Diagnosis of Maroteaux-Lamy syndrome by the use of radiolabelled oligosaccharides as substrates for the determination of arylsulphatase B activity

Decoding the consecutive lysosomal degradation of 3-O-sulfate containing heparan sulfate by Arylsulfatase G (ARSG)

The lysosomal degradation of heparan sulfate is mediated by the concerted action of nine different enzymes. Within this degradation pathway, Arylsulfatase G (ARSG) is critical for removing 3-O-sulfate from glucosamine, and mutations in ARSG are causative for Usher syndrome type IV. We developed a specific ARSG enzyme assay using sulfated monosaccharide substrates, which reflect derivatives of its natural substrates. These sulfated compounds were incubated with ARSG, and resulting products were analyzed by reversed-phase HPLC after chemical addition of the fluorescent dyes 2-aminoacridone or 2-aminobenzoic acid, respectively. We applied the assay to further characterize ARSG regarding its hydrolytic specificity against 3-O-sulfated monosaccharides containing additional sulfate-groups and N-acetylation. The application of recombinant ARSG and cells overexpressing ARSG as well as isolated lysosomes from wild-type and Arsg knockout mice validated the utility of our assay. We further exploited the assay to determine the sequential action of the different sulfatases involved in the lysosomal catabolism of 3-O-sulfated glucosamine residues of heparan sulfate. Our results confirm and extend the characterization of the substrate specificity of ARSG and help to determine the sequential order of the lysosomal catabolic breakdown of (3-O-)sulfated heparan sulfate.

Case Report: Reinterpretation and Reclassification of ARSB:p.Arg159Cys Variant Identified in an Emirati Patient With Hearing Loss Caused by a Pathogenic Variant in the CDH23 Gene

Arylsulfatase B is an enzyme present in the lysosomes that involves in the breakdown of large sugar molecules known as glycosaminoglycans (GAGs). Arylsulfatase B chemically modifies two GAGs, namely, dermatan sulfate and chondroitin sulfate, by removing the sulfate group. Mutations in the gene encoding the arylsulfataseB enzyme causes lysosomal storage disorder, mucopolysaccharidosis type VI (MPS VI), or Maroteaux-Lamy syndrome. In this study, we report a case of congenital hearing loss with mild pigmentary changes in the retina, indicative of Usher syndrome, and a missense variant reported as likely pathogenic for MPS VI. Sequencing results identified a pathogenic missense variant p.Arg1746Gln in the CDH23 gene. However, another missense variant ARSB:p.Arg159Cys was reported as likely pathogenic to the treating physician. Mutations in ARSB gene have been associated with MPS VI. Subsequently, ARSB enzyme activity was found low twice in dried blood spot (DBS), suggestive of MPS VI. The patient did not have the clinical features of MPS VI, but considering the wide clinical spectrum, progressive nature of MPS VI, and the fact that a treatment for MPS VI is available to prevent disease progression, further biochemical, enzymatic, and in silico studies were performed to confirm the pathogenicity of this variant. In silico tools predicted this variant to be pathogenic. However, the results of urine and serum GAGs and ARSB enzyme levels measured from patient's fibroblast were found normal. Based on clinical and biochemical findings, ARSB:p.Arg159Cys is likely benign and did not support the diagnosis of MPS VI. However, CDH23:p.Arg1746Gln, a pathogenic variant, supports the underlying cause of hearing loss. This study highlights the importance of a robust correlation between genetic results and clinical presentation, and biochemical and enzymatic studies, to achieve a differential diagnosis.

Mucopolysaccharidosis enzyme production by bone marrow and dental pulp derived human mesenchymal stem cells

Mucopolysaccharidoses (MPS) are inherited metabolic disorders that arise from a complete loss or a reduction in one of eleven specific lysosomal enzymes. MPS children display pathology in multiple cell types leading to tissue and organ failure and early death. Mesenchymal stem cells (MSCs) give rise to many of the cell types affected in MPS, including those that are refractory to current treatment protocols such as hematopoietic stem cell (HSC) based therapy. In this study we compared multiple MPS enzyme production by bone marrow derived (hBM) and dental pulp derived (hDP) MSCs to enzyme production by HSCs. hBM MSCs produce significantly higher levels of MPS I, II, IIIA, IVA, VI and VII enzyme than HSCs, while hDP MSCs produce significantly higher levels of MPS I, IIIA, IVA, VI and VII enzymes. Higher transfection efficiency was observed in MSCs (89%) compared to HSCs (23%) using a lentiviral vector. Over-expression of four different lysosomal enzymes resulted in up to 9303-fold and up to 5559-fold greater levels in MSC cell layer and media respectively. Stable, persistent transduction of MSCs and sustained over-expression of MPS VII enzyme was observed in vitro. Transduction of MSCs did not affect the ability of the cells to differentiate down osteogenic, adipogenic or chondrogenic lineages, but did partially delay differentiation down the non-mesodermal neurogenic lineage.

Mucopolysaccharidosis type VI phenotypes-genotypes and antibody response to galsulfase

Background

Mucopolysaccharidosis type VI (Maroteaux-Lamy syndrome; MPS VI) is an autosomal recessive lysosomal storage disorder in which deficiency of N-acetylgalactosamine 4-sulfatase (arylsulfatase B; ARSB) leads to the storage of glycosaminoglycans (GAGs) in connective tissue. The genotype-phenotype correlation has been addressed in several publications but the picture is not complete. Since 2007, enzyme-replacement therapy (ERT) has been available for patients with MPS VI in the Netherlands. The purpose of our study was to learn more about the genotype-phenotype correlations in MPS VI and the antibody response to ERT with galsulfase (recombinant human arylsulfatase B).

Methods

We identified ARSB mutations in 12 patients and used site-directed mutagenesis to study their effect. Antibody levels to galsulfase were measured using ELISA and a semi-quantitative immunoprecipitation method. We assessed the in vitro inhibitory effect of antibodies on galsulfase uptake and their effect on clinical outcome.

Results

Five patients had a rapidly progressive phenotype and seven a slowly progressive phenotype. In total 9 pathogenic mutations were identified including 4 novel mutations (N301K, V332G, A237D, and c.1142 + 2 T > C) together composing 8 pathogenic genotypes. Most mutations appeared not to affect the synthesis of ARSB (66 kD precursor), but to hamper its maturation (43 kD ARSB). Disease severity was correlated with urinary GAG excretion. All patients developed antibodies to galsulfase within 26 weeks of treatment. It was demonstrated that these antibodies can inhibit the uptake of galsulfase in vitro.

Conclusions

The clinical phenotypes and the observed defects in the biosynthesis of ARSB show that some of the mutations that we identified are clearly more severe than others. Patients receiving galsulfase as enzyme-replacement therapy can develop antibodies towards the therapeutic protein. Though most titers are modest, they can exceed a level at which they potentially affect the clinical outcome of enzyme-replacement therapy.

Enzymatic Screening and Diagnosis of Lysosomal Storage Diseases

Lysosomal storage diseases (LSDs) are a group of more than 50 genetic disorders. Clinical symptoms are caused by the deficiency of specific enzyme (enzymes) function and resultant substrate accumulation in the lysosomes, which leads to impaired cellular function and progressive tissue and organ dysfunction. Measurement of lysosomal enzyme activity plays an important role in the clinical diagnosis of LSDs. The major enzymatic testing methods include fluorometric assays using artificial 4-methylumbelliferyl (4-MU) substrates, spectrophotometric assays and radioactive assays with radiolabeled natural substrates. As many effective treatment options have become available, presymptomatic diagnosis and early intervention are imperative. Many methods were developed in the past decade for newborn screening (NBS) of selective LSDs in dried blood spot (DBS) specimens. Modified fluorometric assays with 4-MU substrates, MS/MS or LC-MS/MS multiplex enzyme assays, digital microfluidic fluorometric assays, and immune-quantification assays for enzyme contents have been reported in NBS of LSDs, each with its own advantages and limitations. Active technical validation studies and pilot screening studies have been conducted or are ongoing. These studies have provided insight in the efficacy of various methodologies. In this review, technical aspects of the enzyme assays used in clinical diagnosis and NBS are summarized. The important findings from pilot NBS studies are also reviewed.

Expert recommendations for the laboratory diagnosis of MPS VI

Mucopolysaccharidosis VI (MPS VI) is a lysosomal storage disease caused by a deficiency of N-acetylgalactosamine 4-sulfatase (arylsulfatase B, ASB). This enzyme is required for the degradation of dermatan sulfate. In its absence, dermatan sulfate accumulates in cells and is excreted in large quantities in urine. Specific therapeutic intervention is available; however, accurate and timely diagnosis is crucial for maximal benefit. To better understand the current practices for diagnosis and to establish diagnostic guidelines, an international MPS VI laboratory diagnostics scientific summit was held in February of 2011 in Miami, Florida. The various steps in the diagnosis of MPS VI were discussed including urinary glycosaminoglycan (uGAG) analysis, enzyme activity analysis, and molecular analysis. The following conclusions were reached. Dilute urine samples pose a significant problem for uGAG analysis and MPS VI patients can be missed by quantitative uGAG testing alone as dermatan sulfate may not always be excreted in large quantities. Enzyme activity analysis is universally acknowledged as a key component of diagnosis; however, several caveats must be considered and the appropriate use of reference enzymes is essential. Molecular analysis supports enzyme activity test results and is essential for carrier testing, subsequent genetic counseling, and prenatal testing. Overall the expert panel recommends caution in the use of uGAG screening alone to rule out or confirm the diagnosis of MPS VI and acknowledges enzyme activity analysis as a critical component of diagnosis. Measurement of another sulfatase enzyme to exclude multiple sulfatase deficiency was recommended prior to the initiation of therapy. When feasible, the use of molecular testing as part of the diagnosis is encouraged. A diagnostic algorithm for MPS VI is provided.

Tandem mass spectrometry for the direct assay of lysosomal enzymes in dried blood spots: application to screening newborns for mucopolysaccharidosis VI (Maroteaux-Lamy syndrome)

We report a new assay of N-acetylgalactosamine-4-sulfatase (aryl sulfatase B) activity in dried blood spots (DBS) for the early detection of mucopolysaccharidosis VI (Maroteaux-Lamy syndrome) in newborn screening. The assay uses a synthetic substrate consisting of N-acetylgalactosamine-4-sulfate moiety glycosidically linked to a hydrophobic residue and furnished with a tert-butyloxycarbamido group as a marker for specific mass spectrometric fragmentation. Incubation with aryl sulfatase B present in DBS converts the substrate to a desulfated product which is detected by electrospray tandem mass spectrometry and quantified using a homologous internal standard. Assay and workup procedures were optimized to be compatible with the work flow in newborn screening laboratories. Analysis of DBS from human newborns showed clear distinction of aryl sulfatase B activity from 89 healthy individuals where it ranged between 1.4 and 16.9 μmol/(h L of blood), with an average activity of 7.4 μmol/(h L of blood), and an MPS-VI patient that had an activity of 0.12 μmol/(h L of blood). Results are also reported for the aryl sulfatase B assay in DBS from groups of normal felines and felines affected with MPS-VI.

Mucopolysaccharidosis VI

Mucopolysaccharidosis VI (MPS VI) is a lysosomal storage disease with progressive multisystem involvement, associated with a deficiency of arylsulfatase B leading to the accumulation of dermatan sulfate. Birth prevalence is between 1 in 43,261 and 1 in 1,505,160 live births. The disorder shows a wide spectrum of symptoms from slowly to rapidly progressing forms. The characteristic skeletal dysplasia includes short stature, dysostosis multiplex and degenerative joint disease. Rapidly progressing forms may have onset from birth, elevated urinary glycosaminoglycans (generally >100 microg/mg creatinine), severe dysostosis multiplex, short stature, and death before the 2nd or 3rd decades. A more slowly progressing form has been described as having later onset, mildly elevated glycosaminoglycans (generally <100 microg/mg creatinine), mild dysostosis multiplex, with death in the 4th or 5th decades. Other clinical findings may include cardiac valve disease, reduced pulmonary function, hepatosplenomegaly, sinusitis, otitis media, hearing loss, sleep apnea, corneal clouding, carpal tunnel disease, and inguinal or umbilical hernia. Although intellectual deficit is generally absent in MPS VI, central nervous system findings may include cervical cord compression caused by cervical spinal instability, meningeal thickening and/or bony stenosis, communicating hydrocephalus, optic nerve atrophy and blindness. The disorder is transmitted in an autosomal recessive manner and is caused by mutations in the ARSB gene, located in chromosome 5 (5q13-5q14). Over 130 ARSB mutations have been reported, causing absent or reduced arylsulfatase B (N-acetylgalactosamine 4-sulfatase) activity and interrupted dermatan sulfate and chondroitin sulfate degradation. Diagnosis generally requires evidence of clinical phenotype, arylsulfatase B enzyme activity <10% of the lower limit of normal in cultured fibroblasts or isolated leukocytes, and demonstration of a normal activity of a different sulfatase enzyme (to exclude multiple sulfatase deficiency). The finding of elevated urinary dermatan sulfate with the absence of heparan sulfate is supportive. In addition to multiple sulfatase deficiency, the differential diagnosis should also include other forms of MPS (MPS I, II IVA, VII), sialidosis and mucolipidosis. Before enzyme replacement therapy (ERT) with galsulfase (Naglazyme), clinical management was limited to supportive care and hematopoietic stem cell transplantation. Galsulfase is now widely available and is a specific therapy providing improved endurance with an acceptable safety profile. Prognosis is variable depending on the age of onset, rate of disease progression, age at initiation of ERT and on the quality of the medical care provided.

Development of a functional bioassay for arylsulfatase B using the natural substrates of the enzyme

A functional bioassay has been developed for measuring the intracellular activity of recombinant human arylsulfatase B (rhASB) on its natural glycosaminoglycan (GAG) substrates, dermatan sulfate (DS), and chondroitin sulfate (CS) when the enzyme is taken up into cultured ASB-deficient human fibroblasts (GM00519). The enzyme ASB is a lysosomal exohydrolase, cleaving sulfate from the N-acetylgalactosamine-4-sulfate (GalNAc-4S) residue at the nonreducing terminal of GAG structures. ASB-deficient cells accumulate DS and CS, which may be partially hydrolyzed by other lysosomal hydrolases, with the reactions stopping if a GalNAc-4S residue is reached on the nonreducing end of the oligosaccharide. When rhASB is added to the culture medium, the enzyme is taken up and translocates to the lysosomes and the intracellular DS and CS are depleted, demonstrating that the uptake of rhASB is able to restore lysosomal function in an in vitro cell-based assay. The accumulation and depletion of DS and CS are measured by digesting the residual intracellular DS and CS content with chondroitin ABC lyase and monitoring a characteristic disaccharide digestion product by laser-induced fluorescence-capillary zone electrophoresis (LIF-CZE). In the proposed assay format, GM00519 cells are cultured 5 weeks postconfluence to accumulate DS/CS, followed by incubation with rhASB (1-20 pM) for 5 days, and the CS/DS depletion profiles are compared between samples. The assay measures depletion of DS/CS independently of their molecular size or processing state; in this approach, all DS- and CS-like substances accumulating in the absence of ASB activity are considered to be natural substrates of the enzyme.

Early diagnosis of Maroteaux-Lamy syndrome in two patients with accelerated growth and advanced bone maturation

The diagnosis of mucopolysaccharidosis (MPS) VI is usually not made before the age of 2 or 3 years when the main clinical signs of dwarfed stature, skeletal deformities, coarsening facies, stiff joints and hepatosplenomegaly described in textbooks are recognisable. Conversely, accelerated growth with advanced bone age, a precocious feature of this condition although not suggestive of a storage disorder, is usually neither recognised, nor adequately interpreted. We report on two infants with MPS VI who presented with these two "unexpected" features within the 1st year of life.

CONCLUSION

Recognition of precocious excessive growth in a mucopolysaccharidosis enables an early diagnosis, the prime responsibility of the clinician, in order to propose early treatment like bone marrow transplantation or active recombinant sulphatase therapy, and appropriate genetic counselling.

Regulation of N-acetylgalactosamine 4-sulfatase expression in retrovirus-transduced feline mucopolysaccharidosis type VI muscle cells

As a preliminary step toward muscle-mediated gene therapy in the mucopolysaccharidosis (MPS) type VI cat, we have analyzed the transcriptional regulation of feline N-acetylgalactosamine 4-sulfatase (f4S) gene expression from various retroviral constructs in primary cultures of muscle cells. Two retroviral constructs were made containing the f4S cDNA under the transcriptional control of the human polypeptide chain-elongation factor 1alpha (EF1alpha) gene promoter or the cytomegalovirus (CMV) immediate-early promoter. Two further retroviral constructs were made with the murine muscle creatine kinase (mck) enhancer sequence upstream of the internal promoter. Virus made from each construct was used to transduce feline MPS VI myoblasts. The mck enhancer significantly upregulated f4S gene expression from both the EF1alpha promoter and the CMV promoter in transduced myoblasts and in differentiated myofibers. The highest level of 4S activity was observed in myoblasts and myofibers transduced with the retroviral construct Lmckcmv4S, in which the f4S gene is under the transcriptional regulation of the mck enhancer and CMV immediate-early promoter. Lmckcmv4S-transduced myofibers demonstrated correction of glycosaminoglycan storage and contained a 58-fold elevated level of 4S activity compared with normal myofibers. Recombinant f4S secreted from Lmckcmv4S-transduced myofibers was endocytosed by feline MPS VI myofibers, leading to correction of the biochemical storage phenotype.

Evaluation of fibroblast-mediated gene therapy in a feline model of mucopolysaccharidosis type VI

Fibroblast-mediated ex vivo gene therapy was evaluated in the N-acetylgalactosamine 4-sulfatase (4S) deficient mucopolysaccharidosis type VI (MPS VI) cat. Skin biopsies were obtained at birth from severely affected MPS VI kittens and used to initiate fibroblast outgrowths for retroviral transduction with the 4S cDNA. 4S gene expression in transduced cells was under the transcriptional control of the MoMLV long terminal repeat promoter or the cytomegalovirus (CMV) immediate-early promoter. Characterisation of gene-transduced fibroblasts demonstrated the cells to be over-expressing 4S activity. Twenty-four to forty million autologous, gene-corrected fibroblasts were implanted under the renal capsule of three MPS VI kittens at 8-16 weeks of age. Transient, low levels of 4S activity were detected in peripheral blood leukocytes shortly after implantation but were not detectable within 3-8 weeks' post-implantation. Long-term biochemical and clinical evaluation of these cats demonstrated identical disease progression to that previously described in untreated, clinically severe MPS VI cats.

Mild feline mucopolysaccharidosis type VI. Identification of an N-acetylgalactosamine-4-sulfatase mutation causing instability and increased specific activity

The missense mutation, L476P, in the N-acetylgalactosamine 4-sulfatase (4S) gene, has previously been shown to be associated with a severe feline mucopolysaccharidosis type VI (MPS VI) phenotype. The present study describes a second mutation, D520N, in the same MPS VI cat colony, which is inherited independently of L476P and is associated with a clinically mild MPS VI phenotype in D520N/L476P compound heterozygous cats. Biochemical and clinical assessment of L476P homozygous, D520N/L476P compound heterozygous, and D520N homozygous cats demonstrated that the entire range of clinical phenotypes, from severe MPS VI, to mild MPS VI, to normal are clustered within a narrow range of residual 4S activity from 0. 5% to 4.6% of normal levels. When overexpressed in CHO-KI cells, the secreted form of D520N 4S was inactivated in neutral pH conditions. In addition, intracellular D520N 4S protein was rapidly degraded and corresponded to 37%, 14.5%, and 0.67% of normal 4S protein levels in the microsomal, endosomal, and lysosomal compartments, respectively. However, the specific activity of lysosomal D520N 4S was elevated 22. 5-fold when compared with wild-type 4S. These results suggest that the D520N mutation causes a rapid degradation of 4S protein. The effect of this is partially ameliorated as a result of a significant elevation in the specific activity of mutant D520N 4S reaching the lysosomal compartment.

Sulfatases, trapping of the sulfated enzyme intermediate by substituting the active site formylglycine

Sulfatases contain an active site formylglycine residue that is generated by post-translational modification. Crystal structures of two lysosomal sulfatases revealed significant similarity to the catalytic site of alkaline phosphatase containing a serine at the position of formylglycine. To elucidate the catalytic mechanism of sulfate ester hydrolysis, the formylglycine of arylsulfatases A and B was substituted by serine. These mutants upon incubation with substrate were covalently sulfated at the introduced serine. This sulfated enzyme intermediate was stable at pH 5. At alkaline pH it was slowly hydrolyzed. These characteristics are analogous to that of alkaline phosphatase which forms a phosphoserine intermediate that is stable at pH 5, but is hydrolyzed at alkaline pH. In wild-type sulfatases the hydroxyl needed for formation of the sulfated enzyme intermediate is provided by the aldehyde hydrate of the formylglycine. The second, non-esterified hydroxyl of the aldehyde hydrate is essential for rapid desulfation of the enzyme at acidic pH, which most likely occurs by elimination. The lack of this second hydroxyl in the serine mutants explains the trapping of the sulfated enzyme intermediate. Thus, in acting as a geminal diol the formylglycine residue allows for efficient ester hydrolysis in an acidic milieu.

Two mutations within a feline mucopolysaccharidosis type VI colony cause three different clinical phenotypes

Mucopolysaccharidosis type VI (MPS VI) is a lysosomal storage disease caused by a deficiency of N-acetylgalactosamine-4-sulfatase (4S). A feline MPS VI model used to demonstrate efficacy of enzyme replacement therapy is due to the homozygous presence of an L476P mutation in 4-sulfatase. An additional mutation, D520N, inherited independently from L476P and recently identified in the same family of cats, has resulted in three clinical phenotypes. L476P homozygotes exhibit dwarfism and facial dysmorphia due to epiphyseal dysplasia, abnormally low leukocyte 4S/betahexosaminidase ratios, dermatan sulfaturia, lysosomal inclusions in most tissues including chondrocytes, corneal clouding, degenerative joint disease, and abnormal leukocyte inclusions. Similarly, D520N/D520N and L476P/D520N cats have abnormally low leukocyte 4S/betahexosaminidase ratios, mild dermatan sulfaturia, lysosomal inclusions in some chondrocytes, and abnormal leukocyte inclusions. However, both have normal growth and appearance. In addition, L476P/D520N cats have a high incidence of degenerative joint disease. We conclude that L476P/D520N cats have a very mild MPS VI phenotype not previously described in MPS VI humans. The study of L476P/D520N and D520N/ D520N genotypes will improve understanding of genotype to phenotype correlations and the pathogenesis of skeletal dysplasia and joint disease in MPS VI, and will assist in development of therapies to prevent lysosomal storage in chondrocytes.

Feline mucopolysaccharidosis type VI: correction of glycosaminoglycan storage in myoblasts by retrovirus-mediated transfer of the feline N-acetylgalactosamine 4-sulfatase gene

Mucopolysaccharidosis type VI (MPS VI) is an autosomal recessive lysosomal storage disorder characterised by the deficiency of N-acetylgalactosamine 4-sulfatase (4S). MPS VI has also been described in the cat. As an initial step toward muscle-mediated gene therapy in the MPS VI cat, we have made two retroviral constructs (pLf4S and pLf4SSN) that transduce the feline 4S gene. Both constructs were designed to express the feline 4S sequence from the viral long terminal repeat promoter. In addition pLf4SSN expressed the neomycin resistance gene from the SV40 early promoter. Amphotrophic virus was produced for each construct and used to transduce feline MPS VI myoblasts. Lf4S- and Lf4SSN-transduced MPS VI feline myoblasts demonstrated correction of glycosaminoglycan storage and contained 55-fold and 3.5-fold elevated levels of 4S activity when compared with normal feline myoblasts respectively. Recombinant feline 4S (rf4S) secreted by Lf4S-transduced MPS VI myoblasts was shown to be endocytosed by MPS VI feline cells via the mannose-6-phosphate receptor system, leading to metabolic correction. The results from this study demonstrate that muscle-mediated gene replacement therapy may be a viable method for achieving circulating levels of recombinant f4S (rf4S) in the MPS VI cat.

Expression, purification and characterization of recombinant caprine N-acetylglucosamine-6-sulphatase

Mucopolysaccharidosis type IIID or Sanfilippo D syndrome is a lysosomal storage disorder caused by the deficiency of N-acetylglucosamine-6-sulphatase (Glc6S). In addition to human patients, a Nubian goat with this disorder has been described and the caprine Glc6S (cGlc6S) cDNA cloned. In this study, the full-length cGlc6S cDNA was inserted into the expression vector, pEFNeo, which placed the cGlc6S cDNA under the transcriptional control of the human polypeptide chain elongation factor promoter. The pEFNeo expression vector also contains the human growth hormone polyadenylation signal and the genes encoding resistance to ampicillin and G418. The cGlc6S expression construct was electroporated into Chinese hamster ovary (CHO-K1) cells, and stably transfected clones were isolated. One clone, CHOrcGlc6S.17, which secreted the highest Glc6S activity into the culture medium, was selected and cultured in cell factories. The secreted recombinant cGlc6S (rcGlc6S) precursor was purified to homogeneity from conditioned medium by a two-column procedure which consisted of a Cu2+-chelating Sepharose column followed by TSK G3000SW gel filtration. The native molecular mass of rcFlc6S was estimated to be 102 kDa and the subunit size was 94 kDa. The kinetic properties of cGlc6S were similar to those of human Glc6S isolated from liver. rcGlc6S was endocytosed by fibroblasts from patients with mucopolysaccharidosis type IIID via the mannose 6-phosphate receptor-mediated pathway resulting in correction of the storage phenotype of these cells.

Immortalization and characterization of a cell line exhibiting a severe multiple sulphatase deficiency phenotype

Multiple sulphatase deficiency (MSD) is a rare genetic defect that causes a simultaneous deficiency of all known sulphatases. All available evidence suggests that the deficient gene product is normally responsible for the post-translational modification of a conserved cysteine residue to 2-amino-3-oxopropionic acid and that this modification is necessary for sulphatase activity. MSD often has an enzymically mild phenotype, with significant levels of residual sulphatase activity being detectable. Here we identify an MSD cell line in which the residual activity of the sulphatases assayed was generally very low. To characterize the phenotype of this cell line further, immortalized lines were established after transformation with simian virus 40 (SV40) T antigen. Immortalized cell lines representing normal and MSD phenotypes were then transduced with a retroviral vector carrying the gene encoding human N-acetylgalactosamine-4-sulphatase. Analysis of N-acetylgalactosamine-4-sulphatase protein synthesis and enzyme activity showed that transduced cell lines expressed large amounts of enzyme and that the specific activity of this enzyme was approx. 0.5-1.5% of normal, confirming that this cell line defines a severe phenotype for MSD. N-Acetylgalactosamine-4-sulphatase purified from a transduced MSD cell line seemed normal on denaturing PAGE. Kinetic analysis of the purified enzyme suggests that the residual activity is due to small amounts of normal enzyme rather than unmodified enzyme with low levels of residual activity. These cell lines and the availability of large amounts of inactive N-acetylgalactosamine-4-sulphatase from MSD cells should facilitate the further study of this disorder.

Structure of a human lysosomal sulfatase

BACKGROUND

. Sulfatases catalyze the hydrolysis of sulfuric acid esters from a wide variety of substrates including glycosaminoglycans, glycolipids and steroids. There is sufficient common sequence similarity within the class of sulfatase enzymes to indicate that they have a common structure. Deficiencies of specific lysosomal sulfatases that are involved in the degradation of glycosamino-glycans lead to rare inherited clinical disorders termed mucopolysaccharidoses. In sufferers of multiple sulfatase deficiency, all sulfatases are inactive because an essential post-translational modification of a specific active-site cysteine residue to oxo-alanine does not occur. Studies of this disorder have contributed to location and characterization of the sulfatase active site. To understand the catalytic mechanism of sulfatases, and ultimately the determinants of their substrate specificities, we have determined the structure of N-acetylgalactosamine-4-sulfatase.

RESULTS

. The crystal structure of the enzyme has been solved and refined at 2.5 resolution using data recorded at both 123K and 273K. The structure has two domains, the larger of which belongs to the alpha/beta class of proteins and contains the active site. The enzyme active site in the crystals contains several hitherto undescribed features. The active-site cysteine residue, Cys91, is found as the sulfate derivative of the aldehyde species, oxo-alanine. The sulfate is bound to a previously undetected metal ion, which we have identified as calcium. The structure of a vanadate-inhibited form of the enzyme has also been solved, and this structure shows that vanadate has replaced sulfate in the active site and that the vanadate is covalently linked to the protein. Preliminary data is presented for crystals soaked in the monosaccharide N-acetylgalactosamine, the structure of which forms a product complex of the enzyme.

CONCLUSIONS

. The structure of N-acetylgalactosamine-4-sulfatase reveals that residues conserved amongst the sulfatase family are involved in stabilizing the calcium ion and the sulfate ester in the active site. This suggests an archetypal fold for the family of sulfatases. A catalytic role is proposed for the post-translationally modified highly conserved cysteine residue. Despite a lack of any previously detectable sequence similarity to any protein of known structure, the large sulfatase domain that contains the active site closely resembles that of alkaline phosphatase: the calcium ion in sulfatase superposes on one of the zinc ions in alkaline phosphatase and the sulfate ester of Cys91 superposes on the phosphate ion found in the active site of alkaline phosphatase.

Feline mucopolysaccharidosis type VI. Characterization of recombinant N-acetylgalactosamine 4-sulfatase and identification of a mutation causing the disease

Mucopolysaccharidosis type VI (MPS VI) is an autosomal recessive disease caused by a deficiency of N-acetylgalactosamine 4-sulfatase (4S) leading to the lysosomal accumulation and urinary excretion of dermatan sulfate. MPS VI has also been described in the Siamese cat. As an initial step toward enzyme replacement therapy with recombinant feline 4S (rf4S) in MPS VI cats, the feline 4S cDNA was isolated and expressed in CHO-KI cells and rf4S was immunopurified from the culture medium. SDS-polyacrylamide gel electrophoresis analysis showed that the precursor form of immunopurified rf4S was a 66-kDa polypeptide that underwent maturation to a 43-44-kDa polypeptide. Endocytosis of rf4S by cultured feline MPS VI myoblasts was predominantly mediated by a mannose 6-phosphate receptor and resulted in the correction of dermatan sulfate storage. The mutation causing feline MPS VI was identified as a base substitution at codon 476, altering a leucine codon to a proline (L476P). The L476P allele displayed no detectable 4S activity when expressed in CHO-KI cells and was observed only as a "precursor" polypeptide that was not secreted into the medium. Identification of the mutation has allowed the development of a rapid PCR-based screening method to genotype individuals within the cat colony.

Targeted disruption of the arylsulfatase B gene results in mice resembling the phenotype of mucopolysaccharidosis VI

Mucopolysaccharidosis VI (MPS VI) is a lysosomal storage disease with autosomal recessive inheritance caused by a deficiency of the enzyme arylsulfatase B (ASB), which is involved in degradation of dermatan sulfate and chondroitin 4-sulfate. A MPS VI mouse model was generated by targeted disruption of the ASB gene. Homozygous mutant animals exhibit ASB enzyme deficiency and elevated urinary secretion of dermatan sulfate. They develop progressive symptoms resembling those of MPS VI in humans. Around 4 weeks of age facial dysmorphia becomes overt, long bones are shortened, and pelvic and costal abnormalities are observed. Major alterations in bone formation with perturbed cartilaginous tissues in newborns and widened, perturbed, and persisting growth plates in adult animals are seen. All major parenchymal organs show storage of glycosaminoglycans preferentially in interstitial cells and macrophages. Affected mice are fertile and mortality is not elevated up to 15 months of age. This mouse model will be a valuable tool for studying pathogenesis of MPS VI and may help to evaluate therapeutical approaches for lysosomal storage diseases.

Identification, expression, and biochemical characterization of N-acetylgalactosamine-4-sulfatase mutations and relationship with clinical phenotype in MPS-VI patients

Maroteaux-Lamy syndrome, or mucopolysaccharidosis type VI (MPS-VI), is a lysosomal storage disorder characterized by the defective degradation of dermatan sulfate due to the deficiency of N-acetylgalactosamine-4-sulfatase (4S). The clinical severity of MPS-VI ranges in a continuum from mildly affected to severely affected patients. Mutations in MPS-VI patient samples were identified by chemical cleavage and direct DNA sequencing of PCR products derived from patient cDNA. Five amino acid substitutions were identified (T92M, R95Q, Y210C, H393P, and L498P), individually introduced into the wild-type 4S cDNA by site-directed in vitro mutagenesis, and transfected into Chinese hamster ovary cells. Three of the five mutations (R95Q, Y210C, and H393P) were observed in >1 of 25 unrelated MPS-VI patients; however, the mutations were not found in 20 control individuals. The residual 4S activity and protein (biochemical phenotype) were determined for each mutant in order to confirm their identity as mutations and to dissect the contribution of each mutant allele to the overall clinical phenotype of the patient. For each patient, the combined biochemical phenotypes of the two 4S mutant alleles demonstrated a good correspondence with the observed clinical phenotype (with the possible exception of a patient who was a compound heterozygote for T92M and L498P). This preliminary correspondence between genotype and the phenotype in MPS-VI may, with further refinement, contribute to the assessment of therapeutic approaches for MPS-VI patients.

Expression, purification and characterization of recombinant human N-acetylgalactosamine-6-sulphatase

Full-length cDNA sequences encoding human N-acetylgalactosamine-6-sulphatase were stably expressed in Chinese hamster ovary cells under the transcriptional control of the human polypeptide chain elongation factor 1 alpha gene promoter. A clonal cell line overexpressing recombinant N-acetylgalactosamine-6-sulphatase to a level of approx. 3 mg/l of culture medium was isolated. The secreted precursor enzyme was purified to homogeneity by a two-column procedure with an overall yield of 53% of the activity. The physical and catalytic parameters of the recombinant enzyme were similar to those of the mature form isolated from liver. On SDS/PAGE and gel filtration, recombinant N-acetylgalactosamine-6-sulphatase had a native molecular mass of 58-60 kDa. Recombinant N-acetylgalactosamine-6-sulphatase was endocytosed by mucopolysaccharidosis IVA fibroblasts via the mannose-6-phosphate receptor-mediated pathway and was efficiently localized to lysosomes.

Two site-directed mutations abrogate enzyme activity but have different effects on the conformation and cellular content of the N-acetylgalactosamine 4-sulphatase protein

The sulphatase family of enzymes have regions of sequence similarity, but relatively little is known about either the structure-function relationships of sulphatases, or the role of highly conserved amino acids. The sequence of amino acids CTPSR at position 91-95 of 4-sulphatase has been shown to be highly conserved in all of the sequenced sulphatase enzymes. The cysteine at amino acid 91 of 4-sulphatase was selected for mutation analysis due to its potential role in either the active site, substrate-binding site or part of a key structural domain of 4-sulphatase and due to the absence of naturally occurring mutations in this residue in mucopolysaccharidosis type VI (MPS VI) patients. Two mutations, C91S and C91T, altering amino acid 91 of 4-sulphatase were generated and expressed in Chinese hamster ovary cells. Biochemical analysis of protein from a C91S cell line demonstrated no detectable 4-sulphatase enzyme activity but a relatively normal level of 4-sulphatase polypeptide (180% of the wild-type control protein level). Epitope detection, using a panel of ten monoclonal antibodies, demonstrated that the C91S polypeptide had a similar immunoreactivity to wild-type 4-sulphatase, suggesting that the C91S substitution does not induce a major structural change in the protein. Reduced catalytic activity associated with normal levels of 4-sulphatase protein have not been observed in any of the MPS VI patients tested and all show evidence of structural modification of 4-sulphatase protein with the same panel of antibodies [Brooks, McCourt, Gibson, Ashton, Shutter and Hopwood (1991) Am. J. Hum. Genet. 48, 710-719]. The loss of enzyme activity without a detectable protein conformation change suggests that Cys-91 may be a critical residue in the catalytic process. In contrast, analysis of protein from a C91T cell line revealed low levels of catalytically inactive 4-sulphatase polypeptide (0.37% of the wild-type control protein level) which had missing or masked epitopes, suggesting an altered protein structure or conformation. Subcellular fractionation studies of the C91T cell line demonstrated a high proportion of 4-sulphatase polypeptide content in organelles characteristic of microsomes. The aberrant intracellular localization and the reduced cellular content of 4-sulphatase polypeptide was consistent with the observed structural modification leading to retention and degradation of the protein within an early vacuolar compartment.

Lysosomal sulfate efflux following glycosaminoglycan degradation: measurements in enzyme-supplemented Maroteaux-Lamy syndrome fibroblasts and isolated lysosomes

Studies using lysosomal membrane vesicles have suggested that efflux of the sulfate that results from lysosomal glycosaminoglycan degradation is carrier-mediated. In this study, glycosaminoglycan degradation and sulfate efflux were examined using cultured skin fibroblasts and lysosomes deficient in the lysosomal enzyme N-acetylgalactosamine-4-sulfatase. Such fibroblasts store dermatan sulfate lysosomally, which could be labelled biosynthetically with Na2(35)SO4. The addition of recombinant N-acetylgalactosamine-4-sulfatase to the media of 35S labelled fibroblasts degraded up to 82% of the stored dermatan [35S] sulfate over a subsequent 96 h chase and released inorganic [35S] sulfate into the medium. In the presence of 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), sulfate was reused to a minor extent in newly synthesized proteoglycan. Isolated granules from recombinant enzyme supplemented fibroblasts degraded stored dermatan [35S]sulfate to sulfate which was rapidly released into the medium at a rate that was reduced by the extra-lysosomal presence of the lysosomal sulfate transport inhibitors SITS, Na2SO4 and Na2MoO4. SITS also inhibited dermatan sulfate turnover, although it had no effect on the action of purified recombinant enzyme in vitro. These data imply that sulfate clearance occurred concomitantly with dermatan sulfate turnover in the lysosome even at high substrate loading, and that lysosome-derived sulfate, while available, is reutilized minimally in synthetic pathways.

Overexpression of N-acetylgalactosamine-4-sulphatase induces a multiple sulphatase deficiency in mucopolysaccharidosis-type-VI fibroblasts

High-titre stocks of an amphotropic retrovirus, constructed so as to express a full-length cDNA encoding the human lysosomal enzyme N-acetylgalactosamine-4-sulphatase (4-sulphatase) from the cytomegalovirus immediate early promoter, were used to infect skin fibroblasts from a clinically severe mucopolysaccharidosis type VI (MPS VI) patient. The infected MPS VI cells showed correction of the enzymic defect with the enzyme being expressed at high levels and in the correct subcellular compartment. Surprisingly this did not result in correction of glycosaminoglycan turnover as measured by accumulation of 35S in metabolically labelled cells. We demonstrate that this is apparently caused by an induced reduction of the activities of other lysosomal sulphatases, presumably due to competition for a sulphatase-specific processing mechanism by the over-expressed 4-sulphatase. The level of steroid sulphatase, which is a microsomal sulphatase, was also reduced. Infection of skin fibroblasts from a second, clinically mildly affected, MPS VI patient with the same virus also resulted in no significant change in the level of glycosaminoglycan storage. However, in this case the cause of the observed phenomenon was less clear. These results are of obvious practical importance when considering gene therapy for a sulphatase deficiency such as MPS VI and also provide possible new avenues for exploration of the processes involved in sulphatase synthesis and genetically determined multiple sulphatase deficiency.

A cDNA clone for human glucosamine-6-sulphatase reveals differences between arylsulphatases and non-arylsulphatases

Glucosamine-6-sulphatase is an exo-hydrolase required for the lysosomal degradation of heparan sulphate and keratan sulphate. Deficiency of glucosamine-6-sulphatase activity leads to the lysosomal storage of the glycosaminoglycan, heparan sulphate and the monosaccharide sulphate N-acetylglucosamine 6-sulphate and the autosomal recessive genetic disorder mucopolysaccharidosis type IIID. Glucosamine-6-sulphatase can be classified as a non-arylsulphatase since, relative to arylsulphatase B, it shows negligible activity toward 4-methylumbelliferyl sulphate. We have isolated human cDNA clones and derived amino acid sequence coding for the entire glucosamine-6-sulphatase protein. The predicted sequence has 552 amino acids with a leader peptide of 36 amino acids and contains 13 potential N-glycosylation sites, of which it is likely that 10 are used. Glucosamine-6-sulphatase shows strong sequence similarity to other sulphatases such as the family of arylsulphatases, although the degree of similarity is not as high as that between members of the arylsulphatase family. This pattern of inter- and intra-family similarity delineates regions and amino acid residues that may be critical for sulphatase function and substrate specificity.

Components and proteolytic processing sites of arylsulfatase B from human placenta

Previous studies have shown that mature arylsulfatase B purified from human sources is composed of two non-identical chains with apparent molecular masses of 43 kDa and 8 kDa. Arylsulfatase B purified from human placenta in the present study, however, included another 7 kDa component that could be detected only by carbohydrate staining on reducing SDS-PAGE employing the Tris-Tricine system. The 43 kDa and 7 kDa components contained a carbohydrate moiety, but the 8 kDa one did not, as demonstrated by periodic acid-Schiff staining, Con-A lectin blotting, endo-glycosidase treatment and in vitro phosphorylation by UDP-N-acetylglucosamine: lysosomal enzyme N-acetylglucosamine 1-phosphotransferase. The purified arylsulfatase B migrated as a single polypeptide of 58 kDa on non-reducing SDS-PAGE, indicating that the three chains are linked by disulfide bonds. In order to determine the origin of the components, N-terminal sequencing of the isolated polypeptides was performed. As a result, the 43, 7 and 8 kDa components were found to commence with Ala-41, Ala-424 and Asp-466, respectively. These results suggest that after removal of the signal peptide, human arylsulfatase B undergoes proteolytic processing on at least two sites during maturation.

Correction of human mucopolysaccharidosis type-VI fibroblasts with recombinant N-acetylgalactosamine-4-sulphatase

A full-length human N-acetylgalactosamine-4-sulphatase (4-sulphatase) cDNA clone was constructed and expressed in CHO-DK1 cells under the transcriptional control of the Rous sarcoma virus long terminal repeat. A clonal cell line expressing high activities of human 4-sulphatase was isolated. The maturation and processing of the human enzyme in this transfected CHO cell line showed it to be identical with that seen in normal human skin fibroblasts. The high-uptake precursor form of the recombinant enzyme was purified from the medium of the transfected cells treated with NH4Cl and was shown to be efficiently endocytosed by control fibroblasts and by fibroblasts from a mucopolysaccharidosis type-VI (MPS VI) patient. Enzyme uptake was inhibitable by mannose 6-phosphate. After uptake, the enzyme was processed normally in both normal and MPS VI fibroblasts and was shown both to correct the enzymic defect and to initiate degradation of [35S]sulphated dermatan sulphate in MPS VI fibroblasts. The stabilities of the recombinant enzyme and enzyme from human fibroblasts appeared to be similar after uptake. However, endocytosed enzyme has a significantly shorter half-life than endogenous human enzyme. The purified precursor 4-sulphatase had a similar pH optimum and catalytic parameters to the mature form of 4-sulphatase isolated from human liver.

Mucopolysaccharidosis type VI: identification of three mutations in the arylsulfatase B gene of patients with the severe and mild phenotypes provides molecular evidence for genetic heterogeneity

Mucopolysaccharidosis type VI (MPS VI; Maroteaux-Lamy disease) results from the deficient activity of the lysosomal enzyme, arylsulfatase B (ASB; N-acetylgalactosamine-4-sulfatase E.C.3.1.6.1). The enzymatic defect leads to the accumulation of the glycosaminoglycan, dermatan sulfate, primarily in connective tissue and reticuloendothelial cell lysosomes. Although MPS VI patients have normal intelligence and no neurologic abnormalities, the disease is clinically heterogeneous: severely affected individuals expire in childhood or early adolescence while those with the mild or intermediate phenotypes have a slower, milder disease course and a longer life span. The recent isolation of the full-length cDNA-encoding human ASB permitted an investigation of the molecular lesions underlying the phenotypic heterogeneity in MPS VI. The ASB cDNA-coding sequences were determined from two unrelated MPS VI patients with the severe (proband 1) and mild (proband 2) phenotypes. These patients had about 2% and 7% of normal ASB activity in cultured fibroblasts, respectively. Proband 1 was homoallelic for a T-to-C transition in nucleotide (nt) 349, which predicted a cysteine-to-arginine substitution in the ASB polypeptide at residue 117 (C117R). Proband 2 was heteroallelic, having a T-to-C transition in nt 707, which predicted a leucine-to-proline replacement at ASB residue 236 (L236P), and having a G-to-A transition in nt 1214, which predicted a cysteine-to-tyrosine substitution at ASB residue 405 (C405Y). These mutations did not occur in three other unrelated MPS VI patients or in 120 ASB alleles from normal individuals, indicating that they were not polymorphisms. The identification of these three ASB mutations documents the first evidence of molecular heterogeneity in MPS VI and provides an initial basis for genotype/phenotype correlations in this lysosomal storage disease.

Human glucosamine-6-sulphatase deficiency. Diagnostic enzymology towards heparin-derived trisaccharide substrates

Glucosamine-6-sulphatase (6S) activity towards a series of radiolabelled heparin-derived trisaccharide substrates was determined in cultured human skin fibroblast and leucocyte homogenates, and in urine supernatants of normal individuals and patients affected with 6S deficiency [Sanfilippo D syndrome; mucopolysaccharidosis (MPS) type IIID]. The N-sulphated and N-acetylated derivatives of the trisaccharide substrate O-(alpha-glucosamine 6-sulphate)-(1----4)-L-O-(alpha-iduronic acid 2-sulphate)-(1----4)-D-O-2,5-anhydro[1-3H]mannitol 6-sulphate (GlcNH6S-IdoA2S-anM6S) were prepared by enzymic digestion of a pentasulphated tetrasaccharide isolated following the HNO2 deamination of heparin. Purified lysosomal enzymes and MPS-patient skin fibroblasts were used along with chemical degradation to confirm the structure of each of the substrates that were utilized to study the interaction of the enzyme activities required to degrade the highly sulphated regions of heparan sulphate. Human liver, skin fibroblast and urine 6S activities were separated by chromatofocusing into at least four and possibly up to six individual activities. 6S activities present in each of the tissues generally had similar catalytic properties, including Km values, pH optima and inhibition with NaCl, Na2SO4 and NaH2PO4. Leucocyte and skin fibroblast 6S activities towards GlcNAc6S-IdoA2S-anM6S were maximal at pH 4.1 and 3.9 respectively, with Km values of 2.8 microM and 0.9-1.7 microM respectively. Urine 6S activity towards GlcNAc6S-IdoA2S-anM6S was stimulated 30-fold by BSA at pH 3.9, which shifted the pH optimum from 5.1 to 4.2 and decreased the Km value at pH 4.2 from 4.0 microM to 0.5 microM. Residual 6S activity present in the skin fibroblast homogenates from MPS IIID patients was characterized for activity towards GlcNAc6S-IdoA2S-anM6S and observed to have similar pH optima and Km values to normal skin fibroblast 6S activities, although the residual 6S activity was less than 1% of the normal control range.

An N-acetylgalactosamine-4-sulfatase mutation (delta G238) results in a severe Maroteaux-Lamy phenotype

Maroteaux-Lamy syndrome (mucopolysaccharidosis type VI, MPS VI) is an autosomally inherited lysosomal storage disorder caused by a deficiency of N-acetylgalactosamine-4-sulfatase (EC 3.1.6.1; 4-sulfatase). In order to determine the gene defect in a clinically severe MPS VI patient, polymerase chain reaction (PCR) products were generated from the patient's fibroblast mRNA and also from a 4-sulfatase cDNA clone and subjected to the chemical cleavage technique to detect mismatched bases, which were then identified by direct DNA sequencing of the PCR products. The patient was homozygous for an early frameshift mutation caused by the deletion of a G at position 238 (delta G238), which produces a truncated 4-sulfatase with an altered amino acid sequence from amino acid 80 to a premature stop codon at codon 113 relative to the normal 4-sulfatase reading frame of 533 amino acids. Since the mutation occurs only 40 amino acids past the signal peptidase cleavage site, it is most likely that this will result in a protein with no 4-sulfatase activity. This is consistent with the severe clinical presentation and the absence of 4-sulfatase enzyme activity or mutant 4-sulfatase protein in the patient. The patient was also found to be homozygous for two polymorphisms, i.e., a G to A transition at nucleotide 1072 resulting in a valine358 to methionine substitution (V358M) and a salient A to G transition in the third base of the proline397 codon at nucleotide 1191.

Human liver N-acetylgalactosamine 6-sulphatase. Purification and characterization

Human N-acetylgalactosamine 6-sulphatase (EC 3.1.6.14), which is involved in the lysosomal degradation of the glycosaminoglycans keratan sulphate and chondroitin 6-sulphate, was purified more than 130,000-fold in 2.8% yield from liver by an eight-step column procedure. One major form was identified with a pI of 5.7 and a native molecular mass of 62 kDa by gel filtration. When analysed by SDS/PAGE, dithioerythritol-reduced enzyme contained polypeptides of molecular masses 57 kDa, 39 kDa and 19 kDa, whereas non-reduced enzyme contained a major polypeptide of molecular mass 70 kDa. It is proposed that active enzyme contains either the 57 kDa polypeptide or disulphide-linked 39 kDa and 19 kDa polypeptides. Minor amounts of other enzyme forms separated during the chromatofocusing step and the Blue A-agarose step were not further characterized. Purified N-acetylgalactosamine 6-sulphatase was inactive towards 4-methylumbelliferyl sulphate, but was active, with pH optima of 3.5-4.0, towards 6-sulphated oligosaccharide substrates. Km values of 12.5 and 50 microM and Vmax. values of 1.5 and 0.09 mumol/min per mg were determined with oligosaccharide substrates derived from chondroitin 6-sulphate and keratan sulphate respectively. Sulphate, phosphate and chloride ions were inhibitors of enzyme activity towards both substrates, with 50 microM-Na2SO4 giving 50% inhibition towards the chondroitin 6-sulphate trisaccharide substrate.

Sanfilippo D syndrome: correction of glucosamine-6-sulphatase deficiency following fibroblast culture in Chang's media

The de-O-sulphation of alpha-linked glucosamine-6-sulphate residues in heparan sulphate requires a specific sulphatase, glucosamine-6-sulphatase, which has been shown to be deficient in tissues of Sanfilippo D, or mucopolysaccharidosis type IIID (MPS IIID), patients. MPS IIID fibroblasts cultured in Basal Eagle's medium supplemented with either fetal calf serum or heat-inactivated fetal calf serum, MDCB or Ultraserg media had residual glucosamine-6-sulphatase activities towards a heparin-derived trisaccharide substrate, O-(alpha-N-acetylglucosamine-6-sulphate)-(1----4)-L-O-(alpha- iduronic acid-2-sulphate)-(1----4)-D-O-2,5-anhydro[1-3H]mannitol-6- sulphate, GlcNAc6S-IdoA2S-anM6S, which were less than 1 per cent of the normal range for fibroblasts cultured in Basal Eagle's medium supplemented with fetal calf serum. However, the glucosamine-6-sulphatase activities of MPS IIID fibroblasts grown in Chang's medium were similar to the activities in normal control fibroblasts which were cultured in Basal Eagle's medium. These results indicate that caution is required for prenatal diagnosis of MPS IIID patients using chorionic villi or amniotic cells cultured in Chang's medium.

Analysis of N-acetylgalactosamine-4-sulfatase protein and kinetics in mucopolysaccharidosis type VI patients

A sensitive and specific, monoclonal antibody-based immunoquantification assay has facilitated determination of the N-acetylgalactosamine-4-sulfatase (4-sulfatase) protein content in cultured fibroblasts from normal controls and mucopolysaccharidosis type VI (MPS VI) patients. The assay enabled the quantification of 4-sulfatase protein by using a panel of seven monoclonal antibodies and has shown that fibroblasts from 16 MPS VI patients contained less than or equal to 5% of the level determined for normal controls. Fibroblasts from the most severely affected patients contained the lowest levels of 4-sulfatase protein, usually with few epitopes detected, while fibroblasts from mildly affected patients had higher levels of 4-sulfatase protein, with all seven epitopes detected. The pattern of epitope expression is proposed to reflect the conformational changes in the 4-sulfatase protein that arise from different mutations in the 4-sulfatase gene. Immunoquantification in combination with a specific and highly sensitive 4-sulfated trisaccharide-based assay of enzyme activity in these MPS VI patient fibroblasts enabled the determination of residual 4-sulfatase catalytic efficiency (kcat/Km). The capacity of fibroblasts to degrade substrate (catalytic capacity) was calculated as the product of 4-sulfatase catalytic efficiency and the content of 4-sulfatase in fibroblasts. One patient, 2357, with no clinical signs of MPS VI but with reduced 4-sulfatase activity and protein (both 5% of normal) and dermatansulfaturia, had 5% of normal catalytic capacity. The other 15 MPS VI patient fibroblasts had 0%-1.4% of the catalytic capacity of fibroblasts from normal controls and were representative of the spectrum of MPS VI clinical phenotypes, from severe to mild.(ABSTRACT TRUNCATED AT 250 WORDS)

A specific fluorogenic assay for N-acetylgalactosamine-4-sulphatase activity using immunoadsorption

A method combining immune capture and enzyme detection by fluorochemistry has been developed for the diagnostic assay of N-acetylgalactosamine-4-sulphatase (4-sulphatase). The procedure uses a monoclonal antibody 4-S 4.1 to immunoadsorb 4-sulphatase specifically from complex protein samples containing other sulphatases, and 4-methylumbelliferyl sulphate to detect captured 4-sulphatase. The assay provides an accurate and simple method for the diagnosis of Maroteaux-Lamy syndrome (Mucopolysaccharidosis type VI).

Human liver iduronate-2-sulphatase. Purification, characterization and catalytic properties

Human iduronate-2-sulphatase (EC 3.1.6.13), which is involved in the lysosomal degradation of the glycosaminoglycans heparan sulphate and dermatan sulphate, was purified more than 500,000-fold in 5% yield from liver with a six-step column procedure, which consisted of a concanavalin A-Sepharose-Blue A-agarose coupled step, chromatofocusing, gel filtration on TSK HW 50S-Fractogel, hydrophobic separation on phenyl-Sepharose CL-4B and size separation on TSK G3000SW Ultrapac. Two major forms were identified. Form A and form B, with pI values of 4.5 and less than 4.0 respectively, separated at the chromatofocusing step in approximately equal amounts of recovered enzyme activity. By gel-filtration methods form A had a native molecular mass in the range 42-65 kDa. When analysed by SDS/PAGE, dithioerythritol-reduced and non-reduced form A and form B consistently contained polypeptides of molecular masses 42 kDa and 14 kDa. Iduronate-2-sulphatase was purified from human kidney, placenta and lung, and form A was shown to have similar native molecular mass and subunit components to those observed for liver enzyme. Both forms of liver iduronate-2-sulphatase were active towards a variety of substrates derived from heparin and dermatan sulphate. Kinetic parameters (Km and Kcat) of form A were determined with a variety of substrates matching structural aspects of the physiological substrates in vivo, namely heparan sulphate, heparin and dermatan sulphate. Substrate with 6-sulphate esters on the aglycone residue adjacent to the iduronic acid 2-sulphate residue being attack were hydrolysed with catalytic efficiencies up to 200 times above that observed for the simplest disaccharide substrate without a 6-sulphated aglycone residue. The effect of incubation pH on enzyme activity towards the variety of substrates evaluated was complex and dependent on substrate aglycone structure, substrate concentration, buffer type and the presence of other proteins. Sulphate and phosphate ions and a number of substrate and product analogues were potent inhibitor of form A and form B enzyme activities.

Human N-acetylgalactosamine-4-sulphatase biosynthesis and maturation in normal, Maroteaux-Lamy and multiple-sulphatase-deficient fibroblasts

The biosynthesis and maturation of N-acetylgalactosamine-4-sulphatase (4-sulphatase) was studied in normal fibroblasts and in fibroblasts from patients with either mucopolysaccharidosis type VI (MPS VI; Maroteaux-Lamy syndrome) or multiple sulphatase deficiency (MSD). Fibroblasts were incubated in culture medium containing [3H]leucine or [35S]methionine, and radiolabelled 4-sulphatase was isolated by immunoaffinity chromatography using 4-sulphatase-specific monoclonal antibodies. In normal fibroblasts a precursor of 66 kDa, detected intracellularly after 3 h and in NH4Cl-induced secretions, was processed intracellularly, within an additional 3 h, to a polypeptide of 57 kDa composed of disulphide-linked polypeptides of 43 kDa and 8 kDa. All fibroblast lines obtained from MPS VI patients, exhibiting clinical characteristics ranging from no clearly recognized symptoms to the severe classical phenotype, incorporated radioactivity into immune-purified 4-sulphatase at a rate less than 10% of that seen in normal fibroblasts. Maturation of the residual 4-sulphatase showed, variously, features which may be indicative of delayed intracellular transport, decreased intracellular stability, failure of lysosomal targetting or resistance to enzyme processing. Although some features of the residual enzyme synthesis and maturation were consistent with the patient's clinical phenotype, this was infrequent. The maturation of 4-sulphatase in fibroblasts from MSD patients was indistinguishable from that in normal fibroblasts, and the half-life of 4-sulphatase in these fibroblasts, determined after a 24 h pulse and prolonged chase, was only slightly less than that in normal fibroblasts.

Immunoquantification of the low abundance lysosomal enzyme N-acetylgalactosamine 4-sulphatase

The low abundance lysosomal enzyme N-acetylgalactosamine 4-sulphatase (4-sulphatase) has been quantified using a microimmunopurification step and a monoclonal-based ELISA detection system. The assay is similar in principle to a two-site ELISA but uses a single monoclonal antibody against one epitope to bind 4-sulphatase in two separate assay steps. The sensitivity of this assay is sufficient to allow the quantification of 4-sulphatase in human cultured skin fibroblasts derived from normal controls and patients deficient in 4-sulphatase activity (mucopolysaccharidosis type VI or Maroteaux-Lamy syndrome). The results obtained suggest a range of mucopolysaccharidosis type VI or 4-sulphatase deficient mutants, from those expressing little or no quantifiable 4-sulphatase protein to those examples with quantifiable levels of 4-sulphatase protein which is enzymically inactive. Phenotypic variability in patients with a 4-sulphatase deficiency may therefore be partially attributed to a range of protein expressions. The method should allow the determination of 4-sulphatase specific activity in mucopolysaccharidosis type VI patients.

Human liver glucuronate 2-sulphatase. Purification, characterization and catalytic properties

Human glucuronate 2-sulphatase (GAS), which is involved in the degradation of the glycosaminoglycans heparan sulphate and chondroitin 6-sulphate, was purified almost 2,000,000-fold to homogeneity in 8% yield from liver with a four-step six-column procedure, which consists of a concanavalin A-Sepharose/Blue A-agarose coupled step, a DEAE-Sephacel/octyl-Sepharose coupled step, CM-Sepharose chromatography and gel-permeation chromatography. Although more than 90% of GAS activity had a pI of greater than 7.5, other forms with pI values of 5.8, 5.3, 4.7 and less than 4.0 were also present. The pI greater than 7.5 form of GAS had a native molecular mass of 63 kDa. SDS/polyacrylamide-gel-electrophoretic analysis resulted in two polypeptide subunits of molecular mass 47 and 19.5 kDa. GAS was active towards disaccharide substrates derived from heparin [O-(beta-glucuronic acid 2-sulphate)-(1----4)-O-(2,5)-anhydro[1-3H]mannitol 6-sulphate (GSMS)] and chondroitin 6-sulphate [O-(beta-glucuronic acid 2-sulphate-(1----3)-O-(2,5)-anhydro[1-3H]talitol 6-sulphate (GSTS)]. GAS activity towards GSMS and GSTS was at pH optima of 3.2 and 3.0 respectively with apparent Km values of 0.3 and 0.6 microM respectively and corresponding Vmax values of 12.8 and 13.7 mumol/min per mg of protein respectively. Sulphate and phosphate ions are potent inhibitors of enzyme activity. Cu2+ ions stimulated, whereas EDTA inhibited enzyme activity. It was concluded that GAS is required together with a series of other exoenzyme activities in the lysosomal degradation of glycosaminoglycans containing glucuronic acid 2-sulphate residues.

Chromosomal localization of ARSB, the gene for human N-acetylgalactosamine-4-sulphatase

A deficiency of N-acetylgalactosamine-4-sulphatase (G4S, gene symbol ARSB), results in the accumulation of undegraded substrate and the lysosomal storage disorder, Maroteaux-Lamy syndrome (mucopolysaccharidosis type VI). In situ hybridization using an 3H-labelled human G4S genomic DNA fragment to human metaphase chromosomes localized ARSB to chromosome 5q13-5q14. This location is consistent with, an refines, previous chromosomal assignments based on the expression of human G4S in somatic cell hybrids.

Human N-acetylgalactosamine-4-sulphate sulphatase. Purification, monoclonal antibody production and native and subunit Mr values

Initial purification of N-acetylgalactosamine-4-sulphate sulphatase from human liver homogenates containing approx. 1 mg of enzyme in 26 g of soluble proteins was achieved by a six-column chromatography procedure and yielded approx. 40 micrograms of a single major protein species. Enzyme thus prepared was used to produce N-acetylgalactosamine-4-sulphate sulphatase-specific monoclonal antibodies. The use of a monoclonal antibody linked to a solid support facilitated the purification of approx. 0.5 mg of N-acetylgalactosamine-4-sulphate sulphatase from a similar liver homogenate. Moreover the enzyme isolated contained a single protein species, shown by SDS/polyacrylamide-gel electrophoresis to have an Mr of 57,000, which dissociated into subunits of Mr 43,000 and 13,000 in the presence of reducing agents. Essentially identical enzyme preparations were isolated from homogenates of human kidney and lung and from concentrated human urine. The native protein Mr of enzyme from human liver and kidney was assessed by gel-permeation chromatography to be 43,000 on Ultrogel AcA and Bio-Gel P-150. The liver N-acetylgalactosamine-4-sulphate sulphatase was shown to have pH optima of approx. 4 and 5.5 with the oligosaccharide substrate (GalNAc4S-GlcA-GalitolNAc4S) and fluorogenic substrate (methylumbelliferyl sulphate) respectively. Km values of 60 microM and 4 mM and Vmax. values of 2 and 20 mumol/min per mg were determined with the oligosaccharide and fluorogenic substrates respectively.

Human liver N-acetylglucosamine-6-sulphate sulphatase. Catalytic properties

Kinetic parameters (Km and kcat.) of the two major forms (A and B) and a minor form (C) of human liver N-acetylglucosamine-6-sulphate sulphatase [Freeman, Clements & Hopwood (1987) Biochem. J. 246, 347-354] were determined with a variety of substrates matching structural aspects of the physiological substrates in vivo, namely heparin, heparan sulphate and keratan sulphate. Enzyme activity is highly specific towards glucosamine 6-sulphate or glucose 6-sulphate residues. More structurally complex substrates, in which several aspects of the aglycone structure of the natural substrate were maintained, are hydrolysed with catalytic efficiencies up to 3900 times above that observed for the monosaccharide substrate N-acetylglucosamine 6-sulphate. Forms A and B both desulphate substrates derived from keratan sulphate and heparin. Aglycone structures that influence substrate binding and/or enzyme activity were penultimate-residue 6-carboxy and 2-sulphate ester groups for heparin-derived substrates and penultimate-residue 6-sulphate ester groups for keratan sulphate-derived substrates. The 4-hydroxy group of the N-acetylglucosamine 6-sulphate or the 2-sulphaminoglucosamine 6-sulphate under enzymic attack is involved in the catalytic mechanism. The presence of a 2-amino group in place of a 2-acetamido or a 2-sulphoamino group considerably decreases the catalytic efficiency of the sulphatase, particularly in the absence of a penultimate-aglycone-residue 6-carboxy group. Both forms A and B are exo-enzymes, since activity towards internal sulphate ester bonds was not observed. The effect of incubation pH on enzyme activity towards the variety of substrates evaluated was complex and dependent on substrate aglycone structure. The presence of aglycone 2-sulphate ester, 6-carboxy group and 6-sulphate ester groups on the glucosamine 6-sulphate residue under attack considerably affects the pH response. Sulphate and phosphate ions are potent inhibitors of enzyme activity.

Human liver N-acetylglucosamine-6-sulphate sulphatase. Purification and characterization

Human N-acetylglucosamine-6-sulphate sulphatase was purified at least 50,000-fold to homogeneity in 78% yield from liver with a simple three-step four-column procedure, which consists of a concanavalin A-Sepharose/Blue A-agarose coupled step, chromatofocusing and Cu2+-chelating Sepharose chromatography. In all, four forms were isolated and partially characterized. Forms A and B, both with a pI greater than 9.5 and representing 30% and 60% respectively of the recovered enzyme activity, were separated by hydroxyapatite chromatography of the enzyme preparation obtained from the Cu2+-chelating Sepharose step. Both forms A and B had native molecular masses of 75 kDa. When analysed by SDS/polyacrylamide-gel electrophoresis, form A consists of a single polypeptide of molecular mass 78 kDa, whereas form B contained 48 kDa and 32 kDa polypeptide subunits. Neither form A nor form B was taken up from the culture medium into cultured human skin fibroblasts. The two other forms (C and D), with pI values of 5.8 and 5.4 respectively, represented approx. 7% and 3% of the total recovered enzyme activity. The native molecular masses of forms C and D were 94 kDa and approx. 75 kDa respectively. Form C contained three polypeptides with molecular masses of 48, 45 and 32 kDa. N-Acetylglucosamine-6-sulphate sulphatase activity was measured with a radiolabelled disaccharide substrate derived from heparin. The development of this substrate enabled the isolation and characterization of N-acetylglucosamine-6-sulphate sulphatase to proceed efficiently. Forms A, B and C had pH optima of 5.0, Km values of 11.7, 14.2 and 11.1 microM respectively and Vmax. values of 105, 60 and 53 nmol/min per mg of protein respectively. The molecular basis of the multiple forms of this sulphatase is not known. It is postulated that the differences in structure and properties of the four enzyme forms are due to differences in the state of processing of a large subunit.