Characterization of the mutant N-acetylglucosaminylphosphotransferase in I-cell disease and pseudo-Hurler polydystrophy: complementation analysis and kinetic studies

Prenatal diagnosis of I-cell disease in the first and second trimesters

First trimester prenatal diagnosis of I-cell disease (1 case) was based on demonstration of profound deficiency of N-acetylglucosamine 1-phosphotransferase in chorionic villi and in cultured trophoblasts derived from the chorionic villus specimen. Deficiency of this enzyme in cultured amniotic fluid cells obtained via amniocentesis was the basis for prenatal diagnosis of I-cell disease in the second trimester (2 cases). In both procedures, the diagnosis was corroborated by the finding of intracellular deficiency and extracellular elevation of multiple lysosomal enzymes in the fetal cell cultures (trophoblasts and amniotic fluid cells), as well as a significant increase in several lysosomal enzyme activities in the maternal serum.

First trimester prenatal evaluation for I-cell disease by N-acetyl-glucosamine 1-phosphotransferase assay

First trimester prenatal diagnosis was offered to a couple at risk for having a child with I-cell disease (mucolipidosis II). The prenatal evaluation was based for the first time on examination of N-acetylglucosamine 1-phosphotransferase activity, deficiency of which is the primary biochemical defect in both I-cell disease and pseudo-Hurler polydystrophy (mucolipidosis III). Heterozygote levels of this enzyme activity were determined in chorionic villi obtained at 9 weeks of gestation, as well as in cultured trophoblasts derived from this specimen, and led to the diagnosis of an unaffected fetus. This procedure has advantages over that based on detection of abnormal intracellular-extracellular distribution of lysosomal enzyme activities, which is expressed only in homozygotes and fully expressed only in cell culture specimens.

Altered molecular size of N-acetylglucosamine 1-phosphotransferase in I-cell disease and pseudo-Hurler polydystrophy

The size of the mutant N-acetylglucosamine 1-phosphotransferase in Golgi membranes from fibroblasts of patients with I-cell disease and classical pseudo-Hurler polydystrophy, which comprised one complementation group characterized by deficiency towards both artificial and natural acceptor substrates, was significantly smaller than the normal enzyme, 151-174 kDa compared with 225-278 kDa. The size of the mutant enzyme from cell lines of patients with variant forms of pseudo-Hurler polydystrophy, which comprised another complementation group characterized by normal activity towards mono- and oligo-saccharide substrates, was significantly larger than the normal enzyme, ranging from 321 to 356 kDa in two families and from 528 to 547 kDa in a third family. These findings suggest that the mutations in I-cell disease and classical pseudo-Hurler polydystrophy result in a missing enzyme component, which renders the enzyme catalytically inefficient toward any type of acceptor substrate. In contrast, the mutations in the variant forms of pseudo-Hurler polydystrophy produce a larger enzyme molecule which is active toward small substrates but is incapable of binding natural lysosomal glycoprotein substrates.