Identification of a novel A2 allele through nt543 substitution.

<bold>Background: </bold>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).<bold>Methods: </bold>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.<bold>Results: </bold>The 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.<bold>Conclusion: </bold>The 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. [ABSTRACT FROM AUTHOR]