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

Serology and molecular genetic analysis of two unrelated individuals with the same novel CisAB blood type

OBJECTIVE

This study aims to report two unrelated individuals with the same novel CisAB blood type and confirm this rare blood type using a comprehensive approach that combines serological and molecular biology techniques.

METHODS

Peripheral blood samples were collected from two patients and their family members. ABO blood typing and antibody detection were performed using conventional tube methods. Molecular biology techniques were employed to amplify and sequence the 6th and 7th exons of the ABO gene, with reference to gene mutation databases provided by NCBI and ISBT.

RESULTS

The genotypes of the two unrelated individuals were identical and were confirmed as a new genotype through ISBT gene database comparison. Serological testing results showed different antigen reaction patterns, especially in terms of reverse typing. Gene sequencing identified a series of mutation points, and both unrelated individuals and one of their daughters had mutations at 297 A>G, 526 C>G, 657 C>T, 703 G>A, 803 G>C, and 930 G>A. According to the comprehensive results from The Blood Group Antigen Gene Mutation Database provided by NCBI, the genotype was determined as Bw37. However, based on the results from Names for ABO (ISBT 001) blood group alleles v1.1 171023, the sequencing results indicated a novel mutation combination not found in the ISBT database. Considering the serological reactions of all three individuals, the final determination was CisAB.

CONCLUSIONS

This study confirmed the novel CisAB blood type in two individuals through the comprehensive application of serology and molecular biology techniques. The identified gene mutation points were not recorded in known databases, emphasizing the uniqueness of CisAB blood types. This research provides important insights into the genetic basis of ABO subtypes and the characteristics of CisAB blood types, and the relevant results have been submitted to the ISBT website for further research.

One novel single nucleotide polymorphism c.424A>G on A1.02 allele in ABO glycosyltransferases leads to Aweak phenotype

BACKGROUND

The dysfunction of the ABO glycosyltransferase (GT) enzyme, which is caused by mutations in the ABO gene, can lead to weak ABO phenotypes. In this study, we have discovered a novel weak ABO subgroup allele and investigated the underlying mechanism to causing its Aweak phenotype.

MATERIALS AND METHODS

The ABO phenotyping and genotyping were performed by serological studies and direct DNA sequencing of ABO gene. The role of the novel single nucleotide polymorphism (SNP) was evaluated by 3D model, predicting protein structure changes, and in vitro expression assay. The total glycosyltransferase transfer capacity in supernatant of transfected cells was examined.

RESULTS

The results of serological showed the subject was Aweak phenotype. A novel SNP c.424A > G (p. M142V) based on ABO*A1.02 was identified, and the genotype of the subject was AW-var/O.01 according to the gene analysis. In silico analysis showed that the SNP c.424A > G on the A allele may change the local conformation by damaging the hydrogen bonds and reduce the stability of GT. In vitro expression study showed that SNP p.M142V impaired H to A antigen conversion, although it did not affect the generation of A glycosyltransferase (GTA).

CONCLUSIONS

One novel AW allele was identified and the SNP c.424A > G (p.M142V) can cause the Aweak phenotype through damaging the hydrogen bonds and reducing stability of the GTA.

B subgroup detection in a small hospital transfusion service

The ABO blood group system includes phenotypes, or subgroups, that differ in the amount of A and B antigens present on the red blood cells (RBCs). These subgroups also differ in the A, B, or H substances present in secretions (for individuals who have the secretor phenotype). B subgroups are very rare and are less frequently reported than A subgroups. Usually, B subgroups are discovered during serologic testing when there is a discrepancy between RBC and serum grouping results. Subgroups of B are usually identified by a reference laboratory using molecular and adsorption-elution methods. This report details a case of a young, healthy, pregnant woman with a B subgroup detected by a small transfusion service using adsorption-elution methods. Serology and genotyping of the ABO gene was performed at a reference laboratory where the serology was consistent with a B subgroup, but no changes were identified in ABO gene sequencing. It is important to correctly identify B subgroups in donors and recipients to help resolve ABO discrepancies and potentially prevent ABO incompatibility in blood transfusion, thus minimizing transfusion reactions.

The ABO blood group system includes phenotypes, or subgroups, that differ in the amount of A and B antigens present on the red blood cells (RBCs). These subgroups also differ in the A, B, or H substances present in secretions (for individuals who have the secretor phenotype). B subgroups are very rare and are less frequently reported than A subgroups. Usually, B subgroups are discovered during serologic testing when there is a discrepancy between RBC and serum grouping results. Subgroups of B are usually identified by a reference laboratory using molecular and adsorption-elution methods. This report details a case of a young, healthy, pregnant woman with a B subgroup detected by a small transfusion service using adsorption-elution methods. Serology and genotyping of the ABO gene was performed at a reference laboratory where the serology was consistent with a B subgroup, but no changes were identified in ABO gene sequencing. It is important to correctly identify B subgroups in donors and recipients to help resolve ABO discrepancies and potentially prevent ABO incompatibility in blood transfusion, thus minimizing transfusion reactions.

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.

Two novel mutations p. L319V and p. L91P in ABO glycosyltransferases lead to Ael and Bel phenotypes

BACKGROUND

Mutations of the ABO gene may cause the dysfunction of ABO glycosyltransferase (GT) that can result in weak ABO phenotypes. Here, we identified two novel weak ABO subgroup alleles and explored their mechanisms that caused A_el and B_el phenotypes.

MATERIALS AND METHODS

The ABO phenotyping and genotyping were performed by serological studies and direct DNA sequencing of the ABO gene. The role of the novel mutations were evaluated by a three-dimensional model, predicting protein structure changes, and in vitro expression assay. The total glycosyltransferase transfer capacity in supernatant of transfected cells was examined.

RESULTS

We identified a mutation c. 955C>G (p. L319V) of A allele in an A_el subject and a mutation c. 272T>C (p. L91P) of B allele in a B_el subject. In silico analysis showed that the mutation p. L319V of the A allele and p. L91P of the B allele may change the local conformation of GT and impair the catalysis of H to A or B antigen conversion. In vitro expression study showed that mutation p. L319V impaired H to A antigen conversion, although it did not affect the expression of glycosyltransferase A.

CONCLUSIONS

Two novel "el"-type ABO subgroup alleles were identified. Both of the two novel mutations can change the local conformation of GTs and reduce protein stability. GTA mutation p. L319V can impair the conversion from H to A antigen and causes the A_el phenotype.

Establishing Blood Group Genotyping to Resolve ABO Discrepancies in Iran

ABO discrepancies are recognized when the reactions obtained in the forward type do not "match" the reactions obtained in the reverse type. Discrepant results are often caused by a variant ABO gene. Molecular analysis is required to confirm the type of subgroups and discrepancy. In this study ABO genotyping was performed on a series of blood donors and patients to determine their definite blood groups. We examined 100 samples with ABO discrepancies from blood donors and patients referred to Tehran Blood Transfusion Center between October 2015 and August 2016. ABO genotyping was performed on all samples with allele specific PCR for differentiation of A, B and O alleles. Exon 6 and 7 of ABO gene were sequenced to confirm the results. The genotyping of donor/patients samples with discrepant results of ABO blood typing consisted of 61 cases of A₂ and A₂B, 3 cases of B 302 and 4 cases of Aw06. Genotyping of 6 samples that had extra antibody in their serum (AB blood group) confirmed the cell type reaction results. 6 samples that had shown a very weak reaction with anti-AB (similar to O blood group) and had no anti-A in their serum were genotyped as O ₁ O ₂. Blood group genotyping laboratory provides an efficient service for evaluation of ABO discrepancies and resolve the problems encountered in serology reactions.

p.R180C mutation of glycosyltransferase B leads to B subgroup, an in vitro and in silico study

BACKGROUND AND OBJECTIVES

Dysfunctional glycosyltransferase A or B may lead to incomplete glycosylation of H antigen and atypical ABO blood group with weak A or B phenotypes, posing challenges for blood typing for transfusion.

MATERIALS AND METHODS

Serological studies and ABO gene analysis were performed. Flow cytometry was performed on HeLa cells transfected glycosyltransferase B expressing plasmids. Agglutination of transfected cells and total glycosyltransferase B transfer capacity were examined. Molecular dynamics simulations were used to explore possible dynamic conformational changes around the binding pocket.

RESULTS

We identified a mutation c.538C>T (p. R180C) of B allele in a Chinese donor and his father with ABw phenotype. In vitro expression study showed that mutation p.R180C, although not affecting expression of glycosyltransferase B, impaired H to B antigen conversion. The in silico analyses found that the residue Arg180 on the internal loop next to the entry of the binding pocket may have its long side chain salt-bridged with the highly flexible C-terminal carboxyl and contribute to the catalysis of H to B antigen conversion.

CONCLUSION

The p.R180C mutation impairs the conversion from H to B antigen and leads to weak B phenotype. Dynamic interaction between Arg180 and C-terminal of glycosyltransferase B may stabilize its binding with UDP-galactose and facilitate H/B antigen conversion.

Protein stability changes of the novel p.Arg180Cys mutant A glycosyltransferase resulted in a weak A phenotype

A novel A subgroup allele (c.538C>T p.Arg180Cys) showing weak A phenotype was found in a 30-year-old Korean woman with ABO discrepancy. Using 3D structural analysis, protein stability prediction and flow cytometric analysis of ABO antigen expression on HeLa cells transfected with plasmids containing the p.Arg180Cys mutant, we found that the Arg180 residue in the loop region of the A glycosyltransferases (GTA) structure plays significant role in stabilizing its closed conformation, which is required for substrate binding and catalysis study.

Structures of complexes of a metal-independent glycosyltransferase GT6 from Bacteroides ovatus with UDP-N-acetylgalactosamine (UDP-GalNAc) and its hydrolysis products

Mammalian members of glycosyltransferase family 6 (GT6) of the CAZy database have a GT-A fold containing a conserved Asp-X-Asp (DXD) sequence that binds an essential metal cofactor. Bacteroides ovatus GT6a represents a GT6 clade found in more than 30 Gram-negative bacteria that is similar in sequence to the catalytic domains of mammalian GT6, but has an Asn(95)-Ala-Asn(97) (NXN) sequence substituted for the DXD motif and metal-independent catalytic activity. Co-crystals of a low activity mutant of BoGT6a (E192Q) with UDP-GalNAc contained protein complexes with intact UDP-GalNAc and two forms with hydrolysis products (UDP plus GalNAc) representing an initial closed complex and later open form primed for product release. Two cationic residues near the C terminus of BoGT6a, Lys(231) and Arg(243), interact with the diphosphate moiety of UDP-GalNAc, but only Lys(231) interacts with the UDP product and may function in leaving group stabilization. The amide group of Asn(95), the first Asn of the NXN motif, interacts with the ribose moiety of the substrate. This metal-independent GT6 resembles its metal-dependent homologs in undergoing conformational changes on binding UDP-GalNAc that arise from structuring the C terminus to cover this substrate. It appears that in the GT6 family, the metal cofactor functions specifically in binding the UDP moiety in the donor substrate and transition state, actions that can be efficiently performed by components of the polypeptide chain.

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.

Development of a laboratory project to determine human ABO genotypes-Limitations lead to further student explorations

A multiplex allele-specific PCR analysis was developed to identify six "common" genotypes: AA, AO, BB, BO, OO, and AB. This project included a pre-laboratory exercise that provided active learning experiences and developed critical thinking skills. This laboratory resulted in many successful analyses, which were verified by student knowledge of their phenotypes. However, the design was found to be deficient for the analysis of variants. The limitations in the original allied-health multiplex design were verified through a student designed problem-based laboratory project in an advanced level biochemistry class. Variants were further analyzed in an undergraduate research project using SSCP analyses. The topic of ABO genotyping provides several opportunities for student-centered explorations.

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.

Glycosyltransferases: structures, functions, and mechanisms

Glycosyltransferases catalyze glycosidic bond formation using sugar donors containing a nucleoside phosphate or a lipid phosphate leaving group. Only two structural folds, GT-A and GT-B, have been identified for the nucleotide sugar-dependent enzymes, but other folds are now appearing for the soluble domains of lipid phosphosugar-dependent glycosyl transferases. Structural and kinetic studies have provided new insights. Inverting glycosyltransferases utilize a direct displacement S(N)2-like mechanism involving an enzymatic base catalyst. Leaving group departure in GT-A fold enzymes is typically facilitated via a coordinated divalent cation, whereas GT-B fold enzymes instead use positively charged side chains and/or hydroxyls and helix dipoles. The mechanism of retaining glycosyltransferases is less clear. The expected two-step double-displacement mechanism is rendered less likely by the lack of conserved architecture in the region where a catalytic nucleophile would be expected. A mechanism involving a short-lived oxocarbenium ion intermediate now seems the most likely, with the leaving phosphate serving as the base.