Correction of sickle cell disease in transgenic mouse models by gene therapy

Deletion of a Negatively Acting Sequence in a Chimeric GATA-1 Enhancer-Long Terminal Repeat Greatly Increases Retrovirally Mediated Erythroid Expression

The locus control region of the beta-globin gene cluster has been used previously to direct erythroid expression of globin genes from retroviral vectors for the purpose of gene therapy. Short erythroid regulatory elements represent a potentially valuable alternative to the locus control region. Among them, the GATA-1 enhancer HS2 was used to replace the retroviral enhancer within the 3'-long terminal repeat (LTR) of the retroviral vector SFCM, converting it into an erythroid-specific regulatory element. In this work, we have functionally studied an additional GATA-1 enhancer, HS1. HS1 participates in the transcriptional autoregulation of GATA-1 through an essential GATA-binding site that is footprinted in vivo. In this work we identified within HS1 a new in vivo footprinted region, and we showed that this sequence indeed binds a nuclear protein in vitro. Addition of HS1 to HS2 within the LTR of SFCM significantly improves the expression of a reporter gene. The deletion of the newly identified footprinted sequence in the retroviral construct further increases expression up to a level almost equal to that of the wild type retroviral LTR, without loss of erythroid specificity, suggesting that this sequence may act as a negative regulatory element. An improved vector backbone, MDeltaN, allows even better expression from the new GATA cassette. These results suggest that substantial improvement of overall expression can be achieved by the combination of multiple changes in both regulatory elements and vectors.

Efficient gene transfer into rhesus repopulating hematopoietic stem cells using a simian immunodeficiency virus-based lentiviral vector system

High-titer, HIV-1-based lentiviral vector particles were found to transduce cytokine-mobilized rhesus macaque CD34(+) cells and clonogenic progenitors very poorly (< 1%), reflecting the postentry restriction in rhesus cells to HIV infection. To overcome this barrier, we developed a simian immunodeficiency virus (SIV)-based vector system. A single exposure to a low concentration of amphotropic pseudotyped SIV vector particles encoding the green fluorescent protein (GFP) resulted in gene transfer into 68% +/- 1% of rhesus bulk CD34(+) cells and 75% +/- 1% of clonogenic progenitors. Polymerase chain reaction (PCR) analysis of DNA from individual hematopoietic colonies confirmed these relative transduction efficiencies. To evaluate SIV vector-mediated stem cell gene transfer in vivo, 3 rhesus macaques underwent transplantation with transduced, autologous cytokine-mobilized peripheral blood CD34(+) cells following myeloablative conditioning. Hematopoietic reconstitution was rapid, and an average of 18% +/- 8% and 15% +/- 7% GFP-positive granulocytes and monocytes, respectively, were observed 4 to 6 months after transplantation, consistent with the average vector copy number of 0.19 +/- 0.05 in peripheral blood leukocytes as determined by real-time PCR. Vector insertion site analysis demonstrated polyclonal reconstitution with vector-containing cells. SIV vectors appear promising for evaluating gene therapy approaches in nonhuman primate models.

Mechanisms of Homogeneous Nucleation of Polymers of Sickle Cell Anemia Hemoglobin in Deoxy State

The primary pathogenic event of sickle cell anemia is the polymerization of the mutant hemoglobin (Hb) S within the red blood cells, occurring when HbS is in deoxy state in the venous circulation. Polymerization is known to start with nucleation of individual polymer fibers, followed by growth and branching via secondary nucleation, yet the mechanisms of nucleation of the primary fibers have never been subjected to dedicated tests. We implement a technique for direct determination of rates and induction times of primary nucleation of HbS fibers, based on detection of emerging HbS polymers using optical differential interference contrast microscopy after laser photolysis of CO-HbS. We show that: (i). nucleation throughout these determinations occurs homogeneously and not on foreign substrates; (ii). individual nucleation events are independent of each other; (iii). the nucleation rates are of the order of 10(6)-10(8)cm(-3)s(-1); (iv). nucleation induction times agree with an a priori prediction based on Zeldovich's theory; (v). in the probed parameter space, the nucleus contains 11 or 12 molecules. The nucleation rate values are comparable to those leading to erythrocyte sickling in vivo and suggest that the mechanisms deduced from in vitro experiments might provide physiologically relevant insights. While the statistics and dynamics of nucleation suggest mechanisms akin to those for small-molecule and protein crystals, the nucleation rate values are nine to ten orders of magnitude higher than those known for protein crystals. These high values cannot be rationalized within the current understanding of the nucleation processes.

A stable murine-based RD114 retroviral packaging line efficiently transduces human hematopoietic cells

Several barriers exist to high-efficiency transfer of therapeutic genes into human hematopoietic stem cells (HSCs) using complex oncoretroviral vectors. Human clinical trials to date have used Moloney leukemia virus-based amphotropic and gibbon ape leukemia virus-based envelopes in stable retroviral packaging lines. However, retroviruses pseudotyped with these envelopes have low titers due to the inability to concentrate viral supernatants efficiently by centrifugation without damaging the virus and low transduction efficiencies because of low-level expression of viral target receptors on human HSC. The RD114 envelope from the feline endogenous virus has been shown to transduce human CD34+ cells using transient packaging systems and to be concentrated to high titers by centrifugation. Stable packaging systems have potential advantages over transient systems because greater and more reproducible viral productions can be attained. We have, therefore, constructed and tested a stable RD114-expressing packaging line capable of high-level transduction of human CD34+ cells. Viral particles from this cell line were concentrated up to 100-fold (up to 10(7) viral particles/ml) by ultracentrifugation. Human hematopoietic progenitors from cord blood and sickle cell CD34+ cells were efficiently transduced with a Neo(R)-containing vector after a single exposure to concentrated RD114-pseudotyped virus produced from this cell line. Up to 78% of progenitors from transduced cord blood CD34+ cells and 51% of progenitors from sickle cell CD34+ cells expressed the NeoR gene. We also show transfer of a human beta-globin gene into progenitor cells from CD34+ cells from sickle cell patients with this new RD114 stable packaging system. The results indicate that this packaging line may eventually be useful in human clinical trials of globin gene therapy.

Hematopoietic stem cell gene therapy: selecting only the best

Hematopoietic stem cell (HSC) gene therapy can potentially cure a variety of human hematopoietic diseases, such as sickle cell disease. Selection and expansion of gene-corrected HSCs has now been accomplished for the first time using HSC from large animals - dogs and humans - with a novel drug-resistance gene, MGMT, which is not expressed in normal HSCs (see the related articles beginning on pages 1561 and 1581). Highly efficient lentiviral transfer and expression of MGMT into relatively few HSCs led to repopulation of most of the hematopoietic compartment with gene-corrected cells following suitable drug treatment. This selection system may be useful in human clinical trials to permit gene therapy in autologous and allogeneic bone marrow transplantation settings.

Prospects and implications of using chromatin insulators in gene therapy and transgenesis

Gene therapy has emerged from the idea of inserting a wild-type copy of a gene in order to restore the proper expression and function of a damaged gene. Initial efforts have focused on finding the proper vector and delivery method to introduce a corrected gene to the affected tissue or cell type. Even though these first attempts are clearly promising, several problems remain unsolved. A major problem is the influence of chromatin structure on transgene expression. To overcome chromatin-dependent repressive transgenic states, researchers have begun to use chromatin regulatory elements to drive transgene expression. Insulators or chromatin boundaries are able to protect a transgene against chromatin position effects at their genomic integration sites, and they are able to maintain transgene expression for long periods of time. Therefore, these elements may be very useful tools in gene therapy applications for ensuring high-level and stable expression of transgenes.

Correction of a mouse model of sickle cell disease: lentiviral/antisickling beta-globin gene transduction of unmobilized, purified hematopoietic stem cells

Although sickle cell anemia was the first hereditary disease to be understood at the molecular level, there is still no adequate long-term treatment. Allogeneic bone marrow transplantation is the only available cure, but this procedure is limited to a minority of patients with an available, histocompatible donor. Autologous transplantation of bone marrow stem cells that are transduced with a stably expressed, antisickling globin gene would benefit a majority of patients with sickle cell disease. Therefore, the development of a gene therapy protocol that corrects the disease in an animal model and is directly translatable to human patients is critical. A method is described in which unmobilized, highly purified bone marrow stem cells are transduced with a minimum amount of self-inactivating (SIN) lentiviral vector containing a potent antisickling beta-globin gene. These cells, which were transduced in the absence of cytokine stimulation, fully reconstitute irradiated recipients and correct the hemolytic anemia and organ pathology that characterize the disease in humans. The mean increase of hemoglobin concentration was 46 g/L (4.6 g/dL) and the average lentiviral copy number was 2.2; therefore, a 21-g/L /vector copy increase (2.1-g/dL) was achieved. This transduction protocol may be directly translatable to patients with sickle cell disease who cannot tolerate current bone marrow mobilization procedures and may not safely be exposed to large viral loads.

Plasticity of marrow-derived stem cells

Bone marrow (BM) contains hematopoietic stem cells (HSCs), which differentiate into every type of mature blood cell; endothelial cell progenitors; and marrow stromal cells, also called mesenchymal stem cells (MSCs), which can differentiate into mature cells of multiple mesenchymal tissues including fat, bone, and cartilage. Recent findings indicate that adult BM also contains cells that can differentiate into additional mature, nonhematopoietic cells of multiple tissues including epithelial cells of the liver, kidney, lung, skin, gastrointestinal (GI) tract, and myocytes of heart and skeletal muscle. Experimental results obtained in vitro and in vivo are the subject of this review. The emphasis is on how these experiments were performed and under what conditions differentiation from bone marrow to epithelial and neural cells occurs. Questions arise regarding whether tissue injury is necessary for this differentiation and the mechanisms by which it occurs. We also consider which bone marrow subpopulations are capable of this differentiation. Only after we have a better understanding of the mechanisms involved and of the cells required for this differentiation will we be able to fully harness adult stem cell plasticity for clinical purposes.

Advances in retroviral transduction of hematopoietic stem cells for the gene therapy of atherosclerosis

PURPOSE OF REVIEW

Atherosclerosis is a chronic inflammatory disease that is the primary cause of morbidity and mortality in the developed world. Many studies have shown that macrophages and T-cells play critical roles in multiple aspects of the pathogenesis of the disease. Given that these cells are ultimately derived from bone marrow precursors, the concept of performing gene therapy for atherosclerosis through the retroviral transduction of hematopoietic stem cells has received much attention. This review will highlight recent advances that will help bring this goal closer.

RECENT FINDINGS

The clinical application of retroviral gene transfer into hematopoietic stem cells has been hampered, in part, by the absence of vectors that can direct long-lasting, cell-type specific gene expression. In this review we will detail recent developments in the design of novel retroviral and lentiviral vectors that appear to overcome these problems, offering approaches to express therapeutic genes in specific cell-types within atherosclerotic lesions. We will also highlight advances in our understanding of the pathogenesis of atherosclerosis that may offer new gene therapeutic targets.

SUMMARY

The use of retroviral transduction of hematopoietic stem cells for treatment of patients with atherosclerosis still remains a long-term goal. However, the recent development of retroviral vectors capable of directing expression to specific cell types within the lesion will allow more targeted therapeutic strategies to be devised. In addition, these vectors will provide powerful experimental tools to further our understanding of the pathogenesis of the disease.

Gene therapy with viral vectors

A key factor in the success of gene therapy is the development of gene delivery systems that are capable of efficient gene transfer in a broad variety of tissues, without causing any pathogenic effect. Currently, viral vectors based on many different viruses have been developed, and their performance and pathogenicity has been evaluated in animal models. The results of these studies form the basis for the first clinical trials for correcting genetic disorders using retroviral, adenoviral, and adeno-associated viral vectors. Even though the results of these trials are encouraging, vector development is still required to improve and refine future treatment of hereditary disorders.

Chemical genomic profiling of biological networks using graph theory and combinations of small molecule perturbations

Genome-wide measurements of multiple experimental samples yield rich fingerprints for comparison and interpretation. Here, a two-dimensional matrix of the cellular effects of all possible pairwise combinations of 24 small molecules, each with a different structure and bioactivity, was used to profile otherwise isogenic deletion strains of the yeast Saccharomyces cerevisiae. Using principles from graph theory, we derived a discrete model of the data for each strain by encoding the information in the form of a binary adjacency matrix. This matrix was used to construct a graph composed of nodes representing small molecules and edges connecting combinations that inhibited cell cycle progression. Computation of a set of graph theoretic descriptors for each chemical genetic network provided a topological fingerprint that showed genotype-dependent fluctuations. Because the structure of the genetic network determines the structure of the chemical genetic network, multidimensional chemical genomic profiling can be used for the characterization of perturbations in biological networks or the networks themselves. This application of small molecules could be useful for discerning the molecular basis of highly complex biological phenotypes, including those involved in the susceptibility to or etiology of human disease.

A bicistronic SIN-lentiviral vector containing G156A MGMT allows selection and metabolic correction of hematopoietic protoporphyric cell lines

BACKGROUND

Erythropoietic protoporphyria (EPP) is an inherited disease characterised by a ferrochelatase (FECH) deficiency, the latest enzyme of the heme biosynthetic pathway, leading to the accumulation of toxic protoporphyrin in the liver, bone marrow and spleen. We have previously shown that a successful gene therapy of a murine model of the disease was possible with lentiviral vectors even in the absence of preselection of corrected cells, but lethal irradiation of the recipient was necessary to obtain an efficient bone marrow engraftment. To overcome a preconditioning regimen, a selective growth advantage has to be conferred to the corrected cells.

METHODS

We have developed a novel bicistronic lentiviral vector that contains the human alkylating drug resistance mutant O(6)-methylguanine DNA methyltransferase (MGMT G156A) and FECH cDNAs. We tested their capacity to protect hematopoietic cell lines efficiently from alkylating drug toxicity and correct enzymatic deficiency.

RESULTS

EPP lymphoblastoid (LB) cell lines, K562 and cord-blood-derived CD34(+) cells were transduced at a low multiplicity of infection (MOI) with the bicistronic constructs. Resistance to O(6)-benzylguanine (BG)/N,N'-bis(2-chloroethyl)-N-nitrosourea (BCNU) was clearly shown in transduced cells, leading to the survival and expansion of provirus-containing cells. Corrected EPP LB cells were selectively amplified, leading to complete restoration of enzymatic activity and the absence of protoporphyrin accumulation.

CONCLUSIONS

This study demonstrates that a lentiviral vector including therapeutic and G156A MGMT genes followed by BG/BCNU exposure can lead to a full metabolic correction of deficient cells. This vector might form the basis of new EPP mouse gene therapy protocols without a preconditioning regimen followed by in vivo selection of corrected hematopoietic stem cells.

Novel therapeutic approaches in sickle cell disease

In this update, selected clinical features of sickle cell disease and their management are reviewed. In addition, the current status of interventions that have curative potential for sickle cell disease is discussed, with particular attention focused on indications, methodology, recent results, and challenges to wider clinical application. In Section I, Dr. Nienhuis describes recent improvements in vector technology, safety, and replacement gene expression that are creating the potential for clinical application of this technology. In Section II, Dr. Vichinsky reviews our current understanding of the pathophysiology and treatment of pulmonary injury in sickle cell disease. The acute and chronic pulmonary complications of sickle cell disease, modulators and predictors of severity, and conventional and novel treatment of these complications are discussed. In Section III, Dr. Walters reviews the current status of hematopoietic cell transplantation for sickle cell disease. Newer efforts to expand its availability by identifying alternate sources of stem cells and by reducing the toxicity of transplantation are discussed.

Successful treatment of murine beta-thalassemia using in vivo selection of genetically modified, drug-resistant hematopoietic stem cells

Successful gene therapy of beta-thalassemia will require replacement of the abnormal erythroid compartment with erythropoiesis derived from genetically corrected, autologous hematopoietic stem cells (HSCs). However, currently attainable gene transfer efficiencies into human HSCs are unlikely to yield sufficient numbers of corrected cells for a clinical benefit. Here, using a murine model of beta-thalassemia, we demonstrate for the first time that selective enrichment in vivo of transplanted, drug-resistant HSCs can be used therapeutically and may therefore be a useful approach to overcome limiting gene transfer. We used an oncoretroviral vector to transfer a methylguanine methyltransferase (MGMT) drug-resistance gene into normal bone marrow cells. These cells were transplanted into beta-thalassemic mice given nonmyeloablative pretransplantation conditioning with temozolomide (TMZ) and O6-benzylguanine (BG). A majority of mice receiving 2 additional courses of TMZ/BG demonstrated in vivo selection of the drug-resistant cells and amelioration of anemia, compared with untreated control animals. These results were extended using a novel gamma-globin/MGMT dual gene lentiviral vector. Following drug treatment, normal mice that received transduced cells had an average 67-fold increase in gamma-globin expressing red cells. These studies demonstrate that MGMT-based in vivo selection may be useful to increase genetically corrected cells to therapeutic levels in patients with beta-thalassemia.

Gene therapy: theoretical and bioethical concepts

Gene therapy holds great promise. Somatic gene therapy has the potential to treat a wide range of disorders, including inherited conditions, cancers, and infectious diseases. Early progress has already been made in the treatment of a range of disorders. Ethical issues surrounding somatic gene therapy are primarily those concerned with safety. Germline gene therapy is theoretically possible but raises serious ethical concerns concerning future generations.

Performance- and safety-enhanced lentiviral vectors containing the human interferon-beta scaffold attachment region and the chicken beta-globin insulator

Retroviral vectors are the most efficient means of stable gene delivery to hematopoietic stem cells (HSCs). However, transgene expression from retroviral vectors is frequently subject to the negative influence of chromosomal sequences flanking the site of integration. Toward the development of autonomous transgene expression cassettes, we inserted the human interferon-beta scaffold attachment region (IFN-SAR) and the chicken beta-globin 5' DNase I hypersensitive site 4 (5'HS4) insulator both separately and together into a series of self-inactivating (SIN) lentiviral vector backbones. Transduced cells of the human CD34+ hematopoietic progenitor line KG1a-pooled populations as well as individual clones harboring single integrants--were analyzed for reporter expression during culture periods of up to 4 months. Vectors carrying both the 5'HS4 insulator and the IFN-SAR consistently outperformed control vectors without inserts as well as vectors carrying either element alone. The performance of a set of vectors containing the murine stem cell virus long terminal repeat as an internal promoter was subsequently assessed during in vitro monocytic differentiation of transduced primary human CD34+ cord blood cells. Similar to what was observed in the KG1a hematopoietic progenitor cell model, optimal reporter expression in primary monocytes was obtained with the vector bearing both regulatory elements. These findings indicate that the 5'HS4/IFN-SAR combination is particularly effective at maintaining open chromatin domains permissive for high-level transgene expression at early and late stages of hematopoietic development, and thus could be of utility in HSC-directed retroviral vector-mediated gene transfer applications.

Pathophysiological-based approaches to treatment of sickle cell disease

Sickle hemoglobin (HbS), as a result of its polymer-related and oxidant effects, damages the sickle erythrocyte, provokes inflammation, and causes endothelial injury. All these elements cause the phenotype of sickle cell disease. Novel treatments inhibit HbS polymerization by inducing fetal hemoglobin expression, prevent or repair erythrocyte dehydration by slowing cellular potassium and water loss, and replace HbS-producing erythroid progenitors by stem cell transplantation. Future treatment prospects include gene therapy, interruption of the interaction of sickle cells with the endothelium, inhibition of oxidative damage, and protection of an injured endothelium.

Testing for hematologic disorders and complications

This review summarizes state-of-the-art and emerging techniques in the antenatal diagnosis of fetal anemia and hemoglobinopathies. Fetal anemia may result from hemolytic disease, hemorrhage, suppression of erythropoiesis, infection (eg, parvovirus B19), or trauma. The clinical laboratory plays an essential role in the evaluation of these disorders by way of the use of various hematologic, biochemical, serologic, cytometric, and molecular genetics methods. Hemoglobinopathies are the most common class of single gene disorders worldwide. The authors have used the example of homozygous alpha-thalassemia major (Hb Barts disease) as a paradigmatic case for antenatal hemoglobinopathy screening. Perhaps the most familiar indication for hematologic screening in pregnancy is HDFN, most commonly in pregnancies previously sensitized to the RhD antigen. All pregnant women, regardless of their past medical or obstetric history or previous antibody screens, should have ABO/Rh blood typing and a red cell antibody screen performed at the first prenatal visit. Long-established methods for assaying FMH (KB method), microcytosis (hemogram with red cell indices), and blood group incompatibility (direct antigen test, serologies) remain critical for rapid, sensitive diagnosis. Analysis of fetal free DNA in maternal plasma holds the promise for rapid, ultrasensitive, and noninvasive detection of many fetal hematologic disorders.

Lentivirus-mediated gene transfer of uroporphyrinogen III synthase fully corrects the porphyric phenotype in human cells

Congenital erythropoietic porphyria (CEP) is an inherited disease due to a deficiency in the uroporphyrinogen III synthase, the fourth enzyme of the heme biosynthesis pathway. It is characterized by accumulation of uroporphyrin I in the bone marrow, peripheral blood and other organs. The prognosis of CEP is poor, with death often occurring early in adult life. For severe transfusion-dependent cases, when allogeneic cell transplantation cannot be performed, the autografting of genetically modified primitive/stem cells may be the only alternative. In vitro gene transfer experiments have documented the feasibility of gene therapy via hematopoietic cells to treat this disease. In the present study lentiviral transduction of porphyric cell lines and primary CD34(+) cells with the therapeutic human uroporphyrinogen III synthase (UROS) cDNA resulted in both enzymatic and metabolic correction, as demonstrated by the increase in UROS activity and the suppression of porphyrin accumulation in transduced cells. Very high gene transfer efficiency (up to 90%) was achieved in both cell lines and CD34(+) cells without any selection. Expression of the transgene remained stable over long-term liquid culture. Furthermore, gene expression was maintained during in vitro erythroid differentiation of CD34(+) cells. Therefore the use of lentiviral vectors is promising for the future treatment of CEP patients by gene therapy.

Development of gene therapy for hemoglobin disorders

The hemoglobin disorders, severe beta-thalassemia and sickle cell anemia, are prevalent monogenetic disorders which cause severe morbidity and mortality worldwide. Gene therapy approaches to these disorders envision stem cell targeted gene transfer, autologous transplantation of gene-corrected stem cells, and functional, phenotypically corrective globin gene expression in developing erythroid cells. Lentiviral vector systems potentially appear to afford adequately efficient gene transfer into stem cells and are capable, with appropriate genetic engineering, of transferring a globin gene with the regulatory elements required to achieve high-level, erythroid-specific expression. Herein are results obtained in use of lentiviral vectors to insert a gamma-globin gene into murine stem cells with phenotypic correction of the thalassemia phenotype. Further, we have developed a drug-selection system for genetically modified stem cells based on a mutant form of methylguanine, methyltransferase, which allows selective amplification of genetically modified stem cells with phenotypic correction even in the absence of myeloablation prior to stem cell transplantation. These advances provide essential preclinical data which build toward the development of effective gene therapy for the severe hemoglobin disorders.

A novel murine model of Cooley anemia and its rescue by lentiviral-mediated human beta-globin gene transfer

Patients affected by beta-thalassemia major require lifelong transfusions because of insufficient or absent production of the beta chain of hemoglobin (Hb). A minority of patients are cured by allogeneic bone marrow transplantation. In the most severe of the hitherto available mouse models of beta-thalassemia, a model for human beta-thalassemia intermedia, we previously demonstrated that globin gene transfer in bone marrow cells is curative, stably raising Hb levels from 8.0-8.5 to 11.0-12.0 g/dL in long-term chimeras. To fully assess the therapeutic potential of gene therapy in the context of a lethal anemia, we now have created an adult model of beta(0)-thalassemia major. In this novel model, mice engrafted with beta-globin-null (Hbb(th3/th3)) fetal liver cells succumb to ineffective erythropoiesis within 60 days. These mice rapidly develop severe anemia (2-4 g/dL), massive splenomegaly, extramedullary hematopoiesis (EMH), and hepatic iron overload. Remarkably, most mice (11 of 13) treated by lentivirus-mediated globin gene transfer were rescued. Long-term chimeras with an average 1.0-2.4 copies of the TNS9 vector in their hematopoietic and blood cells stably produced up to 12 g/dL chimeric Hb consisting of mu alpha(2):hu beta(2) tetramers. Pathologic analyses indicated that erythroid maturation was restored, while EMH and iron overload dramatically decreased. Thus, we have established an adult animal model for the most severe of the hemoglobinopathies, Cooley anemia, which should prove useful to investigate both genetic and pharmacologic treatments. Our findings demonstrate the remarkable efficacy of lentivirus-mediated globin gene transfer in treating a fulminant blood disorder and strongly support the efficacy of gene therapy in the severe hemoglobinopathies.

Current developments in the design of onco-retrovirus and lentivirus vector systems for hematopoietic cell gene therapy

Over the past dozen years, the majority of clinical gene therapy trials for inherited genetic diseases and cancer therapy have been performed using murine onco-retrovirus as the gene delivery vector. The earliest systems used were relatively inefficient in both the rates of transduction and expression of the transgene. Formidable obstacles inherent in the cell biology and/or the immunology of the target cell systems limited the efficacy of gene therapy for many target diseases. Development of novel retrovirus gene transfer systems that are in progress have begun to overcome these obstacles. Evidence of this progress is the recent successful functional correction of the immune T and B lymphocyte deficiency in patients with X-linked severe combined immunodeficiency (X-SCID) and adenosine deaminase (ADA)-deficient SCID following onco-retrovirus vector ex vivo transduction of autologous marrow stem cells [Science 296 (2002) 2410; Science 288 (2000) 669; N. Engl. J. Med. 346 (2002) 1185]. These achievements of prolonged clinical benefit from gene therapy were tempered by the finding of insertional mutageneses in two of the treated X-SCID patients [N. Engl. J. Med. 348 (2003) 255].

ICA-17043, a novel Gardos channel blocker, prevents sickled red blood cell dehydration in vitro and in vivo in SAD mice

A prominent feature of sickle cell anemia is the presence of dehydrated red blood cells (RBCs) in circulation. Loss of potassium (K(+)), chloride (Cl(-)), and water from RBCs is thought to contribute to the production of these dehydrated cells. One main route of K(+) loss in the RBC is the Gardos channel, a calcium (Ca(2+))-activated K(+) channel. Clotrimazole (CLT), an inhibitor of the Gardos channel, has been shown to reduce RBC dehydration in vitro and in vivo. We have developed a chemically novel compound, ICA-17043, that has greater potency and selectivity than CLT in inhibiting the Gardos channel. ICA-17043 blocked Ca(2+)-induced rubidium flux from human RBCs with an IC(50) value of 11 +/- 2 nM (CLT IC(50) = 100 +/- 12 nM) and inhibited RBC dehydration with an IC(50) of 30 +/- 20 nM. In a transgenic mouse model of sickle cell disease (SAD), treatment with ICA-17043 (10 mg/kg orally, twice a day) for 21 days showed a marked and constant inhibition of the Gardos channel activity (with an average inhibition of 90% +/- 27%, P <.005), an increase in RBC K(+) content (from 392 +/- 19.9 to 479.2 +/- 40 mmol/kg hemoglobin [Hb], P <.005), a significant increase in hematocrit (Hct) (from 0.435 +/- 0.007 to 0.509 +/- 0.022 [43.5% +/- 0.7% to 50.9% +/- 2.2%], P <.005), a decrease in mean corpuscular hemoglobin concentration (MCHC) (from 340 +/- 9.0 to 300 +/- 15 g/L [34.0 +/- 0.9 to 30 +/- 1.5 g/dL], P <.05), and a left-shift in RBC density curves. These data indicate that ICA-17043 is a potent inhibitor of the Gardos channel and ameliorates RBC dehydration in the SAD mouse.

The degree of phenotypic correction of murine beta -thalassemia intermedia following lentiviral-mediated transfer of a human gamma-globin gene is influenced by chromosomal position effects and vector copy number

Increased fetal hemoglobin (HbF) levels diminish the clinical severity of beta-thalassemia and sickle cell anemia. A treatment strategy using autologous stem cell-targeted gene transfer of a gamma-globin gene may therefore have therapeutic potential. We evaluated oncoretroviral- and lentiviral-based gamma-globin vectors for expression in transduced erythroid cell lines. Compared with gamma-globin, oncoretroviral vectors containing either a beta-spectrin or beta-globin promoter and the alpha-globin HS40 element, a gamma-globin lentiviral vector utilizing the beta-globin promoter and elements from the beta-globin locus control region demonstrated a higher probability of expression. This lentiviral vector design was evaluated in lethally irradiated mice that received transplants of transduced bone marrow cells. Long-term, stable erythroid expression of human gamma-globin was observed with levels of vector-encoded gamma-globin mRNA ranging from 9% to 19% of total murine alpha-globin mRNA. The therapeutic efficacy of the vector was subsequently evaluated in a murine model of beta-thalassemia intermedia. The majority of mice that underwent transplantation expressed significant levels of chimeric m(alpha)(2)h(gamma)(2) molecules (termed HbF), the amount of which correlated with the degree of phenotypic improvement. A group of animals with a mean HbF level of 21% displayed a 2.5 g/dL (25 g/L) improvement in Hb concentration and normalization of erythrocyte morphology relative to control animals. gamma-Globin expression and phenotypic improvement was variably lower in other animals due to differences in vector copy number and chromosomal position effects. These data establish the potential of using a gamma-globin lentiviral vector for gene therapy of beta-thalassemia.

Side effects of retroviral gene transfer into hematopoietic stem cells

Recent conceptual and technical improvements have resulted in clinically meaningful levels of gene transfer into repopulating hematopoietic stem cells. At the same time, evidence is accumulating that gene therapy may induce several kinds of unexpected side effects, based on preclinical and clinical data. To assess the therapeutic potential of genetic interventions in hematopoietic cells, it will be important to derive a classification of side effects, to obtain insights into their underlying mechanisms, and to use rigorous statistical approaches in comparing data. We here review side effects related to target cell manipulation; vector production; transgene insertion and expression; selection procedures for transgenic cells; and immune surveillance. We also address some inherent differences between hematopoiesis in the most commonly used animal model, the laboratory mouse, and in humans. It is our intention to emphasize the need for a critical and hypothesis-driven analysis of "transgene toxicology," in order to improve safety, efficiency, and prognosis for the yet small but expanding group of patients that could benefit from gene therapy.

Transduced CD34+ cells from adrenoleukodystrophy patients with HIV-derived vector mediate long-term engraftment of NOD/SCID mice

X-linked adrenoleukodystrophy (ALD), an inherited demyelinating disorder of the central nervous system, can be corrected by allogeneic bone marrow transplantation, likely due to the turnover of brain macrophages that are bone marrow derived. ALD is characterized by an accumulation of very long chain fatty acids (VLCFA) due to the deficiency of an ATP binding cassette transporter that imports these fatty acids in peroxisomes. Murine retroviral transduction results in metabolic correction of ALD CD34(+) cells in vitro but reinfusion of these cells into ALD patients would not provide clinical benefit owing to the absence of selective advantage conferred by transgene expression. High-efficiency transduction of ALD CD34(+) peripheral blood mobilized cells was achieved using an HIV-based vector driving ALD gene expression under the elongation factor 1 alpha promoter and a protocol without prestimulation of CD34(+) cells with cytokines prior to transduction to preserve their stem cell properties. Efficient expression of the ALD gene was demonstrated in monocytes/macrophages derived from cultures of transduced ALD CD34(+) cells and in long-term culture initiating cells. VLCFA metabolism was corrected in transduced CD34(+), CFU-derived, and LTC-derived cells, indicating that the vector-encoded ALD protein was fully functional. Transplantation of transduced ALD CD34(+) cells into NOD/SCID mice resulted in long-term expression of ALD protein in monocytes/macrophages derived from engrafted stem cells.

Doxycycline-regulated lentiviral vector system with a novel reverse transactivator rtTA2S-M2 shows a tight control of gene expression in vitro and in vivo

Regulated expression of therapeutic genes is required for long-term gene therapy applications for many disorders. Here we describe a doxycycline (dox)-regulated lentiviral vector system consisting of two HIV-1-based self-inactivating viruses. One of the vectors is constitutively expressing a novel improved version of the tetracycline reverse transactivator rtTA2(S)-M2 and the other has a rtTA responsive promoter driving the expression of beta-galactosidase gene (lacZ). The rtTA2(S)-M2 has highly improved properties with respect to specificity, stability and inducibility. Functionality of the system by dox was confirmed after in vitro cotransduction of Chinese hamster ovary and human endothelial hybridoma (EAhy926) cells. Regulation of the system showed tight control of the gene expression. Dose dependence for dox was seen with concentrations that can be obtained in vivo with doses normally used in clinical practice. LacZ expression could be switched on/off during long-term (3 months) culturing of cotransduced cells. The system was next tested in vivo after cotransduction into rat brain and studying expression of the lacZ gene in dox-treated and control rats. Nested RT-PCR confirmed that the tight control of the gene expression was achieved in vivo. Also, X-gal staining showed positive cells in the dox-treated rats, but not in the controls 10 days after cotransduction with 4 days preceding treatment with dox. It is concluded that our doxycycline-regulated vector system shows significant potential for long-term gene therapy treatments.

Spontaneous circulation of myeloid-lymphoid-initiating cells and SCID-repopulating cells in sickle cell crisis

The only curative therapy for sickle cell disease (SCD) is allogeneic hematopoietic stem cell (HSC) transplantation. Gene therapy approaches for autologous HSC transplantation are being developed. Although earlier engraftment is seen when cells from GCSF-mobilized blood are transplanted than when bone marrow is transplanted, administration of GCSF to patients with SCD can cause significant morbidity. We tested whether primitive hematopoietic progenitors are spontaneously mobilized in the blood of patients with SCD during acute crisis (AC-SCD patients). The frequency of myeloid-lymphoid-initiating cells (ML-ICs) and SCID-repopulating cells (SRCs) was significantly higher in blood from AC-SCD patients than in blood from patients with steady-state SCD or from normal donors. The presence of SRCs in peripheral blood was not associated with detection of long-term culture-initiating cells, consistent with the notion that SRCs are more primitive than long-term culture-initiating cells. As ML-ICs and SRCs were both detected in blood of AC-SCD patients only, these assays may both measure primitive progenitors. The frequency of ML-ICs also correlated with increases in stem cell factor, GCSF, and IL-8 levels in AC-SCD compared with steady-state SCD and normal-donor sera. Because significant numbers of ML-ICs and SRCs are mobilized in the blood without exogenous cytokine treatment during acute crisis of SCD, collection of peripheral blood progenitors during crisis may yield a source of autologous HSCs suitable for ex-vivo correction by gene therapy approaches and subsequent transplantation.

LCR-regulated transgene expression levels depend on the Oct-1 site in the AT-rich region of beta -globin intron-2

Human beta-globin transgenes regulated by the locus control region (LCR) express at all integration sites in transgenic mice. For such LCR activity at ectopic sites, the 5'HS3 element requires the presence of the AT-rich region (ATR) in beta-globin intron-2. Here, we examine the dependence of 5'HS3 LCR activity on transcription factor binding sites in the ATR. In vitro DNaseI footprint analysis and electrophoretic mobility shift assays of the ATR identified an inverted double Gata-1 site composed of 2 noncanonical sequences (GATT and GATG) and an Oct-1 consensus site. Mutant Oct-1, Gata-1, or double mutant sites were created in the ATR of the BGT50 construct composed of a 5'HS3 beta/gamma-globin hybrid transgene. Transgenes with double mutant sites expressed at all sites of integration, but mean expression levels in transgenic mice were reduced from 64% per copy (BGT50) to 37% (P <.05). Mutation of the inverted double Gata-1 site had no effect at 61% per copy expression levels. In contrast, mutation of the Oct-1 site alone reduced per-copy expression levels to 31% (P <.05). We conclude that the ability of 5'HS3 to activate expression from all transgene integration sites is dependent on sequences in the ATR that are not bound at high affinity by transcription factors. In addition, the Oct-1 site in the ATR is required for high-level 5'HS3 beta/gamma-globin transgene expression and should be retained in LCRbeta-globin expression cassettes designed for gene therapy.

Gene therapy of apolipoprotein E-deficient mice using a novel macrophage-specific retroviral vector

The use of retroviral gene transfer into hematopoietic stem cells for human gene therapy has been hampered by the absence of retroviral vectors that can generate long-lasting, lineage-specific gene expression. We developed self-inactivating retroviral vectors that incorporate gene-regulatory elements from the macrophage-restricted human CD68 gene. Through the transplantation of transduced murine hematopoietic stem cells (HSCs), we show that a vector incorporating a 342-base pair (bp) fragment of 5' flanking sequence from the CD68 gene, in addition to the CD68 first intron, was able to direct macrophage-specific expression of an enhanced green fluorescent protein (EGFP) reporter gene in inflammatory cell exudates and lymphoid organs in vivo. Levels of EGFP expression generated by this vector were greater than those generated by a standard Moloney murine leukemia retroviral vector, and they were stable for at least a year after transplantation of transduced HSCs. To evaluate the ability of this vector to generate therapeutically useful levels of gene expression, we transplanted apolipoprotein E (ApoE)-deficient HSCs transduced with a virus encoding ApoE into ApoE-deficient mice. Macrophages from these mice expressed levels of ApoE that were comparable to those from wild-type mice, and vector-driven expression of ApoE in macrophages was sufficient to reverse both hypercholesterolemia and atherosclerotic lesion development. The future application of this retroviral vector should provide a powerful tool to further elucidate macrophage function and for human gene therapy.

Prolonged multilineage clonal hematopoiesis in a rhesus recipient of CD34 positive cells marked with a RD114 pseudotyped oncoretroviral vector

The ability to efficiently transfer a gene into repopulating hematopoietic stem cells would create many therapeutic opportunities. We have evaluated the ability of particles bearing an alternative envelope protein, that of the feline endogenous virus (RD114), to transduce stem cells in a nonhuman primate autologous transplantation model using rhesus macaques. We have previously shown this pseudotyped vector to be superior to the amphotropic vector at transducing cells in umbilical cord blood capable of establishing hematopoiesis in immunodeficient mice. Gene transfer efficiency as reflected by the number of genetically modified cells in hematopoietic tissues varied among the five monkeys studied from low levels (<1%) in three animals to much higher levels in two (20-60%). An animal that exhibited extremely high levels for several weeks was found by vector genome insertion site analysis to have reconstitution predominantly with a single clone of cells. This variability among animals is in keeping with computer simulations of reconstitution with limiting numbers of stem cells genetically modified at about 10% efficiency. Our studies provide insights into the biology of hematopoietic reconstitution and suggest approaches for increasing stem cell targeted gene transfer efficiency.

Hemoglobinopathies

The outlook for patients with sickle cell disease has improved steadily during the last two decades. In spite of these improvements, curative therapies are currently available only to a small minority of patients. The main theme of this chapter is to describe new therapeutic options that are at different stages of development that might result in further improvements in the outlook for patients with these disorders.

Dr. Joseph DeSimone and his colleagues had previously made the important observation that the hypomethylating agent 5-azacytidine can reverse the switch from adult to fetal hemoglobin in adult baboons. Although similar activity was demonstrated in patients with sickle cell disease and β-thalassemia, concern about the toxicity of 5-azacytidine prevented its widespread use in these disorders. In Section I, Dr. DeSimone discusses the role of DNA methylation in globin gene regulation and describe recent clinical experience with decitabine (an analogue of 5-azacytidine) in patients with sickle cell disease. These encouraging studies demonstrate significant fetal hemoglobin inducing activity of decitabine in patients who fail to respond to hydroxyurea.

In Section II, Dr. George Atweh continues the same theme by describing recent progress in the study of butyrate, another inducer of fetal hemoglobin, in patients with sickle cell disease and β-thalassemia. The main focus of his section is on the use of a combination of butyrate and hydroxyurea to achieve higher levels of fetal hemoglobin that might be necessary for complete amelioration of the clinical manifestations of these disorders. Dr. Atweh also describes novel laboratory studies that shed new light on the mechanisms of fetal hemoglobin induction by butyrate.

In Section III, Dr. Ronald Nagel discusses the different available transgenic sickle mice as experimental models for human sickle cell disease. These experimental models have already had a significant impact on our understanding of the pathophysiology of sickle cell disease. Dr. Nagel describes more recent studies in which transgenic sickle mice provide the first proof of principle that globin gene transfer into hematopoietic stem cells inhibits in vivo sickling and ameliorates the severity of the disease.

Although stroke in adult patients with sickle cell disease is not as common as in children, adult hematologists, like their pediatric colleagues, need to make management decisions in adult patients with a stroke or a history of stroke. Dr. Robert Adams has led several large clinical studies that investigated the role of transfusions in the prevention of stroke in children with sickle cell disease. Much less is known, however, about the prevention of first or subsequent strokes in adult patients with sickle cell disease. In Section IV, Dr. Adams provides some general guidelines for the management of adult patients with stroke while carefully distinguishing between recommendations that are evidence-based and those that are anecdotal in nature.

Large-scale production of pseudotyped lentiviral vectors using baculovirus GP64

Unlike oncoretroviruses, lentiviral vectors can insert large genes and can target both dividing and nondividing cells; thus they hold unique promise as gene transfer agents. To enhance target range, the native lentiviral envelope glycoprotein is replaced (pseudotyped) with vesicular stomatitis virus G (VSVG), and the genes of interest are packaged in nonreplicating vectors by transient transfection with three plasmids. However, because of cytotoxic effects of VSVG expression in producer cells (293T cells) it has been difficult to generate a packaging cell line, required for even modest scale-up of vector production. Here we introduce a pseudotyped lentivirus vector using the baculovirus GP64 envelope glycoprotein. Compared with VSVG, GP64 vectors exhibited a similar broad tropism and similar native titers. GP64-pseudotyped vectors were usually highly concentrated without much loss of titer. Because, unlike VSVG, GP64 expression does not kill cells, we generated 293T-based cell lines constitutively expressing GP64. Our results demonstrate that the baculovirus GP64 protein is an attractive alternative to VSVG for viral vectors used in the large-scale production of high-titer virus required for clinical and commercial applications.

High-level erythroid lineage-directed gene expression using globin gene regulatory elements after lentiviral vector-mediated gene transfer into primitive human and murine hematopoietic cells

Lentiviral vectors efficiently transduce primitive human hematopoietic cells and are capable of transferring complex genomes. Vectors were designed with hypersensitive sites containing regulatory elements from the beta-globin locus control region linked to the beta-globin gene promoter to drive expression of the enhanced green fluorescent protein marker to facilitate analysis of the pattern of gene expression in various hematopoietic lineages. Such vectors gave higher level, induced expression in mouse erythroleukemia cells than a previously described vector that utilized an enhancer from the alpha locus and the ankyrin-1 promoter [Moreau-Gaudry, F., Xia, P., Jiang, G., Perelman, N.P., Bauer, G., Ellis, J., Surinya, K.H., Mavilio, F., Shen, C.K., and Malik, P. (2001). Blood 98, 2664-2672]. The addition of gamma-globin intron sequences further augmented vector expression. Expression was also effectively targeted to the erythroid lineage in cultured human cells from peripheral blood and in mouse red blood cells in vivo, although lower levels of expression were also observed in other lineages. Thus, these newly described vectors provide a means to achieve high-level gene expression, predominantly in erythroid cells, an outcome that may have potential therapeutic application.

Current and future genetic screening for male infertility

Although much of male infertility is currently unexplained, it is likely that underlying defects in critical genes or entire gene pathways are responsible. Because powerful technologies exist to bypass severe male-factor infertility, improving the diagnosis of genetic infertility is important for the infertile couple, not only to explain the problem but also to inform them of conditions potentially transmissible to offspring.

Efficient transduction and long-term retroviral expression of the melanoma-associated tumor antigen tyrosinase in CD34(+) cord blood-derived dendritic cells

Differentiation of genetically modified CD34(+) hematopoietic stem cells into dendritic cells (DCs) will contribute to the development of immunotherapeutic anticancer protocols. Retroviral vectors that have been used for the transduction of CD34(+) cells face the problem of gene silencing when integrated into the genome of repopulating stem cells. We reasoned that a high copy number of retroviral DNA sequences might overcome silencing of transgene expression during expansion and differentiation of progenitor cells into functional DCs. To prove this, we utilized a retroviral vector with bicistronic expression of the melanoma-associated antigen tyrosinase and the enhanced green fluorescent protein (EGFP). Human cord blood CD34(+) cells were transduced with vesicular stomatitis virus G-protein (VSV-G) pseudotyped Moloney murine leukemia virus (MoMuLV) particles using 100-150 multiplicity of infection. During expansion of transduced cells with immature phenotype, transgene expression was strongly silenced, but upon differentiation into mature DCs, residual transgene expression was retained. Intracellular processing of the provirally expressed tyrosinase was tested in a chromium release assay utilizing a cytotoxic T cell clone specific for a HLA-A*0201-restricted tyrosinase peptide. We suggest that retroviral transduction of tumor-associated antigens in hematopoietic progenitor cells and subsequent differentiation into DCs is a suitable basis for the development of potent anti-tumor vaccines.

Permanent and panerythroid correction of murine beta thalassemia by multiple lentiviral integration in hematopoietic stem cells

Achieving long-term pancellular expression of a transferred gene at therapeutic level in a given hematopoietic lineage remains an important goal of gene therapy. Advances have recently been made in the genetic correction of the hemoglobinopathies by means of lentiviral vectors and large locus control region (LCR) derivatives. However, panerythroid beta globin gene expression has not yet been achieved in beta thalassemic mice because of incomplete transduction of the hematopoietic stem cell compartment and position effect variegation of proviruses integrated at a single copy per genome. Here, we report the permanent, panerythroid correction of severe beta thalassemia in mice, resulting from a homozygous deletion of the beta major globin gene, by transplantation of syngeneic bone marrow transduced with an HIV-1-derived [beta globin gene/LCR] lentiviral vector also containing the Rev responsive element and the central polypurine tract/DNA flap. The viral titers produced were high enough to achieve transduction of virtually all of the hematopoietic stem cells in the graft with an average of three integrated proviral copies per genome in all transplanted mice; the transduction was sustained for >7 months in both primary and secondary transplants, at which time approximately 95% of the red blood cells in all mice contained human beta globin contributing to 32 +/- 4% of all beta-like globin chains. Hematological parameters approached complete phenotypic correction, as assessed by hemoglobin levels and reticulocyte and red blood cell counts. All circulating red blood cells became and remained normocytic and normochromic, and their density was normalized. Free alpha globin chains were completely cleared from red blood cell membranes, splenomegaly abated, and iron deposit was almost eliminated in liver sections. These findings indicate that virtually complete transduction of the hematopoietic stem cell compartment can be achieved by high-titer lentiviral vectors and that position effect variegation can be mitigated by multiple events of proviral integration to yield balanced, panerythroid expression. These results provide a solid foundation for the initiation of human clinical trials in beta thalassemia patients.

Advances in lentiviral vector design for gene-modification of hematopoietic stem cells

Lentiviral vectors are more efficient at transducing quiescent hematopoietic stem cells than murine retroviral vectors. This characteristic is due to multiple karyophilic components of the lentiviral vector pre-integration complex. Lentiviral vectors are also able to carry more complex payloads than murine retroviral vectors, making it possible to deliver expression cassettes that direct either constitutive or targeted expression in various hematopoietic stem cell progeny.

Development of virus vectors for gene therapy of beta chain hemoglobinopathies: flanking with a chromatin insulator reduces gamma-globin gene silencing in vivo

We have previously described the development of oncoretrovirus vectors for human gamma-globin using a truncated beta-globin promoter, modified gamma-globin cassette, and alpha-globin enhancer. However, one of these vectors is genetically unstable, and both vectors exhibit variable expression patterns in cultured cells, common characteristics of oncoretrovirus vectors for globin genes. To address these problems, we identified and removed the vector sequences responsible for genetic instability and flanked the resultant vector with the chicken beta-globin HS4 chromatin insulator to protect expression from chromosomal position effects. After determining that flanking with the cHS4 element allowed higher, more uniform levels of gamma-globin expression in MEL cell lines, we tested these vectors using a mouse bone marrow transduction and transplantation model. When present, the gamma-globin cassettes from the uninsulated vectors were expressed in only 2% to 5% of red blood cells (RBCs) long term, indicating they are highly sensitive to epigenetic silencing. In contrast, when present the gamma-globin cassette from the insulated vector was expressed in 49% +/- 20% of RBCs long term. RNase protection analysis indicated that the insulated gamma-globin cassette was expressed at 23% +/- 16% per copy of mouse alpha-globin in transduced RBCs. These results demonstrate that flanking a globin vector with the cHS4 insulator increases the likelihood of expression nearly 10-fold, which in turn allows for gamma-globin expression approaching the therapeutic range for sickle cell anemia and beta thalassemia.

Feasibility of using autologous transplantation to evaluate hematopoietic stem cell-based gene therapy strategies in transgenic mouse models of human disease

Histoincompatibility between murine donors and recipients of bone marrow (BM) transplants reduces engraftment, and this compromises assessment of hematopoietic stem cells (HSCs) in certain transgenic mice. To study HSCs in the S+S-Antilles mouse model of human sickle cell disease (SCD), we developed an autotransplant protocol. Initial experiments showed no differences between S+S-Antilles mice and normal C57BL/6 (+/+) mice in their radiosensitivity or baseline hematopoietic progenitor numbers. The kinetics of red blood cell (RBC) replacement post-transplant in +/+ recipients of mixtures of transgenic and +/+ BM cells also showed no competitive advantage of the +/+ cells. BM cells were then aspirated from mice 4 days after 5-fluorouracil treatment, transduced with a green fluorescent protein (GFP)-encoding retrovirus, and transplanted into the same recipients that, just before transplant, were irradiated with 800 cGy. We subsequently detected high levels of GFP(+) RBCs (21-79%) and white blood cells (WBCs; 35-88%) in the blood for 11 months and showed that transduced HSCs regenerated in the primary mice also repopulated secondary mice. These findings provide a generally applicable protocol for performing autotransplants in mice and forecast the potential utility of this approach in assessing HSC-based gene therapy protocols in transgenic mouse models of many human diseases.