Type II sialidosis: review of the clinical spectrum and identification of a new splicing defect with chitotriosidase assessment in two patients.

Sialidosis is a lysosomal storage disease caused by the deficiency of alpha-N-acetyl neuraminidase-1 (NEU1). Sialidosis is classified into two main clinical variants: Type I, the milder form of the disease, and Type II, which can in turn be subdivided into three forms: congenital, infantile and juvenile. We report herein the clinical, biochemical and molecular characterisation of two patients with Type II sialidosis exhibiting the congenital (P1) and infantile forms (P2). We also review clinical data on the rare Type II forms of sialidosis in the hope of improving understanding of the disorder and facilitating its diagnosis. The genetic characterization of the two patients showed one known [c. 679G > A (p.G227R)] NEU1 missense mutation (detected in P2), and the new c.807 + 1G > A splicing defect (detected in P1), a genetic lesion that is extremely rare in this disease. Interestingly, P2 presented an extremely elevated level of chitotriosidase in plasma. This is the first pathological detection of chitotriosidase in sialidosis patients.

A Brazilian galactosialidosis patient given renal transplantation: a case report.

We report a Brazilian girl who was diagnosed as having galactosialidosis (deficiency of protective protein/cathepsin A; PPCA deficiency; GS) at the age of 2 years 6 months during an extensive investigation for renal failure. She was found to have low levels of both ß-galactosidase and α-neuraminidase in fibroblasts and to be a carrier of two novel mutations in the PPGB gene (p.G57V and p.R396W). She received a renal allograft at the age of 3 years 4 months. Transplantation was successful and graft function remains excellent after 6 years. However, the patient shows signs of progression of her primary disease. To our knowledge, she is the first GS patient to be given renal transplantation worldwide. We propose that renal transplantation should be considered as a therapeutic option for the treatment of severe renal complications of GS.

Gangliosides as apoptotic signals in ER stress response.

Renewed attention has been given lately to gangliosides and to their function as intracellular messengers of the adaptive responses to stress. Gangliosides are vital components of cell membranes; therefore, deleterious consequences can result from changes in their chemical composition and concentration, that is, membrane dynamics and structure can be altered as can the behavior of other membrane proteins. The importance of gangliosides in human health is evident in neurodegenerative diseases associated with defects in their degradation. As key modulators of intracellular calcium flux, gangliosides are involved in cellular processes downstream of calcium signaling. In this review, we focus on the effect of ganglioside accumulation on the endoplasmic reticulum calcium homeostasis and on the integrity of the mitochondrial membranes. We discuss how these events elicit an apoptotic program that ultimately leads to cell death. Owing to interorganelle crosstalk, these events are not necessarily self-contained, and gangliosides may serve as the common factor.

Substrate reduction reduces gangliosides in postnatal cerebrum-brainstem and cerebellum in GM1 gangliosidosis mice.

II3NeuAc-GgOse4Cer (GM1) gangliosidosis is an incurable lysosomal storage disease caused by a deficiency in acid beta-galactosidase (beta-gal), resulting in the accumulation of ganglioside GM1 and its asialo derivative GgOse4Cer (GA1) in the central nervous system, primarily in the brain. In this study, we investigated the effects of N-butyldeoxygalacto-nojirimycin (N B-DGJ), an imino sugar that inhibits ganglioside biosynthesis, in normal C57BL/6J mice and in beta-gal knockout (beta-gal-/-) mice from postnatal day 9 (p-9) to p-15. This is a period of active cerebellar development and central nervous system (CNS) myelinogenesis in the mouse and would be comparable to late-stage embryonic and early neonatal development in humans. N B-DGJ significantly reduced total ganglioside and GM1 content in cerebrum-brainstem (C-BS) and in cerebellum of normal and beta-gal-/- mice. N B-DGJ had no adverse effects on body weight or C-BS/cerebellar weight, water content, or thickness of the external cerebellar granule cell layer. Sphingomyelin was increased in C-BS and cerebellum, but no changes were found for cerebroside (a myelin-enriched glycosphingolipid), neutral phospholipids, or GA1 in the treated mice. Our findings indicate that the effects of N B-DGJ in the postnatal CNS are largely specific to gangliosides and suggest that N B-DGJ may be an effective early intervention therapy for GM1 gangliosidosis and other ganglioside storage disorders.

Four novel mutations in patients from the Middle East with the infantile form of GM1-gangliosidosis.

GM1-gangliosidosis is a lysosomal storage disorder caused by a deficiency of beta-galactosidase. It is mainly characterized by progressive neurodegeneration and in its most severe infantile form it leads to death before the age of four. We have performed molecular analysis of five patients with the infantile form of GM1-gangliosidosis originating from the Middle East (two from Saudi Arabia and three from the United Arab Emirates). We have identified four novel mutations and one previously reported mutation in the GLB1 gene. The first novel mutation found in the homoallelic state in a patient from Saudi Arabia, is a c.171C>G transversion in exon 2 which creates a premature stop codon. Northern blot analysis in fibroblasts from the patient showed no mRNA and expression studies in COS-1 cells showed complete absence of the 85kDa precursor protein and no catalytic activity. The second novel mutation is a splicing error in intron 2, c.245+1G>A. This mutation was found in the heteroallelic state in a patient from Saudi Arabia, the second mutation being the previously described c.145C>T mutation. The third novel mutation is a missense mutation in exon 4, c.451G>T, found in the homoallelic state in a patient from the United Arab Emirates. Expression studies of this mutation in COS-1 cells showed complete absence of the 85kDa precursor protein and no catalytic activity. The fourth novel mutation is a splicing mutation in intron 8, c.914+4A>G, found in the homoallelic state in two siblings from the United Arab Emirates. This study has revealed genetic heterogeneity of the beta-galactosidase deficiency in the Arabic population [corrected]

New mutations in two Dutch patients with early infantile galactosialidosis.

Galactosialidosis is an autosomal recessive lysosomal storage disease caused by a combined deficiency of lysosomal beta-galactosidase and neuraminidase as a result of a primary defect in the protective protein/cathepsin A (PPCA). We report the first 2 Dutch cases of early infantile galactosialidosis, both presenting with neonatal ascites. The defect was identified in urine, leukocytes, and fibroblasts. Residual activity was determined with a modified assay for cathepsin A and was <5% in leukocytes and <1% in fibroblasts. Histological examination of the placenta in case 1 showed extensive vacuolization in all cell types. Northern blot analysis of RNA isolated from the patients' cultured fibroblasts showed substantially decreased levels of the PPCA transcript, which nevertheless had the correct size of 2 kb. Mutation analysis of both mRNA and genomic DNA from the patients identified two novel mutations in the PPCA locus. Case 1 was a compound heterozygote, with a single missense mutation in one allele, which resulted in Gly57Ser amino acid substitution, and a single C insertion at nucleotide position 899 in the second allele, which gave rise to a frame shift and premature termination codon. Case 2 was homozygous for the same C899 insertion found in case 1.

Central nervous system inflammation is a hallmark of pathogenesis in mouse models of GM1 and GM2 gangliosidosis.

Mouse models of the GM2 gangliosidoses [Tay-Sachs, late onset Tay-Sachs (LOTS), Sandhoff] and GM1 gangliosidosis have been studied to determine whether there is a common neuro-inflammatory component to these disorders. During the disease course, we have: (i) examined the expression of a number of inflammatory markers in the CNS, including MHC class II, CD68, CD11b (CR3), 7/4, F4/80, nitrotyrosine, CD4 and CD8; (ii) profiled cytokine production [tumour necrosis factor alpha (TNF alpha), transforming growth factor (TGF beta 1) and interleukin 1 beta (IL1 beta)]; and (iii) studied blood-brain barrier (BBB) integrity. The kinetics of apoptosis and the expression of Fas and TNF-R1 were also assessed. In all symptomatic mouse models, a progressive increase in local microglial activation/expansion and infiltration of inflammatory cells was noted. Altered BBB permeability was evident in Sandhoff and GM1 mice, but absent in LOTS mice. Progressive CNS inflammation coincided with the onset of clinical signs in these mouse models. Substrate reduction therapy in the Sandhoff mouse model slowed the rate of accumulation of glycosphingolipids in the CNS, thus delaying the onset of the inflammatory process and disease pathogenesis. These data suggest that inflammation may play an important role in the pathogenesis of the gangliosidoses.

Novel mutations in lysosomal neuraminidase identify functional domains and determine clinical severity in sialidosis.

Lysosomal neuraminidase is the key enzyme for the intralysosomal catabolism of sialylated glycoconjugates and is deficient in two neurodegenerative lysosomal disorders, sialidosis and galactosialidosis. Here we report the identification of eight novel mutations in the neuraminidase gene of 11 sialidosis patients with various degrees of disease penetrance. Comparison of the primary structure of human neuraminidase with the primary and tertiary structures of bacterial sialidases indicated that most of the single amino acid substitutions occurred in functional motifs or conserved residues. On the basis of the subcellular distribution and residual catalytic activity of the mutant neuraminidases we assigned the mutant proteins to three groups: (i) catalytically inactive and not lysosomal; (ii) catalytically inactive, but localized in lysosome; and (iii) catalytically active and lysosomal. In general, there was a close correlation between the residual activity of the mutant enzymes and the clinical severity of disease. Patients with the severe infantile type II disease had mutations from group I, whereas patients with a mild form of type I disease had at least one mutation from group III. Mutations from the second group were mainly found in juvenile type II patients with intermediate clinical severity. Overall, our findings explain the clinical heterogeneity observed in sialidosis and may help in the assignment of existing or new allelic combinations to specific phenotypes.

Lysosomal neuraminidase. Catalytic activation in insect cells is controlled by the protective protein/cathepsin A.

Lysosomal N-Acetyl-alpha-neuraminidase is active in complex with the protective protein/cathepsin A (PPCA) and beta-galactosidase. The interaction with PPCA is essential for the correct intracellular routing and lysosomal localization of neuraminidase, but the mechanism of its catalytic activation is unclear. To investigate this process, we have used the baculovirus expression system to co-express neuraminidase and PPCA precursors in insect cells, which resulted in high enzymatic activity of neuraminidase. Both the 34- and 20-kDa PPCA subunits were required for the activation. We further demonstrated that when expressed alone, the neuraminidase precursor remained dimeric (114 kDa) and had low enzymatic activity, but when co-expressed with PPCA and beta-galactosidase, it multimerized in a complex of approximately 1350 kDa, together with the other two proteins. The fully active neuraminidase co-precipitated with full-length PPCA and beta-galactosidase precursors. However, when co-expressed with the individual PPCA subunits, neuraminidase co-precipitated only with the small 20-kDa polypeptide, which therefore must contain a neuraminidase-binding site. Our finding suggests a model of activation of neuraminidase dependent on its oligomerization at acidic pH that is mediated by interaction with PPCA.

Processing of lysosomal beta-galactosidase. The C-terminal precursor fragment is an essential domain of the mature enzyme.

Lysosomal beta-D-galactosidase (beta-gal), the enzyme deficient in the autosomal recessive disorders G(M1) gangliosidosis and Morquio B, is synthesized as an 85-kDa precursor that is C-terminally processed into a 64-66-kDa mature form. The released approximately 20-kDa proteolytic fragment was thought to be degraded. We now present evidence that it remains associated to the 64-kDa chain after partial proteolysis of the precursor. This polypeptide was found to copurify with beta-gal and protective protein/cathepsin A from mouse liver and Madin-Darby bovine kidney cells and was immunoprecipitated from human fibroblasts but not from fibroblasts of a G(M1) gangliosidosis and a galactosialidosis patient. Uptake of wild-type protective protein/cathepsin A by galactosialidosis fibroblasts resulted in a significant increase of mature and active beta-gal and its C-terminal fragment. Expression in COS-1 cells of mutant cDNAs encoding either the N-terminal or the C-terminal domain of beta-gal resulted in the synthesis of correctly sized polypeptides without catalytic activity. Only when co-expressed, the two subunits associate and become catalytically active. Our results suggest that the C terminus of beta-gal is an essential domain of the catalytically active enzyme and provide evidence that lysosomal beta-galactosidase is a two-subunit molecule. These data may give new significance to mutations in G(M1) gangliosidosis patients found in the C-terminal part of the molecule.

beta-galactosidase gene mutations affecting the lysosomal enzyme and the elastin-binding protein in GM1-gangliosidosis patients with cardiac involvement.

GM1-gangliosidosis is a lysosomal storage disorder caused by deficiency of acid beta-galactosidase (GLB1). We report five new beta-galactosidase gene mutations in nine Italian patients and one fetus, segregating in seven unrelated families. Six of the eight patients with the infantile, severe form of the disease presented cardiac involvement, a feature rarely associated with GM1-gangliosidosis. Molecular analysis of the patients' RNA and DNA identified two new RNA splicing defects, three new and three previously described amino acid substitutions. Interestingly, all patients with cardiac involvement were homozygous for one of these mutations: R59H, Y591C, Y591N, or IVS14-2A>G. In contrast, all other patients were compound heterozygous for one of the following mutations: R201H, R482H, G579D, IVS8+2T>C. Although we could not directly correlate the presence of cardiac abnormalities with specific genetic lesions, the mutations identified in patients with cardiomyopathy fell in the GLB1 cDNA region common to the lysosomal enzyme and the Hbeta-Gal-related protein, also known as the elastin binding protein (EBP). Consequently, both molecules are affected by the mutations, and they may contribute differently to the occurrence of specific clinical manifestations.

Enhanced proteasomal degradation of mutant human thiopurine S-methyltransferase (TPMT) in mammalian cells: mechanism for TPMT protein deficiency inherited by TPMT*2, TPMT*3A, TPMT*3B or TPMT*3C.

Inheritance of the TPMT*2, TPMT*3A and TPMT*3C mutant alleles is associated with deficiency of thiopurine S-methyltransferase (TPMT) activity in humans. However, unlike TPMT*2 and TPMT*3A, the catalytically active protein coded by TPMT*3C does not undergo enhanced proteolysis when heterologously expressed in yeast, making it unclear why this common mutant allele should be associated with inheritance of TPMT-deficiency. To further elucidate the mechanism for TPMT deficiency associated with these alleles, we characterized TPMT proteolysis following heterologous expression of wild-type and mutant proteins in mammalian cells. When expressed in COS-1 cells, proteins encoded by TPMT*2, TPMT*3A, and TPMT*3C cDNAs had significantly reduced steady-state levels and shorter degradation half-lives compared with the wild-type protein. Similarly, in rabbit reticulocyte lysate (RRL), these mutant TPMT proteins were degraded significantly faster than the wild-type protein. Thus, enhanced proteolysis of TPMT*3C protein in mammalian cells is in contrast to its stability in yeast, but consistent with TPMT-deficiency in humans. Proteolysis was ATP-dependent and sensitive to proteasomal inhibitors MG115, MG132 and lactacystin, but not to calpain inhibitor II. We conclude that all of these mutant TPMT proteins undergo enhanced proteolysis in mammalian cells, through an ATP-dependent proteasomal pathway, leading to low or undetectable levels of TPMT protein in humans who inherit these mutant alleles.

Six novel beta-galactosidase gene mutations in Brazilian patients with GM1-gangliosidosis.

GM1-gangliosidosis is a lysosomal storage disease caused by a deficiency of acid beta-galactosidase. Three clinical forms are recognized-infantile, juvenile, and adult-based on age of onset and severity of the symptoms. We have performed molecular analysis of a large cohort of GM1 patients (19 Brazilian and one Uruguayan), using nonradioactive single-strand conformation polymorphism (SSCP) and restriction enzyme analysis of genomic DNA. Six novel mutations (R121S, V240M, D491N, 638-641insT, 895-896insC, 1622-1627insG) and two previously described point mutations (R59H, R208C) were identified. Together they accounted for 90% of the disease alleles of the patients. Two mutations, 1622-1627insG and R59H, were present in 18 of 20 patients. In addition, four polymorphisms (L10P, L12L, R521C, S532G) were identified. All cases reported are infantile GM1 gangliosidosis. This report constitutes the most comprehensive molecular study to date of this disorder in infantile patients. Since GM1-gangliosidosis is the most common lysosomal storage disorder in Southern Brazil, molecular diagnosis will be important for genetic counseling, carrier detection and prenatal diagnosis in index families.

Correction of murine galactosialidosis by bone marrow-derived macrophages overexpressing human protective protein/cathepsin A under control of the colony-stimulating factor-1 receptor promoter.

Galactosialidosis (GS) is a human neurodegenerative disease caused by a deficiency of lysosomal protective protein/cathepsin A (PPCA). The GS mouse model resembles the severe human condition, resulting in nephropathy, ataxia, and premature death. To rescue the disease phenotype, GS mice were transplanted with bone marrow from transgenic mice overexpressing human PPCA specifically in monocytes/macrophages under the control of the colony stimulating factor-1 receptor promoter. Transgenic macrophages infiltrated and resided in all organs and expressed PPCA at high levels. Correction occurred in hematopoietic tissues and nonhematopoietic organs, including the central nervous system. PPCA-expressing perivascular and leptomeningeal macrophages were detected throughout the brain of recipient mice, although some neuronal cells, such as Purkinje cells, continued to show storage and died. GS mice crossed into the transgenic background reflected the outcome of bone marrow-transplanted mice, but the course of neuronal degeneration was delayed in this model. These studies present definite evidence that macrophages alone can provide a source of corrective enzyme for visceral organs and may be beneficial for neuronal correction if expression levels are sufficient.

Lack of PPCA expression only partially coincides with lysosomal storage in galactosialidosis mice: indirect evidence for spatial requirement of the catalytic rather than the protective function of PPCA.

Protective protein/cathepsin A (PPCA) is a pleiotropic lysosomal enzyme that complexes with beta-galactosidase and neuraminidase, and possesses serine carboxypeptidase activity. Its deficiency in man results in the neurodegenerative lysosomal storage disorder galactosialidosis (GS). The mouse model of this disease resembles the human early onset phenotype and results in severe nephropathy and ataxia. To understand better the pathophysiology of the disease, we compared the occurrence of lysosomal PPCA mRNA and protein in normal adult mouse tissues with the incidence of lysosomal storage in PPCA(-/-) mice. PPCA expression was markedly variable among different tissues. Most sites that produced both mRNA and protein at high levels in normal mice showed extensive and overt storage in the knockout mice. However, this correlation was not consistent as some cells that normally expressed high levels of PPCA were unaffected in their storage capability in the PPCA(-/-) mice. In addition, some normally low expressing cells accumulated large amounts of undegraded products in the GS mouse. This apparent discrepancy may reflect a requirement for the catalytic rather than the protective function of PPCA and/or the presence of cell-specific substrates in certain cell types. A detailed map showing the cellular distribution of PPCA in nomal mouse tissues as well as the sites of lysosomal storage in deficient mice is critical for accurate assessment of the effects of therapeutic interventions.

Transport of human lysosomal neuraminidase to mature lysosomes requires protective protein/cathepsin A.

Human lysosomal N-acetyl-alpha-neuraminidase is deficient in two lysosomal storage disorders, sialidosis, caused by structural mutations in the neuraminidase gene, and galactosialidosis, in which a primary defect of protective protein/cathepsin A (PPCA) leads to a combined deficiency of neuraminidase and beta-D-galactosidase. These three glycoproteins can be isolated in a high molecular weight multi-enzyme complex, and the enzymatic activity of neuraminidase is contingent on its interaction with PPCA. To explain the unusual need of neuraminidase for an auxiliary protein, we examined, in transfected COS-1 cells, the effect of PPCA expression on post-translational modification, turnover and intracellular localization of neuraminidase. In pulse-chase studies, we show that the enzyme is synthesized as a 46 kDa glycoprotein, which is poorly phosphorylated, does not undergo major proteolytic processing and is secreted. Importantly, its half-life is not altered by the presence of PPCA. However, neuraminidase associates with the PPCA precursor shortly after synthesis, since the latter protein co-precipitates with neuraminidase using anti-neuraminidase antibodies. We further demonstrate by subcellular fractionation of transfected cells that neuraminidase segregates to mature lysosomes only when accompanied by wild-type PPCA, but not by transport-impaired PPCA mutants. These data suggest a novel role for PPCA in the activation of lysosomal neuraminidase, that of an intracellular transport protein.