2 Gaucher Disease11This is manuscript 8587-MEM from The Scripps Research Institute. Supported by National Institutes of Health Grants DK36639 and RR00833 and the Sam Stein and Rose Stein Charitable Trust Fund

Parkin-mediated ubiquitination of mutant glucocerebrosidase leads to competition with its substrates PARIS and ARTS

Background

Parkinson’s disease (PD) is a movement neurodegenerative disorder characterized by death of dopaminergic neurons in the substantia nigra pars compacta of the brain that leads to movement impairments including bradykinesia, resting tremor, postural instability and rigidity. Mutations in several genes have been associated with familial PD, such as parkin, pink, DJ-1, LRKK2 and α-synuclein. Lately, mutations in the GBA gene were recognized as a major cause for the development of PD.

Mutations in the GBA gene, which encodes for lysosomal β-glucocerebrosidase (GCase), lead to Gaucher disease (GD), an autosomal recessive sphingolipidosis characterized by accumulation of glucosylceramide, mainly in monocyte-derived cells. It is a heterogeneous disease, with Type 1 patients that do not present any primary neurological signs, and Type 2 or Type 3 patients who suffer from a neurological disease. The propensity of type 1 GD patients and carriers of GD mutations to develop PD is significantly higher than that of the non-GD population.

We have shown in the past that parkin and mutant GCase, expressed in heterologous systems, interact with each other, and that normal but not mutant parkin mediates K48-dependent proteasomal degradation of mutant GCase variants.

Methods

We tested possible competition between mutant GCase and PARIS or ARTS on the E3 ubiquitin ligase parkin, using coimmunoprecipitation assays and quantitative real-time PCR.

Results

We show that endogenous mutant GCase variants associate with parkin and undergo parkin-dependent degradation. Mutant GCase competes with the known parkin substrates PARIS and ARTS, whose accumulation leads to apoptosis. Dopaminergic cells expressing mutant GCase are more susceptible to apoptotic stimuli than dopaminergic cells expressing normal GCase, present increased cleavage of caspase 3 and caspase 9 levels and undergo cell death.

Conclusions

Our results imply that presence of mutant GCase leads to accumulation of parkin substrates like PARIS and ARTS, which may cause apoptotic death of cells.

Complex haemostatic abnormalities as a cause of bleeding after neurosurgery in a patient with Gaucher disease

We report a treatment-naïve patient with Gaucher disease (GD) who experienced repeated bleeding after three neurosurgeries for a brain tumour, identified as an oligoastrocytoma. The patient had normal values on basic haemostatic tests: prothrombin time, 75-105%; activated partial thromboplastin time, 30.3-34 s; and mild thrombocytopaenia, 96-115 × 10(9 )cells/l. However, additional tests showed mild von Willebrand factor (vWF) deficiency (vWF antigen, 56%; vWF ristocetin cofactor, 49%; factor VIII [FVIII], 54%) and abnormal collagen-mediated platelet aggregation (0.45-0.55). Bleeding control was achieved after vWF/FVIII concentrate and platelet transfusions. This case raises questions about the safe platelet count and basic haemostatic tests for assessing bleeding risk in patients with GD prior to surgery. In patients with GD, a minimum haemostatic evaluation should include platelet count and basic haemostatic tests such as fibrinogen, prothrombin time, activated partial thromboplastin time as well as platelet function tests and assessing vWF and FVIII levels. Specific coagulation factors or platelet function deficiencies should be corrected with factor concentrates or platelet transfusions.

Reduced cathepsins B and D cause impaired autophagic degradation that can be almost completely restored by overexpression of these two proteases in Sap C-deficient fibroblasts

Saposin (Sap) C deficiency, a rare variant form of Gaucher disease, is due to mutations in the Sap C coding region of the prosaposin (PSAP) gene. Sap C is required as an activator of the lysosomal enzyme glucosylceramidase (GCase), which catalyzes glucosylceramide (GC) degradation. Deficit of either GCase or Sap C leads to the accumulation of undegraded GC and other lipids in lysosomes of monocyte/macrophage lineage. Recently, we reported that Sap C mutations affecting a cysteine residue result in increased autophagy. Here, we characterized the basis for the autophagic dysfunction. We analyzed Sap C-deficient and GCase-deficient fibroblasts and observed that autophagic disturbance was only associated with lack of Sap C. By a combined fluorescence microscopy and biochemical studies, we demonstrated that the accumulation of autophagosomes in Sap C-deficient fibroblasts is not due to enhanced autophagosome formation but to delayed degradation of autolysosomes caused, in part, to decreased amount and reduced enzymatic activity of cathepsins B and D. On the contrary, in GCase-deficient fibroblasts, the protein level and enzymatic activity of cathepsin D were comparable with control fibroblasts, whereas those of cathepsin B were almost doubled. Moreover, the enhanced expression of both these lysosomal proteases in Sap C-deficient fibroblasts resulted in close to functional autophagic degradation. Our data provide a novel example of altered autophagy as secondary event resulting from insufficient lysosomal function.

Ambroxol as a pharmacological chaperone for mutant glucocerebrosidase

Gaucher disease (GD) is characterized by accumulation of glucosylceramide in lysosomes due to mutations in the GBA1 gene encoding the lysosomal hydrolase β-glucocerebrosidase (GCase). The disease has a broad spectrum of phenotypes, which were divided into three different Types; Type 1 GD is not associated with primary neurological disease while Types 2 and 3 are associated with central nervous system disease. GCase molecules are synthesized on endoplasmic reticulum (ER)-bound polyribosomes, translocated into the ER and following modifications and correct folding, shuttle to the lysosomes. Mutant GCase molecules, which fail to fold correctly, undergo ER associated degradation (ERAD) in the proteasomes, the degree of which is one of the factors that determine GD severity.

Several pharmacological chaperones have already been shown to assist correct folding of mutant GCase molecules in the ER, thus facilitating their trafficking to the lysosomes. Ambroxol, a known expectorant, is one such chaperone. Here we show that ambroxol increases both the lysosomal fraction and the enzymatic activity of several mutant GCase variants in skin fibroblasts derived from Type 1 and Type 2 GD patients.

Gaucher disease paradigm: from ERAD to comorbidity

Mutations in the GBA gene, encoding the lysosomal acid beta-glucocerebrosidase (GCase), lead to deficient activity of the enzyme in the lysosomes, to glucosylceramide accumulation and to development of Gaucher disease (GD). More than 280 mutations in the GBA gene have been directly associated with GD. Mutant GCase variants present variable levels of endoplasmic reticulum (ER) retention, due to their inability to correctly fold, and undergo ER-associated degradation (ERAD) in the proteasomes. The degree of ER retention and proteasomal degradation is one of the factors that determine GD severity. In the present review, we discuss ERAD of mutant GCase variants and its possible consequences in GD patients and in carriers of GD mutations.

The enigma of the E326K mutation in acid β-glucocerebrosidase

A large number of mutations, and several polymorphisms, have been characterized in the GBA gene, encoding the lysosomal enzyme glucocerebrosidase, the activity of which is impaired in Gaucher disease. In this communication we summarize published and new data concerning biochemical characterization of the E326K amino acid change (1093G>A in the GBA1 cDNA) in tissue culture and its association with Parkinson disease, suggesting it is a disease causing mutation and not merely a polymorphism in the GBA gene.

Haemostatic abnormalities in treatment-naïve patients with Type 1 Gaucher's disease

There is a paucity of data on the effects of enzyme replacement therapy (ERT) on the coagulation abnormalities and platelet function of patients with Gaucher's disease (GDPs) and much of this data are controversial. This study investigates the haemostatic parameters in treatment-naïve GDPs and the effects of ERT. 31 Serbian treatment-naïve type 1 GDPs (M/F 17/14; median age 49 years, splenectomized 9/31) were studied. The complete blood count, prothrombin time (PT), activated partial tromboplastin time (aPTT) and coagulation factors were measured using the standard methods. Platelet aggregation was assessed with a whole-blood aggregometer. Splenic volumes were assessed using computer tomography. Twenty-one patients were treated with ERT (Imiglucerase). The haemostatic parameters were assessed after 6, 12 and 24 months (ERT(6, 12, 24)). Initially bleeding episodes were registered in 10/31 GDPs. Median platelet count was 108 × 10(9)/L; 22/31 GDPs were thrombocytopenic. The PT and aPTT values were abnormal in 16/31 and 13/31 GDPs, respectively. Platelet aggregation abnormalities were recorded in 19/31GDPs. Median platelet aggregation was reduced in response to adenosine-diphosphate 5 µmol/L (ADP(5) 0.46) and collagen 5 µmol/L (Col(5) 0.47). Splenic volume inversely correlated with the platelet count and a reduced response to arachidonic acid (AA), Col(5) and ADP(5) (p < 0.05). The splenectomized GDPs had a significantly lower platelet aggregation to Col(10) (p < 0.05). Bleeding GDPs had a significantly lower platelet count, higher chitotriosidase levels and a greater splenic volume compared to non-bleeding patients (p < 0.01). ERT: The number of bleeding GDPs had significantly decreased by ERT(6) (1/10; p < 0.01). The platelet count had significantly increased by ERT(6) (ERT(6) 180 × 10(9)/L, p < 0.01). The PT increased significantly from ERT(0) to ERT(24) (PT(0) 65%, PT(24) 81%; p < 0.05). The von Willebrand factor had increased significantly by ERT(6) and ERT(24) (ERT(0) 56%, ERT(6) 70%, ERT(12) 70%, ERT(24) 86%; p < 0.01). The number of GDPs with abnormal platelet aggregation had decreased significantly by ERT(6) (10/19; p < 0.05). Platelet aggregation on ADP(10) and AA significantly increased by ERT(6) (ADP(10): ERT(0) 0.75, ERT(6) 0.8 p < 0.01; AA: ERT(0) 0.7, ERT(6) 0.8 p < 0.05). In conclusion, platelet dysfunction and coagulation abnormalities were found in a considerable number of our GDPs. The absence of severe bleeding episodes suggests that the haemostatic system is sufficiently balanced and therefore the exact mechanism of the etiology of these abnormalities need to be fully clarified. ERT resulted in the cessation of bleeding and marked increase in platelet count, PT, vWF and platelet aggregation.

Interaction between parkin and mutant glucocerebrosidase variants: a possible link between Parkinson disease and Gaucher disease

Gaucher disease (GD), a sphingolipidosis characterized by impaired activity of the lysosomal enzyme glucocerebrosidase (GCase), results from mutations in the GCase-encoding gene, GBA. We have shown that mutant GCase variants present variable degrees of endoplasmic reticulum (ER) retention and undergo ER-associated degradation (ERAD) in the proteasome. Furthermore, the degree of ERAD of mutant GCase variants correlates with and is one of the factors that determine GD severity. An association between GD and Parkinson disease (PD) has been demonstrated by the concurrence of PD in GD patients and the identification of GCase mutations in probands with sporadic PD. One of the genes involved in PD is PARK2, encoding the E3 ubiquitin ligase parkin. Parkin functions in the ERAD of misfolded ER proteins, and it is upregulated by unfolded protein response. Loss of parkin function leads to the accumulation of its substrates, which is deleterious to dopaminergic neurons in PD. We, therefore, tested the possibility that the association between GD and PD reflects the fact that parkin acts as an E3 ligase of mutant GCase variants. Our results showed that mutant GCase variants associate with parkin. Normal parkin, but not its RING finger mutants, affects the stability of mutant GCase variants. Parkin also promotes the accumulation of mutant GCase in aggresome-like structures and is involved in K48-mediated polyubiquitination of GCase mutants, indicating its function as its E3 ligase. We suggest that involvement of parkin in the degradation of mutant GCase explains the concurrence of GD and PD.

Guidance on the use of miglustat for treating patients with type 1 Gaucher disease

Type 1 Gaucher disease (GD) is a progressive lysosomal storage disorder due to an autosomal recessive deficiency of glucocerebrosidase. Clinical manifestations include anemia, thrombocytopenia, hepatosplenomegaly, and bone and pulmonary disease. Intravenous enzyme replacement (ERT) with imiglucerase is the accepted standard for treatment of symptomatic patients. More than 3,500 patients worldwide have received ERT with well-documented beneficial effects on the hematological, visceral, skeletal, and pulmonary manifestations, and with resultant improvement in health-related quality of life. Miglustat, an imino sugar that reversibly inhibits glucosylceramide synthase and reduces intracellular substrate burden, is an oral treatment for patients with type 1 GD that was recently approved in the United States for symptomatic patients with mild to moderate clinical manifestations for whom ERT is not an option. Because responses to miglustat are slower and less robust than those observed with ERT, and because miglustat is associated with significant side effects, clinicians who care for patients with GD should become familiar with the limited indications for miglustat use and the circumstances when it may be prescribed appropriately. This review article and position statement represents the current opinion of American physicians with extensive expertise in GD regarding patient management in the context of the availability of standard imiglucerase treatment and the recent introduction of miglustat.

Enzyme replacement in Gaucher disease

The development of enzyme replacement therapy for Gaucher disease was a triumph of translational medicine. What were the key steps in its development? What are the controversies surrounding its use?

Mutations in the gene encoding cytosolic beta-glucosidase in Gaucher disease

Patients with Gaucher disease have a deficiency of the lysosomal acid beta-glucosidase. The phenotypes of genotypically identical patients with Gaucher disease may differ markedly. We have examined the possibility that polymorphisms in another beta-glucosidase are responsible for this variability in the phenotype. Sequence analysis of the gene encoding cytosolic beta-glucosidase (GBA3) from 4 chromosomes revealed the presence of 4 single-nucleotide substitutions: c.316 G -->A (D106N), c.1353A-->G (Y451Y), c.1368T-->A (Y456X), and c.1540 to 1541AG -->T in the 3' untranslated region. We examined the DNA from 62 patients with Gaucher disease who were homozygous for the 1226A-->G (N370S) mutation and from 542 control subjects from various populations for these polymorphisms. Six of the possible 16 haplotypes were found, and none was over- or underrepresented among patients with the severe Gaucher disease phenotypes compared with those from patients with mild phenotypes.

Transient expression of wild-type and mutant glucocerebrosidases in hybrid vaccinia expression system

Gaucher disease, the most prevalent lysosomal storage disease, is characterised by a significant phenotypic variation caused by more than 150 mutations. In order to verify pathogenicity of mutations found in the Czech Gaucher population, the vaccinia expression system was used. The wild-type human beta-glucocerebrosidase cDNA and cDNAs carrying the mutations 72delC, 1326insT, 1263del55, S196P, N370S, L444P, G202E, D409H, T369M, L444P+V460V, and D409H+T369M were expressed in Gaucher fibroblast cell line (L444P/S107L), BSC40, and HeLa G cells. The enzymatic activity and immunological reactivity were analysed. Only beta-glucocerebrosidase-deficient fibroblasts were suitable for expression using plasmid transfection. The expressed beta-glucosidase activity of mutant glucocerebrosidases was in good correlation with the presumed severity of the mutations.

Molecular analysis and clinical findings in the Spanish Gaucher disease population: putative haplotype of the N370S ancestral chromosome

Gaucher disease (GD) is an autosomal recessive disorder caused by mutations in the lysosomal beta-glucocerebrosidase (GBA) gene. As the disease is particularly prevalent among Ashkenazi Jews, most studies have been carried out on this ethnic group. In the current study, we present a mutation analysis of the GBA gene in Spanish patients together with the clinical findings. We conducted a systematic analysis in 53 unrelated GD patients. The GBA gene was initially scanned for nine previously described mutations by ASO hybridization or restriction analysis after PCR amplification. The remaining unidentified alleles were screened by nonisotopic PCR-SSCP analysis and sequenced. This approach allowed the identification of 101 of 106 GD alleles (95.3%) involving 24 different mutations, 11 of which are described for the first time: G113E (455G-->A), T134P (517A-->C), G389E (1283G-->A), P391L (1289C-->T), N392I (1292A-->T), Y412H (1351T-->G), W(-4)X (108G-->A), Q169X (662C-->T), R257X (886C-->T), 500insT, and IVS5+1G-->T. Most mutations are present in one or few GD chromosomes. However, two mutations, N370S (1226A-->G) and L444P (1448T-->C), are very frequent and account for 66.1% of the total number of alleles. Linkage disequilibrium was detected between these two mutations and an intragenic polymorphism, indicating that expansion of founder alleles occurred in both cases. Analysis of several microsatellite markers close to the GBA gene allowed us to establish the putative haplotype of the ancestral N370S chromosome.

Prevalence of glucocerebrosidase mutations in the Israeli Ashkenazi Jewish population

Gaucher disease is the most prevalent inherited disease among Ashkenazi Jews. It is very heterogeneous due to a large number of mutations within the glucocerebrosidase gene, whose impaired activity is the cause for this disease. Aiming at determining Gaucher carrier frequency among the Ashkenazi Jewish population in Israel, 1,208 individuals were molecularly diagnosed for six mutations known to occur among Ashkenazi Jewish Gaucher patients, using the newly developed Pronto Gaucher kit. The following mutations were tested: N370S, 84GG, IVS2+1, D409H, L444P, and V394L. Molecular testing of these mutations also allows identification of the recTL allele. The results indicated that Gaucher carrier frequency is 1:17 within the tested population. The prevalence of N370S carriers is 1:17.5. This implies that approximately 1:1225 Ashkenazi Jews will be homozygous for the N370S mutation. Actually, in our study of 1,208 individuals one was found to be homozygous for the N370S mutation. The actual number of known Ashkenazi Jewish Gaucher patients with this genotype is much lower than that expected according to the frequency of the N370S mutation, suggesting a low penetrance of this mutation. Results of loading experiments in cells homozygous for the N370S mutation, as well as cells homozygous for the L444P and the D409H mutations, exemplified this phenomenon.

Communicating hydrocephalus in a patient with Gaucher's disease type 3

The case of a 3-year-old male with type 3 Gaucher's disease, whose genotype for the beta-glucosidase gene was D409H/unknown mutation, is presented. After the onset of visceral and neurologic signs during infancy, a radiologic investigation at 3 years of age revealed communicating hydrocephalus, an unusual complication of Gaucher's disease. A ventriculoperitoneal shunt operation led to clinical and radiologic improvement. The possibility of this complication should be considered in the treatment of patients with Gaucher's disease.

A far upstream, cell type-specific enhancer of the mouse thrombospondin 3 gene is located within intron 6 of the adjacent metaxin gene

Thrombospondin 3 (TSP3) is a secreted, pentameric glycoprotein whose regulation of expression and function are not well understood. Mouse Thbs3 is located just downstream from the divergently transcribed metaxin gene (Mtx), which encodes an outer mitochondrial membrane import protein. Although Thbs3 and Mtx share a common promoter region, previous studies showed that Mtx is regulated by proximal elements that had little effect on Thbs3 expression. In this study, transient transfection of rat chondrosarcoma cells and NIH-3T3 fibroblasts demonstrated that Thbs3 is regulated in a cell type-specific manner by a position- and orientation-independent far upstream enhancer located within intron 6 of Mtx. Despite its greater proximity to the transcription start site of Mtx, the Thbs3 enhancer did not have a significant effect on Mtx expression. Two DNA-protein complexes, which were both required for activity, were identified when nuclear extracts were assayed with a probe containing the enhancer sequence. The protein in one of these complexes was identified as Sp1, while the other DNA-protein complex remains uncharacterized. A 6-kilobase pair promoter containing the enhancer was able to direct specific expression of the E. coli lacZ gene in transgenic mice, whereas a 2-kilobase pair promoter that lacked the enhancer was inactive. Thus, despite their close proximity, the genes of the Mtx/Thbs3 gene cluster are regulated independently.

Gaucher's Disease. Pathological features

Gaucher's disease is the most common lysosomal storage disease. The pathological features are a consequence of the progressive accumulation of glucosylceramide within mononuclear phagocytes. A wide variety of gross and microscopic anatomical changes are seen, primarily in the bone marrow, liver, spleen and bones. It is probable that cellular reactions to the presence of Gaucher cells ('lipid-engorged' macrophages) contribute to the tissue damage observed in this disease, although only a few investigations have been undertaken to elucidate what, if any, other mechanisms may play a contributory role in defining individual disease outcome. The general clinico-pathological features of Gaucher's disease are reviewed herein, with exclusion of the central nervous system and skin involvement, which are covered elsewhere.

Neuronopathic forms of Gaucher's disease

Neuronopathic Gaucher patients may have a wide variety of clinical manifestations and natural history, and can present with a range of degrees of severity of systemic disease and neurological deficit. The brain pathology of these patients has been well described, but the mechanism by which glucocerebrosidase deficiency leads to neuronal dysfunction is not yet understood. The almost 20 different mutations of the glucocerebrosidase gene that have been described in Type 2 and 3 Gaucher patients poorly predict the phenotype of individual patients. Enzyme replacement therapy (ERT), often at high doses, has been shown to reverse most of the systemic manifestations of this disease, but can rarely reverse the neurological deficits. Therefore, other forms of treatment, such as gene therapy or a more efficient and direct enzyme delivery to neurons, are being devised.

Metaxin is a component of a preprotein import complex in the outer membrane of the mammalian mitochondrion

Metaxin, a novel gene located between the glucocerebrosidase and thrombospondin 3 genes in the mouse, is essential for survival of the postimplantation mouse embryo. In this study, the subcellular location, domain structure, and biochemical function of metaxin were investigated. Anti-recombinant metaxin antibodies recognized 35- and 70-kDa proteins in mitochondria from various tissues; the 35-kDa protein is consistent in size with the predicted translation product of metaxin cDNA. When metaxin cDNA was transfected into COS cells, immunofluorescence staining demonstrated that the protein is located in mitochondria. Metaxin contains a putative mitochondrial outer membrane signal anchor domain at its C terminus, and a truncated form of metaxin lacking this signal anchor domain had a reduced association with mitochondria. In addition, metaxin was highly susceptible to proteases in intact mitochondria. We therefore conclude that metaxin is a mitochondrial protein that extends into the cytosol while anchored into the outer membrane at its C terminus. In its N-terminal region, metaxin shows significant sequence identity to Tom37, a component of the outer membrane portion of the mitochondrial preprotein translocation apparatus in Saccharomyces cerevisiae, but important structural differences, including apparently different mechanisms of targeting to membranes, also exist between the two proteins. Given the similar subcellular locations of metaxin and Tom37, the possible role of metaxin in mitochondrial preprotein import was investigated. Antibodies against metaxin, when preincubated with mitochondria, partially inhibited the uptake of radiolabeled preadrenodoxin into mitochondria. Metaxin is therefore the second mammalian component of the protein translocation apparatus of the mitochondrial outer membrane to be characterized at the molecular level and the first for which an inherited mutation has been described. The early embryonic lethal phenotype of mice lacking metaxin demonstrates that efficient import of proteins into mitochondria is crucial for cellular survival. The characterization of metaxin provides an opportunity to elucidate similarities and possible differences in the mechanisms of protein import between fungi and mammals and in the phenotypes of fungi and mammals lacking mitochondrial import receptors.