AAV-mediated gene delivery attenuates neuroinflammation in feline Sandhoff disease

Intravenous gene therapy improves lifespan and clinical outcomes in feline Sandhoff Disease

Sandhoff Disease (SD), a fatal neurodegenerative disorder, is caused by the absence of ß-hexosaminidase (Hex) and subsequent accumulation of GM2 ganglioside in lysosomes. Previous studies have led to adeno-associated virus (AAV) gene therapy for children with GM2 gangliosidosis in both expanded access and Phase I/II clinical trials via intracranial and/or cerebrospinal fluid-based delivery. The current study investigated intravenous (IV) gene therapy of SD cats, treated at one month of age with a bicistronic AAV vector. While untreated SD cats lived to 4.3±0.2 months, cats treated with low and high doses lived to 8.3±1.2 and 12.4±2.7 months, respectively. In-life assessments revealed clear clinical benefit of AAV treatment, with the most dramatic improvement seen in the reduction of overt full-body tremors. Cerebrospinal fluid levels of aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) were decreased, indicating a reduction of cell damage within the central nervous system. Magnetic resonance imaging (MRI) and spectroscopy (MRS) acquired on a 7 Tesla scanner indicated that structural pathology and metabolite abnormalities are partially normalized by AAV treatment. Dose-dependent reduction of GM2 ganglioside storage and increases in Hex activity were most substantial in the caudal regions of the brain and in the spinal cord. Immunohistochemistry revealed reduction in neuroinflammatory cell populations and partial correction of myelin deficits. These results support the dose-dependent efficacy of AAV delivered IV for significant restoration of clinical metrics and Hex function in a feline model of SD.

Delivery of Adeno-Associated Virus Vectors to the Central Nervous System for Correction of Single Gene Disorders

Genetic disorders of the central nervous system (CNS) comprise a significant portion of disability in both children and adults. Several preclinical animal models have shown effective adeno-associated virus (AAV) mediated gene transfer for either treatment or prevention of autosomal recessive genetic disorders. Owing to the intricacy of the human CNS and the blood-brain barrier, it is difficult to deliver genes, particularly since the expression of any given gene may be required in a particular CNS structure or cell type at a specific time during development. In this review, we analyzed delivery methods for AAV-mediated gene therapy in past and current clinical trials. The delivery routes analyzed were direct intraparenchymal (IP), intracerebroventricular (ICV), intra-cisterna magna (CM), lumbar intrathecal (IT), and intravenous (IV). The results demonstrated that the dose used in these routes varies dramatically. The average total doses used were calculated and were 1.03 × 1013 for IP, 5.00 × 1013 for ICV, 1.26 × 1014 for CM, and 3.14 × 1014 for IT delivery. The dose for IV delivery varies by patient weight and is 1.13 × 1015 IV for a 10 kg infant. Ultimately, the choice of intervention must weigh the risk of an invasive surgical procedure to the toxicity and immune response associated with a high dose vector.

Sphingolipids and their role in health and disease in the central nervous system

Sphingolipids (SLs) are lipids derived from sphingosine, and their metabolism involves a broad and complex network of reactions. Although SLs are widely distributed in the body, it is well known that they are present in high concentrations within the central nervous system (CNS). Under physiological conditions, their abundance and distribution in the CNS depend on brain development and cell type. Consequently, SLs metabolism impairment may have a significant impact on the normal CNS function, and has been associated with several disorders, including sphingolipidoses, Parkinson's, and Alzheimer's. This review summarizes the main SLs characteristics and current knowledge about synthesis, catabolism, regulatory pathways, and their role in physiological and pathological scenarios in the CNS.

Gene Therapy for Rare Neurological Disorders

There are over 7 000 diseases that are individually rare, but collectively affect missions of people worldwide. They are very commonly neurologic single-gene disorders. Recent advances in recombinant adeno-associated virus (rAAV) vectors have enabled breakthroughs, including FDA-approved gene therapies for Inherited Retinal Dystrophy due to RPE65 mutation and spinal muscular atrophy. A range of other gene therapies for rare neurologic diseases are at various stages of development. Future development of gene editing technologies promises further to broaden the potential for more patients with these disorders to benefit from innovative therapies.

Natural history of Tay-Sachs disease in sheep

Tay-Sachs disease (TSD) is a fatal neurodegenerative disease caused by a deficiency of the enzyme β-N-acetylhexosaminidase A (HexA). TSD naturally occurs in Jacob sheep is the only experimental model of TSD. TSD in sheep recapitulates neurologic features similar to juvenile onset and late onset TSD patients. Due to the paucity of human literature on pathology of TSD, a better natural history in the sheep TSD brain, which is on the same order of magnitude as a child's, is necessary for evaluating therapy and characterizing the pathological events that occur. To provide clinicians and researchers with a clearer understanding of longitudinal pathology in patients, we compare spectrum of clinical signs and brain pathology in mildly symptomatic (3-months), moderately symptomatic (6-months), or severely affected TSD sheep (humane endpoint at ~9-months of age). Increased GM2 ganglioside in the CSF of TSD sheep and a TSD specific biomarker on MRS (taurine) correlate with disease severity. Microglial activation and reactive astrocytes were observed globally on histopathology in TSD sheep with a widespread reduction in oligodendrocyte density. Myelination is reduced primarily in the forebrain illustrated by loss of white matter on MRI. GM2 and GM3 ganglioside were increased and distributed differently in various tissues. The study of TSD in the sheep model provides a natural history to shed light on the pathophysiology of TSD, which is of utmost importance due to novel therapeutics being assessed in human patients.

Adeno-Associated Vector-Delivered CRISPR/SaCas9 System Reduces Feline Leukemia Virus Production In Vitro

Feline leukemia virus (FeLV) is a retrovirus of cats worldwide. High viral loads are associated with progressive infection and the death of the host, due to FeLV-associated disease. In contrast, low viral loads, an effective immune response, and a better clinical outcome can be observed in cats with regressive infection. We hypothesize that by lowering viral loads in progressively infected cats, using CRISPR/SaCas9-assisted gene therapy, the cat’s immune system may be permitted to direct the infection towards a regressive outcome. In a step towards this goal, the present study evaluates different adeno-associated vectors (AAVs) for their competence in delivering a gene editing system into feline cells, followed by investigations of the CRISPR/SaCas9 targeting efficiency for different sites within the FeLV provirus. Nine natural AAV serotypes, two AAV hybrid strains, and Anc80L65, an in silico predicted AAV ancestor, were tested for their potential to infect different feline cell lines and feline primary cells. AAV-DJ revealed superior infection efficiency and was thus employed in subsequent transduction experiments. The introduction of double-strand breaks, using the CRISPR/SaCas9 system targeting 12 selected FeLV provirus sites, was confirmed by T7 endonuclease 1 (T7E1), as well as Tracking of Indels by Decomposition (TIDE) analysis. The highest percentage (up to 80%) of nonhomologous end-joining (NHEJ) was found in the highly conserved gag and pol regions. Subsequent transduction experiments, using AAV-DJ, confirmed indel formation and showed a significant reduction in FeLV p27 antigen for some targets. The targeting of the FeLV provirus was efficient when using the CRISPR/SaCas9 approach in vitro. Whether the observed extent of provirus targeting will be sufficient to provide progressively FeLV-infected cats with the means to overcome the infection needs to be further investigated in vivo.

White Matter Pathology as a Barrier to Gangliosidosis Gene Therapy

The gangliosidoses are a family of neurodegenerative lysosomal storage diseases that have recently seen promising advances in gene therapy. White matter deficits are well established components of gangliosidosis pathology that are now receiving more attention because they are partially refractory to correction by gene therapy. After a brief synopsis of normal myelinogenesis, this review outlines current viewpoints on the origin of white matter deficits in the gangliosidoses and potential obstacles to treating them effectively by gene therapy. Dysmyelinogenesis (failure of myelin sheaths to form properly) is proposed as the predominant contributor to white matter pathology, but precise mechanistic details are not well understood. The involvement of neuronal storage deficits may extend beyond secondary demyelination (destruction of myelin due to axonal loss) and contribute to dysmyelinogenesis. Preclinical studies in animal models of the gangliosidoses have substantially improved lifespan and quality of life, leading to the initiation of several clinical trials. However, improvement of white matter pathology has lagged behind other metrics and few evidence-based explanations have been proposed to date. Research groups in the field are encouraged to include myelin-specific investigations in future gene therapy work to address this gap in knowledge.

Therapeutic benefit after intracranial gene therapy delivered during the symptomatic stage in a feline model of Sandhoff disease

Sandhoff disease (SD) is an autosomal recessive lysosomal storage disease caused by defects in the β-subunit of β-N-acetylhexosaminidase (Hex), the enzyme that catabolizes GM2 ganglioside (GM2). Hex deficiency causes neuronal storage of GM2 and related glycoconjugates, resulting in progressive neurodegeneration and death, typically in infancy. No effective treatment exists for human patients. Adeno-associated virus (AAV) gene therapy led to improved clinical outcome and survival of SD cats treated before the onset of disease symptoms. Most human patients are diagnosed after clinical disease onset, so it is imperative to test AAV gene therapy in symptomatic SD cats to provide a realistic indication of therapeutic benefits that can be expected in humans. In this study, AAVrh8 vectors injected into the thalamus and deep cerebellar nuclei of symptomatic SD cats resulted in widespread central nervous system enzyme distribution, although a substantial burden of storage material remained. Cats treated in the early symptomatic phase showed delayed disease progression and a significant survival increase versus untreated cats. Treatment was less effective when administered later in the disease course, although therapeutic benefit was still possible. Results are encouraging for the treatment of human patients and provide support for the development AAV gene therapy for human SD.

GM2 Gangliosidoses: Clinical Features, Pathophysiological Aspects, and Current Therapies

GM2 gangliosidoses are a group of pathologies characterized by GM2 ganglioside accumulation into the lysosome due to mutations on the genes encoding for the β-hexosaminidases subunits or the GM2 activator protein. Three GM2 gangliosidoses have been described: Tay-Sachs disease, Sandhoff disease, and the AB variant. Central nervous system dysfunction is the main characteristic of GM2 gangliosidoses patients that include neurodevelopment alterations, neuroinflammation, and neuronal apoptosis. Currently, there is not approved therapy for GM2 gangliosidoses, but different therapeutic strategies have been studied including hematopoietic stem cell transplantation, enzyme replacement therapy, substrate reduction therapy, pharmacological chaperones, and gene therapy. The blood-brain barrier represents a challenge for the development of therapeutic agents for these disorders. In this sense, alternative routes of administration (e.g., intrathecal or intracerebroventricular) have been evaluated, as well as the design of fusion peptides that allow the protein transport from the brain capillaries to the central nervous system. In this review, we outline the current knowledge about clinical and physiopathological findings of GM2 gangliosidoses, as well as the ongoing proposals to overcome some limitations of the traditional alternatives by using novel strategies such as molecular Trojan horses or advanced tools of genome editing.

Adeno-associated virus-binding antibodies detected in cats living in the Northeastern United States lack neutralizing activity

Cats are a critical pre-clinical model for studying adeno-associated virus (AAV) vector-mediated gene therapies. A recent study has described the high prevalence of anti-AAV neutralizing antibodies among domestic cats in Switzerland. However, our knowledge of pre-existing humoral immunity against various AAV serotypes in cats is still limited. Here, we show that, although antibodies binding known AAV serotypes (AAV1 to AAV11) are prevalent in cats living in the Northeastern United States, these antibodies do not necessarily neutralize AAV infectivity. We analyzed sera from 35 client-owned, 20 feral, and 30 specific pathogen-free (SPF) cats for pre-existing AAV-binding antibodies against the 11 serotypes. Antibody prevalence was 7 to 90% with an overall median of 50%. The AAV-binding antibodies showed broad reactivities with other serotypes. Of 44 selected antibodies binding AAV2, AAV6 or AAV9, none exhibited appreciable neutralizing activities. Instead, AAV6 or AAV9-binding antibodies showed a transduction-enhancing effect. AAV6-binding antibodies were highly prevalent in SPF cats (83%), but this was primarily due to cross-reactivity with preventive vaccine-induced anti-feline panleukopenia virus antibodies. These results indicate that prevalent pre-existing immunity in cats is not necessarily inhibitory to AAV and highlight a substantial difference in the nature of AAV-binding antibodies in cats living in geographically different regions.

7T MRI Predicts Amelioration of Neurodegeneration in the Brain after AAV Gene Therapy

GM1 gangliosidosis (GM1) is a fatal neurodegenerative lysosomal storage disease that occurs most commonly in young children, with no effective treatment available. Long-term follow-up of GM1 cats treated by bilateral thalamic and deep cerebellar nuclei (DCN) injection of adeno-associated virus (AAV)-mediated gene therapy has increased lifespan to 8 years of age, compared with an untreated lifespan of ~8 months. Due to risks associated with cerebellar injection in humans, the lateral ventricle was tested as a replacement route to deliver an AAVrh8 vector expressing feline β-galactosidase (β-gal), the defective enzyme in GM1. Treatment via the thalamus and lateral ventricle corrected storage, myelination, astrogliosis, and neuronal morphology in areas where β-gal was effectively delivered. Oligodendrocyte number increased, but only in areas where myelination was corrected. Reduced AAV and β-gal distribution were noted in the cerebellum with subsequent increases in storage, demyelination, astrogliosis, and neuronal degeneration. These postmortem findings were correlated with endpoint MRI and magnetic resonance spectroscopy (MRS). Compared with the moderate dose with which most cats were treated, a higher AAV dose produced superior survival, currently 6.5 years. Thus, MRI and MRS can predict therapeutic efficacy of AAV gene therapy and non-invasively monitor cellular events within the GM1 brain.

Brain endothelial specific gene therapy improves experimental Sandhoff disease

In Tay-Sachs and Sandhoff disease, a deficiency of the lysosomal enzyme β-hexosaminidase causes GM2 and other gangliosides to accumulate in neurons and triggers neurodegeneration. Although the pathology centers on neurons, β-hexosaminidase is mainly expressed outside of neurons, suggesting that gene therapy of these diseases should target non-neuronal cells to reconstitute physiological conditions. Here, we tested in Hexb^-/- mice, a model of Sandhoff disease, to determine whether endothelial expression of the genes for human β-hexosaminidase subunit A and B (HEXA, HEXB) is able to reduce disease symptoms and prolong survival of the affected mice. The brain endothelial selective vectors AAV-BR1-CAG-HEXA and AAV-BR1-CAG-HEXB transduced brain endothelial cells, which subsequently released β-hexosaminidase enzyme. In vivo intravenous administration of the gene vectors to adult and neonatal mice prolonged survival. They improved neurological function and reduced accumulation of the ganglioside GM2 and the glycolipid GA2 as well as astrocytic activation. Overall, the data demonstrate that endothelial cells are a suitable target for intravenous gene therapy of GM2 gangliosidoses and possibly other lysosomal storage disorders.

Large animal models contribute to the development of therapies for central and peripheral nervous system dysfunction in patients with lysosomal storage diseases

Lysosomal storage diseases (LSDs) are a group of 70 monogenic disorders characterized by the lysosomal accumulation of a substrate. As a group, LSDs affect ~1 in 5000 live births; however, each individual storage disease is rare, limiting the ability to perform natural history studies or to perform clinical trials. Perhaps in no other biomedical field have naturally occurring large animal (canine, feline, ovine, caprine, and bovine) models been so essential for understanding the fundamentals of disease pathogenesis and for developing safe and effective therapies. These models were critical for the development of hematopoietic stem cell transplantation in α- and β- mannosidosis, fucosidosis, and the mucopolysaccharidoses; enzyme replacement therapy for fucosidosis, the mucopolysaccharidoses, and neuronal ceroid lipofuscinosis; and small molecule therapy in Niemann-Pick type C disease. However, their most notable contributions to the biomedical field are in the development of gene therapy for LSDs. Adeno-associated viral vectors to treat nervous system disease have been evaluated in the large animal models of α-mannosidosis, globoid cell leukodystrophy, GM1 and GM2 gangliosidosis, the mucopolysaccharidoses, and neuronal ceroid lipofuscinosis. This review article will summarize the large animal models available for study as well as their contributions to the development of central and peripheral nervous system dysfunction in LSDs.

A Safe and Reliable Technique for CNS Delivery of AAV Vectors in the Cisterna Magna

Global gene delivery to the CNS has therapeutic importance for the treatment of neurological disorders that affect the entire CNS. Due to direct contact with the CNS, cerebrospinal fluid (CSF) is an attractive route for CNS gene delivery. A safe and effective route to achieve global gene distribution in the CNS is needed, and administration of genes through the cisterna magna (CM) via a suboccipital puncture results in broad distribution in the brain and spinal cord. However, translation of this technique to clinical practice is challenging due to the risk of serious and potentially fatal complications in patients. Herein, we report development of a gene therapy delivery method to the CM through adaptation of an intravascular microcatheter, which can be safely navigated intrathecally under fluoroscopic guidance. We examined the safety, reproducibility, and distribution/transduction of this method in sheep using a self-complementary adeno-associated virus 9 (scAAV9)-GFP vector. This technique was used to treat two Tay-Sachs disease patients (30 months old and 7 months old) with AAV gene therapy. No adverse effects were observed during infusion or post-treatment. This delivery technique is a safe and minimally invasive alternative to direct infusion into the CM, achieving broad distribution of AAV gene transfer to the CNS.

Lysosomal Glycosphingolipid Storage Diseases

Glycosphingolipids are cell-type-specific components of the outer leaflet of mammalian plasma membranes. Gangliosides, sialic acid–containing glycosphingolipids, are especially enriched on neuronal surfaces. As amphi-philic molecules, they comprise a hydrophilic oligosaccharide chain attached to a hydrophobic membrane anchor, ceramide. Whereas glycosphingolipid formation is catalyzed by membrane-bound enzymes along the secretory pathway, degradation takes place at the surface of intralysosomal vesicles of late endosomes and lysosomes catalyzed in a stepwise fashion by soluble hydrolases and assisted by small lipid-binding glycoproteins. Inherited defects of lysosomal hydrolases or lipid-binding proteins cause the accumulation of undegradable material in lysosomal storage diseases (GM1 and GM2 gangliosidosis; Fabry, Gaucher, and Krabbe diseases; and metachromatic leukodystrophy). The catabolic processes are strongly modified by the lipid composition of the substrate-carrying membranes, and the pathological accumulation of primary storage compounds can trigger an accumulation of secondary storage compounds (e.g., small glycosphingolipids and cholesterol in Niemann-Pick disease).

Substrate Reduction Therapy for Sandhoff Disease through Inhibition of Glucosylceramide Synthase Activity

Neuronopathic glycosphingolipidoses are a sub-group of lysosomal storage disorders for which there are presently no effective therapies. Here, we evaluated the potential of substrate reduction therapy (SRT) using an inhibitor of glucosylceramide synthase (GCS) to decrease the synthesis of glucosylceramide (GL1) and related glycosphingolipids. The substrates that accumulate in Sandhoff disease (e.g., ganglioside GM2 and its nonacylated derivative, lyso-GM2) are distal to the drug target, GCS. Treatment of Sandhoff mice with a GCS inhibitor that has demonstrated CNS access (Genz-682452) reduced the accumulation of GL1 and GM2, as well as a variety of disease-associated substrates in the liver and brain. Concomitant with these effects was a significant decrease in the expression of CD68 and glycoprotein non-metastatic melanoma B protein (Gpnmb) in the brain, indicating a reduction in microgliosis in the treated mice. Moreover, using in vivo imaging, we showed that the monocytic biomarker translocator protein (TSPO), which was elevated in Sandhoff mice, was normalized following Genz-682452 treatment. These positive effects translated in turn into a delay (∼28 days) in loss of motor function and coordination, as measured by rotarod latency, and a significant increase in longevity (∼17.5%). Together, these results support the development of SRT for the treatment of gangliosidoses, particularly in patients with residual enzyme activity.

Pre-existing antibodies to candidate gene therapy vectors (adeno-associated vector serotypes) in domestic cats

Adeno-associated virus (AAV) vectors represent promising candidates for gene therapy; however, pre-existing neutralizing antibodies (NAb) may reduce AAV vector delivery efficiency. In this study, the presence of AAV NAb was investigated in cats, which serve as a larger and outbred animal model for the prediction of gene therapy outcomes in humans but also in cats.Serum/plasma samples from 230 client-owned Swiss cats and 20 specified pathogen-free cats were investigated for NAb to AAV1, AAV2, AAV5, AAV6, AAV7, AAV8 and AAV9 using in vitro transduction inhibition and a beta-galactosidase assay. NAb to all tested AAV serotypes were found. Of the client-owned cats, 53% had NAb to one or more of the AAV serotypes. NAb (≥1:10) were found at frequencies of 5% (AAV6) to 28% (AAV7). The highest titers were found against AAV7 (≥1:160). The NAb prevalence to AAV2, AAV7 and AAV9 differed geographically. Regarding titers ≥1:10 against single AAV serotypes, age, breed and sex of the cats were not associated with the NAb prevalence. Cats with titers ≥1:20 against AAV2 and titers ≥1:40 against AAV7 were significantly younger than cats with low/no titers, and purebred cats were significantly more likely than non-purebred cats to have NAb to AAV2 (≥1:40). Additionally, regarding NAb to all AAV combined, female cats were significantly more likely than male cats to have NAb titers ≥1:40. Preliminary data using AAV-DJ indicated that less pre-existing NAb to the hybrid AAV-DJ can be expected compared to the wild-type AAV serotypes. AAV NAb will need to be taken into account for future in vivo gene therapy studies in cats.