Two patients from Turkey with a novel variant in the GM2A gene and review of the literature.

OBJECTIVES: GM2 gangliosidosis is a rare form of inborn errors of metabolism including Tay-Sachs disease, Sandhoff disease, and GM2 activator deficiency. GM2 activator protein deficiency is an ultra-rare form of GM2 gangliosidosis. To date, 16 cases of GM2 activator protein deficiency have been reported in the literature, and among them, 11 cases were the infantile form of the disease. Here we report the first two patients from Turkey with the infantile form of the disease with a novel likely pathogenic variant. CASE PRESENTATION: A boy of eight months old presented to the metabolic department with very mild neurological deterioration, although he had achieved early developmental milestones at the appropriate time. The parents also had a daughter who had lost skills progressively before one year of age. The boy was evaluated and bilateral cherry-red spots were found with no abnormality in either metabolic screening including ß-hexosaminidase or cranial magnetic resonance imaging. A novel homozygous likely pathogenic variant in GM2A was detected in a next-generation sequence panel revealing GM2 activator protein deficiency. His sister was investigated after he was diagnosed with GM2 activator deficiency and it was found that she had the same variant as her brother. CONCLUSIONS: This case report emphasizes that in the event of normal ß-hexosaminidase activity, GM2 activator protein deficiency could be underdiagnosed, and further molecular analysis should be performed. To the best of our knowledge, this boy is one of the youngest patient diagnosed with very mild symptoms. With this novel pathogenic variant, these patients have expanded the mutation spectrum of GM2 activator protein deficiency.

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 ß-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.

Distinct progression patterns of brain disease in infantile and juvenile gangliosidoses: Volumetric quantitative MRI study.

BACKGROUND: GM1-gangliosidosis and GM2-gangliosidosis (Tay-Sachs disease and Sandhoff disease) are unrelenting heritable neurodegenerative conditions of lysosomal ganglioside accumulation. Although progressive brain atrophy is characteristic, longitudinal quantification of specific brain structures has not been systematically studied. OBJECTIVES: The goal of this longitudinal study has been to quantify and track brain MRI volume changes, including specific structure volume changes, at different times in disease progression of childhood gangliosidoses, and to explore quantitative brain MRI volumetry (qMRI) as a non-invasive marker of disease progression for future treatment trials. METHODS: Brain qMRI studies were performed in 14 patients with gangliosidoses (9 infantile, 5 juvenile) yearly. Cerebellar cortex and white matter, caudate, putamen, corpus callosum, ventricles, total brain, and intracranial volumes were measured, as well as total brain volume. Age-matched controls were available for the patients with the juvenile phenotype. RESULTS: The infantile phenotype of all gangliosidoses showed a consistent pattern of macrocephaly and rapidly increasing intracranial MRI volume with both (a) brain tissue volume (cerebral cortex and other smaller structures) and (b) ventricular volume (P<0.01 for all). In contrast to apparent enlargement of the total brain volume, and chiefly the enlarged cerebral cortex, a subset of smaller brain substructures generally decreased in size: the corpus callosum, caudate and putamen became smaller with time. The volume of cerebellar cortex also decreased in patients with infantile GM1-gangliosidosis and juvenile GM1- and GM2-gangliosidosis; however, infantile GM2-gangliosidosis cerebellar cortex was the exception, increasing in size. Elevated intracranial pressure (estimated by lumbar spinal pressure) was a common finding in infantile disease and showed continued increases as the disease progressed, yet lacked MRI signs of hydrocephalus except for increasing ventricular size. Notably, in patients with juvenile gangliosidosis, macrocephaly and elevated intracranial pressure were absent and total brain volume decreased with time compared to controls (P=0.004). CONCLUSIONS: The disease course of infantile versus juvenile gangliosidoses is clearly distinguished by the rate of brain disease progression as characterized by qMRI. Assessments by qMRI represent a robust non-invasive method for monitoring CNS changes in the clinical course of gangliosidoses and is ideally suited to monitor effects of novel CNS-directed therapies in future clinical trials.

GM2 Gangliosidosis in Shiba Inu Dogs with an In-Frame Deletion in HEXB.

Consistent with a tentative diagnosis of neuronal ceroid lipofuscinosis (NCL), autofluorescent cytoplasmic storage bodies were found in neurons from the brains of 2 related Shiba Inu dogs with a young-adult onset, progressive neurodegenerative disease. Unexpectedly, no potentially causal NCL-related variants were identified in a whole-genome sequence generated with DNA from 1 of the affected dogs. Instead, the whole-genome sequence contained a homozygous 3 base pair (bp) deletion in a coding region of HEXB. The other affected dog also was homozygous for this 3-bp deletion. Mutations in the human HEXB ortholog cause Sandhoff disease, a type of GM2 gangliosidosis. Thin-layer chromatography confirmed that GM2 ganglioside had accumulated in an affected Shiba Inu brain. Enzymatic analysis confirmed that the GM2 gangliosidosis resulted from a deficiency in the HEXB encoded protein and not from a deficiency in products from HEXA or GM2A, which are known alternative causes of GM2 gangliosidosis. We conclude that the homozygous 3-bp deletion in HEXB is the likely cause of the Shiba Inu neurodegenerative disease and that whole-genome sequencing can lead to the early identification of potentially disease-causing DNA variants thereby refocusing subsequent diagnostic analyses toward confirming or refuting candidate variant causality.

Rare Variant of GM2 Gangliosidosis through Activator-Protein Deficiency.

GM2 gangliosidosis, AB variant, is a very rare form of GM2 gangliosidosis due to a deficiency of GM2 activator protein. We report on two patients with typical clinical features suggestive of GM2 gangliosidosis, but normal results for hexosaminidase A and hexosaminidase B as well as their corresponding genes. Genetic analysis of the gene encoding the activator protein, the GM2A gene, elucidated the cause of the disease, adding a novel mutation to the spectrum of GM2 AB variant. This report points out that in typical clinical constellations with normal enzyme results, genetic diagnostic for activator protein defects should be performed.

Direct Intracranial Injection of AAVrh8 Encoding Monkey ß-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 ß-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 ß-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 ß-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/ß 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/ß, 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/ß intracranial injection among different species, despite encoding for self-proteins.

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.

Late onset GM2 gangliosidosis presenting with motor neuron disease: an autopsy case.

Adult-onset GM2 gangliosidosis is very rare and only three autopsy cases have been reported up to now. We report herein an autopsy case of adult-onset GM2 gangliosidosis. The patient developed slowly progressive motor neuron disease-like symptoms after longstanding mood disorder and cognitive dysfunction. He developed gait disturbance and weakness of lower limbs at age 52 years. Because of progressive muscle weakness and atrophy, he became bed-ridden at age 65. At age of 68, he died. His neurological findings presented slight cognitive disturbance, slight manic state, severe muscle weakness, atrophy of four limbs and no extrapyramidal signs and symptoms, and cerebellar ataxia. Neuropathologically, mild neuronal loss and abundant lipid deposits were noted in the neuronal cytoplasm throughout the nervous system, including peripheral autonomic neurons. The most outstanding findings were marked neuronal loss and distended neurons in the anterior horn of the spinal cord, which supports his clinical symptomatology of lower motor neuron disease in this case. The presence of lipofuscin, zebra bodies and membranous cytoplasmic bodies (MCB) and the increase of GM2 ganglioside by biochemistry led to diagnosis of GM2 gangliosidosis.

GM2 gangliosidosis in British Jacob sheep.

GM2 gangliosidosis (Tay-Sachs disease) was diagnosed in 6- to 8-month-old pedigree Jacob lambs from two unrelated flocks presenting clinically with progressive neurological dysfunction of 10 day's to 8 week's duration. Clinical signs included hindlimb ataxia and weakness, recumbency and proprioceptive defects. Histopathological examination of the nervous system identified extensive neuronal cytoplasmic accumulation of material that stained with periodic acid--Schiff and Luxol fast blue. Electron microscopy identified membranous cytoplasmic bodies within the nervous system. Serum biochemistry detected a marked decrease in hexosaminidase A activity in the one lamb tested, when compared with the concentration in age matched controls and genetic analysis identified a mutation in the sheep hexa allele G444R consistent with Tay-Sachs disease in Jacob sheep in North America. The identification of Tay-Sachs disease in British Jacob sheep supports previous evidence that the mutation in North American Jacob sheep originated from imported UK stock.

Mutations in B4GALNT1 (GM2 synthase) underlie a new disorder of ganglioside biosynthesis.

Glycosphingolipids are ubiquitous constituents of eukaryotic plasma membranes, and their sialylated derivatives, gangliosides, are the major class of glycoconjugates expressed by neurons. Deficiencies in their catabolic pathways give rise to a large and well-studied group of inherited disorders, the lysosomal storage diseases. Although many glycosphingolipid catabolic defects have been defined, only one proven inherited disease arising from a defect in ganglioside biosynthesis is known. This disease, because of defects in the first step of ganglioside biosynthesis (GM3 synthase), results in a severe epileptic disorder found at high frequency amongst the Old Order Amish. Here we investigated an unusual neurodegenerative phenotype, most commonly classified as a complex form of hereditary spastic paraplegia, present in families from Kuwait, Italy and the Old Order Amish. Our genetic studies identified mutations in B4GALNT1 (GM2 synthase), encoding the enzyme that catalyzes the second step in complex ganglioside biosynthesis, as the cause of this neurodegenerative phenotype. Biochemical profiling of glycosphingolipid biosynthesis confirmed a lack of GM2 in affected subjects in association with a predictable increase in levels of its precursor, GM3, a finding that will greatly facilitate diagnosis of this condition. With the description of two neurological human diseases involving defects in two sequentially acting enzymes in ganglioside biosynthesis, there is the real possibility that a previously unidentified family of ganglioside deficiency diseases exist. The study of patients and animal models of these disorders will pave the way for a greater understanding of the role gangliosides play in neuronal structure and function and provide insights into the development of effective treatment therapies.

[Molecular pathogenesis and therapeutic approach of GM2 gangliosidosis].

Tay-Sachs and Sandhoff diseases (GM2 gangliosidoses) are autosomal recessive lysosomal storage diseases caused by gene mutations in HEXA and HEXB, each encoding human lysosomal ß-hexosaminidase α-subunits and ß-subunits, respectively. In Tay-Sachs disease, excessive accumulation of GM2 ganglioside (GM2), mainly in the central nervous system, is caused by a deficiency of the HexA isozyme (αß heterodimer), resulting in progressive neurologic disorders. In Sandhoff disease, combined deficiencies of HexA and HexB (ßß homodimer) cause not only the accumulation of GM2 but also of oligosaccharides carrying terminal N-acetylhexosamine residues (GlcNAc-oligosaccharides), resulting in systemic manifestations including hepatosplenomegaly as well as neurologic symptoms. Hence there is little clinically effective treatment for these GM2 gangliosidoses. Recent studies on the molecular pathogenesis in Sandhoff disease patients and disease model mice have shown the involvement of microglial activation and chemokine induction in neuroinflammation and neurodegeneration in this disease. Experimental and therapeutic approaches, including recombinant enzyme replacement, have been performed using Sandhoff disease model mice, suggesting the future application of novel techniques to treat GM2 gangliosidoses (Hex deficiencies), including Sandhoff disease as well as Tay-Sachs disease. In this study, we isolated astrocytes and microglia from the neonatal brain of Sandhoff disease model mice and demonstrated abnormalities of glial cells. Moreover, we demonstrated the therapeutic effect of an intracerebroventricular administration of novel recombinant human HexA carrying a high content of M6P residue in Sandhoff disease model mice.

GM2 gangliosidosis in an adult pet rabbit.

A 1.5-year-old neutered male rabbit was presented with chronic nasal discharge and ataxia. Rapid progression of neurological signs was noted subsequent to general anaesthesia and the rabbit was humanely destroyed due to the poor prognosis. At necropsy examination there were no gross changes affecting the brain or spinal cord. Microscopical examination revealed that the perikarya of numerous neurons in the brain and spinal cord were distended by the intracytoplasmic accumulation of pale, finely granular to vacuolar material. Transmission electron microscopy showed this to be composed of concentric membranous cytoplasmic bodies. Thin layer chromatography revealed elevation of GM2 ganglioside in the brain of this rabbit compared with that of an unaffected control rabbit. Enzymatically, there was markedly reduced activity of tissue ß-hexosaminidase A in brain and liver tissue from the rabbit. This was a result of an almost complete absence of the enzymatic activity of the α-subunit of that enzyme. These findings are consistent with sphingolipidosis comparable with human GM2 gangliosidosis variant B1.

Highly phosphomannosylated enzyme replacement therapy for GM2 gangliosidosis.

OBJECTIVE: Novel recombinant human lysosomal ß-hexosaminidase A (HexA) was developed for enzyme replacement therapy (ERT) for Tay-Sachs and Sandhoff diseases, ie, autosomal recessive GM2 gangliosidoses, caused by HexA deficiency. METHODS: A recombinant human HexA (Om4HexA) with a high mannose 6-phosphate (M6P)-type-N-glycan content, which was produced by a methylotrophic yeast strain, Ogataea minuta, overexpressing the OmMNN4 gene, was intracerebroventricularly (ICV) administered to Sandhoff disease model mice (Hexb⁻/⁻ mice) at different doses (0.5-2.5 mg/kg), and then the replacement and therapeutic effects were examined. RESULTS: The Om4HexA was widely distributed across the ependymal cell layer, dose-dependently restored the enzyme activity due to uptake via cell surface cation-independent M6P receptor (CI-M6PR) on neural cells, and reduced substrates, including GM2 ganglioside (GM2), asialo GM2 (GA2), and oligosaccharides with terminal N-acetylglucosamine residues (GlcNAc-oligosaccharides), accumulated in brain parenchyma. A significant inhibition of chemokine macrophage inflammatory protein-1 α (MIP-1α) induction was also revealed, especially in the hindbrain (< 63%). The decrease in central neural storage correlated with an improvement of motor dysfunction as well as prolongation of the lifespan. INTERPRETATION: This lysosome-directed recombinant human enzyme drug derived from methylotrophic yeast has the high therapeutic potential to improve the motor dysfunction and quality of life of the lysosomal storage diseases (LSDs) patients with neurological manifestations. We emphasize the importance of neural cell surface M6P receptor as a delivery target of neural cell-directed enzyme replacement therapy (NCDERT) for neurodegenerative metabolic diseases.

Pathology of GM2 gangliosidosis in Jacob sheep.

The G(M2) gangliosidoses are a group of lysosomal storage diseases caused by defects in the genes coding for the enzyme hexosaminidase or the G(M2) activator protein. Four Jacob sheep from the same farm were examined over a 3-year period for a progressive neurologic disease. Two lambs were 6-month-old intact males and 2 were 8-month-old females. Clinical findings included ataxia in all 4 limbs, proprioceptive deficits, and cortical blindness. At necropsy, the nervous system appeared grossly normal. Histologically, most neurons within the brain, spinal cord, and peripheral ganglia were enlarged, and the cytoplasm was distended by foamy to granular material that stained positively with Luxol fast blue and Sudan black B stains. Other neuropathologic findings included widespread astrocytosis, microgliosis, and scattered spheroids. Electron microscopy revealed membranous cytoplasmic bodies within the cytoplasm of neurons. Biochemical and molecular genetic studies confirmed the diagnosis of G(M2) gangliosidosis. This form of G(M2) gangliosidosis in Jacob sheep is very similar to human Tay-Sachs disease and is potentially a useful animal model.

Thymic alterations in GM2 gangliosidoses model mice.

BACKGROUND: Sandhoff disease is a lysosomal storage disorder characterized by the absence of ß-hexosaminidase and storage of GM2 ganglioside and related glycolipids. We have previously found that the progressive neurologic disease induced in Hexb(-/-) mice, an animal model for Sandhoff disease, is associated with the production of pathogenic anti-glycolipid autoantibodies. METHODOLOGY/PRINCIPAL FINDINGS: In our current study, we report on the alterations in the thymus during the development of mild to severe progressive neurologic disease. The thymus from Hexb(-/-) mice of greater than 15 weeks of age showed a marked decrease in the percentage of immature CD4(+)/CD8(+) T cells and a significantly increased number of CD4(+)/CD8(-) T cells. During involution, the levels of both apoptotic thymic cells and IgG deposits to T cells were found to have increased, whilst swollen macrophages were prominently observed, particularly in the cortex. We employed cDNA microarray analysis to monitor gene expression during the involution process and found that genes associated with the immune responses were upregulated, particularly those expressed in macrophages. CXCL13 was one of these upregulated genes and is expressed specifically in the thymus. B1 cells were also found to have increased in the thy mus. It is significant that these alterations in the thymus were reduced in FcRγ additionally disrupted Hexb(-/-) mice. CONCLUSIONS/SIGNIFICANCE: These results suggest that the FcRγ chain may render the usually poorly immunogenic thymus into an organ prone to autoimmune responses, including the chemotaxis of B1 cells toward CXCL13.

GM2 gangliosidosis variant 0 (Sandhoff-like disease) in a family of toy poodles.

BACKGROUND: GM2 gangliosidosis variant 0 (human Sandhoff disease) is a lysosomal storage disorder caused by deficiencies of acid ß-hexosaminidase (Hex) A and Hex B because of an abnormality of the ß-subunit, a common component in these enzyme molecules, which is coded by the HEXB gene. OBJECTIVE: To describe the clinical, pathological, biochemical, and magnetic resonance imaging (MRI) findings of Sandhoff-like disease identified in a family of Toy Poodles. ANIMALS: Three red-haired Toy Poodles demonstrated clinical signs including motor disorders and tremor starting between 9 and 12 months of age. The animals finally died of neurological deterioration between 18 and 23 months of age. There were some lymphocytes with abnormal cytoplasmic vacuoles detected. METHODS: Observational case study. RESULTS: The common MRI finding was diffuse T2-hyperintensity of the subcortical white matter in the cerebrum. Bilateral T2-hyperintensity and T1-hypointensity in the nucleus caudatus, and atrophic findings of the cerebrum and cerebellum, were observed in a dog in the late stage. Histopathologically, swollen neurons with pale to eosinophilic granular materials in the cytoplasm were observed throughout the central nervous system. Biochemically, GM2 ganglioside had accumulated in the brain, and Hex A and Hex B were deficient in the brain and liver. Pedigree analysis demonstrated that the 3 affected dogs were from the same family line. CONCLUSIONS AND CLINICAL IMPORTANCE: The Sandhoff-like disease observed in this family of Toy Poodles is the 2nd occurrence of the canine form of this disease and the 1st report of its identification in a family of dogs.

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.

Neural precursor cell cultures from GM2 gangliosidosis animal models recapitulate the biochemical and molecular hallmarks of the brain pathology.

In this work we showed that genotype-related patterns of hexosaminidase activity, isoenzyme composition, gene expression and ganglioside metabolism observed during embryonic and postnatal brain development are recapitulated during the progressive stages of neural precursor cell (NPC) differentiation to mature glia and neurons in vitro. Further, by comparing NPCs and their differentiated progeny established from Tay-Sachs (TS) and Sandhoff (SD) animal models with the wild-type counterparts, we studied the events linking the accumulation of undegraded substrates to hexosaminidase activity. We showed that similarly to what observed in brain tissues in TS NPCs and progeny, the stored GM2 was partially converted by sialidase to GA2, which can be then degraded in the lysosomes to its components. The latter can be used in a salvage pathway for the formation of GM3. Interestingly, results obtained from ganglioside feeding assays and from measurement of lysosomal sialidase activity suggest that a similar pathway might work also in the SD model.