Saposin A: second cerebrosidase activator protein

Insertion in the mRNA of a metachromatic leukodystrophy patient with sphingolipid activator protein-1 deficiency

The lysosomal catabolism of sulfatide requires arylsulfatase A and a specific sphingolipid activator protein, SAP-1. While most patients with metachromatic leukodystrophy have mutations in the gene for arylsulfatase A, some patients have deficient SAP-1, as determined by immunological techniques. We now describe the molecular findings in a patient who died at 22 years of age with SAP-1 deficiency. The DNA polymerase chain reaction was used to amplify regions of cDNA which were subcloned in M13 phage DNA and sequenced by the dideoxy chain-termination method. The patient was found to have a 33-base-pair insertion between nucleotides 777 and 778 (numbered from the A of the ATG initiation codon). No other changes were found in the coding sequence of the cDNA from this patient. At the site of the insertion some normal people have an additional 9 base pairs, which correspond to the last 9 nucleotides at the 3' end of the insertion. The cDNAs from the second-cousin parents were amplified and sequenced, and in both two alleles were identified, one with the 33-base-pair insertion and one with no insertion. Two brothers were found to have only the normal alleles and a sister was found to have the 33-base-pair insertion and a normal allele. The findings confirm studies performed on leukocyte extracts demonstrating normal antigen levels in the two brothers and a lower level in the sister. The presence of 11 additional amino acids in the coding region of mature SAP-1 in this patient causes significant changes in the hydropathy profile compatible with the previous findings at the protein level.

Advances in the molecular genetics of metachromatic leukodystrophy

Metachromatic leukodystrophy is a lysosomal storage disorder caused by the deficiency of arylsulphatase A (EC 3.1.6.1). This results in the intralysosomal storage of cerebroside sulphate, which leads to a progressive demyelination of the nervous system. The patients usually die within a few years from the onset of symptoms. Clinically, there are different forms of the disease and the molecular basis for this heterogeneity is unknown. The gene for arylsulphatase A has recently been cloned and provides a necessary tool for the exact description of the molecular defects occurring in the different forms of metachromatic leukodystrophy. Metachromatic leukodystrophy can also be caused by the deficiency of an arylsulphatase A activator protein (sphingolipid activator protein B). The cDNA for the precursor of this protein has been isolated and a mutant cDNA of one patient has been analysed. A substantial arylsulphatase A deficiency can also occur in healthy individuals, a phenotype termed pseudodeficiency. Two concurrent mutations have been identified in this low arylsulphatase A activity allele. This permitted the development of a rapid assay which allows the detection of the pseudodeficiency allele. Bone marrow transplantation has been tried in several metachromatic leukodystrophy patients and there is evidence that this treatment might slow or even halt the progression of the disease. A final conclusion as to whether bone marrow transplantation is a suitable therapy for metachromatic leukodystrophy cannot be drawn yet.

Acid beta-glucosidase: enzymology and molecular biology of Gaucher disease

Human lysosomal beta-glucosidase (D-glucosyl-acylsphingosine glucohydrolase, EC 3.2.1.45) is a membrane-associated enzyme that cleaves the beta-glucosidic linkage of glucosylceramide (glucocerebroside), its natural substrate, as well as synthetic beta-glucosides. Experiments with cultured cells suggest that in vivo this glycoprotein requires interaction with negatively charged lipids and a small acidic protein, SAP-2, for optimal glucosylceramide hydrolytic rates. In vitro, detergents (Triton X-100 or bile acids) or negatively charged ganglioside or phospholipids and one of several "activator proteins" increase hydrolytic rate of lipid and water-soluble substrates. Using such in vitro assay systems and active site-directed covalent inhibitors, kinetic and structural properties of the active site have been elucidated. The defective activity of this enzyme leads to the variants of Gaucher disease, the most prevalent lysosomal storage disease. The nonneuronopathic (type 1) and neuronopathic (types 2 and 3) variants of this inherited (autosomal recessive) disease but panethnic, but type 1 is most prevalent in the Ashkenazi Jewish population. Several missense mutations, identified in the structural gene for lysosomal beta-glucosidase from Gaucher disease patients, are presumably casual to the specifically altered posttranslational oligosaccharide processing or stability of the enzyme as well as the altered in vitro kinetic properties of the residual enzyme from patient tissues.