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 nonsense variant in RHAG underlies a Nordic Rhnull phenotype

BACKGROUND AND OBJECTIVES

The extremely rare Rhnull phenotype is characterized by the absence of all Rh antigens on erythrocytes. It is divided into the regulator and amorph types based on the underlying genetic background. The more common regulator type depends on critical variants silencing RHAG, which encodes RhAG glycoprotein, necessary for RhD/RhCE expression. Rhnull cells have altered expression of glycophorin B and LW glycoprotein.

MATERIALS AND METHODS

Four unrelated Rhnull individuals were investigated. Serological testing was performed according to standard blood bank practice. RHD/RHCE and S/s allele-specific Polymerase chain reaction (PCR) genotyping was done on genomic DNA using in-house PCR assays. RHAG, and in some cases also RHD/RHCE, were sequenced. Initial s phenotyping results triggered additional serological investigation.

RESULTS

Anti-Rh29 was identified in all four individuals. Extended typing with anti-S and anti-s showed that the three samples predicted to type as s+ failed to react with 2 of 5 anti-s. Sequence analysis of all 10 RHAG exons and the immediate intron/exon boundaries revealed a single nucleotide variant in the 3'-end of intron 6, c.946 -2a>g in all samples. RHD/RHCE showed no alterations.

CONCLUSION

A novel Nordic Rhnull allele was identified. In addition, it was shown that s+ Rhnull red blood cells are not only U- but also have qualitative changes in their s antigen expression.

SID: a new carbohydrate blood group system based on a well-characterized but still mysterious antigen of great pathophysiologic interest

The high-prevalence blood group antigen, Sd^a, had been puzzling blood bankers and transfusionists for at least a decade when it was reported in 1967. The characteristic mix of agglutinates and free red blood cells (RBCs), caused by anti-Sd^a, is seen with the RBCs from 90 percent of individuals of European descent. However, only 2-4 percent of individuals are truly Sd(a-) and may produce anti-Sd^a. The antibodies, generally considered insignificant, may cause hemolytic transfusion reactions with high-expressing Sd(a+) RBCs (e.g., the unusual Cad phenotype, which can also be polyagglutinable). The Sd^a glycan, GalNAcβ1-4(NeuAcα2-3)Gal-R, is produced in the gastrointestinal and urinary systems, while its origin on RBCs is more controversial. According to current theory, Sd^a is likely to be passively adsorbed in low amounts, except in Cad individuals, where it has been found on erythroid proteins and at higher levels. The long-standing hypothesis that B4GALNT2 encodes the Sd^a synthase was confirmed in 2019, since homozygosity for a variant allele with rs7224888:C produces a non-functional enzyme associated with most cases of the Sd(a-) phenotype. Thereby, the SID blood group system was acknowledged as number 038 by the International Society of Blood Transfusion. Although the genetic background of Sd(a-) was settled, questions remain. The genetic background of the Cad phenotype has not yet been determined, and the source of the RBC-carried Sd^a is unknown. Furthermore, the interest of Sd^a stretches beyond transfusion medicine. Some tantalizing examples are lowered antigen levels in malignant tissue compared with normal tissue and interference with infectious agents like Escherichia coli, influenza virus, and malaria parasites.

International Society of Blood Transfusion Working Party on Red Cell Immunogenetics and Blood Group Terminology Report of Basel and three virtual business meetings: Update on blood group systems

BACKGROUND AND OBJECTIVES

Under the ISBT, the Working Party (WP) for Red Cell Immunogenetics and Blood Group Terminology is charged with ratifying blood group systems, antigens and alleles. This report presents the outcomes from four WP business meetings, one located in Basel in 2019 and three held as virtual meetings during the COVID-19 pandemic in 2020 and 2021.

MATERIALS AND METHODS

As in previous meetings, matters pertaining to blood group antigen nomenclature were discussed. New blood group systems and antigens were approved and named according to the serologic, genetic, biochemical and cell biological evidence presented.

RESULTS

Seven new blood group systems, KANNO (defined numerically as ISBT 037), SID (038), CTL2 (039), PEL (040), MAM (041), EMM (042) and ABCC1 (043) were ratified. Two (039 and 043) were de novo discoveries, and the remainder comprised reported antigens where the causal genes were previously unknown. A further 15 blood group antigens were added to the existing blood group systems: MNS (002), RH (004), LU (005), DI (010), SC (013), GE (020), KN (022), JMH (026) and RHAG (030).

CONCLUSION

The ISBT now recognizes 378 antigens, of which 345 are clustered within 43 blood group systems while 33 still have an unknown genetic basis. The ongoing discovery of new blood group systems and antigens underscores the diverse and complex biology of the red cell membrane. The WP continues to update the blood group antigen tables and the allele nomenclature tables. These can be found on the ISBT website (http://www.isbtweb.org/working-parties/red-cell-immunogenetics-and-blood-group-terminology/).

Noninvasive Prenatal Testing in Immunohematology-Clinical, Technical and Ethical Considerations

Hemolytic disease of the fetus and newborn (HDFN), as well as fetal and neonatal alloimmune thrombocytopenia (FNAIT), represent two important disease entities that are caused by maternal IgG antibodies directed against nonmaternally inherited antigens on the fetal blood cells. These antibodies are most frequently directed against the RhD antigen on red blood cells (RBCs) or the human platelet antigen 1a (HPA-1a) on platelets. For optimal management of pregnancies where HDFN or FNAIT is suspected, it is essential to determine the RhD or the HPA-1a type of the fetus. Noninvasive fetal RhD typing is also relevant for identifying which RhD-negative pregnant women should receive antenatal RhD prophylaxis. In this review, we will give an overview of the clinical indications and technical challenges related to the noninvasive analysis of fetal RBCs or platelet types. In addition, we will discuss the ethical implications associated with the routine administration of antenatal RhD to all pregnant RhD-negative women and likewise the ethical challenges related to making clinical decisions concerning the mother that have been based on samples collected from the (presumptive) father, which is a common practice when determining the risk of FNAIT.

Glycoproteomic and Phenotypic Elucidation of B4GALNT2 Expression Variants in the SID Histo-Blood Group System

The Sda histo-blood group antigen (GalNAcβ1-4(NeuAcα2-3)Galβ-R) is implicated in various infections and constitutes a potential biomarker for colon cancer. Sd(a−) individuals (2−4% of Europeans) may produce anti-Sda, which can lead to incompatible blood transfusions, especially if donors with the high-expressing Sd(a++)/Cad phenotype are involved. We previously reported the association of B4GALNT2 mutations with Sd(a−), which established the SID blood-group system. The present study provides causal proof underpinning this correlation. Sd(a−) HEK293 cells were transfected with different B4GALNT2 constructs and evaluated by immunostaining and glycoproteomics. The predominant SIDnull candidate allele with rs7224888:T>C (p.Cys406Arg) abolished Sda synthesis, while this antigen was detectable as N- or O-glycans on glycoproteins following transfection of wildtype B4GALNT2. Surprisingly, two rare missense variants, rs148441237:A>G and rs61743617:C>T, found in a Sd(a−) compound heterozygote, gave results similar to wildtype. To elucidate on whether Sd(a++)/Cad also depends on B4GALNT2 alterations, this gene was sequenced in five individuals. No Cad-specific changes were identified, but a detailed erythroid Cad glycoprotein profile was obtained, especially for glycophorin-A (GLPA) O-glycosylation, equilibrative nucleoside transporter 1 (S29A1) O-glycosylation, and band 3 anion transport protein (B3AT) N-glycosylation. In conclusion, the p.Cys406Arg β4GalNAc-T2 variant causes Sda-deficiency in humans, while the enigmatic Cad phenotype remains unresolved, albeit further characterized.

Recommendation for validation and quality assurance of non-invasive prenatal testing for foetal blood groups and implications for IVD risk classification according to EU regulations

BACKGROUND AND OBJECTIVES

Non-invasive assays for predicting foetal blood group status in pregnancy serve as valuable clinical tools in the management of pregnancies at risk of detrimental consequences due to blood group antigen incompatibility. To secure clinical applicability, assays for non-invasive prenatal testing of foetal blood groups need to follow strict rules for validation and quality assurance. Here, we present a multi-national position paper with specific recommendations for validation and quality assurance for such assays and discuss their risk classification according to EU regulations.

MATERIALS AND METHODS

We reviewed the literature covering validation for in-vitro diagnostic (IVD) assays in general and for non-invasive foetal RHD genotyping in particular. Recommendations were based on the result of discussions between co-authors.

RESULTS

In relation to Annex VIII of the In-Vitro-Diagnostic Medical Device Regulation 2017/746 of the European Parliament and the Council, assays for non-invasive prenatal testing of foetal blood groups are risk class D devices. In our opinion, screening for targeted anti-D prophylaxis for non-immunized RhD negative women should be placed under risk class C. To ensure high quality of non-invasive foetal blood group assays within and beyond the European Union, we present specific recommendations for validation and quality assurance in terms of analytical detection limit, range and linearity, precision, robustness, pre-analytics and use of controls in routine testing. With respect to immunized women, different requirements for validation and IVD risk classification are discussed.

CONCLUSION

These recommendations should be followed to ensure appropriate assay performance and applicability for clinical use of both commercial and in-house assays.

A novel ABO allele with a 21-bp duplication identified in two unrelated European individuals with weak A expression

OBJECTIVES

To carry out genetic and serological analyses of a Swiss blood donor and a Danish patient carrying an aberrant ABO phenotype with weak A expression.

BACKGROUND

ABO is the most clinically important blood group system but also one of the most complex. The system antigens are determined by carbohydrate structures generated by A and B glycosyltransferases encoded by the ABO gene. Genetic variants of ABO may encode a glycosyltransferase with reduced activity, leading to weak expression of A antigen.

METHODS

Samples from two individuals were examined using genetic testing and extended immunohaematological evaluation, including standard serological methods, flow cytometry and analysis of plasma glycosyltransferase activity.

RESULTS

Both individuals were serologically determined to be A_weak B. Genetic testing revealed that both were heterozygous for a novel ABO*A1.01-like allele with an in-frame duplication of 21 nucleotides in exon 7 (c.543_563dup), leading to the insertion of seven amino acids (QDVSMRR). Flow cytometric testing of native red blood cells (RBCs) showed very weak A antigen expression. This was in accordance with the enzyme activity test.

CONCLUSION

In summary, we describe a novel A allele with a duplication of 21 nucleotides in exon 7 that significantly decreases the enzyme activity and leads to very weak expression of A antigen. (200 words).

The P1PK blood group system: revisited and resolved

This update on the P1PK blood group system (Hellberg Å, Westman JS, Thuresson B, Olsson ML. P1PK: the blood group system that changed its name and expanded. Immunohematology 2013;29:25-33) provides recent findings concerning the P1PK blood group system that have both challenged and confirmed old theories. The glycosphingolipids can no longer be considered the sole carriers of the antigens in this system because the P1 antigen has been detected on human red blood cell glycoproteins. New indications suggest that P1Pk synthase activity truly depends on the DXD motif, and the genetic background and molecular mechanism behind the common P1 and P2 phenotypes were found to depend on transcriptional regulation. Transcription factors bind the P1 allele selectively to a motif around rs5751348 in a regulatory region of A4GALT, which enhances transcription of the gene. Nonetheless, unexplained differences in antigen expression between individuals remain.

Missense mutations in the C-terminal portion of the B4GALNT2-encoded glycosyltransferase underlying the Sd(a-) phenotype

Sd^a is a high-frequency carbohydrate histo-blood group antigen, GalNAcβ1-4(NeuAcα2-3)Galβ, implicated in pathogen invasion, cancer, xenotransplantation and transfusion medicine. Complete lack of this glycan epitope results in the Sd(a−) phenotype observed in 4% of individuals who may produce anti-Sd^a. A candidate gene (B4GALNT2), encoding a Sd^a-synthesizing β-1,4-N-acetylgalactosaminyltransferase (β4GalNAc-T2), was cloned in 2003 but the genetic basis of human Sd^a deficiency was never elucidated. Experimental and bioinformatic approaches were used to identify and characterize B4GALNT2 variants in nine Sd(a−) individuals. Homozygosity for rs7224888:T > C dominated the cohort (n = 6) and causes p.Cys466Arg, which targets a highly conserved residue located in the enzymatically active domain and is judged deleterious to β4GalNAc-T2. Its allele frequency was 0.10–0.12 in different cohorts. A Sd(a−) compound heterozygote combined rs7224888:T > C with a splice-site mutation, rs72835417:G > A, predicted to alter splicing and occurred at a frequency of 0.11–0.12. Another compound heterozygote had two rare nonsynonymous variants, rs148441237:A > G (p.Gln436Arg) and rs61743617:C > T (p.Arg523Trp), in trans. One sample displayed no differences compared to Sd(a+). When investigating linkage disequilibrium between B4GALNT2 variants, we noted a 32-kb block spanning intron 9 to the intergenic region downstream of B4GALNT2. This block includes RP11-708H21.4, a long non-coding RNA recently reported to promote tumorigenesis and poor prognosis in colon cancer. The expression patterns of B4GALNT2 and RP11-708H21.4 correlated extremely well in >1000 cancer cell lines. In summary, we identified a connection between variants of the cancer-associated B4GALNT2 gene and Sd^a, thereby establishing a new blood group system and opening up for the possibility to predict Sd(a+) and Sd(a‒) phenotypes by genotyping.

An update on the GLOB blood group system (and former GLOB collection)

The main change that has occurred in the GLOB blood group system since the GLOB review published in this journal in 2013 is the addition of an antigen. The high-prevalence PX2 antigen, originally recognized as the x2 glycosphingolipid, is expressed on red blood cells of most individuals and is elevated in the rare PP1Pk-negative p blood group phenotype. P synthase, encoded by B3GALNT1, was found to elongate paragloboside to PX2 by adding the terminal β3GalNAc moiety. Hence, PX2 was moved from the GLOB collection to the GLOB system. The presence of naturally-occurring anti-PX2 was noted in P1k and P2k individuals exhibiting nonfunctional P synthase. Although the clinical significance of this specificity remains unclear, a recommendation to avoid transfusing Pk patients with p phenotype blood has been made. Currently, 13 mutations at the highly conserved B3GALNT1 locus have been found to abolish P synthase function and are recognized as null alleles by the International Society of Blood Transfusion. A new allele with a missense mutation but resulting in normal expression of P has been assigned GLOB*02. Finally, the GLOB collection was made obsolete after the move of LKE antigen to the 901 series.

A novel RHCE*02 allele, containing the single-nucleotide change c.460A>G, encodes weakened expression of C and e antigens

We report a novel RHCE*02 allele in a Swedish blood donor that is characterized by the change c.460A>G (Arg154Gly). The blood donor's red blood cells showed variable reactivity with different monoclonal anti-C and anti-e and antigen strength was markedly weakened. We believe that these changes represent both a quantitative and qualitative alteration of the antigens encoded by this allele.

Identification of the Molecular and Genetic Basis of PX2, a Glycosphingolipid Blood Group Antigen Lacking on Globoside-deficient Erythrocytes

The x2 glycosphingolipid is expressed on erythrocytes from individuals of all common blood group phenotypes and elevated on cells of the rare P/P1/P(k)-negative p blood group phenotype. Globoside or P antigen is synthesized by UDP-N-acetylgalactosamine:globotriaosyl-ceramide 3-β-N-acetylgalactosaminyltransferase encoded by B3GALNT1. It is the most abundant non-acid glycosphingolipid on erythrocytes and displays the same terminal disaccharide, GalNAcβ3Gal, as x2. We encountered a patient with mutations in B3GALNT1 causing the rare P-deficient P1 (k) phenotype and whose pretransfusion plasma was unexpectedly incompatible with p erythrocytes. The same phenomenon was also noted in seven other unrelated P-deficient individuals. Thin-layer chromatography, mass spectrometry, and flow cytometry were used to show that the naturally occurring antibodies made by p individuals recognize x2 and sialylated forms of x2, whereas x2 is lacking on P-deficient erythrocytes. Overexpression of B3GALNT1 resulted in synthesis of both P and x2. Knockdown experiments with siRNA against B3GALNT1 diminished x2 levels. We conclude that x2 fulfills blood group criteria and is synthesized by UDP-N-acetylgalactosamine: globotriaosylceramide 3-β-N-acetylgalactosaminyltransferase. Based on this linkage, we proposed that x2 joins P in the GLOB blood group system (ISBT 028) and is renamed PX2 (GLOB2). Thus, in the absence of a functional P synthase, neither P nor PX2 are formed. As a consequence, naturally occurring anti-P and anti-PX2 can be made. Until the clinical significance of anti-PX2 is known, we also recommend that rare P1 (k) or P2 (k) erythrocyte units are preferentially selected for transfusion to P(k) patients because p erythrocytes may pose a risk for hemolytic transfusion reactions due to their elevated PX2 levels.

Two previously proposed P1/P2-differentiating and nine novel polymorphisms at the A4GALT (Pk) locus do not correlate with the presence of the P1 blood group antigen

Background

The molecular genetics of the P blood group system and the absence of P1 antigen in the p phenotype are still enigmatic. One theory proposes that the same gene encodes for both the P1 and P^k glycosyltransferases, but no polymorphisms in the coding region of the P^k gene explain the P₁/P₂ phenotypes. We investigated the potential regulatory regions up- and downstream of the A4GALT (P^k) gene exons.

Results

P₁ (n = 18) and P₂ (n = 9) samples from donors of mainly Swedish descent were analysed by direct sequencing of PCR-amplified 5'- and 3'-fragments surrounding the P^k coding region. Seventy-eight P₁ and P₂ samples were investigated with PCR using allele-specific primers (ASP) for two polymorphisms previously proposed as P₂-related genetic markers (-551_-550insC, -160A>G). Haplotype analysis of single nucleotide polymorphisms was also performed with PCR-ASP. In ~1.5 kbp of the 3'-untranslated region one new insertion and four new substitutions compared to a GenBank sequence (AL049757) were found. In addition to the polymorphisms at positions -550 and -160, one insertion, two deletions and one substitution were found in ~1.0 kbp of the 5'-upstream region. All 20 P₂ samples investigated with PCR-ASP were homozygous for -550insC. However, so were 18 of the 58 P₁ samples investigated. Both the 20 P₂ and the 18 P₁ samples were also homozygous for -160G.

Conclusion

The proposed P₂-specific polymorphisms, -551_-550insC and -160G, found in P₂ samples in a Japanese study were found here in homozygous form in both P₁ and P₂ donors. Since P² is the null allele in the P blood group system it is difficult to envision how these mutations would cause the P₂ phenotype. None of the novel polymorphisms reported in this study correlated with P₁/P₂ status and the P1/p mystery remains unsolved.

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.