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.