A new missense variant in exon 7 of the ABO gene, c.662G>A, in a family with B w phenotype.

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Abstract

1 BACKGROUND
Weak expression of ABO antigens is encountered in the clinical laboratory occasionally, and subgroups of A are more commonly observed in Europeans than subgroups of B. To date, weakly expressing B variant phenotypes have been associated with 38 different alleles according to ISBT (https://www.isbtweb.org/resource/001aboalleles.html). This number is an underrepresentation since there have been several reports of aberrant B expression due to variant alleles since the last update of the ISBT allele table. The current study was initiated by an unusual blood group typing result in a 55-year-old male patient of Czech origin and previously reported as an abstract.1

2 BRIEF METHODS
Blood grouping was performed according to standard blood banking practice, initially using an automatic analyzer (Galileo, Immucor) followed by confirmation with manual gel (BioRad; DG-Gel) and tube agglutination techniques. Initial genotyping analysis was done using a PCR-SSP kit (Innotrain), microarray (BloodChip Reference, Progenika) and subsequently verified by expanded PCR-ASP and PCR-RFLP as described previously.2, 3 ABO exons 1–7 and splice sites were amplified and analyzed, together with the product(s) of PCR-ASP for exons 6–7, by Sanger sequencing.4 A single nucleotide variation (SNV) was detected, and the localization of the affected amino acid is visualized in a 3D-model of ABO glycosyltransferase by Cn3D (v.4.3.1, www.ncbi.nih.gov) and a detailed view obtained by AlphaFold.5, 6 Flow cytometry testing with monoclonal ABO reagents was performed as described previously.7

3 RESULTS
The proband's red blood cells (RBCs) initially typed as group O but the plasma typing gave negative or weak reactions with test RBCs of group B, depending on the method used, Table 1. An ABO*B.01/O.01.01 genotype was revealed, normally consistent with group B. Screening for selected A and B subgroup allele markers was negative.2 After informed consent, samples from family members were drawn and further investigation was performed.
In samples from the proband, his sister and niece, sequence analysis revealed heterozygosity for a SNV in ABO exon 7, c.662G>A (no rs number available) in an otherwise normal ABO*B.01 allele. Significantly weakened B antigen expression was observed in all three individuals. An overview of serological testing and genetic results is shown in Table 1.

SNV c.662G>A encodes an amino acid change, p.Gly221Asp. The glycine residue is completely evolutionarily conserved among the members of the GT6 family of glycosyltransferases8 and centrally located in the enzyme, seven amino acids away from the DVD motif (pp. 211–213) that coordinates the Mn2+ ion and the UDP part of the UDP-galactose donor substrate (Figure 1A). However, it is not directly interfering with the catalytic site. Instead, the change of the small neutral glycine to the bulkier and charged aspartic acid is predicted to abolish selected hydrogen bonds and is therefore hypothesized to destabilize the protein conformation (Figure 1B).5, 6
Original languageEnglish
Pages (from-to)E55-E58
JournalTransfusion
Volume62
Issue number10
DOIs
Publication statusPublished - 2022 Oct

Subject classification (UKÄ)

  • Hematology

Keywords

  • ABO Blood-Group System/genetics
  • Alleles
  • Exons/genetics
  • Genotype
  • Mutation, Missense
  • Phenotype

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