A novel cis AB allele derived from a B allele through a single point mutation

BACKGROUND: The very rare cis AB phenotype, first described in 1964, corresponds to a special ABO allele encoding a glycosyltransferase that is capable of synthesizing both A and B substances. Until now, gene sequences of only two cis AB alleles were partially characterized. One involved the A(1)*02 allele with a single nonsynonymous substitution at codon 268, whereas the second arose from a single nonsynonymous substitution at codon 266 in exon 7 of a B-1*01 allele. STUDY DESIGN AND METHODS: A cis AB phenotype was identified in a French family. The serologic characteristics of this phenotype were determined. The cis AB allele was characterized from exon 6 to exon 7 by cloning and sequencing. RESULTS: The cis AB.tlse*01 allele is identical to B1*01 except for a single point mutation at nucleotide position 700, where a T replaces a C, implying a change of amino acid 234 (the B1*01 proline being replaced by a serine). CONCLUSION: The cis AB.tlse*01 allele clearly differs from all previously reported ABO, including the two previous cis AB described.