Identification of a novel A2 allele through nt543 substitution

Abstract

Background: ABO 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).

Methods: Based on standard ABO serological test, the subject was identified as A₂ phenotype. Direct sequencing and ABO gene cloning were performed to analyze the allele.

Results: The subject had one A^1v allele (ABO*A1.02) and one O allele. The haplotype sequencing analysis of each allelic clone demonstrated that allele 1 was A^1v (ABO*A1.02) allele with nt543 variation (543 G > C) and allele 2 was O^1v allele (ABO*O.01.02) with nt261 deletion and nt220 variation.

Conclusion: The 543 G > C nucleotide substitution of the present A^1v allele (ABO*A1.02) shares the same sequence variation site with A^x 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 A_x to that expected of A₂, so there could be another novel underlying mechanism to be investigated.

Keywords: 543 G>C; A(1v) (ABO*A1.02) allele; A(2) phenotype; Blood group.