A pseudodeficiency allele common in non-Jewish Tay-Sachs carriers: implications for carrier screening.

Deficiency of beta-hexosaminidase A (Hex A) activity typically results in Tay-Sachs disease.
However, healthy subjects found to be deficient in Hex A activity (i. e., pseudodeficient) by means of in vitro biochemical tests have been described.
We analyzed the HEXA gene of one pseudodeficient subject and identified both a C739-to-T substitution that changes Arg247----Trp on one allele and a previously identified Tay-Sachs disease mutation on the second allele.
Six additional pseudodeficient subjects were found to have the C739-to-T mutation.
This allele accounted for 32% (20/62) of non-Jewish enzyme-defined Tay-Sachs disease carriers but for none of 36 Jewish enzyme-defined carriers who did not have one of three known mutations common to this group.
The C739-to-T allele, together with a " true " Tay-Sachs disease allele, causes Hex A pseudodeficiency.
Given both the large proportion of non-Jewish carriers with this allele and that standard biochemical screening cannot differentiate between heterozygotes for the C739-to-T mutations and Tay-Sachs disease carriers, DNA testing for this mutation in at-risk couples is essential.

