Myoblast gene therapy in canine mucopolysaccharidosis. I: Abrogation by an immune response to alpha-L-iduronidase

Ex vivo gene therapy for lysosomal storage disorders: future perspectives

INTRODUCTION

Lysosomal storage disorders (LSD) are a group of monogenic rare diseases caused by pathogenic variants in genes that encode proteins related to lysosomal function. These disorders are good candidates for gene therapy for different reasons: they are monogenic, most of lysosomal proteins are enzymes that can be secreted and cross-correct neighboring cells, and small quantities of these proteins are able to produce clinical benefits in many cases. Ex vivo gene therapy allows for autologous transplant of modified cells from different sources, including stem cells and hematopoietic precursors.

AREAS COVERED

Here, we summarize the main gene therapy and genome editing strategies that are currently being used as ex vivo gene therapy approaches for lysosomal disorders, highlighting important characteristics, such as vectors used, strategies, types of cells that are modified and main results in different disorders.

EXPERT OPINION

Clinical trials are already ongoing, and soon approved therapies for LSD based on ex vivo gene therapy approaches should reach the market.

Current therapies for Morquio A syndrome and their clinical outcomes

INTRODUCTION

Morquio A syndrome is characterized by a unique skeletal dysplasia, leading to short neck and trunk, pectus carinatum, laxity of joints, kyphoscoliosis, and tracheal obstruction. Cervical spinal cord compression/inability, a restrictive and obstructive airway, and/or bone deformity and imbalance of growth, are life-threatening to Morquio A patients, leading to a high morbidity and mortality. It is critical to review the current therapeutic approaches with respect to their efficacy and limitations.

AREAS COVERED

Patients with progressive skeletal dysplasia often need to undergo orthopedic surgical interventions in the first two decades of life. Recently, we have treated four patients with a new surgery to correct progressive tracheal obstruction. Enzyme replacement therapy (ERT) has been approved clinically. Cell-based therapies such as hematopoietic stem cell therapy (HSCT) and gene therapy are typically one-time, permanent treatments for enzyme deficiencies. We report here on four Morquio A patients treated with HSCT approved in Japan and followed for at least ten years after treatment. Gene therapy is under investigation on mouse models but not yet available as a therapeutic option.

EXPERT OPINION

ERT and HSCT in combination with surgical intervention(s) are a therapeutic option for Morquio A; however, the approach for bone and cartilage lesion remains an unmet challenge.

Recent advances in gene therapy for lysosomal storage disorders

Lysosomal storage disorders (LSDs) are a group of genetic diseases that result in metabolic derangements of the lysosome. Most LSDs are due to the genetic absence of a single catabolic enzyme, causing accumulation of the enzyme’s substrate within the lysosome. Over time, tissue-specific substrate accumulations result in a spectrum of symptoms and disabilities that vary by LSD. LSDs are promising targets for gene therapy because delivery of a single gene into a small percentage of the appropriate target cells may be sufficient to impact the clinical course of the disease. Recently, there have been several significant advancements in the potential for gene therapy of these disorders, including the first human trials. Future clinical trials will build upon these initial attempts, with an improved understanding of immune system responses to gene therapy, the obstacle that the blood–brain barrier poses for neuropathic LSDs, as well other biological barriers that, when overcome, may facilitate gene therapy for LSDs. In this manuscript, we will highlight the recent innovations in gene therapy for LSDs and discuss the clinical limitations that remain to be overcome, with the goal of fostering an understanding and further development of this important field.

Factors influencing the use and interpretation of animal models in the development of parenteral drug delivery systems

Depending upon the drug and drug delivery platform, species-specific physiological differences can lead to errors in the interspecies extrapolation of drug performance. This manuscript provides an overview of the species-specific physiological variables that can influence the performance of parenteral dosage forms such as in situ forming delivery systems, nanoparticles, microspheres, liposomes, targeted delivery systems, lipophilic solutions, and aqueous suspensions. Also discussed are those factors that can influence the partitioning of therapeutic compounds into tumors, the central nervous system and the lymphatics. Understanding interspecies differences in the movement and absorption of molecules is important to the interpretation of data generated through the use of animal models when studying parenteral drug delivery.

Cell microencapsulation: a potential tool for the treatment of neuronopathic lysosomal storage diseases

Lysosomal storage disorders (LSD) are monogenic diseases caused by the deficiency of different lysosomal enzymes that degrade complex substrates such as glycosaminoglycans, sphingolipids, and others. As a consequence there is multisystemic storage of these substrates. Most treatments for these disorders are based in the fact that most of these enzymes are soluble and can be internalized by adjacent cells via mannose-6-phosphate receptor. In that sense, these disorders are good candidates to be treated by somatic gene therapy based on cell microencapsulation. Here, we review the existing data about this approach focused on the LSD treatments, the advantages and limitations faced by these studies.

Large animal models of neurological disorders for gene therapy

he development of therapeutic interventions for genetic disorders and diseases that affect the central nervous system (CNS) has proven challenging. There has been significant progress in the development of gene therapy strategies in murine models of human disease, but gene therapy outcomes in these models do not always translate to the human setting. Therefore, large animal models are crucial to the development of diagnostics, treatments, and eventual cures for debilitating neurological disorders. This review focuses on the description of large animal models of neurological diseases such as lysosomal storage diseases, Parkinsons disease, Huntingtons disease, and neuroAIDS. The review also describes the contributions of these models to progress in gene therapy research.

Gene therapy for mucopolysaccharidosis

Mucopolysaccharidoses (MPS) are due to deficiencies in activities of lysosomal enzymes that degrade glycosaminoglycans. Some attempts at gene therapy for MPS in animal models have involved intravenous injection of vectors derived from an adeno-associated virus (AAV), adenovirus, retrovirus or a plasmid, which primarily results in expression in liver and secretion of the relevant enzyme into blood. Most vectors can correct disease in liver and spleen, although correction in other organs including the brain requires high enzyme activity in the blood. Alternative approaches are to transduce hematopoietic stem cells, or to inject a vector locally into difficult-to-reach sites such as the brain. Gene therapy holds great promise for providing a long-lasting therapeutic effect for MPS if safety issues can be resolved.

Correction of clinical manifestations of canine mucopolysaccharidosis I with neonatal retroviral vector gene therapy

Mucopolysaccharidosis I (MPS I) (Hurler syndrome) is due to deficient alpha-L-iduronidase (IDUA) activity and is the most common of the MPS disorders. Neonatal MPS I dogs were injected intravenously (IV) with a gamma retroviral vector containing a complete long-terminal repeat (LTR) and an internal human alpha(1)-antitrypsin (hAAT) promoter upstream of the canine IDUA complementary DNA (cDNA). This resulted in stable serum IDUA activity of 366 +/- 344 units (U)/ml (28-fold normal) for up to 1.8 years, which likely derived primarily from secretion of IDUA by transduced liver cells. Retroviral vector (RV)-treated dogs had >18% of normal IDUA activity in organs and had decreased severity and/or incidence of hernias, chest deformities, joint disease, facial dysmorphia, corneal clouding, valvular heart disease, and aortic dilatation as compared with untreated MPS I dogs. The marked reduction that was observed in lysosomal storage in the brain of RV-treated dogs may have been due in part to expression from the LTR of the vector in cells in the brain. This possibility will be explored in future studies, because the potential for insertional mutagenesis has raised concerns about using vectors with an intact LTR. If proven safe, this gene therapy technique may be utilized in treating children with Hurler syndrome.

Development of MPS IVA mouse (Galnstm(hC79S.mC76S)slu) tolerant to human N-acetylgalactosamine-6-sulfate sulfatase

Mucopolysaccharidosis IVA (MPS IVA) is an autosomal recessive disease caused by N-acetylgalactosamine-6-sulfate sulfatase (GALNS) deficiency. In recent studies of enzyme replacement therapy for animal models with lysosomal storage diseases, cellular and humoral immune responses to the injected enzymes have been recognized as major impediments to effective treatment. To study the long-term effectiveness and side effects of therapies in the absence of immune responses, we have developed an MPS IVA mouse model, which has many similarities to human MPS IVA and is tolerant to human GALNS protein. We used a construct containing both a transgene (cDNA) expressing inactive human GALNS in intron 1 and an active site mutation (C76S) in adjacent exon 2 and thereby introduced both the inactive cDNA and the C76S mutation into the murine Galns by targeted mutagenesis. Affected homozygous mice have no detectable GALNS enzyme activity and accumulate glycosaminoglycans in multiple tissues including visceral organs, brain, cornea, bone, ligament and bone marrow. At 3 months, lysosomal storage is marked within hepatocytes, reticuloendothelial Kupffer cells, and cells of the sinusoidal lining of the spleen, neurons and meningeal cells. The bone storage is also obvious, with lysosomal distention in osteoblasts and osteocytes lining the cortical bone, in chondrocytes and in the sinus lining cells in bone marrow. Ubiquitous expression of the inactive human GALNS was also confirmed by western blot using the anti-GALNS monoclonal antibodies newly produced, which resulted in tolerance to immune challenge with human enzyme. The newly generated MPS IVA mouse model should provide a good model to evaluate long-term administration of enzyme replacement.

Liver-directed neonatal gene therapy prevents cardiac, bone, ear, and eye disease in mucopolysaccharidosis I mice

Mucopolysaccharidosis I (MPS I) due to deficient alpha-L-iduronidase (IDUA) activity results in accumulation of glycosaminoglycans in many cells. Gene therapy could program liver to secrete enzyme with mannose 6-phosphate (M6P), and enzyme in blood could be taken up by other cells via the M6P receptor. Newborn MPS I mice were injected with 10(9) (high dose) or 10(8) (low dose) transducing units/kg of a retroviral vector (RV) expressing canine IDUA. Most animals achieved stable expression of IDUA in serum at 1240 +/- 147 and 110 +/- 31 units/ml, respectively. At 8 months, untreated MPS I mice had aortic insufficiency, increased bone mineral density (BMD), and reduced responses to sound and light. In contrast, MPS I mice that received high-dose RV had normal echocardiograms, BMD, auditory-evoked brain-stem responses, and electroretinograms. This is the first report of complete correction of these clinical manifestations in any model of mucopolysaccharidosis. Biochemical and pathologic evaluation confirmed that storage was reduced in these organs. Mice that received low-dose RV and achieved 30 units/ml of serum IDUA activity had no or only partial improvement. We conclude that high-dose neonatal gene therapy with an RV reduces some major clinical manifestations of MPS I in mice, but low dose is less effective.

Production of MPS VII mouse (Gus(tm(hE540A x mE536A)Sly)) doubly tolerant to human and mouse beta-glucuronidase

Mucopolysaccharidosis VII (MPS VII, Sly syndrome) is an autosomal recessive lysosomal storage disease caused by beta-glucuronidase (GUS) deficiency. A naturally occurring mouse model of that disease has been very useful for studying experimental approaches to therapy. However, immune responses can complicate evaluation of the long-term benefits of enzyme replacement or gene therapy delivered to adult MPS VII mice. To make this model useful for studying the long-term effectiveness and side effects of experimental therapies delivered to adult mice, we developed a new MPS VII mouse model, which is tolerant to both human and murine GUS. To achieve this, we used homologous recombination to introduce simultaneously a human cDNA transgene expressing inactive human GUS into intron 9 of the murine Gus gene and a targeted active site mutation (E536A) into the adjacent exon 10. When the heterozygote products of germline transmission were bred to homozygosity, the homozygous mice expressed no GUS enzyme activity but expressed inactive human GUS protein highly and were tolerant to immune challenge with human enzyme. Expression of the mutant murine Gus gene was reduced to about 10% of normal levels, but the inactive murine GUS enzyme also conferred tolerance to murine GUS. This MPS VII mouse model should be useful to evaluate therapeutic responses in adult mice receiving repetitive doses of enzyme or mice receiving gene therapy as adults. Heterozygotes expressed only 9.5-26% of wild-type levels of murine GUS instead of the expected 50%, indicating a dominant-negative effect of the mutant enzyme monomers on the activity of GUS tetramers in different tissues. Corrective gene therapy in this model should provide high enough levels of expression of normal GUS monomers to overcome the dominant negative effect of mutant monomers on newly synthesized GUS tetramers in most tissues.

Induction of tolerance to a recombinant human enzyme, acid alpha-glucosidase, in enzyme deficient knockout mice

When knockout mice are used to test the efficacy of recombinant human proteins, the animals often develop antibodies to the enzyme, precluding long-term pre-clinical studies. This has been a problem with a number of models, for example, the evaluation of gene or enzyme replacement therapies in a knockout model of glycogen storage disease type II (GSDII; Pompe syndrome). In this disease, the lack of acid alpha-glucosidase (GAA) results in lysosomal accumulation of glycogen, particularly in skeletal and cardiac muscle. Here, we report that in a GAA-deficient mouse model of GSDII, low levels of transgene-encoded human GAA expressed in skeletal muscle or liver dramatically blunt or abolish the immune response to human recombinant protein. Of two low expression transgenic lines, only the liver-expressing line exhibited a profound GAA deficiency in skeletal muscle and heart indistinguishable from that in the original knockouts. The study suggests that the induction of tolerance in animal models of protein deficiencies could be achieved by restricting the expression of a gene of interest to a particular, carefully chosen tissue.

Hurler's syndrome, West's syndrome, and vitamin D-dependent rickets

Mucopolysaccharidosis I is a metabolic disease of autosomal recessive inheritance caused by deficient activity of alpha-L-iduronidase. The clinical phenotype presents a wide spectrum of signs in the first year of life. We report a child with clinical features and laboratory data consistent with mucopolysaccharidosis I who precociously developed hydrocephalus and flexion spasms with hypsarrythmia in the electroencephalographic registration characteristic of West's syndrome. His radiologic and biochemical data suggested vitamin D-dependent rickets. To our knowledge, this is the first report of a patient demonstrating an association among mucopolysaccharidosis 1, West's syndrome, and vitamin D-dependent rickets.

Avoiding untoward immune responses to novel gene products

For successful therapeutic transfection of a missing (or corrective) gene, by the use of liposomes or other delivery systems, it is essential that the patient be immunosuppressed to the corresponding immunogenic, transgenic protein prior to its transfection. We have developed a method for antigen-specific, long-term suppression of de novo induction of both antibodies and cytotoxic T lymphocytes--in spite of repeated administration of the antigen in question over extended periods. The method consists of converting the antigen to its tolerogenic derivative by coupling it to an appropriate number of molecules of monomethoxypolyethylene glycol (mPEG).

Active site mutant transgene confers tolerance to human beta-glucuronidase without affecting the phenotype of MPS VII mice

Mucopolysaccharidosis type VII (MPS VII; Sly syndrome) is an autosomal recessive lysosomal storage disorder due to an inherited deficiency of beta-glucuronidase. A naturally occurring mouse model for this disease was discovered at The Jackson Laboratory and shown to be due to homozygosity for a 1-bp deletion in exon 10 of the gus gene. The murine model MPS VII (gus(mps/mps)) has been very well characterized and used extensively to evaluate experimental strategies for lysosomal storage diseases, including bone marrow transplantation, enzyme replacement therapy, and gene therapy. To enhance the value of this model for enzyme and gene therapy, we produced a transgenic mouse expressing the human beta-glucuronidase cDNA with an amino acid substitution at the active site nucleophile (E540A) and bred it onto the MPS VII (gus(mps/mps)) background. We demonstrate here that the mutant mice bearing the active site mutant human transgene retain the clinical, morphological, biochemical, and histopathological characteristics of the original MPS VII (gus(mps/mps)) mouse. However, they are now tolerant to immune challenge with human beta-glucuronidase. This "tolerant MPS VII mouse model" should be useful for preclinical trials evaluating the effectiveness of enzyme and/or gene therapy with the human gene products likely to be administered to human patients with MPS VII.

Canine models for human genetic neurodegenerative diseases

1. Canine models of human neurodegenerative disorders are uncommon. However, the similarity between canines and humans in body sizes and physiology provides an exceptional opportunity to use these models to study human diseases. 2. The authors will present a review on the neurological deficits that have been observed in canine models of genetic neurodegenerative diseases, and summarize the current gene therapy treatments being developed for some of these conditions.

Adeno-associated virus vector transduction of vascular smooth muscle cells in vivo

Adeno-associated virus (AAV) vectors might offer solutions for restenosis and angiogenesis by transducing nondividing cells and providing long-term gene expression. We investigated the feasibility of vascular cell transduction by AAV vectors in an in vivo rabbit carotid artery model. Time course of gene expression, inflammatory reaction to the vector, and effects of varying viral titer, exposure time, and intraluminal pressures on gene expression were examined. Recombinant AAV vectors with an Rous sarcoma virus promoter and alkaline phosphatase reporter gene were injected intraluminally into transiently isolated carotid segments. Following transduction, gene expression increased significantly over 14 days and then remained stable to 28 days, the last time point examined. Medial vascular smooth muscle cells were the main cell type transduced even with an intact endothelial layer. Increasing the viral titer and intraluminal pressure both enhanced transduction efficiency to achieve a mean of 34 +/- 7% of the subintimal layer of smooth muscle cells expressing gene product. A mild inflammatory reaction, composed of T cells with only rare macrophages, with minimal intimal thickening was demonstrated in 40% of transduced vessels; inflammatory cells were not detected in sham-operated control arteries. These findings demonstrate that AAV is a promising vector for intravascular applications in coronary and peripheral vascular diseases.

Gene transfer into canine myoblasts

We have developed and characterized cultures of healthy and dystrophic canine myoblasts for the evaluation of various gene transfer protocols. The number of desmin-positive myoblasts was elevated (>>80%) in cultures of myoblasts obtained from different muscle territories, the diaphragm muscle giving rise to the purest cultures. Myoblasts from dogs turned out to be a very convenient source of well transfectable and transducible cells. Transfection with plasmid DNA allowed efficient transgene expression (50% of beta-galactosidase positive cells and about 375 ng luciferase/mg protein after transfection with a calcium phosphate-precipitated plasmid). Infection with high concentrations of adenoviral and retroviral vectors allowed transgene (beta-galactosidase or mini-dystrophin) detection in about 75 to 90% of the canine cells. Therefore, primary dog myoblast cultures represent a useful in vitro model for viral and non-viral gene delivery, as well as for functional evaluation and cell grafting with applications in genetic diseases, vaccination or production of circulating therapeutic proteins.

Gene therapy for canine alpha-L-iduronidase deficiency: in utero adoptive transfer of genetically corrected hematopoietic progenitors results in engraftment but not amelioration of disease

Canine alpha-L-iduronidase (iduronidase) deficiency is a model of the human lysosomal storage disorder mucopolysaccharidosis type I (MPS I). We used this canine model to evaluate the therapeutic potential of hematopoietic stem cell (HSC) gene therapy for enzyme deficiencies. In previous studies, iduronidase-deficient dogs infused with autologous marrow cells genetically modified to express iduronidase had long-term engraftment with provirally marked cells, but there was no evidence of proviral iduronidase expression or clinical improvement. The presence of humoral and cellular immune responses against iduronidase apparently abrogated the therapeutic potential of HSC gene therapy in these experiments. To evaluate HSC gene therapy for canine MPS I in the absence of a confounding immune response, we have now performed in utero adoptive transfer of iduronidase-transduced MPS I marrow cells into preimmune fetal pups. In three separate experiments, 17 midgestation fetal pups were injected with 0.5-1.5 x 10(7) normal or MPS I allogeneic long-term marrow culture (LTMC) cells transduced with neo(r)- or iduronidase-containing retroviral vectors. Nine normal and three MPS I pups survived the neonatal period and demonstrated engraftment of provirally marked progenitors at levels of up to 12% for up to 12 months. However, the proportion of provirally marked circulating leukocytes was approximately 1%. Neither iduronidase enzyme nor proviral-specific transcripts were detected in blood or marrow leukocytes of any MPS I dog. Humoral immune responses to iduronidase were not detected in neonates, even after "boosting" with autologous iduronidase-transduced LTMC cells. All MPS I dogs died at 8-11 months of age from complications of MPS I disease with no evidence of amelioration of MPS I disease. Our results suggest that iduronidase-transduced primitive hematopoietic progenitors can engraft in fetal recipients, contribute to hematopoiesis, and induce immunologic nonresponsiveness to iduronidase in MPS I dogs. However, the therapeutic potential of HSC gene transfer in this model of iduronidase deficiency appears to be limited by poor maintenance of proviral iduronidase gene expression and relatively low levels of genetically corrected circulating leukocytes.

Enzyme replacement in murine mucopolysaccharidosis type VII: neuronal and glial response to beta-glucuronidase requires early initiation of enzyme replacement therapy

We have previously shown that mucopolysaccharidosis type VII (MPS VII) mice receiving six weekly injections of recombinant beta-glucuronidase from birth had improved cognitive ability and reduced central nervous system lysosomal storage. However, a single beta-glucuronidase injection at 5 wk of age did not correct neuronal storage. We define the age at which central nervous system storage in MPS VII mice becomes resistant to beta-glucuronidase therapy and determine the effect of enzyme on other tissues by comparing the histology of mice begun on therapy at various times after birth. MPS VII mice received injections on the day of birth and then weekly for 5 wk with 16,000U/g beta-glucuronidase had reduced lysosomal storage in brain. The same therapy begun on d 14 of life or thereafter failed to correct neuronal storage, even when treatment was continued for six doses. Glial responsiveness or accessibility to enzyme also depended on early treatment. In contrast, leptomeningeal, osteoblast, and retinal pigment epithelial storage reduction depended on enzyme dose rather than age at initiation of therapy. Fixed tissue macrophage storage was reduced in all treated MPS VII mice, even those receiving a single dose. These observations indicate that fixed tissue macrophages in MPS VII mice remain sensitive to enzyme replacement therapy well into adulthood although neurons are responsive or accessible to enzyme therapy early in life. Because early initiation of enzyme replacement is important to achieve a central nervous system response, these studies emphasize the importance of newborn screening for lysosomal storage diseases so that early treatment can maximize the likelihood of a favorable therapeutic response.

Sustained gene expression in transplanted skin fibroblasts in rats

Retrovirus-mediated gene transfer into adult skin fibroblasts has provided measurable amounts of therapeutic proteins in animal models. However, the major problem emerging from these experiments was a limited time of vector encoded gene expression once transduced cells were engrafted. We hypothesized that sustained transduced gene expression in quiescent fibroblasts in vivo might be obtained by using a fibronectin (Fn) promoter. Fibronectin plays a key role in cell adhesion, migration and wound healing and is up-regulated in quiescent fibroblasts. Retroviral vectors containing human adenosine deaminase (ADA) cDNA linked to rat fibronectin promoter (LNFnA) or viral LTR promoter (LASN) were compared for their ability to express ADA from transduced primary rat skin fibroblasts in vivo. Skin grafts formed from fibroblasts transduced with LNFnA showed strong human ADA enzyme activity from 1 week to 3 months. In contrast, skin grafts containing LASN-transduced fibroblasts tested positive for human ADA for weeks 1 and 2, were faintly positive at week 3 and showed no human ADA expression at 1, 2 and 3 months. Thus, a fibronectin promoter provided sustained transduced gene expression at high levels for at least 3 months in transplanted rat skin fibroblasts, perhaps permitting the targeting of this tissue for human gene therapy.

Evaluation of fibroblast-mediated gene therapy in a feline model of mucopolysaccharidosis type VI

Fibroblast-mediated ex vivo gene therapy was evaluated in the N-acetylgalactosamine 4-sulfatase (4S) deficient mucopolysaccharidosis type VI (MPS VI) cat. Skin biopsies were obtained at birth from severely affected MPS VI kittens and used to initiate fibroblast outgrowths for retroviral transduction with the 4S cDNA. 4S gene expression in transduced cells was under the transcriptional control of the MoMLV long terminal repeat promoter or the cytomegalovirus (CMV) immediate-early promoter. Characterisation of gene-transduced fibroblasts demonstrated the cells to be over-expressing 4S activity. Twenty-four to forty million autologous, gene-corrected fibroblasts were implanted under the renal capsule of three MPS VI kittens at 8-16 weeks of age. Transient, low levels of 4S activity were detected in peripheral blood leukocytes shortly after implantation but were not detectable within 3-8 weeks' post-implantation. Long-term biochemical and clinical evaluation of these cats demonstrated identical disease progression to that previously described in untreated, clinically severe MPS VI cats.

Mobilization and transduction of peripheral blood progenitor cells in patients with mucopolysaccharidosis I

Mucopolysaccharidosis type I (MPS I) results from a deficiency of alpha-L-iduronidase enzyme (IDUA), an enzyme responsible for the catabolism of glycosaminoglycans. Genetically modified progenitor cells may permit a therapeutic effect similar to that obtained from allogeneic BMT without the associated risks. To that end, CD34+ peripheral blood hematopoietic progenitor cells from patients with MPS I were mobilized using G-CSF, collected by apheresis, and enriched using avidin-biotin separation techniques. These cells were cultured in a hollow fiber bioreactor and transduced with a retroviral vector (LP1CD) containing the cDNA for human IDUA and a murine dihydrofolate reductase (DHFR) enzyme. Approximately 4%-16% of the colonies expressed methotrexate drug resistance. Expression of the IDUA enzyme in the progenitor cells was initially high and declined after approximately 10 days of culture. These results indicate that PBPC from patients with MPS I can be mobilized, isolated, enriched, and transduced with a therapeutic gene.

Development of engineered cells for implantation in gene therapy

Human gene therapy is based on the technology of genetic engineering of cells, either through ex vivo or in vivo methods of gene transfer. Many autologous cell types have been successfully modified to deliver recombinant gene products. An alternate form of gene therapy based on genetic modification of non-autologous cells is described. Protection within immuno-isolating devices would allow implantation of well-established recombinant cell lines in different allogeneic hosts, potentially offering a more cost-effective approach to gene therapy. Implantation with microencapsulated fibroblasts and myoblasts has resulted in successful recombinant product delivery in vivo. Correction of disease phenotypes in animal models of human genetic diseases has also been achieved. Cell types such as myoblasts which can differentiate terminally within the implantation device are particularly promising for the future development of this method of gene therapy.

Stomach Implant for Long-Term Erythropoietin Expression in Rats

To approach the goal of consistent long-term erythropoietin (Epo) expression in vivo, we developed an implantation procedure in which transduced autologous vascular smooth muscle was introduced into rats in a chamber created from a polytetrafluoroethylene (PTFE) ring placed under the serosa of the stomach. The implant became vascularized and permitted the long-term survival of smooth muscle cells expressing Epo. Hematocrits of treated animals increased rapidly and monitored over 12 months gave a mean value of 56.0 ± 4.0% (P < .001; n = 9), increased from a presurgery mean of 42.3 ± 1.6%. Hemoglobin levels rose from a presurgery mean of 15.2 ± 0.4 g/dL and for 12 months were significantly elevated with a mean value of 19.5 ± 1.3 g/dL (P < .001; n = 9). The hematocrit and hemoglobin levels of control animals receiving human adenosine deaminase (ADA)–expressing cells were not significantly different from baseline (P > .05; n = 5). In response to tissue oxygenation, kidney, and (to a lesser extent) liver are specific organs that synthesize Epo. Treated animals showed downregulation of endogenous Epo mRNA in kidney over a 12-month period. The PTFE implant provides sustained gene delivery, is safe, and is minimally invasive. It allows easy engraftment of transduced cells and may be applied generally to the systemic delivery of therapeutic proteins such as hormones and clotting factors.

© 1998 by The American Society of Hematology.

Stomach Implant for Long-Term Erythropoietin Expression in Rats

To approach the goal of consistent long-term erythropoietin (Epo) expression in vivo, we developed an implantation procedure in which transduced autologous vascular smooth muscle was introduced into rats in a chamber created from a polytetrafluoroethylene (PTFE) ring placed under the serosa of the stomach. The implant became vascularized and permitted the long-term survival of smooth muscle cells expressing Epo. Hematocrits of treated animals increased rapidly and monitored over 12 months gave a mean value of 56.0 ± 4.0% (P &lt; .001; n = 9), increased from a presurgery mean of 42.3 ± 1.6%. Hemoglobin levels rose from a presurgery mean of 15.2 ± 0.4 g/dL and for 12 months were significantly elevated with a mean value of 19.5 ± 1.3 g/dL (P &lt; .001; n = 9). The hematocrit and hemoglobin levels of control animals receiving human adenosine deaminase (ADA)–expressing cells were not significantly different from baseline (P &gt; .05; n = 5). In response to tissue oxygenation, kidney, and (to a lesser extent) liver are specific organs that synthesize Epo. Treated animals showed downregulation of endogenous Epo mRNA in kidney over a 12-month period. The PTFE implant provides sustained gene delivery, is safe, and is minimally invasive. It allows easy engraftment of transduced cells and may be applied generally to the systemic delivery of therapeutic proteins such as hormones and clotting factors.

© 1998 by The American Society of Hematology.

Feline mucopolysaccharidosis type VI: correction of glycosaminoglycan storage in myoblasts by retrovirus-mediated transfer of the feline N-acetylgalactosamine 4-sulfatase gene

Mucopolysaccharidosis type VI (MPS VI) is an autosomal recessive lysosomal storage disorder characterised by the deficiency of N-acetylgalactosamine 4-sulfatase (4S). MPS VI has also been described in the cat. As an initial step toward muscle-mediated gene therapy in the MPS VI cat, we have made two retroviral constructs (pLf4S and pLf4SSN) that transduce the feline 4S gene. Both constructs were designed to express the feline 4S sequence from the viral long terminal repeat promoter. In addition pLf4SSN expressed the neomycin resistance gene from the SV40 early promoter. Amphotrophic virus was produced for each construct and used to transduce feline MPS VI myoblasts. Lf4S- and Lf4SSN-transduced MPS VI feline myoblasts demonstrated correction of glycosaminoglycan storage and contained 55-fold and 3.5-fold elevated levels of 4S activity when compared with normal feline myoblasts respectively. Recombinant feline 4S (rf4S) secreted by Lf4S-transduced MPS VI myoblasts was shown to be endocytosed by MPS VI feline cells via the mannose-6-phosphate receptor system, leading to metabolic correction. The results from this study demonstrate that muscle-mediated gene replacement therapy may be a viable method for achieving circulating levels of recombinant f4S (rf4S) in the MPS VI cat.

Enzyme replacement therapy from birth in a feline model of mucopolysaccharidosis type VI

We report evidence of a dose responsive effect of enzyme replacement therapy in mucopolysaccharidosis type VI cats from birth, at the clinical, biochemical, and histopathological level. Cats treated with weekly, intravenous recombinant human N-acetylgalactosamine-4-sulfatase at 1 and 5 mg/kg, were heavier, more flexible, had greatly reduced or no spinal cord compression, and had almost normal urinary glycosaminoglycan levels. There was near normalization or complete reversal of lysosomal storage in heart valve, aorta, skin, dura, liver, and brain perivascular cells. No reduction in lysosomal vacuolation was observed in cartilage or cornea; however, articular cartilage was thinner and external ear pinnae were larger in some treated cats. Degenerative joint changes were not obviously delayed in treated cats. Skeletal pathology was reduced, with more normalized bone dimensions and with more uniform bone density and trabecular pattern clearly visible on radiographs by 5 to 6 mo; however, differences between 1 and 5 mg/kg dose rates were not clearly distinguishable. At a dose of 0.2 mg/kg, disease was not significantly altered in the majority of parameters examined. Lysosomal storage was present in all tissues examined in the midterm mucopolysaccharidosis type VI fetus and increased rapidly in extent and severity from birth.