Molecular genetic analysis for the B(3) allele

Review of ABO Expression and Variations based on Transcriptional Regulation of the ABO Blood Group Gene

Background and Summary

We review the transcriptional regulation of ABO expression and discuss variants in the promoter and erythroid cell-specific regulatory region in individuals with weak ABO phenotypes such as Bm, Am, B3, and A3. We also review the molecular mechanisms responsible for variations in ABO expression in development and disease including the cell type-specific expression of ABO during erythroid cell differentiation, and reduction of A- or B-antigens in cancer cells or on red blood cells in patients with leukemia. Although the relationship between ABO blood group antigens and diseases has been characterized, the physiological significance of the ABO blood group system remains unclear.

Key Messages

This review discusses accumulated knowledge of the ABO gene regulation and potential reasons for conservation of ABO during evolution.

A historical overview of advances in molecular genetic/genomic studies of the ABO blood group system

In 1990, 90 years after the discovery of ABO blood groups by Karl Landsteiner, my research team at the Molecular Biology Laboratory of the now-defunct Biomembrane Institute elucidated the molecular genetic basis of the ABO polymorphism. Henrik Clausen, Head of the Immunology Laboratory, initiated the project by isolating human group A transferase (AT), whose partial amino acid sequence was key to its success. Sen-itiroh Hakomori, the Scientific Director, provided all the institutional support. The characterization started from the 3 major alleles (A1, B, and O), and proceeded to the alleles of A2, A3, Ax and B3 subgroups and also to the cis-AB and B(A) alleles, which specify the expression of A and B antigens by single alleles. In addition to the identification of allele-specific single nucleotide polymorphism (SNP) variations, we also experimentally demonstrated their functional significance in glycosyltransferase activity and sugar specificity of the encoded proteins. Other scientists interested in blood group genes later characterized more than 250 ABO alleles. However, recent developments in next-generation sequencing have enabled the sequencing of millions of human genomes, transitioning from the era of genetics to the era of genomics. As a result, numerous SNP variations have been identified in the coding and noncoding regions of the ABO gene, making ABO one of the most studied loci for human polymorphism. As a tribute to Dr. Hakomori's scientific legacy, a historical overview in molecular genetic/genomic studies of the human ABO gene polymorphism is presented, with an emphasis on early discoveries made at his institute.

Micro-Droplet Platform for Exploring the Mechanism of Mixed Field Agglutination in B3 Subtype

B3 is the most common subtype of blood group B in the Taiwanese population, and most of the B3 individuals in the Taiwanese population have the IVS3 + 5 G > A (rs55852701) gene variation. Additionally, a typical mixed field agglutination is observed when the B3 subtype is tested with anti-B antibody or anti-AB antibody. The molecular biology of the gene variation in the B3 subtype has been identified, however, the mechanism of the mixed field agglutination caused by the type B3 blood samples is still unclear. Therefore, the purpose of this study was to understand the reason for the mixed field agglutination caused by B3. A micro-droplet platform was used to observe the agglutination of type B and type B3 blood samples in different blood sample concentrations, antibody concentrations, and at reaction times. We found that the agglutination reaction in every droplet slowed down with an increase in the dilution ratio of blood sample and antibody, whether type B blood or type B3 blood was used. However, as the reaction time increased, the complete agglutination in the droplet was seen in type B blood, while the mixed field agglutination still occurred in B3 within 1 min. In addition, the degree of agglutination was similar in each droplet, which showed high reproducibility. As a result, we inferred that there are two types of cells in the B3 subtype that simultaneously create a mixed field agglutination, rather than each red blood cell carrying a small amount of antigen, resulting in less agglutination.

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.

Rapid rare ABO blood typing using a single PCR based on a multiplex SNaPshot reaction

BACKGROUND

ABO subgroups would be considered when discrepancies in ABO grouping occur. Serological methods including adsorption-elution test, salivary ABH inhibition test, and anti-A1 (lectin) saline method could be used. However, these serological methods are laboring and obscure. Therefore, reliable and affordable method to assess the ABO subgroups is of particular interest.

METHODS

To solve this problem, the multiplex SNaPshot-based assays were designed to determine rare A and B subgroups. Primers used as probes for determination of rare ABO blood groups known in Taiwanese population were designed. Many ABO subtype samples were used to validate the accuracy and reproducibility of our SNaPshot panel.

RESULTS

A panel of primer probes were successfully designed in determining 8 SNP sites (261, 539, 838, 820, 745, 664, IVS6 +5, and 829 in exon 6 and 7) for A phenotype and 6 SNP sites (261, 796, IVS3 +5, 247, 523, and 502 in exon 2, 6 and 7 and intron 3) for B phenotype. SNaPshot analysis for defining blood group A alleles (A₁, A₂, A₃, A_m and A_el) and blood group B alleles (B₁, B₃, B_w and B_el) was therefore available.

CONCLUSION

SNaPshot analysis could be used in reference laboratories for typing known rare subgroups of A and B without DNA cloning and traditional sequencing. Moreover, this method would help to construct databases of genotyped blood donors, and it potentially plays a role in determining fetal-maternal ABO incompatibility.

An exonic missense mutation c.28G>A is associated with weak B blood group by affecting RNA splicing of the ABO gene

BACKGROUND

The amino acid substitutions caused by ABO gene mutations are usually predicted to impact glycosyltransferase's function or its biosynthesis. Here we report an ABO exonic missense mutation that affects B-antigen expression by decreasing the mRNA level of the ABO gene rather than the amino acid change.

STUDY DESIGN AND METHODS

Serologic studies including plasma total GTB transfer capacity were performed. The exon sequences of the ABO gene were analyzed by Sanger sequencing. B³¹⁰ cDNA with c.28G>A (p.G10R) mutation was expressed in HeLa cells and total GTB transfer capacity in cell supernatant was measured. Flow cytometry was performed on these HeLa cells after transfection, and agglutination of Hela-B_weak cells was also examined. The mRNA of the ABO gene was analyzed by direct sequencing and real-time reverse transcriptase-polymerase chain reaction. A minigene construct was prepared to evaluate the potential of splicing.

RESULTS

While plasma total GTB transfer capacity was undetectable in this B₃ -like individual, the relative percentage of antigen-expressing cells and mean fluorescence index of the B_weak red blood cells (RBCs) were 19 and 14% of normal B RBCs, respectively. There was no significant difference of total GTB transfer capacity in cell supernatant and B-antigen expression on cell surfaces between HeLa cells transfected with B³¹⁰ cDNA and B cDNA. The mRNA expression level of B³¹⁰ in peripheral whole blood was significantly reduced. The amount of splicing is significantly lower in c.28G>A construct compared to that in wild-type construct after transfection in K562 cells.

CONCLUSION

ABO c.28G>A mutation may cause B₃ -like subgroup by affecting RNA splicing of the ABO gene.

Blood group B gene is barely expressed in in vitro erythroid culture of Bm-derived CD34+ cells without an erythroid cell-specific regulatory element

BACKGROUND AND OBJECTIVES

Previously, a weak phenotype Am or Bm was assumed to be caused by a reduction of A or B gene expression in bone marrow cells, but not in mucus-secreting cells. However, ABO expression has not been examined in erythroid progenitor cells of Am or Bm individuals.

MATERIALS AND METHODS

We carried out in vitro erythroid differentiation of CD34(+) cells from peripheral blood of a Bm individual harbouring a 3.0-kb deletion including an erythroid cell-specific regulatory element, named the +5.8-kb site, in intron 1 of the human ABO blood group gene.

RESULTS

During the in vitro differentiation of CD34(+) cells from this Bm individual into erythroid cells, B-antigens were not detectable on the cultured cells by flow cytometric analysis, and allele-specific RT-PCR consistently detected the transcripts from the O allele, but not from the B allele. Moreover, chromatin immunoprecipitation assay demonstrated that both RUNX1 and GATA-2 or GATA-1 were bound to the +5.8-kb site in cultured erythroid cells expressing ABO.

CONCLUSION

It is likely that the +5.8-kb site enhances transcription from the ABO promoter in erythroid cells through binding of RUNX1 and GATA-2 or GATA-1.

A Novel ABO Gene Variant Leads to Discrepant Results in Forward/Reverse and Molecular Blood Grouping

BACKGROUND

Discrepant results in antigen and reverse ABO blood typing are often caused by a variant ABO gene. Molecular analysis can help to characterize such variants. Here, we describe the identification of a novel ABO gene variant in a patient with aberrant ABO phenotype and discrepant genotyping results.

METHODS

A patient with discrepant results in automated forward and reverse ABO phenotyping was further investigated by serological (gel and tube technique) and molecular (commercial and inhouse PCR-SSP, DNA sequencing) methods. A PCR-SSP system was established to screen the novel mutation in 1,820 blood donors.

RESULTS

Standard serological tests confirmed blood group O, however, only anti-B isoagglutinins were present. A monoclonal anti-AB antibody detected very weak agglutination in gel technique. Standard ABO genotyping using PCR-SSP led to discrepant results (O(1)/O(1) or O(1)/A) depending on the test system used. ABO exon re-sequencing identified a novel missense mutation in exon 6 at position 248A>G (Asp83Gly) in the binding region of PCR-SSP primers for the detection of 261G alleles. Blood donors with regular ABO blood groups were all negative for the 248G allele designated Aw34.

CONCLUSION

The novel ABO gene variant Aw34 is associated with very weak A antigen expression and absent anti-A isoagglutinins. The mutation is located in exon 6 close to the O(1)-specific 261G deletion in the binding region of PCR-SSP primers. Presumably, depending on the primer concentration used in commercial ABO genotyping kits, the mutation could lead to a false-negative reaction.

Deletion of the RUNX1 binding site in the erythroid cell-specific regulatory element of the ABO gene in two individuals with the Am phenotype

BACKGROUND AND OBJECTIVES

An erythroid cell-specific regulatory element, referred to as the +5·8-kb site, had been identified in the first intron of the human ABO blood group gene. Subsequent studies revealed that either a 5·8-kb deletion including the +5·8-kb site or disruption of a GATA factor binding motif at the site was present in all Bm and ABm individuals examined. We investigated the molecular mechanism of the Am phenotype, which is analogous to the Bm phenotype.

MATERIALS AND METHODS

Genomic DNAs were prepared from peripheral blood of two Am individuals, and the nucleotide sequences were investigated using PCR and direct sequencing. Electrophoretic mobility shift assay (EMSA) and promoter assay with K562 cells were carried out.

RESULTS

A novel 23-bp nucleotide deletion was found at the +5·8-kb site in both individuals. EMSAs demonstrated binding of the transcription factor RUNX1 to the nucleotides within the deletion. Promoter assays showed that the deletion reduced the transcriptional activity of the +5·8-kb site.

CONCLUSION

Deletion of the 23-bp nucleotides including the RUNX1 binding site decreases transcription of the A allele, resulting in the reduction in A antigen expression in the Am phenotype.

Mutation of the GATA site in the erythroid cell-specific regulatory element of the ABO gene in a Bm subgroup individual

BACKGROUND

The ABO blood group is important in blood transfusion. Recently, an erythroid cell-specific regulatory element has been identified in the first intron of ABO using luciferase reporter assays with K562 cells. The erythroid cell-specific regulatory activity of the element was dependent upon GATA-1 binding. In addition, partial deletion of Intron 1 including the element was observed in genomic DNAs obtained from 111 Bm and ABm individuals, except for one, whereas the deletion was never found among 1005 individuals with the common phenotypes.

STUDY DESIGN AND METHODS

In this study, further investigation was performed to reveal the underlying mechanism responsible for reduction of B antigen expression in the exceptional Bm individual. Peptide nucleic acid-clamping polymerase chain reaction was carried out to amplify the B-related allele, followed by sequence determination. Electrophoretic mobility assays and promoter assays were performed to examine whether a nucleotide substitution reduced the binding of a transcription factor and induced loss of function of the element.

RESULTS

Sequence determination revealed one point mutation of the GATA motif in the element. The electrophoretic mobility shift assays showed that the mutation abolished the binding of GATA transcription factors, and the promoter assays demonstrated complete loss of enhancer activity of the element.

CONCLUSION

These observations suggest that the mutation in the GATA motif of the erythroid-specific regulatory element may diminish the binding of GATA transcription factors and down regulate transcriptional activity of the element on the B allele, leading to reduction of B antigen expression in erythroid lineage cells of the Bm individual.

Expression of ABO blood-group genes is dependent upon an erythroid cell–specific regulatory element that is deleted in persons with the Bm phenotype

The ABO blood group is of great importance in blood transfusion and organ transplantation. However, the mechanisms regulating human ABO gene expression remain obscure. On the basis of DNase I–hypersensitive sites in and upstream of ABO in K562 cells, in the present study, we prepared reporter plasmid constructs including these sites. Subsequent luciferase assays indicated a novel positive regulatory element in intron 1. This element was shown to enhance ABO promoter activity in an erythroid cell–specific manner. Electrophoretic mobility–shift assays demonstrated that it bound to the tissue-restricted transcription factor GATA-1. Mutation of the GATA motifs to abrogate binding of this factor reduced the regulatory activity of the element. Therefore, GATA-1 appears to be involved in the cell-specific activity of the element. Furthermore, we found that a partial deletion in intron 1 involving the element was associated with Bm phenotypes. Therefore, it is plausible that deletion of the erythroid cell–specific regulatory element could down-regulate transcription in the Bm allele, leading to reduction of B-antigen expression in cells of erythroid lineage, but not in mucus-secreting cells. These results support the contention that the enhancer-like element in intron 1 of ABO has a significant function in erythroid cells.

Genetic and mechanistic evaluation for the mixed-field agglutination in B3 blood type with IVS3+5G>A ABO gene mutation

BACKGROUND

The ABO blood type B(3) is the most common B subtype in the Chinese population with a frequency of 1/900. Although IVS3+5G>A (rs55852701) mutation of B gene has been shown to associate with the development of B(3) blood type, genetic and mechanistic evaluation for the unique mixed-field agglutination phenotype has not yet been completely addressed.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we analyzed 16 cases of confirmed B(3) individuals and found that IVS3+5G>A attributes to all cases of B(3). RT-PCR analyses revealed the presence of at least 7 types of aberrant B(3) splicing transcripts with most of the transcripts causing early termination and producing non-functional protein during translation. The splicing transcript without exon 3 that was predicted to generate functional B(3) glycosyltransferase lacking 19 amino acids at the N-terminal segment constituted only 0.9% of the splicing transcripts. Expression of the B(3) cDNA with exon 3 deletion in the K562 erythroleukemia cells revealed that the B(3) glycosyltransferase had only 40% of B(1) activity in converting H antigen to B antigen. Notably, the typical mixed-field agglutination of B(3)-RBCs can be mimicked by adding anti-B antibody to the K562-B(3) cells.

CONCLUSIONS/SIGNIFICANCE

This study thereby demonstrates that both aberrant splicing of B transcripts and the reduced B(3) glycosyltransferase activity contribute to weak B expression and the mixed-field agglutination of B(3), adding to the complexity for the regulatory mechanisms of ABO gene expression.

Molecular basis of blood group expression

Antigen diversity arises from changes at the gene level that range from single nucleotide polymorphisms (SNPs) to intra- and inter-genic exchanges, inversions, insertions, and deletions. Nucleotide changes often result in amino acid difference from the wild-type gene product and with those changes new blood group antigens arise. Alternatively, there is loss of expression altogether, which is deemed the 'null' phenotype. Near complete knowledge of the genetic changes underlying the expression of blood group antigens will lead to the reality that red cell genotyping as a test-of-record. The importance of molecular testing in immunohematology necessitates appropriate training and competency programs to ensure that the highly skilled staff has the appropriate knowledge background. This review summarizes the core mechanisms for gene expression and provides a compilation of the molecular basis for blood group expression.

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.

[A case of ABO*Ael02/O04 genotype with typical phenotype O]

Ael is a rare blood type which has the least amount of A antigen among A subgroups. It can be detected by special tests performed to resolve the discrepancy between red cell and serum typing in routine serological typing. The presence of A antigen on Ael red cell is demonstrable only by adsorption and elution tests. An Ael individual does not secret A substance in the saliva and may have anti-A antibody in the serum which is usually less reactive with the reagent red cells than anti-B antibody. In Korea, Ael02 has been reported more frequently than other Ael alleles. We report a case of Ael02/O04 who presented as typical phenotype O with strong anti-A and anti-B antibodies and no A antigen detected even by adsorption and elution tests. The case has been proved to be Ael02/O04 by direct sequencing analysis. In individuals with history of discrepancies in the results of ABO phenotyping, ABO genotyping is needed for an accurate evaluation of their blood type.

ABO transcript levels in peripheral blood and erythropoietic culture show different allele-related patterns independent of the CBF/NF-Y enhancer motif and multiple novel allele-specific variations in the 5'- and 3'-noncoding regions

BACKGROUND

Mechanisms regulating the ABO gene are unclear, especially in the hematopoietic compartment. The number of 43-bp repeats in the CBF/NF-Y-binding enhancer region is considered to have a major influence on transcription.

STUDY DESIGN AND METHODS

Transcript levels in peripheral blood and in erythropoietic culture of CD34+ cells from marrow donors were measured with TaqMan assays. The 5'-regulatory region and 3'-downstream sequences were investigated to determine if allelic variations occur.

RESULTS

Surprisingly, transcripts from A(1) and A(2) alleles could not be detected in peripheral blood, although transcripts from B/O(1)/O(1v)/O(2) alleles were readily observed. Sequencing of approximately 4 kb upstream and 1.8 kb downstream of the coding region showed multiple novel allele-specific and allele-related motifs. No correlation between these sequence variations and transcript levels was found, however. Contradictory to the results with peripheral blood, in erythropoietic culture of CD34+ cells from healthy marrow donors transcripts from A(1) and A(2) alleles were found at higher levels than transcripts from B/O(1)/O(1v) alleles.

CONCLUSION

These data do not support previous suggestions that nonsense-mutated O(1)/O(1v) transcripts are eliminated first. Furthermore, our results contradict the notion that the number of repeats in the upstream CBF/NF-Y-binding enhancer region, which contains four 43-bp repeats in A(2)/B/O(1)/O(1v) but only one 43-bp unit in A(1)/O(2) alleles, determines the transcription rate. The reason for the remarkable discrepancy between blood and marrow remains to be elucidated.

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.

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.

Identification of a novel A1v-O1v hybrid allele with G829A mutation in a chimeric individual of AelBel phenotype

BACKGROUND

Many A and B suballeles responsible for ABO subgroup formation have been identified. Some of these minor alleles have mutations in the ABO gene coding sequence. Most of these mutations are due to single-nucleotide substitution and lead to amino acid alteration. Several alleles at the ABO locus appear to be caused by crossing over between dissimilar alleles.

STUDY DESIGN AND METHODS

Blood samples were collected from an individual with AelBel phenotype and her family members. Sequencing of the seven ABO exons was performed on these samples. The following was performed for the samples from the AelBel proposita: cloning and sequencing of the genomic DNA of the ABO gene, reverse transcription-polymerase chain reaction (PCR) analysis of cDNA transcript of the ABO gene, sequence-specific priming (SSP)-PCR analysis and direct sequencing of the ABO gene, SSP-PCR DNA typing of generic HLA-ABC and HLA-DRB, and short-tandem repeat (STR)-PCR typing on 15 autosomal, 2 X-chromosomal, and 6 Y-chromosomal loci.

RESULTS

The proposita with AelBel phenotype has blood group chimerism with a major group of A1v-O1v/O1(O01) and a minor group of B(B101)/O1v(O02). Additional haplotypes on HLA-ABC, HLA-DR-B, STR loci, and Y-chromosome STR loci were present on the proposita. The paternal genotype is B(B101)/O1(O01) and the maternal genotype is A1v(A102)/O1v(O02). No other siblings have the A1v-O1v hybrid allele. Parentage was confirmed with the paternity STR-PCR test. Full-length cDNA transcripts of the B(B101) allele and alternately spliced cDNA transcripts of the hybrid A1v-O1v, O(1), and O1v alleles were cloned from the proposita. The A1v-O1v hybrid gene contains two missense mutations: C467T and G829A, resulting in Pro156Leu and Val277Met substitution.

CONCLUSION

Formation of the A1v-O1v hybrid allele appears to result from de novo recombination in the germ line of the mother during meiosis. G829A with Val277Met appears to be responsible for the decrease in A-transferase activity and Ael phenotypic expression in the proposita. The chimeric minor population of B(B101)/O1v is responsible for Bel phenotypic expression in the proposita.

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

BACKGROUND

Weak blood group A and B phenotypes are correlated with ABO glycosyltransferases exhibiting single-amino-acid changes and/or C-terminal modifications.

STUDY DESIGN AND METHODS

A healthy donor diagnosed as having weak A antigen expression and his two children were subjected to extensive ABO typing. HeLa cells were used to transfect ABO expression plasmids.

RESULTS

The donor's red blood cells were type A(weak)B and his serum sample contained weakly reactive anti-A(1) antibodies. A single T>C transition identified at the +2 position of the start codon of an ABO*A101 allele predicted the disruption of this methionine codon. In the transfection studies, a significant reduction of A activity was observed on HeLa cells transfected with a plasmid containing the variant ABO*A allele. Coexpression of the respective antithetical ABO*B101 wild-type construct further reduced cell surface A antigen expression. Similar expression results were obtained with ABO constructs in which the Met(1) start codon and five alternative start sites at codons 20, 26, 43, 53, and 69 had successively been interrupted.

CONCLUSION

The donor's weak blood group A phenotype most likely resulted from expression of an N-truncated A transferase triggered by alternative translation start sites in the transmembrane domain or stem region.

Molecular genetic analysis for Ax phenotype of the ABO blood group system in Chinese

BACKGROUND AND OBJECTIVES

To elucidate the molecular genetic background of the Ax phenotype in the Chinese population.

MATERIALS AND METHODS

The ABO genes of eight Ax phenotype samples, four Ax and four AxB, were amplified by polymerase chain reaction (PCR) and were cloned, along with those of 10 random A(1) Chinese subjects. We analysed the ABO gene transcript structure and the sequences of two exons and one intron at the ABO locus.

RESULTS

Among the four Ax phenotype samples, we identified one Ax02, two Ax03 and one novel Ax allele with the 543G > T mutation in the A102 background. Two of five family members also carry the allele. Of the four AxB phenotypes, one was designated as cis-AB-1/B101; the other three were shown to carry one B allele and one O with the nt261G deletion. The B alleles of the latter three were identical to B101 except for single point mutation at nt700C > G, nt640A > G and nt641T > C, respectively. The novel B101-like alleles were first associated with A(weak)B phenotypes.

CONCLUSIONS

Two ABO*B(A) alleles and an Ax allele clearly differ from all previously reported ABO alleles, suggesting that the molecular genetic background of Ax is heterogeneous in the Chinese population.

Missense mutations outside the catalytic domain of the ABO glycosyltransferase can cause weak blood group A and B phenotypes

BACKGROUND

Only little is known about the impact of amino acid substitutions outside an enzyme's active site on A and B transferase activity.

STUDY DESIGN AND METHODS

A panel of blood group A- and B-specific plasmids containing the six known missense mutations of the coding sequence upstream of exon 6 of the ABO gene were constructed. HeLa cells were used to transfect ABO expression plasmids.

RESULTS

Expression of ABO variants containing single or multiple missense mutations in HeLa cells resulted in a significant decrease in the percentage of antigen-expressing cells (up to 29%) and in mean fluorescence intensity (MFI; up to 50%) compared to transfection with ABO*A101 or ABO*B101. Coexpression of the respective antithetical wild-type construct (ABO*A101 and ABO*B101, respectively) further reduced cell surface expression of variant ABO constructs in regard to the percentage of expressing cells (up to 53% decrease) and MFI (up to 59% decrease).

CONCLUSION

Weak A and B subgroups can arise from transferases with amino acid changes in the N-terminal domain, particularly in AB phenotypes, where normal A1 or B1 glycosyltransferases compete for the same substrates.

What a difference 2 nucleotides make: a short review of ABO genetics

ABO is the most important blood group system for transfusion and solid organ transplantation, but it is only over the past 15 years that the techniques for studying its molecular basis became mainstream. Many of its common and rare alleles are now well characterized and by using various expression systems, their effects on the resulting glycosyltransferases are being appreciated. As progress has been made in genetics and glycobiology, so too do reagents used to routinely type red blood cells in the clinical laboratory evolve. Monoclonal reagents are now widely used. This has created difficulties in nomenclature to describe subtype phenotypes as the names of some of these uncommon phenotypes were based on the red blood cell agglutination pattern using polyclonal reagents. In this brief review a discussion of the wild-type ABO allele and the enzymes it encodes is followed by a description of a selection of unusual and fascinating alleles-some that encode enzymes that create both A and B antigens and others that result from hybridization events. A short section on the techniques of ABO allele investigation describes some of the current methodologies used in both research and clinical laboratories.

The Abantu phenotype in the ABO blood group system is due to a splice-site mutation in a hybrid between a new O1-like allelic lineage and the A2 allele

BACKGROUND AND OBJECTIVES

Many phenotypic variations in the expression of blood group A have been explained by variations in gene structure, but unresolved samples are frequently encountered in the reference laboratory. Among ABO subgroups, A(bantu) has the highest frequency in a specified population. The molecular basis of this phenotype is now described.

MATERIALS AND METHODS

Blood from Black donors phenotyped as A(bantu) was subjected to genomic ABO screening and direct sequencing of polymerase chain reaction (PCR)-amplified ABO exons 1-7 and introns 2-6. Total RNA was extracted and ABO cDNA was synthesized by reverse transcription (RT)-PCR. Control material comprised Black South African, Swedish, Jordanian and Brazilian blood samples with common phenotypes.

RESULTS

Genomic ABO typing indicated the presence of an A(2) allele in each A(bantu) donor, in combination with an O allele. No previously reported mutations associated with weak A or B expression were found. Direct sequencing indicated the common A(2) sequence with a single nucleotide deletion (AGGT>AGT) at the exon 4/intron 4 junction, predicted either to disrupt the reading frame (resulting in a premature stop codon) or to cause erroneous splicing (resulting in the exclusion of exon 4 from the mRNA). O mRNA, but no transcripts from the A(bantu) allele, could be detected. Surprisingly, the splice-site mutation was also found in approximately 5% of O alleles in Black South Africans, but not in other blood donors, or in non-O(1) alleles. Utilizing intron polymorphisms, the A(bantu) allele was shown to be a recombination between a new allelic lineage (O(1bantu)) and A(2), with a cross-over region near exon 5.

CONCLUSION

The A(bantu) phenotype is caused by an O(1bantu)-A(2) hybrid at the ABO locus.

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.

A novel Bvar allele (547 G>A) demonstrates differential expression depending on the co-inherited ABO allele

BACKGROUND AND OBJECTIVES

Genetic analysis of group B donors in Korea was performed.

MATERIALS AND METHODS

Exons 6 and 7 were sequenced in 12 phenotypically B3 donors 6 B3, 6 A1B3.

RESULTS

Consensus sequences all B3 and 2/6 A1B3 donors were present. Four A1B3 donors demonstrated a novel B allele, B(var), in the context of A101/ or A102/B(var) genotypes. Family studies based on an A1B3 donor with the B(var) allele and on another unrelated subject with identical genotype and phenotype revealed B(var)/O01 genotypes with full B-antigen expression.

CONCLUSIONS

B(var) allele is subject to differential expression, depending on the co-inherited ABO allele.

ABO exon and intron analysis in individuals with the AweakB phenotype reveals a novel O1v-A2 hybrid allele that causes four missense mutations in the A transferase

Background

Since the cloning in 1990 of cDNA corresponding to mRNA transcribed at the blood-group ABO locus, polymorphisms due to ethnic and/or phenotypic variations have been reported. Some subgroups have been explained at the molecular level, but unresolved samples are frequently encountered in the reference laboratory.

Results

ABO blood grouping discrepancies were investigated serologically and by ABO genotyping [duplex polymerase-chain-reaction (PCR) – restriction-fragment-length-polymorphism (RFLP) and PCR – allele-specific-primer (ASP) across intron 6] and DNA sequencing of the ABO gene and its proposed regulatory elements. Blood samples from five individuals living in Portugal, Switzerland, Sweden and the USA were analysed. These individuals were confirmed to be of Black ethnic origin and had the unusual A_weakB phenotype but appeared to have the A²B genotype without previously reported mutations associated with weak A or B expression. Sequencing of this A allele (having 467C>T and 1061delC associated with the common A² [A201] allele) revealed three mutations regularly encountered in the O^1v [O02] allele: 106C>T (Val36Phe), 188G>A (Arg63His), 220C>T (Pro74Ser) in exons 3, 4 and 5, respectively. The additional presence of 46G>A (Ala16Thr) was noted, whilst 189C>T that normally accompanies 188G>A in O^1v was missing, as were all O^1v-related mutations in exons 6 and 7 (261delG, 297A>G, 646T>A, 681G>A, 771C>T and 829G>A). On screening other samples, 46G>A was absent, but two new O alleles were found, a Jordanian O¹ and an African O^1v allele having 188G>A but lacking 189C>T. Sequencing of introns 2, 3, 4 and 5 in common alleles (A¹ [A101], A², B [B101], O¹, O^1vand O² [O03]) revealed 7, 12, 17 and 8 polymorphic positions, respectively, suggesting that alleles could be defined by intronic sequences. These polymorphic sites allowed definition of a breakpoint in intron 5 where the O^1v-related sequence was fused with A² to form the new hybrid. Intron 6 has previously been sequenced. Four new mutations were detected in the hybrid allele and these were subsequently also found in intron 6 of A² alleles in other Black African samples.

Conclusions

A novel O^1v-A² hybrid was defined by ABO exon/intron analysis in five unrelated individuals of African descent with the A_weakB blood group phenotype.

The nature of diversity and diversification at the ABO locus

In this study we analyzed the complete genomic sequences, except intron 1, and 2 regulatory regions of 6 common (ABO*A101, ABO*A201, ABO*B101, ABO*O01, ABO*O02, and ABO*O03) and 18 rare ABO alleles, 3 of which were new. This was done by phylogenetic analysis and correlating sequence data with the ABO phenotypes. The study revealed multiple polymorphisms in noncoding regions. The intron-based phylogenetic analysis revealed 5 main lineages: ABO*A, ABO*B, ABO*O01, ABO*O02, and ABO*O03. The genomic sequences of most rare ABO alleles differed slightly from those of the common alleles. Singular mutations or hybrid alleles were most common, but a few exhibited mosaic sequence pattern containing multiple exon and/or intron motifs from other ABO lineages. Thus, both an accumulation of mutations as well as an assortment of the mutations by recombination seems to be responsible for the ABO gene diversity. The prevalence of replacement mutations indicates positive selection for allelic diversity. Phenotype-genotype correlation showed that sequence variations within the complete coding sequence can affect A- and B-antigen expression. All variant ABO*A/B alleles and one new ABO*O03-like allele were associated with weak ABO phenotypes. These findings are suggestive of the requirement of a comprehensive coding sequence database for sequence-based phenotype prediction.

Molecular genetic analysis of the Bel phenotype

BACKGROUND AND OBJECTIVES

In addition to the common ABO phenotypes, numerous phenotypes with a weak expression of the A or B antigens on the red blood cells have been found. This study describes the molecular genetic analysis of the Bel phenotype in Taiwanese individuals.

MATERIALS AND METHODS

The exon 6-7 region of the ABO gene of an individual with the Bel phenotype was amplified by the polymerase chain reaction (PCR), cloned, and the sequences of the exons and their adjacent splice sites were analysed. A PCR-based restriction fragment length polymorphism (RFLP) analysis was designed to detect the 502C>T nucleotide change identified in the Bel allele. Six unrelated individuals with the Bel phenotype were analysed, and samples from 40 randomly selected individuals with the common B phenotype were also assessed.

RESULTS

All six unrelated Taiwanese individuals with the Bel phenotype were shown to possess a B gene with the 502C>T mutation. The mutation was not detected in the general group B population. The 502C>T nucleotide change predicts an amino acid alteration of Arg168-->Trp in the encoded B transferase.

CONCLUSIONS

The results suggest a new molecular basis, a 502C>T missense mutation in the B allele, for the Bel phenotype and an association of the Bel502C>T allele with the Bel phenotype in the Taiwanese population.

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

BACKGROUND

The ABO system includes many variant subgroups. Some of them are difficult to identify serologically, leading to mistyping of blood groups. For example, Bel is often typed as O blood group.

STUDY DESIGN AND METHODS

DNA sequencing and a molecular approach were explored to accurately determine the genotypes of Bel subgroups. Seven Bel blood donors and 106 individuals with other blood groups were analyzed serologically and molecularly.

RESULTS

The serologic results of these seven Bel blood donors showed that their RBCs do not react with anti-B or anti-A,B, and their B antigen was detected by adsorption and elution methods. Sequencing results for exons 6 and 7 of ABO genes showed a new Bel allele with a C>T substitution at nucleotide position 502 in exon 7 of the ABO gene in all seven cases but not in other blood groups. Consequently, an amplification-created restriction site protocol was designed to detect the 502C>T genotype in Bel subgroup cases.

CONCLUSION

A novel 502C>T mutation was found in the Bel subgroup in Taiwan and successfully developed a rapid and accurate molecular protocol to detect this mutation. To our knowledge, the new Bel allele that was found is unique in Taiwanese residents.

Molecular genetic analysis for the Ae1 and A3 alleles

BACKGROUND

In addition to the common ABO phenotypes, numerous phenotypes with a weak expression of the A or B antigens on RBCs have been found. This study describes the molecular genetic analysis of the Ael and the A3 phenotypes.

STUDY DESIGN AND METHODS

The seven-exon regions of the ABO genes of Ael and A3B individuals were amplified by PCR and cloned, and the sequences of the exons and their adjacent splice sites were analyzed. Samples from 30 randomly selected A1 individuals were also assessed.

RESULTS

The A gene with wild-type coding sequence was demonstrated in the Ael propositus, but all the six unrelated Taiwanese people with the Ael or AelB phenotype were shown to possess an A allele with the G-->A mutation at the +5 position of intron 6 (IVS6+5G-->A). RT-PCR analysis showed that the complete A transcript structure was absent in the Ael RNA samples. The A3B individual possessed an A gene with an 838C-->T missense mutation.

CONCLUSION

The results suggest an association of the Ael*IVS6+5G-->A allele with the Ael phenotype in Taiwanese people. The mechanism defining how the Ael*IVS6+5G-->A allele leads to the Ael phenotype awaits elucidation.

Transcription starting from an alternative promoter leads to the expression of the human ABO histo-blood group antigen

BACKGROUND

Using the 5'-rapid amplification of cDNA ends technique with the ex vivo culture of AC133-CD34+ cells, a transcription start site was recently identified approximately 0.7 kb upstream from the transcription start sites previously determined. The transcripts from the alternative starting exon 1a were demonstrated in the cells of both erythroid and epithelial lineages. Because the nucleotide sequence of exon 1a does not contain an ATG codon, we examined whether transcription starting from exon 1a leads to production of a functional glycosyltransferase.

STUDY DESIGN AND METHODS

Stable transfection experiments into the human gastric cancer MKN28 cells were performed using the various A transferase expression plasmids.

RESULTS

Large amounts of A antigens were demonstrated on the cells transfected with the A transferase expression plasmid containing the entire cDNA from exon 1a or the 5'-truncated cDNA leading to the production of the N-truncated protein with deletion of the cytoplasmic tail and a portion of the transmembrane domain. However, negligible amounts of A antigens were observed on the cells transfected with the A transferase expression plasmids containing the 5'-truncated cDNA leading to the production of the N-truncated proteins without the cytoplasmic tail and the transmembrane domain.

CONCLUSION

This study suggests that a functional A transferase could be produced by the transcription from exon 1a.

Systematic analysis of the ABO gene diversity within exons 6 and 7 by PCR screening reveals new ABO alleles

BACKGROUND

Mutations critical for ABO blood group phenotypes have predominantly been found in exons 6 and 7 of the ABO gene, both of which encode the catalytic domain of ABO glycosyltransferase. To design rapid and reliable ABO genotyping assays, a profound knowledge of the prevalent alleles is required and a reliable sequence database needs to be established.

STUDY DESIGN AND METHODS

A PCR screening system was established consisting of 102 different PCRs, each specific for a single nucleotide (nt) variation. The primer mixes were developed to walk from the 5' to the 3' end of exons 6 and 7 of the ABO gene to screen for nt mutations at 50 known polymorphic sites. A total of 109 unrelated individuals with common and rare ABO characteristics were screened. All blood samples in which the PCR results were inconclusive or inconsistent with the ABO phenotypes were subjected to sequence analysis of exons 6 and 7.

RESULTS

The results of PCR screening were conclusive and consistent with the ABO phenotypes in 90 cases. In the remaining 19 cases, PCR screening revealed unusual allele combinations or amplification results that were incompatible with known ABO allele combinations or subgroups predicted by serologic analysis. In these 19 cases, sequencing revealed new ABO alleles (one ABO*Ael allele, one ABO*B(A) allele and two ABO*O alleles) in two individuals with common and seven individuals with variant ABO phenotypes.

CONCLUSION

This PCR screening strategy is an effective tool for obtaining deeper insight into the ABO gene diversity and diversification and may be useful to increase the quality of the ABO sequence database.