The Le(a) and Le(b) human blood group antigens are synthesized in tissues producing exocrine secretions; they also circulate in plasma, where they are adsorbed by erythrocytes. They are synthesized by two fucosyltransferases, encoded by Lewis (FUT3) and secretor (FUT2) loci. This genetic model has been challenged because some erythrocyte Lewis-negative individuals express Lewis antigens in saliva. To define the molecular basis of this apparent discrepancy, we sequenced FUT3 in Lewis-negative individuals. We identified two single base pair changes. One, termed L1, yields a Leu-20-->Arg substitution in the enzyme's transmembrane domain. When expressed in COS-7 cells, enzyme substrate affinities are essentially identical to those of wild type. However, the mutant enzyme is found at substantially reduced levels in transfected cells. This suggests that the L1 mutation may alter the Golgi membrane anchoring of the enzyme. It was found alone in double dose in 10 of 30 erythrocyte Lewis-negative individuals, nine of whom express Lewis antigens in saliva. Therefore, L1 can account for erythrocyte/saliva-discrepant Lewis typing results. The L2 mutation creates an Ile-356-->Lys change in the enzyme's catalytic domain and inactivates the enzyme. It was found in double dose in 18 of 19 individuals bearing the double erythrocyte and salivary Lewis deficiency and can account for this phenotype
