PRENATAL AND POSTNATAL STUDIES OF A LATE INFANTILE GM2 GANGLIOSIDOSISIN A FAMILY OF SYRIAN ORIGIN - A POSSIBLE B1 VARIANT

We describe late infantile Tay-Sachs disease with high residual hexosa minidase A activity in two siblings of a Syrian Druze family. The pati ents' leukocytes had 26% of normal hexosaminidase A activity when test ed with the conventional fluorogenic substrate liferyl-2-acetamido-2-d eoxy-beta-D-glucopyranoside (4-MUG) and only about 10% when assayed wi th the sulfated substrate, -umbelliferal-beta-N-acetyl-glucoseamine-6- sulfate (4-MUGS). According to the standard procedure of the heterozyg ote screening program (employing 4-MUG and heat inactivation), the par ents were not diagnosed as an at-risk couple since the father was clas sified as a noncarrier. However, both parents' levels were clearly wit hin the carrier range on the basis of 4-MUGS. The unique catalytic cha racteristics of the patients' enzyme forward the assumption that the a ffected sibs are B1 variants. The parents' enzymatic levels, together with their known consanguinity, might indicate that these patients are homozygotes for the rare mutation and not genetic compounds as has be en documented for most of the infantile B1 variants. To the best of ou r knowledge this is the first reported case of B1 variant in a child o f that extraction.