Galactosialidosis: historic aspects and overview of investigated and emerging treatment options

AAV-mediated gene therapy for sialidosis

Sialidosis (mucolipidosis I) is a glycoprotein storage disease, clinically characterized by a spectrum of systemic and neurological phenotypes. The primary cause of the disease is deficiency of the lysosomal sialidase NEU1, resulting in accumulation of sialylated glycoproteins/oligosaccharides in tissues and body fluids. Neu1-/- mice recapitulate the severe, early-onset forms of the disease, affecting visceral organs, muscles, and the nervous system, with widespread lysosomal vacuolization evident in most cell types. Sialidosis is considered an orphan disorder with no therapy currently available. Here, we assessed the therapeutic potential of AAV-mediated gene therapy for the treatment of sialidosis. Neu1-/- mice were co-injected with two scAAV2/8 vectors, expressing human NEU1 and its chaperone PPCA. Treated mice were phenotypically indistinguishable from their WT controls. NEU1 activity was restored to different extent in most tissues, including the brain, heart, muscle, and visceral organs. This resulted in diminished/absent lysosomal vacuolization in multiple cell types and reversal of sialyl-oligosacchariduria. Lastly, normalization of lysosomal exocytosis in the cerebrospinal fluids and serum of treated mice, coupled to diminished neuroinflammation, were measures of therapeutic efficacy. These findings point to AAV-mediated gene therapy as a suitable treatment for sialidosis and possibly other diseases, associated with low NEU1 expression.

AAV-mediated gene therapy for Sialidosis

Sialidosis is a glycoprotein storage disease caused by deficiency of the lysosomal sialidase NEU1, which leads to pathogenic accumulation of sialylated glycoproteins and oligosaccharides in tissues and body fluids. The disease belongs to the group of orphan disorders with no therapy currently available. Here, we have tested the therapeutic potential of AAV-mediated gene therapy for the treatment of sialidosis in a mouse model of the disease. One-month-old Neu1 -/- mice were co-injected with two scAAV2/8 vectors, expressing NEU1 and its chaperone PPCA, and sacrificed at 3 months post-injection. Treated mice were phenotypically indistinguishable from their WT controls. Histopathologically, they showed diminished or absent vacuolization in cells of visceral organs, including the kidney, as well as the choroid plexus and other areas of the brain. This was accompanied by restoration of NEU1 activity in most tissues, reversal of sialyl-oligosacchariduria, and normalization of lysosomal exocytosis in the CSF and serum of treated mice. AAV injection prevented the occurrence of generalized fibrosis, which is a prominent contributor of disease pathogenesis in Neu1 -/- mice and likely in patients. Overall, this therapeutic strategy holds promise for the treatment of sialidosis and may be applicable to adult forms of human idiopathic fibrosis with low NEU1 expression.

Structure of the immunoregulatory sialidase NEU1

Sialic acids linked to glycoproteins and glycolipids are important mediators of cell and protein recognition events. These sugar residues are removed by neuraminidases (sialidases). Neuraminidase-1 (sialidase-1 or NEU1) is a ubiquitously expressed mammalian sialidase located in lysosomes and on the cell membrane. Because of its modulation of multiple signaling processes, it is a potential therapeutic target for cancers and immune disorders. Genetic defects in NEU1 or in its protective protein cathepsin A (PPCA, CTSA) cause the lysosomal storage diseases sialidosis and galactosialidosis. To further our understanding of this enzyme's function at the molecular level, we determined the three-dimensional structure of murine NEU1. The enzyme oligomerizes through two self-association interfaces and displays a wide substrate-binding cavity. A catalytic loop adopts an inactive conformation. We propose a mechanism of activation involving a conformational change in this loop upon binding to its protective protein. These findings may facilitate the development of selective inhibitor and agonist therapies.

Models to study basic and applied aspects of lysosomal storage disorders

The lack of available treatments and fatal outcome in most lysosomal storage disorders (LSDs) have spurred research on pathological mechanisms and novel therapies in recent years. In this effort, experimental methodology in cellular and animal models have been developed, with aims to address major challenges in many LSDs such as patient-to-patient variability and brain condition. These techniques and models have advanced knowledge not only of LSDs but also for other lysosomal disorders and have provided fundamental insights into the biological roles of lysosomes. They can also serve to assess the efficacy of classical therapies and modern drug delivery systems. Here, we summarize the techniques and models used in LSD research, which include both established and recently developed in vitro methods, with general utility or specifically addressing lysosomal features. We also review animal models of LSDs together with cutting-edge technology that may reduce the need for animals in the study of these devastating diseases.

Facial features of lysosomal storage disorders

INTRODUCTION

The use of facial recognition technology has diversified the diagnostic toolbelt for clinicians and researchers for the accurate diagnoses of patients with rare and challenging disorders. Specific identifiers in patient images can be grouped using artificial intelligence to allow the recognition of diseases and syndromes with similar features. Lysosomal storage disorders are rare, and some have prominent and unique features that may be used to train the accuracy of facial recognition software algorithms. Noteworthy features of lysosomal storage disorders (LSDs) include facial features such as prominent brows, wide noses, thickened lips, mouth, and chin, resulting in coarse and rounded facial features.

AREAS COVERED

We evaluated and report the prevalence of facial phenotypes in patients with different LSDs, noting two current examples when artificial intelligence strategies have been utilized to identify distinctive facies.

EXPERT OPINION

Specific LSDs, including Gaucher disease, Mucolipidosis IV and Fabry disease have recently been distinguished using facial recognition software. Additional lysosomal disorders LSDs lysosomal storage disorders with unique and distinguishable facial features also merit evaluation using this technology. These tools may ultimately aid in the identification of specific LSDs and shorten the diagnostic odyssey for patients with these rare and under-recognized disorders.

AAV-mediated gene therapy for galactosialidosis: A long-term safety and efficacy study

AAV-mediated gene therapy holds promise for the treatment of lysosomal storage diseases (LSDs), some of which are already in clinical trials. Yet, ultra-rare subtypes of LSDs, such as some glycoproteinoses, have lagged. Here, we report on a long-term safety and efficacy preclinical study conducted in the murine model of galactosialidosis, a glycoproteinosis caused by a deficiency of protective protein/cathepsin A (PPCA). One-month-old Ctsa -/- mice were injected intravenously with a high dose of a self-complementary AAV2/8 vector expressing human CTSA in the liver. Treated mice, examined up to 12 months post injection, appeared grossly indistinguishable from their wild-type littermates. Sustained expression of scAAV2/8-CTSA in the liver resulted in the release of the therapeutic precursor protein in circulation and its widespread uptake by cells in visceral organs and the brain. Increased cathepsin A activity resolved lysosomal vacuolation throughout the affected organs and sialyl-oligosacchariduria. No signs of hyperplasia or inflammation were detected in the liver up to a year of age. Clinical chemistry panels, blood cell counts, and T cell immune responses were normal in all treated animals. These results warrant a close consideration of this gene therapy approach for the treatment of galactosialidosis, an orphan disease with no cure in sight.

Hearing loss in inherited metabolic disorders: A systematic review

Inherited metabolic disorders (IMDs) have been observed in individuals with hearing loss (HL), but IMDs are rarely the cause of syndromic HL. With early diagnosis, management of HL is more effective and cortical reorganization is possible with hearing aids or cochlear implants. This review describes relationships between IMDs and HL in terms of incidence, etiology of HL, pathophysiology, and treatment. Forty types of IMDs are described in the literature, mainly in case reports. Management and prognosis are noted where existing. We also describe IMDs with HL given age of occurrence of HL. Reviewing the main IMDs that are associated with HL may provide an additional clinical tool with which to better diagnose syndromic HL.

Clinical Exome Sequencing Enables Congenital Sialidosis Type II Diagnosis in Two Siblings Presenting with Unreported Clinical Features from a Rare Homozygous Sequence Variant p.(Tyr370Cys) in NEU1

Sialidosis is a rare autosomal recessive disease that presents with progressive lysosomal storage of sialylated glycopeptides and oligosaccharides caused by homozygous or compound heterozygous sequence variants in the neuraminidase 1 (NEU1) gene. These sequence variants can lead to sialidosis type I and II; the latter is the most severe and presents prenatally or at early age. However, sialidosis diagnosis is challenging, especially in those health systems with limited resources of developing countries. Consequently, it is necessary to dip into high-throughput molecular diagnostic tools to allow for an accurate diagnosis with better cost-effectiveness and turnaround time. We report a 4-member pedigree segregating an ultrarare missense variant, c.1109A>G; p.Tyr370Cys, in NEU1 as detected by whole-exome sequencing. Two short-lived siblings, who presented with previously unreported clinical features from such a homozygous sequence variant, were diagnosed with sialidosis type II. Additionally, we present a novel molecular model exhibiting the consequences of the variant in the sialidase-1 tridimensional structure. This study allowed us to provide a definitive diagnosis for our patients, increase our understanding of this pathogenic variant, and improve genetic counseling.

A sialidosis type I cohort and a quantitative approach to multimodal ophthalmic imaging of the macular cherry-red spot

AIM

To describe the ophthalmologic findings on the largest cohort of patients with sialidosis type I due to deficiency of the lysosomal sialidase, neuraminidase 1 (NEU1) and to introduce a quantitative neuroretinal image analysis approach to the associated 'macular cherry-red spot'.

METHODS

Seven patients with sialidosis type I (mutations in NEU1) and one with galactosialidosis (mutations in CTSA) were included. All patients underwent detailed ophthalmological examinations. The reflectivity of macular optical coherence tomography (OCT) was measured using greyscale analysis (Fiji) and compared with age-matched healthy volunteers. Four patients were evaluated over a time of 1.5+0.5 years.

RESULTS

The mean age of the patients at their first visit was 27.5+9.8 years. All patients had a macular cherry-red spot, clear corneas and visually non-significant lenticular opacities. The mean visual acuity was LogMar 0.4 (20/50)+0.4 (20/20 to 20/125). Six patients had good visual function. Optic atrophy was present in two individuals with reduced acuity. A significant increase in macular reflectivity was present in all patients compared to age-matched controls (p<0.0001).

CONCLUSION

Most of our patients (75%) have preserved visual acuity, even in adulthood. The presence of optic atrophy is associated with poor visual acuity. Increased macular reflectivity by OCT greyscale measurements is noted in all patients, although the underlying biological basis is unknown. These findings complement the current methods for examining and monitoring disease progression, especially in patients for whom visualisation of the cherry-red spot is not entirely clear.

TRIAL REGISTRATION NUMBER

NCT00029965.

Galactose in human metabolism, glycosylation and congenital metabolic diseases: Time for a closer look

Galactose is an essential carbohydrate for cellular metabolism, as it contributes to energy production and storage in several human tissues while also being a precursor for glycosylation. Galactosylated glycoconjugates, such as glycoproteins, keratan sulfate-containing proteoglycans and glycolipids, exert a plethora of biological functions, including structural support, cellular adhesion, intracellular signaling and many more. The biological relevance of galactose is further entailed by the number of pathogenic conditions consequent to defects in galactosylation and galactose homeostasis. The growing number of rare congenital disorders involving galactose along with its recent therapeutical applications are drawing increasing attention to galactose metabolism. In this review, we aim to draw a comprehensive overview of the biological functions of galactose in human cells, including its metabolism and its role in glycosylation, and to provide a systematic description of all known congenital metabolic disorders resulting from alterations of its homeostasis.

Sialylation of host proteins as targetable risk factor for COVID-19 susceptibility and spreading: A hypothesis

Individuals infected with the severe acute respiratory syndrome (SARS)‐related coronavirus 2 (SARS‐CoV‐2) develop a critical and even fatal disease, called Coronavirus disease‐19 (COVID‐19), that eventually evolves into acute respiratory distress syndrome. The gravity of the SARS‐CoV‐2 pandemic, the escalating number of confirmed cases around the world, the many unknowns related to the virus mode of action, and the heterogenous outcome of COVID‐19 disease in the population ask for the rapid development of alternative approaches, including repurposing of existing drugs, that may dampen virus infectivity. SARS‐CoV‐2 infects human cells through interaction with sialylated receptors at the surface of epithelial cells, such as angiotensin‐converting enzyme 2 (ACE2). Glycan composition on virus entry receptors has been shown to influence the rate of infection of SARS‐CoV‐2 and spreading of virions has recently been linked to altered lysosomal exocytosis. These processes could concurrently involve the lysosomal system and its glycosidases. We hypothesize that modulating the activity of one of them, the lysosomal sialidase NEU1, could impinge on both the sialylation status of ACE2 and other host receptors as well as the extent of lysosomal exocytosis. Thus NEU1‐controlled pathways may represent therapeutic targets, which could impact on SARS‐CoV‐2 susceptibility, infectivity, and spread.

Galactosialidosis: preclinical enzyme replacement therapy in a mouse model of the disease, a proof of concept

Galactosialidosis is a rare lysosomal storage disease caused by a congenital defect of protective protein/cathepsin A (PPCA) and secondary deficiency of neuraminidase-1 and β-galactosidase. PPCA is a lysosomal serine carboxypeptidase that functions as a chaperone for neuraminidase-1 and β-galactosidase within a lysosomal multi-protein complex. Combined deficiency of the three enzymes leads to accumulation of sialylated glycoproteins and oligosaccharides in tissues and body fluids and manifests in a systemic disease pathology with severity mostly correlating with the type of mutation(s) and age of onset of the symptoms. Here, we describe a proof-of-concept, preclinical study toward the development of enzyme replacement therapy for galactosialidosis, using a recombinant human PPCA. We show that the recombinant enzyme, taken up by patient-derived fibroblasts, restored cathepsin A, neuraminidase-1, and β-galactosidase activities. Long-term, bi-weekly injection of the recombinant enzyme in a cohort of mice with null mutation at the PPCA (CTSA) locus (PPCA -/- ), a faithful model of the disease, demonstrated a dose-dependent, systemic internalization of the enzyme by cells of various organs, including the brain. This resulted in restoration/normalization of the three enzyme activities, resolution of histopathology, and reduction of sialyloligosacchariduria. These positive results underscore the benefits of a PPCA-mediated enzyme replacement therapy for the treatment of galactosialidosis.

Lipophagy and Lipolysis Status in Lipid Storage and Lipid Metabolism Diseases

This review discusses how lipophagy and cytosolic lipolysis degrade cellular lipids, as well as how these pathway ys communicate, how they affect lipid metabolism and energy homeostasis in cells and how their dysfunction affects the pathogenesis of lipid storage and lipid metabolism diseases. Answers to these questions will likely uncover novel strategies for the treatment of aforementioned human diseases, but, above all, will avoid destructive effects of high concentrations of lipids-referred to as lipotoxicity-resulting in cellular dysfunction and cell death.

The Role of Hematopoietic Cell Transplant in the Glycoprotein Diseases

The glycoprotein disorders are a group of lysosomal storage diseases (α-mannosidosis, aspartylglucosaminuria, β-mannosidosis, fucosidosis, galactosialidosis, sialidosis, mucolipidosis II, mucolipidosis III, and Schindler Disease) characterized by specific lysosomal enzyme defects and resultant buildup of undegraded glycoprotein substrates. This buildup causes a multitude of abnormalities in patients including skeletal dysplasia, inflammation, ocular abnormalities, liver and spleen enlargement, myoclonus, ataxia, psychomotor delay, and mild to severe neurodegeneration. Pharmacological treatment options exist through enzyme replacement therapy (ERT) for a few, but therapies for this group of disorders is largely lacking. Hematopoietic cell transplant (HCT) has been explored as a potential therapeutic option for many of these disorders, as HCT introduces functional enzyme-producing cells into the bone marrow and blood along with the engraftment of healthy donor cells in the central nervous system (presumably as brain macrophages or a type of microglial cell). The outcome of HCT varies widely by disease type. We report our institutional experience with HCT as well as a review of the literature to better understand HCT and outcomes for the glycoprotein disorders.

Cathepsins in the Pathophysiology of Mucopolysaccharidoses: New Perspectives for Therapy

Cathepsins (CTSs) are ubiquitously expressed proteases normally found in the endolysosomal compartment where they mediate protein degradation and turnover. However, CTSs are also found in the cytoplasm, nucleus, and extracellular matrix where they actively participate in cell signaling, protein processing, and trafficking through the plasma and nuclear membranes and between intracellular organelles. Dysregulation in CTS expression and/or activity disrupts cellular homeostasis, thus contributing to many human diseases, including inflammatory and cardiovascular diseases, neurodegenerative disorders, diabetes, obesity, cancer, kidney dysfunction, and others. This review aimed to highlight the involvement of CTSs in inherited lysosomal storage disorders, with a primary focus to the emerging evidence on the role of CTSs in the pathophysiology of Mucopolysaccharidoses (MPSs). These latter diseases are characterized by severe neurological, skeletal and cardiovascular phenotypes, and no effective cure exists to date. The advance in the knowledge of the molecular mechanisms underlying the activity of CTSs in MPSs may open a new challenge for the development of novel therapeutic approaches for the cure of such intractable diseases.

Conventional and Unconventional Therapeutic Strategies for Sialidosis Type I

Congenital deficiency of the lysosomal sialidase neuraminidase 1 (NEU1) causes the lysosomal storage disease, sialidosis, characterized by impaired processing/degradation of sialo-glycoproteins and sialo-oligosaccharides, and accumulation of sialylated metabolites in tissues and body fluids. Sialidosis is considered an ultra-rare clinical condition and falls into the category of the so-called orphan diseases, for which no therapy is currently available. In this study we aimed to identify potential therapeutic modalities, targeting primarily patients affected by type I sialidosis, the attenuated form of the disease. We tested the beneficial effects of a recombinant protective protein/cathepsin A (PPCA), the natural chaperone of NEU1, as well as pharmacological and dietary compounds on the residual activity of mutant NEU1 in a cohort of patients’ primary fibroblasts. We observed a small, but consistent increase in NEU1 activity, following administration of all therapeutic agents in most of the fibroblasts tested. Interestingly, dietary supplementation of betaine, a natural amino acid derivative, in mouse models with residual NEU1 activity mimicking type I sialidosis, increased the levels of mutant NEU1 and resolved the oligosacchariduria. Overall these findings suggest that carefully balanced, unconventional dietary compounds in combination with conventional therapeutic approaches may prove to be beneficial for the treatment of sialidosis type I.

Nomenclature of Genetically Determined Myoclonus Syndromes: Recommendations of the International Parkinson and Movement Disorder Society Task Force

Genetically determined myoclonus disorders are a result of a large number of genes. They have wide clinical variation and no systematic nomenclature. With next-generation sequencing, genetic diagnostics require stringent criteria to associate genes and phenotype. To improve (future) classification and recognition of genetically determined movement disorders, the Movement Disorder Society Task Force for Nomenclature of Genetic Movement Disorders (2012) advocates and renews the naming system of locus symbols. Here, we propose a nomenclature for myoclonus syndromes and related disorders with myoclonic jerks (hyperekplexia and myoclonic epileptic encephalopathies) to guide clinicians in their diagnostic approach to patients with these disorders. Sixty-seven genes were included in the nomenclature. They were divided into 3 subgroups: prominent myoclonus syndromes, 35 genes; prominent myoclonus syndromes combined with another prominent movement disorder, 9 genes; disorders that present usually with other phenotypes but can manifest as a prominent myoclonus syndrome, 23 genes. An additional movement disorder is seen in nearly all myoclonus syndromes: ataxia (n = 41), ataxia and dystonia (n = 6), and dystonia (n = 5). However, no additional movement disorders were seen in related disorders. Cognitive decline and epilepsy are present in the vast majority. The anatomical origin of myoclonus is known in 64% of genetic disorders: cortical (n = 34), noncortical areas (n = 8), and both (n = 1). Cortical myoclonus is commonly seen in association with ataxia, and noncortical myoclonus is often seen with myoclonus-dystonia. This new nomenclature of myoclonus will guide diagnostic testing and phenotype classification. © 2019 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Lysosomal Glycosphingolipid Storage Diseases

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

Drug delivery systems and novel formulations to improve treatment of rare corneal disease

As the field of ocular drug delivery grows so does the potential for novel drug discovery or reformulation in lesser-known diseases of the eye. In particular, rare corneal diseases are an interesting area of research because drug delivery is limited to the outermost tissue of the eye. This review will highlight the opportunities and challenges of drug reformulation and alternative treatment approaches for rare corneal diseases. The barriers to effective drug delivery and proposed solutions in development will be discussed along with an overview of corneal rare disease resources, their current treatments and ophthalmic drug delivery systems that could benefit such cases. The regulatory considerations for effective translation of orphan-designated products will also be discussed.

Dental Management of a Young Child Affected by Galactosialidosis and a Gigantic Abdominal Growth

Galactosialidosis (GS) is a rare form of lysosomal storage disease that involves a broad spectrum of skeletal and soft tissue abnormalities. We report here on a 4-year 7-month-old boy with mild mental retardation, exhibiting multiple caries cavities and associated infectious foci and macroglossia. A huge abdominal enlargement due to peritoneal ascites was evident. Behavioral management and patient positioning on the dental chair represented a true challenge. The patient was treated under general anesthesia. However, life-threatening postoperative complications occurred because of the impossibility of extubating the patient. A very careful preanesthetic assessment is crucial in children affected by general conditions associated with airway anomalies, such as GS.

Emptying the stores: lysosomal diseases and therapeutic strategies

Lysosomal storage disorders (LSDs) - designated as 'orphan' diseases - are inborn errors of metabolism caused by defects in genes that encode proteins involved in various aspects of lysosomal homeostasis. For many years, LSDs were viewed as unattractive targets for the development of therapies owing to their low prevalence. However, the development and success of the first commercial biologic therapy for an LSD - enzyme replacement therapy for type 1 Gaucher disease - coupled with regulatory incentives rapidly catalysed commercial interest in therapeutically targeting LSDs. Despite ongoing challenges, various therapeutic strategies for LSDs now exist, with many agents approved, undergoing clinical trials or in preclinical development.