Stem Cell Applications in Lysosomal Storage Disorders: Progress and Ongoing Challenges

Are transplantable stem cells required for adult hematopoiesis?

Stem cell gene transfer—efficacy and safety in large animal studies

In this issue, Banikazemi and colleagues' landmark trial on Fabry disease provides a crucial path toward lowering the morbidity of this disease. Among other findings, they determined that hemizygou...

Human neurological disorders such as Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis (ALS), Alzheimer's disease, multiple sclerosis (MS), stroke, and spinal cord injury are caused by a loss of neurons and glial cells in the brain or spinal cord. Cell replacement therapy and gene transfer to the diseased or injured brain have provided the basis for the development of potentially powerful new therapeutic strategies for a broad spectrum of human neurological diseases. However, the paucity of suitable cell types for cell replacement therapy in patients suffering from neurological disorders has hampered the development of this promising therapeutic approach. In recent years, neurons and glial cells have successfully been generated from stem cells such as embryonic stem cells, mesenchymal stem cells, and neural stem cells, and extensive efforts by investigators to develop stem cell-based brain transplantation therapies have been carried out. We review here notable experimental and preclinical studies previously published involving stem cell-based cell and gene therapies for Parkinson's disease, Huntington's disease, ALS, Alzheimer's disease, MS, stroke, spinal cord injury, brain tumor, and lysosomal storage diseases and discuss the future prospects for stem cell therapy of neurological disorders in the clinical setting. There are still many obstacles to be overcome before clinical application of cell therapy in neurological disease patients is adopted: 1) it is still uncertain what kind of stem cells would be an ideal source for cellular grafts, and 2) the mechanism by which transplantation of stem cells leads to an enhanced functional recovery and structural reorganization must to be better understood. Steady and solid progress in stem cell research in both basic and preclinical settings should support the hope for development of stem cell-based cell therapies for neurological diseases.

The Promise and Perils of Stem Cell Therapeutics

Introduction: Mucopolysaccharidoses (MPS) are lysosomal storage disorders caused by the deficiency of enzymes that are responsible for the stepwise degradation of complex carbohydrates, the glycosaminoglycans. Whereas in the past the treatment of MPS consisted mainly of palliative care, enzyme replacement therapy (ERT) is now possible for some MPS disorders, and in the future many other therapeutic options will become available.Areas covered: This review, based on personal experience and the currently available literature, will give an overview on the efficacy and limitations of ERT and will discuss new therapeutic approaches, such as anti-inflammatory drugs, substrate reduction therapy, chaperones and gene therapy.Expert opinion: The therapeutic strategies available nowadays for MPS patients, namely ERT and hematopoietic stem cell transplantation, have their limitations, particularly in regard to the bone and CNS manifestation. Small molecules such as substrate inhibitors or chaperones that are capable of passing the blood–brain barrier are being developed. The use of anti-inflammatory drugs also seems to be an alternative therapeutic option. Before gene therapy can be seen as a routine treatment for MPS disorders, many issues, in particular regarding safety, have to be addressed.

Glycogen storage disease type II (GSDII) or Pompe disease is an inherited disease of glycogen metabolism caused by a lack of functional lysosomal acid alpha-glucosidase (GAA). Affected individuals store glycogen in lysosomes resulting in fatal hypertrophic cardiomyopathy and respiratory failure in the most severe form. Even if enzyme replacement therapy (ERT) has already proven some efficacy, its results remain heterogeneous in skeletal muscle, especially in cross reactive immunological material (CRIM)-negative patients. We investigated for the first time the use of hematopoietic stem cell (HSC) gene therapy in a murine model of GSDII. Deficient HSC were transduced with a lentiviral vector expressing human GAA or enhanced green fluorescent protein (GFP) under the control of the retroviral MND promoter and transplanted into lethally irradiated GSDII mice. Animals were then subjected to an ERT protocol for 5 weeks and monitored for metabolic correction and GAA-induced immune reaction. GAA was expressed as a correctly processed protein, allowing a complete enzymatic correction in transduced deficient cells without toxicity. Seventeen weeks after transplantation, a partial restoration of the GAA enzymatic activity was observed in bone marrow and peripheral blood cells of GSDII mice, allowing a significant glycogen clearance in skeletal muscle. ERT induced a robust antibody response in GFP-transplanted mice, whereas no immune reaction could be detected in GAA-transplanted mice. Lentiviral vector-mediated HSC gene therapy leads to a partial metabolic correction and induces a tolerance to ERT in GSDII mice. This strategy could enhance the efficacy of ERT in CRIM-negative Pompe patients.

BackgroundLysosomal storage disorders are a heterogeneous group of approximately 50 monogenically inherited orphan conditions. A defect leads to the storage of complex molecules in the lysosome, and patients develop a complex multisystemic phenotype of high morbidity often associated with premature death. More than 30 years ago the Orphan Drug Act of 1983 passed the United States legislation intended to facilitate the development of drugs for rare disorders.We directed our efforts in assessing which lysosomal diseases had drug development pressure and what distinguished those with successful development and approvals from diseases not treated or without orphan drug designation.MethodsAnalysis of the FDA database for orphan drug designations through descriptive and comparative statistics.ResultsBetween 1983 and 2013, fourteen drugs for seven conditions received FDA approval. Overall, orphan drug status was designated 70 times for 20 conditions. Approved therapies were enzyme replacement therapies (N = 10), substrate reduction therapies (N = 1), small molecules facilitating lysosomal substrate transportation (N = 3). FDA approval was significantly associated with a disease prevalence higher than 0.5/100,000 (p = 0.00742) and clinical development programs that did not require a primary neurological endpoint (p = 0.00059). Orphan drug status was designated for enzymes, modified enzymes, fusion proteins, chemical chaperones, small molecules leading to substrate reduction, or facilitating subcellular substrate transport, stem cells as well as gene therapies.ConclusionsDrug development focused on more common diseases. Primarily neurological diseases were neglected. Small clinical trials with either somatic or biomarker endpoints were successful. Enzyme replacement therapy was the most successful technology. Four factors played a key role in successful orphan drug development or orphan drug designations: 1) prevalence of disease 2) endpoints 3) regulatory precedent, and 4) technology platform. Successful development seeded further innovation.

Gene therapy using autologous hematopoietic stem cells (HSC) that are corrected with the normal gene may have a beneficial effect on blood cell production or function, without the immunologic complications of allogeneic HSC transplantation. Childhood immunological diseases are highly favorable candidates for responses to gene therapy using HSC. Hemoglobinopathies, lysosomal and metabolic disorders and defects of hematopoietic stem and progenitor cells should also be ameliorated by gene therapy using autologous HSC. At present, gene therapy has been beneficial for patients with XSCID, ADA-deficient SCID and chronic granulomatous disease. The principle that partial marrow conditioning increases engraftment of gene-corrected HSC has been demonstrated. Clinical trials are being developed in Europe and the United States to treat several other genetic blood cell disorders. This progress is tempered by the serious complication observed in XSCID patients developing T lymphoproliferative disease. New methods for gene transfer (lentiviral and foamy viral vectors, semi-viral systems and gene correction) may retain or further increase the efficacy and decrease the risks from gene therapy using HSC. Ultimately, the relative benefits and risks of autologous gene therapy will be weighed against other available options (for example, allogeneic HSCT) to determine the treatment of choice.

Lysosomal storage diseases (LSDs) are rare inherited metabolic disorders characterized by a dysfunction in lysosomes, leading to waste material accumulation and severe organ damage. Enzyme replacement therapy (ERT) and haematopoietic stem cell transplant (HSCT) have been exploited as potential treatments for LSDs but pre-clinical and clinical studies have shown in some cases limited efficacy. Intravenous ERT is able to control the damage of visceral organs but cannot prevent nervous impairment. Depending on the disease type, HSCT has important limitations when performed for early variants, unless treatment occurs before disease onset. In the attempt to overcome these issues, gene therapy has been proposed as a valuable therapeutic option, either ex vivo, with target cells genetically modified in vitro, or in vivo, by inserting the genetic material with systemic or intra-parenchymal, in situ administration. In particular, the use of autologous haematopoietic stem cells (HSC) transduced with a viral vector containing a healthy copy of the mutated gene would allow supra-normal production of the defective enzyme and cross correction of target cells in multiple tissues, including the central nervous system. This review will provide an overview of the most recent scientific advances in HSC-based gene therapy approaches for the treatment of LSDs with particular focus on metachromatic leukodystrophy (MLD) and mucopolysaccharidosis type I (MPS-I).

Stem cells can be either totipotent, pluripotent, multipotent or unipotent. Totipotent cells have the capability to produce all cell types of the developing organism, including both embryonic and extraembryonic tissues. The Hematopoietic Stem Cells (HSC) are the first defined adult stem cells (ASC) that give rise to all blood cells and immune system. Use of HSCs for treatment of hematologic malignancies, which is also called bone marrow (BM) transplantation or peripheral blood stem cells (PBSC) transplantation is the pioneer of cellular therapy and translational research. However, stem cell research field is developing so fast that, innovative approaches using HSCs for treatment of refractory diseases are growing rapidly. Hematopoietic stem cell transplantation (HSCT) has been widely used to achieve cure in different hematological diseases. Applications include the treatment of marrow failure syndromes, leukemia, lymphoma, multiple myeloma (MM), certain inherited blood disorders, autoimmune diseases and as an enzyme replacement in metabolic disorders. Innovative approaches such as haploidentical stem cell transplantation, new monoclonal antibodies and immunotherapies as well as Chimeric Antigen Receptor T-cell (CAR-T cell) therapies are on the way as promising treatment options especially for patients with refractory hematologic malignancies and even in solid tumors. However, there are still some challenges remaining before some of these therapies are translated into clinical application. In this paper, HSCs including its properties, niches, clinical usage and its contribution to modern medicine today and in the future will be discussed.

Enzyme replacement therapy (ERT) has been shown to be effective at reducing the accumulation of undegraded substrates in lysosomal storage diseases. Most ERT studies have been performed with recombinant proteins that are mixtures of phosphorylated and non-phosphorylated enzyme. Because different cell types use different receptors to take up phosphorylated or non-phosphorylated enzyme, it is difficult to determine which form of enzyme contributed to the clinical response. Here we compare the uptake, distribution, and efficacy of highly phosphorylated and non-phosphorylated beta-glucuronidase (GUSB) in the MPS VII mouse. Highly phosphorylated murine GUSB was efficiently taken up by a wide range of tissues. In contrast, non-phosphorylated murine GUSB was taken up primarily by tissues of the reticuloendothelial (RE) system. Although the tissue distribution was different, the half-lives of both enzymes in any particular tissue were similar. Both preparations of enzyme were capable of preventing the accumulation of lysosomal storage in cell types they targeted. An important difference in clinical efficacy emerged in that phosphorylated GUSB was more efficient than non-phosphorylated enzyme at preventing the hearing loss associated with this disease. These data suggest that both forms of enzyme contribute to the clinical responses of ERT in MPS VII mice but that enzyme preparations containing phosphorylated GUSB are more broadly effective than non-phosphorylated enzyme.

Severe combined immunodeficiency (SCID) due to deficiency of the purine metabolic enzyme adenosine deaminase (ADA) is a fatal childhood immunodeficiency disease. Immune reconstitution by transplantation with HLA-identical bone marrow is the treatment of choice. For patients not candidates for bone marrow transplantation, we propose to attempt immune reconstitution by using infusions of autologous T lymphocytes expanded in tissue culture and genetically corrected by insertion of a normal ADA gene using retroviral-mediated gene transfer. The vector is LASN, in which the human ADA gene is promoted by the LTR while the NeoR gene is driven by the SV40 early gene promoter. The packaging line is PA317. The protocol is designed to have two parts. In Part 1, autologous gene-corrected T lymphocytes would be infused repeatedly in low numbers in order to build an immune repertoire of T cells and also to obtain information as to how long gene corrected T cells survive in vivo. In Part 2A, the gene-corrected T cells would be selected in G418 and/or 2'deoxyadenosine and reinfused into the patient at monthly intervals for approximately 6 months. The goals would be essentially the same as in Part 1. In Part 2B, the number of gene-corrected T cells would be escalated in half-log increments to the predicted therapeutic level (probably around 1 x 10(9)/kg). 1-3 x 10(9)/kg gene-corrected cells would be infused several times and the patients would be monitored in order to determine if significant clinical improvement has occurred.

Sanfilippo syndrome (mucopolysaccharidosis type III; MPS III) is a devastating neurodegenerative lysosomal storage disorder of childhood. There is currently no cure or effective treatment available for MPS III. There are however many therapies in early development, including gene therapies, enzyme replacement, chaperone and substrate reduction. With Sanfilippo MPSIII Type A, there is currently a large focus on gene therapy evaluating different vectors across many different groups. Less research appears to be focused on types C and D. Phoenix Nest (PN) was formed by a parent (of a child with MPSIIIC) and a pharmaceutical researcher. Initially the goal was to create a virtual company that would then be able to apply for STTR and SBIR grants to work on treatments for MPS types C and D. In the space of 2 years we have built up collaborations with leading academics and industry and submitted multiple grant proposals. We were able to secure our first STTR recently and this sets the stage for our growth. All of our resources are spent on supporting research and development with minimal overhead as we leverage collaborative researchers and tools to ensure we reach our goal. We will describe our strategy and propose that alongside forming foundations, a patient driven company may be a useful vehicle to push for more translational research in a rare disease. We would encourage other parents and researchers to start a small company and learn from this experience. Due to the limited pool of funding for this disorder, enhanced collaboration between foundations, academics and companies facilitated by small companies like PN may prevent unnecessary redundancies and broaden the impact of the ongoing research efforts. Ultimately our goal is to deliver a treatment to the patient. We will summarize the current state of research for Sanfilippo syndrome and describe the path we have taken and the pros and cons of forming a company along with some recommendations. As we are just beginning PN we see our own collaboration as an experiment that if successful could be a useful model for other rare diseases.

Quiescence regulators for hematopoietic stem cell