Successful expression of beta-galactosidase and factor IX transgenes in fetal and neonatal sheep after ultrasound-guided percutaneous adenovirus vector administration into the umbilical vein

Gene modification therapies for hereditary diseases in the fetus

Proof-of-principle disease models have demonstrated the feasibility of an intrauterine gene modification therapy (in utero gene therapy (IUGT)) approach to hereditary diseases as diverse as coagulation disorders, haemoglobinopathies, neurogenetic disorders, congenital metabolic, and pulmonary diseases. Gene addition, which requires the delivery of an integrating or episomal transgene to the target cell nucleus to be transcribed, and gene editing, where the mutation is corrected within the gene of origin, have both been used successfully to increase normal protein production in a bid to reverse or arrest pathology in utero. While most experimental models have employed lentiviral, adenoviral, and adeno-associated viral vectors engineered to efficiently enter target cells, newer models have also demonstrated the applicability of non-viral lipid nanoparticles. Amelioration of pathology is dependent primarily on achieving sustained therapeutic transgene expression, silencing of transgene expression, production of neutralising antibodies, the dilutional effect of the recipient's growth on the mass of transduced cells, and the degree of pre-existing cellular damage. Safety assessment of any IUGT strategy will require long-term postnatal surveillance of both the fetal recipient and the maternal bystander for cell and genome toxicity, oncogenic potential, immune-responsiveness, and germline mutation. In this review, we discuss advances in the field and the push toward clinical translation of IUGT.

Clinical Design and Analysis Strategies for the Development of Gene Therapies: Considerations for Quantitative Drug Development in the Age of Genetic Medicine

Cell and gene therapies have shown enormous promise across a range of diseases in recent years. Numerous adoptive cell therapy modalities as well as systemic and direct-to-target tissue gene transfer administrations are currently in clinical development. The clinical trial design, development, reporting, and analysis of novel cell and gene therapies can differ significantly from established practices for small molecule drugs and biologics. Here, we discuss important quantitative considerations and key competencies for drug developers in preclinical requirements, trial design, and lifecycle planning for gene therapies. We argue that the unique development path of gene therapies requires practicing quantitative drug developers-statisticians, pharmacometricians, pharmacokineticists, epidemiologists, and medical and translational science leads-to exercise active collaboration and cross-functional learning across development stages.

Mechanistic Insights into Factor VIII Immune Tolerance Induction via Prenatal Cell Therapy in Hemophilia A

Purpose of Review

Prenatal stem cell and gene therapy approaches are amongst the few therapies that can promise the birth of a healthy infant with specific known genetic diseases. This review describes fetal immune cell signaling and its potential influence on donor cell engraftment, and summarizes mechanisms of central T cell tolerance to peripherally-acquired antigen in the context of prenatal therapies for Hemophilia A.

Recent Findings

During early gestation, different subsets of antigen presenting cells take up peripherally-acquired, non-inherited antigens and induce the deletion of antigen-reactive T-cell precursors in the thymus, demonstrating the potential for using prenatal cell and gene therapies to induce central tolerance to FVIII in the context of prenatal diagnosis/therapy of Hemophilia A.

Summary

Prenatal cell and gene therapies are promising approaches to treat several genetic disorders including Hemophilia A and B. Understanding the mechanisms of how FVIII-specific tolerance is achieved during ontogeny could help develop novel therapies for HA and better approaches to overcome FVIII inhibitors.

In utero stem cell transplantation and gene therapy: rationale, history, and recent advances toward clinical application

Recent advances in high-throughput molecular testing have made it possible to diagnose most genetic disorders relatively early in gestation with minimal risk to the fetus. These advances should soon allow widespread prenatal screening for the majority of human genetic diseases, opening the door to the possibility of treatment/correction prior to birth. In addition to the obvious psychological and financial benefits of curing a disease in utero, and thereby enabling the birth of a healthy infant, there are multiple biological advantages unique to fetal development, which provide compelling rationale for performing potentially curative treatments, such as stem cell transplantation or gene therapy, prior to birth. Herein, we briefly review the fields of in utero transplantation (IUTx) and in utero gene therapy and discuss the biological hurdles that have thus far restricted success of IUTx to patients with immunodeficiencies. We then highlight several recent experimental breakthroughs in immunology, hematopoietic/marrow ontogeny, and in utero cell delivery, which have collectively provided means of overcoming these barriers, thus setting the stage for clinical application of these highly promising therapies in the near future.

Hemophilia A: an ideal disease to correct in utero

Hemophilia A (HA) is the most frequent inheritable defect of the coagulation proteins. The current standard of care for patients with HA is prophylactic factor infusion, which is comprised of regular (2-3 times per week) intravenous infusions of recombinant or plasma-derived FVIII to maintain hemostasis. While this treatment has greatly increased the quality of life and lengthened the life expectancy for many HA patients, its high cost, the need for lifelong infusions, and the fact that it is unavailable to roughly 75% of the world's HA patients make this type of treatment far from ideal. In addition, this lifesaving therapy suffers from a high risk of treatment failure due to immune response to the infused FVIII. There is thus a need for novel treatments, such as those using stem cells and/or gene therapy, which have the potential to mediate long-term correction or permanent cure following a single intervention. In the present review, we discuss the clinical feasibility and unique advantages that an in utero approach to treating HA could offer, placing special emphasis on a new sheep model of HA we have developed and on the use of mesenchymal stromal cells (MSC) as cellular vehicles for delivering the FVIII gene.

Adrenal gland infection by serotype 5 adenovirus requires coagulation factors

Recombinant, replication-deficient serotype 5 adenovirus infects the liver upon in vivo, systemic injection in rodents. This infection requires the binding of factor X to the capsid of this adenovirus. Another organ, the adrenal gland is also infected upon systemic administration of Ad, however, whether this infection is dependent on the cocksackie adenovirus receptor (CAR) or depends on the binding of factor X to the viral capsid remained to be determined. In the present work, we have used a pharmacological agent (warfarin) as well as recombinant adenoviruses lacking the binding site of Factor X to elucidate this mechanism in mice. We demonstrate that, as observed in the liver, adenovirus infection of the adrenal glands in vivo requires Factor X. Considering that the level of transduction of the adrenal glands is well-below that of the liver and that capsid-modified adenoviruses are unlikely to selectively infect the adrenal glands, we have used single-photon emission computed tomography (SPECT) imaging of gene expression to determine whether local virus administration (direct injection in the kidney) could increase gene transfer to the adrenal glands. We demonstrate that direct injection of the virus in the kidney increases gene transfer in the adrenal gland but liver transduction remains important. These observations strongly suggest that serotype 5 adenovirus uses a similar mechanism to infect liver and adrenal gland and that selective transgene expression in the latter is more likely to be achieved through transcriptional targeting.

Treatment of Hemophilia A in Utero and Postnatally using Sheep as a Model for Cell and Gene Delivery

Hemophilia A represents the most common inheritable deficiency of the coagulation proteins. Current state-of- the-art treatment consists of frequent prophylactic infusions of plasma-derived or recombinant FVIII protein to maintain hemostasis, and has greatly increased life expectancy and quality of life for many hemophilia A patients. This treatment approach is, however, far from ideal, due to the need for lifelong intravenous infusions, the high treatment cost, and the fact that it is unavailable to a large percentage of the world's hemophiliacs. There is thus a need for novel treatments that can promise long-term or permanent correction. In contrast to existing protein based therapeutics, gene therapy offers to provide a permanent cure following few, or even a single, treatment. In the present paper, we review ongoing work towards this end, focusing on studies we have performed in a large animal model. Some of the key topics covered in this review include the unique opportunities sheep offer as a model system, the re-establishment and clinical and molecular characterization of a line of sheep with severe hemophilia A, the advantages and feasibility of treating a disease like hemophilia A in utero, and the use of Mesenchymal Stem Cells (MSC) as cellular delivery vehicles for the FVIII gene. The review finishes with a brief discussion of our recent success correcting ovine hemophilia A with a postnatal transplant with gene-modified MSC, and the limitations of this approach that remain to be overcome.

The case for intrauterine gene therapy

Single-gene disorders can cause perinatal mortality or severe permanent morbidity. Intrauterine gene therapy seeks to correct the genetic defect in the early stages of pathogenesis through delivery of a vector system expressing the therapeutic transgene to the fetus. Advantages of intrauterine gene therapy include prevention of irreversible organ damage, potentially inducing central tolerance and wider bio-distribution, including the brain after delivery of vector. Already, proof-of-cure has been demonstrated in knockout animal models for several diseases. Long-term outcomes pertaining to efficacy and durability of transgene expression and safety are under investigation in clinically relevant non-human primate models. Bystander effects in the mother from transplacental vector trafficking require further assessment. In this chapter, we discuss the candidate diseases amenable to intrauterine gene therapy, current state-of-the-art evidence, and potential clinical applications.

Animal models for prenatal gene therapy: the sheep model

Large animal experiments are vital in the field of prenatal gene therapy, to allow translation from small animals into man. Sheep provide many advantages for such experiments. They have been widely used in research into fetal physiology and pregnancy and the sheep fetus is a similar size to that in the human. Sheep are tolerant to in utero manipulations such as fetoscopy or even hysterotomy, and they are cheaper and easier to maintain than non-human primates. In this chapter, we describe the animal husbandry involved in generating time-mated sheep pregnancies, the large number of injection routes in the fetus that can be achieved using ultrasound or fetoscopic-guided injection, and laparotomy when these more minimally invasive routes of injection are not feasible.

Fetal gene therapy: recent advances and current challenges

INTRODUCTION

Fetal gene therapy (FGT) can potentially be applied to perinatally lethal monogenic diseases for rescuing clinically severe phenotypes, increasing the probability of intact neurological and other key functions at birth, or inducing immune tolerance to a transgenic protein to facilitate readministration of the vector/protein postnatally. As the field is still at an experimental stage, there are several important considerations regarding the practicality and the ethics of FGT.

AREAS COVERED

Here, through a review of FGT studies, the authors discuss the role and applications of FGT, the progress made with animal models that simulate human development, possible adverse effects in the recipient fetus and the mother and factors that affect clinical translation.

EXPERT OPINION

Although there are valid safety and ethical concerns, the authors argue that there may soon be enough convincing evidence from non-human primate models to take the next step towards clinical trials in the near future.

Organ targeted prenatal gene therapy--how far are we?

Prenatal gene therapy aims to deliver genes to cells and tissues early in prenatal life, allowing correction of a genetic defect, before long-term tissue damage has occurred. In contrast to postnatal gene therapy, prenatal application can target genes to a large population of dividing stem cells, and the smaller fetal size allows a higher vector-to-target cell ratio to be achieved. Early-gestation delivery may allow the development of immune tolerance to the transgenic protein which would facilitate postnatal repeat vector administration if needed. Targeting particular organs will depend on manipulating the vector to achieve selective tropism and on choosing the most appropriate gestational age and injection method for fetal delivery. Intra-amniotic injection reaches the skin, and other organs that are bathed in the fluid however since gene transfer to the lung and gut is usually poor more direct injection methods will be needed. Delivery to the liver and blood can be achieved by systemic delivery via the umbilical vein or peritoneal cavity. Gene transfer to the central nervous system in the fetus is difficult but newer vectors are available that transduce neuronal tissue even after systemic delivery.

Recombinant adeno-associated virus-mediated in utero gene transfer gives therapeutic transgene expression in the sheep

Somatic in utero gene therapy aims to treat congenital diseases where pathology develops in perinatal life, thereby preventing permanent damage. The aim of this study was to determine whether delivery of self-complementary (sc) adeno-associated virus (AAV) vector in utero would provide therapeutic long-term transgene expression in a large animal model. We performed ultrasound-guided intraperitoneal injection of scAAV2/8-LP1-human Factor IX (hFIX)co (1 × 10(12) vector genomes/kg) in early (n = 4) or late (n = 2) gestation fetal sheep. The highest mean hFIX levels were detected 3 weeks after injection in late gestation (2,055 and 1,687.5 ng/ml, n = 2) and 3 days after injection in early gestation (435 ng/ml, n = 1). Plasma hFIX levels then dropped as fetal liver and lamb weights increased, although low levels were detected 6 months after late gestation injection (75 and 52.5 ng/ml, n = 2). The highest vector levels were detected in the fetal liver and other peritoneal organs; no vector was present in fetal gonads. hFIX mRNA was detectable only in hepatic tissues after early and late gestation injection. Liver function tests and bile acid levels were normal up to a year postnatal; there was no evidence of liver pathology. No functional antibodies to hFIX protein or AAV vector were detectable, although lambs mounted an antibody response after injection of hFIX protein and Freund's adjuvant. In conclusion, hFIX expression is detectable up to 6 months after delivery of scAAV vector to the fetal sheep using a clinically applicable method. This is the first study to show therapeutic long-term hFIX transgene expression after in utero gene transfer in a large animal model.

Fetal muscle gene therapy/gene delivery in large animals

Gene delivery to the fetal muscles is a potential strategy for the early treatment of muscular dystrophies. In utero muscle gene therapy can also be used to treat other genetic disorders such as hemophilia, where the missing clotting proteins may be secreted from the treated muscle. In the past few years, studies in small animal models have raised the hopes that a phenotypic cure can be obtained after fetal application of gene therapy. Studies of efficacy and safety in large animals are, however, essential before clinical application can be considered in the human fetus. For this reason, the development of clinically applicable strategies for the delivery of gene therapy to the fetal muscles is of prime importance. In this chapter, we describe the protocols for in utero ultrasound-guided gene delivery to the ovine fetal muscle in early gestation. In particular, procedures to inject skeletal muscle groups such as the thigh and thoracic musculature and targeting the diaphragm in the fetus are described in detail.

Fetal gene therapy: opportunities and risks

Advances in human prenatal medicine and molecular genetics have allowed the diagnosis of many genetic diseases early in gestation. In-utero transplantation of allogeneic hematopoietic stem cells (HSC) has been successfully used as a therapy in different animal models and recently also in human fetuses. Unfortunately, clinical success of this novel treatment is limited by the lack of donor cell engraftment in non-immunocompromised hosts and is thus restricted to diseases where the fetus is affected by severe immunodeficiency. Gene therapy using genetically modified autologous HSC circumvents allogeneic HLA barriers and constitutes one of the most promising new approaches to correct genetic deficits in the fetus. Recent developments of strategies to overcome failure of efficient transduction of quiescent hematopoietic cells include the use of new vector constructs and transduction protocols. These improvements open new perspectives for gene therapy in general and for prenatal gene transfer in particular. The fetus may be especially susceptible for successful gene therapy due to the immunologic naiveté of the immature hematopoietic system during gestation, precluding an immune reaction towards the transgene. Ethical issues, in particular those regarding treatment safety, must be taken into account before clinical trials with fetal gene therapy in human pregnancies can be initiated.

Factors determining the risk of inadvertent retroviral transduction of male germ cells after in utero gene transfer in sheep

The possibility of permanent genetic changes to the germline is central to the bioethics of in utero gene therapy (IUGT) because of the concern of inadvertent potentially deleterious alterations to the gene pool. Despite presumed protection of the male germline due to early germ cell (GC) compartmentalization, we reported that GCs within the developing ovine testes are transduced at low levels after retrovirus-mediated IUGT, thus underscoring the need for a thorough understanding of GC development in clinically predictive models to determine the optimal time to perform IUGT and avoid germline modification. In the present studies, we used the fetal sheep model to analyze GCs for phenotype, location, proliferation, and incidence of transduction after IUGT at various fetal ages to learn when during development the nascent germline is likely to be at greatest risk of retrovirus-mediated alteration. Our studies show that although GCs were transduced at all injection ages, the levels of transduction varied by nearly 700-fold as a function of the age at transfer. After remaining largely quiescent as they migrated to/settled within nascent sex cords, GCs began active cycling before cord closure was complete, suggesting this is likely the point at which they would be most susceptible to retroviral transduction.Furthermore, we observed that compartmentalization of GCs continued into early postnatal life, suggesting the male germline may be vulnerable to low-level inadvertent retroviral vector modification throughout fetal life, but that this risk can be minimized by performing IUGT later in gestation.

Perinatal stem-cell and gene therapy for hemoglobinopathies

Most genetic diseases of the lymphohematopoietic system, including hemoglobinopathies, can now be diagnosed early in gestation. However, as yet, prenatal treatment is not available. Postnatal therapy by hematopoietic stem cell (HSC) transplantation from bone marrow, mobilized peripheral blood, or umbilical cord blood is possible for several of these diseases, in particular for the hemoglobinopathies, but is often limited by a lack of histocompatible donors, severe treatment-associated morbidity, and preexisting organ damage that developed before birth. In-utero transplantation of allogeneic HSC has been performed successfully in various animal models and recently in humans. However, the clinical success of this novel treatment is limited to diseases in which the fetus is affected by severe immunodeficiency. The lack of donor cell engraftment in nonimmunocompromised hosts is thought to be due to immunologic barriers, as well as to competitive fetal marrow population by host HSCs. Among the possible strategies to circumvent allogeneic HLA barriers, the use of gene therapy by genetically corrected autologous HSCs in the fetus is one of the most promising approaches. The recent development of strategies to overcome failure of efficient transduction of quiescent hematopoietic cells using new vector constructs and transduction protocols opens new perspectives for gene therapy in general, as well as for prenatal gene transfer in particular. The fetus might be especially susceptible for successful gene therapy approaches because of the developing, expanding hematopoietic system during gestation and the immunologic naiveté early in gestation, precluding immune reaction towards the transgene by inducing tolerance. Ethical issues, in particular regarding treatment safety, must be addressed more closely before clinical trials with fetal gene therapy in human pregnancies can be initiated.

Gene therapy for the fetus: is there a future?

Gene therapy uses the intracellular delivery of genetic material for the treatment of disease. A wide range of diseases - including cancer, vascular and neurodegenerative disorders and inherited genetic diseases - are being considered as targets for this therapy in adults. There are particular reasons why fetal application might prove better than application in the adult for treatment, or even prevention of early-onset genetic disorders such as cystic fibrosis and Duchenne muscular dystrophy. Research shows that gene transfer to the developing fetus targets rapidly expanding populations of stem cells, which are inaccessible after birth, and indicates that the use of integrating vector systems results in permanent gene transfer. In animal models of congenital disease such as haemophilia, studies show that the functionally immature fetal immune system does not respond to the product of the introduced gene, and therefore immune tolerance can be induced. This means that treatment could be repeated after birth, if that was necessary to continue to correct the disease. For clinicians and parents, fetal gene therapy would give a third choice following prenatal diagnosis of inherited disease, where termination of pregnancy or acceptance of an affected child are currently the only options. Application of this therapy in the fetus must be safe, reliable and cost-effective. Recent developments in the understanding of genetic disease, vector design, and minimally invasive delivery techniques have brought fetal gene therapy closer to clinical practice. However more research needs to be done in before it can be introduced as a therapy.

Evaluation of prenatal intra-amniotic LAMB3 gene delivery in a mouse model of Herlitz disease

Prenatal gene therapy has been considered for Herlitz junctional epidermolysis bullosa (H-JEB), a lethal genodermatosis caused by the absence of any of the three subunits of laminin-5, resulting from birth in widespread blistering and erosions of skin and mucosae. To investigate this strategy in an animal model, adenovirus type 5- and adeno-associated virus (AAV) type 2-derived vectors carrying a beta-galactosidase reporter gene or LAMB3 cDNA encoding the beta3 chain of laminin-5 were generated, tested for stability in amniotic fluid and evaluated in vitro on murine H-JEB keratinocytes, and in vivo by prenatal injection into the amniotic cavities of laminin-5 beta3-deficient mice. The different vectors were administered individually or combined at maximum doses on day 14 post coitum. Adenoviral vectors infected preferentially the foetal epidermis, whereas AAV delivered the transgene mainly to mucous membranes of the airways and the upper digestive tract. The LAMB3 transgene was expressed in target epithelia of newborn laminin-5 beta3-deficient mice, and the transgenic beta3 chain was shown to assemble with its endogenous partner chains, resulting in detectable amounts of laminin-5 in the basement membranes of skin and mucosae and in a lower extent of tissue separation in the skin. However, only combined delivery of the two vector types led to a minor increase of the life span of H-JEB mice. Failure to rescue diseased animals was, at least in part, due to abandonment of any conspicuous pup by the heterozygous mother. This is the first study of a prenatal gene therapy approach to a heritable blistering disorder. Although our findings indicate that prenatal combined administration of adenoviral and adeno-associated LAMB3 vectors provides therapeutic benefit to H-JEB mice, this animal model appears unsuitable for long-term investigations of the therapeutic concept.

Utility of intraperitoneal administration as a route of AAV serotype 5 vector-mediated neonatal gene transfer

BACKGROUND

Gene transfer into a fetus or neonate can be a fundamental approach for treating genetic diseases, particularly disorders that have irreversible manifestations in adulthood. Although the potential utility of this technique has been suggested, the advantages of neonatal gene transfer have not been widely investigated. Here, we tested the usefulness of neonatal gene transfer using adeno-associated virus (AAV) vectors by comparing the administration routes and vector doses.

METHODS

To determine the optimal administration route, neonates were subjected to intravenous (i.v.) or intraperitoneal (i.p.) injections of AAV5-based vectors encoding the human coagulation factor IX (hfIX) gene, and the dose response was examined. To determine the distribution of transgene expression, vectors encoding lacZ or luciferase (luc) genes were used and assessed by X-gal staining and in vivo imaging, respectively. After the observation period, the vector distribution across tissues was quantified.

RESULTS

The factor IX concentration was higher in i.p.-injected mice than in i.v.-injected mice. All transgenes administered by i.p. injection were more efficiently expressed in neonates than in adults. The expression was confined to the peritoneal tissue. Interestingly, a sex-related difference was observed in transgene expression in adults, whereas this difference was not apparent in neonates.

CONCLUSIONS

AAV vector administration to neonates using the i.p. route was clearly advantageous in obtaining robust transgene expression. Vector genomes and transgene expression were observed mainly in the peritoneal tissue. These findings indicate the advantages of neonatal gene therapy and would help in designing strategies for gene therapy using AAV vectors.

Clinically applicable procedure for gene delivery to fetal gut by ultrasound-guided gastric injection: toward prenatal prevention of early-onset intestinal diseases

Targeting gene therapy vectors to the fetal intestinal tract could provide a novel means toward prevention of the early postnatal intestinal pathology of cystic fibrosis and other conditions, such as congenital enteropathy, that cause intestinal failure. Among these conditions, cystic fibrosis is by far the most common lethal genetic disease. It is caused by a functional absence or deficiency of the cystic fibrosis transmembrane conductance regulator and manifests in the gut as meconium ileus. Prenatal treatment of genetic disease may avoid early-onset tissue damage and immune sensitization, and may target cells that are less accessible in the adult. We investigated gene transfer to the fetal gut, using a minimally invasive injection technique. First-generation replication-deficient adenoviral vectors encoding the beta-galactosidase gene and transduction-enhancing agents were injected into the stomach of early-gestation fetal sheep (n = 8, 60 days of gestation; term, 145 days) under ultrasound guidance. Reporter gene expression was observed 2 days after injection in the villi of the gastrointestinal epithelia after 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside staining and beta-galactosidase immunohistochemistry of fetal tissues. Expression of beta-galactosidase, as measured by enzyme-linked immunosorbent assay, was enhanced after pretreatment of the fetal gut with sodium caprate, which opens tight junctions, and after adenovirus complexation with DEAE-dextran, which confers a positive charge to the virus. Instillation of the fluorocarbon perflubron after virus delivery resulted in tissue transduction from the fetal stomach to the colon. Using a clinically relevant technique, we have demonstrated widespread gene transfer to the fetal gastrointestinal epithelia.

Differentiation and regeneration of adrenal tissues: An initial step toward regeneration therapy for steroid insufficiency

In animal experiments, adrenal cortical tissue has been successfully regenerated through xenotransplantation of cloned adrenocortical cells, suggesting that the intraadrenal stem cells required for such tissue formation may be present in the adrenal cortex. Stable expression of Ad4BP/SF-1, a key factor for adrenal and gonadal development and steroidogenesis, has been shown to direct embryonic stem cells toward the steroidogenic lineage. However, this steroidogenic capacity was very limited since progesterone was only produced in the presence of an exogenous substrate. Bone marrow mesenchymal cells are thought to contain pluripotent progenitor cells, which differentiate into multiple lineages. We have demonstrated that adenovirus-mediated forced expression of SF-1 in long-term cultured bone marrow cells can produce steroidogenic cells with the capacity for de novo synthesis of various steroid hormones in response to ACTH. This discovery may represent the first step in autologous cell transplantation therapy for patients with steroid hormone deficiency.

Targeting the respiratory muscles of fetal sheep for prenatal gene therapy for Duchenne muscular dystrophy

OBJECTIVE

Duchenne muscular dystrophy (DMD) is a lethal degenerative muscular disease. Fetal gene therapy may correct the primary genetic defect. Our aim was to achieve expression of a reporter gene in the respiratory muscles of early gestation fetal sheep.

STUDY DESIGN

An adenovirus vector containing the beta-galactosidase reporter gene (AdRSVbetagal) was injected into the thoracic musculature (n = 3) and pleural cavity (n = 6) of fetal sheep (61-67 days' gestation) under ultrasound guidance. Tissues were harvested after 48 hours and site and intensity of beta-galactosidase expression were assessed.

RESULTS

Limited transgene expression observed after a single injection was improved by multiple injections, but remained localized. Ultrasound-guided creation of a hydrothorax led to an increase in the intensity of beta-galactosidase expression (ELISA). X-gal staining and immunohistochemistry showed that vector spread was confined to the innermost intercostal musculature.

CONCLUSION

Ultrasound-guided injection can deliver gene therapy vectors to the fetal pleural cavity and achieve transduction of the respiratory muscles.

Fetal gene transfer using lentiviral vectors and the potential for germ cell transduction in rhesus monkeys (Macaca mulatta)

Genetic modification of germ cells in somatic gene therapy protocols is a concern, particularly with fetal approaches. This study focused on the potential for germ cell gene transfer post-fetal gene delivery using a human immunodeficiency virus type 1 (HIV-1)-derived lentiviral vector pseudotyped with the vesicular stomatitis virus-glycoprotein (VSV-G). Rhesus monkey fetuses (n = 47) were administered vector supernatant (10(7) infectious particles per fetus) via the intraperitoneal (IP), intrapulmonary (Ipu), or intracardiac routes (Ica) under ultrasound guidance. Tissue harvests were performed near term or 3 months postnatal age, and genomic DNA obtained to analyze for vector sequences from collected sections of gonads and gonadal cells obtained by laser capture microdissection (germ cells, stroma, epithelium). Results indicated no evidence of germ cell gene transfer in fetuses or infants with Ipu or Ica routes of administration. However, evidence of the transgene (1.33 +/- 0.78 enhanced green fluorescent protein [EGFP] copies per copy epsilon-globin) was found in females, but not males, when using the IP administration approach (p < 0.05). The highest EGFP copies were detected on the surface epithelium (p < 0.05). The results of these studies suggest that the HIV-1-derived lentiviral vector pseudotyped with VSV-G may transduce a subpopulation of gonadal cells in female fetuses with IP administration, whereas no evidence of gene transfer was shown to occur in males or with organ-targeting approaches.

In utero gene therapy: current challenges and perspectives

Over the past few years, considerable progress in prenatal diagnosis and surgery combined with improvements in vector design vindicate a reappraisal of the feasibility of in utero gene therapy for serious monogenetic diseases. As adult gene therapy gathers pace, several apparent obstacles to its application as a treatment may be overcome by pre- or early postnatal treatment. This review will examine the concepts and practice of prenatal vector administration. We aim to highlight the advantages of early therapeutic intervention focusing on diseases that could benefit greatly from a prenatal gene therapy approach. We will pay special attention to the strategies and vectors that are most likely to be used for this application and will speculate on their expected developments for the near future.

No evidence for germ-line transmission following prenatal and early postnatal AAV-mediated gene delivery

BACKGROUND

Recombinant adeno-associated viruses have been used successfully in a number of pre-clinical and clinical gene therapy studies. Since there is a broad consensus that gene therapy must not lead to germ-line transmission, the potential of such vectors for inadvertent gene transfer into germ cells deserves special attention. This applies in particular to pre- or perinatal vector application which has been considered for diseases presenting with morbidity already at birth.

METHODS

AAV serotype 2 derived vectors carrying a beta-galactosidase reporter gene or human clotting factor IX cDNA were injected intraperitoneally or via a yolk sac vein into mouse fetuses or administered intravascularly to newborn mice. Tissue samples of the treated animals including the gonads as well as sperm DNA, obtained by differential lysis of one testis of each male animal, and the offspring of all treated mice were investigated for the presence of vector DNA by nested PCR. In positive samples, the copy number of the vector was determined by quantitative real-time PCR.

RESULTS

AAV vectors administered intraperitoneally or intravascularly to fetal or newborn mice reached the gonads of these animals and persisted there for time periods greater than one year. Intravascular injection of the vector resulted more frequently in gene transfer to the gonads than intraperitoneal injection. Vector copy numbers in the gonads ranged from 0.3 to 74 per 10(4) cell equivalents. However, neither in isolated sperm DNA from the treated animals nor in their offspring were vector sequences detectable.

CONCLUSIONS

These data suggest the risk of inadvertent germ-line transmission following prenatal or early postnatal AAV type 2 mediated gene delivery to be very low.

Successful in utero gene transfer using a gene gun in midgestational mouse fetuses

BACKGROUND/PURPOSE

In utero gene therapy offers a number of potential advantages over postnatal gene therapy. A latest method of gene transfer to fetuses in utero uses a new tool called a gene gun. The gene gun is less invasive and simpler than other in utero methods. The current study was designed to determine whether the gene gun is an effective tool for transferring genes to mouse fetuses in utero.

METHODS

Using a gene gun, we transferred plasmids that included enhanced green fluorescent protein (EGFP) genes and cytomegalo virus promoters to the abdominal skin of 40 A/J fetal mice at each of 3 gestational ages (13, 14, or 15 days). Four or 5 days after gene transfer, the number of surviving fetuses was counted, and a color image of EGFP in the skin was analyzed for gene transfer rates by fluorescence microscopy. Survival rates were analyzed using Fisher's Exact test.

RESULTS

The mean survival rate was 89.2% (107 of 120) in gene transfer fetuses and 91.7% (55 of 60) in controls. There is no difference in survival rate between gene transfer fetus and control. The highest gene transfer rate was 100% (37 of 37) at the gestational age of 14 days. The rate was 97.1% (34 of 35) at gestational ages of 13 and 15 days.

CONCLUSIONS

The results of this study show that in utero gene transfer by gene gun is a less-invasive technique, and the gene gun is an effective tool transferring genes to mouse fetuses in utero.

The Hopes and Fears of In Utero Gene Therapy for Genetic Disease—A Review

Somatic gene delivery in utero is a novel approach to gene therapy for genetic disease. It is based on the concept that application of gene therapy vectors to the fetus in utero may prevent the development of early disease related tissue damage, may allow targeting of otherwise inaccessible organs, tissues and still expanding stem cell populations and may also provide postnatal tolerance against the therapeutic transgenic protein. This review outlines the hypothesis and scientific background of in utero gene therapy and addresses some of the frequently expressed concerns raised by this still experimental, potentially preventive gene therapy approach. We describe and discuss the choice of vectors, of animal models and routes of administration to the fetus. We address potential risk factors of prenatal gene therapy such as vector toxicity, inadvertent germ line modification, developmental aberration and oncogenesis as well as specific risks of this procedure for the fetus and mother and discuss their ethical implications.

Cellular characterization of the tropism of recombinant adenovirus for the adrenal glands

BACKGROUND

Recombinant adenoviruses are widely used in gene therapy clinical trials. A particular tropism for the adrenal glands has been reported but the precise cellular base for this tropism has not been determined.

MATERIALS AND METHODS

Recombinant adenoviruses were injected intravenously into Balb/c nu/nu or C57BL/6 mice. Seventy-two hours later, the animals were sacrificed and the adrenal glands and livers collected. The glands were sectioned and analyzed using immunohistochemical methods to detect adenoviral epitopes and transgene expression. Total RNA were extracted from the liver and adrenal glands of some animals and subjected to real-time RT-PCR.

RESULTS

The only cell type infected in the adrenal glands of Balb/c nu/nu or C57BL/6 mice is the adrenocortical cells in the zona fasciculata. Quantitatively, the relative level of gene expression in the adrenal gland is comparable but lower than that measured in the liver.

CONCLUSIONS

Systemic injection of recombinant adenovirus could be used as a procedure to restore adrenal steroidogenesis in clinical gene therapy protocols. In addition, our study suggest that adrenal dysfunction should be considered when criteria are established to assess the safety of gene therapy formulations administered systemically.

Long-term transgene expression by administration of a lentivirus-based vector to the fetal circulation of immuno-competent mice

Inefficient gene transfer, inaccessibility of stem cell compartments, transient gene expression, and adverse immune and inflammatory reactions to vector and transgenic protein are major barriers to successful in vivo application of gene therapy for most genetic diseases. Prenatal gene therapy with integrating vectors may overcome these problems and prevent early irreparable organ damage. To this end, high-dose attenuated VSV-G pseudotyped equine infectious anaemia virus (EIAV) encoding beta-galactosidase under the CMV promoter was injected into the fetal circulation of immuno-competent MF1 mice. We saw prolonged, extensive gene expression in the liver, heart, brain and muscle, and to a lesser extent in the kidney and lung of postnatal mice. Progressive clustered hepatocyte staining suggests clonal expansion of cells stably transduced. We thus provide proof of principle for efficient gene delivery and persistent transgene expression after prenatal application of the EIAV vector and its potential for permanent correction of genetic diseases.

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.

Ultrasound-guided percutaneous delivery of adenoviral vectors encoding the beta-galactosidase and human factor IX genes to early gestation fetal sheep in utero

In utero gene therapy may provide treatment of genetic diseases before significant organ damage, allow permanent genetic correction by reaching stem cell populations, and provide immune tolerance against the therapeutic transgenes and vectors. We have used percutaneous ultrasound-guided injection as a minimally invasive fetal procedure. First-generation adenoviruses encoding the nuclear localizing beta-galactosidase reporter gene or the human factor IX (hFIX) gene, or colloidal carbon were delivered via the umbilical vein (UV, n = 4), heart (intracardiac [IC], n = 2), liver parenchyma (intrahepatic [HE], n = 11), peritoneal cavity (intraperitoneal [IP], n = 14), skeletal musculature ([intramuscular [IM], n = 11), or the amniotic cavity (intraamniotic [IA], n = 14) to early-gestation fetal sheep (0.3 gestation = day 33-61). Postmortem analysis was performed at 2, 9, or 28 days after injection. Although fetal survival was between 77% and 91% for IP, HE, IA, and IM routes, no fetuses survived UV or IC procedures. The hFIX levels reaching 1900 and 401 ng/ml (IP), 30 ng/ml (HE), 66.5 and 39 ng/ml (IA), and 83 and 65.5 ng/ml (IM), respectively, were determined 2 days after injection and decreased at birth to 16.5 ng/ml (IP), 7 ng/ml (HE), 4.5 ng/ml (IA), and 4 and 0 ng/ml (IM). Polymerase chain reaction (PCR) and immunohistochemistry showed broadest hFIX transgene spread and highest localised beta-galactosidase expression, respectively, after IP administration. Antibodies were observed against vector but not against hFIX.

In utero gene transfer of human factor IX to fetal mice can induce postnatal tolerance of the exogenous clotting factor

The fundamental hypotheses behind fetal gene therapy are that it may be possible (1) to achieve immune tolerance of transgene product and, perhaps, vector; (2) to target cells and tissues that are inaccessible in adult life; (3) to transduce a high percentage of rapidly proliferating cells, and in particular stem cells, with relatively low absolute virus doses leading to clonal transgene amplification by integrating vectors; and (4) to prevent early disease manifestation of genetic diseases. This study provides evidence vindicating the first hypothesis; namely, that intravascular prenatal administration of an adenoviral vector carrying the human factor IX (hFIX) transgene can induce immune tolerance of the transgenic protein. Following repeated hFIX protein injection into adult mice, after prenatal vector injection, we found persistence of blood hFIX and absence of hFIX antibodies in 5 of 9 mice. Furthermore, there was substantial hFIX expression after each of 2 reinjections of vector without detection of hFIX antibodies. In contrast, all adult mice that had not been treated prenatally showed a rapid loss of the injected hFIX and the development of high hFIX antibody levels, both clear manifestations of a strong immune reaction.

Possible mechanism of gene transfer into early to mid-gestational mouse fetuses by tail vein injection

Our aim is to develop a simple gene transfer method into egg cylinder and mid-gestational murine embryos. We examined whether plasmid/lipid complexes injected into the tail veins of pregnant transgenic mice can be transferred to fetuses at E 4.5-13.5. When pregnant CETZ-17 mice carrying a transgene consisting of a ubiquitous promoter, floxed EGFP/CAT and the LacZ gene, were injected with a Cre expression vector DNA/lipid complex, Cre-mediated excision of the transgenes, as evaluated by X-gal staining, occurred in 10-50% of fetuses treated at E 11.5-13.5. Although younger embryos remained unstained, PCR analysis revealed low levels of the Cre vector DNA and recombined transgene. To examine the fate of a solution given intravenously, we injected trypan blue or fluorescence-labeled plasmid DNA/lipid complexes into females at E 5.5-11.5 and E 6.5, respectively. Both collected in the visceral endoderm (VE) lineage, but were undetectable in the embryo proper. These findings suggest that substances in maternal blood are delivered to post-implantation embryos via cells of the VE lineage and placenta, but that most are trapped in the VE. If significantly improved, gene transfer to fetuses by injection into the maternal circulation may become a promising tool in fetal gene therapy and embryological studies.

Efficient transduction of liver and muscle after in utero injection of lentiviral vectors with different pseudotypes

In this study we investigate the efficacy of lentiviral vectors of different pseudotypes for gene transfer to tissues of the preimmune fetus. BALB/c fetuses at 14-15 days' gestation received lentiviral vectors carrying the transgene lacZ under the control of the human cytomegalovirus (CMV) promoter by intramuscular (i.m.) or intrahepatic (i.h.) injection. We pseudotyped the lentiviral vectors with vesicular stomatitis virus (VSV-G), with Mokola virus, or with Ebola virus envelope glycoproteins. We harvested the pups at time points between 5 days and 9 months following injection and performed a detailed histologic assessment. The efficiency and distribution of transduction after in utero administration was highly dependent upon the route of administration and the pseudotype of vector used. Biodistribution studies showed widespread distribution of vector sequences in multiple tissues, albeit at very low levels, and transduced cells were found in significant numbers only in liver, heart, and muscle. Overall, VSV-G was the most efficient in transducing hepatocytes, whereas Mokola and Ebola were more efficient in transducing myocytes. Transduction of cardiomyocytes was observed after both i.m. and i.h. injection of all three vectors. Our findings of long-term transduction of skeletal myocytes and cardiomyocytes after in utero administration suggest a novel strategy for the treatment of congenital muscular dystrophies.

On the scientific and ethical issues of fetal somatic gene therapy

Fetal somatic gene therapy is often seen as an ethically particularly controversial field of gene therapy. This review outlines the hypothesis and scientific background of in utero gene therapy and addresses some of the frequently raised questions and concerns in relation to this still experimental, potentially preventive gene therapy approach. We discuss here the choice of vectors, of animal models and routes of administration to the fetus. We address the relation of fetal gene therapy to abortion, to post-implantation selection and postnatal gene therapy and the concerns of inadvertent germ-line modification. Our views on the specific risks of prenatal gene therapy and on the particular prerequisites that have to be met before human application can be considered are presented.

Sustained delivery of therapeutic concentrations of human clotting factor IX--a comparison of adenoviral and AAV vectors administered in utero

BACKGROUND

Prenatal somatic gene therapy has been considered for genetic disorders presenting with morbidity at birth. Haemophilia is associated with an increased risk of catastrophic perinatal bleeding complications such as intracranial haemorrhage, which could be prevented by gene transfer in utero. Prenatal gene therapy may be more promising than postnatal treatment, as the fetus may be more amenable to uptake and integration of therapeutic DNA and the immaturity of its immune system may permit life-long immune tolerance of the transgenic protein, thus avoiding the dominant problem in haemophilia treatment, the formation of inhibitory antibodies.

METHODS

Adenovirus serotype 5-derived or AAV serotype 2-derived vectors carrying human clotting factor IX (hfIX) cDNA or a reporter gene were administered intramuscularly, intraperitoneally or intravascularly to late-gestation mouse fetuses. Both vector types were evaluated with respect to the kinetics of hfIX delivery to the systemic circulation and possible immune responses against the vector or the transgene product.

RESULTS

Mice treated in utero by intramuscular injection of an adenoviral vector carrying hfIX cDNA exhibited high-level gene expression at birth and therapeutic--albeit continuously decreasing--plasma concentrations of hfIX over the entire 6 months of the study. Adenoviral vector spread to multiple organs was detected by polymerase chain reaction (PCR). Intramuscular, intraperitoneal or intravascular application of AAV vectors carrying hfIX cDNA led to much lower plasma concentrations of hfIX shortly after birth, which appeared to decline during the first month of life but stabilized in some of the mice at detectable levels. No signs of immune responses were found, either against the different viral vectors or against hfIX.

CONCLUSION

This study demonstrates for the first time that sustained systemic delivery of a therapeutic protein can be achieved by prenatal gene transfer. It thus shows the feasibility of gene therapy in utero and provides a basis for considering this concept as a preventive therapeutic strategy for haemophilia and perhaps also for other plasma protein deficiencies.

Fibroblast-mediated delivery of growth factor complementary DNA into mouse joints induces chondrogenesis but avoids the disadvantages of direct viral gene transfer

OBJECTIVE

To assess the advantages and disadvantages of a direct adenoviral and a cell-mediated approach to the induction of cartilage formation in joints by transfer of growth factor genes.

METHODS

Adenoviral vectors carrying insulin-like growth factor 1 (IGF-1) or bone morphogenetic protein 2 (BMP-2) complementary DNA were constructed and applied to primary human and murine chondrocytes or fibroblasts. Transgene expression was quantified by enzyme-linked immunosorbent assay. Direct injection of these vectors or AdLacZ, a reporter gene vector, into mouse knee joints was compared with the transplantation of syngeneic fibroblasts (infected ex vivo with the same vectors) with respect to virus spread, immune response, and cartilage formation by use of histologic, immunohistochemical, and molecular analyses.

RESULTS

AdIGF-1 and AdBMP-2 efficiently infected all cell types tested. Human cells secreted biologically relevant levels of protein over a period of at least 28 days. Direct transfer of AdLacZ into mouse knee joints resulted in positively stained synovial tissues, whereas AdLacZ-infected fibroblasts settled on the surface of the synovial membranes. Inadvertent spread of vector DNA into the liver, lung, and spleen was identified by nested polymerase chain reaction in all mice that had received the vector directly; this rarely occurred following fibroblast-mediated gene transfer. Direct injection of AdBMP-2 induced the synthesis of new cartilage in periarticular mesenchyme, accompanied by extensive osteophyte formation. When AdBMP-2 was administered by injecting ex vivo-infected fibroblasts, cartilage formation was observed only in regions near the injected cells. AdIGF-1 treatment did not lead to morphologic changes. Importantly, fibroblast-mediated gene transfer avoided the strong immune response to adenovirus that was elicited following direct application of the vector.

CONCLUSION

Our results indicate that cell-mediated gene transfer provides sufficient BMP-2 levels in the joint to induce cartilage formation while avoiding inadvertent vector spread and immune reactions.

Induction of stable prenatal tolerance to beta-galactosidase by in utero gene transfer into preimmune sheep fetuses

The successful transduction of hematopoietic stem cells and long-term (28 months) transgene expression within the hematopoietic system following the direct injection of high-titer retroviral vectors into preimmune fetal sheep was previously demonstrated. The present studies extended these analyses for 40 months postinjection and evaluated whether the longevity of transgene expression in this model system was the result of induction of prenatal tolerance to the transgene product. The intraperitoneal injection of retroviral vectors into preimmune sheep fetuses transduces thymic epithelial cells thought to present antigen and thus define self during immune system development. To directly demonstrate induction of tolerance, postnatal sheep were boosted with purified beta-galactosidase and showed that the peripheral blood lymphocytes from in utero-transduced sheep exhibited significantly lower stimulation indices to transduced autologous cells than did control animals and that the in utero-transduced sheep had a reduced ability to mount an antibody response to the vector-encoded beta-galactosidase protein compared with control sheep. Collectively, our results provide evidence that the direct injection of retroviral vectors into preimmune sheep fetuses induces cellular and humoral tolerance to the vector/transgene products and provide an explanation for the duration and stability of transgene expression seen in this model. These results also suggest that even relatively low levels of gene transfer in utero may render the recipient tolerant to the exogenous gene and thus potentially permit the successful postnatal treatment of the recipient. (Blood. 2001;97:3417-3423)

New ideas for medical treatment of congenital adrenal hyperplasia

During the past 50 years since the discovery of cortisone therapy as an effective treatment for CAH, many advances have been made in the management of 21-hydroxylase deficiency. Despite these advances, the clinical management of patients with CAH is often complicated by abnormal growth and development, iatrogenic Cushing's syndrome, inadequately treated hyperandrogenism, and infertility. New treatment approaches to classic CAH represent potential solutions to these unresolved issues. At the National Institutes of Health, a long-term randomized clinical trial is investigating a new treatment regimen: a reduced hydrocortisone dose, an antiandrogen, and an aromatase inhibitor. Peripheral blockade of androgens may also be helpful in the adult woman with CAH and PCOS. Other promising new treatment approaches include LHRH agonist-induced pubertal delay with or without growth hormone therapy, alternative glucocorticoid preparations or dose schedules, CRH antagonist treatment, and gene therapy. The applicability and success of these new approaches await the results of current research.

In utero delivery of adeno-associated viral vectors: intraperitoneal gene transfer produces long-term expression

Recombinant adeno-associated viruses (rAAV) are promising gene transfer vectors that produce long-term expression without toxicity. To investigate future approaches for in utero gene delivery, the efficacy and safety of prenatal administration of rAAV were determined. Using luciferase as a reporter, expression was assessed by whole-body imaging and by analysis of luciferase activity in tissue extracts, at the time of birth and monthly thereafter. Transgene expression was detected in all injected animals. Highest levels of luciferase activity were detected at birth in the peritoneum and liver, while the heart, brain, and lung demonstrated low-level expression. In vivo luciferase imaging revealed persistent peritoneal expression for 18 months after in utero injection and provided a sensitive whole-body assay, useful in identifying tissues for subsequent analyses. There was no detectable hepatocellular injury. Antibodies that reacted with either luciferase or rAAV were not found. AAV sequences were not detected in germ-line tissues of injected animals or in tissues of their progeny. In utero AAV-mediated gene transfer in this animal model demonstrates that novel therapeutic vectors and strategies can be rapidly tested in vivo and that rAAV may be developed to ameliorate genetic diseases with perinatal morbidity and mortality.

Rhesus monkey model for fetal gene transfer: studies with retroviral- based vector systems

Many life-threatening conditions that can be diagnosed early in gestation may be treatable in utero using gene therapy. In order to determine in utero gene transfer efficiency and safety, studies were conducted with fetal rhesus monkeys as a model for the human. Included in these studies were Moloney murine leukemia virus (MLV)-based amphotropic retrovirus, vesicular stomatitis virus-G (VSV-G) pseudotyped MLV, and a VSV-G pseudotyped HIV-1-based vector, all expressing the enhanced green fluorescent protein (EGFP) as a reporter gene and driven by a cytomegalovirus-immediate early promoter (N = 16). Rhesus monkey fetuses were administered viral vector supernatant preparations by the intraperitoneal (ip) (N = 14) or intrahepatic (ih) (N = 2) routes via ultrasound guidance at 55 +/- 5 days gestation (late first trimester; term 165 +/- 10 days). Fetuses were monitored sonographically, specimens were collected prenatally and postnatally, and tissue harvests were performed at birth or 3 or 6 months postnatal age (3-10 months post-gene transfer). PCR analyses demonstrated that transduced cells were present at approximately 1.2% in peripheral blood mononuclear cells from fetuses administered amphotropic MLV, <0.5% in fetuses receiving MLV/VSV-G, and approximately 4.2% for the lentiviral vector, which decreased to 2% at birth. Hematopoietic progenitors showed that overall (mean of all time points assessed), approximately 25% of the collected colonies were positive for the EGFP transgene with the lentiviral vector, which was significantly greater than results achieved with the MLV-based vector systems (4-9%; P < or = 0.001-0.016). At necropsy, 0.001-10% of the total genomic DNA was positive for EGFP in most tissues for all groups. EGFP-positive fluorescent cells were found in cell suspensions of thymus, liver, spleen, lymph nodes, cerebral cortex, and bone marrow (0.5-6%). Overall, the results of these studies have shown: (1) healthy infants expressing vector sequences up to 10 months post-gene transfer, (2) fetal primate administration of retroviral vectors results in gene transfer to multiple organ systems, (3) the highest level of gene transfer to hematopoietic progenitors was observed with the lentiviral vector system, and (4) there was no evidence of transplacental transfer of vector sequences into the dams. The rhesus monkey is an important preclinical primate model system for exploring gene transfer approaches for future applications in humans.

From gene transfer to gene therapy in lysosomal storage diseases affecting the central nervous system

Gene transfer into the central nervous system (CNS) is one of the foremost scientific challenges today. To give a brief survey of possible approaches to gene therapy in diseases affecting the CNS, we have selected the lysosomal storage diseases (LDS), which are an excellent model of both early-onset infantile neurological forms and late-onset adult psychiatric forms. Lysosomal storage diseases represent a group of about 50 monogenic metabolic disorders resulting from a deficiency in intralysosomal enzymes involved in macromolecule catabolism. The clinical severity, including neuropsychiatric symptoms, and the absence of an efficient therapy for the majority of these disorders prompted the various trials of gene therapy now in progress. Most of the genes encoding the normal lysosomal enzymes have been cloned, and the size of the corresponding cDNAs is generally compatible with their transfer by recombinant vectors. New vectors with improved immunogenicity, transduction efficacy, insert capacity, and specificity of targeting are under development. Here we discuss several gene therapy strategies for the correction of LSD-induced anomalies in the CNS. Interesting results have been obtained by animal model brain, which raises hopes that large-scale clinical trials may soon be started.