Molecular consequences of the pathogenic mutation in feline GM1 gangliosidosis

Tracking footprints of artificial and natural selection signatures in breeding and non-breeding cats

Stray non-breeding cats (stray) represent the largest heterogeneous cat population subject to natural selection, while populations of the Siamese (SIAM) and Oriental Shorthair (OSH) breeds developed through intensive artificial selection for aesthetic traits. Runs of homozygosity (ROH) and demographic measures are useful tools to discover chromosomal regions of recent selection and to characterize genetic diversity in domestic cat populations. To achieve this, we genotyped 150 stray and 26 household non-breeding cats (household) on the Illumina feline 63 K SNP BeadChip and compared them to SIAM and OSH. The 50% decay value of squared correlation coefficients (r²) in stray (0.23), household (0.25), OSH (0.24) and SIAM (0.25) corresponded to a mean marker distance of 1.12 Kb, 4.55 Kb, 62.50 Kb and 175.07 Kb, respectively. The effective population size (N_e) decreased in the current generation to 55 in stray, 11 in household, 9 in OSH and 7 in SIAM. In the recent generation, the increase in inbreeding per generation (ΔF) reached its maximum values of 0.0090, 0.0443, 0.0561 and 0.0710 in stray, household, OSH and SIAM, respectively. The genomic inbreeding coefficient (F_ROH) based on ROH was calculated for three length categories. The F_ROH was between 0.014 (F_ROH60) and 0.020 (F_ROH5) for stray, between 0.018 (F_ROH60) and 0.024 (F_ROH5) for household, between 0.048 (F_ROH60) and 0.069 (F_ROH5) for OSH and between 0.053 (F_ROH60) and 0.073 (F_ROH5) for SIAM. We identified nine unique selective regions for stray through genome-wide analyses for regions with reduced heterozygosity based on F_ST statistics. Genes in these regions have previously been associated with reproduction (BUB1B), motor/neurological behavior (GPHN, GABRB3), cold-induced thermogenesis (DIO2, TSHR), immune system development (TSHR), viral carcinogenesis (GTF2A1), host immune response against bacteria, viruses, chemoattractant and cancer cells (PLCB2, BAHD1, TIGAR), and lifespan and aging (BUB1B, FGF23). In addition, we identified twelve unique selective regions for OSH containing candidate genes for a wide range of coat colors and patterns (ADAMTS20, KITLG, TYR, TYRO3-a MITF regulator, GPNMB, FGF7, RAB38) as well as congenital heart defects (PDE4D, PKP2) and gastrointestinal disorders (NLGN1, ALDH1B1). Genes in stray that represent unique selective events indicate, at least in part, natural selection for environmental adaptation and resistance to infectious disease, and should be the subject of future research. Stray cats represent an important genetic resource and have the potential to become a research model for disease resistance and longevity, which is why we recommend preserving semen before neutering.

Intravenous delivery of adeno-associated viral gene therapy in feline GM1 gangliosidosis

GM1 gangliosidosis is a fatal neurodegenerative disease caused by a deficiency of lysosomal β-galactosidase. In its most severe form, GM1 gangliosidosis causes death by 4 years of age, and no effective treatments exist. Previous work has shown that injection of the brain parenchyma with an adeno-associated viral vector provides pronounced therapeutic benefit in a feline GM1 model. To develop a less invasive treatment for the brain and increase systemic biodistribution, intravenous injection of AAV9 was evaluated. AAV9 expressing feline β-galactosidase was intravenously administered at 1.5x1013 vector genomes/kilogram body weight to six GM1 cats at approximately 1 month of age. The animals were divided into two cohorts: 1) a long-term group, which was followed to humane endpoint, and 2) a short-term group, which was analyzed 16-weeks post treatment. Clinical assessments included neurological exams, cerebrospinal fluid and urine biomarkers, and 7-Telsa magnetic resonance imaging and spectroscopy. Postmortem analysis included β-galactosidase and virus distribution, histological analysis, and ganglioside content. Untreated GM1 animals survived 8.0 ± 0.6 months while intravenous treatment increased survival to an average of 3.5 years (n = 2) with substantial improvements in quality of life and neurologic function. Neurological abnormalities, which in untreated animals progress to the inability to stand and debilitating neurological disease by 8 months of age, were mild in all treated animals. Cerebrospinal fluid biomarkers were normalized, indicating decreased central nervous system cell damage in the treated animals. Urinary glycosaminoglycans decreased to normal levels in the long-term cohort. Magnetic resonance imaging and spectroscopy showed partial preservation of the brain in treated animals, which was supported by postmortem histological evaluation. β-galactosidase activity was increased throughout the central nervous system, reaching carrier levels in much of the cerebrum and normal levels in the cerebellum, spinal cord and cerebrospinal fluid. Ganglioside accumulation was significantly reduced by treatment. Peripheral tissues such as heart, skeletal muscle, and sciatic nerve also had normal β-galactosidase activity in treated GM1 cats. GM1 histopathology was largely corrected with treatment. There was no evidence of tumorigenesis or toxicity. Restoration of β-galactosidase activity in the central nervous system and peripheral organs by intravenous gene therapy led to profound increases in lifespan and quality of life in GM1 cats. This data supports the promise of intravenous gene therapy as a safe, effective treatment for GM1 gangliosidosis.

A GM1 gangliosidosis mutant mouse model exhibits activated microglia and disturbed autophagy

GM1 gangliosidosis is a rare lysosomal storage disease caused by a deficiency of β-galactosidase due to mutations in the GLB1 gene. We established a C57BL/6 mouse model with Glb1G455R mutation using CRISPR/Cas9 genome editing. The β-galactosidase enzyme activity of Glb1G455R mice measured by fluorometric assay was negligible throughout the whole body. Mutant mice displayed no marked phenotype at eight weeks. After 16 weeks, GM1 ganglioside accumulation in the brain of mutant mice was observed by immunohistochemical staining. Meanwhile, a declining performance in behavioral tests was observed among mutant mice from 16 to 32 weeks. As the disease progressed, the neurological symptoms of mutant mice worsened, and they then succumbed to the disease by 47 weeks of age. We also observed microglia activation and proliferation in the cerebral cortex of mutant mice at 16 and 32 weeks. In these activated microglia, the level of autophagy regulator LC3 was up-regulated but the mRNA level of LC3 was normal. In conclusion, we developed a novel murine model that mimicked the chronic phenotype of human GM1. This Glb1G455R murine model is a practical in vivo model for studying the pathogenesis of GM1 gangliosidosis and exploring potential therapies.

GM1 Gangliosidosis: Mechanisms and Management

The lysosomal storage disorder, GM1 gangliosidosis (GM1), is a neurodegenerative condition resulting from deficiency of the enzyme β-galactosidase (β-gal). Mutation of the GLB1 gene, which codes for β-gal, prevents cleavage of the terminal β-1,4-linked galactose residue from GM1 ganglioside. Subsequent accumulation of GM1 ganglioside and other substrates in the lysosome impairs cell physiology and precipitates dysfunction of the nervous system. Beyond palliative and supportive care, no FDA-approved treatments exist for GM1 patients. Researchers are critically evaluating the efficacy of substrate reduction therapy, pharmacological chaperones, enzyme replacement therapy, stem cell transplantation, and gene therapy for GM1. A Phase I/II clinical trial for GM1 children is ongoing to evaluate the safety and efficacy of adeno-associated virus-mediated GLB1 delivery by intravenous injection, providing patients and families with hope for the future.

Intermittent enzyme replacement therapy with recombinant human β-galactosidase prevents neuraminidase 1 deficiency

Mutations in the galactosidase β 1 (GLB1) gene cause lysosomal β-galactosidase (β-Gal) deficiency and clinical onset of the neurodegenerative lysosomal storage disease, GM1 gangliosidosis. β-Gal and neuraminidase 1 (NEU1) form a multienzyme complex in lysosomes along with the molecular chaperone, protective protein cathepsin A (PPCA). NEU1 is deficient in the neurodegenerative lysosomal storage disease sialidosis, and its targeting to and stability in lysosomes strictly depend on PPCA. In contrast, β-Gal only partially depends on PPCA, prompting us to investigate the role that β-Gal plays in the multienzyme complex. Here, we demonstrate that β-Gal negatively regulates NEU1 levels in lysosomes by competitively displacing this labile sialidase from PPCA. Chronic cellular uptake of purified recombinant human β-Gal (rhβ-Gal) or chronic lentiviral-mediated GLB1 overexpression in GM1 gangliosidosis patient fibroblasts coincides with profound secondary NEU1 deficiency. A regimen of intermittent enzyme replacement therapy dosing with rhβ-Gal, followed by enzyme withdrawal, is sufficient to augment β-Gal activity levels in GM1 gangliosidosis patient fibroblasts without promoting NEU1 deficiency. In the absence of β-Gal, NEU1 levels are elevated in the GM1 gangliosidosis mouse brain, which are restored to normal levels following weekly intracerebroventricular dosing with rhβ-Gal. Collectively, our results highlight the need to carefully titrate the dose and dosing frequency of β-Gal augmentation therapy for GM1 gangliosidosis. They further suggest that intermittent intracerebroventricular enzyme replacement therapy dosing with rhβ-Gal is a tunable approach that can safely augment β-Gal levels while maintaining NEU1 at physiological levels in the GM1 gangliosidosis brain.

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.

Intracerebroventricular enzyme replacement therapy with β-galactosidase reverses brain pathologies due to GM1 gangliosidosis in mice

Autosomal recessive mutations in the galactosidase β1 (GLB1) gene cause lysosomal β-gal deficiency, resulting in accumulation of galactose-containing substrates and onset of the progressive and fatal neurodegenerative lysosomal storage disease, GM1 gangliosidosis. Here, an enzyme replacement therapy (ERT) approach in fibroblasts from GM1 gangliosidosis patients with recombinant human β-gal (rhβ-gal) produced in Chinese hamster ovary cells enabled direct and precise rhβ-gal delivery to acidified lysosomes. A single, low dose (3 nm) of rhβ-gal was sufficient for normalizing β-gal activity and mediating substrate clearance for several weeks. We found that rhβ-gal uptake by the fibroblasts is dose-dependent and saturable and can be competitively inhibited by mannose 6-phosphate, suggesting cation-independent, mannose 6-phosphate receptor–mediated endocytosis from the cell surface. A single intracerebroventricularly (ICV) administered dose of rhβ-gal (100 μg) resulted in broad bilateral biodistribution of rhβ-gal to critical regions of pathology in a mouse model of GM1 gangliosidosis. Weekly ICV dosing of rhβ-gal for 8 weeks substantially reduced brain levels of ganglioside and oligosaccharide substrates and reversed well-established secondary neuropathology. Of note, unlike with the ERT approach, chronic lentivirus-mediated GLB1 overexpression in the GM1 gangliosidosis patient fibroblasts caused accumulation of a prelysosomal pool of β-gal, resulting in activation of the unfolded protein response and endoplasmic reticulum stress. This outcome was unsurprising in light of our in vitro biophysical findings for rhβ-gal, which include pH-dependent and concentration-dependent stability and dynamic self-association. Collectively, our results highlight that ICV-ERT is an effective therapeutic intervention for managing GM1 gangliosidosis potentially more safely than with gene therapy approaches.

Polyethylene glycol-b-poly(lactic acid) polymersomes as vehicles for enzyme replacement therapy

AIM

Polymersomes are created to deliver an enzyme-based therapy to the brain in lysosomal storage disease patients.

MATERIALS & METHODS

Polymersomes are formed via the injection method using poly(ethylene glycol)-b-poly(lactic acid) (PEGPLA) and bound to apolipoprotein E, to create a brain-targeted delivery vehicle.

RESULTS

Polymersomes have a smallest average diameter of 145 ± 21 nm and encapsulate β-galactosidase at 72.0 ± 12.2% efficiency. PEGPLA polymersomes demonstrate limited release at physiologic pH (7.4), with a burst release at the acidic pH (4.8) of the lysosome. PEGPLA polymersomes facilitate delivery of active β-galactosidase to an in vitro model of GM1 gangliosidosis.

CONCLUSION

The foundation has been laid for testing of PEGPLA polymersomes to deliver enzymatic treatments to the brain in lysosomal storage disorders for the first time.

The practical use of genome sequencing data in the management of a feline colony pedigree

Background

A higher prevalence of inherited disorders among companion animals are often rooted in their historical restricted artificial selection for a variety of observed phenotypes that eventually decreased genetic diversity. Cats have been afflicted with many inherited diseases due to domestication and intense breed selection. Advances in sequencing technology have generated a more comprehensive way to access genetic information from an individual, allowing identification of putative disease-causing variants and in practice a means to avoid their spread and thus better pedigree management. We examine variants in three domestic shorthair cats and then calculated overall genetic diversity to extrapolate the benefits of this data for breeding programs within a feline colony.

Results

We generated whole genome sequence (WGS) data for three related cats that belong to a large feline pedigree colony. Genome-wide coverage ranged from 27-32X, from which we identified 18 million variants in total. Previously known disease-causing variants were screened in our cats, but none carry any of these known disease alleles. Loss of function (LoF) variants, that are in genes associated with a detrimental phenotype in human or mice were chosen for further evaluation on the comparative impact inferred. A set of LoF variants were observed in four genes, each with predicted detrimental phenotypes as a result. However, none of our cats displayed the expected disease phenotypes. Inbreeding coefficients and runs of homozygosity were also evaluated as a measure of genetic diversity. We find low inbreeding coefficients and total runs of homozygosity, thus suggesting pedigree management of genetic relatedness is acceptable.

Conclusions

The use of WGS of a small sampling among a large feline colony has enabled us to identify possible disease-causing variants, their genotype state and measure pedigree management of genetic diversity. We contend a limited but strategic sampling of feline colony individuals using WGS can inform veterinarians of future health anomalies and guide breeding practices to ensure healthy genetic diversity.

Electronic supplementary material

The online version of this article (doi:10.1186/s12917-017-1144-y) contains supplementary material, which is available to authorized users.

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.

GM1 gangliosidosis in a Japanese domestic cat: a new variant identified in Hokkaido, Japan

A male Japanese domestic cat with retarded growth in Hokkaido, Japan, showed progressive motor dysfunction, such as ataxia starting at 3 months of age and tremors, visual disorder and seizure after 4 months of age. Finally, the cat died of neurological deterioration at 9 months of age. Approximately half of the peripheral blood lymphocytes had multiple abnormal vacuoles. Magnetic resonance imaging showed bisymmetrical hyperintensity in the white matter of the parietal and occipital lobes in the forebrain on T2-weighted and fluid-attenuated inversion recovery images, and mild encephalatrophy of the olfactory bulbs and temporal lobes. The activity of lysosomal acid β-galactosidase in leukocytes was negligible, resulting in the biochemical diagnosis of GM1 gangliosidosis. Histologically, swollen neurons characterized by accumulation of pale, slightly granular cytoplasmic materials were observed throughout the central nervous system. Dysmyelination or demyelination and gemistocytic astrocytosis were observed in the white matter. Ultrastructually, membranous cytoplasmic bodies were detected in the lysosomes of neurons. However, genetic analysis did not identify the c.1448G>C mutation, which is the single known mutation of feline GM1 gangliosidosis, suggesting that the cat was affected with a new variant of the feline disease.

MRI/MRS as a surrogate marker for clinical progression in GM1 gangliosidosis

Background GM1 gangliosidosis is a lysosomal storage disorder caused by mutations in GLB1, encoding β-galactosidase. The range of severity is from type I infantile disease, lethal in early childhood, to type III adult onset, resulting in gradually progressive neurological symptoms in adulthood. The intermediate group of patients has been recently classified as having type II late infantile subtype with onset of symptoms at one to three years of age or type II juvenile subtype with symptom onset at 2-10 years. To characterize disease severity and progression, six Late infantile and nine juvenile patients were evaluated using magnetic resonance imaging (MRI), and MR spectroscopy (MRS). Since difficulties with ambulation (gross motor function) and speech (expressive language) are often the first reported symptoms in type II GM1, patients were also scored in these domains. Deterioration of expressive language and ambulation was more rapid in the late infantile patients. Fourteen MRI scans in six Late infantile patients identified progressive atrophy in the cerebrum and cerebellum. Twenty-six MRI scans in nine juvenile patients revealed greater variability in extent and progression of atrophy. Quantitative MRS demonstrated a deficit of N-acetylaspartate in both the late infantile and juvenile patients with greater in the late infantile patients. This correlates with clinical measures of ambulation and expressive language. The two subtypes of type II GM1 gangliosidosis have different clinical trajectories. MRI scoring, quantitative MRS and brain volume correlate with clinical disease progression and may serve as important minimally-invasive outcome measures for clinical trials.

Bis(monoacylglycero)phosphate: a secondary storage lipid in the gangliosidoses

Bis(monoacylglycero)phosphate (BMP) is a negatively charged glycerophospholipid with an unusual sn-1;sn-1' structural configuration. BMP is primarily enriched in endosomal/lysosomal membranes. BMP is thought to play a role in glycosphingolipid degradation and cholesterol transport. Elevated BMP levels have been found in many lysosomal storage diseases (LSDs), suggesting an association with lysosomal storage material. The gangliosidoses are a group of neurodegenerative LSDs involving the accumulation of either GM1 or GM2 gangliosides resulting from inherited deficiencies in β-galactosidase or β-hexosaminidase, respectively. Little information is available on BMP levels in gangliosidosis brain tissue. Our results showed that the content of BMP in brain was significantly greater in humans and in animals (mice, cats, American black bears) with either GM1 or GM2 ganglioside storage diseases, than in brains of normal subjects. The storage of BMP and ganglioside GM2 in brain were reduced similarly following adeno-associated viral-mediated gene therapy in Sandhoff disease mice. We also found that C22:6, C18:0, and C18:1 were the predominant BMP fatty acid species in gangliosidosis brains. The results show that BMP accumulates as a secondary storage material in the brain of a broad range of mammals with gangliosidoses.

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.

Polycystic kidneys and GM2 gangliosidosis-like disease in neonatal springboks (Antidorcas marsupialis)

Clinical, gross, histopathologic, electron microscopic findings and enzymatic analysis of 4 captive, juvenile springboks (Antidorcas marsupialis) showing both polycystic kidneys and a storage disease are described. Springbok offspring (4 of 34; 12%) were affected by either one or both disorders in a German zoo within a period of 5 years (2008-2013). Macroscopic findings included bilaterally severely enlarged kidneys displaying numerous cysts in 4 animals and superior brachygnathism in 2 animals. Histopathologically, kidneys of 4 animals displayed cystic dilation of the renal tubules. In addition, abundant cytoplasmic vacuoles with a diameter ranging from 2 to 10 μm in neurons of the central and peripheral nervous system, hepatocytes, thyroid follicular epithelial cells, pancreatic islets of Langerhans and renal tubular cells were found in 2 springbok neonates indicative of an additional storage disease. Ultrastructurally, round electron-lucent vacuoles, up to 4 μm in diameter, were present in neurons. Enzymatic analysis of liver and kidney tissue of 1 affected springbok revealed a reduced activity of total hexosaminidase (Hex) with relatively increased HexA activity at the same level of total Hex, suggesting a hexosaminidase defect. Pedigree analysis suggested a monogenic autosomal recessive inheritance for both diseases. In summary, related springboks showed 2 different changes resembling both polycystic kidney and a GM2 gangliosidosis similar to the human Sandhoff disease. Whether the simultaneous occurrence of these 2 entities represents an incidental finding or has a genetic link needs to be investigated in future studies.

DNA testing in neurologic diseases

DNA testing is available for a growing number of hereditary diseases in neurology and other specialties. In addition to guiding breeding decisions, DNA tests are important tools in the diagnosis of diseases, particularly in conditions for which clinical signs are relatively nonspecific. DNA testing also can provide valuable insight into the risk of hereditary disease when decisions about treating comorbidities are being made. Advances in technology and bioinformatics will make broad screening for potential disease-causing mutations available soon. As DNA tests come into more common use, it is critical that clinicians understand the proper application and interpretation of these test results.

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.

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.

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.

Cats: a gold mine for ophthalmology

Over 200 hereditary diseases have been identified and reported in the cat, several of which affect the eye, with homology to human hereditary disease. Compared with traditional murine models, the cat demonstrates more features in common with humans, including many anatomic and physiologic similarities, longer life span, increased size, and a genetically more heterogeneous background. The development of genomic resources in the cat has facilitated mapping and further characterization of feline models. During recent years, the wealth of knowledge in feline ophthalmology and neurophysiology has been extended to include new diseases of significant interest for comparative ophthalmology. This makes the cat an extremely valuable animal species to utilize for further research into disease processes affecting both cats and humans. This is especially true in the advancement and study of new treatment regimens and for extended therapeutic trials. Groups of feline eye diseases reviewed in the following are lysosomal storage disorders, congenital glaucoma, and neuroretinal degenerations. Each has important implications for human ophthalmic research.

Mutation analysis of GM1 gangliosidosis in a Siamese cat from Japan in the 1960s

GM1 gangliosidosis is a fatal, progressive neurodegenerative lysosomal storage disease caused by mutations of the β-galactosidase (GLB1) gene. In feline GM1 gangliosidosis, a pathogenic mutation (c.1448G>C) of the feline GLB1 gene was identified in Siamese and Korat cats previously diagnosed with the disease in the USA and Italy, respectively. The present study demonstrated the same mutation in a Siamese cat that had been diagnosed with GM1 gangliosidosis in Japan in the 1960s. The mutation was confirmed using DNA extracted from stored paraffin-embedded brain tissue by a direct sequencing method and a polymerase chain reaction-restriction fragment length polymorphism assay. This pathogenic mutation seems to have been distributed around the world.

Identification of Bangladeshi domestic cats with GM1 gangliosidosis caused by the c.1448G>C mutation of the feline GLB1 gene: case study

GM1 gangliosidosis is a fatal, progressive neurodegenerative lysosomal storage disease caused by mutations in the β-galactosidase (GLB1) gene. In feline GM1 gangliosidosis, a pathogenic mutation (c.1448G>C) in the feline GLB1 gene was identified in Siamese cats in the United States and Japan and in Korat cats in Western countries. The present study found the homozygous c.1448G>C mutation in 2 apparent littermate native kittens in Bangladesh that were exhibiting neurological signs. This is the first identification of GM1 gangliosidosis in native domestic cats in Southeast Asia. This pathogenic mutation seems to have been present in the domestic cat population in the Siamese region and may have been transferred to pure breeds such as Siamese and Korat cats originating in this region.

Evaluation of N-nonyl-deoxygalactonojirimycin as a pharmacological chaperone for human GM1 gangliosidosis leads to identification of a feline model suitable for testing enzyme enhancement therapy

Deficiencies of lysosomal β-D-galactosidase can result in GM1 gangliosidosis, a severe neurodegenerative disease characterized by massive neuronal storage of GM1 ganglioside in the brain. Currently there are no available therapies that can even slow the progression of this disease. Enzyme enhancement therapy utilizes small molecules that can often cross the blood brain barrier, but are also often competitive inhibitors of their target enzyme. It is a promising new approach for treating diseases, often caused by missense mutations, associated with dramatically reduced levels of functionally folded enzyme. Despite a number of positive reports based on assays performed with patient cells, skepticism persists that an inhibitor-based treatment can increase mutant enzyme activity in vivo. To date no appropriate animal model, i.e., one that recapitulates a responsive human genotype and clinical phenotype, has been reported that could be used to validate enzyme enhancement therapy. In this report, we identify a novel enzyme enhancement-agent, N-nonyl-deoxygalactonojirimycin, that enhances the mutant β-galactosidase activity in the lysosomes of a number of patient cell lines containing a variety of missense mutations. We then demonstrate that treatment of cells from a previously described, naturally occurring feline model (that biochemically, clinically and molecularly closely mimics GM1 gangliosidosis in humans) with this molecule, results in a robust enhancement of their mutant lysosomal β-galactosidase activity. These data indicate that the feline model could be used to validate this therapeutic approach and determine the relationship between the disease stage at which this therapy is initiated and the maximum clinical benefits obtainable.

Unfolded protein response is not activated in the mucopolysaccharidoses but protein disulfide isomerase 5 is deregulated

Mucopolysaccharidoses (MPSs) are lysosomal storage diseases (LSDs) caused by defects in lysosomal enzymes involved in the catabolism of glycosaminoglycans. The pathogenesis of these disorders is still not completely known, although inflammation and oxidative stress appear to be common mechanisms, as in all LSDs. Recently, it was hypothesized that endoplasmic reticulum (ER) stress followed by an unfolded protein response (UPR) could be another common pathogenetic mechanism in LSDs. The aim of the present study was to verify if the UPR was elicited in the mucopolysaccharidoses and if the mechanism was MPS type- and mutation-dependent. To this end, we analyzed the UPR in vitro, in fibroblasts from patients with different types of mucopolysaccharidoses (MPS I, II, IIIA, IIIB, IVA) and in vivo, in the murine MPS IIIB model. In both cases we found no changes in mRNA levels of several UPR-related genes, such as the spliced or unspliced form of Xbp-1, Bip, Chop, Edem1, Edem2, Edem3. Therefore, we report here that the unfolded protein response of the ER is not triggered either in vitro or in vivo; accordingly, cytotoxicity assays indicated that affected fibroblasts are no more sensitive to apoptosis induction than normal cells. However, our results show that in most of the analyzed MPS fibroblasts the expression of a poorly known protein belonging to the family of the protein disulfide isomerases, namely Pdia5, is upregulated; here we discuss if its upregulation could be an early event of ER stress possibly related to the severity of the damage induced in the mutant proteins.

Characterization of the biosynthesis, processing and kinetic mechanism of action of the enzyme deficient in mucopolysaccharidosis IIIC

Heparin acetyl-CoA:alpha-glucosaminide N-acetyltransferase (N-acetyltransferase, EC 2.3.1.78) is an integral lysosomal membrane protein containing 11 transmembrane domains, encoded by the HGSNAT gene. Deficiencies of N-acetyltransferase lead to mucopolysaccharidosis IIIC. We demonstrate that contrary to a previous report, the N-acetyltransferase signal peptide is co-translationally cleaved and that this event is required for its intracellular transport to the lysosome. While we confirm that the N-acetyltransferase precursor polypeptide is processed in the lysosome into a small amino-terminal alpha- and a larger ß- chain, we further characterize this event by identifying the mature amino-terminus of each chain. We also demonstrate this processing step(s) is not, as previously reported, needed to produce a functional transferase, i.e., the precursor is active. We next optimize the biochemical assay procedure so that it remains linear as N-acetyltransferase is purified or protein-extracts containing N-acetyltransferase are diluted, by the inclusion of negatively charged lipids. We then use this assay to demonstrate that the purified single N-acetyltransferase protein is both necessary and sufficient to express transferase activity, and that N-acetyltransferase functions as a monomer. Finally, the kinetic mechanism of action of purified N-acetyltransferase was evaluated and found to be a random sequential mechanism involving the formation of a ternary complex with its two substrates; i.e., N-acetyltransferase does not operate through a ping-pong mechanism as previously reported. We confirm this conclusion by demonstrating experimentally that no acetylated enzyme intermediate is formed during the reaction.

1-Deoxy-D-galactonojirimycins with dansyl capped N-substituents as β-galactosidase inhibitors and potential probes for GM1 gangliosidosis affected cell lines

Two simple and reliably accessible intermediates, N-carboxypentyl- and N-aminohexyl-1-deoxy-D-galactonojirimycin were employed for the synthesis of a set of terminally N-dansyl substituted derivatives. Reaction of the terminal carboxylic acid of N-carboxypentyl-1-deoxy-D-galactonojirimycin with N-dansyl-1,6-diaminohexane provided the chain-extended fluorescent derivative. Employing bis(6-dansylaminohexyl)amine, the corresponding branched di-N-dansyl compound was obtained. Partially protected N-aminohexyl-1-deoxy-D-galactonojirimycin served as intermediate for two additional chain-extended fluorescent 1-deoxy-D-galactonojirimycin (1-DGJ) derivatives featuring terminal dansyl groups in the N-alkyl substituent. These new compounds are strong inhibitors of d-galactosidases and may serve as leads en route to pharmacological chaperones for GM1-gangliosidosis.

Removal of potentially confounding phenotypes from a Siamese-derived feline glaucoma breeding colony

Feline breeding colonies face genetic constraints involving founder effects. A Siamese-founded colony used to study primary congenital glaucoma displayed coat colors additional to the Siamese coat. Genes affecting pigment can exhibit pleiotropy on ocular development and function. To remove potentially confounding phenotypes from our colony, we documented the source and frequency of the Siamese allele at the gene for tyrosinase (TYR), the dilution allele at melanophilin (MLPH), and the brown allele at tyrosinase-related protein 1 (TYRP1). We used PCR-RFLP diagnostics to genotype cats in our colony for the published alleles. A commercially acquired phenotypically normal tom was the source of the dilute allele. A founding Siamese queen was the source of the brown allele. Founders also were blood-typed and screened for disease-associated alleles segregating in Siamese cats at 3 loci (ASB, GLB1, and CEP290). Siamese founders were normal at all loci except ASB, at which both animals carried the hypomorpic allele. Current stock is being managed to limit production of glaucomatous cats with brown, dilute, or Siamese phenotypes or homozygosity for the ASB hypomorphic allele. Genotyping will aid in the elimination of these alleles. The clinical effect of these phenotypes and alleles on the glaucoma phenotype is uncertain, but their elimination will remove potentially confounding effects. In conclusion, when founding a colony, stock should be selected or screened to limit potentially confounding phenotypes. When studying the immune, nervous, and visual systems, screening stock for alleles known to be associated with coat color may be warranted.

A sensitive fluorescence-based assay for monitoring GM2 ganglioside hydrolysis in live patient cells and their lysates

Enzyme enhancement therapy, utilizing small molecules as pharmacological chaperones, is an attractive approach for the treatment of lysosomal storage diseases that are associated with protein misfolding. However, pharmacological chaperones are also inhibitors of their target enzyme. Thus, a major concern with this approach is that, despite enhancing protein folding within, and intracellular transport of the functional mutant enzyme out of the endoplasmic reticulum, the chaperone will continue to inhibit the enzyme in the lysosome, preventing substrate clearance. Here we demonstrate that the in vitro hydrolysis of a fluorescent derivative of lyso-GM2 ganglioside, like natural GM2 ganglioside, is specifically carried out by the beta-hexosaminidase A isozyme, requires the GM2 activator protein as a co-factor, increases when the derivative is incorporated into anionic liposomes and follows similar Michaelis-Menten kinetics. This substrate can also be used to differentiate between lysates from normal and GM2 activator-deficient cells. When added to the growth medium of cells, the substrate is internalized and primarily incorporated into lysosomes. Utilizing adult Tay-Sachs fibroblasts that have been pre-treated with the pharmacological chaperone Pyrimethamine and subsequently loaded with this substrate, we demonstrate an increase in both the levels of mutant beta-hexosaminidase A and substrate-hydrolysis as compared to mock-treated cells.

Impact of beta-galactosidase mutations on the expression of the canine lysosomal multienzyme complex

beta-galactosidase (GLB1) forms a functional lysosomal multienzyme complex with lysosomal protective protein (PPCA) and neuraminidase 1 (NEU1) which is important for its intracellular processing and activity. Mutations in the beta-galactosidase gene cause the lysosomal storage disease G(M1)-gangliosidosis. In order to identify additional molecular changes associated with the presence of beta-galactosidase mutations, the expression of canine lysosomal multienzyme complex components in GLB1(+/+), GLB1(+/-) and GLB1(-/-) fibroblasts was investigated by quantitative RT-PCR, Western blot and enzymatic assays. Quantitative RT-PCR revealed differential regulation of total beta-galactosidase, beta-galactosidase variants and protective protein for beta-galactosidase gene (PPGB) in GLB1(+/-) and GLB1(-/-) compared to GLB1(+/+) fibroblasts. Furthermore, it was shown that PPGB levels gradually increased with the number of mutant beta-galactosidase alleles while no change in the NEU1 expression was observed. This is the first study that simultaneously examine the effect of GLB1(+/+), GLB1(+/-) and GLB1(-/-) genotypes on the expression of lysosomal multienzyme complex components. The findings reveal a possible adaptive process in GLB1 homozygous mutant and heterozygous individuals that could facilitate the design of efficient therapeutic strategies.

Identification and characterization of ambroxol as an enzyme enhancement agent for Gaucher disease

Gaucher disease (GD), the most prevalent lysosomal storage disease, is caused by a deficiency of glucocerebrosidase (GCase). The identification of small molecules acting as agents for enzyme enhancement therapy is an attractive approach for treating different forms of GD. A thermal denaturation assay utilizing wild type GCase was developed to screen a library of 1,040 Food and Drug Administration-approved drugs. Ambroxol (ABX), a drug used to treat airway mucus hypersecretion and hyaline membrane disease in newborns, was identified and found to be a pH-dependent, mixed-type inhibitor of GCase. Its inhibitory activity was maximal at neutral pH, found in the endoplasmic reticulum, and undetectable at the acidic pH of lysosomes. The pH dependence of ABX to bind and stabilize the enzyme was confirmed by monitoring the rate of hydrogen/deuterium exchange at increasing guanidine hydrochloride concentrations. ABX treatment significantly increased N370S and F213I mutant GCase activity and protein levels in GD fibroblasts. These increases were primarily confined to the lysosome-enriched fraction of treated cells, a finding confirmed by confocal immunofluorescence microscopy. Additionally, enhancement of GCase activity and a reduction in glucosylceramide storage was verified in ABX-treated GD lymphoblasts (N370S/N370S). Hydrogen/deuterium exchange mass spectrometry revealed that upon binding of ABX, amino acid segments 243-249, 310-312, and 386-400 near the active site of GCase are stabilized. Consistent with its mixed-type inhibition of GCase, modeling studies indicated that ABX interacts with both active and non-active site residues. Thus, ABX has the biochemical characteristics of a safe and effective enzyme enhancement therapy agent for the treatment of patients with the most common GD genotypes.

Neurodegenerative lysosomal storage disease in European Burmese cats with hexosaminidase beta-subunit deficiency

GM2 gangliosidosis is a fatal, progressive neuronopathic lysosomal storage disease resulting from a deficiency of beta-N-acetylhexosaminidase (EC 3.2.1.52) activity. GM2 gangliosidosis occurs with varying degrees of severity in humans and in a variety of animals, including cats. In the current research, European Burmese cats presented with clinical neurological signs and histopathological features typical of a lysosomal storage disease. Thin layer chromatography revealed substantial storage of GM2 ganglioside in brain tissue of affected cats, and assays with a synthetic fluorogenic substrate confirmed the absence of hexosaminidase activity. When the hexosaminidase beta-subunit cDNA was sequenced from affected cats, a 91 base pair deletion constituting the entirety of exon 12 was documented. Subsequent sequencing of introns 11 and 12 revealed a 15 base pair deletion at the 3' end of intron 11 that included the preferred splice acceptor site, generating two minor transcripts from cryptic splice acceptor sites in affected Burmese cats. In the cerebral cortex of affected cats, hexosaminidase beta-subunit mRNA levels were approximately 1.5 times higher than normal (P<0.001), while beta-subunit protein levels were substantially reduced on Western blots.

Diltiazem, a L-type Ca(2+) channel blocker, also acts as a pharmacological chaperone in Gaucher patient cells

Recently, inhibition of L-type Ca(2+) channels, using either Diltiazem or Verapamil, has been reported to partially restore mutant glucocerebrosidase activity in cells from patients with Gaucher disease homozygous for the N370S or L444P alleles, as well as cells from patients with two other lysosomal storage diseases. It was hypothesized that these drugs act on the endoplasmic reticulum, increasing its folding efficiency, inhibited due to altered calcium homeostasis. Several other laboratories have reported that cells carrying either the N370S or the F213I alleles are amenable to enzyme enhancement therapy with pharmacological chaperones, whereas cells homozygous for L444P respond poorly. We found that Verapamil treatment does not enhance mutant enzyme activity in any of the cell lines tested, while Diltiazem moderately increases activity in normal cells, and in N370S/N370S and F213I/L444P, but not in L444P/L444P Gaucher cells, or in either of two adult Tay-Sachs disease cell lines. Since the mode of action of pharmacological chaperones and Diltiazem are believed to be different, we examined the possibility that they could act in concert. Diltiazem co-administered with known chaperones failed to increase enzyme activities above that reached by chaperone-treatment alone in any of the patient cell lines. Thus, we re-examined the possibility that Diltiazem acts as a pharmacological chaperone. We found that, at the acidic pH of lysosomes, Diltiazem was not an inhibitor, nor did its presence increase the heat stability of glucocerebrosidase. However, at neutral pH, found in the endoplasmic reticulum, Diltiazem exhibited both of these properties. Thus Diltiazem exhibits the biochemical characteristics of a glucocerebrosidase pharmacological chaperone.