Sandhoff disease: defective glycosaminoglycan catabolism in cultured fibroblasts and its correction by beta-N-acetylhexosaminidase

Metabolomics profiling reveals profound metabolic impairments in mice and patients with Sandhoff disease

Sandhoff disease (SD) results from mutations in the HEXB gene, subsequent deficiency of N-acetyl-β-hexosaminidase (Hex) and accumulation of GM2 gangliosides. SD leads to progressive neurodegeneration and early death. However, there is a lack of established SD biomarkers, while the pathogenesis etiology remains to be elucidated. To identify potential biomarkers and unveil the pathogenic mechanisms, metabolomics analysis with reverse phase liquid chromatography (RPLC) was conducted. A total of 177, 112 and 119 metabolites were found to be significantly dysregulated in mouse liver, mouse brain and human hippocampus samples, respectively (p < .05, ID score > 0.5). Principal component analysis (PCA) analysis of the metabolites showed clear separation of metabolomics profiles between normal and diseased individuals. Among these metabolites, dipeptides, amino acids and derivatives were elevated, indicating a robust protein catabolism. Through pathway enrichment analysis, we also found alterations in metabolites associated with neurotransmission, lipid metabolism, oxidative stress and inflammation. In addition, N-acetylgalactosamine 4-sulphate, key component of glycosaminoglycans (GAG) was significantly elevated, which was also confirmed by biochemical assays. Collectively, these results indicated major shifts of energy utilization and profound metabolic impairments, contributing to the pathogenesis mechanisms of SD. Global metabolomics profiling may provide an innovative tool for better understanding the disease mechanisms, and identifying potential diagnostic biomarkers for SD.

Mucopolysaccharidosis-like phenotype in feline Sandhoff disease and partial correction after AAV gene therapy

Sandhoff disease (SD) is a fatal neurodegenerative disease caused by a mutation in the enzyme β-N-acetylhexosaminidase. Children with infantile onset SD develop seizures, loss of motor tone and swallowing problems, eventually reaching a vegetative state with death typically by 4years of age. Other symptoms include vertebral gibbus and cardiac abnormalities strikingly similar to those of the mucopolysaccharidoses. Isolated fibroblasts from SD patients have impaired catabolism of glycosaminoglycans (GAGs). To evaluate mucopolysaccharidosis-like features of the feline SD model, we utilized radiography, MRI, echocardiography, histopathology and GAG quantification of both central nervous system and peripheral tissues/fluids. The feline SD model exhibits cardiac valvular and structural abnormalities, skeletal changes and spinal cord compression that are consistent with accumulation of GAGs, but are much less prominent than the severe neurologic disease that defines the humane endpoint (4.5±0.5months). Sixteen weeks after intracranial AAV gene therapy, GAG storage was cleared in the SD cat cerebral cortex and liver, but not in the heart, lung, skeletal muscle, kidney, spleen, pancreas, small intestine, skin, or urine. GAG storage worsens with time and therefore may become a significant source of pathology in humans whose lives are substantially lengthened by gene therapy or other novel treatments for the primary, neurologic disease.

Analysis of the host transcriptome from demyelinating spinal cord of murine coronavirus-infected mice

Persistent infection of the mouse central nervous system (CNS) with mouse hepatitis virus (MHV) induces a demyelinating disease pathologically similar to multiple sclerosis and is therefore used as a model system. There is little information regarding the host factors that correlate with and contribute to MHV-induced demyelination. Here, we detail the genes and pathways associated with MHV-induced demyelinating disease in the spinal cord. High-throughput sequencing of the host transcriptome revealed that demyelination is accompanied by numerous transcriptional changes indicative of immune infiltration as well as changes in the cytokine milieu and lipid metabolism. We found evidence that a Th1-biased cytokine/chemokine response and eicosanoid-derived inflammation accompany persistent MHV infection and that antigen presentation is ongoing. Interestingly, increased expression of genes involved in lipid transport, processing, and catabolism, including some with known roles in neurodegenerative diseases, coincided with demyelination. Lastly, expression of several genes involved in osteoclast or bone-resident macrophage function, most notably TREM2 and DAP12, was upregulated in persistently infected mouse spinal cord. This study highlights the complexity of the host antiviral response, which accompany MHV-induced demyelination, and further supports previous findings that MHV-induced demyelination is immune-mediated. Interestingly, these data suggest a parallel between bone reabsorption by osteoclasts and myelin debris clearance by microglia in the bone and the CNS, respectively. To our knowledge, this is the first report of using an RNA-seq approach to study the host CNS response to persistent viral infection.

Enzyme replacement therapy for lysosomal diseases: lessons from 20 years of experience and remaining challenges

In 1964, Christian de Duve first suggested that enzyme replacement might prove therapeutic for lysosomal storage diseases (LSDs). Early efforts identified the major obstacles, including the inability to produce large quantities of the normal enzymes, the lack of animal models for proof-of-concept studies, and the potentially harmful immune responses to the "foreign" normal enzymes. Subsequently, the identification of receptor-mediated targeting of lysosomal enzymes, the cloning and overexpression of human lysosomal genes, and the generation of murine models markedly facilitated the development of enzyme replacement therapy (ERT). However, ERT did not become a reality until the early 1990s, when its safety and effectiveness were demonstrated for the treatment of type 1 Gaucher disease. Today, ERT is approved for six LSDs, and clinical trials with recombinant human enzymes are ongoing in several others. Here, we review the lessons learned from 20 years of experience, with an emphasis on the general principles for effective ERT and the remaining challenges.

Inherited metabolic disorders involving the eye: a clinico-biochemical perspective

The diagnosis of inborn errors of metabolism is challenging for most physicians. Improvements in medical technology and greater knowledge of the human genome are resulting in significant changes in the diagnosis, classification, and treatment of inherited metabolic disorders (IMDs). Many known inborn errors of metabolism will be recognised earlier or treated differently because of these changes. It is important that physicians recognise the clinical signs of IMDs and know when to propose advanced laboratory testing or referral to a higher centre for better patient management. Ocular manifestations occur in various metabolic disorders. Although there is an extensive understanding of many inborn errors of metabolism at the biochemical, molecular, and metabolic levels, little is known about their pathogenesis. In particular, how systemic metabolic disease contributes to ocular defects remains to be elucidated in IMDs. The occurrence of eye abnormalities could be due to direct toxic mechanisms of abnormal metabolic products or accumulation of normal metabolites by errors of synthetic pathways or by deficient energy metabolism. A detailed ophthalmological assessment is essential. Definitive diagnosis and management of patients with IMDs is ideally carried out by a combination of specialists, including an ophthalmologist, paediatrician, biochemist, and medical geneticist. Recent advances in the diagnosis and treatment of IMDs have substantially improved the prognosis for many of these conditions.

Central nervous system therapy for lysosomal storage disorders

✓ Most lysosomal storage disorders are characterized by progressive central nervous system impairment, with or without systemic involvement. Affected individuals have an array of symptoms related to brain dysfunction, the most devastating of which is neurodegeneration following a period of normal development. The blood–brain barrier has represented a significant impediment to developing therapeutic approaches to treat brain disease, but novel approaches—including enzyme replacement, small-molecule, gene, and cell-based therapies—have given children afflicted by these conditions and those who care for them hope for the future.

Enzyme replacement and enhancement therapies for lysosomal diseases

Although first suggested by de Duve in 1964, enzyme replacement therapy (ERT) for lysosomal storage diseases did not become a reality until the early 1990s when its safety and effectiveness were demonstrated in type 1 Gaucher disease. Today, ERT is a reality for Gaucher disease, Fabry disease and mucopolysaccharidosis type I (MPS I), and clinical trials with recombinant human enzymes are ongoing in Pompe disease, MPS II and MPS VI, and are about to begin in Neimann-Pick B disease. In addition to ERT, enzyme enhancement therapy (EET) offers a novel therapeutic strategy to increase the residual function of mutant proteins. EET employs small molecules as 'pharmacological chaperones' to rescue misfolded and/or unstable mutant enzymes or proteins that have residual function. EET also offers the possibility of treating neurodegenerative lysosomal disorders since these small therapeutic molecules may cross the blood-brain barrier. The current status of ERT and the prospects for EET for lysosomal storage diseases are reviewed.

Enzyme replacement and enhancement therapies: lessons from lysosomal disorders

The past decade has witnessed remarkable advances in our ability to treat inherited metabolic disorders, especially the lysosomal storage diseases, a group of more than 40 disorders, each of which is caused by the deficiency of a lysosomal enzyme or protein. During the past few years, both enzyme replacement and enhancement therapies have been developed to treat these disorders. This review discusses the successes and shortcomings of these therapeutic strategies, and the contributions that they have made to treating lysosomal storage diseases.

Hexosaminidase inhibitors as new drug candidates for the therapy of osteoarthritis

BACKGROUND

Articular cartilage from patients with osteoarthritis is characterized by a decreased concentration and reduced size of glycosaminoglycans. Degeneration of the cartilage matrix is a multifactorial process, which is due in part to accelerated glycosaminoglycan catabolism. Recently, we have demonstrated that hexosaminidase represents the dominant glycosaminoglycan-degrading glycosidase released by chondrocytes into the extracellular compartment and is the dominant glycosidase in synovial fluid from patients with osteoarthritis. Inhibition of hexosaminidase activity may represent a novel approach to the prevention of cartilage matrix glycosaminoglycan degradation and a potentially new strategy to treat osteoarthritis.

RESULTS

We have synthesized and investigated a series of iminocyclitols designed as transition-state analog inhibitors of human hexosaminidase, and demonstrated that the five-membered iminocyclitol 4 expresses the strongest inhibitory activity with K(i)=24 nM. Inhibition of hexosaminidase activity in human cultured articular chondrocytes and human chondrosarcoma cells with iminocyclitol 4 resulted in accumulation of hyaluronic acid and sulfated glycosaminoglycans in the cell-associated fraction. Similarly, incubation of human cartilage tissue with iminocyclitol 4 resulted in an accumulation of glycosaminoglycans in the pericellular compartment.

CONCLUSIONS

Inhibition of hexosaminidase activity represents a new strategy for preventing or even reversing cartilage degradation in patients with osteoarthritis.

Mice lacking both subunits of lysosomal beta-hexosaminidase display gangliosidosis and mucopolysaccharidosis

The GM2 gangliosidoses, Tay-Sachs and Sandhoff diseases, are caused by mutations in the HEXA (alpha-subunit) and HEXB (beta-subunit) genes, respectively. Each gene encodes a subunit for the heterodimeric lysosomal enzyme, beta-hexosaminidase A (alpha beta), as well as for the homodimers beta-hexosaminidase B (beta beta) and S (alpha alpha). In this study, we have produced mice that have both Hexa and Hexb genes disrupted through interbreeding Tay-Sachs (Hexa-/-) and Sandhoff (Hexb-/-) disease model mice. Lacking both the alpha and beta-subunits these 'double knockout' mice displayed a total deficiency of all forms of lysosomal beta-hexosaminidase including the small amount of beta-hexosaminidase S present in the Sandhoff disease model mice. More surprisingly, these mice showed the phenotypic, pathologic and biochemical features of the mucopolysaccharidoses, lysosomal storage diseases caused by the accumulation of glycosaminoglycans. The mucopolysaccharidosis phenotype is not seen in the Tay-Sachs or Sandhoff disease model mice or in the corresponding human patients. This result demonstrates that glycosaminoglycans are crucial substrates for beta-hexosaminidase and that their lack of storage in Tay-Sachs and Sandhoff diseases is due to functional redundancy in the beta-hexosaminidase enzyme system.

Screening for lysosomal disorders

Patients at any age who develop regression of learned skills, onset of dementia, loss of motor control and organ enlargement should be considered for lysosomal screening. Morphological and biochemical screening methods may reinforce the clinical suspicion, but they are not diagnostic. A widespread use of enzyme assays that appear to be related to the clinical problems is recommended.

Morquio syndrome (MPS IVA) and hypophosphatasia in a Hutterite kindred

A patient is described who has Morquio syndrome (MPS IVA). He is a member of the Hutterite Brethren and genealogic analysis discloses a high inbreeding coefficient for the proband. The proband's sibship is segregating two autosomal recessive disorders, ie, MPS IVA and infantile hypophosphatasia. Two other families each have one or the other of these diseases but not both. The three families are distantly related.

Diagnosis of Tay-Sachs disease by estimation of beta-N-acetylhexosaminidase activity using a radiolabeled hyaluronic acid-derived trisaccharide substrate

We have prepared a new radiolabeled substrate (N-[3H]acetylglucosamine-glucuronic acid-N-[3H]acetylglucosamine), from hyaluronic acid, for an assay of beta-N-acetylhexosaminidase activity. Using this substrate, we found a striking deficiency of beta-N-acetylhexosaminidase activity in cultured skin fibroblasts and in liver homogenates from patients with Tay-Sachs disease. DEAE-cellulose chromatography at pH 6.0 revealed that both isoenzymes A and B of beta-N-acetylhexosaminidase from normal liver participated in the catabolism of hyaluronic acid. There were, however, major differences in substrate specificities between isoenzymes A and B. Our results indicate that this substrate should be useful for enzymatic diagnosis of Tay-Sachs disease.

Impaired degradation of chondroitin sulfate in GM2-gangliosidosis

We have prepared a new radiolabeled substrate, derived from chondroitin 6-sulfate oligosaccharide, for the assaying of chondroitin sulfate degradation by beta-N-acetylgalactosaminidase. Using this substrate, we found a striking deficiency of beta-N-acetylgalactosaminidase activity in the cultured skin fibroblasts of patients with Sandhoff disease and Tay-Sachs disease. DEAE-cellulose chromatography at pH 6.0 revealed that both isoenzymes A and B of beta-N-acetylgalactosaminidases from normal human liver participated in the catabolism of chondroitin 6-sulfate. However, there were major differences in substrate specificity between isoenzyme A and isoenzyme B.

Degradation of keratan sulfate by beta-N-acetylhexosaminidases in GM2-gangliosidosis

We have prepared a new substrate from a keratan sulfate-derived-oligosaccharide (2-acetamido-2-deoxyglucosyl-(1--3)-[1-3H] Galactitol), which is necessary to measure beta-N-acetylhexosaminidase activity. This substrate was prepared from a cornea keratan sulfate by digestion with endo-beta-galactosidase, followed by isolation of disaccharide on gel filtration chromatography and chemical desulfation. Using this substrate, we found that a striking deficiency of beta-N-acetylhexosaminidase activity was present in the skin fibroblasts of patients with Sandhoff disease but not in Tay-Sachs disease. Both beta-N-acetyl-hexosaminidase A & H contributed to the catabolism of keratan sulfate.

Cleavage of the (1 goes to 3)-2-acetamido-2-deoxy-beta-D-glucopyranosyl linkage present in keratan sulfate. The A and B isoenzymes of human liver hexosaminidase (EC 3.2.1.30)

The disaccharide 2-acetamido-2-deoxy-beta-D-glucopyranosyl-(1 goes to 3)-D-[1-3H]-galactitol, prepared from keratan sulfate, was rapidly hydrolyzed by the A and B isoenzymes of normal human liver hexosaminidase (EC 3.2.1.30), and by the B isoenzyme prepared from the liver of a patient who had died of Tay-Sachs disease. The disaccharide substrate was also hydrolyzed by extracts of normal, cultured-skin fibroblasts, and fibroblasts of patients with Tay-Sachs disease, whereas it was not hydrolyzed by fibroblast extracts of patients with Sandhoff disease. Thus, effective degradation of keratan sulfate, secondary to a defect of the beta subunits present in the A and B isoenzymes of hexosaminidase, may contribute to the appearance of skeletal lesions in patients affected by Sandhoff disease.

The mucopolysaccharidoses: biochemistry and clinical symptoms

The mucopolysaccharidoses are a group of genetic diseases which are characterized by an excessive intralysosomal accumulation of partially degraded mucopolysaccharides. This storage is caused by the inactivity of one of eleven enzymes that are required for the degradation of the different types of mucopolysaccharides. There is a rough correlation between phenotype and chemical nature of the storage material. Similar clinical pictures, however, may be caused by an inactivity of different enzymes. Conversely, different clinical expressions of the defect of a single enzyme may be attributed to allelic mutations. The recent development of specific assay procedures for the respective enzymes allows 1. an early genotype-specific diagnosis of affected patients, 2. prenatal diagnosis of the metabolic defect in families at risk, 3. to prognosticate the course of the disease at least in some instances, and 4. genetic counseling for members of affected families. At present, there is no specific therapy. Attempts of enzyme replacement therapy are still at an experimental stage.

Degradation of keratan sulphate by beta-N-acetylhexosaminidases A and B

Enzymic cleavage of beta-N-acetylglucosamine residues of keratan sulphate was studied in vitro by using substrate a [3H]glucosamine-labelled desulphated keratan sulphate with N-acetylglucosamine residues at the non-reducing end. Both lysosomal beta-N-acetylhexosaminidases A and B are proposed to participate in the degradation of keratan sulphate on the basis of the following observations. Homogenates of fibroblasts from patients with Sandhoff disease, but not those from patients with Tay--Sachs disease, were unable to release significant amounts of N-acetyl[3H]glucosamine. On isoelectric focusing of beta-N-acetylhexosaminidase from human liver the peaks of keratan sulphate-degrading activity coincided with the activity towards p-nitrophenyl beta-N-acetylglucosaminide. A monospecific antibody against the human enzyme reacted with both enzyme forms and precipitated the keratan sulphate-degrading activity. Both isoenzymes had the same apparent Km of 4mM, but the B form was approximately twice as active as the A form when compared with the activity towards a chromogenic substrate. Differences were noted in the pH--activity profiles of both isoenzymes. Thermal inactivation of isoenzyme B was less pronounced towards the polymeric substrate than towards the p-nitrophenyl derivative.

Selective noncompetitive assimilation of bovine testicular beta-galactosidase and bovine liver beta-glucuronidase by generalized gangliosidosis fibroblasts

Bovine liver beta-glucuronidase and testicular beta-galactosidase were assimilated by generalized gangliosidosis fibroblasts at respectively rates of 90 and 464 times the rate of assimilation of horseradish peroxidase. Assimilation of either of the two enzymes by the fibroblasts was saturable, suggesting the participation of receptor-mediated adsorptive endocytosis for internalization. The rate of assimilation of either enzyme was not affected by high levels of the other enzyme, suggesting that distinct receptors for each enzyme occur on the fibroblasts' cell surface. Furthermore, although assimilation of beta-galactosidase was inhibited by mannose, methyl mannosides, mannosyl alpha 1 leads to 2 mannose, and mannose-6-phosphate, these compounds did not detectably inhibit the assimilation of beta-glucuronidase. These results suggest that testicular beta-galactosidase was assimilated by the well-established phosphomannosyl recognition system. However, liver beta-glucuronidase was assimilated by a distinct, noncompeting, and as yet undefined, recognition system.

Basic findings and current developments in sphingolipidoses

Sphingolipidoses are caused by recessively inherited deficiencies of lysosomal hydrolases. The clinical backgrounds of and current biochemical and genetic approaches to the different forms and variants of gangliosidoses, trihexosylceramidosis (Fabry's disease), galactosylceramidosis (Krabbe's disease), sulfatidoses (metachromatic leukodystrophies), glucosylceramidosis (Gaucher's disease), sphingomyelinoses (Niemann-Pick disease) and ceramidosis (Farber's disease) are presented.

Biochemistry and genetics of gangliosidoses

The gangliosidoses comprise an-ever increasing number of biochemically and phenotypically variant diseases. In most of them an autosomal recessive inherited deficiency of a lysosomal hydrolase results in the fatal accumulation of glucolipids (predominantly in the nervous tissue) and of oligosaccharides. The structure, substrate specificity, immunological properties of and genetic studies on the relevant glycosidases, ganglioside GM1 beta-galactosidase and beta-hexosaminidase isoenzymes, are reviewed in this paper. Contrary to general expectation, only a poor correlation is observed between the severity of the disease and residual activity of the defective enzyme when measured with synthetic or natural substrates in the presence of detergents. For the understanding of variant diseases and for their pre- and postnatal diagnosis, the necessity of studying the substrate specificity of normal and mutated enzymes under conditions similar to the in vivo situation, e.g., with natural substrates in the presence of appropriate activator proteins, is stressed. The possibility that detergents may have adverse affects on the substrate specificity of the enzymes is discussed for the beta-hexosaminidases. The significance of activator proteins for the proper interaction of lipid substrates and water-soluble hydrolases is illustrated by the fatal glycolipid storage resulting from an activator protein deficiency in the AB variant of GM2-gangliosidosis. Recent somatic complementation studies have revealed the existence of a presumably post-translational modification factor necessary for the expression of ganglioside GM1 beta-galactosidase activity. This factor is deficient in a group of variants of GM1-glangliosidosis. Among the possible reasons for the variability of enzyme activity levels in heterozygotes and patients, allelic mutations, formation of hybrid enzymes, and the existence of patients as compound heterozygotes are discussed. All these may result in the production of mutant enzymes with an altered specificity for a variety of natural substrates.

Metabolic correction of fucosidosis fibroblasts by human alpha-L-fucosidase

Human alpha-L-fucosidase, purified from placenta, was taken up from the culture medium by skin fibroblasts from patients with fucosidosis (alpha-L-fucosidase deficiency). The rate of uptake was low (uptake coefficient = 6 X 10(-4) ml.mg-1.h-1). Intracellular alpha-L-fucosidase activity was directly proportional to enzyme in the medium up to an activity of at least 40 nmoles/min/ml. No evidence for saturation of specific cell-surface receptors was seen. However, uptake was reduced by 75% by 1 mM mannose-6-phosphate and by 50% by 1mM glucose-6-phosphate, suggesting that uptake may be mediated by a receptor recognising a phosphorylated sugar or an analagous compound. Enzyme taken up by the cells was most active in subcellular fractions enriched with lysosomes and had an isozyme pattern, by isoelectric focusing, identical to that of the original enzyme preparation. Fucosidosis fibroblasts were shown to accumulate low molecular-weight, fucose-containing compounds to a level several times greater than control cells. This stored material was eluted from Sephadex G-25 as an asymmetrical peak with an elution volume of approximately twice the void volume of the column. Addition of placental alpha-L-fucosidase to the culture medium of fucosidosis fibroblasts prevented excessive accumulation of fucose-containing material and accelerated the breakdown of material accumulated prior to enzyme uptake.

Concanavalin A promotes the uptake of lysosomal hydrolases by human fibroblasts

Human placental hexosaminidase B and beta-galactosidase are taken up very poorly by human fibroblasts in culture. However, if fibroblasts manifesting genetically determined deficiencies of these lysosomal hydrolases are first treated with concanavalin A, then enzyme uptake is markedly increased. Enzyme activity which becomes associated with concanavalin A-treated fibroblasts maintained at 4 degrees C can be greatly removed by treatment with haptene sugar, while enzyme activity which becomes associated with cells maintained at 37 degrees C is refractory to haptens treatment. These results are interpreted as an initial binding of enzyme to concanvalin A molecules located at the cell surface, followed by an active cellular process leading to internalization of the lectin-enzyme complexes.

A difference in the specificities of human liver N-acetyl-beta-hexosaminidases A and B detected by their activities towards glycosaminoglycan oligosaccharides

N-Acetyl-beta-hexosaminidases A and B differ in their activities towards oligosaccharides prepared from glycosaminoglycans. Trisaccharides from hyaluronic acid and desulphated chondroitin 4-sulphate were hydrolysed by N-acetyl-beta-hexosaminidase A, but not by N-acetyl-beta-hexosaminidase B.

A sensitive procedure for the diagnosis of N-acetyl-galactosamine-6-sulfate sulfatase deficiency in classical Morquio's disease

The trisaccharide 6-sulfo-N-acetylgalactosamine-glucuronic acid-6-sulfo-N-acetyl-[1-3H]galactosaminitol was used as a substrate for the determination of N-acetylgalactosamine-6-sulfate sulfatase activity. The amount of liberated sulfate was measured indirectly by separating monosulfated reaction products from the substrate on Dowex 1 X 2 microcolumns in a simple two step procedure. Fibroblast homogenates from patients with various genotypes, except classical Morquio's disease, released 410 +/- 90 pmol sulfate/h/mg cell protein. The enzyme exhibited a pH optimum of pH 4.8 and a KM of about 1 X 10(-4) mol/1. It was strongly inhibited by phosphate, sulfate and chloride ions. In three cell lines from patients with classical Morquio's disease a residual activity between 1 and 2% of the mean normal activity was found. All cell lines tested released sulfate from 6-sulfo-N-acetylglucosamine-glucuronic acid-[1-3H]-anhydromannitol. Cell extracts from cultured amniotic fluid cells exhibited a N-acetylgalactosamine-6-sulfate sulfatase activity between 120 and 320 pmol/h/mg protein. An enzyme activity of 370 +/- 100 pmol sulfate/h/mg protein was found in peripheral leucocytes from healthy donors. The determination of N-acetyl-galactosamine-6-sulfate sulfatase activity in one family with an affected patient indicated that the enzyme deficiency is also expressed in leucocytes.

Cell disease: desialylation of beta-hexosaminidase and its effect on uptake by fibroblasts

The pinocytosis by fibroblasts of beta-hexosaminidase (EC 3.2.1.30) excreted by cultured skin fibroblasts from a patient with I-cell disease was not enhanced by neuraminidase treatment of the enzyme. The uptake of sialic acid-rich normal plasma beta-hexosaminidase was minimal and neuraminidase treatment did not appreciably enhance uptake. In contrast, sialic acid-rich normal seminal fluid beta-hexosaminidase was readily pinocytosed regardless of neuraminidase treatment. Thus the presence of sialic acid on beta-hexosaminidase does not influence uptake and a neuraminidase deficiency in I-cell disease may not be directly responsible for excessive extracellular enzyme.

The mucopolysaccharidoses: inborn errors of glycosaminoglycan catabolism

The mucopolysaccharidoses are genetic disorders of glycosaminoglycan metabolism. Patients with these diseases accumulate within the lysosomes of most tissues excessive amounts of dermatan and/or heparan sulfates, or of keratan sulfate. The clinical consequences of such glycosaminoglycan storage range from skeletal abnormalities to cardiovascular problems, and to motor and mental retardation. In all mucopolysaccharidoses, except Morquio disease, an excessive accumulation of sulfate-labeled glycosaminoglycans has been demonstrated in fibroblasts cultured from the patient's skin. It was subsequently shown that this was due to the deficiency of specific proteins which were named "corrective factors", because their addition to the culture medium effected a normalization of the impaired glycosaminoglycan catabolism in the respective mucopolysaccharidosis fibroblasts. The investigation of the function of the corrective factors, and other studies, led to the identification of the enzymatic defect in each of the mucopolysaccharidoses. Seven lysosomal enzyme deficiencies are now recognized among this group of disorders. A classification of the diseases, according to the mutant gene products, reveals that there is considerable phenotypic variation not only between diseases, but also within several disease types. With the availability of the appropriate enzyme assays, the previous difficulties in diagnosing these disorders have now been overcome. Methods are also available for the prenatal diagnosis, and the detection of heterozygous individuals, in most of the mucopolysaccharidoses. Although correction of the metabolic defect through enzyme replacement has been achieved in tissue culture, many problems remain to be solved before such therapy may become applicable in the patients themselves.

The mucopolysaccharidoses (a review)

The mucopolysaccharidoses are a group of genetic diseases characterized by storage of incompletely degraded glycosaminoglycans. Such storage causes marked distortion of many tissues with consequent severe somatic changes and mental retardation. Storage of glycosaminoglycans results from markedly diminished activity of specific hydrolases requisite for the normal degradation of glycosaminoglycans. The specific enzymic defects have been identified in nine different diseases. In some cases evidence has been obtained indicating the existence of additional allelic diseases based on the same enzyme. The knowledge obtained from these studies has made prenatal diagnosis possible and has led to the possibility that therapy may be undertaken utilizing enzyme replacement.