The developmental stage determines the distribution and duration of gene expression after early intra-amniotic gene transfer using lentiviral vectors

Recent Genome-Editing Approaches toward Post-Implanted Fetuses in Mice

Genome editing, as exemplified by the CRISPR/Cas9 system, has recently been employed to effectively generate genetically modified animals and cells for the purpose of gene function analysis and disease model creation. There are at least four ways to induce genome editing in individuals: the first is to perform genome editing at the early preimplantation stage, such as fertilized eggs (zygotes), for the creation of whole genetically modified animals; the second is at post-implanted stages, as exemplified by the mid-gestational stages (E9 to E15), for targeting specific cell populations through in utero injection of viral vectors carrying genome-editing components or that of nonviral vectors carrying genome-editing components and subsequent in utero electroporation; the third is at the mid-gestational stages, as exemplified by tail-vein injection of genome-editing components into the pregnant females through which the genome-editing components can be transmitted to fetal cells via a placenta-blood barrier; and the last is at the newborn or adult stage, as exemplified by facial or tail-vein injection of genome-editing components. Here, we focus on the second and third approaches and will review the latest techniques for various methods concerning gene editing in developing fetuses.

In utero delivery of mRNA to the heart, diaphragm and muscle with lipid nanoparticles

Nanoparticle-based drug delivery systems have the potential to revolutionize medicine, but their low vascular permeability and rapid clearance by phagocytic cells have limited their medical impact. Nanoparticles delivered at the in utero stage can overcome these key limitations due to the high rate of angiogenesis and cell division in fetal tissue and the under-developed immune system. However, very little is known about nanoparticle drug delivery at the fetal stage of development. In this report, using Ai9 CRE reporter mice, we demonstrate that lipid nanoparticle (LNP) mRNA complexes can deliver mRNA in utero, and can access and transfect major organs, such as the heart, the liver, kidneys, lungs and the gastrointestinal tract with remarkable efficiency and low toxicity. In addition, at 4 weeks after birth, we demonstrate that 50.99 ± 5.05%, 36.62 ± 3.42% and 23.7 ± 3.21% of myofiber in the diaphragm, heart and skeletal muscle, respectively, were transfected. Finally, we show here that Cas9 mRNA and sgRNA complexed to LNPs were able to edit the fetal organs in utero. These experiments demonstrate the possibility of non-viral delivery of mRNA to organs outside of the liver in utero, which provides a promising strategy for treating a wide variety of devastating diseases before birth.

Molecular and Cellular In Utero Therapy

Significant advances in maternal-fetal medicine and gene sequencing technology have fostered a new frontier of in utero molecular and cellular therapeutics, including gene editing, enzyme replacement therapy, and stem cell transplantation to treat single-gene disorders with limited postnatal treatment strategies. In utero therapies take advantage of unique developmental properties of the fetus to allow for the correction of monogenic disorders before irreversible disease pathology develops. While early preclinical studies in animal models are encouraging, more studies are needed to further evaluate their safety and efficacy prior to widespread clinical use.

Administration of Drugs/Gene Products to the Respiratory System: A Historical Perspective of the Use of Inert Liquids

The present review is a historical perspective of methodology and applications using inert liquids for respiratory support and as a vehicle to deliver biological agents to the respiratory system. As such, the background of using oxygenated inert liquids (considered a drug when used in the lungs) opposed to an oxygen-nitrogen gas mixture for respiratory support is presented. The properties of these inert liquids and the mechanisms of gas exchange and lung function alterations using this technology are described. In addition, published preclinical and clinical trial results are discussed with respect to treatment modalities for respiratory diseases. Finally, this forward-looking review provides a comprehensive overview of potential methods for administration of drugs/gene products to the respiratory system and potential biomedical applications.

Intrauterine Fetal Gene Therapy: Is That the Future and Is That Future Now?

Researchers are looking into techniques to intervene sooner and earlier in the disease process thanks to advances in disease genetics, etiologies, and prenatal diagnosis. We conducted a literature search in PubMed-indexed journals to provide an overview of the evolution of gene therapy, rationale for prenatal gene therapy, uses and risks of gene therapy, and ethical issues following the usage of gene therapy. Recent animal research has revealed that transmitting genetic material to a growing fetus through viral and non-viral vectors is conceivable besides proving how gene-editing technology is achieved by various mechanisms that utilize zinc finger nucleases, TAL effector nucleases, and clustered short palindromic repeats-Cas9 complex. This review offers an overview of the current knowledge in the field of prenatal gene therapy, as well as potential future research avenues. In addition, it weighs the risks of prenatal gene therapy, such as oncogenesis, genetic mutation transfer from mother to child, and fetal disruption, against the expected benefits, such as preventing the development of severe early-onset illness symptoms, targeting previously inaccessible organs, and establishing tolerance to the therapeutic transgenic protein, all of which lead to permanent somatic gene correction.

This review discusses the scientific, ethical, legal, and sociological implications of these groundbreaking genetic disease prevention techniques, as well as the parameters that must be satisfied for a future clinical application to be considered.

Society for Maternal-Fetal Medicine Special Statement: Beyond the scalpel: in utero fetal gene therapy and curative medicine

With the recent advances in gene editing with systems such as CRISPR-Cas9, precise genome editing in utero is on the horizon. Sickle cell disease is an excellent candidate for in utero fetal gene therapy, because the disease is monogenic, causes irreversible harm, and has life-limiting morbidity. Gene therapy has recently been proven to be effective in an adolescent patient. Several hurdles still impede the progress for fetal gene therapy in humans, including an incomplete understanding of the fetal immune system, unclear maternal immune responses to in utero gene therapy, risks of off-target effects from gene editing, gestational age constraints, and ethical questions surrounding fetal genetic intervention. However, none of these barriers appears insurmountable, and the journey to in utero gene therapy for sickle cell disease and other conditions should be well underway.

Prenatal Gene Therapy for Metabolic Disorders

Gene therapy has traditionally involved the delivery of exogenous genetic material to a cell-most commonly to replace defective genes causing monogenic disorders. This allows cells to produce proteins that are otherwise absent in sufficient quantities, ideally for a therapeutic purpose. Since its inception over 40 years ago, the field of gene therapy has significantly expanded and now includes targeted gene editing strategies, including, but not limited to, clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9), transcription activator-like effector nucleases (TALENs), and zinc-finger nucleases (ZFNs).

The Future of Fetal Surgery

The field of fetal medicine has evolved significantly over the past several decades. Our ability to identify and treat the unborn patient has been shaped by advancements in imaging technology, genetic diagnosis, an improved understanding of fetal physiology, and the development and optimization of in utero surgical techniques. The future of the field will be shaped by medical innovators pushing for the continued refinement of minimally invasive surgical technique, the application of pioneering technologies such as robotic surgery and in utero stem cell and gene therapies, and the development of innovative ex utero fetal support systems.

In utero adenine base editing corrects multi-organ pathology in a lethal lysosomal storage disease

In utero base editing has the potential to correct disease-causing mutations before the onset of pathology. Mucopolysaccharidosis type I (MPS-IH, Hurler syndrome) is a lysosomal storage disease (LSD) affecting multiple organs, often leading to early postnatal cardiopulmonary demise. We assessed in utero adeno-associated virus serotype 9 (AAV9) delivery of an adenine base editor (ABE) targeting the Idua G→A (W392X) mutation in the MPS-IH mouse, corresponding to the common IDUA G→A (W402X) mutation in MPS-IH patients. Here we show efficient long-term W392X correction in hepatocytes and cardiomyocytes and low-level editing in the brain. In utero editing was associated with improved survival and amelioration of metabolic, musculoskeletal, and cardiac disease. This proof-of-concept study demonstrates the possibility of efficiently performing therapeutic base editing in multiple organs before birth via a clinically relevant delivery mechanism, highlighting the potential of this approach for MPS-IH and other genetic diseases.

A narrative review of in utero gene therapy: advances, challenges, and future considerations

The field of in utero gene therapy (IUGT) represents a crossroad of technologic advancements and medical ethical boundaries. Several strategies have been developed for IUGT focusing on either modifying endogenous genes, replacing missing genes, or modifying gene transcription products. The list of candidate diseases such as hemoglobinopathies, cystic fibrosis, lysosomal storage disorders continues to grow with new strategies being developed as our understanding of their respective underlying molecular pathogenesis increases. Treatment in utero has several distinct advantages to postnatal treatment. Biologic and physiologic phenomena enable the delivery of a higher effective dose, generation of immune tolerance, and the prevention of phenotypic onset for genetic diseases. Therapeutic technology for IUGT including CRISPR-Cas9 systems, zinc finger nucleases (ZFN), and peptide nucleic acids (PNAs) has already shown promise in animal models and early postnatal clinical trials. While the ability to detect fetal diagnoses has dramatically improved with developments in ultrasound and next-generation sequencing, treatment options remain experimental, with several translational gaps remaining prior to implementation in the clinical realm. Complicating this issue, the potential diseases targeted by this approach are often debilitating and would otherwise prove fatal if not treated in some manner. The leap from small animals to large animals, and subsequently, to humans will require further vigorous testing of safety and efficacy.

The Future of In Utero Gene Therapy

Significant advances in the safety and efficacy of gene therapy have sparked a new frontier in therapeutics for genetic diseases as evidenced by the greater than 700 active gene therapy investigational new drug applications reported by the NIH and the US Food and Drug Association. Although postnatal gene therapy trials are encouraging, limitations to effective therapy including an immune barrier and initiation of treatment after disease onset can exist. Advances in prenatal diagnostics provide hope that many genetic abnormalities will be able to be diagnosed before birth. Prenatal gene therapy has the potential to take advantage of normal developmental properties of the fetus and overcome some of the current limitations to efficient postnatal gene therapy. The rationale for prenatal gene therapy includes the small fetal size, the tolerogenic fetal immune system, the presence of highly proliferative and accessible stem/progenitor cells of multiple organs, and, ultimately, the ability to treat diseases in which irreversible pathology begins prior to birth. This rationale is based on and supported by a number of published animal studies. Unique ethical considerations exist in the context of prenatal gene therapy, including the importance of rigorous evaluation of the effect of the therapy on fetal germ cells and developing organs as well as the mother. To date, animal studies have not demonstrated any significant germline or maternal effect of prenatal gene therapy. Finally, practical considerations of future clinical prenatal gene therapy will include, but not be limited to, determining the initial target disease characteristics and the importance of non-directive prenatal counseling of families carrying a fetus with a genetic diagnosis.

Delivery technologies for in utero gene therapy

Advances in prenatal imaging, molecular diagnostic tools, and genetic screening have unlocked the possibility to treat congenital diseases in utero prior to the onset of clinical symptoms. While fetal surgery and in utero stem cell transplantation can be harnessed to treat specific structural birth defects and congenital hematological disorders, respectively, in utero gene therapy allows for phenotype correction of a wide range of genetic disorders within the womb. However, key challenges to realizing the broad potential of in utero gene therapy are biocompatibility and efficiency of intracellular delivery of transgenes. In this review, we outline the unique considerations to delivery of in utero gene therapy components and highlight advances in viral and non-viral delivery platforms that meet these challenges. We also discuss specialized delivery technologies for in utero gene editing and provide future directions to engineer novel delivery modalities for clinical translation of this promising therapeutic approach.

Gene and Stem Cell Therapies for Fetal Care: A Review

Importance

Gene and stem cell therapies have become viable therapeutic options for many postnatal disorders. For select conditions, prenatal application would provide improved outcomes. The fetal state allows for several theoretical advantages over postnatal therapy, including immune immaturity and cellular niche accessibility.

Observations

Advances in prenatal diagnostic accuracy and surgical precision, as well as improvements in stem cell and gene therapy methods, have made prenatal gene and stem cell therapy realistic. Studies in mouse models and early human trials demonstrate the feasibility of these approaches. Additional efforts are under way to streamline fetal applications of stem cell and gene therapy while carefully considering best ethical practice and following established regulatory pathways.

Conclusions and Relevance

Fetal stem cell and gene therapy bring important therapeutic opportunities for select disorders that present in the fetal and neonatal periods. While this field is in its infancy, these therapies are starting to be available clinically, and clinicians should be aware of their benefits and challenges.

In utero gene editing for monogenic lung disease

Monogenic lung diseases that are caused by mutations in surfactant genes of the pulmonary epithelium are marked by perinatal lethal respiratory failure or chronic diffuse parenchymal lung disease with few therapeutic options. Using a CRISPR fluorescent reporter system, we demonstrate that precisely timed in utero intra-amniotic delivery of CRISPR-Cas9 gene editing reagents during fetal development results in targeted and specific gene editing in fetal lungs. Pulmonary epithelial cells are predominantly targeted in this approach, with alveolar type 1, alveolar type 2, and airway secretory cells exhibiting high and persistent gene editing. We then used this in utero technique to evaluate a therapeutic approach to reduce the severity of the lethal interstitial lung disease observed in a mouse model of the human SFTPCI73T mutation. Embryonic expression of SftpcI73T alleles is characterized by severe diffuse parenchymal lung damage and rapid demise of mutant mice at birth. After in utero CRISPR-Cas9-mediated inactivation of the mutant SftpcI73T gene, fetuses and postnatal mice showed improved lung morphology and increased survival. These proof-of-concept studies demonstrate that in utero gene editing is a promising approach for treatment and rescue of monogenic lung diseases that are lethal at birth.

In utero CRISPR-mediated therapeutic editing of metabolic genes

In utero gene editing has the potential to prenatally treat genetic diseases that result in significant morbidity and mortality before or shortly after birth. We assessed the viral vector-mediated delivery of CRISPR-Cas9 or base editor 3 in utero, seeking therapeutic modification of Pcsk9 or Hpd in wild-type mice or the murine model of hereditary tyrosinemia type 1, respectively. We observed long-term postnatal persistence of edited cells in both models, with reduction of plasma PCSK9 and cholesterol levels following in utero Pcsk9 targeting and rescue of the lethal phenotype of hereditary tyrosinemia type 1 following in utero Hpd targeting. The results of this proof-of-concept work demonstrate the possibility of efficiently performing gene editing before birth, pointing to a potential new therapeutic approach for selected congenital genetic disorders.

Fetal stem cell and gene therapy

Advances in our understanding of stem cells, gene editing, prenatal imaging and fetal interventions have opened up new opportunities for the treatment of congenital diseases either through in-utero stem cell transplantation or in-utero gene therapy. Improvements in ultrasound-guided access to the fetal vasculature have also enhanced the safety and efficacy of cell delivery. The fetal environment offers accessible stem cell niches, localized cell populations with large proliferative potential, and an immune system that is able to acquire donor-specific tolerance. In-utero therapy seeks to take advantage of these factors and has the potential to cure diseases prior to the onset of symptoms, a strategy that offers substantial social and economic benefits. In this article, we examine previous studies in animal models as well as clinical attempts at in-utero therapy. We also discuss the barriers to successful in-utero therapy and future strategies for overcoming these obstacles.

Induction of Immune Tolerance to Foreign Protein via Adeno-Associated Viral Vector Gene Transfer in Mid-Gestation Fetal Sheep

A major limitation to adeno-associated virus (AAV) gene therapy is the generation of host immune responses to viral vector antigens and the transgene product. The ability to induce immune tolerance to foreign protein has the potential to overcome this host immunity. Acquisition and maintenance of tolerance to viral vector antigens and transgene products may also permit repeat administration thereby enhancing therapeutic efficacy. In utero gene transfer (IUGT) takes advantage of the immunologic immaturity of the fetus to induce immune tolerance to foreign antigens. In this large animal study, in utero administration of AAV6.2, AAV8 and AAV9 expressing green fluorescent protein (GFP) to ~60 day fetal sheep (term: ~150 days) was performed. Transgene expression and postnatal immune tolerance to GFP and viral antigens were assessed. We demonstrate 1) hepatic expression of GFP 1 month following in utero administration of AAV6.2.GFP and AAV8.GFP, 2) in utero recipients of either AAV6.2.GFP or AAV8.GFP fail to mount an anti-GFP antibody response following postnatal GFP challenge and lack inflammatory cellular infiltrates at the intramuscular site of immunization, 3) a serotype specific anti-AAV neutralizing antibody response is elicited following postnatal challenge of in utero recipients of AAV6.2 or AAV8 with the corresponding AAV serotype, and 4) durable hepatic GFP expression was observed up to 6 months after birth in recipients of AAV8.GFP but expression was lost between 1 and 6 months of age in recipients of AAV6.2.GFP. The current study demonstrates, in a preclinical large animal model, the potential of IUGT to achieve host immune tolerance to the viral vector transgene product but also suggests that a single exposure to the vector capsid proteins at the time of IUGT is inadequate to induce tolerance to viral vector antigens.

Targeting the gastrointestinal tract with viral vectors: state of the art and possible applications in research and therapy

While there is a large body of preclinical data on the use of viral vectors in gene transfer, relatively little is known about viral gene transfer in the gastrointestinal tract. Viral vector technology is especially underused in the field of neurogastroenterology when compared to brain research. This review provides an overview of the studies employing viral vectors-in particular retroviruses, adenoviruses and adeno-associated viruses-to transduce different cell types in the intestine. Early work mainly focused on mucosal transduction, but had limited success due to the harsh luminal conditions in the gastrointestinal tract and the high turnover rate of enterocytes. More recently, several studies have successfully employed viral gene transfer to target the enteric nervous system and its progenitors. Although several hurdles still need to be overcome, in particular on how to augment transduction efficiency and specific cell targeting, viral vector technology holds strong potential not only as a valid research tool in fundamental gastroenterological research but also as a therapeutic agent in translational (bio)medical research.

Life-Long Transgene Expression in Skeletal Muscle Without Transduction of Satellite Cells Following Embryonic Myogenic Progenitor Transduction by Lentivirus Administered in Utero

Embryologic events in mammalian myogenesis remain to be fully defined. Recent evidence supports the presence of a common progenitor arising in the dermomyotome that gives rise to both embryologic and adult muscle and postnatal myogenic stem cells (satellite cells). In this study, we utilize the technique of early intra-amniotic gene transfer to target nascent muscle progenitors as they traverse the primitive streak before formation of the dermomyotome. This technique robustly transduced both epaxial and hypaxial muscle groups. Marker gene expression is observed in up to 100% muscle fibers in the lower extremities and is sustained for the lifetime of the mouse. We next analyzed transduced muscle for satellite cell transduction using highly sensitive methodology. Surprisingly, despite high levels of sustained transgene expression in muscle fibers, satellite cells lacked the marker transgene. Our data suggest that dermatomyotome is a heterogeneous structure and that not all myogenic progenitors of dermatomyotome give rise to satellite cells.

Developmental stage determines efficiency of gene transfer to muscle satellite cells by in utero delivery of adeno-associated virus vector serotype 2/9

Efficient gene transfer to muscle stem cells (satellite cells) has not been achieved despite broad transduction of skeletal muscle by systemically administered adeno-associated virus serotype 2/9 (AAV-9) in mice. We hypothesized that cellular migration during fetal development would make satellite cells accessible for gene transfer following in utero intravascular injection. We injected AAV-9 encoding green fluorescent protein (GFP) marker gene into the vascular space of mice ranging in ages from post-coital day 12 (E12) to postnatal day 1 (P1). Satellite cell transduction was examined using: immunohistochemistry and confocal microscopy, satellite cell migration assay, myofiber isolation and FACS analysis. GFP positive myofibers were detected in all mature skeletal muscle groups and up to 100% of the myofibers were transduced. We saw gestational variation in cardiac and skeletal muscle expression. E16 injection resulted in 27.7 ± 10.0% expression in satellite cells, which coincides with the timing of satellite cell migration, and poor satellite cell expression before and after satellite cell migration (E12 and P1). Our results demonstrate that efficient gene expression is achieved in differentiated myofibers and satellite cells after injection of AAV-9 in utero. These findings support the potential of prenatal gene transfer for muscle based treatment strategies.

In utero lung gene transfer using adeno-associated viral and lentiviral vectors in mice

Virus-mediated gene transfer to the fetal lung epithelium holds considerable promise for the therapeutic management of prenatally diagnosed, potentially life-threatening inherited lung diseases. In this study we hypothesized that efficient and life-long lung transduction can be achieved by in utero gene therapy, using viral vectors. To facilitate diffuse entry into the lung, viral vector was injected into the amniotic sac of C57BL/6 mice on embryonic day 16 (term, ∼ 20 days) in a volume of 10 μl. Vectors investigated included those based on adeno-associated virus (AAV) (serotypes 5, 6.2, 9, rh.64R1) and vesicular stomatitis virus G glycoprotein (VSV-G)-pseudotyped HIV-1-based lentivirus (LV). All vectors expressed green fluorescent protein (GFP) under the transcriptional control of various promoters including chicken β-actin (CB) or cytomegalovirus (CMV) for AAV and CMV or MND (myeloproliferative sarcoma virus enhancer, negative control region deleted) for LV. Pulmonary GFP gene expression was detected by fluorescence stereoscopic microscopy and immunohistochemistry for up to 9 months after birth. At equivalent vector doses (mean, 12 × 10(10) genome copies per fetus) three AAV vectors resulted in long-term (up to 9 months) pulmonary epithelium transduction. AAV2/6.2 transduced predominantly cells of the conducting airway epithelium, although transduction decreased 2 months after vector delivery. AAV2/9-transduced cells of the alveolar epithelium with a type 1 pneumocyte phenotype for up to 6 months. Although minimal levels of GFP expression were observed with AAV2/5 up to 9 months, the transduced cells immunostained positive for F480 and were retrievable by bronchoalveolar lavage, confirming an alveolar macrophage phenotype. No GFP expression was observed in lung epithelial cells after AAV2/rh.64R1 and VSV-G-LV vector-mediated gene transfer. We conclude that these experiments demonstrate that prenatal lung gene transfer with AAV vectors engineered to target pulmonary epithelial cells may provide sustained long-term levels of transgene expression, supporting the therapeutic potential of prenatal gene transfer for the treatment of congenital lung diseases.

Stem cell and genetic therapies for the fetus

The prenatal diagnosis and management of congenital disease has made significant progress over the previous decade. Currently, fetal therapy (including open surgery and fetoscopic intervention) provides therapeutic options for a range of congenital anomalies; however, it is restricted to the treatment of fetal pathophysiology. Improvements in prenatal screening and the early diagnosis of genetic disease allow for preemptive treatment of anticipated postnatal disease by stem cell or genetic therapy. While currently awaiting clinical application, in utero stem cell therapy has made significant advances in overcoming the engraftment and immunologic barriers in both murine and pre-clinical large animal models. Likewise, proof in principle for fetal gene therapy has been demonstrated in rodent and large animal systems as a method to prevent the onset of inherited genetic disease; however, safety and ethical risks still need to be addressed prior to human application. In this review, we examine the current status and future direction of stem cell and genetic therapy for the fetus.

Intra-amniotic transient transduction of the periderm with a viral vector encoding TGFβ3 prevents cleft palate in Tgfβ3(-/-) mouse embryos

Cleft palate is a developmental defect resulting from the failure of embryonic palatal shelves to fuse with each other at a critical time. Immediately before and during palatal fusion (E13-E15 in mice), transforming growth factor β3 (TGFβ3) is expressed in the palatal shelf medial edge epithelium (MEE) and plays a pivotal role in palatal fusion. Using Tgfβ3(-/-) mice, which display complete penetrance of the cleft palate phenotype, we tested the hypothesis that intra-amniotic gene transfer could be used to prevent cleft palate formation by restoring palatal midline epithelial function. An adenoviral vector encoding Tgfβ3 was microinjected into the amniotic sacs of mouse embryos at successive developmental stages. Transduced Tgfβ3(-/-) fetuses showed efficient recovery of palatal fusion with mesenchymal confluence following injection at E12.5 (100%), E13.5 (100%), E14.5 (82%), and E15.5 (75%). Viral vectors injected into the amniotic sac transduced the most superficial and transient peridermal cell layer but not underlying basal epithelial cells. TGFβ3 transduction of the peridermdal cell layer was sufficient to induce adhesion, fusion, and disappearance of the palatal shelf MEE in a cell nonautonomous manner. We propose that intra-amniotic gene transfer approaches have therapeutic potential to prevent cleft palate in utero, especially those resulting from palatal midline epithelial dysfunction.

Animal models for prenatal gene therapy: choosing the right model

Testing in animal models is an essential requirement during development of prenatal gene therapy for -clinical application. Some information can be derived from cell lines or cultured fetal cells, such as the efficiency of gene transfer and the vector dose that might be required. Fetal tissues can also be maintained in culture for short periods of time and transduced ex vivo. Ultimately, however, the use of animals is unavoidable since in vivo experiments allow the length and level of transgene expression to be measured, and provide an assessment of the effect of the delivery procedure and the gene therapy on fetal and neonatal development. The choice of animal model is determined by the nature of the disease and characteristics of the animal, such as its size, lifespan, and immunology, the number of fetuses and their development, parturition, and the length of gestation and the placentation. The availability of a disease model is also critical. In this chapter, we discuss the various animal models that can be used and consider how their characteristics can affect the results obtained. The projection to human application and the regulatory hurdles are also presented.

Risks, benefits and ethical, legal, and societal considerations for translation of prenatal gene therapy to human application

The still experimental nature of prenatal gene therapy carries a certain degree of risk, both for the pregnant mother as well as for the fetus. Some of the risks are procedural hazards already known from more conventional fetal medicine interventions. Others are more specific to gene therapy such as the potential for interference with normal fetal development, the possibility of inadvertent germ line gene transfer, and the danger of oncogenesis. This chapter reviews the potential risks in relation to the expected benefits of prenatal gene therapy. It discusses the scientific, ethical, legal, and social implications of this novel preventive approach to genetic disease and outlines preconditions to be met in preparation for a potential future clinical application.

Animal models for prenatal gene therapy: rodent models for prenatal gene therapy

Fetal gene transfer has been studied in various animal models, including rabbits, guinea pigs, cats, dogs, and nonhuman primate; however, the most common model is the rodent, particularly the mouse. There are numerous advantages to mouse models, including a short gestation time of around 20 days, large litter size usually of more than six pups, ease of colony maintenance due to the small physical size, and the relatively low expense of doing so. Moreover, the mouse genome is well defined, there are many transgenic models particularly of human monogenetic disorders, and mouse-specific biological reagents are readily available. One criticism has been that it is difficult to perform procedures on the fetal mouse with suitable accuracy. Over the past decade, accumulation of technical expertise and development of technology such as high-frequency ultrasound have permitted accurate vector delivery to organs and tissues. Here, we describe our experiences of gene transfer to the fetal mouse with and without ultrasound guidance from mid to late gestation. Depending upon the vector type, the route of delivery and the age of the fetus, specific or widespread gene transfer can be achieved, making fetal mice excellent models for exploratory biodistribution studies.

Use of manipulated stem cells for prenatal therapy

Prenatal stem cell therapy has broad potential for therapeutic application. "Stem cells" of interest include multipotent adult-derived stem cells, cord blood, amniotic fluid, or fetal stem cells, and embryonic or induced pluripotent stem cells. Potential manipulations of stem cells prior to their administration may include harvest, processing, enrichment, expansion, and genetic transduction. A complete description of the methodology related to all of the above is well beyond the scope of this chapter. In the interest of practical application and proven efficacy, we limit our description to adult-derived hematopoietic stem cells (HSCs) and their application to in utero transplantation with or without HSC-targeted gene transfer.

Recent advances in fetal gene therapy

Over the first decade of this new millennium gene therapy has demonstrated clear clinical benefits in several diseases for which conventional medicine offers no treatment. Clinical trials of gene therapy for single gene disorders have recruited predominantly young patients since older subjects may have suffered irrevocablepathological changes or may not be available because the disease is lethal relatively early in life. The concept of fetal gene therapy is an extension of this principle in that diseases in which irreversible changes occur at or beforebirth can be prevented by gene supplementation or repair in the fetus or associated maternal tissues. This article ccnsiders the enthusiasm and skepticism held for fetal gene therapy and its potential for clinical application. It coversa spectrum of candidate diseases for fetal gene therapy including Pompe disease, Gaucher disease, thalassemia, congenital protein C deficiency and cystic fibrosis. It outlines successful and not-so-successful examples of fetal gene therapy in animal models. Finally the application and potential of fetal gene transfer as a fundamental research tool for developmental biology and generation of somatic transgenic animals is surveyed.

Early intra-amniotic gene transfer using lentiviral vector improves skin blistering phenotype in a murine model of Herlitz junctional epidermolysis bullosa

Mutations of the LAMB3 gene cause a lethal form of junctional epidermolysis bullosa (JEB). We hypothesized that early intra-amniotic gene transfer in a severe murine model of JEB would improve or correct the skin phenotype. Time-dated fetuses from heterozygous LAMB3(IAP) breeding pairs underwent ultrasound guided intra-amniotic injection of lentiviral vector encoding the murine LAMB3 gene at embryonic day 8 (E8). Gene expression was monitored by immunohistochemistry. The transgenic laminin-β3 chain was shown to assemble with its endogenous partner chains, resulting in detectable amounts of laminin-332 in the basement membrane zone of skin and mucosa. Ultrastructually, the restoration of ∼60% of hemidesmosomal structures was also noted. Although we could correct the skin phenotype in 11.9% of homozygous LAMB3(IAP) mice, none survived beyond 48 h. However, skin transplants from treated E18 homozygous LAMB3(IAP) fetuses maintained normal appearance for 6 months with persistence of normal assembly of laminin-332. These results demonstrate for the first time long-term phenotypic correction of the skin pathology in a severe model of JEB by in vivo prenatal gene transfer. Although survival remained limited due to the limitations of this mouse model, this study supports the potential for treatment of JEB by prenatal gene transfer.

Animal models for prenatal gene therapy: the nonhuman primate model

Intrauterine gene therapy (IUGT) potentially enables the treatment and possible cure of monogenic -diseases that cause severe fetal damage. The main benefits of this approach will be the ability to correct the disorder before the onset of irreversible pathology and inducing central immune tolerance to the vector and transgene if treatment is instituted in early gestation. Cure has been demonstrated in small animal models, but because of the significant differences in immune ontogeny and the much shorter gestation compared to humans, it is unlikely that questions of long-term efficacy and safety will be adequately addressed in rodents. The nonhuman primate (NHP) allows investigation of key issues, in particular, the different outcomes in early and late-gestation IUGT associated with different stages of immune maturity, longevity of transgene expression, and delayed-onset adverse events in treated offspring and mothers including insertional mutagenesis. Here, we describe a model based on the Macaca fascicularis using ultrasound and fetoscopic approaches to systemic vector delivery and the processes involved in vector administration and longitudinal analyses.

Early gestational gene transfer with targeted ATP7B expression in the liver improves phenotype in a murine model of Wilson's disease

The ideal gene therapy for metabolical liver disorders would target hepatocytes before the onset of disease and be durable, non-toxic and non-immunogenic. Early gestational gene transfer can achieve such goals. Here, we demonstrate that prenatal gene transfer of human Atp7b reduces liver pathology and improves biochemical markers in Atp7b(-/-) mice, a murine model of Wilson's disease (WD). Following prenatal injection of lentivirus vector containing the human Atp7b gene under the transcriptional control of a liver-specific promoter, the full-length ATP7B was detectable in mouse livers for the entire duration of experiments (20 weeks after birth). In contrast to a marked pathology in non-injected animals, livers from age-matched treated mice consistently demonstrated normal gross and histological morphology. Hepatic copper content was decreased in the majority of treated mice, although remaining copper levels varied. Improvement of hepatic copper metabolism was further apparent from the presence of copper-bound ceruloplasmin in the sera and normalization of the mRNA levels for HMG CoA-reductase. With this approach, the complete loss of copper transport function can be ameliorated, as evident from phenotypical improvement in treated Atp7b(-/-) mice. This study provides proof of principle for in utero gene therapy in WD and other liver-based enzyme deficiencies.

Jaagsiekte sheep retrovirus pseudotyped lentiviral vector-mediated gene transfer to fetal ovine lung

Viral vector-mediated gene transfer to the postnatal respiratory epithelium has, in general, been of low efficiency due to physical and immunological barriers, non-apical location of cellular receptors critical for viral uptake and limited transduction of resident stem/progenitor cells. These obstacles may be overcome using a prenatal strategy. In this study, HIV-1-based lentiviral vectors (LVs) pseudotyped with the envelope glycoproteins of Jaagsiekte sheep retrovirus (JSRV-LV), baculovirus GP64 (GP64-LV), Ebola Zaire-LV or vesicular stomatitis virus (VSVg-LV) and the adeno-associated virus-2/6.2 (AAV2/6.2) were compared for in utero transfer of a green fluorescent protein (GFP) reporter gene to ovine lung epithelium between days 65 and 78 of gestation. GFP expression was examined on day 85 or 136 of gestation (term is ∼145 days). The percentage of the respiratory epithelial cells expressing GFP in fetal sheep that received the JSRV-LV (3.18 × 10(8)-6.85 × 10(9) viral particles per fetus) was 24.6±0.9% at 3 weeks postinjection (day 85) and 29.9±4.8% at 10 weeks postinjection (day 136). Expression was limited to the surface epithelium lining fetal airways <100 μm internal diameter. Fetal airways were amenable to VSVg-LV transduction, although the percentage of epithelial expression was low (6.6±0.6%) at 1 week postinjection. GP64-LV, Ebola Zaire-LV and AAV2/6.2 failed to transduce the fetal ovine lung under these conditions. These data demonstrate that prenatal lung gene transfer with LV engineered to target apical surface receptors can provide sustained and high levels of transgene expression and support the therapeutic potential of prenatal gene transfer for the treatment of congenital lung diseases.

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.

Genetic modification of airway progenitors after lentiviral gene delivery to the amniotic fluid of murine fetuses

Lentiviral vectors with the firefly luciferase or enhanced green fluorescent protein (EGFP) transgenes were delivered to the amniotic fluid of murine fetuses at Embryonic Day (E) 14.5 or E16.5. Whole-body imaging of luciferase recipients after birth demonstrated transgene expression in the peritoneal and thoracic regions. Organ imaging showed luciferase expression in lung, skin, stomach, and/or intestine. Histological immunofluorescence analysis of EGFP recipients demonstrated that small clusters (≤ three cells) of EGFP-positive epithelial cells were present in the large and small airways of recipients at up to 7 months (n = 11). There was no difference in the frequency of transgene expression in mice injected at E14.5 or E16.5 in respiratory or nonrespiratory organs. Analysis of the bronchoalveolar duct junctions on tissue sections of recipient mice identified multiple EGFP-positive epithelial cells. Cells coexpressing EGFP, Clara cell 10-kd protein, and surfactant protein C (SPC) were also found in lungs, consistent with the transduction of bronchoalveolar stem cells. Next, naphthalene lung injury in both luciferase and EGFP recipients was performed to determine whether transduced cells could contribute to tissue repair. In luciferase recipients, the whole-body luciferase signal increased 2- to 20-fold at 2 weeks after naphthalene treatment. Remarkably, immunohistological analysis of the lungs of EGFP recipients after lung injury repair demonstrated repopulation of airways with long stretches of EGFP-positive epithelial cells (n = 4). Collectively, these data demonstrate that lentiviral gene delivery to the amniotic fluid of murine fetuses genetically modifies long-lived epithelial progenitors capable of contributing to lung injury repair.

Robust in vivo transduction of nervous system and neural stem cells by early gestational intra amniotic gene transfer using lentiviral vector

Presently, in vivo methods to efficiently and broadly transduce all major cell types throughout both the central (CNS) and peripheral adult nervous system (PNS) are lacking. In this study, we hypothesized that during early fetal development neural cell populations, including neural stem cells (NSCs), may be accessible for gene transfer via the open neural groove. To test this hypothesis, we injected lentiviral vectors encoding a green fluorescent protein (GFP) marker gene into the murine amniotic cavity at embryonic day 8. This method (i) efficiently and stably transduced the entire nervous system for at least 80% of the lifespan of the mice, (ii) transduced all major neural cell types, and (iii) transduced adult NSCs of the subventricular zone (SVZ) and subgranular zones (SGZs). This simple approach has broad applications for the study of gene function in nervous system development and adult NSCs and may have future clinical applications for treatment of genetic disorders of the nervous system.