Gene therapy of human severe combined immunodeficiency (SCID)-X1 disease

Self-selection by genetically modified committed lymphocyte precursors reverses the phenotype of JAK3-deficient mice without myeloablation

Janus kinase 3 (JAK3) is an essential component of cytokine receptor signal transduction pathways required for normal lymphocyte development and function. JAK3 deficiency in both mice and humans results in severe combined immunodeficiency (SCID) and increased susceptibility to opportunistic infections. We have previously shown that JAK3 gene transfer into irradiated recipients could restore immune function. However, since this toxic conditioning would be undesirable for infants in a clinical application, we have tested whether immune function could be restored in nonmyeloablated JAK3-deficient (-/-) mice. Murine JAK3 retroviral vectors were transduced into hematopoietic stem cells from the livers of newborn JAK3(-/-) mice. These cells were then injected intraperitoneally into nonirradiated JAK3(-/-) neonates. Transduced cells were detectable in these mice at time points 4 to 6 months after injection and resulted in significant correction of T and B lymphocyte numbers and circulating immunoglobulin (Ig) levels. After immune challenge with a dose of influenza A virus that was lethal to nonmanipulated JAK3(-/-) mice, mice injected with transduced cells showed development of circulating virus-specific IgG and enhanced survival. This work shows that the large selective advantage for JAK3-corrected lymphoid cells may be sufficient to overcome the need for myeloablative conditioning in JAK3 gene therapy protocols.

Genomics and gene therapy. Artificial chromosomes coming to life

One of the biggest obstacles to gene therapy is the delivery of the therapeutic gene to the target tissue so that it is appropriately expressed. In his Perspective, Willard looks at the potential advantages of using a human artificial chromosome to maintain expression of a therapeutic gene and discusses some of the hurdles yet to be overcome before this gene delivery system can be tried out in the clinic.

The notch ligand jagged-1 represents a novel growth factor of human hematopoietic stem cells

The Notch ligand, Jagged-1, plays an essential role in tissue formation during embryonic development of primitive organisms. However, little is known regarding the role of Jagged-1 in the regulation of tissue-specific stem cells or its function in humans. Here, we show that uncommitted human hematopoietic cells and cells that comprise the putative blood stem cell microenvironment express Jagged-1 and the Notch receptors. Addition of a soluble form of human Jagged-1 to cultures of purified primitive human blood cells had modest effects in augmenting cytokine-induced proliferation of progenitors. However, intravenous transplantation of cultured cells into immunodeficient mice revealed that human (h)Jagged-1 induces the survival and expansion of human stem cells capable of pluripotent repopulating capacity. Our findings demonstrate that hJagged-1 represents a novel growth factor of human stem cells, thereby providing an opportunity for the clinical utility of Notch ligands in the expansion of primitive cells capable of hematopoietic reconstitution.

[X-linked lymphoproliferative syndrome (XLP syndrome): from clinic to gene]

X-linked lymphoproliferative syndrome (XLP, also known as Duncan's disease) is characterised by an extreme sensitivity to Epstein Barr virus (EBV), resulting in a complex phenotype manifested by severe or fatal mononucleosis, acquired hyogammaglobulinemia and malignant lymphoma. The gene responsible for XLP has recently been identified by a positional cloning and a functional cloning approach and encodes a small cytoplasmic protein involved in signal transduction of T and NK cells. The identification of the XLP gene will permit direct diagnosis of XLP in families with a single affected male. Recent progress in immunobiology and genetics of this primary immunodeficiency disease are presented.

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.

Gene therapy: trials and tribulations

The art and science of gene therapy has received much attention of late. The tragic death of 18-year-old Jesse Gelsinger, a volunteer in a Phase I clinical trial, has overshadowed the successful treatment of three children suffering from a rare but fatal immunological disease. In the light of the success and tragedy, it is timely to consider the challenges faced by gene therapy--a novel form of molecular medicine that may be poised to have an important impact on human health in the new millennium.

Characterization of chemokine receptors expressed in primitive blood cells during human hematopoietic ontogeny

Chemokines are capable of regulating a variety of fundamental processes of hematopoietic cells that include proliferation, differentiation, and migration. To evaluate potential chemokine signaling pathways important to the regulation of primitive human hematopoietic cells, we examined chemokine receptor expression of highly purified subpopulations of uncommitted human blood cells. CXCR1-, CXCR2-, CXCR4-, and CCR5-expressing cells were detected by flow cytometry among human blood subsets depleted of lineage-restricted cells (Lin(-)) derived from adult bone marrow, mobilized peripheral blood, cord blood (CB), and circulating fetal blood. Although these chemokine receptors could be detected on Lin(-) cells throughout human development, only CXCR4 could be detected in CD34(-)CD38(-)Lin(-) and CD34(+)CD38(-)Lin(-) subfractions enriched for stem cell function, suggesting that independent of ontogeny, CXCR4-mediated signals are critical to primitive hematopoiesis. Distinct to other stages of human hematopoietic development, primitive CB cells expressed higher levels of CXCR1, CXCR2, CCR5, and CXCR4 on both CD34(-)CD38(-)Lin(-) and CD34(+)CD38(-)Lin(-) subsets. Isolation of these fractions revealed expression of additional chemokine receptors CCR7, CCR8, and Bonzo (STRL133), whereas BOB (GPR15) could not be detected. Our study illustrates that rare uncommitted hematopoietic cells express chemokine receptors not previously associated with primitive human blood cells. Based on these results, we suggest that signaling pathways mediated by chemokine receptors identified here may play a fundamental role in hematopoietic stem cell regulation and provide alternative receptor targets for retroviral pseudotyping for genetic modification of repopulating cells.

Progress in the use of gene transfer methods to treat genetic blood diseases

A report by French physician-scientists suggests a successful application of gene transfer methods in the treatment of two children with severe combined immunodeficiency (SCID) due to defective interleukin 2 receptor common gamma chain. The protocol used in this clinical trial was derived from a number of preclinical and basic studies leading to improved transduction of hematopoietic stem and primitive progenitor cells using retrovirus vectors. These improvements have also been shown to impact transduction of a long-lived progenitor cell in a chemotherapy protocol in cancer patients. The improved results of these human trials come during a period of increased scrutiny and criticism of human gene therapy trials, due, in part, to significant toxicities in some trials using adenovirus-based vectors. The potential efficacy versus toxicity of phase I trials of human gene therapy is also under question. After many years of research, however, there appears to be real evidence that genetic diseases may be successfully treated by gene transfer techniques. Future clinical studies should be based on continued progress in the understanding of the toxicology of gene delivery systems, vector technology, and target cell manipulation.

TGF-beta-based immunotherapy for cancer: breaching the tumor firewall

Many malignant cells secrete transforming growth factor-beta (TGF-beta), a potent immunosuppresant, suggesting that TGF-beta production may represent a significant tumor escape mechanism from host immunosurveillance. Establishment of a leukocyte subpopulation with disrupted TGF-beta signaling in the tumor-bearing host offers a potential means for immunotherapy of cancer. Downregulation of TGF-beta secretion in tumor cells results in restoration of immunogenicity in the host, while T-cell insensitivity to TGF-beta results in accelerated differentiation and autoimmunity, elements of which may be required in order to combat self-antigen-expressing tumors in a tolerized host. The rationale, approaches, and potential pitfalls of this strategy will be discussed.

X-linked severe combined immunodeficiency: from molecular cause to gene therapy within seven years

X-linked severe combined immunodeficiency (XSCID) is the most common form of SCID. The discovery of the genetic defect in this disease, namely mutations in the gene encoding the common cytokine receptor gamma chain, gammac, was reported just over seven years ago. In the subsequent period, a tremendous amount of knowledge about the biology and function of this protein has been generated. Moreover, gammac-knockout mice have been generated and their immune systems successfully reconstituted by gene therapy. Furthermore, initial attempts at using gene therapy to treat patients with XSCID have been successful for more than ten months, making this disease perhaps the most promising to date for treatment with such a strategy.

In vivo competitive studies between normal and common gamma chain-defective bone marrow cells: implications for gene therapy

Corrective gene transfer into hematopoietic stem cells (HSCs) is being investigated as therapy for X-linked severe combined immunodeficiency (XSCID) and it is hoped that selective advantage of gene-corrected HSCs will help in achieving full immune reconstitution after treatment. Lines of evidence from the results of allogeneic bone marrow transplantation in patients with XSCID support this hypothesis that, however, has not been rigorously tested in an experimental system. We studied the competition kinetics between normal and XSCID bone marrow (BM) cells using a murine bone marrow transplantation (BMT) model. For easy chimerism determination, we used genetic marking with retrovirus-mediated expression of the enhanced green fluorescent protein (EGFP). We found that XSCID BM cells were able to compete with normal BM cells for engraftment of myeloid lineages in a dose-dependent manner, whereas we observed selective repopulation of T, B, and NK cells deriving from normal BM cells. This was true despite the evidence of competitive engraftment of XSCID lineage marker-negative/c-Kit-positive (Lin-/c-Kit+) cells in the bone marrow of treated animals. From these results we extrapolate that genetic correction of XSCID HSCs will result in selective advantage of gene-corrected lymphoid lineages with consequent restoration of lymphocyte populations and high probability of clinical benefit.

[Mechanisms of repair and radiation-induced mutagenesis in higher eukaryotes]

Cells of higher eukaryotes possess several very efficient systems for the repair of radiation-induced lesions in DNA. Different strategies have been adopted at the cellular level to remove or even tolerate various types of lesions in order to assure survival and limit the mutagenic consequences. In mammalian cells, the main DNA repair systems comprise direct reversion of damage, excision of damage and exchange mechanisms with intact DNA. Among these, the direct ligation of single strand breaks (SSB) by a DNA ligase and the multi-enzymatic repair systems of mismatch repair, base and nucleotide excision repair as well as the repair of double strand breaks (DSB) by homologous recombination or non homologous end-joining are the most important systems. Most of these processes are error-free except the non homologous end-joining pathway used mainly for the repair of DSB. Moreover, certain lesions can be tolerated by more or less accurately acting polymerases capable of performing translesional DNA syntheses. The DNA repair systems are intimately integrated in the network of cellular regulation. Some of their components are DNA damage inducible. Radiation-induced mutagenesis is largely due to unrepaired DNA damage but also involves error-prone repair processes like the repair of DSB by non-homologous end-joining. Generally, mammalian cells are well prepared to repair radiation-induced lesions. However, some questions remain to be asked about mechanistic details and efficiencies of the systems for removing certain types of radiation-damage and about their order and timing of action. The answers to these questions would be important for radioprotection as well as radiotherapy.

Retrovirus-mediated transduction of primary ZAP-70-deficient human T cells results in the selective growth advantage of gene-corrected cells: implications for gene therapy

Humans lacking the ZAP-70 protein tyrosine kinase present with an absence of CD8+ T cells and defective CD4+ T cells in the periphery. This severe combined immunodeficiency is fatal unless treated by allogeneic bone marrow transplantation. However, in the absence of suitable marrow donors, the development of alternative forms of therapy is desirable. Because lymphocytes are long-lived, it is possible that introduction of the wild-type ZAP-70 gene into CD4+ ZAP-70-deficient T cells will restore their immune function in vivo. Initial investigations evaluating the feasibility of gene therapy for ZAP-70 deficiency were performed using HTL V-I-transformed lymphocytes. Although transformation was useful in circumventing problems associated with the maintenance of ZAP-70-deficient T cells and low gene transfer levels, the presence of HTL V-I precluded any biological studies. Here, we investigated a retrovirus-mediated approach for the correction of primary T cells derived from two ZAP-70-deficient patients. Upon introduction of the wild-type ZAP-70 gene, TCR-induced MAPK activation, IL-2 secretion and proliferation were restored to approximately normal levels. Importantly, this gain-of-function was associated with a selective growth advantage of gene-corrected cells, thereby indicating the feasibility of a gene therapy-based strategy.

Bone Marrow Transplantation for Non-Malignant Disease

This article reviews the experience in hematopoietic stem cell transplantation (HSCT) for non-malignant disease. HSCT has long been applied as treatment of life-threatening congenital immunodeficiency and metabolic diseases. In Section I, Dr. Parkman reviews that experience for severe combined immunodeficiency, Wiscott-Aldrich syndrome, hyper IGM syndrome, Chédiak-Higashi disease and hereditary lymphohistiocytosis. The value of HSCT in genetic metabolic diseases such as osteopetrosis, osteogenesis imperfecta and the storage diseases are reviewed. In Section II, Dr. Walters reviews the experience over the last decade with allogeneic stem cell transplantation in patients with thalassemia major and sickle cell disease. In Section III, Dr. Sullivan reviews the more recent investigations using stem cell transplantation in patients with advanced autoimmune diseases such as systemic sclerosis, systemic lupus erythematosus, multiple sclerosis and juvenile rheumatoid arthritis. The pathogenesis and outcome with conventional care of these patients, the selection criteria and current results for HSCT, and the future directions in clinical research and patient care using this modality are addressed.

Congenital Hemorrhagic Disorders: New Insights into the Pathophysiology and Treatment of Hemophilia

The diagnostic and treatment strategies related to hemophilia are rapidly evolving. This article focuses on some of the issues of importance. Diagnostic advances in molecular genetics are reviewed by Dr. Ginsburg in Section I, including the current state of knowledge regarding the mutations responsible for hemophilia, with reference to the potential clinical applications of DNA diagnosis and prenatal testing.

Within the area of new therapeutic approaches in hemophilia, recombinant factor VIII and factor IX concentrates, their use and availability are addressed by Dr. Lusher in Section II as well as the use of so-called “primary prophylaxis” with the aim of decreasing long-term hemophilia athropathy. The use of radionuclide synovectomy as replacement for more invasive methods is also reviewed.

Various approaches to the ongoing challenge of the management of hemophilia patients with inhibitors against factor VIII and factor IX are reviewed by Dr. Hedner in Section III, including the principles for immune tolerance induction and the use of recombinant factor VIIa to induce hemostasis in bleeding patients with inhibitors.

In Section IV, gene therapy in hemophilia is reviewed by Dr. High, who focuses on recent developments in the rapidly moving field of gene therapy for hemophilia. Three phase I trials of gene therapy for hemophilia were initiated in 1999, and additional proposed trials are currently in the regulatory review process. Certain aspects of the pathophysiology of hemophilia make it an attractive model for a gene-based approach to treatment. These include latitude in choice of target tissue, a wide therapeutic window, the availability of small and large animal models of the disease, and the ease of determining therapeutic efficacy. Since there is very little published information regarding the ongoing trials, this section reviews the approaches being used, the published pre-clinical data, and considerations affecting clinical trial design in hemophilia gene therapy.

Gene Therapy 2000

This article reviews 1) the use of gene transfer methods to genetically manipulate hematopoietic stem cell targets, 2) recent advances in technology that are addressing problems that have prevented widespread successful translation of gene transfer approaches for the cure of disease, and 3) recent regulatory issues related to human gene therapy trials.

In Section I, Dr. Nienhuis describes the use of alternative viral envelopes and vector systems to improve efficiency of transduction of hematopoietic stem cells. Major limitations of stem cell transduction are related to low levels of viral receptors on the stem cells of large animal species and the low frequency of cycling stem cells in the bone marrow. Attempts to circumvent these limitations by exploiting non-oncoretroviral vectors and pseudotyping of Moloney vectors with alternative envelopes are discussed.

In Section II, Dr. Hawley addresses new strategies to improve the expression of transgenes in cells derived from long-term reconstituting hematopoietic stem cells. Transgene silencing in transduced hematopoietic stem cells remains an obstacle to gene therapy for some gene sequences. New generations of retroviral backbones designed to both improve expression and reduce silencing in primary cells are explored.

In Section III, Drs. Smith and Cornetta update regulatory issues related to human gene therapy trials. Increased scrutiny of human trials has led to changes in requirements and shifts in emphasis of existing regulations, which apply to human gene therapy trials. The current Food and Drug Administration's structure and regulations and the roles of the Recombinant DNA Advisory Committee of the NIH and other sponsors and partners in gene therapy trials are reviewed.

Congenital Hemorrhagic Disorders: New Insights into the Pathophysiology and Treatment of Hemophilia

The diagnostic and treatment strategies related to hemophilia are rapidly evolving. This article focuses on some of the issues of importance. Diagnostic advances in molecular genetics are reviewed by Dr. Ginsburg in Section I, including the current state of knowledge regarding the mutations responsible for hemophilia, with reference to the potential clinical applications of DNA diagnosis and prenatal testing.

Within the area of new therapeutic approaches in hemophilia, recombinant factor VIII and factor IX concentrates, their use and availability are addressed by Dr. Lusher in Section II as well as the use of so-called “primary prophylaxis” with the aim of decreasing long-term hemophilia athropathy. The use of radionuclide synovectomy as replacement for more invasive methods is also reviewed.

Various approaches to the ongoing challenge of the management of hemophilia patients with inhibitors against factor VIII and factor IX are reviewed by Dr. Hedner in Section III, including the principles for immune tolerance induction and the use of recombinant factor VIIa to induce hemostasis in bleeding patients with inhibitors.

In Section IV, gene therapy in hemophilia is reviewed by Dr. High, who focuses on recent developments in the rapidly moving field of gene therapy for hemophilia. Three phase I trials of gene therapy for hemophilia were initiated in 1999, and additional proposed trials are currently in the regulatory review process. Certain aspects of the pathophysiology of hemophilia make it an attractive model for a gene-based approach to treatment. These include latitude in choice of target tissue, a wide therapeutic window, the availability of small and large animal models of the disease, and the ease of determining therapeutic efficacy. Since there is very little published information regarding the ongoing trials, this section reviews the approaches being used, the published pre-clinical data, and considerations affecting clinical trial design in hemophilia gene therapy.

Bone Marrow Transplantation for Non-Malignant Disease

This article reviews the experience in hematopoietic stem cell transplantation (HSCT) for non-malignant disease. HSCT has long been applied as treatment of life-threatening congenital immunodeficiency and metabolic diseases. In Section I, Dr. Parkman reviews that experience for severe combined immunodeficiency, Wiscott-Aldrich syndrome, hyper IGM syndrome, Chédiak-Higashi disease and hereditary lymphohistiocytosis. The value of HSCT in genetic metabolic diseases such as osteopetrosis, osteogenesis imperfecta and the storage diseases are reviewed. In Section II, Dr. Walters reviews the experience over the last decade with allogeneic stem cell transplantation in patients with thalassemia major and sickle cell disease. In Section III, Dr. Sullivan reviews the more recent investigations using stem cell transplantation in patients with advanced autoimmune diseases such as systemic sclerosis, systemic lupus erythematosus, multiple sclerosis and juvenile rheumatoid arthritis. The pathogenesis and outcome with conventional care of these patients, the selection criteria and current results for HSCT, and the future directions in clinical research and patient care using this modality are addressed.

Gene Therapy 2000

This article reviews 1) the use of gene transfer methods to genetically manipulate hematopoietic stem cell targets, 2) recent advances in technology that are addressing problems that have prevented widespread successful translation of gene transfer approaches for the cure of disease, and 3) recent regulatory issues related to human gene therapy trials.

In Section I, Dr. Nienhuis describes the use of alternative viral envelopes and vector systems to improve efficiency of transduction of hematopoietic stem cells. Major limitations of stem cell transduction are related to low levels of viral receptors on the stem cells of large animal species and the low frequency of cycling stem cells in the bone marrow. Attempts to circumvent these limitations by exploiting non-oncoretroviral vectors and pseudotyping of Moloney vectors with alternative envelopes are discussed.

In Section II, Dr. Hawley addresses new strategies to improve the expression of transgenes in cells derived from long-term reconstituting hematopoietic stem cells. Transgene silencing in transduced hematopoietic stem cells remains an obstacle to gene therapy for some gene sequences. New generations of retroviral backbones designed to both improve expression and reduce silencing in primary cells are explored.

In Section III, Drs. Smith and Cornetta update regulatory issues related to human gene therapy trials. Increased scrutiny of human trials has led to changes in requirements and shifts in emphasis of existing regulations, which apply to human gene therapy trials. The current Food and Drug Administration's structure and regulations and the roles of the Recombinant DNA Advisory Committee of the NIH and other sponsors and partners in gene therapy trials are reviewed.