Cell and Gene Therapies for Mucopolysaccharidoses: Base Editing and Therapeutic Delivery to the CNS

Molecular Trojan Horses for treating lysosomal storage diseases

Lysosomal storage diseases (LSDs) are caused by monogenic mutations in genes encoding for proteins related to the lysosomal function. Lysosome plays critical roles in molecule degradation and cell signaling through interplay with many other cell organelles, such as mitochondria, endoplasmic reticulum, and peroxisomes. Even though several strategies (i.e., protein replacement and gene therapy) have been attempted for LSDs with promising results, there are still some challenges when hard-to-treat tissues such as bone (i.e., cartilages, ligaments, meniscus, etc.), the central nervous system (mostly neurons), and the eye (i.e., cornea, retina) are affected. Consistently, searching for novel strategies to reach those tissues remains a priority. Molecular Trojan Horses have been well-recognized as a potential alternative in several pathological scenarios for drug delivery, including LSDs. Even though molecular Trojan Horses refer to genetically engineered proteins to overcome the blood-brain barrier, such strategy can be extended to strategies able to transport and deliver drugs to specific tissues or cells using cell-penetrating peptides, monoclonal antibodies, vesicles, extracellular vesicles, and patient-derived cells. Only some of those platforms have been attempted in LSDs. In this paper, we review the most recent efforts to develop molecular Trojan Horses and discuss how this strategy could be implemented to enhance the current efficacy of strategies such as protein replacement and gene therapy in the context of LSDs.

Advances in therapies for neurological lysosomal storage disorders

Lysosomal Storage Disorders (LSDs) are a diverse group of inherited, monogenic diseases caused by functional defects in specific lysosomal proteins. The lysosome is a cellular organelle that plays a critical role in catabolism of waste products and recycling of macromolecules in the body. Disruption to the normal function of the lysosome can result in the toxic accumulation of storage products, often leading to irreparable cellular damage and organ dysfunction followed by premature death. The majority of LSDs have no curative treatment, with many clinical subtypes presenting in early infancy and childhood. Over two-thirds of LSDs present with progressive neurodegeneration, often in combination with other debilitating peripheral symptoms. Consequently, there is a pressing unmet clinical need to develop new therapeutic interventions to treat these conditions. The blood-brain barrier is a crucial hurdle that needs to be overcome in order to effectively treat the central nervous system (CNS), adding considerable complexity to therapeutic design and delivery. Enzyme replacement therapy (ERT) treatments aimed at either direct injection into the brain, or using blood-brain barrier constructs are discussed, alongside more conventional substrate reduction and other drug-related therapies. Other promising strategies developed in recent years, include gene therapy technologies specifically tailored for more effectively targeting treatment to the CNS. Here, we discuss the most recent advances in CNS-targeted treatments for neurological LSDs with a particular emphasis on gene therapy-based modalities, such as Adeno-Associated Virus and haematopoietic stem cell gene therapy approaches that encouragingly, at the time of writing are being evaluated in LSD clinical trials in increasing numbers. If safety, efficacy and improved quality of life can be demonstrated, these therapies have the potential to be the new standard of care treatments for LSD patients.

Neurological Disease Modeling Using Pluripotent and Multipotent Stem Cells: A Key Step towards Understanding and Treating Mucopolysaccharidoses

Despite extensive research, the links between the accumulation of glycosaminoglycans (GAGs) and the clinical features seen in patients suffering from various forms of mucopolysaccharidoses (MPSs) have yet to be further elucidated. This is particularly true for the neuropathology of these disorders; the neurological symptoms are currently incurable, even in the cases where a disease-specific therapeutic approach does exist. One of the best ways to get insights on the molecular mechanisms driving that pathogenesis is the analysis of patient-derived cells. Yet, not every patient-derived cell recapitulates relevant disease features. For the neuronopathic forms of MPSs, for example, this is particularly evident because of the obvious inability to access live neurons. This scenario changed significantly with the advent of induced pluripotent stem cell (iPSC) technologies. From then on, a series of differentiation protocols to generate neurons from iPSC was developed and extensively used for disease modeling. Currently, human iPSC and iPSC-derived cell models have been generated for several MPSs and numerous lessons were learnt from their analysis. Here we review most of those studies, not only listing the currently available MPS iPSC lines and their derived models, but also summarizing how they were generated and the major information different groups have gathered from their analyses. Finally, and taking into account that iPSC generation is a laborious/expensive protocol that holds significant limitations, we also hypothesize on a tempting alternative to establish MPS patient-derived neuronal cells in a much more expedite way, by taking advantage of the existence of a population of multipotent stem cells in human dental pulp to establish mixed neuronal and glial cultures.

Efficient CRISPR/Cas9 nickase-mediated genome editing in an in vitro model of mucopolysaccharidosis IVA

Mucopolysaccharidosis IVA (MPS IVA) is a lysosomal storage disorder (LSD) caused by mutations in gene encoding for GALNS enzyme. Lack of GALNS activity leads to the accumulation of glycosaminoglycans (GAGs) keratan sulfate and chondroitin 6-sulfate. Although enzyme replacement therapy has been approved since 2014 for MPS IVA, still there is an unmet medical need to have improved therapies for this disorder. CRISPR/Cas9-based gene therapy has been tested for several LSDs with encouraging findings, but to date it has not been assayed on MPS IVA. In this work, we validated for the first time the use of CRISPR/Cas9, using a Cas9 nickase, for the knock-in of an expression cassette containing GALNS cDNA in an in vitro model of MPS IVA. The results showed the successful homologous recombination of the expression cassette into the AAVS1 locus, as well as a long-term increase in GALNS activity reaching up to 40% of wild-type levels. We also observed normalization of lysosomal mass, total GAGs, and oxidative stress, which are some of the major findings regarding the pathophysiological events in MPS IVA. These results represent a proof-of-concept of the use of CRISPR/Cas9 nickase strategy for the development of a novel therapeutic alternative for MPS IVA.

Gene editing strategies to treat lysosomal disorders: The example of mucopolysaccharidoses

Lysosomal storage disorders are a group of progressive multisystemic hereditary diseases with a combined incidence of 1:4,800. Here we review the clinical and molecular characteristics of these diseases, with a special focus on Mucopolysaccharidoses, caused primarily by the lysosomal storage of glycosaminoglycans. Different gene editing techniques can be used to ameliorate their symptoms, using both viral and nonviral delivery methods. Whereas these are still being tested in animal models, early results of phase I/II clinical trials of gene therapy show how this technology may impact the future treatment of these diseases. Hurdles related to specific hard-to-reach organs, such as the central nervous system, heart, joints, and the eye must be tackled. Finally, the regulatory framework necessary to advance into clinical practice is also discussed.

Novel therapies for mucopolysaccharidosis type III

Mucopolysaccharidosis type III (MPS III) or Sanfilippo disease is an orphan inherited lysosomal storage disease and one of the most common MPS subtypes. The classical presentation is an infantile-onset neurodegenerative disease characterised by intellectual regression, behavioural and sleep disturbances, loss of ambulation, and early death. Unlike other MPS, no disease-modifying therapy has yet been approved. Here, we review the numerous approaches of curative therapy developed for MPS III from historical ineffective haematopoietic stem cell transplantation and substrate reduction therapy to the promising ongoing clinical trials based on enzyme replacement therapy or adeno-associated or lentiviral vectors mediated gene therapy. Preclinical studies are presented alongside the most recent translational first-in-man trials. In addition, we present experimental research with preclinical mRNA and gene editing strategies. Lessons from animal studies and clinical trials have highlighted the importance of an early therapy before extensive neuronal loss. A disease-modifying therapy for MPS III will undoubtedly mandate development of new strategies for early diagnosis.

Mucopolysaccharidoses I and II: Brief Review of Therapeutic Options and Supportive/Palliative Therapies

Purpose. Mucopolysaccharidoses (MPS) are group of inherited lysosomal storage diseases caused by mutations of enzymes involved in catalyzing different glycosaminoglycans (GAGs). MPS I and MPS II exhibit both somatic and neurological symptoms with a relatively high disease incidence. Hematopoietic stem cell therapy (HSCT) and intravenous enzyme replacement therapy (ERT) have had a significant impact on the treatment and comprehension of disease. This review is aimed at providing a comprehensive evaluation of the pros and cons of HSCT and ERT, as well as an up-to-date knowledge of new drugs under development. In addition, multiple disease management strategies for the uncontrollable manifestations of MPS I and MPS II to improve patients' quality of life are presented. Findings. Natural history of MPS I and MPS II shows that somatic and neurological symptoms occur earlier in severe forms of MPS I than in MPS II. ERT increases life expectancy and alleviates some of the somatic symptoms, but musculoskeletal, ophthalmological, and central nervous system (CNS) manifestations are not controlled. Additionally, life-long treatment burdens and immunogenicity restriction are unintended consequences of ERT application. HSCT, another treatment method, is effective in controlling the CNS symptoms and hence has been adopted as the standard treatment for severe types of MPS I. However, it is ineffective in MPS II, which can be explained by the relatively late diagnosis. In addition, several factors such as transplant age limits or graft-versus-host disease in HSCT have limited its application for patients. Novel therapies, including BBB-penetrable-ERT, gene therapy, and substrate reduction therapy, are under development to control currently unmanageable manifestations. BBB-penetrable-ERT is being studied comprehensively in the hopes of being used in the near future as a method to effectively control CNS symptoms. Gene therapy has the potential to “cure” the disease with a one-time treatment rather than just alleviate symptoms, which makes it an attractive treatment strategy. Several clinical studies on gene therapy reveal that delivering genes directly into the brain achieves better results than intravenous administration in patients with neurological symptoms. Considering new drugs are still in clinical stage, disease management with close monitoring and supportive/palliative therapy is of great importance for the time being. Proper rehabilitation therapy, including physical and occupational therapy, surgical intervention, or medications, can benefit patients with uncontrolled musculoskeletal, respiratory, ophthalmological, and neurological manifestations.

Genome Editing for Mucopolysaccharidoses

Genome editing holds the promise of one-off and potentially curative therapies for many patients with genetic diseases. This is especially true for patients affected by mucopolysaccharidoses as the disease pathophysiology is amenable to correction using multiple approaches. Ex vivo and in vivo genome editing platforms have been tested primarily on MSPI and MPSII, with in vivo approaches having reached clinical testing in both diseases. Though we still await proof of efficacy in humans, the therapeutic tools established for these two diseases should pave the way for other mucopolysaccharidoses. Herein, we review the current preclinical and clinical development studies, using genome editing as a therapeutic approach for these diseases. The development of new genome editing platforms and the variety of genetic modifications possible with each tool provide potential applications of genome editing for mucopolysaccharidoses, which vastly exceed the potential of current approaches. We expect that in a not-so-distant future, more genome editing-based strategies will be established, and individual diseases will be treated through multiple approaches.