A novel A allele with 664G>A mutation identified in a family with the Am phenotype

Abstract

The Am phenotype has been characterized as a weak expression of the A antigen on red blood cells but the presence of a normal quantity of the A antigen in saliva. This study describes a molecular genetic analysis of members of an Am family. The eight exon regions of the ABO genes of the Am proposita were amplified by polymerase chain reaction and cloned, and their sequences were analyzed. The alpha-1,3-N-acetylgalactosaminyltransferase (A-transferase) activities of the Am serum and the expressed Am transferase were analyzed. An A gene with a 664G>A mutation, which predicts an amino acid alteration of Val222Met, was identified in the Am proposita. This Am664A allele was demonstrated in other three family members with the Am phenotype. The A-transferase activity was virtually undetectable in the Am sera, and the expressed Am transferase showed weak A-transferase activity, when compared with the expressed A1 transferase, in assays that use acceptor substrates mimicking the Type 2 H structure and Type 1 H structure. A novel A allele with 664G>A mutation was demonstrated in a pedigree with the Am phenotype. The mechanism leading to the formation of the Am phenotype still awaits elucidation.