Unusual thermolability properties of beta-hexosaminidase: studies of enzyme from cultured cells and clinical implications.

Tay-Sachs disease (TSD) is a neurodegenerative genetic disorder caused by a deficiency of beta-hexosaminidase A (Hex A) activity. To diagnose TSD and to screen for TSD heterozygosity, laboratories use an assay that exploits the differential thermolability of the major beta-hexosaminidase isoenzymes, Hex A and Hex B. At 50-52 degrees C Hex A is labile, and Hex B is stable. We previously noted that the stability of leukocyte Hex B at 52 degrees C varied significantly, depending on the sample concentration in the incubation mixture. We have now examined this phenomenon in enzyme from cultured cells used for prenatal and postnatal diagnostic testing. We found that fibroblast Hex A and Hex B behave similarly to the leukocyte isoenzymes. In control and TSD fibroblasts there was a linear correlation between Hex B thermostability and sample concentration; at lower sample concentrations Hex B was less stable than at higher concentrations. Dialysis of the samples prior to heat treatment did not change the thermostability properties of Hex B, indicating that the change in stability is not due to a soluble low molecular weight substance. Cultured amniotic fluid cell and chorionic villus cell Hex B had a similar, but less pronounced, instability at low sample concentrations. Therefore, the unusual thermolability properties of Hex B, first detected for leukocyte Hex B, were noted in multiple tissues. Based on these data, we suggest that the concentration of cell extract be stringently controlled when the heat-inactivation method is used for the pre- or postnatal diagnosis of TSD, and that supplementation with non-thermolability-based beta-hexosaminidase assays should be employed as needed.

Unusual thermolability properties of leukocyte beta-hexosaminidase: implications in screening for carriers of Tay-Sachs disease.

Tay-Sachs disease (TSD), an autosomal recessive neurodegenerative condition, is the result of a deficiency of beta-hexosaminidase A (hex A). Heterozygotic individuals are screened by analysis for hex A and hex B activities; the percent of hex A is the critical determinant of carrier vs noncarrier status. Most laboratories use a heat-inactivation assay that exploits the differential thermolability of the isoenzymes. However, we have found a reciprocal relation between the apparent leukocyte hex A activity and the amount of the sample used in the assay; i.e., a significant increase in the percent of hex A activity with decreasing amounts of sample. Three sets of data indicate that this phenomenon was caused by an effect on the hex B isoenzyme and not on hex A. This variation in hex A activity with sample amount was not observed when a hex A-specific substrate was used. This phenomenon was also not seen in assays of leukocytes from carriers for Sandhoff disease, a condition associated with a reduction in the amount of hex B. Finally, when leukocytes from a TSD homozygote, containing almost no hex A, were analyzed, marked increases in the percent of hex A were observed with decreasing sample concentrations. These data indicate that misdiagnoses could result from variations in sample concentrations used for TSD carrier testing and support the view that the leukocyte concentrations used for these assays should be standardized.