A unique 502C>T mutation in exon 7 of ABO gene associated with the Bel phenotype in Taiwan

ABO genotyping with next-generation sequencing to resolve heterogeneity in donors with serology discrepancies

BACKGROUND

ABO subtypes are characterized by the alteration of antigens present and their expression levels on red blood cells and many are linked to genetic changes in the ABO gene. Weakened expression of antigens should be identified to prevent transfusion reactions or ABO-incompatible transplantations. Genotyping can be applied to identify subtypes to complement serologic testing. Next-generation sequencing (NGS) has shown to provide sensitive and accurate genotyping results as well as valuable cis/trans information. Here we took advantage of NGS and applied it to resolve serology discrepancies in ABO typing.

STUDY DESIGN AND METHODS

In this study, we customized capture probes targeting the entire ABO gene and sequenced on MiSeq Illumina. The subtype-causing variants were identified, and cis/trans association to ABO alleles was determined. The results from NGS, serology, and Sanger sequencing were compared.

RESULTS

Four control samples typed A, B, O, and AB were correctly genotyped. Of 24 serologically discrepant samples, subtype-causing variations were found in 20 cases, with two unresolved and two identified as weakening of ABO antibody in reverse. The types of variations include 17 known subtype alleles, one novel variant, one novel large deletion, and one microchimerism. Haplotypes encompassing Exons 6 and 7 of ABO were reconstructed in 17 of the 20 samples.

CONCLUSION

This study demonstrated a full coverage of ABO by capture-based panel, phasing analysis with NGS in ABO genotyping resolved heterogeneity with novel allele and microchimerism findings. This approach provided a more precise method for subtyping and thereby leading to safer transfusion.

Structural basis for red cell phenotypic changes in newly identified, naturally occurring subgroup mutants of the human blood group B glycosyltransferase

BACKGROUND

Four amino-acid-changing polymorphisms differentiate the blood group A and B alleles. Multiple missense mutations are associated with weak expression of A and B antigens but the structural changes causing subgroups have not been studied.

STUDY DESIGN AND METHODS

Individuals or families having serologically weak B antigen on their red cells were studied. Alleles were characterized by sequencing of exons 1 through 7 in the ABO gene. Single crystal X-ray diffraction, three-dimensional-structure molecular modeling, and enzyme kinetics showed the effects of the B allele mutations on the glycosyltransferases.

RESULTS

Seven unrelated individuals with weak B phenotypes possessed seven different B alleles, five of which are new and result in substitution of highly conserved amino acids: M189V, I192T, F216I, D262N, and A268T. One of these (F216I) was due to a hybrid allele resulting from recombination between B and O(1v) alleles. The two other alleles were recently described in other ethnic groups and result in V175M and L232P. The first crystal-structure determination (A268T) of a subgroup glycosyltransferase and molecular modeling (F216I, D262N, L232P) indicated conformational changes in the enzyme that could explain the diminished enzyme activity. The effect of three mutations could not be visualized since they occur in a disordered loop.

CONCLUSION

The genetic background for B(w) phenotypes is very heterogeneous but usually arises through seemingly random missense mutations throughout the last ABO exon. The targeted amino acid residues, however, are well conserved during evolution. Based on analysis of the resulting structural changes in the glycosyltransferase, the mutations are likely to disrupt molecular bonds of importance for enzymatic function.

Three missense mutations, including a novel 860C>T transition, and allelic enhancement phenomenon associated with ABO blood subgroups A in Taiwan

BACKGROUND

It was estimated that approximately 25 percent of Taiwanese residents were ABO blood group A. Many subgroups of A, however, revealed ambiguous serologic typing results. This study aimed to delineate the molecular basis of the A3, Ax, and weak A phenotypes.

STUDY DESIGN AND METHODS

Serologic analyses including adsorption and elution assay, serum transferases activity assay, and saliva test were performed to determine the unique phenotypes of these samples. DNA sequencing and polymerase chain reaction-restriction fragment length polymorphism were performed to further investigate the relationships between the genetic characteristics and phenotypic features of these samples.

RESULTS

Three single-nucleotide transitions (745C>T, 820G>A, and a novel 860C>T) were found in nine A3/A3B cases. In addition, the Ax and A3B subjects shared the same 860C>T mutation. This A(x) allele with 860C>T transition expressed A3B phenotype in A(x)/B101 heterozygote but Ax phenotype in A(x)/O01 heterozygote. This allelic enhancement was also observed in the weak A family with Aw05 allele, which was previously not found in Taiwan.

CONCLUSION

This allelic enhancement phenomenon was prone to cause serologic discrepancy between parents and children. Genotyping could help us to resolve this problem. Thus, a novel mutation is reported among Taiwanese blood donors.

Identification of a novel A2 allele derived from the A transferase gene through a nucleotide substitution G539C

BACKGROUND AND OBJECTIVES

The A2 is a very rare phenotype in the ABO blood group system in the Oriental population. It corresponds to a special ABO allele encoding a glycosyltransferase that is capable of synthesizing A2 antigens, which is weaker than the typical A antigen. In this study, we report a novel A2 allele in two unrelated Taiwanese individuals.

MATERIALS AND METHODS

Two individuals were identified as the A2 phenotype based on the standard ABO serological test. For analysing the A2 allele, both direct sequencing and gene cloning of the ABO gene were performed.

RESULTS

The ABO gene of the two A2 individuals was composed of O1 and A2 alleles, and the novel A2 allele has a 539G > C that results in the amino acid change Arg180Pro. The mutation was not detected in the general group A population.

CONCLUSION

We report for the first time that a 539G > C mutation represents a new molecular basis for the A2 blood type. The amino acid substitution from arginine to proline may have effect on the expression of A antigen.

Association of ABO gene mutations resulting in a rare B subgroup

BACKGROUND AND OBJECTIVES

B subgroups are rare and the genetic analysis reported to date has been limited.

MATERIALS AND METHODS

Serological and molecular investigations were performed in blood from a B-subgroup donor.

RESULTS

Red cells did not react with anti-B and anti-AB reagents. However, cells absorbed anti-B. Red cells presented positive reactions with anti-H, and saliva secreted H substance. The molecular study demonstrated a B allele with the substitutions 467C>T, 646T>A, 681G>A, 771C>T, 796C>A, 803G>C, 829G>A and an O allele with the sequence of O02.

CONCLUSIONS

It is probable that the presence in exon 7 of some of the O02 substitutions could have weakened the enzymatic activity of the encoded B transferase.