Evaluation of Histo-Blood Group ABO Genotyping in a Danish Population: Frequency of a Novel O Allele Defined as O2

Traditional blood group ABO serology is based on immunoreactivity with the carbohydrate determinants A, B and H antigens. Recent advances at the DNA level of the ABO genes have provided a molecular genetic model for the ABO polymorphism. This genetic model has to date only been tested on a limited basis. The present study was initiated to evaluate the universality of the proposed genetic model on a larger group of serologically defined ABO phenotypes. Three hundred healthy Danish blood donors were analysed (A: 50, B: 50, AB: 50, O: 150) by PCR amplification followed by diagnostic restriction enzyme cutting. In all cases A, B, and AB at least one allele of correctly predicted status was found. However, in O phenotype individuals, 11 out of 150 carried one allele discordant to the proposed genetic model. This novel O allele (3.7% allele frequency) was further characterized by diagnostic restriction enzyme analysis in two positions divergent between A and B alleles and by DNA sequencing of the two major exons. The novel O allele is termed O² as it typed as B in nucleotide position 526 and as A in positions 703, 796, and 803, in contrast to the most predominant O allele termed O¹, which types as A in all 4 positions. The structural defect in the O² allele appears to be an additional substitution at nucleotide position 802. The results clearly demonstrate that with the addition of the two distinctly different O alleles, O¹, O², the previously proposed molecular genetic basis of the ABO polymorphism is quite valid. More importantly the determined characteristics of these two O alleles have practical implications in ABO genotyping, because it establishes within the limits of the number of samples tested that ABO genotypes can be assessed directly by non-allele specific PCR amplification and restriction enzyme analysis.