We have isolated a 2.1 kb cDNA which encodes human aspartylglucosaminidase (AGA, E.C. 3.5.1.26). The activity of this lysosomal enzyme is deficient in aspartylglucosaminuria (AGU), a recessively inherited lysosomal accumulation disease resulting in severe mental retardation. The polypeptide chain deduced from the AGA cDNA consists of 346 amino acids, has two potential N‐glycosylation sites and 11 cysteine residues. Transient expression of this cDNA in COS‐1 cells resulted in increased expression of immunoprecipitable AGA protein. Direct sequencing of amplified AGA cDNA from an AGU patient revealed a G‐‐‐‐C transition resulting in the substitution of cysteine 163 with serine. This mutation was subsequently found in all the 20 analyzed Finnish AGU patients, in the heterozygous form in all 53 carriers and in none of 67 control individuals, suggesting that it represents the major AGU causing mutation enriched in this isolated population. Since the mutation produces a change in the predicted flexibility of the AGA polypeptide chain and removes an intramolecular S‐S bridge, it most probably explains the deficient enzyme activity found in cells and tissues of AGU patients.

Aspartyucaminuria (AGU) Is an inherited lysosomal storage disoder caused by the deficiency of aspartylguminidase. We have earlier reported a single missense mutation (Cys"*3 -~Ser) to be responsible for 98% of the AGU alleles in the isolated Finnish population, which contains about 90% of the reported AGU patients. Here we describe the spectrum of 10 AGU mutations found in unrelated patients of non-Finnish origin. Since 11 out of 12 AGU patients were homozygotes, con g ity has to be a common denominator in most AGU families. The mutations were distributed over the entire coding region of the aspartylgi cDNA, except in the carboxyl-terminal 17-kDa subunit in which they were clustered within a 46-amino acid region. Based on the character of the mutations, most of them are prone to affect the folding and stability and not to directly affect the active site of the aspartylglucosaminidase enzyme.

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Aspartylglucosaminuria (AGU) is caused by deficient activity of the enzyme aspartylglucosaminidase (AGA). The structural gene for AGA has been assigned to the region 4q21-qter of chromosome 4. We have studied the map position of the AGU locus in relation to other marker loci on the long arm of chromosome 4 using linkage analyses. Restriction fragment length polymorphism alleles for the ADH2, ADH3, EGF, FG alpha and FG beta loci and blood group antigens for the MNS locus were determined in a panel of 12 Finnish AGU families. The heterozygous family members were identified by reduced activity of AGA in lymphocytes. Linkage studies were performed using both pairwise and multipoint analyses. Loose linkage of the AGU locus to the FG and MNS loci was observed (z = 1.16, z = 1.39, respectively). Multipoint analysis to the fixed map [ADH-(0.03)-EGF-(0.35)-FG-(0.11)-MNS] suggests that the location of the AGU locus is 0.05-0.30 recombination units distal to MNS (z = 3.03). The order cen-ADH-EGF-FG-MNS-AGU is 35 times more likely than the next best order cen-ADH-EGF-AGU-FG-MNS.