• Ying-Hao Wen, Tzong-Shi Chiueh, Wei-Ting Wang, Wei-Tzu Lin, and Ding-Ping Chen.
• Department of Laboratory Medicine, Linkou Chang Gung Memorial Hospital, Taoyuan, Taiwan; Graduate Institute of Clinical Medical Sciences, College of Medicine, Chang Gung University, Taoyuan, Taiwan.
• J Formos Med Assoc. 2020 Apr 1; 119 (4): 845-849.

BackgroundABO blood system has many subgroups. In A group, A1 phenotype and A2 phenotype are more common, and A2 is caused by deletion or substitution in A1 allele (ABO*A1.01).MethodsBased on standard ABO serological test, the subject was identified as A2 phenotype. Direct sequencing and ABO gene cloning were performed to analyze the allele.ResultsThe subject had one A1v allele (ABO*A1.02) and one O allele. The haplotype sequencing analysis of each allelic clone demonstrated that allele 1 was A1v (ABO*A1.02) allele with nt543 variation (543 G > C) and allele 2 was O1v allele (ABO*O.01.02) with nt261 deletion and nt220 variation.ConclusionThe 543 G > C nucleotide substitution of the present A1v allele (ABO*A1.02) shares the same sequence variation site with Ax allele (ABO*AW.33) (543 G > T), and both 543 G > C and 543 G > T nucleotide substitutions encode the same amino acid change of tryptophan to cysteine. Mechanism, such as allelic enhancement, has been proposed to explain this controversial phenotype-genotype relationship. But in present case, there has been no B allele to enhance the expression of Ax to that expected of A2, so there could be another novel underlying mechanism to be investigated.Copyright © 2019. Published by Elsevier B.V.

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