Investigation into A antigen expression onO2heterozygous group O-labeled red blood cell units

A 24-base pair deletion in the ABO gene causes a hereditary splice site defect: a novel mechanism underlying ABO blood group O

BACKGROUND

Blood group A and B antigens are synthesized by glycosyltransferases regulated by a complex molecular genetic background. A multibase deletion in the ABO gene was identified in two related blood donors. To define its hereditary character and to evaluate genotype-phenotype associations, a detailed study including 30 family members was conducted.

METHODS AND MATERIALS

ABO phenotyping was performed with agglutination techniques and adsorption-elution tests. The secretor status was determined. Allele-specific sequencing of ABO and genotyping of family members by a mutation-specific polymerase chain reaction were carried out. Functional analysis included cloning of complementary DNA and transfection experiments in HeLa cells. The antigen expression was investigated by flow cytometry and adsorption-elution method.

RESULTS

Sequencing analysis revealed a 24-bp deletion in Exon 5 and the adjacent intronic region of ABO. The alteration was inherited by 16 family members. Nine of them being heterozygous for the mutated allele failed to express A antigen on their erythrocytes as found by routine typing. In particular samples, however, adsorption-elution studies indicated inconclusive results. HeLa cells transfected with aberrant gene transcripts did not express blood group antigen A.

CONCLUSION

The variation causes defects in messenger RNA splicing, most likely inactivating the transferase as observed by serological typing and in vitro expression analysis. These data suggest a novel mechanism associated with blood group O and extend the knowledge of exceptionally rare ABO splice site mutations and deletions. With increased understanding of the molecular bases of ABO, the diagnostics may be further enhanced to ensure the safest possible use of the blood supply.

Blood grouping discrepancies between ABO genotype and phenotype caused by O alleles

PURPOSE OF REVIEW

In the modern transfusion service, analysis of the ABO allele underlying a donor or recipient's A or B subtype phenotype is becoming a mainstream adjunct to the serological investigation. Although an analysis of the ABO gene can be helpful in establishing the nature of the subtype phenotype, numerous confounding factors exist that can lead to a discrepancy between the genotype and the observed phenotype.

RECENT FINDINGS

Although the most common group O alleles share a common crippling polymorphism, a growing number of alleles feature other polymorphisms that render their protein nonfunctional yet are similar enough to the consensus A allele that an errant phenotype would be predicted from the genotype, if the genotyping method was not specifically designed for their detection. Some of these O alleles might actually encode a protein with weak and variable A antigen synthetic ability.

SUMMARY

ABO genotyping can be a powerful asset in the transfusion service, but a thorough knowledge of the confounding factors that can lead to genotype/phenotype discrepancies is required.