Human alpha-N-acetylgalactosaminidase (alpha-NAGA) deficiency: new mutations and the paradox between genotype and phenotype

The Role of Hematopoietic Cell Transplant in the Glycoprotein Diseases

The glycoprotein disorders are a group of lysosomal storage diseases (α-mannosidosis, aspartylglucosaminuria, β-mannosidosis, fucosidosis, galactosialidosis, sialidosis, mucolipidosis II, mucolipidosis III, and Schindler Disease) characterized by specific lysosomal enzyme defects and resultant buildup of undegraded glycoprotein substrates. This buildup causes a multitude of abnormalities in patients including skeletal dysplasia, inflammation, ocular abnormalities, liver and spleen enlargement, myoclonus, ataxia, psychomotor delay, and mild to severe neurodegeneration. Pharmacological treatment options exist through enzyme replacement therapy (ERT) for a few, but therapies for this group of disorders is largely lacking. Hematopoietic cell transplant (HCT) has been explored as a potential therapeutic option for many of these disorders, as HCT introduces functional enzyme-producing cells into the bone marrow and blood along with the engraftment of healthy donor cells in the central nervous system (presumably as brain macrophages or a type of microglial cell). The outcome of HCT varies widely by disease type. We report our institutional experience with HCT as well as a review of the literature to better understand HCT and outcomes for the glycoprotein disorders.

A Novel Homozygous Missense Variant in the NAGA Gene with Extreme Intrafamilial Phenotypic Heterogeneity

Schindler disease is a rare autosomal recessive lysosomal storage disorder caused by a deficiency in alpha-N-acetylgalactosaminidase (α-NAGA) activity due to defects in the NAGA gene. Accumulation of the enzyme's substrates results in clinically heterogeneous symptoms ranging from asymptomatic individuals to individuals with severe neurological manifestations. Here, a 5-year-old Emirati male born to consanguineous parents presented with congenital microcephaly and severe neurological manifestations. Whole genome sequencing revealed a homozygous missense variant (c.838C>A; p.L280I) in the NAGA gene. The allele is a reported SNP in the ExAC database with a 0.0007497 allele frequency. The proband's asymptomatic sister and cousin carry the same genotype in a homozygous state as revealed from the family screening. Due to the extreme intrafamilial heterogeneity of the disease as seen in previously reported cases, we performed further analyses to establish the pathogenicity of this variant. Both the proband and his sister showed abnormal urine oligosaccharide patterns, which is consistent with the diagnosis of Schindler disease. The α-NAGA activity was significantly reduced in the proband and his sister with 5.9% and 12.1% of the mean normal activity, respectively. Despite the activity loss, p.L280I α-NAGA processing and trafficking were not affected. However, protein molecular dynamic simulation analysis revealed that this amino acid substitution is likely to affect the enzyme's natural dynamics and hinders its ability to bind to the active site. Functional analysis confirmed the pathogenicity of the identified missense variant and the diagnosis of Schindler disease. Extreme intrafamilial clinical heterogeneity of the disease necessitates further studies for proper genetic counseling and management.

Pleiotropic Meta-Analysis of Cognition, Education, and Schizophrenia Differentiates Roles of Early Neurodevelopmental and Adult Synaptic Pathways

Susceptibility to schizophrenia is inversely correlated with general cognitive ability at both the phenotypic and the genetic level. Paradoxically, a modest but consistent positive genetic correlation has been reported between schizophrenia and educational attainment, despite the strong positive genetic correlation between cognitive ability and educational attainment. Here we leverage published genome-wide association studies (GWASs) in cognitive ability, education, and schizophrenia to parse biological mechanisms underlying these results. Association analysis based on subsets (ASSET), a pleiotropic meta-analytic technique, allowed jointly associated loci to be identified and characterized. Specifically, we identified subsets of variants associated in the expected ("concordant") direction across all three phenotypes (i.e., greater risk for schizophrenia, lower cognitive ability, and lower educational attainment); these were contrasted with variants that demonstrated the counterintuitive ("discordant") relationship between education and schizophrenia (i.e., greater risk for schizophrenia and higher educational attainment). ASSET analysis revealed 235 independent loci associated with cognitive ability, education, and/or schizophrenia at p < 5 × 10-8. Pleiotropic analysis successfully identified more than 100 loci that were not significant in the input GWASs. Many of these have been validated by larger, more recent single-phenotype GWASs. Leveraging the joint genetic correlations of cognitive ability, education, and schizophrenia, we were able to dissociate two distinct biological mechanisms-early neurodevelopmental pathways that characterize concordant allelic variation and adulthood synaptic pruning pathways-that were linked to the paradoxical positive genetic association between education and schizophrenia. Furthermore, genetic correlation analyses revealed that these mechanisms contribute not only to the etiopathogenesis of schizophrenia but also to the broader biological dimensions implicated in both general health outcomes and psychiatric illness.

Targeted next-generation sequencing analysis in couples at increased risk for autosomal recessive disorders

Background

Many of the genetic childhood disorders leading to death in the pre- or neonatal period or during early childhood follow autosomal recessive modes of inheritance and bear specific challenges for genetic counseling and prenatal diagnostics. Parents are carriers but clinically unaffected, and diseases are rare but have recurrence risks of 25% in the same family. Often, affected children (or fetuses) die before a genetic diagnosis can be established, post-mortem analysis and phenotypic descriptions are insufficient and DNA from affected fetuses or children is not available for later analysis. A genetic diagnosis showing biallelic causative mutations is, however, the requirement for targeted carrier testing in parents and prenatal and preimplantation genetic diagnosis in further pregnancies.

Methods

We undertook targeted next-generation sequencing (NGS) for carrier screening of autosomal recessive lethal disorders in 8 consanguineous and 5 non-consanguineous couples with one or more affected children. We searched for heterozygous variants (non-synonymous coding or splice variants) in parents’ DNA, using a set of 430 genes known to be causative for rare autosomal recessive diseases with poor prognosis, and then filtering for variants present in genes overlapping in both partners. Putative pathogenic variants were tested for cosegregation in affected fetuses or children where material was available.

Results

The diagnosis for the premature death in children was established in 5 of the 13 couples. Out of the 8 couples in which no causative diagnosis could be established 4 consented to undergo further analysis, in two of those a potentially causative variant in a novel candidate gene was identified.

Conclusions

For the families in whom causative variants could be identified, these may now be used for prenatal and preimplantation genetic diagnostics. Our data show that NGS based gene panel sequencing of selected genes involved in lethal autosomal recessive disorders is an effective tool for carrier screening in parents and for the identification of recessive gene defects and offers the possibility of prenatal and preimplantation genetic diagnosis in further pregnancies in families that have experienced deaths in early childhood and /or multiple abortions.

Electronic supplementary material

The online version of this article (10.1186/s13023-018-0763-0) contains supplementary material, which is available to authorized users.

Lysosomal storage diseases

Lysosomes are cytoplasmic organelles that contain a variety of different hydrolases. A genetic deficiency in the enzymatic activity of one of these hydrolases will lead to the accumulation of the material meant for lysosomal degradation. Examples include glycogen in the case of Pompe disease, glycosaminoglycans in the case of the mucopolysaccharidoses, glycoproteins in the cases of the oligosaccharidoses, and sphingolipids in the cases of Niemann-Pick disease types A and B, Gaucher disease, Tay-Sachs disease, Krabbe disease, and metachromatic leukodystrophy. Sometimes, the lysosomal storage can be caused not by the enzymatic deficiency of one of the hydrolases, but by the deficiency of an activator protein, as occurs in the AB variant of GM2 gangliosidosis. Still other times, the accumulated lysosomal material results from failed egress of a small molecule as a consequence of a deficient transporter, as in cystinosis or Salla disease. In the last couple of decades, enzyme replacement therapy has become available for a number of lysosomal storage diseases. Examples include imiglucerase, taliglucerase and velaglucerase for Gaucher disease, laronidase for Hurler disease, idursulfase for Hunter disease, elosulfase for Morquio disease, galsulfase for Maroteaux-Lamy disease, alglucosidase alfa for Pompe disease, and agalsidase alfa and beta for Fabry disease. In addition, substrate reduction therapy has been approved for certain disorders, such as eliglustat for Gaucher disease. The advent of treatment options for some of these disorders has led to newborn screening pilot studies, and ultimately to the addition of Pompe disease and Hurler disease to the Recommended Uniform Screening Panel (RUSP) in 2015 and 2016, respectively.

Recombinant α-NAcetylgalactosaminidase from Marine Bacterium-Modifying A Erythrocyte Antigens

A plasmid based on pET-40b was constructed to synthesize recombinant α-N-acetylgalactosaminidase of the marine bacterium Arenibacter latericius KMM 426T (α-AlNaGal) in Escherichia coli cells. The yield of α-Al- NaGal attains 10 mg/ml with activity of 49.7 ± 1.3 U at 16°C, concentration of inductor 2 mM, and cultivation for 12 h. Techniques such as anion exchange, metal affinity and gel filtration chromatography to purify α-AlNaGal were applied. α-AlNaGal is a homodimer with a molecular weight of 164 kDa. This enzyme is stable at up to 50°C with a temperature range optimum activity of 20-37°C. Furthermore, its activity is independent of the presence of metal ions in the incubation medium. 1H NMR spectroscopy revealed that α-AlNaGal catalyzes the hydrolysis of the O-glycosidic bond with retention of anomeric stereochemistry and possesses a mechanism of action identical to that of other glycoside hydrolases of the 109 family. α-AlNaGal reduces the serological activity of A erythrocytes at pH 7.3. This property of α-AlNaGal can potentially be used for enzymatic conversion of A and AB erythrocytes to blood group O erythrocytes.

Pharmacological chaperones for human α-N-acetylgalactosaminidase

Schindler/Kanzaki disease is an inherited metabolic disease with no current treatment options. This neurologic disease results from a defect in the lysosomal α-N-acetylgalactosaminidase (α-NAGAL) enzyme. In this report, we show evidence that the iminosugar DGJNAc can inhibit, stabilize, and chaperone human α-NAGAL both in vitro and in vivo. We demonstrate that a related iminosugar DGJ (currently in phase III clinical trials for another metabolic disorder, Fabry disease) can also chaperone human α-NAGAL in Schindler/Kanzaki disease. The 1.4- and 1.5-Å crystal structures of human α-NAGAL complexes reveal the different binding modes of iminosugars compared with glycosides. We show how differences in two functional groups result in >9 kcal/mol of additional binding energy and explain the molecular interactions responsible for the unexpectedly high affinity of the pharmacological chaperones. These results open two avenues for treatment of Schindler/Kanzaki disease and elucidate the atomic basis for pharmacological chaperoning in the entire family of lysosomal storage diseases.

The 1.9 a structure of human alpha-N-acetylgalactosaminidase: The molecular basis of Schindler and Kanzaki diseases

alpha-N-acetylgalactosaminidase (alpha-NAGAL; E.C. 3.2.1.49) is a lysosomal exoglycosidase that cleaves terminal alpha-N-acetylgalactosamine residues from glycopeptides and glycolipids. In humans, a deficiency of alpha-NAGAL activity results in the lysosomal storage disorders Schindler disease and Kanzaki disease. To better understand the molecular defects in the diseases, we determined the crystal structure of human alpha-NAGAL after expressing wild-type and glycosylation-deficient glycoproteins in recombinant insect cell expression systems. We measured the enzymatic parameters of our purified wild-type and mutant enzymes, establishing their enzymatic equivalence. To investigate the binding specificity and catalytic mechanism of the human alpha-NAGAL enzyme, we determined three crystallographic complexes with different catalytic products bound in the active site of the enzyme. To better understand how individual defects in the alpha-NAGAL glycoprotein lead to Schindler disease, we analyzed the effect of disease-causing mutations on the three-dimensional structure.

Quantification of free and total sialic acid excretion by LC-MS/MS

BACKGROUND

The main purpose for measuring urinary free sialic acid (FSA) is to diagnose sialic acid (SA) storage diseases. Elevated amounts of conjugated sialic acid (CSA) are observed in several diseases indicating the need to quantify CSA as well. A LC-MS/MS method for quantification of FSA and total sialic acid (TSA) in urine is developed and validated.

METHODS

FSA is analyzed directly after filtration of urine samples. For determination of TSA an enzymatic (neuraminidase) and a chemical (acid) hydrolysis were compared. 13C3-sialic acid was used as internal standard. LC-MS/MS was performed in negative electrospray ionisation mode with multiple reaction monitoring of transitions m/z 308.2-->87.0 (SA) and m/z 311.2-->90.0 (13C3-SA). CSA was calculated by subtracting FSA from TSA.

RESULTS

Limit of detection for FSA and TSA was 0.3 and 1.7 micromol/L, respectively. Limit of quantification for FSA and TSA was 1.0 and 5.0 micromol/L. Intra- and inter-assay variations of FSA were 4.6% and 6.6% (n=10) for FSA and 6.5% and 3.6% (n=10) for TSA. Linearity was tested till 7800 micromol/L (r2=0.9998). Values of SA analyzed after neuraminidase- or acid hydrolysis treatment were comparable. Urine samples from patients with inborn errors of SA (related) metabolism were analyzed and compared with age-related reference values.

CONCLUSION

A method has been developed for routine determination of urinary FSA and TSA. The method is rapid, specific, robust and sensitive. Age-related reference values for FSA, TSA and CSA were determined and improved diagnostic efficacy.

External quality assurance programme for enzymatic analysis of lysosomal storage diseases: a pilot study

Inborn errors of metabolism are rare and laboratories performing diagnostic tests in this field must participate in external quality assurance (EQA) schemes to demonstrate their competence and also to maintain sufficient experience with patient material. EQA schemes for metabolite analyses are available (ERNDIM), but corresponding EQA schemes for enzyme analyses are nonexistent. In this paper we describe a pilot study on lysosomal enzyme testing by four centres in The Netherlands. Quantitative aspects of EQA were studied by interlaboratory comparison of activities of six lysosomal enzymes in a series of buffy coat samples. Interlaboratory variance was enormous. To reduce variance caused by methodological differences, participants reported enzyme activities relative to mean normal values. Beta-D-Galactosidase activities compared well between the participating laboratories (average interlaboratory CV 13%), but for other enzymes large differences were observed, e.g. sphingomyelinase (average CV 38%). Diagnostic proficiency was tested with cultured fibroblasts. In 45 out of a total of 48 tests (12 cell lines, 4 participants) the correct diagnosis was accomplished on the basis of merely biochemical investigations, i.e. without clinical data of the patients. In a survey using blood of a late-onset Pompe disease patient, less conclusive results were obtained. A stable enzyme source was developed for easy distribution. Most lysosomal enzymes were stable upon lyophilization of leukocyte homogenates and during subsequent storage of the freeze-dried material at room temperature, in particular when cryolyoprotectant was added. Shipment of such lyophilized samples is simple and cheap and ideal for an EQA scheme. Our study shows that an EQA programme for enzymatic testing of lysosomal storage diseases is necessary to accomplish reliable diagnostic procedures for lysosomal storage diseases. We recommend that EQA for lysosomal enzymes be implemented through ERNDIM.

Neurologic manifestations of Kanzaki disease

We describe the neurologic findings in a patient with alpha-N-acetylgalactosaminidase deficiency (Kanzaki disease). Clinical and electrophysiologic studies revealed sensory-motor polyneuropathy, and sural nerve pathology showed decreased density of myelinated fibers with axonal degeneration. The patient had mildly impaired intellectual function with abnormal brain MRI and sensory-neuronal hearing impairment with repeated episodes of vertigo attacks. These findings suggest that Kanzaki disease may develop neurologic complications in the CNS and peripheral nervous system.

Structural and immunocytochemical studies on alpha-N-acetylgalactosaminidase deficiency (Schindler/Kanzaki disease)

Alpha-N-acetylgalactosaminidase (alpha-NAGA) deficiency (Schindler/Kanzaki disease) is a clinically and pathologically heterogeneous genetic disease with a wide spectrum including an early onset neuroaxonal dystrophy (Schindler disease) and late onset angiokeratoma corporis diffusum (Kanzaki disease). In alpha-NAGA deficiency, there are discrepancies between the genotype and phenotype, and also between urinary excretion products (sialyl glycoconjugates) and a theoretical accumulated material (Tn-antigen; Gal NAcalpha1-O-Ser/Thr) resulting from a defect in alpha-NAGA. As for the former issue, previously reported genetic, biochemical and pathological data raise the question whether or not E325K mutation found in Schindler disease patients really leads to the severe phenotype of alpha-NAGA deficiency. The latter issue leads to the question of whether alpha-NAGA deficiency is associated with the basic pathogenesis of this disease. To clarify the pathogenesis of this disease, we performed structural and immunocytochemical studies. The structure of human alpha-NAGA deduced on homology modeling is composed of two domains, domain I, including the active site, and domain II. R329W/Q, identified in patients with Kanzaki disease have been deduced to cause drastic changes at the interface between domains I and II. The structural change caused by E325K found in patients with Schindler disease is localized on the N-terminal side of the tenth beta-strand in domain II and is smaller than those caused by R329W/Q. Immunocytochemical analysis revealed that the main lysosomal accumulated material in cultured fibroblasts from patients with Kanzaki disease is Tn-antigen. These data suggest that a prototype of alpha-NAGA deficiency in Kanzaki disease and factors other than the defect of alpha-NAGA may contribute to severe neurological disorders, and Kanzaki disease is thought to be caused by a single enzyme deficiency.

Angiokeratoma corporis diffusum in a Spanish patient with aspartylglucosaminuria

Angiokeratoma corporis diffusum (ACD), initially considered to be synonymous with Fabry's disease, represents a well-known cutaneous marker of some other lysosomal enzyme disorders. Aspartylglucosaminuria (AGU) is a rare hereditary disorder mostly affecting the Finnish population, with only a few sporadic patients of non-Finnish origin. To date, only three patients with AGU have been reported with cutaneous lesions of ACD. A 19-year-old Spanish woman presented with a 10-year history of progressive ACD affecting the limbs, buttocks and trunk. After the age of 6 years she had developed progressive mental deterioration, coarse facies and macroglossia with a scrotal appearance. Peripheral blood smears showed many vacuolated lymphocytes. Enzyme analysis in cultured fibroblasts revealed a decreased activity of aspartylglucosaminidase. By the age of 31 years the patient had developed a bipolar psychosis, polycystic ovarian disease and severe impairment of cognitive skills. This is the first case of AGU detected in a Spanish patient presenting with cutaneous lesions of ACD. To our knowledge, macroglossia with a scrotal appearance and polycystic ovarian disease have not been reported in previous cases of AGU.

Defects in degradation of blood group A and B glycosphingolipids in Schindler and Fabry diseases

Skin fibroblast cultures from patients with inherited lysosomal enzymopathies, alpha-N-acetylgalactosaminidase (alpha-NAGA) and alpha-galactosidase A deficiencies (Schindler and Fabry disease, respectively), and from normal controls were used to study in situ degradation of blood group A and B glycosphingolipids. Glycosphingolipids A-6-2 (GalNAc (alpha 1-->3)[Fuc alpha 1-->2]Gal(beta1-->4)GlcNAc(beta 1-->3)Gal(beta 1--> 4)Glc (beta 1-->1')Cer, IV(2)-alpha-fucosyl-IV(3)-alpha-N-acetylgalactosaminylneolactotetraosylceramide), B-6-2 (Gal(alpha 1-->3)[Fuc alpha 1--> 2] Gal (beta 1-->4)GlcNAc(beta 1-->3)Gal(beta 1-->4)Glc(beta 1-->1')Cer, IV(2)- alpha-fucosyl-IV(3)-alpha-galactosylneolactotetraosylceramide), and globoside (GalNAc(beta 1-->3)Gal(alpha 1-->4)Gal(beta 1-->4)Glc(beta 1-->1') Cer, globotetraosylceramide) were tritium labeled in their ceramide moiety and used as natural substrates. The degradation rate of glycolipid A-6-2 was very low in fibroblasts of all the alpha-NAGA-deficient patients (less than 7% of controls), despite very heterogeneous clinical pictures, ruling out different residual enzyme activities as an explanation for the clinical heterogeneity. Strongly elevated urinary excretion of blood group A glycolipids was detected in one patient with blood group A, secretor status (five times higher than upper limit of controls), in support of the notion that blood group A-active glycolipids may contribute as storage compounds in blood group A patients. When glycolipid B-6-2 was fed to alpha-galactosidase A-deficient cells, the degradation rate was surprisingly high (50% of controls), while that of globotriaosylceramide was reduced to less than 15% of control average, presumably reflecting differences in the lysosomal enzymology of polar glycolipids versus less-polar ones. Relatively high-degree degradation of substrates with alpha-D-Galactosyl moieties hints at a possible contribution of other enzymes.

The 1.9 A structure of alpha-N-acetylgalactosaminidase: molecular basis of glycosidase deficiency diseases

In the lysosome, glycosidases degrade glycolipids, glycoproteins, and oligosaccharides. Mutations in glycosidases cause disorders characterized by the deposition of undegraded carbohydrates. Schindler and Fabry diseases are caused by the incomplete degradation of carbohydrates with terminal alpha-N-acetylgalactosamine and alpha-galactose, respectively. Here we present the X-ray structure of alpha-N-acetylgalactosaminidase (alpha-NAGAL), the glycosidase that removes alpha-N-acetylgalactosamine, and the structure with bound ligand. The active site residues of alpha-NAGAL are conserved in the closely related enzyme a-galactosidase A (alpha-GAL). The structure demonstrates the catalytic mechanisms of both enzymes and reveals the structural basis of mutations causing Schindler and Fabry diseases. As alpha-NAGAL and alpha-GAL produce type O "universal donor" blood from type A and type B blood, the alpha-NAGAL structure will aid in the engineering of improved enzymes for blood conversion.

A new case of alpha-N-acetylgalactosaminidase deficiency with angiokeratoma corporis diffusum, with Ménière's syndrome and without mental retardation

alpha-N-acetylgalactosaminidase (alpha-NAGA) deficiency is a rare hereditary lysosomal storage disease, and only three alpha-NAGA-deficient patients with angiokeratoma corporis diffusum (Kanzaki) have been described. We report a further case in a 47-year-old Japanese woman, the product of a consanguineous marriage. The remarkable findings in this patient were her normal intelligence, Ménière's syndrome, disturbance of peripheral sensory nerves, hearing loss and cardiac hypertrophy. alpha-NAGA enzyme activity in her plasma was 0.77% of the normal value. Other enzyme activities, such as alpha-galactosidase, beta-galactosidase, alpha-L-fucosidase, beta-mannosidase and aspartylglucosaminidase, were within normal limits. A large quantity of amino acid O-glycans was detected in her urine. Gene analysis revealed a novel point mutation (G-->A transition) at nucleotide 11018 (986 in the cDNA) resulting in an Arg-329-Gln substitution. Kanzaki disease has the same enzyme defect as Schindler disease, but the manifestations are quite different.

Human alpha-N-acetylgalactosaminidase (alpha-NAGA) deficiency: no association with neuroaxonal dystrophy?

Two new individuals with alpha-NAGA deficiency are presented. The index patient, 3 years old, has congenital cataract, slight motor retardation and secondary demyelinisation. Screening of his sibs revealed an alpha-NAGA deficiency in his 7-year-old healthy brother who had no clinical or neurological symptoms. Both sibs are homozygous for the E325K mutation, the same genotype that was found in the most severe form of alpha-NAGA deficiency presenting as infantile neuroaxonal dystrophy. Thus, at the age of 7 years the same genotype of alpha-NAGA may present as a 'non-disease' (present healthy case) and can be associated with the vegetative state (the first two patients described with alpha-NAGA deficiency). The clinical heterogeneity among the 11 known individuals with alpha-NAGA deficiency is extreme, with a 'non-disease' (two cases) and infantile neuroaxonal dystrophy (two cases) at the opposite sides of the clinical spectrum. The broad spectrum is completed by a very heterogeneous group of patients with various degrees of epilepsy/behavioural difficulties/psychomotor retardation (four patients) and a mild phenotype in adults without overt neurological manifestations who have angiokeratoma and clear vacuolisation in various cell types (three cases). These observations are difficult to reconcile with a straightforward genotype-phenotype correlation and suggest that factors or genes other than alpha-NAGA contribute to the clinical heterogeneity of the 11 patients with alpha-NAGA deficiency.

Glycoprotein lysosomal storage disorders: alpha- and beta-mannosidosis, fucosidosis and alpha-N-acetylgalactosaminidase deficiency

Glycoproteinoses belong to the lysosomal storage disorders group. The common feature of these diseases is the deficiency of a lysosomal protein that is part of glycan catabolism. Most of the lysosomal enzymes involved in the hydrolysis of glycoprotein carbohydrate chains are exo-glycosidases, which stepwise remove terminal monosaccharides. Thus, the deficiency of a single enzyme causes the blockage of the entire pathway and induces a storage of incompletely degraded substances inside the lysosome. Different mutations may be observed in a single disease and in all cases account for the nonexpression of lysosomal glycosidase activity. Different clinical phenotypes generally characterize a specific disorder, which rather must be described as a continuum in severity, suggesting that other biochemical or environmental factors influence the course of the disease. This review provides details on clinical features, genotype-phenotype correlations, enzymology and biochemical storage of four human glycoprotein lysosomal storage disorders, respectively alpha- and beta-mannosidosis, fucosidosis and alpha-N-acetylgalactosaminidase deficiency. Moreover, several animal disorders of glycoprotein metabolism have been found and constitute valuable models for the understanding of their human counterparts.

Molecular cloning, structural organization, sequence, chromosomal assignment, and expression of the mouse alpha-N-acetylgalactosaminidase gene

Alpha-N-acetylgalactosaminidase (2-acetamido-2-deoxy-alpha-d-galactoside acetamidodeoxy-galactohydrolase, NAGA; EC 3.2.1.49) deficiency is a recently recognized autosomal recessive lysosomal disease. As a prerequisite for the generation of an animal model, the mouse NAGA gene was cloned and characterized. The NAGA gene was assigned to mouse chromosome 15 band E3, syntenic to the region encompassing the human gene, and NAGA-deficient mutant human cells transfected with the cosmid clone containing the mouse NAGA gene expressed NAGA activity. Comparison of the mouse NAGA nucleotide sequence with the human NAGA sequence predicted that the mouse NAGA gene contains an open reading frame of 1245bp, comprising nine coding exons and spanning a genomic region of 8258bp, and a 3' untranslated region of 0.5kb. The 5' untranslated region was determined in primer extension studies to be 235bp in length. Nucleotide identity between the human and mouse NAGA exons ranged from 67.4 to 89.5%, with better matches for exons 1-7 than for 8 and 9. The overall amino acid identity between the mouse and human deduced NAGA polypeptides was 82.0%, between those of mouse and chicken 72.9%. Homology was found to only one other mouse gene, i.e. the alpha-galactosidase A (GALA; EC 3.2.1. 22) gene. The amino acid identity ranged from 51.6 to 62.1% in the polypeptide regions corresponding to NAGA exons 2-7 and GALA exons 1-6, but little, if any, in the remainder. These analyses gave emphasis to the strong conservation of the NAGA gene and its origin from an ancestor common with the GALA gene, with NAGA exons 8 and 9 and GALA exon 7 being the most divergent regions in the evolution of the two genes.