The Lewis ␣(1,3/1,4)-fucosyltransferase, Fuc-TIII, encoded by the FUT3 gene is responsible for the final synthesis of Le a and Le b antigens. Various point mutations have been described explaining the Lewis negative phenotype, Le(a؊b؊), on erythrocytes and secretions. Two of these, T202C and C314T originally described in a Swedish population, have not been found as single isolated point mutations so far. To define the relative contribution of each of these two mutations to the Lewis negative phenotype, we cloned and made chimeric FUT3 constructs separating the T202C mutation responsible for the amino acid change Trp 68 3 Arg, from the C314T mutation leading to the Thr 105 3 Met shift. COS-7 cells were transfected and the expression of Fuc-TIII enzyme activity and the presence of Lewis antigens were determined. There was no decrease in enzyme activity nor of immunofluorescence staining on cells transfected with the construct containing the isolated C314T mutation compared with cells transfected with a wild type FUT3 allele control. No enzyme activity nor immunoreactivity for Lewis antigens was detected in FUT3 constructs containing both mutations in combination. The T202C mutation alone decreased the enzyme activity to less than 1% of the activity of the wild type FUT3 allele. These results demonstrate, that the Trp 68 3 Arg substitution in human Fuc-TIII is the capital amino acid change responsible for the appearance of the Le(a؊b؊) phenotype on human erythrocytes in individuals homozygous for both the T202C and C314T mutations.The Lewis histo-blood group system comprises different complex carbohydrate structures, which participate in different biological processes such as embryogenesis, tissue differentiation, tumor metastasis, inflammation, and bacterial adhesion (1). Le a1 and Le b (2, 3) are the major Lewis antigens found on human erythrocytes. These fucosylated glycosphingolipids are synthesized by exocrine epithelial cells (4) and are secondarily passively adsorbed onto erythrocytes in the peripheral circulation giving these blood cells their Lewis phenotype (5).It was shown as early as the 1950's, that the Lewis phenotype of erythrocytes is influenced by the ABH secretor status of the individual (6), and the erythrocyte phenotype is the result of the epistatic interaction of the Lewis (Le-le) and the salivary ABH secretor (Se-se) loci (7). Fucosyltransferases (Fuc-Ts) encoded by genes at these loci compete and interact with each other and with other glycosyltransferases to determine the individual's final Lewis and secretor phenotypes.Since 1990, seven human fucosyltransferase genes (FUT1-7) have been cloned, sequenced, and characterized according to acceptor specificities (8 -18). The human ␣(1,2)-Fuc-T gene family comprises FUT1 encoding for the H enzyme and FUT2 encoding for the human secretor ␣(1,2)-fucosyltransferase (9). FUT3-7 encode for ␣(1,3)-Fuc-Ts. Two of these human enzymes, respectively (10,14), also express ␣(1,4)-fucosyltransferase activities (15,19,20).Five main missense mutations have be...
