Gene therapy using hematopoietic stem cells: Sisyphus approaches the crest

In vivo selection of human embryonic stem cell-derived cells expressing methotrexate-resistant dihydrofolate reductase

Human embryonic stem cells (hESCs) provide a novel source of hematopoietic and other cell populations suitable for gene therapy applications. Preclinical studies to evaluate engraftment of hESC-derived hematopoietic cells transplanted into immunodeficient mice demonstrate only limited repopulation. Expression of a drug resistance gene, such as Tyr22-dihydrofolate reductase (Tyr22-DHFR), coupled to methotrexate (MTX) chemotherapy has the potential to selectively increase engraftment of gene-modified hESC-derived cells in mouse xenografts. Here, we describe the generation of Tyr22-DHFR – GFP expressing hESCs that maintain pluripotency, produce teratomas and can differentiate into MTXr-hemato-endothelial cells. We demonstrate that MTX administered to nonobese diabetic/severe combined immunodeficient/IL-2Rγc^null (NSG) mice after injection of Tyr22-DHFR-derived cells significantly increases human CD34⁺ and CD45⁺ cell engraftment in the bone marrow (BM) and peripheral blood of transplanted MTX-treated mice. These results demonstrate that MTX treatment supports selective, long-term engraftment of Tyr22-DHFR-cells in vivo, and provides a novel approach for combined human cell and gene therapy.

Genetic modification of hematopoietic stem cells with nonviral systems: past progress and future prospects

Serious unwanted complications provoked by retroviral gene transfer into hematopoietic stem cells (HSCs) have recently raised the need for the development and assessment of alternative gene transfer vectors. Within this context, nonviral gene transfer systems are attracting increasing interest. Their main advantages include low cost, ease of handling and large-scale production, large packaging capacity and, most importantly, biosafety. While nonviral gene transfer into HSCs has been restricted in the past by poor transfection efficiency and transient maintenance, in recent years, biotechnological developments are converting nonviral transfer into a realistic approach for genetic modification of cells of hematopoietic origin. Herein we provide an overview of past accomplishments in the field of nonviral gene transfer into hematopoietic progenitor/stem cells and we point at future challenges. We argue that episomally maintained self-replicating vectors combined with physical methods of delivery show the greatest promise among nonviral gene transfer strategies for the treatment of disorders of the hematopoietic system.

Hematopoietic stem cell transduction by recombinant adeno-associated virus vectors: problems and solutions

Recombinant adeno-associated virus 2 (AAV) vectors have taken center stage owing to their potentially safer profile compared with the more commonly used retroviral and adenoviral vectors in human gene therapy clinical trials. Their remarkable versatility and efficacy in a wide variety of preclinical animal models of human diseases have attracted further attention of a number of investigators. Although two particular cell types, muscle and brain, have been shown to be highly transducible by AAV vectors, controversies abound with reference to the efficacy of these vectors in transducing primary hematopoietic cells. Whereas some investigators have claimed that primitive hematopoietic cells are impervious to AAV vectors, others have reported that AAV vectors are capable of transducing these cells, but only at high vector-to-cell ratios. Still other investigators have reported successful transduction of primitive hematopoietic cells at relatively low vector-to-cell ratios. This review attempts to resolve these controversies, and provides a basis for the optimism that safe and high-efficiency transduction of hematopoietic stem and progenitor cells by AAV vectors is well within reach.

Specific pharmacological dimerization of KDR in lentivirally transduced human hematopoietic cells activates antiapoptotic and proliferative mechanisms

Selective and regulatable expansion of transduced cells could augment gene therapy for many disorders. The activation of modified growth factor receptors via synthetic chemical inducers of dimerization allows for the coordinated growth of transduced cells. This system can also provide information on specific receptor-mediated signaling without interference from other family members. Although several receptor subunits have been investigated in this context, little is known about the precise molecular events associated with dimerizer-initiated signaling. We have constructed and expressed an AP20187-regulated KDR chimeric receptor in human TF1 cells and analyzed activation of this gene switch using functional, biochemical, and microarray analyses. When deprived of natural ligands, GM-CSF, interleukin-3, or erythropoietin, AP20187 prevented apoptosis of transduced TF1 cells, induced dose-dependent proliferation, and supported long-term growth. In addition, AP20187 stimulation activated the signaling molecules associated with mitogen-activated protein kinase and phosphatidyl-inositol 3-kinase/Akt pathways. Microarray analysis determined that a number of transcripts involved in a variety of cellular processes were differentially expressed. Notably, mRNAs affiliated with heat stress, including Hsp70 and Hsp105, were up-regulated. Functional assays showed that Hsp70 and Hsp105 protected transduced TF1 cells from apoptosis and premature senescence, in part through regulation of Akt. These observations delineate specific roles for kinase insert domain-containing receptor, or KDR, signaling and suggest strategies to endow genetically modified cells with a survival advantage enabling the generation of adequate cell numbers for therapeutic outcomes.

Improved gene transfer and normalized enzyme levels in primitive hematopoietic progenitors from patients with mucopolysaccharidosis type I using a bioreactor

BACKGROUND

One of the major barriers to the clinical application of hematopoietic stem cell (HSC) gene therapy has been relatively low gene transfer efficiency. Other inadequacies of current transduction protocols are related to their multi-step procedures, e.g., using tissue-culture flasks, roller bottles or gas-permeable bags for clinical application.

METHODS

In comparison with a conventional bag transduction protocol, a 'closed' hollow-fiber bioreactor system (HBS) was exploited to culture and transduce human peripheral blood CD34(+) progenitor cells (PBPC(MPS)) from patients with mucopolysaccharidosis type I (MPS I) using an amphotropic retroviral vector based on a murine Moloney leukemia virus LN prototype. Both short-term colony-forming cell (CFC) and long-term culture initiating cell (LTCIC) assays were employed to determine transduction frequency and transgene expression in committed progenitor cells and primitive progenitors with multi-lineage potentials.

RESULTS

A novel ultrafiltration-transduction method was established to culture and transduce enzyme-deficient PBPC(MPS) over a 5-day period without loss in viability and CD34 identity (n = 5). Significantly higher transduction efficiencies were achieved in primary CFC that derived from the HBS (5.8-14.2%) in comparison with those from gas-permeable bags (undetectable to 1.7%; p < 0.01). Up to 15-fold higher-than-normal enzyme activity was found in selected PBPC(MPS)-LP1CD transductants. Moreover, higher gene transfer (4.4-fold) and expression in very primitive progenitors were observed in products from the HBS compared with bag experiments as indicated by CFC derived from primitive LTCIC. Remarkably, with relatively modest gene transfer levels in LTCIC from HBS experiments, the expression of the IDUA transgene corrected the enzyme-deficiency in 5-week long-term cultures (LTC).

CONCLUSIONS

MPS I progenitor cells achieved normalized enzyme levels in LTC after transduction in a HBS system. These studies demonstrate the advantages of a bioreactor-transduction system for viral-mediated stem cell gene transfer.

Methotrexate Preconditioning Allows Sufficient Engraftment to Confer Drug Resistance in Mice Transplanted with Marrow Expressing Drug-Resistant Dihydrofolate Reductase Activity

Methotrexate (MTX) is an effective antitumor agent that has been demonstrated to be particularly useful in the treatment of hematopoietic neoplasms but causes substantial hematologic and gastrointestinal toxicity. We previously demonstrated that transplantation with transgenic marrow expressing drug-resistant dihydrofolate reductase (DHFR) into animals preconditioned by irradiation substantially protected recipient mice from the toxic side effects of methotrexate administration. Here we test the use of methotrexate itself as a preconditioning agent for engraftment of drug-resistant transgenic marrow, subsequently conferring drug resistance upon recipient animals. Administration of methotrexate beginning 1 or 2 weeks prior to or on the same day as transplantation with drug-resistant DHFR transgenic marrow did not allow sufficient engraftment to confer drug resistance to most unirradiated recipients. A small number of animals were curiously protected from lethal MTX toxicity but exhibited extremely low hematocrits and were not engrafted with stem cells, as indicated by low engraftment levels assessed in secondary transplant recipients. However, we subsequently found that MTX preconditioning allowed sufficient engraftment of DHFR transgenic marrow to confer drug resistance if MTX administration was withdrawn at the time of bone marrow transplantation (BMT) and withheld until 2 weeks post-transplant. Quantitative molecular analysis of primary and secondary recipients indicated a stem cell engraftment level of approximately 1%, consistent with previous studies demonstrating that a low level of DHFR transgenic cell engraftment was sufficient to confer drug resistance in recipient animals. We conclude that MTX can be used as a preconditioning agent for subsequent engraftment of hematopoietic stem cells, in this case conferring resistance to MTX.

Gene therapy in the treatment of intestinal inflammation

BACKGROUND

Local expression of anti-inflammatory or immunoregulatory genes may offer an alternative treatment of gastrointestinal inflammation.

DISCUSSION

We review the basic requirements for gene therapy, the possible routes of delivery, and the different strategies for specific targeting focusing on gastrointestinal inflammation.

Approaches to utilize mesenchymal progenitor cells as cellular vehicles

Mammalian cells represent a novel vector approach for gene delivery that overcomes major drawbacks of viral and nonviral vectors and couples cell therapy with gene delivery. A variety of cell types have been tested in this regard, confirming that the ideal cellular vector system for ex vivo gene therapy has to comply with stringent criteria and is yet to be found. Several properties of mesenchymal progenitor cells (MPCs), such as easy access and simple isolation and propagation procedures, make these cells attractive candidates as cellular vehicles. In the current work, we evaluated the potential utility of MPCs as cellular vectors with the intent to use them in the cancer therapy context. When conventional adenoviral (Ad) vectors were used for MPC transduction, the highest transduction efficiency of MPCs was 40%. We demonstrated that Ad primary-binding receptors were poorly expressed on MPCs, while the secondary Ad receptors and integrins presented in sufficient amounts. By employing Ad vectors with incorporated integrin-binding motifs (Ad5lucRGD), MPC transduction was augmented tenfold, achieving efficient genetic loading of MPCs with reporter and anticancer genes. MPCs expressing thymidine kinase were able to exert a bystander killing effect on the cancer cell line SKOV3ip1 in vitro. In addition, we found that MPCs were able to support Ad replication, and thus can be used as cell vectors to deliver oncolytic viruses. Our results show that MPCs can foster expression of suicide genes or support replication of adenoviruses as potential anticancer therapeutic payloads. These findings are consistent with the concept that MPCs possess key properties that ensure their employment as cellular vehicles and can be used to deliver either therapeutic genes or viruses to tumor sites.

Hematoprotection by transfer of drug-resistance genes

Myelosuppression represents a major side effect of cytotoxic anti-cancer agents. Infection due to granulocytopenia and the risk of bleeding due to thrombocytopenia compromise the potential of curative and palliative chemotherapy. Considering the many chemotherapeutic agents for which drug resistance genes have been described, and the recent improvements in vector and transduction technology, it seems conceivable that drug resistance gene transfer into a patient's autologous hematopoietic stem or progenitor cells will be able to reduce or abolish chemotherapy-induced myelosuppression.

Gene therapy progress and prospects: gene therapy of lysosomal storage disorders

Despite disappointments with early clinical studies, there is continued interest in the development of gene therapy for the group of metabolic diseases referred to as lysosomal storage disorders (LSDs). The LSDs are monogenic and several small and large, representative animal models of the human diseases are available. Further, the successful reconstitution of only low and unregulated tissue levels of the affected lysosomal enzymes are expected to be sufficient to correct the disease at least in the case of some of the LSDs. For these reasons, they are perceived as good models for the evaluation of different gene delivery vectors and of different strategies for treating chronic genetic diseases by gene transfer. In this review, we will highlight the progress that has been made over the past 2 years in preclinical research for this group of disorders and speculate on future prospects.

Occurrence of leukaemia following gene therapy of X-linked SCID

Recombinant viral vectors have allowed gene transfer to be developed as a promising approach to the treatment of genetic diseases. Recently, gene therapy of children with X-linked severe combined immune deficiency resulted in impressive levels of immune reconstitution--a triumph that was later overshadowed by the development of leukaemia in two patients. What were the causes of this cancer, and how can the therapeutic benefits of gene therapy be achieved while minimizing risk to the patient?

Genetically modified NT2N human neuronal cells mediate long-term gene expression as CNS grafts in vivo and improve functional cognitive outcome following experimental traumatic brain injury

Human Ntera-2 (NT2) cells can be differentiated in vitro into well-characterized populations of NT2N neurons that engraft and mature when transplanted into the adult CNS of rodents and humans. They have shown promise as treatments for neurologic disease, trauma, and ischemic stroke. Although these features suggest that NT2N neurons would be an excellent platform for ex vivo gene therapy in the CNS, stable gene expression has been surprisingly difficult to achieve in these cells. In this report we demonstrate stable, efficient, and nontoxic gene transfer into undifferentiated NT2 cells using a pseudotyped lentiviral vector encoding the human elongation factor 1-alpha promoter and the reporter gene eGFP. Expression of eGFP was maintained when the NT2 cells were differentiated into NT2N neurons after treatment with retinoic acid. When transplanted into the striatum of adult nude mice, transduced NT2N neurons survived, engrafted, and continued to express the reporter gene for long-term time points in vivo. Furthermore, transplantation of NT2N neurons genetically modified to express nerve growth factor significantly attenuated cognitive dysfunction following traumatic brain injury in mice. These results demonstrate that defined populations of genetically modified human NT2N neurons are a practical and effective platform for stable ex vivo gene delivery into the CNS.

Gene therapy in infants with severe combined immunodeficiency

Severe combined immunodeficiencies (SCID) are rare disorders that represent paediatric medical emergencies, as the outcome for affected patients can easily be fatal unless proper treatment is performed. The only curative treatment for SCID is reconstitution of the patient's immunity. For more than 30 years, allogeneic bone marrow transplantation (BMT) has been extremely successful for SCID. However, BMT often results in only incomplete restoration of B cell function in treated patients, especially when haploidentical donors are used. In addition, BMT can be associated with severe complications such as graft-versus-host disease (GVHD). Alternative forms of therapy for SCID are therefore desirable. Genetic correction of peripheral T lymphocytes and/or haematopoietic stem cells (HSCs) by retrovirally mediated gene transfer has been attempted for patients with SCID due to adenosine deaminase deficiency, the first genetic disease targeted in clinical gene therapy trials with very limited success, overall. After these pioneer trials, recent progress has led to significant improvement of gene transfer techniques and better understanding of HSC biology which has culminated in the recent success of a gene therapy trial for patients affected with X-linked SCID (X-SCID). In this trial, patients with X-SCID received autologous bone marrow stem/progenitor cells which had been retrovirally transduced with a therapeutic gene. Based on the current follow-up, the overall efficacy of this gene therapy procedure is to be considered similar to or even better than that achievable by allogeneic BMT, because patients were not exposed to the risks of GVHD. Although these exciting results have clearly demonstrated that gene therapy is a feasible therapeutic option for X-SCID, they have also raised important questions regarding the long-term outcome of this experimental procedure and the possibility of translating this success into applications for other forms of SCID.

Gene delivery using herpes simplex virus vectors

Herpes simplex virus (HSV) is a neurotropic DNA virus with many favorable properties as a gene delivery vector. HSV is highly infectious, so HSV vectors are efficient vehicles for the delivery of exogenous genetic material to cells. Viral replication is readily disrupted by null mutations in immediate early genes that in vitro can be complemented in trans, enabling straightforward production of high-titre pure preparations of non-pathogenic vector. The genome is large (152 Kb) and many of the viral genes are dispensable for replication in vitro, allowing their replacement with large or multiple transgenes. Latent infection with wild-type virus results in episomal viral persistence in sensory neuronal nuclei for the duration of the host lifetime. Transduction with replication-defective vectors causes a latent-like infection in both neural and non-neural tissue; the vectors are non-pathogenic, unable to reactivate and persist long-term. The latency active promoter complex can be exploited in vector design to achieve long-term stable transgene expression in the nervous system. HSV vectors transduce a broad range of tissues because of the wide expression pattern of the cellular receptors recognized by the virus. Increasing understanding of the processes involved in cellular entry has allowed preliminary steps to be taken towards targeting the tropism of HSV vectors. Using replication-defective HSV vectors, highly encouraging results have emerged from recent pre-clinical studies on models of neurological disease, including glioma, peripheral neuropathy, chronic pain and neurodegeneration. Consequently, HSV vectors encoding appropriate transgenes to tackle these pathogenic processes are poised to enter clinical trials.

Obstacles to human hematopoietic stem cell transduction by recombinant adeno-associated virus 2 vectors

Recombinant adeno-associated virus 2 (AAV) vectors have proven to be a potentially useful alternative to the more commonly used retroviral and adenoviral vectors for gene therapy in humans. Their safety and efficacy in Phase I clinical trials for gene therapy of cystic fibrosis and hemophilia B have been well documented, and their remarkable versatility and efficacy in a wide variety of pre-clinical models of human diseases have catapulted these vectors to the forefront. AAV vectors have been shown to be particularly well suited for transduction of brain and muscle cells. However, controversies exist with regard to their utility as a vector for gene transfer into human hematopoietic stem cells. On the one hand, some investigators have concluded that AAV vectors do not transduce hematopoietic stem cells at all, and others have reported that stem cell transduction requires enormously high vector-to-cell ratios. On the other hand, some investigators have reported high-efficiency transduction of human hematopoietic stem cells at low vector-to cell ratios. This article will provide a historical perspective as well as attempt to elaborate the reasons behind these controversies which have become clearer by studies focused on understanding, at the molecular level, the fundamental aspects of the life cycle of recombinant AAV vectors.

Gene therapy of X-linked severe combined immunodeficiency

Severe combined immunodeficiency (SCID) conditions appear to be the best possible candidates for a gene therapy approach. Transgene expression by lymphocyte precursors should confer to these cells a selective growth advantage that gives rise to long-lived T-lymphocytes. This rationale was used as a basis for a clinical trial of the SCID-X1 disorder caused by common gamma (gamma c) gene mutations. This trial consists of ex vivo retroviral-mediated (MFG-B2 gamma c vector) gammac gene transfer into marrow CD34+ cells in CH-296 fibronectin fragment-coated bags. Up to now, 9 patients with typical SCID-X1 diagnosed within the first year of life and lacking an HLA-identical donor have been enrolled. More than 2 years' assessment of 5 patients and more than 1 year for 7 patients provide evidence for full development of functional, mature T-cells in the absence of any adverse effects. Functional transduced natural killer cells are also detectable, although in low numbers. All but 1 patient with T-cell immunity have also developed immunoglobulin production, which has alleviated the need for intravenous immunoglobulin substitution despite a low detection frequency of transduced B-cells. These 8 patients are doing well and living in a normal environment. This yet successful gene therapy demonstrates that in a setting where transgene expression provides a selective advantage, a clinical benefit can be expected.

Modification of multiple transcriptional regulatory elements in a Moloney murine leukemia virus gene transfer vector circumvents silencing in fibroblast grafts and increases levels of expression of the transferred enzyme

Down-regulation of retroviral vector expression occurs in a number of cell types after transplantation. Although a number of vector elements have been shown to affect expression in specific experimental situations, the results can vary depending on the specific cDNA being expressed, the individual retroviral elements included in vectors, the promoter, or the inclusion of selectable markers. In previous experiments with the lysosomal enzyme beta-glucuronidase, silencing has occurred in more than 95% of transduced cells regardless of the position of the expression unit within the vector, whether a eukaryotic or viral promoter was used, whether a bacterial selectable marker gene was present or not, the target cell type, or the species of the host. It has been a consistent finding that a small number of continuously expressing cells persist for long periods after transplantation. In this study we found that deletion of all the transcriptional regulatory elements from the vector LTR, inclusion of a permissive primer binding site sequence, and use of a eukaryotic housekeeping promoter could greatly increase the number of expressing cells in fibroblast grafts in subcutaneous neo-organs and in the brain. Furthermore, the level of enzyme expression was increased five-fold on a per positive cell basis, indicating that the deleted regulatory elements were exerting a negative effect on expression in the few cells that were positive before modification of the vector. This resulted in more than a 50-fold increase in total activity compared with the previous highest expressing vector.

Gene repair and transposon-mediated gene therapy

The main strategy of gene therapy has traditionally been focused on gene augmentation. This approach typically involves the introduction of an expression system designed to express a specific protein in the transfected cell. Both the basic and clinical sciences have generated enough information to suggest that gene therapy would eventually alter the fundamental practice of modern medicine. However, despite progress in the field, widespread clinical applications and success have not been achieved. The myriad deficiencies associated with gene augmentation have resulted in the development of alternative approaches to treat inherited and acquired genetic disorders. One, derived primarily from the pioneering work of homologous recombination, is gene repair. Simply stated, the process involves targeting the mutation in situ for gene correction and a return to normal gene function. Site-specific genetic repair has many advantages over augmentation although it too is associated with significant limitations. This review outlines the advantages and disadvantages of gene correction. In particular, we discuss technologies based on chimeric RNA/DNA oligonucleotides, single-stranded and triplex-forming oligonucleotides, and small fragment homologous replacement. While each of these approaches is different, they all share a number of common characteristics, including the need for efficient delivery of nucleic acids to the nucleus. In addition, we review the potential application of a novel and exciting nonviral gene augmentation strategy--the Sleeping Beauty transposon system.

Adenosine deaminase deficiency with mosaicism for a "second-site suppressor" of a splicing mutation: decline in revertant T lymphocytes during enzyme replacement therapy

Four patients from 3 Saudi Arabian families had delayed onset of immune deficiency due to homozygosity for a novel intronic mutation, g.31701T>A, in the last splice acceptor site of the adenosine deaminase (ADA) gene. Aberrant splicing mutated the last 4 ADA amino acids and added a 43-residue "tail" that rendered the protein unstable. Mutant complementary DNA (cDNA) expressed in Escherichia coli yielded 1% of the ADA activity obtained with wild-type cDNA. The oldest patient, 16 years old at diagnosis, had greater residual immune function and less elevated erythrocyte deoxyadenosine nucleotides than his 4-year-old affected sister. His T cells and Epstein-Barr virus (EBV) B cell line had 75% of normal ADA activity and ADA protein of normal size. DNA from these cells and his whole blood possessed 2 mutant ADA alleles. Both carried g.31701T>A, but one had acquired a deletion of the 11 adjacent base pair, g.31702-12, which suppressed aberrant splicing and excised an unusual purine-rich tract from the wild-type intron 11/exon 12 junction. During ADA replacement therapy, ADA activity in T cells and abundance of the "second-site" revertant allele decreased markedly. This finding raises an important issue relevant to stem cell gene therapy.

Targeting transgene expression to antigen-presenting cells derived from lentivirus-transduced engrafting human hematopoietic stem/progenitor cells

Hematopoietic stem cells (HSCs) represent an important target for the treatment of various blood disorders. As the source of critical cells within the immune system, genetic modification of HSCs can also be used to modulate immune responses. The effectiveness of HSC-mediated gene therapy largely depends on efficient gene delivery into long-term repopulating progenitors and targeted transgene expression in an appropriate progeny of the transduced pluripotent HSCs. Self-inactivating (SIN) lentiviral vectors have been demonstrated to be capable of transducing mitotically inactive cells, including HSCs, and accommodating a nonviral promoter to control the transgene expression in transduced cells. In this study, we constructed 2 SIN lentiviral vectors, EF.GFP and DR.GFP, to express the green fluorescent protein (GFP) gene controlled solely by the promoter of either a housekeeping gene EF-1alpha or the human HLA-DRalpha gene, which is selectively expressed in antigen-presenting cells (APCs). We demonstrated that both vectors efficiently transduced human pluripotent CD34+ cells capable of engrafting nonobese diabetic/severe combined immunodeficiency (NOD/SCID) mice. When the EF.GFP vector was used, constitutive high-level GFP expression was obtained in all the human HSC progeny detectable in NOD/SCID mice and in subsequent in vitro differentiation assays, indicating that engrafting human HSCs have been transduced. In contrast, the DR.GFP vector mediated transgene expression specifically in human HLA-DR+ cells and highly in differentiated dendritic cells (DCs), which are critical in regulating immunity. Furthermore, human DCs derived from transduced and engrafted human cells potently stimulated allogeneic T-cell proliferation. This study demonstrated successful targeting of transgene expression to APCs/DCs after stable gene transduction of pluripotent HSCs.

Multiple applications for replication-defective herpes simplex virus vectors

Herpes simplex virus (HSV) is a neurotropic DNA virus. The viral genome is large (152 kb), and many genes are dispensable for viral function, allowing insertion of multiple or large transgene expression cassettes. The virus life cycle includes a latent phase, during which the viral genome remains as a stable episomal element within neuronal nuclei for the lifetime of the host, without disturbing normal function. We have exploited these features of HSV to construct a series of nonpathogenic gene therapy vectors that efficiently deliver therapeutic and experimental transgenes to neural and non-neural tissue. Importantly, transgene expression may be sustained long term; reporter gene expression has been demonstrated for over a year in the nervous system. This article discusses the generation of replication-defective HSV vectors and reviews recent studies investigating their use in several animal models of human disease. We have demonstrated correction or prevention of a number of important neurological phenotypes, including neurodegeneration, chronic pain, peripheral neuropathy, and malignancy. In addition, HSV-mediated transduction of non-neurological tissues allows their use as depot sites for synthesis of circulating and locally acting secreted proteins. New applications for this vector system include the genetic modification of stem cell populations; this may become an important means to direct cellular differentiation or deliver therapeutic genes systemically. Replication-defective HSV vectors are an effective and flexible vehicle for the delivery of transgenes to numerous tissues, with multiple applications.

Gene therapy for primary immune deficiencies

Primary immunodeficiency diseases have been important targets of corrective gene transfer approaches since the very early days of gene therapy. The potential for selective survival advantage of gene-corrected cells over populations carrying the mutated, causative gene translates into the possibility of obtaining clinical meaningful results in patients with primary immunodeficiency diseases even if levels of gene transfer are low. This critical prospect has fueled the interest of researchers since the mid-1980s and has recently determined the success of a clinical trial of gene therapy for X-linked severe combined immunodeficiency.

Murine epidermal label-retaining cells isolated by flow cytometry do not express the stem cell markers CD34, Sca-1, or Flk-1

Keratinocyte stem cells are present in the murine epidermis, based on both in vitro and in vivo evidence, and better characterization of these cells remains an active goal. Because keratinocyte stem cells are believed to cycle slowly, a good method for identification is based on their ability to retain nucleoside analog, such as bromodeoxyuridine. Adult stem cells have been identified in other tissues, including hematopoietic, neural, and skeletal muscle, and stem cell surface markers have been characterized. We wanted to determine if cell-surface markers present on both hematopoietic and skeletal muscle stem cells (CD34, Sca-1, and Flk-1) were also present on keratinocyte stem cells, and could be used to identify them. The cell-surface expression of cells that retained bromodeoxyuridine label for at least 21 d was compared with that of nonlabel-retaining cells. Double-labeling for flow cytometric analysis was employed, and label-retaining cells were found to lack expression of the tested markers. Beta1 integrin levels were also evaluated, and although high expression was found on label-retaining cells, it was not specific for these cells.

Baboon mesenchymal stem cells can be genetically modified to secrete human erythropoietin in vivo

Human mesenchymal stem cells (MSCs) are capable of differentiating into multiple mesenchymal lineages including chondrocytes, osteocytes, adipocytes, and marrow stromal cells. Using a nonhuman primate model, we evaluated nonhuman primate MSCs as targets for gene therapy. Baboon MSCs (bMSCs) cultured from bone marrow aspirates appeared as a homogeneous population of spindle-shaped cells. bMSCs were capable of differentiating into adipocytes and osteocytes in vitro and chondrocytes in vivo. bMSCs were genetically modified with a bicistronic vector encoding the human erythropoietin (hEPO) gene and the green fluorescent protein (GFP) gene. Transduction efficiencies ranged from 72 to 99% after incubation of MSCs with retroviral supernatant. Transduced cells produced from 1.83 x 10(5) to 7.12 x 10(5) mIU of hEPO per 10(6) cells per 24 hr in vitro before implantation. To determine the capacity of bMSCs to express hEPO in vivo, transduced bMSCs were injected intramuscularly in NOD/SCID mice. In a separate experiment, transduced bMSCs were loaded into immunoisolatory devices (IIDs) and surgically implanted into either autologous or allogeneic baboon recipients. Human EPO was detected in the serum of NOD/SCID mice for up to 28 days and in the serum of five baboons for between 9 and 137 days. NOD/SCID mice experienced sharp rises in hematocrit after intramuscular injection of hEPO-transduced bMSCs. The baboon that expressed hEPO for 137 days experienced a statistically significant (p < 0.04) rise in its hematocrit. These data demonstrate that nonhuman primate MSCs can be engineered to deliver a secreted and biologically active gene product. Therefore, human MSCs may be an effective target for future human gene therapy trials.

Approaches to gene therapy for human immunodeficiency virus infection

Much progress has been made in developing new and more efficient treatments for human immunodeficiency virus (HIV) infection, the cause of acquired immunodeficiency syndrome (AIDS). However, the scope of the HIV epidemic and the limitations of existing treatments necessitate the continued development of novel treatment strategies. Gene therapy is one such forward-looking strategy. Gene therapy approaches for HIV infection include efforts to interfere with viral replication directly by engineering HIV-resistant cells or indirectly by eliminating infected cells from the body, primarily by eliciting a therapeutic immune response to destroy HIV-infected cells. Although the prospect of gene therapy as a routine treatment for HIV infection remains distant, continuous progress is being made, which should also have implications for gene therapy strategies for a variety of other diseases. This article reviews some of the strategies for investigating the feasibility of gene transfer for the treatment of HIV infection.

Gene transfer into nonhuman primate hematopoietic stem cells: implications for gene therapy

Hematopoietic stem cells (HSCs) are desirable targets for gene therapy because of their self-renewal and multilineage differentiation abilities. Retroviral vectors are extensively used for HSC gene therapy. However, the initial human trials of HSC gene marking and therapy showed that the gene transfer efficiency into human HSCs with retroviral vectors was very low in contrast to the much higher efficiency observed in murine experiments. The more quiescent nature of human HSCs and the lower density of retroviral receptors on them hindered the efficient gene transfer with retroviral vectors. Since nonhuman primates have marked similarity to humans in all aspects including the HSC biology, their models are considered to be important to evaluate and improve gene transfer into human HSCs. Using these models, clinically relevant levels (around 10% or even more) of gene-modified cells in peripheral blood have recently been achieved after gene transfer into HSCs and their autologous transplantation. This has been made possible by improving ex vivo transduction conditions such as introduction of Flt-3 ligand and specific fibronectin fragment (CH-296) into ex vivo culture during transduction, and the use of retroviral vectors pseudotyped with the gibbon ape leukemia virus or feline endogenous retrovirus envelope. Other strategies including the use of lentiviral vectors and in vivo selective expansion of gene-modified cells with the drug resistance gene or selective amplifier gene (also designated the molecular growth switch) are now being tested to further increase the fraction of gene-modified cells using nonhuman primate models. In addition to the high gene transfer efficiency, high-level and long-term expression of transgenes in human HSCs and their progeny is also required for effective HSC gene therapy. For this purpose, other backbones of retroviral vectors such as the murine stem cell virus and cis-DNA elements, such as the ss-globin locus control region and the chromatin insulator, also need to be tested in nonhuman primate models. Nonhuman primate studies will continue to provide an important framework for human HSC gene therapy. Well-designed nonhuman primate studies will also offer unique insights into the HSCs, immune system, and transplantation biology characteristic of large animals.

Sonic hedgehog induces the proliferation of primitive human hematopoietic cells via BMP regulation

A pool of stem cells that arise from the mesoderm during embryogenesis initiates hematopoiesis. However, factors that regulate the expansion of blood stem cells are poorly understood. We show here that cytokine-induced proliferation of primitive human hematopoietic cells could be inhibited with antibodies to hedgehog (Hh). Conversely, Sonic hedgehog (Shh) treatment induced the expansion of pluripotent human hematopoietic repopulating cells detected in immunodeficient mice. Noggin, a specific inhibitor of bone morphogenetic protein 4 (BMP-4), was capable of inhibiting Shh-induced proliferation in a similar manner to anti-Hh; however, anti-Hh had no effect on BMP-4-induced proliferation. Our study shows that Shh functions as a regulator of primitive hematopoietic cells via mechanisms that are dependent on downstream BMP signals.

Viral vectors for gene therapy: the art of turning infectious agents into vehicles of therapeutics

Considered by some to be among the simpler forms of life, viruses represent highly evolved natural vectors for the transfer of foreign genetic information into cells. This attribute has led to extensive attempts to engineer recombinant viral vectors for the delivery of therapeutic genes into diseased tissues. While substantial progress has been made, and some clinical successes are over the horizon, further vector refinement and/or development is required before gene therapy will become standard care for any individual disorder.

Gene therapy for Fabry disease

Fabry disease is an X-linked metabolic disorder caused by a deficiency of alpha-galactosidase A (alpha-Gal A). Lack of this lysosomal hydrolase results in the accumulation of galactose-terminal glycosphingolipids in a number of tissues, including vascular endothelial cells. Premature death is predominantly associated with vascular conditions of the heart, kidneys and brain. Historically, treatment has largely been palliative. Alternative treatments for many lysosomal storage diseases have been developed, including allogeneic organ and bone marrow transplantation, enzyme replacement therapy, and gene therapy. Significant clinical risks still exist with allogeneic transplantations. Alpha-Gal A enzyme replacement therapy has been implemented in clinical trials. This approach has been effective but may have limitations for long-term systemic or cost-effective correction. As an alternative, gene therapy approaches, involving a variety of gene delivery systems, have been pursued for the amelioration of Fabry disease. Fabry disease is a compelling disorder for gene therapy, as target cells are readily accessible and relatively low levels of enzyme correction may suffice to reduce storage. Importantly, metabolic cooperativity effects are also manifested in Fabry disease, wherein corrected cells secrete alpha-Gal A that can correct bystander cells. In addition, a broad therapeutic window probably exists, and mouse models of Fabry disease have been generated to assist studies. As an example, in vitro and in vivo studies using alpha-Gal A-transduced haematopoietic cells from Fabry mice have demonstrated enzymatic correction of recipient cells and dissemination of alpha-Gal A upon transplantation, leading to reduced lipid storage in a number of clinically relevant organs. This corrective enzymatic effect has recently been shown to be even further enhanced upon pre-selection of therapeutically transduced cells prior to transplantation. This review will briefly detail current gene delivery methods and summarize results to date in the context of gene therapy for Fabry disease.

Gene therapy of lymphoid primary immunodeficiencies

Gene therapy offers an attractive option to the most severe forms of primary immunodeficiency diseases. Identification of disease-associated genes as well as advances in the technology of gene transfer into hematopoietic progenitor cells have set the basis for the first clinical trials. Settings characterized by the potential for a selective advantage provided to transduced cells are the first diseases to target. The recent example of successful treatment of severe combined immunodeficiency-X1 (gammac deficiency) illustrates this potential.

Issues in the manufacture and transplantation of genetically modified hematopoietic stem cells

The advent of safe and practical means to correct, enhance or protect blood cells at the genetic level offers tantalizing therapeutic perspectives. At present, gene delivery using a replication-defective retrovirus is the most efficient method to stably transduce hematopoietic cells. The successful adaptation of retroviral infection to hematopoietic stem cells requires optimized transduction conditions that maximize gene transfer while preserving the cells' potential for engraftment and longterm hematopoiesis. The successful establishment of effective transduction protocols hinges on retrovirus biology as well as stem cell and transplantation biology. Interestingly, the genetic approach could permit novel strategies to promote host repopulation by transplanted stem cells. However, regulated and predictable expression of any transgene integrated at random chromosomal locations cannot be taken for granted. Investigation of the control of transgene expression and prevention of vector silencing will become increasingly important.