Glucosylceramide transfer from lysosomes--the missing link in molecular pathology of glucosylceramidase deficiency: a hypothesis based on existing data

GBA-associated PD: chances and obstacles for targeted treatment strategies

Given the clear role of GBA in the pathogenesis of Parkinson’s disease (PD) and its impact on phenotypical characteristics, this review provides an overview of the current knowledge of GBA-associated PD with a special focus on clinical trajectories and the underlying pathological mechanisms. Importantly, differences and characteristics based on mutation severity are recognized, and current as well as potential future treatment options are discussed. These findings will inform future strategies for patient stratification and cohort enrichment as well as suitable outcome measures when designing clinical trials.

Quantitative assessment of the exposure-efficacy relationship of glucocerebrosidase using Markovian elements in Gaucher patients treated with enzyme replacement therapy

AIMS

The aims of this study are (i) to develop a population pharmacokinetic model of enzyme activity in Gaucher-type 1 (GD1) patients after intravenous administration of enzyme replacement therapy (ERT) and, (ii) to establish an exposure-efficacy relationship for bone marrow infiltration to propose dose adjustments according to patient covariate values.

MATERIALS AND METHODS

A prospective follow-up, semi-experimental multi-centre study was conducted in four hospitals to evaluate the pharmacokinetics, efficacy and safety of ERT in GD1 patients. 25 individuals with 266 glucocerebrosidase (GCase) observations in plasma and leukocytes and 14 individuals with 68 Spanish Magnetic Resonance Imaging (S-MRI) observations were enrolled.

RESULTS

A two concatenated compartments with zero-order endogenous production and first-order distribution (CL₁ =3.85 x10^-1 L/d) and elimination (CL₂ = 1.25 L/d) allowed to describe GCase observations in plasma and leucocytes, respectively. An exponential time-dependency (k_T =6.14 x10^-1 d^-1 ) effect on CL₁ was incorporated. The final exposure-efficacy model was a longitudinal logistic regression model with a first-order Markov element. An Emax function (EC₅₀ =15.73 U/L and Emax=2.33) linked steady-state concentrations of GCase in leucocytes to the probability of transition across the different S-MRI stages.

CONCLUSION

A population pharmacokinetic model successfully characterized the leukocyte activity-time profiles of GCase following intravenous administration of ERT in GD1 patients together with an exposure-efficacy relationship in bone marrow using markovian elements. The information obtained from this study could be of high clinical relevance in individualization of ERT in GD1 patients, as this could lead to anticipate decision-making regarding clinical response in bone and optimal dosing strategy. NONSTANDARD ABBREVIATIONS: -2LL: -2xlog(likelihood); ERT: enzyme replacement therapy; GCase: glucocerebrosidase activity; GD1: Gaucher disease type 1; GOF: goodness-of-fit plots; IIV: inter-individual variability; NLME: non-linear mixed effects modelling; OFV: objective function value; pc-VPC: prediction-corrected visual predictive check; PK: pharmacokinetic; RSE: relative standard error; RUV: residual unexplained variability, S-MRI: Spanish Magnetic Resonance Imaging, TDM: therapeutic drug monitoring.

Mapping Sphingolipid Metabolism Pathways during Phagosomal Maturation

Phagocytosis is an important physiological process, which, in higher organisms, is a means of fighting infections and clearing cellular debris. During phagocytosis, detrimental foreign particles (e.g. pathogens and apoptotic cells) are engulfed by phagocytes (e.g. macrophages), enclosed in membrane-bound vesicles called phagosomes, and transported to the lysosome for eventual detoxification. During this well-choreographed process, the nascent phagosome (also called early phagosome, EP) undergoes a series of spatiotemporally regulated changes in its protein and lipid composition and matures into a late phagosome (LP), which subsequently fuses with the lysosomal membrane to form the phagolysosome. While several elegant proteomic studies have identified the role of unique proteins during phagosomal maturation, the corresponding lipidomic studies are sparse. Recently, we reported a comparative lipidomic analysis between EPs and LPs and showed that ceramides are enriched on the LPs. Further, we found that this ceramide accumulation on LPs was orchestrated by ceramide synthase 2, inhibition of which hampers phagosomal maturation. Following up on this study, here, using biochemical assays, we first show that the increased ceramidase activity on EPs also significantly contributes to the accumulation of ceramides on LPs. Next, leveraging lipidomics, we show that de novo ceramide synthesis does not significantly contribute to the ceramide accumulation on LPs, while concomitant to increased ceramides, glucosylceramides are substantially elevated on LPs. We validate this interesting finding using biochemical assays and show that LPs indeed have heightened glucosylceramide synthase activity. Taken together, our studies provide interesting insights and possible new roles of sphingolipid metabolism during phagosomal maturation.

Deletion of fatty acid amide hydrolase reduces lyso-sulfatide levels but exacerbates metachromatic leukodystrophy in mice

An inherited deficiency of arylsulfatase A (ASA) causes the lysosomal storage disease metachromatic leukodystrophy (MLD) characterized by massive intralysosomal storage of the acidic glycosphingolipid sulfatide and progressive demyelination. Lyso-sulfatide, which differs from sulfatide by the lack of the N-linked fatty acid also accumulates in MLD and is considered a key driver of pathology although its concentrations are far below sulfatide levels. However, the metabolic origin of lyso-sulfatide is unknown. We show here that ASA-deficient murine macrophages and microglial cells express an endo-N-deacylase that cleaves the N-linked fatty acid from sulfatide. An ASA-deficient astrocytoma cell line devoid of this activity was used to identify the enzyme by overexpressing 13 deacylases with potentially matching substrate specificities. Hydrolysis of sulfatide was detected only in cells overexpressing the enzyme fatty acid amide hydrolase (FAAH). A cell-free assay with recombinant FAAH confirmed the novel role of this enzyme in sulfatide hydrolysis. Consistent with the in vitro data, deletion of FAAH lowered lyso-sulfatide levels in a mouse model of MLD. Regardless of the established cytotoxicity of lyso-sulfatide and the anti-inflammatory effects of FAAH inhibition seen in mouse models of several neurological diseases, genetic inactivation of FAAH did not mitigate, but rather exacerbated the disease phenotype of MLD mice. This unexpected finding was reflected by worsening of rotarod performance, increase of anxiety-related exploratory activity, aggravation of peripheral neuropathy and reduced life expectancy. Thus, we conclude that FAAH has a protective function in MLD and may represent a novel therapeutic target for treatment of this fatal condition.

Sphingolipid lysosomal storage diseases: from bench to bedside

Johann Ludwig Wilhelm Thudicum described sphingolipids (SLs) in the late nineteenth century, but it was only in the past fifty years that SL research surged in importance and applicability. Currently, sphingolipids and their metabolism are hotly debated topics in various biochemical fields. Similar to other macromolecular reactions, SL metabolism has important implications in health and disease in most cells. A plethora of SL-related genetic ailments has been described. Defects in SL catabolism can cause the accumulation of SLs, leading to many types of lysosomal storage diseases (LSDs) collectively called sphingolipidoses. These diseases mainly impact the neuronal and immune systems, but other systems can be affected as well. This review aims to present a comprehensive, up-to-date picture of the rapidly growing field of sphingolipid LSDs, their etiology, pathology, and potential therapeutic strategies. We first describe LSDs biochemically and briefly discuss their catabolism, followed by general aspects of the major diseases such as Gaucher, Krabbe, Fabry, and Farber among others. We conclude with an overview of the available and potential future therapies for many of the diseases. We strive to present the most important and recent findings from basic research and clinical applications, and to provide a valuable source for understanding these disorders.

The Mutation Matters: CSF Profiles of GCase, Sphingolipids, α-Synuclein in PDGBA

BACKGROUND

With pathway-specific trials in PD associated with variants in the glucocerebrosidase gene (PD_GBA ) under way, we need markers that confirm the impact of genetic variants in patient-derived biofluids in order to allow patient stratification merely based on genetics and that might serve as biochemical read-out for target engagement.

OBJECTIVE

To explore GBA-pathway-specific biomarker profiles cross-sectionally (TUEPAC-MIGAP, PPMI) and longitudinally (PPMI).

METHODS

We measured enzyme activity of the lysosomal glucocerebrosidase, CSF levels of glucosylceramides (upstream substrate of glucocerebrosidase), CSF levels of ceramides (downstream product of glucocerebrosidase), lactosylceramides, sphingosines, sphingomyelin (by-products) and CSF levels of total α-synuclein in PD_GBA patients compared to PD_{GBA_wildtype} patients.

RESULTS

Cross-sectionally in both cohorts and longitudinally in PPMI: (1) glucocerebrosidase activity was significantly lower in PD_GBA compared to PD_{GBA_wildtype} . (2) CSF levels of upstream substrates (glucosylceramides species) were higher in PD_GBA compared to PD_{GBA_wildtype} . (3) CSF levels of total α-synuclein were lower in PD_GBA compared to PD_{GBA_wildtype} . All of these findings were most pronounced in PD_GBA with severe mutations (PD_{GBA_severe} ). Cross-sectionally in TUEPAC-MIGAP and longitudinally in PPMI, CSF levels of downstream-products (ceramides) were higher in PD_{GBA_severe} . Cross-sectionally in TUEPAC-MIGAP by-products sphinganine and sphingosine-1-phosphate and longitudinally in PPMI species of by-products lactosylceramides and sphingomyelin were higher in PD_{GBA_severe} .

INTERPRETATION

These findings confirm that GBA mutations have a relevant functional impact on biomarker profiles in patients. Bridging the gap between genetics and biochemical profiles now allows patient stratification for clinical trials merely based on mutation status. Importantly, all findings were most prominent in PD_GBA with severe variants. © 2021 International Parkinson and Movement Disorder Society.

Reduced sphingolipid hydrolase activities, substrate accumulation and ganglioside decline in Parkinson's disease

Background

Haploinsufficiency in the Gaucher disease GBA gene, which encodes the lysosomal glucocerebrosidase GBA, and ageing represent major risk factors for developing Parkinson’s disease (PD). Recently, more than fifty other lysosomal storage disorder gene variants have been identified in PD, implicating lysosomal dysfunction more broadly as a key risk factor for PD. Despite the evidence of multiple lysosomal genetic risks, it remains unclear how sphingolipid hydrolase activities, other than GBA, are altered with ageing or in PD. Moreover, it is not fully known if levels of glycosphingolipid substrates for these enzymes change in vulnerable brain regions of PD. Finally, little is known about the levels of complex gangliosides in substantia nigra which may play a significant role in ageing and PD.

Methods

To study sphingolipid hydrolase activities and glycosphingolipid expression in ageing and in PD, two independent cohorts of human substantia nigra tissues were obtained. Fluorescent 4-methylumbelliferone assays were used to determine multiple enzyme activities. The lysosomal GBA and non-lysosomal GBA2 activities were distinguished using the inhibitor NB-DGJ. Sensitive and quantitative normal-phase HPLC was performed to study glycosphingolipid levels. In addition, glycosphingolipid levels in cerebrospinal fluid and serum were analysed as possible biomarkers for PD.

Results

The present study demonstrates, in two independent cohorts of human post-mortem substantia nigra, that sporadic PD is associated with deficiencies in multiple lysosomal hydrolases (e.g. α-galactosidase and β-hexosaminidase), in addition to reduced GBA and GBA2 activities and concomitant glycosphingolipid substrate accumulation. Furthermore, the data show significant reductions in levels of complex gangliosides (e.g. GM1a) in substantia nigra, CSF and serum in ageing, PD, and REM sleep behaviour disorder, which is a strong predictor of PD.

Conclusions

These findings conclusively demonstrate reductions in GBA activity in the parkinsonian midbrain, and for the first time, reductions in the activity of several other sphingolipid hydrolases. Furthermore, significant reductions were seen in complex gangliosides in PD and ageing. The diminished activities of these lysosomal hydrolases, the glycosphingolipid substrate accumulation, and the reduced levels of complex gangliosides are likely major contributors to the primary development of the pathology seen in PD and related disorders with age.

Glucosylsphingosine Promotes α-Synuclein Pathology in Mutant GBA-Associated Parkinson's Disease

Glucocerebrosidase 1 (GBA) mutations responsible for Gaucher disease (GD) are the most common genetic risk factor for Parkinson's disease (PD). Although the genetic link between GD and PD is well established, the underlying molecular mechanism(s) are not well understood. We propose that glucosylsphingosine, a sphingolipid accumulating in GD, mediates PD pathology in GBA-associated PD. We show that, whereas GD-related sphingolipids (glucosylceramide, glucosylsphingosine, sphingosine, sphingosine-1-phosphate) promote α-synuclein aggregation in vitro, glucosylsphingosine triggers the formation of oligomeric α-synuclein species capable of templating in human cells and neurons. Using newly generated GD/PD mouse lines of either sex [Gba mutant (N370S, L444P, KO) crossed to α-synuclein transgenics], we show that Gba mutations predispose to PD through a loss-of-function mechanism. We further demonstrate that glucosylsphingosine specifically accumulates in young GD/PD mouse brain. With age, brains exhibit glucosylceramide accumulations colocalized with α-synuclein pathology. These findings indicate that glucosylsphingosine promotes pathological aggregation of α-synuclein, increasing PD risk in GD patients and carriers.SIGNIFICANCE STATEMENT Parkinson's disease (PD) is a prevalent neurodegenerative disorder in the aging population. Glucocerebrosidase 1 mutations, which cause Gaucher disease, are the most common genetic risk factor for PD, underscoring the importance of delineating the mechanisms underlying mutant GBA-associated PD. We show that lipids accumulating in Gaucher disease, especially glucosylsphingosine, play a key role in PD pathology in the brain. These data indicate that ASAH1 (acid ceramidase 1) and GBA2 (glucocerebrosidase 2) enzymes that mediate glucosylsphingosine production and metabolism are attractive therapeutic targets for treating mutant GBA-associated PD.

Identification of a feedback loop involving β-glucosidase 2 and its product sphingosine sheds light on the molecular mechanisms in Gaucher disease

The lysosomal acid β-glucosidase GBA1 and the non-lysosomal β-glucosidase GBA2 degrade glucosylceramide (GlcCer) to glucose and ceramide in different cellular compartments. Loss of GBA2 activity and the resulting accumulation of GlcCer results in male infertility, whereas mutations in the GBA1 gene and loss of GBA1 activity cause the lipid-storage disorder Gaucher disease. However, the role of GBA2 in Gaucher disease pathology and its relationship to GBA1 is not well understood. Here, we report a GBA1-dependent down-regulation of GBA2 activity in patients with Gaucher disease. Using an experimental approach combining cell biology, biochemistry, and mass spectrometry, we show that sphingosine, the cytotoxic metabolite accumulating in Gaucher cells through the action of GBA2, directly binds to GBA2 and inhibits its activity. We propose a negative feedback loop, in which sphingosine inhibits GBA2 activity in Gaucher cells, preventing further sphingosine accumulation and, thereby, cytotoxicity. Our findings add a new chapter to the understanding of the complex molecular mechanism underlying Gaucher disease and the regulation of β-glucosidase activity in general.

Varied autopsy findings in five treated patients with Gaucher disease and parkinsonism include the absence of Gaucher cells

Enzyme replacement therapy is standard of care for patients with Gaucher disease, as it significantly improves skeletal, visceral, and hematological symptoms. Few pathological studies have documented the extent of pathological findings in treated patients. Autopsy findings in five treated patients, who ultimately developed parkinsonism, ranged from the complete absence of Gaucher pathology to extensive involvement of multiple tissues, without correlation to age, genotype, spleen status, or dose/duration of therapy. Additional autopsies may elucidate modifiers and biomarkers contributing to disease burden and response to therapy.

Parkinson's disease: acid-glucocerebrosidase activity and alpha-synuclein clearance

The role of mutations in the gene GBA1 encoding the lysosomal hydrolase β-glucocerebrosidase for the development of synucleinopathies, such as Parkinson's disease and dementia with Lewy bodies, was only very recently uncovered. The knowledge obtained from the study of carriers or patients suffering from Gaucher disease (a common lysosomal storage disorder because of GBA1 mutations) is of particular importance for understanding the role of the enzyme and its catabolic pathway in the development of synucleinopathies. Decreased activity of β-glucocerebrosidase leads to lysosomal dysfunction and the accumulation of its substrate glucosylceramide and related lipid derivatives. Glucosylceramide is suggested to stabilize toxic oligomeric forms of α-synuclein that negatively influence the activity of β-glucocerebrosidase and to partially block export of newly synthesized β-glucocerebrosidase from the endoplasmic reticulum to late endocytic compartments, amplifying the pathological effects of α-synuclein and ultimately resulting in neuronal cell death. This pathogenic molecular feedback loop and most likely other factors (such as impaired endoplasmic reticulum-associated degradation, activation of the unfolded protein response and dysregulation of calcium homeostasis induced by misfolded GC mutants) are involved in shifting the cellular homeostasis from monomeric α-synuclein towards oligomeric neurotoxic and aggregated forms, which contribute to Parkinson's disease progression. From a therapeutic point of view, strategies aiming to increase either the expression, stability or delivery of the β-glucocerebrosidase to lysosomes are likely to decrease the α-synuclein burden and may be useful for an in depth evaluation at the organismal level. Lysosomes are critical for protein and lipid homeostasis. Recent research revealed that dysfunction of this organelle contributes to the development of neurodegenerative diseases such as Parkinson's disease (PD). Mutations in the lysosomal hydrolase β-glucocerebrosidase (GBA1) are a major risk factor for the development of PD and the molecular events linked to the reduced activity of GBA1 and the pathological accumulation of lipids and α-synuclein are just at the beginning to be understood. New therapeutic concepts in regards to how to increase the expression, stability, or delivery of β-glucocerebrosidase to lysosomes are currently developed. This article is part of a special issue on Parkinson disease.

Reducing GBA2 Activity Ameliorates Neuropathology in Niemann-Pick Type C Mice

The enzyme glucocerebrosidase (GBA) hydrolyses glucosylceramide (GlcCer) in lysosomes. Markedly reduced GBA activity is associated with severe manifestations of Gaucher disease including neurological involvement. Mutations in the GBA gene have recently also been identified as major genetic risk factor for Parkinsonism. Disturbed metabolism of GlcCer may therefore play a role in neuropathology. Besides lysosomal GBA, cells also contain a non-lysosomal glucosylceramidase (GBA2). Given that the two β-glucosidases share substrates, we speculated that over-activity of GBA2 during severe GBA impairment might influence neuropathology. This hypothesis was studied in Niemann-Pick type C (Npc1^-/-) mice showing secondary deficiency in GBA in various tissues. Here we report that GBA2 activity is indeed increased in the brain of Npc1^-/- mice. We found that GBA2 is particularly abundant in Purkinje cells (PCs), one of the most affected neuronal populations in NPC disease. Inhibiting GBA2 in Npc1^-/- mice with a brain-permeable low nanomolar inhibitor significantly improved motor coordination and extended lifespan in the absence of correction in cholesterol and ganglioside abnormalities. This trend was recapitulated, although not to full extent, by introducing a genetic loss of GBA2 in Npc1^-/- mice. Our findings point to GBA2 activity as therapeutic target in NPC.

Treatment of lysosomal storage disorders: successes and challenges

Treatment options for a number of lysosomal storage disorders have rapidly expanded and currently include enzyme replacement therapy, substrate reduction, chaperone treatment, hematopoietic stem cell transplantation, and gene-therapy. Combination treatments are also explored. Most therapies are not curative but change the phenotypic expression of the disease. The effectiveness of treatment varies considerably between the different diseases, but also between sub-groups of patients with a specific lysosomal storage disorder. The heterogeneity of the patient populations complicates the prediction of benefits of therapy, specifically in patients with milder disease manifestations. In addition, there is a lack of data on the natural history of diseases and disease phenotypes. Initial trial data show benefits on relevant short-term endpoints, but the real world situation may reveal different outcomes. Collaborative international studies are much needed to study the long-term clinical efficacy of treatments, and to detect new complications or associated conditions of the diseases. This review summarizes the available treatment modalities for lysosomal storage disorders and the challenges associated with long term clinical care for these patients.

Glucocerebrosidase 2 gene deletion rescues type 1 Gaucher disease

The inherited deficiency of the lysosomal glucocerebrosidase (GBA) due to mutations in the GBA gene results in Gaucher disease (GD). A vast majority of patients present with nonneuronopathic, type 1 GD (GD1). GBA deficiency causes the accumulation of two key sphingolipids, glucosylceramide (GL-1) and glucosylsphingosine (LysoGL-1), classically noted within the lysosomes of mononuclear phagocytes. How metabolites of GL-1 or LysoGL-1 produced by extralysosomal glucocerebrosidase GBA2 contribute to the GD1 pathophysiology is not known. We recently recapitulated hepatosplenomegaly, cytopenia, hypercytokinemia, and the bone-formation defect of human GD1 through conditional deletion of Gba in Mx1-Cre(+):GD1 mice. Here we show that the deletion of Gba2 significantly rescues the GD1 clinical phenotype, despite enhanced elevations in GL-1 and LysoGL-1. Most notably, the reduced bone volume and bone formation rate are normalized. These results suggest that metabolism of GL-1 or LysoGL-1 into downstream bioactive lipids is a major contributor to the bone-formation defect. Direct testing revealed a strong inhibition of osteoblast viability by nanomolar concentrations of sphingosine, but not of ceramide. These findings are consistent with toxicity of high circulating sphingosine levels in GD1 patients, which decline upon enzyme-replacement therapy; serum ceramide levels remain unchanged. Together, complementary results from mice and humans affected with GD1 not only pinpoint sphingosine as being an osteoblast toxin, but also set forth Gba2 as a viable therapeutic target for the development of inhibitors to ameliorate certain disabling consequences of GD1.

Bone disease in patients with Gaucher disease

Gaucher disease is an inborn error of metabolism due to a deficiency of the lysosomal enzyme glucocerebrosidase. As a result of this deficiency, the substrate glucocerebroside accumulates in the liver, spleen, bone and bone marrow. Bone involvement can lead to abnormalities in bone growth, bone remodeling, bone infarcts, aseptic necrosis, osteonecrosis, increased fracture risk and lytic bone lesions. Patients may experience bone pain and bone crises related to bone infarcts. There is evidence of abnormal bone metabolism in both bone resorption and bone formation based upon biochemical abnormalities found in patients. In addition, both immunological and coagulation abnormalities have in part been implicated in the causation of bone disease. Treatment with enzyme replacement therapy and substrate reduction therapy has led to improvement in both the symptoms and the radiographic abnormalities seen in these patients. It is unknown whether these treatments lower fracture risk.

β-Glucosidase 2 (GBA2) activity and imino sugar pharmacology

β-Glucosidase 2 (GBA2) is an enzyme that cleaves the membrane lipid glucosylceramide into glucose and ceramide. The GBA2 gene is mutated in genetic neurological diseases (hereditary spastic paraplegia and cerebellar ataxia). Pharmacologically, GBA2 is reversibly inhibited by alkylated imino sugars that are in clinical use or are being developed for this purpose. We have addressed the ambiguity surrounding one of the defining characteristics of GBA2, which is its sensitivity to inhibition by conduritol B epoxide (CBE). We found that CBE inhibited GBA2, in vitro and in live cells, in a time-dependent fashion, which is typical for mechanism-based enzyme inactivators. Compared with the well characterized impact of CBE on the lysosomal glucosylceramide-degrading enzyme (glucocerebrosidase, GBA), CBE inactivated GBA2 less efficiently, due to a lower affinity for this enzyme (higher KI) and a lower rate of enzyme inactivation (k(inact)). In contrast to CBE, N-butyldeoxygalactonojirimycin exclusively inhibited GBA2. Accordingly, we propose to redefine GBA2 activity as the β-glucosidase that is sensitive to inhibition by N-butyldeoxygalactonojirimycin. Revised as such, GBA2 activity 1) was optimal at pH 5.5-6.0; 2) accounted for a much higher proportion of detergent-independent membrane-associated β-glucosidase activity; 3) was more variable among mouse tissues and neuroblastoma and monocyte cell lines; and 4) was more sensitive to inhibition by N-butyldeoxynojirimycin (miglustat, Zavesca®), in comparison with earlier studies. Our evaluation of GBA2 makes it possible to assess its activity more accurately, which will be helpful in analyzing its physiological roles and involvement in disease and in the pharmacological profiling of monosaccharide mimetics.

The cell biology of disease: lysosomal storage disorders: the cellular impact of lysosomal dysfunction

Lysosomal storage diseases (LSDs) are a family of disorders that result from inherited gene mutations that perturb lysosomal homeostasis. LSDs mainly stem from deficiencies in lysosomal enzymes, but also in some non-enzymatic lysosomal proteins, which lead to abnormal storage of macromolecular substrates. Valuable insights into lysosome functions have emerged from research into these diseases. In addition to primary lysosomal dysfunction, cellular pathways associated with other membrane-bound organelles are perturbed in these disorders. Through selective examples, we illustrate why the term “cellular storage disorders” may be a more appropriate description of these diseases and discuss therapies that can alleviate storage and restore normal cellular function.

Gaucher disease gene GBA functions in immune regulation

Inherited deficiency of acid β-glucosidase (GCase) due to biallelic mutations in the GBA (glucosidase, β, acid) gene causes the classic manifestations of Gaucher disease (GD) involving the viscera, the skeleton, and the lungs. Clinical observations point to immune defects in GD beyond the accumulation of activated macrophages engorged with lysosomal glucosylceramide. Here, we show a plethora of immune cell aberrations in mice in which the GBA gene is deleted conditionally in hematopoietic stem cells (HSCs). The thymus exhibited the earliest and most striking alterations reminiscent of impaired T-cell maturation, aberrant B-cell recruitment, enhanced antigen presentation, and impaired egress of mature thymocytes. These changes correlated strongly with disease severity. In contrast to the profound defects in the thymus, there were only limited cellular defects in peripheral lymphoid organs, mainly restricted to mice with severe disease. The cellular changes in GCase deficiency were accompanied by elevated T-helper (Th)1 and Th2 cytokines that also tracked with disease severity. Finally, the proliferation of GCase-deficient HSCs was inhibited significantly by both GL1 and Lyso-GL1, suggesting that the "supply" of early thymic progenitors from bone marrow may, in fact, be reduced in GBA deficiency. The results not only point to a fundamental role for GBA in immune regulation but also suggest that GBA mutations in GD may cause widespread immune dysregulation through the accumulation of substrates.

Genome-wide association study of N370S homozygous Gaucher disease reveals the candidacy of CLN8 gene as a genetic modifier contributing to extreme phenotypic variation

Mutations in GBA1 gene result in defective acid β-glucosidase and the complex phenotype of Gaucher disease (GD) related to the accumulation of glucosylceramide-laden macrophages. The phenotype is highly variable even among patients harboring identical GBA1 mutations. We hypothesize that modifier gene(s) underlie phenotypic diversity in GD and performed a GWAS study in Ashkenazi Jewish patients with type 1 GD (GD1), homozygous for N370S mutation. Patients were assigned to mild, moderate, or severe disease categories using composite disease severity scoring systems. Whole-genome genotyping for >500,000 SNPs was performed to search for association signals using OQLS algorithm in 139 eligible patients. Several SNPs in linkage disequilibrium within the CLN8 gene locus were associated with the GD1 severity: SNP rs11986414 was associated with GD1 severity at P value 1.26 × 10(-6) . Compared to mild disease, risk allele A at rs11986414 conferred an odds ratio of 3.72 for moderate/severe disease. Loss of function mutations in CLN8 causes neuronal ceroid-lipofuscinosis, but our results indicate that its increased expression may protect against severe GD1. In cultured skin fibroblasts, the relative expression of CLN8 was higher in mild GD compared to severely affected patients, in whom CLN8 risk alleles were overrepresented. In an in vitro cell model of GD, CLN8 expression was increased, which was further enhanced in the presence of bioactive substrate, glucosylsphingosine. Taken together, CLN8 is a candidate modifier gene for GD1 that may function as a protective sphingolipid sensor and/or in glycosphingolipid trafficking. Future studies should explore the role of CLN8 in pathophysiology of GD.

Glucocerebrosidase gene variants in parkinsonian patients with Machado Joseph/spinocerebellar ataxia 3

Machado-Joseph disease/spinocerebellar ataxia type 3 (MJD/SCA3) may rarely presents a parkinsonian phenotype. Considering that mutations in the glucocerebrosidase (GBA) gene have been associated with Parkinson disease, we investigated whether these would be more prevalent in MJD/SCA3 patients with parkinsonian manifestations than in those without them.

METHODS

MJD/SCA3 patients with parkinsonian features were identified and compared to relatives and to a MJD/SCA3 control group with no such features. The GBA gene was sequenced and, in a subset of patients and in normal volunteers, GBA enzyme activity was measured.

RESULTS

We have identified nine index MJD/SCA3 patients with parkinsonian manifestations. Overall, GBA sequence variations were found in 3/9 MJD/SCA3 index cases with parkinsonian manifestations (33%) and in 0/40 MJD/SCA3 controls without parkinsonism (p=0.03, Fisher exact test). The GBA sequence variations found were p.K(-27)R, p.E326K, and p.T369M. The latter two sequence variations were also found in two symptomatic relatives with no parkinsonian manifestations. A MJD/SCA3 relative belonging to the first positive pedigree and carrier of the p.K(-27)R mutation also presented parkinsonian manifestations. GBA activity in MJD/SCA3 patients was similar to those found in the normal control group.

CONCLUSION

Sequence variations at the GBA gene may play a role as a minor, modifying gene of MJD/SCA3 phenotype. This hypothetical role was not related to changes in GBA activity in peripheral leukocytes.

Pulmonary vascular disease in Gaucher disease: clinical spectrum, determinants of phenotype and long-term outcomes of therapy

Pulmonary arterial hypertension (PAH) and hepatopulmonary syndrome (HPS) are rare pulmonary vascular complications of type 1 Gaucher disease (GD1). We examined GBA1 genotype, spleen status, Severity Score Index (SSI), and other patient characteristics as determinants of GD/PAH-HPS phenotype. We also examined the long-term outcomes of imiglucerase enzyme replacement therapy (ERT) +/- adjuvant therapies in 14 consecutive patients. We hypothesized a role of BMPR2 and ALK1 as genetic modifiers underlying GD/PAH-HPS phenotype. Median age at diagnosis of GD1 was 5 yrs (2-22); PAH was diagnosed at median 36 yrs (22-63). There was a preponderance of females (ratio 5:2). ERT was commenced at median 36.5 yrs (16-53) and adjuvant therapy at 36 yrs (24-57). GBA1 genotype was N370S homozygous in two patients, N370S heteroallelic in 12. Median SSI was 15 (7-20). All patients had undergone splenectomy at median age 12 yrs (2-30). In three patients, HPS was the initial presentation, and PAH developed after its resolution; in these three, HPS responded dramatically to ERT. In seven patients, sequencing of the coding regions of BMPR2 and ALK1 was undertaken: 3/7 were heterozygous for BMPR2 polymorphisms; none harbored ALK1 variants. With ERT (± adjuvant therapy), 5/14 improved dramatically, five remained stable, two worsened, and two died prematurely. In this largest series of GD/PAH-HPS patients, there is preponderance of females and N370S heteroallelic GBA1 genotype. Splenectomy appears essential to development of this phenotype. In some patients, HPS precedes PAH. BMPR2 and ALK1 appear not be modifier genes for this rare phenotype of GD. ERT +/- adjuvant therapy improves prognosis of this devastating GD phenotype.

A reappraisal of Gaucher disease-diagnosis and disease management algorithms

Type 1 (non-neuronopathic) Gaucher disease was the first lysosomal storage disorder for which an effective enzyme replacement therapy was developed and it has become a prototype for treatments for related orphan diseases. There are currently four treatment options available to patients with Gaucher disease, nevertheless, almost 25% of Type 1 Gaucher patients do not gain timely access to therapy because of delays in diagnosis after the onset of symptoms. Diagnosis of Gaucher disease by enzyme testing is unequivocal, but the rarity of the disease and nonspecific and heterogeneous nature of Gaucher disease symptoms may impede consideration of this disease in the differential diagnosis. To help promote timely diagnosis and optimal management of the protean presentations of Gaucher disease, a consensus meeting was convened to develop algorithms for diagnosis and disease management for Gaucher disease.

Glucocerebrosidase gene-deficient mouse recapitulates Gaucher disease displaying cellular and molecular dysregulation beyond the macrophage

In nonneuronopathic type 1 Gaucher disease (GD1), mutations in the glucocerebrosidase gene (GBA1) gene result in glucocerebrosidase deficiency and the accumulation of its substrate, glucocerebroside (GL-1), in the lysosomes of mononuclear phagocytes. This prevailing macrophage-centric view, however, does not explain emerging aspects of the disease, including malignancy, autoimmune disease, Parkinson disease, and osteoporosis. We conditionally deleted the GBA1 gene in hematopoietic and mesenchymal cell lineages using an Mx1 promoter. Although this mouse fully recapitulated human GD1, cytokine measurements, microarray analysis, and cellular immunophenotyping together revealed widespread dysfunction not only of macrophages, but also of thymic T cells, dendritic cells, and osteoblasts. The severe osteoporosis was caused by a defect in osteoblastic bone formation arising from an inhibitory effect of the accumulated lipids LysoGL-1 and GL-1 on protein kinase C. This study provides direct evidence for the involvement in GD1 of multiple cell lineages, suggesting that cells other than macrophages may be worthwhile therapeutic targets.

High incidence of cholesterol gallstone disease in type 1 Gaucher disease: characterizing the biliary phenotype of type 1 Gaucher disease

BACKGROUND

In Gaucher disease (GD), lysosomal glucocerebrosidase deficiency results in glucosylceramide accumulation in macrophage lysosomes. Hepatocytes do not accumulate glucosylceramide due in part to biliary secretion. Although gallstones (GS) occur in type 1 Gaucher disease (GD1), the chemical nature of stones, their association with metabolic parameters, and whether bile composition is altered are not understood. We assessed the prevalence of GS, their chemical composition, biliary lipids, and associated metabolic factors.

METHODS

The study cohort comprised 417 patients comprehensively evaluated for GD1 severity. Ascertainment of GS, fasting lipoprotein profile, and bile lipid analyses were performed.

RESULTS

The prevalence of GS in GD1 was 32%. Compared with men, the prevalence of GS was higher in women, increasing from 4.2% and 11.8% at age 20-29 years to 71% and 60% at age >70 years, respectively. Patients with GS were more likely to be asplenic (p < 0.0001), older (p < 0.0001), have higher low-density lipoprotein (LDL) cholesterol (p = 0.002), and more severe GD1 disease compared with those without GS. On multiple logistic regression analysis, factors associated with GS were age (p < 0.001), female sex (p = 0.03), and splenectomy (p = 0.005). Compared with the general population, prevalence of GS was approximately 5-fold higher. Bile lipid analyses revealed cholesterol stones in five patients and pigment stones in one. Bile lipid composition was abnormal and contained glucosylceramide.

CONCLUSIONS

Our results point to a metabolic syndrome in GD1 consisting of a propensity to cholesterol GS, low high-density lipoprotein (HDL) cholesterol, LDL cholesterol, and body mass index (BMI) associated with abnormal biliary lipid secretion.

Expanding spectrum of the association between Type 1 Gaucher disease and cancers: a series of patients with up to 3 sequential cancers of multiple types--correlation with genotype and phenotype

In Gaucher disease (GD), inherited deficiency of lysosomal glucocerebrosidase due to mutations in GBA1 gene results in accumulation of glucosylceramide in tissue macrophages, systemic macrophage activation, and a complex multisystemic phenotype. We and others have reported an increased risk of multiple myeloma and other malignancies in non-neuronopathic Type 1 GD (GD1). Here, we describe a subset of GD1 patients with multiple malignancies. In our cohort of 403 patients with GD1, nine patients (2.2%) developed two or three different types of cancers either consecutively or simultaneously. Patients were characterized by age at diagnosis of GD1, GBA1 genotype, disease severity, age at cancer diagnosis, enzyme replacement therapy (ERT) status, and splenectomy status. Of the nine patients, six developed two types of malignancies and three had three cancers each. Overall, the hematologic malignancies comprised lymphoma/leukemia (4) and multiple myeloma (4). Nonhematologic malignancies included colon (2), lung (2), thyroid (2), and prostate cancer (1). Of the seven patients who received ERT, the first cancer was diagnosed before initiation of ERT in all but one. Asplenic patients were more likely to have single or multiple cancers compared with patients with intact spleens (P < 0.0072 and P < 0.0203, respectively). Our data strengthen the association of GD1 and cancer and suggest that patients may be at risk of developing multiple malignancies. We found an association between splenectomy and multiple cancers in GD1. It will be of interest to determine whether timely ERT and declining rates of splenectomy will translate into declining rates of multiple and single cancers.

Abnormal nonstoring capillary endothelium: a novel feature of Gaucher disease. Ultrastructural study of dermal capillaries

Ultrastructural study of skin biopsies in two cases of Gaucher disease (GD) patients (types II and III) revealed hitherto unknown alteration of the blood capillary endothelial cells (ECs) featured by hypertrophy and numerous subplasmalemmal microvesicles underneath both the apical and basal membranes. There was also prominent apical membrane folding with formation of filiform and large cytoplasmic projections, with occasional transcapillary cytoplasmic bridges. Similar, though less frequently expressed, changes were manifested at the basal membrane by numerous cytoplasmic projections into the subendothelial space. Regressive changes with EC breakdown were rare. Lysosomal storage was always absent. Besides EC hypertrophy, there was also increased EC density in the capillary lumen, leading to pronounced changes in capillary architecture with loose or incomplete EC anchoring. There were also signs of EC sprouting. Some pericytes displayed an increase in size and number of cytoplasmic processes, which often extended into distant pericapillary regions. The spectrum of changes suggests that a significant positive growth effect on EC occurs in GD. The putative mechanisms triggered by GBA1 deficiency leading to EC involvement are discussed. The authors are well aware of the fact the results, based on a nontraditional type of bioptic samples, are preliminary, but they are worth following, as further ultrastructural and functional studies of blood endothelium in GD may open a novel field in molecular cell pathophysiology of the disorder: endothelial dysfunction.

Autopsy case of Gaucher disease type I in a patient on enzyme replacement therapy. Comments on the dynamics of persistent storage process

We report a female patient with Gaucher disease (GD) type I on ERT (imiglucerase) for 5 years, which led to a significant general improvement. Aged 59 years she underwent an episode of altitude sickness followed by sepsis, disseminated intravascular coagulation, and multiorgan failure. She succumbed to a cerebral haemorrhage. Autopsy revealed liver cholestatic cirrhosis and multifocal liver carcinoma with immunophenotype compatible with cholangiocarcinoma. Analysis of the storage process revealed its absence or very low levels in the majority of liver and spleen macrophages. Gaucher cells (GCs) were seen only as occasional aggregates of various sizes in these organs. GCs were seen also in the leptomeninx of the cerebellum and as infrequent perivascular clusters in both the grey and white cerebral matters. Bone marrow was heavily infiltrated with GCs, especially in the adipocyte-rich part. GCs in this location displayed varied degrees of cytoplasmic vacuolation unrelated to the lysosomal compartment, caused by droplets of triglyceride, and interpreted as due to resorption of fragments of altered white adipocytes. All these observations point to the relative efficacy of ERT in covering the standard substrate load, which should not be exceeded as it would lead to the evolution of mature GCs. The results are discussed in relation to our recently published hypothesis on GD cell pathology.

Phenotypic heterogeneity of N370S homozygotes with type I Gaucher disease: an analysis of 798 patients from the ICGG Gaucher Registry

Gaucher disease is a lysosomal storage disorder caused by a deficiency of the enzyme acid beta-glucosidase. The most prevalent mutant genotype in type I Gaucher disease, N370S/N370S, is commonly thought to confer a mild phenotype presenting in adulthood. To characterize a subset of more severely affected N370S homozygotes, we assessed the phenotypes at or near the time of diagnosis of all N370S homozygotes with available data enrolled in the International Collaborative Gaucher Group Gaucher Registry. N370S compound heterozygotes were analyzed for comparison, as they are expected to present with a more severe phenotype. Of 798 N370S homozygotes and 1,278 N370S compound heterozygotes identified, 32% (251/788) and 65% (820/1269), respectively, were diagnosed before age 20 years. At diagnosis, N370S homozygotes as compared to N370S compound heterozygotes had the following clinical characteristics: irreversible skeletal lesions 17% (34/198) for N370S homozygotes versus 26% (76/290) for N370S compound heterozygotes; anaemia 18% (59/327) versus 29% (145/494); thrombocytopenia 52% (170/327) versus 62% (281/453); hepatomegaly 44% (83/190) versus 72% (141/195); splenomegaly 73% (142/194) versus 91% (178/195); and osteopenia or osteoporosis 48.6% (34/70) versus 51% (25/49). Some N370S homozygotes exhibited more severe clinical manifestations: 9% (29/327) had severe thrombocytopenia; 3% (5/190) had severe hepatomegaly; 11% (22/194) had severe splenomegaly; 7% (18/255) reported bone crises; 11% (8/70) had osteoporosis. In conclusion, N370S homozygosity does not consistently confer a mild, adult-onset phenotype. Gaucher disease patients with the N370S/N370S genotype exhibit a high degree of phenotypic heterogeneity and some may be at risk for early disease onset and severe clinical manifestations.

Treatment of lysosomal storage disorders : progress with enzyme replacement therapy

Enzyme replacement therapy (ERT) as treatment for lysosomal storage diseases (LSDs) was suggested as long ago as 1966 by De Duve and Wattiaux. However, it took >35 years to demonstrate the safety and effectiveness of ERT for type 1 Gaucher's disease. An important breakthrough was certainly the enactment of legislation in the US, designed to encourage commercialisation of products developed in academic institutions for pharmaceutical companies to invest in treatments for rare diseases. The principles elaborated in the development of the treatment of Gaucher's disease were subsequently applied to the development of ERT of other LSDs. The safety and effectiveness of ERT for Fabry's disease, mucopolysaccharidoses (MPS) I, MPS II and MPS VI, as well as for Pompe's disease have been demonstrated in well designed clinical trials, and the treatments are now commercially available throughout the world. Several questions remain to be answered. The long-term effectiveness of most of the treatments has not yet been established. What is reversible by ERT and what may not be reversible but is preventable, is not yet clear. The pathology in some tissues, such as the brain, is inaccessible to ERT, indicating that some manifestations of the LSD will not respond to the treatment. The extent of this problem is still unclear. The cost of ERT is very high, creating problems for third-party payers, which has strained reimbursement schemes based on the demonstration of acceptable cost effectiveness. ERT of LSDs represents the most important advance in the treatment of this class of diseases. The information that is currently being collected as part of large-scale observational studies will help to establish the full potential of the treatment.

Accumulation of Glucosylceramide in Murine Testis, Caused by Inhibition of β-Glucosidase 2

One of the hallmarks of male germ cell development is the formation of a specialized secretory organelle, the acrosome. This process can be pharmacologically disturbed in C57BL/6 mice, and thus infertility can be induced, by small molecular sugar-like compounds (alkylated imino sugars). Here the biochemical basis of this effect has been investigated. Our findings suggest that in vivo alkylated imino sugars primarily interact with the non-lysosomal glucosylceramidase. This enzyme cleaves glucosylceramide into glucose and ceramide, is sensitive to imino sugars in vitro, and has been characterized as beta-glucosidase 2 (GBA2). Imino sugars raised the level of glucosylceramide in brain, spleen, and testis, in a dose-dependent fashion. In testis, multiple species of glucosylceramide were similarly elevated, those having long acyl chains (C16-24), as well as those with very long polyunsaturated acyl chains (C28-30:5). Both of these GlcCer species were also increased in the testes from GBA2-deficient mice. When considering that the very long polyunsaturated sphingolipids are restricted to germ cells, these results indicate that in the testis GBA2 is present in both somatic and germ cells. Furthermore, in all mouse strains tested imino sugar treatment caused a rise in testicular glucosylceramide, even in a number of strains, of which the males remain fertile after drug administration. Therefore, it appears that acrosome formation can be derailed by accumulation of glucosylceramide in an extralysosomal localization, and that the sensitivity of male germ cells to glucosylceramide is genetically determined.