Pseudo arylsulfatase-A deficiency in healthy individuals: genetic and biochemical relationship to metachromatic leukodystrophy

Direct tandem mass spectrometric profiling of sulfatides in dry urinary samples for screening of metachromatic leukodystrophy

BACKGROUND

Prediagnostic steps in suspected metachromatic leukodystrophy (MLD) rely on clinical chemical methods other than enzyme assays. We report a new diagnostic method which evaluates changes in the spectrum of molecular types of sulfatides (3-O-sulfogalactosyl ceramides) in MLD urine.

METHODS

The procedure allows isolation of urinary sulfatides by solid-phase extraction on DEAE-cellulose membranes, transportation of a dry membrane followed by elution and tandem mass spectrometry (MS/MS) analysis in the clinical laboratory. Major sulfatide isoforms are normalized to the least variable component of the spectrum, which is the indigenous C18:0 isoform. This procedure does not require the use of specific internal standards and minimizes errors caused by sample preparation and measurement.

RESULTS

Urinary sulfatides were analyzed in a set of 21 samples from patients affected by sulfatidosis. The combined abundance of the five most elevated isoforms, C22:0, C22:0-OH, C24:0, C24:1-OH, and C24:0-OH sulfatides, was found to give the greatest distinction between MLD-affected patients and a control group.

CONCLUSIONS

The method avoids transportation of liquid urine samples and generates stable membrane-bound sulfatide samples that can be stored at ambient temperature. MS/MS sulfatide profiling targeted on the most MLD-representative isoforms is simple with robust results and is suitable for screening.

Arylsulfatase A pseudodeficiency: altered kinetic and heat-inactivation properties

Arylsulfatase A (ASA) pseudodeficiency (PD) was described in clinically healthy individuals with ASA-deficient activity. To confirm that the PD individual in the present study is homozygous for the PD allele without any other mutations, direct solid-phase sequencing was done and the two A-to-G transitions--one at the third N-glycosylation site (N350S) and the other at the first polyadenylation signal (ATTAAC to AGTAAC)--were identified. No other mutations were detected in the entire coding region nor in the intron-exon boundary region of the ASA gene in the PD cells. Kinetic studies to compare the partially purified ASA from controls to that from a homozygote (PD allele) were carried out using p-nitrocatechol sulfate (p-NCS) as a substrate. The apparent Km for the control ASA was 0.6 mM and for the PD enzyme 2.0 mM (p < 0.01). The heat inactivation at 60 degrees C revealed 50% inactivation within 90 min for control ASA and 28 min for PD ASA. At 65 degrees C, the 50% inactivation was reached at 18 min for the control and at 8.5 min for the PD. These results document the decreased affinity of ASA toward p-NCS and increased heat inactivation from a PD individual. Western blot analysis following SDS-PAGE and isoelectric focusing revealed differences in both the molecular weight and the isoelectric point between the control ASA and that of the PD allele. To the best of our knowledge, this is the first report showing the altered properties of ASA from a PD homozygote.

Metachromatic leukodystrophy: subtype genotype/phenotype correlations and identification of novel missense mutations (P148L and P191T) causing the juvenile-onset disease

Metachromatic leukodystrophy (MLD) is a lysosomal storage disease resulting from the deficient activity of arylsulfatase A (ASA) and the accumulation of sulfatides. The disease is characterized by several subtypes, designated by age at onset: the late-infantile-, juvenile-, and adult-onset variants. Mutation analysis of genomic DNA from a proband with each variant was performed to identify and characterize their causative ASA mutations. Two sisters with the infantile-onset disease were homoallelic for the missense mutation D335V, a juvenile-onset proband was heteroallelic for two novel missense mutations, P148L and P191T, and an adult-onset patient was heteroallelic for the H397Y and P426L mutations. The novel mutations were not identified in 108 normal alleles indicating that these base substitutions were not common polymorphisms. To further characterize the mutant gene products, the mutant enzymes were partially purified from cultured fibroblasts and their molecular weights and charges were compared by immunoblotting following SDS-PAGE or isoelectric focusing (IEF). Normal fibroblast ASA had a single, broad band at 54 kDa. The enzyme from the late-infantile-onset patient had distinct bands of 36 and 78 kDa, but lacked the normal 54-kDa species. The juvenile- and adult-onset patients each had a faint band of 54 kDa and several other bands ranging from 29 to 64 kDa. IEF revealed several bands for the partially purified normal enzyme with a relatively narrow pH range around 4.0, whereas numerous bands with a wider range of isoelectric points were observed with the enzymes from the juvenile- and adult-onset fibroblasts. In contrast, the enzyme from the late-infantile-onset proband had four bands with more acidic isoelectric points, none corresponding to those of the normal enzyme. These results document changes in both size and charge of the mutant enzymes from patients with different mutations and MLD subtypes.

Arylsulfatase A pseudodeficiency-associated mutations: population studies and identification of a novel haplotype

Pseudodeficiency of arylsulfatase A is characterized by reduction of arylsulfatase A activity without neurodegeneration, making it an important complication when diagnosing metachromatic leukodystrophy. Two DNA substitutions are associated with arylsulfatase A pseudodeficiency. One, 1788A-->G, results in the loss of an N-glycosylated asparagine in the protein, and the second, 2723A-->G, removes the polyadenylation signal site of the mRNA. Previously, the polyadenylation signal site variant was observed only in the presence of the N-glycosylation site variant, although the latter has been reported to occur in the absence of the polyadenylation signal site variant. We investigated the frequencies of these alleles and their linkage disequilibrium in a number of populations and in psychiatric patients. While the N-glycosylation site variant had a high frequency in the Bantu-speaking people from Southern Africa (0.44), the San of Southern Africa (0.22), African Americans (0.37), and Cheyenne Indians (0.375), the polyadenylation signal site variant was absent in these groups. The mutated polyadenylation signal site was found only in the Caucasian groups surveyed. Two Caucasian sibs were identified with the pseudodeficiency polyadenylation signal site variant in the absence of the N-glycosylation site variant, indicating that linkage disequilibrium between the two polymorphisms is not perfect.

Structural characterization of variant forms of arylsulfatase A that associate with alcoholism

Several electrophoretic forms of human platelet arylsulfatase A (ASA), including variant type IIIa and normal type IV(a), have been identified by nondenaturing polyacrylamide gel electrophoresis. An alcoholic population that we have analyzed is enriched in variant type IIIa compared with nonalcoholic psychiatric and normal controls. Individuals with the IIIa enzyme possess greatly reduced levels of ASA activity. To understand further the structural basis for the differences and their potential biological consequences, the nature of the ASA variant expressed by fibroblasts from different individuals was explored. The electrophoretic patterns of fibroblast ASA from the IIIa and IV(a) individuals differ in degree of phosphorylation. Furthermore, fibroblast ASA from IIIa individuals lacks an N-linked glycan found in ASA from IV(a) individuals. In addition, differences in peptide and/or posttranslational modification unrelated to the N-linked carbohydrate or phosphorylation exist between the fibroblast ASA from IIIa and IV(a) individuals. The finding that both fibroblasts and platelets exhibit related electrophoretic isoform patterns characteristic of the donor's ASA type allows for the use of fibroblasts to study the impact of ethanol on the metabolism of cells possessing different ASA types.

Diagnosis of arylsulfatase A deficiency

Metachromatic leukodystrophy (MLD) is a neurologically devastating autosomal recessive disorder in humans associated with deficient arylsulfatase A activity. However, clinically normal individuals described as being pseudo-arylsulfatase-A deficient also demonstrate the same deficiency. Genotypically, they may be homozygous for the pseudodeficiency mutation (associated with 2 A-->G transitions in the cDNA of arylsulfatase A) or heterozygous with one pseudodeficiency and one MLD allele. Using as examples 2 families in which the pseudo deficiency condition occurs either independently or together with MLD, we demonstrate the utility of a proposed diagnostic protocol to provide complete genotype identification of individuals suffering from arylsulfatase A deficiency. Patient fibroblasts are extracted for DNA and a cytoplasmic fraction, which is used for arylsulfatase A enzyme assay. This will identify an arylsulfatase A-deficient group, which is further analyzed electrophoretically. Cells from the clinically affected patients with MLD are completely deficient in arylsulfatase A activity, whereas those from the pseudodeficient individuals demonstrate a characteristic residual arylsulfatase A activity detectable only after electrophoresis. Within this pseudodeficient group, gene amplification of DNA specific for the A-->G mutations will distinguish between those who are homozygous for the pseudodeficiency allele and those who are compound heterozygous for the pseudodeficiency and MLD alleles. This protocol of complete genotype identification requires only about 10(6) fibroblasts (1 x 100 mm dish) and 2 days to complete. Such variant-specific genotype identification increases accuracy and prognostic value of the diagnosis. It will likely become the preferred choice for diagnosis of genetic disease in the future as more variant-specific mutations are identified at the molecular level.

Identification of a mutation in the arylsulfatase A gene of a patient with adult-type metachromatic leukodystrophy

To analyze the genetic abnormality in a Japanese patient with adult-type metachromatic leukodystrophy (MLD), we first elucidated the genomic organization of the human arylsulfatase A (ASA) gene and then compared the nucleotide sequences of exons and splice junctions of the mutant ASA gene to those of a normal control. We have identified a new mutation, a G-to-A transition in exon 2, which results in amino acid substitution of Asp for 99Gly. In a transient expression study, COS cells transfected with the mutant cDNA carrying 99Gly----Asp did not show an increase of ASA activity, which confirms that the mutation is a cause of adult-type MLD.

Low arylsulphatase A activity and choreoathetotic syndrome in three siblings: differentiation of pseudodeficiency from metachromatic leukodystrophy

We report on a family with a sibship of three children for whom the diagnosis of "an unusual form of metachromatic leukodystrophy (MLD)" had been suggested earlier. The patients had choreiform movements and dystonic posturing accompanied by dysarthria since childhood. The availability of the polymerase chain reaction enabled us to show that the three siblings have a pseudodeficiency genotype (ASAp/ASAp). There was no abnormal sulphatiduria, and we propose that the neurological disease and low arylsulphatase A activity are unrelated to one another in this family. A diagnosis of MLD carries very serious implications, and we recommend that gene amplification by polymerase chain reaction and hybridization with allele-specific oligonucleotide probes should be used to corroborate the diagnosis, especially when there is no abnormal sulphatiduria and when metachromatic material cannot be demonstrated in a sural nerve biopsy.

Deficient glycosylation of arylsulfatase A in pseudo arylsulfatase-A deficiency

Deficient arylsulfatase-A activity is diagnostic of a neurodegenerative human lysosomal storage disease, metachromatic leukodystrophy. Paradoxically, similar enzyme deficiency also occurs in normal individuals, who are known as being pseudo arylsulfatase-A deficient. We showed previously that this phenotype is associated with a structural gene mutation that produces an exceptionally labile enzyme. We now report on the nature and consequence of this mutation. When the mutant arylsulfatase-A is deglycosylated by endoglycosidase H, only one smaller molecular species was generated, instead of the two from the normal enzyme. This is consistent with the loss of one of the two N-linked oligosaccharide side chains known to be present on the wild-type enzyme. Quantitative analysis of mannose and leucine incorporation showed that the mutant enzyme incorporated two- to tenfold less mannose than the normal enzyme on a molar basis. This deficient glycosylation was specific to arylsulfatase-A. Another lysosomal enzyme not affected in this mutation, beta-hexosaminidase, was glycosylated normally in the mutant cells. The remaining single oligosaccharide side chain released from the mutant arylsulfatase-A by pronase digestion was normally processed to complex and high-mannose forms. However, the high-mannose side chains contained 30% fewer phosphorylated residues than those of the normal enzyme. Nevertheless, this reduced level of phosphorylation did not prevent targeting of the mutant enzyme to the lysosomes, a process normally mediated through phosphorylated mannose residues. In conclusion, pseudo arylsulfatase-A deficiency is a unique human mutation associated with reduced glycosylation and phosphorylation of a lysosomal enzyme with the loss of one of the two carbohydrate side chains. The mutation results in greatly reduced enzyme stability, thus indicating a role for oligosaccharides in maintaining enzyme stability within the degradative environment of the lysosomes. However, the residual catalytic activity or subcellular targeting of the mutant enzyme was not affected. These properties probably account for the benign clinical presentation of pseudo arylsulfatase-A deficiency.

Arylsulfatase A pseudodeficiency: loss of a polyadenylylation signal and N-glycosylation site

Metachromatic leukodystrophy is a metabolic disorder caused by the deficiency of arylsulfatase A. Deficiency of this enzyme is also found in apparently healthy individuals, a condition for which the term pseudodeficiency was introduced. The arylsulfatase A (cerebroside-3-sulfate 3-sulfohydrolase; EC 3.1.6.8) (ASA) encoding gene was isolated from an individual homozygous for the ASA pseudodeficiency allele. Sequence analysis revealed two A----G transitions. One changes Arg-350 to serine, which leads to the loss of a utilized N-glycosylation site. This loss explains the smaller size of ASA in ASA pseudodeficiency fibroblasts. The introduction of Ser-350 into normal ASA cDNA does not affect the rate of synthesis, the stability, or the catalytic properties of ASA in stably transfected baby hamster kidney cells. Therefore, the loss of the N-linked oligosaccharide does not contribute to the reduction of ASA activity in ASA pseudodeficiency. The other A----G transition changes the first polyadenylylation signal downstream of the stop codon from AATAAC to AGTAAC. The latter causes a severe deficiency of a 2.1-kilobase (kb) mRNA species. The deficiency of the 2.1-kb RNA species provides an explanation for the diminished synthesis of ASA seen in pseudodeficiency fibroblasts. Amplification of genomic DNA and hybridization with allele-specific oligonucleotides detected both mutations in four unrelated individuals with ASA pseudodeficiency.

Myoclonic epilepsy of Lafora and arylsulphatase A deficiency in the same patient

Very low levels of arylsulphatase A were found in a young patient with the clinical features of Lafora disease, confirmed by muscle biopsy. The deficiency was shown both in leukocytes and cultured fibroblasts. A cerebroside sulphate loading showed that 93% of [14C]cerebroside sulphate taken up by skin fibroblasts from the patient remained unmetabolized after a 24h pulse, ruling out pseudo-arylsulphatase A deficiency. In the healthy parents and siblings of the patient, biochemical data suggested heterozygosity for arylsulphatase A deficiency. The apparent co-inheritance of arylsulphatase A deficiency and Lafora disease in this family might be the consequence of genetic linkage between the two genes.

A family with pseudodeficiency of acid alpha-glucosidase

A unique family is presented which consists of a patient with the juvenile muscular dystrophy form of glycogenosis type II and four healthy individuals, both parents and sisters, with low acid alpha-glucosidase activity. It was almost impossible to distinguish the homozygote from the heterozygous members by lymphocyte assay alone. In cultured skin fibroblasts, acid alpha-glucosidase activity measured with a synthetic substrate was less than 1% of the normal mean value in the patient and about 15% in the parents. The activity toward glycogen was not detectable in the patient and was about 30% of the normal mean value in the parents. These values are also lower than expected in heterozygotes. To explain these results properly, a new mutant allele of acid alpha-glucosidase is proposed. Both parents could be compound heterozygotes for the pseudodeficiency allele and the juvenile form of glycogenosis type II allele.

Pseudo arylsulfatase A deficiency. Biosynthesis of an abnormal arylsulfatase A

Pseudo arylsulfatase A deficiency, an asymptomatic condition, and metachromatic leukodystrophy, a severe neurodegenerative disease, are both associated with profound reductions of arylsulfatase A activity in man. We now report that with metabolic labelling, cultured pseudo deficient cells synthesized about 20% of the normal amount of arylsulfatase A at a reduced rate of apparent synthesis and increased rate of degradation. However, in the presence of ammonium chloride which stimulated secretion of lysosomal enzymes, these cells synthesized about 80% of the normal amount of enzyme protein. Hence, the defect in pseudo arylsulfatase A deficiency is associated with labile arylsulfatase A molecules which can be stabilized if they are diverted from intracellular storage.

Aryl sulfatase A deficiency in psychiatric and neurologic patients

Two hundred ninety-five psychiatric and neurologic patients were randomly screened for aryl sulfatase A (ASA) activity in lymphocyte extracts. Two of these patients showed very low ASA activity, in the range of metachromatic leukodystrophy (MLD)-affected patients. The residual activity in these low ASA patients showed normal enzyme behavior with regard to ASA kinetic features and the ability to catabolize 14C labeled sulfatide by intact fibroblasts. Taking into account that approximately 3% of the general population are homozygous for the pseudo-aryl sulfatase A gene and are clinically unaffected, the data obtained here indicate that the patients studied in this work, as well as most psychiatric patients reported in the literature with low ASA activity, represent the normal ASA polymorphism. Thus, the very low ASA activity patients are in fact homozygous for the pseudo-deficient allele, which does not result in clinical abnormalities. The clinical symptoms in these psychiatric patients and probably other "variant" MLD patients are therefore not related to low ASA activity.

Brain galactolipid content in a patient with pseudoarylsulfatase A deficiency and coincidental diffuse disseminated sclerosis, and in patients with metachromatic, adreno-, and other leukodystrophies

A 4-year old boy died of diffuse disseminated sclerosis (DDS) of the brain and was found to have also pseudoarylsulfatase A deficiency (PASAD) with about 20% residual arylsulfatase A (ASA) and cerebroside sulfatase (CS) activity. The reexamination of lipids did not show any sulfatide accumulation in the patient's organ extracts. Although the residual CS activity in the patient's extracts was clearly demonstrable only after partial purification, it was concluded that this activity protects organ tissues from sulfatide accumulation in PASAD, since in sulfatide lipidosis (metachromatic leukodystrophy, MLD) no residual CS activity was detectable. The study of residual ASA activity in the patient's fibroblasts by gel electrofocusing resulted in an almost normal enzyme microheterogeneity. However, the detailed study of the brain galactolipids in the patient revealed an elevated ratio of sulfatide/galactocerebroside content, despite the decrease of both lipids. In tissues of other patients with severe demyelinating diseases different from DDS and MLD, this galactolipid ratio was also found to be increased, especially in three patients with adrenoleukodystrophy. A general mechanism of this anomaly in severe demyelination is considered.

Attenuated activities and structural alterations of arylsulfatase A in tissues from subjects with pseudo arylsulfatase A deficiency

It had been shown previously that arylsulfatase A activity was attenuated in pseudo arylsulfatase A deficiency fibroblasts and that subunits of the enzyme were smaller than subunits of the enzyme in normal fibroblasts. Attenuated enzyme activity has now been affirmed in other tissues. Subunits of the enzyme from these sources were also found to be smaller with apparent molecular size 59 and 56 kdaltons. Subunits of enzyme in corresponding control tissues were larger and there was heterogeneity in apparent molecular size as follows: fibroblasts, 63 and 59 kdaltons; liver, 63 and 59 kdaltons; kidney, 63 and 58 kdaltons; spleen, 63 and 58 kdaltons; placenta, 62 and 58 kdaltons; and urine, 61 and 57 kdaltons. Attenuated enzyme activity and structurally altered enzyme in pseudo arylsulfatase A deficiency appears to be systemic. However, the reason for reduced amounts of structurally altered enzyme with normal catalytic activity is unresolved.

Biochemical variability of arylsulphatases -A, -B and -C in cultured fibroblasts from patients with multiple sulphatase deficiency

Multiple sulphatase deficiency (MSD) in man is inherited as an autosomal recessive trait and associated with deficient activities of various sulphohydrolases. Cultured fibroblasts from seven different patients were assayed for arylsulphatases-A, -B and -C activities. On the basis of the results, they may be classified into three groups: I, deficient in all three arylsulphatases; II, deficient only in arylsulphatases-A and -C with half or near-normal arylsulphatase-B; electrophoretically, arylsulphatase-A activity bands are undetectable as in metachromatic leukodystrophy; III, same as in II except electrophoretically, the residual arylsulphatase-A is detectable as faint activity bands similar to those in pseudo arylsulphatase-A deficiency. In addition to the variability among different strains, within the same strain of MSD or normal cells, each enzyme activity increased several fold with increasing time in culture. These sources of biochemical variability among and within different cell strains have not been recognized before in the study of this apparently monogenic trait with multiple enzyme deficiencies. They may account for some of the discrepancies reported in the literature on arylsulphatase activities among cultured cells from different multiple sulphatase deficient patients.