H.C. Janse

A fluorimetric enzyme assay for the diagnosis of Morquio disease type A (MPS IV A)

4-Methylumbelliferyl-beta-D-galactopyranoside-6-sulphate was synthesized and used for the determination of galactose-6-sulphate sulphatase activity. Fibroblasts and leucocytes from 12 different Morquio A patients, showed 0.0-2.7% of mean normal galactose-6-sulphate sulphatase activity. Heterozygotes showed intermediate activities. The enzymatic liberation of the fluorochrome from 4-methylumbelliferyl-beta-D-galactopyranoside-6-sulphate requires the sequential action of galactose-6-sulphate sulphatase and beta-galactosidase. Normal beta-galactosidase activity caused nearly complete hydrolysis of non-fluorescing 4-methylumbelliferyl-galactoside, formed during incubation. In cell extracts with a beta-galactosidase deficiency however, a second incubation in the presence of excess beta-galactosidase is needed to avoid underestimation of galactose-6-sulphate sulphatase activity.

A fluorimetric enzyme assay for the diagnosis of Sanfilippo disease C (MPS III C).

SummaryBoth the α- and β-anomers of 4-methylumbelliferyl-D-glucosaminide were synthesized and shown to be substrates for the lysosomal acetyl-CoA: glucosaminideN-acetyltransferase. Using the β-anomer, fibroblasts and leukocytes from 11 different Sanfilippo C patients showed <1% of mean normalN-acetyltransferase activity. Heterozygotes showed intermediate activities. The enzymatic liberation of the fluorochrome from 4-methylumbelliferyl-β-d-glucosaminide requires the sequential action of theN-acetyltransferase and β-hexosaminidase. Normal β-hexosaminidase activity caused complete hydrolysis of the reaction intermediate 4-methylumbelliferyl-β-d-N-acetylglucosaminide formed by theN-acetyltransferase. In cell extracts with a β-hexosaminidase deficiency, however, a second incubation in the presence of excess β-hexosaminidase is needed to avoid underestimation of theN-acetyltransferase activity.

Two genetically different MU-NANA neuraminidases in human leucocytes☆

Human leucocytes contain two different MU-NANA neuraminidases, which can be distinguished by Concanavalin A binding. The Con A binding form is predominant in lymphocytes (more than 80%) and the non-binding form predominates in granulocytes. The pH optima of both these neuraminidases as well as their subcellular localization as determined by Percoll gradient centrifugation suggest that they are both lysosomal. Immunological studies indicate that the Con A binding form is present in a complex with beta-galactosidase whereas the non-binding form is not. Leucocytes from patients with sialidosis or galactosialidosis are deficient in the Con A binding neuraminidase, whereas the non-binding form is normal. In sialolipidosis both forms are normal. These results demonstrate that leucocytes contain at least two genetically different MU-NANA neuraminidases. Thus, the use of leucocytes should be avoided for the diagnosis of sialidosis and galactosialidosis, and isolated lymphocytes should be used to obtain reliable results.

Cathepsin A Deficiency in Galactosialidosis: Studies of Patients and Carriers in 16 Families

Deficiency of lysosomal protective protein/cathepsin A in humans is the primary cause of galactosialidosis, a lysosomal storage disease characterized by combined deficiency of β-galactosidase and neuraminidase. We have investigated 20 galactosialidosis patients and nine of their obligate heterozygous parents. A group of 12 patients with the early infantile type of the disease exhibited practically complete absence of cathepsin A activity, whereas eight patients with either the late infantile or the juvenile/adult type had 2-5% residual activity. Highest levels (5%) were present in two patients with milder clinical manifestations and later onset of the disease. In most fibroblast strains, β-galactosidase activity was 10-15% of normal levels, whereas neuraminidase was reduced to less than 4%. Interestingly, a substantial residual activity (10%) of the latter enzyme was detected in the patient with the mildest phenotype and the highest cathepsin A activity. Heterozygous values for cathepsin A were reduced on average to half of normal levels. However, in two cell strains, the activity was far below control range, and in these cases, neuraminidase activity was severely depressed. Finally, we showed that cathepsin A had considerable activity in chorionic villi and amniocytes, but was deficient in amniocytes from a pregnancy with an affected fetus, indicating the relevance of cathepsin A assay for prenatal diagnosis of galactosialidosis.

Salla disease variant in a Dutch patient. Potential value of polymorphonuclear leucocytes for heterozygote detection

A Dutch child with psychomotor retardation, impaired speech, ataxia, sialic acid storage and vacuolized skin fibroblasts and lymphocytes was diagnosed as having free sialic acid storage disease. Slight corneal opacities, pale optic disks at the fundus oculi and vertebral abnormalities, not earlier reported in Salla disease, were peculiar to this case. Free sialic acid was about tenfold increased in urine and cultured fibroblasts, without changes in the glycoconjugate-bound sialic acid pool. A subsequent pregnancy of the patients mother was monitored by assay of sialic acid in chorionic villi and amniotic fluid. An unaffected foetus was predicted. Sialic acid was also assayed in peripheral blood total leucoyctes, and in mononuclear and polymorphonuclear (PMN) leucocyte subpopulations. Each of these leucocyte fractions from the patient showed 10- to 30-fold increase in sialic acid content. The PMN subpopulation provided the most restricted range of control values and showed slightly increased values for the patients parents. These results suggest that the assay of sialic acid in PMN might be useful for the identification of heterozygotes in sialic acid storage disease. Studies on a larger number of obligate heterozygotes are needed to confirm this observation.

Debranching enzyme in fibroblasts, amniotic fluid cells and chorionic villi: pre- and postnatal diagnosis of glycogenosis type III

Glycogenosis type III is characterized by a deficiency of debranching enzyme in most tissues, and it can be detected by the inability to liberate glucose from limit dextrin. However, using this assay, the deficiency is not expressed in cultured fibroblasts from patients with glycogenosis type III. We have demonstrated that the failure to detect debranching enzyme deficiency in fibroblasts is entirely due to interference of acid alpha-glucosidase, which can also hydrolyse limit dextrin. A method is described to remove specifically acid alpha-glucosidase allowing clear discrimination between fibroblasts from patients and controls, whereas heterozygotes showed intermediate values. The results with amniotic fluid cells and chorionic villi suggest the feasibility of first- and second-trimester prenatal diagnosis of glycogenosis III.

Prenatal diagnosis of Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) was demonstrated in two successive pregnancies by strongly reduced activity of sphingomyelinase in amniotic fluid cells. By contrast, chorionic villi from the first pregnancy had shown normal sphingomyelinase activity. The prenatal diagnosis of NPC in the two fetuses was confirmed, after termination of the pregnancies, by (phospho)lipid analyses of the fetal livers, by the assay of sphingomyelinase in the fetal fibroblasts and by the demonstration of a defective esterification of exogenous cholesterol and of cholesterol accumulation by filipin staining. Retrospective analysis of cultured amniocytes for cholesterol esterification and filipin staining confirmed the feasibility of these methods for prenatal diagnosis. In a recent pregnancy in the same mother the three available methods were applied to amniotic fluid cells and an unaffected child was correctly predicted. Lipid analysis of liver tissue from the patient with NPC and the two fetuses showed a 3-5 times increased level of cholesterol, a 2-3 times increased level of sphingomyelin and a remarkable increase of bis (monoacylglyceryl) phosphate.

Biosynthesis of human α-N-acetylgalactosaminidase: Defective phosphorylation and maturation in infantile α-NAGA deficiency

The biosynthesis of human alpha-N-acetylgalactosaminidase (alpha-NAGA) was studied in normal fibroblasts and in cells from patients with infantile alpha-NAGA deficiency. Normal alpha-NAGA is synthesized as a 52 kDa precursor which matures to a 49 kDa species through phosphorylation and carbohydrate triming. Fibroblasts from the patients synthesize normal amounts of a 52 kDa precursor, however phosphorylation does not occur and this precursor is subsequently degraded intracellularly.