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

Clinical outcome assessments of disease burden and progression in late-onset GM2 gangliosidoses

The late-onset GM2 gangliosidoses, comprising late-onset Tay-Sachs and Sandhoff diseases, are rare, slowly progressive, neurogenetic disorders primarily characterized by neurogenic weakness, ataxia, and dysarthria. The aim of this longitudinal study was to characterize the natural history of late-onset GM2 gangliosidoses using a number of clinical outcome assessments to measure different aspects of disease burden and progression over time, including neurological, functional, and quality of life, to inform the design of future clinical interventional trials. Patients attending the United States National Tay-Sachs & Allied Diseases Family Conference between 2015 and 2019 underwent annual clinical outcome assessments. Currently, there are no clinical outcome assessments validated to assess late-onset GM2 gangliosidoses; therefore, instruments used or designed for diseases with similar features, or to address various aspects of the clinical presentations, were used. Clinical outcome assessments included the Friedreich's Ataxia Rating Scale, the 9-Hole Peg Test, and the Assessment of Intelligibility of Dysarthric Speech. Twenty-three patients participated in at least one meeting visit (late-onset Tay-Sachs, n = 19; late-onset Sandhoff, n = 4). Patients had high disease burden at baseline, and scores for the different clinical outcome assessments were generally lower than would be expected for the general population. Longitudinal analyses showed slow, but statistically significant, neurological progression as evidenced by worsening scores on the 9-Hole Peg Test (2.68%/year, 95% CI: 0.13-5.29; p = 0.04) and the Friedreich's Ataxia Rating Scale neurological examination (1.31 points/year, 95% CI: 0.26-2.35; p = 0.02). Time since diagnosis to study entry correlated with worsening scores on the 9-Hole Peg Test (r = 0.728; p < 0.001), Friedreich's Ataxia Rating Scale neurological examination (r = 0.727; p < 0.001), and Assessment of Intelligibility of Dysarthric Speech intelligibility (r = -0.654; p = 0.001). In summary, patients with late-onset GM2 gangliosidoses had high disease burden and slow disease progression. Several clinical outcome assessments suitable for clinical trials showed only small changes and standardized effect sizes (change/standard deviation of change) over 4 years. These longitudinal natural history study results illustrate the challenge of identifying responsive endpoints for clinical trials in rare, slowly progressive, neurogenerative disorders where arguably the treatment goal is to halt or decrease the rate of decline rather than improve clinical status. Furthermore, powering such a study would require a large sample size and/or a long study duration, neither of which is an attractive option for an ultra-rare disease with no available treatment. These findings support the development of potentially more sensitive late-onset GM2 gangliosidoses-specific rating instruments and/or surrogate endpoints for use in future clinical trials.

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.

Therapeutic Strategies For Tay-Sachs Disease

Tay-Sachs disease (TSD) is an autosomal recessive disease that features progressive neurodegenerative presentations. It affects one in 100,000 live births. Currently, there is no approved therapy or cure. This review summarizes multiple drug development strategies for TSD, including enzyme replacement therapy, pharmaceutical chaperone therapy, substrate reduction therapy, gene therapy, and hematopoietic stem cell replacement therapy. In vitro and in vivo systems are described to assess the efficacy of the aforementioned therapeutic strategies. Furthermore, we discuss using MALDI mass spectrometry to perform a high throughput screen of compound libraries. This enables discovery of compounds that reduce GM2 and can lead to further development of a TSD therapy.

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.

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.

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.

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.

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.

Recombinant Adeno-Associated Virus Gene Therapy in Light of Luxturna (and Zolgensma and Glybera): Where Are We, and How Did We Get Here?

The recent market approvals of recombinant adeno-associated virus (rAAV) gene therapies in Europe and the United States are landmark achievements in the history of modern science. These approvals are also anticipated to herald the emergence of a new class of therapies for monogenic disorders, which had hitherto been considered untreatable. These events can be viewed as stemming from the convergence of several important historical trends: the study of basic virology, the development of genomic technologies, the imperative for translational impact of National Institutes of Health-funded research, and the development of economic models for commercialization of rare disease therapies. In this review, these historical trends are described and the key developments that have enabled clinical rAAV gene therapies are discussed, along with an overview of the current state of the field and future directions.

A possible biomarker of neurocytolysis in infantile gangliosidoses: aspartate transaminase

Gangliosidoses (GM1 and GM2 gangliosidosis) are rare, autosomal recessive progressive neurodegenerative lysosomal storage disorders caused by defects in the degradation of glycosphingolipids. We aimed to investigate clinical, biochemical and molecular genetic spectrum of Turkish patients with infantile gangliosidoses and examined the potential role of serum aspartate transaminase levels as a biomarker. We confirmed the diagnosis of GM1 and GM2 gangliosidosis based on clinical findings with specific enzyme and/or molecular analyses. We retrospectively reviewed serum aspartate transaminase levels of patients with other biochemical parameters. Serum aspartate transaminase level was elevated in all GM1 and GM2 gangliosidosis patients in whom the test was performed, along with normal alanine transaminase. Aspartate transaminase can be a biochemical diagnostic clue for infantile gangliosidoses. It might be a simple but important biomarker for diagnosis, follow up, prognosis and monitoring of the response for the future therapies in these patients.

Evaluating hydrophobic galactonoamidines as transition state analogs for enzymatic β-galactoside hydrolysis

A spectroscopic examination of six galactonoamidines with inhibition constants and efficacy in the low nanomolar concentration range (Ki = 6-11 nM, IC50 = 12-36 nM) suggested only two of them as putative transition state analogs for the hydrolysis of β-galactosides by β-galactosidase (A. oryzae). A rationale for the experimental results was elaborated using docking and molecular dynamics studies. An analysis of the combined observations reveals several common factors of the compounds suggested as transition state analogs (TSAs): the putative TSAs have a similar orientation in the active site; show conserved positioning of the glycon; display a large number of H-bond interactions toward the catalytically active amino acid residues via their glycon; and exhibit hydrophobic interactions at the outer rim of the active site with small changes of the position and orientation of their respective aglycons.

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.

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.

AAV-mediated gene delivery attenuates neuroinflammation in feline Sandhoff disease

Sandhoff disease (SD) is a lysosomal storage disorder characterized by the absence of hydrolytic enzyme β-N-acetylhexosaminidase (Hex), which results in storage of GM2 ganglioside in neurons and unremitting neurodegeneration. Neuron loss initially affects fine motor skills, but rapidly progresses to loss of all body faculties, a vegetative state, and death by five years of age in humans. A well-established feline model of SD allows characterization of the disease in a large animal model and provides a means to test the safety and efficacy of therapeutic interventions before initiating clinical trials. In this study, we demonstrate a robust central nervous system (CNS) inflammatory response in feline SD, primarily marked by expansion and activation of the microglial cell population. Quantification of major histocompatibility complex II (MHC-II) labeling revealed significant up-regulation throughout the CNS with areas rich in white matter most severely affected. Expression of the leukocyte chemokine macrophage inflammatory protein-1 alpha (MIP-1α) was also up-regulated in the brain. SD cats were treated with intracranial delivery of adeno-associated viral (AAV) vectors expressing feline Hex, with a study endpoint 16weeks post treatment. AAV-mediated gene delivery repressed the expansion and activation of microglia and normalized MHC-II and MIP-1α levels. These data reiterate the profound inflammatory response in SD and show that neuroinflammation is abrogated after AAV-mediated restoration of enzymatic activity.

Diffusion tensor imaging analysis of the brain in the canine model of Krabbe disease

PURPOSE

The goal of this study was to compare the diffusion tensor imaging (DTI) metrics from an end-stage canine Krabbe brain evaluated by MR imaging ex vivo to those of a normal dog brain. We hypothesized that the white matter of the canine Krabbe brain would show decreased fractional anisotropy (FA) values and increased apparent diffusion coefficient (ADC) and radial diffusivity (RD) values.

METHODS

An 11-week-old Krabbe dog was euthanized after disease progression. The brain was removed and was placed in a solution of 10% formalin. MR imaging was performed and compared to the brain images of a normal dog that was similarly fixed post-mortem. Both brains were scanned using similar protocols on a 7 T small-animal MRI system. For each brain, maps of ADC, FA, and RD were calculated for 11 white-matter regions and five control gray-matter regions.

RESULTS

Large decreases in FA values, increases in ADC values, and increases in RD (consistent with demyelination) values, were seen in white matter of the Krabbe brain but not gray matter. ADC values in gray matter of the Krabbe brain were decreased by approximately 29% but increased by approximately 3.6% in white matter of the Krabbe brain. FA values in gray matter were decreased by approximately 3.3% but decreased by approximately 29% in white matter. RD values were decreased by approximately 27.2% in gray matter but increased by approximately 20% in white matter.

CONCLUSION

We found substantial abnormalities of FA, ADC, and RD values in an ex vivo canine Krabbe brain.

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.

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.

Increased glucose utilization and decreased fatty acid metabolism in myotubes from Glmp(gt/gt) mice

Glycosylated lysosomal membrane protein (GLMP) has been reported to enhance the expression from a peroxisome proliferator-activated receptor alpha (PPARα) responsive promoter, but also to be an integral lysosomal membrane protein. Using myotubes established from wild-type and Glmp(gt/gt) mice, the importance of GLMP in skeletal muscle was examined. Glmp(gt/gt) myotubes expressed a more glycolytic phenotype than wild-type myotubes. Myotubes from Glmp(gt/gt) mice metabolized glucose faster and had a larger pool of intracellular glycogen, while oleic acid uptake, storage and oxidation were significantly reduced. Gene expression analyses indicated lower expression of three PPAR-isoforms, a co-regulator of PPAR (PGC1α) and several genes important for lipid metabolism in Glmp(gt/gt) myotubes. However, ablation of GLMP did not seem to substantially impair the response to PPAR agonists. In conclusion, myotubes established from Glmp(gt/gt) mice were more glycolytic than myotubes from wild-type animals, in spite of no differences in muscle fiber types in vivo.

Pressure for drug development in lysosomal storage disorders - a quantitative analysis thirty years beyond the US orphan drug act

Background

Lysosomal storage disorders are a heterogeneous group of approximately 50 monogenically inherited orphan conditions. A defect leads to the storage of complex molecules in the lysosome, and patients develop a complex multisystemic phenotype of high morbidity often associated with premature death. More than 30 years ago the Orphan Drug Act of 1983 passed the United States legislation intended to facilitate the development of drugs for rare disorders.

We directed our efforts in assessing which lysosomal diseases had drug development pressure and what distinguished those with successful development and approvals from diseases not treated or without orphan drug designation.

Methods

Analysis of the FDA database for orphan drug designations through descriptive and comparative statistics.

Results

Between 1983 and 2013, fourteen drugs for seven conditions received FDA approval. Overall, orphan drug status was designated 70 times for 20 conditions. Approved therapies were enzyme replacement therapies (N = 10), substrate reduction therapies (N = 1), small molecules facilitating lysosomal substrate transportation (N = 3). FDA approval was significantly associated with a disease prevalence higher than 0.5/100,000 (p = 0.00742) and clinical development programs that did not require a primary neurological endpoint (p = 0.00059). Orphan drug status was designated for enzymes, modified enzymes, fusion proteins, chemical chaperones, small molecules leading to substrate reduction, or facilitating subcellular substrate transport, stem cells as well as gene therapies.

Conclusions

Drug development focused on more common diseases. Primarily neurological diseases were neglected. Small clinical trials with either somatic or biomarker endpoints were successful. Enzyme replacement therapy was the most successful technology. Four factors played a key role in successful orphan drug development or orphan drug designations: 1) prevalence of disease 2) endpoints 3) regulatory precedent, and 4) technology platform. Successful development seeded further innovation.

A review of gene therapy in canine and feline models of lysosomal storage disorders

Lysosomal storage disorders (LSDs) are inherited diseases that result from the intracellular accumulation of incompletely degraded macromolecules. The majority of LSDs affect both the peripheral and central nervous systems and are not effectively treated by enzyme replacement therapy, substrate reduction therapy, or bone marrow transplantation. Advances in adeno-associated virus and retroviral vector development over the past decade have resurged gene therapy as a promising therapeutic intervention for these monogenic diseases. Animal models of LSDs provide a necessary intermediate to optimize gene therapy protocols and assess the safety and efficacy of treatment prior to initiating human clinical trials. Numerous LSDs are naturally occurring in large animal models and closely reiterate the lesions, biochemical defect, and clinical phenotype observed in human patients, and whose lifetime is sufficiently long to assess the effect on symptoms that develop later in life. Herein, we review that gene therapy in large animal models (dogs and cats) of LSDs improved many manifestations of disease, and may be used in patients in the near future.