Gene therapy for lysosomal storage diseases

GABAergic interneurons contribute to the fatal seizure phenotype of CLN2 disease mice

GABAergic interneuron deficits have been implicated in the epileptogenesis of multiple neurological diseases. While epileptic seizures are a key clinical hallmark of CLN2 disease, a childhood-onset neurodegenerative lysosomal storage disorder caused by a deficiency of tripeptidyl peptidase 1 (TPP1), the etiology of these seizures remains elusive. Given that Cln2 R207X/R207X mice display fatal spontaneous seizures and an early loss of several cortical interneuron populations, we hypothesized that those two events might be causally related. To address this hypothesis, we first generated an inducible transgenic mouse expressing lysosomal membrane-tethered TPP1 (TPP1LAMP1) on the Cln2 R207X/R207X genetic background to study the cell-autonomous effects of cell-type-specific TPP1 deficiency. We crossed the TPP1LAMP1 mice with Vgat-Cre mice to introduce interneuron-specific TPP1 deficiency. Vgat-Cre ; TPP1LAMP1 mice displayed storage material accumulation in several interneuron populations both in cortex and striatum, and increased susceptibility to die after PTZ-induced seizures. Secondly, to test the role of GABAergic interneuron activity in seizure progression, we selectively activated these cells in Cln2 R207X/R207X mice using Designer Receptor Exclusively Activated by Designer Drugs (DREADDs) in in Vgat-Cre : Cln2 R207X/R207X mice. EEG monitoring revealed that DREADD-mediated activation of interneurons via chronic deschloroclozapine administration accelerated the onset of spontaneous seizures and seizure-associated death in Vgat-Cre : Cln2 R207X/R207X mice, suggesting that modulating interneuron activity can exert influence over epileptiform abnormalities in CLN2 disease. Taken together, these results provide new mechanistic insights into the underlying etiology of seizures and premature death that characterize CLN2 disease.

Comparative dose effectiveness of intravenous and intrathecal AAV9.CB7.hIDS, RGX-121, in mucopolysaccharidosis type II mice

Mucopolysaccharidosis type II (MPS II) is an X-linked recessive lysosomal disease caused by iduronate-2-sulfatase (IDS) deficiency, leading to accumulation of glycosaminoglycans (GAGs) and the emergence of progressive disease. Enzyme replacement therapy is the only currently approved treatment, but it leaves neurological disease unaddressed. Cerebrospinal fluid (CSF)-directed administration of AAV9.CB7.hIDS (RGX-121) is an alternative treatment strategy, but it is unknown if this approach will affect both neurologic and systemic manifestations. We compared the effectiveness of intrathecal (i.t.) and intravenous (i.v.) routes of administration (ROAs) at a range of vector doses in a mouse model of MPS II. While lower doses were completely ineffective, a total dose of 1 × 109 gc resulted in appreciable IDS activity levels in plasma but not tissues. Total doses of 1 × 1010 and 1 × 1011 gc by either ROA resulted in supraphysiological plasma IDS activity, substantial IDS activity levels and GAG reduction in nearly all tissues, and normalized zygomatic arch diameter. In the brain, a dose of 1 × 1011 gc i.t. achieved the highest IDS activity levels and the greatest reduction in GAG content, and it prevented neurocognitive deficiency. We conclude that a dose of 1 × 1010 gc normalized metabolic and skeletal outcomes, while neurologic improvement required a dose of 1 × 1011 gc, thereby suggesting the prospect of a similar direct benefit in humans.

Gene therapy for neurodegenerative disorders in children: dreams and realities

Gene therapy encompasses the administration of biological medicinal products containing recombinant nucleic acids, mainly DNA, with the aim of treating or curing diseases. This represents a unique therapeutic strategy to reach the brain, in order to prevent or halt a neurodegenerative process. During the past decade, active multidisciplinary research has started to solve many issues for gene therapy in neurodegenerative disorders in terms of vectors, modes of administration, and expression of the therapeutic DNA. The engineering of hematopoietic stem cells (HSC) with lentivirus vectors for ex vivo gene therapy has demonstrated efficiency in reaching the brain through their transformation into microglial/macrophages cells with a long-term gene expression of the therapeutic vector as an alternative to autologous HSC transplants. Two drugs based on this strategy have been approved to date. The first is for metachromatic leukodystrophy (MLD), a severe lysosomal storage disease, and provides high levels of the deficient enzyme; the second one is for cerebral forms of X-linked adrenoleukodystrophy (X-ALD), and works by halting the neuroinflammation process. However, due to the long-lasting effect of the procedure, the therapy is applicable only to pre- or pauci/oligo-symptomatic patients. In vivo gene therapy via direct injection into the brain or the cerebrospinal fluid, but also by intravenous injection, represents a more efficient approach; however, many challenges remain to be solved despite the approval of two drugs: one for the early infantile form of spinal muscular atrophy (SMA), in which the gene product injected intravenously is able to prevent spinal motoneuron neurodegeneration. The second one, for aromatic L-amino acid decarboxylase (AADC) deficiency, provides the defective enzyme to the basal ganglia via intraparenchymal injection. The production of vectors able to reach the brain target cells with a sufficiently high expression remains a major bottleneck. In parallel, efforts must continue in order to better define (i) the natural history and clinical outcomes of many neurodegenerative disorders with childhood onset, and (ii) the mechanisms involved in the neurodegenerative process. © 2023 Published by Elsevier Masson SAS on behalf of French Society of Pediatrics.

Efficacy of dual intracerebroventricular and intravitreal CLN5 gene therapy in sheep prompts the first clinical trial to treat CLN5 Batten disease

Mutations in the CLN5 gene cause the fatal, pediatric, neurodegenerative disease CLN5 neuronal ceroid lipofuscinosis. Affected children suffer progressive neuronal loss, visual failure and premature death. Presently there is no treatment. This study evaluated dual intracerebroventricular (ICV) and intravitreal (IVT) administration of a self-complementary adeno-associated viral vector encoding ovine CLN5 (scAAV9/oCLN5) into CLN5 affected sheep (CLN5-/-) at various disease stages. CLN5 disease progression was slowed in pre-symptomatic sheep who received a moderate dose of scAAV9/oCLN5, whilst a higher ICV dose treatment in early and advanced symptomatic animals delayed or halted disease progression. Intracranial (brain) volume loss was attenuated in all treatment cohorts, and visual function was also sustained in both the early and advanced symptomatic treated sheep over the 24-month duration of the study. Robust CLN5 protein expression was detected throughout the brain and spinal cord, and improvements in central nervous system and retinal disease correlates were observed. These findings hold translational promise for extending and improving the quality of life in both pre-symptomatic and symptomatic CLN5 patients, and prompted the initiation of the first in-human Phase I/II clinical trial testing ICV/IVT administration of scAAV9 encoding human CLN5 (https://clinicaltrials.gov/; NCT05228145).

Long-term safety and dose escalation of intracerebroventricular CLN5 gene therapy in sheep supports clinical translation for CLN5 Batten disease

CLN5 neuronal ceroid lipofuscinosis (NCL, Batten disease) is a rare, inherited fatal neurodegenerative disorder caused by mutations in the CLN5 gene. The disease is characterised by progressive neuronal loss, blindness, and premature death. There is no cure. This study evaluated the efficacy of intracerebroventricular (ICV) delivery of an adeno-associated viral vector encoding ovine CLN5 (scAAV9/oCLN5) in a naturally occurring sheep model of CLN5 disease. CLN5 affected (CLN5-/-) sheep received low, moderate, or high doses of scAAV9/oCLN5 at three disease stages. The treatment delayed disease progression, extended survival and slowed stereotypical brain atrophy in most animals. Of note, one high dose treated animal only developed mild disease symptomology and survived to 60.1 months of age, triple the natural life expectancy of an untreated CLN5-/- sheep. Eyesight was not preserved at any administration age or dosage. Histopathologic examination revealed that greater transduction efficiency was achieved through higher ICV doses, and this resulted in greater amelioration of disease pathology. Together with other pre-clinical data from CLN5-/- sheep, the safety and efficacy data from these investigational new drug (IND)-enabling studies supported the initiation of the first in-human CLN5 gene therapy clinical study using the ICV delivery route for the treatment of CLN5 NCL. Clinical Trial Registration: https://clinicaltrials.gov/, identifier NCT05228145.

A Novel Small NPC1 Promoter Enhances AAV-Mediated Gene Therapy in Mouse Models of Niemann-Pick Type C1 Disease

Niemann-Pick disease type C1 (NP-C) is a prematurely lethal genetic lysosomal storage disorder with neurological and visceral pathology resulting from mutations in the NPC1 gene encoding the lysosomal transmembrane protein NPC1. There is currently no cure for NP-C, and the only disease modifying treatment, miglustat, slows disease progression but does not significantly attenuate neurological symptoms. AAV-mediated gene therapy is an attractive option for NP-C, but due to the large size of the human NPC1 gene, there may be packaging and truncation issues during vector manufacturing. One option is to reduce the size of DNA regulatory elements that are essential for gene expression, such as the promoter sequence. Here, we describe a novel small truncated endogenous NPC1 promoter that leads to high gene expression both in vitro and in vivo and compare its efficacy to other commonly used promoters. Following neonatal intracerebroventricular (ICV) injection into the CNS, this novel promoter provided optimal therapeutic efficacy compared to all other promoters including increased survival, improved behavioural phenotypes, and attenuated neuropathology in mouse models of NP-C. Taken together, we propose that this novel promoter can be extremely efficient in designing an optimised AAV9 vector for gene therapy for NP-C.

Polymer-based drug delivery systems under investigation for enzyme replacement and other therapies of lysosomal storage disorders

Lysosomes play a central role in cellular homeostasis and alterations in this compartment associate with many diseases. The most studied example is that of lysosomal storage disorders (LSDs), a group of 60 + maladies due to genetic mutations affecting lysosomal components, mostly enzymes. This leads to aberrant intracellular storage of macromolecules, altering normal cell function and causing multiorgan syndromes, often fatal within the first years of life. Several treatment modalities are available for a dozen LSDs, mostly consisting of enzyme replacement therapy (ERT) strategies. Yet, poor biodistribution to main targets such as the central nervous system, musculoskeletal tissue, and others, as well as generation of blocking antibodies and adverse effects hinder effective LSD treatment. Drug delivery systems are being studied to surmount these obstacles, including polymeric constructs and nanoparticles that constitute the focus of this article. We provide an overview of the formulations being tested, the diseases they aim to treat, and the results observed from respective in vitro and in vivo studies. We also discuss the advantages and disadvantages of these strategies, the remaining gaps of knowledge regarding their performance, and important items to consider for their clinical translation. Overall, polymeric nanoconstructs hold considerable promise to advance treatment for LSDs.

In vivo adenine base editing corrects newborn murine model of Hurler syndrome

Mucopolysaccharidosis type I (MPS I) is a severe disease caused by loss-of-function mutation variants in the α-L-iduronidase (Idua) gene. In vivo genome editing represents a promising strategy to correct Idua mutations, and has the potential to permanently restore IDUA function over the lifespan of patients. Here, we used adenine base editing to directly convert A > G (TAG>TGG) in a newborn murine model harboring the Idua-W392X mutation, which recapitulates the human condition and is analogous to the highly prevalent human W402X mutation. We engineered a split-intein dual-adeno-associated virus 9 (AAV9) adenine base editor to circumvent the package size limit of AAV vectors. Intravenous injection of the AAV9-base editor system into MPS IH newborn mice led to sustained enzyme expression sufficient for correction of metabolic disease (GAGs substrate accumulation) and prevention of neurobehavioral deficits. We observed a reversion of the W392X mutation in 22.46 ± 6.74% of hepatocytes, 11.18 ± 5.25% of heart and 0.34 ± 0.12% of brain, along with decreased GAGs storage in peripheral organs (liver, spleen, lung and kidney). Collectively, these data showed the promise of a base editing approach to precisely correct a common genetic cause of MPS I in vivo and could be broadly applicable to the treatment of a wide array of monogenic diseases.

Nanomolar β-glucosidase and β-galactosidase inhibition by enantiomeric α-1-C-alkyl-1,4-dideoxy-1,4-imino-arabinitol derivatives

A series of α-1-C-alkyl DAB (1,4-dideoxy-1,4-imino-d-arabinitol) and LAB (1,4-dideoxy-1,4-imino-l-arabinitol) derivatives with aryl substituents have been designed as analogues of broussonetine W (12), and assayed as glycosidase inhibitors. While the inhibition spectrum of α-1-C-alkyl DAB derivative 16 showed a good correlation to that of broussonetine W (12), introduction of substituents on the terminal aryl (17a-f) or hydroxyl groups at C-1' position of the alkyl chains (18a-e) decreased their α-glucosidase inhibitions but greatly improved their inhibitions of bovine liver β-glucosidase and β-galactosidase. Furthermore, epimerization of C-1' configurations of compounds 18a-e clearly lowered their inhibition potency of bovine liver β-glucosidase and β-galactosidase. Notably, some of the α-1-C-alkyl DAB derivatives were also found to have potent human lysosome β-glucosidase inhibitions. In contrast, enantiomers of compounds 18a-e and 1'-epi-18a-e generally showed increased α-glucosidase inhibitions, but sharply decreased bovine liver β-glucosidase and β-galactosidase inhibitions. Molecular docking calculations unveiled the novel two set of binding modes for each series of compounds; introduction of C-1' hydroxyl altered the conformations of the pyrrolidine rings and orientation of their long chains, resulting in improved accommodation in the hydrophobic grooves. The compounds reported herein are very potent β-glucosidase and β-galactosidase inhibitions with novel binding mode; and the structure-activity relationship provides guidance for design and development of more pyrrolidine pharmacological chaperones for lysosomal storage diseases.

A novel strategy for delivering Niemann-Pick type C2 proteins across the blood-brain barrier using the brain endothelial-specific AAV-BR1 virus

Treating central nervous system (CNS) diseases is complicated by the incapability of numerous therapeutics to cross the blood-brain barrier (BBB), mainly composed of brain endothelial cells (BECs). Genetically modifying BECs into protein factories that supply the CNS with recombinant proteins is a promising approach to overcome this hindrance, especially in genetic diseases, like Niemann Pick disease type C2 (NPC2), where both CNS and peripheral cells are affected. Here, we investigated the potential of the BEC-specific adeno-associated viral vector (AAV-BR1) encoding NPC2 for expression and secretion from primary BECs cultured in an in vitro BBB model with mixed glial cells, and in healthy BALB/c mice. Transduced primary BECs had significantly increased NPC2 gene expression and secreted NPC2 after viral transduction, which significantly reversed cholesterol deposition in NPC2 deficient fibroblasts. Mice receiving an intravenous injection with AAV-BR1-NCP2-eGFP were sacrificed 8 weeks later and examined for its biodistribution and transgene expression of eGFP and NPC2. AAV-BR1-NPC2-eGFP was distributed mainly to the brain and lightly to the heart and lung, but did not label other organs including the liver. eGFP expression was primarily found in BECs throughout the brain but occasionally also in neurons suggesting transport of the vector across the BBB, a phenomenon also confirmed in vitro. NPC2 gene expression was up-regulated in the brain, and recombinant NPC2 protein expression was observed in both transduced brain capillaries and neurons. Our findings show that AAV-BR1 transduction of BECs is possible and that it may denote a promising strategy for future treatment of NPC2.

Gene Expression Analysis in gla-Mutant Zebrafish Reveals Enhanced Ca2+ Signaling Similar to Fabry Disease

Fabry disease (FD) is an X-linked inborn metabolic disorder due to partial or complete lysosomal α-galactosidase A deficiency. FD is characterized by progressive renal insufficiency and cardio- and cerebrovascular involvement. Restricted access on Gb3-independent tissue injury experimental models has limited the understanding of FD pathophysiology and delayed the development of new therapies. Accumulating glycosphingolipids, mainly Gb3 and lysoGb3, are Fabry specific markers used in clinical follow up. However, recent studies suggest there is a need for additional markers to monitor FD clinical course or response to treatment. We used a gla-knockout zebrafish (ZF) to investigate alternative biomarkers in Gb3-free-conditions. RNA sequencing was used to identify transcriptomic signatures in kidney tissues discriminating gla-mutant (M) from wild type (WT) ZF. Gene Ontology (GO) and KEGG pathways analysis showed upregulation of immune system activation and downregulation of oxidative phosphorylation pathways in kidneys from M ZF. In addition, upregulation of the Ca²⁺ signaling pathway was also detectable in M ZF kidneys. Importantly, disruption of mitochondrial and lysosome-related pathways observed in M ZF was validated by immunohistochemistry. Thus, this ZF model expands the pathophysiological understanding of FD, the Gb3-independent effects of gla mutations could be used to explore new therapeutic targets for FD.

Liposomal formulations for treating lysosomal storage disorders

Lysosomal storage disorders (LSD) are a group of rare life-threatening diseases caused by a lysosomal dysfunction, usually due to the lack of a single enzyme required for the metabolism of macromolecules, which leads to a lysosomal accumulation of specific substrates, resulting in severe disease manifestations and early death. There is currently no definitive cure for LSD, and despite the approval of certain therapies, their effectiveness is limited. Therefore, an appropriate nanocarrier could help improve the efficacy of some of these therapies. Liposomes show excellent properties as drug carriers, because they can entrap active therapeutic compounds offering protection, biocompatibility, and selectivity. Here, we discuss the potential of liposomes for LSD treatment and conduct a detailed analysis of promising liposomal formulations still in the preclinical development stage from various perspectives, including treatment strategy, manufacturing, characterization, and future directions for implementing liposomal formulations for LSD.

Cross-species efficacy of enzyme replacement therapy for CLN1 disease in mice and sheep

CLN1 disease, also called infantile neuronal ceroid lipofuscinosis (NCL) or infantile Batten disease, is a fatal neurodegenerative lysosomal storage disorder resulting from mutations in the CLN1 gene encoding the soluble lysosomal enzyme palmitoyl-protein thioesterase 1 (PPT1). Therapies for CLN1 disease have proven challenging because of the aggressive disease course and the need to treat widespread areas of the brain and spinal cord. Indeed, gene therapy has proven less effective for CLN1 disease than for other similar lysosomal enzyme deficiencies. We therefore tested the efficacy of enzyme replacement therapy (ERT) by administering monthly infusions of recombinant human PPT1 (rhPPT1) to PPT1-deficient mice (Cln1-/-) and CLN1R151X sheep to assess how to potentially scale up for translation. In Cln1-/- mice, intracerebrovascular (i.c.v.) rhPPT1 delivery was the most effective route of administration, resulting in therapeutically relevant CNS levels of PPT1 activity. rhPPT1-treated mice had improved motor function, reduced disease-associated pathology, and diminished neuronal loss. In CLN1R151X sheep, i.c.v. infusions resulted in widespread rhPPT1 distribution and positive treatment effects measured by quantitative structural MRI and neuropathology. This study demonstrates the feasibility and therapeutic efficacy of i.c.v. rhPPT1 ERT. These findings represent a key step toward clinical testing of ERT in children with CLN1 disease and highlight the importance of a cross-species approach to developing a successful treatment strategy.

Galactomannan-Decorated Lipidic Nanocarrier for Gene Supplementation Therapy in Fabry Disease

Gene supplementation therapy with plasmid DNA (pDNA) represents one of the most promising strategies for the treatment of monogenic diseases such as Fabry disease (FD). In the present work, we developed a solid lipid nanoparticles (SLN)-based non-viral vector with a size below 100 nm, and decorated with galactomannan (GM) to target the liver as an α-Galactosidase A (α-Gal A) production factory. After the physicochemical characterization of the GM-SLN vector, cellular uptake, transfection efficacy and capacity to increase α-Gal A activity were evaluated in vitro in a liver cell line (Hep G2) and in vivo in an animal model of FD. The vector showed efficient internalization and it was highly efficient in promoting protein synthesis in Hep G2 cells. Additionally, the vector did not show relevant agglutination of erythrocytes and lacked hemolytic activity. After the systemic administration to Fabry mice, it achieved clinically relevant α-Gal A activity levels in plasma, liver, and other organs, importantly in heart and kidneys, two of the most damaged organs in FD. This work shows the potential application of GM-decorated lipidic nanocarries for the treatment of FD by pDNA-based gene augmentation.

Aggregation chimeras provide evidence of in vivo intercellular correction in ovine CLN6 neuronal ceroid lipofuscinosis (Batten disease)

The neuronal ceroid lipofuscinoses (NCLs; Batten disease) are fatal, mainly childhood, inherited neurodegenerative lysosomal storage diseases. Sheep affected with a CLN6 form display progressive regionally defined glial activation and subsequent neurodegeneration, indicating that neuroinflammation may be causative of pathogenesis. In this study, aggregation chimeras were generated from homozygous unaffected normal and CLN6 affected sheep embryos, resulting in seven chimeric animals with varied proportions of normal to affected cells. These sheep were classified as affected-like, recovering-like or normal-like, based on their cell-genotype ratios and their clinical and neuropathological profiles. Neuropathological examination of the affected-like animals revealed intense glial activation, prominent storage body accumulation and severe neurodegeneration within all cortical brain regions, along with vision loss and decreasing intracranial volumes and cortical thicknesses consistent with ovine CLN6 disease. In contrast, intercellular communication affecting pathology was evident at both the gross and histological level in the normal-like and recovering-like chimeras, resulting in a lack of glial activation and rare storage body accumulation in only a few cells. Initial intracranial volumes of the recovering-like chimeras were below normal but progressively recovered to about normal by two years of age. All had normal cortical thicknesses, and none went blind. Extended neurogenesis was evident in the brains of all the chimeras. This study indicates that although CLN6 is a membrane bound protein, the consequent defect is not cell intrinsic. The lack of glial activation and inflammatory responses in the normal-like and recovering-like chimeras indicate that newly generated cells are borne into a microenvironment conducive to maturation and survival.

Natural history of retinal degeneration in ovine models of CLN5 and CLN6 neuronal ceroid lipofuscinoses

Neuronal ceroid lipofuscinoses (NCL; Batten disease) are a group of inherited neurodegenerative diseases with a common set of symptoms including cognitive and motor decline and vision loss. Naturally occurring sheep models of CLN5 and CLN6 disease display the key clinical features of NCL, including a progressive loss of vision. We assessed retinal histology, astrogliosis, and lysosomal storage accumulation in CLN5 affected (CLN5-/-) and CLN6 affected (CLN6-/-) sheep eyes and age-matched controls at 3, 6, 12, and 18 months of age to determine the onset and progression of retinal pathology in NCL sheep. The retina of CLN5-/- sheep shows progressive atrophy of the outer retinal layers, widespread gliosis, and accumulation of lysosomal storage in retinal ganglion cells late in disease. In contrast, CLN6-/- retina shows significant atrophy of all retinal layers, progressive gliosis, and earlier accumulation of lysosomal storage. This study has highlighted the differential vulnerability of retinal layers and the time course of retinal atrophy in two distinct models of NCL disease. This data will be valuable in determining potential targets for ocular therapies and the optimal timing of these therapies for protection from retinal dysfunction and degeneration in NCL.

A Mutation-Agnostic Hematopoietic Stem Cell Gene Therapy for Metachromatic Leukodystrophy

Metachromatic leukodystrophy (MLD) is a rare genetic disorder caused by mutations in the Arylsulfatase-A (ARSA) gene. The enzyme plays a key role in sulfatide metabolism in brain cells, and its deficiency leads to neurodegeneration. The clinical manifestations of MLD include stagnation and decline of motor and cognitive function, leading to premature death with limited standard treatment options. Here, we describe a mutation-agnostic hematopoietic stem and progenitor cell (HSPC) gene therapy using CRISPR-Cas9 and AAV6 repair template as a prospective treatment option for MLD. Our strategy achieved efficient insertions and deletions (>87%) and a high level of gene integration (>47%) at the ARSA locus in human bone marrow-derived HSPCs, with no detectable off-target editing. As a proof of concept, we tested our mutation-agnostic therapy in HSPCs derived from two MLD patients with distinct mutations and demonstrated restoration of ARSA enzyme activity (>30-fold improvement) equivalent to healthy adults. In summary, our investigation enabled a mutation-agnostic therapy for MLD patients with proven efficacy and strong potential for clinical translation.

Clinical Trials for Gene Therapy in Lysosomal Diseases With CNS Involvement

There are over 70 known lysosomal storage disorders (LSDs), most caused by mutations in genes encoding lysosomal hydrolases. Central nervous system involvement is a hallmark of the majority of LSDs and, if present, generally determines the prognosis of the disease. Nonetheless, brain disease is currently poorly targeted by available therapies, including systemic enzyme replacement therapy, mostly (but not only) due to the presence of the blood–brain barrier that restricts the access of orally or parenterally administered large molecules into the brain. Thus, one of the greatest and most exciting challenges over coming years will be to succeed in developing effective therapies for the treatment of central nervous system manifestations in LSDs. Over recent years, gene therapy (GT) has emerged as a promising therapeutic strategy for a variety of inherited neurodegenerative diseases. In LSDs, the ability of genetically corrected cells to cross-correct adjacent lysosomal enzyme-deficient cells in the brain after gene transfer might enhance the diffusion of the recombinant enzyme, making this group of diseases a strong candidate for such an approach. Both in vivo (using the administration of recombinant adeno-associated viral vectors) and ex vivo (auto-transplantation of lentiviral vector-modified hematopoietic stem cells-HSCs) strategies are feasible. Promising results have been obtained in an ever-increasing number of preclinical studies in rodents and large animal models of LSDs, and these give great hope of GT successfully correcting neurological defects, once translated to clinical practice. We are now at the stage of treating patients, and various clinical trials are underway, to assess the safety and efficacy of in vivo and ex vivo GT in several neuropathic LSDs. In this review, we summarize different approaches being developed and review the current clinical trials related to neuropathic LSDs, their results (if any), and their limitations. We will also discuss the pitfalls and the remaining challenges.

Mechanisms of demyelination and neurodegeneration in globoid cell leukodystrophy

Globoid cell leukodystrophy (GLD), also known as Krabbe disease, is a lysosomal storage disorder causing extensive demyelination in the central and peripheral nervous systems. GLD is caused by loss-of-function mutations in the lysosomal hydrolase, galactosylceramidase (GALC), which catabolizes the myelin sphingolipid galactosylceramide. The pathophysiology of GLD is complex and reflects the expression of GALC in a number of glial and neural cell types in both the central and peripheral nervous systems (CNS and PNS), as well as leukocytes and kidney in the periphery. Over the years, GLD has garnered a wide range of scientific and medical interests, especially as a model system to study gene therapy and novel preclinical therapeutic approaches to treat the spontaneous murine model for GLD. Here, we review recent findings in the field of Krabbe disease, with particular emphasis on novel aspects of GALC physiology, GLD pathophysiology, and therapeutic strategies.

A molecular genetics view on Mucopolysaccharidosis Type II

Mucopolysaccharidosis Type II (MPS II) is an X-linked recessive genetic disorder that primarily affects male patients. With an incidence of 1 in 100,000 male live births, the disease is one of the orphan diseases. MPS II symptoms are caused by mutations in the lysosomal iduronate-2-sulfatase (IDS) gene. The mutations cause a loss of enzymatic performance and result in the accumulation of glycosaminoglycans (GAGs), heparan sulfate and dermatan sulfate, which are no longer degradable. This inadvertent accumulation causes damage in multiple organs and leads either to a severe neurological course or to an attenuated course of the disease, although the exact relationship between mutation, extent of GAG accumulation and disease progression is not yet fully understood. This review is intended to present current diagnostic procedures and therapeutic interventions. In times when the genetic profile of patients plays an increasingly important role in the assessment of therapeutic success and future drug design, we chose to further elucidate the impact of genetic diversity within the IDS gene on disease phenotype and potential implications in current diagnosis, prognosis and therapy. We report recent advances in the structural biological elucidation of I2S enzyme that that promises to improve our future understanding of the molecular damage of the hundreds of IDS gene variants and will aid damage prediction of novel mutations in the future.

Intravitreal gene therapy protects against retinal dysfunction and degeneration in sheep with CLN5 Batten disease

Neuronal ceroid lipofuscinoses (NCL; Batten disease) are a group of inherited neurodegenerative diseases primarily affecting children. A common feature across most NCLs is the progressive loss of vision. We performed intravitreal injections of self-complementary AAV9 vectors packaged with either ovine CLN5 or CLN6 into one eye of 3-month-old CLN5-/- or CLN6-/- animals, respectively. Electroretinography (ERG) was performed every month following treatment, and retinal histology was assessed post-mortem in the treated compared to untreated eye. In CLN5-/- animals, ERG amplitudes were normalised in the treated eye whilst the untreated eye declined in a similar manner to CLN5 affected controls. In CLN6-/- animals, ERG amplitudes in both eyes declined over time although the treated eye showed a slower decline. Post-mortem examination revealed significant attenuation of retinal atrophy and lysosomal storage body accumulation in the treated eye compared with the untreated eye in CLN5-/- animals. This proof-of-concept study provides the first observation of efficacious intravitreal gene therapy in a large animal model of NCL. In particular, the single administration of AAV9-mediated intravitreal gene therapy can successfully ameliorate retinal deficits in CLN5-/- sheep. Combining ocular gene therapy with brain-directed therapy presents a promising treatment strategy to be used in future sheep trials aiming to halt neurological and retinal disease in CLN5 Batten disease.

Gene Therapy for Mucopolysaccharidosis Type II-A Review of the Current Possibilities

Mucopolysaccharidosis type II (MPS II) is a lysosomal storage disorder based on a mutation in the IDS gene that encodes iduronate 2-sulphatase. As a result, there is an accumulation of glycosaminoglycans-heparan sulphate and dermatan sulphate-in almost all body tissues, which leads to their dysfunction. Currently, the primary treatment is enzyme replacement therapy, which improves the course of the disease by reducing somatic symptoms, including hepatomegaly and splenomegaly. The enzyme, however, does not cross the blood-brain barrier, and no improvement in the function of the central nervous system has been observed in patients with the severe form of the disease. An alternative method of treatment that solves typical problems of enzyme replacement therapy is gene therapy, i.e., delivery of the correct gene to target cells through an appropriate vector. Much progress has been made in applying gene therapy for MPS II, from cellular models to human clinical trials. In this article, we briefly present the history and basics of gene therapy and discuss the current state of knowledge about the methods of this therapy in mucopolysaccharidosis type II.

α-Galactosidase A Augmentation by Non-Viral Gene Therapy: Evaluation in Fabry Disease Mice

Fabry disease (FD) is a monogenic X-linked lysosomal storage disorder caused by a deficiency in the lysosomal enzyme α-Galactosidase A (α-Gal A). It is a good candidate to be treated with gene therapy, in which moderately low levels of enzyme activity should be sufficient for clinical efficacy. In the present work we have evaluated the efficacy of a non-viral vector based on solid lipid nanoparticles (SLN) to increase α-Gal A activity in an FD mouse model after intravenous administration. The SLN-based vector incremented α-Gal A activity to about 10%, 15%, 20% and 14% of the levels of the wild-type in liver, spleen, heart and kidney, respectively. In addition, the SLN-based vector significantly increased α-Gal A activity with respect to the naked pDNA used as a control in plasma, heart and kidney. The administration of a dose per week for three weeks was more effective than a single-dose administration. Administration of the SLN-based vector did not increase liver transaminases, indicative of a lack of toxicity. Additional studies are necessary to optimize the efficacy of the system; however, these results reinforce the potential of lipid-based nanocarriers to treat FD by gene therapy.

Sphingolipid lysosomal storage diseases: from bench to bedside

Johann Ludwig Wilhelm Thudicum described sphingolipids (SLs) in the late nineteenth century, but it was only in the past fifty years that SL research surged in importance and applicability. Currently, sphingolipids and their metabolism are hotly debated topics in various biochemical fields. Similar to other macromolecular reactions, SL metabolism has important implications in health and disease in most cells. A plethora of SL-related genetic ailments has been described. Defects in SL catabolism can cause the accumulation of SLs, leading to many types of lysosomal storage diseases (LSDs) collectively called sphingolipidoses. These diseases mainly impact the neuronal and immune systems, but other systems can be affected as well. This review aims to present a comprehensive, up-to-date picture of the rapidly growing field of sphingolipid LSDs, their etiology, pathology, and potential therapeutic strategies. We first describe LSDs biochemically and briefly discuss their catabolism, followed by general aspects of the major diseases such as Gaucher, Krabbe, Fabry, and Farber among others. We conclude with an overview of the available and potential future therapies for many of the diseases. We strive to present the most important and recent findings from basic research and clinical applications, and to provide a valuable source for understanding these disorders.

Using Bibliometric Analysis and Machine Learning to Identify Compounds Binding to Sialidase-1

Rare diseases impact hundreds of millions of individuals worldwide. However, few therapies exist to treat the rare disease population because financial resources are limited, the number of patients affected is low, bioactivity data is often nonexistent, and very few animal models exist to support preclinical development efforts. Sialidosis is an ultrarare lysosomal storage disorder in which mutations in the NEU1 gene result in the deficiency of the lysosomal enzyme sialidase-1. This enzyme catalyzes the removal of sialic acid moieties from glycoproteins and glycolipids. Therefore, the defective or deficient protein leads to the buildup of sialylated glycoproteins as well as several characteristic symptoms of sialidosis including visual impairment, ataxia, hepatomegaly, dysostosis multiplex, and developmental delay. In this study, we used a bibliometric tool to generate links between lysosomal storage disease (LSD) targets and existing bioactivity data that could be curated in order to build machine learning models and screen compounds in silico. We focused on sialidase as an example, and we used the data curated from the literature to build a Bayesian model which was then used to score compound libraries and rank these molecules for in vitro testing. Two compounds were identified from in vitro testing using microscale thermophoresis, namely sulfameter (K _d 2.15 ± 1.02 μM) and mexenone (K _d 8.88 ± 4.02 μM), which validated our approach to identifying new molecules binding to this protein, which could represent possible drug candidates that can be evaluated further as potential chaperones for this ultrarare lysosomal disease for which there is currently no treatment. Combining bibliometric and machine learning approaches has the ability to assist in curating small molecule data and model building, respectively, for rare disease drug discovery. This approach also has the capability to identify new compounds that are potential drug candidates.

The rapidly evolving view of lysosomal storage diseases

Lysosomal storage diseases are a group of metabolic disorders caused by deficiencies of several components of lysosomal function. Most commonly affected are lysosomal hydrolases, which are involved in the breakdown and recycling of a variety of complex molecules and cellular structures. The understanding of lysosomal biology has progressively improved over time. Lysosomes are no longer viewed as organelles exclusively involved in catabolic pathways, but rather as highly dynamic elements of the autophagic-lysosomal pathway, involved in multiple cellular functions, including signaling, and able to adapt to environmental stimuli. This refined vision of lysosomes has substantially impacted on our understanding of the pathophysiology of lysosomal disorders. It is now clear that substrate accumulation triggers complex pathogenetic cascades that are responsible for disease pathology, such as aberrant vesicle trafficking, impairment of autophagy, dysregulation of signaling pathways, abnormalities of calcium homeostasis, and mitochondrial dysfunction. Novel technologies, in most cases based on high-throughput approaches, have significantly contributed to the characterization of lysosomal biology or lysosomal dysfunction and have the potential to facilitate diagnostic processes, and to enable the identification of new therapeutic targets.

Revisiting gene delivery to the brain: silencing and editing

Neurodegenerative disorders, ischemic brain diseases, and brain tumors are debilitating diseases that severely impact a person's life and could possibly lead to their demise if left untreated. Many of these diseases do not respond to small molecule therapeutics and have no effective long-term therapy. Gene therapy offers the promise of treatment or even a cure for both genetic and acquired brain diseases, mediated by either silencing or editing disease-specific genes. Indeed, in the last 5 years, significant progress has been made in the delivery of non-coding RNAs as well as gene-editing formulations to the brain. Unfortunately, the delivery is a major limiting factor for the success of gene therapies. Both viral and non-viral vectors have been used to deliver genetic information into a target cell, but they have limitations. Viral vectors provide excellent transduction efficiency but are associated with toxic effects and have limited packaging capacity; however, non-viral vectors are less toxic and show a high packaging capacity at the price of low transfection efficiency. Herein, we review the progress made in the field of brain gene therapy, particularly in the design of non-toxic and trackable non-viral vectors, capable of controlled release of genes in response to internal/external triggers, and in the delivery of formulations for gene editing. The application of these systems in the context of various brain diseases in pre-clinical and clinical tests will be discussed. Such promising approaches could potentially pave the way for clinical realization of brain gene therapies.

Current Clinical Applications of In Vivo Gene Therapy with AAVs

Hereditary diseases are caused by mutations in genes, and more than 7,000 rare diseases affect over 30 million Americans. For more than 30 years, hundreds of researchers have maintained that genetic modifications would provide effective treatments for many inherited human diseases, offering durable and possibly curative clinical benefit with a single treatment. This review is limited to gene therapy using adeno-associated virus (AAV) because the gene delivered by this vector does not integrate into the patient genome and has a low immunogenicity. There are now five treatments approved for commercialization and currently available, i.e., Luxturna, Zolgensma, the two chimeric antigen receptor T cell (CAR-T) therapies (Yescarta and Kymriah), and Strimvelis (the gammaretrovirus approved for adenosine deaminase-severe combined immunodeficiency [ADA-SCID] in Europe). Dozens of other treatments are under clinical trials. The review article presents a broad overview of the field of therapy by in vivo gene transfer. We review gene therapy for neuromuscular disorders (spinal muscular atrophy [SMA]; Duchenne muscular dystrophy [DMD]; X-linked myotubular myopathy [XLMTM]; and diseases of the central nervous system, including Alzheimer’s disease, Parkinson’s disease, Canavan disease, aromatic l-amino acid decarboxylase [AADC] deficiency, and giant axonal neuropathy), ocular disorders (Leber congenital amaurosis, age-related macular degeneration [AMD], choroideremia, achromatopsia, retinitis pigmentosa, and X-linked retinoschisis), the bleeding disorder hemophilia, and lysosomal storage disorders.

Pre-clinical Gene Therapy with AAV9/AGA in Aspartylglucosaminuria Mice Provides Evidence for Clinical Translation

Aspartylglucosaminuria (AGU) is an autosomal recessive lysosomal storage disease caused by loss of the enzyme aspartylglucosaminidase (AGA), resulting in AGA substrate accumulation. AGU patients have a slow but progressive neurodegenerative disease course, for which there is no approved disease-modifying treatment. In this study, AAV9/AGA was administered to Aga^−/− mice intravenously (i.v.) or intrathecally (i.t.), at a range of doses, either before or after disease pathology begins. At either treatment age, AAV9/AGA administration led to (1) dose dependently increased and sustained AGA activity in body fluids and tissues; (2) rapid, sustained, and dose-dependent elimination of AGA substrate in body fluids; (3) significantly rescued locomotor activity; (4) dose-dependent preservation of Purkinje neurons in the cerebellum; and (5) significantly reduced gliosis in the brain. Treated mice had no abnormal neurological phenotype and maintained body weight throughout the whole experiment to 18 months old. In summary, these results demonstrate that treatment of Aga^−/− mice with AAV9/AGA is effective and safe, providing strong evidence that AAV9/AGA gene therapy should be considered for human translation. Further, we provide a direct comparison of the efficacy of an i.v. versus i.t. approach using AAV9, which should greatly inform the development of similar treatments for other related lysosomal storage diseases.

Gene therapy for inherited metabolic diseases

Over the last two decades, gene therapy has been successfully translated to many rare diseases. The number of clinical trials is rapidly expanding and some gene therapy products have now received market authorisation in the western world. Inherited metabolic diseases (IMD) are orphan diseases frequently associated with a severe debilitating phenotype with limited therapeutic perspective. Gene therapy is progressively becoming a disease-changing therapeutic option for these patients. In this review, we aim to summarise the development of this emerging field detailing the main gene therapy strategies, routes of administration, viral and non-viral vectors and gene editing tools. We discuss the respective advantages and pitfalls of these gene therapy strategies and review their application in IMD, providing examples of clinical trials with lentiviral or adeno-associated viral gene therapy vectors in rare diseases. The rapid development of the field and implementation of gene therapy as a realistic therapeutic option for various IMD in a short term also require a good knowledge and understanding of these technologies from physicians to counsel the patients at best.

Mechanistic Insights into the Chaperoning of Human Lysosomal-Galactosidase Activity: Highly Functionalized Aminocyclopentanes and C-5a-Substituted Derivatives of 4-epi-Isofagomine

Glycosidase inhibitors have shown great potential as pharmacological chaperones for lysosomal storage diseases. In light of this, a series of new cyclopentanoid β-galactosidase inhibitors were prepared and their inhibitory and pharmacological chaperoning activities determined and compared with those of lipophilic analogs of the potent β-d-galactosidase inhibitor 4-epi-isofagomine. Structure-activity relationships were investigated by X-ray crystallography as well as by alterations in the cyclopentane moiety such as deoxygenation and replacement by fluorine of a “strategic” hydroxyl group. New compounds have revealed highly promising activities with a range of β-galactosidase-compromised human cell lines and may serve as leads towards new pharmacological chaperones for G_M1-gangliosidosis and Morquio B disease.

Highlights on Genomics Applications for Lysosomal Storage Diseases

Lysosomal storage diseases (LSDs) are a heterogeneous group of rare multisystem genetic disorders occurring mostly in infancy and childhood, characterized by a gradual accumulation of non-degraded substrates inside the lysosome. Although the cellular pathogenesis of LSDs is complex and still not fully understood, the approval of disease-specific therapies and the rapid emergence of novel diagnostic methods led to the implementation of extensive national newborn screening (NBS) programs in several countries. In the near future, this will help the development of standardized workflows aimed to more timely diagnose these conditions. Hereby, we report an overview of LSD diagnostic process and treatment strategies, provide an update on the worldwide NBS programs, and discuss the opportunities and challenges arising from genomics applications in screening, diagnosis, and research.

Translating CRISPR-Cas Therapeutics: Approaches and Challenges

CRISPR-Cas clinical trials have begun, offering a first glimpse at how DNA and RNA targeting could enable therapies for many genetic and epigenetic human diseases. The speedy progress of CRISPR-Cas from discovery and adoption to clinical use is built on decades of traditional gene therapy research and belies the multiple challenges that could derail the successful translation of these new modalities. Here, we review how CRISPR-Cas therapeutics are translated from technological systems to therapeutic modalities, paying particular attention to the therapeutic cascade from cargo to delivery vector, manufacturing, administration, pipelines, safety, and therapeutic target profiles. We also explore potential solutions to some of the obstacles facing successful CRISPR-Cas translation. We hope to illuminate how CRISPR-Cas is brought from the academic bench toward use in the clinic.

Efficient engraftment of genetically modified cells is necessary to ameliorate central nervous system involvement of murine model of mucopolysaccharidosis type II by hematopoietic stem cell targeted gene therapy

Mucopolysaccharidosis type II (MPS II) is a lysosomal storage disease (LSD) caused by a deficiency of the iduronate-2-sulfatase (IDS) that catabolizes glycosaminoglycans (GAGs). Abnormal accumulations of GAGs in somatic cells lead to various manifestations including central nervous system (CNS) disease. Enzyme replacement therapy (ERT) and hematopoietic stem cell transplantation (HSCT) are the currently available therapy for MPS II, but both therapies fail to improve CNS manifestations. We previously showed that hematopoietic stem cell targeted gene therapy (HSC-GT) with lethal irradiation improved CNS involvement in a murine model of MPS II which lacks the gene coding for IDS. However, the strong preconditioning, with lethal irradiation, would cause a high rate of morbidity and mortality. Therefore, we tested milder preconditioning procedures with either low dose irradiation or low dose irradiation plus an anti c-kit monoclonal antibody (ACK2) to assess CNS effects in mice with MPS II after HSC-GT. Mice from all the HSC-GT groups displayed super-physiological levels of IDS enzyme activity and robust reduction of abnormally accumulated GAGs to the wild type mice levels in peripheral organs. However, only the mice treated with lethal irradiation showed significant cognitive function improvement as well as IDS elevation and GAG reduction in the brain. These results suggest that an efficient engraftment of genetically modified cells for HSC-GT requires strong preconditioning to ameliorate CNS involvement in cases with MPS II.

Macrophages Expressing GALC Improve Peripheral Krabbe Disease by a Mechanism Independent of Cross-Correction

Many therapies for lysosomal storage disorders rely on cross-correction of lysosomal enzymes. In globoid cell leukodystrophy (GLD), mutations in GALC cause psychosine accumulation, inducing demyelination, a neuroinflammatory "globoid" reaction and neurodegeneration. The efficiency of GALC cross-correction in vivo, the role of the GALC substrate galactosylceramide, and the origin of psychosine are poorly understood. Using a novel GLD model, we show that cross-correction does not occur efficiently in vivo and that Galc-deficient Schwann cells autonomously produce psychosine. Furthermore, macrophages require GALC to degrade myelin, as Galc-deficient macrophages are transformed into globoid cells by exposure to galactosylceramide and produce a more severe GLD phenotype. Finally, hematopoietic stem cell transplantation in patients reduces globoid cells in nerves, suggesting that the phagocytic response of healthy macrophages, rather than cross-correction, contributes to the therapeutic effect. Thus, GLD may be caused by at least two mechanisms: psychosine-induced demyelination and secondary neuroinflammation from galactosylceramide storage in macrophages.

Systemic enzyme delivery by blood-brain barrier-penetrating SapC-DOPS nanovesicles for treatment of neuronopathic Gaucher disease

Background

Enzyme replacement therapy (ERT) can positively affect the visceral manifestations of lysosomal storage diseases (LSDs). However, the exclusion of the intravenous ERT agents from the central nervous system (CNS) prevents direct therapeutic effects.

Methods

Using a neuronopathic Gaucher disease (nGD) mouse model, CNS-ERT was created using a systemic, non-invasive, and CNS-selective delivery system based on nanovesicles of saposin C (SapC) and dioleoylphosphatidylserine (DOPS) to deliver to CNS cells and tissues the corrective, functional acid β-glucosidase (GCase).

Findings

Compared to free GCase, human GCase formulated with SapC-DOPS nanovesicles (SapC-DOPS-GCase) was more stable in serum, taken up into cells, mostly by a mannose receptor-independent pathway, and resulted in higher activity in GCase-deficient cells. In contrast to free GCase, SapC-DOPS-GCase nanovesicles penetrated through the blood-brain barrier into the CNS. The CNS targeting was mediated by surface phosphatidylserine (PS) of blood vessel and brain cells. Increased GCase activity and reduced GCase substrate levels were found in the CNS of SapC-DOPS-GCase-treated nGD mice, which showed profound improvement in brain inflammation and neurological phenotypes.

Interpretation

This first-in-class CNS-ERT approach provides considerable promise of therapeutic benefits for neurodegenerative diseases.

Funding

This study was supported by the National Institutes of Health grants R21NS 095047 to XQ and YS, R01NS 086134 and UH2NS092981 in part to YS; Cincinnati Children's Hospital Medical Center Research Innovation/Pilot award to YS and XQ; Gardner Neuroscience Institute/Neurobiology Research Center Pilot award to XQ and YS, Hematology-Oncology Programmatic Support from University of Cincinnati and New Drug State Key Project grant 009ZX09102-205 to XQ.

In Vivo Gene Therapy for Mucopolysaccharidosis Type III (Sanfilippo Syndrome): A New Treatment Horizon

For most lysosomal storage diseases (LSDs), there is no cure. Gene therapy is an attractive tool for treatment of LSDs caused by deficiencies in secretable lysosomal enzymes, in which neither full restoration of normal enzymatic activity nor transduction of all cells of the affected organ is necessary. However, some LSDs, such as mucopolysaccharidosis type III (MPSIII) diseases or Sanfilippo syndrome, represent a difficult challenge because patients suffer severe neurodegeneration with mild somatic alterations. The disease's main target is the central nervous system (CNS) and enzymes do not efficiently cross the blood-brain barrier (BBB) even if present at very high concentration in circulation. No specific treatment has been approved for MPSIII. In this study, we discuss the adeno-associated virus (AAV) vector-mediated gene transfer strategies currently being developed for MPSIII disease. These strategies rely on local delivery of AAV vectors to the CNS either through direct intraparenchymal injection at several sites or through delivery to the cerebrospinal fluid (CSF), which bathes the whole CNS, or exploit the properties of certain AAV serotypes capable of crossing the BBB upon systemic administration. Although studies in small and large animal models of MPSIII diseases have provided evidence supporting the efficacy and safety of all these strategies, there are considerable differences between the different routes of administration in terms of procedure-associated risks, vector dose requirements, sensitivity to the effect of circulating neutralizing antibodies that block AAV transduction, and potential toxicity. Ongoing clinical studies should shed light on which gene transfer strategy leads to highest clinical benefits while minimizing risks. The development of all these strategies opens a new horizon for treatment of not only MPSIII and other LSDs but also of a wide range of neurological diseases.

Gene therapy to the blood-brain barrier with resulting protein secretion as a strategy for treatment of Niemann Picks type C2 disease

Treatment of many diseases affecting the central nervous system (CNS) is complicated by the inability of several therapeutics to cross the blood-brain barrier (BBB). Genetically modifying brain capillary endothelial cells (BCECs) denotes an approach to overcome the limitations of the BBB by turning BCECs into recombinant protein factories. This will result in protein secretion toward both the brain and peripheral circulation, which is particularly relevant in genetic diseases, like lysosomal storage diseases (LSD), where cells are ubiquitously affected both in the CNS and the periphery. Here we investigated transfection of primary rat brain capillary endothelial cells (rBCECs) for synthesis and secretion of recombinant NPC2, the protein deficient in the lysosomal cholesterol storage disease Niemann Pick type C2. We demonstrate prominent NPC2 gene induction and protein secretion in 21% of BCECs in non-mitotic monocultures with a biological effect on NPC2-deficient fibroblasts as verified from changes in filipin III staining of cholesterol deposits. By comparison the transfection efficiency was 75% in HeLa-cells, known to persist in a mitotic state. When co-cultured with primary rat astrocytes in conditions with maintained BBB properties 7% BCECs were transfected, clearly suggesting that induction of BBB properties with polarized conditions of the non-mitotic BCECs influences the transfection efficacy and secretion directionality. In conclusion, non-viral gene therapy to rBCECs leads to protein secretion and signifies a method for NPC2 to target cells inside the CNS otherwise inaccessible because of the presence of the BBB. However, obtaining high transfection efficiencies is crucial in order to achieve sufficient therapeutic effects. Cover Image for this issue: https://doi.org/10.1111/jnc.15050.

Mucopolysaccharidosis Type II: One Hundred Years of Research, Diagnosis, and Treatment

Mucopolysaccharidosis type II (MPS II, Hunter syndrome) was first described by Dr. Charles Hunter in 1917. Since then, about one hundred years have passed and Hunter syndrome, although at first neglected for a few decades and afterwards mistaken for a long time for the similar disorder Hurler syndrome, has been clearly distinguished as a specific disease since 1978, when the distinct genetic causes of the two disorders were finally identified. MPS II is a rare genetic disorder, recently described as presenting an incidence rate ranging from 0.38 to 1.09 per 100,000 live male births, and it is the only X-linked-inherited mucopolysaccharidosis. The complex disease is due to a deficit of the lysosomal hydrolase iduronate 2-sulphatase, which is a crucial enzyme in the stepwise degradation of heparan and dermatan sulphate. This contributes to a heavy clinical phenotype involving most organ-systems, including the brain, in at least two-thirds of cases. In this review, we will summarize the history of the disease during this century through clinical and laboratory evaluations that allowed its definition, its correct diagnosis, a partial comprehension of its pathogenesis, and the proposition of therapeutic protocols. We will also highlight the main open issues related to the possible inclusion of MPS II in newborn screenings, the comprehension of brain pathogenesis, and treatment of the neurological compartment.

A novel gene editing system to treat both Tay-Sachs and Sandhoff diseases

The GM2-gangliosidoses are neurological diseases causing premature death, thus developing effective treatment protocols is urgent. GM2-gangliosidoses result from deficiency of a lysosomal enzyme β-hexosaminidase (Hex) and subsequent accumulation of GM2 gangliosides. Genetic changes in HEXA, encoding the Hex α subunit, or HEXB, encoding the Hex β subunit, causes Tay-Sachs disease and Sandhoff disease, respectively. Previous studies have showed that a modified human Hex μ subunit (HEXM) can treat both Tay-Sachs and Sandhoff diseases by forming a homodimer to degrade GM2 gangliosides. To this end, we applied this HEXM subunit in our PS813 gene editing system to treat neonatal Sandhoff mice. Through AAV delivery of the CRISPR system, a promoterless HEXM cDNA will be integrated into the albumin safe harbor locus, and lysosomal enzyme will be expressed and secreted from edited hepatocytes. Four months after the i.v. of AAV vectors, plasma MUGS and MUG activities reached up to 144- and 17-fold of wildtype levels (n=10, p<0.0001), respectively. More importantly, MUGS and MUG activities in the brain also increased significantly compared with untreated Sandhoff mice (p<0.001). Further, HPLC-MS/MS analysis showed that GM2 gangliosides in multiple tissues, except the brain, of treated mice were reduced to normal levels. Rotarod analysis showed that coordination and motor memory of treated mice were improved (p<0.05). Histological analysis of H&E stained tissues showed reduced cellular vacuolation in the brain and liver of treated Sandhoff mice. These results demonstrate the potential of developing a treatment of in vivo genome editing for Tay-Sachs and Sandhoff patients.

In Vivo Editing of Macrophages through Systemic Delivery of CRISPR-Cas9-Ribonucleoprotein-Nanoparticle Nanoassemblies

Macrophages are key effectors of host defense and metabolism, making them promising targets for transient genetic therapy. Gene editing through delivery of the Cas9-ribonucleoprotein (RNP) provides multiple advantages over gene delivery-based strategies for introducing CRISPR machinery to the cell. There are, however, significant physiological, cellular, and intracellular barriers to the effective delivery of the Cas9 protein and guide RNA (sgRNA) that have to date, restricted in vivo Cas9 protein-based approaches to local/topical delivery applications. Herein we describe a new nanoassembled platform featuring co-engineered nanoparticles and Cas9 protein that has been developed to provide efficient Cas9-sgRNA delivery and concomitant CRISPR editing through systemic tail-vein injection into mice, achieving >8% gene editing efficiency in macrophages of the liver and spleen.

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

Although individually uncommon, rare diseases collectively account for a considerable proportion of disease impact worldwide. A group of rare genetic diseases called the mucopolysaccharidoses (MPSs) are characterized by accumulation of partially degraded glycosaminoglycans cellularly. MPS results in varied systemic symptoms and in some forms of the disease, neurodegeneration. Lack of treatment options for MPS with neurological involvement necessitates new avenues of therapeutic investigation. Cell and gene therapies provide putative alternatives and when coupled with genome editing technologies may provide long term or curative treatment. Clustered regularly interspaced short palindromic repeats (CRISPR)-based genome editing technology and, more recently, advances in genome editing research, have allowed for the addition of base editors to the repertoire of CRISPR-based editing tools. The latest versions of base editors are highly efficient on-targeting deoxyribonucleic acid (DNA) editors. Here, we describe a number of putative guide ribonucleic acid (RNA) designs for precision correction of known causative mutations for 10 of the MPSs. In this review, we discuss advances in base editing technologies and current techniques for delivery of cell and gene therapies to the site of global degeneration in patients with severe neurological forms of MPS, the central nervous system, including ultrasound-mediated blood-brain barrier disruption.

Effects of gene therapy on cardiovascular symptoms of lysosomal storage diseases

Lysosomal storage diseases (LSDs) are inherited conditions caused by impaired lysosomal function and consequent substrate storage, leading to a range of clinical manifestations, including cardiovascular disease. This may lead to significant symptoms and even cardiac failure, which is an important cause of death among patients. Currently available treatments do not completely correct cardiac involvement in the LSDs. Gene therapy has been tested as a therapeutic alternative with promising results for the heart disease. In this review, we present the results of different approaches of gene therapy for LSDs, mainly in animal models, and its effects in the heart, focusing on protocols with cardiac functional analysis.

AAV-Mediated Progranulin Delivery to a Mouse Model of Progranulin Deficiency Causes T Cell-Mediated Toxicity

Adeno-associated virus-mediated gene replacement is emerging as a safe and effective means of correcting single-gene mutations affecting the CNS. AAV-mediated progranulin gene (GRN) delivery has been proposed as a treatment for GRN-deficient frontotemporal dementia and neuronal ceroid lipofuscinosis, and recent studies using intraparenchymal AAV-Grn delivery to brain have shown moderate success in histopathologic and behavioral rescue in mouse models. Here, we used AAV9 to deliver GRN to the lateral ventricle to achieve widespread expression in the Grn null mouse brain. We found that, despite a global increase in progranulin, overexpression resulted in dramatic and selective hippocampal toxicity and degeneration affecting neurons and glia. Hippocampal degeneration was preceded by T cell infiltration and perivascular cuffing. GRN delivery with an ependymal-targeting AAV for selective secretion of progranulin into the cerebrospinal fluid similarly resulted in T cell infiltration, as well as ependymal hypertrophy. Interestingly, overexpression of GRN in wild-type animals also provoked T cell infiltration. These results call into question the safety of GRN overexpression in the CNS, with evidence for both a region-selective immune response and cellular proliferative response. Our results highlight the importance of careful consideration of target gene biology and cellular response to overexpression prior to progressing to the clinic.

Intravenous administration of scAAV9-Hexb normalizes lifespan and prevents pathology in Sandhoff disease mice

Sandhoff disease (SD) is a rare inherited disorder caused by a deficiency of β-hexosaminidase activity which is fatal because no effective treatment is available. A mouse model of Hexb deficiency reproduces the key pathognomonic features of SD patients with severe ubiquitous lysosomal dysfunction, GM2 accumulation, neuroinflammation and neurodegeneration, culminating in death at 4 months. Here, we show that a single intravenous neonatal administration of a self-complementary adeno-associated virus 9 vector (scAAV9) expressing the Hexb cDNA in SD mice is safe and sufficient to prevent disease development. Importantly, we demonstrate for the first time that this treatment results in a normal lifespan (over 700 days) and normalizes motor function assessed by a battery of behavioral tests, with scAAV9-treated SD mice being indistinguishable from wild-type littermates. Biochemical analyses in multiple tissues showed a significant increase in hexosaminidase A activity, which reached 10-15% of normal levels. AAV9 treatment was sufficient to prevent GM2 and GA2 storage almost completely in the cerebrum (less so in the cerebellum), as well as thalamic reactive gliosis and thalamocortical neuron loss in treated Hexb-/- mice. In summary, this study demonstrated a widespread protective effect throughout the entire CNS after a single intravenous administration of the scAAV9-Hexb vector to neonatal SD mice.

Gene therapy for mucopolysaccharidoses: in vivo and ex vivo approaches

Mucopolysaccharidoses (MPS) are a group of lysosomal storage disorders caused by a deficiency in lysosomal enzymes catalyzing the stepwise degradation of glycosaminoglycans (GAGs). The current therapeutic strategies of enzyme replacement therapy and allogeneic hematopoietic stem cell transplantation have been reported to reduce patient morbidity and to improve their quality of life, but they are associated with persistence of residual disease burden, in particular at the neurocognitive and musculoskeletal levels. This indicates the need for more efficacious treatments capable of effective and rapid enzyme delivery to the affected organs, especially the brain and the skeleton. Gene therapy (GT) strategies aimed at correcting the genetic defect in patient cells could represent a significant improvement for the treatment of MPS when compared with conventional approaches. While in-vivo GT strategies foresee the administration of viral vector particles directly to patients with the aim of providing normal complementary DNA to the affected cells, ex-vivo GT approaches are based on the ex-vivo transduction of patient cells that are subsequently infused back. This review provides insights into the state-of-art accomplishments made with in vivo and ex vivo GT-based approaches in MPS and provide a vision for the future in the medical community.

Getting the Most: Enhancing Efficacy by Promoting Erythropoiesis and Thrombopoiesis after Gene Therapy in Mice with Hurler Syndrome

Novel strategies are needed to solve the conundrum of achieving clinical efficacy with high vector copy numbers (VCNs) in hematopoietic stem cells (HSCs) while attempting to minimize the potential risk of oncogenesis in lentiviral vector (LV)-mediated gene therapy clinical trials. We previously reported the benefits of reprogramming erythroid-megakaryocytic (EMK) cells for high-level lysosomal enzyme production with less risk of activating oncogenes in HSCs. Herein, using a murine model of mucopolysaccharidosis type I (MPS I) with a deficiency of α-L-iduronidase (IDUA), we sought to determine the transgene minimum effective doses (MEDs) in major organs, and if a transient increase of IDUA-containing red blood cells and platelets by repeated phlebotomy would provide further therapeutic benefits in diseased mice after EMK-restricted LV-mediated gene therapy. The MEDs for complete metabolic correction ranged from 0.1 to 2 VCNs in major visceral organs, which were dramatically reduced to 0.005–0.1 VCN by one cycle of stress induction and were associated with a further reduction of pathological deficits in mice with 0.005 VCN. This work provides a proof of concept that transiently stimulating erythropoiesis and thrombopoiesis can further improve therapeutic benefits in HSC-mediated gene therapy for MPS I, a repeatable and reversible approach to enhance clinical efficacy in the treatment of lysosomal storage diseases.

Lysosomal storage diseases

Lysosomal storage diseases (LSDs) are a group of over 70 diseases that are characterized by lysosomal dysfunction, most of which are inherited as autosomal recessive traits. These disorders are individually rare but collectively affect 1 in 5,000 live births. LSDs typically present in infancy and childhood, although adult-onset forms also occur. Most LSDs have a progressive neurodegenerative clinical course, although symptoms in other organ systems are frequent. LSD-associated genes encode different lysosomal proteins, including lysosomal enzymes and lysosomal membrane proteins. The lysosome is the key cellular hub for macromolecule catabolism, recycling and signalling, and defects that impair any of these functions cause the accumulation of undigested or partially digested macromolecules in lysosomes (that is, 'storage') or impair the transport of molecules, which can result in cellular damage. Consequently, the cellular pathogenesis of these diseases is complex and is currently incompletely understood. Several LSDs can be treated with approved, disease-specific therapies that are mostly based on enzyme replacement. However, small-molecule therapies, including substrate reduction and chaperone therapies, have also been developed and are approved for some LSDs, whereas gene therapy and genome editing are at advanced preclinical stages and, for a few disorders, have already progressed to the clinic.