Prenatal diagnosis of Tay-Sachs disease: studies on the reliability of hexosaminidase levels in amniotic fluid

Detection of Tay-Sachs disease carriers among individuals with thermolabile hexosaminidase B

The determination of hexosaminidases A and B in most programmes for Tay-Sachs disease carrier detection is based on their different heat sensitivity (hexosaminidase A is the heat labile isoenzyme). This routine cannot be employed for individuals who also possess a thermolabile hexosaminidase B. In Israel, 0.6% of the screened samples have a labile hexosaminidases B (about 110 each year) and the assessment of their hexosaminidase A activity has hitherto been based on isoenzyme separation by ion exchange chromatography. The latter requires relative large serum samples, and the individuals must usually be reappointed. In order to avoid the thermal treatment we have used the alternative technique, which employs two substrates with different specificities for the two isoenzymes: 1. The fluorogenic substance, 4-methylumbelliferyl-N-acetyl-glucopyranoside, which measures total hexosaminidase activity and 2. the derivative, 4-methylumbelliferyl-N-acetyl glucosamine-6-sulphate, which is considerably more specific toward hexosaminidase A. Hexosaminidase A activity was expressed as a ratio of total activities (the ratio of the assay with 4-methylumbelliferyl-N-acetyl glucosamine-6-sulphate to that with 4-methyllumbelliferyl-N-acetyl-glucopyranoside). Using the results from 65 obligate heterozygotes for Tay-Sachs disease, we established our reference ranges for assigning the genotypes with respect to the Tay-Sachs gene. Comparison of the results from 182 unrelated and randomly chosen sera screened by the ratio method and by heat inactivation, showed a very high correlation (r = 0.996). Sixty eight sera with thermolabile hexosaminidase B were tested by ion exchange chromatography and by the double substrate method, and they yielded identical diagnoses with regard to the Tay-Sachs locus.(ABSTRACT TRUNCATED AT 250 WORDS)

Cultured amniotic fluid cells for prenatal diagnosis of lysosomal storage disorders: a methodological study

The influence of culture conditions on the ultrastructure and enzyme activities of amniotic fluid cells are reported. Morphological changes were determined as a function of the number of lysosomal-like inclusion bodies per cell, and these results correlated to the activity of beta-hexosaminidase, alpha-mannosidase, beta-glucuronidase, arylsulphatase C and 5' nucleotidase. The parameters examined were pH of the culture media, type of media, increasing cell passage and day of harvest. Our results indicate that enzyme activities are less sensitive to changes in culture conditions as compared to ultrastructural changes. We therefore recommend that in order to obtain reliable ultrastructural results for the diagnosis of storage disorders, cultures should be grown in MEM as the culture medium, the pH of the medium carefully monitored to remain below pH 7.4, examining the cultures no later than the eighth cell passage and no later than the 10th day after subculture.

Prenatal diagnosis for Tay-Sachs disease using chorionic villus sampling

Prenatal diagnosis for Tay-Sachs disease was performed on 25 patients using chorionic villus sampling (CVS). Nineteen were diagnosed as normal, and six were affected. Normal villus extracts had both hexosaminidase (hex) A and B activity, as determined by Cellogel and polyacrylamide gel electrophoresis, while extracts from affected fetuses had only hex B activity. Compared to cultured amniotic fluid cells or fibroblasts, villi contained less hex A. Hex A levels in fresh villi and cultured trophoblasts were roughly comparable.

Prenatal diagnosis of atypical Tay-Sachs disease by chorionic villi sampling

We studied a family at risk for atypical TSD in which the index case showed, clinically, a late onset and a gradual psychomotor deterioration and biochemically, a residual hex. A activity in leucocytes. Two prenatal diagnoses of affected fetuses were made in this family. The first one on amniotic cells, the second one on trophoblast biopsy samples. Both of them were confirmed after abortion on cultured cells. Prenatal diagnosis of TSD, even of some atypical forms is possible using trophoblast biopsy, but formal confirmation should be obtained on cultured trophoblasts.