A weak blood group A phenotype caused by a translation-initiator mutation in the ABO gene

Serological and molecular characterization of novel ABO variants including an interesting B(A) subgroup

BACKGROUND

ABO grouping is the most important pretransfusion testing that is directly related to the safety of blood transfusion. A weak ABO subgroup is one of the important causes of an ABO grouping discrepancy. Here, we investigated the characterization of four novel ABO variants including a novel B(A) subgroup.

STUDY DESIGN AND METHODS

RBCs were phenotyped by standard serology methods. The full coding regions of the ABO gene and the erythroid cell-specific regulatory elements in intron one were sequenced. The effect of the possible splice site variant was predicted by Alamut software. The 3D structural modeling of three relative B(A) enzymes (p.Met214Thr, p.Met214Val, and p.Met214Leu) were performed by PyMOL software.

RESULTS

Four novel ABO alleles were identified with weak ABO expression in this study, in which two would lead to premature terminations, and two resulted in amino acid changes. In silico analysis revealed that the splice site variant c.155G>T had the potential to alter splice transcripts. 3D structural view shown that the variant amino acid position 214 was spatially adjacent to the donor recognition pocket residues (266Met and 268Ala) and just next to the 211DVD213 motif. The size of the side chain of Thr and Val is the smallest, Leu is medium, and Met is the largest, and the size changes in the critical position 214 may affect the donor recognition pocket.

CONCLUSION

Four ABO subgroup alleles were newly linked to different kinds of ABO variants and the possible mechanism through which they produce weak ABO subgroups was analyzed in silico.

Truncated glycosyltransferase coding regions in novel ABO alleles give rise to weak A or B blood group expression and discrepant typing results

BACKGROUND

Correct ABO blood-group matching between donor and patient is crucial for safe transfusions. We investigated the underlying reason causing inconclusive ABO serology in samples referred to our laboratory.

STUDY DESIGN AND METHODS

Flow cytometric analysis, ABO genotyping, and sequencing were used to characterize ABO-discrepant blood samples (n = 13). ABO gene variants were inserted in a GFP-containing bicistronic vector to assess A/B expression following overexpression in HeLa cells.

RESULTS

Seven novel alleles with nonsense mutations predicted to truncate the encoded ABO glycosyltransferases were identified. While these variants could represent O alleles, serology showed signs of ABO glycosyltransferase activity. ABO*A1.01-related alleles displayed remarkably characteristic percentages of A-positive cells for samples with the same variant: c.42C>A (p.Cys14*; 10%), c.102C>A (p.Tyr34*; 31%-32%, n = 2), c.106dup (p.Val36Glyfs*21; 16%-17%, n = 3) or c.181_182ins (p.Leu61Argfs*21; 12%-13%, n = 2). Transfection studies confirmed significantly decreased A expression compared to wild type. The remaining variants were found on ABO*B.01 background: c.1_5dup (pGly3Trpfs*20), c.15dup (p.Arg6Alafs*51) or c.496del (p.Thr166Profs*26). Although the absence of plasma anti-B was noted overall, B antigen expression was barely detected on erythrocytes. Overexpression confirmed decreased B in two variants compared to wildtype while c.1_5dup only showed a non-significant downward trend.

CONCLUSION

Samples displaying aberrant ABO serology revealed seven principally interesting alleles. Despite the presence of truncating mutations, normally resulting in null alleles, low levels of ABO antigens were detectable where alterations affected ABO exons 1-4 but not exon 7. This is compatible with the previously proposed concept that alternative start codons in early exons can be used to initiate the translation of functional ABO glycosyltransferase.

A Novel Mutation Eliminates GATA-1 and RUNX1-Mediated Promoter Activity in Galactosyltransferase Gene

Introduction

Mutations in the promoter region and exons of ABO gene may cause changes in the expression of blood group antigens, often showing a weak ABO phenotype. Here, we identified a novel weak ABO subgroup allele that caused B_el phenotype and explored its mechanisms.

Methods

The ABO phenotype of subjects (Chinese Han nationality) was classified by serological method. The plasma activity of erythrocyte glycosyltransferase was detected by the phosphate coupling method. ABO subtype genotyping was performed by PCR-SSP and exon sequencing. The activity of the promoter was evaluated by a dual-luciferase reporter assay.

Results

We identified a mutation exon 1 c.15_16insTGTTG of the B allele in a B_el subject. Genealogical investigation showed that the mutation was inherited from her mother. The mutation was located in the promoter region of the ABO gene. The dual-luciferase reporter assay showed that the mutation inactivated GATA-1 and RUNX1-mediated activity of the ABO gene promoter, leading to a decrease in the expression and activity of B glycosyltransferase.

Conclusion

A novel B_var ABO subgroup allele was identified. The novel mutation can reduce the promoter activity that activated by GATA-1 and RUNX1, subsequently causing the B_el phenotype.

Mechanism evaluation for an amino acid substitution p.Y246C of B-glycosyltransferase enzyme with Bweak phenotype

BACKGROUND

The amino acid substitutions caused by ABO gene variants are usually predicted to impact the glycosyltransferase function. Here, the effect of an amino acid substitution in the vicinity of the catalytic active region of the B-glycosyltransferase was explored in vitro and in silico study, which is important for further recognizing the ABO subgroup.

METHODS

The ABO serological tests were performed by the routine methods. The ABO genotype was analyzed by polymerase chain reaction and sequenced bidirectionally. The haplotype of the variant allele was separated using single-strand amplification and sequencing with allele-specific primers. Stably expression cell lines with variant were constructed for study in vitro. 3D structure of the B-glycosyltransferase (GTB) variant was simulated by PyMOL software. The free energy change (ΔΔG) was calculated by FoldX.

RESULTS

A variant c.737A > G was identified in a Chinese individual with Bweak phenotype, which led to an amino acid substitution p.Y246C in the vicinity of the catalytic active region of GTB enzyme. The stably expression cell lines with variant and wild type were successfully established and showed that the variant caused a decrease in protein levels and/or enzyme activity. The 3D structural of the GTB modelling found the amino acid substitution p.Y246C caused the hydrogen bond of the protein changes. Meanwhile, the free energy change (ΔΔG) value predicted the destabilizing effect on the variant GTB.

DISCUSSION

The p.Y246C variant in the vicinity of the enzyme active centre reduced the antigen expression because of greatly destabilizing effect on the GTB variant.

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.

Genomic analysis of blood samples with serologic ABO discrepancy identifies 12 novel alleles in a Chinese Han population

OBJECTIVES

This study aimed at identifying new ABO alleles from155 unrelated blood samples with potential ABO discrepancy in a Chinese Han population of 835 144 donors.

BACKGROUND

Serological strategies and genotyping are crucial for the precise determination of ABO discrepancy.

METHODS

Their ABO phenotypes and plasma glycosyltransferase activity were determined by standard forward and reverse typing and dilution tests. The genomic DNA of the ABO gene was amplified by polymerase chain reaction and sequenced. The frequency of ABO subgroup alleles associated with ABO discrepancy was analysed.

RESULTS

Serological analysis indicated that 53, 96 and 6 samples with ABO discrepancy were identified in the A, B and O subgroups, respectively. Genetic analysis revealed 12 novel alleles among the 46 associated with serologic ABO discrepancy. The majority of novel alleles was obtained from point mutations or single base insertion in Exons 6 to 7 of the ABO gene. The most frequent alleles were ABO*cisAB.01 (14/53, 26.42%) and ABO*A2.05 (7/53, 13.2%) in the A subgroup and ABO*BA.02 (34/96, 35.42%) and ABO*BEL.11 (15/96, 15.62%) in the B subgroup. Samples with the same ABO subgroup allele displayed different phenotypes, such as ABO*AX.13, ABO*BW.03, ABO*BW.12, ABO*BW.15, ABO*BEL.03, ABO*BEL.10 and ABO*BEL.11.

CONCLUSION

This study identified 12 novel alleles among the 46 associated with serologic ABO discrepancies. ABO genotyping is needed for the accurate evaluation of blood phenotype to improve the safety of blood transfusion.

Molecular Basis of ABO Variants Including Identification of 16 Novel ABO Subgroup Alleles in Chinese Han Population

Introduction

The characteristic of ABO blood subgroup is crucial for elucidating the mechanisms of such variant phenotypes and offering useful information in blood transfusion.

Methods

In total, 211 ABO variants including part of available family members were investigated in this study. The phenotypes of these individuals were typed with serologic methods. The full coding regions of ABO gene and the erythroid cell-specific regulatory elements in intron 1 of them were amplified with polymerase chain reaction and then directly sequenced. The novel alleles were confirmed by cloning and sequencing. Phylogenetic tree was made using CLUSTAL W software. 3D structural analyses of the glycosyltransferases (GTs) with some typical mutations were performed by PyMOL software.

Results

Forty-eight distinctly rare ABO alleles were identified in 211 Chinese variant individuals, including 16 novel ABO alleles. All of the alleles were categorized as 5 groups: 16 ABO*A alleles, 23 ABO*B alleles, 4 ABO*BA alleles, 4 ABO*cisAB alleles, and 1 ABO*O alleles. ABO*A2.08 and ABO*BA.02 were the relatively predominant A and B subgroup alleles, respectively. According to the phylogenetic tree, 28 alleles (5 common alleles and 23 alleles identified in our laboratory) were classified into 3 major allelic lineages. The structural analysis of 3D homology modeling predicted reduced protein stability of the mutant GTs and may explain the reduced ABO antigen expression.

Conclusions

The molecular basis of ABO variants was analyzed, and 16 novel ABO alleles were identified. The results extended the information of ABO variants and provided a basis for better transfusion strategies and helped to improve blood transfusion safety.

Blood group ABO gene-encoded A transferase catalyzes the biosynthesis of FORS1 antigen of FORS system upon Met69Thr/Ser substitution

Blood group A/B glycosyltransferases (AT/BTs) and Forssman glycolipid synthase (FS) are encoded by the evolutionarily related ABO (A/B alleles) and GBGT1 genes, respectively. AT/BT and FS catalyze the biosynthesis of A/B and Forssman (FORS1) oligosaccharide antigens that are responsible for the distinct blood group systems of ABO and FORS. Using genetic engineering, DNA transfection, and immunocytochemistry and immunocytometry, we have previously shown that the eukaryotic expression construct encoding human AT, whose LeuGlyGly tripeptide at codons 266 to 268 was replaced with FS-specific GlyGlyAla tripeptide, induced weak appearance of FORS1 antigen. Recently, we have shown that the human AT complementary DNA constructs deleting exons 3 or 4, but not exons 2 or 5, induced moderate expression of FORS1 antigen. The constructs containing both the GlyGlyAla substitution and the exon 3 or 4 deletion exhibited an increased FS activity. Here, we report another molecular mechanism in which an amino acid substitution at codon 69 from methionine to threonine or serine (Met69Thr/Ser) also modified enzymatic specificity and permitted FORS1 biosynthesis. Considering that codon 69 is the first amino acid of exon 5 and that the cointroduction of Met69Thr and GlyGlyAla substitutions also enhanced FS activity, the methionine substitutions may affect enzyme structure in a mode similar to the exon 3 or 4 deletion but distinct from the GlyGlyAla substitution.

Non-AUG start codons responsible for ABO weak blood group alleles on initiation mutant backgrounds

Histo-blood group ABO gene polymorphism is crucial in transfusion medicine. We studied the activity and subcellular distribution of ABO gene-encoded A glycosyltransferases with N-terminal truncation. We hypothesized that truncated enzymes starting at internal methionines drove the synthesis of oligosaccharide A antigen in those already described alleles that lack a proper translation initiation codon. Not only we tested the functionality of the mutant transferases by expressing them and assessing their capacity to drive the appearance of A antigen on the cell surface, but we also analyzed their subcellullar localization, which has not been described before. The results highlight the importance of the transmembrane domain because proteins deprived of it are not able to localize properly and deliver substantial amounts of antigen on the cell surface. Truncated proteins with their first amino acid well within the luminal domain are not properly localized and lose their enzymatic activity. Most importantly, we demonstrated that other codons than AUG might be used to start the protein synthesis rather than internal methionines in translation-initiation mutants, explaining the molecular mechanism by which transferases lacking a classical start codon are able to synthesize A/B antigens.

Molecular genetic analysis of ABO blood group variations reveals 29 novel ABO subgroup alleles

BACKGROUND

Identifying genetic variants of the ABO gene may reveal new biologic mechanisms underlying variant phenotypes of the ABO blood group. We report the molecular genetic analysis of 322 apparently unrelated ABO subgroup individuals in an estimated 2.1 million donors.

STUDY DESIGN AND METHODS

We performed phenotype investigations by serology studies, analyzed the DNA sequence of the ABO gene by direct sequencing or sequencing after cloning, and evaluated promoter activity by reporter assays.

RESULTS

In 62 rare ABO alleles, we identified 29 novel ABO subgroup alleles in 43 apparently unrelated subgroup individuals and their four available pedigrees. Of these alleles, one was a deletion-mutation allele, four were hybrid alleles, and 24 were point-mutation alleles. Most of the point mutations were detected in Exons 6 to 7, while several others were also detected in Exons 1 to 5 or splicing regions. One ABO promoter mutation, -35 to -18 del, was found and verified to reduce promoter activity, as determined by dual luciferase assays. Two mutations, 7G>T and 52C>T, carrying the premature terminal codons E3X and R18X in the 5'-region, were found to be associated with the very weak ABO subgroups "Ael" and "Bel."

CONCLUSION

Twenty-nine ABO subgroup alleles were newly linked to different kinds of ABO variations. We provide the first evidence that promoter abnormality is involved in the formation of weak ABO phenotypes. We also described the first naturally occurring ABO alleles with premature terminal codons in the 5'-region that led to Ael and Bel phenotypes.

Establishment of a simple detection system for blood group ABO-specific transferase activity in DNA-transfected cells

A/B-transferase is a glycosyltransferase that transfers a sugar substrate onto H-antigen resulting in the synthesis of glycoproteins and glycolipids termed A/B-antigens. The ABO blood group (ABO) gene encoding A/B-transferase possesses numerous polymorphisms affecting the specificity and/or activity of the enzyme. The relationship between genotype and phenotype is very complicated, except for those of some critical polymorphisms. In order to establish a system for evaluating the effect of each polymorphism on the transferase function, an A- or B-transferase cDNA expressing vector was introduced into HeLa cells, a cell line that do not possess endogenous A/B-transferase activity. We successfully detected substrate-specific transferase activity in the cells and in the culture medium. Furthermore, in three different assays, each corresponding A- or B-antigen was detected in the transfectants with high sensitivity. Accordingly, the present study demonstrates a possibility that A/B-transferase variants may be characterized by using this method.

The M142T mutation causes B3 phenotype: three cases and an in vitro expression study

The B3 phenotype is the most common B subtype in Korea. The B305 allele (425 T>C, M142T) was first reported in 2 Chinese individuals; however, it has not yet been reported in the Koreans, and the impact of the M142T mutation on the expression of the B3 phenotype has also not been studied. To resolve an ABO discrepancy between a group O neonate and her group O father and A(1)B(3) mother, blood samples from these individuals and other family members were referred to our laboratory for ABO gene analysis. The B305 allele was discovered in the neonate (B305/O01), her mother (A102/ B305), and her maternal aunt (B305/O02), while her father was typed as O01/O02. Transient transfection experiments were performed in HeLa cells using the B305 allele synthesized by site-directed mutagenesis; flow cytometric analysis revealed that this transfect expressed 35.5% of the total B antigen produced by the B101 allele transfect. For comparison, Bx01 allele transfects were also created, and they expressed 11.4% of the total B antigen expressed on the surface of B101 transfects. These experiments demonstrate that the M142T (425 T>C) mutation is responsible for the B subtype phenotype produced by the B305 allele.

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.

Weak blood group B phenotypes may be caused by variations in the CCAAT-binding factor/NF-Y enhancer region of the ABO gene

BACKGROUND

Binding of CCAAT-binding factor NF-Y (CBF/NF-Y) to a 43-bp repeat unit in the minisatellite region in the 5' region of the ABO gene (CBF/NF-Y enhancer region) plays an important role in regulating the transcription of ABO genes. The common ABO alleles were found to have CBF/NF-Y enhancer regions with specific numbers of 43-bp minisatellite repeats.

MATERIAL AND METHODS

Blood samples from four healthy blood donors with weak B phenotypes were subjected to extensive ABO genotyping, including nucleotide sequencing of the 5' regulatory region containing the CBF/NF-Y enhancer.

RESULTS

The coding region of the ABO genes exhibited common ABO*B101-heterozygous genotypes in all samples, but unexpected variations were observed in the CBF/NF-Y enhancer region. In two cases, the CBF/NF-Y enhancer motifs did not exhibit the expected ABO allele dependency. One, an AB(weak) sample was heterozygous for ABO*A101 and ABO*B101 but homozygous for the ABO*B101-specific CBF/NF-Y motif. The second had a common ABO*B101/ABO*O01 genotype but was heterozygous for ABO*A101- and ABO*O01-specific enhancer motifs. In the other two samples, novel CBF/NF-Y motifs were found. One contained a shortened version of an otherwise ABO*B101-specific CBF/NF-Y motif, and the other had a single-base substitution located 12 bp upstream from the beginning of the first 43-bp repeat of an ABO*B101-specific CBF/NF-Y enhancer sequence.

CONCLUSION

The frequency of variations in the CBF/NF-Y region of the ABO gene in these samples with presumably common ABO*B101 alleles suggests that weak blood group B phenotypes may be caused by sequence variations in the CBF/NF-Y regulatory region.

Blood group genotyping in Germany

BACKGROUND

Molecular methods for blood group genotyping became available more than 10 years ago as one major aspect of immunogenetics. Since then, the clinical applications have been expanded and refined. Their implementation varies considerably among different health-care systems, notably between North America and Europe.

STUDY DESIGN

This summary is based on studies published mostly during the last 3 years and on workshop reports from the German and Swiss transfusion societies. It represents an edited transcript of the author's presentation given at the Workshop on Molecular Methods in Immunohematology organized by the Food and Drug Administration (FDA) in Bethesda on September 25, 2006.

RESULTS

Current applications of blood group genotyping in Germany, Switzerland, and Austria are detailed: weak D testing in patients and pregnant women; blood group genotyping in perinatal care, in patients who received a transfusion, and in patients with immunohematologic problems; RHD genotyping in donors for DEL and D(+/-) chimera; and RHD zygosity testing.

CONCLUSION

Since around 2000, molecular tests for blood groups have been widely offered as a routine service. Many samples are shipped to reference laboratories in the German-speaking countries with the specific request for such testing. The advent of Conformité Européenne (CE)-labeled test kits renders it technically and legally possible, within the specifications of the CE-certification process for in vitro diagnostic devices in the European Union, to replace several blood group serology tasks by genotyping.