Gene transfer corrects acute GM2 gangliosidosis--potential therapeutic contribution of perivascular enzyme flow

Creation of an in vitro model of GM1 gangliosidosis by CRISPR/Cas9 knocking-out the GLB1 gene in SH-SY5Y human neuronal cell line

GM1 gangliosidosis is one type of hereditary error of metabolism that occurs due to the absence or reduction of β-galactosidase enzyme content in the lysosome of cells, including neurons. In vitro, the use of neural cell lines could facilitate the study of this disease. By creating a cell model of GM1 gangliosidosis on the SH-SY5Y human nerve cell line, it is possible to understand the main role of this enzyme in breaking down lipid substrate and other pathophysiologic phenomena this disease. To knock-out the human GLB1 gene, guides targeting exons 14 and 16 of the GLB1 gene were designed using the CRISPOR and CHOP-CHOP websites, and high-efficiency guides were selected for cloning in the PX458 vector. After confirming the cloning, the vectors were transformed into DH5α bacteria and then the target vector was extracted and transfected into human nerve cells (SH-SY5Y cell line) by electroporation. After 48 h, GFP+ cells were sorted using the FACS technique and homozygous (compound heterozygous) single cells were isolated using the serial dilution method and sequencing was done to confirm them. Finally, gap PCR tests, X-gal and Periodic acid-Schiff (PAS) staining, and qPCR were used to confirm the knock-out of the human GLB1 gene. Additionally, RNA sequencing data analysis from existing data of the Gene Expression Omnibus (GEO) was used to find the correlation of GLB1 with other genes, and then the top correlated genes were tested for further evaluation of knock-out effects. The nonviral introduction of two guides targeting exons 14 and 16 of the GLB1 gene into SH-SY5Y cells led to the deletion of a large fragment with a size of 4.62 kb. In contrast to the non-transfected cell, X-gal staining resulted in no blue color in GLB1 gene knock-out cells indicating the absence of β-galactosidase enzyme activity in these cells. Real-time PCR (qPCR) results confirmed the RNA-Seq analysis outcomes on the GEO data set and following the GLB1 gene knock-out, the expression of its downstream genes, NEU1 and CTSA, has been decreased. It has been also shown that the downregulation of GLB1-NEU1-CTSA complex gene was involved in suppressed proliferation and invasion ability of knock-out cells. This study proved that using dual guide RNA can be used as a simple and efficient tool for targeting the GLB1 gene in nerve cells and the knockout SH-SY5Y cells can be used as a model investigation of basic and therapeutic surveys for GM1 gangliosidosis disease.

Life-Limiting Peripheral Organ Dysfunction in Feline Sandhoff Disease Emerges after Effective CNS Gene Therapy

OBJECTIVE

GM2 gangliosidosis is usually fatal by 5 years of age in its 2 major subtypes, Tay-Sachs and Sandhoff disease. First reported in 1881, GM2 gangliosidosis has no effective treatment today, and children succumb to the disease after a protracted neurodegenerative course and semi-vegetative state. This study seeks to further develop adeno-associated virus (AAV) gene therapy for human translation.

METHODS

Cats with Sandhoff disease were treated by intracranial injection of vectors expressing feline β-N-acetylhexosaminidase, the enzyme deficient in GM2 gangliosidosis.

RESULTS

Hexosaminidase activity throughout the brain and spinal cord was above normal after treatment, with highest activities at the injection sites (thalamus and deep cerebellar nuclei). Ganglioside storage was reduced throughout the brain and spinal cord, with near complete clearance in many regions. While untreated cats with Sandhoff disease lived for 4.4 ± 0.6 months, AAV-treated cats lived to 19.1 ± 8.6 months, and 3 of 9 cats lived >21 months. Correction of the central nervous system was so effective that significant increases in lifespan led to the emergence of otherwise subclinical peripheral disease, including megacolon, enlarged stomach and urinary bladder, soft tissue spinal cord compression, and patellar luxation. Throughout the gastrointestinal tract, neurons of the myenteric and submucosal plexuses developed profound pathology, demonstrating that the enteric nervous system was inadequately treated.

INTERPRETATION

The vector formulation in the current study effectively treats neuropathology in feline Sandhoff disease, but whole-body targeting will be an important consideration in next-generation approaches. ANN NEUROL 2023;94:969-986.

Efficacy of Adeno-Associated Virus Serotype 9-Mediated Gene Therapy for AB-Variant GM2 Gangliosidosis

GM2 gangliosidoses are a group of neurodegenerative lysosomal storage disorders that are characterized by the accumulation of GM2 gangliosides (GM2), leading to rapid neurological decline and death. The hydrolysis of GM2 requires the specific synthesis, processing, and combination of products of three genes-HEXA, HEXB, and GM2A-within the cell's lysosomes. Mutations in these genes result in Tay-Sachs disease, Sandhoff disease, or AB-variant GM2 gangliosidosis (ABGM2), respectively. ABGM2, the rarest of the three types, is characterized by a mutation in the GM2A gene, which encodes the GM2 activator (GM2A) protein. Being a monogenic disease, gene therapy is a plausible and likely effective method of treatment for ABGM2. This study aimed at assessing the effects of administering a one-time intravenous treatment of single-stranded Adeno-associated virus serotype 9 (ssAAV9)-GM2A viral vector at a dose of 1 × 1014 vector genomes (vg) per kilogram per mouse in an ABGM2 mouse model (Gm2a-/-). ssAAV9-GM2A was administered at 1-day (neonatal) or 6-weeks of age (adult-stage). The results demonstrated that, in comparison to Gm2a-/- mice that received a vehicle injection, the treated mice had reduced GM2 accumulation within the central nervous system and had long-term persistence of vector genomes in the brain and liver. This proof-of-concept study is a step forward towards the development of a clinically therapeutic approach for the treatment of patients with ABGM2.

Therapeutic advantages of combined gene/cell therapy strategies in a murine model of GM2 gangliosidosis

Genetic deficiency of β-N-acetylhexosaminidase (Hex) functionality leads to accumulation of GM2 ganglioside in Tay-Sachs disease and Sandhoff disease (SD), which presently lack approved therapies. Current experimental gene therapy (GT) approaches with adeno-associated viral vectors (AAVs) still pose safety and efficacy issues, supporting the search for alternative therapeutic strategies. Here we leveraged the lentiviral vector (LV)-mediated intracerebral (IC) GT platform to deliver Hex genes to the CNS and combined this strategy with bone marrow transplantation (BMT) to provide a timely, pervasive, and long-lasting source of the Hex enzyme in the CNS and periphery of SD mice. Combined therapy outperformed individual treatments in terms of lifespan extension and normalization of the neuroinflammatory/neurodegenerative phenotypes of SD mice. These benefits correlated with a time-dependent increase in Hex activity and a remarkable reduction in GM2 storage in brain tissues that single treatments failed to achieve. Our results highlight the synergic mode of action of LV-mediated IC GT and BMT, clarify the contribution of treatments to the therapeutic outcome, and inform on the realistic threshold of corrective enzymatic activity. These results have important implications for interpretation of ongoing experimental therapies and for design of more effective treatment strategies for GM2 gangliosidosis.

A novel brain-penetrant oral UGT8 inhibitor decreases in vivo galactosphingolipid biosynthesis in murine Krabbe disease

Krabbe disease is a rare, inherited neurodegenerative disease due to impaired lysosomal β-galactosylceramidase (GALC) activity and formation of neurotoxic β-galactosylsphingosine ('psychosine'). We investigated substrate reduction therapy with a novel brain-penetrant inhibitor of galactosylceramide biosynthesis, RA 5557, in twitcher mice that lack GALC activity and model Krabbe disease. This thienopyridine derivative selectively inhibits uridine diphosphate-galactose glycosyltransferase 8 (UGT8), the final step in the generation of galactosylceramides which are precursors of sulphatide and, in the pathological lysosome, the immediate source of psychosine. Administration of RA 5557, reduced pathologically elevated psychosine concentrations (72-86%) in the midbrain and cerebral cortex in twitcher mice: the inhibitor decreased galactosylceramides by about 70% in midbrain and cerebral cortex in mutant and wild type animals. Exposure to the inhibitor significantly decreased several characteristic inflammatory response markers without causing apparent toxicity to myelin-producing cells in wild type and mutant mice; transcript abundance of oligodendrocyte markers MBP (myelin basic protein) and murine UGT8 was unchanged. Administration of the inhibitor before conception and during several breeding cycles to mice did not impair fertility and gave rise to healthy offspring. Nevertheless, given the unchanged lifespan, it appears that GALC has critical functions in the nervous system beyond the hydrolysis of galactosylceramide and galactosylsphingosine. Our findings support further therapeutic exploration of orally active UGT8 inhibitors in Krabbe disease and related galactosphingolipid disorders. The potent thienopyridine derivative with effective target engagement here studied appears to have an acceptable safety profile in vivo; judicious dose optimization will be needed to ensure efficacious clinical translation.

AAV gene therapy for Tay-Sachs disease

Tay-Sachs disease (TSD) is an inherited neurological disorder caused by deficiency of hexosaminidase A (HexA). Here, we describe an adeno-associated virus (AAV) gene therapy expanded-access trial in two patients with infantile TSD (IND 18225) with safety as the primary endpoint and no secondary endpoints. Patient TSD-001 was treated at 30 months with an equimolar mix of AAVrh8-HEXA and AAVrh8-HEXB administered intrathecally (i.t.), with 75% of the total dose (1 × 1014 vector genomes (vg)) in the cisterna magna and 25% at the thoracolumbar junction. Patient TSD-002 was treated at 7 months by combined bilateral thalamic (1.5 × 1012 vg per thalamus) and i.t. infusion (3.9 × 1013 vg). Both patients were immunosuppressed. Injection procedures were well tolerated, with no vector-related adverse events (AEs) to date. Cerebrospinal fluid (CSF) HexA activity increased from baseline and remained stable in both patients. TSD-002 showed disease stabilization by 3 months after injection with ongoing myelination, a temporary deviation from the natural history of infantile TSD, but disease progression was evident at 6 months after treatment. TSD-001 remains seizure-free at 5 years of age on the same anticonvulsant therapy as before therapy. TSD-002 developed anticonvulsant-responsive seizures at 2 years of age. This study provides early safety and proof-of-concept data in humans for treatment of patients with TSD by AAV gene therapy.

Treatment of GM2 Gangliosidosis in Adult Sandhoff Mice using an Intravenous Self-Complementary Hexosaminidase Vector

BACKGROUND

GM2 gangliosidosis is a neurodegenerative, lysosomal storage disease caused by the deficiency of β-hexosaminidase A enzyme (HexA), an α/β-subunit heterodimer. A novel variant of the human hexosaminidase α-subunit, coded by HEXM, has previously been shown to form a stable homodimer, HexM, that hydrolyzes GM2 gangliosides (GM2) in vivo.

MATERIALS & METHODS

The current study assessed the efficacy of intravenous (IV) delivery of a self-complementary adeno-associated virus serotype 9 (scAAV9) vector incorporating the HEXM transgene, scAAV9/HEXM, including the outcomes based on the dosages provided to the Sandhoff (SD) mice. Six-week-old SD mice were injected with either 2.5E+12 vector genomes (low dose, LD) or 1.0E+13 vg (high dose, HD). We hypothesized that when examining the dosage comparison for scAAV9/HEXM in adult SD mice, the HD group would have more beneficial outcomes than the LD cohort. Assessments included survival, behavioral outcomes, vector biodistribution, and enzyme activity within the central nervous system.

RESULTS

Toxicity was observed in the HD cohort, with 8 of 14 mice dying within one month of the injection. As compared to untreated SD mice, which have typical survival of 16 weeks, the LD cohort and the remaining HD mice had a significant survival benefit with an average/median survival of 40.6/34.5 and 55.9/56.7 weeks, respectively. Significant behavioral, biochemical and molecular benefits were also observed. The second aim of the study was to investigate the effects of IV mannitol infusions on the biodistribution of the LD scAAV9/HEXM vector and the survival of the SD mice. Increases in both the biodistribution of the vector as well as the survival benefit (average/median of 41.6/49.3 weeks) were observed.

CONCLUSION

These results demonstrate the potential benefit and critical limitations of the treatment of GM2 gangliosidosis using IV delivered AAV vectors.

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.

Pronounced Therapeutic Benefit of a Single Bidirectional AAV Vector Administered Systemically in Sandhoff Mice

The GM2 gangliosidoses, Tay-Sachs disease (TSD) and Sandhoff disease (SD), are fatal lysosomal storage disorders caused by mutations in the HEXA and HEXB genes, respectively. These mutations cause dysfunction of the lysosomal enzyme β-N-acetylhexosaminidase A (HexA) and accumulation of GM2 ganglioside (GM2) with ensuing neurodegeneration, and death by 5 years of age. Until recently, the most successful therapy was achieved by intracranial co-delivery of monocistronic adeno-associated viral (AAV) vectors encoding Hex alpha and beta-subunits in animal models of SD. The blood-brain barrier crossing properties of AAV9 enables systemic gene therapy; however, the requirement of co-delivery of two monocistronic AAV vectors to overexpress the heterodimeric HexA protein has prevented the use of this approach. To address this need, we developed multiple AAV constructs encoding simultaneously HEXA and HEXB using AAV9 and AAV-PHP.B and tested their therapeutic efficacy in 4- to 6-week-old SD mice after systemic administration. Survival and biochemical outcomes revealed superiority of the AAV vector design using a bidirectional CBA promoter with equivalent dose-dependent outcomes for both capsids. AAV-treated mice performed normally in tests of motor function, CNS GM2 ganglioside levels were significantly reduced, and survival increased by >4-fold with some animals surviving past 2 years of age.

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.

Beneficial Effects of Acetyl-DL-Leucine (ADLL) in a Mouse Model of Sandhoff Disease

Sandhoff disease is a rare neurodegenerative lysosomal storage disease associated with the storage of GM2 ganglioside in late endosomes/lysosomes. Here, we explored the efficacy of acetyl-DL-leucine (ADLL), which has been shown to improve ataxia in observational studies in patients with Niemann-Pick Type C1 and other cerebellar ataxias. We treated a mouse model of Sandhoff disease (Hexb^-/-) (0.1 g/kg/day) from 3 weeks of age with this orally available drug. ADLL produced a modest but significant increase in life span, accompanied by improved motor function and reduced glycosphingolipid (GSL) storage in the forebrain and cerebellum, in particular GA2. ADLL was also found to normalize altered glucose and glutamate metabolism, as well as increasing autophagy and the reactive oxygen species (ROS) scavenger, superoxide dismutase (SOD1). Our findings provide new insights into metabolic abnormalities in Sandhoff disease, which could be targeted with new therapeutic approaches, including ADLL.

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.

The 100 Most-Cited Articles About Convection-Enhanced Delivery to the Brain: A Bibliometric Analysis

BACKGROUND

Convection-enhanced delivery (CED) overcomes the blood-brain barrier to deliver therapy within the central nervous system. Our aim was to evaluate citation and other bibliometric characteristics of the 100 most-cited articles about CED to the brain to better understand the state of research efforts in the field.

METHODS

Elsevier's Scopus database was searched for the 100 most-cited articles that focused on CED to the brain. Articles were dichotomized as either primarily basic science (BSc) or clinical (CL) articles. Various bibliometric parameters were summarized, and BSc and CL articles were compared.

RESULTS

Of the 100 most-cited articles, 64 (64%) were BSc and 36 (36%) were CL. The most common indications reported were brain tumors (59%) and Parkinson disease (5%). Overall median values were as follows: citation count, 102 (range, 70-933); citation rate per year, 9.0 (range, 3.7-49.4); number of authors, 5 (range, 1-25); and publication year, 2006 (range, 1994-2015). Articles were published in a total of 48 different journals, and predominately originated in the United States (n = 78, 78%). BSc and CL articles were statistically comparable in terms of bibliometric parameters.

CONCLUSIONS

In the 100 most-cited articles about CED to the brain, there were more BSc articles compared with CL articles; however, they were comparable with respect to the reported bibliometric parameters. Given that the peak year of publication of these articles was more than a decade ago, we anticipate that the field will shift toward more CL articles once effective therapies to be delivered via CED are discovered.

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.

New Approaches to Tay-Sachs Disease Therapy

Tay-Sachs disease belongs to the group of autosomal-recessive lysosomal storage metabolic disorders. This disease is caused by β-hexosaminidase A (HexA) enzyme deficiency due to various mutations in α-subunit gene of this enzyme, resulting in GM2 ganglioside accumulation predominantly in lysosomes of nerve cells. Tay-Sachs disease is characterized by acute neurodegeneration preceded by activated microglia expansion, macrophage and astrocyte activation along with inflammatory mediator production. In most cases, the disease manifests itself during infancy, the “infantile form,” which characterizes the most severe disorders of the nervous system. The juvenile form, the symptoms of which appear in adolescence, and the most rare form with late onset of symptoms in adulthood are also described. The typical features of Tay-Sachs disease are muscle weakness, ataxia, speech, and mental disorders. Clinical symptom severity depends on residual HexA enzymatic activity associated with some mutations. Currently, Tay-Sachs disease treatment is based on symptom relief and, in case of the late-onset form, on the delay of progression. There are also clinical reports of substrate reduction therapy using miglustat and bone marrow or hematopoietic stem cell transplantation. At the development stage there are methods of Tay-Sachs disease gene therapy using adeno- or adeno-associated viruses as vectors for the delivery of cDNA encoding α and β HexA subunit genes. Effectiveness of this approach is evaluated in α or β HexA subunit defective model mice or Jacob sheep, in which Tay-Sachs disease arises spontaneously and is characterized by the same pathological features as in humans. This review discusses the possibilities of new therapeutic strategies in Tay-Sachs disease therapy aimed at preventing neurodegeneration and neuroinflammation.

Efficacy of a Bicistronic Vector for Correction of Sandhoff Disease in a Mouse Model

G_M2 gangliosidoses are a family of severe neurodegenerative disorders resulting from a deficiency in the β-hexosaminidase A enzyme. These disorders include Tay-Sachs disease and Sandhoff disease, caused by mutations in the HEXA gene and HEXB gene, respectively. The HEXA and HEXB genes are required to produce the α and β subunits of the β-hexosaminidase A enzyme, respectively. Using a Sandhoff disease mouse model, we tested for the first time the potential of a comparatively lower dose (2.04 × 10¹³ vg/kg) of systemically delivered single-stranded adeno-associated virus 9 expressing both human HEXB and human HEXA cDNA under the control of a single promoter with a P2A-linked bicistronic vector design to correct the neurological phenotype. A bicistronic design allows maximal overexpression and secretion of the Hex A enzyme. Neonatal mice were injected with either this ssAAV9-HexB-P2A-HexA vector or a vehicle solution via the superficial temporal vein. An increase in survival of 56% compared with vehicle-injected controls and biochemical analysis of the brain tissue and serum revealed an increase in enzyme activity and a decrease in brain G_M2 ganglioside buildup. This is a proof-of-concept study showing the “correction efficacy” of a bicistronic AAV9 vector delivered intravenously for G_M2 gangliosidoses. Further studies with higher doses are warranted.

Genetics and Therapies for GM2 Gangliosidosis

Tay-Sachs disease, caused by impaired β-N-acetylhexosaminidase activity, was the first GM2 gangliosidosis to be studied and one of the most severe and earliest lysosomal diseases to be described. The condition, associated with the pathological build-up of GM2 ganglioside, has acquired almost iconic status and serves as a paradigm in the study of lysosomal storage diseases. Inherited as a classical autosomal recessive disorder, this global disease of the nervous system induces developmental arrest with regression of attained milestones; neurodegeneration progresses rapidly to cause premature death in young children. There is no effective treatment beyond palliative care, and while the genetic basis of GM2 gangliosidosis is well established, the molecular and cellular events, from diseasecausing mutations and glycosphingolipid storage to disease manifestations, remain to be fully delineated. Several therapeutic approaches have been attempted in patients, including enzymatic augmentation, bone marrow transplantation, enzyme enhancement, and substrate reduction therapy. Hitherto, none of these stratagems has materially altered the course of the disease. Authentic animal models of GM2 gangliodidosis have facilitated in-depth evaluation of innovative applications such as gene transfer, which in contrast to other interventions, shows great promise. This review outlines current knowledge pertaining the pathobiology as well as potential innovative treatments for the GM2 gangliosidoses.

Adeno-Associated Virus Gene Therapy in a Sheep Model of Tay-Sachs Disease

Tay-Sachs disease (TSD) is a fatal neurodegenerative disorder caused by a deficiency of the enzyme hexosaminidase A (HexA). TSD also occurs in sheep, the only experimental model of TSD that has clinical signs of disease. The natural history of sheep TSD was characterized using serial neurological evaluations, 7 Tesla magnetic resonance imaging, echocardiograms, electrodiagnostics, and cerebrospinal fluid biomarkers. Intracranial gene therapy was also tested using AAVrh8 monocistronic vectors encoding the α-subunit of Hex (TSD α) or a mixture of two vectors encoding both the α and β subunits separately (TSD α + β) injected at high (1.3 × 10¹³ vector genomes) or low (4.2 × 10¹² vector genomes) dose. Delay of symptom onset and/or reduction of acquired symptoms were noted in all adeno-associated virus-treated sheep. Postmortem evaluation showed superior HexA and vector genome distribution in the brain of TSD α + β sheep compared to TSD α sheep, but spinal cord distribution was low in all groups. Isozyme analysis showed superior HexA formation after treatment with both vectors (TSD α + β), and ganglioside clearance was most widespread in the TSD α + β high-dose sheep. Microglial activation and proliferation in TSD sheep-most prominent in the cerebrum-were attenuated after gene therapy. This report demonstrates therapeutic efficacy for TSD in the sheep brain, which is on the same order of magnitude as a child's brain.

Systemic Gene Transfer of a Hexosaminidase Variant Using an scAAV9.47 Vector Corrects GM2 Gangliosidosis in Sandhoff Mice

GM2 gangliosidosis is a group of neurodegenerative diseases caused by β-hexosaminidase A (HexA) enzyme deficiency. There is currently no cure. HexA is composed of two similar, nonidentical subunits, α and β, which must interact with the GM2 activator protein (GM2AP), a substrate-specific cofactor, to hydrolyze GM2 ganglioside. Mutations in either subunit or the activator can result in the accumulation of GM2 ganglioside within neurons throughout the central nervous system. The resulting neuronal cell death induces the primary symptoms of the disease: motor impairment, seizures, and sensory impairments. This study assesses the long-term effects of gene transfer in a Sandhoff (β-subunit knockout) mouse model. The study utilized a modified human β-hexosaminidase α-subunit (μ-subunit) that contains critical sequences from the β-subunit that enables formation of a stable homodimer (HexM) and interaction with GM2AP to hydrolyze GM2 ganglioside. We investigated a self-complementary adeno-associated viral (scAAV) vector expressing HexM, through intravenous injections of the neonatal mice. We monitored one cohort for 8 weeks and another cohort long-term for survival benefit, behavioral, biochemical, and molecular analyses. Untreated Sandhoff disease (SD) control mice reached a humane endpoint at approximately 15 weeks, whereas treated mice had a median survival age of 40 weeks, an approximate 2.5-fold survival advantage. On behavioral tests, the treated mice outperformed their knockout age-matched controls and perform similarly to the heterozygous controls. Through the enzymatic and GM2 ganglioside analyses, we observed a significant decrease in the GM2 ganglioside level, even though the enzyme levels were not significantly increased. Molecular analyses revealed a global distribution of the vector between brain and spinal cord regions. In conclusion, the neonatal delivery of a novel viral vector expressing the human HexM enzyme is effective in ameliorating the SD mouse phenotype for long-term. Our data could have implications not only for treatment of SD but also for Tay-Sachs disease (α-subunit deficiency) and similar brain disorders.

Novel Vector Design and Hexosaminidase Variant Enabling Self-Complementary Adeno-Associated Virus for the Treatment of Tay-Sachs Disease

GM2 gangliosidosis is a family of three genetic neurodegenerative disorders caused by the accumulation of GM2 ganglioside (GM2) in neuronal tissue. Two of these are due to the deficiency of the heterodimeric (α-β), "A" isoenzyme of lysosomal β-hexosaminidase (HexA). Mutations in the α-subunit (encoded by HEXA) lead to Tay-Sachs disease (TSD), whereas mutations in the β-subunit (encoded by HEXB) lead to Sandhoff disease (SD). The third form results from a deficiency of the GM2 activator protein (GM2AP), a substrate-specific cofactor for HexA. In their infantile, acute forms, these diseases rapidly progress with mental and psychomotor deterioration resulting in death by approximately 4 years of age. After gene transfer that overexpresses one of the deficient subunits, the amount of HexA heterodimer formed would empirically be limited by the availability of the other endogenous Hex subunit. The present study used a new variant of the human HexA α-subunit, μ, incorporating critical sequences from the β-subunit that produce a stable homodimer (HexM) and promote functional interactions with the GM2AP- GM2 complex. We report the design of a compact adeno-associated viral (AAV) genome using a synthetic promoter-intron combination to allow self-complementary (sc) packaging of the HEXM gene. Also, a previously published capsid mutant, AAV9.47, was used to deliver the gene to brain and spinal cord while having restricted biodistribution to the liver. The novel capsid and cassette design combination was characterized in vivo in TSD mice for its ability to efficiently transduce cells in the central nervous system when delivered intravenously in both adult and neonatal mice. This study demonstrates that the modified HexM is capable of degrading long-standing GM2 storage in mice, and it further demonstrates the potential of this novel scAAV vector design to facilitate widespread distribution of the HEXM gene or potentially other similar-sized genes to the nervous system.

Direct Intracranial Injection of AAVrh8 Encoding Monkey β-N-Acetylhexosaminidase Causes Neurotoxicity in the Primate Brain

GM2 gangliosidoses, including Tay-Sachs disease and Sandhoff disease, are lysosomal storage disorders caused by deficiencies in β-N-acetylhexosaminidase (Hex). Patients are afflicted primarily with progressive central nervous system (CNS) dysfunction. Studies in mice, cats, and sheep have indicated safety and widespread distribution of Hex in the CNS after intracranial vector infusion of AAVrh8 vectors encoding species-specific Hex α- or β-subunits at a 1:1 ratio. Here, a safety study was conducted in cynomolgus macaques (cm), modeling previous animal studies, with bilateral infusion in the thalamus as well as in left lateral ventricle of AAVrh8 vectors encoding cm Hex α- and β-subunits. Three doses (3.2 × 10¹² vg [n = 3]; 3.2 × 10¹¹ vg [n = 2]; or 1.1 × 10¹¹ vg [n = 2]) were tested, with controls infused with vehicle (n = 1) or transgene empty AAVrh8 vector at the highest dose (n = 2). Most monkeys receiving AAVrh8-cmHexα/β developed dyskinesias, ataxia, and loss of dexterity, with higher dose animals eventually becoming apathetic. Time to onset of symptoms was dose dependent, with the highest-dose cohort producing symptoms within a month of infusion. One monkey in the lowest-dose cohort was behaviorally asymptomatic but had magnetic resonance imaging abnormalities in the thalami. Histopathology was similar in all monkeys injected with AAVrh8-cmHexα/β, showing severe white and gray matter necrosis along the injection track, reactive vasculature, and the presence of neurons with granular eosinophilic material. Lesions were minimal to absent in both control cohorts. Despite cellular loss, a dramatic increase in Hex activity was measured in the thalamus, and none of the animals presented with antibody titers against Hex. The high overexpression of Hex protein is likely to blame for this negative outcome, and this study demonstrates the variations in safety profiles of AAVrh8-Hexα/β intracranial injection among different species, despite encoding for self-proteins.

Animal models of GM2 gangliosidosis: utility and limitations

GM2 gangliosidosis, a subset of lysosomal storage disorders, is caused by a deficiency of the glycohydrolase, β-N-acetylhexosaminidase, and includes the closely related Tay–Sachs and Sandhoff diseases. The enzyme deficiency prevents the normal, stepwise degradation of ganglioside, which accumulates unchecked within the cellular lysosome, particularly in neurons. As a result, individuals with GM2 gangliosidosis experience progressive neurological diseases including motor deficits, progressive weakness and hypotonia, decreased responsiveness, vision deterioration, and seizures. Mice and cats are well-established animal models for Sandhoff disease, whereas Jacob sheep are the only known laboratory animal model of Tay–Sachs disease to exhibit clinical symptoms. Since the human diseases are relatively rare, animal models are indispensable tools for further study of pathogenesis and for development of potential treatments. Though no effective treatments for gangliosidoses currently exist, animal models have been used to test promising experimental therapies. Herein, the utility and limitations of gangliosidosis animal models and how they have contributed to the development of potential new treatments are described.

The GM1 and GM2 Gangliosidoses: Natural History and Progress toward Therapy

The gangliosidoses are lysosomal storage disorders caused by accumulation of GM1 or GM2 gangliosides. GM1 gangliosidosis has both central nervous system and systemic findings; while, GM2 gangliosidosis is restricted primarily to the central nervous system. Both disorders have autosomal recessive modes of inheritance and a continuum of clinical presentations from a severe infantile form to a milder, chronic adult form. Both are devastating diseases without cure or specific treatment however, with the use of supportive aggressive medical management, the lifespan and quality of life has been extended for both diseases. Naturally occurring and engineered animal models that mimic the human diseases have enhanced our understanding of the pathogenesis of disease progression. Some models have shown significant improvement in symptoms and lifespan with enzyme replacement, substrate reduction, and anti-inflammatory treatments alone or in combination. More recently gene therapy has shown impressive results in large and small animal models. Treatment with FDA-approved glucose analogs to reduce the amount of ganglioside substrate is used as off-label treatments for some patients. Therapies also under clinical development include small molecule chaperones and gene therapy.

Construction of a hybrid β-hexosaminidase subunit capable of forming stable homodimers that hydrolyze GM2 ganglioside in vivo

Tay-Sachs or Sandhoff disease result from mutations in either the evolutionarily related HEXA or HEXB genes encoding respectively, the α- or β-subunits of β-hexosaminidase A (HexA). Of the three Hex isozymes, only HexA can interact with its cofactor, the GM2 activator protein (GM2AP), and hydrolyze GM2 ganglioside. A major impediment to establishing gene or enzyme replacement therapy based on HexA is the need to synthesize both subunits. Thus, we combined the critical features of both α- and β-subunits into a single hybrid µ-subunit that contains the α-subunit active site, the stable β-subunit interface and unique areas in each subunit needed to interact with GM2AP. To facilitate intracellular analysis and the purification of the µ-homodimer (HexM), CRISPR-based genome editing was used to disrupt the HEXA and HEXB genes in a Human Embryonic Kidney 293 cell line stably expressing the µ-subunit. In association with GM2AP, HexM was shown to hydrolyze a fluorescent GM2 ganglioside derivative both in cellulo and in vitro. Gene transfer studies in both Tay-Sachs and Sandhoff mouse models demonstrated that HexM expression reduced brain GM2 ganglioside levels.

Copackaged AAV9 Vectors Promote Simultaneous Immune Tolerance and Phenotypic Correction of Pompe Disease

Pompe disease is a progressive neuromuscular disorder caused by lysosomal accumulation of glycogen from a deficiency in acid alpha-glucosidase (GAA). Replacement of the missing enzyme is available by repeated protein infusions; however, efficacy is limited by immune response and inability to restore enzymatic function in the central nervous system. An alternative therapeutic option is adeno-associated virus (AAV)-mediated gene therapy, which results in widespread gene transfer and prolonged transgene expression. Both enzyme replacement therapy (ERT) and gene therapy can elicit anti-GAA immune reactions that dampen their effectiveness and pose life-threatening risks to patient safety. To modulate the immune responses related to gene therapy, we show that a human codon-optimized GAA (coGAA) driven by a liver-specific promoter (LSP) using AAV9 is capable of promoting immune tolerance in a Gaa(-/-) mouse model. Copackaging AAV9-LSP-coGAA with the tissue-restricted desmin promoter (AAV9-DES-coGAA) demonstrates the necessary cell autonomous expression in cardiac muscle, skeletal muscle, peripheral nerve, and the spinal cord. Simultaneous high-level expression in liver led to the expansion of GAA-specific regulatory T-cells (Tregs) and induction of immune tolerance. Transfer of Tregs into naïve recipients prevented pathogenic allergic reactions after repeated ERT challenges. Copackaged AAV9 also attenuated preexisting humoral and cellular immune responses, which enhanced the biochemical correction. Our data present a therapeutic design in which simultaneous administration of two copackaged AAV constructs may provide therapeutic benefit and resolve immune reactions in the treatment of multisystem disorders.

AAV-mediated gene delivery in a feline model of Sandhoff disease corrects lysosomal storage in the central nervous system

Sandhoff disease (SD) is an autosomal recessive neurodegenerative disease caused by a mutation in the gene for the β-subunit of β-N-acetylhexosaminidase (Hex), resulting in the inability to catabolize ganglioside GM2 within the lysosomes. SD presents with an accumulation of GM2 and its asialo derivative GA2, primarily in the central nervous system. Myelin-enriched glycolipids, cerebrosides and sulfatides, are also decreased in SD corresponding with dysmyelination. At present, no treatment exists for SD. Previous studies have shown the therapeutic benefit of adeno-associated virus (AAV) vector-mediated gene therapy in the treatment of SD in murine and feline models. In this study, we treated presymptomatic SD cats with AAVrh8 vectors expressing feline Hex in the thalamus combined with intracerebroventricular (Thal/ICV) injections. Treated animals showed clearly improved neurologic function and quality of life, manifested in part by prevention or attenuation of whole-body tremors characteristic of untreated animals. Hex activity was significantly elevated, whereas storage of GM2 and GA2 was significantly decreased in tissue samples taken from the cortex, cerebellum, thalamus, and cervical spinal cord. Treatment also increased levels of myelin-enriched cerebrosides and sulfatides in the cortex and thalamus. This study demonstrates the therapeutic potential of AAV for feline SD and suggests a similar potential for human SD patients.

Myelin abnormalities in the optic and sciatic nerves in mice with GM1-gangliosidosis

GM1-gangliosidosis is a glycosphingolipid lysosomal storage disease involving accumulation of GM1 and its asialo form (GA1) primarily in the brain. Thin-layer chromatography and X-ray diffraction were used to analyze the lipid content/composition and the myelin structure of the optic and sciatic nerves from 7- and 10-month old β-galactosidase (β-gal) +/? and β-gal −/− mice, a model of GM1gangliosidosis. Optic nerve weight was lower in the β-gal −/− mice than in unaffected β-gal +/? mice, but no difference was seen in sciatic nerve weight. The levels of GM1 and GA1 were significantly increased in both the optic nerve and sciatic nerve of the β-gal −/− mice. The content of myelin-enriched cerebrosides, sulfatides, and plasmalogen ethanolamines was significantly lower in optic nerve of β-gal −/− mice than in β-gal +/? mice; however, cholesteryl esters were enriched in the β-gal −/− mice. No major abnormalities in these lipids were detected in the sciatic nerve of the β-gal −/− mice. The abnormalities in GM1 and myelin lipids in optic nerve of β-gal −/− mice correlated with a reduction in the relative amount of myelin and periodicity in fresh nerve. By contrast, the relative amount of myelin and periodicity in the sciatic nerves from control and β-gal −/− mice were indistinguishable, suggesting minimal pathological involvement in sciatic nerve. Our results indicate that the greater neurochemical pathology observed in the optic nerve than in the sciatic nerve of β-gal −/− mice is likely due to the greater glycolipid storage in optic nerve.

Accuracy of direct magnetic resonance imaging-guided placement of drug infusion cannulae

An intraoperative MRI (iMRI)-compatible system has been developed for direct placement of convection-enhanced delivery (CED) cannulae using real-time imaging. To establish the precision and feasibility of this technology, the authors analyzed findings in patients who underwent direct iMRI CED cannula placement. Three consecutive patients underwent iMRI-guided placement of CED infusion cannulae (6 cannulae) for treatment of diffuse intrinsic brainstem glioma (2 patients) or Parkinson's disease (1 patient). Convective infusion cannulae were guided to the target using the ClearPoint iMRI-based navigation platform (MRI Interventions, Inc.). Placement accuracy was analyzed. Real-time iMRI during infusion cannula insertion allowed for monitoring of trajectory accuracy during placement. During cannula insertion, no reinsertions or changes due to errors in targeting were necessary. The mean radial error was 1.0 ± 0.5 mm (± SD). There was no correlation between the total length of the planned trajectory and the radial error (Pearson's coefficient: -0.40; p = 0.5). The mean anteroposterior and lateral errors were 0.9 ± 0.5 and 0.3 ± 0.2 mm, respectively. The mean in-plane distance error was 1.0 ± 0.4 mm. The mean tip error (scalar distance between the planned target and actual tip) was 1.9 ± 0.9 mm. There was no correlation between the length of the planned trajectory and any of the measured errors. No complications were associated with cannula placement. Real-time iMRI-based targeting and monitoring of infusion cannula placement is a safe, effective, and accurate technique that should enable more selective perfusion of brain regions.

Long-term correction of Sandhoff disease following intravenous delivery of rAAV9 to mouse neonates

G(M2) gangliosidoses are severe neurodegenerative disorders resulting from a deficiency in β-hexosaminidase A activity and lacking effective therapies. Using a Sandhoff disease (SD) mouse model (Hexb(-/-)) of the G(M2) gangliosidoses, we tested the potential of systemically delivered adeno-associated virus 9 (AAV9) expressing Hexb cDNA to correct the neurological phenotype. Neonatal or adult SD and normal mice were intravenously injected with AAV9-HexB or -LacZ and monitored for serum β-hexosaminidase activity, motor function, and survival. Brain G(M2) ganglioside, β-hexosaminidase activity, and inflammation were assessed at experimental week 43, or an earlier humane end point. SD mice injected with AAV9-LacZ died by 17 weeks of age, whereas all neonatal AAV9-HexB-treated SD mice survived until 43 weeks (P < 0.0001) with only three exhibiting neurological dysfunction. SD mice treated as adults with AAV9-HexB died between 17 and 35 weeks. Neonatal SD-HexB-treated mice had a significant increase in brain β-hexosaminidase activity, and a reduction in G(M2) ganglioside storage and neuroinflammation compared to adult SD-HexB- and SD-LacZ-treated groups. However, at 43 weeks, 8 of 10 neonatal-HexB injected control and SD mice exhibited liver or lung tumors. This study demonstrates the potential for long-term correction of SD and other G(M2) gangliosidoses through early rAAV9 based systemic gene therapy.

Widespread correction of central nervous system disease after intracranial gene therapy in a feline model of Sandhoff disease

Sandhoff disease (SD) is caused by deficiency of N-acetyl-β-hexosaminidase (Hex) resulting in pathological accumulation of GM2 ganglioside in lysosomes of the central nervous system (CNS) and progressive neurodegeneration. Currently, there is no treatment for SD, which often results in death by the age of five years. Adeno-associated virus (AAV) gene therapy achieved global CNS Hex restoration and widespread normalization of storage in the SD mouse model. Using a similar treatment approach, we sought to translate the outcome in mice to the feline SD model as an important step toward human clinical trials. Sixteen weeks after four intracranial injections of AAVrh8 vectors, Hex activity was restored to above normal levels throughout the entire CNS and in cerebrospinal fluid, despite a humoral immune response to the vector. In accordance with significant normalization of a secondary lysosomal biomarker, ganglioside storage was substantially improved, but not completely cleared. At the study endpoint, 5-month-old AAV-treated SD cats had preserved neurological function and gait compared with untreated animals (humane endpoint, 4.4±0.6 months) demonstrating clinical benefit from AAV treatment. Translation of widespread biochemical disease correction from the mouse to the feline SD model provides optimism for treatment of the larger human CNS with minimal modification of approach.

Biomarkers for disease progression and AAV therapeutic efficacy in feline Sandhoff disease

The GM2 gangliosidoses, Tay-Sachs disease (TSD) and Sandhoff disease (SD), are progressive neurodegenerative disorders that are caused by a mutation in the enzyme β-N-acetylhexosaminidase (Hex). Due to the recent emergence of novel experimental treatments, biomarker development has become particularly relevant in GM2 gangliosidosis as an objective means to measure therapeutic efficacy. Here we describe blood, cerebrospinal fluid (CSF), magnetic resonance imaging (MRI), and electrodiagnostic methods for evaluating disease progression in the feline SD model and application of these approaches to assess AAV-mediated gene therapy. SD cats were treated by intracranial injections of the thalami combined with either the deep cerebellar nuclei or a single lateral ventricle using AAVrh8 vectors encoding feline Hex. Significantly altered in untreated SD cats, blood and CSF based biomarkers were largely normalized after AAV gene therapy. Also reduced after treatment were expansion of the lysosomal compartment in peripheral blood mononuclear cells and elevated activity of secondary lysosomal enzymes. MRI changes characteristic of the gangliosidoses were documented in SD cats and normalized after AAV gene therapy. The minimally invasive biomarkers reported herein should be useful to assess disease progression of untreated SD patients and those in future clinical trials.

Copackaging of multiple adeno-associated viral vectors in a single production step

Limiting factors in large preclinical and clinical studies utilizing adeno-associated virus (AAV) for gene therapy are focused on the restrictive packaging capacity, the overall yields, and the versatility of the production methods for single AAV vector production. Furthermore, applications where multiple vectors are needed to provide long expression cassettes, whether because of long cDNA sequences or the need of different regulatory elements, require that each vector be packaged and characterized separately, directly affecting labor and cost associated with such manufacturing strategies. To overcome these limitations, we propose a novel method of vector production that allows for the packaging of multiple expression cassettes in a single transfection step. Here we combined two expression cassettes in predetermined ratios before transfection and empirically demonstrate that the output vector recapitulates the predicted ratios. Titration by quantitative polymerase chain reaction of AAV vector genome copies using shared or unique genetic elements allowed for delineation of the individual vector contribution to the total preparation that showed the predicted differential packaging outcomes. By copackaging green fluorescent protein (GFP) and mCherry constructs, we demonstrate that both vector genome and infectious titers reiterated the ratios utilized to produce the constructs by transfection. Copackaged therapeutic constructs that only differ in transcriptional elements produced a heterogeneous vector population of both constructs in the predefined ratios. This study shows feasibility and reproducibility of a method that allows for two constructs, differing in either transgene or transcription elements, to be efficiently copackaged and characterized simultaneously, reducing cost of manufacturing and release testing.

Gene therapy for the nervous system: challenges and new strategies

Current clinical treatments for central nervous system (CNS) diseases, such as Parkinson's disease and glioblastoma do not halt disease progression and have significant treatment morbidities. Gene therapy has the potential to "permanently" correct disease by bringing in a normal gene to correct a mutant gene deficiency, knocking down mRNA of mutant alleles, and inducing cell-death in cancer cells using transgenes encoding apoptosis-inducing proteins. Promising results in clinical trials of eye disease (Leber's congenital aumorosis) and Parkinson's disease have shown that gene-based neurotherapeutics have great potential. The recent development of genome editing technology, such as zinc finger nucleases, TALENS, and CRISPR, has made the ultimate goal of gene correction a step closer. This review summarizes the challenges faced by gene-based neurotherapeutics and the current and recent strategies designed to overcome these barriers. We have chosen the following challenges to focus on in this review: (1) delivery vehicles (both virus and nonviral), (2) use of promoters for vector-mediated gene expression in CNS, and (3) delivery across the blood-brain barrier. The final section (4) focuses on promising pre-clinical/clinical studies of neurotherapeutics.

Sustained normalization of neurological disease after intracranial gene therapy in a feline model

Progressive debilitating neurological defects characterize feline G(M1) gangliosidosis, a lysosomal storage disease caused by deficiency of lysosomal β-galactosidase. No effective therapy exists for affected children, who often die before age 5 years. An adeno-associated viral vector carrying the therapeutic gene was injected bilaterally into two brain targets (thalamus and deep cerebellar nuclei) of a feline model of G(M1) gangliosidosis. Gene therapy normalized β-galactosidase activity and storage throughout the brain and spinal cord. The mean survival of 12 treated G(M1) animals was >38 months, compared to 8 months for untreated animals. Seven of the eight treated animals remaining alive demonstrated normalization of disease, with abrogation of many symptoms including gait deficits and postural imbalance. Sustained correction of the G(M1) gangliosidosis disease phenotype after limited intracranial targeting by gene therapy in a large animal model suggests that this approach may be useful for treating the human version of this lysosomal storage disorder.

Reversibility of neuropathology in Tay-Sachs-related diseases

The GM2 gangliosidoses are progressive neurodegenerative disorders due to defects in the lysosomal β-N-acetylhexosaminidase system. Accumulation of β-hexosaminidases A and B substrates is presumed to cause this fatal condition. An authentic mouse model of Sandhoff disease (SD) with pathological characteristics resembling those noted in infantile GM2 gangliosidosis has been described. We have shown that expression of β-hexosaminidase by intracranial delivery of recombinant adeno-associated viral vectors to young adult SD mice can prevent many features of the disease and extends lifespan. To investigate the nature of the neurological injury in GM2 gangliosidosis and the extent of its reversibility, we have examined the evolution of disease in the SD mouse; we have moreover explored the effects of gene transfer delivered at key times during the course of the illness. Here we report greatly increased survival only when the therapeutic genes are expressed either before the disease is apparent or during its early manifestations. However, irrespective of when treatment was administered, widespread and abundant expression of β-hexosaminidase with consequent clearance of glycoconjugates, α-synuclein and ubiquitinated proteins, and abrogation of inflammatory responses and neuronal loss was observed. We also show that defects in myelination occur in early life and cannot be easily resolved when treatment is given to the adult brain. These results indicate that there is a limited temporal opportunity in which function and survival can be improved-but regardless of resolution of the cardinal pathological features of GM2 gangliosidosis, a point is reached when functional deterioration and death cannot be prevented.

Ganglioside storage diseases: on the road to management

Although the biochemical and genetic basis for the GM1 and GM2 gangliosidoses has been known for decades, effective therapies for these diseases remain in early stages of development. The difficulty with many therapeutic strategies for treating the gangliosidoses comes largely from their inability to remove stored ganglioside once it accumulates in central nervous system (CNS) neurons and glia. This chapter highlights advances made using substrate reduction therapy and gene therapy in reducing CNS ganglioside storage. Information obtained from mouse and feline models provides insight on therapeutic strategies that could be effective in human clinical trials. In addition, information is presented showing how a calorie-restricted diet might facilitate therapeutic drug delivery to the CNS. The development of multiple new therapeutic approaches offers hope that longer-term management of these diseases can be achieved. It is also clear that multiple therapeutic strategies will likely be needed to provide the most complete management.

Eliglustat tartrate, a prototypic glucosylceramide synthase inhibitor

Eliglustat tartrate is a highly specific inhibitor of glucosylceramide synthase, developed for the treatment glucosylceramide-based glycosphingolipidoses. Eliglustat is in late clinical development for Gaucher disease type 1. Phase II and III clinical trials have demonstrated clinical efficacy for eliglustat as a stand-alone agent for newly diagnosed patients that are naïve to prior therapy and for patients who have been previously treated with enzyme replacement therapy. Importantly, the reported toxicity of eliglustat has been limited. Eliglustat will be submitted for the US FDA and EMA review in late 2013. Several structurally unrelated glucosylceramide synthase inhibitors have been identified and are in various stages of development, some of which cross the blood-brain barrier. Targeting glucosylceramide synthesis is also a promising approach for the treatment of type 2 diabetes mellitus, autosomal dominant polycystic kidney disease and certain cancers.

Convection-enhanced drug delivery to the brain: therapeutic potential and neuropathological considerations

Convection-enhanced delivery (CED) describes a direct method of drug delivery to the brain through intraparenchymal microcatheters. By establishing a pressure gradient at the tip of the infusion catheter in order to exploit bulk flow through the interstitial spaces of the brain, CED offers a number of advantages over conventional drug delivery methods-bypass of the blood-brain barrier, targeted distribution through large brain volumes and minimization of systemic side effects. Despite showing early promise, CED is yet to fulfill its potential as a mainstream strategy for the treatment of neurological disease. Substantial research effort has been dedicated to optimize the technology for CED and identify the parameters, which govern successful drug distribution. It seems likely that successful clinical translation of CED will depend on suitable catheter technology being used in combination with drugs with optimal physicochemical characteristics, and on neuropathological analysis in appropriate preclinical models. In this review, we consider the factors most likely to influence the success or failure of CED, and review its application to the treatment of high-grade glioma, Parkinson's disease (PD) and Alzheimer's disease (AD).

Fetal Brain-directed AAV Gene Therapy Results in Rapid, Robust, and Persistent Transduction of Mouse Choroid Plexus Epithelia

Fetal brain-directed gene addition represents an under-appreciated tool for investigating novel therapeutic approaches in animal models of central nervous system diseases with early prenatal onset. Choroid plexuses (CPs) are specialized neuroectoderm-derived structures that project into the brain's ventricles, produce cerebrospinal fluid (CSF), and regulate CSF biochemical composition. Targeting the CP may be advantageous for adeno-associated viral (AAV) gene therapy for central nervous system disorders due to its immunoprivileged location and slow rate of epithelial turnover. Yet the capacity of AAV vectors to transduce CP has not been delineated precisely. We performed intracerebroventricular injections of recombinant AAV serotype 5-green fluorescent protein (rAAV5-GFP) or rAAV9-GFP in embryonic day 15 (E15) embryos of CD-1 and C57BL/6 pregnant mice and quantified the percentages of GFP expression in CP epithelia (CPE) from lateral and fourth ventricles on E17, postnatal day 2 (P2), and P22. AAV5 was selective for CPE and showed significantly higher transduction efficiency in C57BL/6 mice (P = 0.0128). AAV9 transduced neurons and glial cells in both the mouse strains, in addition to CPE. We documented GFP expression in CPE on E17, within just 48 hours of rAAV administration to the fetal lateral ventricle, and expression by both the serotypes persisted at P130. Our results indicate that prenatal administration of rAAV5 and rAAV9 enables rapid, robust, and sustained transduction of mouse CPE and buttress the rationale for experimental therapeutics targeting the CP.Molecular Therapy-Nucleic Acids (2013) 2, e101; doi:10.1038/mtna.2013.27; published online 25 June 2013.

Therapeutic response in feline sandhoff disease despite immunity to intracranial gene therapy

Salutary responses to adeno-associated viral (AAV) gene therapy have been reported in the mouse model of Sandhoff disease (SD), a neurodegenerative lysosomal storage disease caused by deficiency of β-N-acetylhexosaminidase (Hex). While untreated mice reach the humane endpoint by 4.1 months of age, mice treated by a single intracranial injection of vectors expressing human hexosaminidase may live a normal life span of 2 years. When treated with the same therapeutic vectors used in mice, two cats with SD lived to 7.0 and 8.2 months of age, compared with an untreated life span of 4.5 ± 0.5 months (n = 11). Because a pronounced humoral immune response to both the AAV1 vectors and human hexosaminidase was documented, feline cDNAs for the hexosaminidase α- and β-subunits were cloned into AAVrh8 vectors. Cats treated with vectors expressing feline hexosaminidase produced enzymatic activity >75-fold normal at the brain injection site with little evidence of an immune infiltrate. Affected cats treated with feline-specific vectors by bilateral injection of the thalamus lived to 10.4 ± 3.7 months of age (n = 3), or 2.3 times as long as untreated cats. These studies support the therapeutic potential of AAV vectors for SD and underscore the importance of species-specific cDNAs for translational research.

Characterization of inducible models of Tay-Sachs and related disease

Tay-Sachs and Sandhoff diseases are lethal inborn errors of acid β-N-acetylhexosaminidase activity, characterized by lysosomal storage of GM2 ganglioside and related glycoconjugates in the nervous system. The molecular events that lead to irreversible neuronal injury accompanied by gliosis are unknown; but gene transfer, when undertaken before neurological signs are manifest, effectively rescues the acute neurodegenerative illness in Hexb−/− (Sandhoff) mice that lack β-hexosaminidases A and B. To define determinants of therapeutic efficacy and establish a dynamic experimental platform to systematically investigate cellular pathogenesis of GM2 gangliosidosis, we generated two inducible experimental models. Reversible transgenic expression of β-hexosaminidase directed by two promoters, mouse Hexb and human Synapsin 1 promoters, permitted progression of GM2 gangliosidosis in Sandhoff mice to be modified at pre-defined ages. A single auto-regulatory tetracycline-sensitive expression cassette controlled expression of transgenic Hexb in the brain of Hexb−/− mice and provided long-term rescue from the acute neuronopathic disorder, as well as the accompanying pathological storage of glycoconjugates and gliosis in most parts of the brain. Ultimately, late-onset brainstem and ventral spinal cord pathology occurred and was associated with increased tone in the limbs. Silencing transgenic Hexb expression in five-week-old mice induced stereotypic signs and progression of Sandhoff disease, including tremor, bradykinesia, and hind-limb paralysis. As in germline Hexb−/− mice, these neurodegenerative manifestations advanced rapidly, indicating that the pathogenesis and progression of GM2 gangliosidosis is not influenced by developmental events in the maturing nervous system.

Sandhoff and Tay-Sachs disease are devastating neurological diseases associated with developmental regression, blindness, seizures, and death in infants and young children. These disorders are caused by mutations in β-hexosaminidase genes, which result in neuronal accumulation of certain lipids, glycosphingolipids, inside the lysosomes of neurons. It is not yet known how accumulation of lipids affects neuronal function, and although promising treatments such as gene therapy are in development, currently none has been clinically approved. We aimed to develop genetic models that allow manipulation of β-hexosaminidase expression over time. Two inducible strains of mice were created in which acute Sandhoff disease could be “turned on” by the addition of doxycycline in the diet. Once induced in the adult mouse, the disease progressed relentlessly and was apparently independent of the rapid developmental processes that occur in the fetal and neonatal brain, resembling disease course in the germline Hexb−/− mouse. These transgenic inducible strains of Sandhoff disease mice provide a dynamic platform with which to explore the pathophysiological sequelae immediately after loss of neuronal lysosomal β-hexosaminidase activity.