producing pluripotent stem cells without embryos at all. In Part 2, we present the evidence demonstrating the certainty of adult mammalian neurogenesis; in Parts 3 and 4, we describe neurosurgical applications of stem cell technology; and in Part 5, we discuss the philosophical and ethical issues surrounding stem cell therapy, as well as future areas of exploration.

A single EBV-based vector for stable episomal maintenance and expression of GFP in human embryonic stem cells

Aim: Stable expression of transgenes in stem cells has been a challenge due to the nonavailability of efficient transfection methods and the inability of transgenes to support sustained gene expression. Several methods have been reported to stably modify both embryonic and adult stem cells. These methods rely on integration of the transgene into the genome of the host cell, which could result in an expression pattern dependent on the number of integrations and the genomic locus of integration. To overcome this issue, site-specific integration methods mediated by integrase, adeno-associated virus or via homologous recombination have been used to generate stable human embryonic stem cell (hESC) lines. In this study, we describe a vector that is maintained episomally in hESCs. Methods: The vector used in this study is based on components derived from the Epstein–Barr virus, containing the Epstein–Barr virus nuclear antigen 1 expression cassette and the OriP origin of replication. The vector also expresses the drug-resistance marker gene hygromycin, which allows for selection and long-term maintenance of cells harboring the plasmid. Results: Using this vector system, we show sustained expression of green fluorescent protein in undifferentiated hESCs and their differentiating embryoid bodies. In addition, the stable hESC clones show comparable expression with and without drug selection. Consistent with this observation, bulk-transfected adipose tissue-derived mesenchymal stem cells showed persistent marker gene expression as they differentiate into adipocytes, osteoblasts and chondroblasts. Conclusions: Episomal vectors offer a fast and efficient method to create hESC reporter lines, which in turn allows one to test the effect of overexpression of various genes on stem cell growth, proliferation and differentiation.

Derive and conquer: sourcing and differentiating stem cells for therapeutic applications

Although great progress has been made in the isolation and culture of stem cells, the future of stem-cell-based therapies and their productive use in drug discovery and regenerative medicine depends on two key factors: finding reliable sources of multipotent and pluripotent cells and the ability to control their differentiation to generate desired derivatives. It is essential for clinical applications to establish reliable sources of pathogen-free human embryonic stem cells (ESCs) and develop suitable differentiation techniques. Here, we address some of the problems associated with the sourcing of human ESCs and discuss the current status of stem-cell differentiation technology.

Ablation of tumor-derived stem cells transplanted to the central nervous system by genetic modification of embryonic stem cells with a suicide gene

Embryonic stem cell (ESC)-based therapies open new possibilities as regenerative medicine for the treatment of human disease, but the presence of small numbers of undifferentiated ESCs within the transplant could lead to the development of tumors. The safety of ESC transplants would be enhanced if uncontrolled cell growth could be suppressed, using external stimuli. A lentiviral vector carrying the herpes simplex virus thymidine kinase (HSVtk) and green fluorescent protein (GFP) genes was used to genetically modify murine ESCs (HSVtk+GFP+ ESCs). In the presence of ganciclovir (GCV), 100% of HSVtk+GFP+ ESCs were killed in vitro, and 100% of flank tumors derived from HSVtk+GFP+ ESCs were eliminated. When CNS tumors were produced by the HSVtk+GFP+ ESCs, the tumor mass was completely eliminated on GCV treatment for 1 week. After GCV treatment for 3 weeks, histologic analysis showed no residual tumor cells and TaqMan realtime polymerase chain reaction analysis showed no genomic HSVtk copies or HSVtk mRNA. These data demonstrate that it is possible to use ex vivo gene transfer to modify ESCs with conditional genetic elements that can be activated in vivo to control undifferentiated ESC outgrowth and to eliminate transduced ESCs that have escaped growth control after ESC-mediated therapy to the CNS.