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A Novel cis-AB Allele Derived from the A Transferase Gene by Nucleotide Substitution C796A

Address correspondence to Chien-Feng Sun, M.D., Department of Clinical Pathology, Linkou Medical Center, Chang Gung Memorial Hospital, 5 Fu-Shin Street, Kweishan, Taoyuan 333, Taiwan, ROC; tel 886 3 328 1200 ext.2554; fax 886 3 397 1827; e-mail suncgj{at}adm.cgmh.org.tw.

The cis-AB is a very rare phenotype in the ABO blood group system. It corresponds to a special ABO allele encoding a glycosyltransferase that is capable of synthesizing both A and B antigens. Until now, gene sequences of only 3 cis-AB alleles were characterized. One was the A^1v allele with a nucleotide substitution G803C at codon 268; the second was the B allele with a nucleotide substitution A796C at codon 266; and the third arose from a point mutation C700T at codon 234 in exon 7 of the B transferase gene. In this study, we found a novel cis-AB allele when performing paternity tests in Chang Gung Memorial Hospital in Taiwan. Although his father was O blood type, a serologically AB blood type child was confirmed as being his father’s offspring on the basis of 16 microsatellite markers (99.97% plausibility for the child and father). Exons 6 and 7 of the child’s ABO alleles were characterized by direct sequencing and gene cloning. The results showed that the child has one O¹ allele and the second allele is almost identical to A^1*02 allele except for a single point mutation at nucleotide position 796, where an A replaces a C and leads to a change of leucine to methionine at amino acid 266. This implies that the child’s O¹ allele was inherited from his father and the other allele was inherited from his mother. In conclusion, the novel cis-AB allele reported here is derived from the A transferase gene through a nucleotide substitution C796A, which differs from the 3 previously reported cis-AB alleles.

Keywords: blood typing, ABO blood group, glycosyltransferase, paternity testing, rare phenotype