Deficient glycosylation of arylsulfatase A in pseudo arylsulfatase-A deficiency

Association of alcoholism with the N-glycosylation polymorphism of pseudodeficient human arylsulfatase A

The IIIa and IIIb electrophoretic variants of arylsulfatase A (EC 3.1.6.8) are 12 times more prevalent in alcoholic than in nonalcoholic populations. These variant enzymes, found in a subset of alcoholics, possess the pseudodeficient Asn350-Ser mutation of arylsulfatase A and, consequently, lack an N-linked glycan unit. These genetically determined variants of arylsulfatase A show reduced intracellular half-life, and cells from such individuals possess reduced enzymic activity. We propose that this polymorphism is an underlying genetic and biochemical factor contributing to the neuropathology and/or addiction pathway of this disease.

Complications in the genotypic molecular diagnosis of pseudo arylsulfatase A deficiency

Metachromatic leukodystrophy (MLD) is a severe neurodegenerative disease associated with deficient arylsulfatase A activity. Biochemical confirmation of this disorder has been complicated by a clinically normal but enzymatically deficient variant, pseudo arylsulfatase-A deficiency (PD). The PD mutation is associated with two A-->G transitions in the arylsulfatase A gene. They can be detected simultaneously with a recently developed 3'-mismatch polymerase chain reaction, hence providing a rapid method for genotypic identification and resolving ambiguities of carrier identification based solely on enzyme analyses. However, we now report further genotypic complexities in the molecular diagnosis of PD due to the occurrence of another variant in which only one of the two A-->G mutations of the PD allele was present. This variant confers reduced but readily detectable enzyme activity and behaves as a silent allele in the 3'-mismatch polymerase chain reaction, thus leading to conflicting and erroneous genotype assignments in a family in which both variants and MLD co-exist. The inconsistency was resolved after pedigree validation and further molecular analyses in which the two A-->G mutations were assayed separately with allele-specific oligonucleotides. Because arylsulfatase A analysis is one of the most commonly requested lysosomal enzyme assays and the PD mutant allele frequency is high in the general population, complexities as described in this family may be a recurrent problem that can be solved only with combined enzymatic and detailed molecular analyses.

The structural basis for the electrophoretic isoforms of normal and variant human platelet arylsulphatase A

Human platelet arylsulphatase A (ASA) exhibits a multiple banding pattern when examined by PAGE. The isoform pattern (IVa) of the enzyme obtained from normal subjects differs from variants (IIIa, IIIb, IVb) which are primarily found in alcoholic patients. Alkaline phosphatase and endo-N-acetylglucosaminidase H treatments, as well as ion-exchange chromatography, demonstrate that the isoforms of ASA arise because of charge heterogeneity caused by phosphoglycan moieties. The isoform with the slowest mobility in PAGE lacks phosphate substituents. Based upon mannose-6-phosphate-receptor affinity chromatography it can be concluded that most, if not all, of the enzyme-linked phosphate is in the form of 6-O-phospho-D-mannosyl units. Lectin affinity chromatography and peptide-N-glycosidase F treatment followed by SDS/PAGE and Western-blot analysis indicate that normal platelet ASA contains two oligomannose and/or hybrid glycan moieties of which one, or both, possess a 6-O-L-fucosyl substituent on the asparagine-linked N-acetylglucosamine residue. Comparative analysis indicates that the variant IIIa and IIIb types of ASA differ from the IVa ASA with regard to the level of glycan phosphorylation and glycan structure, as well as the polypeptide structure.

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.